Network Working Group                                    Pierre Francois
Internet-Draft                                  Institute IMDEA Networks
Intended status: Informational                            Bruno Decraene
Expires: April 25, 2013                                   France Telecom August 1, 2014                                           Orange
                                                         Cristel Pelsser
                                               Internet Initiative Japan
                                                             Keyur Patel
                                                       Clarence Filsfils
                                                           Cisco Systems
                                                        October 22, 2012
                                                        January 28, 2014

                     Graceful BGP session shutdown


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

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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Packet loss upon manual eBGP session shutdown  . . . . . . . .  5
   4.  Practices to avoid packet losses . . . . . . . . . . . . . . .  5
     4.1.  Improving availability of alternate paths  . . . . . . . .  5
     4.2.  Make before break convergence: g-shut  . . . . . . . . . .  6
       4.2.1.  eBGP g-shut  . . . . . . . . . . . . . . . . . . . . .  6
       4.2.2.  iBGP g-shut  . . . . . . . . . . . . . . . . . . . . .  7
       4.2.3.  Router g-shut  . . . . . . . . . . . . . . . . . . . .  7
   5.  Forwarding modes and transient forwarding loops during
       convergence  . . . . . . . . . . . . . . . . . . . . . . . . .  8
   6.  Link Up cases  . . . . . . . . . . . . . . . . . . . . . . . .  8
     6.1.  Unreachability local to the ASBR . . . . . . . . . . . . .  8
     6.2.  iBGP convergence . . . . . . . . . . . . . . . . . . . . .  9
   7.  IANA assigned g-shut BGP community . . . . . . . . . . . . . .  9
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 10
   9.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 10
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
   Appendix A.  Alternative techniques with limited applicability . . 11
     A.1.  Multi Exit Discriminator tweaking  . . . . . . . . . . . . 11
     A.2.  IGP distance Poisoning . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11

1.  Introduction

   Routing changes in BGP can be caused by planned, 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

   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
   trigger, in both involved ASes, rerouting to the alternate path,
   while allowing routers to keep using old paths until alternate ones
   are learned, installed in the RIB and in the FIB.  This ensures that
   routers always have a valid route available during the convergence

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

   Still, it explains why reserving 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
   router reconfiguration at maintenance time.

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

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.

   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, the FIB of routers 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 ASBR-to-ASBR 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

   All solutions that increase the availability of alternate BGP paths
   at routers performing packet lookups in BGP tables such as
   [BestExternal] and [AddPath] help in reducing the LoC bound with
   manual shutdown of eBGP sessions.

   One of such solutions increasing diversity in such a way that, at any
   single step of the convergence process following the eBGP session
   shutdown, a BGP router does not receive a message withdrawing the
   only path it currently knows for a given NLRI, allows for a
   simplified g-shut procedure.

   Note that the LoC for the inbound traffic of the maintained router,
   induced by a lack of alternate path propagation within the iBGP
   topology of a neighboring AS is not under the control of the operator
   performing the maintenance.  The part of the procedure aimed at
   avoiding LoC for incoming paths can thus be applied even if no LoC
   are expected for the outgoing paths.

4.2.  Make before break convergence: g-shut

   This section describes configurations and actions to be performed to
   perform a for
   the graceful shutdown procedure for of eBGP peering links.

   The goal of this procedure is to let the paths being shutdown
   visible, but with a lower LOCAL_PREF value, while alternate paths
   spread through the iBGP topology.  Instead of withdrawing the path,
   routers of an AS will keep on using it until they become aware of
   alternate paths.

4.2.1.  eBGP g-shut  Pre-configuration

   On each ASBR supporting the g-shut procedure, an outbound BGP route
   policy is applied on all iBGP sessions of the ASBR, that:
   o    matches the g-shut community
   o    sets the LOCAL_PREF attribute of the paths tagged with the
        g-shut community to a low value
   o    removes the g-shut community from the paths.
   o    optionally, adds an AS specific g-shut community on these paths
        to indicate that these are to be withdrawn soon.  If some
        ingress ASBRs reset the LOCAL_PREF attribute, this AS specific
        g-shut community will be used to override other LOCAL_PREF
        preference changes.

