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  Network Working Group                                    B. Decraene
  Internet Draft                                        France Telecom
  Category: Informational                                  P. Francois
                                                                   UCL
  Expiration Date: August 2008                              C. Pelsser
                                                                   NTT
                                                              Z. Ahmad
                                              Orange Business Services
                                               A. J. Elizondo Armengol
                                                        Telefonica I+D
                                                         February 2008

         Requirements for the graceful shutdown of BGP sessions

        draft-decraene-bgp-graceful-shutdown-requirements-00.txt


Status of this Memo

   By submitting this Internet-Draft, each author represents that any
   applicable patent or other IPR claims of which he or she is aware
   have been or will be disclosed, and any of which he or she becomes
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   Internet-Drafts are working documents of the Internet Engineering
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   http://www.ietf.org/ietf/1id-abstracts.txt

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   http://www.ietf.org/shadow.html.

Abstract

   The BGP protocol is heavily used in Service Provider networks both
   for Internet and BGP/MPLS VPN services. For resiliency purposes,
   redundant routers and BGP sessions can be deployed to reduce the
   consequences of an AS Border Router or BGP session breakdown on
   customers' or peers' traffic. However simply taking down a BGP
   session for maintenance purposes may still induce connectivity losses
   during the BGP convergence. This is no more satisfactory for new
   applications (e.g. voice over IP, on line gaming, VPN). Therefore, a
   solution is required for the graceful shutdown of a (set of) BGP

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   session(s) in order to limit the amount of traffic loss during a
   planned shutdown. This document expresses requirements for such a
   solution.


Table of Contents

   1.    Conventions used in this document...........................3
   2.    Introduction................................................3
   3.    Problem statement...........................................3
   3.1.  Example of undesirable BGP routing behavior.................4
   3.2.  Causes of packet loss.......................................5
   4.    Terminology.................................................5
   5.    Goals and requirements......................................6
   6.    Reference Topologies........................................7
   6.1.  E-BGP topologies............................................7
   6.1.1. 1 ASBR in AS1 connected to two ASBRs in the neighboring AS2.7
   6.1.2. 2 ASBRs in AS1 connected to 2 ASBRs in AS2..................8
   6.1.3. 2 ASBRs in AS2 each connected to two different ASes.........8
   6.2.  I-BGP topologies............................................9
   6.2.1. iBGP Full-Mesh..............................................9
   6.2.2. Route Reflector............................................10
   6.2.3. hierarchical Route Reflector...............................10
   6.2.4. Confederations.............................................11
   7.    Security Considerations....................................12
   8.    IANA Considerations........................................12
   9.    References.................................................13
   10.   Acknowledgments............................................13
   11.   Author's Addresses.........................................13























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1. Conventions used in this document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119.

2. Introduction

   The BGP protocol is heavily used in Service Provider networks both
   for Internet and BGP/MPLS VPN services. For resiliency purposes,
   redundant routers and BGP sessions can be deployed to reduce the
   consequences of an AS Border Router or BGP session breakdown on
   customers' or peers' traffic.

   We place ourselves in the context where a Service Provider needs to
   shut down one or multiple BGP peering link(s) or a whole ASBR. If an
   alternate path is available, the requirement is to avoid or reduce
   customer or peer traffic loss during the BGP convergence. Indeed, as
   an alternate path is available in the Autonomous System (AS), it
   should be made possible to reroute the customer or peer traffic on
   the backup path before the BGP session(s) is/are torn down and the
   forwarding is interrupted on the nominal path.

   Currently, the [BGP] and [MP-BGP] do not include any operation to
   reduce or prevent traffic loss in case of planned maintenance
   requiring the shutdown of a forwarding resource. When a BGP session
   is taken down, BGP behaves as if it was a sudden link or router
   failure. Besides, the introduction of Route Reflectors as per [BGP
   RR] to solve scalability issues bound to iBGP full-meshes has
   worsened the duration of routing convergence: some route reflectors
   may hide the back up path and depending on RR topology more iBGP hops
   may be involved in the iBGP convergence. On the other hand, some
   protocols are already considering such graceful shutdown procedure
   (e.g. [GMPLS G-Shut]).

   Note that these planned maintenance operations cannot be addressed by
   Graceful Restart extensions [BGP GR] as GR only applies when the
   forwarding is preserved during the control plane restart. On the
   contrary, Graceful Shutdown applies when the forwarding is
   interrupted.

