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rfc:rfc2961

Network Working Group L. Berger Request for Comments: 2961 LabN Consulting, LLC Category: Standards Track D. Gan

                                               Juniper Networks, Inc.
                                                           G. Swallow
                                                  Cisco Systems, Inc.
                                                               P. Pan
                                               Juniper Networks, Inc.
                                                           F. Tommasi
                                                         S. Molendini
                                                  University of Lecce
                                                           April 2001
             RSVP Refresh Overhead Reduction Extensions

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 (2001).  All Rights Reserved.

Abstract

 This document describes a number of mechanisms that can be used to
 reduce processing overhead requirements of refresh messages,
 eliminate the state synchronization latency incurred when an RSVP
 (Resource ReserVation Protocol) message is lost and, when desired,
 refreshing state without the transmission of whole refresh messages.
 The same extensions also support reliable RSVP message delivery on a
 per hop basis.  These extension present no backwards compatibility
 issues.

Berger, et al. Standards Track [Page 1] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

Table of Contents

 1      Introduction and Background ................................2
 1.1    Trigger and Refresh Messages ...............................4
 2      Refresh-Reduction-Capable Bit ..............................4
 3      RSVP Bundle Message ........................................5
 3.1    Bundle Header ..............................................5
 3.2    Message Formats ............................................6
 3.3    Sending RSVP Bundle Messages ...............................7
 3.4    Receiving RSVP Bundle Messages .............................8
 4      MESSAGE_ID Extension .......................................8
 4.1    Modification of Standard Message Formats ...................9
 4.2    MESSAGE_ID Objects ........................................10
 4.3    MESSAGE_ID_ACK and MESSAGE_ID_NACK Objects ................11
 4.4    Ack Message Format ........................................11
 4.5    MESSAGE_ID Object Usage ...................................12
 4.6    MESSAGE_ID_ACK Object and MESSAGE_ID_NACK Object Usage ....14
 4.7    Multicast Considerations ..................................15
 4.7.1  Reference RSVP/Routing Interface ..........................16
 4.8    Compatibility .............................................16
 5      Summary Refresh Extension .................................17
 5.1    MESSAGE_ID LIST, SRC_LIST and MCAST_LIST Objects ..........18
 5.2    Srefresh Message Format ...................................24
 5.3    Srefresh Message Usage ....................................25
 5.4    Srefresh NACK .............................................28
 5.5    Preserving RSVP Soft State ................................28
 5.6    Compatibility .............................................29
 6      Exponential Back-Off Procedures ...........................29
 6.1    Outline of Operation ......................................30
 6.2    Time Parameters ...........................................30
 6.3    Retransmission Algorithm ..................................31
 6.4    Performance Considerations ................................31
 7      Acknowledgments ...........................................31
 8      Security Considerations ...................................32
 9      References ................................................32
 10     Authors' Addresses ........................................33
 11     Full Copyright Statement...................................34

1. Introduction and Background

 Standard RSVP [RFC2205] maintains state via the generation of RSVP
 refresh messages.  Refresh messages are used to both synchronize
 state between RSVP neighbors and to recover from lost RSVP messages.
 The use of Refresh messages to cover many possible failures has
 resulted in a number of operational problems.  One problem relates to
 scaling, another relates to the reliability and latency of RSVP
 Signaling.

Berger, et al. Standards Track [Page 2] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

 The scaling problems are linked to the resource requirements (in
 terms of processing and memory) of running RSVP.  The resource
 requirements increase proportionally with the number of sessions.
 Each session requires the generation, transmission, reception and
 processing of RSVP Path and Resv messages per refresh period.
 Supporting a large number of sessions, and the corresponding volume
 of refresh messages, presents a scaling problem.
 The reliability and latency problem occurs when a non-refresh RSVP
 message is lost in transmission.  Standard RSVP [RFC2205] recovers
 from a lost message via RSVP refresh messages.  In the face of
 transmission loss of RSVP messages, the end-to-end latency of RSVP
 signaling is tied to the refresh interval of the node(s) experiencing
 the loss.  When end-to-end signaling is limited by the refresh
 interval, the delay incurred in the establishment or the change of a
 reservation may be beyond the range of what is acceptable for some
 applications.
 One way to address the refresh volume problem is to increase the
 refresh period, "R" as defined in Section 3.7 of [RFC2205].
 Increasing the value of R provides linear improvement on transmission
 overhead, but at the cost of increasing the time it takes to
 synchronize state.
 One way to address the reliability and latency of RSVP Signaling is
 to decrease the refresh period R.  Decreasing the value of R
 increases the probability that state will be installed in the face of
 message loss, but at the cost of increasing refresh message rate and
 associated processing requirements.
 An additional issue is the time to deallocate resources after a tear
 message is lost.  RSVP does not retransmit ResvTear or PathTear
 messages.  If the sole tear message transmitted is lost, then
 resources will only be deallocated once the "cleanup timer" interval
 has passed.  This may result in resources being allocated for an
 unnecessary period of time.  Note that even when the refresh period
 is adjusted, the "cleanup timer" must still expire since tear
 messages are not retransmitted.
 The extensions defined in this document address both the refresh
 volume and the reliability issues with mechanisms other than
 adjusting refresh rate.  The extensions are collectively referred to
 as the "Refresh Overhead Reduction" or the "Refresh Reduction"
 extensions.  A Bundle message is defined to reduce overall message
 handling load.  A MESSAGE_ID object is defined to reduce refresh
 message processing by allowing the receiver to more readily identify
 an unchanged message.  A MESSAGE_ACK object is defined which can be
 used to detect message loss and support reliable RSVP message

Berger, et al. Standards Track [Page 3] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

 delivery on a per hop basis.  A summary refresh message is defined to
 enable refreshing state without the transmission of whole refresh
 messages, while maintaining RSVP's ability to indicate when state is
 lost and to adjust to changes in routing.
 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 [RFC2119].

1.1. Trigger and Refresh Messages

 This document categorizes RSVP messages into two types: trigger and
 refresh messages.  Trigger messages are those RSVP messages that
 advertise state or any other information not previously transmitted.
 Trigger messages include messages advertising new state, a route
 change that alters a reservation path, or a modification to an
 existing RSVP session or reservation.  Trigger messages also include
 those messages that include changes in non-RSVP processed objects,
 such as changes in the Policy or ADSPEC objects.
 Refresh messages represent previously advertised state and contain
 exactly the same objects and same information as a previously
 transmitted message, and are sent over the same path.  Only Path and
 Resv messages can be refresh messages.  Refresh messages are
 identical to the corresponding previously transmitted message, with
 some possible exceptions.  Specifically, the checksum field, the
 flags field and the INTEGRITY object may differ in refresh messages.

