[Docs] [txt|pdf] [draft-ietf-mpls-l...] [Diff1] [Diff2]

PROPOSED STANDARD

Network Working Group                                     A. Farrel, Ed.
Request for Comments: 3479                          Movaz Networks, Inc.
Category: Standards Track                                  February 2003


       Fault Tolerance for the Label Distribution Protocol (LDP)

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 (2003).  All Rights Reserved.

IESG Note

   This specification includes procedures for failure detection and
   failover for a TCP connection carrying MPLS LDP control traffic, so
   that it can be switched to a new TCP connection.  It does not provide
   a general approach to using multiple TCP connections to provide this
   kind of fault tolerance.  The specification lacks adequate guidance
   for the timer and retry value choices related to the TCP connection
   fault tolerance procedures.  The specification should not serve as a
   model for TCP connection fault tolerance design for any future
   document, and users are advised to test configurations based on this
   specification very carefully for problems such as premature
   failovers.

Abstract

   Multiprotocol Label Switching (MPLS) systems will be used in core
   networks where system downtime must be kept to an absolute 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 Label Distribution Protocol (LDP), the switching
   hardware and TCP, are implementation specific.  This document
   identifies issues in the LDP specification in RFC 3036, "LDP
   Specification", that make it difficult to implement an FT LSR using
   the current LDP protocols, and defines enhancements to the LDP
   specification to ease such FT LSR implementations.



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   The issues and extensions described here are equally applicable to
   RFC 3212, "Constraint-Based LSP Setup Using LDP" (CR-LDP).

Table of Contents

   1. Conventions and Terminology used in this document..........3
   2. Contributing Authors.......................................4
   3. Introduction...............................................4
      3.1. Fault Tolerance for MPLS..............................4
      3.2. Issues with LDP.......................................5
   4. Overview of LDP FT Enhancements............................7
      4.1. Establishing an FT LDP Session........................8
           4.1.1 Interoperation with Non-FT LSRs.................8
      4.2. TCP Connection Failure................................9
           4.2.1 Detecting TCP Connection Failures...............9
           4.2.2 LDP Processing after Connection Failure.........9
      4.3. Data Forwarding During TCP Connection Failure........10
      4.4. FT LDP Session Reconnection..........................10
      4.5. Operations on FT Labels..............................11
      4.6. Check-Pointing.......................................11
           4.6.1 Graceful Termination...........................12
      4.7. Label Space Depletion and Replenishment..............13
      4.8. Tunneled LSPs........................................13
   5. FT Operations.............................................14
      5.1. FT LDP Messages......................................14
           5.1.1 Sequence Numbered FT Label Messages............14
           5.1.2 FT Address Messages............................15
           5.1.3 Label Resources Available Notifications........15
      5.2. FT Operation ACKs....................................17
      5.3. Preservation of FT State.............................17
      5.4. FT Procedure After TCP Failure.......................19
           5.4.1 FT LDP Operations During TCP Failure...........20
      5.5. FT Procedure After TCP Re-connection.................21
           5.5.1 Re-Issuing FT Messages.........................22
   6. Check-Pointing Procedures.................................22
      6.1 Check-Pointing with the Keepalive Message.............23
      6.2 Quiesce and Keepalive.................................23
   7. Changes to Existing Messages..............................24
      7.1. LDP Initialization Message...........................24
      7.2. LDP Keepalive Messages...............................25
      7.3. All Other LDP Session Messages.......................25
   8. New Fields and Values.....................................26
      8.1. Status Codes.........................................26
      8.2. FT Session TLV.......................................27
      8.3. FT Protection TLV....................................29
      8.4. FT ACK TLV...........................................32
      8.5. FT Cork TLV..........................................33
   9. Example Use...............................................34



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      9.1. Session Failure and Recovery - FT Procedures.........34
      9.2. Use of Check-Pointing With FT Procedures.............37
      9.3. Temporary Shutdown With FT Procedures................38
      9.4. Temporary Shutdown With FT Procedures
           and Check-Pointing...................................40
      9.5. Check-Pointing Without FT Procedures.................42
      9.6. Graceful Shutdown With Check-Pointing
           But No FT Procedures.................................44
   10. Security Considerations..................................45
   11. Implementation Notes.....................................47
      11.1. FT Recovery Support on Non-FT LSRs..................47
      11.2. ACK generation logic................................47
            11.2.1 Ack Generation Logic When Using
                   Check-Pointing...............................47
      11.3 Interactions With Other Label Distribution
           Mechanisms...........................................48
   12. Acknowledgments..........................................48
   13. Intellectual Property Consideration......................49
   14. References...............................................49
      14.1. Normative References................................49
      14.2. Informative References..............................50
   15. Authors' Addresses.......................................50
   16. Full Copyright Statement.................................52

1. Conventions and Terminology used in this document

   Definitions of key words and terms applicable to LDP and CR-LDP are
   inherited from [RFC3212] and [RFC3036].

   The term "FT Label" is introduced in this document to indicate a
   label for which some fault tolerant operation is used.  A "non-FT
   Label" is not fault tolerant and is handled as specified in
   [RFC3036].

   The term "Sequence Numbered FT Label" is used to indicate an FT label
   which is secured using the sequence number in the FT Protection TLV
   described in this document.

   The term "Check-Pointable FT Label" is used to indicate an FT label
   which is secured by using the check-pointing techniques described in
   this document.

   The extensions to LDP specified in this document are collectively
   referred to as the "LDP FT enhancements".

   Within the context of this document, "Check-Pointing" refers to a
   process of message exchanges that confirm receipt and processing (or
   secure storage) of specific protocol messages.



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   When talking about the individual bits in the 16-bit FT Flag Field,
   the words "bit" and "flag" are used interchangeably.

   In the examples quoted, the following notation is used:  Ln : An LSP.
   For example L1.  Pn : An LDP peer.  For example P1.

   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 BCP 14, RFC 2119
   [RFC2119].

2. Contributing Authors

   This document was the collective work of several individuals over a
   period of several years.  The text and content of this document was
   contributed by the editor and the co-authors listed in section 15,
   "Authors' Addresses".

3. Introduction

   High Availability (HA) is typically claimed by equipment vendors when
   their hardware achieves availability levels of at least 99.999% (five
   9s).  To implement this, the equipment must be capable of recovering
   from local hardware and software failures through a process known as
   fault tolerance (FT).

   The usual approach to FT involves provisioning backup copies of
   hardware and/or software.  When a primary copy fails, processing is
   switched to the backup copy.  This process, called failover, should
   result in minimal disruption to the Data Plane.

   In an FT system, backup resources are sometimes provisioned on a
   one-to-one basis (1:1), sometimes as one-to-many (1:n), and
   occasionally as many-to-many (m:n).  Whatever backup provisioning is
   made, the system must switch to the backup automatically on failure
   of the primary, and the software and hardware state in the backup
   must be set to replicate the state in the primary at the point of
   failure.

3.1.  Fault Tolerance for MPLS

   MPLS is a technology that will be used in core networks where system
   downtime must be kept to an absolute minimum.  Many MPLS LSRs may,
   therefore, exploit FT hardware or software to provide high
   availability of core networks.






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   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 a physical connection between LSRs.

   -  failure/hot-swap of the switching fabric in an LSR.

   -  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 are 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.  The last two example types of fault require action in the
   Control Plane to recover from the fault without disrupting traffic in
   the Data Plane.  This is possible because many recent router
   architectures separate the Control and Data Planes such that
   forwarding can continue unaffected by recovery action in the Control
   Plane.

3.2.  Issues with LDP

   LDP uses TCP to provide reliable connections between LSRs over which
   they exchange protocol messages to distribute labels and 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
   (for example, 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 hard 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, 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.



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   This document describes a solution which 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.

   -  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 check-pointing.

   -  solicitation of up-to-date acknowledgement (check-pointing) of
      previous LDP messages to ensure the current state is flushed to
      disk/NVRAM, with an additional option that allows an LDP partner
      to request that state is flushed in both directions if graceful
      shutdown is required.

   -  re-issuing lost messages after failover to ensure that LSP/label
      state is correctly recovered after reconnection of the LDP
      session.

   The issues and objectives described above are equally applicable to
   CR-LDP.

   Other objectives of this document are to:

   -  offer backward-compatibility with LSRs that do not implement these
      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.

   -  avoid full state refresh solutions (such as those present in RSVP:
      see [RFC2205], [RFC2961], [RFC3209] and [RFC3478]) whether they be
      continual, or limited to post-failover recovery.

   Note that this document concentrates on the preservation of label
   state for labels exchanged between a pair of adjacent LSRs when the
   TCP connection between those LSRs is lost.  This is a requirement for
   Fault Tolerant operation of LSPs, but a full implementation of end-
   to-end protection for LSPs requires that this be combined with other
   techniques that are outside the scope of this document.

   In particular, this document does not attempt to describe how to
   modify the routing of an LSP or the resources allocated to a label or
   LSP, which is covered by [RFC3214].  This document also does not



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   address how to provide automatic layer 2 or layer 3 protection
   switching for a label or LSP, which is a separate area for study.

   This specification does not preclude an implementation from
   attempting (or require it to attempt) to use the FT behavior
   described here to recover from a preemptive failure of a connection
   on a non-FT system due to, for example, a partial system crash.
   Note, however, that there are potential issues too numerous to list
   here - not least the likelihood that the same crash will immediately
   occur when processing the restored data.

4. Overview of LDP FT Enhancements

   The LDP FT enhancements consist of the following main elements, which
   are described in more detail in the sections that follow.

   -  The presence of an FT Session TLV on the LDP Initialization
      message indicates that an LSR supports some form of protection or
      recovery from session failure.  A flag bit within this TLV (the S
      bit) indicates that the LSR supports the LDP FT enhancements on
      this session.  Another flag (the C bit) indicates that the check-
      pointing procedures are to be used.

   -  An FT Reconnect Flag in the FT Session TLV (the R bit) indicates
      whether an LSR has preserved FT Label state across a failure of
      the TCP connection.

