Internet Engineering Task Force (IETF) F. Le Faucheur
Request for Comments: 5945 Cisco
Category: Informational J. Manner
ISSN: 2070-1721 Aalto University
D. Wing
Cisco
A. Guillou
SFR
October 2010
Resource Reservation Protocol (RSVP) Proxy Approaches
Abstract
The Resource Reservation Protocol (RSVP) can be used to make end-to-
end resource reservations in an IP network in order to guarantee the
quality of service required by certain flows. RSVP assumes that both
the data sender and receiver of a given flow take part in RSVP
signaling. Yet, there are use cases where resource reservation is
required, but the receiver, the sender, or both, is not RSVP-capable.
This document presents RSVP proxy behaviors allowing RSVP routers to
initiate or terminate RSVP signaling on behalf of a receiver or a
sender that is not RSVP-capable. This allows resource reservations
to be established on a critical subset of the end-to-end path. This
document reviews conceptual approaches for deploying RSVP proxies and
discusses how RSVP reservations can be synchronized with application
requirements, despite the sender, receiver, or both not participating
in RSVP. This document also points out where extensions to RSVP (or
to other protocols) may be needed for deployment of a given RSVP
proxy approach. However, such extensions are outside the scope of
this document. Finally, practical use cases for RSVP proxy are
described.
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Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
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). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see 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/rfc5945.
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RFC 5945 RSVP Proxy Approaches October 2010
Table of Contents
1. Introduction ....................................................3
2. RSVP Proxy Behaviors ............................................6
2.1. RSVP Receiver Proxy ........................................6
2.2. RSVP Sender Proxy ..........................................7
3. Terminology .....................................................7
4. RSVP Proxy Approaches ...........................................9
4.1. Path-Triggered Receiver Proxy ..............................9
4.1.1. Mechanisms for Maximizing the Reservation Span .....11
4.2. Path-Triggered Sender Proxy for Reverse Direction .........15
4.3. Inspection-Triggered Proxy ................................18
4.4. STUN-Triggered Proxy ......................................21
4.5. Application_Entity-Controlled Proxy .......................23
4.5.1. Application_Entity-Controlled Sender Proxy
Using "RSVP over GRE" ..............................26
4.5.2. Application_Entity-Controlled Proxy via Co-Location 28
4.6. Policy_Server-Controlled Proxy ............................29
4.7. RSVP-Signaling-Triggered Proxy ............................32
4.8. Reachability Considerations ...............................33
5. Security Considerations ........................................34
6. Acknowledgments ................................................36
7. References .....................................................36
7.1. Normative References ......................................36
7.2. Informative References ....................................37
Appendix A. Use Cases for RSVP Proxies ...........................40
A.1. RSVP-Based VoD Admission Control in Broadband
Aggregation Networks ......................................40
A.2. RSVP-Based Voice/Video Connection Admission Control
(CAC) in Enterprise WAN ...................................43
A.3. RSVP Proxies for Mobile Access Networks ...................44
A.4. RSVP Proxies for Reservations in the Presence of IPsec
Gateways ..................................................46
1. Introduction
Guaranteed Quality of Service (QoS) for some applications with tight
requirements (such as voice or video) may be achieved by reserving
resources in each node on the end-to-end path. The main IETF
protocol for these resource reservations is the Resource Reservation
Protocol (RSVP), as specified in [RFC2205]. RSVP does not require
that all intermediate nodes support RSVP; however, it assumes that
both the sender and the receiver of the data flow support RSVP.
There are environments where it would be useful to be able to reserve
resources for a flow on at least a subset of the flow path even when
the sender or the receiver (or both) is not RSVP-capable (for
example, from the sender to the network edge, or from edge to edge,
or from the network edge to the receiver).
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Since the data sender or receiver may be unaware of RSVP, there are
two types of RSVP proxies. When the sender is not using RSVP, an
entity in the network must operate on behalf of the data sender, and
in particular, generate RSVP Path messages, and eventually receive,
process, and sink Resv messages. We refer to this entity as the RSVP
Sender Proxy. When the receiver is not using RSVP, an entity in the
network must receive Path messages sent by a data sender (or by an
RSVP Sender Proxy), sink those, and return Resv messages on behalf of
the data receiver(s). We refer to this entity as the RSVP Receiver
Proxy. The RSVP proxies need to be on the data path in order to
establish the RSVP reservation; note, however, that some of the
approaches described in this document allow the RSVP proxies to be
controlled/triggered by an off-path entity.
The flow sender and receiver generally have at least some (if not
full) awareness of the application producing or consuming that flow.
Hence, the sender and receiver are in a natural position to
synchronize the establishment, maintenance, and teardown of the RSVP
reservation with the application requirements. Similarly, they are
in a natural position to determine the characteristics of the
reservation (bandwidth, QoS service, etc.) that best match the
application requirements. For example, before completing the
establishment of a multimedia session, the endpoints may decide to
establish RSVP reservations for the corresponding flows. Similarly,
when the multimedia session is torn down, the endpoints may decide to
tear down the corresponding RSVP reservations. For instance,
[RFC3312] discusses how RSVP reservations can be very tightly
synchronized by endpoints that uses the Session Initiation Protocol
(SIP) ([RFC3261]) for session control.
When RSVP reservation establishment, maintenance, and teardown are to
be handled by RSVP proxies on behalf of an RSVP sender or receiver, a
key challenge for the RSVP proxy is to determine when the RSVP
reservations need to be established, maintained, and torn down, and
to determine what the characteristics are (bandwidth, QoS, etc.) of
the required RSVP reservations matching the application requirements.
We refer to this problem as the synchronization of RSVP reservations
with application-level requirements.
The IETF Next Steps in Signaling (NSIS) working group has specified a
new QoS signaling protocol: the QoS NSIS Signaling Layer Protocol
(NSLP) ([RFC5974]). This protocol also includes the notion of proxy
operation, and terminating QoS signaling on nodes that are not the
actual data senders or receivers (see Section 4.8, "Proxy Mode", of
[RFC5974]. This is the same concept as the proxy operation for RSVP
discussed in this document. One difference, though, is that the NSIS
framework does not consider multicast resource reservations, which
RSVP provides today.
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Section 2 introduces the notion of RSVP Sender Proxy and RSVP
Receiver Proxy. Section 3 defines useful terminology. Section 4
then presents several fundamental RSVP proxy approaches, discussing
how they achieve the necessary synchronization of RSVP reservations
with application-level requirements. Appendix A includes more
detailed use cases for the proxies in various real-life deployment
environments.
It is important to keep in mind that the strongly recommended RSVP
deployment model remains end-to-end as assumed in [RFC2205] with RSVP
support on the sender and the receiver. The end-to-end model allows
the most effective synchronization between the reservation and
application requirements. Also, when compared to the end-to-end RSVP
model, the use of RSVP proxies involves additional operational burden
and/or imposes some topological constraints. The additional
operational burden comes in particular from additional configuration
needed to activate the RSVP proxies and to help them identify for
which senders/receivers a proxy behavior is required and for which
senders/receivers it is not (so that an RSVP proxy does not perform
establishment of reservations on behalf of devices that are capable
of doing so themselves but would then be prevented -- without
notification -- from doing so by the RSVP proxy). The additional
topological constraints come in particular from the requirement to
have one RSVP Receiver Proxy on the path from any sender to every
non-RSVP-capable device (so that a non-RSVP-capable device is always
taken care of by an RSVP proxy) and the objective to have only one
such Receiver Proxy on the path from any sender to every non-RSVP-
capable device (so that an RSVP Receiver Proxy does not short-circuit
another RSVP Receiver Proxy closer to the non-RSVP-capable device,
thereby reducing the span of the RSVP reservation and the associated
benefits). In the case of the Path-Triggered Receiver Proxy
approach, the operational burden and topological constraints can be
significantly alleviated using the mechanisms discussed in
Section 4.1.1.
It is also worth noting that RSVP operations on end-systems are
considerably simpler than on a router, and consequently that RSVP
implementations on end-systems are very lightweight (particularly
considering modern end-systems' capabilities, including mobile and
portable devices). For example, end-system RSVP implementations are
reported to only consume low tens of kilobytes of code space. Hence,
this document should not be seen as an encouragement to depart from
the end-to-end RSVP model. Its purpose is only to allow RSVP
deployment in special environments where RSVP just cannot be used on
some senders and/or some receivers for reasons specific to the
environment.
