Graceful Restart Mechanism for BGP with MPLS
RFC 4781

Network Working Group                                         Y. Rekhter
Request for Comments: 4781                                   R. Aggarwal
Category: Standards Track                               Juniper Networks
                                                            January 2007

              Graceful Restart Mechanism for BGP with MPLS

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2007).

Abstract

   A mechanism for BGP that helps minimize the negative effects on
   routing caused by BGP restart has already been developed and is
   described in a separate document ("Graceful Restart Mechanism for
   BGP").  This document extends this mechanism to minimize the negative
   effects on MPLS forwarding caused by the Label Switching Router's
   (LSR's) control plane restart, and specifically by the restart of its
   BGP component when BGP is used to carry MPLS labels and the LSR is
   capable of preserving the MPLS forwarding state across the restart.

   The mechanism described in this document is agnostic with respect to
   the types of the addresses carried in the BGP Network Layer
   Reachability Information (NLRI) field.  As such, it works in
   conjunction with any of the address families that could be carried in
   BGP (e.g., IPv4, IPv6, etc.).

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Table of Contents

   1. Introduction ....................................................2
      1.1. Specification of Requirements ..............................3
   2. General Requirements ............................................3
   3. Capability Advertisement ........................................4
   4. Procedures for the Restarting LSR ...............................4
      4.1. Case 1 .....................................................4
      4.2. Case 2 .....................................................5
      4.3. Case 3 .....................................................5
   5. Alternative Procedures for the Restarting LSR ...................6
   6. Procedures for a Neighbor of a Restarting LSR ...................6
   7. Comparison between Alternative Procedures for the
      Restarting LSR ..................................................7
   8. Security Considerations .........................................8
   9. Acknowledgments .................................................9
   10. References .....................................................9
      10.1. Normative References ......................................9
      10.2. Informative References ....................................9

1.  Introduction

   In the case where a Label Switching Router (LSR) could preserve its
   MPLS forwarding state across restart of its control plane, and
   specifically its BGP component, and BGP is used to carry MPLS labels
   (e.g., as specified in [RFC3107]), it may be desirable not to perturb
   the LSPs going through that LSR (and specifically, the LSPs
   established by BGP) after failure or restart of the BGP component of
   the control plane.  In this document, we describe a mechanism that
   allows this goal to be accomplished.  The mechanism described in this
   document works in conjunction with the mechanism specified in
   [RFC4724].  The mechanism described in this document places no
   restrictions on the types of addresses (address families) that it can
   support.

   The mechanism described in this document is applicable to all LSRs,
   both those with the ability to preserve forwarding state during BGP
   restart and those without it (although the latter need to implement
   only a subset of this mechanism).  Supporting a subset of the
   mechanism described here by the LSRs that cannot preserve their MPLS
   forwarding state across the restart would not reduce the negative
   impact on MPLS traffic caused by their control plane restart.
   However, the impact would be minimized if their neighbor(s) are
   capable of preserving the forwarding state across the restart of
   their control plane, and if they implement the mechanism described
   here.  The subset includes all the procedures described in this
   document, except the procedures in Sections 4.1, 4.2, 4.3, and 5.

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   For the sake of brevity, by "MPLS forwarding state" we mean one of
   the following mappings:
      <incoming label -> (outgoing label, next hop)>
      <Forwarding Equivalence Class (FEC) -> (outgoing label, next hop)>
      <incoming label -> label pop, next hop>
      <incoming label, label pop>

   In the context of this document, the forwarding state that is
   referred to in [RFC4724] means MPLS forwarding state, as defined
   above.  The term "next hop" refers to the next hop as advertised in
   BGP.