   -  An FT Reconnection Timeout, exchanged on the LDP Initialization
      message, that indicates the maximum time peer LSRs will preserve
      FT Label state after a failure of the TCP connection.

   -  An FT Protection TLV used to identify operations that affect LDP
      labels.  All LDP messages carrying the FT Protection TLV need to
      be secured (e.g. to NVRAM) and ACKed to the sending LDP peer so
      that the state for Sequence Numbered FT Labels can be correctly
      recovered after LDP session reconnection.

      Note that the implementation within an FT system is left open by
      this document.  An implementation could choose to secure entire
      messages relating to Sequence Numbered FT Labels, or it could
      secure only the relevant state information.

   -  Address advertisement may also be secured by use of the FT
      Protection TLV.  This enables recovery after LDP session
      reconnection without the need to re-advertise what may be a very
      large number of addresses.





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   -  The FT Protection TLV may also be used on the Keepalive message to
      flush acknowledgement of all previous FT operations.  This enables
      a check-point for future recovery, either in mid-session or prior
      to graceful shutdown of an LDP session.  This procedure may also
      be used to check-point all (that is both FT and non-FT) operations
      for future recovery.

4.1.  Establishing an FT LDP Session

   In order that the extensions to LDP [RFC3036] described in this
   document can be used successfully on an LDP session between a pair of
   LDP peers, they MUST negotiate that the LDP FT enhancements are to be
   used on the LDP session.

   This is done on the LDP Initialization message exchange using a new
   FT Session TLV.  Presence of this TLV indicates that the peer wants
   to support some form of protection or recovery processing.  The S bit
   within this TLV indicates that the peer wants to support the LDP FT
   enhancements on this LDP session.  The C bit indicates that the peer
   wants to support the check-pointing functions described in this
   document.  The S and C bits may be set independently.

   The relevant LDP FT enhancements MUST be supported on an LDP session
   if both LDP peers include an FT Session TLV on the LDP Initialization
   message and have the same setting of the S or C bit.

   If either LDP Peer does not include the FT Session TLV LDP
   Initialization message, or if there is no match of S and C bits
   between the peers, the LDP FT enhancements MUST NOT be used during
   this LDP session.  Use of LDP FT enhancements by a sending LDP peer
   in these cases MUST be interpreted by the receiving LDP peer as a
   serious protocol error causing the session to be terminated.

   An LSR MAY present different FT/non-FT behavior on different TCP
   connections, even if those connections are successive instantiations
   of the LDP session between the same LDP peers.

4.1.1 Interoperation with Non-FT LSRs

   The FT Session TLV on the LDP Initialization message carries the U-
   bit.  If an LSR does not support any protection or recovery
   mechanisms, it will ignore this TLV.  Since such partners also do not
   include the FT Session TLV, all LDP sessions to such LSRs will not
   use the LDP FT enhancements.

   The rest of this document assumes that the LDP sessions under
   discussion are between LSRs that support the LDP FT enhancements,
   except where explicitly stated otherwise.



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4.2.  TCP Connection Failure

4.2.1 Detecting TCP Connection Failures

   TCP connection failures may be detected and reported to the LDP
   component in a variety of ways.  These should all be treated in the
   same way by the LDP component.

   -  Indication from the management component that a TCP connection or
      underlying resource is no longer active.

   -  Notification from a hardware management component of an interface
      failure.

   -  Sockets keepalive timeout.

   -  Sockets send failure.

   -  New (incoming) Socket opened.

   -  LDP protocol timeout.

4.2.2 LDP Processing after Connection Failure

   If the LDP FT enhancements are not in use on an LDP session, the
   action of the LDP peers on failure of the TCP connection is as
   specified in [RFC3036].

   All state information and resources associated with non-FT Labels
   MUST be released on the failure of the TCP connection, including
   deprogramming the non-FT Label from the switching hardware.  This is
   equivalent to the behavior specified in [RFC3036].

   If the LDP FT enhancements are in use on an LDP session, both LDP
   peers SHOULD preserve state information and resources associated with
   FT Labels exchanged on the LDP session.  Both LDP peers SHOULD use a
   timer to release the preserved state information and resources
   associated with FT-labels if the TCP connection is not restored
   within a reasonable period.  The behavior when this timer expires is
   equivalent to the LDP session failure behavior described in
   [RFC3036].

   The FT Reconnection Timeout each LDP peer intends to apply to the LDP
   session is carried in the FT Session TLV on the LDP Initialization
   messages.  Both LDP peers MUST use the value that corresponds to the
   lesser timeout interval of the two proposed timeout values from the
   LDP Initialization exchange, where a value of zero is treated as
   positive infinity.



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4.3.  Data Forwarding During TCP Connection Failure

   An LSR that implements the LDP FT enhancements SHOULD preserve the
   programming of the switching hardware across a failover.  This
   ensures that data forwarding is unaffected by the state of the TCP
   connection between LSRs.

   It is an integral part of FT failover processing in some hardware
   configurations that some data packets might be lost.  If data loss is
   not acceptable to the applications using the MPLS network, the LDP FT
   enhancements described in this document SHOULD NOT be used.

4.4.  FT LDP Session Reconnection

   When a new TCP connection is established, the LDP peers MUST exchange
   LDP Initialization messages.  When a new TCP connection is
   established after failure, the LDP peers MUST re-exchange LDP
   Initialization messages.

   If an LDP peer includes the FT Session TLV with the S bit set in the
   LDP Initialization message for the new instantiation of the LDP
   session, it MUST also set the FT Reconnect Flag according to whether
   it has been able to preserve label state.  The FT Reconnect Flag is
   carried in the FT Session TLV.

   If an LDP peer has preserved all state information for previous
   instantiations of the LDP session, then it SHOULD set the FT
   Reconnect Flag to 1 in the FT Session TLV.  Otherwise, it MUST set
   the FT Reconnect Flag to 0.

   If either LDP peer sets the FT Reconnect Flag to 0, or omits the FT
   Session TLV, both LDP peers MUST release any state information and
   resources associated with the previous instantiation of the LDP
   session between the same LDP peers, including FT Label state and
   Addresses.  This ensures that network resources are not permanently
   lost by one LSR if its LDP peer is forced to undergo a cold start.

   If an LDP peer changes any session parameters (for example, the label
   space bounds) from the previous instantiation, the nature of any
   preserved labels may have changed.  In particular, previously
   allocated labels may now be out of range.  For this reason, session
   reconnection MUST use the same parameters as were in use on the
   session before the failure.  If an LDP peer notices that the
   parameters have been changed by the other peer, it SHOULD send a
   Notification message with the 'FT Session parameters changed' status
   code.





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   If both LDP peers set the FT Reconnect Flag to 1, both LDP peers MUST
   use the procedures indicated in this document to complete any label
   operations on Sequence Numbered FT Labels that were interrupted by
   the LDP session failure.

   If an LDP peer receives an LDP Initialization message with the FT
   Reconnect Flag set before it sends its own Initialization message,
   but has retained no information about the previous version of the
   session, it MUST respond with an Initialization message with the FT
   Reconnect Flag clear.  If an LDP peer receives an LDP Initialization
   message with the FT Reconnect Flag set in response to an
   Initialization message that it has sent with the FT Reconnect Flag
   clear, it MUST act as if no state was retained by either peer on the
   session.

4.5.  Operations on FT Labels

   Label operations on Sequence Numbered FT Labels are made Fault
   Tolerant by providing acknowledgement of all LDP messages that affect
   Sequence Numbered FT Labels.  Acknowledgements are achieved by means
   of sequence numbers on these LDP messages.

   The message exchanges used to achieve acknowledgement of label
   operations and the procedures used to complete interrupted label
   operations are detailed in section 5, "FT Operations".

   Using these acknowledgements and procedures, it is not necessary for
   LDP peers to perform a complete re-synchronization of state for all
   Sequence Numbered FT Labels, either on re-connection of the LDP
   session between the LDP peers or on a timed basis.

4.6.  Check-Pointing

   Check-pointing is a useful feature that allows nodes to reduce the
   amount of processing that they need to do to acknowledge LDP
   messages.  The C bit in the FT Session TLV is used to indicate that
   check-pointing is supported.

   Under the normal operation on Sequence Numbered FT Labels,
   acknowledgments may be deferred during normal processing and only
   sent periodically.  Check-pointing may be used to flush
   acknowledgement from a peer by including a sequence number on a
   Keepalive message requesting acknowledgement of that message and all
   previous messages.  In this case, all Sequence Numbered FT Labels are
   Check-Pointable FT Labels.






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   If the S bit is not agreed upon, check-pointing may still be used.
   In this case it is used to acknowledge all messages exchanged between
   the peers, and all labels are Check-Pointable FT Labels.

   This offers an approach where acknowledgements need not be sent to
   every message or even frequently, but are only sent as check-points
   in response to requests carried on Keepalive messages.  Such an
   approach may be considered optimal in systems that do not show a high
   degree of change over time (such as targeted LDP sessions) and that
   are prepared to risk loss of state for the most recent LDP exchanges.
   More dynamic systems (such as LDP discovery sessions) are more likely
   to want to acknowledge state changes more frequently so that the
   maximum amount of state can be preserved over a failure.

   Note that an important consideration of this document is that nodes
   acknowledging messages on a one-for-one basis, nodes deferring
   acknowledgements, and nodes relying on check-pointing, should all
   interoperate seamlessly and without protocol negotiation beyond
   session initialization.

   Further discussion of this feature is provided in section 5, "FT
   Operations".

4.6.1 Graceful Termination

   A feature that builds on check-pointing is graceful termination.

   In some cases, such as controlled failover or software upgrade, it is
   possible for a node to know in advance that it is going to terminate
   its session with a peer.

   In these cases the node that intends terminating the session can
   flush acknowledgement using a check-point request as described above.
   The sender SHOULD not send further label or address-related messages
   after requesting shutdown check-pointing in order to preserve the
   integrity of its saved state.

   This, however, only provides for acknowledgement in one direction,
   and the node that is being terminated also requires verification that
   it has secured all state sent by its peer.  This is achieved by a
   three-way hand shake of the check-point which is requested by an
   additional TLV (the Cork TLV) in the Keepalive message.

