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Versions: 00 01 02 03 04 05 RFC 4781

Network Working Group               Yakov Rekhter (Juniper Networks)
Internet Draft                     Rahul Aggarwal (Juniper Networks)
Expiration Date: February 2006


              Graceful Restart Mechanism for BGP with MPLS

                draft-ietf-mpls-bgp-mpls-restart-05.txt



Status of this Memo

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   By submitting this Internet-Draft, each author represents that any
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Abstract

   A mechanism for BGP that helps minimize the negative effects on
   routing caused by BGP restart has already been developed an is
   described in a separate document ("Graceful Restart Mechanism for
   BGP"). This document extends this mechanism to also 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 famililies that could be carried
   in BGP (e.g., IPv4, IPv6, etc...)

   The mechanism described in this document is applicable to all LSRs,
   both those with the ability to preserve their forwarding state during
   BGP restart and those without (although the latter need to implement
   only a subset of the mechanism described in this document).
   Supporting a subset of the mechanism described here by the LSRs that
   can not preserve their MPLS forwarding state across the restart would
   not reduce the negative impact on MPLS traffic caused by their
   control plane restart, but it would minimize the impact if their
   neighbor(s) are capable of preserving the forwarding state across the
   restart of their control plane and implement the mechanism described
   here.


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].















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1. Introduction

   For the sake of brevity in the context of this document by "MPLS
   forwarding state" we mean either <incoming label -> (outgoing label,
   next hop)>, or <Forwarding Equivalence Class (FEC) -> (outgoing
   label, next hop)>, or <incoming label -> label pop, next hop>, or
   <incoming label, label pop> mapping. In the context of this document
   the forwarding state that is referred to in [1] means MPLS forwarding
   state, as defined above. In the context of this document the term
   "next hop" refers to the next hop as advertised in BGP.

   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 of 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 [1].
   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 (although the latter need to implement only
   a subset of the mechanism described in this document).  Supporting a
   subset of the mechanism described here by the LSRs that can not
   preserve their MPLS forwarding state across the restart would not
   reduce the negative impact on MPLS traffic caused by their control
   plane restart, but it would minimize the impact if their neighbor(s)
   are capable of preserving the forwarding state across the restart of
   their control plane and 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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2. General requirements

   First of all an LSR MUST implement the Graceful Restart Mechanism for
   BGP, as specified in [1]. 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 can not 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 just by the BGP component of the control plane, but by other
   protocol components as well (e.g., LDP, RSVP-TE), and 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.

   After the LSR restarts, it MUST follow the procedures as specified in
   [1]. 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.

















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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 [1]. The Subsequent Address Family Identifier (SAFI)
   in the advertised capability MUST indicate that the Network Layer
   Reachability Information (NLRI) field carries not just addressing
   Information, but labels as well (see [RFC3107] as 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 here, 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
   "Procedures for the Restarting Speaker" of [1]), then in addition to
   the procedures specified in [1], 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
   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).





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4.2. Case 2

   The following applies when (a) the best route selected by the LSR was
   received either without a label, or 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 just
   <incoming label, (outgoing label, next hop)> mapping, but also the
   Forwarding Equivalence Class (FEC) associated with this mapping.  In
   such case the LSR would search its MPLS forwarding state for an entry
   that (a) indicates Label pop (means no outgoing label), (b) 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 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), 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. Both of these bindings though would have the
   same outgoing label (and the same next hop).












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6. Procedures for a neighbor of a restarting LSR

   The neighbor of a restarting LSR (the receiving router in terminology
   used in [1]) follows the procedures specified in [1].  In addition,
   the neighbor treats the MPLS labels received from the restarting LSR
   the same way as it treats the routes received from the restarting LSR
   (both prior and after the restart).

   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 mis-routing 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 relative to the procedures described in
   Section 5.

   Procedures described in Section 5 requires twice as many labels as
   the procedures 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



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   Section 4 for some AFI/SAFI(s) and procedures described in Section 5
   for other AFI/SAFI(s).



8. Security Consideration

   The security considerations pertaining to the original BGP protocol
   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, but where the intruder
      sets the  Forwarding State (F) bit (as defined in [1]) 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 mis-routing of data to
   other than intended destinations and it is conceivable that these
   behaviors may be deliberately exploited to either 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 for at least the Restart Time.



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9. Intellectual Property Considerations

   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed to
   pertain to the implementation or use of the technology described in
   this document or the extent to which any license under such rights
   might or might not be available; nor does it represent that it has
   made any independent effort to identify any such rights.  Information
   on the procedures with respect to rights in RFC documents can be
   found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use of
   such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository at
   http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-
   ipr@ietf.org.


10. Copyright Notice

   Copyright (C) The Internet Society (2005).

   This document is subject to the rights, licenses and restrictions
   contained in BCP 78, and except as set forth therein, the authors
   retain all their rights.

   This document and the information contained herein are provided on an
   "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
   OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
   ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
   INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
   INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
   WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.











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11. Acknowledgments

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


12. Normative References

   [1] "Graceful Restart Mechanism for BGP", work in progress

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

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

   [RFC2026] Bradner, S., "The Internet Standards Process -- Revision
   3", RFC2026


13. Non-normative References

   [RFC3107] Rekhter, Y., Rosen, E., "Carrying Label Information in
   BGP-4", RFC3107


14. Author Information


Yakov Rekhter
Juniper Networks
1194 N.Mathilda Ave
Sunnyvale, CA 94089
e-mail: yakov@juniper.net

Rahul Aggarwal
Juniper Networks
1194 N.Mathilda Ave
Sunnyvale, CA 94089
e-mail: rahul@juniper.net









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