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rfc:rfc8326

Internet Engineering Task Force (IETF) P. Francois, Ed. Request for Comments: 8326 Individual Contributor Category: Standards Track B. Decraene, Ed. ISSN: 2070-1721 Orange

                                                            C. Pelsser
                                                 Strasbourg University
                                                              K. Patel
                                                          Arrcus, Inc.
                                                           C. Filsfils
                                                         Cisco Systems
                                                            March 2018
                   Graceful BGP Session Shutdown

Abstract

 This document standardizes a new well-known BGP community,
 GRACEFUL_SHUTDOWN, to signal the graceful shutdown of paths.  This
 document also describes operational procedures that use this
 well-known community to reduce the amount of traffic lost when BGP
 peering sessions are about to be shut down deliberately, e.g., for
 planned maintenance.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 7841.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 https://www.rfc-editor.org/info/rfc8326.

Francois, et al. Standards Track [Page 1] RFC 8326 Graceful BGP Session Shutdown March 2018

Copyright Notice

 Copyright (c) 2018 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (https://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
 2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
 3.  Packet Loss upon Manual EBGP Session Shutdown . . . . . . . .   4
 4.  Procedure for EBGP Graceful Shutdown  . . . . . . . . . . . .   4
   4.1.  Pre-configuration . . . . . . . . . . . . . . . . . . . .   5
   4.2.  Operations at Maintenance Time  . . . . . . . . . . . . .   5
   4.3.  BGP Implementation Support for Graceful Shutdown  . . . .   6
 5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
 6.  Security Considerations . . . . . . . . . . . . . . . . . . .   6
 7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   6
   7.1.  Normative References  . . . . . . . . . . . . . . . . . .   6
   7.2.  Informative References  . . . . . . . . . . . . . . . . .   7
 Appendix A.  Alternative Techniques with Limited Applicability  .   8
   A.1.  Multi-Exit Discriminator Tweaking . . . . . . . . . . . .   8
   A.2.  IGP Distance Poisoning  . . . . . . . . . . . . . . . . .   8
 Appendix B.  Configuration Examples . . . . . . . . . . . . . . .   8
   B.1.  Cisco IOS XR  . . . . . . . . . . . . . . . . . . . . . .   9
   B.2.  BIRD  . . . . . . . . . . . . . . . . . . . . . . . . . .   9
   B.3.  OpenBGPD  . . . . . . . . . . . . . . . . . . . . . . . .  10
 Appendix C.  Beyond EBGP Graceful Shutdown  . . . . . . . . . . .  10
   C.1.  IBGP Graceful Shutdown  . . . . . . . . . . . . . . . . .  10
   C.2.  EBGP Session Establishment  . . . . . . . . . . . . . . .  10
 Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  12
 Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  12

Francois, et al. Standards Track [Page 2] RFC 8326 Graceful BGP Session Shutdown March 2018

1. Introduction

 Routing changes in BGP can be caused by planned maintenance
 operations.  This document defines a well-known community [RFC1997],
 called 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.
 This document discusses operational procedures to be applied in order
 to reduce or eliminate loss of packets during a maintenance
 operation.  Loss comes from transient lack of reachability during BGP
 convergence that follows the shutdown of an EBGP peering session
 between two Autonomous System Border Routers (ASBRs).
 This document presents procedures for the cases where the forwarding
 plane is impacted by the maintenance, hence for 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.  In both Autonomous
 Systems (ASes), these procedures trigger rerouting to alternate paths
 if they exist within the AS while allowing the use of the old path
 until alternate ones are learned.  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] as best possible without changing BGP.
 Other maintenance cases, such as the shutdown of an IBGP session or
 the establishment of an EBGP session, are out of scope for this
 document.  For informational purposes, they are briefly discussed in
 Appendix C.
 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
 "OPTIONAL" in this document are to be interpreted as described in
 BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
 capitals, as shown here.

Francois, et al. Standards Track [Page 3] RFC 8326 Graceful BGP Session Shutdown March 2018

2. Terminology

 graceful shutdown initiator
    A router on which the session shutdown is performed for the
    maintenance.
 graceful shutdown receiver
    A router that has a BGP session to be shut down with the graceful
    shutdown initiator.

