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Network Working Group                                          A. FarrelRequest for Comments: 3612                            Old Dog ConsultingCategory: Informational                                   September 2003Applicability Statement for Restart Mechanismsfor the Label Distribution Protocol (LDP)Status of this Memo   This memo provides information for the Internet community.  It does   not specify an Internet standard of any kind.  Distribution of this   memo is unlimited.Copyright Notice   Copyright (C) The Internet Society (2003).  All Rights Reserved.Abstract   This document provides guidance on when it is advisable to implement   some form of Label Distribution Protocol (LDP) restart mechanism and   which approach might be more suitable.  The issues and extensions   described in this document are equally applicable toRFC 3212,   "Constraint-Based LSP Setup Using LDP".1.  Introduction   Multiprotocol Label Switching (MPLS) systems are used in core   networks where system downtime must be kept to a minimum.  Similarly,   where MPLS is at the network edges (e.g., in Provider Edge (PE)   routers) [RFC2547], system downtime must also be kept to a minimum.   Many MPLS Label Switching Routers (LSRs) may, therefore, exploit   Fault Tolerant (FT) hardware or software to provide high availability   of the core networks.   The details of how FT is achieved for the various components of an FT   LSR, including the switching hardware and the TCP stack, are   implementation specific.  How the software module itself chooses to   implement FT for the state created by the LDP is also implementation   specific.  However, there are several issues in the LDP specification   [RFC3036] that make it difficult to implement an FT LSR using the LDP   protocols without some extensions to those protocols.   Proposals have been made in [RFC3478] and [RFC3479] to address these   issues.Farrel                       Informational                      [Page 1]

RFC 3612        Applicability for LDP Restart Mechanisms  September 20032.  Requirements of an LDP FT System   Many MPLS LSRs may exploit FT hardware or software to provide high   availability (HA) of core networks.  In order to provide HA, an MPLS   system needs to be able to survive a variety of faults with minimal   disruption to the Data Plane, including the following fault types:   -  failure/hot-swap of the switching fabric in an LSR,   -  failure/hot-swap of a physical connection between LSRs,   -  failure of the TCP or LDP stack in an LSR,   -  software upgrade to the TCP or LDP stacks in an LSR.   The first two examples of faults listed above may be confined to the   Data Plane.  Such faults can be handled by providing redundancy in   the Data Plane which is transparent to LDP operating in the Control   Plane.  However, the failure of the switching fabric or a physical   link may have repercussions in the Control Plane since signaling may   be disrupted.   The third example may be caused by a variety of events including   processor or other hardware failure, and software failure.   Any of the last three examples may impact the Control Plane and will   require action in the Control Plane to recover.  Such action should   be designed to avoid disrupting traffic in the Data Plane.  Since   many recent router architectures can separate the Control and Data   Planes, it is possible that forwarding can continue unaffected by   recovery action in the Control Plane.   In other scenarios, the Data and Control Planes may be impacted by a   fault, but the needs of HA require the coordinated recovery of the   Data and Control Planes to a state that existed before the fault.   The provision of protection paths for MPLS LSP and the protection of   links, IP routes or tunnels through the use of protection LSPs is   outside the scope of this document.  See [RFC3469] for further   information.3.  General Considerations   In order for the Data and Control Plane states to be successfully   recovered after a fault, procedures are required to ensure that the   state held on a pair of LDP peers (at least one of which was affectedFarrel                       Informational                      [Page 2]

RFC 3612        Applicability for LDP Restart Mechanisms  September 2003   directly by the fault) are synchronized.  Such procedures must be   implemented in the Control Plane software modules on the peers using   Control Plane protocols.   The required actions may operate fully after the failure (reactive   recovery) or may contain elements that operate before the fault in   order to minimize the actions taken after the fault (proactive   recovery).  It is rare to implement actions that operate solely in   advance of the failure and do not require any further processing   after the failure (preventive recovery) - this is because of the   dynamic nature of signaling protocols and the unpredictability of   fault timing.   Reactive recovery actions may include full re-signaling of state and   re-synchronization of state between peers and synchronization based   on checkpointing.   Proactive recovery actions may include hand-shaking state transitions   and checkpointing.4.  Specific Issues with the LDP Protocol   LDP uses TCP to provide reliable connections between LSRs to exchange   protocol messages to distribute labels and to set up LSPs.  A pair of   LSRs that have such a connection are referred to as LDP peers.   TCP enables LDP to assume reliable transfer of protocol messages.   This means that some of the messages do not need to be acknowledged   (e.g., Label Release).   LDP is defined such that if the TCP connection fails, the LSR should   immediately tear down the LSPs associated with the session between   the LDP peers, and release any labels and resources assigned to those   LSPs.   It is notoriously difficult to provide a Fault Tolerant   implementation of TCP.  To do so might involve making copies of all   data sent and received.  This is an issue familiar to implementers of   other TCP applications, such as BGP.   During failover affecting the TCP or LDP stacks, therefore, the TCP   connection may be lost.  Recovery from this position is made worse by   the fact that LDP control messages may have been lost during the   connection failure.  Since these messages are unconfirmed, it is   possible that LSP or label state information will be lost.Farrel                       Informational                      [Page 3]