2. Refresh-Reduction-Capable Bit

 To indicate support for the refresh overhead reduction extensions, an
 additional capability bit is added to the common RSVP header, which
 is defined in [RFC2205].
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |  Vers | Flags |   Msg Type    |         RSVP Checksum         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Send_TTL    |  (Reserved)   |         RSVP Length           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Flags: 4 bits
       0x01: Refresh (overhead) reduction capable

Berger, et al. Standards Track [Page 4] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

         When set, indicates that this node is willing and capable of
         receiving all the messages and objects described in this
         document.  This includes the Bundle message described in
         Section 3, the MESSAGE_ID objects and Ack messages described
         in Section 4, and the MESSAGE_ID LIST objects and Srefresh
         message described in Section 5.  This bit is meaningful only
         between RSVP neighbors.
 Nodes supporting the refresh overhead reduction extensions must also
 take care to recognize when a next hop stops sending RSVP messages
 with the Refresh-Reduction-Capable bit set.  To cover this case,
 nodes supporting the refresh overhead reduction extensions MUST
 examine the flags field of each received RSVP message.  If the flag
 changes from indicating support to indicating non-support then,
 unless configured otherwise, Srefresh messages (described in Section
 5) MUST NOT be used for subsequent state refreshes to that neighbor
 and Bundle messages (Section 3) MUST NOT be sent to that neighbor.
 Note, a node that supports reliable RSVP message delivery (Section 4)
 but not Bundle and Srefresh messages, will not set the Refresh-
 Reduction-Capable bit.

3. RSVP Bundle Message

 An RSVP Bundle message consists of a bundle header followed by a body
 consisting of a variable number of standard RSVP messages.  A Bundle
 message is used to aggregate multiple RSVP messages within a single
 PDU.  The term "bundling" is used to avoid confusion with RSVP
 reservation aggregation.  The following subsections define the
 formats of the bundle header and the rules for including standard
 RSVP messages as part of the message.

3.1. Bundle Header

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Vers  | Flags |   Msg type    |         RSVP checksum         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Send_TTL    |  (Reserved)   |         RSVP length           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    The format of the bundle header is identical to the format of the
    RSVP common header [RFC2205].  The fields in the header are as
    follows:
    Vers: 4 bits
       Protocol version number.  This is version 1.

Berger, et al. Standards Track [Page 5] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

    Flags: 4 bits
       0x01: Refresh (overhead) reduction capable
         See Section 2.
       0x02-0x08: Reserved
    Msg type: 8 bits
       12 = Bundle
    RSVP checksum: 16 bits
       The one's complement of the one's complement sum of the entire
       message, with the checksum field replaced by zero for the
       purpose of computing the checksum.  An all-zero value means
       that no checksum was transmitted.  Because individual sub-
       messages may carry their own checksum as well as the INTEGRITY
       object for authentication, this field MAY be set to zero.  Note
       that when the checksum is not computed, the header of the
       bundle message will not be covered by any checksum.  If the
       checksum is computed, individual sub-messages MAY set their own
       checksum to zero.
    Send_TTL: 8 bits
       The IP TTL value with which the message was sent.  This is used
       by RSVP to detect a non-RSVP hop by comparing the Send_TTL with
       the IP TTL in a received message.
    RSVP length: 16 bits
       The total length of this RSVP Bundle message in bytes,
       including the bundle header and the sub-messages that follow.

3.2. Message Formats

 An RSVP Bundle message must contain at least one sub-message.  A
 sub-message MAY be any message type except for another Bundle
 message.

Berger, et al. Standards Track [Page 6] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    | Vers  | Flags |      12       |         RSVP checksum         |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |   Send_TTL    |  (Reserved)   |         RSVP length           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    //                   First sub-message                         //
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    //                   More sub-messages..                       //
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

3.3. Sending RSVP Bundle Messages

 Support for RSVP Bundle messages is optional.  While message bundling
 helps in scaling RSVP, by reducing processing overhead and bandwidth
 consumption, a node is not required to transmit every standard RSVP
 message in a Bundle message.  A node MUST always be ready to receive
 standard RSVP messages.
 RSVP Bundle messages can only be sent to RSVP neighbors that support
 bundling.  Methods for discovering such information include: (1)
 manual configuration and (2) observing the Refresh-Reduction-Capable
 bit (see Section 2) in the received RSVP messages.  RSVP Bundle
 messages MUST NOT be used if the RSVP neighbor does not support RSVP
 Bundle messages.
 RSVP Bundle messages are sent hop by hop between RSVP-capable nodes
 as "raw" IP datagrams with protocol number 46.  The IP source address
 is an address local to the system that originated the Bundle message.
 The IP destination address is the RSVP neighbor for which the sub-
 messages are intended.
 RSVP Bundle messages SHOULD NOT be sent with the Router Alert IP
 option in their IP headers.  This is because Bundle messages are
 addressed directly to RSVP neighbors.
 Each RSVP Bundle message MUST occupy exactly one IP datagram, which
 is approximately 64K bytes.  If it exceeds the MTU, the datagram is
 fragmented by IP and reassembled at the recipient node.
 Implementations may choose to limit each RSVP Bundle message to the
 MTU size of the outgoing link, e.g., 1500 bytes.  Implementations
 SHOULD also limit the amount of time that a message is delayed in
 order to be bundled.  Different limits may be used for trigger and

Berger, et al. Standards Track [Page 7] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

 standard refresh messages.  Trigger messages SHOULD be delayed a
 minimal amount of time.  Refresh messages may be delayed up to their
 refresh interval.  Note that messages related to the same Resv or
 Path state should not be delayed at different intervals in order to
 preserve ordering.
 If the RSVP neighbor is not known or changes in next hops cannot be
 identified via routing, Bundle messages MUST NOT be used.  Note that
 when the routing next hop is not RSVP capable it will typically not
 be possible to identify changes in next hop.
 Any message that will be handled by the RSVP neighbor indicated in a
 Bundle Message's destination address may be included in the same
 message.  This includes all RSVP messages that would be sent out a
 point-to-point link.  It includes any message, such as a Resv,
 addressed to the same destination address.  It also includes Path and
 PathTear messages when the next hop is known to be the destination
 and changes in next hops can be detected.  Path and PathTear messages
 for multicast sessions MUST NOT be sent in Bundle messages when the
 outgoing link is not a point-to-point link or when the next hop does
 not support the refresh overhead reduction extensions.

3.4. Receiving RSVP Bundle Messages

 If the local system does not recognize or does not wish to accept a
 Bundle message, the received messages shall be discarded without
 further analysis.
 The receiver next compares the Send_TTL with which a Bundle message
 is sent to the IP TTL with which it is received.  If a non-RSVP hop
 is detected, the number of non-RSVP hops is recorded.  It is used
 later in processing of sub-messages.
 Next, the receiver verifies the version number and checksum of the
 RSVP Bundle message and discards the message if any mismatch is
 found.
 The receiver then starts decapsulating individual sub-messages.  Each
 sub-message has its own complete message length and authentication
 information.  With the exception of using the Send_TTL from the
 header of the Bundle message, each sub-message is processed as if it
 was received individually.

4. MESSAGE_ID Extension

 Three new objects are defined as part of the MESSAGE_ID extension.
 The objects are the MESSAGE_ID object, the MESSAGE_ID_ACK object, and
 the MESSAGE_ID_NACK objects.  The first two objects are used to

Berger, et al. Standards Track [Page 8] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

 support acknowledgments and reliable RSVP message delivery.  The last
 object is used to support the summary refresh extension described in
 Section 5.  The MESSAGE_ID object can also be used to simply provide
 a shorthand indication of when the message carrying the object is a
 refresh message.  Such information can be used by the receiving node
 to reduce refresh processing requirements.
 Message identification and acknowledgment is done on a per hop basis.
 All types of MESSAGE_ID objects contain a message identifier.  The
 identifier MUST be unique on a per object generator's IP address
 basis.  No more than one MESSAGE_ID object may be included in an RSVP
 message.  Each message containing a MESSAGE_ID object may be
 acknowledged via a MESSAGE_ID_ACK object, when so indicated.
 MESSAGE_ID_ACK and MESSAGE_ID_NACK objects may be sent piggy-backed
 in unrelated RSVP messages or in RSVP Ack messages.  RSVP messages
 carrying any of the three object types may be included in a bundle
 message.  When included, each object is treated as if it were
 contained in a standard, non-bundled, RSVP message.

