Internet Engineering Task Force (IETF) J. Rosenberg
Request for Comments: 5245 jdrosen.net
Obsoletes: 4091, 4092 April 2010
Category: Standards Track
ISSN: 2070-1721
Interactive Connectivity Establishment (ICE):
A Protocol for Network Address Translator (NAT) Traversal for
Offer/Answer Protocols
Abstract
This document describes a protocol for Network Address Translator
(NAT) traversal for UDP-based multimedia sessions established with
the offer/answer model. This protocol is called Interactive
Connectivity Establishment (ICE). ICE makes use of the Session
Traversal Utilities for NAT (STUN) protocol and its extension,
Traversal Using Relay NAT (TURN). ICE can be used by any protocol
utilizing the offer/answer model, such as the Session Initiation
Protocol (SIP).
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5245.
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Copyright Notice
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 6
2. Overview of ICE . . . . . . . . . . . . . . . . . . . . . . . 7
2.1. Gathering Candidate Addresses . . . . . . . . . . . . . . 9
2.2. Connectivity Checks . . . . . . . . . . . . . . . . . . . 11
2.3. Sorting Candidates . . . . . . . . . . . . . . . . . . . 12
2.4. Frozen Candidates . . . . . . . . . . . . . . . . . . . . 13
2.5. Security for Checks . . . . . . . . . . . . . . . . . . . 14
2.6. Concluding ICE . . . . . . . . . . . . . . . . . . . . . 14
2.7. Lite Implementations . . . . . . . . . . . . . . . . . . 16
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 16
4. Sending the Initial Offer . . . . . . . . . . . . . . . . . . 19
4.1. Full Implementation Requirements . . . . . . . . . . . . 19
4.1.1. Gathering Candidates . . . . . . . . . . . . . . . . 19
4.1.1.1. Host Candidates . . . . . . . . . . . . . . . . . 20
4.1.1.2. Server Reflexive and Relayed Candidates . . . . . 20
4.1.1.3. Computing Foundations . . . . . . . . . . . . . . 22
4.1.1.4. Keeping Candidates Alive . . . . . . . . . . . . 22
4.1.2. Prioritizing Candidates . . . . . . . . . . . . . . . 22
4.1.2.1. Recommended Formula . . . . . . . . . . . . . . . 23
4.1.2.2. Guidelines for Choosing Type and Local
Preferences . . . . . . . . . . . . . . . . . . . 23
4.1.3. Eliminating Redundant Candidates . . . . . . . . . . 25
4.1.4. Choosing Default Candidates . . . . . . . . . . . . . 25
4.2. Lite Implementation Requirements . . . . . . . . . . . . 25
4.3. Encoding the SDP . . . . . . . . . . . . . . . . . . . . 26
5. Receiving the Initial Offer . . . . . . . . . . . . . . . . . 28
5.1. Verifying ICE Support . . . . . . . . . . . . . . . . . . 28
5.2. Determining Role . . . . . . . . . . . . . . . . . . . . 29
5.3. Gathering Candidates . . . . . . . . . . . . . . . . . . 30
5.4. Prioritizing Candidates . . . . . . . . . . . . . . . . . 30
5.5. Choosing Default Candidates . . . . . . . . . . . . . . . 31
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5.6. Encoding the SDP . . . . . . . . . . . . . . . . . . . . 31
5.7. Forming the Check Lists . . . . . . . . . . . . . . . . . 31
5.7.1. Forming Candidate Pairs . . . . . . . . . . . . . . . 31
5.7.2. Computing Pair Priority and Ordering Pairs . . . . . 34
5.7.3. Pruning the Pairs . . . . . . . . . . . . . . . . . . 34
5.7.4. Computing States . . . . . . . . . . . . . . . . . . 34
5.8. Scheduling Checks . . . . . . . . . . . . . . . . . . . . 37
6. Receipt of the Initial Answer . . . . . . . . . . . . . . . . 39
6.1. Verifying ICE Support . . . . . . . . . . . . . . . . . . 39
6.2. Determining Role . . . . . . . . . . . . . . . . . . . . 39
6.3. Forming the Check List . . . . . . . . . . . . . . . . . 40
6.4. Performing Ordinary Checks . . . . . . . . . . . . . . . 40
7. Performing Connectivity Checks . . . . . . . . . . . . . . . 40
7.1. STUN Client Procedures . . . . . . . . . . . . . . . . . 40
7.1.1. Creating Permissions for Relayed Candidates . . . . . 40
7.1.2. Sending the Request . . . . . . . . . . . . . . . . . 40
7.1.2.1. PRIORITY and USE-CANDIDATE . . . . . . . . . . . 41
7.1.2.2. ICE-CONTROLLED and ICE-CONTROLLING . . . . . . . 41
7.1.2.3. Forming Credentials . . . . . . . . . . . . . . . 41
7.1.2.4. DiffServ Treatment . . . . . . . . . . . . . . . 42
7.1.3. Processing the Response . . . . . . . . . . . . . . . 42
7.1.3.1. Failure Cases . . . . . . . . . . . . . . . . . . 42
7.1.3.2. Success Cases . . . . . . . . . . . . . . . . . . 43
7.1.3.2.1. Discovering Peer Reflexive Candidates . . . . 43
7.1.3.2.2. Constructing a Valid Pair . . . . . . . . . . 44
7.1.3.2.3. Updating Pair States . . . . . . . . . . . . 45
7.1.3.2.4. Updating the Nominated Flag . . . . . . . . . 46
7.1.3.3. Check List and Timer State Updates . . . . . . . 46
7.2. STUN Server Procedures . . . . . . . . . . . . . . . . . 46
7.2.1. Additional Procedures for Full Implementations . . . 47
7.2.1.1. Detecting and Repairing Role Conflicts . . . . . 47
7.2.1.2. Computing Mapped Address . . . . . . . . . . . . 48
7.2.1.3. Learning Peer Reflexive Candidates . . . . . . . 49
7.2.1.4. Triggered Checks . . . . . . . . . . . . . . . . 49
7.2.1.5. Updating the Nominated Flag . . . . . . . . . . . 50
7.2.2. Additional Procedures for Lite Implementations . . . 51
8. Concluding ICE Processing . . . . . . . . . . . . . . . . . . 51
8.1. Procedures for Full Implementations . . . . . . . . . . . 51
8.1.1. Nominating Pairs . . . . . . . . . . . . . . . . . . 51
8.1.1.1. Regular Nomination . . . . . . . . . . . . . . . 52
8.1.1.2. Aggressive Nomination . . . . . . . . . . . . . . 52
8.1.2. Updating States . . . . . . . . . . . . . . . . . . . 53
8.2. Procedures for Lite Implementations . . . . . . . . . . . 54
8.2.1. Peer Is Full . . . . . . . . . . . . . . . . . . . . 54
8.2.2. Peer Is Lite . . . . . . . . . . . . . . . . . . . . 55
8.3. Freeing Candidates . . . . . . . . . . . . . . . . . . . 56
8.3.1. Full Implementation Procedures . . . . . . . . . . . 56
8.3.2. Lite Implementation Procedures . . . . . . . . . . . 56
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9. Subsequent Offer/Answer Exchanges . . . . . . . . . . . . . . 56
9.1. Generating the Offer . . . . . . . . . . . . . . . . . . 57
9.1.1. Procedures for All Implementations . . . . . . . . . 57
9.1.1.1. ICE Restarts . . . . . . . . . . . . . . . . . . 57
9.1.1.2. Removing a Media Stream . . . . . . . . . . . . . 58
9.1.1.3. Adding a Media Stream . . . . . . . . . . . . . . 58
9.1.2. Procedures for Full Implementations . . . . . . . . . 58
9.1.2.1. Existing Media Streams with ICE Running . . . . . 58
9.1.2.2. Existing Media Streams with ICE Completed . . . . 59
9.1.3. Procedures for Lite Implementations . . . . . . . . . 59
9.1.3.1. Existing Media Streams with ICE Running . . . . . 59
9.1.3.2. Existing Media Streams with ICE Completed . . . . 60
9.2. Receiving the Offer and Generating an Answer . . . . . . 60
9.2.1. Procedures for All Implementations . . . . . . . . . 60
9.2.1.1. Detecting ICE Restart . . . . . . . . . . . . . . 60
9.2.1.2. New Media Stream . . . . . . . . . . . . . . . . 61
9.2.1.3. Removed Media Stream . . . . . . . . . . . . . . 61
9.2.2. Procedures for Full Implementations . . . . . . . . . 61
9.2.2.1. Existing Media Streams with ICE Running and no
remote-candidates . . . . . . . . . . . . . . . . 61
9.2.2.2. Existing Media Streams with ICE Completed and
no remote-candidates . . . . . . . . . . . . . . 61
9.2.2.3. Existing Media Streams and remote-candidates . . 61
9.2.3. Procedures for Lite Implementations . . . . . . . . . 62
9.3. Updating the Check and Valid Lists . . . . . . . . . . . 63
9.3.1. Procedures for Full Implementations . . . . . . . . . 63
9.3.1.1. ICE Restarts . . . . . . . . . . . . . . . . . . 63
9.3.1.2. New Media Stream . . . . . . . . . . . . . . . . 63
9.3.1.3. Removed Media Stream . . . . . . . . . . . . . . 64
9.3.1.4. ICE Continuing for Existing Media Stream . . . . 64
9.3.2. Procedures for Lite Implementations . . . . . . . . . 64
10. Keepalives . . . . . . . . . . . . . . . . . . . . . . . . . 65
11. Media Handling . . . . . . . . . . . . . . . . . . . . . . . 66
11.1. Sending Media . . . . . . . . . . . . . . . . . . . . . . 66
11.1.1. Procedures for Full Implementations . . . . . . . . . 66
11.1.2. Procedures for Lite Implementations . . . . . . . . . 67
11.1.3. Procedures for All Implementations . . . . . . . . . 67
11.2. Receiving Media . . . . . . . . . . . . . . . . . . . . . 67
12. Usage with SIP . . . . . . . . . . . . . . . . . . . . . . . 68
12.1. Latency Guidelines . . . . . . . . . . . . . . . . . . . 68
12.1.1. Offer in INVITE . . . . . . . . . . . . . . . . . . . 68
12.1.2. Offer in Response . . . . . . . . . . . . . . . . . . 70
12.2. SIP Option Tags and Media Feature Tags . . . . . . . . . 70
12.3. Interactions with Forking . . . . . . . . . . . . . . . . 70
12.4. Interactions with Preconditions . . . . . . . . . . . . . 70
12.5. Interactions with Third Party Call Control . . . . . . . 71
13. Relationship with ANAT . . . . . . . . . . . . . . . . . . . 71
14. Extensibility Considerations . . . . . . . . . . . . . . . . 72
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15. Grammar . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
15.1. "candidate" Attribute . . . . . . . . . . . . . . . . . . 73
15.2. "remote-candidates" Attribute . . . . . . . . . . . . . . 75
15.3. "ice-lite" and "ice-mismatch" Attributes . . . . . . . . 75
15.4. "ice-ufrag" and "ice-pwd" Attributes . . . . . . . . . . 76
15.5. "ice-options" Attribute . . . . . . . . . . . . . . . . . 76
16. Setting Ta and RTO . . . . . . . . . . . . . . . . . . . . . 76
16.1. RTP Media Streams . . . . . . . . . . . . . . . . . . . . 77
16.2. Non-RTP Sessions . . . . . . . . . . . . . . . . . . . . 78
17. Example . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
18. Security Considerations . . . . . . . . . . . . . . . . . . . 85
18.1. Attacks on Connectivity Checks . . . . . . . . . . . . . 86
18.2. Attacks on Server Reflexive Address Gathering . . . . . . 88
18.3. Attacks on Relayed Candidate Gathering . . . . . . . . . 89
18.4. Attacks on the Offer/Answer Exchanges . . . . . . . . . . 89
18.5. Insider Attacks . . . . . . . . . . . . . . . . . . . . . 90
18.5.1. The Voice Hammer Attack . . . . . . . . . . . . . . . 90
18.5.2. STUN Amplification Attack . . . . . . . . . . . . . . 90
18.6. Interactions with Application Layer Gateways and SIP . . 91
19. STUN Extensions . . . . . . . . . . . . . . . . . . . . . . . 92
19.1. New Attributes . . . . . . . . . . . . . . . . . . . . . 92
19.2. New Error Response Codes . . . . . . . . . . . . . . . . 93
20. Operational Considerations . . . . . . . . . . . . . . . . . 93
20.1. NAT and Firewall Types . . . . . . . . . . . . . . . . . 93
20.2. Bandwidth Requirements . . . . . . . . . . . . . . . . . 93
20.2.1. STUN and TURN Server Capacity Planning . . . . . . . 93
20.2.2. Gathering and Connectivity Checks . . . . . . . . . . 94
20.2.3. Keepalives . . . . . . . . . . . . . . . . . . . . . 94
20.3. ICE and ICE-lite . . . . . . . . . . . . . . . . . . . . 95
20.4. Troubleshooting and Performance Management . . . . . . . 95
20.5. Endpoint Configuration . . . . . . . . . . . . . . . . . 95
21. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 96
21.1. SDP Attributes . . . . . . . . . . . . . . . . . . . . . 96
21.1.1. candidate Attribute . . . . . . . . . . . . . . . . . 96
21.1.2. remote-candidates Attribute . . . . . . . . . . . . . 96
21.1.3. ice-lite Attribute . . . . . . . . . . . . . . . . . 97
21.1.4. ice-mismatch Attribute . . . . . . . . . . . . . . . 97
21.1.5. ice-pwd Attribute . . . . . . . . . . . . . . . . . . 98
21.1.6. ice-ufrag Attribute . . . . . . . . . . . . . . . . . 98
21.1.7. ice-options Attribute . . . . . . . . . . . . . . . . 98
21.2. STUN Attributes . . . . . . . . . . . . . . . . . . . . . 99
21.3. STUN Error Responses . . . . . . . . . . . . . . . . . . 99
22. IAB Considerations . . . . . . . . . . . . . . . . . . . . . 99
22.1. Problem Definition . . . . . . . . . . . . . . . . . . . 100
22.2. Exit Strategy . . . . . . . . . . . . . . . . . . . . . . 100
22.3. Brittleness Introduced by ICE . . . . . . . . . . . . . . 101
22.4. Requirements for a Long-Term Solution . . . . . . . . . . 102
22.5. Issues with Existing NAPT Boxes . . . . . . . . . . . . . 102
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23. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 102
24. References . . . . . . . . . . . . . . . . . . . . . . . . . 103
24.1. Normative References . . . . . . . . . . . . . . . . . . 103
24.2. Informative References . . . . . . . . . . . . . . . . . 104
Appendix A. Lite and Full Implementations . . . . . . . . . . . 107
Appendix B. Design Motivations . . . . . . . . . . . . . . . . . 108
B.1. Pacing of STUN Transactions . . . . . . . . . . . . . . . 108
B.2. Candidates with Multiple Bases . . . . . . . . . . . . . 109
B.3. Purpose of the and Attributes . . . 111
B.4. Importance of the STUN Username . . . . . . . . . . . . . 111
B.5. The Candidate Pair Priority Formula . . . . . . . . . . . 113
B.6. The remote-candidates Attribute . . . . . . . . . . . . . 113
B.7. Why Are Keepalives Needed? . . . . . . . . . . . . . . . 114
B.8. Why Prefer Peer Reflexive Candidates? . . . . . . . . . . 115
B.9. Why Send an Updated Offer? . . . . . . . . . . . . . . . 115
B.10. Why Are Binding Indications Used for Keepalives? . . . . 115
B.11. Why Is the Conflict Resolution Mechanism Needed? . . . . 116
1. Introduction
RFC 3264 [RFC3264] defines a two-phase exchange of Session
Description Protocol (SDP) messages [RFC4566] for the purposes of
establishment of multimedia sessions. This offer/answer mechanism is
used by protocols such as the Session Initiation Protocol (SIP)
[RFC3261].
Protocols using offer/answer are difficult to operate through Network
Address Translators (NATs). Because their purpose is to establish a
flow of media packets, they tend to carry the IP addresses and ports
of media sources and sinks within their messages, which is known to
be problematic through NAT [RFC3235]. The protocols also seek to
create a media flow directly between participants, so that there is
no application layer intermediary between them. This is done to
reduce media latency, decrease packet loss, and reduce the
operational costs of deploying the application. However, this is
difficult to accomplish through NAT. A full treatment of the reasons
for this is beyond the scope of this specification.
Numerous solutions have been defined for allowing these protocols to
operate through NAT. These include Application Layer Gateways
(ALGs), the Middlebox Control Protocol [RFC3303], the original Simple
Traversal of UDP Through NAT (STUN) [RFC3489] specification, and
Realm Specific IP [RFC3102] [RFC3103] along with session description
extensions needed to make them work, such as the Session Description
Protocol (SDP) [RFC4566] attribute for the Real Time Control Protocol
(RTCP) [RFC3605]. Unfortunately, these techniques all have pros and
cons which, make each one optimal in some network topologies, but a
poor choice in others. The result is that administrators and
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implementors are making assumptions about the topologies of the
networks in which their solutions will be deployed. This introduces
complexity and brittleness into the system. What is needed is a
single solution that is flexible enough to work well in all
situations.
This specification defines Interactive Connectivity Establishment
(ICE) as a technique for NAT traversal for UDP-based media streams
(though ICE can be extended to handle other transport protocols, such
as TCP [ICE-TCP]) established by the offer/answer model. ICE is an
extension to the offer/answer model, and works by including a
multiplicity of IP addresses and ports in SDP offers and answers,
which are then tested for connectivity by peer-to-peer connectivity
checks. The IP addresses and ports included in the SDP and the
connectivity checks are performed using the revised STUN
specification [RFC5389], now renamed to Session Traversal Utilities
for NAT. The new name and new specification reflect its new role as
a tool that is used with other NAT traversal techniques (namely ICE)
rather than a standalone NAT traversal solution, as the original STUN
specification was. ICE also makes use of Traversal Using Relays
around NAT (TURN) [RFC5766], an extension to STUN. Because ICE
exchanges a multiplicity of IP addresses and ports for each media
stream, it also allows for address selection for multihomed and dual-
stack hosts, and for this reason it deprecates RFC 4091 [RFC4091] and
[RFC4092].
2. Overview of ICE
In a typical ICE deployment, we have two endpoints (known as AGENTS
in RFC 3264 terminology) that want to communicate. They are able to
communicate indirectly via some signaling protocol (such as SIP), by
which they can perform an offer/answer exchange of SDP [RFC3264]
messages. Note that ICE is not intended for NAT traversal for SIP,
which is assumed to be provided via another mechanism [RFC5626]. At
the beginning of the ICE process, the agents are ignorant of their
own topologies. In particular, they might or might not be behind a
NAT (or multiple tiers of NATs). ICE allows the agents to discover
enough information about their topologies to potentially find one or
more paths by which they can communicate.
Figure 1 shows a typical environment for ICE deployment. The two
endpoints are labelled L and R (for left and right, which helps
visualize call flows). Both L and R are behind their own respective
NATs though they may not be aware of it. The type of NAT and its
properties are also unknown. Agents L and R are capable of engaging
in an offer/answer exchange by which they can exchange SDP messages,
whose purpose is to set up a media session between L and R.
Typically, this exchange will occur through a SIP server.
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In addition to the agents, a SIP server and NATs, ICE is typically
used in concert with STUN or TURN servers in the network. Each agent
can have its own STUN or TURN server, or they can be the same.
+-------+
| SIP |
+-------+ | Srvr | +-------+
| STUN | | | | STUN |
| Srvr | +-------+ | Srvr |
| | / \ | |
+-------+ / \ +-------+
/ \
/ \
/ \
/ \
/ <- Signaling -> \
/ \
/ \
+--------+ +--------+
| NAT | | NAT |
+--------+ +--------+
/ \
/ \
/ \
+-------+ +-------+
| Agent | | Agent |
| L | | R |
| | | |
+-------+ +-------+
Figure 1: ICE Deployment Scenario
The basic idea behind ICE is as follows: each agent has a variety of
candidate TRANSPORT ADDRESSES (combination of IP address and port for
a particular transport protocol, which is always UDP in this
specification)) it could use to communicate with the other agent.
These might include:
o A transport address on a directly attached network interface
o A translated transport address on the public side of a NAT (a
"server reflexive" address)
o A transport address allocated from a TURN server (a "relayed
address").
Potentially, any of L's candidate transport addresses can be used to
communicate with any of R's candidate transport addresses. In
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practice, however, many combinations will not work. For instance, if
L and R are both behind NATs, their directly attached interface
addresses are unlikely to be able to communicate directly (this is
why ICE is needed, after all!). The purpose of ICE is to discover
which pairs of addresses will work. The way that ICE does this is to
systematically try all possible pairs (in a carefully sorted order)
until it finds one or more that work.
2.1. Gathering Candidate Addresses
In order to execute ICE, an agent has to identify all of its address
candidates. A CANDIDATE is a transport address -- a combination of
IP address and port for a particular transport protocol (with only
UDP specified here). This document defines three types of
candidates, some derived from physical or logical network interfaces,
others discoverable via STUN and TURN. Naturally, one viable
candidate is a transport address obtained directly from a local
interface. Such a candidate is called a HOST CANDIDATE. The local
interface could be ethernet or WiFi, or it could be one that is
obtained through a tunnel mechanism, such as a Virtual Private
Network (VPN) or Mobile IP (MIP). In all cases, such a network
interface appears to the agent as a local interface from which ports
(and thus candidates) can be allocated.
If an agent is multihomed, it obtains a candidate from each IP
address. Depending on the location of the PEER (the other agent in
the session) on the IP network relative to the agent, the agent may
be reachable by the peer through one or more of those IP addresses.
Consider, for example, an agent that has a local IP address on a
private net 10 network (I1), and a second connected to the public
Internet (I2). A candidate from I1 will be directly reachable when
communicating with a peer on the same private net 10 network, while a
candidate from I2 will be directly reachable when communicating with
a peer on the public Internet. Rather than trying to guess which IP
address will work prior to sending an offer, the offering agent
includes both candidates in its offer.
Next, the agent uses STUN or TURN to obtain additional candidates.
These come in two flavors: translated addresses on the public side of
a NAT (SERVER REFLEXIVE CANDIDATES) and addresses on TURN servers
(RELAYED CANDIDATES). When TURN servers are utilized, both types of
candidates are obtained from the TURN server. If only STUN servers
are utilized, only server reflexive candidates are obtained from
them. The relationship of these candidates to the host candidate is
shown in Figure 2. In this figure, both types of candidates are
discovered using TURN. In the figure, the notation X:x means IP
address X and UDP port x.
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To Internet
|
|
| /------------ Relayed
Y:y | / Address
+--------+
| |
| TURN |
| Server |
| |
+--------+
|
|
| /------------ Server
X1':x1'|/ Reflexive
+------------+ Address
| NAT |
+------------+
|
| /------------ Local
X:x |/ Address
+--------+
| |
| Agent |
| |
+--------+
Figure 2: Candidate Relationships
When the agent sends the TURN Allocate request from IP address and
port X:x, the NAT (assuming there is one) will create a binding
X1':x1', mapping this server reflexive candidate to the host
candidate X:x. Outgoing packets sent from the host candidate will be
translated by the NAT to the server reflexive candidate. Incoming
packets sent to the server reflexive candidate will be translated by
the NAT to the host candidate and forwarded to the agent. We call
the host candidate associated with a given server reflexive candidate
the BASE.
Note: "Base" refers to the address an agent sends from for a
particular candidate. Thus, as a degenerate case host candidates
also have a base, but it's the same as the host candidate.
When there are multiple NATs between the agent and the TURN server,
the TURN request will create a binding on each NAT, but only the
outermost server reflexive candidate (the one nearest the TURN
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server) will be discovered by the agent. If the agent is not behind
a NAT, then the base candidate will be the same as the server
reflexive candidate and the server reflexive candidate is redundant
and will be eliminated.
The Allocate request then arrives at the TURN server. The TURN
server allocates a port y from its local IP address Y, and generates
an Allocate response, informing the agent of this relayed candidate.
The TURN server also informs the agent of the server reflexive
candidate, X1':x1' by copying the source transport address of the
Allocate request into the Allocate response. The TURN server acts as
a packet relay, forwarding traffic between L and R. In order to send
traffic to L, R sends traffic to the TURN server at Y:y, and the TURN
server forwards that to X1':x1', which passes through the NAT where
it is mapped to X:x and delivered to L.
When only STUN servers are utilized, the agent sends a STUN Binding
request [RFC5389] to its STUN server. The STUN server will inform
the agent of the server reflexive candidate X1':x1' by copying the
source transport address of the Binding request into the Binding
response.
2.2. Connectivity Checks
Once L has gathered all of its candidates, it orders them in highest
to lowest priority and sends them to R over the signaling channel.
The candidates are carried in attributes in the SDP offer. When R
receives the offer, it performs the same gathering process and
responds with its own list of candidates. At the end of this
process, each agent has a complete list of both its candidates and
its peer's candidates. It pairs them up, resulting in CANDIDATE
PAIRS. To see which pairs work, each agent schedules a series of
CHECKS. Each check is a STUN request/response transaction that the
client will perform on a particular candidate pair by sending a STUN
request from the local candidate to the remote candidate.