RFC 3612        Applicability for LDP Restart Mechanisms  September 2003   At the very least, the solution to this problem must include a change   to the basic requirements of LDP so that the failure of an LDP   session does not require that associated LDP or forwarding state be   torn down.   Any changes made to LDP in support of recovery processing must meet   the following requirements:   -  offer backward-compatibility with LSRs that do not implement the      extensions to LDP,   -  preserve existing protocol rules described in [RFC3036] for      handling unexpected duplicate messages and for processing      unexpected messages referring to unknown LSPs/labels.   Ideally, any solution applicable to LDP should be equally applicable   to CR-LDP.5.  Summary of the Features of LDP FT   LDP Fault Tolerance extensions are described in [RFC3479].  This   approach involves:   -  negotiation between LDP peers of the intent to support extensions      to LDP that facilitate recovery from failover without loss of      LSPs,   -  selection of FT survival on a per LSP/label basis or for all      labels on a session,   -  sequence numbering of LDP messages to facilitate acknowledgement      and checkpointing,   -  acknowledgement of LDP messages to ensure that a full handshake is      performed on those messages either frequently (such as per      message) or less frequently as in checkpointing,   -  solicitation of up-to-date acknowledgement (checkpointing) of      previous LDP messages to ensure the current state is secured, with      an additional option that allows an LDP partner to request that      state is flushed in both directions if graceful shutdown is      required,   -  a timer to control how long LDP and forwarding state should be      retained after the LDP session failure, but before being discarded      if LDP communications are not re-established,Farrel                       Informational                      [Page 4]

RFC 3612        Applicability for LDP Restart Mechanisms  September 2003   -  exchange of checkpointing information on LDP session recovery to      establish what state has been retained by recovering LDP peers,   -  re-issuing lost messages after failover to ensure that LSP/label      state is correctly recovered after reconnection of the LDP      session.   The FT procedures in [RFC3479] concentrate on the preservation of   label state for labels exchanged between a pair of adjacent LSRs when   the TCP connection between those LSRs is lost.  There is no intention   within these procedures to support end-to-end protection for LSPs.6.  Summary of the Features of LDP Graceful Restart   LDP graceful restart extensions are defined in [RFC3478].  This   approach involves:   -  negotiation between LDP peers of the intent to support extensions      to LDP that facilitate recovery from failover without loss of      LSPs,   -  a mechanism whereby an LSR that restarts can relearn LDP state by      resynchronization with its peers,   -  use of the same mechanism to allow LSRs recovering from an LDP      session failure to resynchronize LDP state with their peers      provided that at least one of the LSRs has retained state across      the failure or has itself resynchronized state with its peers,   -  a timer to control how long LDP and forwarding state should be      retained after the LDP session failure, but before being discarded      if LDP communications are not re-established,   -  a timer to control the length of the resynchronization period      between adjacent peers should be completed.   The procedures in [RFC3478] are applicable to all LSRs, both those   with the ability to preserve forwarding state during LDP restart and   those without.  LSRs that can not preserve their MPLS forwarding   state across the LDP restart would impact MPLS traffic during   restart.  However, by implementing a subset of the mechanisms in   [RFC3478] they can minimize the impact if their neighbor(s) are   capable of preserving their forwarding state across the restart of   their LDP sessions or control planes by implementing the mechanism in   [RFC3478].Farrel                       Informational                      [Page 5]

