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Versions: 00 01 02 03

Internet Engineering Task Force                                J. Uttaro
Internet-Draft                                                      AT&T
Intended status: Standards Track                                 E. Chen
Expires: May 17, 2014                                      Cisco Systems
                                                             B. Decraene
                                                                  Orange
                                                              J. Scudder
                                                        Juniper Networks
                                                       November 13, 2013


              Support for Long-lived BGP Graceful Restart
                  draft-uttaro-idr-bgp-persistence-03

Abstract

   In this document we introduce a new BGP capability termed "Long-lived
   Graceful Restart Capability" so that stale routes can be retained for
   a longer time upon session failure.  In addition a new BGP community
   "LLGR_STALE" is introduced for marking stale routes retained for a
   longer time.  We also specify that such long-lived stale routes be
   treated as the least-preferred, and their advertisements be limited
   to BGP speakers that have advertised the new capability.  Use of this
   extension is not advisable in all cases, and we provide guidelines to
   help determine if it is.

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 May 17, 2014.









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Copyright Notice

   Copyright (c) 2013 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.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   4
   2.  Definitions . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Protocol Extensions . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Long-lived Graceful Restart Capability  . . . . . . . . .   5
     3.2.  LLGR_STALE Community  . . . . . . . . . . . . . . . . . .   6
     3.3.  NO_LLGR Community . . . . . . . . . . . . . . . . . . . .   7
   4.  Operation . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.1.  Use of Graceful Restart Capability  . . . . . . . . . . .   7
     4.2.  Session Resets  . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  Processing LLGR_STALE Routes  . . . . . . . . . . . . . .  10
     4.4.  Route Selection . . . . . . . . . . . . . . . . . . . . .  10
     4.5.  Multicast VPN . . . . . . . . . . . . . . . . . . . . . .  10
     4.6.  Errors  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     4.7.  Optional Partial Deployment Procedure . . . . . . . . . .  13
     4.8.  Procedures When BGP is the PE-CE Protocol in a VPN  . . .  13
   5.  Deployment Considerations . . . . . . . . . . . . . . . . . .  14
     5.1.  When BGP is the PE-CE Protocol in a VPN . . . . . . . . .  15
     5.2.  Risks of Depreferencing Routes  . . . . . . . . . . . . .  16
   6.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   7.  Examples of Operation . . . . . . . . . . . . . . . . . . . .  18
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  20
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  20
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  21
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  21
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  22
     11.2.  Informative References . . . . . . . . . . . . . . . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  23

1.  Introduction




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   Historically, routing protocols in general and BGP in particular have
   been designed with a focus on correctness, where a key part of
   "correctness" is for each network element's forwarding state to
   converge toward the current state of the network as quickly as
   possible.  For this reason, the protocol was designed to remove state
   advertised by routers which went down (from a BGP perspective) as
   quickly as possible.  Over time, this has been relaxed somewhat,
   notably by BGP Graceful Restart [RFC4724]; however, the paradigm has
   remained one of attempting to rapidly remove "stale" state from the
   network.

   Over time, two phenomena have arisen that call into question the
   underlying assumptions of this paradigm.  The first is the widespread
   adoption of tunneled forwarding infrastructures, for example MPLS.
   Such infrastructures eliminate the risk of some types of forwarding
   loops that can arise in hop-by-hop forwarding, and thus reduce one of
   the motivations for strong consistency between forwarding elements.
   The second is the increasing use of BGP as a transport for data less
   closely associated with packet forwarding than was originally the
   case.  Examples include the use of BGP for autodiscovery (VPLS
   [RFC4761]) and filter programming (FLOWSPEC [RFC5575]).  In these
   cases, BGP data takes on a character more akin to configuration than
   to traditional routing.

   The observations above motivate a desire to offer network operators
   the ability to choose to retain BGP data for a longer period than has
   hitherto been possible when the BGP control plane fails for some
   reason.  Although the semantics of BGP Graceful Restart [RFC4724] are
   close to those desired, several gaps exist, most notably in maximum
   time for which "stale" information can be retained -- Graceful
   Restart imposes a 4095 second upper bound.

   In this document we introduce a new BGP capability termed "Long-lived
   Graceful Restart Capability" so that stale information can be
   retained for a longer time across a session reset.  We also introduce
   a new BGP community, "LLGR_STALE", to mark such information.  Such
   stale information is to be treated as least-preferred, and its
   advertisement limited to BGP speakers that support the new
   capability.  Where possible, we reference the semantics of BGP
   Graceful Restart [RFC4724] rather than specifying similar semantics
   in this document.