The basic principle of the connectivity checks is simple:
1. Sort the candidate pairs in priority order.
2. Send checks on each candidate pair in priority order.
3. Acknowledge checks received from the other agent.
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With both agents performing a check on a candidate pair, the result
is a 4-way handshake:
L R
- -
STUN request -> \ L's
<- STUN response / check
<- STUN request \ R's
STUN response -> / check
Figure 3: Basic Connectivity Check
It is important to note that the STUN requests are sent to and from
the exact same IP addresses and ports that will be used for media
(e.g., RTP and RTCP). Consequently, agents demultiplex STUN and RTP/
RTCP using contents of the packets, rather than the port on which
they are received. Fortunately, this demultiplexing is easy to do,
especially for RTP and RTCP.
Because a STUN Binding request is used for the connectivity check,
the STUN Binding response will contain the agent's translated
transport address on the public side of any NATs between the agent
and its peer. If this transport address is different from other
candidates the agent already learned, it represents a new candidate,
called a PEER REFLEXIVE CANDIDATE, which then gets tested by ICE just
the same as any other candidate.
As an optimization, as soon as R gets L's check message, R schedules
a connectivity check message to be sent to L on the same candidate
pair. This accelerates the process of finding a valid candidate, and
is called a TRIGGERED CHECK.
At the end of this handshake, both L and R know that they can send
(and receive) messages end-to-end in both directions.
2.3. Sorting Candidates
Because the algorithm above searches all candidate pairs, if a
working pair exists it will eventually find it no matter what order
the candidates are tried in. In order to produce faster (and better)
results, the candidates are sorted in a specified order. The
resulting list of sorted candidate pairs is called the CHECK LIST.
The algorithm is described in Section 4.1.2 but follows two general
principles:
o Each agent gives its candidates a numeric priority, which is sent
along with the candidate to the peer.
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o The local and remote priorities are combined so that each agent
has the same ordering for the candidate pairs.
The second property is important for getting ICE to work when there
are NATs in front of L and R. Frequently, NATs will not allow
packets in from a host until the agent behind the NAT has sent a
packet towards that host. Consequently, ICE checks in each direction
will not succeed until both sides have sent a check through their
respective NATs.
The agent works through this check list by sending a STUN request for
the next candidate pair on the list periodically. These are called
ORDINARY CHECKS.
In general, the priority algorithm is designed so that candidates of
similar type get similar priorities and so that more direct routes
(that is, through fewer media relays and through fewer NATs) are
preferred over indirect ones (ones with more media relays and more
NATs). Within those guidelines, however, agents have a fair amount
of discretion about how to tune their algorithms.
2.4. Frozen Candidates
The previous description only addresses the case where the agents
wish to establish a media session with one COMPONENT (a piece of a
media stream requiring a single transport address; a media stream may
require multiple components, each of which has to work for the media
stream as a whole to be work). Typically (e.g., with RTP and RTCP),
the agents actually need to establish connectivity for more than one
flow.
The network properties are likely to be very similar for each
component (especially because RTP and RTCP are sent and received from
the same IP address). It is usually possible to leverage information
from one media component in order to determine the best candidates
for another. ICE does this with a mechanism called "frozen
candidates".
Each candidate is associated with a property called its FOUNDATION.
Two candidates have the same foundation when they are "similar" -- of
the same type and obtained from the same host candidate and STUN
server using the same protocol. Otherwise, their foundation is
different. A candidate pair has a foundation too, which is just the
concatenation of the foundations of its two candidates. Initially,
only the candidate pairs with unique foundations are tested. The
other candidate pairs are marked "frozen". When the connectivity
checks for a candidate pair succeed, the other candidate pairs with
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the same foundation are unfrozen. This avoids repeated checking of
components that are superficially more attractive but in fact are
likely to fail.
While we've described "frozen" here as a separate mechanism for
expository purposes, in fact it is an integral part of ICE and the
ICE prioritization algorithm automatically ensures that the right
candidates are unfrozen and checked in the right order.
2.5. Security for Checks
Because ICE is used to discover which addresses can be used to send
media between two agents, it is important to ensure that the process
cannot be hijacked to send media to the wrong location. Each STUN
connectivity check is covered by a message authentication code (MAC)
computed using a key exchanged in the signaling channel. This MAC
provides message integrity and data origin authentication, thus
stopping an attacker from forging or modifying connectivity check
messages. Furthermore, if the SIP [RFC3261] caller is using ICE, and
their call forks, the ICE exchanges happen independently with each
forked recipient. In such a case, the keys exchanged in the
signaling help associate each ICE exchange with each forked
recipient.
2.6. Concluding ICE
ICE checks are performed in a specific sequence, so that high-
priority candidate pairs are checked first, followed by lower-
priority ones. One way to conclude ICE is to declare victory as soon
as a check for each component of each media stream completes
successfully. Indeed, this is a reasonable algorithm, and details
for it are provided below. However, it is possible that a packet
loss will cause a higher-priority check to take longer to complete.
In that case, allowing ICE to run a little longer might produce
better results. More fundamentally, however, the prioritization
defined by this specification may not yield "optimal" results. As an
example, if the aim is to select low-latency media paths, usage of a
relay is a hint that latencies may be higher, but it is nothing more
than a hint. An actual round-trip time (RTT) measurement could be
made, and it might demonstrate that a pair with lower priority is
actually better than one with higher priority.
Consequently, ICE assigns one of the agents in the role of the
CONTROLLING AGENT, and the other of the CONTROLLED AGENT. The
controlling agent gets to nominate which candidate pairs will get
used for media amongst the ones that are valid. It can do this in
one of two ways -- using REGULAR NOMINATION or AGGRESSIVE NOMINATION.
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With regular nomination, the controlling agent lets the checks
continue until at least one valid candidate pair for each media
stream is found. Then, it picks amongst those that are valid, and
sends a second STUN request on its NOMINATED candidate pair, but this
time with a flag set to tell the peer that this pair has been
nominated for use. This is shown in Figure 4.
L R
- -
STUN request -> \ L's
<- STUN response / check
<- STUN request \ R's
STUN response -> / check
STUN request + flag -> \ L's
<- STUN response / check
Figure 4: Regular Nomination
Once the STUN transaction with the flag completes, both sides cancel
any future checks for that media stream. ICE will now send media
using this pair. The pair an ICE agent is using for media is called
the SELECTED PAIR.
In aggressive nomination, the controlling agent puts the flag in
every STUN request it sends. This way, once the first check
succeeds, ICE processing is complete for that media stream and the
controlling agent doesn't have to send a second STUN request. The
selected pair will be the highest-priority valid pair whose check
succeeded. Aggressive nomination is faster than regular nomination,
but gives less flexibility. Aggressive nomination is shown in
Figure 5.
L R
- -
STUN request + flag -> \ L's
<- STUN response / check
<- STUN request \ R's
STUN response -> / check
Figure 5: Aggressive Nomination
Once all of the media streams are completed, the controlling endpoint
sends an updated offer if the candidates in the m and c lines for the
media stream (called the DEFAULT CANDIDATES) don't match ICE's
SELECTED CANDIDATES.
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Once ICE is concluded, it can be restarted at any time for one or all
of the media streams by either agent. This is done by sending an
updated offer indicating a restart.
2.7. Lite Implementations
In order for ICE to be used in a call, both agents need to support
it. However, certain agents will always be connected to the public
Internet and have a public IP address at which it can receive packets
from any correspondent. To make it easier for these devices to
support ICE, ICE defines a special type of implementation called LITE
(in contrast to the normal FULL implementation). A lite
implementation doesn't gather candidates; it includes only host
candidates for any media stream. Lite agents do not generate
connectivity checks or run the state machines, though they need to be
able to respond to connectivity checks. When a lite implementation
connects with a full implementation, the full agent takes the role of
the controlling agent, and the lite agent takes on the controlled
role. When two lite implementations connect, no checks are sent.
For guidance on when a lite implementation is appropriate, see the
discussion in Appendix A.
It is important to note that the lite implementation was added to
this specification to provide a stepping stone to full
implementation. Even for devices that are always connected to the
public Internet, a full implementation is preferable if achievable.
3. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [RFC2119].
Readers should be familiar with the terminology defined in the offer/
answer model [RFC3264], STUN [RFC5389], and NAT Behavioral
requirements for UDP [RFC4787].
This specification makes use of the following additional terminology:
Agent: As defined in RFC 3264, an agent is the protocol
implementation involved in the offer/answer exchange. There are
two agents involved in an offer/answer exchange.
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Peer: From the perspective of one of the agents in a session, its
peer is the other agent. Specifically, from the perspective of
the offerer, the peer is the answerer. From the perspective of
the answerer, the peer is the offerer.
Transport Address: The combination of an IP address and transport
protocol (such as UDP or TCP) port.
Candidate: A transport address that is a potential point of contact
for receipt of media. Candidates also have properties -- their
type (server reflexive, relayed or host), priority, foundation,
and base.
Component: A component is a piece of a media stream requiring a
single transport address; a media stream may require multiple
components, each of which has to work for the media stream as a
whole to work. For media streams based on RTP, there are two
components per media stream -- one for RTP, and one for RTCP.
Host Candidate: A candidate obtained by binding to a specific port
from an IP address on the host. This includes IP addresses on
physical interfaces and logical ones, such as ones obtained
through Virtual Private Networks (VPNs) and Realm Specific IP
(RSIP) [RFC3102] (which lives at the operating system level).
Server Reflexive Candidate: A candidate whose IP address and port
are a binding allocated by a NAT for an agent when it sent a
packet through the NAT to a server. Server reflexive candidates
can be learned by STUN servers using the Binding request, or TURN
servers, which provides both a relayed and server reflexive
candidate.
Peer Reflexive Candidate: A candidate whose IP address and port are
a binding allocated by a NAT for an agent when it sent a STUN
Binding request through the NAT to its peer.
Relayed Candidate: A candidate obtained by sending a TURN Allocate
request from a host candidate to a TURN server. The relayed
candidate is resident on the TURN server, and the TURN server
relays packets back towards the agent.
Base: The base of a server reflexive candidate is the host candidate
from which it was derived. A host candidate is also said to have
a base, equal to that candidate itself. Similarly, the base of a
relayed candidate is that candidate itself.
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Foundation: An arbitrary string that is the same for two candidates
that have the same type, base IP address, protocol (UDP, TCP,
etc.), and STUN or TURN server. If any of these are different,
then the foundation will be different. Two candidate pairs with
the same foundation pairs are likely to have similar network
characteristics. Foundations are used in the frozen algorithm.
Local Candidate: A candidate that an agent has obtained and included
in an offer or answer it sent.
Remote Candidate: A candidate that an agent received in an offer or
answer from its peer.
Default Destination/Candidate: The default destination for a
component of a media stream is the transport address that would be
used by an agent that is not ICE aware. For the RTP component,
the default IP address is in the c line of the SDP, and the port
is in the m line. For the RTCP component, it is in the rtcp
attribute when present, and when not present, the IP address is in
the c line and 1 plus the port is in the m line. A default
candidate for a component is one whose transport address matches
the default destination for that component.
Candidate Pair: A pairing containing a local candidate and a remote
candidate.
Check, Connectivity Check, STUN Check: A STUN Binding request
transaction for the purposes of verifying connectivity. A check
is sent from the local candidate to the remote candidate of a
candidate pair.
Check List: An ordered set of candidate pairs that an agent will use
to generate checks.
Ordinary Check: A connectivity check generated by an agent as a
consequence of a timer that fires periodically, instructing it to
send a check.
Triggered Check: A connectivity check generated as a consequence of
the receipt of a connectivity check from the peer.
Valid List: An ordered set of candidate pairs for a media stream
that have been validated by a successful STUN transaction.
Full: An ICE implementation that performs the complete set of
functionality defined by this specification.
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Lite: An ICE implementation that omits certain functions,
implementing only as much as is necessary for a peer
implementation that is full to gain the benefits of ICE. Lite
implementations do not maintain any of the state machines and do
not generate connectivity checks.
Controlling Agent: The ICE agent that is responsible for selecting
the final choice of candidate pairs and signaling them through
STUN and an updated offer, if needed. In any session, one agent
is always controlling. The other is the controlled agent.
Controlled Agent: An ICE agent that waits for the controlling agent
to select the final choice of candidate pairs.
Regular Nomination: The process of picking a valid candidate pair
for media traffic by validating the pair with one STUN request,
and then picking it by sending a second STUN request with a flag
indicating its nomination.
Aggressive Nomination: The process of picking a valid candidate pair
for media traffic by including a flag in every STUN request, such
that the first one to produce a valid candidate pair is used for
media.
Nominated: If a valid candidate pair has its nominated flag set, it
means that it may be selected by ICE for sending and receiving
media.
Selected Pair, Selected Candidate: The candidate pair selected by
ICE for sending and receiving media is called the selected pair,
and each of its candidates is called the selected candidate.
4. Sending the Initial Offer
In order to send the initial offer in an offer/answer exchange, an
agent must (1) gather candidates, (2) prioritize them, (3) eliminate
redundant candidates, (4) choose default candidates, and then (5)
formulate and send the SDP offer. All but the last of these five
steps differ for full and lite implementations.
4.1. Full Implementation Requirements
4.1.1. Gathering Candidates
An agent gathers candidates when it believes that communication is
imminent. An offerer can do this based on a user interface cue, or
based on an explicit request to initiate a session. Every candidate
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is a transport address. It also has a type and a base. Four types
are defined and gathered by this specification -- host candidates,
server reflexive candidates, peer reflexive candidates, and relayed
candidates. The server reflexive candidates are gathered using STUN
or TURN, and relayed candidates are obtained through TURN. Peer
reflexive candidates are obtained in later phases of ICE, as a
consequence of connectivity checks. The base of a candidate is the
candidate that an agent must send from when using that candidate.
4.1.1.1. Host Candidates
The first step is to gather host candidates. Host candidates are
obtained by binding to ports (typically ephemeral) on a IP address
attached to an interface (physical or virtual, including VPN
interfaces) on the host.
For each UDP media stream the agent wishes to use, the agent SHOULD
obtain a candidate for each component of the media stream on each IP
address that the host has. It obtains each candidate by binding to a
UDP port on the specific IP address. A host candidate (and indeed
every candidate) is always associated with a specific component for
which it is a candidate. Each component has an ID assigned to it,
called the component ID. For RTP-based media streams, the RTP itself
has a component ID of 1, and RTCP a component ID of 2. If an agent
is using RTCP, it MUST obtain a candidate for it. If an agent is
using both RTP and RTCP, it would end up with 2*K host candidates if
an agent has K IP addresses.
The base for each host candidate is set to the candidate itself.
4.1.1.2. Server Reflexive and Relayed Candidates
Agents SHOULD obtain relayed candidates and SHOULD obtain server
reflexive candidates. These requirements are at SHOULD strength to
allow for provider variation. Use of STUN and TURN servers may be
unnecessary in closed networks where agents are never connected to
the public Internet or to endpoints outside of the closed network.
In such cases, a full implementation would be used for agents that
are dual stack or multihomed, to select a host candidate. Use of
TURN servers is expensive, and when ICE is being used, they will only
be utilized when both endpoints are behind NATs that perform address
and port dependent mapping. Consequently, some deployments might
consider this use case to be marginal, and elect not to use TURN
servers. If an agent does not gather server reflexive or relayed
candidates, it is RECOMMENDED that the functionality be implemented
and just disabled through configuration, so that it can be re-enabled
through configuration if conditions change in the future.
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If an agent is gathering both relayed and server reflexive
candidates, it uses a TURN server. If it is gathering just server
reflexive candidates, it uses a STUN server.
The agent next pairs each host candidate with the STUN or TURN server
with which it is configured or has discovered by some means. If a
STUN or TURN server is configured, it is RECOMMENDED that a domain
name be configured, and the DNS procedures in [RFC5389] (using SRV
records with the "stun" service) be used to discover the STUN server,
and the DNS procedures in [RFC5766] (using SRV records with the
"turn" service) be used to discover the TURN server.
This specification only considers usage of a single STUN or TURN
server. When there are multiple choices for that single STUN or TURN
server (when, for example, they are learned through DNS records and
multiple results are returned), an agent SHOULD use a single STUN or
TURN server (based on its IP address) for all candidates for a
particular session. This improves the performance of ICE. The
result is a set of pairs of host candidates with STUN or TURN
servers. The agent then chooses one pair, and sends a Binding or
Allocate request to the server from that host candidate. Binding
requests to a STUN server are not authenticated, and any ALTERNATE-
SERVER attribute in a response is ignored. Agents MUST support the
backwards compatibility mode for the Binding request defined in
[RFC5389]. Allocate requests SHOULD be authenticated using a long-
term credential obtained by the client through some other means.
Every Ta milliseconds thereafter, the agent can generate another new
STUN or TURN transaction. This transaction can either be a retry of
a previous transaction that failed with a recoverable error (such as
authentication failure), or a transaction for a new host candidate
and STUN or TURN server pair. The agent SHOULD NOT generate
transactions more frequently than one every Ta milliseconds. See
Section 16 for guidance on how to set Ta and the STUN retransmit
timer, RTO.
The agent will receive a Binding or Allocate response. A successful
Allocate response will provide the agent with a server reflexive
candidate (obtained from the mapped address) and a relayed candidate
in the XOR-RELAYED-ADDRESS attribute. If the Allocate request is
rejected because the server lacks resources to fulfill it, the agent
SHOULD instead send a Binding request to obtain a server reflexive
candidate. A Binding response will provide the agent with only a
server reflexive candidate (also obtained from the mapped address).
The base of the server reflexive candidate is the host candidate from
which the Allocate or Binding request was sent. The base of a
relayed candidate is that candidate itself. If a relayed candidate
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is identical to a host candidate (which can happen in rare cases),
the relayed candidate MUST be discarded.
4.1.1.3. Computing Foundations
Finally, the agent assigns each candidate a foundation. The
foundation is an identifier, scoped within a session. Two candidates
MUST have the same foundation ID when all of the following are true:
o they are of the same type (host, relayed, server reflexive, or
peer reflexive).
o their bases have the same IP address (the ports can be different).
o for reflexive and relayed candidates, the STUN or TURN servers
used to obtain them have the same IP address.
o they were obtained using the same transport protocol (TCP, UDP,
etc.).
Similarly, two candidates MUST have different foundations if their
types are different, their bases have different IP addresses, the
STUN or TURN servers used to obtain them have different IP addresses,
or their transport protocols are different.
4.1.1.4. Keeping Candidates Alive
Once server reflexive and relayed candidates are allocated, they MUST
be kept alive until ICE processing has completed, as described in
Section 8.3. For server reflexive candidates learned through a
Binding request, the bindings MUST be kept alive by additional
Binding requests to the server. Refreshes for allocations are done
using the Refresh transaction, as described in [RFC5766]. The
Refresh requests will also refresh the server reflexive candidate.
4.1.2. Prioritizing Candidates
The prioritization process results in the assignment of a priority to
each candidate. Each candidate for a media stream MUST have a unique
priority that MUST be a positive integer between 1 and (2**31 - 1).
This priority will be used by ICE to determine the order of the
connectivity checks and the relative preference for candidates.
An agent SHOULD compute this priority using the formula in
Section 4.1.2.1 and choose its parameters using the guidelines in
Section 4.1.2.2. If an agent elects to use a different formula, ICE
will take longer to converge since both agents will not be
coordinated in their checks.
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4.1.2.1. Recommended Formula
When using the formula, an agent computes the priority by determining
a preference for each type of candidate (server reflexive, peer
reflexive, relayed, and host), and, when the agent is multihomed,
choosing a preference for its IP addresses. These two preferences
are then combined to compute the priority for a candidate. That
priority is computed using the following formula:
priority = (2^24)*(type preference) +
(2^8)*(local preference) +
(2^0)*(256 - component ID)
The type preference MUST be an integer from 0 to 126 inclusive, and
represents the preference for the type of the candidate (where the
types are local, server reflexive, peer reflexive, and relayed). A
126 is the highest preference, and a 0 is the lowest. Setting the
value to a 0 means that candidates of this type will only be used as
a last resort. The type preference MUST be identical for all
candidates of the same type and MUST be different for candidates of
different types. The type preference for peer reflexive candidates
MUST be higher than that of server reflexive candidates. Note that
candidates gathered based on the procedures of Section 4.1.1 will
never be peer reflexive candidates; candidates of these type are
learned from the connectivity checks performed by ICE.
The local preference MUST be an integer from 0 to 65535 inclusive.
It represents a preference for the particular IP address from which
the candidate was obtained, in cases where an agent is multihomed.
65535 represents the highest preference, and a zero, the lowest.
When there is only a single IP address, this value SHOULD be set to
65535. More generally, if there are multiple candidates for a
particular component for a particular media stream that have the same
type, the local preference MUST be unique for each one. In this
specification, this only happens for multihomed hosts. If a host is
multihomed because it is dual stack, the local preference SHOULD be
set equal to the precedence value for IP addresses described in RFC
3484 [RFC3484].
The component ID is the component ID for the candidate, and MUST be
between 1 and 256 inclusive.
4.1.2.2. Guidelines for Choosing Type and Local Preferences
One criterion for selection of the type and local preference values
is the use of a media intermediary, such as a TURN server, VPN
server, or NAT. With a media intermediary, if media is sent to that
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candidate, it will first transit the media intermediary before being
received. Relayed candidates are one type of candidate that involves
a media intermediary. Another are host candidates obtained from a
VPN interface. When media is transited through a media intermediary,
it can increase the latency between transmission and reception. It
can increase the packet losses, because of the additional router hops
that may be taken. It may increase the cost of providing service,
since media will be routed in and right back out of a media
intermediary run by a provider. If these concerns are important, the
type preference for relayed candidates SHOULD be lower than host
candidates. The RECOMMENDED values are 126 for host candidates, 100
for server reflexive candidates, 110 for peer reflexive candidates,
and 0 for relayed candidates. Furthermore, if an agent is multihomed
and has multiple IP addresses, the local preference for host
candidates from a VPN interface SHOULD have a priority of 0.
Another criterion for selection of preferences is IP address family.
ICE works with both IPv4 and IPv6. It therefore provides a
transition mechanism that allows dual-stack hosts to prefer
connectivity over IPv6, but to fall back to IPv4 in case the v6
networks are disconnected (due, for example, to a failure in a 6to4
relay) [RFC3056]. It can also help with hosts that have both a
native IPv6 address and a 6to4 address. In such a case, higher local
preferences could be assigned to the v6 addresses, followed by the
6to4 addresses, followed by the v4 addresses. This allows a site to
obtain and begin using native v6 addresses immediately, yet still
fall back to 6to4 addresses when communicating with agents in other
sites that do not yet have native v6 connectivity.
Another criterion for selecting preferences is security. If a user
is a telecommuter, and therefore connected to a corporate network and
a local home network, the user may prefer their voice traffic to be
routed over the VPN in order to keep it on the corporate network when
communicating within the enterprise, but use the local network when
communicating with users outside of the enterprise. In such a case,
a VPN address would have a higher local preference than any other
address.
Another criterion for selecting preferences is topological awareness.
This is most useful for candidates that make use of intermediaries.
In those cases, if an agent has preconfigured or dynamically
discovered knowledge of the topological proximity of the
intermediaries to itself, it can use that to assign higher local
preferences to candidates obtained from closer intermediaries.
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4.1.3. Eliminating Redundant Candidates
Next, the agent eliminates redundant candidates. A candidate is
redundant if its transport address equals another candidate, and its
base equals the base of that other candidate. Note that two
candidates can have the same transport address yet have different
bases, and these would not be considered redundant. Frequently, a
server reflexive candidate and a host candidate will be redundant
when the agent is not behind a NAT. The agent SHOULD eliminate the
redundant candidate with the lower priority.
4.1.4. Choosing Default Candidates
A candidate is said to be default if it would be the target of media
from a non-ICE peer; that target is called the DEFAULT DESTINATION.
If the default candidates are not selected by the ICE algorithm when
communicating with an ICE-aware peer, an updated offer/answer will be
required after ICE processing completes in order to "fix up" the SDP
so that the default destination for media matches the candidates
selected by ICE. If ICE happens to select the default candidates, no
updated offer/answer is required.
An agent MUST choose a set of candidates, one for each component of
each in-use media stream, to be default. A media stream is in-use if
it does not have a port of zero (which is used in RFC 3264 to reject
a media stream). Consequently, a media stream is in-use even if it
is marked as a=inactive [RFC4566] or has a bandwidth value of zero.
It is RECOMMENDED that default candidates be chosen based on the
likelihood of those candidates to work with the peer that is being
contacted. It is RECOMMENDED that the default candidates are the
relayed candidates (if relayed candidates are available), server
reflexive candidates (if server reflexive candidates are available),
and finally host candidates.