RFC 3612        Applicability for LDP Restart Mechanisms  September 20037.  Applicability Considerations   This section considers the applicability of fault tolerance schemes   within LDP networks and considers issues that might lead to the   choice of one method or another.  Many of the points raised below   should be viewed as implementation issues rather than specific   drawbacks of either solution.7.1.  General Applicability   The procedures described in [RFC3478] and [RFC3479] are intended to   cover two distinct scenarios.  In Session Failure, the LDP peers at   the ends of a session remain active, but the session fails and is   restarted.  Note that session failure does not imply failure of the   data channel even when using an in-band control channel.  In Node   Failure, the session fails because one of the peers has been   restarted (or at least, the LDP component of the node has been   restarted).  These two scenarios have different implications for the   ease of retention of LDP state within an individual LSR, and are   described in sections below.   These techniques are only applicable in LDP networks where at least   one LSR has the capability to retain LDP signaling state and the   associated forwarding state across LDP session failure and recovery.   In [RFC3478], the LSRs retaining state do not need to be adjacent to   the failed LSR or session.   If traffic is not to be impacted, both LSRs at the ends of an LDP   session must at least preserve forwarding state.  Preserving LDP   state is not a requirement to preserve traffic.   [RFC3479] requires that the LSRs at both ends of the session   implement the procedures that it describes.  Thus, either traffic is   preserved and recovery resynchronizes state, or no traffic is   preserved and the LSP fails.   Further, to use the procedures of [RFC3479] to recover state on a   session, both LSRs must have a mechanism for maintaining some session   state and a way of auditing the forwarding state and the   resynhcronized control state.   [RFC3478] is scoped to support preservation of traffic if both LSRs   implement the procedures that it describes.  Additionally, it   functions if only one LSR on the failed session supports retention of   forwarding state, and implements the mechanisms in the document.  In   this case, traffic will be impacted by the session failure, but the   forwarding state will be recovered on session recovery.  Further, in   the event of simultaneous failures, [RFC3478] is capable ofFarrel                       Informational                      [Page 6]

RFC 3612        Applicability for LDP Restart Mechanisms  September 2003   relearning and redistributing state across multiple LSRs by combining   its mechanisms with the usual LDP message exchanges of [RFC3036].7.2.  Session Failure   In Session Failure, an LDP session between two peers fails and is   restarted.  There is no restart of the LSRs at either end of the   session and LDP continues to function on those nodes.   In these cases, it is simple for LDP implementations to retain the   LDP state associated with the failed session and to associate the   state with the new session when it is established.  Housekeeping may   be applied to determine that the failed session is not returning and   to release the old LDP state.  Both [RFC3478] and [RFC3479] handle   this case.   Applicability of [RFC3478] and [RFC3479] to the Session Failure   scenario should be considered with respect to the availability of the   data plane.   In some cases the failure of the LDP session may be independent of   any failure of the physical (or virtual) link(s) between adjacent   peers; for example, it might represent a failure of the TCP/IP stack.   In these cases, the data plane is not impacted and both [RFC3478] and   [RFC3479] are applicable to preserve or restore LDP state.   LDP signaling may also operate out of band; that is, it may use   different links from the data plane.  In this case, a failure of the   LDP session may be a result of a failure of the control channel, but   there is no implied failure of the data plane.  For this scenario   [RFC3478] and [RFC3479] are both applicable to preserve or restore   LDP state.   In the case where the failure of the LDP session also implies the   failure of the data plane, it may be an implementation decision   whether LDP peers retain forwarding state, and for how long.  In such   situations, if forwarding state is retained, and if the LDP session   is re-established, both [RFC3478] and [RFC3479] are applicable to   preserve or restore LDP state.   When the data plane has been disrupted an objective of a recovery   implementation might be to restore data traffic as quickly as   possible.Farrel                       Informational                      [Page 7]