3. Packet Loss upon Manual EBGP Session Shutdown

 Packets can be lost during the BGP convergence following a manual
 shut down of an EBGP session for two reasons.
 First, some routers can have no path 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
 extension defined in [RFC7911] is not used and a) the paths are not
 selected as best by the ASBRs that receive them on an EBGP session or
 b) Route Reflectors do not propagate the paths further in the IBGP
 topology because they do not select them as best.
 Second, the FIB can be inconsistent between routers within the AS,
 and packets toward affected prefixes can loop and be dropped unless
 encapsulation is used within the AS.
 This document only addresses the first reason.

4. Procedure for EBGP Graceful Shutdown

 This section describes configurations and actions to be performed for
 the graceful shutdown of EBGP peering links.
 The goal of this procedure is to retain the paths to be shut down
 between the peers, but with a lower LOCAL_PREF value, allowing the
 paths to remain in use while alternate paths are selected and
 propagated, rather than simply withdrawing the paths.  The LOCAL_PREF
 value SHOULD be lower than any of the alternative paths.  The
 RECOMMENDED value is 0.
 Note that some alternative techniques with limited applicability are
 discussed in Appendix A for informational purposes.

Francois, et al. Standards Track [Page 4] RFC 8326 Graceful BGP Session Shutdown March 2018

4.1. Pre-configuration

 On each ASBR supporting the graceful shutdown receiver procedure, an
 inbound BGP route policy is applied on all EBGP sessions of the ASBR.
 That policy:
 o  matches the GRACEFUL_SHUTDOWN community.
 o  sets the LOCAL_PREF attribute of the paths tagged with the
    GRACEFUL_SHUTDOWN community to a low value.
 For informational purposes, examples of configurations are provided
 in Appendix B.

4.2. Operations at Maintenance Time

 On the graceful shutdown initiator, at maintenance time, the
 operator:
 o  applies an outbound BGP route policy on the EBGP session to be
    shutdown.  This policy tags the paths propagated over the session
    with the GRACEFUL_SHUTDOWN community.  This will trigger the BGP
    implementation to re-advertise all active routes previously
    advertised and tag them with the GRACEFUL_SHUTDOWN community.
 o  applies an inbound BGP route policy on the EBGP session to be
    shutdown.  This policy tags the paths received over the session
    with the GRACEFUL_SHUTDOWN community and sets LOCAL_PREF to a low
    value.
 o  waits for route re-advertisement over the EBGP session and for BGP
    routing convergence on both ASBRs.
 o  shuts down the EBGP session, optionally using [RFC8203] to
    communicate the reason for the shutdown.
 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 gracefully
 shut down the routes originated by this router (e.g., BGP aggregates
 redistributed from other protocols, including static routes).  This
 can be performed by tagging these routes with the GRACEFUL_SHUTDOWN
 community and setting LOCAL_PREF to a low value.

Francois, et al. Standards Track [Page 5] RFC 8326 Graceful BGP Session Shutdown March 2018

4.3. BGP Implementation Support for Graceful Shutdown

 BGP Implementers SHOULD provide configuration knobs that utilize the
 GRACEFUL_SHUTDOWN community to inform BGP neighbors in preparation
 for an impending neighbor shutdown.  Implementation details are
 outside the scope of this document.

5. IANA Considerations

 IANA previously assigned the community value 0xFFFF0000 to the
 'planned-shut' community in the "BGP Well-known Communities"
 registry.  IANA has changed the name 'planned-shut' to
 'GRACEFUL_SHUTDOWN' and updated the reference to point to this
 document.

6. Security Considerations

 By providing the 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 that some prefixes are undergoing
 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 graceful shutdown community.

7. References

7.1. Normative References

 [RFC1997]  Chandra, R., Traina, P., and T. Li, "BGP Communities
            Attribute", RFC 1997, DOI 10.17487/RFC1997, August 1996,
            <https://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,
            <https://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,
            <https://www.rfc-editor.org/info/rfc6198>.

Francois, et al. Standards Track [Page 6] RFC 8326 Graceful BGP Session Shutdown March 2018

 [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
            2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
            May 2017, <https://www.rfc-editor.org/info/rfc8174>.