4.2. Lite Implementation Requirements
Lite implementations only utilize host candidates. A lite
implementation MUST, for each component of each media stream,
allocate zero or one IPv4 candidates. It MAY allocate zero or more
IPv6 candidates, but no more than one per each IPv6 address utilized
by the host. Since there can be no more than one IPv4 candidate per
component of each media stream, if an agent has multiple IPv4
addresses, it MUST choose one for allocating the candidate. If a
host is dual stack, it is RECOMMENDED that it allocate one IPv4
candidate and one global IPv6 address. With the lite implementation,
ICE cannot be used to dynamically choose amongst candidates.
Therefore, including more than one candidate from a particular scope
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is NOT RECOMMENDED, since only a connectivity check can truly
determine whether to use one address or the other.
Each component has an ID assigned to it, called the component ID.
For RTP-based media streams, the RTP itself has a component ID of 1,
and RTCP a component ID of 2. If an agent is using RTCP, it MUST
obtain candidates for it.
Each candidate is assigned a foundation. The foundation MUST be
different for two candidates allocated from different IP addresses,
and MUST be the same otherwise. A simple integer that increments for
each IP address will suffice. In addition, each candidate MUST be
assigned a unique priority amongst all candidates for the same media
stream. This priority SHOULD be equal to:
priority = (2^24)*(126) +
(2^8)*(IP precedence) +
(2^0)*(256 - component ID)
If a host is v4-only, it SHOULD set the IP precedence to 65535. If a
host is v6 or dual stack, the IP precedence SHOULD be the precedence
value for IP addresses described in RFC 3484 [RFC3484].
Next, an agent chooses a default candidate for each component of each
media stream. If a host is IPv4 only, there would only be one
candidate for each component of each media stream, and therefore that
candidate is the default. If a host is IPv6 or dual stack, the
selection of default is a matter of local policy. This default
SHOULD be chosen such that it is the candidate most likely to be used
with a peer. For IPv6-only hosts, this would typically be a globally
scoped IPv6 address. For dual-stack hosts, the IPv4 address is
RECOMMENDED.
4.3. Encoding the SDP
The process of encoding the SDP is identical between full and lite
implementations.
The agent will include an m line for each media stream it wishes to
use. The ordering of media streams in the SDP is relevant for ICE.
ICE will perform its connectivity checks for the first m line first,
and consequently media will be able to flow for that stream first.
Agents SHOULD place their most important media stream, if there is
one, first in the SDP.
There will be a candidate attribute for each candidate for a
particular media stream. Section 15 provides detailed rules for
constructing this attribute. The attribute carries the IP address,
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port, and transport protocol for the candidate, in addition to its
properties that need to be signaled to the peer for ICE to work: the
priority, foundation, and component ID. The candidate attribute also
carries information about the candidate that is useful for
diagnostics and other functions: its type and related transport
addresses.
STUN connectivity checks between agents are authenticated using the
short-term credential mechanism defined for STUN [RFC5389]. This
mechanism relies on a username and password that are exchanged
through protocol machinery between the client and server. With ICE,
the offer/answer exchange is used to exchange them. The username
part of this credential is formed by concatenating a username
fragment from each agent, separated by a colon. Each agent also
provides a password, used to compute the message integrity for
requests it receives. The username fragment and password are
exchanged in the ice-ufrag and ice-pwd attributes, respectively. In
addition to providing security, the username provides disambiguation
and correlation of checks to media streams. See Appendix B.4 for
motivation.
If an agent is a lite implementation, it MUST include an "a=ice-lite"
session-level attribute in its SDP. If an agent is a full
implementation, it MUST NOT include this attribute.
The default candidates are added to the SDP as the default
destination for media. For streams based on RTP, this is done by
placing the IP address and port of the RTP candidate into the c and m
lines, respectively. If the agent is utilizing RTCP, it MUST encode
the RTCP candidate using the a=rtcp attribute as defined in RFC 3605
[RFC3605]. If RTCP is not in use, the agent MUST signal that using
b=RS:0 and b=RR:0 as defined in RFC 3556 [RFC3556].
The transport addresses that will be the default destination for
media when communicating with non-ICE peers MUST also be present as
candidates in one or more a=candidate lines.
ICE provides for extensibility by allowing an offer or answer to
contain a series of tokens that identify the ICE extensions used by
that agent. If an agent supports an ICE extension, it MUST include
the token defined for that extension in the ice-options attribute.
The following is an example SDP message that includes ICE attributes
(lines folded for readability):
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v=0
o=jdoe 2890844526 2890842807 IN IP4 10.0.1.1
s=
c=IN IP4 192.0.2.3
t=0 0
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=audio 45664 RTP/AVP 0
b=RS:0
b=RR:0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 10.0.1.1 8998 typ host
a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ srflx raddr
10.0.1.1 rport 8998
Once an agent has sent its offer or its answer, that agent MUST be
prepared to receive both STUN and media packets on each candidate.
As discussed in Section 11.1, media packets can be sent to a
candidate prior to its appearance as the default destination for
media in an offer or answer.
5. Receiving the Initial Offer
When an agent receives an initial offer, it will check if the offerer
supports ICE, determine its own role, gather candidates, prioritize
them, choose default candidates, encode and send an answer, and for
full implementations, form the check lists and begin connectivity
checks.
5.1. Verifying ICE Support
The agent will proceed with the ICE procedures defined in this
specification if, for each media stream in the SDP it received, the
default destination for each component of that media stream appears
in a candidate attribute. For example, in the case of RTP, the IP
address and port in the c and m lines, respectively, appear in a
candidate attribute and the value in the rtcp attribute appears in a
candidate attribute.
If this condition is not met, the agent MUST process the SDP based on
normal RFC 3264 procedures, without using any of the ICE mechanisms
described in the remainder of this specification with the following
exceptions:
1. The agent MUST follow the rules of Section 10, which describe
keepalive procedures for all agents.
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2. If the agent is not proceeding with ICE because there were
a=candidate attributes, but none that matched the default
destination of the media stream, the agent MUST include an a=ice-
mismatch attribute in its answer.
3. If the default candidates were relayed candidates learned through
a TURN server, the agent MUST create permissions in the TURN
server for the IP addresses learned from its peer in the SDP it
just received. If this is not done, initial packets in the media
stream from the peer may be lost.
5.2. Determining Role
For each session, each agent takes on a role. There are two roles --
controlling and controlled. The controlling agent is responsible for
the choice of the final candidate pairs used for communications. For
a full agent, this means nominating the candidate pairs that can be
used by ICE for each media stream, and for generating the updated
offer based on ICE's selection, when needed. For a lite
implementation, being the controlling agent means selecting a
candidate pair based on the ones in the offer and answer (for IPv4,
there is only ever one pair), and then generating an updated offer
reflecting that selection, when needed (it is never needed for an
IPv4-only host). The controlled agent is told which candidate pairs
to use for each media stream, and does not generate an updated offer
to signal this information. The sections below describe in detail
the actual procedures followed by controlling and controlled nodes.
The rules for determining the role and the impact on behavior are as
follows:
Both agents are full: The agent that generated the offer which
started the ICE processing MUST take the controlling role, and the
other MUST take the controlled role. Both agents will form check
lists, run the ICE state machines, and generate connectivity
checks. The controlling agent will execute the logic in
Section 8.1 to nominate pairs that will be selected by ICE, and
then both agents end ICE as described in Section 8.1.2. In
unusual cases, described in Appendix B.11, it is possible for both
agents to mistakenly believe they are controlled or controlling.
To resolve this, each agent MUST select a random number, called
the tie-breaker, uniformly distributed between 0 and (2**64) - 1
(that is, a 64-bit positive integer). This number is used in
connectivity checks to detect and repair this case, as described
in Section 7.1.2.2.
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One agent full, one lite: The full agent MUST take the controlling
role, and the lite agent MUST take the controlled role. The full
agent will form check lists, run the ICE state machines, and
generate connectivity checks. That agent will execute the logic
in Section 8.1 to nominate pairs that will be selected by ICE, and
use the logic in Section 8.1.2 to end ICE. The lite
implementation will just listen for connectivity checks, receive
them and respond to them, and then conclude ICE as described in
Section 8.2. For the lite implementation, the state of ICE
processing for each media stream is considered to be Running, and
the state of ICE overall is Running.
Both lite: The agent that generated the offer which started the ICE
processing MUST take the controlling role, and the other MUST take
the controlled role. In this case, no connectivity checks are
ever sent. Rather, once the offer/answer exchange completes, each
agent performs the processing described in Section 8 without
connectivity checks. It is possible that both agents will believe
they are controlled or controlling. In the latter case, the
conflict is resolved through glare detection capabilities in the
signaling protocol carrying the offer/answer exchange. The state
of ICE processing for each media stream is considered to be
Running, and the state of ICE overall is Running.
Once roles are determined for a session, they persist unless ICE is
restarted. An ICE restart (Section 9.1) causes a new selection of
roles and tie-breakers.
5.3. Gathering Candidates
The process for gathering candidates at the answerer is identical to
the process for the offerer as described in Section 4.1.1 for full
implementations and Section 4.2 for lite implementations. It is
RECOMMENDED that this process begin immediately on receipt of the
offer, prior to alerting the user. Such gathering MAY begin when an
agent starts.
5.4. Prioritizing Candidates
The process for prioritizing candidates at the answerer is identical
to the process followed by the offerer, as described in Section 4.1.2
for full implementations and Section 4.2 for lite implementations.
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5.5. Choosing Default Candidates
The process for selecting default candidates at the answerer is
identical to the process followed by the offerer, as described in
Section 4.1.4 for full implementations and Section 4.2 for lite
implementations.
5.6. Encoding the SDP
The process for encoding the SDP at the answerer is identical to the
process followed by the offerer for both full and lite
implementations, as described in Section 4.3.
5.7. Forming the Check Lists
Forming check lists is done only by full implementations. Lite
implementations MUST skip the steps defined in this section.
There is one check list per in-use media stream resulting from the
offer/answer exchange. To form the check list for a media stream,
the agent forms candidate pairs, computes a candidate pair priority,
orders the pairs by priority, prunes them, and sets their states.
These steps are described in this section.
5.7.1. Forming Candidate Pairs
First, the agent takes each of its candidates for a media stream
(called LOCAL CANDIDATES) and pairs them with the candidates it
received from its peer (called REMOTE CANDIDATES) for that media
stream. In order to prevent the attacks described in Section 18.5.2,
agents MAY limit the number of candidates they'll accept in an offer
or answer. A local candidate is paired with a remote candidate if
and only if the two candidates have the same component ID and have
the same IP address version. It is possible that some of the local
candidates won't get paired with remote candidates, and some of the
remote candidates won't get paired with local candidates. This can
happen if one agent doesn't include candidates for the all of the
components for a media stream. If this happens, the number of
components for that media stream is effectively reduced, and
considered to be equal to the minimum across both agents of the
maximum component ID provided by each agent across all components for
the media stream.
In the case of RTP, this would happen when one agent provides
candidates for RTCP, and the other does not. As another example, the
offerer can multiplex RTP and RTCP on the same port and signals that
it can do that in the SDP through an SDP attribute [RFC5761].
However, since the offerer doesn't know if the answerer can perform
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such multiplexing, the offerer includes candidates for RTP and RTCP
on separate ports, so that the offer has two components per media
stream. If the answerer can perform such multiplexing, it would
include just a single component for each candidate - for the combined
RTP/RTCP mux. ICE would end up acting as if there was just a single
component for this candidate.
The candidate pairs whose local and remote candidates are both the
default candidates for a particular component is called,
unsurprisingly, the default candidate pair for that component. This
is the pair that would be used to transmit media if both agents had
not been ICE aware.
In order to aid understanding, Figure 6 shows the relationships
between several key concepts -- transport addresses, candidates,
candidate pairs, and check lists, in addition to indicating the main
properties of candidates and candidate pairs.
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+------------------------------------------+
| |
| +---------------------+ |
| |+----+ +----+ +----+ | +Type |
| || IP | |Port| |Tran| | +Priority |
| ||Addr| | | | | | +Foundation |
| |+----+ +----+ +----+ | +ComponentiD |
| | Transport | +RelatedAddr |
| | Addr | |
| +---------------------+ +Base |
| Candidate |
+------------------------------------------+
* *
* *************************************
* *
+-------------------------------+
.| |
| Local Remote |
| +----+ +----+ +default? |
| |Cand| |Cand| +valid? |
| +----+ +----+ +nominated?|
| +State |
| |
| |
| Candidate Pair |
+-------------------------------+
* *
* ************
* *
+------------------+
| Candidate Pair |
+------------------+
+------------------+
| Candidate Pair |
+------------------+
+------------------+
| Candidate Pair |
+------------------+
Check
List
Figure 6: Conceptual Diagram of a Check List
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5.7.2. Computing Pair Priority and Ordering Pairs
Once the pairs are formed, a candidate pair priority is computed.
Let G be the priority for the candidate provided by the controlling
agent. Let D be the priority for the candidate provided by the
controlled agent. The priority for a pair is computed as:
pair priority = 2^32*MIN(G,D) + 2*MAX(G,D) + (G>D?1:0)
Where G>D?1:0 is an expression whose value is 1 if G is greater than
D, and 0 otherwise. Once the priority is assigned, the agent sorts
the candidate pairs in decreasing order of priority. If two pairs
have identical priority, the ordering amongst them is arbitrary.
5.7.3. Pruning the Pairs
This sorted list of candidate pairs is used to determine a sequence
of connectivity checks that will be performed. Each check involves
sending a request from a local candidate to a remote candidate.
Since an agent cannot send requests directly from a reflexive
candidate, but only from its base, the agent next goes through the
sorted list of candidate pairs. For each pair where the local
candidate is server reflexive, the server reflexive candidate MUST be
replaced by its base. Once this has been done, the agent MUST prune
the list. This is done by removing a pair if its local and remote
candidates are identical to the local and remote candidates of a pair
higher up on the priority list. The result is a sequence of ordered
candidate pairs, called the check list for that media stream.
In addition, in order to limit the attacks described in
Section 18.5.2, an agent MUST limit the total number of connectivity
checks the agent performs across all check lists to a specific value,
and this value MUST be configurable. A default of 100 is
RECOMMENDED. This limit is enforced by discarding the lower-priority
candidate pairs until there are less than 100. It is RECOMMENDED
that a lower value be utilized when possible, set to the maximum
number of plausible checks that might be seen in an actual deployment
configuration. The requirement for configuration is meant to provide
a tool for fixing this value in the field if, once deployed, it is
found to be problematic.
5.7.4. Computing States
Each candidate pair in the check list has a foundation and a state.
The foundation is the combination of the foundations of the local and
remote candidates in the pair. The state is assigned once the check
list for each media stream has been computed. There are five
potential values that the state can have:
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Waiting: A check has not been performed for this pair, and can be
performed as soon as it is the highest-priority Waiting pair on
the check list.
In-Progress: A check has been sent for this pair, but the
transaction is in progress.
Succeeded: A check for this pair was already done and produced a
successful result.
Failed: A check for this pair was already done and failed, either
never producing any response or producing an unrecoverable failure
response.
Frozen: A check for this pair hasn't been performed, and it can't
yet be performed until some other check succeeds, allowing this
pair to unfreeze and move into the Waiting state.
As ICE runs, the pairs will move between states as shown in Figure 7.
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+-----------+
| |
| |
| Frozen |
| |
| |
+-----------+
|
|unfreeze
|
V
+-----------+ +-----------+
| | | |
| | perform | |
| Waiting |-------->|In-Progress|
| | | |
| | | |
+-----------+ +-----------+
/ |
// |
// |
// |
/ |
// |
failure // |success
// |
/ |
// |
// |
// |
V V
+-----------+ +-----------+
| | | |
| | | |
| Failed | | Succeeded |
| | | |
| | | |
+-----------+ +-----------+
Figure 7: Pair State FSM
The initial states for each pair in a check list are computed by
performing the following sequence of steps:
1. The agent sets all of the pairs in each check list to the Frozen
state.
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2. The agent examines the check list for the first media stream (a
media stream is the first media stream when it is described by
the first m line in the SDP offer and answer). For that media
stream:
* For all pairs with the same foundation, it sets the state of
the pair with the lowest component ID to Waiting. If there is
more than one such pair, the one with the highest priority is
used.
One of the check lists will have some number of pairs in the Waiting
state, and the other check lists will have all of their pairs in the
Frozen state. A check list with at least one pair that is Waiting is
called an active check list, and a check list with all pairs Frozen
is called a frozen check list.
The check list itself is associated with a state, which captures the
state of ICE checks for that media stream. There are three states:
Running: In this state, ICE checks are still in progress for this
media stream.
Completed: In this state, ICE checks have produced nominated pairs
for each component of the media stream. Consequently, ICE has
succeeded and media can be sent.
Failed: In this state, the ICE checks have not completed
successfully for this media stream.
When a check list is first constructed as the consequence of an
offer/answer exchange, it is placed in the Running state.
ICE processing across all media streams also has a state associated
with it. This state is equal to Running while ICE processing is
under way. The state is Completed when ICE processing is complete
and Failed if it failed without success. Rules for transitioning
between states are described below.
5.8. Scheduling Checks
Checks are generated only by full implementations. Lite
implementations MUST skip the steps described in this section.
An agent performs ordinary checks and triggered checks. The
generation of both checks is governed by a timer that fires
periodically for each media stream. The agent maintains a FIFO
queue, called the triggered check queue, which contains candidate
pairs for which checks are to be sent at the next available
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opportunity. When the timer fires, the agent removes the top pair
from the triggered check queue, performs a connectivity check on that
pair, and sets the state of the candidate pair to In-Progress. If
there are no pairs in the triggered check queue, an ordinary check is
sent.
Once the agent has computed the check lists as described in
Section 5.7, it sets a timer for each active check list. The timer
fires every Ta*N seconds, where N is the number of active check lists
(initially, there is only one active check list). Implementations
MAY set the timer to fire less frequently than this. Implementations
SHOULD take care to spread out these timers so that they do not fire
at the same time for each media stream. Ta and the retransmit timer
RTO are computed as described in Section 16. Multiplying by N allows
this aggregate check throughput to be split between all active check
lists. The first timer fires immediately, so that the agent performs
a connectivity check the moment the offer/answer exchange has been
done, followed by the next check Ta seconds later (since there is
only one active check list).
When the timer fires and there is no triggered check to be sent, the
agent MUST choose an ordinary check as follows:
o Find the highest-priority pair in that check list that is in the
Waiting state.
o If there is such a pair:
* Send a STUN check from the local candidate of that pair to the
remote candidate of that pair. The procedures for forming the
STUN request for this purpose are described in Section 7.1.2.
* Set the state of the candidate pair to In-Progress.
o If there is no such pair:
* Find the highest-priority pair in that check list that is in
the Frozen state.
* If there is such a pair:
+ Unfreeze the pair.
+ Perform a check for that pair, causing its state to
transition to In-Progress.
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* If there is no such pair:
+ Terminate the timer for that check list.
To compute the message integrity for the check, the agent uses the
remote username fragment and password learned from the SDP from its
peer. The local username fragment is known directly by the agent for
its own candidate.
6. Receipt of the Initial Answer
This section describes the procedures that an agent follows when it
receives the answer from the peer. It verifies that its peer
supports ICE, determines its role, and for full implementations,
forms the check list and begins performing ordinary checks.
When ICE is used with SIP, forking may result in a single offer
generating a multiplicity of answers. In that case, ICE proceeds
completely in parallel and independently for each answer, treating
the combination of its offer and each answer as an independent offer/
answer exchange, with its own set of pairs, check lists, states, and
so on. The only case in which processing of one pair impacts another
is freeing of candidates, discussed below in Section 8.3.
6.1. Verifying ICE Support
The logic at the offerer is identical to that of the answerer as
described in Section 5.1, with the exception that an offerer would
not ever generate a=ice-mismatch attributes in an SDP.
In some cases, the answer may omit a=candidate attributes for the
media streams, and instead include an a=ice-mismatch attribute for
one or more of the media streams in the SDP. This signals to the
offerer that the answerer supports ICE, but that ICE processing was
not used for the session because a signaling intermediary modified
the default destination for media components without modifying the
corresponding candidate attributes. See Section 18 for a discussion
of cases where this can happen. This specification provides no
guidance on how an agent should proceed in such a failure case.
6.2. Determining Role
The offerer follows the same procedures described for the answerer in
Section 5.2.
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6.3. Forming the Check List
Formation of check lists is performed only by full implementations.
The offerer follows the same procedures described for the answerer in
Section 5.7.
6.4. Performing Ordinary Checks
Ordinary checks are performed only by full implementations. The
offerer follows the same procedures described for the answerer in
Section 5.8.
7. Performing Connectivity Checks
This section describes how connectivity checks are performed. All
ICE implementations are required to be compliant to [RFC5389], as
opposed to the older [RFC3489]. However, whereas a full
implementation will both generate checks (acting as a STUN client)
and receive them (acting as a STUN server), a lite implementation
will only receive checks, and thus will only act as a STUN server.
7.1. STUN Client Procedures
These procedures define how an agent sends a connectivity check,
whether it is an ordinary or a triggered check. These procedures are
only applicable to full implementations.
7.1.1. Creating Permissions for Relayed Candidates
If the connectivity check is being sent using a relayed local
candidate, the client MUST create a permission first if it has not
already created one previously. It would have created one previously
if it had told the TURN server to create a permission for the given
relayed candidate towards the IP address of the remote candidate. To
create the permission, the agent follows the procedures defined in
[RFC5766]. The permission MUST be created towards the IP address of
the remote candidate. It is RECOMMENDED that the agent defer
creation of a TURN channel until ICE completes, in which case
permissions for connectivity checks are normally created using a
CreatePermission request. Once established, the agent MUST keep the
permission active until ICE concludes.
7.1.2. Sending the Request
The check is generated by sending a Binding request from a local
candidate to a remote candidate. [RFC5389] describes how Binding
requests are constructed and generated. A connectivity check MUST
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utilize the STUN short-term credential mechanism. Support for
backwards compatibility with RFC 3489 MUST NOT be used or assumed
with connectivity checks. The FINGERPRINT mechanism MUST be used for
connectivity checks.
ICE extends STUN by defining several new attributes, including
PRIORITY, USE-CANDIDATE, ICE-CONTROLLED, and ICE-CONTROLLING. These
new attributes are formally defined in Section 19.1, and their usage
is described in the subsections below. These STUN extensions are
applicable only to connectivity checks used for ICE.
7.1.2.1. PRIORITY and USE-CANDIDATE
An agent MUST include the PRIORITY attribute in its Binding request.
The attribute MUST be set equal to the priority that would be
assigned, based on the algorithm in Section 4.1.2, to a peer
reflexive candidate, should one be learned as a consequence of this
check (see Section 7.1.3.2.1 for how peer reflexive candidates are
learned). This priority value will be computed identically to how
the priority for the local candidate of the pair was computed, except
that the type preference is set to the value for peer reflexive
candidate types.
The controlling agent MAY include the USE-CANDIDATE attribute in the
Binding request. The controlled agent MUST NOT include it in its
Binding request. This attribute signals that the controlling agent
wishes to cease checks for this component, and use the candidate pair
resulting from the check for this component. Section 8.1.1 provides
guidance on determining when to include it.
7.1.2.2. ICE-CONTROLLED and ICE-CONTROLLING
The agent MUST include the ICE-CONTROLLED attribute in the request if
it is in the controlled role, and MUST include the ICE-CONTROLLING
attribute in the request if it is in the controlling role. The
content of either attribute MUST be the tie-breaker that was
determined in Section 5.2. These attributes are defined fully in
Section 19.1.
7.1.2.3. Forming Credentials
A Binding request serving as a connectivity check MUST utilize the
STUN short-term credential mechanism. The username for the
credential is formed by concatenating the username fragment provided
by the peer with the username fragment of the agent sending the
request, separated by a colon (":"). The password is equal to the
password provided by the peer. For example, consider the case where
agent L is the offerer, and agent R is the answerer. Agent L
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included a username fragment of LFRAG for its candidates and a
password of LPASS. Agent R provided a username fragment of RFRAG and
a password of RPASS. A connectivity check from L to R utilizes the
username RFRAG:LFRAG and a password of RPASS. A connectivity check
from R to L utilizes the username LFRAG:RFRAG and a password of
LPASS. The responses utilize the same usernames and passwords as the
requests (note that the USERNAME attribute is not present in the
response).
7.1.2.4. DiffServ Treatment
If the agent is using Diffserv Codepoint markings [RFC2475] in its
media packets, it SHOULD apply those same markings to its
connectivity checks.
7.1.3. Processing the Response
When a Binding response is received, it is correlated to its Binding
request using the transaction ID, as defined in [RFC5389], which then
ties it to the candidate pair for which the Binding request was sent.
This section defines additional procedures for processing Binding
responses specific to this usage of STUN.
7.1.3.1. Failure Cases
If the STUN transaction generates a 487 (Role Conflict) error
response, the agent checks whether it included the ICE-CONTROLLED or
ICE-CONTROLLING attribute in the Binding request. If the request
contained the ICE-CONTROLLED attribute, the agent MUST switch to the
controlling role if it has not already done so. If the request
contained the ICE-CONTROLLING attribute, the agent MUST switch to the
controlled role if it has not already done so. Once it has switched,
the agent MUST enqueue the candidate pair whose check generated the
487 into the triggered check queue. The state of that pair is set to
Waiting. When the triggered check is sent, it will contain an ICE-
CONTROLLING or ICE-CONTROLLED attribute reflecting its new role.