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2. RSVP Proxy Behaviors
This section discusses the two types of proxies: the RSVP Sender
Proxy operating on behalf of data senders, and the RSVP Receiver
Proxy operating for data receivers. The concepts presented in this
document are not meant to deprecate the traditional [RFC2205] RSVP
end-to-end model: end-to-end RSVP reservations are still expected to
be used whenever possible. However, RSVP proxies are intended to
facilitate RSVP deployment where end-to-end RSVP signaling is not
possible.
2.1. RSVP Receiver Proxy
With conventional end-to-end RSVP operations, RSVP reservations are
controlled by receivers of data. After a data sender has sent an
RSVP Path message towards the intended recipient(s), each recipient
that requires a reservation generates a Resv message. If, however, a
data receiver is not running the RSVP protocol, the last-hop RSVP
router will still send the Path message to the data receiver, which
will silently drop this message as an IP packet with an unknown
protocol number.
In order for reservations to be made in such a scenario, one of the
RSVP routers on the data path determines that the data receiver will
not be participating in the resource reservation signaling and
performs RSVP Receiver Proxy functionality on behalf of the data
receiver. This is illustrated in Figure 1. Various mechanisms by
which the RSVP proxy router can gain the required information are
discussed later in the document.
|****| *** *** |**********| |----|
| S |---------*r*----------*r*---------| RSVP |----------| R |
|****| *** *** | Receiver | |----|
| Proxy |
|**********|
===================RSVP==============>
***********************************************************>
|****| RSVP-capable |----| non-RSVP-capable ***
| S | Sender | R | Receiver *r* regular RSVP
|****| |----| *** router
***> unidirectional media flow
==> segment of flow path protected by RSVP reservation
Figure 1: RSVP Receiver Proxy
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2.2. RSVP Sender Proxy
With conventional end-to-end RSVP operations, if a data sender is not
running the RSVP protocol, a resource reservation cannot be set up; a
data receiver alone cannot reserve resources without Path messages
first being received. Thus, even if the data receiver is running
RSVP, it still needs some node on the data path to send a Path
message towards the data receiver.
In that case, an RSVP node on the data path determines that it should
generate Path messages to allow the receiver to set up the resource
reservation. This node is referred to as the RSVP Sender Proxy and
is illustrated in Figure 2. This case presents additional challenges
over the Receiver Proxy case, since the RSVP Sender Proxy must be
able to generate all the information in the Path message (such as the
SENDER_TSPEC object) without the benefit of having previously
received any RSVP message. An RSVP Receiver Proxy, by contrast, only
needs to formulate an appropriate Resv message in response to an
incoming Path message. Mechanisms to operate an RSVP Sender Proxy
are discussed later in this document.
|----| |**********| *** *** |****|
| S |---------| RSVP |---------*r*----------*r*----------| R |
|----| | Sender | *** *** |****|
| Proxy |
|**********|
================RSVP==================>
***********************************************************>
|----| non-RSVP-capable |****| RSVP-capable ***
| S | Sender | R | Receiver *r* regular RSVP
|----| |****| *** router
***> unidirectional media flow
==> segment of flow path protected by RSVP reservation
Figure 2: RSVP Sender Proxy
3. Terminology
o On-Path: located on the data path of the actual flow of
application data (regardless of where it is located with respect
to the application-level signaling path).
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o Off-Path: not On-Path.
o RSVP-capable (or RSVP-aware): supporting the RSVP protocol as per
[RFC2205].
o RSVP Receiver Proxy: an RSVP-capable router performing, on behalf
of a receiver, the RSVP operations that would normally be
performed by an RSVP-capable receiver if end-to-end RSVP signaling
were used. Note that while RSVP is used upstream of the RSVP
Receiver Proxy, RSVP is not used downstream of the RSVP Receiver
Proxy.
o RSVP Sender Proxy: an RSVP-capable router performing, on behalf of
a sender, the RSVP operations that would normally be performed by
an RSVP-capable sender if end-to-end RSVP signaling were used.
Note that while RSVP is used downstream of the RSVP Sender Proxy,
RSVP is not used upstream of the RSVP Sender Proxy.
o Regular RSVP Router: an RSVP-capable router that is not behaving
as an RSVP Receiver Proxy or as an RSVP Sender Proxy.
o Application-level signaling: signaling between entities operating
above the IP layer and that are aware of the QoS requirements for
actual media flows. SIP ([RFC3261]) and the Real Time Streaming
Protocol (RTSP) ([RFC2326]) are examples of application-level
signaling protocols. The Session Description Protocol (SDP)
([RFC4566]) is an example of a protocol that can be used by the
application-level signaling protocol and from which some of the
RSVP reservation parameters (addresses, ports, and bandwidth)
might be derived. RSVP is clearly not an application-level
signaling protocol.
The roles of the RSVP Receiver Proxy, RSVP Sender Proxy, and regular
RSVP router are all relative to a given unidirectional flow. A given
router may act as the RSVP Receiver Proxy for a flow, as the RSVP
Sender Proxy for another flow, and as a regular RSVP router for yet
another flow.
Some application-level signaling protocols support negotiation of QoS
reservations for a media stream. For example, with [RFC3312],
resource reservation requirements are explicitly signaled during
session establishment using SIP and SDP. Also, [RFC5432] defines a
mechanism to negotiate which resource reservation mechanism is to be
used for a particular media stream. Clearly, these reservation
negotiation mechanisms can be invoked and operate effectively when
both ends support RSVP (and obviously RSVP proxies are not used).
When both ends do not support RSVP (and RSVP proxies are used at both
ends), these mechanisms will simply not be invoked. In the case
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where one end supports RSVP and the other does not (and is helped by
an RSVP proxy), the application-level signaling entity supporting the
non-RSVP-capable end might use the reservation negotiation mechanisms
in such a way that the non-RSVP-capable end (helped by an RSVP proxy)
appears to the remote end as an RSVP-capable device. This will
ensure that the RSVP-capable end is not discouraged from using RSVP
because the remote end is not RSVP-capable. In the case of SIP, the
application-level entity may achieve this by taking advantage of the
"segmented" status type of [RFC3312] and/or by taking advantage of a
SIP [RFC3261] Back-to-Back User Agent (B2BUA).
4. RSVP Proxy Approaches
This section discusses fundamental RSVP proxy approaches.
4.1. Path-Triggered Receiver Proxy
In this approach, it is assumed that the sender is RSVP-capable and
takes full care of the synchronization between application
requirements and RSVP reservations. With this approach, the RSVP
Receiver Proxy uses the RSVP Path messages generated by the sender as
the cue for establishing the RSVP reservation on behalf of the
receiver. The RSVP Receiver Proxy is effectively acting as a slave
making reservations (on behalf of the receiver) under the sender's
control. This changes somewhat the usual RSVP reservation model
where reservations are normally controlled by receivers. Such a
change greatly facilitates operations in the scenario of interest
here, which is where the receiver is not RSVP-capable. Indeed, it
allows the RSVP Receiver Proxy to remain application-unaware by
taking advantage of the application awareness and RSVP awareness of
the sender.
With the Path-Triggered RSVP Receiver Proxy approach, the RSVP router
may be configured to use receipt of a regular RSVP Path message as
the trigger for RSVP Receiver Proxy behavior.
On receipt of the RSVP Path message, the RSVP Receiver Proxy:
1. establishes the RSVP Path state as per regular RSVP processing.
2. identifies the downstream interface towards the receiver.
3. sinks the Path message.
4. behaves as if a Resv message (whose details are discussed below)
was received on the downstream interface. This includes
performing admission control on the downstream interface,
establishing a Resv state (in case of successful admission
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control), and forwarding the Resv message upstream, sending
periodic refreshes of the Resv message and tearing down the
reservation if the Path state is torn down.
In order to build the Resv message, the RSVP Receiver Proxy can take
into account information received in the Path message. For example,
the RSVP Receiver Proxy may compose a FLOWSPEC object for the Resv
message that mirrors the SENDER_TSPEC object in the received Path
message (as an RSVP-capable receiver would typically do).