   The expected deployment model for this extension is that it will only
   be invoked for certain address families.  This is discussed in more
   detail in the Deployment Considerations section (Section 5).  When
   used, its use may be combined with that of traditional Graceful
   Restart, in which case it is invoked only after the traditional
   Graceful Restart interval has elapsed, or it may be invoked



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   immediately.  Apart from the potential to greatly extend the timer,
   the most obvious difference between Long-Lived and traditional
   Graceful Restart is that in the Long-Lived version, routes are
   "depreferenced", that is, treated as least-preferred, whereas in the
   traditional version, route preference is not affected.  The design
   choice to treat Long-Lived Stale routes as least-preferred was
   informed by the expectation that they might be retained for a
   (potentially) almost unbounded period of time, whereas in the
   traditional Graceful Restart case, stale routes are retained for only
   a brief interval.  In the GR case, the tradeoff between advertising
   new route status (at the cost of routing churn) and not advertising
   it (at the cost of suboptimal or incorrect route selection) is
   resolved in favor of not advertising, and in the LLGR case, it is
   resolved in favor of advertising new state.

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].

2.  Definitions

   Depreference, Depreferenced:  A route is said to be depreferenced if
     it has its route selection preference reduced in reaction to some
     event.

   GR:  Abbreviation for "Graceful Restart" [RFC4724], also sometimes
     referred to herein as "conventional Graceful Restart" or
     "conventional GR" to distinguish it from the "Long-lived Graceful
     Restart" defined by this document.

   Helper:  Or "helper router".  During Graceful Restart or Long-lived
     Graceful Restart, the router that detects a session failure and
     applies the listed procedures.  [RFC4724] refers to this as the
     "receiving speaker".

   LLGR:  Abbreviation for "Long-lived Graceful Restart".

   LLST:  Abbreviation for "Long-lived Stale Time".

   Route:  We use "route" to mean any information encoded as a BGP NLRI
     and set of path attributes.  As discussed above, the connection
     between such routes and installation of forwarding state may be
     quite remote.

3.  Protocol Extensions




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   A new BGP capability and two new BGP communities are introduced.

3.1.  Long-lived Graceful Restart Capability

   The "Long-lived Graceful Restart Capability" is a new BGP capability
   [RFC5492] that can be used by a BGP speaker to indicate its ability
   to preserve its state according to the procedures of this document.
   This capability MUST be advertised in conjunction with the Graceful
   Restart capability [RFC4724], see the "Use of Graceful Restart
   Capability" section (Section 4.1).

   The capability value consists of one or more tuples <AFI, SAFI,
   Flags, Long-lived Stale Time> as follows:

      +--------------------------------------------------+
      | Address Family Identifier (16 bits)              |
      +--------------------------------------------------+
      | Subsequent Address Family Identifier (8 bits)    |
      +--------------------------------------------------+
      | Flags for Address Family (8 bits)                |
      +--------------------------------------------------+
      | Long-lived Stale Time (24 bits)                  |
      +--------------------------------------------------+
      | ...                                              |
      +--------------------------------------------------+
      | Address Family Identifier (16 bits)              |
      +--------------------------------------------------+
      | Subsequent Address Family Identifier (8 bits)    |
      +--------------------------------------------------+
      | Flags for Address Family (8 bits)                |
      +--------------------------------------------------+
      | Long-lived Stale Time (24 bits)                  |
      +--------------------------------------------------+


   The meaning of the fields are as follows:

      Address Family Identifier (AFI), Subsequent Address Family
      Identifier (SAFI):



         The AFI and SAFI, taken in combination, indicate that the BGP
         speaker has the ability to preserve its forwarding state for
         the address family during a subsequent BGP restart.  Routes may
         be explicitly associated with a particular AFI and SAFI using
         the encoding of [RFC4760] or implicitly associated with
         <AFI=IPv4, SAFI=Unicast> if using the encoding of [RFC4271].



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      Flags for Address Family:



         This field contains bit flags relating to routes that were
         advertised with the given AFI and SAFI.

             0 1 2 3 4 5 6 7
            +-+-+-+-+-+-+-+-+
            |F|   Reserved  |
            +-+-+-+-+-+-+-+-+




         The most significant bit is used to indicate whether the state
         for routes that were advertised with the given AFI and SAFI has
         indeed been preserved during the previous BGP restart.  When
         set (value 1), the bit indicates that the state has been
         preserved.  This bit is called the "F bit" since it was
         historically used to indicate preservation of Forwarding State.
         Use of the F bit is detailed in the Session Resets section
         (Section 4.2).

         The remaining bits are reserved and MUST be set to zero by the
         sender and ignored by the receiver.

      Long-lived Stale Time:



         This time (in seconds) specifies how long stale information
         (for the AFI/SAFI) may be retained (possibly in conjunction
         with the period specified by the "Restart Time" in the Graceful
         Restart Capability, if present).

3.2.  LLGR_STALE Community

   We introduce a new BGP community [RFC1997] "LLGR_STALE" (value: TBD).
   It can be used to mark stale routes retained for a longer period of
   time.  Such long-lived stale routes are to be handled according to
   the procedures specified in the Operation section (Section 4).

   An implementation MAY allow users to configure policies that accept,
   reject, or modify routes based on the presence or absence of this
   community.





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3.3.  NO_LLGR Community

   We introduce a new BGP community "NO_LLGR" (value: TBD).  It can be
   used to mark routes which a BGP speaker does not want treated
   according to these procedures, as detailed in the Operation section
   (Section 4).

   An implementation MAY allow users to configure policies that accept,
   reject, or modify routes based on the presence or absence of this
   community.