   Further discussion of this feature is provided in section 5, "FT
   Operations".






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4.7.  Label Space Depletion and Replenishment

   When an LDP peer is unable to satisfy a Label Request message because
   it has no more available labels, it sends a Notification message
   carrying the status code 'No label resources'.  This warns the
   requesting LDP peer that subsequent Label Request messages are also
   likely to fail for the same reason.  This message does not need to be
   acknowledged for FT purposes since Label Request messages sent after
   session recovery will receive the same response.  However, the LDP
   peer that receives a 'No label resources' Notification stops sending
   Label Request messages until it receives a 'Label resources
   available' Notification message.  Since this unsolicited Notification
   might get lost during session failure, it may be protected using the
   procedures described in this document.

   An alternative approach allows that an implementation may always
   assume that labels are available when a session is re-established.
   In this case, it is possible that it may throw away the 'No label
   resources' information from the previous incarnation of the session
   and may send a batch of LDP messages on session re-establishment that
   will fail and that it could have known would fail.

   Note that the sender of a 'Label resources available' Notification
   message may choose whether to add a sequence number requesting
   acknowledgement.  Conversely, the receiver of 'Label resources
   available' Notification message may choose to acknowledge the message
   without actually saving any state.

   This is an implementation choice made possible by making the FT
   parameters on the Notification message optional.  Implementations
   will interoperate fully if they take opposite approaches, but
   additional LDP messages may be sent unnecessarily on session
   recovery.

4.8.  Tunneled LSPs

   The procedures described in this document can be applied to LSPs that
   are tunnels and to LSPs that are carried by tunnels.  Recall that
   tunneled LSPs are managed by a single LDP session that runs end to
   end, while the tunnel is managed by a different LDP session for each
   hop along the path.  Nevertheless, a break in one of the sessions
   that manages the tunnel is likely to correspond with a break in the
   session that manages the tunneled LSP.  This is certainly the case
   when the LDP exchanges share a failed link, but need not be the case
   if the LDP messages have been routed along a path that is different
   from that of the tunnel, or if the failure in the tunnel is caused by
   an LDP software failure at a transit LSR.




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   In order that the forwarding path of a tunneled LSP be preserved, the
   forwarding path of the tunnel itself must be preserved.  This means
   that the tunnel must not be torn down if there is any session failure
   along its path.  To achieve this, the label exchanges between each
   pair of LDP peers along the path of the tunnel must use one of the
   procedures in this document or in [RFC3478].

   It is perfectly acceptable to mix the restart procedures used for the
   tunnel and the tunneled LSP.  For example, the tunnel could be set up
   using just check-pointing because it is a stable LSP, but the
   tunneled LSPs might use full FT procedures so that they can recover
   active state.

   Lastly, it is permissible to carry tunneled LSPs that do not have FT
   protection in an LSP that has FT protection.

5. FT Operations

   Once an FT LDP session has been established, using the S bit in the
   FT Session TLV on the Session Initialization message as described in
   section 4.1, "Establishing an FT LDP Session", both LDP peers MUST
   apply the procedures described in this section for FT LDP message
   exchanges.

   If the LDP session has been negotiated to not use the LDP FT
   enhancements, these procedures MUST NOT be used.

5.1.  FT LDP Messages

5.1.1 Sequence Numbered FT Label Messages

   A label is identified as being a Sequence Numbered FT Label if the
   initial Label Request or Label Mapping message relating to that label
   carries the FT Protection TLV.

   It is a valid implementation option to flag all labels as Sequence
   Numbered FT Labels.  Indeed this may be a preferred option for
   implementations wishing to use Keepalive messages carrying the FT
   Protection TLV to achieve periodic saves of the complete label
   forwarding state.

   If a label is a Sequence Numbered FT Label, all LDP messages
   affecting that label MUST carry the FT Protection TLV so that the
   state of the label can be recovered after a failure of the LDP
   session.






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   A further valid option is for no labels to be Sequence Numbered FT
   Labels.  In this case, check-pointing using the Keepalive message
   applies to all messages exchanged on the session.

5.1.1.1  Scope of FT Labels

   The scope of the FT/non-FT status of a label is limited to the LDP
   message exchanges between a pair of LDP peers.

   In Ordered Control, when the message is forwarded downstream or
   upstream, the TLV may be present or absent according to the
   requirements of the LSR sending the message.

   If a platform-wide label space is used for FT Labels, an FT Label
   value MUST NOT be reused until all LDP FT peers to which the label
   was passed have acknowledged the withdrawal of the FT Label, either
   by an explicit LABEL WITHDRAW/LABEL RELEASE, exchange or implicitly
   if the LDP session is reconnected after failure but without the FT
   Reconnect Flag set.  In the event that a session is not re-
   established within the Reconnection Timeout, a label MAY become
   available for re-use if it is not still in use on some other session.

5.1.2 FT Address Messages

   If an LDP session uses the LDP FT enhancements, both LDP peers MUST
   secure Address and Address Withdraw messages using FT Operation ACKs,
   as described below.  This avoids any ambiguity over whether an
   Address is still valid after the LDP session is reconnected.

   If an LSR determines that an Address message it sent on a previous
   instantiation of a recovered LDP session is no longer valid, it MUST
   explicitly issue an Address Withdraw for that address when the
   session is reconnected.

   If the FT Reconnect Flag is not set by both LDP peers upon
   reconnection of an LDP session (i.e. state has not been preserved),
   both LDP peers MUST consider all Addresses to have been withdrawn.
   The LDP peers SHOULD issue new Address messages for all their valid
   addresses, as specified in [RFC3036].

5.1.3 Label Resources Available Notifications

   In LDP, it is possible that a downstream LSR may not have labels
   available to respond to a Label Request.  In this case, as specified
   in RFC 3036, the downstream LSR must respond with a Notification - No
   Label Resources message.  The upstream LSR then suspends asking for
   new labels until it receives a Notification - Label Resources
   Available message from the downstream LSR.



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   When the FT extensions are used on a session, implementations may
   choose whether or not to secure the label resource state of their
   peer.  This choice impacts the number of LDP messages that will be
   incorrectly routed to a peer with depleted resources on session re-
   establishment, but does not otherwise impact interoperability.

   For full preservation of state:

   -  The downstream LSR must preserve the label availability state
      across a failover so that it remembers to send Notification -
      Label Resources Available when the resources become available.

   -  The upstream LSR must recall the label availability state across
      failover so that it can optimize not sending Label Requests when
      it recovers.

   -  The downstream LSR must use sequence numbers on Notification -
      Label Resources Available so that it can check that LSR A has
      received the message and clear its secured state, or resend the
      message if LSR A recovers without having received it.

   However, the following options also exist:

   -  The downstream LSR may choose to not include a sequence number on
      Notification - Label Resources Available.  This means that on
      session re-establishment it does not know what its peer thinks the
      LSR's resource state is, because the Notification may or may not
      have been delivered.  Such an implementation MUST begin recovered
      sessions by sending an additional Notification - Label Resources
      Available to reset its peer.

   -  The upstream node may choose not to secure information about its
      peer's resource state.  It would acknowledge a Notification -
      Label Resources Available, but would not save the information.
      Such an implementation MUST assume that its peer's resource state
      has been reset to Label Resources Available when the session is
      re-established.

   If the FT Reconnect Flag is not set by both LDP peers upon
   reconnection of an LDP session (i.e. state has not been preserved),
   both LDP peers MUST consider the label availability state to have
   been reset as if the session had been set up for the first time.









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5.2.  FT Operation ACKs

   Handshaking of FT LDP messages is achieved by use of ACKs.
   Correlation between the original message and the ACK is by means of
   the FT Sequence Number contained in the FT Protection TLV, and passed
   back in the FT ACK TLV.  The FT ACK TLV may be carried on any LDP
   message that is sent on the TCP connection between LDP peers.

   An LDP peer maintains a separate FT sequence number for each LDP
   session in which it participates.  The FT Sequence number is
   incremented by one for each FT LDP message (i.e. containing the FT
   Protection TLV) issued by this LSR on the FT LDP session with which
   the FT sequence number is associated.

   When an LDP peer receives a message containing the FT Protection TLV,
   it MUST take steps to secure this message (or the state information
   derived from processing the message).  Once the message is secured,
   it MUST be ACKed.  However, there is no requirement on the LSR to
   send this ACK immediately.

   ACKs may be accumulated to reduce the message flow between LDP peers.
   For example, if an LSR received FT LDP messages with sequence numbers
   1, 2, 3, 4, it could send a single ACK with sequence number 4 to ACK
   receipt, securing of all these messages.  There is no protocol reason
   why the number of ACKs accumulated, or the time for which an ACK is
   deferred, should not be allowed to become relatively large.

   ACKs MUST NOT be sent out of sequence, as this is incompatible with
   the use of accumulated ACKs.  Duplicate ACKs (that is two successive
   messages that acknowledge the same sequence number) are acceptable.

   If an LDP peer discovers that its sequence number space for a
   specific session is full of un-acknowledged sequence numbers (because
   its partner on the session has not acknowledged them in a timely
   way), it cannot allocate a new sequence number for any further FT LPD
   message.  It SHOULD send a Notification message with the status code
   'FT Seq Numbers Exhausted'.

5.3.  Preservation of FT State

   If the LDP FT enhancements are in use on an LDP session, each LDP
   peer SHOULD NOT release the state information and resources
   associated with FT Labels exchanged on that LDP session when the TCP
   connection fails.  This is contrary to [RFC3036], but allows label
   operations on FT Labels to be completed after re-connection of the
   TCP connection.





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   Both LDP peers on an LDP session that is using the LDP FT
   enhancements SHOULD preserve the state information and resources they
   hold for that LDP session as described below.

   -  An upstream LDP peer SHOULD release the resources (in particular
      bandwidth) associated with a Sequence Numbered FT Label when it
      initiates a Label Release or Label Abort message for the label.
      The upstream LDP peer MUST preserve state information for the
      Sequence Numbered FT Label, even if it releases the resources
      associated with the label, as it may need to reissue the label
      operation if the TCP connection is interrupted.