Operation of the Path-Triggered Receiver Proxy in the case of a
successful reservation is illustrated in Figure 3.
|****| *** *** |**********| |----|
| S |---------*r*----------*r*---------| RSVP |----------| R |
|****| *** *** | Receiver | |----|
| Proxy |
|**********|
---Path---> ----Path----> ---Path---->
<--Resv---> <---Resv----- <--Resv----
==================RSVP===============>
**********************************************************>
|****| RSVP-capable |----| Non-RSVP-capable ***
| S | Sender | R | Receiver *r* regular RSVP
|****| |----| *** router
***> media flow
==> segment of flow path protected by RSVP reservation
Figure 3: Path-Triggered RSVP Receiver Proxy
In case the reservation establishment is rejected (for example,
because of an admission control failure on a regular RSVP router on
the path between the RSVP-capable sender and the RSVP Receiver
Proxy), a ResvErr message will be generated as per conventional RSVP
operations and will travel downstream towards the RSVP Receiver
Proxy. While this ensures that the RSVP Receiver Proxy is aware of
the reservation failure, conventional RSVP procedures do not cater to
the notification of the sender of the reservation failure. Operation
of the Path-Triggered RSVP Receiver Proxy in the case of an admission
control failure is illustrated in Figure 4.
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|****| *** *** |**********| |----|
| S |---------*r*----------*r*---------| RSVP |----------| R |
|****| *** *** | Receiver | |----|
| Proxy |
|**********|
---Path---> ----Path----> ---Path---->
<---Resv----- <--Resv------
---ResvErr---> --ResvErr--->
===================RSVP===============>
**********************************************************>
|****| RSVP-capable |----| Non-RSVP-capable ***
| S | Sender | R | Receiver *r* regular RSVP
|****| |----| *** router
***> media flow
==> segment of flow path protected by RSVP reservation
Figure 4: Path-Triggered RSVP Receiver Proxy with Failure
Since, as explained above, in this scenario involving the RSVP
Receiver Proxy, synchronization between an application and an RSVP
reservation is generally performed by the sender, notifying the
sender of reservation failure is needed. [RFC5946] specifies RSVP
extensions allowing such sender notification in the case of
reservation failure in the presence of a Path-Triggered RSVP Receiver
Proxy.
4.1.1. Mechanisms for Maximizing the Reservation Span
The presence in the flow path of a Path-Triggered RSVP Receiver Proxy
(for a given flow) that strictly behaves as described previously
would cause the Path message to be terminated and a Resv message to
be generated towards the sender. When the receiver is indeed not
RSVP-capable and there is no other RSVP Receiver Proxy downstream on
the flow path, this achieves the best achievable result of
establishing an RSVP reservation as far downstream as the RSVP
Receiver Proxy.
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However, if the eventual receiver was in fact RSVP-capable, it would
be prevented from participating in RSVP signaling, since it does not
receive any Path message. As a result, the RSVP reservation would
only span a subset of the path it could actually span. A similar
sub-optimality would exist with multiple Receiver Proxies in the path
of the flow: the first Receiver Proxy may prevent the Path message
from reaching the second one and therefore prevent the reservation
from extending down to the second Receiver Proxy.
It is desirable that, in the presence of Path-Triggered RSVP Receiver
Proxies and of a mix of RSVP-capable and non-RSVP-capable receivers,
the RSVP reservation spans as much of the flow path as possible.
This can be achieved dynamically (avoiding tedious specific
configuration), using the mechanisms described in Sections 4.1.1.1
and 4.1.1.2.
4.1.1.1. Dynamic Discovery of Downstream RSVP Functionality
When generating a proxy Resv message upstream, a Receiver Proxy may
be configured to perform dynamic discovery of downstream RSVP
functionality. To that end, when generating the proxy Resv message
upstream, the Receiver Proxy forwards the Path message downstream
instead of terminating it. This allows an RSVP-capable receiver (or
a downstream Receiver Proxy) to respond to the Path with an upstream
Resv message. On receipt of a Resv message, the Receiver Proxy
internally converts its state from a proxied reservation to a regular
midpoint RSVP behavior. From then on, everything proceeds as if the
RSVP router had behaved as a regular RSVP router at reservation
establishment (as opposed to having behaved as an RSVP Receiver Proxy
for that flow).
The RSVP Receiver Proxy behavior for dynamic discovery of downstream
RSVP functionality is illustrated in Figure 5 and is also discussed
in Section 4.1 of [RFC5946].
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|****| *** |**********| |----|
| S |---------*r*---------| RSVP |---| R1 |
|****| *** | Receiver | |----|
| Proxy |
| |
| | |****|
| |------------| R2 |
|**********| |****|
---Path---> --Path--->
(R1) (R1) \-------Path-->
/ (R1)
<--Resv--- <---Resv---
================RSVP===>
**************************************>
---Path---> --Path--->
(R2) (R2) \-------------Path---->
/ (R2)
<--Resv--- <---Resv---
<----Resv---
================RSVP===========================>
***********************************************>
|****| RSVP-capable |----| non-RSVP-capable |****| RSVP-capable
| S | Sender | R | Receiver | R | Receiver
|****| |----| |****|
***
*r* regular RSVP
*** router
(R1) = Path message contains a Session object whose destination is R1
***> media flow
==> segment of flow path protected by RSVP reservation
Figure 5: Dynamic Discovery of Downstream RSVP Functionality
This dynamic discovery mechanism has the benefit that new (or
upgraded) RSVP endpoints will automatically and seamlessly be able to
take advantage of end-to-end reservations, without impacting the
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ability of a Receiver Proxy to proxy RSVP for other, non-RSVP-capable
endpoints. This mechanism also achieves the goal of automatically
discovering the longest possible RSVP-supporting segment in a network
with multiple Receiver Proxies along the path. This mechanism
dynamically adjusts to any topology and routing change. Also, this
mechanism dynamically handles the situation in which a receiver was
RSVP-capable and for some reason (e.g., software downgrade) no longer
is. Finally, this approach requires no new RSVP protocol extensions
and no configuration changes to the Receiver Proxy as new RSVP-
capable endpoints come and go.
The only identified drawbacks to this approach are:
o If admission control fails on the segment between the Receiver
Proxy and the RSVP-capable receiver, the receiver will get a
ResvErr and can take application-level signaling steps to
terminate the call. However, the Receiver Proxy has already sent
a Resv upstream for this flow, so the sender will see a "false"
reservation that is not truly end-to-end. The actual admission
control status will resolve itself in a short while, but the
sender will need to roll back any permanent action (such as
billing) that may have been taken on receipt of the phantom Resv.
Note that if the second receiver is also a Receiver Proxy that is
not participating in application signaling, it will convert the
received ResvErr into a PathErr that will be received by the
sender.
o If there is no RSVP-capable receiver (or other Receiver Proxy)
downstream of the Receiver Proxy, then the Path messages sent by
the Receiver Proxy every RSVP refresh interval (e.g., 30 seconds
by default) will never be responded to. However, these messages
consume a small amount of bandwidth, and in addition would install
some RSVP state on RSVP-capable midpoint nodes downstream of the
first Receiver Proxy. This is seen as a very minor sub-
optimality. We also observe that such resources would be consumed
anyways if the receiver was RSVP-capable. Still, if deemed
necessary, to mitigate this, the Receiver Proxy can tear down any
unanswered downstream Path state and stop sending Path messages
for the flow (or only send them at much lower frequency) as
further discussed in [RFC5946].
4.1.1.2. Selective Receiver Proxy and Sender Control of Receiver Proxy
An RSVP Receiver Proxy can be selective about the sessions that it
terminates, based on local policy decision. For example, an edge
router functioning as a Receiver Proxy may behave as a proxy only for
Path messages that are actually going to exit the domain in question,
and not for Path messages that are transiting through it but stay
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within the domain. As another example, the Receiver Proxy may be
configurable to only proxy for flows addressed to a given destination
address or destination address ranges (for which end devices are
known to not be RSVP-capable).
The decision to proxy a Resv for a Path may also be based on
information signaled from the sender in the Path message. For
example, the sender may identify the type of application or flow in
the Application Identity policy element ([RFC2872]) in the Path, and
the Receiver Proxy may be configured to proxy for only certain types
of flows. Or, if the sender knows (for example, through application
signaling) that the receiver is RSVP-capable, the sender can include
an indication in a policy element to any Receiver Proxy that it ought
not to terminate the Path (or conversely, if the receiver is known
not to support RSVP, the sender could include an indication to
Receiver Proxies that they ought to generate a proxy Resv message).
The Receiver Proxy Control policy element specified in Section 4.2 of
[RFC5946] can be used for that purpose.
