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Versions: (draft-berger-rsvp-refresh-reduct) 00 01 02 03 04 RFC 2961

Network Working Group                                         Lou Berger
Internet Draft                                      LabN Consulting, LLC
Expiration Date: April 2000
                                                             Der-Hwa Gan
                                                  Juniper Networks, Inc.

                                                          George Swallow
                                                     Cisco Systems, Inc.

                                                                Ping Pan
                                                       Bell Labs, Lucent

                                                            October 1999


                   RSVP Refresh Reduction Extensions


                 draft-ietf-rsvp-refresh-reduct-01.txt

Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

     The list of current Internet-Drafts can be accessed at
     http://www.ietf.org/ietf/1id-abstracts.txt

     The list of Internet-Draft Shadow Directories can be accessed at
     http://www.ietf.org/shadow.html.

Abstract

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




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Contents

 1      Introduction and Background  ...............................   3
 1.1    Trigger and Refresh Messages  ..............................   4
 2      RSVP Bundle Message  .......................................   5
 2.1    Bundle Header  .............................................   5
 2.2    Message Formats  ...........................................   6
 2.3    Sending RSVP Bundle Messages  ..............................   7
 2.4    Receiving RSVP Bundle Messages  ............................   8
 2.5    Forwarding RSVP Bundle Messages  ...........................   8
 2.6    Bundle-Capable Bit  ........................................   8
 3      MESSAGE_ID Extension  ......................................   9
 3.1    MESSAGE_ID Objects   .......................................  10
 3.2    MESSAGE_ID_ACK and MESSAGE_ID_NACK Objects  ................  11
 3.3    Ack Message Format  ........................................  12
 3.4    MESSAGE_ID Object Usage  ...................................  12
 3.5    MESSAGE_ID_ACK Object and MESSAGE_ID_NACK Object Usage  ....  14
 3.6    Multicast Considerations  ..................................  15
 3.6.1  Reference RSVP/Routing Interface  ..........................  15
 3.7    Compatibility  .............................................  16
 4      Summary Refresh Extension  .................................  16
 4.1    MESSAGE_ID LIST, SRC_LIST and MCAST_LIST Objects  ..........  18
 4.2    Srefresh Message Format  ...................................  21
 4.3    Srefresh Message Usage  ....................................  22
 4.4    Srefresh NACK  .............................................  25
 4.5    Compatibility  .............................................  25
 5      Reference Exponential Back-Off Procedures  .................  26
 5.1    Outline of Operation  ......................................  26
 5.2    Time Parameters  ...........................................  27
 5.3    Example Retransmission Algorithm  ..........................  28
 6      Acknowledgments  ...........................................  29
 7      Security Considerations  ...................................  29
 8      References  ................................................  29
 9      Authors' Addresses  ........................................  30




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Changes from previous version

o  Integrated feedback from working group chairs.
o  No substantive changes were made to how extensions operate.
o  Renamed Message_ID field to Message_Identifier.
o  Broke MESSAGE_ID class into MESSAGE_ID, MESSAGE_ID_ACK and
   MESSAGE_ID_LIST classes.
o  Removed some text left over from previous versions and removed
   reference to UDP encapsulation.
o  Many other editorial and clarification related changes.


1. Introduction and Background

   The resource requirements (in terms of CPU processing and memory) for
   running RSVP on a router increases proportionally with the number of
   sessions.  Supporting a large number of sessions can present scaling
   problems.

   This document describes mechanisms to help alleviate one set of
   scaling issues.  RSVP Path and Resv messages must be periodically
   refreshed to maintain state.  The approach described effectively
   reduces the volume of messages which must be periodically sent and
   received, as well as the resources required to process refresh
   messages.

   The described mechanisms also address issues of latency and
   reliability of RSVP Signaling.  The latency and reliability problem
   occurs when a non-refresh RSVP message is lost in transmission.
   Standard RSVP [RFC2205] maintains state via the generation of 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 establishment or
   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 latency and reliability of RSVP Signaling is
   to decrease the refresh period R.  Decreasing the value of R provides
   increased 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.