   A successful approach of such mechanism should minimize the loss of
   traffic in most foreseen maintenance situations.

3. Problem statement

   As per [BGP], when one (or many) BGP session(s) are shut down to
   perform a link or router maintenance operation, a BGP NOTIFICATION
   message is sent to the peer and the session is then closed. A
   protocol convergence is then triggered both by the local router and

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   by the peer. Alternate paths to the destination are selected, if
   known. If those alternates paths are not known prior to the BGP
   session shutdown, additional BGP convergence steps are required in
   each AS to search for an alternate path.

   This behavior is not satisfactory in a maintenance situation because
   the traffic that was directed towards the removed next-hops may be
   lost until the end of the BGP convergence. As it is a planned
   operation, a make before break solution should be made possible.

   As maintenance operations are frequent in large networks
   [Reliability], the global availability of the network is
   significantly impaired by this BGP maintenance issue.

3.1. Example of undesirable BGP routing behavior

   To illustrate these problems, let us consider the following example
   where one customer router "CUST" is dual-attached to two SP routers
   "ASBR1" and "ASBR2".
   ASBR1 and ASBR2 are in the same AS and owned by the same service
   provider. Both are iBGP client of the route reflector R1.

                                '
                          AS1   '      AS2
                                '

                          /-----------ASBR1---
                         /                     \
                        /                       \
                    CUST                         R1
                        \                       /
                 Z/z     \                     /
                          \-----------ASBR2---

                                '
                          AS1   '      AS2
                                '

   Before the maintenance, packets for destination Z/z use the CUST-
   ASBR1 link because R1 selects ASBR1's route based on the IGP cost.

   Let's assume the service provider wants to shutdown the ASBR1-CUST
   link for maintenance purposes. Currently, when the shutdown is
   performed on ASBR1, the following steps are performed:
     1.  ASBR1 sends a withdraw to its route reflector R1 for the prefix
        Z/z.
     2. R1 runs its decision process, selects the route from ASBR2 and
        advertises the new path to ASBR1.
     3. ASBR1 runs its decision process and recovers the reachability of
        Z/z.


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   Traffic is lost between step 1 when ASBR1 looses its route and step 3
   when it discovers a new path.

   Note that this is a simplified description for illustrative purpose.
   In a bigger AS, multiple steps of BGP convergence may be required to
   find and select the best alternate path (e.g. ASBR1 is chosen based
   on a higher local pref, hierarchical route reflectors are used...).
   When multiple BGP routers are involved and plenty of prefixes are
   affected, the recovery process can take longer than applications
   requirements.

3.2. Causes of packet loss

   The loss of packets during the maintenance has two main causes:
   - lack of an alternate path on some routers
   - transient routing inconsistency.

   Some routers may lack an alternate path because another router is
   hiding the backup path. This router can be a route reflector only
   propagating the best path. Or the backup ASBR does not advertise the
   backup path because it prefers the nominal path. This lack of
   knowledge of the alternate path is the first target of this
   requirement draft.

   Transient routing inconsistencies happen during iBGP convergence
   because all routers are not updating their RIB at the same time. This
   can lead to forwarding loops and then packet drops. This can be
   avoided by performing only one IP lookup on BGP routes in each AS and
   by using tunnels (e.g. MPLS LSP) to send packets between ASBRs.

4. Terminology

   g-shut initiator: the router on which the session(s) shutdown is
   (are) performed for the maintenance.

   g-shut neighbor: a router that peers with the g-shut initiator
   via (one of) the session(s) undergoing maintenance.

   Affected prefixes: a prefix initially reached via the peering
   link(s) undergoing maintenance.

   Affected router: a router reaching an affected prefix via a
   peering link undergoing maintenance.

   Initiator AS: the autonomous system of the g-shut initiator
   router.

   Neighbor AS(es): the autonomous system(s) of the g-shut neighbor
   router(s).



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5. Goals and requirements

   When a BGP session of the router under maintenance is shut down, the
   router removes the routes and then triggers the BGP convergence on
   its BGP peers. The goal of BGP graceful shutdown is to initiate the
   BGP convergence to find the alternate paths before the nominal paths
   are removed. As a result, before the nominal BGP session is shut
   down, all routers learn and use the alternate paths. Then the nominal
   BGP session can be shut down.

   As a result, provided an alternate path is available in the AS, the
   packets are rerouted before the BGP session termination and fewer
   packets (possibly none) are lost during the BGP convergence process
   since at any time, all routers have a valid path.

