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RFC 4495 - A Resource Reservation Protocol (RSVP) Extension for


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Network Working Group                                            J. Polk
Request for Comments: 4495                                   S. Dhesikan
Updates: 2205                                              Cisco Systems
Category: Standards Track                                       May 2006

       A Resource Reservation Protocol (RSVP) Extension for the
              Reduction of Bandwidth of a Reservation Flow

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   This document proposes an extension to the Resource Reservation
   Protocol (RSVPv1) to reduce the guaranteed bandwidth allocated to an
   existing reservation.  This mechanism can be used to affect
   individual reservations, aggregate reservations, or other forms of
   RSVP tunnels.  This specification is an extension of RFC 2205.

Table of Contents

   1. Introduction ....................................................2
      1.1. Conventions Used in This Document ..........................4
   2. Individual Reservation Reduction Scenario .......................4
   3. RSVP Aggregation Overview .......................................6
      3.1. RSVP Aggregation Reduction Scenario ........................8
   4. Requirements for Reservation Reduction ..........................9
   5. RSVP Bandwidth Reduction Solution ..............................10
      5.1. Partial Preemption Error Code .............................11
      5.2. Error Flow Descriptor .....................................11
      5.3. Individual Reservation Flow Reduction .....................11
      5.4. Aggregation Reduction of Individual Flows .................12
      5.5. RSVP Flow Reduction Involving IPsec Tunnels ...............12
      5.6. Reduction of Multiple Flows at Once .......................13
   6. Backwards Compatibility ........................................13
   7. Security Considerations ........................................14
   8. IANA Considerations ............................................15
   9. Acknowledgements ...............................................15
   10. References ....................................................15
      10.1. Normative References .....................................15
      10.2. Informative References ...................................16
   Appendix A. Walking through the Solution ..........................17

1.  Introduction

   This document proposes an extension to the Resource Reservation
   Protocol (RSVP) [1] to allow an existing reservation to be reduced in
   allocated bandwidth in lieu of tearing that reservation down when
   some of that reservation's bandwidth is needed for other purposes.
   Several examples exist in which this mechanism may be utilized.

   The bandwidth allotted to an individual reservation may be reduced
   due to a variety of reasons such as preemption, etc.  In such cases,
   when the entire bandwidth allocated to a reservation is not required,
   the reservation need not be torn down.  The solution described in
   this document allows endpoints to negotiate a new (lower) bandwidth
   that falls at or below the specified new bandwidth maximum allocated
   by the network.  Using a voice session as an example, this indication
   in RSVP could lead endpoints, using another protocol such as Session
   Initiation Protocol (SIP) [9], to signal for a lower-bandwidth codec
   and retain the reservation.

   With RSVP aggregation [2], two aggregate flows with differing
   priority levels may traverse the same router interface.  If that
   router interface reaches bandwidth capacity and is then asked to
   establish a new reservation or increase an existing reservation, the

   router has to make a choice: deny the new request (because all
   resources have been utilized) or preempt an existing lower-priority
   reservation to make room for the new or expanded reservation.

   If the flow being preempted is an aggregate of many individual flows,
   this has greater consequences.  While [2] clearly does not terminate
   all the individual flows if an aggregate is torn down, this event
   will cause packets to be discarded during aggregate reservation
   reestablishment.  This document describes a method where only the
   minimum required bandwidth is taken away from the lower-priority
   aggregated reservation and the entire reservation is not preempted.
   This has the advantage that only some of the microflows making up the
   aggregate are affected.  Without this extension, all individual flows
   are affected and the deaggregator will have to attempt the
   reservation request with a reduced bandwidth.

   RSVP tunnels utilizing IPsec [8] also require an indication that the
   reservation must be reduced to a certain amount (or less).  RSVP
   aggregation with IPsec tunnels is being defined in [11], which should
   be able to take advantage of the mechanism created here in this
   specification.

   Note that when this document refers to a router interface being
   "full" or "at capacity", this does not imply that all of the
   bandwidth has been used, but rather that all of the bandwidth
   available for reservation(s) via RSVP under the applicable policy has
   been used.  Policies for real-time traffic routinely reserve capacity
   for routing and inelastic applications, and may distinguish between
   voice, video, and other real-time applications.

