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Network Working Group                                    Pierre Francois
Internet-Draft                          Universite catholique de Louvain
Expires: September 16, 2008                               Bruno Decraene
                                                          France Telecom
                                                         Cristel Pelsser
                                                         NTT Corporation
                                                          March 15, 2008


                     Graceful BGP session shutdown
                      draft-francois-bgp-gshut-00

Status of this Memo

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Abstract

   This draft describes operational procedures aimed at reducing the
   amount of traffic lost during planned maintenances of routers,
   involving the shutdown of BGP peering sessions.









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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Packet loss upon manual eBGP session shutdown  . . . . . . . .  4
   4.  Practices to avoid packet losses . . . . . . . . . . . . . . .  4
     4.1.  Improving availability of alternate paths  . . . . . . . .  4
     4.2.  Graceful shutdown procedures for eBGP sessions . . . . . .  5
       4.2.1.  Outbound traffic . . . . . . . . . . . . . . . . . . .  5
       4.2.2.  Inbound traffic  . . . . . . . . . . . . . . . . . . .  5
     4.3.  Graceful shutdown procedures for iBGP sessions . . . . . .  7
   5.  Forwarding modes and forwarding loops  . . . . . . . . . . . .  7
   6.  Dealing with Internet policies . . . . . . . . . . . . . . . .  8
   7.  Effect of the g-shut procedure on the convergence  . . . . . .  8
     7.1.  Maintenance of an eBGP session . . . . . . . . . . . . . .  9
       7.1.1.  Propagation on the other eBGP sessions of the
               g-shut initiator . . . . . . . . . . . . . . . . . . .  9
       7.1.2.  Propagation on the other iBGP sessions of the
               g-shut initiator . . . . . . . . . . . . . . . . . . .  9
       7.1.3.  Propagation of updates in an iBGP full-mesh  . . . . .  9
       7.1.4.  Propagation of updates from iBGP to iBGP in a RR
               hierarchy  . . . . . . . . . . . . . . . . . . . . . .  9
     7.2.  Maintenance of an iBGP session . . . . . . . . . . . . . . 10
     7.3.  Applicability of the g-shut procedures . . . . . . . . . . 10
     7.4.  Summary of operations  . . . . . . . . . . . . . . . . . . 10
       7.4.1.  Pre-configuration  . . . . . . . . . . . . . . . . . . 11
       7.4.2.  Operations at maintenance time . . . . . . . . . . . . 11
   8.  Techniques with limited applicability  . . . . . . . . . . . . 11
     8.1.  In-filter reconfiguration  . . . . . . . . . . . . . . . . 11
     8.2.  Multi Exit Discriminator tweaking  . . . . . . . . . . . . 12
     8.3.  IGP distance Poisoning . . . . . . . . . . . . . . . . . . 12
   9.  IANA considerations  . . . . . . . . . . . . . . . . . . . . . 13
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 13
   Intellectual Property and Copyright Statements . . . . . . . . . . 15















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1.  Introduction

   Routing changes in BGP can be caused by planned, manual, maintenance
   operations.  This document discusses operational procedures to be
   applied in order to reduce or eliminate losses of packets during the
   maintenance.  These losses come from the transient lack of
   reachability during the BGP convergence following the shutdown of an
   eBGP peering session between two Autonomous System Border Routers
   (ASBR).

   This document presents procedures for the cases where the forwarding
   plane is impacted by the maintenance, hence when the use of Graceful
   Restart does not apply.

   The procedures described in this document can be applied to reduce or
   avoid packet loss for outbound and inbound traffic flows initially
   forwarded along the peering link to be shut down.  These procedures
   allow routers to keep using old paths until alternate ones are
   learned, ensuring that routers always have a valid route available
   during the convergence process.

   The goal of the document is to meet the requirements described in
   [REQS] at best without changing the BGP protocol or BGP
   implementations.

   Still, it explains why standardizing a community value for the
   purpose of BGP session graceful shutdown would reduce the management
   overhead bound with the solution.  It would also allow vendors to
   provide an automatic graceful shutdown mechanism that does not
   require any configuration at maintenance time.


2.  Terminology

   g-shut initiator : a router on which the session shutdown is
   performed for the maintenance.

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

   Initiator AS : the Autonomous System of the g-shut initiator.

   Neighbor AS : the Autonomous System of the g-shut neighbor.

   Affected path / Nominal / pre-convergence path : a BGP path via the
   peering link(s) undergoing the maintenance.  This path will no longer
   exist after the shutdown.