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   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 adjusting the refresh period
   has no impact on this issues 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.  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 delivery.  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 or when next hops change.

   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 altered the reservation paths, or a reservation
   modification by a downstream router.  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
   the exception of the INTEGRITY object, the flags in the MESSAGE_ID
   object and the RSVP Checksum.  The checksum, the flags and the
   INTEGRITY object are allowed to differ in refresh messages.






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2. 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 aggregated 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.


2.1. 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.

      Flags: 4 bits

         0x01: Bundle capable

           If set, indicates to RSVP neighbors that this node is willing
           and capable of receiving bundle messages.  This bit is
           meaningful only between adjacent RSVP neighbors.

         0x02-0x08: Reserved

      Msg type: 8 bits

         12 = Bundle








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      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.  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 IP TTL that a
         Bundle message sent to the TTL in the 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.


2.2. 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.

   Empty RSVP Bundle messages SHOULD NOT be sent.  A Bundle message MUST
   NOT include another RSVP Bundle message as a sub-message.

       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..                       //
      |                                                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+





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2.3. 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 MAY be sent to RSVP neighbors that support the
   message.  Methods for discovering such information include: (1)
   manual configuration and (2) observing the Bundle-capable bit (see
   the description that follows) in the received RSVP messages.  RSVP
   Bundle messages MUST not be used if the RSVP neighbor does not
   support RSVP Bundle messages.  If the RSVP neighbor is not known or
   changes in next hops cannot be identified via routing, Bundle
   messages MUST NOT be sent.  Note that when the routing next hop is
   not RSVP capable it will typically not be possible to identify
   changes in next hop.

   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.  If it
   exceeds the MTU, the datagram is fragmented by IP and reassembled at
   the recipient node.  A single RSVP Bundle message MUST NOT exceed the
   maximum IP datagram size, which is approximately 64K bytes.
   Implementations may choose to limit each RSVP Bundle message to the
   MTU size of the outgoing link, e.g. 1500 bytes.

   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 know 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 is
   not Bundle capable.





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2.4. Receiving RSVP Bundle Messages

   If the local system does not recognize or does not wish to accept an
   Bundle message, the received messages shall be discarded without
   further analysis.

   The receiver next compares the IP TTL with which a Bundle message is
   sent to the 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.  Each sub-message is processed as if it was received
   individually.


2.5. Forwarding RSVP Bundle Messages

   When an RSVP router receives a Bundle messages which is not addressed
   to one of it's IP addresses, it SHALL forward the message.  Non-RSVP
   routers will treat RSVP Bundle messages as any other IP datagram.


2.6. Bundle-Capable Bit


   To support message bundling, 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           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+










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      Flags: 4 bits

         0x01: Bundle capable

           When set, indicates to that this node is willing and capable
           of receiving Bundle messages.  This bit is meaningful only
           between RSVP neighbors.


3. 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
   support acknowledgments and reliable RSVP message delivery.  The last
   object is used to support the summary refresh extension described in
   Section 4.  The MESSAGE_ID object can also be used to simply provide
   a shorthand indication of when a message represents new state.  Such
   information can be used on the receiving node to reduce refresh
   processing requirements.

   Message identification and acknowledgment is done on a hop-by-hop
   basis.  Acknowledgment is handled independent of SESSION or message
   type.  Both types of MESSAGE_ID objects contain a message identifier.
   The identifier MUST be unique on a per source IP address basis across
   messages sent by an RSVP node and received by a particular node.  No
   more than one MESSAGE_ID object may be included in an RSVP message.
   Each message containing an MESSAGE_ID object may be acknowledged via
   a MESSAGE_ID_ACK object.  MESSAGE_ID_ACK and MESSAGE_ID_NACK objects
   may be sent piggy-backed in unrelated RSVP messages or in RSVP Ack
   messages.

   All three object types may be included in a bundle sub-message.  When
   included, the object is treated as if it were contained in a
   standard, non-bundled, RSVP message.  When present, 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.