4.1. Modification of Standard Message Formats

 The MESSAGE_ID, MESSAGE_ID_ACK and MESSAGE_ID_NACK objects may be
 included in the standard RSVP messages, as defined in [RFC2205].
 When included, one or more MESSAGE_ID_ACK or MESSAGE_ID_NACK objects
 MUST immediately follow the INTEGRITY object.  When no INTEGRITY
 object is present, the MESSAGE_ID_ACK or MESSAGE_ID_NACK objects MUST
 immediately follow the message or sub-message header.  Only one
 MESSAGE_ID object MAY be included in a message or sub-message and it
 MUST follow any present MESSAGE_ID_ACK or MESSAGE_ID_NACK objects.
 When no MESSAGE_ID_ACK or MESSAGE_ID_NACK objects are present, the
 MESSAGE_ID object MUST immediately follow the INTEGRITY object.  When
 no INTEGRITY object is present, the MESSAGE_ID object MUST
 immediately follow the message or sub-message header.
 The ordering of the ACK objects for all standard RSVP messages is:
 <Common Header>  [ <INTEGRITY> ]
                  [ [<MESSAGE_ID_ACK> | <MESSAGE_ID_NACK>] ... ]
                  [ <MESSAGE_ID> ]

Berger, et al. Standards Track [Page 9] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

4.2. MESSAGE_ID Objects

 MESSAGE_ID Class = 23
 MESSAGE_ID object
    Class = MESSAGE_ID Class, C_Type = 1
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Flags     |                      Epoch                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Message_Identifier                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Flags: 8 bits
       0x01 = ACK_Desired flag
         Indicates that the sender requests the receiver to send an
         acknowledgment for the message.
    Epoch: 24 bits
       A value that indicates when the Message_Identifier sequence has
       reset.  SHOULD be randomly generated each time a node reboots
       or the RSVP agent is restarted.  The value SHOULD NOT be the
       same as was used when the node was last operational.  This
       value MUST NOT be changed during normal operation.
    Message_Identifier: 32 bits
       When combined with the message generator's IP address, the
       Message_Identifier field uniquely identifies a message.  The
       values placed in this field change incrementally and only
       decrease when the Epoch changes or when the value wraps.

Berger, et al. Standards Track [Page 10] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

4.3. MESSAGE_ID_ACK and MESSAGE_ID_NACK Objects

 MESSAGE_ID_ACK Class = 24
 MESSAGE_ID_ACK object
    Class = MESSAGE_ID_ACK Class, C_Type = 1
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Flags     |                      Epoch                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Message_Identifier                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Flags: 8 bits
       No flags are currently defined.  This field MUST be zero on
       transmission and ignored on receipt.
    Epoch: 24 bits
       The Epoch field copied from the message being acknowledged.
    Message_Identifier: 32 bits
       The Message_Identifier field copied from the message being
       acknowledged.
 MESSAGE_ID_NACK object
    Class = MESSAGE_ID_ACK Class, C_Type = 2
       Definition is the same as the MESSAGE_ID_ACK object.

4.4. Ack Message Format

 Ack messages carry one or more MESSAGE_ID_ACK or MESSAGE_ID_NACK
 objects.  They MUST NOT contain any MESSAGE_ID objects.  Ack messages
 are sent between neighboring RSVP nodes.  The IP destination address
 of an Ack message is the unicast address of the node that generated
 the message(s) being acknowledged.  For messages with RSVP_HOP
 objects, such as Path and Resv messages, the address is found in the
 RSVP_HOP object.  For other messages, such as ResvConf, the
 associated IP address is the source address in the IP header.  The IP
 source address is an address of the node that sends the Ack message.

Berger, et al. Standards Track [Page 11] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

 The Ack message format is as follows:
   <ACK Message> ::= <Common Header> [ <INTEGRITY> ]
                     <MESSAGE_ID_ACK> | <MESSAGE_ID_NACK>
                     [ [<MESSAGE_ID_ACK> | <MESSAGE_ID_NACK>] ... ]
   For Ack messages, the Msg Type field of the Common Header MUST be
   set to 13.
   Section 4.6 provides guidance on when an Ack message should be used
   and when MESSAGE_ID objects should be sent piggy-backed in other
   RSVP messages.

4.5. MESSAGE_ID Object Usage

 The MESSAGE_ID object may be included in any RSVP message other than
 the Ack and Bundle messages.  The MESSAGE_ID object is always
 generated and processed over a single hop between RSVP neighbors.
 The IP address of the object generator, i.e., the node that creates
 the object, is represented in a per RSVP message type specific
 fashion.  For messages with RSVP_HOP objects, such as Path and Resv
 messages, the generator's IP address is found in the RSVP_HOP object.
 For other messages, such as ResvConf message, the generator's IP
 address is the source address in the IP header.  Note that MESSAGE_ID
 objects can only be used in a Bundle sub-messages, but not in a
 Bundle message.  As is always the case with the Bundle message, each
 sub-message is processed as if it was received individually.  This
 includes processing of MESSAGE_ID objects.
 The Epoch field contains a generator selected value.  The value is
 used to indicate when the sender resets the values used in the
 Message_Identifier field.  On startup, a node SHOULD randomly select
 a value to be used in the Epoch field.  The node SHOULD ensure that
 the selected value is not the same as was used when the node was last
 operational.  The value MUST NOT be changed unless the node or the
 RSVP agent is restarted.
 The Message_Identifier field contains a generator selected value.
 This value, when combined with the generator's IP address, identifies
 a particular RSVP message and the specific state information it
 represents.  The combination of Message_Identifier and Epoch can also
 be used to detect out of order messages.  When a node is sending a
 refresh message with a MESSAGE_ID object, it SHOULD use the same
 Message_Identifier value that was used in the RSVP message that first
 advertised the state being refreshed.  When a node is sending a
 trigger message, the Message_Identifier value MUST have a value that
 is greater than any other value previously used with the same Epoch
 field value.  A value is considered to have been used when it has

