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Versions: 00 01 02 03 04 05 06 07 08 09 10 11 12 13 RFC 4724

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.

   In the following sections, "Restarting Speaker" refers to a router
   whose BGP has restarted, and "Receiving Speaker" refers to a router
   that peers with the restarting speaker.

   Consider that the Graceful Restart Capability for an address family
   is advertised by the Restarting Speaker, and is understood by the
   Receiving Speaker, and a BGP session between them is established.
   The following sections detail the procedures that shall be followed
   by the Restarting Speaker as well as the Receiving Speaker once the
   Restarting Speaker restarts.


6.1. Procedures for the Restarting Speaker

   When the Restarting Speaker restarts, if possible it shall retain the
   forwarding state for the BGP routes in the Loc-RIB, and shall mark
   them as stale.  It should not differentiate between stale and other
   information during forwarding.

   To re-establish the session with its peer, the Restarting Speaker
   must set the "Restart State" bit in the Graceful Restart Capability
   of the OPEN message.  Unless allowed via configuration, the
   "Forwarding State" bit for an address family in the capability can be
   set only if the forwarding state has indeed been preserved for that



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   address family during the restart.

   Once the session between the Restarting Speaker and the Receiving
   Speaker is re-established, the Restarting Speaker will receive and
   process BGP messages from its peers. However, it shall defer route
   selection for an address family until it receives the End-of-RIB
   marker from all its peers (excluding the ones with the "Restart
   State" bit set in the received capability and excluding the ones
   which do not advertise the graceful restart capability).  It is noted
   that prior to route selection, the speaker has no routes to advertise
   to its peers and no routes to update the forwarding state.

   In situations where both IGP and BGP have restarted, it might be
   advantageous to wait for IGP to converge before the BGP speaker
   performs route selection.

   After the BGP speaker performs route selection, the forwarding state
   of the speaker shall be updated and any previously marked stale
   information shall be removed. The Adj-RIB-Out can then be advertised
   to its peers. Once the initial update is complete for an address
   family (including the case that there is no routing update to send),
   the End-of-RIB marker shall be sent.

   To put an upper bound on the amount of time a router defers its route
   selection, an implementation must support a (configurable) timer that
   imposes this upper bound.


6.2. Procedures for the Receiving Speaker

   When the Restarting Speaker restarts, the Receiving Speaker may or
   may not detect the termination of the TCP session with the Restarting
   Speaker, depending on the underlying TCP implementation, whether or
   not [BGP-AUTH] is in use, and the specific circumstances of the
   restart.  In case it does not detect the TCP reset and still
   considers the BGP session as being established, it shall treat the
   subsequent open connection from the peer as an indication of TCP
   reset and act accordingly (when the Graceful Restart Capabilty has
   been received from the peer).

   When the Receiving Speaker detects TCP reset for a BGP session with a
   peer that has advertised the Graceful Restart Capability, it shall
   retain the routes received from the peer for all the address families
   that were previously received in the Graceful Restart Capability, and
   shall mark them as stale routing information. To deal with possible
   consecutive restarts, a route (from the peer) previously marked as
   stale shall be deleted. The router should not differentiate between
   stale and other routing information during forwarding.



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   In re-establishing the session, the "Restart State" bit in the
   Graceful Restart Capability of the OPEN message sent by the Receiving
   Speaker shall not be set unless the Receiving Speaker has restarted.
   The presence and the setting of the "Forwarding State" bit for an
   address family depends upon the actual forwarding state and
   configuration.

   If the session does not get re-established within the "Restart Time"
   that the peer advertised previously, the Receiving Speaker shall
   delete all the stale routes from the peer that it is retaining.

   Once the session is re-established, if the "Forwarding State" bit for
   an address family is not set in the received Graceful Restart
   Capability, or if the capability is not received for an address
   family, the Receiving Speaker shall immediately remove all the stale
   routes from the peer that it is retaining for that address family.

   The Receiving Speaker shall send the End-of-RIB marker once it
   completes the initial update for an address family (including the
   case that it has no routes to send) to the peer.

   The Receiving Speaker shall replace the stale routes by the routing
   updates received from the peer. Once the End-of-RIB marker for an
   address family is received from the peer, it shall immediately remove
   any routes from the peer that are still marked as stale for that
   address family.

   To put an upper bound on the amount of time a router retains the
   stale routes, an implementation may support a (configurable) timer
   that imposes this upper bound.


7. Deployment Considerations

   While the procedures described in this document would help minimize
   the effect of routing flaps, it is noted, however, that when a BGP
   Graceful-Restart capable router restarts, there is a potential for
   transient routing loops or blackholes in the network if routing
   information changes before the involved routers complete routing
   updates and convergence. Also, depending on the network topology, if
   not all IBGP speakers are Graceful-Restart capable, there could be an
   increased exposure to transient routing loops or blackholes when the
   Graceful-Restart procedures are exercised.

   The Restart Time, the upper bound for retaining routes and the upper
   bound for deferring route selection may need to be tuned as more
   deployment experience is gained.




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   Finally, it is noted that there is little benefit deploying BGP
   Graceful-Restart in an AS whose IGPs and BGP are tightly coupled
   (i.e., BGP and IGPs would both restart), and IGPs have no similar
   Graceful-Restart capability.


8. Security Considerations

   Since with this proposal a new connection can cause an old one to be
   terminated, it might seem to open the door to denial of service
   attacks.  However, it is noted that unauthenticated BGP is already
   known to be vulnerable to denials of service through attacks on the
   TCP transport.  The TCP transport is commonly protected through use
   of [BGP-AUTH]. Such authentication will equally protect against
   denials of service through spurious new connections.

   It is thus concluded that this proposal does not change the
   underlying security model (and issues) of BGP-4.


9. Acknowledgments

   The authors would like to thank Alvaro Retana, Satinder Singh, David
   Ward, Naiming Shen and Bruce Cole for their review and comments.


10. References

   [BGP-4]   Rekhter, Y., and T. Li, "A Border Gateway Protocol 4 (BGP-
   4)", RFC 1771, March 1995.

   [BGP-MP] Bates, T., Chandra, R., Katz, D., and Rekhter, Y.,
   "Multiprotocol Extensions for BGP-4", RFC 2283, March 1998.

   [BGP-CAP] Chandra, R., Scudder, J., "Capabilities Advertisement with
   BGP-4", RFC 2842, May 2000.

   [BGP-AUTH] Heffernan A., "Protection of BGP Sessions via the TCP MD5
   Signature Option", RFC 2385, August 1998.












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11. Author Information

   Srihari R. Sangli
   Procket Networks, Inc.
   1100 Cadillac Court
   Milpitas, CA 95035
   e-mail: srihari@procket.com

   Yakov Rekhter
   Juniper Networks, Inc.
   1194 N. Mathilda Avenue
   Sunnyvale, CA 94089
   e-mail: yakov@juniper.net

   Rex Fernando
   Procket Networks, Inc.
   1100 Cadillac Court
   Milpitas, CA 95035
   e-mail: rex@procket.com

   John G. Scudder
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134
   e-mail: jgs@cisco.com

   Enke Chen
   Redback Networks, Inc.
   350 Holger Way
   San Jose, CA 95134
   e-mail: enke@redback.com




















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