   Explicit Congestion Notification (ECN) [10] is an indication that the
   transmitting endpoint must reduce its transmission.  It does not
   provide sufficient indication to tell the endpoint by how much the
   reduction should be.  Hence the application may have to attempt
   multiple times before it is able to drop its bandwidth utilization
   below the available limit.  Therefore, while we consider ECN to be
   very useful for elastic applications, it is not sufficient for the
   purpose of inelastic application where an indication of bandwidth
   availability is useful for codec selection.

   Section 2 discusses the individual reservation flow problem, while
   Section 3 discusses the aggregate reservation flow problem space.
   Section 4 lists the requirements for this extension.  Section 5
   details the protocol changes necessary in RSVP to create a
   reservation reduction indication.  And finally, the appendix provides
   a walk-through example of how this extension modifies RSVP
   functionality in an aggregate scenario.

   This document updates RFC 2205 [1], as this mechanism affects the
   behaviors of the ResvErr and ResvTear indications defined in that
   document.

1.1.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [4].

2.  Individual Reservation Reduction Scenario

   Figure 1 is a network topology that is used to describe the benefit
   of bandwidth reduction in an individual reservation.

               +------------+            +------------+
               |     |Int 1 |            |Int 7 |     |
   Flow 1===>  |     +----- |            |------+     | Flow 1===>
               | R1  |Int 2 |===========>|Int 8 | R2  |
               |     |      |:::::::::::>|      |     |
   Flow 2:::>  |     +----- |            |------+     | Flow 2:::>
               |     |Int 3 |            |Int 9 |     |
               +------------+            +------------+

                   Figure 1.  Simple Reservation Flows

         Legend/Rules:

         - Flow 1 priority = 300
         - Flow 2 priority = 100
         - Both flows are shown in the same direction (left to
           right).  Corresponding flows in the reverse direction are
           not shown for diagram simplicity

   RSVP is a reservation establishment protocol in one direction only.
   This split-path philosophy is because the routed path from one device
   to the other in one direction might not be the routed path for
   communicating between the same two endpoints in the reverse
   direction.  End-systems must request 2 one-way reservations if that
   is what is needed for a particular application (like voice calls).
   Please refer to [1] for the details on how this functions.  This
   example only describes the reservation scenario in one direction for
   simplicity's sake.

   Figure 1 depicts 2 routers (R1 and R2) initially with only one flow
   (Flow 1).  The flows are forwarded from R1 to R2 via Int 2.  For this
   example, let us say that Flow 1 and Flow 2 each require 80 units of
   bandwidth (such as for the codec G.711 with no silence suppression).

   Let us also say that the RSVP bandwidth limit for Int 2 of R1 is 100
   units.

   As described in [3], a priority indication is established for each
   flow.  In fact, there are two priority indications:

      1) one to establish the reservation, and

      2) one to defend the reservation.

   In this example, Flow 1 and Flow 2 have an 'establishing' and a
   'defending' priority of 300 and 100, respectively.  Flow 2 will have
   a higher establishing priority than Flow 1 has for its defending
   priority.  This means that when Flow 2 is signaled, and if no
   bandwidth is available at the interface, Flow 1 will have to
   relinquish bandwidth in favor of the higher-priority request of Flow
   2.  The priorities assigned to a reservation are always end-to-end,
   and not altered by any routers in transit.

   Without the benefit of this specification, Flow 1 will be preempted.
   This specification makes it possible for the ResvErr message to
   indicate that 20 units are still available for a reservation to
   remain up (the interface's 100 units maximum minus Flow 2's 80
   units).  The reservation initiating node (router or end-system) for
   Flow 1 has the opportunity to renegotiate (via call signaling) for
   acceptable parameters within the existing and available bandwidth for
   the flow (for example, it may decide to change to using a codec such
   as G.729)

   The problems avoided with the partial failure of the flow are:

   - Reduced packet loss, which results as Flow 1 attempts to
     reestablish the reservation for a lower bandwidth.

   - Inefficiency caused by multiple attempts until Flow 1 is able to
     request bandwidth equal to or lower than what is available.  If
     Flow 1 is established with much less than what is available then it
     leads to inefficient use of available bandwidth.