Berger, et al. Standards Track [Page 12] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

 been sent in any message using the associated IP address with the
 same Epoch field value.
 The ACK_Desired flag is set when the MESSAGE_ID object generator
 wants a MESSAGE_ID_ACK object sent in response to the message.  Such
 information can be used to ensure reliable delivery of RSVP messages
 in the face of network loss.  Nodes setting the ACK_Desired flag
 SHOULD retransmit unacknowledged messages at a more rapid interval
 than the standard refresh period until the message is acknowledged or
 until a "rapid" retry limit is reached.  Rapid retransmission rate
 MUST be based on the exponential exponential back-off procedures
 defined in section 6.  The ACK_Desired flag will typically be set
 only in trigger messages.  The ACK_Desired flag MAY be set in refresh
 messages.  Issues relate to multicast sessions are covered in a later
 section.
 Nodes processing incoming MESSAGE_ID objects SHOULD check to see if a
 newly received message is out of order and can be ignored.  Out of
 order messages SHOULD be ignored, i.e., silently dropped.  Out of
 order messages can be identified by examining the values in the Epoch
 and Message_Identifier fields.  To determine ordering, the received
 Epoch value must match the value previously received from the message
 sender.  If the values differ then the receiver MUST NOT treat the
 message as out of order.  When the Epoch values match and the
 Message_Identifier value is less than the largest value previously
 received from the sender, then the receiver SHOULD check the value
 previously received for the state associated with the message.  This
 check should be performed for any message that installs or changes
 state.  (Includes at least: Path, Resv, PathTear, ResvTear, PathErr
 and ResvErr.)  If no local state information can be associated with
 the message, the receiver MUST NOT treat the message as out of order.
 If local state can be associated with the message and the received
 Message_Identifier value is less than the most recently received
 value associated with the state, the message SHOULD be treated as
 being out of order.
 Note that the 32-bit Message_Identifier value MAY wrap.  To cover the
 wrap case, the following expression may be used to test if a newly
 received Message_Identifier value is less than a previously received
 value:
     if ((int) old_id - (int) new_id > 0) {
        new value is less than old value;
     }
 MESSAGE_ID objects of messages that are not out of order SHOULD be
 used to aid in determining if the message represents new state or a
 state refresh.  Note that state is only refreshed in Path and Resv

Berger, et al. Standards Track [Page 13] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

 messages.  If the received Epoch values differs from the value
 previously received from the message sender, the message is a trigger
 message and the receiver MUST fully process the message.  If a Path
 or Resv message contains the same Message_Identifier value that was
 used in the most recently received message for the same session and,
 for Path messages, SENDER_TEMPLATE then the receiver SHOULD treat the
 message as a state refresh.  If the Message_Identifier value is
 greater than the most recently received value, the receiver MUST
 fully processes the message.  When fully processing a Path or Resv
 message, the receiver MUST store the received Message_Identifier
 value as part of the local Path or Resv state for future reference.
 Nodes receiving a non-out of order message containing a MESSAGE_ID
 object with the ACK_Desired flag set, SHOULD respond with a
 MESSAGE_ID_ACK object.  Note that MESSAGE_ID objects received in
 messages containing errors, i.e., are not syntactically valid,  MUST
 NOT be acknowledged.  PathErr and ResvErr messages SHOULD be treated
 as implicit acknowledgments.

4.6. MESSAGE_ID_ACK Object and MESSAGE_ID_NACK Object Usage

 The MESSAGE_ID_ACK object is used to acknowledge receipt of messages
 containing MESSAGE_ID objects that were sent with the ACK_Desired
 flag set.  A MESSAGE_ID_ACK object MUST NOT be generated in response
 to a received MESSAGE_ID object when the ACK_Desired flag is not set.
 The MESSAGE_ID_NACK object is used as part of the summary refresh
 extension.  The generation and processing of MESSAGE_ID_NACK objects
 is described in further detail in Section 5.4.
 MESSAGE_ID_ACK and MESSAGE_ID_NACK objects MAY be sent in any RSVP
 message that has an IP destination address matching the generator of
 the associated MESSAGE_ID object.  This means that the objects will
 not typically be included in the non hop-by-hop Path, PathTear and
 ResvConf messages.  When no appropriate message is available, one or
 more objects SHOULD be sent in an Ack message.  Implementations
 SHOULD include MESSAGE_ID_ACK and MESSAGE_ID_NACK objects in standard
 RSVP messages when possible.
 Implementations SHOULD limit the amount of time that an object is
 delayed in order to be piggy-backed or sent in an Ack message.
 Different limits may be used for MESSAGE_ID_ACK and MESSAGE_ID_NACK
 objects.  MESSAGE_ID_ACK objects are used to detect link transmission
 losses.  If an ACK object is delayed too long, the corresponding
 message will be retransmitted.  To avoid such retransmission, ACK
 objects SHOULD be delayed a minimal amount of time.  A delay time
 equal to the link transit time MAY be used.  MESSAGE_ID_NACK objects
 may be delayed an independent and longer time, although additional

Berger, et al. Standards Track [Page 14] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

 delay increases the amount of time a desired reservation is not
 installed.

4.7. Multicast Considerations

 Path and PathTear messages may be sent to IP multicast destination
 addresses.  When the destination is a multicast address, it is
 possible that a single message containing a single MESSAGE_ID object
 will be received by multiple RSVP next hops.  When the ACK_Desired
 flag is set in this case, acknowledgment processing is more complex.
 There are a number of issues to be addressed including ACK implosion,
 number of acknowledgments to be expected and handling of new
 receivers.
 ACK implosion occurs when each receiver responds to the MESSAGE_ID
 object at approximately the same time.  This can lead to a
 potentially large number of MESSAGE_ID_ACK objects being
 simultaneously delivered to the message generator.  To address this
 case, the receiver MUST wait a random interval prior to acknowledging
 a MESSAGE_ID object received in a message destined to a multicast
 address.  The random interval SHOULD be between zero (0) and a
 configured maximum time.  The configured maximum SHOULD be set in
 proportion to the refresh and "rapid" retransmission interval, i.e,
 such that the maximum time before sending an acknowledgment does not
 result in retransmission.  It should be noted that ACK implosion is
 being addressed by spreading acknowledgments out in time, not by ACK
 suppression.
 A more fundamental issue is the number of acknowledgments that the
 upstream node, i.e., the message generator, should expect.  The
 number of acknowledgments that should be expected is the same as the
 number of RSVP next hops.  In the router-to-router case, the number
 of next hops can often be obtained from routing.  When hosts are
 either the upstream node or the next hops, the number of next hops
 will typically not be readily available.  Another case where the
 number of RSVP next hops will typically not be known is when there
 are non-RSVP routers between the message generator and the RSVP next
 hops.
 When the number of next hops is not known, the message generator
 SHOULD only expect a single response.  The result of this behavior
 will be special retransmission handling until the message is
 delivered to at least one next hop, then followed by standard RSVP
 refreshes.  Refresh messages will synchronize state with any next
 hops that don't receive the original message.

Berger, et al. Standards Track [Page 15] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

4.7.1. Reference RSVP/Routing Interface

 When using the MESSAGE_ID extension with multicast sessions it is
 preferable for RSVP to obtain the number of next hops from routing
 and to be notified when that number changes.  The interface between
 routing and RSVP is purely an implementation issue.  Since RSVP
 [RFC2205] describes a reference routing interface, a version of the
 RSVP/routing interface updated to provide number of next hop
 information is presented.  See [RFC2205] for previously defined
 parameters and function description.
    o    Route Query
         Mcast_Route_Query( [ SrcAddress, ] DestAddress,
                            Notify_flag )
                            -> [ IncInterface, ] OutInterface_list,
                            NHops_list
    o    Route Change Notification
         Mcast_Route_Change( ) -> [ SrcAddress, ] DestAddress,
                           [ IncInterface, ] OutInterface_list,
                           NHops_list
    NHops_list provides the number of multicast group members
    reachable via each OutInterface_list entry.