1.1.  Specification of Requirements

   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.  General Requirements

   First of all, an LSR MUST implement the Graceful Restart Mechanism
   for BGP, as specified in [RFC4724].  Second, the LSR SHOULD be
   capable of preserving its MPLS forwarding state across the restart of
   its control plane (including the restart of BGP).  Third, for the
   <Forwarding Equivalence Class (FEC) -> label> bindings distributed
   via BGP, the LSR SHOULD be able either (a) to reconstruct the same
   bindings as the LSR had prior to the restart (see Section 4), or (b)
   to create new <FEC -> label> bindings after restart, while
   temporarily maintaining MPLS forwarding state corresponding to both
   the bindings prior to the restart, as well as to the newly created
   bindings (see Section 5).  Fourth, as long as the LSR retains the
   MPLS forwarding state that the LSR preserved across the restart, the
   labels from that state cannot be used to create new local label
   bindings (but could be used to reconstruct the existing bindings, as
   per procedures in Section 4).  Finally, for each next hop, if the
   next hop is reachable via a Label Switched Path (LSP), then the
   restarting LSR MUST be able to preserve the MPLS forwarding state
   associated with that LSP across the restart.

   In the scenario where label binding on an LSR is created/maintained
   not only by the BGP component of the control plane, but also by other
   protocol components (e.g., LDP, RSVP-TE), and where the LSR supports
   restart of the individual components of the control plane that
   create/maintain label binding (e.g., restart of BGP, but no restart
   of LDP), the LSR MUST be able to preserve across the restart the
   information about which protocol has assigned which labels.

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   After the LSR restarts, it MUST follow the procedures as specified in
   [RFC4724].  In addition, if the LSR is able to preserve its MPLS
   forwarding state across the restart, the LSR SHOULD advertise this to
   its neighbors by appropriately setting the Flag for Address Family
   field in the Graceful Restart Capability for all applicable AFI/SAFI
   pairs.

3.  Capability Advertisement

   An LSR that supports the mechanism described in this document
   advertises this to its peer by using the Graceful Restart Capability,
   as specified in [RFC4724].  The Subsequent Address Family Identifier
   (SAFI) in the advertised capability MUST indicate that the Network
   Layer Reachability Information (NLRI) field carries not only
   addressing Information, but also labels (see [RFC3107] for an example
   of where NLRI carries labels).

4.  Procedures for the Restarting LSR

   Procedures in this section apply when a restarting LSR is able to
   reconstruct the same <FEC -> label> bindings as the LSR had prior to
   the restart.

   The procedures described in this section are conceptual and do not
   have to be implemented precisely as described, as long as the
   implementations support the described functionality and their
   externally visible behavior is the same.

   Once the LSR completes its route selection (as specified in Section
   4.1, "Procedures for the Restarting Speaker", of [RFC4724]), then in
   addition to the those procedures, the LSR performs one of the
   following:

4.1.  Case 1

   The following applies when (a) the best route selected by the LSR was
   received with a label, (b) that label is not an Implicit NULL, and
   (c) the LSR advertises this route with itself as the next hop.

   In this case, the LSR searches its MPLS forwarding state (the one
   preserved across the restart) for an entry with <outgoing label, next
   hop> equal to the one in the received route.  If such an entry is
   found, the LSR no longer marks the entry as stale.  In addition, if
   the entry is of type <incoming label, (outgoing label, next hop)>
   rather than <Forwarding Equivalence Class (FEC), (outgoing label,
   next hop)>, the LSR uses the incoming label from the entry when
   advertising the route to its neighbors.  If the found entry has no
   incoming label, or if no such entry is found, the LSR allocates a new

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   label when advertising the route to its neighbors (assuming that
   there are neighbors to which the LSR has to advertise the route with
   a label).

4.2.  Case 2

   The following applies when (a) the best route selected by the LSR was
   received either without a label, with an Implicit NULL label, or the
   route is originated by the LSR; (b) the LSR advertises this route
   with itself as the next hop; and (c) the LSR has to generate a (non-
   Implicit NULL) label for the route.