   -  An upstream LDP peer MUST release the state information and
      resources associated with a Sequence Numbered FT Label when it
      receives an acknowledgement to a Label Release or Label Abort
      message that it sent for the label, or when it sends a Label
      Release message in response to a Label Withdraw message received
      from the downstream LDP peer.

   -  A downstream LDP peer SHOULD NOT release the resources associated
      with a Sequence Numbered FT Label when it sends a Label Withdraw
      message for the label as it has not yet received confirmation that
      the upstream LDP peer has ceased to send data using the label.
      The downstream LDP peer MUST NOT release the state information it
      holds for the label as it may yet have to reissue the label
      operation if the TCP connection is interrupted.

   -  A downstream LDP peer MUST release the resources and state
      information associated with a Sequence Numbered FT Label when it
      receives an acknowledgement to a Label Withdraw message for the
      label.

   -  When the FT Reconnection Timeout expires, an LSR SHOULD release
      all state information and resources from previous instantiations
      of the (permanently) failed LDP session.

   -  Either LDP peer MAY elect to release state information based on
      its internal knowledge of the loss of integrity of the state
      information or an inability to pend (or queue) LDP operations (as
      described in section 5.4.1, "LDP Operations During TCP Failure")
      during a TCP failure.  That is, the peer is not required to wait
      for the duration of the FT Reconnection Timeout before releasing
      state; the timeout provides an upper limit on the persistence of
      state.  However, in the event that a peer releases state before
      the expiration of the Reconnection Timeout, it MUST NOT re-use any
      label that was in use on the session until the Reconnection
      Timeout has expired.




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   -  When an LSR receives a Status TLV with the E-bit set in the status
      code, which causes it to close the TCP connection, the LSR MUST
      release all state information and resources associated with the
      session.  This behavior is mandated because it is impossible for
      the LSR to predict the precise state and future behavior of the
      partner LSR that set the E-bit without knowledge of the
      implementation of that partner LSR.

      Note that the 'Temporary Shutdown' status code does not have the
      E-bit set, and MAY be used during maintenance or upgrade
      operations to indicate that the LSR intends to preserve state
      across a closure and re-establishment of the TCP session.

   -  If an LSR determines that it must release state for any single FT
      Label during a failure of the TCP connection on which that label
      was exchanged, it MUST release all state for all labels on the LDP
      session.

   The release of state information and resources associated with non-FT
   labels is as described in [RFC3036].

   Note that a Label Release and the acknowledgement to a Label Withdraw
   may be received by a downstream LSR in any order.  The downstream LSR
   MAY release its resources upon receipt of the first message and MUST
   release its resources upon receipt of the second message.

5.4.  FT Procedure After TCP Failure

   When an LSR discovers or is notified of a TCP connection failure it
   SHOULD start an FT Reconnection Timer to allow a period for re-
   connection of the TCP connection between the LDP peers.

   The RECOMMENDED default value for this timer is 5 seconds.  During
   this time, failure must be detected and reported, new hardware may
   need to be activated, software state must be audited, and a new TCP
   session must be set up.

   Once the TCP connection between LDP peers has failed, the active LSR
   SHOULD attempt to re-establish the TCP connection.  The mechanisms,
   timers and retry counts to re-establish the TCP connection are an
   implementation choice.  It is RECOMMENDED that any attempt to re-
   establish the connection should take into account the failover
   processing necessary on the peer LSR, the nature of the network
   between the LDP peers, and the FT Reconnection Timeout chosen on the
   previous instantiation of the TCP connection (if any).






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   If the TCP connection cannot be re-established within the FT
   Reconnection Timeout period, the LSR detecting this timeout SHOULD
   release all state preserved for the failed LDP session.  If the TCP
   connection is subsequently re-established (for example, after a
   further Hello exchange to set up a new LDP session), the LSR MUST set
   the FT Reconnect Flag to 0 if it released the preserved state
   information on this timeout event.

   If the TCP connection is successfully re-established within the FT
   Reconnection Timeout, both peers MUST re-issue LDP operations that
   were interrupted by (that is, un-acknowledged as a result of) the TCP
   connection failure.  This procedure is described in section 5.5, "FT
   Procedure After TCP Re-connection".

   The Hold Timer for an FT LDP Session (see [RFC3036] section 2.5.5)
   SHOULD be ignored while the FT Reconnection Timer is running.  The
   hold timer SHOULD be restarted when the TCP connection is re-
   established.

5.4.1 FT LDP Operations During TCP Failure

   When the LDP FT enhancements are in use for an LDP session, it is
   possible for an LSR to determine that it needs to send an LDP message
   to an LDP peer, but that the TCP connection to that peer is currently
   down.  These label operations affect the state of FT Labels preserved
   for the failed TCP connection, so it is important that the state
   changes are passed to the LDP peer when the TCP connection is
   restored.

   If an LSR determines that it needs to issue a new FT LDP operation to
   an LDP peer to which the TCP connection is currently failed, it MUST
   pend the operation (e.g. on a queue) and complete that operation with
   the LDP peer when the TCP connection is restored, unless the label
   operation is overridden by a subsequent additional operation during
   the TCP connection failure (see section 5.5, "FT Procedure After TCP
   Re-connection").

   If, during TCP Failure, an LSR determines that it cannot pend an
   operation which it cannot simply fail (for example, a Label Withdraw,
   Release or Abort operation), it MUST NOT attempt to re-establish the
   previous LDP session.  The LSR MUST behave as if the Reconnection
   Timer expired and release all state information with respect to the
   LDP peer.  An LSR may be unable (or unwilling) to pend operations;
   for instance, if a major routing transition occurred while TCP was
   inoperable between LDP peers, it might result in excessively large
   numbers of FT LDP Operations.  An LSR that releases state before the
   expiration of the Reconnection Timeout MUST NOT re-use any label that
   was in use on the session until the Reconnection Timeout has expired.



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   In ordered operation, received FT LDP operations that cannot be
   correctly forwarded because of a TCP connection failure MAY be
   processed immediately (provided sufficient state is kept to forward
   the label operation) or pended for processing when the onward TCP
   connection is restored and the operation can be correctly forwarded
   upstream or downstream.  Operations on existing FT Labels SHOULD NOT
   be failed during TCP session failure.

   It is RECOMMENDED that Label Request operations for new FT Labels not
   be pended awaiting the re-establishment of TCP connection that is
   awaiting recovery at the time the LSR determines that it needs to
   issue the Label Request message.  Instead, such Label Request
   operations SHOULD be failed and, if necessary, a notification message
   containing the 'No LDP Session' status code sent upstream.

   Label Requests for new non-FT Labels MUST be rejected during TCP
   connection failure, as specified in [RFC3036].

5.5.  FT Procedure After TCP Re-connection

   The FT operation handshaking described above means that all state
   changes for Sequence Numbered FT Labels and Address messages are
   confirmed or reproducible at each LSR.

   If the TCP connection between LDP peers fails but is re-connected
   within the FT Reconnection Timeout, and both LSRs have indicated they
   will be re-establishing the previous LDP session, both LDP peers on
   the connection MUST complete any label operations for Sequence
   Numbered FT Labels that were interrupted by the failure and re-
   connection of the TCP connection.

   The procedures for FT Reconnection Timeout MAY have been invoked as a
   result of either LDP peer being unable (or unwilling) to pend
   operations which occurred during the TCP Failure (as described in
   section 5.4.1, "LDP Operations During TCP Failure").

   If, for any reason, an LSR has been unable to pend operations with
   respect to an LDP peer, as described in section 5.4.1, "LDP
   Operations During TCP Failure", the LSR MUST set the FT Reconnect
   Flag to 0 on re-connection to that LDP peer indicating that no FT
   state has been preserved.

   Label operations are completed using the following procedure.








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5.5.1 Re-Issuing FT Messages

   Upon restoration of the TCP connection between LDP peers, any LDP
   messages for Sequence Numbered FT Labels that were lost because of
   the TCP connection failure are re-issued.  The LDP peer that receives
   a re-issued message processes the message as if received for the
   first time.

   "Net-zero" combinations of messages need not be re-issued after re-
   establishment of the TCP connection between LDP peers.  This leads to
   the following rules for re-issuing messages that are not ACKed by the
   LDP peer on the LDP Initialization message exchange after re-
   connection of the TCP session.

   -  A Label Request message MUST be re-issued unless a Label Abort
      would be re-issued for the same Sequence Numbered FT Label.

   -  A Label Mapping message MUST be re-issued unless a Label Withdraw
      message would be re-issued for the same Sequence Numbered FT
      Label.

   -  All other messages on the LDP session that were sent and carried
      the FT Protection TLV MUST be re-issued if an acknowledgement was
      not previously been received.

   Any FT Label operations that were pended (see section 5.4.1, "LDP
   Operations During TCP Failure") during the TCP connection failure
   MUST also be issued upon re-establishment of the LDP session, except
   where they form part of a "net-zero" combination of messages
   according to the above rules.

   The determination of "net-zero" FT Label operations according to the
   above rules MAY be performed on pended messages prior to the re-
   establishment of the TCP connection in order to optimize the use of
   queue resources.  Messages that were sent to the LDP peer before the
   TCP connection failure, or pended messages that were paired with
   them, MUST NOT be subject to such optimization until an FT ACK TLV is
   received from the LDP peer.  This ACK allows the LSR to identify
   which messages were received by the LDP peer prior to the TCP
   connection failure.

6. Check-Pointing Procedures

   Check-Pointing can be selected independently from the FT procedures
   described above by using the C bit in the FT Session TLV on the
   Session Initialization message.  Note, however, that check-pointing
   is an integral part of the FT procedures.  Setting the S and the C
   bit will achieve the same function as setting just the S bit.



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   If the C bit is set, but the S bit is not set, no label is a Sequence
   Numbered FT Label.  Instead, all labels are Check-Pointable FT
   Labels.  Check-Pointing is used to synchronize all label exchanges.
   No message, apart from the check-point request and acknowledgement,
   carries an active sequence number.  (Note that the Session
   Initialization message may carry a sequence number to confirm that
   the check-point is still in place).

