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Network Working Group                                          J. Uttaro
Internet-Draft                                                      AT&T
Intended status: Standards Track                              A. Simpson
Expires: April 22, 2012                                   Alcatel-Lucent
                                                               R. Shakir
                                                                     C&W
                                                             C. Filsfils
                                                            P. Mohapatra
                                                           Cisco Systems
                                                             B. Decraene
                                                          France Telecom
                                                              J. Scudder
                                                              Y. Rekhter
                                                        Juniper Networks
                                                        October 20, 2011


                            BGP Persistence
                  draft-uttaro-idr-bgp-persistence-00

Abstract

   For certain AFI/SAFI combinations it is desirable that a BGP speaker
   be able to retain routing state learned over a session that has
   terminated.  By maintaining routing state forwarding may be
   preserved.  This technique works effectively as long as the AFI/SAFI
   is primarily used to realize services that do not depend on
   exchanging BGP routing state with peers or customers.  There may be
   exceptions based upon the amount and frequency of route exchange that
   allow for this technique.  Generally the BGP protocol tightly couples
   the viability of a session and the routing state that is learned over
   it.  This is driven by the history of the protocol and it's
   application in the internet space as a vehicle to exchange routing
   state between administrative authorities.  This document addresses
   new services whose requirements for persistence diverge from the
   Internet routing point of view.

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



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   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 April 22, 2012.

Copyright Notice

   Copyright (c) 2011 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
   (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.































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Requirements Language  . . . . . . . . . . . . . . . . . .  4
   2.  Communities  . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.1.  PERSIST  . . . . . . . . . . . . . . . . . . . . . . . . .  5
     2.2.  DO_NOT_PERSIST . . . . . . . . . . . . . . . . . . . . . .  5
     2.3.  STALE  . . . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.  Configuration (Persistence Timer, PERSIST and
       DO_NOT_PERSIST Community)  . . . . . . . . . . . . . . . . . .  6
     3.1.  Settings for Different Applications  . . . . . . . . . . .  6
   4.  Operation  . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.1.  Attaching the STALE Community Value and Propagation of
           Paths  . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.2.  Forwarding . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.3.  Example Behaviour  . . . . . . . . . . . . . . . . . . . .  8
   5.  Deployment Considerations  . . . . . . . . . . . . . . . . . .  9
   6.  Applications . . . . . . . . . . . . . . . . . . . . . . . . . 10
     6.1.  Persistence in L2VPN (VPLS/VPWS) . . . . . . . . . . . . . 10
     6.2.  Persistence in L3VPN . . . . . . . . . . . . . . . . . . . 11
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 14
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   10. Normative References . . . . . . . . . . . . . . . . . . . . . 18
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 19


























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1.  Introduction

   In certain scenarios, a BGP speaker may maintain forwarding in spite
   of BGP session termination.  Currently all routing state learned
   between two speakers is flushed upon either normal or abnormal
   session termination.  There are techniques that are useful for
   maintaining routing when a session abnormally terminates i.e BGR
   Graceful RestartR ( RFC 4724 ) or normal termination such as
   increasing timers but they do not change the fundamental problem.
   The technique of BGP persistence works effectively as long as the
   expectation is that there is a decoupling of session viability and
   the correct service delivery, and the delivery uses the routing state
   learned over that session.  This document proposes a modification to
   BGP's behavior by enabling persistence of BGP learned routing state
   in spite of normal or abnormal session termination.

1.1.  Requirements Language

   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].






























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2.  Communities

   This memo defines three new communities that are used to identify the
   capability of a path to persist and whether or not that path is live
   or stale.

2.1.  PERSIST

   This memo defines a new transitive BGP community, PERSIST, with value
   TBD (to be assigned by IANA).  Attaching of the PERSIST community
   SHOULD be controlled by configuration.  Attaching the PERSIST
   community indicates that the peer should maintain forwarding in the
   case of a session failure.  The functionality SHOULD default to being
   disabled.

2.2.  DO_NOT_PERSIST

   This memo defines a new transitive BGP community, DO_NOT_PERSIST,
   with value TBD (to be assigned by IANA).  Attaching of the
   DO_NOT_PERSIST community SHOULD be controlled by configuration.  The
   functionality SHOULD default to being disabled.

2.3.  STALE

   This memo defines a new transitive BGP community, STALE, with value
   TBD (to be assigned by IANA).  Attaching of the STALE community is
   limited to a path that currently has the PERSIST community attached
























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3.  Configuration (Persistence Timer, PERSIST and DO_NOT_PERSIST
    Community)

   Persistence must be configured on a per session basis.  A speaker
   configures the ability to persist independently of it's peer.  There
   is no negotiation between the peers.  A timer must be configured
   indicating the time to persist stale state from a peer where the
   session is no longer viable.  This timer is designated as the
   persist-timer.  A speaker must also attach persistence community
   value indicating if a path to a route should persist.

