draft-ietf-idr-restart-03.txt

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Network Working Group            Srihari R. Sangli (Procket Networks)
Internet Draft                       Yakov Rekhter (Juniper Networks)
Expiration Date: October 2002         Rex Fernando (Procket Networks)
                                      John G. Scudder (Cisco Systems)
                                         Enke Chen (Redback Networks)


                   Graceful Restart Mechanism for BGP

                     draft-ietf-idr-restart-03.txt


1. 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.


2. Abstract

   This document proposes a mechanism for BGP that would help minimize
   the negative effects on routing caused by BGP restart. An End-of-RIB
   marker is specified and can be used to convey routing convergence
   information.  A new BGP capability, termed "Graceful Restart
   Capability", is defined which would allow a BGP speaker to express
   its ability to preserve forwarding state during BGP restart. Finally,
   procedures are outlined for temporarily retaining routing information
   across a TCP transport reset.







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3. Introduction

   Usually when BGP on a router restarts, all the BGP peers detect that
   the session went down, and then came up. This "down/up" transition
   results in a "routing flap" and causes BGP route re-computation,
   generation of BGP routing updates and flap the forwarding tables. It
   could spread across multiple routing domains. Such routing flaps may
   create transient forwarding blackholes and/or transient forwarding
   loops. They also consume resources on the control plane of the
   routers affected by the flap. As such they are detrimental to the
   overall network performance.

   This document proposes a mechanism for BGP that would help minimize
   the negative effects on routing caused by BGP restart. An End-of-RIB
   marker is specified and can be used to convey routing convergence
   information.  A new BGP capability, termed "Graceful Restart
   Capability", is defined which would allow a BGP speaker to express
   its ability to preserve forwarding state during BGP restart. Finally,
   procedures are outlined for temporarily retaining routing information
   across a TCP transport reset.


4. Marker for End-of-RIB

   An UPDATE message with empty withdrawn NLRI is specified as the End-
   Of-RIB Marker that can be used by a BGP speaker to indicate to its
   peer the completion of the initial routing update after the session
   is established. For IPv4 unicast address family, the End-Of-RIB
   Marker is an UPDATE message with the minimum length [BGP-4].  For any
   other address family, it is an UPDATE message that contains only
   MP_UNREACH_NLRI [BGP-MP] with no withdrawn routes for that <AFI, Sub-
   AFI>.

   Although the End-of-RIB Marker is specified for the purpose of BGP
   graceful restart, it is noted that the generation of such a marker
   upon completion of the initial update would be useful for routing
   convergence in general, and thus the practice is recommended.

   In addition, it would be beneficial for routing convergence if a BGP
   speaker can indicate to its peer up-front that it will generate the
   End-Of-RIB marker, regardless of its ability to preserve its
   forwarding state during BGP restart. This can be accomplished using
   the Graceful Restart Capability described in the next section.








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5. Graceful Restart Capability

   The Graceful Restart Capability is a new BGP capability [BGP-CAP]
   that can be used by a BGP speaker to indicate its ability to preserve
   its forwarding state during BGP restart. It can also be used to
   convey to its peer its intention of generating the End-Of-RIB marker
   upon the completion of its initial routing updates.

   This capability is defined as follows:

       Capability code: 64

       Capability length: variable

       Capability value: Consists of the "Restart Flags" field,
       "Restart Time" field, and zero or more of the tuples <AFI,
       Sub-AFI, Flags for address family> as follows.


           +--------------------------------------------------+
           | Restart Flags (4 bits)                           |
           +--------------------------------------------------+
           | Restart Time in seconds (12 bits)                |
           +--------------------------------------------------+
           | Address Family Identifier (16 bits)              |
           +--------------------------------------------------+
           | Subsequent Address Family Identifier (8 bits)    |
           +--------------------------------------------------+
           | Flags for Address Family (8 bits)                |
           +--------------------------------------------------+
           | ...                                              |
           +--------------------------------------------------+
           | Address Family Identifier (16 bits)              |
           +--------------------------------------------------+
           | Subsequent Address Family Identifier (8 bits)    |
           +--------------------------------------------------+
           | Flags for Address Family (8 bits)                |
           +--------------------------------------------------+


   The use and meaning of the fields are as follows:

     Restart Flags:

       This field contains bit flags related to restart.

       The most significant bit is defined as the Restart State bit
       which can be used to avoid possible deadlock caused by waiting



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       for the End-of-RIB marker when multiple BGP speakers peering
       with each other restart. When set (value 1), this bit indicates
       that the BGP speaker has restarted, and its peer should not wait
       for the End-of-RIB marker from the speaker before advertising
       routing information to the speaker.

       The remaining bits are reserved.

     Restart Time:

       This is the estimated time (in seconds) it will take for the BGP
       session to be re-established after a restart. This can be used to
       speed up routing convergence by its peer in case that the BGP
       speaker does not come back after a restart.

     Address Family Identifier (AFI):

       This field carries the identity of the Network Layer protocol
       for which the Graceful Restart support is advertised. Presently
       defined values for this field are specified in RFC1700 (see
       the Address Family Numbers section).

     Subsequent Address Family Identifier (Sub-AFI):

       This field provides additional information about the type of
       the Network Layer Reachability Information carried in the
       attribute.

     Flags for Address Family:

       This field contains bit flags for the <AFI, Sub-AFI>.

       The most significant bit is defined as the Forwarding State
       bit which can be used to indicate if the forwarding state for
       the <AFI, Sub-AFI> has indeed been preserved during the previous
       BGP restart. When set (value 1), the bit indicates that the
       forwarding state has been preserved.

       The remaining bits are reserved.


   The advertisement of this capability by a BGP speaker also implies
   that it will generate the End-of-RIB marker (for all address families
   exchanged) upon completion of its initial routing update to its peer.
   The value of the "Restart Time" field is irrelevant in the case that
   the capability does not carry any <AFI, Sub-AFI>.





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6. Operation

   A BGP speaker may advertise the Graceful Restart Capability for an
   address family to its peer only if it has the ability to preserve its
   forwarding state for the address family when BGP restarts.

   Even if the speaker does not have the ability to preserve its
   forwarding state for any address family during BGP restart, it is
   still recommended that the speaker advertise the Graceful Restart
   Capability to its peer to indicate its intention of generating the
   End-of-RIB marker upon the completion of its initial routing updates.

   The End-of-RIB marker should be sent by a BGP speaker to its peer
   once it completes the initial routing update (including the case when
   there is no update to send) for an address family after the BGP
   session is established.

   It is noted that the normal BGP procedures MUST be followed when the
   TCP session terminates due to the sending or receiving of a BGP
   NOTIFICATION message.

   In general the Restart Time SHOULD NOT be greater than the HOLDTIME
   carried in the OPEN.