Network Working Group J. Hautakorpi
Request for Comments: 5318 G. Camarillo
Category: Informational Ericsson
December 2008
The Session Initiation Protocol (SIP)
P-Refused-URI-List Private-Header (P-Header)
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
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Abstract
This document specifies the Session Initiation Protocol (SIP)
P-Refused-URI-List Private-Header (P-Header). This P-Header is used
in the Open Mobile Alliance's (OMA) Push to talk over Cellular (PoC)
system. It enables URI-list servers to refuse the handling of
incoming URI lists that have embedded URI lists. This P-Header also
makes it possible for the URI-list server to inform the client about
the embedded URI list that caused the rejection and the individual
URIs that form such a URI list.
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Table of Contents
1. Introduction ....................................................2
2. Terminology .....................................................2
3. Usage Scenario ..................................................3
4. Overview of Operation ...........................................6
5. Syntax of P-Refused-URI-List Header Field .......................6
6. Response Generation .............................................7
7. Message Sequence Example ........................................7
8. Applicability ...................................................9
9. Security Considerations ........................................10
10. IANA Considerations ...........................................11
11. Acknowledgements ..............................................11
12. References ....................................................11
12.1. Normative References .....................................11
12.2. Informative References ...................................12
1. Introduction
The Open Mobile Alliance (OMA) has specified the Push to talk over
Cellular (PoC) service, which uses the Session Initiation Protocol
(SIP) [3] and Uniform Resource Identifier (URI)-list services [5]
(more information about OMA PoC can be found at [8]).
OMA PoC needs a mechanism for servers to refuse the handling of
incoming URI lists when these have embedded URI lists. Such a
mechanism is intended to be used to establish a particular type of
PoC session called an ad-hoc PoC group session.
The remainder of this document is organized as follows. Section 3
describes the scenario where the mechanism will be used. Section 4
provides an overview of the mechanism, which includes a new P-Header
called P-Refused-URI-List. Section 5 defines the syntax of this new
P-Header. Section 6 specifies how to use the P-Header. Section 7
provides a usage example. Section 8 describes the applicability of
the P-Header. Security considerations are discussed in Section 9
and, finally, the IANA considerations are stated in Section 10.
2. 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 [1].
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3. Usage Scenario
An ad-hoc PoC group session is a type of multi-party PoC session.
The originator of a particular ad-hoc PoC group session chooses in an
ad-hoc manner (e.g., selecting from an address book) the set of
desired participants. In order to establish the ad-hoc PoC group
session, the originator sends an INVITE request with a URI list that
contains the URIs of those participants.
The PoC network, following the procedures defined in [6], receives
such an INVITE request and generates an individual INVITE request
towards each of the URIs in the URI list.
In previous versions of the OMA PoC service, the originator of an
ad-hoc PoC group session was only allowed to populate the initial URI
list with URIs identifying individual PoC users. Later versions of
the service allow the originator to also include URI lists whose
entries represent URI lists. That is, the initial URI list contains
entries that are URI lists themselves. The expected service behavior
then is that the members of the embedded URI lists are invited to
join the ad-hoc PoC group session.
Figure 1 illustrates the desired behavior. The originator (not
shown) places the URI list friends@example.org, along with the URI
alice@example.com, in the initial URI list. The PoC network resolves
friends@example.org into its members, bob@example.org and
carol@example.net, and sends INVITE requests to all the recipients.
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2. INVITE
+------------------>
| alice@example.com
|
|
+-------------+
| |
1. INVITE | | 3. INVITE
------------------>| PoC Network |---------------->
alice@example.com | | bob@example.org
friends@example.org | |
+-------------+
|
|
|
| 4. INVITE
+------------------>
carol@example.net
Figure 1: PoC Expected Behavior
The PoC network in Figure 1 consists of PoC servers, which are SIP
entities that can behave as proxies or B2BUAs (Back-to-Back User
Agents). There are two types of logical PoC servers: controlling and
participating.
In an ad-hoc PoC group session, there is always exactly one
controlling PoC server. The controlling PoC server of an ad-hoc PoC
group session resolves an incoming URI list and sends INVITEs to the
members of the list. The controlling PoC server also functions as
the focus of the session. Every participant in an ad-hoc PoC group
has an associated participating PoC server, which resides in the home
domain of the participant.
Figure 2 shows how the PoC servers of the PoC network behave in the
scenario shown in Figure 1. An originating PoC user agent sends an
INVITE request (1) with a URI list to its participating PoC server.
The participating PoC server of the originator receives the INVITE
request, assumes the role of controlling PoC server for the ad-hoc
PoC group session, and sends an INVITE request to each of the URIs in
the URI list.
