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INFORMATIONAL

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.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   publication of this document.  Please review these documents
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   described in the Simplified BSD License.

   This document may contain material from IETF Documents or IETF
   Contributions published or made publicly available before November
   10, 2008.  The person(s) controlling the copyright in some of this
   material may not have granted the IETF Trust the right to allow
   modifications of such material outside the IETF Standards Process.
   Without obtaining an adequate license from the person(s) controlling
   the copyright in such materials, this document may not be modified
   outside the IETF Standards Process, and derivative works of it may
   not be created outside the IETF Standards Process, except to format
   it for publication as an RFC or to translate it into languages other
   than English.









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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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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,
                   <http://ethesis.helsinki.fi>.

   [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", <http://www.w3.org/TR/ws-arch/>.
























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Appendix A.  Use Cases for RSVP Proxies

A.1.  RSVP-Based VoD Admission Control in Broadband Aggregation Networks

   As broadband services for residential customers are becoming more and
   more prevalent, next-generation aggregation networks are being
   deployed in order to aggregate traffic from broadband users (whether
   attached via Digital Subscriber Line technology, aka DSL; Fiber To
   The Home/Curb, aka FTTx; Cable; or other broadband access
   technology).  Video on Demand (VoD) services, which may be offered to
   broadband users, present significant capacity planning challenges for
   the aggregation network for a number of reasons.  First, each VoD
   stream requires significant dedicated sustained bandwidth (typically
   2-4 Mb/s in Standard Definition TV and 6-12 Mb/s in High Definition
   TV).  Secondly, the VoD codec algorithms are very sensitive to packet
   loss.  Finally, the load resulting from such services is very hard to
   predict (e.g., it can vary quite suddenly with blockbuster titles
   made available as well as with promotional offerings).  As a result,
   transport of VoD streams on the aggregation network usually translate
   into a strong requirement for admission control.  The admission
   control solution protects the quality of established VoD sessions by
   rejecting the additional excessive session attempts during
   unpredictable peaks, during link or node failures, or a combination
   of those factors.

   RSVP can be used in the aggregation network for admission control of
   the VoD sessions.  However, since customer premises equipment such as
   Set Top Boxes (STBs) (which behave as the receiver for VoD streams)
   often do not support RSVP, the last IP hop in the aggregation network
   can behave as an RSVP Receiver Proxy.  This way, RSVP can be used
   between VoD pumps and the last IP hop in the aggregation network to
   perform accurate admission control of VoD streams over the resources
   set aside for VoD in the aggregation network (typically a certain
   percentage of the bandwidth of any link).  As VoD streams are
   unidirectional, a simple Path-Triggered RSVP Receiver Proxy (as
   described in Section 4.1) is all that is required in this use case.

   Figure 14 illustrates operation of RSVP-based admission control of
   VoD sessions in an aggregation network involving RSVP support on the
   VoD pump (the senders) and the RSVP Receiver proxy on the last IP hop
   of the aggregation network.  All the customer premises equipment
   remains RSVP-unaware.









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                         |-------------|
                         | VoD  SRM    |
                         |             |
                 ////////|             |\\\\\\\\\\\\\\
                /        |-------------|              \
               /                                       \
              /                                         \
             /                                           \
            /                                             \
           /                                               \
      |****|   ***   ***   ***   |********|   |-----|    |---|
      |VoD |---*r*---*r*---*r*---|RSVP    |---|DSLAM|~~~~|STB|--TV
      |Pump|   ***   ***   ***   |Receiver|   |-----|    |---|
      |****|                     |Proxy   |
                                 |********|

               <---Aggregation Net----------->

           ************************************************>

           ==============RSVP================>


   SRM Session Resource Manager

   ***                       |---|
   *r* regular RSVP          |STB| Set Top Box
   *** router                |---|

   ***> VoD media flow

   ==>  segment of flow path protected by RSVP reservation

   /\   VoD Application-level signaling (e.g., RTSP)

                Figure 14: VoD Use Case with Receiver Proxy

   In the case where the VoD pumps are not RSVP-capable, an
   Application_Entity-Controlled Sender Proxy via the "RSVP over GRE"
   approach (as described in Section 4.5.1) can also be implemented on
   the VoD Controller or Session Resource Manager (SRM) devices
   typically involved in VoD deployments.  Figure 15 illustrates
   operation of RSVP-based admission control of VoD sessions in an
   aggregation network involving such an Application_Entity-Controlled
   Source Proxy combined with an RSVP Receiver Proxy on the last IP hop
   of the aggregation network.  All the customer premises equipment, as
   well as the VoD pumps, remain RSVP-unaware.