   From the above goals we can derive the following requirements:

   a)   A mechanism to advertise the maintenance action to all affected
   routers is REQUIRED. Such mechanism may be either implicit or
   explicit. Note that affected routers can be located both in the local
   AS and in neighboring ASes.

   b)   An Internet wide convergence is OPTIONAL. However if the
   initiator AS and the neighbor AS(es) have a backup path, they MUST be
   able to gracefully converge before the nominal path is shut down.

   c)   The proposed solution SHOULD be applicable to any kind of BGP
   sessions (e-BGP, i-BGP, i-BGP route reflector client, e-BGP
   confederations, e-BGP multi hop, multiProtocol BGP extension...) and
   any address family. If a BGP implementation allows closing a sub-set
   of AFIs carried in a MP-BGP session, this mechanism MAY be applicable
   to this sub-set of AFIs.

   Depending on the session type (eBGP, iBGP...), there may be some
   variations in the proposed solution in order to fit the requirements.

   The following cases should be handled in priority:
   - The shutdown of an inter-AS link and therefore the shutdown of an
   eBGP session.
   - The shutdown of an AS Border Router and therefore the shutdown of
   all its BGP sessions
   - The shutdown of a customer access router and all of its BGP
   sessions. In VPN as per [VPN], this router is called a CE and the use
   of others protocols than BGP on the PE-CE access link should also be
   considered (static routes, RIPv2, OSPF, IS-IS...).

   d)   The proposed solution SHOULD not change the BGP convergence
   behavior for the ASes exterior to the maintenance process. An
   incremental deployment on a per AS or per BGP session basis MUST be
   made possible. In case of partial deployment the proposed solution
   SHOULD incrementally improve the maintenance process. The solution

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   SHOULD bring improvements even when one of the two ASes does not
   support graceful shutdown. In particular, large Service Providers may
   not be able to upgrade all of the deployed customer premises access
   routers (CPE).

   e)   Redistribution or advertisement of (static) IP routes into BGP
   SHOULD also be covered.

   f)   The proposed solution MAY be designed in order to avoid
   transient forwarding loops. Indeed, forwarding loops increase packet
   transit delay and may lead to link saturation.

   g)   The specific procedure SHOULD end when the BGP session is
   closed. The procedure SHOULD be reverted, either automatically or
   manually, when the session is re-established. During this reversion
   procedure, it SHOULD be ensured that the backup path is not removed
   from the routing tables of the effected nodes before it learns the
   nominal path. In the end, once the planned maintenance is finished
   and the shutdown resource becomes available again, the nominal BGP
   routing MUST be reestablished.


   The metrics to evaluate and compare the proposed solutions are, in
   decreasing order of importance:
   - The duration of the remaining loss of connectivity;
   - The applicability to a wide range of BGP and network topologies,
   especially those described in section 6;
   - The duration of transient forwarding loops;
   - The additional load introduced in BGP (eg BGP messages sent to peer
   routers, peer ASes, the Internet).

6. Reference Topologies

   In order to benchmark the proposed solutions, some typical BGP
   topologies are detailed in this section. The solution drafts
   should state its applicability for each of these possible
   topologies.

   However, solutions SHOULD be applicable to all possible BGP
   topologies and not only to these below examples.

6.1. E-BGP topologies

6.1.1. 1 ASBR in AS1 connected to two ASBRs in the neighboring AS2

   In this topology we have an asymmetric protection scheme between
   AS1 and AS2:
   - On AS2 side, two different routers are used to connect to AS1.
   - On AS1 side, one single router with two BGP sessions is used.



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                   '
             AS1   '      AS2
                   '
             /----------- ASBR2.1
            /      '
           /       '
        ASBR1.1    '
           \       '
            \      '
             \----------- ASBR2.2
                   '
                   '
         AS1       '      AS2
                   '

   The requirements of section 5 should be applicable to:
   - Maintenance of one of the routers of AS2;
   - Maintenance of one link between AS1 and AS2, performed either
     on an AS1 or AS2 router.

   Note that in case of maintenance of the whole router, all its BGP
   session needs to be shutdown.

6.1.2. 2 ASBRs in AS1 connected to 2 ASBRs in AS2

   In this topology we have a symmetric protection scheme between
   AS1 and AS2: on both sides, two different routers are used to
   connect AS1 to AS2.