   It is an implementation matter to decide the ordering of received
   messages and check-point requests to ensure that check-point
   acknowledgements are secured.

   If the S and C bits are both set, or only the S bit is set, check-
   pointing applies only to Sequence Numbered FT Labels and to address
   messages.

   The set of all messages check-pointed in this way is called the
   Check-Pointable Messages.

6.1 Check-Pointing with the Keepalive Message

   If an LSR receives a FT Protection TLV on a Keepalive message, this
   is a request to flush the acknowledgements for all previously
   received Check-Pointable Messages on the session.

   As soon as the LSR has completed securing the Check-Pointable
   Messages (or state changes consequent on those messages) received
   before the Keepalive, it MUST send an acknowledgement to the sequence
   number of the Keepalive message.

   In the case where the FT procedures are in use and acknowledgements
   have been stored up, this may occur immediately upon receipt of the
   Keepalive.

   An example message flow showing this use of the Keepalive message to
   perform a periodic check-point of state is shown in section 9.2, "Use
   of Check-Pointing With FT Procedures".

   An example message flow showing the use of check-pointing without the
   FT procedures is shown in section 9.5, "Check-Pointing Without FT
   Procedures".

6.2 Quiesce and Keepalive

   If the Keepalive Message also contains the FT Cork TLV, this
   indicates that the peer LSR wishes to quiesce the session prior to a
   graceful restart.




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   It is RECOMMENDED that upon receiving a Keepalive with the FT CORK
   TLV, an LSR should cease to send any further label or address related
   messages on the session until it has been disconnected and
   reconnected, other than messages generated while processing and
   securing previously unacknowledged messages received from the peer
   requesting the quiesce.  It should also attempt to complete this
   processing and return a Keepalive with the FT ACK TLV as soon as
   possible in order to allow the session to be quiesced.

   An example message flow showing this use of the FT Cork TLV to
   achieve a three-way handshake of state synchronization between two
   LDP peers is given in section 9.4, "Temporary Shutdown With FT
   Procedures and Check-Pointing".

7. Changes to Existing Messages

7.1.  LDP Initialization Message

   The LDP FT enhancements add the following optional parameters to a
   LDP Initialization message:

      Optional Parameter    Length     Value

      FT Session TLV        4          See Below
      FT ACK TLV            4          See Below

   The encoding for these TLVs is found in Section 8, "New Fields and
   Values".

   FT Session TLV
      If present, specifies the FT behavior of the LDP session.

   FT ACK TLV
      If present, specifies the last FT message that the sending LDP
      peer was able to secure prior to the failure of the previous
      instantiation of the LDP session.  This TLV is only present if the
      FT Reconnect flag is set in the FT Session TLV, in which case this
      TLV MUST be present.













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7.2.  LDP Keepalive Messages

   The LDP FT enhancements add the following optional parameters to a
   LDP Keepalive message:

      Optional Parameter     Length     Value

      FT Protection TLV      4          See below
      FT Cork TLV            0          See below
      FT ACK TLV             4          See below

   The encoding for these TLVs is found in Section 8, "New Fields and
   Values".

   FT Protection TLV
      If present, specifies the FT Sequence Number for the LDP message.
      When present on a Keepalive message, this indicates a solicited
      flush of the acknowledgements to all previous LDP messages
      containing sequence numbers and issued by the sender of the
      Keepalive on the same session.

   FT Cork TLV
      Indicates that the remote LSR wishes to quiesce the LDP session.
      See section 5, "FT Operations", for the recommended action in such
      cases.

   FT ACK TLV
      If present, specifies the most recent FT message that the sending
      LDP peer has been able to secure.

7.3.  All Other LDP Session Messages

   The LDP FT enhancements add the following optional parameters to all
   other message types that flow on an LDP session after the LDP
   Initialization message

      Optional Parameter    Length     Value

      FT Protection TLV      4          See below
      FT ACK TLV             4          See below

   The encoding for these TLVs is found in section 8, "New Fields and
   Values".

   FT Protection TLV
      If present, specifies the FT Sequence Number for the LDP message.





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   FT ACK TLV
      If present, identifies the most recent FT LDP message ACKed by the
      sending LDP peer.

8. New Fields and Values

8.1.  Status Codes

   The following new status codes are defined to indicate various
   conditions specific to the LDP FT enhancements.  These status codes
   are carried in the Status TLV of a Notification message.

   The "E" column is the required setting of the Status Code E-bit; the
   "Status Data" column is the value of the 30-bit Status Data field in
   the Status Code TLV.

   Note that the setting of the Status Code F-bit is at the discretion
   of the LSR originating the Status TLV.  However, it is RECOMMENDED
   that the F-bit is not set on Notification messages containing status
   codes except 'No LDP Session' because the duplication of messages
   SHOULD be restricted to being a per-hop behavior.

   Status Code                 E   Status Data

   No LDP Session              0   0x0000001A
   Zero FT seqnum              1   0x0000001B
   Unexpected TLV /            1   0x0000001C
      Session Not FT
   Unexpected TLV /            1   0x0000001D
      Label Not FT
   Missing FT Protection TLV   1   0x0000001E
   FT ACK sequence error       1   0x0000001F
   Temporary Shutdown          0   0x00000020

   FT Seq Numbers Exhausted    1   0x00000021
   FT Session parameters /     1   0x00000022
      changed
   Unexpected FT Cork TLV      1   0x00000023

   The 'Temporary Shutdown' status code SHOULD be used in place of the
   'Shutdown' status code (which has the E-bit set) if the LSR that is
   shutting down wishes to inform its LDP peer that it expects to be
   able to preserve FT Label state and return to service before the FT
   Reconnection Timer expires.







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8.2.  FT Session TLV

   LDP peers can negotiate whether the LDP session between them supports
   FT extensions by using a new OPTIONAL parameter, the FT Session TLV,
   on LDP Initialization Messages.

   The FT Session TLV is encoded as follows.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |1|0| FT Session TLV (0x0503)   |      Length (= 12)            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     FT Flags                  |      Reserved                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                FT Reconnect Timeout (in milliseconds)         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Recovery Time (in milliseconds)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   FT Flags
      FT Flags: A 16 bit field that indicates various attributes the FT
      support on this LDP session.  This field is formatted as follows:

      0                   1
      0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |R|         Reserved    |S|A|C|L|
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   R: FT Reconnect Flag.
      Set to 1 if the sending LSR has preserved state and resources for
      all FT-labels since the previous LDP session between the same LDP
      peers, and is otherwise set to 0.  See section 5.4, "FT Procedures
      After TCP Failure", for details of how this flag is used.

      If the FT Reconnect Flag is set, the sending LSR MUST include an
      FT ACK TLV on the LDP Initialization message.

   S: Save State Flag.
      Set to 1 if the use of the FT Protection TLV is supported on
      messages other than the KeepAlive message used for check-pointing
      (see the C bit).  I.e., the S bit indicates that some label on the
      session may be a Sequence Numbered FT Label.

   A: All-Label Protection Required
      Set to 1 if all labels on the session MUST be treated as Sequence
      Numbered FT Labels.  This removes from a node the option of



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      treating some labels as FT Labels and some labels as non-FT
      Labels.

      Passing this information may be considered helpful to a peer since
      it may allow it to make optimizations in its processing.

      The A bit only has meaning if the S bit is set.

   C: Check-Pointing Flag.
      Set to 1 to indicate that the check-Pointing procedures in this
      document are in use.

      If the S bit is also set to 1 then the C bit indicates that
      check-pointing is applied only to Sequence Numbered FT Labels.

      If the S bit is set to 0 (zero) then the C bit indicates that
      check-pointing applies to all labels - all labels are Check-
      Pointable FT Labels.

   L: Learn From Network Flag.
      Set to 1 if the Fault Recovery procedures of [RFC3478] are to be
      used to re-learn state from the network.

      It is not valid for all of the S, C and L bits to be zero.

      It is not valid for both the L and either the S or C bits to be
      set to 1.

      All other bits in this field are currently reserved and SHOULD be
      set to zero on transmission and ignored upon receipt.

      The following table summarizes the settings of these bits.

      S   A   C   L    Comments
      =========================
      0   x   0   0    Invalid
      0   0   0   1    See [RFC3478]
      0   1   0   1    Invalid
      0   x   1   0    Check-Pointing of all labels
      0   x   1   1    Invalid
      1   0   0   0    Full FT on selected labels
      1   1   0   0    Full FT on all labels
      1   x   0   1    Invalid
      1   x   1   0    Same as (S=1,A=x,C=0,L=0)
      1   x   1   1    Invalid.






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   FT Reconnection Timeout
      If the S bit or C bit in the FT Flags field is set, this indicates
      the period of time the sending LSR will preserve state and
      resources for FT Labels exchanged on the previous instantiation of
      an FT LDP session that has recently failed.  The timeout is
      encoded as a 32-bit unsigned integer number of milliseconds.

      A value of zero in this field means that the sending LSR will
      preserve state and resources indefinitely.

      See section 4.4 for details of how this field is used.

      If the L bit is set to 1 in the FT Flags field, the meaning of
      this field is defined in [RFC3478].

   Recovery Time
      The Recovery Time only has meaning if the L bit is set in the FT
      Flags.  The meaning is defined in [RFC3478].

8.3.  FT Protection TLV

   LDP peers use the FT Protection TLV to indicate that an LDP message
   contains an FT label operation.

   The FT Protection TLV MUST NOT be used in messages flowing on an LDP
   session that does not support the LDP FT enhancements.  Its presence
   in such messages SHALL be treated as a protocol error by the
   receiving LDP peer which SHOULD send a Notification message with the
   'Unexpected TLV Session Not FT' status code.  LSRs that do not
   recognize this TLV SHOULD respond with a Notification message with
   the 'Unknown TLV' status code.

   The FT Protection TLV MAY be carried on an LDP message transported on
   the LDP session after the initial exchange of LDP Initialization
   messages.  In particular, this TLV MAY optionally be present on the
   following messages:

   -  Label Request Messages in downstream on-demand distribution mode.

   -  Label Mapping messages in downstream unsolicited mode.