   In this case, the LSR searches its MPLS forwarding state for an entry
   that indicates that the LSR has to perform label pop, and the next
   hop equal to the next hop of the route in consideration.  If such an
   entry is found, then the LSR uses the incoming label from the entry
   when advertising the route to its neighbors.  If no such entry is
   found, the LSR allocates a new label when advertising the route to
   its neighbors.

   The description in the above paragraph assumes that the LSR generates
   the same label for all the routes with the same next hop.  If this is
   not the case and the LSR generates a unique label per each such
   route, then the LSR needs to preserve across the restart not only
   <incoming label, (outgoing label, next hop)> mapping, but also the
   Forwarding Equivalence Class (FEC) associated with this mapping.  In
   such a case the LSR would search its MPLS forwarding state for an
   entry that (a) indicates label pop (means no outgoing label), (b)
   indicates that the next hop equal to the next hop of the route, and
   (c) has the same FEC as the route.  If such an entry is found, then
   the LSR uses the incoming label from the entry when advertising the
   route to its neighbors.  If no such entry is found, the LSR allocates
   a new label when advertising the route to its neighbors.

4.3.  Case 3

   The following applies when the LSR does not set BGP next hop to self.

   In this case, the LSR, when advertising its best route for a
   particular NLRI, just uses the label that was received with that
   route.  And if the route was received with no label, the LSR
   advertises the route with no label as well.  Either way, the LSR does
   not allocate a label for that route.

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5.  Alternative Procedures for the Restarting LSR

   In this section, we describe an alternative to the procedures
   described in Section "Procedures for the restarting LSR".

   Procedures in this section apply when a restarting LSR does not
   reconstruct the same <FEC -> label> bindings as the LSR had prior to
   the restart, but instead creates new <FEC -> label> bindings after
   restart, while temporarily maintaining MPLS forwarding state
   corresponding to both the bindings prior to the restart, as well as
   to the newly created bindings.

   The procedures described in this section require that for the use by
   BGP graceful restart, the LSR SHOULD have (at least) as many
   unallocated labels as labels allocated for the <FEC -> label>
   bindings distributed by BGP.  The latter forms the MPLS forwarding
   state that the LSR managed to preserve across the restart.  The
   former is used for allocating labels after the restart.

   To create (new) local label bindings after the restart, the LSR uses
   unallocated labels (this is pretty much the normal procedure).

   The LSR SHOULD retain the MPLS forwarding state that the LSR
   preserved across the restart at least until the LSR sends an
   End-of-RIB marker to all of its neighbors (by that time the LSR
   already completed its route selection process, and also advertised
   its Adj-RIB-Out to its neighbors).  The LSR MAY retain the forwarding
   state even a bit longer (the amount of extra time MAY be controlled
   by configuration on the LSR), so as to allow the neighbors to receive
   and process the routes that have been advertised by the LSR.  After
   that, the LSR SHOULD delete the MPLS forwarding state that it
   preserved across the restart.

   Note that while an LSR is in the process of restarting, the LSR may
   have not one, but two local label bindings for a given BGP route --
   one that was retained from prior to restart, and another that was
   created after the restart.  Once the LSR completes its restart, the
   former will be deleted.  However, both of these bindings would have
   the same outgoing label (and the same next hop).

6.  Procedures for a Neighbor of a Restarting LSR

   The neighbor of a restarting LSR (the receiving router terminology
   used in [RFC4724]) follows the procedures specified in [RFC4724].  In
   addition, the neighbor treats the MPLS labels received from the
   restarting LSR the same way that it treats the routes received from
   the restarting LSR (both prior and after the restart).

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   Replacing the stale routes by the routing updates received from the
   restarting LSR involves replacing/updating the appropriate MPLS
   labels.

   In addition, if the Flags in the Graceful Restart Capability received
   from the restarting LSR indicate that the LSR wasn't able to retain
   its MPLS state across the restart, the neighbor SHOULD immediately
   remove all the NLRI and the associated MPLS labels that it previously
   acquired via BGP from the restarting LSR.