   Note that in the case where an AS is aggregating multiple routes
   under a covering prefix, it is recommended to filter out the g-shut
   community from the resulting aggregate BGP route.  By doing so, the
   setting of the g-shut community on one of the aggregated routes will
   not let the entire aggregate inherit the community.  Not doing so
   would let the entire aggregate undergo the g-shut behavior.  Operations at maintenance time

   On the g-shut initiator, upon maintenance time, it is required to:
   o  apply an outbound BGP route policy on the maintained eBGP session
      to tag the paths propagated over the session with the g-shut
      community.  This will trigger the BGP implementation to re-
      advertise all active routes previously advertised, and tag them
      with the g-shut community.
   o  apply an inbound BGP route policy on the maintained eBGP session
      to tag the paths received over the session with the g-shut

   o  wait for convergence to happen.
   o  perform a BGP session shutdown.  BGP implementation support for G-Shut

   A BGP router implementation MAY provide features aimed at automating
   the application of the graceful shutdown procedures described above.

   Upon a session shutdown specified as graceful by the operator, a BGP
   implementation supporting a g-shut feature SHOULD:

   1.   On the eBGP side, update all the paths propagated over the
        corresponding eBGP session, tagging the GSHUT community to them.
        Any subsequent update sent to the session being gracefully shut
        down would be tagged with the GSHUT community.
   2.   On the iBGP side, lower the LOCAL_PREF value of the paths
        received over the eBGP session being shut down, upon their
        propagation over iBGP sessions.  Optionally, also tag these
        paths with an AS specific g-shut community.  Note that
        alternatively, the LOCAL_PREF of the paths received over the
        eBGP session can be lowered on the g-shut initiator itself,
        instead of only when propagating over its iBGP sessions.
   3.   Optionally shut down the session after a configured time.
   4.   Prevent the GSHUT community from being inherited by a path that
        would aggregate some paths tagged with the GSHUT community.
        This behavior avoids the GSHUT procedure to be applied to the
        aggregate upon the graceful shutdown of one of its covered

   A BGP implementation supporting a g-shut feature SHOULD also
   automatically install the BGP policies that are supposed to be
   configured, as decribed in Section for sessions over which
   g-shut is to be supported.

4.2.2.  iBGP g-shut

   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 prefixes advertised on this g-shut iBGP session, then the
   shutdown of an iBGP session does not lead to transient

4.2.3.  Router g-shut

   In the case of a shutdown of a router, a reconfiguration of the
   outbound BGP route policies 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 transient forwarding loops during convergence

   The g-shut procedure or the solutions improving the availability of
   alternate paths, do not change the fact that BGP convergence and the
   subsequent FIB updates are runned run independently on each router of the
   ASes.  If the AS applying the solution does not rely on encapsulation
   to forward packets from the Ingress Border Router to the Egress
   Border Router, then transient forwarding loops and consequent packet
   losses can occur during the convergence process.  If zero LoC is
   required, encapsulation is required between ASBRs of the AS.

6.  Link Up cases

   We identify two potential causes for transient packet losses upon an
   eBGP link up event.  The first one is local to the g-no-shut
   initiator, the second one is due to the BGP convergence following the
   injection of new best paths within the iBGP topology.

6.1.  Unreachability local to the ASBR

   An ASBR that selects as best a path received over a newly brought up
   eBGP session may transiently drop traffic.  This can typically happen
   when the nexthop attribute differs from the IP address of the eBGP
   peer, and the receiving ASBR has not yet resolved the MAC address
   associated with the IP address of that "third party" nexthop.

   A BGP speaker implementation could avoid such losses by ensuring that
   "third party" nexthops are resolved before installing paths using
   these in the RIB.

   If the link up event corresponds to an eBGP session that is being
   manually brought up, over an already up multi-access link, then the
   operator can ping third party nexthops that are expected to be used
   before actually bringing the session up, or ping directed broadcast
   the subnet IP address of the link.  By proceeding like this, the MAC
   addresses associated with these third party nexthops will be resolved
   by the g-no-shut initiator.

6.2.  iBGP convergence

   Corner cases leading to LoC can occur during an eBGP link up event.