4.2. Path-Triggered Sender Proxy for Reverse Direction
In this approach, it is assumed that one endpoint is RSVP-capable and
takes full care of the synchronization between application
requirements and RSVP reservations. This endpoint is the sender for
one flow direction (which we refer to as the "forward" direction) and
is the receiver for the flow in the opposite direction (which we
refer to as the "reverse" direction).
With the Path-Triggered Sender Proxy for Reverse Direction approach,
the RSVP proxy uses the RSVP signaling generated by the receiver (for
the reverse direction) as the cue for initiating RSVP signaling for
the reservation in the reverse direction. More precisely, the RSVP
proxy can take the creation (or maintenance or teardown) of a Path
state by the receiver as the cue to create (or maintain or tear down,
respectively) a Path state towards the receiver. Thus, the RSVP
proxy is effectively acting as a Sender Proxy for the reverse
direction under the control of the receiver (for the reverse
direction). Note that this assumes a degree of symmetry, for
example, in terms of bandwidth for the two directions of the flow (as
is currently typical for IP telephony).
The signaling flow for the Path-Triggered Sender Proxy for Reverse
Direction is illustrated in Figure 6.
Path messages generated by the receiver need to transit via the RSVP
Sender Proxy that is on the path from the sender to the receiver. In
some topologies, this will always be the case: for example, where the
sender is on a stub network hanging off the RSVP Sender Proxy or
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where there is no asymmetric routing (such that if an RSVP Sender
Proxy is on the path from receiver to sender, then it is also on the
path from sender to receiver). In some topologies (such as those
involving asymmetric routing), this may not always happen naturally.
Measures to ensure this does happen in these topologies are outside
the scope of this document.
|****| *** *** |**********| |----|
| R |---------*r*----------*r*---------| RSVP |----------| S |
|****| *** *** | Sender | |----|
| Proxy |
|**********|
---Path---> ----Path----> ---Path---->
<--Path---> <---Path----- <--Path----
---Resv---> ----Resv----> ---Resv---->
<================RSVP==================
<**********************************************************
|****| RSVP-capable |----| Non-RSVP-capable ***
| R | Receiver for | S | Sender for *r* regular RSVP
|****| reverse direction |----| reverse direction *** router
***> media flow
==> segment of flow path protected by RSVP reservation
in reverse direction
Figure 6: Path-Triggered Sender Proxy for Reverse Direction
Of course, the RSVP proxy may simultaneously (and typically will)
also act as the Path-Triggered Receiver Proxy for the forward
direction, as defined in Section 4.1. Such an approach is most
useful in situations involving RSVP reservations in both directions
for symmetric flows. This is illustrated in Figure 7.
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|****| *** *** |----------| |----|
|S/R |---------*r*----------*r*---------| RSVP |----------|S/R |
|****| *** *** | Receiver | |----|
| & Sender |
| Proxy |
|----------|
---Path---> ----Path----> ---Path---->
<--Resv---> <---Resv----- <--Resv----
<--Path---> <---Path----- <--Path----
---Resv---> ----Resv----> ---Resv---->
================RSVP==================>
<================RSVP==================
**********************************************************>
<**********************************************************
|****| RSVP-capable |----| Non-RSVP-capable ***
|S/R | Sender and |S/R | Sender and *r* regular RSVP
|****| Receiver |----| Receiver *** router
***> media flow
==> segment of flow path protected by RSVP reservation
in forward and in reverse direction
Figure 7: Path Triggered Receiver and Sender Proxy
With the Path-Triggered Sender Proxy for Reverse Direction approach,
the RSVP router may be configurable to use receipt of a regular RSVP
Path message as the trigger for Sender Proxy for Reverse Direction
behavior.
On receipt of the RSVP Path message for the forward direction, the
RSVP Sender Receiver Proxy:
1. sinks the Path message.
2. behaves as if a Path message for the reverse direction (whose
details are discussed below) had been received by the Sender
Proxy. This includes establishing the corresponding Path state,
forwarding the Path message downstream, sending periodic
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refreshes of the Path message, and tearing down the Path in the
reverse direction when the Path state in the forward direction is
torn down.
In order to build the Path message for the reverse direction, the
RSVP Sender Proxy can take into account information in the received
Path message for the forward direction. For example, the RSVP Sender
Proxy may mirror the SENDER_TSPEC object in the received Path
message.
We observe that this approach does not require any extensions to the
existing RSVP protocol.
In the case where reservations are required in both directions (as
shown in Figure 7), the RSVP-capable device simply needs to behave as
a regular RSVP sender and RSVP receiver. It need not be aware that
an RSVP proxy happens to be used, and the Path message it sent for
the forward reservation also acts as the trigger for establishment of
the reverse reservation. However, in the case where a reservation is
only required in the reverse direction (as shown in Figure 6), the
RSVP-capable device has to generate Path messages in order to trigger
the reverse-direction reservation even if no reservation is required
in the forward direction. Although this is not in violation of
[RFC2205], it may not be the default behavior of an RSVP-capable
device and therefore may need a behavioral change specifically to
facilitate operation of the Path-Triggered Sender Proxy for Reverse
Direction.
4.3. Inspection-Triggered Proxy
In this approach, it is assumed that the RSVP proxy is on the data
path of "packets of interest", that it can inspect such packets on
the fly as they transit through it, and that it can infer information
from these packets of interest to determine what RSVP reservations
need to be established, as well as when and with what characteristics
(possibly also using some configured information).
One example of "packets of interest" could be application-level
signaling. An RSVP proxy capable of inspecting SIP signaling for a
multimedia session or RTSP signaling for video streaming can obtain
from such signaling information about when a multimedia session is up
or when a video is going to be streamed. It can also identify the
addresses and ports of senders and receivers and can determine the
bandwidth of the corresponding flows. It can also determine when the
reservation is no longer needed and tear it down. Thus, such an RSVP
proxy can determine all necessary information to synchronize RSVP
reservations to application requirements. This is illustrated in
Figure 8.
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|-------------|
| Application |
| Signaling |
| Entity |
|-------------|
/ \
/ \
/ \
|----| |********| *** |********| |----|
| S |--------| RSVP |------*r*--------| RSVP |----------| R |
|----| | Proxy | *** | Proxy | |----|
|********| |********|
=======RSVP=======>
********************************************************>
|----| Non-RSVP-capable |----| Non-RSVP-capable ***
| S | Sender | R | Receiver *r* regular RSVP
|----| |----| *** router
application-level signaling
***> media flow
==> segment of flow path protected by RSVP reservation
Figure 8: Inspection-Triggered RSVP Proxy
Another example of "packets of interest" could be transport control
messages (e.g., the Real-time Transport Control Protocol (RTCP)
[RFC3550]) traveling alongside the application flow itself (i.e.,
media packets). An RSVP proxy capable of detecting the transit of
packets from a particular flow can attempt to establish a reservation
corresponding to that flow. Characteristics of the reservation may
be derived by various methods such as from configuration, flow
measurement, or a combination of those. However, these methods
usually come with their respective operational drawbacks:
configuration involves an operational cost and may hinder
introduction of new applications, and measurement is reactive so that
accurate reservation may lag actual traffic.
In the case of reservation failure, the Inspection-Triggered RSVP
Proxy does not have a direct mechanism for notifying the application
(since it is not participating itself actively in application
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signaling) so that the application is not in a position to take
appropriate action (for example, terminate the corresponding
session). To mitigate this problem, the Inspection-Triggered RSVP
Proxy may differently mark the Differentiated Services codepoint
(DSCP) ([RFC2474]) of flows for which an RSVP reservation has been
successfully proxied from the flows for which a reservation is not in
place. In some situations, the Inspection-Triggered Proxy might be
able to modify the "packets of interest" (e.g., application signaling
messages) to convey some hint to applications that the corresponding
flows cannot be guaranteed by RSVP reservations.
With the Inspection-Triggered Proxy approach, the RSVP proxy is
effectively required to attempt to build application awareness by
traffic inspection and then is somewhat limited in the actions it can
take in case of reservation failure. Depending on the "packets of
interest" used by the RSVP proxy to trigger the reservation, there is
a risk that the RSVP proxy will end up establishing a reservation for
a media flow that actually never starts. However, this can be
mitigated by the timing out and tearing down of an unnecessary
reservation by the RSVP proxy when no corresponding media flow is
observed. This flow observation and timeout approach can also be
used to tear down reservations that were rightfully established for a
flow but are no longer needed because the flow stopped.
