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rfc:rfc3479

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.

Farrel Standards Track [Page 1] RFC 3479 Fault Tolerance for the LDP February 2003

 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

Farrel Standards Track [Page 2] RFC 3479 Fault Tolerance for the LDP February 2003

    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.

Farrel Standards Track [Page 3] RFC 3479 Fault Tolerance for the LDP February 2003

 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.

Farrel Standards Track [Page 4] RFC 3479 Fault Tolerance for the LDP February 2003

 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:
  1. failure/hot-swap of a physical connection between LSRs.
  1. failure/hot-swap of the switching fabric in an LSR.
  1. failure of the TCP or LDP stack in an LSR.
  1. 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.

Farrel Standards Track [Page 5] RFC 3479 Fault Tolerance for the LDP February 2003

 This document describes a solution which involves:
  1. negotiation between LDP peers of the intent to support extensions

to LDP that facilitate recovery from failover without loss of

    LSPs.
  1. selection of FT survival on a per LSP/label basis.
  1. 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.
  1. 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.
  1. 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:
  1. offer backward-compatibility with LSRs that do not implement these

extensions to LDP.

  1. preserve existing protocol rules described in [RFC3036] for

handling unexpected duplicate messages and for processing

    unexpected messages referring to unknown LSPs/labels.
  1. 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

Farrel Standards Track [Page 6] RFC 3479 Fault Tolerance for the LDP February 2003

 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.
  1. 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.
  1. 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.
  1. 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.
  1. 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.
  1. 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.

Farrel Standards Track [Page 7] RFC 3479 Fault Tolerance for the LDP February 2003

  1. 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.

Farrel Standards Track [Page 8] RFC 3479 Fault Tolerance for the LDP February 2003

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.
  1. Indication from the management component that a TCP connection or

underlying resource is no longer active.

  1. Notification from a hardware management component of an interface

failure.

  1. Sockets keepalive timeout.
  1. Sockets send failure.
  1. New (incoming) Socket opened.
  1. 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.

Farrel Standards Track [Page 9] RFC 3479 Fault Tolerance for the LDP February 2003

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.

Farrel Standards Track [Page 10] RFC 3479 Fault Tolerance for the LDP February 2003

 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.

Farrel Standards Track [Page 11] RFC 3479 Fault Tolerance for the LDP February 2003

 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".

Farrel Standards Track [Page 12] RFC 3479 Fault Tolerance for the LDP February 2003

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.

Farrel Standards Track [Page 13] RFC 3479 Fault Tolerance for the LDP February 2003

 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.

Farrel Standards Track [Page 14] RFC 3479 Fault Tolerance for the LDP February 2003

 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.

Farrel Standards Track [Page 15] RFC 3479 Fault Tolerance for the LDP February 2003

 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:
  1. 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.
  1. The upstream LSR must recall the label availability state across

failover so that it can optimize not sending Label Requests when

    it recovers.
  1. 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:
  1. 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.
  1. 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.

Farrel Standards Track [Page 16] RFC 3479 Fault Tolerance for the LDP February 2003

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.

Farrel Standards Track [Page 17] RFC 3479 Fault Tolerance for the LDP February 2003

 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.
  1. 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.
  1. 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.
  1. 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.
  1. 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.
  1. When the FT Reconnection Timeout expires, an LSR SHOULD release

all state information and resources from previous instantiations

    of the (permanently) failed LDP session.
  1. 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.

Farrel Standards Track [Page 18] RFC 3479 Fault Tolerance for the LDP February 2003

  1. 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.
  1. 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).

Farrel Standards Track [Page 19] RFC 3479 Fault Tolerance for the LDP February 2003

 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.

Farrel Standards Track [Page 20] RFC 3479 Fault Tolerance for the LDP February 2003

 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.

Farrel Standards Track [Page 21] RFC 3479 Fault Tolerance for the LDP February 2003

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.
  1. A Label Request message MUST be re-issued unless a Label Abort

would be re-issued for the same Sequence Numbered FT Label.

  1. A Label Mapping message MUST be re-issued unless a Label Withdraw

message would be re-issued for the same Sequence Numbered FT

    Label.
  1. 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.

Farrel Standards Track [Page 22] RFC 3479 Fault Tolerance for the LDP February 2003

 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.

