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Versions: 00 01 02 03 04 05 RFC 4781

Network Working Group               Yakov Rekhter (Juniper Networks)
Internet Draft                     Rahul Aggarwal (Redback Networks)
Expiration Date: April 2003


              Graceful Restart Mechanism for BGP with MPLS

                draft-ietf-mpls-bgp-mpls-restart-02.txt


Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups. Note that other
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   Internet-Drafts are draft documents valid for a maximum of six months
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   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
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Abstract

   A mechanism for BGP that would help minimize the negative effects on
   routing caused by BGP restart is described in "Graceful Restart
   Mechanism for BGP" (see [1]). 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...)




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   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.

   The mechanism makes minimalistic assumptions on what has to be
   preserved across restart - the mechanism assumes that only the actual
   MPLS forwarding state has to be preserved; the mechanism does not
   require any of the BGP-related state to be preserved across the
   restart.


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].


Summary for Sub-IP Area

   (This section to be removed before publication.)


0.1 Summary

   This document describes a mechanism that helps to minimize the
   negative effects on MPLS forwarding caused by LSR's control plane
   restart, and specifically by the restart of its BGP component in the
   case where BGP is used to carry MPLS labels and LSR is capable of
   preserving its MPLS forwarding state across the restart.













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0.2 Related documents

   See the Reference Section


0.3 Where does it fit in the Picture of the Sub-IP Work

   This work fits squarely in MPLS box.


0.4 Why is it Targeted at this WG

   The specifications on carrying MPLS Labels in BGP is a product of the
   MPLS WG. This document specifies procedures to minimize the negative
   effects on MPLS forwarding caused by the restart of the control plane
   BGP module in the case where BGP is used to carry MPLS labels. Since
   the procedures described in this document are directly related to
   MPLS forwarding and carrying MPLS labels in BGP, it would be logical
   to target this document at the MPLS WG.


0.5 Justification

   The WG should consider this document, as it allows to minimize the
   negative effects on MPLS forwarding caused by the restart of the
   control plane BGP module in the case where BGP is used to carry MPLS
   labels.


1. Motivation

   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 <address prefix -> (outgoing label, next hop)>
   mapping. In the context of this document the forwarding state that is
   referred to in [1] means MPLS forwarding state.

   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
   (as specified in [2]), it may be desirable not to perturb the LSPs
   going through that LSR (and specifically, the LSPs established by
   BGP). In this document, we describe a mechanism that allows to
   accomplish this goal. 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.




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   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.


2. Assumptions

   First of all we assume that an LSR implements the Graceful Restart
   Mechanism for BGP, as specified in [1]. Second, we assume that the
   LSR is capable of preserving its MPLS forwarding state across the
   restart of its control plane (including the restart of BGP).

   The mechanism makes minimalistic assumptions on what has to be
   preserved across restart - the mechanism assumes that only the actual
   MPLS forwarding state has to be preserved; the mechanism does not
   require any of the BGP-related state to be preserved 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 needs to preserve across the restart the
   information about which protocol has assigned which labels.


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 [2]).









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4. Procedures for the restarting LSR

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

   Once the restarting 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 restarting LSR
   performs one of the following:


4.1. Case 1

   The following applies when (a) the best route selected by the
   restarting 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 restarting 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 <prefix, (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 just picks up some unused 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
   restarting LSR was received either without a label, or with an
   Implicit NULL label, or the route is originated by the restarting
   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



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   found, the LSR just picks up some unused label when advertising the
   route to its neighbors.

   The description in the above paragraph assumes that the restarting
   LSR generates the same label for all the routes with the same next
   hop. If this is not the case, and the restarting 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 prefix 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 prefix 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 just picks up some unused label when
   advertising the route to its neighbors.


4.3. Case 3

   The following applies when the restarting LSR does not set BGP Next
   Hop to self.

   In this case the restarting 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.


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".

   The procedures described in this section assume that the restarting
   LSR has (at least) as many unallocated as allocated labels.  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.

   After the LSR restarts, it follows the procedures as specified in
   [1]. In addition, if the LSR is able to preserve its MPLS forwarding
   state across the restart, the LSR advertises this to its neighbors by
   appropriately setting the Flag field in the Graceful Restart
   Capability.

   To create local label bindings the LSR uses unallocated labels (this
   is pretty much the normal procedure). That means that as long as the



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   LSR retains the MPLS forwarding state that the LSR preserved across
   the restart, the labels from that state are not used for creating
   local label bindings.

   The restarting 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 restarting LSR MAY retain the
   forwarding state even a bit longer, as to allow the neighbors to
   receive and process the routes that have been advertised by the
   restarting LSR. After that, the restarting LSR MAY 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).


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,



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   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.


7. Security Consideration

   The security considerations pertaining to the original BGP protocol
   remain relevant.

   In addition, the mechanism described here renders LSRs that implement
   it 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



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   label. To reduce this issue, this document requires that labels are
   not re-used until for at least the Restart Time.



8. Intellectual Property Considerations

   This section is taken from Section 10.4 of [RFC2026].

   The IETF takes no position regarding the validity or scope of any
   intellectual property 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; neither does it represent that it
   has made any effort to identify any such rights.  Information on the
   IETF's procedures with respect to rights in standards-track and
   standards-related documentation can be found in BCP-11.  Copies of
   claims of rights made available for publication 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 implementors or users of this specification can
   be obtained from the IETF Secretariat.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights which may cover technology that may be required to practice
   this standard.  Please address the information to the IETF Executive
   Director.

   The IETF has been notified of intellectual property rights claimed in
   regard to some or all of the specification contained in this
   document.  For more information consult the online list of claimed
   rights.


9. Copyright Notice

   Copyright (C) The Internet Society (date). All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implmentation may be prepared, copied, published and
   distributed, in whole or in part, without restriction of any kind,
   provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the  purpose of



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   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS 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.



10. 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.


11. Normative References

   [1] "Graceful Restart Mechanism for BGP", draft-ietf-idr-
   restart-01.txt

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

   [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










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12. Author Information


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

Rahul Aggarwal
Redback Networks
350 Holger Way
San Jose, CA 95134
e-mail: rahul@redback.com





































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