4.8. Compatibility

 All nodes sending messages with the Refresh-Reduction-Capable bit set
 will support the MESSAGE_ID Extension.  There are no backward
 compatibility issues raised by the MESSAGE_ID Class with nodes that
 do not set the Refresh-Reduction-Capable bit.  The MESSAGE_ID Class
 has an assigned value whose form is 0bbbbbbb.  Per RSVP [RFC2205],
 classes with values of this form must be rejected with an "Unknown
 Object Class" error by nodes not supporting the class.  When the
 receiver of a MESSAGE_ID object does not support the class, a
 corresponding error message will be generated.  The generator of the
 MESSAGE_ID object will see the error and then MUST re-send the
 original message without the MESSAGE_ID object.  In this case, the
 message generator MAY still choose to retransmit messages at the
 "rapid" retransmission interval.  Lastly, since the MESSAGE_ID_ACK
 class can only be issued in response to the MESSAGE_ID object, there
 are no possible issues with this class or Ack messages.  A node MAY
 support the MESSAGE_ID Extension without supporting the other refresh
 overhead reduction extensions.

Berger, et al. Standards Track [Page 16] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

5. Summary Refresh Extension

 The summary refresh extension enables the refreshing of RSVP state
 without the transmission of standard Path or Resv messages.  The
 benefits of the described extension are that it reduces the amount of
 information that must be transmitted and processed in order to
 maintain RSVP state synchronization.  Importantly, the described
 extension preserves RSVP's ability to handle non-RSVP next hops and
 to adjust to changes in routing.  This extension cannot be used with
 Path or Resv messages that contain any change from previously
 transmitted messages, i.e., are trigger messages.
 The summary refresh extension builds on the previously defined
 MESSAGE_ID extension.  Only state that was previously advertised in
 Path and Resv messages containing MESSAGE_ID objects can be refreshed
 via the summary refresh extension.
 The summary refresh extension uses the objects and the ACK message
 previously defined as part of the MESSAGE_ID extension, and a new
 Srefresh message.  The new message carries a list of
 Message_Identifier fields corresponding to the Path and Resv trigger
 messages that established the state.  The Message_Identifier fields
 are carried in one of three Srefresh related objects.  The three
 objects are the MESSAGE_ID LIST object, the MESSAGE_ID SRC_LIST
 object, and the MESSAGE_ID MCAST_LIST object.
 The MESSAGE_ID LIST object is used to refresh all Resv state, and
 Path state of unicast sessions.  It is made up of a list of
 Message_Identifier fields that were originally advertised in
 MESSAGE_ID objects.  The other two objects are used to refresh Path
 state of multicast sessions.  A node receiving a summary refresh for
 multicast path state will at times need source and group information.
 These two objects provide this information.  The objects differ in
 the information they contain and how they are sent.  Both carry
 Message_Identifier fields and corresponding source IP addresses.  The
 MESSAGE_ID SRC_LIST is sent in messages addressed to the session's
 multicast IP address.  The MESSAGE_ID MCAST_LIST object adds the
 group address and is sent in messages addressed to the RSVP next hop.
 The MESSAGE_ID MCAST_LIST is normally used on point-to-point links.
 An RSVP node receiving an Srefresh message, matches each listed
 Message_Identifier field with installed Path or Resv state.  All
 matching state is updated as if a normal RSVP refresh message has
 been received.  If matching state cannot be found, then the Srefresh
 message sender is notified via a refresh NACK.

Berger, et al. Standards Track [Page 17] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

 A refresh NACK is sent via the MESSAGE_ID_NACK object.  As described
 in the previous section, the rules for sending a MESSAGE_ID_NACK
 object are the same as for sending a MESSAGE_ID_ACK object.  This
 includes sending MESSAGE_ID_NACK object both piggy-backed in
 unrelated RSVP messages or in RSVP ACK messages.

5.1. MESSAGE_ID LIST, SRC_LIST and MCAST_LIST Objects

 MESSAGE_ID LIST object
 MESSAGE_ID_LIST Class = 25
    Class = MESSAGE_ID_LIST Class, C_Type = 1
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Flags     |                      Epoch                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Message_Identifier                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                 :                             |
    //                                :                            //
    |                                 :                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                       Message_Identifier                      |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Flags: 8 bits
       No flags are currently defined.  This field MUST be zero on
       transmission and ignored on receipt.
    Epoch: 24 bits
       The Epoch field from the MESSAGE_ID object corresponding to the
       trigger message that advertised the state being refreshed.
    Message_Identifier: 32 bits
       The Message_Identifier field from the MESSAGE_ID object
       corresponding to the trigger message that advertised the state
       being refreshed.  One or more Message_Identifiers may be
       included.

Berger, et al. Standards Track [Page 18] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

 IPv4/MESSAGE_ID SRC_LIST object
    Class = MESSAGE_ID_LIST Class, C_Type = 2
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Flags     |                      Epoch                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                              Source_                          |
    |                      Message_Identifier_Tuple                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                 :                             |
    //                                :                            //
    |                                 :                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                              Source_                          |
    |                      Message_Identifier_Tuple                 |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Where a Source_Message_Identifier_Tuple consists of:
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Message_Identifier                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Source_IP_Address (4 bytes)                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Berger, et al. Standards Track [Page 19] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

 IPv6/MESSAGE_ID SRC_LIST object
    Class = MESSAGE_ID_LIST Class, C_Type = 3
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Flags     |                      Epoch                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                            IPv6_Source_                       |
    |                      Message_Identifier_Tuple                 |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                 :                             |
    //                                :                            //
    |                                 :                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                            IPv6_Source_                       |
    |                      Message_Identifier_Tuple                 |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Where a IPv6 Source_Message_Identifier_Tuple consists of:
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Message_Identifier                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                      IPv6 Source_IP_Address                   |
    |                            (16 Bytes)                         |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Flags: 8 bits
       No flags are currently defined.  This field MUST be zero on
       transmission and ignored on receipt.
    Epoch: 24 bits
       The Epoch field from the MESSAGE_ID object corresponding to the
       trigger message that advertised the state being refreshed.

Berger, et al. Standards Track [Page 20] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

    Message_Identifier
       The Message_Identifier field from the MESSAGE_ID object
       corresponding to the trigger message that advertised the Path
       state being refreshed.  One or more Message_Identifiers may be
       included.  Each Message_Identifier MUST be followed by the
       source IP address corresponding to the sender described in the
       Path state being refreshed.
    Source_IP_Address
       The IP address corresponding to the sender of the Path state
       being refreshed.
    IPv4/MESSAGE_ID MCAST_LIST object
    Class = MESSAGE_ID_LIST Class, C_Type = 4
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Flags     |                      Epoch                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                             Multicast_                        |
    |                        Message_Identifier_                    |
    |                               Tuple                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                 :                             |
    //                                :                            //
    |                                 :                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                             Multicast_                        |
    |                        Message_Identifier_                    |
    |                               Tuple                           |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 Where a Multicast_Message_Identifier_Tuple consists of:
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Message_Identifier                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                    Source_IP_Address (4 bytes)                |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                 Destination_IP_Address (4 bytes)              |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Berger, et al. Standards Track [Page 21] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