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   Affected prefix : a prefix initially reached via an affected path.

   Affected router : a router having an affected prefix.

   Backup / alternate / post-convergence path : a path toward an
   affected prefix that will be selected as the best path by an affected
   router, when the link is shut down and the BGP convergence is
   completed.

   Transient alternate path : a path towards an affected prefix that may
   be transiently selected as best by an affected router during the
   convergence process but that is not a post-convergence path.

   Loss of Connectivity (LoC) : the state when a router has no path
   towards an affected prefix.


3.  Packet loss upon manual eBGP session shutdown

   Packets can be lost during a manual shutdown of an eBGP session for
   two reasons.

   First, routers involved in the convergence process can transiently
   lack of paths towards an affected prefix, and drop traffic destined
   to this prefix.  This is because alternate paths can be hidden by
   nodes of an AS.  This happens when the paths are not selected as best
   by the ASBR that receive them on an eBGP session, or by Route
   Reflectors that do not propagate them further in the iBGP topology
   because they do not select them as best.

   Second, within the AS, routers' FIB can be transiently inconsistent
   during the BGP convergence and packets towards affected prefixes can
   loop and be dropped.  Note that these loops only happen when BR-to-BR
   encapsulation is not used within the AS.

   This document only addresses the first reason.


4.  Practices to avoid packet losses

   This section describes means for an ISP to reduce the transient loss
   of packets upon a manual shutdown of a BGP session.

4.1.  Improving availability of alternate paths

   Using advertise-best-external on ASBRs improves the availability of
   alternate paths.  Hence it reduces the LoC duration for the outbound
   traffic of the ISP upon an eBGP Session shutdown.



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   All solutions that improve the availability of paths beyond the Route
   Reflectors barrier also help in reducing the LoC.  These have been
   discussed for years but are not yet available, probably due to their
   implementation complexity.

4.2.  Graceful shutdown procedures for eBGP sessions

   This section aims at describing a procedure to be applied to reduce
   the LoC with readily available BGP features, and without assuming
   particular iBGP design for the AS performing the maintenance and in
   the neighboring ASes.

4.2.1.  Outbound traffic

   This section discusses a mean to render the affected paths less
   desirable by the BGP decision process of affected routers, still
   allowing these to be used during the convergence while alternate
   paths are propagated to the affected routers.

   A decrease of the local-pref value of the affected paths can be
   issued in order to render the affected paths less preferable, at the
   highest possible level of the BGP Decision Process.

   This operation can be performed by reconfiguring the out-filters
   associated with the iBGP sessions established by the g-shut
   initiator.

   The modification of the filters MUST supplant any other rule
   affecting the local-pref value of the old paths.

   Compared to using an in-filter of the eBGP session to be shut down,
   the modification of the out-filters will not let the g-shut initiator
   switch to another path, as the input to the BGP decision process of
   that router does not change.  As a consequence, the g-shut initiator
   will not send a withdraw message over its iBGP sessions when it
   receives an alternate path over an iBGP session.  It will however
   modify the local-pref of the affected paths so that upstream routers
   will switch to alternate ones.

   When the actual shutdown of the session is performed, the g-shut
   initiator will itself switch to the alternate paths.

4.2.2.  Inbound traffic

   The solution described for the outbound traffic can be applied at the
   neighbor AS.  This can be done either "manually" or by using a
   community value dedicated to this task.




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4.2.2.1.  Phone call

   The operator performing the maintenance of the eBGP session can
   contact the operator at the other side of the peering link, and let
   him apply the procedure described above for its own outbound traffic.

4.2.2.2.  Community tagging

   A community value (referred to as GSHUT community in this document)
   can be agreed upon by neighboring ASes.  A path tagged with this
   community must be considered as soon to be affected by a maintenance
   operation.

4.2.2.2.1.  Pre-Configuration

   A g-shut neighbor is pre-configured to set a low local-pref value for
   the paths received over eBGP sessions which are tagged with the GSHUT
   community.

   This rule must supplant any other rule affecting the local-pref value
   of the paths.

   This local-pref reconfiguration SHOULD be performed at the out-
   filters of the iBGP sessions of the g-shut neighbor.  That is, the
   g-shut neighbor does not take into account this low local-pref in its
   own BGP best path selection.  As described in Section 4.2.1 this
   avoids sending the withdraw messages that can lead to LoC.