4.  Operation

   A BGP speaker MAY use BGP Capabilities Advertisements [RFC5492] to
   advertise the "Long-lived Graceful Restart Capability" to indicate
   its ability to retain state and perform related procedures specified
   in this document.  The setting of the parameters for an AFI/SAFI
   depends on the properties of the BGP speaker, network scale, and
   local configuration.

   In the presence of the "Long-lived Graceful Restart Capability", the
   procedures specified in [RFC4724] and
   [I-D.ietf-idr-bgp-gr-notification] continue to apply unless
   explicitly revised by this document.

4.1.  Use of Graceful Restart Capability

   The Graceful Restart capability MUST be advertised in conjunction
   with the LLGR capability.  If it is not so advertised, the LLGR
   capability MUST be disregarded.  The purpose for mandating that both
   be used in conjunction is to enable reuse of certain base mechanisms
   that are common to both "flavors", notably origination, collection
   and processing of EoR, as well as the finite state machine
   modifications and connection reset logic introduced by GR.

   We observe that if support for conventional Graceful Restart is not
   desired for the session, the conventional GR phase can be skipped by
   omitting all AFI/SAFI from the GR capability, advertising a Restart
   Time of zero, or both.  The Session Resets section (Section 4.2)
   discusses the interaction of conventional and long-lived GR.

4.2.  Session Resets

   BGP Graceful Restart [RFC4724], updated by
   [I-D.ietf-idr-bgp-gr-notification], defines conditions under which a
   BGP session can reset and have its associated routes retained.  If
   such a reset occurs for a session for which the LLGR Capability has
   also been exchanged, the following procedures apply.



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   If the Graceful Restart Capability that was received does not list
   all AFI/SAFI supported by the session, then for those non-listed AFI/
   SAFI the GR "Restart Time" shall be deemed zero.  Similarly, if the
   received LLGR Capability does not list all AFI/SAFI supported by the
   session, then for those non-listed AFI/SAFI the "Long-lived Stale
   Time" shall be deemed zero.

   The following text in Section 4.2 of the GR specification [RFC4724]
   no longer applies:

      If the session does not get re-established within the "Restart
      Time" that the peer advertised previously, the Receiving Speaker
      MUST delete all the stale routes from the peer that it is
      retaining.

   and the following procedures are specified instead:

   After the session goes down and before the session is re-established,
   the stale routes for an AFI/SAFI MUST be retained.  The interval for
   which they are retained is limited by the sum of the "Restart Time"
   in the received Graceful Restart Capability and the "Long-lived Stale
   Time" in the received Long-lived Graceful Restart Capability.  These
   timers MAY be modified by local configuration.

   If the value of the "Restart Time" or the "Long-lived Stale Time" is
   zero, the duration of the corresponding period would be zero seconds.
   So, for example, if the "Restart Time" is zero and the "Long-lived
   Stale Time" is nonzero, only the procedures particular to LLGR would
   apply.  Conversely, if the "Long-lived Stale Time" is zero and the
   "Restart Time" is nonzero, only the procedures of GR would apply.  If
   both are zero, none of these procedures would apply, only those of
   the base BGP specification (although EoR would still be used as
   detailed in [RFC4724]).  And finally, if both are nonzero, then the
   procedures would be applied serially -- first those of GR, then those
   of LLGR.  We observe that during the first interval, while the
   procedures of GR are in effect, route preference would not be
   affected, while during the second interval, while LLGR procedures are
   in effect, routes would be treated as least-preferred as specified
   elsewhere in this document.

   Once the "Restart Time" period ends (including the case that the
   "Restart Time" is zero), the LLGR period is said to have begun and
   the following procedures MUST be performed:

   o  The helper router MUST start a timer for the "Long-lived Stale
      Time".  If the timer for the "Long-lived Stale Time" expires
      before the session is re-established, the helper MUST delete all
      the stale routes from the neighbor that it is retaining.



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   o  The helper router MUST attach the LLGR_STALE community for the
      stale routes being retained.  Note that this requirement implies
      that the routes would need to be readvertised, to disseminate the
      modified community.

   o  If any of the routes from the peer have been marked with the
      NO_LLGR community, either as sent by the peer, or as the result of
      a configured policy, they MUST NOT be retained, but MUST be
      removed as per the normal operation of [RFC4271].

   o  The helper router MUST perform the procedures listed under
      Section 4.3.

   Once the session is re-established, the procedures specified in
   [RFC4724] apply for the stale routes irrespective of whether the
   stale routes are retained during the "Restart Time" period or the
   "Long-lived Stale Time" period.  However, in the case of consecutive
   restarts (i.e, the session goes down before the EoR is received) the
   previously marked stale routes MUST NOT be deleted before the timer
   for the "Long-lived Stale Time" expires.

   Similarly to [RFC4724], once the session is re-established, if the F
   bit for a specific address family is not set in the newly received
   LLGR Capability, or if a specific address family is not included in
   the newly received LLGR Capability, or if the LLGR and accompanying
   GR Capability are not received in the re-established session at all,
   then the Helper MUST immediately remove all the stale routes from the
   peer that it is retaining for that address family.