 IPv6/MESSAGE_ID MCAST_LIST object
    Class = MESSAGE_ID_LIST Class, C_Type = 5
     0                   1                   2                   3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |     Flags     |                      Epoch                    |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                                                               |
    |                                                               |
    |                           IPv6 Multicast_                     |
    |                        Message_Identifier_                    |
    |                               Tuple                           |
    |                                                               |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                 :                             |
    //                                :                            //
    |                                 :                             |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                                                               |
    |                                                               |
    |                           IPv6 Multicast_                     |
    |                        Message_Identifier_                    |
    |                               Tuple                           |
    |                                                               |
    |                                                               |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

Berger, et al. Standards Track [Page 22] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

 Where a IPv6 Multicast_Message_Identifier_Tuple consists of:
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                        Message_Identifier                     |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                      IPv6 Source_IP_Address                   |
    |                            (16 Bytes)                         |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |                                                               |
    |                     IPv6 Destination_IP_Address               |
    |                            (16 Bytes)                         |
    |                                                               |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    Flags: 8 bits
       No flags are currently defined.  This field MUST be zero on
       transmission and ignored on receipt.
    Epoch: 24 bits
       The Epoch field from the MESSAGE_ID object corresponding to the
       trigger message that advertised the state being refreshed.
    Message_Identifier: 32 bits
       The Message_Identifier field from the MESSAGE_ID object
       corresponding to the trigger message that advertised the Path
       state being refreshed.  One or more Message_Identifiers may be
       included.  Each Message_Identifier MUST be followed by the
       source IP address corresponding to the sender of the Path state
       being refreshed, and the destination IP address of the session.
    Source_IP_Address
       The IP address corresponding to the sender of the Path state
       being refreshed.
    Destination_IP_Address
       The destination IP address corresponding to the session of the
       Path state being refreshed.

Berger, et al. Standards Track [Page 23] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

5.2. Srefresh Message Format

 Srefresh messages carry one or more MESSAGE_ID LIST, MESSAGE_ID
 SRC_LIST, and MESSAGE_ID MCAST_LIST objects.  MESSAGE_ID LIST and
 MESSAGE_ID MCAST_LIST objects MAY be carried in the same Srefresh
 message.  MESSAGE_ID SRC_LIST can not be combined in Srefresh
 messages with the other objects.  A single Srefresh message MAY
 refresh both Path and Resv state.
 Srefresh messages carrying Message_Identifier fields corresponding to
 Path state are normally sent with a destination IP address equal to
 the address carried in the corresponding SESSION objects.  The
 destination IP address MAY be set to the RSVP next hop when the next
 hop is known to be RSVP capable and either (a) the session is unicast
 or (b) the outgoing interface is a point-to-point link.  Srefresh
 messages carrying Message_Identifier fields corresponding to Resv
 state MUST be sent with a destination IP address set to the Resv
 state's previous hop.
 Srefresh messages sent to a multicast session's destination IP
 address, MUST contain MESSAGE_ID SRC_LIST objects and MUST NOT
 include any MESSAGE_ID LIST or MESSAGE_ID MCAST_LIST objects.
 Srefresh messages sent to the RSVP next hop MAY contain either or
 both MESSAGE_ID LIST and MESSAGE_ID MCAST_LIST objects, but MUST NOT
 include any MESSAGE_ID SRC_LIST objects.
 The source IP address of an Srefresh message is an address of the
 node that generates the message.  The source IP address MUST match
 the address associate with the MESSAGE_ID objects when they were
 included in a standard RSVP message.  As previously mentioned, the
 source address associated with a MESSAGE_ID object is represented in
 a per RSVP message type specific fashion.  For messages with RSVP_HOP
 objects, such as Path and Resv messages, the address is found in the
 RSVP_HOP object.  For other messages, such as ResvConf message, the
 associated IP address is the source address in the IP header.
 Srefresh messages that are addressed to a session's destination IP
 address MUST be sent with the Router Alert IP option in their IP
 headers.  Srefresh messages addressed directly to RSVP neighbors
 SHOULD NOT be sent with the Router Alert IP option in their IP
 headers.
 Each Srefresh message MUST occupy exactly one IP datagram.  If it
 exceeds the MTU, the datagram is fragmented by IP and reassembled at
 the recipient node.  Srefresh messages MAY be sent within an RSVP
 Bundle messages.  Although this is not expected since Srefresh

Berger, et al. Standards Track [Page 24] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

 messages can carry a list of Message_Identifier fields within a
 single object.  Implementations may choose to limit each Srefresh
 message to the MTU size of the outgoing link, e.g., 1500 bytes.
 The Srefresh message format is:
 <Srefresh Message> ::= <Common Header> [ <INTEGRITY> ]
                       [ [<MESSAGE_ID_ACK> | <MESSAGE_ID_NACK>] ... ]
                       [ <MESSAGE_ID> ]
                       <srefresh list> | <source srefresh list>
 <srefresh list> ::= <MESSAGE_ID LIST> | <MESSAGE_ID MCAST_LIST>
                       [ <srefresh list> ]
 <source srefresh list> ::= <MESSAGE_ID SRC_LIST>
                              [ <source srefresh list> ]
 For Srefresh messages, the Msg Type field of the Common Header MUST
 be set to 15.

5.3. Srefresh Message Usage

 An Srefresh message may be generated to refresh Resv and Path state.
 If an Srefresh message is used to refresh some particular state, then
 the generation of a standard refresh message for that particular
 state SHOULD be suppressed.  A state's refresh interval is not
 affected by the use of Srefresh message based refreshes.
 When generating an Srefresh message, a node SHOULD refresh as much
 Path and Resv state as is possible by including the information from
 as many MESSAGE_ID objects in the same Srefresh message.  Only the
 information from MESSAGE_ID objects that meet the source and
 destination IP address restrictions, as described in Sections 5.2,
 may be included in the same Srefresh message.  Identifying Resv state
 that can be refreshed using the same Srefresh message is fairly
 straightforward.  Identifying which Path state may be included is a
 little more complex.
 Only state that was previously advertised in Path and Resv messages
 containing MESSAGE_ID objects can be refreshed via an Srefresh
 message.  Srefresh message based refreshes must preserve the state
 synchronization properties of Path or Resv message based refreshes.
 Specifically, the use of Srefresh messages MUST NOT result in state
 being timed-out at the RSVP next hop.  The period at which state is
 refreshed when using Srefresh messages MAY be shorter than the period
 that would be used when using Path or Resv message based refreshes,
 but it MUST NOT be longer.