4.2.2.2.2.  Operational action upon maintenance

   Upon the manual shutdown, the output filter associated with the
   maintained eBGP session will be modified on the g-shut initiator so
   as to tag all the paths advertised over the session with the GSHUT
   community.

4.2.2.2.3.  Transitivity of the community

   If the GSHUT community is an extended community, it SHOULD be set non
   transitive.

   If a normal community is used, this community SHOULD be removed from
   the path by the ASBR of the peer receiving it.  If not, the GSHUT
   community MAY be removed from the path by all the ASBRs of the
   neighboring AS, before propagating the path to other peers.

   Not propagating the community further in the Internet reduces the
   amount of BGP churn and avoids rerouting in distant ASes that would
   also recognize this community value.  In other words, it helps



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   concealing the convergence at the maintenance location.

   There are cases where an interdomain exploration is to be performed
   to recover the reachability, e.g., in the case of a shutdown in
   confederations where the alternate paths will be found in another AS
   of the confederation.  In such scenarios, the community value SHOULD
   be allowed to transit through the confederation but MAY be removed
   from the paths advertised outside of the confederation.

   When the local-pref value of a path is conserved upon its propagation
   from one AS of the confederation to the other, there is no need to
   have the GSHUT community be propagated throughout that confederation.

4.2.2.2.4.  Easing the configuration for G-SHUT

   From a configuration burden viewpoint, it would be much easier to
   have the GSHUT community value be standardized.

   First, on the g-shut initiator, an operator would have a single
   configuration rule to be applied at the maintenance time, which would
   not depend on the identity of its peer.  This would make the
   maintenance operations less error prone.

   Second, on the g-shut neighbor, the same filter related to g-shut can
   be applied to all iBGP sessions, at the g-shut neighbor.

4.3.  Graceful shutdown procedures for iBGP sessions

   If the iBGP topology is viable after the maintenance of the session,
   i.e, if all BGP speakers of the AS have an iBGP signaling path for
   all affected prefixes after the convergence, then the shutdown of an
   iBGP session does not lead to transient unreachability.

   However, in the case of a shutdown of a router, a reconfiguration of
   the out-filters of the g-shut initiator should be performed to set a
   low local-pref value for the paths originated by the g-shut initiator
   (e.g, BGP aggregates redistributed from other protocols, including
   static routes).

   This behavior is equivalent to the recommended behavior for paths
   "redistributed" from eBGP sessions to iBGP sessions in the case of
   the shutdown of an ASBR.


5.  Forwarding modes and forwarding loops

   If the AS applying the solution does not rely on encapsulation to
   forward packets from the Ingress Border Router to the Egress Border



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   Router, then transient forwarding loops and consequent packet losses
   can occur during the convergence process, even if the procedure
   described above is applied.

   Using the out-filter reconfiguration avoids the forwarding loops
   between the g-shut initiator and its upstream neighbors.  Indeed,
   when this reconfiguration is applied, the g-shut initiator keeps
   using its own external path and lets the upstream routers converge to
   the alternate ones.  During this phase, no forwarding loops can occur
   between the g-shut initiator and its upstream neighbors as the g-shut
   neighbor keeps using the affected paths via its eBGP peering links.
   When the first step is finished, all the upstream routers have
   switched to alternate paths and the transition performed by the
   g-shut initiator will be loopfree.  Transient forwarding loops
   between other routers will not be avoided with this procedure.


6.  Dealing with Internet policies

   A side gain of the maintenance solution is that it can be used to
   reduce the churn implied by a shutdown of an eBGP session.

   For this, it is recommended to apply the filters modifying the local-
   pref value of the paths to values strictly lower but as close as
   possible to the local-pref values of the post-convergence paths.

   For example, if a peering link is shut down between a provider and
   one of its customers, and another peering link with this customer
   remains active, then the value of the local-pref of the old paths
   SHOULD be decreased to the smallest possible value of the 'customer'
   local_pref range, minus 1.  Thus, routers will not transiently switch
   to paths received from shared-cost peers or providers, which could
   lead to the sending of withdraw messages over eBGP sessions with
   shared-cost peers and providers.

   Proceeding like this reduces both BGP churn and traffic shifting as
   routers will less likely switch to transient paths.

   In our example, it also prevents transient unreachabilities in the
   neighboring AS that are due to the sending of "abrupt" withdraw
   messages to shared-cost peers and providers.


7.  Effect of the g-shut procedure on the convergence

   This section describes the effect of applying the solution.





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7.1.  Maintenance of an eBGP session

   This section describes the effect of applying the solution for the
   shutdown of an eBGP session.