Note, however, that the tie-breaker value MUST NOT be reselected.
A change in roles will require an agent to recompute pair priorities
(Section 5.7.2), since those priorities are a function of controlling
and controlled roles. The change in role will also impact whether
the agent is responsible for selecting nominated pairs and generating
updated offers upon conclusion of ICE.
Agents MAY support receipt of ICMP errors for connectivity checks.
If the STUN transaction generates an ICMP error, the agent sets the
state of the pair to Failed. If the STUN transaction generates a
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STUN error response that is unrecoverable (as defined in [RFC5389])
or times out, the agent sets the state of the pair to Failed.
The agent MUST check that the source IP address and port of the
response equal the destination IP address and port to which the
Binding request was sent, and that the destination IP address and
port of the response match the source IP address and port from which
the Binding request was sent. In other words, the source and
destination transport addresses in the request and responses are
symmetric. If they are not symmetric, the agent sets the state of
the pair to Failed.
7.1.3.2. Success Cases
A check is considered to be a success if all of the following are
true:
o The STUN transaction generated a success response.
o The source IP address and port of the response equals the
destination IP address and port to which the Binding request was
sent.
o The destination IP address and port of the response match the
source IP address and port from which the Binding request was
sent.
7.1.3.2.1. Discovering Peer Reflexive Candidates
The agent checks the mapped address from the STUN response. If the
transport address does not match any of the local candidates that the
agent knows about, the mapped address represents a new candidate -- a
peer reflexive candidate. Like other candidates, it has a type,
base, priority, and foundation. They are computed as follows:
o Its type is equal to peer reflexive.
o Its base is set equal to the local candidate of the candidate pair
from which the STUN check was sent.
o Its priority is set equal to the value of the PRIORITY attribute
in the Binding request.
o Its foundation is selected as described in Section 4.1.1.3.
This peer reflexive candidate is then added to the list of local
candidates for the media stream. Its username fragment and password
are the same as all other local candidates for that media stream.
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However, the peer reflexive candidate is not paired with other remote
candidates. This is not necessary; a valid pair will be generated
from it momentarily based on the procedures in Section 7.1.3.2.2. If
an agent wishes to pair the peer reflexive candidate with other
remote candidates besides the one in the valid pair that will be
generated, the agent MAY generate an updated offer which includes the
peer reflexive candidate. This will cause it to be paired with all
other remote candidates.
7.1.3.2.2. Constructing a Valid Pair
The agent constructs a candidate pair whose local candidate equals
the mapped address of the response, and whose remote candidate equals
the destination address to which the request was sent. This is
called a valid pair, since it has been validated by a STUN
connectivity check. The valid pair may equal the pair that generated
the check, may equal a different pair in the check list, or may be a
pair not currently on any check list. If the pair equals the pair
that generated the check or is on a check list currently, it is also
added to the VALID LIST, which is maintained by the agent for each
media stream. This list is empty at the start of ICE processing, and
fills as checks are performed, resulting in valid candidate pairs.
It will be very common that the pair will not be on any check list.
Recall that the check list has pairs whose local candidates are never
server reflexive; those pairs had their local candidates converted to
the base of the server reflexive candidates, and then pruned if they
were redundant. When the response to the STUN check arrives, the
mapped address will be reflexive if there is a NAT between the two.
In that case, the valid pair will have a local candidate that doesn't
match any of the pairs in the check list.
If the pair is not on any check list, the agent computes the priority
for the pair based on the priority of each candidate, using the
algorithm in Section 5.7. The priority of the local candidate
depends on its type. If it is not peer reflexive, it is equal to the
priority signaled for that candidate in the SDP. If it is peer
reflexive, it is equal to the PRIORITY attribute the agent placed in
the Binding request that just completed. The priority of the remote
candidate is taken from the SDP of the peer. If the candidate does
not appear there, then the check must have been a triggered check to
a new remote candidate. In that case, the priority is taken as the
value of the PRIORITY attribute in the Binding request that triggered
the check that just completed. The pair is then added to the VALID
LIST.
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7.1.3.2.3. Updating Pair States
The agent sets the state of the pair that *generated* the check to
Succeeded. Note that, the pair which *generated* the check may be
different than the valid pair constructed in Section 7.1.3.2.2 as a
consequence of the response. The success of this check might also
cause the state of other checks to change as well. The agent MUST
perform the following two steps:
1. The agent changes the states for all other Frozen pairs for the
same media stream and same foundation to Waiting. Typically, but
not always, these other pairs will have different component IDs.
2. If there is a pair in the valid list for every component of this
media stream (where this is the actual number of components being
used, in cases where the number of components signaled in the SDP
differs from offerer to answerer), the success of this check may
unfreeze checks for other media streams. Note that this step is
followed not just the first time the valid list under
consideration has a pair for every component, but every
subsequent time a check succeeds and adds yet another pair to
that valid list. The agent examines the check list for each
other media stream in turn:
* If the check list is active, the agent changes the state of
all Frozen pairs in that check list whose foundation matches a
pair in the valid list under consideration to Waiting.
* If the check list is frozen, and there is at least one pair in
the check list whose foundation matches a pair in the valid
list under consideration, the state of all pairs in the check
list whose foundation matches a pair in the valid list under
consideration is set to Waiting. This will cause the check
list to become active, and ordinary checks will begin for it,
as described in Section 5.8.
* If the check list is frozen, and there are no pairs in the
check list whose foundation matches a pair in the valid list
under consideration, the agent
+ groups together all of the pairs with the same foundation,
and
+ for each group, sets the state of the pair with the lowest
component ID to Waiting. If there is more than one such
pair, the one with the highest priority is used.
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7.1.3.2.4. Updating the Nominated Flag
If the agent was a controlling agent, and it had included a USE-
CANDIDATE attribute in the Binding request, the valid pair generated
from that check has its nominated flag set to true. This flag
indicates that this valid pair should be used for media if it is the
highest-priority one amongst those whose nominated flag is set. This
may conclude ICE processing for this media stream or all media
streams; see Section 8.
If the agent is the controlled agent, the response may be the result
of a triggered check that was sent in response to a request that
itself had the USE-CANDIDATE attribute. This case is described in
Section 7.2.1.5, and may now result in setting the nominated flag for
the pair learned from the original request.
7.1.3.3. Check List and Timer State Updates
Regardless of whether the check was successful or failed, the
completion of the transaction may require updating of check list and
timer states.
If all of the pairs in the check list are now either in the Failed or
Succeeded state:
o If there is not a pair in the valid list for each component of the
media stream, the state of the check list is set to Failed.
o For each frozen check list, the agent
* groups together all of the pairs with the same foundation, and
* for each group, sets the state of the pair with the lowest
component ID to Waiting. If there is more than one such pair,
the one with the highest priority is used.
If none of the pairs in the check list are in the Waiting or Frozen
state, the check list is no longer considered active, and will not
count towards the value of N in the computation of timers for
ordinary checks as described in Section 5.8.
7.2. STUN Server Procedures
An agent MUST be prepared to receive a Binding request on the base of
each candidate it included in its most recent offer or answer. This
requirement holds even if the peer is a lite implementation.
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The agent MUST use a short-term credential to authenticate the
request and perform a message integrity check. The agent MUST
consider the username to be valid if it consists of two values
separated by a colon, where the first value is equal to the username
fragment generated by the agent in an offer or answer for a session
in-progress. It is possible (and in fact very likely) that an
offerer will receive a Binding request prior to receiving the answer
from its peer. If this happens, the agent MUST immediately generate
a response (including computation of the mapped address as described
in Section 7.2.1.2). The agent has sufficient information at this
point to generate the response; the password from the peer is not
required. Once the answer is received, it MUST proceed with the
remaining steps required, namely, 7.2.1.3, 7.2.1.4, and 7.2.1.5 for
full implementations. In cases where multiple STUN requests are
received before the answer, this may cause several pairs to be queued
up in the triggered check queue.
An agent MUST NOT utilize the ALTERNATE-SERVER mechanism, and MUST
NOT support the backwards-compatibility mechanisms to RFC 3489. It
MUST utilize the FINGERPRINT mechanism.
If the agent is using Diffserv Codepoint markings [RFC2475] in its
media packets, it SHOULD apply those same markings to its responses
to Binding requests. The same would apply to any layer 2 markings
the endpoint might be applying to media packets.
7.2.1. Additional Procedures for Full Implementations
This subsection defines the additional server procedures applicable
to full implementations.
7.2.1.1. Detecting and Repairing Role Conflicts
Normally, the rules for selection of a role in Section 5.2 will
result in each agent selecting a different role -- one controlling
and one controlled. However, in unusual call flows, typically
utilizing third party call control, it is possible for both agents to
select the same role. This section describes procedures for checking
for this case and repairing it.
An agent MUST examine the Binding request for either the ICE-
CONTROLLING or ICE-CONTROLLED attribute. It MUST follow these
procedures:
o If neither ICE-CONTROLLING nor ICE-CONTROLLED is present in the
request, the peer agent may have implemented a previous version of
this specification. There may be a conflict, but it cannot be
detected.
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o If the agent is in the controlling role, and the ICE-CONTROLLING
attribute is present in the request:
* If the agent's tie-breaker is larger than or equal to the
contents of the ICE-CONTROLLING attribute, the agent generates
a Binding error response and includes an ERROR-CODE attribute
with a value of 487 (Role Conflict) but retains its role.
* If the agent's tie-breaker is less than the contents of the
ICE-CONTROLLING attribute, the agent switches to the controlled
role.
o If the agent is in the controlled role, and the ICE-CONTROLLED
attribute is present in the request:
* If the agent's tie-breaker is larger than or equal to the
contents of the ICE-CONTROLLED attribute, the agent switches to
the controlling role.
* If the agent's tie-breaker is less than the contents of the
ICE-CONTROLLED attribute, the agent generates a Binding error
response and includes an ERROR-CODE attribute with a value of
487 (Role Conflict) but retains its role.
o If the agent is in the controlled role and the ICE-CONTROLLING
attribute was present in the request, or the agent was in the
controlling role and the ICE-CONTROLLED attribute was present in
the request, there is no conflict.
A change in roles will require an agent to recompute pair priorities
(Section 5.7.2), since those priorities are a function of controlling
and controlled roles. The change in role will also impact whether
the agent is responsible for selecting nominated pairs and generated
updated offers upon conclusion of ICE.
The remaining sections in Section 7.2.1 are followed if the server
generated a successful response to the Binding request, even if the
agent changed roles.
7.2.1.2. Computing Mapped Address
For requests being received on a relayed candidate, the source
transport address used for STUN processing (namely, generation of the
XOR-MAPPED-ADDRESS attribute) is the transport address as seen by the
TURN server. That source transport address will be present in the
XOR-PEER-ADDRESS attribute of a Data Indication message, if the
Binding request was delivered through a Data Indication. If the
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Binding request was delivered through a ChannelData message, the
source transport address is the one that was bound to the channel.
7.2.1.3. Learning Peer Reflexive Candidates
If the source transport address of the request does not match any
existing remote candidates, it represents a new peer reflexive remote
candidate. This candidate is constructed as follows:
o The priority of the candidate is set to the PRIORITY attribute
from the request.
o The type of the candidate is set to peer reflexive.
o The foundation of the candidate is set to an arbitrary value,
different from the foundation for all other remote candidates. If
any subsequent offer/answer exchanges contain this peer reflexive
candidate in the SDP, it will signal the actual foundation for the
candidate.
o The component ID of this candidate is set to the component ID for
the local candidate to which the request was sent.
This candidate is added to the list of remote candidates. However,
the agent does not pair this candidate with any local candidates.
7.2.1.4. Triggered Checks
Next, the agent constructs a pair whose local candidate is equal to
the transport address on which the STUN request was received, and a
remote candidate equal to the source transport address where the
request came from (which may be the peer reflexive remote candidate
that was just learned). The local candidate will either be a host
candidate (for cases where the request was not received through a
relay) or a relayed candidate (for cases where it is received through
a relay). The local candidate can never be a server reflexive
candidate. Since both candidates are known to the agent, it can
obtain their priorities and compute the candidate pair priority.
This pair is then looked up in the check list. There can be one of
several outcomes:
o If the pair is already on the check list:
* If the state of that pair is Waiting or Frozen, a check for
that pair is enqueued into the triggered check queue if not
already present.
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* If the state of that pair is In-Progress, the agent cancels the
in-progress transaction. Cancellation means that the agent
will not retransmit the request, will not treat the lack of
response to be a failure, but will wait the duration of the
transaction timeout for a response. In addition, the agent
MUST create a new connectivity check for that pair
(representing a new STUN Binding request transaction) by
enqueueing the pair in the triggered check queue. The state of
the pair is then changed to Waiting.
* If the state of the pair is Failed, it is changed to Waiting
and the agent MUST create a new connectivity check for that
pair (representing a new STUN Binding request transaction), by
enqueueing the pair in the triggered check queue.
* If the state of that pair is Succeeded, nothing further is
done.
These steps are done to facilitate rapid completion of ICE when
both agents are behind NAT.
o If the pair is not already on the check list:
* The pair is inserted into the check list based on its priority.
* Its state is set to Waiting.
* The pair is enqueued into the triggered check queue.
When a triggered check is to be sent, it is constructed and processed
as described in Section 7.1.2. These procedures require the agent to
know the transport address, username fragment, and password for the
peer. The username fragment for the remote candidate is equal to the
part after the colon of the USERNAME in the Binding request that was
just received. Using that username fragment, the agent can check the
SDP messages received from its peer (there may be more than one in
cases of forking), and find this username fragment. The
corresponding password is then selected.
7.2.1.5. Updating the Nominated Flag
If the Binding request received by the agent had the USE-CANDIDATE
attribute set, and the agent is in the controlled role, the agent
looks at the state of the pair computed in Section 7.2.1.4:
o If the state of this pair is Succeeded, it means that the check
generated by this pair produced a successful response. This would
have caused the agent to construct a valid pair when that success
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response was received (see Section 7.1.3.2.2). The agent now sets
the nominated flag in the valid pair to true. This may end ICE
processing for this media stream; see Section 8.
o If the state of this pair is In-Progress, if its check produces a
successful result, the resulting valid pair has its nominated flag
set when the response arrives. This may end ICE processing for
this media stream when it arrives; see Section 8.
7.2.2. Additional Procedures for Lite Implementations
If the check that was just received contained a USE-CANDIDATE
attribute, the agent constructs a candidate pair whose local
candidate is equal to the transport address on which the request was
received, and whose remote candidate is equal to the source transport
address of the request that was received. This candidate pair is
assigned an arbitrary priority, and placed into a list of valid
candidates called the valid list. The agent sets the nominated flag
for that pair to true. ICE processing is considered complete for a
media stream if the valid list contains a candidate pair for each
component.
8. Concluding ICE Processing
This section describes how an agent completes ICE.
8.1. Procedures for Full Implementations
Concluding ICE involves nominating pairs by the controlling agent and
updating of state machinery.
8.1.1. Nominating Pairs
The controlling agent nominates pairs to be selected by ICE by using
one of two techniques: regular nomination or aggressive nomination.
If its peer has a lite implementation, an agent MUST use a regular
nomination algorithm. If its peer is using ICE options (present in
an ice-options attribute from the peer) that the agent does not
understand, the agent MUST use a regular nomination algorithm. If
its peer is a full implementation and isn't using any ICE options or
is using ICE options understood by the agent, the agent MAY use
either the aggressive or the regular nomination algorithm. However,
the regular algorithm is RECOMMENDED since it provides greater
stability.
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8.1.1.1. Regular Nomination
With regular nomination, the agent lets some number of checks
complete, each of which omit the USE-CANDIDATE attribute. Once one
or more checks complete successfully for a component of a media
stream, valid pairs are generated and added to the valid list. The
agent lets the checks continue until some stopping criterion is met,
and then picks amongst the valid pairs based on an evaluation
criterion. The criteria for stopping the checks and for evaluating
the valid pairs is entirely a matter of local optimization.
When the controlling agent selects the valid pair, it repeats the
check that produced this valid pair (by enqueuing the pair that
generated the check into the triggered check queue), this time with
the USE-CANDIDATE attribute. This check should succeed (since the
previous did), causing the nominated flag of that and only that pair
to be set. Consequently, there will be only a single nominated pair
in the valid list for each component, and when the state of the check
list moves to completed, that exact pair is selected by ICE for
sending and receiving media for that component.
Regular nomination provides the most flexibility, since the agent has
control over the stopping and selection criteria for checks. The
only requirement is that the agent MUST eventually pick one and only
one candidate pair and generate a check for that pair with the USE-
CANDIDATE attribute present. Regular nomination also improves ICE's
resilience to variations in implementation (see Section 14). Regular
nomination is also more stable, allowing both agents to converge on a
single pair for media without any transient selections, which can
happen with the aggressive algorithm. The drawback of regular
nomination is that it is guaranteed to increase latencies because it
requires an additional check to be done.
8.1.1.2. Aggressive Nomination
With aggressive nomination, the controlling agent includes the USE-
CANDIDATE attribute in every check it sends. Once the first check
for a component succeeds, it will be added to the valid list and have
its nominated flag set. When all components have a nominated pair in
the valid list, media can begin to flow using the highest priority
nominated pair. However, because the agent included the USE-
CANDIDATE attribute in all of its checks, another check may yet
complete, causing another valid pair to have its nominated flag set.
ICE always selects the highest-priority nominated candidate pair from
the valid list as the one used for media. Consequently, the selected
pair may actually change briefly as ICE checks complete, resulting in
a set of transient selections until it stabilizes.
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8.1.2. Updating States
For both controlling and controlled agents, the state of ICE
processing depends on the presence of nominated candidate pairs in
the valid list and on the state of the check list. Note that, at any
time, more than one of the following cases can apply:
o If there are no nominated pairs in the valid list for a media
stream and the state of the check list is Running, ICE processing
continues.
o If there is at least one nominated pair in the valid list for a
media stream and the state of the check list is Running:
* The agent MUST remove all Waiting and Frozen pairs in the check
list and triggered check queue for the same component as the
nominated pairs for that media stream.
* If an In-Progress pair in the check list is for the same
component as a nominated pair, the agent SHOULD cease
retransmissions for its check if its pair priority is lower
than the lowest-priority nominated pair for that component.
o Once there is at least one nominated pair in the valid list for
every component of at least one media stream and the state of the
check list is Running:
* The agent MUST change the state of processing for its check
list for that media stream to Completed.
* The agent MUST continue to respond to any checks it may still
receive for that media stream, and MUST perform triggered
checks if required by the processing of Section 7.2.
* The agent MUST continue retransmitting any In-Progress checks
for that check list.
* The agent MAY begin transmitting media for this media stream as
described in Section 11.1.
o Once the state of each check list is Completed:
* The agent sets the state of ICE processing overall to
Completed.
* If an agent is controlling, it examines the highest-priority
nominated candidate pair for each component of each media
stream. If any of those candidate pairs differ from the
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default candidate pairs in the most recent offer/answer
exchange, the controlling agent MUST generate an updated offer
as described in Section 9. If the controlling agent is using
an aggressive nomination algorithm, this may result in several
updated offers as the pairs selected for media change. An
agent MAY delay sending the offer for a brief interval (one
second is RECOMMENDED) in order to allow the selected pairs to
stabilize.
o If the state of the check list is Failed, ICE has not been able to
complete for this media stream. The correct behavior depends on
the state of the check lists for other media streams:
* If all check lists are Failed, ICE processing overall is
considered to be in the Failed state, and the agent SHOULD
consider the session a failure, SHOULD NOT restart ICE, and the
controlling agent SHOULD terminate the entire session.
* If at least one of the check lists for other media streams is
Completed, the controlling agent SHOULD remove the failed media
stream from the session in its updated offer.
* If none of the check lists for other media streams are
Completed, but at least one is Running, the agent SHOULD let
ICE continue.
8.2. Procedures for Lite Implementations
Concluding ICE for a lite implementation is relatively
straightforward. There are two cases to consider:
The implementation is lite, and its peer is full.
The implementation is lite, and its peer is lite.
The effect of ICE concluding is that the agent can free any allocated
host candidates that were not utilized by ICE, as described in
Section 8.3.
8.2.1. Peer Is Full
In this case, the agent will receive connectivity checks from its
peer. When an agent has received a connectivity check that includes
the USE-CANDIDATE attribute for each component of a media stream, the
state of ICE processing for that media stream moves from Running to
Completed. When the state of ICE processing for all media streams is
Completed, the state of ICE processing overall is Completed.
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The lite implementation will never itself determine that ICE
processing has failed for a media stream; rather, the full peer will
make that determination and then remove or restart the failed media
stream in a subsequent offer.
8.2.2. Peer Is Lite
Once the offer/answer exchange has completed, both agents examine
their candidates and those of its peer. For each media stream, each
agent pairs up its own candidates with the candidates of its peer for
that media stream. Two candidates are paired up when they are for
the same component, utilize the same transport protocol (UDP in this
specification), and are from the same IP address family (IPv4 or
IPv6).
o If there is a single pair per component, that pair is added to the
Valid list. If all of the components for a media stream had one
pair, the state of ICE processing for that media stream is set to
Completed. If all media streams are Completed, the state of ICE
processing is set to Completed overall. This will always be the
case for implementations that are IPv4 only.
o If there is more than one pair per component:
* The agent MUST select a pair based on local policy. Since this
case only arises for IPv6, it is RECOMMENDED that an agent
follow the procedures of RFC 3484 [RFC3484] to select a single
pair.
* The agent adds the selected pair for each component to the
valid list. As described in Section 11.1, this will permit
media to begin flowing. However, it is possible (and in fact
likely) that both agents have chosen different pairs.
* To reconcile this, the controlling agent MUST send an updated
offer as described in Section 9.1.3, which will include the
remote-candidates attribute.
* The agent MUST NOT update the state of ICE processing when the
offer is sent. If this subsequent offer completes, the
controlling agent MUST change the state of ICE processing to
Completed for all media streams, and the state of ICE
processing overall to Completed. The states for the controlled
agent are set based on the logic in Section 9.2.3.
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8.3. Freeing Candidates
8.3.1. Full Implementation Procedures
The procedures in Section 8 require that an agent continue to listen
for STUN requests and continue to generate triggered checks for a
media stream, even once processing for that stream completes. The
rules in this section describe when it is safe for an agent to cease
sending or receiving checks on a candidate that was not selected by
ICE, and then free the candidate.
When ICE is used with SIP, and an offer is forked to multiple
recipients, ICE proceeds in parallel and independently with each
answerer, all using the same local candidates. Once ICE processing
has reached the Completed state for all peers for media streams using
those candidates, the agent SHOULD wait an additional three seconds,
and then it MAY cease responding to checks or generating triggered
checks on that candidate. It MAY free the candidate at that time.
Freeing of server reflexive candidates is never explicit; it happens
by lack of a keepalive. The three-second delay handles cases when
aggressive nomination is used, and the selected pairs can quickly
change after ICE has completed.
8.3.2. Lite Implementation Procedures
A lite implementation MAY free candidates not selected by ICE as soon
as ICE processing has reached the Completed state for all peers for
all media streams using those candidates.
9. Subsequent Offer/Answer Exchanges
Either agent MAY generate a subsequent offer at any time allowed by
RFC 3264 [RFC3264]. The rules in Section 8 will cause the
controlling agent to send an updated offer at the conclusion of ICE
processing when ICE has selected different candidate pairs from the
default pairs. This section defines rules for construction of
subsequent offers and answers.
Should a subsequent offer be rejected, ICE processing continues as if
the subsequent offer had never been made.
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9.1. Generating the Offer
9.1.1. Procedures for All Implementations
9.1.1.1. ICE Restarts
An agent MAY restart ICE processing for an existing media stream. An
ICE restart, as the name implies, will cause all previous states of
ICE processing to be flushed and checks to start anew. The only
difference between an ICE restart and a brand new media session is
that, during the restart, media can continue to be sent to the
previously validated pair.
An agent MUST restart ICE for a media stream if:
o The offer is being generated for the purposes of changing the
target of the media stream. In other words, if an agent wants to
generate an updated offer that, had ICE not been in use, would
result in a new value for the destination of a media component.
o An agent is changing its implementation level. This typically
only happens in third party call control use cases, where the
entity performing the signaling is not the entity receiving the
media, and it has changed the target of media mid-session to
another entity that has a different ICE implementation.
These rules imply that setting the IP address in the c line to
0.0.0.0 will cause an ICE restart. Consequently, ICE implementations
MUST NOT utilize this mechanism for call hold, and instead MUST use
a=inactive and a=sendonly as described in [RFC3264].
To restart ICE, an agent MUST change both the ice-pwd and the ice-
ufrag for the media stream in an offer. Note that it is permissible
to use a session-level attribute in one offer, but to provide the
same ice-pwd or ice-ufrag as a media-level attribute in a subsequent
offer. This is not a change in password, just a change in its
representation, and does not cause an ICE restart.