Farrel Standards Track [Page 23] RFC 3479 Fault Tolerance for the LDP February 2003

 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.

Farrel Standards Track [Page 24] RFC 3479 Fault Tolerance for the LDP February 2003

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.

Farrel Standards Track [Page 25] RFC 3479 Fault Tolerance for the LDP February 2003

 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.

Farrel Standards Track [Page 26] RFC 3479 Fault Tolerance for the LDP February 2003

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

Farrel Standards Track [Page 27] RFC 3479 Fault Tolerance for the LDP February 2003

    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.

Farrel Standards Track [Page 28] RFC 3479 Fault Tolerance for the LDP February 2003

 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:
  1. Label Request Messages in downstream on-demand distribution mode.
  1. Label Mapping messages in downstream unsolicited mode.
  1. Keepalive messages used to request flushing of acknowledgement of

all previous messages that contained this TLV.

Farrel Standards Track [Page 29] RFC 3479 Fault Tolerance for the LDP February 2003

 If a label is to be a Sequence Numbered FT Label, then the Protection
 TLV MUST be present:
  1. on the Label Request message in downstream on-demand distribution

mode.

  1. on the Label Mapping message in in downstream unsolicited

distribution mode.

  1. 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:
  1. on Notification messages on the session that have the status code

'Label Resources Available'.

  1. on Keepalive messages.

Farrel Standards Track [Page 30] RFC 3479 Fault Tolerance for the LDP February 2003

 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.

Farrel Standards Track [Page 31] RFC 3479 Fault Tolerance for the LDP February 2003

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.

Farrel Standards Track [Page 32] RFC 3479 Fault Tolerance for the LDP February 2003

 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:
  1. 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.
  1. 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).
  1. 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.
  1. 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

Farrel Standards Track [Page 33] RFC 3479 Fault Tolerance for the LDP February 2003

 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,-)
                                          -------------------------->

Farrel Standards Track [Page 34] RFC 3479 Fault Tolerance for the LDP February 2003

 (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.

Farrel Standards Track [Page 35] RFC 3479 Fault Tolerance for the LDP February 2003

 (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.

Farrel Standards Track [Page 36] RFC 3479 Fault Tolerance for the LDP February 2003

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.

Farrel Standards Track [Page 37] RFC 3479 Fault Tolerance for the LDP February 2003

 (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,-)
               <---------------------------

Farrel Standards Track [Page 38] RFC 3479 Fault Tolerance for the LDP February 2003

 (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.

Farrel Standards Track [Page 39] RFC 3479 Fault Tolerance for the LDP February 2003

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,-)
               <---------------------------

Farrel Standards Track [Page 40] RFC 3479 Fault Tolerance for the LDP February 2003

 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.

Farrel Standards Track [Page 41] RFC 3479 Fault Tolerance for the LDP February 2003

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)
               <---------------------------

Farrel Standards Track [Page 42] RFC 3479 Fault Tolerance for the LDP February 2003

 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.

Farrel Standards Track [Page 43] RFC 3479 Fault Tolerance for the LDP February 2003

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)
               --------------------------->

Farrel Standards Track [Page 44] RFC 3479 Fault Tolerance for the LDP February 2003

 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:
  1. 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.
  1. 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.

Farrel Standards Track [Page 45] RFC 3479 Fault Tolerance for the LDP February 2003

  1. 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.

Farrel Standards Track [Page 46] RFC 3479 Fault Tolerance for the LDP February 2003

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:
  1. An LSR secures received messages in order and tracks the sequence

number of the most recently secured message, Sr.

  1. On each LDP KeepAlive that the LSR sends, it attaches an FT ACK

TLV listing Sr.

  1. 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.

Farrel Standards Track [Page 47] RFC 3479 Fault Tolerance for the LDP February 2003

 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.

Farrel Standards Track [Page 48] RFC 3479 Fault Tolerance for the LDP February 2003

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.

Farrel Standards Track [Page 49] RFC 3479 Fault Tolerance for the LDP February 2003

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

Farrel Standards Track [Page 50] RFC 3479 Fault Tolerance for the LDP February 2003

 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

Farrel Standards Track [Page 51] RFC 3479 Fault Tolerance for the LDP February 2003

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.

Farrel Standards Track [Page 52]

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