7.1.1.  Propagation on the other eBGP sessions of the g-shut initiator

   Nothing is propagated on the other eBGP sessions when the out-filters
   reconfiguration step is applied.  The reconfiguration is indeed only
   defined for its iBGP sessions.

   The reconfiguration of the iBGP out-filters will trigger the
   reception of alternate paths at the g-shut initiator.  As the eBGP
   in-filters have not been modified at that step, the old paths are
   still preferred by the g-shut initiator.

7.1.2.  Propagation on the other iBGP sessions of the g-shut initiator

   During the out-filter reconfiguration, path updates are propagated
   with a reduced local-pref value for the affected paths.  As a
   consequence, Route Reflectors and distant ASBRs select and propagate
   alternate paths through the iBGP topology as they no longer select
   the old paths as best.

   When the shut-down is performed, for each affected prefix, the g-shut
   initiator propagates on its iBGP sessions:

   .  The alternate path, if the best path was received over an eBGP
   sessions.

   .  A withdraw, if the best path was received over an iBGP sessions.

7.1.3.  Propagation of updates in an iBGP full-mesh

   No transient LoC can occur if a reconfiguration of the iBGP out-
   filters on the g-shut initiator is performed.

7.1.4.  Propagation of updates from iBGP to iBGP in a RR hierarchy

   Upon the reception of the update of a primary path with a lower
   local-pref value, Route Reflectors will either propagate the update,
   or select an alternate path and propagate it within their RR iBGP
   full-mesh, to their own Route Reflectors in the case of a multiple
   level Route Reflector hierarchy, and to their clients.

   During the convergence process controlled with the described
   procedure, some corner case timings can trigger transient
   unreachabilities.



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   A typical example for such transient unreachability for a given
   prefix is the following :

        1.  A Route Reflector RR1 only knew about the primary path upon
        the shutdown.

        2.  A member of its RR full-mesh, RR2, propagates an update of
        the old path with a lower local-pref.

        3.  Another member of its RR full-mesh, RR3 processes the
        update, selects an alternate path, and propagates an update in
        the mesh.

        4.  RR2 receives the alternate path, selects it as best, and
        hence withdraws the updated old path on the iBGP session of the
        mesh.

        5.  If for any reason, RR1 receives and processes the withdraw
        generated in step 4 before processing the update generated in
        step 3, RR1 transiently suffers from unreachability for the
        affected prefix.

   In such corner cases, the solution improves the iBGP convergence
   behavior/LoC but does not ensure 0 packet loss, as we cannot define a
   simple solution relying only on a reconfiguration of the filters of
   the g-shut initiator.  Improving the availability of alternate paths
   in Route Reflectors, using advertise best external, or any flavor of
   add-path [AddPath], seems to be the most pragmatic solution to these
   corner cases.

7.2.  Maintenance of an iBGP session

   If the shutdown does not temper with the viability of the iBGP
   topology, the described procedure is sufficient to avoid LoC.

7.3.  Applicability of the g-shut procedures

   The applicability of the procedure described in this draft to the
   cases presented in [REQS] can be shown by combining the effects
   described in this section.  A complete case by case analysis will be
   provided in the next versions of the draft.

7.4.  Summary of operations

   This section summarizes the configurations and actions to be
   performed to support the g-shut procedure for eBGP peering links.





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7.4.1.  Pre-configuration

   On each ASBR supporting the g-shut procedure, set-up an out-filter
   applied on all iBGP sessions of the ASBR, that :

   . sets the local-pref of the paths tagged with the g-shut community
   to a low value

   . removes the g-shut community from the path.

7.4.2.  Operations at maintenance time

   On the g-shut initiator :

   .  Apply an in-filter on the maintained BGP session to tag the paths
   received over the session with the g-shut community.

   .  Apply an out-filter on the maintained BGP session to tag the paths
   propagated over the session with the g-shut community.

   .  Wait for convergence to happen.

   .  Perform a BGP session shutdown.


8.  Techniques with limited applicability

8.1.  In-filter reconfiguration

   An In-filter reconfiguration on the eBGP session undergoing the
   maintenance could be performed instead of out-filter reconfigurations
   on the iBGP sessions of the g-shut initiator.

   Upon the application of the maintenance procedure, if the g-shut
   initiator has an alternate path in its Adj-Rib-In, it will switch to
   it directly.

   If this new path was advertised by an eBGP neighbor of the g-shut
   initiator, the g-shut initiator will send a BGP Path Update message
   over its iBGP and eBGP sessions.