An agent sets the rest of the fields in the SDP for this media stream
as it would in an initial offer of this media stream (see
Section 4.3). Consequently, the set of candidates MAY include some,
none, or all of the previous candidates for that stream and MAY
include a totally new set of candidates gathered as described in
Section 4.1.1.
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9.1.1.2. Removing a Media Stream
If an agent removes a media stream by setting its port to zero, it
MUST NOT include any candidate attributes for that media stream and
SHOULD NOT include any other ICE-related attributes defined in
Section 15 for that media stream.
9.1.1.3. Adding a Media Stream
If an agent wishes to add a new media stream, it sets the fields in
the SDP for this media stream as if this was an initial offer for
that media stream (see Section 4.3). This will cause ICE processing
to begin for this media stream.
9.1.2. Procedures for Full Implementations
This section describes additional procedures for full
implementations, covering existing media streams.
The username fragments, password, and implementation level MUST
remain the same as used previously. If an agent needs to change one
of these, it MUST restart ICE for that media stream.
Additional behavior depends on the state ICE processing for that
media stream.
9.1.2.1. Existing Media Streams with ICE Running
If an agent generates an updated offer including a media stream that
was previously established, and for which ICE checks are in the
Running state, the agent follows the procedures defined here.
An agent MUST include candidate attributes for all local candidates
it had signaled previously for that media stream. The properties of
that candidate as signaled in SDP -- the priority, foundation, type,
and related transport address -- SHOULD remain the same. The IP
address, port, and transport protocol, which fundamentally identify
that candidate, MUST remain the same (if they change, it would be a
new candidate). The component ID MUST remain the same. The agent
MAY include additional candidates it did not offer previously, but
which it has gathered since the last offer/answer exchange, including
peer reflexive candidates.
The agent MAY change the default destination for media. As with
initial offers, there MUST be a set of candidate attributes in the
offer matching this default destination.
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9.1.2.2. Existing Media Streams with ICE Completed
If an agent generates an updated offer including a media stream that
was previously established, and for which ICE checks are in the
Completed state, the agent follows the procedures defined here.
The default destination for media (i.e., the values of the IP
addresses and ports in the m and c lines used for that media stream)
MUST be the local candidate from the highest-priority nominated pair
in the valid list for each component. This "fixes" the default
destination for media to equal the destination ICE has selected for
media.
The agent MUST include candidate attributes for candidates matching
the default destination for each component of the media stream, and
MUST NOT include any other candidates.
In addition, if the agent is controlling, it MUST include the
a=remote-candidates attribute for each media stream whose check list
is in the Completed state. The attribute contains the remote
candidates from the highest-priority nominated pair in the valid list
for each component of that media stream. It is needed to avoid a
race condition whereby the controlling agent chooses its pairs, but
the updated offer beats the connectivity checks to the controlled
agent, which doesn't even know these pairs are valid, let alone
selected. See Appendix B.6 for elaboration on this race condition.
9.1.3. Procedures for Lite Implementations
9.1.3.1. Existing Media Streams with ICE Running
This section describes procedures for lite implementations for
existing streams for which ICE is running.
A lite implementation MUST include all of its candidates for each
component of each media stream in an a=candidate attribute in any
subsequent offer. These candidates are formed identically to the
procedures for initial offers, as described in Section 4.2.
A lite implementation MUST NOT add additional host candidates in a
subsequent offer. If an agent needs to offer additional candidates,
it MUST restart ICE.
The username fragments, password, and implementation level MUST
remain the same as used previously. If an agent needs to change one
of these, it MUST restart ICE for that media stream.
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9.1.3.2. Existing Media Streams with ICE Completed
If ICE has completed for a media stream, the default destination for
that media stream MUST be set to the remote candidate of the
candidate pair for that component in the valid list. For a lite
implementation, there is always just a single candidate pair in the
valid list for each component of a media stream. Additionally, the
agent MUST include a candidate attribute for each default
destination.
Additionally, if the agent is controlling (which only happens when
both agents are lite), the agent MUST include the a=remote-candidates
attribute for each media stream. The attribute contains the remote
candidates from the candidate pairs in the valid list (one pair for
each component of each media stream).
9.2. Receiving the Offer and Generating an Answer
9.2.1. Procedures for All Implementations
When receiving a subsequent offer within an existing session, an
agent MUST reapply the verification procedures in Section 5.1 without
regard to the results of verification from any previous offer/answer
exchanges. Indeed, it is possible that a previous offer/answer
exchange resulted in ICE not being used, but it is used as a
consequence of a subsequent exchange.
9.2.1.1. Detecting ICE Restart
If the offer contained a change in the a=ice-ufrag or a=ice-pwd
attributes compared to the previous SDP from the peer, it indicates
that ICE is restarting for this media stream. If all media streams
are restarting, then ICE is restarting overall.
If ICE is restarting for a media stream:
o The agent MUST change the a=ice-ufrag and a=ice-pwd attributes in
the answer.
o The agent MAY change its implementation level in the answer.
An agent sets the rest of the fields in the SDP for this media stream
as it would in an initial answer to this media stream (see
Section 4.3). Consequently, the set of candidates MAY include some,
none, or all of the previous candidates for that stream and MAY
include a totally new set of candidates gathered as described in
Section 4.1.1.
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9.2.1.2. New Media Stream
If the offer contains a new media stream, the agent sets the fields
in the answer as if it had received an initial offer containing that
media stream (see Section 4.3). This will cause ICE processing to
begin for this media stream.
9.2.1.3. Removed Media Stream
If an offer contains a media stream whose port is zero, the agent
MUST NOT include any candidate attributes for that media stream in
its answer and SHOULD NOT include any other ICE-related attributes
defined in Section 15 for that media stream.
9.2.2. Procedures for Full Implementations
Unless the agent has detected an ICE restart from the offer, the
username fragments, password, and implementation level MUST remain
the same as used previously. If an agent needs to change one of
these it MUST restart ICE for that media stream by generating an
offer; ICE cannot be restarted in an answer.
Additional behaviors depend on the state of ICE processing for that
media stream.
9.2.2.1. Existing Media Streams with ICE Running and no remote-
candidates
If ICE is running for a media stream, and the offer for that media
stream lacked the remote-candidates attribute, the rules for
construction of the answer are identical to those for the offerer as
described in Section 9.1.2.1.
9.2.2.2. Existing Media Streams with ICE Completed and no remote-
candidates
If ICE is Completed for a media stream, and the offer for that media
stream lacked the remote-candidates attribute, the rules for
construction of the answer are identical to those for the offerer as
described in Section 9.1.2.2, except that the answerer MUST NOT
include the a=remote-candidates attribute in the answer.
9.2.2.3. Existing Media Streams and remote-candidates
A controlled agent will receive an offer with the a=remote-candidates
attribute for a media stream when its peer has concluded ICE
processing for that media stream. This attribute is present in the
offer to deal with a race condition between the receipt of the offer,
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and the receipt of the Binding response that tells the answerer the
candidate that will be selected by ICE. See Appendix B.6 for an
explanation of this race condition. Consequently, processing of an
offer with this attribute depends on the winner of the race.
The agent forms a candidate pair for each component of the media
stream by:
o Setting the remote candidate equal to the offerer's default
destination for that component (e.g., the contents of the m and c
lines for RTP, and the a=rtcp attribute for RTCP)
o Setting the local candidate equal to the transport address for
that same component in the a=remote-candidates attribute in the
offer.
The agent then sees if each of these candidate pairs is present in
the valid list. If a particular pair is not in the valid list, the
check has "lost" the race. Call such a pair a "losing pair".
The agent finds all the pairs in the check list whose remote
candidates equal the remote candidate in the losing pair:
o If none of the pairs are In-Progress, and at least one is Failed,
it is most likely that a network failure, such as a network
partition or serious packet loss, has occurred. The agent SHOULD
generate an answer for this media stream as if the remote-
candidates attribute had not been present, and then restart ICE
for this stream.
o If at least one of the pairs is In-Progress, the agent SHOULD wait
for those checks to complete, and as each completes, redo the
processing in this section until there are no losing pairs.
Once there are no losing pairs, the agent can generate the answer.
It MUST set the default destination for media to the candidates in
the remote-candidates attribute from the offer (each of which will
now be the local candidate of a candidate pair in the valid list).
It MUST include a candidate attribute in the answer for each
candidate in the remote-candidates attribute in the offer.
9.2.3. Procedures for Lite Implementations
If the received offer contains the remote-candidates attribute for a
media stream, the agent forms a candidate pair for each component of
the media stream by:
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o Setting the remote candidate equal to the offerer's default
destination for that component (e.g., the contents of the m and c
lines for RTP, and the a=rtcp attribute for RTCP).
o Setting the local candidate equal to the transport address for
that same component in the a=remote-candidates attribute in the
offer.
It then places those candidates into the Valid list for the media
stream. The state of ICE processing for that media stream is set to
Completed.
Furthermore, if the agent believed it was controlling, but the offer
contained the remote-candidates attribute, both agents believe they
are controlling. In this case, both would have sent updated offers
around the same time. However, the signaling protocol carrying the
offer/answer exchanges will have resolved this glare condition, so
that one agent is always the 'winner' by having its offer received
before its peer has sent an offer. The winner takes the role of
controlled, so that the loser (the answerer under consideration in
this section) MUST change its role to controlled. Consequently, if
the agent was going to send an updated offer since, based on the
rules in Section 8.2.2, it was controlling, it no longer needs to.
Besides the potential role change, change in the Valid list, and
state changes, the construction of the answer is performed
identically to the construction of an offer as described in
Section 9.1.3.
9.3. Updating the Check and Valid Lists
9.3.1. Procedures for Full Implementations
9.3.1.1. ICE Restarts
The agent MUST remember the highest-priority nominated pairs in the
Valid list for each component of the media stream, called the
previous selected pairs, prior to the restart. The agent will
continue to send media using these pairs, as described in
Section 11.1. Once these destinations are noted, the agent MUST
flush the valid and check lists, and then recompute the check list
and its states as described in Section 5.7.
9.3.1.2. New Media Stream
If the offer/answer exchange added a new media stream, the agent MUST
create a new check list for it (and an empty Valid list to start of
course), as described in Section 5.7.
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9.3.1.3. Removed Media Stream
If the offer/answer exchange removed a media stream, or an answer
rejected an offered media stream, an agent MUST flush the Valid list
for that media stream. It MUST terminate any STUN transactions in
progress for that media stream. An agent MUST remove the check list
for that media stream and cancel any pending ordinary checks for it.
9.3.1.4. ICE Continuing for Existing Media Stream
The valid list is not affected by an updated offer/answer exchange
unless ICE is restarting.
If an agent is in the Running state for that media stream, the check
list is updated (the check list is irrelevant if the state is
completed). To do that, the agent recomputes the check list using
the procedures described in Section 5.7. If a pair on the new check
list was also on the previous check list, and its state was Waiting,
In-Progress, Succeeded, or Failed, its state is copied over.
Otherwise, its state is set to Frozen.
If none of the check lists are active (meaning that the pairs in each
check list are Frozen), the full-mode agent sets the first pair in
the check list for the first media stream to Waiting, and then sets
the state of all other pairs in that check list for the same
component ID and with the same foundation to Waiting as well.
Next, the agent goes through each check list, starting with the
highest-priority pair. If a pair has a state of Succeeded, and it
has a component ID of 1, then all Frozen pairs in the same check list
with the same foundation whose component IDs are not 1 have their
state set to Waiting. If, for a particular check list, there are
pairs for each component of that media stream in the Succeeded state,
the agent moves the state of all Frozen pairs for the first component
of all other media streams (and thus in different check lists) with
the same foundation to Waiting.
9.3.2. Procedures for Lite Implementations
If ICE is restarting for a media stream, the agent MUST start a new
Valid list for that media stream. It MUST remember the pairs in the
previous Valid list for each component of the media stream, called
the previous selected pairs, and continue to send media there as
described in Section 11.1. The state of ICE processing for each
media stream MUST change to Running, and the state of ICE processing
MUST change to Running.
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10. Keepalives
All endpoints MUST send keepalives for each media session. These
keepalives serve the purpose of keeping NAT bindings alive for the
media session. These keepalives MUST be sent regardless of whether
the media stream is currently inactive, sendonly, recvonly, or
sendrecv, and regardless of the presence or value of the bandwidth
attribute. These keepalives MUST be sent even if ICE is not being
utilized for the session at all. The keepalive SHOULD be sent using
a format that is supported by its peer. ICE endpoints allow for
STUN-based keepalives for UDP streams, and as such, STUN keepalives
MUST be used when an agent is a full ICE implementation and is
communicating with a peer that supports ICE (lite or full). An agent
can determine that its peer supports ICE by the presence of
a=candidate attributes for each media session. If the peer does not
support ICE, the choice of a packet format for keepalives is a matter
of local implementation. A format that allows packets to easily be
sent in the absence of actual media content is RECOMMENDED. Examples
of formats that readily meet this goal are RTP No-Op [NO-OP-RTP], and
in cases where both sides support it, RTP comfort noise [RFC3389].
If the peer doesn't support any formats that are particularly well
suited for keepalives, an agent SHOULD send RTP packets with an
incorrect version number, or some other form of error that would
cause them to be discarded by the peer.
If there has been no packet sent on the candidate pair ICE is using
for a media component for Tr seconds (where packets include those
defined for the component (RTP or RTCP) and previous keepalives), an
agent MUST generate a keepalive on that pair. Tr SHOULD be
configurable and SHOULD have a default of 15 seconds. Tr MUST NOT be
configured to less than 15 seconds. Alternatively, if an agent has a
dynamic way to discover the binding lifetimes of the intervening
NATs, it can use that value to determine Tr. Administrators
deploying ICE in more controlled networking environments SHOULD set
Tr to the longest duration possible in their environment.
If STUN is being used for keepalives, a STUN Binding Indication is
used [RFC5389]. The Indication MUST NOT utilize any authentication
mechanism. It SHOULD contain the FINGERPRINT attribute to aid in
demultiplexing, but SHOULD NOT contain any other attributes. It is
used solely to keep the NAT bindings alive. The Binding Indication
is sent using the same local and remote candidates that are being
used for media. Though Binding Indications are used for keepalives,
an agent MUST be prepared to receive a connectivity check as well.
If a connectivity check is received, a response is generated as
discussed in [RFC5389], but there is no impact on ICE processing
otherwise.
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An agent MUST begin the keepalive processing once ICE has selected
candidates for usage with media, or media begins to flow, whichever
happens first. Keepalives end once the session terminates or the
media stream is removed.
11. Media Handling
11.1. Sending Media
Procedures for sending media differ for full and lite
implementations.
11.1.1. Procedures for Full Implementations
Agents always send media using a candidate pair, called the selected
candidate pair. An agent will send media to the remote candidate in
the selected pair (setting the destination address and port of the
packet equal to that remote candidate), and will send it from the
local candidate of the selected pair. When the local candidate is
server or peer reflexive, media is originated from the base. Media
sent from a relayed candidate is sent from the base through that TURN
server, using procedures defined in [RFC5766].
If the local candidate is a relayed candidate, it is RECOMMENDED that
an agent create a channel on the TURN server towards the remote
candidate. This is done using the procedures for channel creation as
defined in Section 11 of [RFC5766].
The selected pair for a component of a media stream is:
o empty if the state of the check list for that media stream is
Running, and there is no previous selected pair for that component
due to an ICE restart
o equal to the previous selected pair for a component of a media
stream if the state of the check list for that media stream is
Running, and there was a previous selected pair for that component
due to an ICE restart
o equal to the highest-priority nominated pair for that component in
the valid list if the state of the check list is Completed
If the selected pair for at least one component of a media stream is
empty, an agent MUST NOT send media for any component of that media
stream. If the selected pair for each component of a media stream
has a value, an agent MAY send media for all components of that media
stream.
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Note that the selected pair for a component of a media stream may not
equal the default pair for that same component from the most recent
offer/answer exchange. When this happens, the selected pair is used
for media, not the default pair. When ICE first completes, if the
selected pairs aren't a match for the default pairs, the controlling
agent sends an updated offer/answer exchange to remedy this
disparity. However, until that updated offer arrives, there will not
be a match. Furthermore, in very unusual cases, the default
candidates in the updated offer/answer will not be a match.
11.1.2. Procedures for Lite Implementations
A lite implementation MUST NOT send media until it has a Valid list
that contains a candidate pair for each component of that media
stream. Once that happens, the agent MAY begin sending media
packets. To do that, it sends media to the remote candidate in the
pair (setting the destination address and port of the packet equal to
that remote candidate), and will send it from the local candidate.
11.1.3. Procedures for All Implementations
ICE has interactions with jitter buffer adaptation mechanisms. An
RTP stream can begin using one candidate, and switch to another one,
though this happens rarely with ICE. The newer candidate may result
in RTP packets taking a different path through the network -- one
with different delay characteristics. As discussed below, agents are
encouraged to re-adjust jitter buffers when there are changes in
source or destination address of media packets. Furthermore, many
audio codecs use the marker bit to signal the beginning of a
talkspurt, for the purposes of jitter buffer adaptation. For such
codecs, it is RECOMMENDED that the sender set the marker bit
[RFC3550] when an agent switches transmission of media from one
candidate pair to another.
11.2. Receiving Media
ICE implementations MUST be prepared to receive media on each
component on any candidates provided for that component in the most
recent offer/answer exchange (in the case of RTP, this would include
both RTP and RTCP if candidates were provided for both).
It is RECOMMENDED that, when an agent receives an RTP packet with a
new source or destination IP address for a particular media stream,
that the agent re-adjust its jitter buffers.
RFC 3550 [RFC3550] describes an algorithm in Section 8.2 for
detecting synchronization source (SSRC) collisions and loops. These
algorithms are based, in part, on seeing different source transport
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addresses with the same SSRC. However, when ICE is used, such
changes will sometimes occur as the media streams switch between
candidates. An agent will be able to determine that a media stream
is from the same peer as a consequence of the STUN exchange that
proceeds media transmission. Thus, if there is a change in source
transport address, but the media packets come from the same peer
agent, this SHOULD NOT be treated as an SSRC collision.
12. Usage with SIP
12.1. Latency Guidelines
ICE requires a series of STUN-based connectivity checks to take place
between endpoints. These checks start from the answerer on
generation of its answer, and start from the offerer when it receives
the answer. These checks can take time to complete, and as such, the
selection of messages to use with offers and answers can affect
perceived user latency. Two latency figures are of particular
interest. These are the post-pickup delay and the post-dial delay.
The post-pickup delay refers to the time between when a user "answers
the phone" and when any speech they utter can be delivered to the
caller. The post-dial delay refers to the time between when a user
enters the destination address for the user and ringback begins as a
consequence of having successfully started ringing the phone of the
called party.
Two cases can be considered -- one where the offer is present in the
initial INVITE and one where it is in a response.
12.1.1. Offer in INVITE
To reduce post-dial delays, it is RECOMMENDED that the caller begin
gathering candidates prior to actually sending its initial INVITE.
This can be started upon user interface cues that a call is pending,
such as activity on a keypad or the phone going offhook.
If an offer is received in an INVITE request, the answerer SHOULD
begin to gather its candidates on receipt of the offer and then
generate an answer in a provisional response once it has completed
that process. ICE requires that a provisional response with an SDP
be transmitted reliably. This can be done through the existing
Provisional Response Acknowledgment (PRACK) mechanism [RFC3262] or
through an optimization that is specific to ICE. With this
optimization, provisional responses containing an SDP answer that
begins ICE processing for one or more media streams can be sent
reliably without RFC 3262. To do this, the agent retransmits the
provisional response with the exponential backoff timers described in
RFC 3262. Retransmits MUST cease on receipt of a STUN Binding
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request for one of the media streams signaled in that SDP (because
receipt of a Binding request indicates the offerer has received the
answer) or on transmission of the answer in a 2xx response. If the
peer agent is lite, there will never be a STUN Binding request. In
such a case, the agent MUST cease retransmitting the 18x after
sending it four times (ICE will actually work even if the peer never
receives the 18x; however, experience has shown that sending it is
important for middleboxes and firewall traversal). If no Binding
request is received prior to the last retransmit, the agent does not
consider the session terminated. Despite the fact that the
provisional response will be delivered reliably, the rules for when
an agent can send an updated offer or answer do not change from those
specified in RFC 3262. Specifically, if the INVITE contained an
offer, the same answer appears in all of the 1xx and in the 2xx
response to the INVITE. Only after that 2xx has been sent can an
updated offer/answer exchange occur. This optimization SHOULD NOT be
used if both agents support PRACK. Note that the optimization is
very specific to provisional response carrying answers that start ICE
processing; it is not a general technique for 1xx reliability.
Alternatively, an agent MAY delay sending an answer until the 200 OK;
however, this results in a poor user experience and is NOT
RECOMMENDED.
Once the answer has been sent, the agent SHOULD begin its
connectivity checks. Once candidate pairs for each component of a
media stream enter the valid list, the answerer can begin sending
media on that media stream.
However, prior to this point, any media that needs to be sent towards
the caller (such as SIP early media [RFC3960]) MUST NOT be
transmitted. For this reason, implementations SHOULD delay alerting
the called party until candidates for each component of each media
stream have entered the valid list. In the case of a PSTN gateway,
this would mean that the setup message into the PSTN is delayed until
this point. Doing this increases the post-dial delay, but has the
effect of eliminating 'ghost rings'. Ghost rings are cases where the
called party hears the phone ring, picks up, but hears nothing and
cannot be heard. This technique works without requiring support for,
or usage of, preconditions [RFC3312], since it's a localized
decision. It also has the benefit of guaranteeing that not a single
packet of media will get clipped, so that post-pickup delay is zero.
If an agent chooses to delay local alerting in this way, it SHOULD
generate a 180 response once alerting begins.
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12.1.2. Offer in Response
In addition to uses where the offer is in an INVITE, and the answer
is in the provisional and/or 200 OK response, ICE works with cases
where the offer appears in the response. In such cases, which are
common in third party call control [RFC3725], ICE agents SHOULD
generate their offers in a reliable provisional response (which MUST
utilize RFC 3262), and not alert the user on receipt of the INVITE.
The answer will arrive in a PRACK. This allows for ICE processing to
take place prior to alerting, so that there is no post-pickup delay,
at the expense of increased call setup delays. Once ICE completes,
the callee can alert the user and then generate a 200 OK when they
answer. The 200 OK would contain no SDP, since the offer/answer
exchange has completed.
Alternatively, agents MAY place the offer in a 2xx instead (in which
case the answer comes in the ACK). When this happens, the callee
will alert the user on receipt of the INVITE, and the ICE exchanges
will take place only after the user answers. This has the effect of
reducing call setup delay, but can cause substantial post-pickup
delays and media clipping.
12.2. SIP Option Tags and Media Feature Tags
[RFC5768] specifies a SIP option tag and media feature tag for usage
with ICE. ICE implementations using SIP SHOULD support this
specification, which uses a feature tag in registrations to
facilitate interoperability through signaling intermediaries.
12.3. Interactions with Forking
ICE interacts very well with forking. Indeed, ICE fixes some of the
problems associated with forking. Without ICE, when a call forks and
the caller receives multiple incoming media streams, it cannot
determine which media stream corresponds to which callee.
With ICE, this problem is resolved. The connectivity checks which
occur prior to transmission of media carry username fragments, which
in turn are correlated to a specific callee. Subsequent media
packets that arrive on the same candidate pair as the connectivity
check will be associated with that same callee. Thus, the caller can
perform this correlation as long as it has received an answer.
12.4. Interactions with Preconditions
Quality of Service (QoS) preconditions, which are defined in RFC 3312
[RFC3312] and RFC 4032 [RFC4032], apply only to the transport
addresses listed as the default targets for media in an offer/answer.
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If ICE changes the transport address where media is received, this
change is reflected in an updated offer that changes the default
destination for media to match ICE's selection. As such, it appears
like any other re-INVITE would, and is fully treated in RFCs 3312 and
4032, which apply without regard to the fact that the destination for
media is changing due to ICE negotiations occurring "in the
background".
Indeed, an agent SHOULD NOT indicate that QoS preconditions have been
met until the checks have completed and selected the candidate pairs
to be used for media.
ICE also has (purposeful) interactions with connectivity
preconditions [SDP-PRECON]. Those interactions are described there.
Note that the procedures described in Section 12.1 describe their own
type of "preconditions", albeit with less functionality than those
provided by the explicit preconditions in [SDP-PRECON].
12.5. Interactions with Third Party Call Control
ICE works with Flows I, III, and IV as described in [RFC3725]. Flow
I works without the controller supporting or being aware of ICE.
Flow IV will work as long as the controller passes along the ICE
attributes without alteration. Flow II is fundamentally incompatible
with ICE; each agent will believe itself to be the answerer and thus
never generate a re-INVITE.
The flows for continued operation, as described in Section 7 of RFC
3725, require additional behavior of ICE implementations to support.