Berger, et al. Standards Track [Page 25] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

 The particular approach used to trigger Srefresh message based
 refreshes is implementation specific.  Some possibilities are
 triggering Srefresh message generation based on each state's refresh
 period or, on a per interface basis, periodically generating Srefresh
 messages to refresh all state that has not been refreshed within the
 state's refresh interval.  Other approaches are also possible.  A
 default Srefresh message generation interval of 30 seconds is
 suggested for nodes that do not dynamically calculate a generation
 interval.
 When generating an Srefresh message, there are two methods for
 identifying which Path state may be refreshed in a specific message.
 In both cases, the previously mentioned refresh interval and source
 IP address restrictions must be followed.  The primary method is to
 include only those sessions that share the same destination IP
 address in the same Srefresh message.
 The secondary method for identifying which Path state may be
 refreshed within a single Srefresh message is an optimization.  This
 method MAY be used when the next hop is known to support RSVP and
 when either (a) the session is unicast or (b) the outgoing interface
 is a point-to-point link.  This method MUST NOT be used when the next
 hop is not known to support RSVP or when the outgoing interface is to
 a multi-access network and the session is to a multicast address.
 The use of this method MAY be administratively configured.  When
 using this method, the destination address in the IP header of the
 Srefresh message is usually the next hop's address.  When the use of
 this method is administratively configured, the destination address
 should be the well known group address 224.0.0.14.  When the outgoing
 interface is a point-to-point link, all Path state associated with
 sessions advertised out the interface SHOULD be included in the same
 Srefresh message.  When the outgoing interface is not a point-to-
 point link, all unicast session Path state SHOULD be included in the
 same Srefresh message.
 Identifying which Resv state may be refreshed within a single
 Srefresh message is based simply on the source and destination IP
 addresses.  Any state that was previously advertised in Resv messages
 with the same IP addresses as an Srefresh message MAY be included.
 After identifying the Path and Resv state that can be included in a
 particular Srefresh message, the message generator adds to the
 message MESSAGE_ID information matching each identified state's
 previously used object.  For all Resv state and for Path state of
 unicast sessions, the information is added to the message in a
 MESSAGE_ID LIST object that has a matching Epoch value.  (Note only
 one Epoch value will be in use during normal operation.)  If no
 matching object exists, then a new MESSAGE_ID LIST object is created.

Berger, et al. Standards Track [Page 26] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

 Path state of multicast sessions may be added to the same message
 when the destination address of the Srefresh message is the RSVP next
 hop and the outgoing interface is a point-to-point link.  In this
 case the information is added to the message in a MESSAGE_ID
 MCAST_LIST object that has a matching Epoch value.  If no matching
 object exists, then a new MESSAGE_ID MCAST_LIST object is created.
 When the destination address of the message is a multicast address,
 then identified information is added to the message in a MESSAGE_ID
 SRC_LIST object that has a matching Epoch value.  If no matching
 object exists, then a new MESSAGE_ID SRC_LIST object is created.
 Once the Srefresh message is composed, the message generator
 transmits the message out the proper interface.
 Upon receiving an Srefresh message, the node MUST attempt to identify
 matching installed Path or Resv state.  Matching is done based on the
 source address in the IP header of the Srefresh message, the object
 type and each Message_Identifier field.  If matching state can be
 found, then the receiving node MUST update the matching state
 information as if a standard refresh message had been received.  If
 matching state cannot be identified, then an Srefresh NACK MUST be
 generated corresponding to the unmatched Message_Identifier field.
 Message_Identifier fields received in MESSAGE_ID LIST objects may
 correspond to any Resv state or to Path state of unicast sessions.
 Message_Identifier fields received in MESSAGE_ID SRC_LIST or
 MCAST_LIST objects correspond to Path state of multicast sessions.
 An additional check must be performed to determine if a NACK should
 be generated for unmatched Message_Identifier fields associated with
 Path state of multicast sessions, i.e., fields that were carried in
 MESSAGE_ID SRC_LIST or MCAST_LIST objects.  The receiving node must
 check to see if the node would forward data packets originated from
 the source corresponding to the unmatched field.  This check,
 commonly known as an RPF check, is performed based on the source and
 group information carried in the MESSAGE_ID SRC_LIST and MCAST_LIST
 objects.  In both objects the IP address of the source is listed
 immediately after the corresponding Message_Identifier field.  The
 group address is listed immediately after the source IP address in
 MESSAGE_ID MCAST_LIST objects.  The group address is the message's
 destination IP address when MESSAGE_ID SRC_LIST objects are used.
 The receiving node only generates an Srefresh NACK when the node
 would forward packets to the identified group from the listed sender.
 If the node would forward multicast data packets from a listed sender
 and there is a corresponding unmatched Message_Identifier field, then
 an appropriate Srefresh NACK MUST be generated.  If the node would
 not forward packets to the identified group from a listed sender, a
 corresponding unmatched Message_Identifier field is silently ignored.

Berger, et al. Standards Track [Page 27] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

5.4. Srefresh NACK

 Srefresh NACKs are used to indicate that a received
 Message_Identifier field carried in MESSAGE_ID LIST, SRC_LIST, or
 MCAST_LIST object does not match any installed state.  This may occur
 for a number of reasons including, for example, a route change.  An
 Srefresh NACK is encoded in a MESSAGE_ID_NACK object.  When
 generating an Srefresh NACK, the epoch and Message_Identifier fields
 of the MESSAGE_ID_NACK object MUST have the same value as was
 received.  MESSAGE_ID_NACK objects are transmitted as described in
 Section 4.6.
 Received MESSAGE_ID_NACK objects indicate that the object generator
 does not have any installed state matching the object.  Upon
 receiving a MESSAGE_ID_NACK object, the receiver performs an
 installed Path or Resv state lookup based on the Epoch and
 Message_Identifier values contained in the object.  If matching state
 is found, then the receiver MUST transmit the matching state via a
 standard Path or Resv message.  If the receiver cannot identify any
 installed state, then no action is required.

5.5. Preserving RSVP Soft State

 As discussed in [RFC2205], RSVP uses soft state to address a large
 class of potential errors.  RSVP does this by periodically sending a
 full representation of installed state in Resv and Path messages.
 Srefresh messages are used in place of the periodic sending of
 standard Path and Resv refresh messages.  While this provides scaling
 benefits and protects against common network events such as packet
 loss or routing change, it does not provide exactly the same error
 recovery properties.  An example error that could potentially be
 recovered from via standard messages but not with Srefresh messages
 is internal corruption of state.  This section recommends two methods
 that can be used to better preserve RSVP's soft state error recovery
 mechanism.  Both mechanisms are supported using existing protocol
 messages.
 The first mechanism uses a checksum or other algorithm to detect a
 previously unnoticed change in internal state.  This mechanism does
 not protect against internal state corruption.  It just covers the
 case where a trigger message should have been sent, but was not.
 When sending a Path or Resv trigger message, a node should run a
 checksum or other algorithm, such as [MD5], over the internal state
 and store the result.  The choice of algorithm is an administrative
 decision.  Periodically the node should rerun the algorithm and
 compare the new result with the stored result.  If the values differ,
 then a corresponding standard Path or Resv refresh message should be

Berger, et al. Standards Track [Page 28] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

 sent and the new value should be stored.  The recomputation period
 should be set based on the computation resources of the node and the
 reliability requirements of the network.
 The second mechanism is simply to periodically send standard Path and
 Resv refresh messages.  Since this mechanism uses standard refresh
 messages, it can recover from the same set of errors as standard
 RSVP.  When using this mechanism, the period that standard refresh
 messages are sent must be longer than the interval that Srefresh
 messages are generated in order to gain the benefits of using the
 summary refresh extension.  When a standard refresh message is sent,
 a corresponding summary refresh SHOULD NOT be sent during the same
 refresh period.  When a node supports the periodic generation of
 standard refresh messages while Srefreshes are being used, the
 frequency of generation of standard refresh messages relative to the
 generation of summary refreshes SHOULD be configurable by the network
 administrator.