   -  Keepalive messages used to request flushing of acknowledgement of
      all previous messages that contained this TLV.








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   If a label is to be a Sequence Numbered FT Label, then the Protection
   TLV MUST be present:

   -  on the Label Request message in downstream on-demand distribution
      mode.

   -  on the Label Mapping message in in downstream unsolicited
      distribution mode.

   -  on all subsequent messages concerning this label.

   Here 'subsequent messages concerning this label' means any message
   whose Label TLV specifies this label or whose Label Request Message
   ID TLV specifies the initial Label Request message.

   If a label is not to be a Sequence Numbered FT Label, then the
   Protection TLV MUST NOT be present on any of these messages that
   relate to the label.  The presence of the FT TLV on a message
   relating to a non-FT Label SHALL be treated as a protocol error by
   the receiving LDP peer which SHOULD send a notification message with
   the 'Unexpected TLV Label Not FT' status code.

   Where a Label Withdraw or Label Release message contains only an FEC
   TLV and does not identify a single specific label, the FT TLV should
   be included in the message if any label affected by the message is a
   Sequence Numbered FT Label.  If there is any doubt as to whether an
   FT TLV should be present, it is RECOMMENDED that the sender add the
   TLV.

   When an LDP peer receives a Label Withdraw Message or Label Release
   message that contains only a FEC, it SHALL accept the FT TLV if it is
   present regardless of the FT status of the labels that it affects.

   If an LDP session is an FT session as determined by the presence of
   the FT Session TLV, with the S bit set on the LDP Initialization
   messages, the FT Protection TLV MUST be present on all Address
   messages on the session.

   If the session is an FT session, the FT Protection TLV may also
   optionally be present:

   -  on Notification messages on the session that have the status code
      'Label Resources Available'.

   -  on Keepalive messages.






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   The FT Protection TLV is encoded as follows.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|0| FT Protection (0x0203)    |      Length (= 4)             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      FT Sequence Number                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   FT Sequence Number
      The sequence number for this Sequence Numbered FT Label operation.
      The sequence number is encoded as a 32-bit unsigned integer.  The
      initial value for this field on a new LDP session is 0x00000001
      and is incremented by one for each FT LDP message issued by the
      sending LSR on this LDP session.  This field may wrap from
      0xFFFFFFFF to 0x00000001.

      This field MUST be reset to 0x00000001 if either LDP peer does not
      set the FT Reconnect Flag upon re-establishment of the TCP
      connection.

      See section 5.2, "FT Operation Acks" for details of how this field
      is used.

      The special use of 0x00000000 is discussed in the section 8.4, "FT
      ACK TLV" below.

   If an LSR receives an FT Protection TLV on a session that does not
   support the FT LDP enhancements, it SHOULD send a Notification
   message to its LDP peer containing the 'Unexpected TLV, Session Not
   FT' status code.  LSRs that do not recognize this TLV SHOULD respond
   with a Notification message with the 'Unknown TLV' status code.

   If an LSR receives an FT Protection TLV on an operation affecting a
   label that it believes is a non-FT Label, it SHOULD send a
   Notification message to its LDP peer containing the 'Unexpected TLV,
   Label Not FT' status code.

   If an LSR receives a message without the FT Protection TLV affecting
   a label that it believes is a Sequence Numbered FT Label, it SHOULD
   send a Notification message to its LDP peer containing the 'Missing
   FT Protection TLV' status code.

   If an LSR receives an FT Protection TLV containing a zero FT Sequence
   Number, it SHOULD send a Notification message to its LDP peer
   containing the 'Zero FT Seqnum' status code.




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8.4.  FT ACK TLV

   LDP peers use the FT ACK TLV to acknowledge FT Label operations.

   The FT ACK TLV MUST NOT be used in messages flowing on an LDP session
   that does not support the LDP FT enhancements.  Its presence on such
   messages SHALL be treated as a protocol error by the receiving LDP
   peer.

   The FT ACK TLV MAY be present on any LDP message exchanged on an LDP
   session after the initial LDP Initialization messages.  It is
   RECOMMENDED that the FT ACK TLV be included in all FT Keepalive
   messages in order to ensure that the LDP peers do not build up a
   large backlog of unacknowledged state information.

   The FT ACK TLV is encoded as follows.

    0                   1                   2                  3
   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|0|   FT ACK (0x0504)         |      Length (= 4)             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      FT ACK Sequence Number                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   FT ACK Sequence Number
      The sequence number for the most recent FT label message that the
      sending LDP peer has received from the receiving LDP peer and
      secured against failure of the LDP session.  It is not necessary
      for the sending peer to have fully processed the message before
      ACKing it.  For example, an LSR MAY ACK a Label Request message as
      soon as it has securely recorded the message, without waiting
      until it can send the Label Mapping message in response.

      ACKs are cumulative.  Receipt of an LDP message containing an FT
      ACK TLV with an FT ACK Sequence Number of 12 is treated as the
      acknowledgement of all messages from 1 to 12 inclusive (assuming
      the LDP session started with a sequence number of 1).

      This field MUST be set to 0 if the LSR sending the FT ACK TLV has
      not received any FT label operations on this LDP session.  This
      applies to LDP sessions, to new LDP peers or after an LSR
      determines that it must drop all state for a failed TCP
      connection.

      See section 5.2, "FT Operation Acks" for details of how this field
      is used.




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   If an LSR receives an FT ACK TLV that contains an FT ACK Sequence
   Number that is less than the previously received FT ACK Sequence
   Number (remembering to take account of wrapping), it SHOULD send a
   Notification message to its LDP peer containing the 'FT ACK Sequence
   Error' status code.

8.5.  FT Cork TLV

   LDP peers use the FT Cork TLV on FT Keepalive messages to indicate
   that they wish to quiesce the LDP session prior to a controlled
   shutdown and restart, for example during control-plane software
   upgrade.

   The FT Cork TLV is encoded as follows.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|0|   FT Cork (0x0505)        |      Length (= 0)             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Upon receipt of a Keepalive message with the FT Cork TLV and the FT
   Protection TLV, an LSR SHOULD perform the following actions:

   -  Process and secure any messages from the peer LSR that have
      sequence numbers less than (accounting for wrap) that contained in
      the FT Protection TLV on the Keepalive message.

   -  Send a Keepalive message back to the peer containing the FT Cork
      TLV and the FT ACK TLV specifying the FT ACK sequence number
      equal to that in the original Keepalive message (i.e. ACKing all
      messages up to that point).

   -  If this LSR has not yet received an FT ACK to all the messages it
      has sent containing the FT Protection TLV, then also include an FT
      Protection TLV on the Keepalive sent to the peer LSR.  This tells
      the remote peer that the local LSR has saved state prior to
      quiesce but is still awaiting confirmation that the remote peer
      has saved state.

   -  Cease sending any further state changing messages on this LDP
      session until it has been disconnected and recovered.

   On receipt of a Keepalive message with the FT Cork TLV and an FT ACK
   TLV that acknowledges the previously sent Keepalive that carried the
   FT Cork TLV, an LSR knows that quiesce is complete.  If the received
   Keepalive also carries the FT Protection TLV, the LSR must respond
   with a further Keepalive to complete the 3-way handshake.  It SHOULD



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   now send a "Temporary Shutdown" Notification message, disconnect the
   TCP session and perform whatever control plane actions required this
   session shutdown.

   An example of such a 3-way handshake for controlled shutdown is given
   in section section 9.4, "Temporary Shutdown With FT Procedures and
   Check-Pointing".

   If an LSR receives a message that should not carry the FT Cork TLV,
   or if the FT Cork TLV is used on a Keepalive message without one of
   the FT Protection or FT ACK TLVs present, it SHOULD send a
   Notification message to its LDP peer containing the 'Unexpected FT
   Cork TLV' status code.

9. Example Use

   Consider two LDP peers, P1 and P2, implementing LDP over a TCP
   connection that connects them, and the message flow shown below.

   The parameters shown on each message below are as follows:

      message (label, senders FT sequence number, FT ACK number)

      A "-" for FT ACK number means that the FT ACK TLV is not included
      on that message.  "n/a" means that the parameter in question is
      not applicable to that type of message.

   In the diagrams below, time flows from top to bottom.  The relative
   position of each message shows when it is transmitted.  See the notes
   for a description of when each message is received, secured for FT or
   processed.

9.1.  Session Failure and Recovery - FT Procedures

   notes         P1                         P2
   =====         ==                         ==
   (1)           Label Request(L1,27,-)
                 --------------------------->
                 Label Request(L2,28,-)
                 --------------------------->
   (2)                Label Request(L3,93,27)
                 <---------------------------
   (3)                                      Label Request(L1,123,-)
                                            -------------------------->
                                            Label Request(L2,124,-)
                                            -------------------------->





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   (4)                                           Label Mapping(L1,57,-)
                                            <--------------------------
                      Label Mapping(L1,94,28)
                 <---------------------------
   (5)                                           Label Mapping(L2,58,-)
                                            <--------------------------
                       Label Mapping(L2,95,-)
                 <---------------------------
   (6)           Address(n/a,29,-)
                 --------------------------->
   (7)           Label Request(L4,30,-)
                 --------------------------->
   (8)           Keepalive(n/a,-,94)
                 --------------------------->
   (9)                   Label Abort(L3,96,-)
                 <---------------------------
   (10)          ===== TCP Session lost =====
                   :
   (11)            :                            Label Withdraw(L1,59,-)
                   :                        <--------------------------
                   :
   (12)          === TCP Session restored ===

                 LDP Init(n/a,n/a,94)
                 --------------------------->
                         LDP Init(n/a,n/a,29)
                 <---------------------------
   (13)          Label Request(L4,30,-)
                 --------------------------->
   (14)                Label Mapping(L2,95,-)
                 <---------------------------
                        Label Abort(L3,96,30)
                 <---------------------------
   (15)               Label Withdraw(L1,97,-)
                 <---------------------------

   Notes:
   ======

   (1)  Assume that the LDP session has already been initialized.  P1
        issues 2 new Label Requests using the next sequence numbers.

