Network Working Group                                    Pierre                                        P. Francois
Internet-Draft                                    Individual Contributor
Intended status: Informational                               B. Decraene
Expires: December 27, 2017 January 4, 2018                                          Orange
                                                              C. Pelsser
                                                   Strasbourg University
                                                                K. Patel
                                                            Arrcus, Inc.
                                                             C. Filsfils
                                                           Cisco Systems
                                                           June 25,
                                                            July 3, 2017

                     Graceful BGP session shutdown
                      draft-ietf-grow-bgp-gshut-08
                      draft-ietf-grow-bgp-gshut-09

Abstract

   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.  It defines a
   well-known BGP community, called g-shut, GRACEFUL_SHUTDOWN, to signal the
   graceful shutdown of paths to other Autonomous Systems. paths.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on December 27, 2017. January 4, 2018.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Packet loss upon manual EBGP session shutdown . . . . . . . .   3   4
   4.  Practices to avoid packet losses  . . . . . . . . . . . . . .   4
     4.1.  Improving availability of alternate paths . . . . . . . .   4
     4.2.  Make before break convergence: g-shut . . . . . . graceful shutdown  . . . .   4
   5.   5
     4.3.  Forwarding modes and transient forwarding loops during
           convergence . . . . . . . . . . . . . . . . . . . . . . .   5
   5.  EBGP graceful shutdown procedure  . . . . . . . . . . . . . .   5
     5.1.  Pre-configuration . . . . . . . . . . . . . . . . . . . .   5
     5.2.  Operations at maintenance time  . . . . . . . . .   7
   6.  Link Up cases . . . .   6
     5.3.  BGP implementation support for g-Shut . . . . . . . . . .   6
   6.  Beyond EBGP graceful shutdown . . . . . . . . . . . . . . . .   7
     6.1.  Unreachability local to the ASBR  IBGP graceful shutdown  . . . . . . . . . . . . . . . . .   7
     6.2.  iBGP convergence  Link Up cases . . . . . . . . . . . . . . . . . . . . . .   7
   7.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   8
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   8   9
   9.  Acknowledgments . . . . . . . . . . . . . . . . . . . . . . .   9
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .   9
     10.1.  Normative References . . . . . . . . . . . . . . . . . .   9
     10.2.  Informative References . . . . . . . . . . . . . . . . .   9
   Appendix A.  Alternative techniques with limited applicability  .   9  10
     A.1.  Multi Exit Discriminator tweaking . . . . . . . . . . . .  10
     A.2.  IGP distance Poisoning  . . . . . . . . . . . . . . . . .  10
   Appendix B.  Configuration Examples . . . . . . . . . . . . . . .  10
     B.1.  Cisco IOS XR  . . . . . . . . . . . . . . . . . . . . . .  11
     B.2.  BIRD  . . . . . . . . . . . . . . . . . . . . . . . . . .  11
     B.3.  OpenBGPD  . . . . . . . . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  10  12

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
   (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
   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
   process.

   The goal of the document is to meet the requirements described in
   [RFC6198] at best, without changing the BGP protocol.

   This document defines a well-known community [RFC1997], called
   g-shut,
   GRACEFUL_SHUTDOWN, for the purpose of reducing the management
   overhead of gracefully shutting down BGP sessions.  The well-known
   community allows implementers to provide an automated graceful
   shutdown mechanism that does not require any router reconfiguration
   at maintenance time.

   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 [RFC2119].

2.  Terminology

   g-shut

   graceful shutdown initiator: a router on which the session shutdown
   is performed for the maintenance.

   g-shut neighbor:

   graceful shutdown receiver: a router that has a BGP session, to be
   shutdown, with the g-shut graceful shutdown initiator.

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

   Neighbor

   Receiver AS: the Autonomous System of the g-shut neighbor. graceful shutdown receiver.

   Loss of Connectivity (LoC: the state when a router has no path
   towards toward
   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 toward 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 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 toward 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
   [I-D.ietf-idr-best-external] and [RFC7911] 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 graceful shutdown procedure.

   Note that the LoC for the inbound traffic of the maintained router,
   induced by a lack of alternate path propagation within the iBGP IBGP
   topology of a neighboring receiver 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 for
   the graceful shutdown of BGP sessions.

   The goal of this procedure is to let, in both ASes, retain the paths being to be shutdown visible,
   between the peers, but with a lower LOCAL_PREF value, allowing the
   paths to remain in use while alternate paths spread through the iBGP topology.  Instead of are selected and
   propagated, rather than simply withdrawing the
   path, routers paths.

   Section 5 describes configurations and actions to be performed for
   the graceful shutdown of an BGP sessions.