                       '
                 AS1   '      AS2
                       '
          ASBR1.1----------- ASBR2.1
                       '
                       '
                       '
                       '
                       '
          ASBR1.2----------- ASBR2.2
                       '
             AS1       '      AS2
                       '

   The requirements of section 5 should be applicable to:
   - Maintenance of any of the ASBR routers (in AS1 or AS2);
   - Maintenance of one link between AS1 and AS2 performed either on
     an AS1 or AS2 router.

6.1.3. 2 ASBRs in AS2 each connected to two different ASes




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   In this topology at least three ASes are involved. Depending on
   which routes are exchanged between these ASes, some protection
   for some of the traffic may be possible.

                       '
                 AS1   '      AS2
                       '
          ASBR1.1----------- ASBR2.1
             |         '
             |         '
        '''''|''''''''''
             |         '
             |         '
          ASBR3.1----------- ASBR2.2
                       '
             AS3       '      AS2


   The requirements of section 5 do not translate as easily as in
   the two previous topologies because we do not require propagating
   the maintenance advertisement outside of the two ASes involved in
   an eBGP session.
   For instance if ASBR2.2 requires a maintenance affecting ASBR3.1,
   then ASBR3.1 will be notified. However we do not require for ASBR1.1
   to be notified of the maintenance of the eBGP session between
   ASBR3.1-ASBR2.2.

6.2. I-BGP topologies

   We describe here some frequent i-BGP topologies that SHOULD be
   supported.
   Indeed maintenance of an e-BGP session needs to be propagated
   within the AS so the solution may depend on the specific i-BGP
   topology.

6.2.1. iBGP Full-Mesh
















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   In this topology we have a full mesh of iBGP sessions:

           P1 ------ P2
           | \     / |
           |  \   /  |
           |   \ /   |     AS1
           |   / \   |
           |  /   \  |
         ASBR1.1---ASBR1.2
            \        /
             \      /
        ''''''\''''/''''''''''''
               \  /      AS2
               ASBR2.1

   When the session between ASBR1.1 and ASBR2.1 undergoes
   maintenance, it is required that all i-BGP peers of ASBR1.1
   reroute traffic to ASBR1.2 before the session between ASBR1.1 and
   ASBR2.1 is shut down.



6.2.2. Route Reflector

   In this topology, route reflectors are used to limit the number of
   i-BGP sessions.

           P1 RR----- P2 RR
           | \       / |
           |  \     /  |
           |   \   /   |     AS1
           |    \ /    |
           |    / \    |
           |   /   \   |
           |  /     \  |
         ASBR1.1     ASBR1.2
            \          /
             \        /
        ''''''\''''''/''''''''''''
               \    /
                \  /         AS2
               ASBR2.1

   When the session between ASBR1.1 and ASBR2.1 undergoes
   maintenance, it is required that all BGP routers of AS1 reroute
   traffic to ASBR1.2 before the session between ASBR1.1 and ASBR2.1
   is shut down.

6.2.3. hierarchical Route Reflector

   In this topology, hierarchical route reflectors are used to limit
   the number of i-BGP sessions.

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        P1/hRR --------  P2/hRR
           |               |
           |               |
           |               |   AS1
           |               |
           |               |

         P3/RR           P4/RR
           |               |
           |               |
           |               |   AS1
           |               |
           |               |
         ASBR1.1         ASBR1.2
            \             /
             \           /
        ''''''\'''''''''/''''''''''''
               \       /
                \     /        AS2
                 ASBR2.1

   When the session between ASBR1.1 and ASBR2.1 undergoes
   maintenance, it is required that all BGP routers of AS1 reroute
   traffic to ASBR1.2 before the session between ASBR1.1 and ASBR2.1
   is shut down.

6.2.4. Confederations

   In this topology, a confederation of ASs is used to limit the number
   of i-BGP sessions. Moreover, RRs may be present in the member ASs of
   the confederation.
   Confederations may be run with different sub-options. Regarding the
   IGP, each member AS can run its own IGP or they can all share the
   same IGP. Regarding BGP, local_pref may or may not cross the member
   AS boundaries.
   A solution should support the shutdown of eBGP sessions between
   member-ASs in the confederation in addition to the shutdown of eBGP
   sessions between a member-AS and an AS outside of the confederation.