3.  RSVP Aggregation Overview

   The following network overview is to help visualize the concerns that
   this specification addresses in RSVP aggregates.  Figure 2 consists
   of 10 routers (the boxes) and 11 flows (1, 2, 3, 4, 5, 9, A, B, C, D,
   and E).  Initially, there will be 5 flows per aggregate (Flow 9 will
   be introduced to cause the problem we are addressing in this
   document), with 2 aggregates (X and Y); Flows 1 through 5 in
   aggregate X and Flows A through E in aggregate Y.  These 2 aggregates
   will cross one router interface utilizing all available capacity (in
   this example).

   RSVP aggregation (per [2]) is no different from an individual
   reservation with respect to being unidirectional.

           Aggregator of X                             Deaggregator of X
                |                                          |
                V                                          V
             +------+   +------+            +------+   +------+
    Flow 1-->|      |   |      |            |      |   |      |-->Flow 1
    Flow 2-->|      |   |      |            |      |   |      |-->Flow 2
    Flow 3-->|      |==>|      |            |      |==>|      |-->Flow 3
    Flow 4-->|      | ^ |      |            |      | ^ |      |-->Flow 4
    Flow 5-->|      | | |      |            |      | | |      |-->Flow 5
    Flow 9   |  R1  | | |  R2  |            |  R3  | | |  R4  |   Flow 9
             +------+ | +------+            +------+ | +------+
                      |   ||                  ||     |
            Aggregate X-->||    Aggregate X   ||<--Aggregate X
                          ||        |         ||
               +--------------+     |      +--------------+
               |       |Int 7 |     |      |Int 1 |       |
               |       +----- |     V      |------+       |
               |   R10 |Int 8 |===========>|Int 2 | R11   |
               |       |      |:::::::::::>|      |       |
               |       +----- |     ^      |------+       |
               |       |Int 9 |     |      |Int 3 |       |
               +--------------+     |      +--------------+
                          ..        |        ..
           Aggregate Y--->..    Aggregate Y  ..<---Aggregate Y
                     |    ..                 ..     |
            +------+ | +------+            +------+ | +------+
   Flow A-->|      | | |      |            |      | | |      |-->Flow A
   Flow B-->|      | V |      |            |      | V |      |-->Flow B
   Flow C-->|      |::>|      |            |      |::>|      |-->Flow C
   Flow D-->|      |   |      |            |      |   |      |-->Flow D
   Flow E-->|  R5  |   |  R6  |            |  R7  |   |  R8  |-->Flow E
            +------+   +------+            +------+   +------+
               ^                                         ^
               |                                         |
       Aggregator of Y                              Deaggregator of Y

                 Figure 2.  Generic RSVP Aggregate Topology

         Legend/Rules:

         - Aggregate X priority = 100
         - Aggregate Y priority = 200
         - All boxes are routers
         - Both aggregates are shown in the same direction (left to
           right).  Corresponding aggregates in the reverse direction
           are not shown for diagram simplicity.

      The path for aggregate X is:

         R1 => R2 => R10 => R11 => R3 => R4

      where aggregate X starts in R1, and deaggregates in R4.

      Flows 1, 2, 3, 4, 5, and 9 communicate through aggregate A.

      The path for aggregate Y is:

         R5 ::> R6 ::> R10 ::> R11 ::> R7 ::> R8

      where aggregate Y starts in R5, and deaggregates in R8.

      Flows A, B, C, D, and E communicate through aggregate B.

   Both aggregates share one leg or physical link: between R10 and R11,
   thus they share one outbound interface: Int 8 of R10, where
   contention of resources may exist.  That link has an RSVP capacity of
   800 kbps.  RSVP signaling (messages) is outside the 800 kbps in this
   example, as is any session signaling protocol like SIP.