RFC 3612        Applicability for LDP Restart Mechanisms  September 20037.3.  Controlled Session Failure   In some circumstances, the LSRs may know in advance that an LDP   session is going fail (e.g., perhaps a link is going to be taken out   of service).   [RFC3036] includes provision for controlled shutdown of a session.   [RFC3478] and [RFC3479] allow resynchronization of LDP state upon   re-establishment of the session.   [RFC3479] offers the facility to both checkpoint all LDP states   before the shut-down, and to quiesce the session so that no new state   changes are attempted between the checkpoint and the shut-down.  This   means that on recovery, resynchronization is simple and fast.   [RFC3478] resynchronizes all state on recovery regardless of the   nature of the shut-down.7.4.  Node Failure   Node Failure describes events where a whole node is restarted or   where the component responsible for LDP signaling is restarted.  Such   an event will be perceived by the LSR's peers as session failure, but   the restarting node sees the restart as full re-initialization.   The basic requirement is that the forwarding state is retained,   otherwise the data plane will necessarily be interrupted.  If   forwarding state is not retained, it may be relearned from the saved   control state in [RFC3479].  [RFC3478] does not utilize or expect a   saved control state.  If a node restarts without preserved forwarding   state it informs its neighbors, which immediately delete all label-   FEC bindings previously received from the restarted node.   The ways to retain a forwarding and control state are numerous and   implementation specific.  It is not the purpose of this document to   espouse one mechanism or another, nor even to suggest how this might   be done.  If state has been preserved across the restart,   synchronization with peers can be carried out as though recovering   from Session Failure as in the previous section.  Both [RFC3478] and   [RFC3479] support this case.   How much control state is retained is largely an implementation   choice, but [RFC3479] requires that at least small amount of per-   session control state be retained.  [RFC3478] does not require or   expect control state to be retained.   It is also possible that the restarting LSR has not preserved any   state.  In this case, [RFC3479] is of no help.  [RFC3478] however,Farrel                       Informational                      [Page 8]

RFC 3612        Applicability for LDP Restart Mechanisms  September 2003   allows the restarting LSR to relearn state from each adjacent peer   through the processes for resynchronizing after Session Failure.   Further, in the event of simultaneous failure of multiple adjacent   nodes, the nodes at the edge of the failure zone can recover state   from their active neighbors and distribute it to the other recovering   LSRs without any failed LSR having to have saved state.7.5.  Controlled Node Failure   In some cases (hardware repair, software upgrade, etc.), node failure   may be predictable.  In these cases all sessions with peers may be   shutdown and existing state retention may be enhanced by special   actions.   [RFC3479] checkpointing and quiesce may be applied to all sessions so   that state is up-to-date.   As above, [RFC3478] does not require that state is retained by the   restarting node, but can utilize it if it is.7.6.  Speed of Recovery   Speed of recovery is impacted by the amount of signaling required.   If forwarding state is preserved on both LSRs on the failed session,   then the recovery time is constrained by the time to resynchronize   the state between the two LSRs.   [RFC3479] may resynchronize very quickly.  In a stable network, this   resolves to a handshake of a checkpoint.  At the most,   resynchronization involves this handshake plus an exchange of   messages to handle state changes since the checkpoint was taken.   Implementations that support only the periodic checkpointing subset   of [RFC3479] are more likely to have additional state to   resynchronize.   [RFC3478] must resynchronize state for all label mappings that have   been retained.  At the same time, resources that have been retained   by a restarting upstream LSR but are not actually required, because   they have been released by the downstream LSR (perhaps because it was   in the process of releasing the state), they must be held for the   full resynchronization time to ensure that they are not needed.   The impact of recovery time will vary according to the use of the   network.  Both [RFC3478] and [RFC3479] allow advertisement of new   labels while resynchronization is in progress.  Issues to consider   are re-availability of falsely retained resources and conflict   between retained label mappings and newly advertised ones.  This mayFarrel                       Informational                      [Page 9]

RFC 3612        Applicability for LDP Restart Mechanisms  September 2003   cause incorrect forwarding of data (since labels are advertised from   downstream), an LSR upstream of a failure may continue to forward   data for one FEC on an old label while the recovering downstream LSR   might re-assign that label to another FEC and advertise it.  For this   reason, restarting LSRs may choose to not advertise new labels until   resynchronization with their peers has completed, or may decide to   use special techniques to cover the short period of overlap between   resynchronization and new LSP setup.7.7.  Scalability   Scalability is largely the same issue as speed of recovery and is   governed by the number of LSPs managed through the failed session(s).   Note that there are limits to how small the resynchronization time in   [RFC3478] may be made given the capabilities of the LSRs, the   throughput on the link between them, and the number of labels that   must be resynchronized.   Impact on normal operation should also be considered.   [RFC3479] requires acknowledgement of all messages.  These   acknowledgements may be deferred as for checkpointing described insection 4, or may be frequent.  Although acknowledgements can be   piggy-backed on other state messages, an option for frequent   acknowledgement is to send a message solely for the purpose of   acknowledging a state change message.  Such an implementation would   clearly be unwise in a busy network.   [RFC3478] has no impact on normal operations.7.8.  Rate of Change of LDP State   Some networks do not show a high degree of change over time, such as   those using targeted LDP sessions; others change the LDP forwarding   state frequently, perhaps reacting to changes in routing information   on LDP discovery sessions.   Rate of change of LDP state exchanged over an LDP session depends on   the application for which the LDP session is being used.  LDP   sessions used for exchanging <FEC, label> bindings for establishing   hop by hop LSPs will typically exchange state reacting to IGP   changes.  Such exchanges could be frequent.  On the other hand, LDP   sessions established for exchanging MPLS Layer 2 VPN FECs will   typically exhibit a smaller rate of state exchange.Farrel                       Informational                     [Page 10]