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        ASBR1C.1 ---------- ASBR1C.2
           |                   |
           |                   |
           |       AS1C        |
           |                   |
           |                   |
        """|"""""""""""""""""""|"""
           |        "          |
         ASBR1A.2   "        ASBR1B.2
           |        "          |
           |        "          |
           |  AS1A  "   AS1B   |             AS1
           |        "          |
           |        "          |
         ASBR1A.1   "         ASBR1B.1
            \       "         /
             \      "        /
        ''''''\'''''''''''''/''''''''''''
               \           /
                \         /                   AS2
                  ASBR2.1


   In the above figure, member-AS AS1A, AS1B, AS1C belong to a
   confederation of ASs in AS1. AS1A and AS1B are connected to AS2.

   In normal operation, for the traffic toward AS2,
   . AS1A sends the traffic directly to AS2 through ASBR1A.1
   . AS1B sends the traffic directly to AS2 through ASBR1B.1
   . AS1C load balances the traffic between AS1A and AS1B

   When the session between ASBR1A.1 and ASBR2.1 undergoes
   maintenance, it is required that all BGP routers of AS1 reroute
   traffic to ASBR1B.1 before the session between ASBR1A.1 and
   ASBR2.1 is shut down.

7. Security Considerations

   Security considerations MUST be addressed by the proposed
   solutions.

   One AS SHOULD not be able to use the graceful shutdown procedure
   to selectively influence routing decision in the peer AS (inbound
   TE) outside the case of the planned maintenance. In the case the
   proposed solution allows this, the peer AS SHOULD have means to
   detect such behavior.

8. IANA Considerations

   This document has no actions for IANA.

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9. References

9.1. Normative References

   [BGP] Y. Rekhter, T. Li,
         "A Border Gateway protocol 4 (BGP)", RFC 4271, January 2006.

   [MP-BGP] T. Bates, R. Chandra, D. Katz, Y. Rekhter
         "Multiprotocol Extensions for BGP-4", RFC 4760 January 2007.

   [BGP RR] T. Bates, E. Chen, R. Chandra
         "BGP Route Reflection: An Alternative to Full Mesh Internal BGP
   (IBGP)", RFC 4456 April 2006.

   [BGP GR] S. Sangli, E. Chen, R. Fernando, J. Scudder, Y. Rekhter
         "Graceful Restart Mechanism for BGP", RFC 4724 January 2007.

   [VPN] E. Rosen, Y. Rekhter
         "BGP/MPLS IP Virtual Private Networks (VPNs)", RFC 4364
   February 2006.

9.2. Informative References

   [GMPLS G-Shut] Z. Ali, J.P. Vasseur, A. Zamfir and J. Newton
          "Graceful Shutdown in MPLS and Generalized MPLS Traffic
   Engineering Networks" January 2008, internet draft, draft-ietf-ccamp-
   mpls-graceful-shutdown-05.txt, work in progress.

   [Reliability] Network Strategy Partners, LLC.
         "Reliable IP Nodes: A prerequisite to profitable IP services",
   November 2002. http://www.nspllc.com/NewPages/Reliable_IP_Nodes.pdf

10.     Acknowledgments

   This draft is mostly an updated version of draft-dubois-bgp-pm-
   reqs-02.txt.

   Authors would like to thank Nicolas Dubois, Benoit Fondeviole,
   Christian Jacquenet, Olivier Bonaventure, Steve Uhlig, Xavier
   Vinet, Tomonori Takeda, Vincent Gillet, and Jean-Louis le Roux
   for the useful discussions on this subject, their review and
   comments.

   This draft has been partly sponsored by the European project IST
   AGAVE.

11.     Author's Addresses

   Bruno Decraene
   France Telecom

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   38-40 rue du General Leclerc
   92794 Issy Moulineaux cedex 9
   France
   Email: bruno.decraene@orange-ftgroup.com

   Pierre Francois
   Universite catholique de Louvain
   Place Ste Barbe, 2
   Louvain-la-Neuve  1348
   BE
   Email: francois@info.ucl.ac.be

   Cristel Pelsser
   NTT Corporation
   9-11, Midori-Cho 3 Chrome
   Musashino-Shi, Tokyo 180-8585
   Japan
   Email: pelsser.cristel@lab.ntt.co.jp

   Zubair Ahmad
   Orange Business Services
   13775 McLearen Road, Oak Hill VA 20171
   USA
   Email: zubair.ahmad@ orange-ftgroup.com

   Antonio Jose Elizondo Armengol
   Division de Analisis Tecnologicos
   Technology Analysis Division
   Telefonica I+D
   C/ Emilio Vargas 6
   28043, Madrid
   E-mail: ajea@tid.es

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