   If a "Long-lived Stale Time" timer is running for a peer, it MUST NOT
   be updated (other than by manual operator intervention) until the
   peer has established and synchronized a new session.  The session is
   termed "synchronized" once the EoR has been received from the peer.

   The value of the "Long-lived Stale Time" in the capability received
   from a neighbor MAY be reduced by local configuration.

   While the session is down, the expiration of the "Long-lived Stale
   Time" timer is treated analogously to the expiration of the "Restart
   Time" timer in Graceful Restart.  However, the timer continues to run
   once the session has re-established.  The timer is not stopped, nor
   updated, until EoR is received from the peer.  If the timer expires
   during synchronization with the peer, any stale routes that the peer
   has not refreshed, are removed.  If the session subsequently resets
   prior to becoming synchronized, any remaining routes should be
   removed immediately.





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4.3.  Processing LLGR_STALE Routes

   A BGP speaker that has advertised the "Long-lived Graceful Restart
   Capability" to a neighbor MUST perform the following upon receiving a
   route from that neighbor with the "LLGR_STALE" community, or upon
   attaching the "LLGR_STALE" community itself per Section 4.2:

   o  Treat the route as the least-preferred in route selection (see
      below).  See the Risks of Depreferencing Routes section
      (Section 5.2) for a discussion of potential risks inherent in
      doing this.

   o  The route SHOULD NOT be advertised to any neighbor from which the
      Long-lived Graceful Restart Capability has not been received.  The
      exception is described in the Optional Partial Deployment
      Procedure section (Section 4.7).  Note that this requirement
      implies that such routes should be withdrawn from any such
      neighbor.

   o  The "LLGR_STALE" community MUST NOT be removed when the route is
      further advertised.

4.4.  Route Selection

   In this document, when we refer to treating a route as least-
   preferred, this means the route MUST be treated as less preferred
   than any other route that is not so treated.  When performing route
   selection between two routes both of which are least-preferred,
   normal tie-breaking applies.  Note that this would only be expected
   to happen if the only routes available for selection were least-
   preferred -- in all other cases, such routes would have been
   eliminated from consideration.

4.5.  Multicast VPN

   If LLGR is being used in a network that carries Multicast VPN (MVPN)
   traffic ([RFC6513],[RFC6514]), special considerations apply.

   [RFC6513] defines the notion of the "Upstream PE" and the "Upstream
   Multicast Hop" (UMH) for a particular multicast flow.  To determine
   the Upstream PE and/or the UMH for a particular flow, a particular
   set of comparable BGP routes (the "UMH-eligible" routes for that
   flow, as defined in [RFC6513]) is considered, and the "best" one
   (according to the BGP bestpath selection algorithm) is chosen.  The
   UMH-eligible routes are routes with AFI/SAFI 1/1, 1/2, 2/1, or 2/2.
   When a router detects a change in the Upstream PE or UMH for a given
   flow, the router may modify its data plane state for that flow.  For
   example, the router may begin to discard any packets of the flow that



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   it believes have arrived from the previously chosen Upstream PE or
   UMH.  The assumption is that the newly chosen Upstream PE and/or UMH
   will make the corresponding changes, if necessary, to their own data
   plane states.  In addition, if a router detects a change in the
   Uptream PE or UMH for a given flow, it may originate or readvertise
   (with different attributes) certain of the BGP MCAST-VPN routes
   (routes with SAFI 5) that are defined in [RFC6514].  The assumption
   is that the MCAST-VPN routes will be properly distributed by BGP to
   other routers that have data plane states for the given flow, i.e.,
   that BGP will converge so that all routers handle the flow in a
   consistent manner.

   However, if detection of a change to the Upstream PE or UMH is based
   entirely on stale routes, one cannot assume that BGP will converge;
   rather one must assume that the UMH-eligible routes and the MCAST-VPN
   routes are not being properly distributed.  Since the purpose of the
   LLGR procedures is to try to keep the data flowing (by "freezing" the
   data plane states) when the control plane updates are not being
   properly distributed, it does not seem appropriate to react to
   changes that are based entirely on stale routes.  Therefore, the
   following rules MUST be applied when a router is computing or
   recomputing the Upstream PE and/or the UMH for a given multicast
   flow:

   o  STALE routes (i.e., UMH-eligible routes with the LLGR_STALE
      attribute) are less preferable than non-STALE routes.

   o  If all the UMH-eligible routes for a given flow are STALE, then
      the Upstream PE and/or UMH for that flow is considered to be
      "stale".

   o  If the Upstream PE or UMH for a given multicast flow has already
      been determined, and the result of a new computation yields a new
      Upstream PE or UMH, but the Upstream PE or UMH is "stale" (as
      defined just above), then the Upstream PE and/or UMH for that flow
      MUST be left unchanged.

   o  If the Upstream PE or UMH for a given multicast flow has not
      already been determined, but is now determined to be STALE, the
      multicast flow is considered to have no reachable Upstream PE and/
      or UMH.