   If this new path was received over an iBGP session, the g-shut
   initiator will select that path and directly propagate a withdraw
   message over the iBGP sessions for which it is not acting as a Route
   Reflector.  There can be iBGP topologies where the iBGP peers of the
   g-shut initiator do not know an alternate path, and hence may drop
   traffic.




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   Also, applying an In-filter reconfiguration on the eBGP session
   undergoing the maintenance may lead to transient LoC, in full-mesh
   iBGP topologies if

        a.  An ASBR of the initiator AS, ASBR1 did not initially select
        its own external path as best, and

        b.  An ASBR of the initiator AS, ASBR2 propagates an Update
        message along its iBGP sessions upon the reception of ASBR1's
        update following the in-filter reconfiguration on the g-shut
        initiator, and

        c.  ASBR1 receives the update message, runs its Decision Process
        and hence propagates a withdraw of its external path after
        having selected ASBR2's path as best, and

        d.  An impacted router of the AS processes the withdraw of ASBR1
        before processing the update from ASBR2.

   Applying a reconfiguration of the out-filters prevents such transient
   unreachabilities.

   Indeed, when the g-shut initiator propagates an update of the old
   path first, the sending of the withdraw by ASBR2 does not trigger
   unreachability in other nodes as the old path is still available.
   Indeed, even though it receives alternate paths, the g-shut initiator
   keeps using its old path as best as the in-filter of the maintained
   eBGP session has not been modified yet.

   Applying the out-filter reconfiguration also prevents packet loops
   between the g-shut initiator and its direct neighbors when
   encapsulation is not used between the ASBRs of the AS.

8.2.  Multi Exit Discriminator tweaking

   The MED attribute of the paths to be avoided can be increased so as
   to force the routers in the neighboring AS to select other paths.

   The solution only works if the alternate paths are as good as the
   initial ones with respect to the Local-Pref value and the AS Path
   Length value.  In the other cases, increasing the MED value will not
   have an impact on the decision process of the routers in the
   neighboring AS.

8.3.  IGP distance Poisoning

   The distance to the BGP nexthop corresponding to the maintained
   session can be increased in the IGP so that the old paths will be



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   less preferred during the application of the IGP distance tie-break
   rule.  However, this solution only works for the paths whose
   alternates are as good as the old paths with respect to their Local-
   Pref value, their AS Path length, and their MED value.

   Also, this poisoning cannot be applied when nexthop self is used as
   there is no nexthop specific to the maintained session to poison in
   the IGP.


9.  IANA considerations

   Applying the g-shut procedure is rendered much easier when the g-shut
   community value is to be standardized.  Hence this draft suggests to
   reserve a community value, e.g., 0xFFFF0005, for this purpose.


10.  Security Considerations

   By providing the g-shut service to a neighboring AS, an ISP provides
   means to this neighbor to lower the local-pref value assigned to the
   paths received from this neighbor.

   The neighbor could abuse the technique and do inbound traffic
   engineering by declaring some prefixes as undergoing a maintenance so
   as to switch traffic to another peering link.

   If this behavior is not tolerated by the ISP, it SHOULD monitor the
   use of the g-shut community by this neighbor.


11.  References

   [AddPath]  D. Walton, A. Retana, and E. Chen, "Advertisement of
              Multiple Paths in BGP", draft-walton-bgp-add-paths-05.txt
              (work in progress).

   [REQS]     Decraene, B., Francois, P., Pelsser, C., and Z. Ahmad,
              "Requirements for the graceful shutdown of BGP sessions",
               draft-decraene-bgp-graceful-shutdown-requirements-00.txt
              , December 2007.










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Authors' Addresses

   Pierre Francois
   Universite catholique de Louvain
   Place Ste Barbe, 2
   Louvain-la-Neuve  1348
   BE

   Email: pierre.francois@uclouvain.be
   URI:   http://inl.info.ucl.ac.be/pfr


   Bruno Decraene
   France Telecom
   38-40 rue du General Leclerc
   92794 Issi Moulineaux cedex 9
   FR

   Email: bruno.decraene@orange-ftgroup.com


   Cristel Pelsser
   NTT Corporation
   9-11, Midori-Cho 3 Chrome
   Musashino-Shi, Tokyo  180-8585
   JP

   Email: pelsser.cristel@lab.ntt.co.jp























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Pierre Francois, et al.  Expires September 16, 2008            [Page 15]


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