3.1.  Settings for Different Applications

   The setting of the persist-timer should be based upon the field of
   use.  BGP is used in a many different applications that each bring a
   unique requirement for retaining state.  The following is not meant
   as a comprehensive listing but to suggest timer settings for a subset
   of AFI/SAFIs.

   L2VPN  This AFI/SAFI requires the exchange of routing state in order
      to establish PWs to realize a VPLS VPN, or a VPWS PW.  This AFI/
      SAFI does not require exchange of routing state with a customer
      and there is no eBGP session established.  The persist-timer
      should be set to a large value on the order of days to infinity.

   L3VPN  This AFI/SAFI requires the exchange of routing state to create
      a private VPN.  This AFI/SAFI requires exchange of state with
      customers via eBGP and is dynamic.  The SP needs to consider the
      possibility that stale state may not reflect the latest route
      updates and therefore may be incorrect from the customer
      perspective.  The persist-timer should be set to a large value on
      the order of hours to a few days. this is built upon the notion
      some incorrectness is preferable to a large outage.


















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4.  Operation

   Assuming a session failure has occurred a BGP persistent router must
   retain local forwarding state for those paths that are Persistent/
   Stale and propagate paths to downstream speakers that indicate that a
   given path is now stale.

4.1.  Attaching the STALE Community Value and Propagation of Paths

   The following rules must be followed.

   o  Identify paths learned over a failed session that have the PERSIST
      capable community value attached.

   o  For those paths attach the STALE community value and propagate to
      all peers.

   o  For those paths learned over the failed session that do not have
      PERSIST capable community value or are marked with the
      DO_NOT_PERSIST community follow BGP rules and generate withdrawals
      to all peers for those paths.

4.2.  Forwarding

   The following rules must be followed to ensure valid forwarding:

   o  All forwarding state must be retained i.e labels for BGP labeled
      unicast.

   o  Forwarding must ensure that the Next Hop to a "stale" route is
      viable.

   o  Forwarding to a "stale" route is only used if there are no other
      paths available to that route.  In other words an active path
      always wins regardless of path selection.  "Stale" state is always
      considered to be less preferred when compared with an active path.

   o  Forwarding should be retained through an advertisement.  When the
      session is re-established forwarding should only change if the new
      state is either different or better in terms of path selection.  A
      make before break strategy should be employed.

   o  Stale state may be retained indefinitely or may be programmed to
      expire via configuration.

   o  The Receiving Speaker MUST replace the stale routes by the routing
      updates received from the peer.  Once the End-of-RIB marker for an
      address family is received from the peer, it MUST immediately



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      remove any paths from the peer that are still marked as stale for
      that address family.

   o  There is no restriction on whether the session is internal or
      external.

4.3.  Example Behaviour

   Upon session establishment a speaker S2 may receive paths from S1
   that are marked with PERSIST, DO_NOT_PERSIST or neither.  Assume S2
   is also peered with a downstream speaker S3..  Implementations MUST
   follow the specifications outlined below for.

   Upon recognition of the failure to S1, S2 will identify paths that
   had been marked with PERSIST, DO_NOT_PERSIST or neither learned from
   S1.  S2 MUST implement the following behavior:

 if ( P1 is tagged with PERSIST ) {

 Retain Forwarding
   Attach the STALE Community to all paths that were marked with PERSIST
   Advertise STALE paths to all peers including S3
 }
 else ( P1 is marked with DO_NOT_PERSIST || not marked )

 Tear down the forwarding structure for P1
 Follow normal BGP rules i.e Best path, withdrawal etc.

 fi






















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5.  Deployment Considerations

   BGP Persistence as described in this document is useful within a
   single autonomous system or across autonomous systems.















































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6.  Applications

   This technique may be useful in a wide array of applications where
   routing state is either fairly static or, the state is localized
   within a routing context.  Some applications that come immediately to
   mind are L2 and L3 VPN.

6.1.  Persistence in L2VPN (VPLS/VPWS)

   VPLS/VPWS VPNs use BGP to exchange routing state between two PEs.
   This exchange allows for the creation of a PW within a VPN context
   between those PEs.  By definition, L2VPN does not exchange any
   routing state with customers via BGP.  BGP persistence is very useful
   here as the state is quite constant.  The only time state is
   exchanged is when a PW endpoint is provisioned, deleted or when a
   speaker reboots.

   Referring to Figure 1, PE1 and PE2 have advertised BGP routing state
   in order to create PWs between PE1 and PE2.  The RRs are only
   responsible to reflect this state between the PEs.  The use of a
   unique RD makes every path unique from the RRs perspective.