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

   Let's consider 3 route reflectors RR1, RR2, RR3.  There is a full
   mesh of iBGP session between them.

        1.  RR1 is initially advertising the current best path to the
        members of its iBGP RR full-mesh.  It propagated that path
        within its RR full-mesh.  RR2 knows only that path towards the
        2.  RR3 receives a new best path orginated originated by the "g-no-shut"
        initiator, being one of its RR clients.  RR3 selects it as best,
        and propagates an UPDATE within its RR full-mesh, i.e., to RR1
        and RR2.
        3.  RR1 receives that path, reruns its decision process, and
        picks this new path as best.  As a result, RR1 withdraws its
        previously announced best-path on the iBGP sessions of its RR
        4.  If, for any reason, RR3 processes the withdraw generated in
        step 3, before processing the update generated in step 2, RR3
        transiently suffers from unreachability for the affected prefix.

   The use of [BestExternal] among the RR of the iBGP full-mesh can
   solve these corner cases by ensuring that within an AS, the
   advertisement of a new route is not translated into the withdraw of a
   former route.

   Indeed, "best-external" ensures that an ASBR does not withdraw a
   previously advertised (eBGP) path when it receives an additional,
   preferred path over an iBGP session.  Also, "best-intra-cluster"
   ensures that a RR does not withdraw a previously advertised (iBGP)
   path to its non clients (e.g. other RRs in a mesh of RR) when it
   receives a new, preferred path over an iBGP session.

7.  IANA assigned g-shut BGP community

   Applying the g-shut procedure is rendered much easier with the use of
   a single g-shut community value which could be used on all eBGP
   sessions, for both inbound and outbound signaling.  The community
   value 0xFFFF0000 has been assigned by IANA for this purpose.

   For Internet routes, a non transitive extended community will be
   reserved from the pool defined in [EXT_POOL].  Using such a community
   type allows for not leaking graceful signaling out of the AS
   boundaries, without the need to explicitly configure filters to strip
   the community off upon path propagation.

8.  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.

   ASes using the regular (transitive) g-shut community SHOULD remove
   the community from neighboring ASes that do not support the g-shut
   procedure.  Doing so prevents malignant remote ASes from using the
   community through intermediate ASes that do not support the feature,
   in order to perform inbound traffic engineering.  ASes using the non-
   transitive extended community do not need to do this as the community
   is non transitive and hence cannot be used by remote ASes.

9.  Acknowledgments

   The authors wish to thank Olivier Bonaventure and Pradosh Mohapatra
   for their useful comments on this work.

10.  References

   [AddPath]  D. Walton, E. Chen, A. Retana, and J. Scudder,
              "Advertisement of Multiple Paths in BGP",
              draft-ietf-idr-add-paths-09.txt (work in progress).

              Marques, P., Fernando, R., Chen, E., Mohapatra, P., and H.
              Gredler, "Advertisement of the best-external route to
              IBGP",  draft-ietf-idr-best-external-05.txt.

   [REQS]     Decraene, B., Francois, P., Pelsser, C., Ahmad, Z.,
              Armengol, A., and T. Takeda, "Requirements for the
              graceful shutdown of BGP sessions", RFC 6198.

   [RFC4360]  Sangli, S., Tappan, D., and Y. Rekhter, "BGP Extended
              Communities Attribute", RFC 4360, February 2006.

              Decraene, B. and P. Francois, "Assigned BGP extended

   [BGPWKC]   "

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

Appendix A.  Alternative techniques with limited applicability

   A few alternative techniques have been considered to provide g-shut
   capabilities but have been rejected due to their limited
   applicability.  This section describe them for possible reference.

A.1.  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.

A.2.  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
   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.

Authors' Addresses

   Pierre Francois
   Institute IMDEA Networks
   Avda. del Mar Mediterraneo, 22
   Leganese  28918


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


   Cristel Pelsser
   Internet Initiative Japan
   Jinbocho Mitsui Bldg.
   1-105 Kanda Jinbo-cho
   Tokyo  101-0051


   Keyur Patel
   Cisco Systems
   170 West Tasman Dr
   San Jose, CA  95134


   Clarence Filsfils
   Cisco Systems
   De kleetlaan 6a
   Diegem  1831