3.1.  RSVP Aggregation Reduction Scenario

   Figure 2 shows an established aggregated reservation (aggregate X)
   between the routers R1 and R4.  This aggregated reservation consists
   of 5 microflows (Flows 1, 2, 3, 4, and 5).  For the sake of this
   discussion, let us assume that each flow represents a voice call and
   requires 80 kb (such as for the codec G.711 with no silence
   suppression).  Aggregate X request is for 400 kbps (80 kbps * 5
   flows).  The priority of the aggregate is derived from the individual
   microflows that it is made up of.  In the simple case, all flows of a
   single priority are bundled as a single aggregate (another priority
   level would be in another aggregate, even if traversing the same path
   through the network).  There may be other ways in which the priority
   of the aggregate is derived, but for this discussion it is sufficient
   to note that each aggregate contains a priority (both hold and
   defending priority).  The means of deriving the priority is out of
   scope for this discussion.

   Aggregate Y, in Figure 2, consists of Flows A, B, C, D, and E and
   requires 400 kbps (80 kbps * 5 flows), and starts at R5 and ends R8.
   This means there are two aggregates occupying all 800 kbps of the
   RSVP capacity.

   When Flow 9 is added into aggregate X, this will occupy 80 kbps more
   than Int 8 on R10 has available (880k offered load vs. 800k capacity)
   [1] and [2] create a behavior in RSVP to deny the entire aggregate Y

   and all its individual flows because aggregate X has a higher
   priority.  This situation is where this document focuses its
   requirements and calls for a solution.  There should be some means to
   signal to all affected routers of aggregate Y that only 80 kbps is
   needed to accommodate another (higher priority) aggregate.  A
   solution that accomplishes this reduction instead of a failure could:

      - reduce significant packet loss of all flows within aggregate Y

   During the re-reservation request period of time no packets will
   traverse the aggregate until it is reestablished.

      - reduces the chances that the reestablishment of the aggregate
        will reserve an inefficient amount of bandwidth, causing the
        likely preemption of more individual flows at the aggregator
        than would be necessary had the aggregator had more information
        (that RSVP does not provide at this time)

   During reestablishment of the aggregation in Figure 2 (without any
   modification to RSVP), R8 would guess at how much bandwidth to ask
   for in the new RESV message.  It could request too much bandwidth,
   and have to wait for the error that not that much bandwidth was
   available; it could request too little bandwidth and have that
   aggregation accepted, but this would mean that more individual flows
   would need to be preempted outside the aggregate than were necessary,
   leading to inefficiencies in the opposite direction.

4.  Requirements for Reservation Reduction

   The following are the requirements to reduce the bandwidth of a
   reservation.  This applies to both individual and aggregate
   reservations:

   Req#1 - MUST have the ability to differentiate one reservation from
           another.  In the case of aggregates, it MUST distinguish one
           aggregate from other flows.

   Req#2 - MUST have the ability to indicate within an RSVP error
           message (generated at the router with the congested
           interface) that a specific reservation (individual or
           aggregate) is to be reduced in bandwidth.

   Req#3 - MUST have the ability to indicate within the same error
           message the new maximum amount of bandwidth that is available
           to be utilized within the existing reservation, but no more.

   Req#4 - MUST NOT produce a case in which retransmitted reduction
           indications further reduce the bandwidth of a reservation.
           Any additional reduction in bandwidth for a specified
           reservation MUST be signaled in a new message.

   RSVP messages are unreliable and can get lost.  This specification
   should not compound any error in the network.  If a reduction message
   were lost, another one needs to be sent.  If the receiver ends up
   receiving two copies to reduce the bandwidth of a reservation by some
   amount, it is likely the router will reduce the bandwidth by twice
   the amount that was actually called for.  This will be in error.

5.  RSVP Bandwidth Reduction Solution

   When a reservation is partially failed, a ResvErr (Reservation Error)
   message is generated just as it is done currently with preemptions.
   The ERROR_SPEC object and the PREEMPTION_PRI object are included as
   well.  Very few additions/changes are needed to the ResvErr message
   to support partial preemptions.  A new error subcode is required and
   is defined in Section 5.1.  The ERROR_SPEC object contained in the
   ResvErr message indicates the flowspec that is reserved.  The
   bandwidth indication in this flowspec SHOULD be less than the
   original reservation request.  This is defined in Section 5.2.

   A comment about RESV messages that do not use reliable transport:
   This document RECOMMENDS that ResvErr messages be made reliable by
   implementing mechanisms in [6].