   Assume that the both RR experience catastrophic failure.

   Case 1 - All BGP speakers are persistent capable.

   The PWs created between PE1 and PE2 persist.  Forwarding
   uninterrupted.

   Case 2 - PE1 and the RRs are persistent capable, PE2 is not.

   In this case the path advertised from PE2 via the RRs is persistent
   at PE1, the PW from PE1 to PE2 is not torn down.  PE2 will remove the
   path from PE1 and tear down the PW from PE2 to PE1.  THe effect is
   that MAC state learned at PE2 is valid as the PW is still valid.  MAC
   state learned at PE1 is removed as the PW is no longer valid.
   Eventually MAC destinations recursed to the PW at PE1 destined for
   PE2 over the valid PW will time out.

   Assume that the RRs are valid but the iBGP sessions are torn down..

   Case 3 - All BGP speakers are persistent capable.

   The PWs created between PE1 and PE2 persist.  Forwarding
   uninterrupted.






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                VPNA                 VPNA
                 PW+++++++++++++++++++PW

                CE1-------PE1--------RR1-------PE2------CE2
                  |                    |
                  |                    |
                  ----------RR2---------

                  <--iBGP---><---iBGP-->

                                 Figure 1

6.2.  Persistence in L3VPN

                            --------RR1-------
                           / A              C \
               CE1 ----- PE1 --Forwarding Path-- PE2 ---- CE2
                           \ B              D /
                            ------- RR2 ------

                                 Figure 2

   In the case of a Layer 3 VPN topology, during the failure of a route
   reflector device at the current time, all routing information
   propagated via BGP is purged from the routing database.  In this
   case, forwarding is interrupted within such a topology due to the
   lack of signalling information, rather than an outage to the
   forwarding path between the PE devices.  With the addition of BGP
   persistence, a complete service outage can be avoided.

   The topology shown in Figure 2 is a simple L3VPN topology consisting
   of two customer edge (CE) devices, along with two provider edge (PE),
   and route reflector (RR) devices.  In this case, where an RFC4364 VPN
   topology is utilised a BGP session exists between PE1 to both RR1 and
   RR2, and from PE2 to RR1 and RR2, in order to propagate the VPN
   topology.

   Case 1: No BGP speakers are persistence capable:

   o  In this scenario, during a simultaneous failure of RR1 and RR2
      (which are extremely likely to share route reflector clients) both
      PE1 and PE2 remove all routing information from the VPN from their
      RIB, and hence a complete service outage is experienced.

   o  Where either sessions A and B, or C and D fail simultaneously,
      routing information from either PE1 (in the case of A and B), or
      PE2 (in the case of C and D) are withdrawn, and a partial service
      topology exists.



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   o  Both of the states described reflect a service outage where the
      forwarding path between the PE devices is not interrupted.

   Case 2: All BGP speakers are persistence capable:

   o  PE1 continues to forward utilising the label information received
      from PE2 via the working forwarding path for the duration of the
      persistence timer (and vice versa).

   o  This condition occurs regardless of the session(s) that fail.  In
      the worst case where sessions A, B, C and D fail simultaneously,
      the network continues to operate in the state in which it was at
      the time of the failure.

   Case 3: PE1 and RR[12] are persistence capable - PE2 is not.

   o  During a failure of BGP session A or B, PE1 will continue to
      forward utilising the routing information received from the RRs
      for PE2 for the duration of the persistence timer.  PE2 will
      continue to forward utilising the routing information received
      from the RRs, again for the duration of the persistence timer.

   o  In the case that either BGP session C or D fails, all routes will
      be withdrawn by RR[12] towards PE1 since these routes are not
      valid to be persisted by the RRs.  The end result of this will be
      that the routes advertised by CE2 into the VPN will be withdrawn.

   o  Where the worst case failure occurs (i.e. sessions A, B, C and D
      fail) the routes advertised by CE1 into the VPN will be
      persistently advertised by the RR devices, whereas those
      advertised by CE2 will be withdrawn.  Clearly in the example shown
      in the figure this results in a service outage, but where multiple
      PE devices exist within a topology, service is maintained for the
      subset of CEs attached to PE devices supporting the persistence
      capability.

   Within the Layer 3 VPN deployment it should be noted that routing
   information is less static than that of the many Layer 2 VPNs since
   typically multiple routes exist within the topology rather than an
   individual MAC address or egress interface per CE device on the PE
   device.  As such, the L3VPN operates with the routing databases in
   the 'core' of the network reflecting those at the time of failure.
   Should there be re-convergence for any path between the PE and CE
   devices, this will result in invalid routing information, should the
   egress PE device not hold alternate routing information for the
   prefixes undergoing such re-convergence.  It is expected that where
   each PE maintains multiple paths to each egress prefix (where an
   alternate path is available), it is expected that the egress PE will



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   forward packets towards an alternative egress PE for the prefix in
   question where the topology is no longer valid.