The Inspection-Triggered approach is also subject to the general
limitations associated with data inspection. This includes being
impeded by encryption or tunneling, or being dependent on some
topology constraints such as relying on the fact that both the
packets of interest and the corresponding flow packets always transit
through the same RSVP proxy.
Nonetheless, this may be a useful approach in specific environments.
Note also that this approach does not require any change to the RSVP
protocol.
With the Inspection-Triggered RSVP Proxy approach, the RSVP router
may be configurable to use and interpret some specific packets of
interest as the trigger for RSVP Receiver Proxy behavior.
When operating off signaling traffic, the Inspection-Triggered RSVP
Proxy may be able to detect from the signaling that the endpoint is
capable of establishing an RSVP reservation (e.g., in the case of
SIP, via the inspection of the [RFC3312]/[RFC4032] precondition), in
which case it would not behave as a proxy for that endpoint. Also,
the Inspection-Triggered RSVP Proxy may inspect RSVP signaling, and
if it sees RSVP signaling for the flow of interest, it can disable
its Sender Proxy behavior for that flow (or that sender).
Optionally, through RSVP signaling inspection, the Sender Proxy might
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also gradually "learn" (possibly with some timeout) which sender is
RSVP-capable and which is not. These mechanisms can facilitate
gradual and dynamic migration from the proxy model towards the end-
to-end RSVP model as more and more endpoints become RSVP-capable.
4.4. STUN-Triggered Proxy
In this approach, the RSVP proxy takes advantage of the application
awareness provided by the Session Traversal Utilities for NAT (STUN)
([RFC5389]) signaling to synchronize RSVP reservations with
application requirements. The STUN signaling is sent from endpoint
to endpoint. This is illustrated in Figure 9. In this approach, a
STUN message triggers the RSVP proxy.
|----| |********| *** |********| |----|
| S |--------| RSVP |------*r*--------| RSVP |----------| R |
|----| | Proxy | *** | Proxy | |----|
|********| |********|
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^>
=======RSVP=======>
********************************************************>
|----| Non-RSVP-capable |----| Non-RSVP-capable ***
| S | Sender | R | Receiver *r* regular RSVP
|----| |----| *** router
^^^> STUN message flow (over same UDP ports as media flow)
==> segment of flow path protected by RSVP reservation
***> RTP media flow
Figure 9: STUN-Triggered Proxy
For unicast flows, [RFC5245] is a widely adopted approach for Network
Address Translator (NAT) traversal. For our purposes of triggering
RSVP proxy behavior, we rely on the Interactive Connectivity
Establishment (ICE) protocol's connectivity check, which is based on
the exchange of STUN Binding Request messages between hosts to verify
connectivity (see Section 2.2 of [RFC5245]). The STUN message could
also include (yet to be specified) STUN attributes to indicate
information such as the bandwidth and application requesting the
flow, which would allow the RSVP proxy agent to create an
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appropriately sized reservation for each flow. Including such new
STUN attributes in the ICE connectivity check messages would
facilitate operation of the RSVP proxy. To ensure RSVP reservations
are only established when needed, the RSVP proxy needs to
distinguish, among all the STUN messages, the ones that reflect (with
high likelihood) an actual upcoming media flow. This can be achieved
by identifying the STUN messages associated with an ICE connectivity
check. In turn, this can be achieved through (some combination of)
the following checks:
o if, as discussed above, new STUN attributes (e.g., conveying the
flow bandwidth) are indeed defined in the future in view of
facilitating STUN-Triggered reservations, then the presence of
these attributes would reveal that the STUN message is part of an
ICE connectivity check.
o the presence of the PRIORITY, USE-CANDIDATE, ICE-CONTROLLED, or
ICE-CONTROLLING attributes reveals that the STUN message is part
of an ICE connectivity check.
o the RSVP proxy may wait for a STUN message containing the USE-
CANDIDATE attribute indicating the selected ICE "path" to trigger
reservation only for the selected "path". This allows the RSVP
proxy to only trigger a reservation for the "path" actually
selected and therefore for the media flow that will actually be
established (for example, when ICE is being used for IPv4/v6 path
selection).
o the RSVP proxy configuration could contain some information
facilitating determination of when to perform RSVP proxy
reservation and when not to. For example, the RSVP proxy
configuration could contain the IP addresses of the STUN servers
such that STUN messages to/from those addresses are known to not
be part of an ICE connectivity check. As another example, the
RSVP proxy configuration could contain information identifying the
set of Differentiated Services codepoint (DSCP) values that the
media flows requiring reservation use, so that STUN messages not
using one of these DSCP values are known to not be part of an ICE
connectivity check.
Despite these checks, there is always a potential risk that the RSVP
proxy will end up establishing a reservation for a media flow that
actually never starts. However, this is limited to situations in
which the end-systems are interested enough in establishing
connectivity for a flow but never transmit. Also, this can be
mitigated by timing out and tear down of an unnecessary reservation
by the RSVP proxy when no corresponding media flow is observed.
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The RSVP proxy agent can inform endpoints of an RSVP reservation
failure implicitly by dropping the ICE connectivity check message or
explicitly by sending ICMP messages back to the endpoint. This
allows reasonably effective synchronization between RSVP reservations
handled by the RSVP proxies and the application running on non-RSVP-
capable endpoints. It also has the benefits of operating through
NATs.
For multicast flows (or certain kinds of unicast flows that don't or
can't use ICE), a STUN Indication message [RFC5389] could be used to
carry the (yet to be defined) STUN attributes mentioned earlier to
indicate the flow bandwidth, thereby providing a benefit similar to
the ICE connectivity check. STUN Indication messages are not
acknowledged by the receiver and have the same scalability as the
underlying multicast flow.
The corresponding extensions to ICE and STUN for such a STUN-
Triggered RSVP Proxy approach are beyond the scope of this document.
They may be defined in the future in a separate document. As the
STUN-Triggered RSVP Proxy approach uses STUN in a way (i.e., to
trigger reservations) that is beyond its initial intended purpose,
the potential security implications need to be considered by the
operator.
ICE connectivity checks are not always used for all flows. When the
STUN-Triggered RSVP Proxy approach is used, it can establish RSVP
reservations for flows for which ICE connectivity is performed.
However, the STUN-Triggered RSVP Proxy will not establish a
reservation for flows for which an ICE connectivity check is not
performed. Those flows either will not benefit from an RSVP
reservation or can benefit from an RSVP reservation established
through other means (end-to-end RSVP, other forms of RSVP proxy).
The STUN-Triggered approach relies on interception and inspection of
STUN messages. Thus, this approach may be impeded by encryption or
tunneling.
4.5. Application_Entity-Controlled Proxy
In this approach, it is assumed that an entity involved in the
application-level signaling controls an RSVP proxy that is located in
the data path of the application flows (i.e., "on-path"). With this
approach, the RSVP proxy does not itself attempt to determine the
application reservation requirements. Instead, the RSVP proxy is
instructed by the entity participating in application-level signaling
to establish, maintain, and tear down reservations as needed by the
application flows. In other words, with this approach, the solution
for synchronizing RSVP signaling with application-level requirements
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is to rely on an application-level signaling entity that controls an
RSVP proxy function that sits in the flow data path. This approach
allows control of an RSVP Sender Proxy, an RSVP Receiver Proxy, or
both.
Operation of the Application_Entity-Controlled Proxy is illustrated
in Figure 10.
|---------| |---------|
/////////| App |////\\\\| App |\\\\\\\\
/ | Entity | | Entity | \
/ |---------| |---------| \
/ // \\ \
/ // \\ \
/ // \\ \
/ // \\ \
/ // \\ \
|----| |********| *** |*********| |----|
| S |----------| |------*r*-------| |---------| R |
|----| | RSVP | *** | RSVP | |----|
| Sender | | Receiver|
| Proxy | | Proxy |
|********| |*********|
=======RSVP=======>
********************************************************>
|----| Non-RSVP-capable |----| Non-RSVP-capable ***
| S | Sender | R | Receiver *r* regular RSVP
|----| |----| *** router
***> media flow
==> segment of flow path protected by RSVP reservation
/\ Application signaling (e.g., SIP)
// RSVP proxy control interface
Figure 10: Application_Entity-Controlled Proxy
As an example, the Application_Entity-Controlled Proxy may be used in
the context of SIP servers ([RFC3261]) or Session Border Controllers
(SBCs) (see [RFC5853] for a description of SBCs) to establish RSVP
reservations for multimedia sessions. In that case, the application
entity may be the signaling component of the SBC.