   The current behavior in RSVP requires a ResvTear message to be
   transmitted upstream when the ResvErr message is transmitted
   downstream (per [1]).  This ResvTear message terminates the
   reservation in all routers upstream of the router where the failure
   occurred.  This document requires that the ResvTear is only generated
   when the reservation is to be completely removed.  In cases where the
   reservation is only to be reduced, routers compliant with this
   specification require that the ResvTear message MUST NOT be sent.

   The appendix has been written to walk through the overall solution to
   the problems presented in Sections 2 and 3.  There is mention of this
   ResvTear transmission behavior in the appendix.

5.1.  Partial Preemption Error Code

   The ResvErr message generated due to preemption includes the
   ERROR_SPEC object as well as the PREEMPTION_PRI object.  The format
   of ERROR_SPEC objects is defined in [1].  The error code listed in
   the ERROR_SPEC object for preemption [5] currently is as follows:

         Errcode = 2 (Policy Control Failure) and
         ErrSubCode = 5 (ERR_PREEMPT)

   The following error code is suggested in the ERROR_SPEC object for
   partial preemption:

      Errcode = 2 (Policy Control Failure) and
      ErrSubCode = 102 (ERR_PARTIAL_PREEMPT)

   There is also an error code in the PREEMPTION-PRI object.  This error
   code takes a value of 1 to indicate that the admitted flow was
   preempted [3].  The same error value of 1 may be used for the partial
   preemption case as well.

5.2.  Error Flow Descriptor

   The error flow descriptor is defined in [1] and [7].  In the case of
   partial failure, the flowspec contained in the error flow descriptor
   indicates the highest average and peak rates that the preempting
   system can accept in the next RESV message.  The deaggregator must
   reduce its reservation to a number less than or equal to that,
   whether by changing codecs, dropping reservations, or some other
   mechanism.

5.3.  Individual Reservation Flow Reduction

   When a router requires part of the bandwidth that has been allocated
   to a reservation be used for another flow, the router engages in the
   partial reduction of bandwidth as described in this document.  The
   router sends a ResvErr downstream to indicate the partial error with
   the error code and subcode as described in section 5.1.  The flowspec
   contained in the ResvErr message will be used to indicate the
   bandwidth that is currently allocated.

   The requesting endpoint that receives the ResvErr can then negotiate
   with the transmitting endpoint to lower the bandwidth requirement (by
   selecting another lower bandwidth codec, for example).  After the
   negotiations, both endpoints will issue the RSVP PATH and RESV
   message with the new, lowered bandwidth.

5.4.  Aggregation Reduction of Individual Flows

   When a partial failure occurs in an aggregation scenario, the
   deaggregator receives the ResvErr message with the reduction
   indication from a router in the path of the aggregate.  It then
   decides whether one or more individual flows from the aggregate are
   to be affected by this ResvErr message.  The following choices are
   possible:

   o  If that (deaggregator) router determines that one or more
      individual flow(s) are to partially failed, then it sends a
      ResvErr message with a reduced bandwidth indication to those
      individual flow(s).  This is as per the descriptions in the
      previous section (5.3).

   o  If that (deaggregator) router determines that one individual flow
      is to be preempted to satisfy the aggregate ResvErr, it determines
      which flow is affected.  That router transmits a new ResvErr
      message downstream per [3].  That same router transmits a ResvTear
      message upstream.  This ResvTear message of an individual flow
      does not tear down the aggregate.  Only the individual flow is
      affected.

   o  If that (deaggregator) router determines that multiple individual
      flows are to be preempted to satisfy the aggregate ResvErr, it
      chooses which flows are affected.  That router transmits a new
      ResvErr message downstream as per [3] to each individual flow.
      The router also transmits ResvTear messages upstream for the same
      individual flows.  These ResvTear messages of an individual flow
      do not tear down the aggregate.  Only the individual flows are
      affected.

   In all cases, the deaggregator lowers the bandwidth requested in the
   Aggregate Resv message to reflect the change.

   Which particular flow or series of flows within an aggregate are
   picked by the deaggregator for bandwidth reduction or preemption is
   outside the scope of this document.