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+-------------+
2. INVITE | Particip. |
+------------------>| PoC server |->
| alice@example.com | example.com |
| +-------------+
|
+-------------+ 3. INVITE +-------------+
| |-------------------->| |
1. INVITE | Controlling | friends@example.org | Particip. |
---------------->| PoC server | | PoC server |->
alice@example.com | | 4. 403 Forbidden | example.org |
friends@example.org| |<--------------------| |
+-------------+ bob@example.org +-------------+
| | carol@example.net ^
| | |
| | 5. INVITE |
| +--------------------------------+
| bob@example.org
|
| +------------+
| 6. INVITE | Particip. |
+------------------>| PoC server |->
carol@example.net | example.net|
+------------+
Figure 2: PoC Network Behavior
The first URI of the list, alice@example.com, identifies a single
user. The second URI of the URI list, friends@example.org,
identifies a URI list. In PoC terminology, friends@example.com
identifies a pre-arranged PoC group. The PoC server at example.org,
which knows the membership of friends@example.com, cannot send INVITE
requests to the members of friends@example.org because that PoC
server does not act as a controlling PoC server for the ad-hoc PoC
group session being established. Instead, it informs the controlling
PoC server that friends@example.org is a list whose members are
bob@example.org and carol@example.net. Upon receiving this
information, the controlling PoC server generates INVITE requests
towards bob@example.org and carol@example.net.
Although not shown in the above example, the participating PoC server
(example.org) can include -- based on policy, presence of the
members, etc. -- just a partial list of URIs of the URI list.
Furthermore, a URI that the participating PoC server returns can be a
URI list.
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At present, there is not a mechanism for a participating PoC server
to inform a controlling PoC server that a URI identifies a list and
the members of that list, nor is there a mechanism to indicate the
URIs contained in the list. This document defines such a mechanism:
the P-Refused-URI-List P-Header.
4. Overview of Operation
When a URI-list server receives an INVITE request with a URI list
containing entries that are URI lists themselves, and the server
cannot handle the request, it returns a 403 (Forbidden) response with
a P-Refused-URI-List P-Header, as shown in Figure 3. The P-Refused-
URI-List P-Header contains the members of the URI list or lists that
caused the rejection of the request. This way, the client can send
requests directly to those member URIs.
+---------+ INVITE request +----------+
| |------------------------------>| |
| | [URI list in a URI list] | URI-list |
| Client | | server |
| | 403 Forbidden | |
| |<------------------------------| |
| | [Content of refused URI list] | |
+---------+ +----------+
Figure 3: Operational Overview
5. Syntax of P-Refused-URI-List Header Field
The following is the augmented Backus-Naur Form (BNF) [4] syntax of
the P-Refused-URI-List P-Header:
P-Refused-URI-List = "P-Refused-URI-List" HCOLON
uri-list-entry
*(COMMA uri-list-entry)
uri-list-entry = ( name-addr / addr-spec )
*( SEMI refused-param )
refused-param = members-param / generic-param
members-param = "members" EQUAL
LDQUOT *( qdtext / quoted-pair ) RDQUOT
The members P-Header parameter MUST contain a cid-url, which is
defined in RFC 2392 [2].
The HCOLON, SEMI, EQUAL, LDQUOT, RDQUOT, and generic-param are
defined in [3].
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6. Response Generation
A 403 (Forbidden) response can contain more than one P-Refused-URI-
List entries. The P-Refused-URI-List header field MUST NOT be used
with any other response. The P-Refused-URI-List P-Header contains
one or more URIs, which were present in the URI list in the incoming
request and could not be handled by the server. Additionally, the
P-Refused-URI-List can optionally carry some or all of the members of
the URI lists identified by those URIs.
The 403 (Forbidden) response MAY contain body parts which contain URI
lists. Those body parts can be referenced by the P-Refused-URI-List
entries through their Content-IDs [2]. If there is a Content-ID
defined in the P-Refused-URI-List, one of the body parts MUST have an
equivalent Content-ID. The format of a URI list is service specific.
This kind of message structure enables clients to determine which URI
relates to which URI list, if the URI-list server is willing to
disclose that information. Furthermore, the information enclosed in
the URI lists enable clients to take further actions to remedy the
rejection situation (e.g., send individual requests to the members of
the URI list).
7. Message Sequence Example
In the following message sequence example, a controlling PoC server
sends an INVITE request to a participating PoC server. The
participating PoC server rejects the request with a 403 (Forbidden)
response. The 403 response has a P-Refused-URI-List P-Header that
carries the members of the rejected URI lists that the participating
PoC server determines to disclose to this controlling PoC server in
the body of the message.
Controlling PoC server Participating PoC server
example.com example.net
| |
| INVITE |
|-------------------------------->|
| |
| 403 Forbidden |
|<--------------------------------|
| |
Figure 4: Message Sequence Example
The INVITE request shown in Figure 4 is as follows (Via header fields
are not shown for simplicity):
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INVITE sip:poc-service@example.net SIP/2.0
Max-Forwards: 70
From: PoC service ;tag=4fxaed73sl
To: PoC service
Call-ID: 7xTn9vxNit65XU7p4@example.com
CSeq: 1 INVITE
Contact:
Require: recipient-list-invite
Content-Type: multipart/mixed;boundary="boundary1"
Content-Length: 538
--boundary1
Content-Type: application/sdp
(SDP not shown)
--boundary1
Content-Type: application/resource-lists+xml
Content-Disposition: recipient-list
--boundary1--
The URIs sip:friends-list@example.net and
sip:colleagues-list@example.net in the example above are actually
references to URI lists (i.e., pre-arranged PoC groups). In the
following response, the URI lists are in the XML resource list format
[7].