In particular, if an agent receives a mid-dialog re-INVITE that
contains no offer, it MUST restart ICE for each media stream and go
through the process of gathering new candidates. Furthermore, that
list of candidates SHOULD include the ones currently being used for
media.
13. Relationship with ANAT
RFC 4091 [RFC4091], the Alternative Network Address Types (ANAT)
Semantics for the SDP grouping framework, and RFC 4092 [RFC4092], its
usage with SIP, define a mechanism for indicating that an agent can
support both IPv4 and IPv6 for a media stream, and it does so by
including two m lines, one for v4 and one for v6. This is similar to
ICE, which allows for an agent to indicate multiple transport
addresses using the candidate attribute. However, ANAT relies on
static selection to pick between choices, rather than a dynamic
connectivity check used by ICE.
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This specification deprecates RFC 4091 and RFC 4092. Instead, agents
wishing to support dual stack will utilize ICE.
14. Extensibility Considerations
This specification makes very specific choices about how both agents
in a session coordinate to arrive at the set of candidate pairs that
are selected for media. It is anticipated that future specifications
will want to alter these algorithms, whether they are simple changes
like timer tweaks or larger changes like a revamp of the priority
algorithm. When such a change is made, providing interoperability
between the two agents in a session is critical.
First, ICE provides the a=ice-options SDP attribute. Each extension
or change to ICE is associated with a token. When an agent
supporting such an extension or change generates an offer or an
answer, it MUST include the token for that extension in this
attribute. This allows each side to know what the other side is
doing. This attribute MUST NOT be present if the agent doesn't
support any ICE extensions or changes.
At this time, no IANA registry or registration procedures are defined
for these option tags. At time of writing, it is unclear whether ICE
changes and extensions will be sufficiently common to warrant a
registry.
One of the complications in achieving interoperability is that ICE
relies on a distributed algorithm running on both agents to converge
on an agreed set of candidate pairs. If the two agents run different
algorithms, it can be difficult to guarantee convergence on the same
candidate pairs. The regular nomination procedure described in
Section 8 eliminates some of the tight coordination by delegating the
selection algorithm completely to the controlling agent.
Consequently, when a controlling agent is communicating with a peer
that supports options it doesn't know about, the agent MUST run a
regular nomination algorithm. When regular nomination is used, ICE
will converge perfectly even when both agents use different pair
prioritization algorithms. One of the keys to such convergence is
triggered checks, which ensure that the nominated pair is validated
by both agents. Consequently, any future ICE enhancements MUST
preserve triggered checks.
ICE is also extensible to other media streams beyond RTP, and for
transport protocols beyond UDP. Extensions to ICE for non-RTP media
streams need to specify how many components they utilize, and assign
component IDs to them, starting at 1 for the most important component
ID. Specifications for new transport protocols must define how, if
at all, various steps in the ICE processing differ from UDP.
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15. Grammar
This specification defines seven new SDP attributes -- the
"candidate", "remote-candidates", "ice-lite", "ice-mismatch", "ice-
ufrag", "ice-pwd", and "ice-options" attributes.
15.1. "candidate" Attribute
The candidate attribute is a media-level attribute only. It contains
a transport address for a candidate that can be used for connectivity
checks.
The syntax of this attribute is defined using Augmented BNF as
defined in RFC 5234 [RFC5234]:
candidate-attribute = "candidate" ":" foundation SP component-id SP
transport SP
priority SP
connection-address SP ;from RFC 4566
port ;port from RFC 4566
SP cand-type
[SP rel-addr]
[SP rel-port]
*(SP extension-att-name SP
extension-att-value)
foundation = 1*32ice-char
component-id = 1*5DIGIT
transport = "UDP" / transport-extension
transport-extension = token ; from RFC 3261
priority = 1*10DIGIT
cand-type = "typ" SP candidate-types
candidate-types = "host" / "srflx" / "prflx" / "relay" / token
rel-addr = "raddr" SP connection-address
rel-port = "rport" SP port
extension-att-name = byte-string ;from RFC 4566
extension-att-value = byte-string
ice-char = ALPHA / DIGIT / "+" / "/"
This grammar encodes the primary information about a candidate: its
IP address, port and transport protocol, and its properties: the
foundation, component ID, priority, type, and related transport
address:
: is taken from RFC 4566 [RFC4566]. It is the
IP address of the candidate, allowing for IPv4 addresses, IPv6
addresses, and fully qualified domain names (FQDNs). When parsing
this field, an agent can differentiate an IPv4 address and an IPv6
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address by presence of a colon in its value - the presence of a
colon indicates IPv6. An agent MUST ignore candidate lines that
include candidates with IP address versions that are not supported
or recognized. An IP address SHOULD be used, but an FQDN MAY be
used in place of an IP address. In that case, when receiving an
offer or answer containing an FQDN in an a=candidate attribute,
the FQDN is looked up in the DNS first using an AAAA record
(assuming the agent supports IPv6), and if no result is found or
the agent only supports IPv4, using an A. If the DNS query
returns more than one IP address, one is chosen, and then used for
the remainder of ICE processing.
: is also taken from RFC 4566 [RFC4566]. It is the port of
the candidate.
: indicates the transport protocol for the candidate.
This specification only defines UDP. However, extensibility is
provided to allow for future transport protocols to be used with
ICE, such as TCP or the Datagram Congestion Control Protocol
(DCCP) [RFC4340].
: is composed of 1 to 32 s. It is an
identifier that is equivalent for two candidates that are of the
same type, share the same base, and come from the same STUN
server. The foundation is used to optimize ICE performance in the
Frozen algorithm.
: is a positive integer between 1 and 256 that
identifies the specific component of the media stream for which
this is a candidate. It MUST start at 1 and MUST increment by 1
for each component of a particular candidate. For media streams
based on RTP, candidates for the actual RTP media MUST have a
component ID of 1, and candidates for RTCP MUST have a component
ID of 2. Other types of media streams that require multiple
components MUST develop specifications that define the mapping of
components to component IDs. See Section 14 for additional
discussion on extending ICE to new media streams.
: is a positive integer between 1 and (2**31 - 1).
: encodes the type of candidate. This specification
defines the values "host", "srflx", "prflx", and "relay" for host,
server reflexive, peer reflexive, and relayed candidates,
respectively. The set of candidate types is extensible for the
future.
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and : convey transport addresses related to the
candidate, useful for diagnostics and other purposes.
and MUST be present for server reflexive, peer
reflexive, and relayed candidates. If a candidate is server or
peer reflexive, and are equal to the base
for that server or peer reflexive candidate. If the candidate is
relayed, and is equal to the mapped address
in the Allocate response that provided the client with that
relayed candidate (see Appendix B.3 for a discussion of its
purpose). If the candidate is a host candidate, and
MUST be omitted.
The candidate attribute can itself be extended. The grammar allows
for new name/value pairs to be added at the end of the attribute. An
implementation MUST ignore any name/value pairs it doesn't
understand.
15.2. "remote-candidates" Attribute
The syntax of the "remote-candidates" attribute is defined using
Augmented BNF as defined in RFC 5234 [RFC5234]. The remote-
candidates attribute is a media-level attribute only.
remote-candidate-att = "remote-candidates" ":" remote-candidate
0*(SP remote-candidate)
remote-candidate = component-ID SP connection-address SP port
The attribute contains a connection-address and port for each
component. The ordering of components is irrelevant. However, a
value MUST be present for each component of a media stream. This
attribute MUST be included in an offer by a controlling agent for a
media stream that is Completed, and MUST NOT be included in any other
case.
15.3. "ice-lite" and "ice-mismatch" Attributes
The syntax of the "ice-lite" and "ice-mismatch" attributes, both of
which are flags, is:
ice-lite = "ice-lite"
ice-mismatch = "ice-mismatch"
"ice-lite" is a session-level attribute only, and indicates that an
agent is a lite implementation. "ice-mismatch" is a media-level
attribute only, and when present in an answer, indicates that the
offer arrived with a default destination for a media component that
didn't have a corresponding candidate attribute.
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15.4. "ice-ufrag" and "ice-pwd" Attributes
The "ice-ufrag" and "ice-pwd" attributes convey the username fragment
and password used by ICE for message integrity. Their syntax is:
ice-pwd-att = "ice-pwd" ":" password
ice-ufrag-att = "ice-ufrag" ":" ufrag
password = 22*256ice-char
ufrag = 4*256ice-char
The "ice-pwd" and "ice-ufrag" attributes can appear at either the
session-level or media-level. When present in both, the value in the
media-level takes precedence. Thus, the value at the session-level
is effectively a default that applies to all media streams, unless
overridden by a media-level value. Whether present at the session or
media-level, there MUST be an ice-pwd and ice-ufrag attribute for
each media stream. If two media streams have identical ice-ufrag's,
they MUST have identical ice-pwd's.
The ice-ufrag and ice-pwd attributes MUST be chosen randomly at the
beginning of a session. The ice-ufrag attribute MUST contain at
least 24 bits of randomness, and the ice-pwd attribute MUST contain
at least 128 bits of randomness. This means that the ice-ufrag
attribute will be at least 4 characters long, and the ice-pwd at
least 22 characters long, since the grammar for these attributes
allows for 6 bits of randomness per character. The attributes MAY be
longer than 4 and 22 characters, respectively, of course, up to 256
characters. The upper limit allows for buffer sizing in
implementations. Its large upper limit allows for increased amounts
of randomness to be added over time.
15.5. "ice-options" Attribute
The "ice-options" attribute is a session-level attribute. It
contains a series of tokens that identify the options supported by
the agent. Its grammar is:
ice-options = "ice-options" ":" ice-option-tag
0*(SP ice-option-tag)
ice-option-tag = 1*ice-char
16. Setting Ta and RTO
During the gathering phase of ICE (Section 4.1.1) and while ICE is
performing connectivity checks (Section 7), an agent sends STUN and
TURN transactions. These transactions are paced at a rate of one
every Ta milliseconds, and utilize a specific RTO. This section
describes how the values of Ta and RTO are computed. This
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computation depends on whether ICE is being used with a real-time
media stream (such as RTP) or something else. When ICE is used for a
stream with a known maximum bandwidth, the computation in
Section 16.1 MAY be followed to rate-control the ICE exchanges. For
all other streams, the computation in Section 16.2 MUST be followed.
16.1. RTP Media Streams
The values of RTO and Ta change during the lifetime of ICE
processing. One set of values applies during the gathering phase,
and the other, for connectivity checks.
The value of Ta SHOULD be configurable, and SHOULD have a default of:
For each media stream i:
Ta_i = (stun_packet_size / rtp_packet_size) * rtp_ptime
1
Ta = MAX (20ms, ------------------- )
k
----
\ 1
> ------
/ Ta_i
----
i=1
where k is the number of media streams. During the gathering phase,
Ta is computed based on the number of media streams the agent has
indicated in its offer or answer, and the RTP packet size and RTP
ptime are those of the most preferred codec for each media stream.
Once an offer and answer have been exchanged, the agent recomputes Ta
to pace the connectivity checks. In that case, the value of Ta is
based on the number of media streams that will actually be used in
the session, and the RTP packet size and RTP ptime are those of the
most preferred codec with which the agent will send.
In addition, the retransmission timer for the STUN transactions, RTO,
defined in [RFC5389], SHOULD be configurable and during the gathering
phase, SHOULD have a default of:
RTO = MAX (100ms, Ta * (number of pairs))
where the number of pairs refers to the number of pairs of candidates
with STUN or TURN servers.
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For connectivity checks, RTO SHOULD be configurable and SHOULD have a
default of:
RTO = MAX (100ms, Ta*N * (Num-Waiting + Num-In-Progress))
where Num-Waiting is the number of checks in the check list in the
Waiting state, and Num-In-Progress is the number of checks in the In-
Progress state. Note that the RTO will be different for each
transaction as the number of checks in the Waiting and In-Progress
states change.
These formulas are aimed at causing STUN transactions to be paced at
the same rate as media. This ensures that ICE will work properly
under the same network conditions needed to support the media as
well. See Appendix B.1 for additional discussion and motivations.
Because of this pacing, it will take a certain amount of time to
obtain all of the server reflexive and relayed candidates.
Implementations should be aware of the time required to do this, and
if the application requires a time budget, limit the number of
candidates that are gathered.
The formulas result in a behavior whereby an agent will send its
first packet for every single connectivity check before performing a
retransmit. This can be seen in the formulas for the RTO (which
represents the retransmit interval). Those formulas scale with N,
the number of checks to be performed. As a result of this, ICE
maintains a nicely constant rate, but becomes more sensitive to
packet loss. The loss of the first single packet for any
connectivity check is likely to cause that pair to take a long time
to be validated, and instead, a lower-priority check (but one for
which there was no packet loss) is much more likely to complete
first. This results in ICE performing sub-optimally, choosing lower-
priority pairs over higher-priority pairs. Implementors should be
aware of this consequence, but still should utilize the timer values
described here.
16.2. Non-RTP Sessions
In cases where ICE is used to establish some kind of session that is
not real time, and has no fixed rate associated with it that is known
to work on the network in which ICE is deployed, Ta and RTO revert to
more conservative values. Ta SHOULD be configurable, SHOULD have a
default of 500 ms, and MUST NOT be configurable to be less than 500
ms.
In addition, the retransmission timer for the STUN transactions, RTO,
SHOULD be configurable and during the gathering phase, SHOULD have a
default of:
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RTO = MAX (500ms, Ta * (number of pairs))
where the number of pairs refers to the number of pairs of candidates
with STUN or TURN servers.
For connectivity checks, RTO SHOULD be configurable and SHOULD have a
default of:
RTO = MAX (500ms, Ta*N * (Num-Waiting + Num-In-Progress))
17. Example
The example is based on the simplified topology of Figure 8.
+-----+
| |
|STUN |
| Srvr|
+-----+
|
+---------------------+
| |
| Internet |
| |
| |
+---------------------+
| |
| |
+---------+ |
| NAT | |
+---------+ |
| |
| |
| |
+-----+ +-----+
| | | |
| L | | R |
| | | |
+-----+ +-----+
Figure 8: Example Topology
Two agents, L and R, are using ICE. Both are full-mode ICE
implementations and use aggressive nomination when they are
controlling. Both agents have a single IPv4 address. For agent L,
it is 10.0.1.1 in private address space [RFC1918], and for agent R,
192.0.2.1 on the public Internet. Both are configured with the same
STUN server (shown in this example for simplicity, although in
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practice the agents do not need to use the same STUN server), which
is listening for STUN Binding requests at an IP address of 192.0.2.2
and port 3478. TURN servers are not used in this example. Agent L
is behind a NAT, and agent R is on the public Internet. The NAT has
an endpoint independent mapping property and an address dependent
filtering property. The public side of the NAT has an IP address of
192.0.2.3.
To facilitate understanding, transport addresses are listed using
variables that have mnemonic names. The format of the name is
entity-type-seqno, where entity refers to the entity whose IP address
the transport address is on, and is one of "L", "R", "STUN", or
"NAT". The type is either "PUB" for transport addresses that are
public, and "PRIV" for transport addresses that are private.
Finally, seq-no is a sequence number that is different for each
transport address of the same type on a particular entity. Each
variable has an IP address and port, denoted by varname.IP and
varname.PORT, respectively, where varname is the name of the
variable.
The STUN server has advertised transport address STUN-PUB-1 (which is
192.0.2.2:3478).
In the call flow itself, STUN messages are annotated with several
attributes. The "S=" attribute indicates the source transport
address of the message. The "D=" attribute indicates the destination
transport address of the message. The "MA=" attribute is used in
STUN Binding response messages and refers to the mapped address.
"USE-CAND" implies the presence of the USE-CANDIDATE attribute.
The call flow examples omit STUN authentication operations and RTCP,
and focus on RTP for a single media stream between two full
implementations.
L NAT STUN R
|RTP STUN alloc. | |
|(1) STUN Req | | |
|S=$L-PRIV-1 | | |
|D=$STUN-PUB-1 | | |
|------------->| | |
| |(2) STUN Req | |
| |S=$NAT-PUB-1 | |
| |D=$STUN-PUB-1 | |
| |------------->| |
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| |(3) STUN Res | |
| |S=$STUN-PUB-1 | |
| |D=$NAT-PUB-1 | |
| |MA=$NAT-PUB-1 | |
| |<-------------| |
|(4) STUN Res | | |
|S=$STUN-PUB-1 | | |
|D=$L-PRIV-1 | | |
|MA=$NAT-PUB-1 | | |
|<-------------| | |
|(5) Offer | | |
|------------------------------------------->|
| | | |RTP STUN
alloc.
| | |(6) STUN Req |
| | |S=$R-PUB-1 |
| | |D=$STUN-PUB-1 |
| | |<-------------|
| | |(7) STUN Res |
| | |S=$STUN-PUB-1 |
| | |D=$R-PUB-1 |
| | |MA=$R-PUB-1 |
| | |------------->|
|(8) answer | | |
|<-------------------------------------------|
| |(9) Bind Req | |Begin
| |S=$R-PUB-1 | |Connectivity
| |D=L-PRIV-1 | |Checks
| |<----------------------------|
| |Dropped | |
|(10) Bind Req | | |
|S=$L-PRIV-1 | | |
|D=$R-PUB-1 | | |
|USE-CAND | | |
|------------->| | |
| |(11) Bind Req | |
| |S=$NAT-PUB-1 | |
| |D=$R-PUB-1 | |
| |USE-CAND | |
| |---------------------------->|
| |(12) Bind Res | |
| |S=$R-PUB-1 | |
| |D=$NAT-PUB-1 | |
| |MA=$NAT-PUB-1 | |
| |<----------------------------|
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|(13) Bind Res | | |
|S=$R-PUB-1 | | |
|D=$L-PRIV-1 | | |
|MA=$NAT-PUB-1 | | |
|<-------------| | |
|RTP flows | | |
| |(14) Bind Req | |
| |S=$R-PUB-1 | |
| |D=$NAT-PUB-1 | |
| |<----------------------------|
|(15) Bind Req | | |
|S=$R-PUB-1 | | |
|D=$L-PRIV-1 | | |
|<-------------| | |
|(16) Bind Res | | |
|S=$L-PRIV-1 | | |
|D=$R-PUB-1 | | |
|MA=$R-PUB-1 | | |
|------------->| | |
| |(17) Bind Res | |
| |S=$NAT-PUB-1 | |
| |D=$R-PUB-1 | |
| |MA=$R-PUB-1 | |
| |---------------------------->|
| | | |RTP flows
Figure 9: Example Flow
First, agent L obtains a host candidate from its local IP address
(not shown), and from that, sends a STUN Binding request to the STUN
server to get a server reflexive candidate (messages 1-4). Recall
that the NAT has the address and port independent mapping property.
Here, it creates a binding of NAT-PUB-1 for this UDP request, and
this becomes the server reflexive candidate for RTP.
Agent L sets a type preference of 126 for the host candidate and 100
for the server reflexive. The local preference is 65535. Based on
this, the priority of the host candidate is 2130706431 and for the
server reflexive candidate is 1694498815. The host candidate is
assigned a foundation of 1, and the server reflexive, a foundation of
2. It chooses its server reflexive candidate as the default
candidate, and encodes it into the m and c lines. The resulting
offer (message 5) looks like (lines folded for clarity):
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v=0
o=jdoe 2890844526 2890842807 IN IP4 $L-PRIV-1.IP
s=
c=IN IP4 $NAT-PUB-1.IP
t=0 0
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=audio $NAT-PUB-1.PORT RTP/AVP 0
b=RS:0
b=RR:0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 $L-PRIV-1.IP $L-PRIV-1.PORT typ
host
a=candidate:2 1 UDP 1694498815 $NAT-PUB-1.IP $NAT-PUB-1.PORT typ
srflx raddr $L-PRIV-1.IP rport $L-PRIV-1.PORT
The offer, with the variables replaced with their values, will look
like (lines folded for clarity):
v=0
o=jdoe 2890844526 2890842807 IN IP4 10.0.1.1
s=
c=IN IP4 192.0.2.3
t=0 0
a=ice-pwd:asd88fgpdd777uzjYhagZg
a=ice-ufrag:8hhY
m=audio 45664 RTP/AVP 0
b=RS:0
b=RR:0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 10.0.1.1 8998 typ host
a=candidate:2 1 UDP 1694498815 192.0.2.3 45664 typ srflx raddr
10.0.1.1 rport 8998
This offer is received at agent R. Agent R will obtain a host
candidate, and from it, obtain a server reflexive candidate (messages
6-7). Since R is not behind a NAT, this candidate is identical to
its host candidate, and they share the same base. It therefore
discards this redundant candidate and ends up with a single host
candidate. With identical type and local preferences as L, the
priority for this candidate is 2130706431. It chooses a foundation
of 1 for its single candidate. Its resulting answer looks like:
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v=0
o=bob 2808844564 2808844564 IN IP4 $R-PUB-1.IP
s=
c=IN IP4 $R-PUB-1.IP
t=0 0
a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh
a=ice-ufrag:9uB6
m=audio $R-PUB-1.PORT RTP/AVP 0
b=RS:0
b=RR:0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 $R-PUB-1.IP $R-PUB-1.PORT typ host
With the variables filled in:
v=0
o=bob 2808844564 2808844564 IN IP4 192.0.2.1
s=
c=IN IP4 192.0.2.1
t=0 0
a=ice-pwd:YH75Fviy6338Vbrhrlp8Yh
a=ice-ufrag:9uB6
m=audio 3478 RTP/AVP 0
b=RS:0
b=RR:0
a=rtpmap:0 PCMU/8000
a=candidate:1 1 UDP 2130706431 192.0.2.1 3478 typ host
Since neither side indicated that it is lite, the agent that sent the
offer that began ICE processing (agent L) becomes the controlling
agent.
Agents L and R both pair up the candidates. They both initially have
two pairs. However, agent L will prune the pair containing its
server reflexive candidate, resulting in just one. At agent L, this
pair has a local candidate of $L_PRIV_1 and remote candidate of
$R_PUB_1, and has a candidate pair priority of 4.57566E+18 (note that
an implementation would represent this as a 64-bit integer so as not
to lose precision). At agent R, there are two pairs. The highest
priority has a local candidate of $R_PUB_1 and remote candidate of
$L_PRIV_1 and has a priority of 4.57566E+18, and the second has a
local candidate of $R_PUB_1 and remote candidate of $NAT_PUB_1 and
priority 3.63891E+18.
Agent R begins its connectivity check (message 9) for the first pair
(between the two host candidates). Since R is the controlled agent
for this session, the check omits the USE-CANDIDATE attribute. The
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host candidate from agent L is private and behind a NAT, and thus
this check won't be successful, because the packet cannot be routed
from R to L.
When agent L gets the answer, it performs its one and only
connectivity check (messages 10-13). It implements the aggressive
nomination algorithm, and thus includes a USE-CANDIDATE attribute in
this check. Since the check succeeds, agent L creates a new pair,
whose local candidate is from the mapped address in the Binding
response (NAT-PUB-1 from message 13) and whose remote candidate is
the destination of the request (R-PUB-1 from message 10). This is
added to the valid list. In addition, it is marked as selected since
the Binding request contained the USE-CANDIDATE attribute. Since
there is a selected candidate in the Valid list for the one component
of this media stream, ICE processing for this stream moves into the
Completed state. Agent L can now send media if it so chooses.
Soon after receipt of the STUN Binding request from agent L (message
11), agent R will generate its triggered check. This check happens
to match the next one on its check list -- from its host candidate to
agent L's server reflexive candidate. This check (messages 14-17)
will succeed. Consequently, agent R constructs a new candidate pair
using the mapped address from the response as the local candidate
(R-PUB-1) and the destination of the request (NAT-PUB-1) as the
remote candidate. This pair is added to the Valid list for that
media stream. Since the check was generated in the reverse direction
of a check that contained the USE-CANDIDATE attribute, the candidate
pair is marked as selected. Consequently, processing for this stream
moves into the Completed state, and agent R can also send media.
18. Security Considerations
There are several types of attacks possible in an ICE system. This
section considers these attacks and their countermeasures. These
countermeasures include:
o Using ICE in conjunction with secure signaling techniques, such as
SIPS.
o Limiting the total number of connectivity checks to 100, and
optionally limiting the number of candidates they'll accept in an
offer or answer.
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18.1. Attacks on Connectivity Checks
An attacker might attempt to disrupt the STUN connectivity checks.
Ultimately, all of these attacks fool an agent into thinking
something incorrect about the results of the connectivity checks.
The possible false conclusions an attacker can try and cause are:
False Invalid: An attacker can fool a pair of agents into thinking a
candidate pair is invalid, when it isn't. This can be used to
cause an agent to prefer a different candidate (such as one
injected by the attacker) or to disrupt a call by forcing all
candidates to fail.
False Valid: An attacker can fool a pair of agents into thinking a
candidate pair is valid, when it isn't. This can cause an agent
to proceed with a session, but then not be able to receive any
media.
False Peer Reflexive Candidate: An attacker can cause an agent to
discover a new peer reflexive candidate, when it shouldn't have.
This can be used to redirect media streams to a Denial-of-Service
(DoS) target or to the attacker, for eavesdropping or other
purposes.