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   The ordering of the ACK objects are:
   <Common Header>  [ <INTEGRITY> ]
                    [ <MESSAGE_ID_ACK> | <MESSAGE_ID_NACK> ... ]
                    [ <MESSAGE_ID> ]


3.1. MESSAGE_ID Objects


   MESSAGE_ID Class = 166 (Value to be assigned by IANA of form
   10bbbbbb)

   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

            0x80 = Summary_Capable flag

             Indicates that the sender supports the summary refresh
             extension.  This flag MUST be set if the node supports the
             summary refresh extension.  See Section 4.5 for description
             of handling by receiver.

            0x40 = 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.
         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.  This



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         field is ordered and only decreases in value when the Epoch
         changes or the value wraps.


3.2. MESSAGE_ID_ACK and MESSAGE_ID_NACK Objects

   MESSAGE_ID_ACK Class = 167 (Value to be assigned by IANA of form
   10bbbbbb)

   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

            0x80 = Summary_Capable flag

             Indicates that the sender supports the summary refresh
             extension.  This flag MUST be set if the node supports the
             summary refresh extension.  See Section 4.5 for description
             of handling by receiver.

         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 same as MESSAGE_ID_ACK object.







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3.3. 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 hop-by-hop between 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 and Bundle
   messages, 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.

   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 (This is a suggested value, the permanent value is to be
   assigned by IANA).


3.4. MESSAGE_ID Object Usage

   The MESSAGE_ID object may be included in any RSVP message other than
   the Ack message.  The MESSAGE_ID object is always generated and
   processed hop-by-hop.  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 and
   Bundle messages, the generator's IP address is the source address in
   the IP header.  Note that MESSAGE_ID objects can be used in both a
   Bundle message and its sub-messages.  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.  This information is used by the receiver
   to detect out of order messages.  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.



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   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.  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 previously used value.  A value is considered to have been used
   when it has been sent in any message using the associated IP address.
   Note that this 32-bit value MAY wrap.

   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 error and
   confirm messages and to support fast refreshes 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 SHOULD be based on
   well known exponential back-off procedures.  See Section 5 for
   details on one exponential back-off retransmission approach.  Note
   that nodes setting the ACK_Desired flag for unicast sessions, do not
   need to track the identify of the next hop since all that is expected
   is an ACK, not an ACK from a specific next hop.  Issues relate to
   multicast sessions are covered in a later section.  The ACK_Desired
   flag will typically be set only in trigger messages.  The ACK_Desired
   flag MAY be set in refresh messages.

   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.



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   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
   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 processes 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.


3.5. 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 received MESSAGE_ID_NACK
   objects is described in further detail in Section 4.4.

   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.









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3.6. 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 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 back-off time does not result in
   retransmission.

   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 usually 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.


3.6.1. 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



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   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.


3.7. Compatibility

   There are no backward compatibility issues raised by the MESSAGE_ID
   Class.  The MESSAGE_ID Class has an assigned value whose form is
   10bbbbbb.  Per RSVP [RFC2205], classes with values of this form must
   be ignored and not forwarded by nodes not supporting the class.  When
   the receiver of a MESSAGE_ID object does not support the class, the
   object will be silently ignored.  The generator of the MESSAGE_ID
   object will not see any acknowledgments and therefore refresh
   messages per standard RSVP.  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.


4. 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



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   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 may only 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.

   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.

   Nodes supporting the described extension can advertise their support
   and detect if an RSVP neighbor also supports the extension.  This is
   accomplished via a flag in the MESSAGE_ID and MESSAGE_ID_ACK objects.











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4.1. MESSAGE_ID LIST, SRC_LIST and MCAST_LIST Objects


   MESSAGE_ID LIST object

   MESSAGE_ID_ACK Class = TBD (Value to be assigned by IANA of form
   0bbbbbbb)

      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.  A variable number of Message_Identifiers may
         be included.











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   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                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      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.  A variable number of
         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.





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      Source_IP_Address: 32 bits

         The IP address corresponding to the sender of the Path state
         being refreshed.