   An LSR, once it creates a binding between a label and a Forwarding
   Equivalence Class (FEC), SHOULD keep the value of the label in this
   binding for as long as the LSR has a route to the FEC in the binding.
   If the route to the FEC disappears and then re-appears again later,
   then this may result in using a different label value, as when the
   route re-appears, the LSR would create a new <label, FEC> binding.

   To minimize the potential misrouting caused by the label change, when
   creating a new <label, FEC> binding, the LSR SHOULD pick up the least
   recently used label.  Once an LSR releases a label, the LSR SHALL NOT
   re-use this label for advertising a <label, FEC> binding to a
   neighbor that supports graceful restart for at least the Restart
   Time, as advertised by the neighbor to the LSR.  This rule SHALL
   apply to any label release at any time.

7.  Comparison between Alternative Procedures for the Restarting LSR

   Procedures described in Section 4 involve more computational overhead
   on the restarting router than do the procedures described in Section
   5.

   Procedures described in Section 5 require twice as many labels as
   those described in Section 4.

   Procedures described in Section 4 cause fewer changes to the MPLS
   forwarding state in the neighbors of the restarting router than the
   procedures described in Section 5.

   In principle, it is possible for an LSR to use procedures described
   in Section 4 for some AFI/SAFI(s) and procedures described in Section
   5 for other AFI/SAFI(s).

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8.  Security Considerations

   The security considerations pertaining to the BGP protocol [RFC4271]
   remain relevant.

   In addition, the mechanism described here renders LSRs that implement
   it vulnerable to additional denial-of-service attacks as follows:

      An intruder may impersonate a BGP peer in order to force a failure
      and reconnection of the TCP connection, where the intruder sets
      the Forwarding State (F) bit (as defined in [RFC4724]) to 0 on
      reconnection.  This forces all labels received from the peer to be
      released.

      An intruder could intercept the traffic between BGP peers and
      override the setting of the Forwarding State (F) bit to be set to
      0.  This forces all labels received from the peer to be released.

   All of these attacks may be countered by use of an authentication
   scheme between BGP peers, such as the scheme outlined in [RFC2385].

   As with BGP carrying labels, a security issue may exist if a BGP
   implementation continues to use labels after expiration of the BGP
   session that first caused them to be used.  This may arise if the
   upstream LSR detects the session failure after the downstream LSR has
   released and re-used the label.  The problem is most obvious with the
   platform-wide label space and could result in misrouting of data to
   destinations other than those intended; and it is conceivable that
   these behaviors may be deliberately exploited, either to obtain
   services without authorization or to deny services to others.

   In this document, the validity of the BGP session may be extended by
   the Restart Time, and the session may be re-established in this
   period.  After the expiry of the Restart Time, the session must be
   considered to have failed, and the same security issue applies as
   described above.

   However, the downstream LSR may declare the session as failed before
   the expiration of its Restart Time.  This increases the period during
   which the downstream LSR might reallocate the label while the
   upstream LSR continues to transmit data using the old usage of the
   label.  To reduce this issue, this document requires that labels are
   not re-used until at least the Restart Time.

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9.  Acknowledgments

   We would like to thank Chaitanya Kodeboyina and Loa Andersson for
   their review and comments.  The approach described in Section 5 is
   based on the idea suggested by Manoj Leelanivas.

10.  References

10.1.  Normative References

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

   [RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
             Signature Option", RFC 2385, August 1998.

   [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.

10.2.  Informative References

   [RFC3107] Rekhter, Y. and E. Rosen, "Carrying Label Information in
             BGP-4", RFC 3107, May 2001.

Authors' Addresses

   Yakov Rekhter
   Juniper Networks
   1194 N.Mathilda Ave
   Sunnyvale, CA 94089

   EMail: yakov@juniper.net

   Rahul Aggarwal
   Juniper Networks
   1194 N.Mathilda Ave
   Sunnyvale, CA 94089

   EMail: rahul@juniper.net

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