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This RSVP proxy approach does not require any extension to the RSVP
protocol. However, it relies on an RSVP proxy control interface
allowing control of the RSVP proxy by an application signaling
entity. This RSVP proxy control interface is beyond the scope of
this document. Candidate protocols for realizing such an interface
include the IETF Network Configuration (NETCONF) Protocol ([RFC4741],
[RFC5277]), the Web Services protocol ([W3C]), the QoS Policy
Information Model (QPIM) ([RFC3644]), and Diameter ([RFC3588]). This
interface can rely on soft states or hard states. Clearly, when hard
states are used, those need to be converted appropriately by the RSVP
proxy entities into the corresponding RSVP soft states. As an
example, [RFC5866] is intended to allow control of RSVP proxy via
Diameter.
In general, the application entity is not expected to maintain
awareness of which RSVP Receiver Proxy is on the path to which
destination. However, in the particular cases where it does so
reliably, we observe that the application entity could control the
RSVP Sender Proxy and Receiver Proxy so that aggregate RSVP
reservations are used between those, instead of one reservation per
flow. For example, these aggregate reservations could be of the
RSVP-AGGREGATE type, as specified in [RFC3175], or of the GENERIC-
AGGREGATE type, as specified in [RFC4860]. Such aggregate
reservations could be used so that a single reservation can be used
for multiple (possibly all) application flows transiting via the same
RSVP Sender Proxy and the same RSVP Receiver Proxy.
For situations in which only the RSVP Sender Proxy has to be
controlled by this interface, the interface may be realized through
the simple use of RSVP itself, over a Generic Routing Encapsulation
(GRE) tunnel from the application entity to the RSVP Sender Proxy.
This particular case is further discussed in Section 4.5.1. Another
particular case of interest is where the application signaling entity
resides on the same device as the RSVP proxy. In that case, this
interface may be trivially realized as an internal API. An example
environment based on this particular case is illustrated in
Section 4.5.2.
The application entity controlling the RSVP proxy (e.g., a SIP Call
Agent) would often be aware of a number of endpoint capabilities, and
it has to be aware of which endpoint can be best "served" by which
RSVP proxy anyways. So it is reasonable to assume that such an
application is aware of whether a given endpoint is RSVP-capable or
not. The application may also consider the QoS preconditions and QoS
mechanisms signaled by an endpoint as per [RFC3312]/[RFC4032] and
[RFC5432]. The information about endpoint RSVP capability can then
be used by the application to decide whether to trigger proxy
behavior or not for a given endpoint. This can facilitate gradual
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and dynamic migration from the proxy model towards the end-to-end
RSVP model as more and more endpoints become RSVP-capable.
In some environments, the application entities (e.g., SIP back-to-
back user agents) that need to control the RSVP proxies would already
be deployed independently of the use, or not, of the
Application_Entity-Controlled Proxy approach. In this case, the
activation of the RSVP proxy approach should not introduce
significant disruption in the application signaling path. In some
environments, additional application entities may need to be deployed
to control the RSVP proxies. In this case, the network operator
needs to consider the associated risks of disruption to the
application signaling path.
4.5.1. Application_Entity-Controlled Sender Proxy Using "RSVP over GRE"
This approach is simply a particular case of the more general
Application_Entity-Controlled Proxy, but where only RSVP Sender
Proxies need to be controlled by the application, and where RSVP is
effectively used as the control protocol between the application-
signaling entity and the RSVP Sender Proxy.
In this approach, the RSVP messages (e.g., RSVP Path message) are
effectively generated by the application entity and logically
"tunneled" to the RSVP Sender Proxy via GRE tunneling. This is to
ensure that the RSVP messages follow the exact same path as the flow
they protect (as required by RSVP operations) on the segment of the
end-to-end path that is to be subject to RSVP reservations.
Figure 11 illustrates such an environment.
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|-------------|
////////////| Application |\\\\\\\\\
/ | Entity | \
/ |-------------| \
/ /=/ \
/ /=/ \
/ /=/ \
/ /=/ \
/ /=/ \
/ /=/ \
/ /=/ \
/ /=/ \
|----| |********| *** |****|
| S |-----------| RSVP |-----------*r*-----------------| R |
|----| | Sender | *** |****|
| Proxy |
|********|
=========RSVP====================>
*****************************************************>
|----| non-RSVP-capable |----| RSVP-capable ***
| S | Sender | R | Receiver *r* regular RSVP
|----| |----| *** router
***> media flow
==> segment of flow path protected by RSVP reservation
/\ Application-level signaling
/=/ GRE-tunneled RSVP (Path messages)
Figure 11: Application_Entity-Controlled Sender Proxy via
"RSVP over GRE"
With the Application_Entity-Controlled Sender Proxy using "RSVP Over
GRE", the application entity:
o generates a Path message on behalf of the sender, corresponding to
the reservation needed by the application, and maintains the
corresponding Path state. The Path message built by the
application entity is exactly the same as would be built by the
actual sender (if it was RSVP-capable), with one single exception,
which is that the application entity puts its own IP address as
the RSVP previous hop. In particular, it is recommended that the
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source address of the Path message built by the application entity
be set to the IP address of the sender (not of the application
entity). This helps ensure that, in the presence of non-RSVP
routers and of load-balancing in the network where the load-
balancing algorithm takes into account the source IP address, the
Path message generated by the application entity follows the exact
same path as the actual stream sourced by the sender.
o encapsulates the Path message into a GRE tunnel whose destination
address is the RSVP Sender Proxy, i.e., an RSVP router sitting on
the data path for the flow (and upstream of the segment that
requires QoS guarantees via RSVP reservation).
o processes the corresponding received RSVP messages (including Resv
messages) as per regular RSVP.
o synchronizes the RSVP reservation state with application-level
requirements and signaling.
Note that since the application entity encodes its own IP address as
the previous RSVP hop inside the [RFC2205] RSVP_HOP object of the
Path message, the RSVP router terminating the GRE tunnel naturally
addresses all the RSVP messages traveling upstream hop-by-hop (such
as Resv messages) to the application entity (without having to
encapsulate those in a reverse-direction GRE tunnel towards the
application entity).
4.5.2. Application_Entity-Controlled Proxy via Co-Location
This approach is simply a particular case of the more general
Application_Entity-Controlled Proxy, but where the application entity
is co-located with the RSVP proxy. As an example, Session Border
Controllers (SBCs) with on-board SIP agents could implement RSVP
proxy functions and make use of such an approach to achieve session
admission control over the SBC-to-SBC segment using RSVP signaling.
Figure 12 illustrates operations of the Application_Entity-Controlled
RSVP Proxy via co-location.
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|---------| |---------|
////////| App |////////\\\\\\\| App |\\\\\\\\\
/ | Entity | | Entity | \
/ | | | | \
|----| |*********| *** |*********| |----|
| S |--------| RSVP |------*r*------| RSVP |---------| R |
|----| | Sender | *** | Receiver| |----|
| Proxy | | Proxy |
|*********| |*********|
=======RSVP======>
*******************************************************>
|----| Non-RSVP-capable |----| Non-RSVP-capable ***
| S | Sender | R | Receiver *r* regular RSVP
|----| |----| *** router
***> media flow
==> segment of flow path protected by RSVP reservation
/\ Application-level signaling
Figure 12: Application_Entity-Controlled Proxy via Co-Location
This RSVP proxy approach does not require any protocol extensions.
We also observe that when multiple sessions are to be established on
paths sharing the same RSVP Sender Proxy and the same RSVP Receiver
Proxy, the RSVP proxies have the option to establish aggregate RSVP
reservations (as defined in ([RFC3175] or [RFC4860]) for a group of
sessions, instead of establishing one RSVP reservation per session.