   (2)  P2 issues a Label Request to P1.  At the time of sending this
        request, P2 has secured the receipt of the label request for L1
        from P1, so it includes an ACK for that message.






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   (3)  P2 processes the Label Requests for L1 and L2 and forwards them
        downstream.  Details of downstream processing are not shown in
        the diagram above.

   (4)  P2 receives a Label Mapping from downstream for L1, which it
        forwards to P1.  It includes an ACK to the Label Request for L2,
        as that message has now been secured and processed.

   (5)  P2 receives the Label Mapping for L2, which it forwards to P1.
        This time it does not include an ACK as it has not received any
        further messages from P1.

   (6)  Meanwhile, P1 sends a new Address Message to P2.

   (7)  P1 also sends a fourth Label Request to P2

   (8)  P1 sends a Keepalive message to P2, on which it includes an ACK
        for the Label Mapping for L1, which is the latest message P1 has
        received and secured at the time the Keepalive is sent.

   (9)  P2 issues a Label Abort for L3.

   (10) At this point, the TCP session goes down.

   (11) While the TCP session is down, P2 receives a Label Withdraw
        Message for L1, which it queues.

   (12) The TCP session is reconnected and P1 and P2 exchange LDP
        Initialization messages on the recovered session, which include
        ACKS for the last message each peer received and secured prior
        to the failure.

   (13) From the LDP Init exchange, P1 determines that it needs to re-
        issue the Label request for L4.

   (14) Similarly, P2 determines that it needs to re-issue the Label
        Mapping for L2 and the Label Abort.

   (15) P2 issues the queued Label Withdraw to P1.












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9.2.  Use of Check-Pointing With FT Procedures

   notes         P1                         P2
   =====         ==                         ==
   (1)           Label Request(L1,27,-)
                 --------------------------->
                 Label Request(L2,28,-)
                 --------------------------->
   (2)                Label Request(L3,93,-)
                 <---------------------------
   (3)                                      Label Request(L1,123,-)
                                            -------------------------->
                                            Label Request(L2,124,-)
                                            -------------------------->
   (4)                                           Label Mapping(L1,57,-)
                                            <--------------------------
                      Label Mapping(L1,94,-)
                 <---------------------------
   (5)                                           Label Mapping(L2,58,-)
                                            <--------------------------
                       Label Mapping(L2,95,-)
                 <---------------------------
   (6)           Address(n/a,29,-)
                 --------------------------->
   (7)           Label Request(L4,30,-)
                 --------------------------->
   (8)           Keepalive(n/a,31,-)
                 --------------------------->
   (9)                   Keepalive(n/a,-,31)
                 <---------------------------
   (10)                                          Keepalive(n/a,59,124)
                                            <---------------------------
   (11)                                     Keepalive(n/a,-,59)
                                            --------------------------->
   Notes:
   ======

   Notes (1) through (7) are as in the previous example except note that
   no acknowledgements are piggy-backed on reverse direction messages.
   This means that at note (8) there are deferred acknowledgements in
   both directions on both links.

   (8)  P1 wishes to synchronize state with P2.  It sends a Keepalive
        message containing an FT Protection TLV with sequence number 31.
        Since it is not interested in P2's perception of the state that
        it has stored, it does not include an FT ACK TLV.





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   (9)  P2 responds at once with a Keepalive acknowledging the sequence
        number on the received Keepalive.  This tells P1 that P2 has
        preserved all state/messages previously received on this
        session.

   (10) The downstream node wishes to synchronize state with P2.  It
        sends a Keepalive message containing an FT Protection TLV with
        sequence number 59.  P3 also takes this opportunity to get up to
        date with its acknowledgements to P2 by including an FT ACK TLV
        acknowledging up to sequence number 124.

   (11) P2 responds at once with a Keepalive acknowledging the sequence
        number on the received Keepalive.

9.3.  Temporary Shutdown With FT Procedures

   notes         P1                         P2
   =====         ==                         ==
   (1)           Label Request(L1,27,-)
                 --------------------------->
                 Label Request(L2,28,-)
                 --------------------------->
   (2)                Label Request(L3,93,27)
                 <---------------------------
   (3)                                      Label Request(L1,123,-)
                                            -------------------------->
                                            Label Request(L2,124,-)
                                            -------------------------->
   (4)                                           Label Mapping(L1,57,-)
                                            <--------------------------
                      Label Mapping(L1,94,28)
                 <---------------------------
   (5)                                           Label Mapping(L2,58,-)
                                            <--------------------------
                       Label Mapping(L2,95,-)
                 <---------------------------
   (6)           Address(n/a,29,-)
                 --------------------------->
   (7)           Label Request(L4,30,-)
                 --------------------------->
   (8)           Keepalive(n/a,-,94)
                 --------------------------->
   (9)                   Label Abort(L3,96,-)
                 <---------------------------







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   (10)          Notification(Temporary shutdown)
                 --------------------------->
                 ===== TCP Session shutdown =====
                   :
   (11)            :                            Label Withdraw(L1,59,-)
                   :                        <--------------------------
                   :
                 ===== TCP Session restored =====
   (12)          LDP Init(n/a,n/a,94)
                 --------------------------->
                         LDP Init(n/a,n/a,29)
                 <---------------------------
   (13)          Label Request(L4,30,-)
                 --------------------------->
   (14)                Label Mapping(L2,95,-)
                 <---------------------------
                        Label Abort(L3,96,30)
                 <---------------------------
   (15)               Label Withdraw(L1,97,-)
                 <---------------------------

   Notes:
   ======

   Notes are as in the previous example except as follows.

   (10) P1 needs to upgrade the software or hardware that it is running.
        It issues a Notification message to terminate the LDP session,
        but sets the status code as 'Temporary shutdown' to inform P2
        that this is not a fatal error, and P2 should maintain FT state.
        The TCP connection may also fail during the period that the LDP
        session is down (in which case it will need to be re-
        established), but it is also possible that the TCP connection
        will be preserved.

















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9.4.  Temporary Shutdown With FT Procedures and Check-Pointing

   notes         P1                         P2
   =====         ==                         ==
   (1)           Label Request(L1,27,-)
                 --------------------------->
                 Label Request(L2,28,-)
                 --------------------------->
   (2)                Label Request(L3,93,-)
                 <---------------------------
                                            Label Request(L1,123,-)
                                            -------------------------->
                                            Label Request(L2,124,-)
                                            -------------------------->
                                                 Label Mapping(L1,57,-)
                                            <--------------------------
   (3)                 Label Mapping(L1,94,-)
                 <---------------------------
                                                 Label Mapping(L2,58,-)
                                            <--------------------------
                       Label Mapping(L2,95,-)
                 <---------------------------
   (4)           Address(n/a,29,-)
                 --------------------------->
   (5)           Label Request(L4,30,-)
                 --------------------------->
   (6)           Keepalive(n/a,31,95) * with FT Cork TLV *
                 --------------------------->
   (7)                   Label Abort(L3,96,-)
                 <---------------------------
   (8)                    Keepalive(n/a,97,31) * with FT Cork TLV *
                 <---------------------------
   (9)           Keepalive(n/a,-,97) * with FT Cork TLV *
                 --------------------------->
   (10)          Notification(Temporary shutdown)
                 --------------------------->
                 ===== TCP Session shutdown =====
                   :
                   :                            Label Withdraw(L1,59,-)
                   :                        <--------------------------
                   :
                 ===== TCP Session restored =====
   (11)          LDP Init(n/a,n/a,96)
                 --------------------------->
                         LDP Init(n/a,n/a,31)
                 <---------------------------
                      Label Withdraw(L1,97,-)
                 <---------------------------



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   Notes:
   ======

   This example operates much as the previous one.  However, at (1),
   (2), (3), (4) and (5), no acknowledgements are made.

   At (6), P1 determines that graceful shutdown is required and sends a
   Keepalive acknowledging all previously received messages and itself
   containing an FT Protection TLV number and the FT Cork TLV.

   The Label abort at (7) crosses with this Keepalive, so at (8) P2
   sends a Keepalive that acknowledges all messages received so far, but
   also includes the FT Protection and FT Cork TLVs to indicate that
   there are still messages outstanding to be acknowledged.

   P1 is then able to complete the 3-way handshake at (9) and close the
   TCP session at (10).

   Upon recovery at (11), there are no messages to be re-sent because
   the KeepAlives flushed the acknowledgements.  The only messages sent
   after recovery is the Label Withdraw that was pended during the TCP
   session failure.





























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9.5.  Check-Pointing Without FT Procedures

   notes         P1                         P2
   =====         ==                         ==
   (1)           Label Request(L1)
                 --------------------------->
   (2)                Label Request(L2)
                 <---------------------------
                                            Label Request(L1)
                                            -------------------------->
                                                 Label Mapping(L1)
                                            <--------------------------
   (3)                 Label Mapping(L1)
                 <---------------------------
   (4)           Keepalive(n/a,12,-)
                 --------------------------->
   (5)           Label Request(L3)
                 --------------------------->
   (6)                    Keepalive(n/a,-,12)
                 <---------------------------
                                            Label Request(L3)
                                            -------------------------->
                                                 Label Mapping(L3)
                                            <--------------------------
   (7)                 Label Mapping(L3)
                 <---------------------------
                 ===== TCP Session failure =====
                   :
                   :
                   :
                 ===== TCP Session restored =====
   (8)          LDP Init(n/a,n/a,23)
                 --------------------------->
                         LDP Init(n/a,n/a,12)
                 <---------------------------
   (9)           Label Request(L3)
                 --------------------------->
                                            Label Request(L3)
                                            -------------------------->
                                                 Label Mapping(L3)
                                            <--------------------------
   (10)                Label Mapping(L3)
                 <---------------------------
   (11)                Label Request(L2)
                 <---------------------------






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   Notes:
   ======

   (1), (2) and (3) show label distribution without FT sequence numbers.

   (4)  A check-Point request from P1.  It carries the sequence number
        of the check-point request.

   (5)  P1 immediately starts a new label distribution request.

   (6)  P2 confirms that it has secured all previous transactions.

   (7)  The subsequent (un-acknowledged) label distribution completes.