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                         |-------------|
                     ////| VoD  SRM    |\\\\\\\\\\\
                    /    |             |           \
                   /     |     +       |            \
                  /      | RSVP Sender |             \
                 /       |Proxy Control|              \
                /        |-------------|               \
               /        /=/                             \
              /        /=/                               \
             /        /=/                                 \
            /        /=/                                   \
           /        /=/                                     \
      |----|  |******|    ***  ***  |********|  |-----|    |---|
      | VoD|--|RSVP  |----*r*--*r*--|RSVP    |--|DSLAM|~~~~|STB|--TV
      |Pump|  |Sender|    ***  ***  |Receiver|  |-----|    |---|
      |----|  |Proxy |              |Proxy   |
              |******|              |********|

               <---Aggregation Net------------->

           ************************************************>

                    =========RSVP==============>


   SRM Systems Resource Manager

   ***                       |---|
   *r* regular RSVP          |STB| Set Top Box
   *** router                |---|

   ***> VoD media flow

   ==>  segment of flow path protected by RSVP reservation

   /    VoD Application-level signaling (e.g., RTSP)

   /=/  GRE-tunneled RSVP (Path messages)

                Figure 15: VoD Use Case with Receiver Proxy
                        and SRM-Based Sender Proxy

   The RSVP proxy entities specified in this document play a significant
   role here since they allow immediate deployment of an RSVP-based
   admission control solution for VoD without requiring any upgrade to
   the huge installed base of non-RSVP-capable customer premises
   equipment.  In one mode described above, they also avoid upgrade of
   non-RSVP-capable VoD pumps.  In turn, this means that the benefits of



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   on-path admission control can be offered to VoD services over
   broadband aggregation networks without network or VoD pump upgrade.
   Those include accurate bandwidth accounting regardless of topology
   (hub-and-spoke, ring, mesh, star, arbitrary combinations) and dynamic
   adjustment to any change in topology (such as failure, routing
   change, additional links, etc.).

A.2.  RSVP-Based Voice/Video Connection Admission Control (CAC) in
      Enterprise WAN

   More and more enterprises are migrating their telephony and
   videoconferencing applications onto IP.  When doing so, there is a
   need for retaining admission control capabilities of existing TDM-
   based (Time-Division Multiplexing) systems to ensure the QoS of these
   applications is maintained even when transiting through the
   enterprise's Wide Area Network (WAN).  Since many of the endpoints
   already deployed (such as IP phones or videoconferencing terminals)
   are not RSVP-capable, RSVP proxy approaches are very useful: they
   allow deployment of an RSVP-based admission control solution over the
   WAN without requiring upgrade of the existing terminals.

   A common deployment architecture for such environments relies on the
   Application_Entity-Controlled Proxy approach as defined in
   Section 4.5.  Routers sitting at the edges of the WAN are naturally
   "on-path" for all inter-campus calls (or sessions) and behave as RSVP
   proxies.  The RSVP proxies establish, maintain, and tear down RSVP
   reservations over the WAN segment for the calls (or sessions) under
   the control of the SIP server/proxy.  The SIP server/proxy
   synchronizes the RSVP reservation status with the status of end-to-
   end calls.  For example, the called IP phone will only be instructed
   to play a ring tone if the RSVP reservation over the corresponding
   WAN segment has been successfully established.

   This architecture allowing RSVP-based admission control of voice and
   video on the enterprise WAN is illustrated in Figure 16.
















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                                 |---------|
                   //////////////| SIP     |\\\\\\\\\\\\
                  /              | Server/ |            \
                 /               | Proxy   |             \
                /                |---------|              \
               /                //       \\                \
              /                //         \\                \
             /                //           \\                \
            /                //             \\                \
           /                //               \\                \
      |-----|      |********|   ***   ***   |********|       |-----|
      | IP  |------| Media  |---*r*---*r*---| Media  |-------|IP   |
      |Phone|      | Relay  |   ***   ***   | Relay  |       |Phone|
      |-----|      |  +     |               |    +   |       |-----|
                   | RSVP   |               | RSVP   |
                   | Proxy  |               | Proxy  |
                   |********|               |********|

        <--campus-->                                <--campus-->
           network                                     network

                        <---------WAN----------->

        <*************> <***********************> <**************>

                       <=========RSVP===========>


   ***
   *r*   Regular RSVP router
   ***

   <***> media flow

   <==>  segment of flow path protected by RSVP reservation

   /\    SIP signaling

   //   control interface between the SIP server/proxy and
        RSVP proxy

                 Figure 16: CAC on Enterprise WAN Use Case

A.3.  RSVP Proxies for Mobile Access Networks

   Mobile access networks are increasingly based on IP technology.  This
   implies that, on the network layer, all traffic, both traditional
   data and streamed data like audio or video, is transmitted as



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   packets.  Increasingly popular multimedia applications would benefit
   from better than best-effort service from the network, a forwarding
   service with strict Quality of Service (QoS) with guaranteed minimum
   bandwidth and bounded delay.  Other applications, such as electronic
   commerce, network control and management, and remote-login
   applications, would also benefit from a differentiated treatment.