4.6. Policy_Server-Controlled Proxy
In this approach, it is assumed that a policy server, which is
located in the control plane of the network, controls an RSVP proxy
that is located in the data path of the application flows (i.e., "on-
path"). In turn, the policy server is triggered by an entity
involved in the application-level signaling. With this approach, the
RSVP proxy does not itself attempt to determine the application
reservation requirements, but instead is instructed by the policy
server to establish, maintain, and tear down reservations as needed
by the application flows. Moreover, the entity participating in
application-level signaling does not attempt to understand the
specific reservation mechanism (i.e., RSVP) or the topology of the
network layer, but instead it simply asks the policy server to
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perform (or tear down) a reservation. In other words, with this
approach, the solution for synchronizing RSVP signaling with
application-level requirements is to rely on an application-level
entity that controls a policy server that, in turn, controls an RSVP
proxy function that sits in the flow data path. This approach allows
control of an RSVP Sender Proxy, an RSVP Receiver Proxy, or both.
Operation of the Policy_Server-Controlled proxy is illustrated in
Figure 13.
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|---------|
/////////////| App |\\\\\\\\\\\\\\
/ | Entity | \
/ |---------| \
/ I \
/ I \
/ |----------| \
/ | Policy | \
/ | Server | \
/ |----------| \
/ // \\ \
/ // \\ \
/ // \\ \
|----| |********| *** |*********| |----|
| S |-----------| |------*r*-----| |----------| R |
|----| | RSVP | *** | RSVP | |----|
| Sender | | Receiver|
| Proxy | | Proxy |
|********| |*********|
=====RSVP========>
**********************************************************>
|----| Non-RSVP-capable |----| Non-RSVP-capable ***
| S | Sender | R | Receiver *r* regular RSVP
|----| |----| *** router
***> media flow
==> segment of flow path protected by RSVP reservation
/\ Application signaling (e.g., SIP)
// RSVP proxy control interface
I Interface between application entity and policy server
Figure 13: Policy_Server-Controlled Proxy
This RSVP proxy approach does not require any extension to the RSVP
protocol. However, as with the Application_Entity-Controlled Proxy
approach presented in Figure 10, this approach relies on an RSVP
proxy control interface allowing control of the RSVP proxy (by the
policy server in this case). This RSVP proxy control interface is
beyond the scope of this document. Considerations about candidate
protocols for realizing such an interface can be found in
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Section 4.5. Again, for situations in which only the RSVP Sender
Proxy has to be controlled by this interface, the interface may be
realized through the simple use of RSVP itself, over a GRE tunnel
from the policy server to the RSVP Sender Proxy. This is similar to
what is presented in Section 4.5.1, except that the "RSVP over GRE"
interface is used in this case by the policy server (instead of the
application entity).
The interface between the application entity and the policy server is
beyond the scope of this document.
4.7. RSVP-Signaling-Triggered Proxy
An RSVP proxy can also be triggered and controlled through extended
RSVP signaling from the remote end that is RSVP-capable (and supports
these RSVP extensions for proxy control). For example, an RSVP-
capable sender could send a new or extended RSVP message explicitly
requesting an RSVP proxy on the path towards the receiver to behave
as an RSVP Receiver Proxy and also to trigger a reverse-direction
reservation, thus also behaving as an RSVP Sender Proxy. The new or
extended RSVP message sent by the sender could also include
attributes (e.g., bandwidth) for the reservations to be signaled by
the RSVP proxy.
The challenges in these explicit signaling schemes include the
following:
o How can the nodes determine when a reservation request ought to be
proxied and when it should not, and accordingly invoke appropriate
signaling procedures?
o How does the node sending the messages explicitly triggering the
proxy know where the proxy is located, e.g., determine an IP
address of the proxy that should reply to the signaling?
o How is all the information needed by a Sender Proxy to generate a
Path message actually communicated to the proxy?
An example of such a mechanism is presented in [QOS-MOBILE]. This
scheme is primarily targeted to local access network reservations
whereby an end host can request resource reservations for both
incoming and outgoing flows only over the access network. This may
be useful in environments where the access network is typically the
bottleneck while the core is comparatively over-provisioned, as may
be the case with a number of radio access technologies. In this
proposal, messages targeted to the proxy are flagged with one bit in
all RSVP messages. Similarly, all RSVP messages sent back by the
proxy are also flagged. The use of such a flag allows
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differentiating between proxied and end-to-end reservations. For
triggering an RSVP Receiver Proxy, the sender of the data sends a
Path message that is marked with the mentioned flag. The Receiver
Proxy is located on the signaling and data path, eventually gets the
Path message, and replies back with a Resv message. A node triggers
an RSVP Sender Proxy with a newly defined Path_Request message, which
instructs the proxy to send Path messages towards the triggering
node. The node then replies back with a Resv. More details can be
found in [QOS-MOBILE].
Such an RSVP-Signaling-Triggered Proxy approach would require RSVP
signaling extensions (that are outside the scope of this document).
However, it could provide more flexibility in the control of the
proxy behavior (e.g., control of reverse reservation parameters) than
would the Path-Triggered approaches defined in Section 4.1 and
Section 4.2.
Through potential corresponding protocol extensions, an RSVP-
Signaling-Triggered Proxy approach could facilitate operation (e.g.,
reduce or avoid the need for associated configuration) in hybrid
environments involving both reservations established end-to-end and
reservations established via RSVP proxies. For example, [QOS-MOBILE]
proposed a mechanism allowing an end-system to control whether a
reservation can be handled by an RSVP proxy on the path, or is to be
established end-to-end.
4.8. Reachability Considerations
There may be situations in which the RSVP Receiver Proxy is reachable
by the sender, while the receiver itself is not. In such situations,
it is possible that the RSVP Receiver Proxy is not always aware that
the receiver is unreachable, and consequently may accept to establish
an RSVP reservation on behalf of that receiver. This would result in
unnecessary reservation establishment and unnecessary network
resource consumption.
This is not considered a significant practical concern for a number
of reasons. First, in many cases, if the receiver is not reachable
from the sender, it will not be reachable for application signaling
either, and so application-level session establishment will not be
possible in the first place. Secondly, where the receiver is
unreachable from the sender but is reachable for application-level
signaling (say, because session establishment is performed through an
off-path SIP agent that uses a different logical topology to
communicate with the receiver), then the sender may detect that the
receiver is unreachable before attempting reservation establishment.
This may be achieved through mechanisms such as ICE's connectivity
check ([RFC5245]). Finally, even if the sender does not detect that
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the receiver is unreachable before triggering the RSVP reservation
establishment, it is very likely that the application will quickly
realize this lack of connectivity (e.g., the human accepting the
phone call on the receiver side will not hear the human's voice on
the sender side) and therefore tear down the session (e.g., hang up
the phone), which in turn will trigger RSVP reservation release.
Nonetheless, it is recommended that network administrators consider
the above in light of their particular environment when deploying
RSVP proxies.
The mirror considerations apply for situations involving an RSVP
Sender Proxy and where the sender cannot reach the destination while
the RSVP Sender Proxy can.
5. Security Considerations
In the environments of concern for this document, RSVP messages are
used to control resource reservations on a segment of the end-to-end
path of flows. The general security considerations associated with
[RFC2205] apply. To ensure the integrity of the associated
reservation and admission control mechanisms, the RSVP cryptographic
authentication mechanisms defined in [RFC2747] and [RFC3097] can be
used. Those protect RSVP messages integrity hop-by-hop and provide
node authentication, thereby protecting against corruption, spoofing
of RSVP messages, and replay. [RSVP-SEC-KEY] discusses key types and
key provisioning methods, as well as their respective applicability
to RSVP authentication.
[RSVP-SEC-KEY] also discusses applicability of IPsec mechanisms
([RFC4302][RFC4303]) and associated key provisioning methods for
security protection of RSVP. This discussion applies to the
protection of RSVP in the presence of RSVP proxies as defined in this
document.
A subset of RSVP messages are signaled with the IP router alert
option ([RFC2113], [RFC2711]). Based on the current security
concerns associated with the use of the IP router alert option, the
applicability of RSVP (and therefore of the RSVP proxy approaches
discussed in this document) is limited to controlled environments
(i.e., environments where the security risks associated with the use
of the IP router alert option are understood and protected against).
The security aspects and common practices around the use of the
current IP router alert option, and consequences of using the IP
router alert option by applications such as RSVP, are discussed in
detail in [RTR-ALERT].
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A number of additional security considerations apply to the use of
RSVP proxies and are discussed below.