   [RFC6514] also defines a set of route types with SAFI 5 ("MCAST-VPN"
   routes).  LLGR can be applied to MCAST-VPN routes.  However, the
   following MCAST-VPN route types require special procedures, as
   specified in this section:

   o  Leaf A-D routes



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   o  C-multicast Shared Tree Join routes
   o  C-multicast Source Tree Join routes

   Routes of these three types are always "targeted" to a particular
   upstream router.  Depending on the situation, the targeted router may
   be the Upstream PE for a given flow or the UMH for a given flow.
   Alternatively, the targeted router may be determined by choosing the
   "best" route (according to the BGP bestpath algorithm) from among a
   set of comparable Intra-AS I-PMSI A-D routes, or from among a set of
   comparable Inter-AS I-PMSI A-D routes, or from among a set of
   comparable S-PMSI A-D routes.  (See [RFC6513], [RFC6514], [RFC6625],
   and draft-ietf-mpls-seamless-mcast for details.)  Once the target is
   chosen, it is identified in an IPv4-address-specific Route Target
   (RT) or an IPv6-address-specific RT that is attached to the route
   before the route is advertised.  If the target for one of these
   routes changes, the value of the attached RT will also change.  This
   in turn may cause the route to be advertised, readvertised, or
   withdrawn on specific BGP sessions.

   For cases where the targeted router is the Upstream PE or the UMH for
   a particular flow, the rules given previously in this section apply.
   For example, if a Leaf A-D route is targeted to a flow's UMH, and all
   the relevant UMH-eligible routes are stale, the UMH is left
   unchanged.  Thus the Leaf A-D route is not readvertised with a new
   RT.

   In those cases where the targeted router for a given Leaf A-D route
   is selected by comparing a set of S-PMSI A-D routes, or where the
   targeted router for a given C-multicast Shared or Source Tree Join
   route is selected by comparing a set of Inter-AS I-PMSI A-D routes,
   the following rules MUST be applied:

   o  STALE routes (i.e., "I/S-PMSI A-D routes" with the LLGR_STALE
      attribute) are less preferable than non-STALE routes.

   o  If all the routes being considered are STALE, then the targeted
      router of the Leaf A-D route or C-multicast Shared or Source Tree
      Join route MUST NOT be changed.

   This prevents a Leaf A-D route or C-multicast route from being
   targeted to a particular router if the relevant I/S-PMSI A-D routes
   from that router are stale.  Since those routes are stale, it is
   likely that the Leaf A-D route or C-multicast route would not make it
   to the targeted router, in which case it is better to maintain the
   existing data plane states than to make changes that presuppose that
   the MCAST-VPN routes will be properly distributed.





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4.6.  Errors

   If the LLGR capability is received without an accompanying GR
   capability, the LLGR capability MUST be ignored, that is, the
   implementation MUST behave as though no LLGR capability had been
   received.

4.7.  Optional Partial Deployment Procedure

   Ideally, all routers in an Autonomous System would support this
   specification before it was enabled.  However, to facilitate
   incremental deployment, stale routes MAY be advertised to neighbors
   that have not advertised the Long-lived Graceful Restart Capability
   under the following conditions:

   o  The neighbors MUST be internal (IBGP or Confederation) neighbors.

   o  The NO_EXPORT community [RFC1997] MUST be attached to the stale
      routes.

   o  The stale routes MUST have their LOCAL_PREF set to zero.  See the
      Risks of Depreferencing Routes section (Section 5.2) for a
      discussion of potential risks inherent in doing this.

   If this strategy for partial deployment is used, the network operator
   should set LOCAL_PREF to zero for all LLGR routes throughout the
   Autonomous System.  This trades off a small reduction in flexibility
   (ordering may not be preserved between competing LLGR routes) for
   consistency between routers which do, and do not, support this
   specification.  Since consistency of route selection can be important
   for preventing forwarding loops, the latter consideration dominates.

4.8.  Procedures When BGP is the PE-CE Protocol in a VPN

   In VPN deployments, for example [RFC4364], BGP is often used as a PE-
   CE protocol.  It may be a practical necessity in such deployments to
   accommodate interoperation with CEs that cannot easily be upgraded to
   support specifications such as this one.  This leads to a problem: in
   this specification, we take pains to ensure that "stale" routing
   information will not leak beyond the perimeter of routers that
   support these procedures, so that it can be depreferenced as
   expected, and we provide a workaround (Section 4.7) for the case
   where one or more IBGP routers are not upgraded.  However, in the VPN
   PE-CE case, the protocol in use is EBGP, and our workaround does not
   work since it relies on the use of LOCAL_PREF, an IBGP-only path
   attribute.





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   We observe that the principal motivation for restricting the
   propagation of "stale" routing information is the desire to prevent
   it from spreading without limit once it exits the "safe" perimeter.
   We further observe that VPN deployments are typically topologically
   constrained, making this concern moot.  For this reason, an
   implementation MAY advertise stale routes over a PE-CE session, when
   explicitly configured to do so.  That is, the second rule listed in
   Section 4.3 MAY be disregarded in such cases.  All other rules
   continue to apply.  Finally, if this exception is used, the
   implementation SHOULD by default attach the NO_EXPORT community to
   the routes in question, as an additional protection against stale
   routes spreading without limit.  Attachment of the NO_EXPORT
   community MAY be disabled by explicit configuration, to accommodate
   exceptional cases.