5.5.  RSVP Flow Reduction Involving IPsec Tunnels

   RFC 2207 (per [8]) specifies how RSVP reservations function in IPsec
   data flows.  The nodes initiating the IPsec flow can be an end-system
   like a computer, or it can router between two end-systems, or it can
   be an in-line bulk encryption device immediately adjacent to a router
   interface; [11] directly addresses this later scenario.

   The methods of identification of an IPsec with reservation flow are
   different from non-encrypted flows, but how the reduction mechanism
   specified within this document functions is not.

   An IPsec with reservation flow is, for all intents and purposes,
   considered an individual flow with regard to how to reduce the
   bandwidth of the flow.  Obviously, an IPsec with reservation flow can
   be a series of individual flows or disjointed best-effort packets
   between two systems.  But to this specification, this tunnel is an
   individual RSVP reservation.

   Anywhere within this specification that mentions an individual
   reservation flow, the same rules of bandwidth reduction and
   preemption MUST apply.

5.6.  Reduction of Multiple Flows at Once

   As a cautionary note, bandwidth SHOULD NOT be reduced across multiple
   reservations at the same time, in reaction to the same reduction
   event.  A router not knowing the impact of reservation bandwidth
   reduction on more than one flow may cause more widespread ill effects
   than is necessary.

   This says nothing to a policy where preemption should or should not
   occur across multiple flows.

6.  Backwards Compatibility

   Backwards compatibility with this extension will result in RSVP
   operating as it does without this extension, and no worse.  The two
   routers involved in this extension are the router that had the
   congested interface and the furthest downstream router that
   determines what to do with the reduction indication.

   In the case of the router that experiences congestion or otherwise
   needs to reduce the bandwidth of an existing reservation:

   - If that router supports this extension:

     #1 - it generates the ResvErr message with the error code
          indicating the reduction in bandwidth.

     #2 - it does not generate the ResvTear message.

   - If that router does not support this extension, it generates both
     ResvErr and ResvTear messages according to [1].

   In the case of the router at the extreme downstream of a reservation
   that receives the ResvErr message with the reduction indication:

   - If that router does support this extension:

     #1 - it processes this error message and applies whatever local
          policy it is configured to do to determine how to reduce the
          bandwidth of this designated flow.

   - If the router does not support this extension:

     #1 - it processes the ResvErr message according to [1] and all
          extensions it is able to understand, but not this extension
          from this document.

   Thus, this extension does not cause ill effects within RSVP if one or
   more routers support this extension, and one or more routers do not
   support this extension.

7.  Security Considerations

   This document does not lessen the overall security of RSVP or of
   reservation flows through an aggregate.

   If this specification is implemented poorly - which is never
   intended, but is a consideration - the following issues may arise:

   1) If the ResvTear messages are transmitted initially (at the same
      time as the ResvErr messages indicating a reduction in bandwidth
      is necessary), all upstream routers will tear down the entire
      reservation.  This will free up the total amount of bandwidth of
      this reservation inadvertently.  This may cause the re-
      establishment of an otherwise good reservation to fail.  This has
      the most severe affects on an aggregate that has many individual
      flows that would have remained operational.

   2) Just as RSVP has the vulnerability of premature termination of
      valid reservations by rogue flows without authentication [12, 13],
      this mechanism will have the same vulnerability.  Usage of RSVP
      authentication mechanisms is encouraged.

8.  IANA Considerations

   The IANA has assigned the following from RFC 4495 (i.e., this
   document):

   The following error code has been defined in the ERROR_SPEC object
   for partial reservation failure under "Errcode = 2 (Policy Control
   Failure)":

      ErrSubCode = 102 (ERR_PARTIAL_PREEMPT)

   The behavior of this ErrSubCode is defined in this document.

9.  Acknowledgements

   The authors would like to thank Fred Baker for contributing text and
   guidance in this effort and to Roger Levesque and Francois Le
   Faucheur for helpful comments.

10.  References

10.1.  Normative References

   [1]  Braden, R., Ed., Zhang, L., Berson, S., Herzog, S., and S.
        Jamin, "Resource ReSerVation Protocol (RSVP) -- Version 1
        Functional Specification", RFC 2205, September 1997.

   [2]  Baker, F., Iturralde, C., Le Faucheur, F., and B. Davie,
        "Aggregation of RSVP for IPv4 and IPv6 Reservations", RFC 3175,
        September 2001.