RFC 3612        Applicability for LDP Restart Mechanisms  September 2003   In [RFC3479], two options exist.  The first uses a frequent (up to   per-message) acknowledgement system which is most likely to be   applicable in a more dynamic system where it is desirable to preserve   the maximum amount of state over a failure to reduce the level of   resynchronization required and to speed the recovery time.   The second option in [RFC3479] uses a less-frequent acknowledgement   scheme known as checkpointing.  This is particularly suitable to   networks where changes are infrequent or bursty.   [RFC3478] resynchronizes all state on recovery regardless of the rate   of change of the network before the failure.  This consideration is   thus not relevant to the choice of [RFC3478].7.9.  Label Distribution Modes   Both [RFC3478] and [RFC3479] are suitable for use with Downstream   Unsolicited label distribution.   [RFC3478] describes Downstream-On-Demand as an area for future study   and is therefore not applicable for a network in which this label   distribution mode is used.  It is possible that future examination of   this issue will reveal that once a label has been distributed in   either distribution mode, it can be redistributed by [RFC3478] upon   session recovery.   [RFC3479] is suitable for use in a network that uses Downstream-On-   Demand label distribution.   In theory, and according to [RFC3036], even in networks configured to   utilize Downstream Unsolicited label distribution, there may be   occasions when the use of Downstream-On-Deman distribution is   desirable.  The use of the Label Request message is not prohibited in   a Downstream Unsolicited label distribution LDP network.   Opinion varies as to whether there is a practical requirement for the   use of the Label Request message in a Downstream Unsolicited label   distribution LDP network.  Current deployment experience suggests   that there is no requirement.7.10.  Implementation Complexity   Implementation complexity has consequences for the implementer and   also for the deployer since complex software is more error prone and   harder to manage.Farrel                       Informational                     [Page 11]

RFC 3612        Applicability for LDP Restart Mechanisms  September 2003   [RFC3479] is a more complex solution than [RFC3478].  In particular,   [RFC3478] does not require any modification to the normal signaling   and processing of LDP state changing messages.   [RFC3479] implementations may be simplified by implementing only the   checkpointing subset of the functionality.7.11.  Implementation Robustness   In addition to the implication for robustness associated with   complexity of the solutions, consideration should be given to the   effects of state preservation on robustness.   If state has become incorrect for whatever reason, then state   preservation may retain incorrect state.  In extreme cases, it may be   that the incorrect state is the cause of the failure in which case   preserving that state would be inappropriate.   When state is preserved, the precise amount that is retained is an   implementation issue.  The basic requirement is that forwarding state   is retained (to preserve the data path) and that that state can be   accessed by the LDP software component.   In both solutions, if the forwarding state is incorrect and is   retained, it will continue to be incorrect.  Both solutions have a   mechanism to housekeep and free the unwanted state after   resynchronization is complete.  [RFC3478] may be better at   eradicating incorrect forwarding state, because it replays all   message exchanges that caused the state to be populated.   In [RFC3478], no more data than the forwarding state needs to have   been saved by the recovering node.  All LDP state may be relearned by   message exchanges with peers.  Whether those exchanges may cause the   same incorrect state to arise on the recovering node is an obvious   concern.   In [RFC3479], the forwarding state must be supplemented by a small   amount of state specific to the protocol extensions.  LDP state may   be retained directly or reconstructed from the forwarding state.  The   same issues apply when reconstructing state but are mitigated by the   fact that this is likely a different code path.  Errors in the   retained state specific to the protocol extensions will persist.7.12.  Interoperability and Backward Compatibility   It is important that new additions to LDP interoperate with existing   implementations at least in provision of the existing levels of   function.Farrel                       Informational                     [Page 12]