   The IETF has two main models for providing differentiated treatment
   of packets in routers.  The Integrated Services (IntServ) model
   [RFC1633], together with the Resource Reservation Protocol (RSVP)
   [RFC2205], [RFC2210], [RFC2961] provides per-flow guaranteed end-to-
   end transmission service.  The Differentiated Services (Diffserv)
   framework [RFC2475] provides non-signaled flow differentiation that
   usually provides, but does not guarantee, proper transmission
   service.

   However, these architectures have potential weaknesses for deployment
   in Mobile Access Networks.  For example, RSVP requires support from
   both communication endpoints, and the protocol may have potential
   performance issues in mobile environments.  Diffserv can only provide
   statistical guarantees and is not well suited for dynamic
   environments.

   Let us consider a scenario, where a fixed network correspondent node
   (CN) would be sending a multimedia stream to an end host behind a
   wireless link.  If the correspondent node does not support RSVP, it
   cannot signal its traffic characteristics to the network and request
   specific forwarding services.  Likewise, if the correspondent node is
   not able to mark its traffic with a proper Differentiated Services
   codepoint (DSCP) to trigger service differentiation, the multimedia
   stream will get only best-effort service, which may result in poor
   visual and audio quality in the receiving application.  Even if the
   connecting wired network is over-provisioned, an end host would still
   benefit from local resource reservations, especially in wireless
   access networks, where the bottleneck resource is most probably the
   wireless link.

   RSVP proxies would be a very beneficial solution to this problem.  It
   would allow distinguishing local network reservations from the end-
   to-end reservations.  The end host does not need to know the access
   network topology or the nodes that will reserve the local resources.
   The access network would do resource reservations for both incoming
   and outgoing flows based on certain criteria, e.g., filters based on
   application protocols.  Another option is that the mobile end host
   makes an explicit reservation that identifies the intention, and the
   access network will find the correct local access network node(s) to
   respond to the reservation.  RSVP proxies would, thus, allow resource
   reservation over the segment that is the most likely bottleneck, the



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   wireless link.  If the wireless access network uses a local mobility
   management mechanism, where the IP address of the mobile node does
   not change during handover, RSVP reservations would follow the mobile
   node movement.

A.4.  RSVP Proxies for Reservations in the Presence of IPsec Gateways

   [RFC4923] discusses how resource reservation can be supported end-to-
   end in a nested VPN environment.  At each VPN level, VPN routers
   behave as [RFC4301] security gateways between a plaintext domain and
   a ciphertext domain.  To achieve end-to-end resource reservation, the
   VPN routers process RSVP signaling on the plaintext side, perform
   aggregation of plaintext reservations, and maintain the corresponding
   aggregate RSVP reservations on the ciphertext side.  Each aggregate
   reservation is established on behalf of multiple encrypted end-to-end
   sessions sharing the same ingress and egress VPN routers.  These
   aggregate reservations can be as specified in [RFC3175] or [RFC4860].

   Section 3 of [RFC4923] discusses the necessary data flows within a
   VPN router to achieve the behavior described in the previous
   paragraph.  Two mechanisms are described to achieve such data flows.
   Section 3.1 presents the case where the VPN router carries data
   across the cryptographic boundary.  Section 3.2 discusses the case
   where the VPN router uses a Network Guard.

   Where such mechanisms are not supported by the VPN routers, the
   approach for end-to-end reservations presented in [RFC4923] cannot be
   deployed.  An alternative approach to support resource reservations
   within the ciphertext core is to use the Application_Entity-
   Controlled Proxy approach (as defined in Section 4.5) in the
   following way:

   o  the RSVP proxies are located inside the ciphertext domain and use
      aggregate RSVP reservations.

   o  the application entity exchange application-level signaling with
      the end-systems in the plaintext domain.

   o  the application entity controls the RSVP proxies in the ciphertext
      domain via an RSVP proxy control interface.

   This is illustrated in Figure 17 in the case where the application is
   SIP-based multimedia communications.