   MESSAGE_ID MCAST_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                    |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             Multicast_                        |
      |                        Message_Identifier_                    |
      |                               Tuple                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                                 :                             |
      //                                :                            //
      |                                 :                             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             Multicast_                        |
      |                        Message_Identifier_                    |
      |                               Tuple                           |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Where a Multicast_Message_Identifier_Tuple consists of:

      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                        Message_Identifier                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                         Source_IP_Address                     |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                      Destination_IP_Address                   |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      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.





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      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.  A variable number of
         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: 32 bits

         The IP address corresponding to the sender of the Path state
         being refreshed.

      Destination_IP_Address: 32 bits

         The destination IP address corresponding to the session of the
         Path state being refreshed.


4.2. 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 destination, 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



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   node that generates the message.  The source IP address MUST match
   the addressed associate with the MESSAGE_ID objects when they were
   included in a standard RSVP message.  As previously mentioned, the
   source address associate 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 and Bundle
   messages, 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 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
   aggregate messages.  Although this is not expected since Srefresh
   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 list> | <source message_id list>

   <message_id list> ::= <MESSAGE_ID LIST> | <MESSAGE_ID MCAST_LIST>
                         [ <message_id list> ]

   <source message_id list> ::= <MESSAGE_ID SRC_LIST>
                                [ <MESSAGE_ID SRC_LIST> ... ]

   For Srefresh messages, the Msg Type field of the Common Header MUST
   be set to 15 (This is a suggested value, the permanent value is to be
   assigned by IANA).


4.3. 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 SHOULD be suppressed.  A
   state's refresh interval is not affected by the use of Srefresh
   message based refreshes.  An Srefresh message MUST NOT be used in
   place of a trigger Path or Resv message, i.e., one that would
   advertise a state change.



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   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 4.2,
   may be included in the same Srefresh message.  Identifying Resv state
   that can refreshed using the same Srefresh message is fairly
   straightforward.  Identifying which Path state may be included is a
   little more complex.

   Independent of the state being refreshed, 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.

   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.

   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.
   When using this method, the destination address in the IP header of
   the Srefresh message is always the next hop's address.  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



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   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 an
   MESSAGE_ID LIST object that has a matching Epoch value.  If no
   matching object exists, then a new MESSAGE_ID LIST object is created.
   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 an 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 an 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



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   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.


4.4. Srefresh NACK

   Srefresh NACKs are used to indicated 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 3.5.

   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.


4.5. Compatibility

   Nodes supporting the summary refresh extension advertise their
   support via the Summary_Capable flag in all MESSAGE_ID and
   MESSAGE_ID_ACK objects transmitted by the node.  Support is also
   implied when a node transmits an Srefresh Message.  This enables
   supporting nodes 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



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   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.

   Nodes supporting the summary refresh extension must also take care to
   recognize when a next hop stops sending MESSAGE_ID and MESSAGE_ID_ACK
   objects with the Summary_Capable flag set.  To cover this case, nodes
   supporting the summary refresh extension MUST examine each
   Summary_Capable flag received in a MESSAGE_ID or MESSAGE_ID_ACK
   object.  Summary_Capable flags received in MESSAGE_ID_NACK objects
   SHOULD be ignored.  If the flag changes from indicating support to
   indicating non-support then Srefresh messages MUST NOT be used for
   subsequent state refreshes to that neighbor.


5. Reference Exponential Back-Off Procedures

   This section is based on [Pan] and provides an example of how to
   implement exponential back-off.  Implementations MAY choose to use
   the described procedures.


5.1. Outline of Operation

   The following is one possible feedback mechanism for exponential
   back-off retransmission of an RSVP message: When sending such a
   message, a node inserts a MESSAGE_ID object with the ACK_Desired flag
   set.  Upon reception, a receiving node acknowledges the arrival of
   the message by sending back an message acknowledgment (that is, a
   corresponding MESSAGE_ID_ACK object.)  When the sending node receives
   this message acknowledgment for a Path or Resv message, it will
   automatically scale back the retransmission rate for these messages
   for the flow.  If the trigger message was for a different message
   type, no other further action is required.