RFC 3612        Applicability for LDP Restart Mechanisms  September 2003   Both [RFC3478] and [RFC3479] do this through rules for handling the   absence of the FT optional negotiation object during session   initialization.   Additionally, [RFC3478] is able to perform limited recovery (i.e.,   redistribution of state) even when only one of the participating LSRs   supports the procedures.  This may offer considerable advantages in   interoperation with legacy implementations.7.13.  Interaction With Other Label Distribution Mechanisms   Many LDP LSRs also run other label distribution mechanisms.  These   include management interfaces for configuration of static label   mappings, other distinct instances of LDP, and other label   distribution protocols.  The last example includes traffic   engineering label distribution protocol that are used to construct   tunnels through which LDP LSPs are established.   As with re-use of individual labels by LDP within a restarting LDP   system, care must be taken to prevent labels that need to be retained   by a restarting LDP session or protocol component from being used by   another label distribution mechanism.  This might compromise data   security, amongst other things.   It is a matter for implementations to avoid this issue through the   use of techniques, such as a common label management component or   segmented label spaces.7.14.  Applicability to CR-LDP   CR-LDP [RFC3212] utilizes Downstream-On-Demand label distribution.   [RFC3478] describes Downstream-On-Demand as an area for future study   and is therefore not applicable for CR-LDP.  [RFC3479] is suitable   for use in a network entirely based on CR-LDP or in one that is mixed   between LDP and CR-LDP.8.  Security Considerations   This document is informational and introduces no new security   concerns.   The security considerations pertaining to the original LDP protocol   [RFC3036] remain relevant.   [RFC3478] introduces the possibility of additional denial-of- service   attacks.  All of these attacks may be countered by use of an   authentication scheme between LDP peers, such as the MD5-based scheme   outlined in [LDP].Farrel                       Informational                     [Page 13]

RFC 3612        Applicability for LDP Restart Mechanisms  September 2003   In MPLS, a data mis-delivery security issue can arise if an LSR   continues to use labels after expiration of the session that first   caused them to be used.  Both [RFC3478] and [RFC3479] are open to   this issue.9.  Intellectual Property Statement   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 inBCP-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.10.  References10.1.  Normative References   [RFC2026]    Bradner, S., "The Internet Standards Process -- Revision                3",BCP 9,RFC 2026, October 1996.   [RFC2119]    Bradner, S., "Key words for use in RFCs to Indicate                Requirement Levels",BCP 14,RFC 2119, March 1997.   [RFC3036]    Andersson, L., Doolan, P., Feldman, N., Fredette, A. and                B. Thomas, "LDP Specification",RFC 3036, January 2001.   [RFC3478]    Leelanivas, M., Rekhter, Y. and R. Aggarwal, "Graceful                Restart Mechanism for LDP",RFC 3478, February 2003.   [RFC3479]    Farrel, A., Editor, "Fault Tolerance for the Label                Distribution Protocol (LDP)",RFC 3479, February 2003.Farrel                       Informational                     [Page 14]

RFC 3612        Applicability for LDP Restart Mechanisms  September 200310.2.  Informative References   [RFC2547]    Rosen, E. and Y. Rekhter, "BGP/MPLS VPNs",RFC 2547,                March 1999.   [RFC3212]    Jamoussi, B., Editor, Andersson, L., Callon, R., Dantu,                R., Wu, L., Doolan, P., Worster, T., Feldman, N.,                Fredette, A., Girish, M., Gray, E., Heinanen, J., Kilty,                T. and A. Malis, "Constraint-Based LSP Setup using LDP",RFC 3212, January 2002.   [RFC3469]    Sharma, V., Ed., and F. Hellstrand, Ed., "Framework for                Multi-Protocol Label Switching (MPLS)-based Recovery",RFC 3469, February 2003.11.  Acknowledgements   The author would like to thank the authors of [RFC3478] and [RFC3479]   for their work on fault tolerance of LDP.  Many thanks to Yakov   Rekhter, Rahul Aggarwal, Manoj Leelanivas and Andrew Malis for their   considered input to this applicability statement.12.  Author's Address   Adrian Farrel   Old Dog Consulting   Phone:  +44 (0) 1978 860944   EMail:  adrian@olddog.co.ukFarrel                       Informational                     [Page 15]

RFC 3612        Applicability for LDP Restart Mechanisms  September 200313.  Full Copyright Statement   Copyright (C) The Internet Society (2003).  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 implementation 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   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 assignees.   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.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Farrel                       Informational                     [Page 16]