   The lack of convergence within a Layer 3 topology during the
   persistent state SHOULD be considered since it may adversely affect
   services, however, an assumption is made that a degraded service is
   preferable to a complete service outage during a large-scale BGP
   control plane failure.











































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7.  Security Considerations

   The security implications of the persistence mechanism defined within
   in this document are akin to those incurred by the maintenance of
   stale routing information within a network.  This is particularly
   relevant when considering the maintenance of routing information that
   is utilised for service segregation - such as MPLS label entries.

   For MPLS VPN services, the effectiveness of the traffic isolation
   between VPNs relies on the correctness of the MPLS labels between
   ingress and egress PEs.  In particular, when an egress PE withdraws a
   label L1 allocated to a VPN1 route, this label MUST not be assigned
   to a VPN route of a different VPN until all ingress PEs stop using
   the old VPN1 route using L1.

   Such a corner case may happen today, if the propagation of VPN routes
   by BGP messages between PEs takes more time than the label re-
   allocation delay on a PE.  Given that we can generally bound worst
   case BGP propagation time to a few minutes (e.g. 2-5), the security
   breach will not occur if PEs are designed to not reallocate a
   previous used and withdrawn label before a few minutes.

   The problem is made worse with BGP GR between PEs as VPN routes can
   be stalled for a longer period of time (e.g. 20 minutes).

   This is further aggravated by the BGP persistent extension proposed
   in this document as VPN routes can be stalled for a much longer
   period of time (e.g. 2 hours, 1 day).

   Therefore, to avoid VPN breach, before enabling BGP persistence, SPs
   needs to check how fast a given label can be reused by a PE, taking
   into account:

   o  The load of the BGP route churn on a PE (in term of number of VPN
      label advertised and churn rate).

   o  The label allocation policy on the PE (possibly depending upon the
      size of pool of the VPN labels (which can be restricted by
      hardware consideration or others MPLS usages), the label
      allocation scheme (e.g. per route or per VRF/CE), the re-
      allocation policy (e.g. least recently used label...)

   In addition to these considerations, the persistence mechanism
   described within this document is considered to be complex to exploit
   maliciously - in order to inject packets into a topology, there is a
   requirement to engineer a specific persistence state between two PE
   devices, whilst engineering label reallocation to occur in a manner
   that results in the two topologies overlapping.  Such allocation is



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   particularly difficult to engineer (since it is typically an internal
   mechanism of an LSR).

















































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8.  IANA Considerations

   IANA shall assigned community values from BGP well-known communities
   registry for the PERSIST, DO-NOT-PERSIST and STALE communities.  No
   additional IANA action is required.














































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9.  Acknowledgements

   We would like to acknowledge Roberto Fragassi (Alcatel-Lucent), John
   Medamana, (AT&T) Han Nguyen (AT&T), Jeffrey Haas (Juniper), Nabil
   Bitar (Verizon), Nicolai Leymann (DT) for their contributions to this
   document.













































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10.  Normative References

   [RFC1997]  Chandrasekeran, R., Traina, P., and T. Li, "BGP
              Communities Attribute", RFC 1997, August 1996.

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

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, January 2006.

   [RFC4364]  Rosen, E. and Y. Rekhter, "BGP/MPLS IP Virtual Private
              Networks (VPNs)", RFC 4364, February 2006.






































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Authors' Addresses

   James Uttaro
   AT&T
   200 S. Laurel Avenue
   Middletown, NJ  07748
   USA

   Email: ju1738@att.com


   Adam Simpson
   Alcatel-Lucent
   600 March Road
   Ottawa, Ontario  K2K 2E6
   Canada

   Email: adam.simpson@alcatel-lucent.com


   Rob Shakir
   Cable&Wireless Worldwide
   London
   UK

   Email: rjs@cw.net
   URI:   http://www.cw.com/


   Clarence Filsfils
   Cisco Systems
   Brussels  1000
   BE

   Email: cf@cisco.com


   Pradosh Mohapatra
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134
   USA

   Email: pmohapat@cisco.com







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   Bruno Decraene
   France Telecom
   38-40 Rue de General Leclerc
   92794 Issy Moulineaux  cedex 9
   France

   Email: bruno.decraene@orange.com


   John Scudder
   Juniper Networks
   1194 N. Mathilda Ave
   Sunnyvale, CA  94089
   USA

   Email: jgs@juniper.net


   Yakov Rekhter
   Juniper Networks

   Email: yakov@juniper.net





























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