   (8)  The session fails and is restarted.  Initialization messages
        confirm the sequence numbers of the secured check-points.

   (9)  P1 recommences the unacknowledged label distribution request.

   (10) P2 recommences an unacknowledged label distribution request.































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9.6.  Graceful Shutdown With Check-Pointing But No FT Procedures

   notes         P1                         P2
   =====         ==                         ==
   (1)           Label Request(L1)
                 --------------------------->
   (2)                Label Request(L2)
                 <---------------------------
                                            Label Request(L1)
                                            -------------------------->
                                                 Label Mapping(L1)
                                            <--------------------------
   (3)                 Label Mapping(L1)
                 <---------------------------
   (4)           Keepalive(n/a,12,23) * With Cork TLV *
                 --------------------------->
   (5)             :
                   :
                   :
   (6)                    Keepalive(n/a,24,12) * With Cork TLV *
                 <---------------------------
   (7)           Keepalive(n/a,-,24) * With Cork TLV *
                 --------------------------->
   (8)           Notification(Temporary shutdown)
                 --------------------------->
                 ===== TCP Session failure =====
                   :
                   :
                   :
                 ===== TCP Session restored =====
   (9)          LDP Init(n/a,n/a,24)
                 --------------------------->
                         LDP Init(n/a,n/a,12)
                 <---------------------------
   (10)          Label Request(L3)
                 --------------------------->
                                            Label Request(L3)
                                            -------------------------->
                                                 Label Mapping(L3)
                                            <--------------------------
   (11)                Label Mapping(L3)
                 <---------------------------
   (12)                Label Mapping(L2)
                 --------------------------->







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   Notes:
   ======

   (1), (2) and (3) show label distribution without FT sequence numbers.

   (4)  A check-point request from P1.  It carries the sequence number
        of the check-point request and a Cork TLV.

   (5)  P1 has sent a Cork TLV so quieces.

   (6)  P2 confirms the check-point and continues the three-way
        handshake by including a Cork TLV itself.

   (7)  P1 completes the three-way handshake.  All operations have now
        been check-pointed and the session is quiesced.

   (8)  The session is gracefully shut down.

   (9)  The session recovers and the peers exchange the sequence numbers
        of the last secured check-points.

   (10) P1 starts a new label distribution request.

   (11) P1 continues processing a previously received label distribution
        request.

10.   Security Considerations

   The LDP FT enhancements inherit similar security considerations to
   those discussed in [RFC3036].

   The LDP FT enhancements allow the re-establishment of a TCP
   connection between LDP peers without a full re-exchange of the
   attributes of established labels, which renders LSRs that implement
   the extensions specified in this document vulnerable to additional
   denial-of-service attacks as follows:

   -  An intruder may impersonate an LDP peer in order to force a
      failure and reconnection of the TCP connection, but where the
      intruder does not set the FT Reconnect Flag upon re-connection.
      This forces all FT labels to be released.

   -  Similarly, an intruder could set the FT Reconnect Flag on re-
      establishment of the TCP session without preserving the state and
      resources for FT labels.






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   -  An intruder could intercept the traffic between LDP peers and
      override the setting of the FT Label Flag to be set to 0 for all
      labels.

   All of these attacks may be countered by use of an authentication
   scheme between LDP peers, such as the MD5-based scheme outlined in
   [RFC3036].

   Alternative authentication schemes for LDP peers are outside the
   scope of this document, but could be deployed to provide enhanced
   security to implementations of LDP and the LDP FT enhancements.

   As with LDP, a security issue may exist if an LDP implementation
   continues to use labels after expiration of the 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-forwarding 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 session may be extended by the
   FT Reconnection Timeout, and the session may be re-established in
   this period.  After the expiry of the Reconnection Timeout, 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 Reconnection Timeout.  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 not be re-used until the Reconnection Timeout has expired.

   A further issue might apply if labels were re-used prior to the
   expiration of the FT Reconnection Timeout, but this is forbidden by
   this document.

   The issue of re-use of labels extends to labels managed through other
   mechanisms including direct configuration through management
   applications and distribution through other label distribution
   protocols.  Avoiding this problem may be construed as an
   implementation issue (see below), but failure to acknowledge it could
   result in the mis-forwarding of data between LSPs established using
   some other mechanism and those recovered using the methods described
   in this document.




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11.   Implementation Notes

11.1. FT Recovery Support on Non-FT LSRs

   In order to take full advantage of the FT capabilities of LSRs in the
   network, it may be that an LSR that does not itself contain the
   ability to recover from local hardware or software faults still needs
   to support the LDP FT enhancements described in this document.

   Consider an LSR, P1, that is an LDP peer of a fully Fault Tolerant
   LSR, P2.  If P2 experiences a fault in the hardware or software that
   serves an LDP session between P1 and P2, it may fail the TCP
   connection between the peers.  When the connection is recovered, the
   LSPs/labels between P1 and P2 can only be recovered if both LSRs were
   applying the FT recovery procedures to the LDP session.

11.2. ACK generation logic

   FT ACKs SHOULD be returned to the sending LSR as soon as is
   practicable in order to avoid building up a large quantity of
   unacknowledged state changes at the LSR.  However, immediate one-
   for-one acknowledgements would waste bandwidth unnecessarily.

   A possible implementation strategy for sending ACKs to FT LDP
   messages is as follows:

   -  An LSR secures received messages in order and tracks the sequence
      number of the most recently secured message, Sr.

   -  On each LDP KeepAlive that the LSR sends, it attaches an FT ACK
      TLV listing Sr.

   -  Optionally, the LSR may attach an FT ACK TLV to any other LDP
      message sent between Keepalive messages if, for example, Sr has
      increased by more than a threshold value since the last ACK sent.

   This implementation combines the bandwidth benefits of accumulating
   ACKs while still providing timely ACKs.

11.2.1 Ack Generation Logic When Using Check-Pointing

   If check-pointing is in use, the LSRs need not be concerned with
   sending ACKs in such a timely manner.

   Check-points are solicitations for acknowledgements conveyed as a
   sequence number in an FT Protection TLV on a Keepalive message.  Such
   check-point requests could be issued on a timer, after a significant
   amount of change, or before controlled shutdown of a session.



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   The use of check-pointing may considerably simplify an implementation
   since it does not need to track the sequence numbers of all received
   LDP messages.  It must, however, still ensure that all received
   messages (or the consequent state changes) are secured before
   acknowledging the sequence number on the Keepalive.

   This approach may be considered optimal in systems that do not show a
   high degree of change over time (such as targeted LDP sessions) and
   that are prepared to risk loss of state for the most recent LDP
   exchanges.  More dynamic systems (such as LDP discovery sessions) are
   more likely to want to acknowledge state changes more frequently so
   that the maximum amount of state can be preserved over a failure.

11.3 Interactions 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 the traffic
   engineering label distribution protocol that is 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 since that 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.

12.   Acknowledgments

   The work in this document is based on the LDP ideas expressed by the
   authors of [RFC3036].

   The ACK scheme used in this document was inspired by the proposal by
   David Ward and John Scudder for restarting BGP sessions now included
   in [BGP-RESTART].

   The authors would also like to acknowledge the careful review and
   comments of Nick Weeds, Piers Finlayson, Tim Harrison, Duncan Archer,
   Peter Ashwood-Smith, Bob Thomas, S. Manikantan, Adam Sheppard,
   Alan Davey, Iftekhar Hussain and Loa Andersson.






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13.   Intellectual Property Consideration

   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.

14.  References

14.1. 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. Aggrawal, "Graceful
                  Restart Mechanism for Label Distribution Protocol",
                  RFC 3478, February 2003.







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14.2. Informative References

   [RFC2205]      Braden, R., Zhang, L., Berson, S., Herzog, S. and S.
                  Jamin, "Resource ReSerVation Protocol (RSVP) --
                  Version 1, Functional Specification", RFC 2205,
                  September 1997.

   [RFC2961]      Berger, L., Gan, D., Swallow, G., Pan, P., Tomassi, F.
                  and S. Molendini, "RSVP Refresh Reduction Extensions",
                  RFC 2961, April 2001.

   [RFC3209]      Awduche, D., Berger, L., Gan, D., Li, T., Srinivasan,
                  V. and G. Swallow, "Extensions to RSVP for LSP
                  Tunnels", RFC 3209, December 2001.

   [RFC3212]      Jamoussi, B., 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.

   [RFC3214]      Ash, G., Lee, Y., Ashwood-Smith, P., Jamoussi, B.,
                  Fedyk, D., Skalecki, D. and L. Li, "LSP Modification
                  Using CR-LDP", RFC 3214, January 2001.

   [BGP-RESTART]  Sangli, S., et al., Graceful Restart Mechanism for
                  BGP, Work in Progress.

15.  Authors' Addresses

   Adrian Farrel (editor)
   Movaz Networks, Inc.
   7926 Jones Branch Drive, Suite 615
   McLean, VA 22102

   Phone:  +1 703-847-1867
   EMail:  afarrel@movaz.com

   Paul Brittain
   Data Connection Ltd.
   Windsor House, Pepper Street,
   Chester, Cheshire
   CH1 1DF, UK

   Phone:   +44-(0)20-8366-1177
   EMail:   pjb@dataconnection.com





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   Philip Matthews
   Hyperchip
   1800 Rene-Levesque Blvd W
   Montreal, Quebec H3H 2H2
   Canada

   Phone:  +1 514-906-4965
   EMail: pmatthews@hyperchip.com

   Eric Gray

   EMail: ewgray@GraIyMage.com

   Jack Shaio
   Vivace Networks
   2730 Orchard Parkway
   San Jose, CA 95134

   Phone: +1 408 432 7623
   EMail: jack.shaio@vivacenetworks.com

   Toby Smith
   Laurel Networks, Inc.
   1300 Omega Drive
   Pittsburgh, PA 15205

   EMail: tob@laurelnetworks.com

   Andrew G. Malis
   Vivace Networks
   2730 Orchard Parkway
   San Jose, CA 95134

   Phone: +1 408 383 7223
   EMail: andy.malis@vivacenetworks.com
















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16.  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 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.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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