With some RSVP proxy approaches, the RSVP proxy operates autonomously
inside an RSVP router. This is the case for the Path-Triggered Proxy
approaches defined in Section 4.1 and in Section 4.2, for the
Inspection-Triggered Proxy approach defined in Section 4.3, for the
STUN-Triggered Proxy approach defined in Section 4.4, and for the
RSVP-Signaling-Triggered approach defined in Section 4.7. Proper
reservation operation assumes that the RSVP proxy can be trusted to
behave correctly in order to control the RSVP reservation as required
and expected by the end-systems. Since the basic RSVP operation
already assumes a trust model where end-systems trust RSVP nodes to
appropriately perform RSVP reservations, the use of an RSVP proxy
that behaves autonomously within an RSVP router is not seen as
introducing any significant additional security threat or as
fundamentally modifying the RSVP trust model.
With some RSVP proxy approaches, the RSVP proxy operates under the
control of another entity. This is the case for the
Application_Entity-Controlled Proxy approach defined in Section 4.5
and for the Policy_Server-Controlled Proxy approach defined in
Section 4.6. This introduces additional security risks since the
entity controlling the RSVP proxy needs to be trusted for proper
reservation operation and also introduces additional authentication
and confidentiality requirements. The exact mechanisms to establish
such trust, authentication, and confidentiality are beyond the scope
of this document, but they may include security mechanisms inside the
protocol used as the control interface between the RSVP proxy and the
entity controlling it, as well as security mechanisms for all the
interfaces involved in the reservation control chain (e.g., inside
the application signaling protocol between the end-systems and the
application entity, and, in the case of the Policy_Server-Controlled
Proxy approach, in the protocol between the application entity and
the policy server).
In some situations, the use of RSVP proxy to control reservations on
behalf of end-systems may actually reduce the security risk (at least
from the network operator viewpoint). This could be the case, for
example, because the routers where the RSVP proxy functionality runs
are less exposed to tampering than end-systems. Such a case is
further discussed in Section 4 of [RFC5946]. This could also be the
case because the use of RSVP proxy allows localization of RSVP
operation within the boundaries of a given administrative domain
(thus easily operating as a controlled environment) while the end-to-
end flow path spans multiple administrative domains.
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6. Acknowledgments
This document benefited from earlier work on the concept of RSVP
proxy including the one documented by Silvano Gai, Dinesh Dutt,
Nitsan Elfassy, and Yoram Bernet. It also benefited from discussions
with Pratik Bose, Chris Christou, and Michael Davenport. Tullio
Loffredo and Massimo Sassi provided the base material for
Section 4.6. Thanks to James Polk, Magnus Westerlund, Dan Romascanu,
Ross Callon, Cullen Jennings, and Jari Arkko for their thorough
review and comments.
7. References
7.1. Normative References
[RFC2205] Braden, B., Zhang, L., Berson, S., Herzog, S., and S.
Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
Functional Specification", RFC 2205, September 1997.
[RFC2210] Wroclawski, J., "The Use of RSVP with IETF Integrated
Services", RFC 2210, September 1997.
[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.,
and W. Weiss, "An Architecture for Differentiated
Services", RFC 2475, December 1998.
[RFC2747] Baker, F., Lindell, B., and M. Talwar, "RSVP Cryptographic
Authentication", RFC 2747, January 2000.
[RFC3097] Braden, R. and L. Zhang, "RSVP Cryptographic
Authentication -- Updated Message Type Value", RFC 3097,
April 2001.
[RFC5245] Rosenberg, J., "Interactive Connectivity Establishment
(ICE): A Protocol for Network Address Translator (NAT)
Traversal for Offer/Answer Protocols", RFC 5245,
April 2010.
[RFC5389] Rosenberg, J., Mahy, R., Matthews, P., and D. Wing,
"Session Traversal Utilities for NAT (STUN)", RFC 5389,
October 2008.
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RFC 5945 RSVP Proxy Approaches October 2010
7.2. Informative References
[QOS-MOBILE] Manner, J., "Provision of Quality of Service in IP-
based Mobile Access Networks", Doctoral
dissertation, University of Helsinki, 2003,
.
[RFC1633] Braden, B., Clark, D., and S. Shenker, "Integrated
Services in the Internet Architecture: an Overview",
RFC 1633, June 1994.
[RFC2113] Katz, D., "IP Router Alert Option", RFC 2113,
February 1997.
[RFC2326] Schulzrinne, H., Rao, A., and R. Lanphier, "Real Time
Streaming Protocol (RTSP)", RFC 2326, April 1998.
[RFC2474] Nichols, K., Blake, S., Baker, F., and D. Black,
"Definition of the Differentiated Services Field (DS
Field) in the IPv4 and IPv6 Headers", RFC 2474,
December 1998.
[RFC2711] Partridge, C. and A. Jackson, "IPv6 Router Alert
Option", RFC 2711, October 1999.
[RFC2872] Bernet, Y. and R. Pabbati, "Application and Sub
Application Identity Policy Element for Use with
RSVP", RFC 2872, June 2000.
[RFC2961] Berger, L., Gan, D., Swallow, G., Pan, P., Tommasi,
F., and S. Molendini, "RSVP Refresh Overhead
Reduction Extensions", RFC 2961, April 2001.
[RFC3175] Baker, F., Iturralde, C., Le Faucheur, F., and B.
Davie, "Aggregation of RSVP for IPv4 and IPv6
Reservations", RFC 3175, September 2001.
[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.
[RFC3312] Camarillo, G., Marshall, W., and J. Rosenberg,
"Integration of Resource Management and Session
Initiation Protocol (SIP)", RFC 3312, October 2002.
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RFC 5945 RSVP Proxy Approaches October 2010
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3588] Calhoun, P., Loughney, J., Guttman, E., Zorn, G., and
J. Arkko, "Diameter Base Protocol", RFC 3588,
September 2003.
[RFC3644] Snir, Y., Ramberg, Y., Strassner, J., Cohen, R., and
B. Moore, "Policy Quality of Service (QoS)
Information Model", RFC 3644, November 2003.
[RFC4032] Camarillo, G. and P. Kyzivat, "Update to the Session
Initiation Protocol (SIP) Preconditions Framework",
RFC 4032, March 2005.
[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
Internet Protocol", RFC 4301, December 2005.
[RFC4302] Kent, S., "IP Authentication Header", RFC 4302,
December 2005.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, December 2005.
[RFC4566] Handley, M., Jacobson, V., and C. Perkins, "SDP:
Session Description Protocol", RFC 4566, July 2006.
[RFC4741] Enns, R., "NETCONF Configuration Protocol", RFC 4741,
December 2006.
[RFC4860] Le Faucheur, F., Davie, B., Bose, P., Christou, C.,
and M. Davenport, "Generic Aggregate Resource
ReSerVation Protocol (RSVP) Reservations", RFC 4860,
May 2007.
[RFC4923] Baker, F. and P. Bose, "Quality of Service (QoS)
Signaling in a Nested Virtual Private Network",
RFC 4923, August 2007.
[RFC5277] Chisholm, S. and H. Trevino, "NETCONF Event
Notifications", RFC 5277, July 2008.
[RFC5432] Polk, J., Dhesikan, S., and G. Camarillo, "Quality of
Service (QoS) Mechanism Selection in the Session
Description Protocol (SDP)", RFC 5432, March 2009.
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[RFC5853] Hautakorpi, J., Camarillo, G., Penfield, R.,
Hawrylyshen, A., and M. Bhatia, "Requirements from
Session Initiation Protocol (SIP) Session Border
Control (SBC) Deployments", RFC 5853, April 2010.
[RFC5866] Sun, D., McCann, P., Tschofenig, H., Tsou, T., Doria,
A., and G. Zorn, "Diameter Quality-of-Service
Application", RFC 5866, May 2010.
[RFC5946] Le Faucheur, F., Manner, J., Narayanan, A., Guillou,
A., and H. Malik, "Resource Reservation Protocol
(RSVP) Extensions for Path-Triggered RSVP Receiver
Proxy", RFC 5946, October 2010.
[RFC5974] Manner, J., Karagiannis, G., and A. McDonald, "NSIS
Signaling Layer Protocol (NSLP) for Quality-of-
Service Signaling", RFC 5974, October 2010.
[RSVP-SEC-KEY] Behringer, M. and F. Le Faucheur, "Applicability of
Keying Methods for RSVP Security", Work in Progress,
June 2009.
[RTR-ALERT] Le Faucheur, F., "IP Router Alert Considerations and
Usage", Work in Progress, October 2009.
[W3C] "World Wide Web Consortium (W3C) - Web Services
Architecture",
RFC, FYI, BCP