4.3.  Forwarding modes and transient forwarding loops during convergence

   The graceful shutdown procedure or the solutions improving the
   availability of alternate paths, do not change the fact that BGP
   convergence and the subsequent FIB updates are run independently on
   each router of the ASes.  If the AS will keep applying the solution does not
   rely on using it until they become aware 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 alternate paths.

4.2.1. the AS.

5.  EBGP g-shut graceful shutdown procedure

   This section describes configurations and actions to be performed for
   the graceful shutdown of EBGP peering links.

4.2.1.1.

5.1.  Pre-configuration

   On each ASBR supporting the g-shut graceful shutdown receiver procedure, an outbound
   inbound BGP route policy is applied on all iBGP EBGP sessions of the ASBR,
   that:

   o  matches the g-shut GRACEFUL_SHUTDOWN community

   o  sets the LOCAL_PREF attribute of the paths tagged with the g-shut
      GRACEFUL_SHUTDOWN 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
   GRACEFUL_SHUTDOWN community from the resulting aggregate BGP route.
   By doing so, the setting of the g-shut GRACEFUL_SHUTDOWN 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
   graceful shutdown behavior.

4.2.1.2.

5.2.  Operations at maintenance time

   On the g-shut graceful shutdown initiator, upon maintenance time, it is
   required to:

   o  apply an outbound BGP route policy on the EBGP session to be
      shutdown.  This policy tags the paths propagated over the session
      with the g-shut GRACEFUL_SHUTDOWN community.  This will trigger the BGP
      implementation to re-advertise all active routes previously
      advertised, and tag them with the g-shut GRACEFUL_SHUTDOWN 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
      GRACEFUL_SHUTDOWN community.

   o  wait for convergence to happen.

   o  shutdown the EBGP session, optionally using
      [I-D.ietf-idr-shutdown] to communicate the reason of the shutdown.

4.2.1.3.

   In the case of a shutdown of the whole router, in addition to the
   graceful shutdown of all EBGP sessions, there is a need to graceful
   shutdown the routes originated by this router (e.g, BGP aggregates
   redistributed from other protocols, including static routes).  This
   can be performed by tagging such routes with the GRACEFUL_SHUTDOWN
   community.

5.3.  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 graceful shutdown feature SHOULD:

   1.  On the EBGP side, update  Update all the paths propagated over the corresponding EBGP
       session, tagging the g-shut GRACEFUL_SHUTDOWN community to them.  Any
       subsequent update sent over the session being gracefully shut
       down would be tagged with the g-shut GRACEFUL_SHUTDOWN community.

   2.  On the iBGP side, lower  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. down.

   3.  Optionally shut down the session after a configured time.

   4.  Prevent the g-shut GRACEFUL_SHUTDOWN 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 prefixes.

   A BGP implementation supporting a g-shut graceful shutdown feature SHOULD
   also automatically install the BGP policies that are supposed to be
   configured, as described in Section 4.2.1.1 5.1 for sessions over which
   g-shut
   graceful shutdown is to be supported.

4.2.2.  iBGP g-shut

6.  Beyond EBGP graceful shutdown

6.1.  IBGP graceful shutdown

   For the shutdown of an iBGP IBGP session, provided the iBGP IBGP topology is
   viable after the maintenance of the session, i.e, if all BGP speakers
   of the AS have an iBGP IBGP signaling path for all prefixes advertised on
   this g-shut iBGP graceful shutdown IBGP session, then the shutdown of an iBGP IBGP
   session does not lead to transient unreachability.  As a consequence,
   no specific
   g-shut graceful shutdown action is required.

4.2.3.  Router g-shut

   In the case of a shutdown of the whole router, in addition to the
   g-shut of all EBGP sessions, there is a need to g-shut the routes
   originated by this router (e.g, BGP aggregates redistributed from
   other protocols, including static routes).  This can be performed by
   tagging such routes with the g-shut community.

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 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.

6.2.  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 graceful no-shut
   initiator, the second one is due to the BGP convergence following the
   injection of new best paths within the iBGP IBGP topology.

6.1.

6.2.1.  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 NEXT_HOP 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. NEXT_HOP.

   A BGP speaker implementation could avoid such losses by ensuring that
   "third party" nexthops NEXT_HOPs 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 NEXT_HOP 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 NEXT_HOP will be resolved
   by the g-no-shut graceful no-shut initiator.

6.2.  iBGP

6.2.2.  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 IBGP session between them.

      1.  RR1 is initially advertising the current best path to the
      members of its iBGP IBGP RR full-mesh.  It propagated that path within
      its RR full-mesh.  RR2 knows only that path toward the prefix.