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         |-------|                                    |-------|
         |SIP    |///////////////////\\\\\\\\\\\\\\\\\|SIP    |
        /|Server/|                                    |Server/|\
       / |Proxy  |                                    |Proxy  | \
      /  |-------|                                    |-------|  \
     /      ^    \\                                  //   ^       \
    /       ^     \\                                //    ^        \
   /        ^      \\                              //     ^         \
 |***|   |------|  |********|   ***   ***   |********|  |------|   |***|
 | S |---|IPsec |--|  ARSVP |---*r*---*r*---| ARSVP  |--|IPsec |---| R |
 |***|   | GW   |  | Sender |   ***   ***   |Receiver|  | GW   |   |***|
         |------|  |  Proxy |               | Proxy  |  |------|
                   |********|               |********|

     ***PT*****> **********************CT****************> ****PT***>

     =====>                                                   =====>
                            =====ARSVP======>


 |****| RSVP-capable      |****| RSVP-capable         ***
 | S  | Sender            | R  | Receiver             *r* regular RSVP
 |****|                   |****|                      *** router

 |------|
 |IPsec | IPsec security gateway
 | GW   |
 |------|

 ARSVP Aggregate RSVP

 ***>  media flow

 ==>   segment of flow path protected by RSVP reservation

 / \   SIP signaling

  ^    Network management interface between SIP server/proxy
       and IPsec security gateway

 //    control interface between SIP server/proxy and ARSVP proxy

 PT    Plaintext network

 CT    Ciphertext network

        Figure 17: RSVP Proxies for Reservations in the Presence of
                              IPsec Gateways



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   Where the sender and receiver are RSVP-capable, they may also use
   RSVP signaling.  This achieves resource reservation on the plaintext
   segments of the end-to-end, i.e.,

   o  from the sender to the ingress IPsec gateway, and

   o  from the egress IPsec gateway to the receiver.

   In this use case, because the VPN routers do not support any RSVP-
   specific mechanism, the end-to-end RSVP signaling is effectively
   hidden by the IPsec gateways on the ciphertext segment of the end-to-
   end path.

   As with the Application_Entity-Controlled Proxy approach (defined in
   Section 4.5), the solution here for synchronizing RSVP signaling with
   application-level signaling is to rely on an application-level
   signaling device that controls an on-path RSVP proxy function.
   However, in this use case, the RSVP proxies are a component of a
   ciphertext network where all user (bearer) traffic is IPsec
   encrypted.  This has a number of implications, including the
   following:

   1.  encrypted flows cannot be identified in the ciphertext domain so
       that network nodes can only classify traffic based on IP address
       and Differentiated Services codepoints (DSCPs).  As a result,
       only aggregate RSVP reservations (such as those specified in
       [RFC3175] or [RFC4860]) can be used.  This is similar to
       [RFC4923].

   2.  Determining the RSVP Sender Proxy and RSVP Receiver Proxy to be
       used for aggregation of a given flow from sender to receiver
       creates a number of challenges.  Details on how this may be
       achieved are beyond the scope of this document.  We observe that,
       as illustrated in Figure 17, this may be facilitated by a network
       management interface between the application entity and the IPsec
       gateways.  For example, this interface may be used by the
       application entity to obtain information about which IPsec
       gateway is on the path of a given end-to-end flow.  Then, the
       application entity may maintain awareness of which RSVP proxy is
       on the ciphertext path between a given pair of IPsec gateways.
       How such awareness is achieved is beyond the scope of this
       document.  We simply observe that such awareness can be easily
       achieved through simple configuration in the particular case
       where a single (physical or logical) RSVP proxy is fronting a
       given IPsec gateway.  We also observe that when awareness of the
       RSVP Receiver Proxy for a particular egress IPsec gateway (or





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       end-to-end flow) is not available, the aggregate reservation may
       be signaled by the RSVP Sender Proxy to the destination address
       of the egress IPsec gateway and then proxied by the RSVP Receiver
       Proxy.

   Different flavors of operations are possible in terms of aggregate
   reservation sizing.  For example, the application entity can initiate
   an aggregate reservation of fixed size a priori and then simply keep
   count of the bandwidth used by sessions and reject sessions that
   would result in excess usage of an aggregate reservation.  The
   application entity could also re-size the aggregate reservations on a
   session-by-session basis.  Alternatively, the application entity
   could re-size the aggregate reservations in step increments typically
   corresponding to the bandwidth requirement of multiple sessions.





































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Authors' Addresses

   Francois Le Faucheur
   Cisco Systems
   Greenside, 400 Avenue de Roumanille
   Sophia Antipolis  06410
   France

   Phone: +33 4 97 23 26 19
   EMail: flefauch@cisco.com


   Jukka Manner
   Aalto University
   Department of Communications and Networking (Comnet)
   P.O. Box 13000
   FIN-00076 Aalto
   Finland

   Phone: +358 9 470 22481
   EMail: jukka.manner@tkk.fi
   URI:   http://www.netlab.tkk.fi/~jmanner/


   Dan Wing
   Cisco Systems
   170 West Tasman Drive
   San Jose, CA  95134
   United States

   EMail: dwing@cisco.com


   Allan Guillou
   SFR
   40-42 Quai du Point du Jour
   Boulogne-Billancourt  92659
   France

   EMail: allan.guillou@sfr.com











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