The content of the 403 (Forbidden) response in Figure 4 is as follows
(Via header fields are not shown for simplicity):
SIP/2.0 403 Forbidden
From: PoC service ;tag=4fxaed73sl
To: PoC service ;tag=814254
Call-ID: 7xTn9vxNit65XU7p4@example.com
CSeq: 1 INVITE
P-Refused-URI-List: sip:friends-list@example.net;
members=
P-Refused-URI-List: sip:colleagues-list@example.net;
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members=
Content-Type: multipart/mixed;boundary="boundary1"
Content-Length: 745
--boundary1
Content-Type: application/resource-lists+xml
Content-Disposition: recipient-list
Content-ID:
--boundary1
Content-Type: application/resource-lists+xml
Content-Disposition: recipient-list
Content-ID:
--boundary1--
Using the message body of the 403 (Forbidden) response above, the
controlling PoC server can determine the members of
sip:friend-list@example.net and sip:colleagues-list@example.net that
the participating PoC server determines to disclose to this
controlling PoC server. Furthermore, the controlling PoC server can
deduce that the participating PoC server has not sent any outgoing
requests, per regular URI-list server procedures.
8. Applicability
The P-Refused-URI-List header field is intended to be used in OMA PoC
networks. This header field is used between PoC servers and carries
information about those URI lists that were rejected by the server
receiving the request.
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The OMA PoC services is designed so that, in a given session, only
one PoC server can resolve incoming URI lists and send INVITEs to
members of these lists. This restriction is not present on services
developed to be used on the public Internet. Therefore, the
P-Refused-URI-List P-Header does not seem to have general
applicability outside the OMA PoC service.
Additionally, the use of the P-Refused-URI-List P-Header requires
special trust relationships between servers that do not typically
exist on the public Internet.
It is important to note that the P-Refused-URI-List is optional and
does not change the basic behavior of a SIP URI-list service. The
P-Refused-URI-List only provides clients with additional information
about the refusal of the request.
9. Security Considerations
It is assumed that the network elements handling the P-Refused-URI-
List P-Header are trusted. Also, attackers are not supposed to have
access to the protocol messages between those elements. This is
because the P-Refused-URI-List is intended to be used in the OMA PoC
environment, which is implemented in the operators' core network; for
more on OMA PoC security assumptions, see [9]. Traffic protection
between network elements is achieved by using IP Security (IPsec) and
sometimes by physically protecting the network.
However, implementors and administrators should be aware of two
special security considerations related to the use of P-Refused-URI-
List:
Eavesdropping: 403 (Forbidden) responses may contain information
about the members of a given URI list. Eavesdroppers can acquire
this information if the 403 (Forbidden) responses are not
encrypted. Therefore, it is RECOMMENDED that either hop-by-hop or
end-to-end encryption (e.g., using TLS or S/MIME, respectively) is
used.
Disclosing information: A rogue entity may be able to acquire
information about the members of a given URI list if the URI-list
server sends information about those URI lists to unauthorized
users. Therefore, it is RECOMMENDED that a URI-list server
discloses the content of that URI-list only to authorized clients.
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10. IANA Considerations
The IANA has made two additions to the Session Initiation Protocol
(SIP) Parameters registry. The following header field has been added
to the Header Fields sub-registry.
Header Name compact Reference
----------------- ------- ---------
P-Refused-URI-List [RFC5318]
The following header field parameter has been added to the Header
Field Parameters and Parameter Values sub-registry.
Predefined
Header Field Parameter Name Values Reference
---------------------------- --------------- --------- ---------
P-Refused-URI-List members No [RFC5318]
11. Acknowledgements
Authors would like to thank Tom Hiller who did a thorough, dedicated
review for this document.
12. References
12.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Levinson, E., "Content-ID and Message-ID Uniform Resource
Locators", RFC 2392, August 1998.
[3] 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.
[4] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.
[5] Camarillo, G. and A. Roach, "Framework and Security
Considerations for Session Initiation Protocol (SIP) URI-List
Services", RFC 5363, October 2008.
[6] Camarillo, G. and A. Johnston, "Conference Establishment Using
Request-Contained Lists in the Session Initiation Protocol
(SIP)", RFC 5366, October 2008.
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12.2. Informative References
[7] Rosenberg, J., "Extensible Markup Language (XML) Formats for
Representing Resource Lists", RFC 4826, May 2007.
[8] Open Mobile Alliance, "OMA PoC System Description: Draft Version
2.0", April 2007.
[9] Open Mobile Alliance, "Push to talk over Cellular (PoC) -
Architecture: Draft Version 2.0", April 2007.
Authors' Addresses
Jani Hautakorpi
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
EMail: Jani.Hautakorpi@ericsson.com
Gonzalo Camarillo
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
EMail: Gonzalo.Camarillo@ericsson.com
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