False Valid on False Candidate: An attacker has already convinced an
agent that there is a candidate with an address that doesn't
actually route to that agent (for example, by injecting a false
peer reflexive candidate or false server reflexive candidate). It
must then launch an attack that forces the agents to believe that
this candidate is valid.
If an attacker can cause a false peer reflexive candidate or false
valid on a false candidate, it can launch any of the attacks
described in [RFC5389].
To force the false invalid result, the attacker has to wait for the
connectivity check from one of the agents to be sent. When it is,
the attacker needs to inject a fake response with an unrecoverable
error response, such as a 400. However, since the candidate is, in
fact, valid, the original request may reach the peer agent, and
result in a success response. The attacker needs to force this
packet or its response to be dropped, through a DoS attack, layer 2
network disruption, or other technique. If it doesn't do this, the
success response will also reach the originator, alerting it to a
possible attack. Fortunately, this attack is mitigated completely
through the STUN short-term credential mechanism. The attacker needs
to inject a fake response, and in order for this response to be
processed, the attacker needs the password. If the offer/answer
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signaling is secured, the attacker will not have the password and its
response will be discarded.
Forcing the fake valid result works in a similar way. The agent
needs to wait for the Binding request from each agent, and inject a
fake success response. The attacker won't need to worry about
disrupting the actual response since, if the candidate is not valid,
it presumably wouldn't be received anyway. However, like the fake
invalid attack, this attack is mitigated by the STUN short-term
credential mechanism in conjunction with a secure offer/answer
exchange.
Forcing the false peer reflexive candidate result can be done either
with fake requests or responses, or with replays. We consider the
fake requests and responses case first. It requires the attacker to
send a Binding request to one agent with a source IP address and port
for the false candidate. In addition, the attacker must wait for a
Binding request from the other agent, and generate a fake response
with a XOR-MAPPED-ADDRESS attribute containing the false candidate.
Like the other attacks described here, this attack is mitigated by
the STUN message integrity mechanisms and secure offer/answer
exchanges.
Forcing the false peer reflexive candidate result with packet replays
is different. The attacker waits until one of the agents sends a
check. It intercepts this request, and replays it towards the other
agent with a faked source IP address. It must also prevent the
original request from reaching the remote agent, either by launching
a DoS attack to cause the packet to be dropped, or forcing it to be
dropped using layer 2 mechanisms. The replayed packet is received at
the other agent, and accepted, since the integrity check passes (the
integrity check cannot and does not cover the source IP address and
port). It is then responded to. This response will contain a XOR-
MAPPED-ADDRESS with the false candidate, and will be sent to that
false candidate. The attacker must then receive it and relay it
towards the originator.
The other agent will then initiate a connectivity check towards that
false candidate. This validation needs to succeed. This requires
the attacker to force a false valid on a false candidate. Injecting
of fake requests or responses to achieve this goal is prevented using
the integrity mechanisms of STUN and the offer/answer exchange.
Thus, this attack can only be launched through replays. To do that,
the attacker must intercept the check towards this false candidate,
and replay it towards the other agent. Then, it must intercept the
response and replay that back as well.
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This attack is very hard to launch unless the attacker is identified
by the fake candidate. This is because it requires the attacker to
intercept and replay packets sent by two different hosts. If both
agents are on different networks (for example, across the public
Internet), this attack can be hard to coordinate, since it needs to
occur against two different endpoints on different parts of the
network at the same time.
If the attacker itself is identified by the fake candidate, the
attack is easier to coordinate. However, if SRTP is used [RFC3711],
the attacker will not be able to play the media packets, but will
only be able to discard them, effectively disabling the media stream
for the call. However, this attack requires the agent to disrupt
packets in order to block the connectivity check from reaching the
target. In that case, if the goal is to disrupt the media stream,
it's much easier to just disrupt it with the same mechanism, rather
than attack ICE.
18.2. Attacks on Server Reflexive Address Gathering
ICE endpoints make use of STUN Binding requests for gathering server
reflexive candidates from a STUN server. These requests are not
authenticated in any way. As a consequence, there are numerous
techniques an attacker can employ to provide the client with a false
server reflexive candidate:
o An attacker can compromise the DNS, causing DNS queries to return
a rogue STUN server address. That server can provide the client
with fake server reflexive candidates. This attack is mitigated
by DNS security, though DNS-SEC is not required to address it.
o An attacker that can observe STUN messages (such as an attacker on
a shared network segment, like WiFi) can inject a fake response
that is valid and will be accepted by the client.
o An attacker can compromise a STUN server by means of a virus, and
cause it to send responses with incorrect mapped addresses.
A false mapped address learned by these attacks will be used as a
server reflexive candidate in the ICE exchange. For this candidate
to actually be used for media, the attacker must also attack the
connectivity checks, and in particular, force a false valid on a
false candidate. This attack is very hard to launch if the false
address identifies a fourth party (neither the offerer, answerer, nor
attacker), since it requires attacking the checks generated by each
agent in the session, and is prevented by SRTP if it identifies the
attacker themself.
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If the attacker elects not to attack the connectivity checks, the
worst it can do is prevent the server reflexive candidate from being
used. However, if the peer agent has at least one candidate that is
reachable by the agent under attack, the STUN connectivity checks
themselves will provide a peer reflexive candidate that can be used
for the exchange of media. Peer reflexive candidates are generally
preferred over server reflexive candidates. As such, an attack
solely on the STUN address gathering will normally have no impact on
a session at all.
18.3. Attacks on Relayed Candidate Gathering
An attacker might attempt to disrupt the gathering of relayed
candidates, forcing the client to believe it has a false relayed
candidate. Exchanges with the TURN server are authenticated using a
long-term credential. Consequently, injection of fake responses or
requests will not work. In addition, unlike Binding requests,
Allocate requests are not susceptible to replay attacks with modified
source IP addresses and ports, since the source IP address and port
are not utilized to provide the client with its relayed candidate.
However, TURN servers are susceptible to DNS attacks, or to viruses
aimed at the TURN server, for purposes of turning it into a zombie or
rogue server. These attacks can be mitigated by DNS-SEC and through
good box and software security on TURN servers.
Even if an attacker has caused the client to believe in a false
relayed candidate, the connectivity checks cause such a candidate to
be used only if they succeed. Thus, an attacker must launch a false
valid on a false candidate, per above, which is a very difficult
attack to coordinate.
18.4. Attacks on the Offer/Answer Exchanges
An attacker that can modify or disrupt the offer/answer exchanges
themselves can readily launch a variety of attacks with ICE. They
could direct media to a target of a DoS attack, they could insert
themselves into the media stream, and so on. These are similar to
the general security considerations for offer/answer exchanges, and
the security considerations in RFC 3264 [RFC3264] apply. These
require techniques for message integrity and encryption for offers
and answers, which are satisfied by the SIPS mechanism [RFC3261] when
SIP is used. As such, the usage of SIPS with ICE is RECOMMENDED.
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18.5. Insider Attacks
In addition to attacks where the attacker is a third party trying to
insert fake offers, answers, or stun messages, there are several
attacks possible with ICE when the attacker is an authenticated and
valid participant in the ICE exchange.
18.5.1. The Voice Hammer Attack
The voice hammer attack is an amplification attack. In this attack,
the attacker initiates sessions to other agents, and maliciously
includes the IP address and port of a DoS target as the destination
for media traffic signaled in the SDP. This causes substantial
amplification; a single offer/answer exchange can create a continuing
flood of media packets, possibly at high rates (consider video
sources). This attack is not specific to ICE, but ICE can help
provide remediation.
Specifically, if ICE is used, the agent receiving the malicious SDP
will first perform connectivity checks to the target of media before
sending media there. If this target is a third-party host, the
checks will not succeed, and media is never sent.
Unfortunately, ICE doesn't help if its not used, in which case an
attacker could simply send the offer without the ICE parameters.
However, in environments where the set of clients is known, and is
limited to ones that support ICE, the server can reject any offers or
answers that don't indicate ICE support.
18.5.2. STUN Amplification Attack
The STUN amplification attack is similar to the voice hammer.
However, instead of voice packets being directed to the target, STUN
connectivity checks are directed to the target. The attacker sends
an offer with a large number of candidates, say, 50. The answerer
receives the offer, and starts its checks, which are directed at the
target, and consequently, never generate a response. The answerer
will start a new connectivity check every Ta ms (say, Ta=20ms).
However, the retransmission timers are set to a large number due to
the large number of candidates. As a consequence, packets will be
sent at an interval of one every Ta milliseconds, and then with
increasing intervals after that. Thus, STUN will not send packets at
a rate faster than media would be sent, and the STUN packets persist
only briefly, until ICE fails for the session. Nonetheless, this is
an amplification mechanism.
It is impossible to eliminate the amplification, but the volume can
be reduced through a variety of heuristics. Agents SHOULD limit the
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total number of connectivity checks they perform to 100.
Additionally, agents MAY limit the number of candidates they'll
accept in an offer or answer.
Frequently, protocols that wish to avoid these kinds of attacks force
the initiator to wait for a response prior to sending the next
message. However, in the case of ICE, this is not possible. It is
not possible to differentiate the following two cases:
o There was no response because the initiator is being used to
launch a DoS attack against an unsuspecting target that will not
respond.
o There was no response because the IP address and port are not
reachable by the initiator.
In the second case, another check should be sent at the next
opportunity, while in the former case, no further checks should be
sent.
18.6. Interactions with Application Layer Gateways and SIP
Application Layer Gateways (ALGs) are functions present in a NAT
device that inspect the contents of packets and modify them, in order
to facilitate NAT traversal for application protocols. Session
Border Controllers (SBCs) are close cousins of ALGs, but are less
transparent since they actually exist as application layer SIP
intermediaries. ICE has interactions with SBCs and ALGs.
If an ALG is SIP aware but not ICE aware, ICE will work through it as
long as the ALG correctly modifies the SDP. A correct ALG
implementation behaves as follows:
o The ALG does not modify the m and c lines or the rtcp attribute if
they contain external addresses.
o If the m and c lines contain internal addresses, the modification
depends on the state of the ALG:
If the ALG already has a binding established that maps an
external port to an internal IP address and port matching the
values in the m and c lines or rtcp attribute, the ALG uses
that binding instead of creating a new one.
If the ALG does not already have a binding, it creates a new
one and modifies the SDP, rewriting the m and c lines and rtcp
attribute.
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Unfortunately, many ALGs are known to work poorly in these corner
cases. ICE does not try to work around broken ALGs, as this is
outside the scope of its functionality. ICE can help diagnose these
conditions, which often show up as a mismatch between the set of
candidates and the m and c lines and rtcp attributes. The ice-
mismatch attribute is used for this purpose.
ICE works best through ALGs when the signaling is run over TLS. This
prevents the ALG from manipulating the SDP messages and interfering
with ICE operation. Implementations that are expected to be deployed
behind ALGs SHOULD provide for TLS transport of the SDP.
If an SBC is SIP aware but not ICE aware, the result depends on the
behavior of the SBC. If it is acting as a proper Back-to-Back User
Agent (B2BUA), the SBC will remove any SDP attributes it doesn't
understand, including the ICE attributes. Consequently, the call
will appear to both endpoints as if the other side doesn't support
ICE. This will result in ICE being disabled, and media flowing
through the SBC, if the SBC has requested it. If, however, the SBC
passes the ICE attributes without modification, yet modifies the
default destination for media (contained in the m and c lines and
rtcp attribute), this will be detected as an ICE mismatch, and ICE
processing is aborted for the call. It is outside of the scope of
ICE for it to act as a tool for "working around" SBCs. If one is
present, ICE will not be used and the SBC techniques take precedence.
19. STUN Extensions
19.1. New Attributes
This specification defines four new attributes, PRIORITY, USE-
CANDIDATE, ICE-CONTROLLED, and ICE-CONTROLLING.
The PRIORITY attribute indicates the priority that is to be
associated with a peer reflexive candidate, should one be discovered
by this check. It is a 32-bit unsigned integer, and has an attribute
value of 0x0024.
The USE-CANDIDATE attribute indicates that the candidate pair
resulting from this check should be used for transmission of media.
The attribute has no content (the Length field of the attribute is
zero); it serves as a flag. It has an attribute value of 0x0025.
The ICE-CONTROLLED attribute is present in a Binding request and
indicates that the client believes it is currently in the controlled
role. The content of the attribute is a 64-bit unsigned integer in
network byte order, which contains a random number used for tie-
breaking of role conflicts.
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The ICE-CONTROLLING attribute is present in a Binding request and
indicates that the client believes it is currently in the controlling
role. The content of the attribute is a 64-bit unsigned integer in
network byte order, which contains a random number used for tie-
breaking of role conflicts.
19.2. New Error Response Codes
This specification defines a single error response code:
487 (Role Conflict): The Binding request contained either the ICE-
CONTROLLING or ICE-CONTROLLED attribute, indicating a role that
conflicted with the server. The server ran a tie-breaker based on
the tie-breaker value in the request and determined that the
client needs to switch roles.
20. Operational Considerations
This section discusses issues relevant to network operators looking
to deploy ICE.
20.1. NAT and Firewall Types
ICE was designed to work with existing NAT and firewall equipment.
Consequently, it is not necessary to replace or reconfigure existing
firewall and NAT equipment in order to facilitate deployment of ICE.
Indeed, ICE was developed to be deployed in environments where the
Voice over IP (VoIP) operator has no control over the IP network
infrastructure, including firewalls and NAT.
That said, ICE works best in environments where the NAT devices are
"behave" compliant, meeting the recommendations defined in [RFC4787]
and [RFC5766]. In networks with behave-compliant NAT, ICE will work
without the need for a TURN server, thus improving voice quality,
decreasing call setup times, and reducing the bandwidth demands on
the network operator.
20.2. Bandwidth Requirements
Deployment of ICE can have several interactions with available
network capacity that operators should take into consideration.
20.2.1. STUN and TURN Server Capacity Planning
First and foremost, ICE makes use of TURN and STUN servers, which
would typically be located in the network operator's data centers.
The STUN servers require relatively little bandwidth. For each
component of each media stream, there will be one or more STUN
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transactions from each client to the STUN server. In a basic voice-
only IPv4 VoIP deployment, there will be four transactions per call
(one for RTP and one for RTCP, for both caller and callee). Each
transaction is a single request and a single response, the former
being 20 bytes long, and the latter, 28. Consequently, if a system
has N users, and each makes four calls in a busy hour, this would
require N*1.7bps. For one million users, this is 1.7 Mbps, a very
small number (relatively speaking).
TURN traffic is more substantial. The TURN server will see traffic
volume equal to the STUN volume (indeed, if TURN servers are
deployed, there is no need for a separate STUN server), in addition
to the traffic for the actual media traffic. The amount of calls
requiring TURN for media relay is highly dependent on network
topologies, and can and will vary over time. In a network with 100%
behave-compliant NAT, it is exactly zero. At time of writing, large-
scale consumer deployments were seeing between 5 and 10 percent of
calls requiring TURN servers. Considering a voice-only deployment
using G.711 (so 80 kbps in each direction), with .2 erlangs during
the busy hour, this is N*3.2 kbps. For a population of one million
users, this is 3.2 Gbps, assuming a 10% usage of TURN servers.
20.2.2. Gathering and Connectivity Checks
The process of gathering of candidates and performing of connectivity
checks can be bandwidth intensive. ICE has been designed to pace
both of these processes. The gathering phase and the connectivity
check phase are meant to generate traffic at roughly the same
bandwidth as the media traffic itself. This was done to ensure that,
if a network is designed to support multimedia traffic of a certain
type (voice, video, or just text), it will have sufficient capacity
to support the ICE checks for that media. Of course, the ICE checks
will cause a marginal increase in the total utilization; however,
this will typically be an extremely small increase.
Congestion due to the gathering and check phases has proven to be a
problem in deployments that did not utilize pacing. Typically,
access links became congested as the endpoints flooded the network
with checks as fast as they can send them. Consequently, network
operators should make sure that their ICE implementations support the
pacing feature. Though this pacing does increase call setup times,
it makes ICE network friendly and easier to deploy.
20.2.3. Keepalives
STUN keepalives (in the form of STUN Binding Indications) are sent in
the middle of a media session. However, they are sent only in the
absence of actual media traffic. In deployments that are not
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utilizing Voice Activity Detection (VAD), the keepalives are never
used and there is no increase in bandwidth usage. When VAD is being
used, keepalives will be sent during silence periods. This involves
a single packet every 15-20 seconds, far less than the packet every
20-30 ms that is sent when there is voice. Therefore, keepalives
don't have any real impact on capacity planning.
20.3. ICE and ICE-lite
Deployments utilizing a mix of ICE and ICE-lite interoperate
perfectly. They have been explicitly designed to do so, without loss
of function.
However, ICE-lite can only be deployed in limited use cases. Those
cases, and the caveats involved in doing so, are documented in
Appendix A.
20.4. Troubleshooting and Performance Management
ICE utilizes end-to-end connectivity checks, and places much of the
processing in the endpoints. This introduces a challenge to the
network operator -- how can they troubleshoot ICE deployments? How
can they know how ICE is performing?
ICE has built-in features to help deal with these problems. SIP
servers on the signaling path, typically deployed in the data centers
of the network operator, will see the contents of the offer/answer
exchanges that convey the ICE parameters. These parameters include
the type of each candidate (host, server reflexive, or relayed),
along with their related addresses. Once ICE processing has
completed, an updated offer/answer exchange takes place, signaling
the selected address (and its type). This updated re-INVITE is
performed exactly for the purposes of educating network equipment
(such as a diagnostic tool attached to a SIP server) about the
results of ICE processing.
As a consequence, through the logs generated by the SIP server, a
network operator can observe what types of candidates are being used
for each call, and what address was selected by ICE. This is the
primary information that helps evaluate how ICE is performing.
20.5. Endpoint Configuration
ICE relies on several pieces of data being configured into the
endpoints. This configuration data includes timers, credentials for
TURN servers, and hostnames for STUN and TURN servers. ICE itself
does not provide a mechanism for this configuration. Instead, it is
assumed that this information is attached to whatever mechanism is
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used to configure all of the other parameters in the endpoint. For
SIP phones, standard solutions such as the configuration framework
[SIP-UA-FRMWK] have been defined.
21. IANA Considerations
This specification registers new SDP attributes, four new STUN
attributes, and one new STUN error response.
21.1. SDP Attributes
This specification defines seven new SDP attributes per the
procedures of Section 8.2.4 of [RFC4566]. The required information
for the registrations is included here.
21.1.1. candidate Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: candidate
Long Form: candidate
Type of Attribute: media-level
Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and provides one of many possible candidate
addresses for communication. These addresses are validated with
an end-to-end connectivity check using Session Traversal Utilities
for NAT (STUN)).
Appropriate Values: See Section 15 of RFC 5245.
21.1.2. remote-candidates Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: remote-candidates
Long Form: remote-candidates
Type of Attribute: media-level
Charset Considerations: The attribute is not subject to the charset
attribute.
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Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and provides the identity of the remote
candidates that the offerer wishes the answerer to use in its
answer.
Appropriate Values: See Section 15 of RFC 5245.
21.1.3. ice-lite Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-lite
Long Form: ice-lite
Type of Attribute: session-level
Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and indicates that an agent has the minimum
functionality required to support ICE inter-operation with a peer
that has a full implementation.
Appropriate Values: See Section 15 of RFC 5245.
21.1.4. ice-mismatch Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-mismatch
Long Form: ice-mismatch
Type of Attribute: session-level
Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and indicates that an agent is ICE capable,
but did not proceed with ICE due to a mismatch of candidates with
the default destination for media signaled in the SDP.
Appropriate Values: See Section 15 of RFC 5245.
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21.1.5. ice-pwd Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-pwd
Long Form: ice-pwd
Type of Attribute: session- or media-level
Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and provides the password used to protect
STUN connectivity checks.
Appropriate Values: See Section 15 of RFC 5245.
21.1.6. ice-ufrag Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-ufrag
Long Form: ice-ufrag
Type of Attribute: session- or media-level
Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and provides the fragments used to construct
the username in STUN connectivity checks.
Appropriate Values: See Section 15 of RFC 5245.
21.1.7. ice-options Attribute
Contact Name: Jonathan Rosenberg, jdrosen@jdrosen.net.
Attribute Name: ice-options
Long Form: ice-options
Type of Attribute: session-level
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Charset Considerations: The attribute is not subject to the charset
attribute.
Purpose: This attribute is used with Interactive Connectivity
Establishment (ICE), and indicates the ICE options or extensions
used by the agent.
Appropriate Values: See Section 15 of RFC 5245.
21.2. STUN Attributes
This section registers four new STUN attributes per the procedures in
[RFC5389].
0x0024 PRIORITY
0x0025 USE-CANDIDATE
0x8029 ICE-CONTROLLED
0x802A ICE-CONTROLLING
21.3. STUN Error Responses
This section registers one new STUN error response code per the
procedures in [RFC5389].
487 Role Conflict: The client asserted an ICE role (controlling
or
controlled) that is in conflict with the role of the server.
22. IAB Considerations
The IAB has studied the problem of "Unilateral Self-Address Fixing",
which is the general process by which a agent attempts to determine
its address in another realm on the other side of a NAT through a
collaborative protocol reflection mechanism [RFC3424]. ICE is an
example of a protocol that performs this type of function.
Interestingly, the process for ICE is not unilateral, but bilateral,
and the difference has a significant impact on the issues raised by
IAB. Indeed, ICE can be considered a B-SAF (Bilateral Self-Address
Fixing) protocol, rather than an UNSAF protocol. Regardless, the IAB
has mandated that any protocols developed for this purpose document a
specific set of considerations. This section meets those
requirements.
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22.1. Problem Definition
>From RFC 3424, any UNSAF proposal must provide:
Precise definition of a specific, limited-scope problem that is to
be solved with the UNSAF proposal. A short-term fix should not be
generalized to solve other problems; this is why "short-term fixes
usually aren't".
The specific problems being solved by ICE are:
Provide a means for two peers to determine the set of transport
addresses that can be used for communication.
Provide a means for a agent to determine an address that is
reachable by another peer with which it wishes to communicate.
22.2. Exit Strategy
>From RFC 3424, any UNSAF proposal must provide:
Description of an exit strategy/transition plan. The better
short-term fixes are the ones that will naturally see less and
less use as the appropriate technology is deployed.
ICE itself doesn't easily get phased out. However, it is useful even
in a globally connected Internet, to serve as a means for detecting
whether a router failure has temporarily disrupted connectivity, for
example. ICE also helps prevent certain security attacks that have
nothing to do with NAT. However, what ICE does is help phase out
other UNSAF mechanisms. ICE effectively selects amongst those
mechanisms, prioritizing ones that are better, and deprioritizing
ones that are worse. Local IPv6 addresses can be preferred. As NATs
begin to dissipate as IPv6 is introduced, server reflexive and
relayed candidates (both forms of UNSAF addresses) simply never get
used, because higher-priority connectivity exists to the native host
candidates. Therefore, the servers get used less and less, and can
eventually be remove when their usage goes to zero.
Indeed, ICE can assist in the transition from IPv4 to IPv6. It can
be used to determine whether to use IPv6 or IPv4 when two dual-stack
hosts communicate with SIP (IPv6 gets used). It can also allow a
network with both 6to4 and native v6 connectivity to determine which
address to use when communicating with a peer.
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22.3. Brittleness Introduced by ICE
>From RFC 3424, any UNSAF proposal must provide:
Discussion of specific issues that may render systems more
"brittle". For example, approaches that involve using data at
multiple network layers create more dependencies, increase
debugging challenges, and make it harder to transition.
ICE actually removes brittleness from existing UNSAF mechanisms. In
particular, classic STUN (as described in RFC 3489 [RFC3489]) has
several points of brittleness. One of them is the discovery process
that requires an agent to try to classify the type of NAT it is
behind. This process is error-prone. With ICE, that discovery
process is simply not used. Rather than unilaterally assessing the
validity of the address, its validity is dynamically determined by
measuring connectivity to a peer. The process of determining
connectivity is very robust.
Another point of brittleness in classic STUN and any other unilateral
mechanism is its absolute reliance on an additional server. ICE
makes use of a server for allocating unilateral addresses, but allows
agents to directly connect if possible. Therefore, in some cases,
the failure of a STUN server would still allow for a call to progress
when ICE is used.
Another point of brittleness in classic STUN is that it assumes that
the STUN server is on the public Internet. Interestingly, with ICE,
that is not necessary. There can be a multitude of STUN servers in a
variety of address realms. ICE will discover the one that has
provided a usable address.
The most troubling point of brittleness in classic STUN is that it
doesn't work in all network topologies. In cases where there is a
shared NAT between each agent and the STUN server, traditional STUN
may not work. With ICE, that restriction is removed.
Classic STUN also introduces some security considerations.
Fortunately, those security considerations are also mitigated by ICE.
Consequently, ICE serves to repair the brittleness introduced in
classic STUN, and does not introduce any additional brittleness into
the system.
The penalty of these improvements is that ICE increases session
establishment times.