   See further discussion in Section 5.1.

5.  Deployment Considerations

   The deployment considerations discussed in [RFC4724] apply to this
   document.  In addition, network operators are cautioned to carefully
   consider the potential disadvantages of deploying these procedures
   for a given AFI/SAFI.  Most notably, if used for an AFI/SAFI that
   conveys traditional reachability information, use of a long-lived
   stale route could result in a loss of connectivity for the covered
   prefix.  This specification takes pains to mitigate this risk where
   possible, by making such routes least-preferred and by restricting
   the scope of such routes to routers that support these procedures
   (or, optionally, a single Autonomous System, see "Optional Partial
   Deployment Procedure", above).  However, according to the normal
   rules of IP forwarding a stale more-specific route, that has no non-
   stale alternate paths available, will still be used instead of a non-
   stale less-specific route.  Networks in which the deployment of these
   procedures would be especially concerning include those which do not
   use "tunneled" forwarding (in other words, those using traditional
   hop-by-hop forwarding).

   Implementations MUST NOT enable these procedures by default.  They
   MUST require affirmative configuration per AFI/SAFI in order to
   enable them.

   The procedures of this document do not alter the route resolvability
   requirement of [RFC4271] Section 9.1.2.1.. Because of this, it will
   commonly be the case that "stale" IBGP routes will only continue to
   be used if the router depicted in the next hop remains resolvable,
   even if its BGP component is down.  Details of IGP fault-tolerance
   strategies are beyond the scope of this document.  In addition to the
   foregoing, it may be advisable to check the viability of the next hop



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   through other means, see for example
   [I-D.ietf-idr-bgp-bestpath-selection-criteria].  This may be
   especially useful in cases where the next hop is known directly at
   the network layer, notably EBGP.

   As discussed in this document, after a BGP session goes down and
   before the session is re-established, stale routes may be retained
   for up to two consecutive periods, controlled by the "Restart Time"
   and the "Long-lived Stale Time", respectively.  During the first
   period routing churn would be prevented but with potential
   blackholing of traffic.  During the second period potential
   blackholing of traffic may be reduced but routing churn would be
   visible throughout the network.  The setting of the relevant
   parameters for a particular application should take into account the
   tradeoffs, the network dynamics and potential failure scenarios.  If
   needed, the first period can be bypassed either by local
   configuration or by setting the "Restart Time" in the Graceful
   Restart Capability to zero and/or not listing the AFI/SAFI in that
   Capability.

   The setting of the F bit (and the "Forwarding State" bit of the
   accompanying GR capability) depends in part on deployment
   considerations.  The F bit can be understood as an indication that
   the Helper should flush associated routes (if the bit is left clear).
   As discussed in the Introduction, an important use case for LLGR is
   for routes that are more akin to configuration than to traditional
   routing.  For such routes, it may make sense to always set the F bit,
   regardless of other considerations.  Likewise, for control-plane-only
   entities such as dedicated route reflectors, that do not participate
   in the forwarding plane, it makes sense to always set the F bit.
   Overall, the rule of thumb is that if loss of state on the restarting
   router can reasonably be expected to cause a forwarding loop or black
   hole, the F bit should be set scrupulously according to whether state
   has been retained.  Specifics of when the F bit is, and is not, set
   is implementation-dependent and may also be controlled by
   configuration.

5.1.  When BGP is the PE-CE Protocol in a VPN

   As discussed in Section 4.8, it may be necessary to advertise stale
   routes to a CE in some VPN deployments, even if the CE does not
   support this specification.  In that case, the network operator
   configuring their PE to advertise such routes should notify the
   operator of the CE receiving the routes, and the CE should be
   configured to depreference the routes.  Typical BGP implementations
   will be able to do this by matching on the LLGR_STALE community, and
   setting the LOCAL_PREF for matching routes to zero, similar to the
   procedure described in Section 4.7.



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5.2.  Risks of Depreferencing Routes

   Depreferencing EBGP routes is considered safe, no different from the
   common practice of applying a routing policy to an EBGP session.
   However, the same is not always true of IBGP.

   Consistent route selection is a fundamental tenet of IBGP correctness
   and safe operation in hop-by-hop routed networks.  When routers
   within an AS apply different criteria in selecting routes, they can
   arrive at inconsistent route selections, potentially with the
   consequence of forming forwarding loops unless some form of tunneled
   forwarding is used to prevent "core" routers from making a
   (potentially inconsistent) forwarding decision based on the IP
   header.

   This specification uses the state of a peering session as an input to
   the selection criteria, depreferencing routes that are associated
   with a session that has gone down but have not yet aged out.  Since
   different routers within an AS might have different notions as to
   whether their respective sessions with a given peer are up or down,
   they might apply different selection criteria to routes from that
   peer.  This could result in a forwarding loop forming between such
   routers.