5.6. Compatibility

 Nodes supporting the summary refresh extension advertise their
 support via the Refresh-Reduction-Capable bit in the RSVP message
 header.  This enables nodes supporting the extension to detect each
 other.  When it is not known if a next hop supports the extension,
 standard Path and Resv message based refreshes MUST be used.  Note
 that when the routing next hop does not support RSVP, it will not
 always be possible to detect if the RSVP next hop supports the
 summary refresh extension.  Therefore, when the routing next hop is
 not RSVP capable the Srefresh message based refresh SHOULD NOT be
 used.  A node MAY be administratively configured to use Srefresh
 messages in all cases when all RSVP nodes in a network are known to
 support the summary refresh extension.  This is useful since when
 operating in this mode, the extension properly adjusts to the case of
 non-RSVP next hops and changes in routing.
 Per section 2, nodes supporting the summary refresh extension must
 also take care to recognize when a next hop stops sending RSVP
 messages with the Refresh-Reduction-Capable bit set.

6. Exponential Back-Off Procedures

 This section is based on [Pan] and provides procedures to implement
 exponential back-off for retransmission of messages awaiting
 acknowledgment, see Section 4.5.  Implementations MUST use the
 described procedures or their equivalent.

Berger, et al. Standards Track [Page 29] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

6.1. Outline of Operation

 The following is one possible mechanism for exponential back-off
 retransmission of an unacknowledged RSVP message: When sending such a
 message, a node inserts a MESSAGE_ID object with the ACK_Desired flag
 set.  The sending node will retransmit the message until a message
 acknowledgment is received or the message has been transmitted a
 maximum number of times.  Upon reception, a receiving node
 acknowledges the arrival of the message by sending back a message
 acknowledgment (that is, a corresponding MESSAGE_ID_ACK object.)
 When the sending node receives the acknowledgment retransmission of
 the message is stopped.  The interval between retransmissions is
 governed by a rapid retransmission timer.  The rapid retransmission
 timer starts at a small interval and increases exponentially until it
 reaches a threshold.

6.2. Time Parameters

 The described procedures make use of the following time parameters.
 All parameters are per interface.
    Rapid retransmission interval Rf:
         Rf is the initial retransmission interval for unacknowledged
         messages.  After sending the message for the first time, the
         sending node will schedule a retransmission after Rf seconds.
         The value of Rf could be as small as the round trip time
         (RTT) between a sending and a receiving node, if known.
    Rapid retry limit Rl:
         Rl is the maximum number of times a message will be
         transmitted without being acknowledged.
    Increment value Delta:
         Delta governs the speed with which the sender increases the
         retransmission interval.  The ratio of two successive
         retransmission intervals is (1 + Delta).
 Suggested default values are an initial retransmission timeout (Rf)
 of 500ms, a power of 2 exponential back-off (Delta = 1) and a retry
 limit (Rl) of 3.

Berger, et al. Standards Track [Page 30] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

6.3. Retransmission Algorithm

 After a sending node transmits a message containing a MESSAGE_ID
 object with the ACK_Desired flag set, it should immediately schedule
 a retransmission after Rf seconds.  If a corresponding MESSAGE_ID_ACK
 object is received earlier than Rf seconds, then retransmission
 SHOULD be canceled.  Otherwise, it will retransmit the message after
 (1 + Delta)*Rf seconds.  The staged retransmission will continue
 until either an appropriate MESSAGE_ID_ACK object is received, or the
 rapid retry limit, Rl, has been reached.
 A sending node can use the following algorithm when transmitting a
 message containing a MESSAGE_ID object with the ACK_Desired flag set:
     Prior to initial transmission initialize: Rk = Rf and Rn = 0
     while (Rn++ < Rl)  {
         transmit the message;
         wake up after Rk seconds;
         Rk = Rk * (1 + Delta);
     }
     /* acknowledged or no reply from receiver for too long: */ do any
     needed clean up; exit;
 Asynchronously, when a sending node receives a corresponding
 MESSAGE_ID_ACK object, it will change the retry count, Rn, to Rl.
 Note that the transmitting node does not advertise the use of the
 described exponential back-off procedures via the TIME_VALUE object.

6.4. Performance Considerations

 The use of exponential back-off retransmission is a new and
 significant addition to RSVP.  It will be important to review related
 operations and performance experience before this document advances
 to Draft Standard.  It will be particularly important to review
 experience with multicast, and any ACK implosion problems actually
 encountered.

7. Acknowledgments

 This document represents ideas and comments from the MPLS-TE design
 team and participants in the RSVP Working Group's interim meeting.
 Thanks to Bob Braden, Lixia Zhang, Fred Baker, Adrian Farrel, Roch
 Guerin, Kireeti Kompella, David Mankins, Henning Schulzrinne, Andreas
 Terzis, Lan Wang and Masanobu Yuhara for specific feedback on the
 various versions of the document.

Berger, et al. Standards Track [Page 31] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

 Portions of this work are based on work done by Masanobu Yuhara and
 Mayumi Tomikawa [Yuhara].

8. Security Considerations

 No new security issues are raised in this document.  See [RFC2205]
 for a general discussion on RSVP security issues.

9. References

 [Pan]     Pan, P., Schulzrinne, H., "Staged Refresh Timers for RSVP,"
           Global Internet'97, Phoenix, AZ, November 1997.
           http://www.cs.columbia.edu/~pingpan/papers/timergi.pdf
 [MD5]     Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321,
           April 1992.
 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119, March 1997.
 [RFC2205] Braden, R., Ed., Zhang, L., Berson, S., Herzog, S. and S.
           Jamin , "Resource ReserVation Protocol -- Version 1
           Functional Specification", RFC 2205, September 1997.
 [Yuhara]  Yuhara, M., and M Tomikawa, "RSVP Extensions for ID-based
           Refreshes", Work in Progress.

Berger, et al. Standards Track [Page 32] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

10. Authors' Addresses

 Lou Berger
 LabN Consulting, LLC
 Phone:  +1 301 468 9228
 EMail:  lberger@labn.net
 Der-Hwa Gan
 Juniper Networks, Inc.
 1194 N. Mathilda Avenue,
 Sunnyvale, CA 94089
 Voice: +1 408 745 2074
 Email:  dhg@juniper.net
 George Swallow
 Cisco Systems, Inc.
 250 Apollo Drive
 Chelmsford, MA 01824
 Phone:  +1 978 244 8143
 EMail:  swallow@cisco.com
 Ping Pan
 Juniper Networks, Inc.
 1194 N. Mathilda Avenue,
 Sunnyvale, CA 94089
 Voice: +1 408 745 3704
 Email:  pingpan@juniper.net
 Franco Tommasi
 University of Lecce, Fac. Ingegneria
 Via Monteroni 73100 Lecce, ITALY
 EMail:  franco.tommasi@unile.it
 Simone Molendini
 University of Lecce, Fac. Ingegneria
 Via Monteroni 73100 Lecce, ITALY
 EMail:  molendini@ultra5.unile.it

Berger, et al. Standards Track [Page 33] RFC 2961 RSVP Refresh Overhead Reduction Extensions April 2001

11. Full Copyright Statement

 Copyright (C) The Internet Society (2001).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Berger, et al. Standards Track [Page 34]

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