Network Working Group P. Francois, Ed. Internet-Draft Individual Contributor Intended status: Informational B. Decraene, Ed. Expires: January 28, 2018 Orange C. Pelsser Strasbourg University K. Patel Arrcus, Inc. C. Filsfils Cisco Systems July 27, 2017 Graceful BGP session shutdown draft-ietf-grow-bgp-gshut-10 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 GRACEFUL_SHUTDOWN, to signal the graceful shutdown of paths. Status of This Memo This Internet-Draft is submitted in full conformance with the provisions of BCP 78 and BCP 79. Internet-Drafts are working documents of the Internet Engineering Task Force (IETF). Note that other groups may also distribute working documents as Internet-Drafts. The list of current Internet- Drafts is at http://datatracker.ietf.org/drafts/current/. Internet-Drafts are draft documents valid for a maximum of six months and may be updated, replaced, or obsoleted by other documents at any time. It is inappropriate to use Internet-Drafts as reference material or to cite them other than as "work in progress." This Internet-Draft will expire on January 28, 2018. Copyright Notice Copyright (c) 2017 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 Francois, et al. Expires January 28, 2018 [Page 1] Internet-Draft Graceful BGP session shutdown July 2017 (http://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 . . . . . . . . . . . . . . . . . . . . . . . . 2 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. Make before break convergence: graceful shutdown . . . . 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 . . . . . . . . . . . . . 6 5.3. BGP implementation support for graceful shutdown . . . . 6 6. Beyond EBGP graceful shutdown . . . . . . . . . . . . . . . . 7 6.1. IBGP graceful shutdown . . . . . . . . . . . . . . . . . 7 6.2. EBGP session establishment . . . . . . . . . . . . . . . 7 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8 8. Security Considerations . . . . . . . . . . . . . . . . . . . 9 9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 9 10. References . . . . . . . . . . . . . . . . . . . . . . . . . 9 10.1. Normative References . . . . . . . . . . . . . . . . . . 9 10.2. Informative References . . . . . . . . . . . . . . . . . 9 Appendix A. Alternative techniques with limited applicability . 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 . . . . . . . . . . . . . . . . . . . . . . . . 11 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 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 loss of packets during the maintenance. These losses come from the transient lack of reachability during the BGP convergence following the shutdown of an Francois, et al. Expires January 28, 2018 [Page 2] Internet-Draft Graceful BGP session shutdown July 2017 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 ASes, rerouting to the alternate path if one exists 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] at best, without changing the BGP protocol. 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. 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 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 shutdown, with the graceful shutdown initiator. Initiator AS: the Autonomous System of the graceful shutdown initiator. Receiver AS: the Autonomous System of the graceful shutdown receiver. Loss of Connectivity (LoC: the state when a router has no path toward an affected prefix. Francois, et al. Expires January 28, 2018 [Page 3] Internet-Draft Graceful BGP session shutdown July 2017 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 paths 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 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 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 forwarding lookups from BGP routes such as [I-D.ietf-idr-best-external] and [RFC7911] help in reducing the LoC bound with the shutdown of EBGP sessions. Any such solution where, 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 graceful shutdown procedure. Note that the LoC for the inbound traffic of graceful shutdown initiator, due to the lack of an alternate path on the graceful shutdown receiver is not under the control of the Initiator AS. The part of the procedure aimed at avoiding LoC for incoming traffic should thus be applied even if no LoC are expected for the outgoing traffic. Francois, et al. Expires January 28, 2018 [Page 4] Internet-Draft Graceful BGP session shutdown July 2017 4.2. Make before break convergence: graceful shutdown The goal of this procedure is to retain the paths to be shutdown 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 must be lower than the one of the alternate path. 0 being the lowest value, it can be used in all cases, except if it already has a special meaning within the AS. Section 5 describes configurations and actions to be performed for the graceful shutdown of 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 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. 5. EBGP graceful shutdown procedure This section describes configurations and actions to be performed for the graceful shutdown of EBGP peering links. 5.1. 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: 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 Note that in the case where an AS is aggregating multiple routes under a covering prefix, it is recommended to filter out the GRACEFUL_SHUTDOWN community from the resulting aggregate BGP route. By doing so, the setting of the 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 graceful shutdown behavior. Francois, et al. Expires January 28, 2018 [Page 5] Internet-Draft Graceful BGP session shutdown July 2017 5.2. Operations at maintenance time On the 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 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 apply 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 wait for convergence to happen. o shutdown the EBGP session, optionally using [I-D.ietf-idr-shutdown] to communicate the reason of 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 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 and setting LOCAL_PREF to a low value. 5.3. BGP implementation support for graceful shutdown 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 graceful shutdown feature SHOULD: 1. Update all the paths propagated over the corresponding EBGP session, tagging the GRACEFUL_SHUTDOWN community to them. Any subsequent update sent over the session being gracefully shut down SHOULD be tagged with the GRACEFUL_SHUTDOWN community. 2. Lower the LOCAL_PREF value of the paths received over the EBGP session being shut down and set the GRACEFUL_SHUTDOWN community. 3. Optionally shut down the session after a configured time. 4. Prevent the GRACEFUL_SHUTDOWN community from being inherited by a path that would aggregate some paths tagged with the GSHUT Francois, et al. Expires January 28, 2018 [Page 6] Internet-Draft Graceful BGP session shutdown July 2017 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 graceful shutdown feature SHOULD also automatically install the BGP policies that are supposed to be configured, as described in Section 5.1 for sessions over which graceful shutdown is to be supported. 6. Beyond EBGP graceful shutdown 6.1. 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. 6.2. 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. 6.2.1. Unreachability local to the ASBR An ASBR that selects as best a path received over a newly established EBGP session 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 in the RIB. Alternatively, the operator (script) may ping third party NEXT_HOPs that are expected to be used before 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. Expires January 28, 2018 [Page 7] Internet-Draft Graceful BGP session shutdown July 2017 6.2.2. IBGP convergence Corner cases leading to LoC can occur during the establishment of an EBGP session. 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 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, 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 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 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 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 GRACEFUL_SHUTDOWN and set this document as the reference. Francois, et al. Expires January 28, 2018 [Page 8] Internet-Draft Graceful BGP session shutdown July 2017 8. 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 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 graceful shutdown community. 9. Acknowledgments The authors wish to thank Olivier Bonaventure, Pradosh Mohapatra, Job Snijders and John Heasley for their useful comments. 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, . [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, March 1997, . [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, . 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. Francois, et al. Expires January 28, 2018 [Page 9] Internet-Draft Graceful BGP session shutdown July 2017 [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, . 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 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 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. Francois, et al. Expires January 28, 2018 [Page 10] Internet-Draft Graceful BGP session shutdown July 2017 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; } B.3. OpenBGPD Francois, et al. Expires January 28, 2018 [Page 11] Internet-Draft Graceful BGP session shutdown July 2017 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 } 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. 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