   For an example of such a forwarding loop, consider the following
   simple topology:

     A ---- B ---- C ------------------------- D
     ^                                         ^
     |                                         |
     R1                                        R2


   In this example, A - D are routers with a full mesh of IBGP sessions
   between them.  The short links have unit cost, the long link has cost
   5.  Routers A and D are AS border routers, each advertising some
   route, R, into the AS -- these are denoted R1 and R2 in the diagram.
   In ordinary operation, it can be seen that routers B and C will
   select R1 for forwarding, and will forward toward A.












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   Suppose that the session between A and B goes down for some reason,
   and stays down long enough for LLGR processing to be invoked on B.
   Then on B, route R1 will be depreferenced, leading to the selection
   of R2 by B.  However, C will continue to prefer R1.  It can be seen
   that in this case, a forwarding loop for packets destined to R would
   form between B and C. (We note that other forwarding loop scenarios
   can be constructed for traditional GR, but are generally considered
   less severe since GR can remain in effect for a much more limited
   interval.)

   The potential benefits of this specification can outweigh the risks
   discussed above, as long as care is exercised in deployment.  The
   cardinal rule to be followed is, if a given set of routes are being
   used within an AS for hop-by-hop forwarding, it is NOT RECOMMENDED to
   enable LLGR procedures.  If tunneled forwarding (such as MPLS) is
   used within the AS, or if routes are being used for purposes other
   than hop-by-hop forwarding, less caution is needed, though the
   operator should still carefully consider the consequences of enabling
   LLGR.

6.  Security Considerations

   The security implications of the LLGR 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 (for example 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 (for example 20 minutes).

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



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   Therefore, to avoid VPN breach, before enabling BGP LLGR, 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 (for example per route or per VRF/CE), the re-
      allocation policy (for example least recently used label...)

   Note that [RFC4781] which defines Graceful Restart Mechanism for BGP
   with MPLS is also applicable to BGP LLGR.

   In addition to these considerations, the LLGR 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 LLGR state between two PE devices,
   whilst engineering label reallocation to occur in a manner that
   results in the two topologies overlapping.  Such allocation is
   particularly difficult to engineer (since it is typically an internal
   mechanism of an LSR).

7.  Examples of Operation

   For illustrative purposes, we present a few examples of how this
   specification might be used in practice.  These examples are neither
   exhaustive nor normative.

   Consider the following scenario: A border router, ASBR1, has an IBGP
   peering with a route reflector, RR1, from which it learns routes.  It
   has an EBGP peering with an external peer, EXT, to which it
   advertises those routes.  The external peer has advertised the GR and
   LLGR Capabilities to ASBR1.  ASBR1 is configured to support GR and
   LLGR on its session with RR1 and EXT.  RR1 advertises a GR Restart
   Time of 1 (second) and a LLST of 3600 (seconds):

   +------------+------------------------------------------------------+
   | Time       | Event                                                |
   +------------+------------------------------------------------------+
   | t          | ASBR1's IBGP session with RR fails. ASBR1 retains    |
   |            | RR's routes according to the rules of GR [RFC4724]   |
   |            |                                                      |
   | t+1        | GR Restart Time expires. ASBR1 transitions RR's      |
   |            | routes to long-lived stale by attaching the          |
   |            | LLGR_STALE community and depreferencing them.        |



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   |            | However, since it has no backup routes, it continues |
   |            | to make use of them. It re-announces them to EXT     |
   |            | with the LLGR_STALE community attached.              |
   |            |                                                      |
   | t+1+3600   | LLST expires. ASBR1 removes RR's stale routes from   |
   |            | its own RIB and sends BGP updates to withdraw them   |
   |            | from EXT.                                            |
   +------------+------------------------------------------------------+


   Next, imagine the same scenario but suppose RR1 advertised a GR
   Restart Time of zero, effectively disabling GR.  Equally, ASBR1 could
   have used local configuration to override RR1's offered Restart Time,
   setting it to a locally-configured value of zero:

   +------------+------------------------------------------------------+
   | Time       | Event                                                |
   +------------+------------------------------------------------------+
   | t          | ASBR1's IBGP session with RR fails. ASBR1            |
   |            | transitions RR's routes to long-lived stale by       |
   |            | attaching the LLGR_STALE community and               |
   |            | depreferencing them. However, since it has no backup |
   |            | routes, it continues to make use of them. It re-     |
   |            | announces them to EXT with the LLGR_STALE community  |
   |            | attached.                                            |
   |            |                                                      |
   | t+0+3600   | LLST expires. ASBR1 removes RR's stale routes from   |
   |            | its own RIB and sends BGP updates to withdraw them   |
   |            | from EXT.                                            |
   +------------+------------------------------------------------------+


   Next, imagine the original scenario, but consider that the ASBR1-RR1
   session comes back up and becomes synchronized 180 seconds after the
   failure was detected:

   +---------+---------------------------------------------------------+
   | Time    | Event                                                   |
   +---------+---------------------------------------------------------+
   | t       | ASBR1's IBGP session with RR fails. ASBR1 retains RR's  |
   |         | routes according to the rules of GR [RFC4724]           |
   |         |                                                         |
   | t+1     | GR Restart Time expires. ASBR1 transitions RR's routes  |
   |         | to long-lived stale by attaching the LLGR_STALE         |
   |         | community and depreferencing them. However, since it    |
   |         | has no backup routes, it continues to make use of them. |
   |         | It re-announces them to EXT with the LLGR_STALE         |
   |         | community attached.                                     |



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   |         |                                                         |
   | t+1+179 | Session is reestablished and resynchronized. ASBR1      |
   |         | removes the LLGR_STALE community from RR1's routes and  |
   |         | re-announces them to EXT with the LLGR_STALE community  |
   |         | removed.                                                |
   +---------+---------------------------------------------------------+


   Finally, imagine the original scenario, but consider that EXT has not
   advertised the LLGR Capability to ASBR1:

   +------------+------------------------------------------------------+
   | Time       | Event                                                |
   +------------+------------------------------------------------------+
   | t          | ASBR1's IBGP session with RR fails. ASBR1 retains    |
   |            | RR's routes according to the rules of GR [RFC4724]   |
   |            |                                                      |
   | t+1        | GR Restart Time expires. ASBR1 transitions RR's      |
   |            | routes to long-lived stale by attaching the          |
   |            | LLGR_STALE community and depreferencing them.        |
   |            | However, since it has no backup routes, it continues |
   |            | to make use of them. It withdraws them from EXT.     |
   |            |                                                      |
   | t+1+3600   | LLST expires. ASBR1 removes RR's stale routes from   |
   |            | its own RIB.                                         |
   +------------+------------------------------------------------------+


8.  Acknowledgements

   We would like to thank Roberto Fragassi, John Medamana, Han Nguyen,
   Jeffrey Haas, Nabil Bitar, Nicolai Leymann, Pranav Mehta, Saikat Ray,
   Martin Djernaes and Eric Rosen for their valuable inputs and
   contributions to the discussions and solutions.

9.  Contributors

    Clarence Filsfils
    Cisco Systems
    Brussels  1000
    Belgium

    Email: cf@cisco.com


    Pradosh Mohapatra
    Cumulus Networks




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    Email: pmohapat@cumulusnetworks.com


    Yakov Rekhter
    Juniper Networks

    Email: yakov@juniper.net


    Eric Rosen
    Cisco Systems

    Email: erosen@cisco.com


    Rob Shakir
    BT

    Email: rob.shakir@bt.com


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

    Email: adam.simpson@alcatel-lucent.com


10.  IANA Considerations

   This document defines a new BGP capability - Long-lived Graceful
   Restart Capability.  The Capability Code needs to be assigned by
   IANA.

   This document introduces a new BGP community "LLGR_STALE" for marking
   the long-lived stale routes, and another community "NO_LLGR" to
   indicate that stale routes should not be retained.  These community
   values need to be assigned by IANA.

11.  References









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11.1.  Normative References

   [I-D.ietf-idr-bgp-gr-notification]
              Patel, K., Fernando, R., Scudder, J., and J. Haas,
              "Notification Message support for BGP Graceful Restart",
              draft-ietf-idr-bgp-gr-notification-01 (work in progress),
              April 2013.

   [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.

   [RFC4724]  Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
              Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724,
              January 2007.

   [RFC4760]  Bates, T., Chandra, R., Katz, D., and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 4760, January
              2007.

   [RFC5492]  Scudder, J. and R. Chandra, "Capabilities Advertisement
              with BGP-4", RFC 5492, February 2009.

   [RFC6513]  Rosen, E. and R. Aggarwal, "Multicast in MPLS/BGP IP
              VPNs", RFC 6513, February 2012.

   [RFC6514]  Aggarwal, R., Rosen, E., Morin, T., and Y. Rekhter, "BGP
              Encodings and Procedures for Multicast in MPLS/BGP IP
              VPNs", RFC 6514, February 2012.

11.2.  Informative References

   [I-D.ietf-idr-bgp-bestpath-selection-criteria]
              Asati, R., "BGP Bestpath Selection Criteria Enhancement",
              draft-ietf-idr-bgp-bestpath-selection-criteria-06 (work in
              progress), February 2013.

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

   [RFC4761]  Kompella, K. and Y. Rekhter, "Virtual Private LAN Service
              (VPLS) Using BGP for Auto-Discovery and Signaling", RFC
              4761, January 2007.



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   [RFC4781]  Rekhter, Y. and R. Aggarwal, "Graceful Restart Mechanism
              for BGP with MPLS", RFC 4781, January 2007.

   [RFC5575]  Marques, P., Sheth, N., Raszuk, R., Greene, B., Mauch, J.,
              and D. McPherson, "Dissemination of Flow Specification
              Rules", RFC 5575, August 2009.

Authors' Addresses

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

   Email: ju1738@att.com


   Enke Chen
   Cisco Systems
   170 W. Tasman Drive
   San Jose, CA  95134
   USA

   Email: enkechen@cisco.com


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

   Email: bruno.decraene@orange.com


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

   Email: jgs@juniper.net








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