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22.4. Requirements for a Long-Term Solution
From RFC 3424, any UNSAF proposal must provide:
... requirements for longer term, sound technical solutions --
contribute to the process of finding the right longer term
solution.
Our conclusions from RFC 3489 remain unchanged. However, we feel ICE
actually helps because we believe it can be part of the long-term
solution.
22.5. Issues with Existing NAPT Boxes
From RFC 3424, any UNSAF proposal must provide:
Discussion of the impact of the noted practical issues with
existing, deployed NA[P]Ts and experience reports.
A number of NAT boxes are now being deployed into the market that try
to provide "generic" ALG functionality. These generic ALGs hunt for
IP addresses, either in text or binary form within a packet, and
rewrite them if they match a binding. This interferes with classic
STUN. However, the update to STUN [RFC5389] uses an encoding that
hides these binary addresses from generic ALGs.
Existing NAPT boxes have non-deterministic and typically short
expiration times for UDP-based bindings. This requires
implementations to send periodic keepalives to maintain those
bindings. ICE uses a default of 15 s, which is a very conservative
estimate. Eventually, over time, as NAT boxes become compliant to
behave [RFC4787], this minimum keepalive will become deterministic
and well-known, and the ICE timers can be adjusted. Having a way to
discover and control the minimum keepalive interval would be far
better still.
23. Acknowledgements
The authors would like to thank Dan Wing, Eric Rescorla, Flemming
Andreasen, Rohan Mahy, Dean Willis, Eric Cooper, Jason Fischl,
Douglas Otis, Tim Moore, Jean-Francois Mule, Kevin Johns, Jonathan
Lennox, and Francois Audet for their comments and input. A special
thanks goes to Bill May, who suggested several of the concepts in
this specification, Philip Matthews, who suggested many of the key
performance optimizations in this specification, Eric Rescorla, who
drafted the text in the introduction, and Magnus Westerlund, for
doing several detailed reviews on the various revisions of this
specification.
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24. References
24.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3605] Huitema, C., "Real Time Control Protocol (RTCP) attribute
in Session Description Protocol (SDP)", RFC 3605,
October 2003.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3264] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model
with Session Description Protocol (SDP)", RFC 3264,
June 2002.
[RFC3556] Casner, S., "Session Description Protocol (SDP) Bandwidth
Modifiers for RTP Control Protocol (RTCP) Bandwidth",
RFC 3556, July 2003.
[RFC3312] Camarillo, G., Marshall, W., and J. Rosenberg,
"Integration of Resource Management and Session Initiation
Protocol (SIP)", RFC 3312, October 2002.
[RFC4032] Camarillo, G. and P. Kyzivat, "Update to the Session
Initiation Protocol (SIP) Preconditions Framework",
RFC 4032, March 2005.
[RFC3262] Rosenberg, J. and H. Schulzrinne, "Reliability of
Provisional Responses in Session Initiation Protocol
(SIP)", RFC 3262, June 2002.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP: Session
Description Protocol", RFC 4566, July 2006.
[RFC4091] Camarillo, G. and J. Rosenberg, "The Alternative Network
Address Types (ANAT) Semantics for the Session Description
Protocol (SDP) Grouping Framework", RFC 4091, June 2005.
[RFC4092] Camarillo, G. and J. Rosenberg, "Usage of the Session
Description Protocol (SDP) Alternative Network Address
Types (ANAT) Semantics in the Session Initiation Protocol
(SIP)", RFC 4092, June 2005.
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[RFC3484] Draves, R., "Default Address Selection for Internet
Protocol version 6 (IPv6)", RFC 3484, February 2003.
[RFC5234] Crocker, D., Ed., and P. Overell, "Augmented BNF for
Syntax Specifications: ABNF", STD 68, RFC 5234, January
2008.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
[RFC5766] Mahy, R., Matthews, P., and J. Rosenberg, "Traversal Using
Relays around NAT (TURN): Relay Extensions to Session
Traversal Utilities for NAT (STUN)", RFC 5766, April 2010.
[RFC5768] Rosenberg, J., "Indicating Support for Interactive
Connectivity Establishment (ICE) in the Session Initiation
Protocol (SIP)", RFC 5768, April 2010.
24.2. Informative References
[RFC3489] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy,
"STUN - Simple Traversal of User Datagram Protocol (UDP)
Through Network Address Translators (NATs)", RFC 3489,
March 2003.
[RFC3235] Senie, D., "Network Address Translator (NAT)-Friendly
Application Design Guidelines", RFC 3235, January 2002.
[RFC3303] Srisuresh, P., Kuthan, J., Rosenberg, J., Molitor, A., and
A. Rayhan, "Middlebox communication architecture and
framework", RFC 3303, August 2002.
[RFC3725] Rosenberg, J., Peterson, J., Schulzrinne, H., and G.
Camarillo, "Best Current Practices for Third Party Call
Control (3pcc) in the Session Initiation Protocol (SIP)",
BCP 85, RFC 3725, April 2004.
[RFC3102] Borella, M., Lo, J., Grabelsky, D., and G. Montenegro,
"Realm Specific IP: Framework", RFC 3102, October 2001.
[RFC3103] Borella, M., Grabelsky, D., Lo, J., and K. Taniguchi,
"Realm Specific IP: Protocol Specification", RFC 3103,
October 2001.
[RFC3424] Daigle, L. and IAB, "IAB Considerations for UNilateral
Self-Address Fixing (UNSAF) Across Network Address
Translation", RFC 3424, November 2002.
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[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains
via IPv4 Clouds", RFC 3056, February 2001.
[RFC3389] Zopf, R., "Real-time Transport Protocol (RTP) Payload for
Comfort Noise (CN)", RFC 3389, September 2002.
[RFC3960] Camarillo, G. and H. Schulzrinne, "Early Media and Ringing
Tone Generation in the Session Initiation Protocol (SIP)",
RFC 3960, December 2004.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
[RFC1918] Rekhter, Y., Moskowitz, R., Karrenberg, D., Groot, G., and
E. Lear, "Address Allocation for Private Internets",
BCP 5, RFC 1918, February 1996.
[RFC4787] Audet, F. and C. Jennings, "Network Address Translation
(NAT) Behavioral Requirements for Unicast UDP", BCP 127,
RFC 4787, January 2007.
[SDP-PRECON]
Andreasen, F., Camarillo, G., Oran, D., and D. Wing,
"Connectivity Preconditions for Session Description
Protocol Media Streams", Work in Progress, March 2010.
[NO-OP-RTP]
Andreasen, F., Oran, D., and D. Wing, "A No-Op Payload
Format for RTP", Work in Progress, May 2007.
[RFC5761] Perkins, C. and M. Westerlund, "Multiplexing RTP Data and
Control Packets on a Single Port", RFC 5761, April 2010.
[RFC4340] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", RFC 4340, March 2006.
[RFC4103] Hellstrom, G. and P. Jones, "RTP Payload for Text
Conversation", RFC 4103, June 2005.
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[RFC5626] Jennings, C., Mahy, R., and F. Audet, "Managing Client-
Initiated Connections in the Session Initiation Protocol
(SIP)", RFC 5626, October 2009.
[RFC5382] Guha, S., Biswas, K., Ford, B., Sivakumar, S., and P.
Srisuresh, "NAT Behavioral Requirements for TCP", BCP 142,
RFC 5382, October 2008.
[SIP-UA-FRMWK]
Petrie, D. and S. Channabasappa, Ed., "A Framework for
Session Initiation Protocol User Agent Profile Delivery",
Work in Progress, February 2010.
[ICE-TCP] Perreault, S., Ed. and J. Rosenberg, "TCP Candidates with
Interactive Connectivity Establishment (ICE)", Work
in Progress, October 2009.
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Appendix A. Lite and Full Implementations
ICE allows for two types of implementations. A full implementation
supports the controlling and controlled roles in a session, and can
also perform address gathering. In contrast, a lite implementation
is a minimalist implementation that does little but respond to STUN
checks.
Because ICE requires both endpoints to support it in order to bring
benefits to either endpoint, incremental deployment of ICE in a
network is more complicated. Many sessions involve an endpoint that
is, by itself, not behind a NAT and not one that would worry about
NAT traversal. A very common case is to have one endpoint that
requires NAT traversal (such as a VoIP hard phone or soft phone) make
a call to one of these devices. Even if the phone supports a full
ICE implementation, ICE won't be used at all if the other device
doesn't support it. The lite implementation allows for a low-cost
entry point for these devices. Once they support the lite
implementation, full implementations can connect to them and get the
full benefits of ICE.
Consequently, a lite implementation is only appropriate for devices
that will *always* be connected to the public Internet and have a
public IP address at which it can receive packets from any
correspondent. ICE will not function when a lite implementation is
placed behind a NAT.
ICE allows a lite implementation to have a single IPv4 host candidate
and several IPv6 addresses. In that case, candidate pairs are
selected by the controlling agent using a static algorithm, such as
the one in RFC 3484, which is recommended by this specification.
However, static mechanisms for address selection are always prone to
error, since they cannot ever reflect the actual topology and can
never provide actual guarantees on connectivity. They are always
heuristics. Consequently, if an agent is implementing ICE just to
select between its IPv4 and IPv6 addresses, and none of its IP
addresses are behind NAT, usage of full ICE is still RECOMMENDED in
order to provide the most robust form of address selection possible.
It is important to note that the lite implementation was added to
this specification to provide a stepping stone to full
implementation. Even for devices that are always connected to the
public Internet with just a single IPv4 address, a full
implementation is preferable if achievable. A full implementation
will reduce call setup times, since ICE's aggressive mode can be
used. Full implementations also obtain the security benefits of ICE
unrelated to NAT traversal; in particular, the voice hammer attack
described in Section 18 is prevented only for full implementations,
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not lite. Finally, it is often the case that a device that finds
itself with a public address today will be placed in a network
tomorrow where it will be behind a NAT. It is difficult to
definitively know, over the lifetime of a device or product, that it
will always be used on the public Internet. Full implementation
provides assurance that communications will always work.
Appendix B. Design Motivations
ICE contains a number of normative behaviors that may themselves be
simple, but derive from complicated or non-obvious thinking or use
cases that merit further discussion. Since these design motivations
are not neccesary to understand for purposes of implementation, they
are discussed here in an appendix to the specification. This section
is non-normative.
B.1. Pacing of STUN Transactions
STUN transactions used to gather candidates and to verify
connectivity are paced out at an approximate rate of one new
transaction every Ta milliseconds. Each transaction, in turn, has a
retransmission timer RTO that is a function of Ta as well. Why are
these transactions paced, and why are these formulas used?
Sending of these STUN requests will often have the effect of creating
bindings on NAT devices between the client and the STUN servers.
Experience has shown that many NAT devices have upper limits on the
rate at which they will create new bindings. Experiments have shown
that once every 20 ms is well supported, but not much lower than
that. This is why Ta has a lower bound of 20 ms. Furthermore,
transmission of these packets on the network makes use of bandwidth
and needs to be rate limited by the agent. Deployments based on
earlier draft versions of this document tended to overload rate-
constrained access links and perform poorly overall, in addition to
negatively impacting the network. As a consequence, the pacing
ensures that the NAT device does not get overloaded and that traffic
is kept at a reasonable rate.
The definition of a "reasonable" rate is that STUN should not use
more bandwidth than the RTP itself will use, once media starts
flowing. The formula for Ta is designed so that, if a STUN packet
were sent every Ta seconds, it would consume the same amount of
bandwidth as RTP packets, summed across all media streams. Of
course, STUN has retransmits, and the desire is to pace those as
well. For this reason, RTO is set such that the first retransmit on
the first transaction happens just as the first STUN request on the
last transaction occurs. Pictorially:
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First Packets Retransmits
| |
| |
-------+------ -------+------
/ \ / \
/ \ / \
+--+ +--+ +--+ +--+ +--+ +--+
|A1| |B1| |C1| |A2| |B2| |C2|
+--+ +--+ +--+ +--+ +--+ +--+
---+-------+-------+-------+-------+-------+------------ Time
0 Ta 2Ta 3Ta 4Ta 5Ta
In this picture, there are three transactions that will be sent (for
example, in the case of candidate gathering, there are three host
candidate/STUN server pairs). These are transactions A, B, and C.
The retransmit timer is set so that the first retransmission on the
first transaction (packet A2) is sent at time 3Ta.
Subsequent retransmits after the first will occur even less
frequently than Ta milliseconds apart, since STUN uses an exponential
back-off on its retransmissions.
B.2. Candidates with Multiple Bases
Section 4.1.3 talks about eliminating candidates that have the same
transport address and base. However, candidates with the same
transport addresses but different bases are not redundant. When can
an agent have two candidates that have the same IP address and port,
but different bases? Consider the topology of Figure 10:
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+----------+
| STUN Srvr|
+----------+
|
|
-----
// \\
| |
| B:net10 |
| |
\\ //
-----
|
|
+----------+
| NAT |
+----------+
|
|
-----
// \\
| A |
|192.168/16 |
| |
\\ //
-----
|
|
|192.168.1.100 -----
+----------+ // \\ +----------+
| | | | | |
| Offerer |---------| C:net10 |-----------| Answerer |
| |10.0.1.100| | 10.0.1.101 | |
+----------+ \\ // +----------+
-----
Figure 10: Identical Candidates with Different Bases
In this case, the offerer is multihomed. It has one IP address,
10.0.1.100, on network C, which is a net 10 private network. The
answerer is on this same network. The offerer is also connected to
network A, which is 192.168/16. The offerer has an IP address of
192.168.1.100 on this network. There is a NAT on this network,
natting into network B, which is another net 10 private network, but
not connected to network C. There is a STUN server on network B.
The offerer obtains a host candidate on its IP address on network C
(10.0.1.100:2498) and a host candidate on its IP address on network A
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(192.168.1.100:3344). It performs a STUN query to its configured
STUN server from 192.168.1.100:3344. This query passes through the
NAT, which happens to assign the binding 10.0.1.100:2498. The STUN
server reflects this in the STUN Binding response. Now, the offerer
has obtained a server reflexive candidate with a transport address
that is identical to a host candidate (10.0.1.100:2498). However,
the server reflexive candidate has a base of 192.168.1.100:3344, and
the host candidate has a base of 10.0.1.100:2498.
B.3. Purpose of the and Attributes
The candidate attribute contains two values that are not used at all
by ICE itself -- and . Why is it present?
There are two motivations for its inclusion. The first is
diagnostic. It is very useful to know the relationship between the
different types of candidates. By including it, an agent can know
which relayed candidate is associated with which reflexive candidate,
which in turn is associated with a specific host candidate. When
checks for one candidate succeed and not for others, this provides
useful diagnostics on what is going on in the network.
The second reason has to do with off-path Quality of Service (QoS)
mechanisms. When ICE is used in environments such as PacketCable
2.0, proxies will, in addition to performing normal SIP operations,
inspect the SDP in SIP messages, and extract the IP address and port
for media traffic. They can then interact, through policy servers,
with access routers in the network, to establish guaranteed QoS for
the media flows. This QoS is provided by classifying the RTP traffic
based on 5-tuple, and then providing it a guaranteed rate, or marking
its Diffserv codepoints appropriately. When a residential NAT is
present, and a relayed candidate gets selected for media, this
relayed candidate will be a transport address on an actual TURN
server. That address says nothing about the actual transport address
in the access router that would be used to classify packets for QoS
treatment. Rather, the server reflexive candidate towards the TURN
server is needed. By carrying the translation in the SDP, the proxy
can use that transport address to request QoS from the access router.
B.4. Importance of the STUN Username
ICE requires the usage of message integrity with STUN using its
short-term credential functionality. The actual short-term
credential is formed by exchanging username fragments in the SDP
offer/answer exchange. The need for this mechanism goes beyond just
security; it is actually required for correct operation of ICE in the
first place.
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Consider agents L, R, and Z. L and R are within private enterprise
1, which is using 10.0.0.0/8. Z is within private enterprise 2,
which is also using 10.0.0.0/8. As it turns out, R and Z both have
IP address 10.0.1.1. L sends an offer to Z. Z, in its answer,
provides L with its host candidates. In this case, those candidates
are 10.0.1.1:8866 and 10.0.1.1:8877. As it turns out, R is in a
session at that same time, and is also using 10.0.1.1:8866 and
10.0.1.1:8877 as host candidates. This means that R is prepared to
accept STUN messages on those ports, just as Z is. L will send a
STUN request to 10.0.1.1:8866 and another to 10.0.1.1:8877. However,
these do not go to Z as expected. Instead, they go to R! If R just
replied to them, L would believe it has connectivity to Z, when in
fact it has connectivity to a completely different user, R. To fix
this, the STUN short-term credential mechanisms are used. The
username fragments are sufficiently random that it is highly unlikely
that R would be using the same values as Z. Consequently, R would
reject the STUN request since the credentials were invalid. In
essence, the STUN username fragments provide a form of transient host
identifiers, bound to a particular offer/answer session.
An unfortunate consequence of the non-uniqueness of IP addresses is
that, in the above example, R might not even be an ICE agent. It
could be any host, and the port to which the STUN packet is directed
could be any ephemeral port on that host. If there is an application
listening on this socket for packets, and it is not prepared to
handle malformed packets for whatever protocol is in use, the
operation of that application could be affected. Fortunately, since
the ports exchanged in SDP are ephemeral and usually drawn from the
dynamic or registered range, the odds are good that the port is not
used to run a server on host R, but rather is the agent side of some
protocol. This decreases the probability of hitting an allocated
port, due to the transient nature of port usage in this range.
However, the possibility of a problem does exist, and network
deployers should be prepared for it. Note that this is not a problem
specific to ICE; stray packets can arrive at a port at any time for
any type of protocol, especially ones on the public Internet. As
such, this requirement is just restating a general design guideline
for Internet applications -- be prepared for unknown packets on any
port.
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B.5. The Candidate Pair Priority Formula
The priority for a candidate pair has an odd form. It is:
pair priority = 2^32*MIN(G,D) + 2*MAX(G,D) + (G>D?1:0)
Why is this? When the candidate pairs are sorted based on this
value, the resulting sorting has the MAX/MIN property. This means
that the pairs are first sorted based on decreasing value of the
minimum of the two priorities. For pairs that have the same value of
the minimum priority, the maximum priority is used to sort amongst
them. If the max and the min priorities are the same, the
controlling agent's priority is used as the tie-breaker in the last
part of the expression. The factor of 2*32 is used since the
priority of a single candidate is always less than 2*32, resulting in
the pair priority being a "concatenation" of the two component
priorities. This creates the MAX/MIN sorting. MAX/MIN ensures that,
for a particular agent, a lower-priority candidate is never used
until all higher-priority candidates have been tried.
B.6. The remote-candidates Attribute
The a=remote-candidates attribute exists to eliminate a race
condition between the updated offer and the response to the STUN
Binding request that moved a candidate into the Valid list. This
race condition is shown in Figure 11. On receipt of message 4, agent
L adds a candidate pair to the valid list. If there was only a
single media stream with a single component, agent L could now send
an updated offer. However, the check from agent R has not yet
generated a response, and agent R receives the updated offer (message
7) before getting the response (message 9). Thus, it does not yet
know that this particular pair is valid. To eliminate this
condition, the actual candidates at R that were selected by the
offerer (the remote candidates) are included in the offer itself, and
the answerer delays its answer until those pairs validate.
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Agent A Network Agent B
|(1) Offer | |
|------------------------------------------>|
|(2) Answer | |
|<------------------------------------------|
|(3) STUN Req. | |
|------------------------------------------>|
|(4) STUN Res. | |
|<------------------------------------------|
|(5) STUN Req. | |
|<------------------------------------------|
|(6) STUN Res. | |
|-------------------->| |
| |Lost |
|(7) Offer | |
|------------------------------------------>|
|(8) STUN Req. | |
|<------------------------------------------|
|(9) STUN Res. | |
|------------------------------------------>|
|(10) Answer | |
|<------------------------------------------|
Figure 11: Race Condition Flow
B.7. Why Are Keepalives Needed?
Once media begins flowing on a candidate pair, it is still necessary
to keep the bindings alive at intermediate NATs for the duration of
the session. Normally, the media stream packets themselves (e.g.,
RTP) meet this objective. However, several cases merit further
discussion. Firstly, in some RTP usages, such as SIP, the media
streams can be "put on hold". This is accomplished by using the SDP
"sendonly" or "inactive" attributes, as defined in RFC 3264
[RFC3264]. RFC 3264 directs implementations to cease transmission of
media in these cases. However, doing so may cause NAT bindings to
timeout, and media won't be able to come off hold.
Secondly, some RTP payload formats, such as the payload format for
text conversation [RFC4103], may send packets so infrequently that
the interval exceeds the NAT binding timeouts.
Thirdly, if silence suppression is in use, long periods of silence
may cause media transmission to cease sufficiently long for NAT
bindings to time out.
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For these reasons, the media packets themselves cannot be relied
upon. ICE defines a simple periodic keepalive utilizing STUN Binding
indications. This makes its bandwidth requirements highly
predictable, and thus amenable to QoS reservations.
B.8. Why Prefer Peer Reflexive Candidates?
Section 4.1.2 describes procedures for computing the priority of
candidate based on its type and local preferences. That section
requires that the type preference for peer reflexive candidates
always be higher than server reflexive. Why is that? The reason has
to do with the security considerations in Section 18. It is much
easier for an attacker to cause an agent to use a false server
reflexive candidate than it is for an attacker to cause an agent to
use a false peer reflexive candidate. Consequently, attacks against
address gathering with Binding requests are thwarted by ICE by
preferring the peer reflexive candidates.
B.9. Why Send an Updated Offer?
Section 11.1 describes rules for sending media. Both agents can send
media once ICE checks complete, without waiting for an updated offer.
Indeed, the only purpose of the updated offer is to "correct" the SDP
so that the default destination for media matches where media is
being sent based on ICE procedures (which will be the highest-
priority nominated candidate pair).
This begs the question -- why is the updated offer/answer exchange
needed at all? Indeed, in a pure offer/answer environment, it would
not be. The offerer and answerer will agree on the candidates to use
through ICE, and then can begin using them. As far as the agents
themselves are concerned, the updated offer/answer provides no new
information. However, in practice, numerous components along the
signaling path look at the SDP information. These include entities
performing off-path QoS reservations, NAT traversal components such
as ALGs and Session Border Controllers (SBCs), and diagnostic tools
that passively monitor the network. For these tools to continue to
function without change, the core property of SDP -- that the
existing, pre-ICE definitions of the addresses used for media -- the
m and c lines and the rtcp attribute -- must be retained. For this
reason, an updated offer must be sent.
B.10. Why Are Binding Indications Used for Keepalives?
Media keepalives are described in Section 10. These keepalives make
use of STUN when both endpoints are ICE capable. However, rather
than using a Binding request transaction (which generates a
response), the keepalives use an Indication. Why is that?
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The primary reason has to do with network QoS mechanisms. Once media
begins flowing, network elements will assume that the media stream
has a fairly regular structure, making use of periodic packets at
fixed intervals, with the possibility of jitter. If an agent is
sending media packets, and then receives a Binding request, it would
need to generate a response packet along with its media packets.
This will increase the actual bandwidth requirements for the 5-tuple
carrying the media packets, and introduce jitter in the delivery of
those packets. Analysis has shown that this is a concern in certain
layer 2 access networks that use fairly tight packet schedulers for
media.
Additionally, using a Binding Indication allows integrity to be
disabled, allowing for better performance. This is useful for large-
scale endpoints, such as PSTN gateways and SBCs.
B.11. Why Is the Conflict Resolution Mechanism Needed?
When ICE runs between two peers, one agent acts as controlled, and
the other as controlling. Rules are defined as a function of
implementation type and offerer/answerer to determine who is
controlling and who is controlled. However, the specification
mentions that, in some cases, both sides might believe they are
controlling, or both sides might believe they are controlled. How
can this happen?
The condition when both agents believe they are controlled shows up
in third party call control cases. Consider the following flow:
A Controller B
|(1) INV() | |
|<-------------| |
|(2) 200(SDP1) | |
|------------->| |
| |(3) INV() |
| |------------->|
| |(4) 200(SDP2) |
| |<-------------|
|(5) ACK(SDP2) | |
|<-------------| |
| |(6) ACK(SDP1) |
| |------------->|
Figure 12: Role Conflict Flow
This flow is a variation on flow III of RFC 3725 [RFC3725]. In fact,
it works better than flow III since it produces fewer messages. In
this flow, the controller sends an offerless INVITE to agent A, which
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responds with its offer, SDP1. The agent then sends an offerless
INVITE to agent B, which it responds to with its offer, SDP2. The
controller then uses the offer from each agent to generate the
answers. When this flow is used, ICE will run between agents A and
B, but both will believe they are in the controlling role. With the
role conflict resolution procedures, this flow will function properly
when ICE is used.
At this time, there are no documented flows that can result in the
case where both agents believe they are controlled. However, the
conflict resolution procedures allow for this case, should a flow
arise that would fit into this category.
Author's Address
Jonathan Rosenberg
jdrosen.net
Monmouth, NJ
US
Email: jdrosen@jdrosen.net
URI: http://www.jdrosen.net
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RFC, FYI, BCP