      2.  RR3 receives a new best path originated by the "g-no-shut" "graceful 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 IBGP sessions of its RR full-mesh.

      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 [I-D.ietf-idr-best-external] among the RR of the iBGP 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 IBGP session.  Also, "best-intra-cluster"
   ensures that a RR does not withdraw a previously advertised (iBGP) (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 IBGP session.

7.  IANA Considerations

   The IANA has assigned the community value 0xFFFF0000 to the planned-
   shut community in the "BGP Well-known Communities" registry.  IANA is
   requested to change the name planned-shut to g-shut GRACEFUL_SHUTDOWN and
   set this document as the reference.

8.  Security Considerations

   By providing the g-shut graceful shutdown service to a neighboring AS, an
   ISP provides means to this neighbor and possibly its downstream ASes
   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 graceful shutdown community by this neighbor.

9.  Acknowledgments

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

10.  References

10.1.  Normative References

   [RFC1997]  Chandra, R., Traina, P., and T. Li, "BGP Communities
              Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996,
              <http://www.rfc-editor.org/info/rfc1997>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC6198]  Decraene, B., Francois, P., Pelsser, C., Ahmad, Z.,
              Elizondo Armengol, A., and T. Takeda, "Requirements for
              the Graceful Shutdown of BGP Sessions", RFC 6198,
              DOI 10.17487/RFC6198, April 2011,
              <http://www.rfc-editor.org/info/rfc6198>.

10.2.  Informative References

   [I-D.ietf-idr-best-external]
              Marques, P., Fernando, R., Chen, E., Mohapatra, P., and H.
              Gredler, "Advertisement of the best external route in
              BGP", draft-ietf-idr-best-external-05 (work in progress),
              January 2012.

   [I-D.ietf-idr-shutdown]
              Snijders, J., Heitz, J., and J. Scudder, "BGP
              Administrative Shutdown Communication", draft-ietf-idr-
              shutdown-10 (work in progress), June 2017.

   [RFC7911]  Walton, D., Retana, A., Chen, E., and J. Scudder,
              "Advertisement of Multiple Paths in BGP", RFC 7911,
              DOI 10.17487/RFC7911, July 2016,
              <http://www.rfc-editor.org/info/rfc7911>.

Appendix A.  Alternative techniques with limited applicability

   A few alternative techniques have been considered to provide g-shut graceful
   shutdown 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 NEXT_HOP 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.

Appendix B.  Configuration Examples

   This appendix is non-normative.

   Example routing policy configurations to honor the GRACEFUL_SHUTDOWN
   well-known BGP community.

B.1.  Cisco IOS XR

   community-set comm-graceful-shutdown
     65535:0
   end-set
   !
   route-policy AS64497-ebgp-inbound
     ! normally this policy would contain much more
     if community matches-any comm-graceful-shutdown then
       set local-preference 0
     endif
   end-policy
   !
   router bgp 64496
    neighbor 2001:db8:1:2::1
     remote-as 64497
     description a fantastic EBGP neighbor
     address-family ipv6 unicast
      send-community-ebgp
      route-policy AS64497-ebgp-inbound in
      route-policy AS65040v6-bgp-out out
     !
    !
   !

B.2.  BIRD

   function honor_graceful_shutdown() {
       if (65535, 0) ~ bgp_community then {
           bgp_local_pref = 0;
       }
   }
   filter AS64497_ebgp_inbound
   {
           # normally this policy would contain much more
           honor_graceful_shutdown();
   }
   protocol bgp peer_64497_1 {
       description "a fantastic EBGP neighbor";
       neighbor 2001:db8:1:2::1 as 64497;
       local as 64496;
       import keep filtered;
       import filter AS64497_ebgp_inbound;
       export filter AS64497_ebgp_outbound;
   }

B.3.  OpenBGPD

   AS 64496
   router-id 192.0.2.1
   neighbor 2001:db8:1:2::1 {
           descr "a fantastic EBGP neighbor"
           remote-as 64497
   }
   # normally this policy would contain much more
   match from any community GRACEFUL_SHUTDOWN set { localpref 0 }

Authors' Addresses

   Pierre Francois
   Individual Contributor

   Email: pfrpfr@gmail.com

   Bruno Decraene
   Orange

   Email: bruno.decraene@orange.com

   Cristel Pelsser
   Strasbourg University

   Email: pelsser@unistra.fr

   Keyur Patel
   Arrcus, Inc.

   Email: keyur@arrcus.com

   Clarence Filsfils
   Cisco Systems

   Email: cfilsfil@cisco.com