7.2. Informative References

 [BEST-EXTERNAL]
            Marques, P., Fernando, R., Chen, E., Mohapatra, P., and H.
            Gredler, "Advertisement of the best external route in
            BGP", Work in Progress, draft-ietf-idr-best-external-05,
            January 2012.
 [RFC7911]  Walton, D., Retana, A., Chen, E., and J. Scudder,
            "Advertisement of Multiple Paths in BGP", RFC 7911,
            DOI 10.17487/RFC7911, July 2016,
            <https://www.rfc-editor.org/info/rfc7911>.
 [RFC8203]  Snijders, J., Heitz, J., and J. Scudder, "BGP
            Administrative Shutdown Communication", RFC 8203,
            DOI 10.17487/RFC8203, July 2017,
            <https://www.rfc-editor.org/info/rfc8203>.

Francois, et al. Standards Track [Page 7] RFC 8326 Graceful BGP Session Shutdown March 2018

Appendix A. Alternative Techniques with Limited Applicability

 A few alternative techniques have been considered to provide graceful
 shutdown capabilities but have been rejected due to their limited
 applicability.  This section describes these techniques for possible
 reference.

A.1. Multi-Exit Discriminator Tweaking

 The Multi-Exit Discriminator (MED) attribute of the paths to be
 avoided can be increased to influence 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 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 BGP "NEXT_HOP self" is
 used, as there is no BGP NEXT_HOP specific to the maintained session
 to poison in the IGP.

Appendix B. Configuration Examples

 This appendix is non-normative.
 This appendix includes examples of routing policy configurations to
 honor the GRACEFUL_SHUTDOWN well-known BGP community.

Francois, et al. Standards Track [Page 8] RFC 8326 Graceful BGP Session Shutdown March 2018

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
   address-family ipv6 unicast
    send-community-ebgp
    route-policy AS64497-ebgp-inbound in
   !
  !
 !

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 {
     neighbor 2001:db8:1:2::1 as 64497;
     local as 64496;
     import keep filtered;
     import filter AS64497_ebgp_inbound;
 }

Francois, et al. Standards Track [Page 9] RFC 8326 Graceful BGP Session Shutdown March 2018

B.3. OpenBGPD

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

Appendix C. Beyond EBGP Graceful Shutdown

C.1. IBGP Graceful Shutdown

 For the shutdown of an IBGP session, provided 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 graceful shutdown IBGP session), then the shutdown
 of an IBGP session does not lead to transient unreachability.  As a
 consequence, no specific graceful shutdown action is required.

C.2. EBGP Session Establishment

 We identify two potential causes for transient packet losses upon the
 establishment of an EBGP session.  The first one is local to the
 startup initiator; the second one is due to the BGP convergence
 following the injection of new best paths within the IBGP topology.

C.2.1. Unreachability Local to the ASBR

 An ASBR that selects a path received over a newly established EBGP
 session as the best path may transiently drop traffic.  This can
 typically happen when the 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
 NEXT_HOP.
 A BGP speaker implementation MAY avoid such losses by ensuring that
 third-party NEXT_HOPs are resolved before installing paths using
 these NEXT_HOPs in the RIB.
 Alternatively, the operator (script) MAY ping third-party NEXT_HOPs
 that are expected to be used prior to establishing the session.  By
 proceeding like this, the MAC addresses associated with these third-
 party NEXT_HOPs are resolved by the startup initiator.

Francois, et al. Standards Track [Page 10] RFC 8326 Graceful BGP Session Shutdown March 2018

C.2.2. IBGP Convergence

 During the establishment of an EBGP session, in some corner cases, a
 router may have no path toward an affected prefix, leading to loss of
 connectivity.
 A typical example for such transient unreachability for a given
 prefix is the following:
    Consider three Route Reflectors (RR): RR1, RR2, RR3.  There is a
    full mesh of IBGP sessions 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 toward the
        prefix.
    2.  RR3 receives a new best path originated by the startup
        initiator, which is 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
        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 [RFC7911] or [BEST-EXTERNAL] 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, advertising the best external route ensures that an ASBR does
 not withdraw a previously advertised (EBGP) path when it receives an
 additional, preferred path over an IBGP session.  Also, advertising
 the best intra-cluster route ensures that an 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.

Francois, et al. Standards Track [Page 11] RFC 8326 Graceful BGP Session Shutdown March 2018

Acknowledgments

 The authors wish to thank Olivier Bonaventure, Pradosh Mohapatra, Job
 Snijders, John Heasley, and Christopher Morrow for their useful
 comments.

Authors' Addresses

 Pierre Francois (editor)
 Individual Contributor
 Email: pfrpfr@gmail.com
 Bruno Decraene (editor)
 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

Francois, et al. Standards Track [Page 12]

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