   [3]  Herzog, S., "Signaled Preemption Priority Policy Element", RFC
        3181, October 2001.

   [4]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, RFC 2119, March 1997.

   [5]  Herzog, S., "RSVP Extensions for Policy Control", RFC 2750,
        January 2000.

   [6]  Berger, L., Gan, D., Swallow, G., Pan, P., Tommasi, F., and S.
        Molendini, "RSVP Refresh Overhead Reduction Extensions", RFC
        2961, April 2001.

   [7]  Wroclawski, J., "The Use of RSVP with IETF Integrated Services",
        RFC 2210, September 1997.

   [8]  Berger, L. and T. O'Malley, "RSVP Extensions for IPSEC Data
        Flows", RFC 2207, September 1997.

10.2.  Informative References

   [9]  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.

   [10] Ramakrishnan, K., Floyd, S., and D. Black, "The Addition of
        Explicit Congestion Notification (ECN) to IP", RFC 3168,
        September 2001.

   [11] Le Faucheur, F., Davie, B., Bose, P., Christou, C., and M.
        Davenport, "Generic Aggregate RSVP Reservations", Work in
        Progress, October 2005.

   [12] Baker, F., Lindell, B., and M. Talwar, "RSVP Cryptographic
        Authentication", RFC 2747, January 2000.

   [13] Braden, R. and L. Zhang, "RSVP Cryptographic Authentication --
        Updated Message Type Value", RFC 3097, April 2001.

Appendix A.  Walking through the Solution

   Here is a concise explanation of roughly how RSVP behaves with the
   solution to the problems presented in Sections 2 and 3 of this
   document.  There is no normative text in this appendix.

   Here is a duplicate of Figure 2 from section 3 of the document body
   (to bring it closer to the detailed description of the solution).

        Aggregator of X                              Deaggregator of X
                |                                          |
                V                                          V
             +------+   +------+            +------+   +------+
    Flow 1-->|      |   |      |            |      |   |      |-->Flow 1
    Flow 2-->|      |   |      |            |      |   |      |-->Flow 2
    Flow 3-->|      |==>|      |            |      |==>|      |-->Flow 3
    Flow 4-->|      | ^ |      |            |      | ^ |      |-->Flow 4
    Flow 5-->|      | | |      |            |      | | |      |-->Flow 5
    Flow 9-->|  R1  | | |  R2  |            |  R3  | | |  R4  |-->Flow 9
             +------+ | +------+            +------+ | +------+
                     |    ||                  ||    |
           Aggregate X--->||    Aggregate X   ||<--Aggregate X
                          ||        |         ||
               +--------------+     |      +--------------+
               |       |Int 7 |     |      |Int 1 |       |
               |       +----- |     V      |------+       |
               |  R10  |Int 8 |===========>|Int 2 |  R11  |
               |       |      |:::::::::::>|      |       |
               |       +----- |     ^      |------+       |
               |       |Int 9 |     |      |Int 3 |       |
               +--------------+     |      +--------------+
                          ..        |        ..
           Aggregate Y--->..    Aggregate Y  ..<---Aggregate Y
                     |    ..                 ..     |
            +------+ | +------+            +------+ | +------+
   Flow A-->|      | | |      |            |      | | |      |-->Flow A
   Flow B-->|      | V |      |            |      | V |      |-->Flow B
   Flow C-->|      |::>|      |            |      |::>|      |-->Flow C
   Flow D-->|      |   |      |            |      |   |      |-->Flow D
   Flow E-->|  R5  |   |  R6  |            |  R7  |   |  R8  |-->Flow E
            +------+   +------+            +------+   +------+
               ^                                         ^
               |                                         |
       Aggregator of Y                              Deaggregator of Y

           Duplicate of Figure 2.  Generic RSVP Aggregate Topology

   Looking at Figure 2, aggregate X (with five 80 kbps flows) traverses:

         R1 ==> R2 ==> R10 ==> R11 ==> R3 ==> R4

   And aggregate Y (with five 80 kbps flows) traverses:

         R5 ::> R6 ::> R10 ::> R11 ::> R7 ::> R8

   Both aggregates are 400 kbps.  This totals 800 kbps at Int 7 in R10,
   which is the maximum bandwidth that RSVP has access to at this
   interface.  Signaling messages still traverse the interface without
   problem.  Aggregate X is at a higher relative priority than aggregate
   Y.  Local policy in this example is for higher relative priority
   flows to preempt lower-priority flows during times of congestion.
   The following points describe the flow when aggregate A is increased
   to include Flow 9.

   o  When Flow 9 (at 80 kbps) is added to aggregate X, R1 will initiate
      the PATH message towards the destination endpoint of the flow.
      This hop-by-hop message will take it through R2, R10, R11, R3, and
      R4, which is the aggregate X path (that was built per [2] from the
      aggregate's initial setup) to the endpoint node.

   o  In response, R4 will generate the RESV (reservation) message
      (defined behavior per [1]).  This RESV from the deaggregator
      indicates an increase bandwidth sufficient to accommodate the
      existing 5 flows (1, 2, 3, 4, and 5) and the new flow (9), as
      stated in [2].

   o  As mentioned before, in this example, Int 8 in R10 can only
      accommodate 800 kbps, and aggregates X and Y have each already
      established 400 kbps flows comprised of five 80 kbps individual
      flows.  Therefore, R10 (the interface that detects a congestion
      event in this example) must make a decision about this new
      congestion generating condition in regard to the RESV message
      received at Int 8.

   o  Local policy in this scenario is to preempt lower-priority
      reservations to place higher-priority reservations.  This would
      normally cause all of aggregate Y to be preempted just to
      accommodate aggregate X's request for an additional 80 kbps.

   o  This document defines how aggregate Y is not completely preempted,
      but reduced in bandwidth by 80 kbps.  This is contained in the
      ResvErr message that R10 generates (downstream) towards R11, R7,
      and R8.  See section 5 for the details of the error message.

   o  Normal operation of RSVP is to have the router that generates a
      ResvErr message downstream to also generate a ResvTear message
      upstream (in the opposite direction, i.e., towards R5).  The
      ResvTear message terminates an individual flow or aggregate flow.
      This document calls for that message not to be sent on any partial
      failure of reservation.

   o  R8 is the deaggregator of aggregate Y.  The deaggregator controls
      all the parameters of an aggregate reservation.  This will be the
      node that reduces the necessary bandwidth of the aggregate as a
      response to the reception of an ResvErr message (from R10)
      indicating such an action is called for.  In this example,
      bandwidth reduction is accomplished by preempting an individual
      flow within the aggregate (perhaps picking on Flow D for
      individual preemption by generating a ResvErr downstream on that
      individual flow).

   o  At the same time, a ResvTear message is transmitted upstream on
      that individual flow (Flow D) by R8.  This will not affect the
      aggregate directly, but is an indication to the routers (and the
      source end-system) which individual flow is to be preempted.

   o  Once R8 preempts whichever individual flow (or 'bandwidth' at the
      aggregate ingress), it transmits a new RESV message for that
      aggregate (Y), not for a new aggregate.  This RESV from the
      deaggregator indicates a decrease in bandwidth sufficient to
      accommodate the remaining 4 flows (A, B, C, and E), which is now
      320 kbps (in this example).

   o  This RESV message travels the entire path of the reservation,
      resetting all routers to this new aggregate bandwidth value.  This
      should be what is necessary to prevent a ResvTear message from
      being generated by R10 towards R6 and R5.

   R5 will not know through this RESV message which individual flow was
   preempted.  If in this example, R8 was given more bandwidth to keep,
   it might have transmitted a bandwidth reduction ResvErr indication
   towards the end-system of Flow D.  In that case, a voice signaling
   protocol (such as SIP) could have attempted a renegotiation of that
   individual flow to a reduced bandwidth (say, but changing the voice
   codec from G.711 to G. 729).  This could have saved Flow D from
   preemption.

Authors' Addresses

   James M. Polk
   Cisco Systems
   2200 East President George Bush Turnpike
   Richardson, Texas 75082 USA

   EMail: jmpolk@cisco.com

   Subha Dhesikan
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA 95134 USA

   EMail: sdhesika@cisco.com

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