   Until the message acknowledgment is received, the sending node will
   retransmit the message.  The interval between retransmissions is
   governed by a rapid retransmission timer.  The rapid retransmission
   timer starts at a small interval which increases exponentially until
   it reaches a threshold.  From that point on, the sending node will
   use a fixed timer to refresh Path and Resv messages and stop re-
   transmitting other messages.  This mechanism is designed so that the
   message load is only slightly larger than in the current
   specification even when the receiving node does not support message
   acknowledgment.



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5.2. 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.  Unless a node
           knows that all receiving nodes support echo-replies, a slightly
           larger configurable value is suggested.


      Slow refresh interval Rs:

           The sending node retransmits Path and Resv messages with this
           interval after it has determined that the receiving node will
           generate MESSAGE_ID_ACK objects.  To reduce the number of
           unnecessary retransmissions in a stable network, Rs can be set
           to a large value.  The value of Rs should be configurable per
           each egress interface.

      Fixed retransmission interval R:

           A node retransmits the trigger message with the interval Rf*(1 +
           Delta)**i until the retransmission interval reaches the fixed
           retransmission interval R or a message acknowledgment has been
           received.  If no acknowledgment has been received, the node
           continues to retransmit Resv and Path messages every R seconds.
           By default R should be the same value as the retransmission
           interval in the current RSVP specification.

      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).











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5.3. Example 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
   retransmission interval has been increased to R.  Once the
   retransmission interval has been increased to R, Path and Resv
   messages will be refreshed within the interval R.  Other messages
   will not be retransmitted.

   The implementation of exponential back-off retransmission is simple.
   A sending node can use the following algorithm after transmitting a
   message containing a MESSAGE_ID object with the ACK_Desired flag set:

       On initial transmission initialize: Rk = Rf

       if (Rk < R)  {
           retransmit the message;
           wake up after Rk seconds;
           Rk = Rk * (1 + Delta);
           exit;
       }
       else  {    /* no reply from receivers for too long: */
           Rk = R;
           if (message is a Path or Resv Message) {
               send out a refresh message;
               wake up after Rk seconds;
               exit;
           }
           else  {
               clean up any associated state and resources;
               exit;
           }
       }

   Asynchronously, when a sending node receives a corresponding
   MESSAGE_ID_ACK object, it will change the retransmission interval Rk
   to Rs and, for non Path or Resv messages, clean up any associated
   state and resources.  Note that the transmitting node does not
   advertise the use of the described exponential back-off procedures
   via the TIME_VALUE object.






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6. 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 Fred Baker, Bob Braden, Roch Guerin, David Mankins, Henning
   Schulzrinne, Andreas Terzis and Masanobu Yuhara for specific feedback
   on the document.

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


7. Security Considerations

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

8. References

[Pan]     Pan, P., Schulzrinne, H., "Staged Refresh Timers for RSVP,"
          Global Internet'97, Phoenix, AZ, November 1997.
          http://www.ctr.columbia.edu/~pan/papers/timergi.ps

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
          Requirement Levels," RFC 2119.

[RFC2205] Braden, R. Ed. et al, "Resource ReserVation Protocol
           -- Version 1 Functional Specification", RFC 2205,
           September 1997.

[Yuhara]  Yuhara, M., Tomikawa, M. "RSVP Extensions for ID-based
          Refreshes,"  Internet Draft,
          draft-yuhara-rsvp-refresh-00.txt, April 1999.


















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9. Authors' Addresses

   Lou Berger
   LabN Consulting, LLC
   Voice:  +1 301 468 9228
   Email:  lberger@labn.net

   Der-Hwa Gan
   Juniper Networks, Inc.
   385 Ravendale Drive
   Mountain View, CA 94043
   Voice:  +1 650 526 8074
   Email:  dhg@juniper.net

   George Swallow
   Cisco Systems, Inc.
   250 Apollo Drive
   Chelmsford, MA 01824
   Voice:  +1 978 244 8143
   Email:  swallow@cisco.com

   Ping Pan
   Bell Labs, Lucent
   101 Crawfords Corner Road, Room 4C-508
   Holmdel, NJ 07733
   USA
   Phone:  +1 732 332 6744
   Email:  pingpan@dnrc.bell-labs.com























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