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Versions: (draft-farrel-mpls-ldp-ft) 00 01 02 03 04 05 06 RFC 3479

MPLS Working Group                                               Editor
Internet Draft                                            Adrian Farrel
Document: draft-ietf-mpls-ldp-ft-06.txt            Movaz Networks, Inc.
Expiration Date: March 2003                              September 2002


  Fault Tolerance for the Label Distribution Protocol (LDP)

                draft-ietf-mpls-ldp-ft-06.txt

Status of this Memo

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

   Internet-Drafts are working documents of the Internet
   Engineering Task Force (IETF), its areas, and its working
   groups. Note that other groups may also distribute
   working documents as Internet-Drafts. Internet-Drafts are
   draft documents valid for a maximum of six months and may
   be updated, replaced, or obsoleted by other documents at
   any time. It is inappropriate to use Internet- Drafts as
   reference material or to cite them other than as "work in
   progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be
   accessed at http://www.ietf.org/shadow.html.

   NOTE: The new TLV type numbers, bit values for flags
   specified in this draft, and new LDP status code values
   are preliminary suggested values and have yet to be
   approved by IANA or the MPLS WG.  See the section "IANA
   Considerations" for further details.

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 proposes enhancements to the
   LDP specification to ease such FT LSR implementations.

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

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draft-ietf-mpls-ldp-ft-06.txt                             September 2002

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. Checkpointing Procedures                                       22
    6.1 Checkpointing with the Keepalive Message                    23
    6.1 Quiesce and Keepalive                                       23
  7. Changes to Existing Messages                                   24
    7.1. LDP Initialization Message                                 24
    7.2. LDP Keepalive Messages                                     24
    7.3. All Other LDP Session Messages                             25
  8. New Fields and Values                                          25
    8.1. Status Codes                                               25
    8.2. FT Session TLV                                             26
    8.3. FT Protection TLV                                          29
    8.4. FT ACK TLV                                                 31
    8.5. FT Cork TLV                                                33
  9. Example Use                                                    34
    9.1. Session Failure and Recovery - FT Procedures               35
    9.2. Use of Check-Pointing With FT Procedures                   37
    9.3. Temporary Shutdown With FT Procedures                      39
    9.4. Temporary Shutdown With FT Procedures and Check-Pointing   41
    9.5. Checkpointing Without FT Procedures                        43
    9.6. Graceful Shutdown With Checkpointing But No FT Procedures  45
  10. Security Considerations                                       46
  11. Implementation Notes                                          48
    11.1. FT Recovery Support on Non-FT LSRs                        48
    11.2. ACK generation logic                                      48
      11.2.1 Ack Generation Logic When Using Check-Pointing         48
    11.3 Interactions With Other Label Distribution Mechanisms      49

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draft-ietf-mpls-ldp-ft-06.txt                             September 2002

  12. Acknowledgments                                               49
  13. Intellectual Property Consideration                           50
  14. Full Copyright Statement                                      50
  15. IANA Considerations                                           50
    15.1. New TLVs                                                  51
    15.2. New Status Codes                                          51
  16. Authors' Addresses                                            52
  17. References                                                    52
    17.1. Normative References                                      52
    17.2. Informative References                                    53


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
   indicated 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 "Checkpointable FT Label" is used to indicate an
   FT label which is secured by using the checkpointing
   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 draft, "Checkpointing" refers
   to a process of messages exchanges that confirm receipt
   and processing (or secure storage) of specific protocol
   messages.

   When talking about the individual bits in the 16-bit FT
   Flag Field, the words "bit" and "flag" is 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 RFC-2119 [RFC2119].








Farrel, et al.                                                  [Page 3]


draft-ietf-mpls-ldp-ft-06.txt                             September 2002

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 16.


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.

   In order to provide HA, an MPLS system needs to be able
   to survive a variety of faults with minimal disruption to
   the Data Plane, including the following fault types:

   -  failure/hot-swap of a physical connection between LSRs

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

   -  failure of the TCP or LDP stack in an LSR

   -  software upgrade to the TCP or LDP stacks in an LSR.






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draft-ietf-mpls-ldp-ft-06.txt                             September 2002

   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 to exchange protocol messages to distribute
   labels and to set up LSPs. A pair of LSRs that have such
   a connection are referred to as LDP peers.

   TCP enables LDP to assume reliable transfer of protocol
   messages. This means that some of the messages do not
   need to be acknowledged (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,
   therefore, the TCP connection may be lost.  Recovery from
   this position is made worse by the fact that LDP control
   messages may have been lost during the connection
   failure.  Since these messages are unconfirmed, it is
   possible that LSP or label state information will be
   lost.

   This draft describes a solution which involves

   - negotiation between LDP peers of the intent to support
     extensions to LDP that facilitate recovery from failover
     without loss of LSPs

   - selection of FT survival on a per LSP/label basis

   - acknowledgement of LDP messages to ensure that a full
     handshake is performed on those messages either frequently
     (such as per message) or less frequently as in check-
     pointing




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draft-ietf-mpls-ldp-ft-06.txt                             September 2002

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

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

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

   Other objectives of this draft are to

   - offer backward-compatibility with LSRs that do not
     implement these extensions to LDP

   - preserve existing protocol rules described in [RFC3036]
     for handling unexpected duplicate messages and for
     processing unexpected messages referring to unknown
     LSPs/labels

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

   Note that this draft 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 is combined
   with other techniques that are outside the scope of this
   draft.

   In particular, this draft 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 draft also does not 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.






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draft-ietf-mpls-ldp-ft-06.txt                             September 2002

4. Overview of LDP FT Enhancements

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

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

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

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

   - An FT Protection TLV used to identify operations that
     affect LDP labels.  All LDP messages carrying the FT
     Protection TLV need to be secured (e.g. to NVRAM) and ACKed
     to the sending LDP peer in order 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 draft.  An implementation could choose to
     secure entire messages relating to Sequence Numbered FT
     Labels, or it could secure only the relevant state
     information.

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

   - 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 checkpoint
     all (that is both FT and non-FT) operations for future
     recovery.










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draft-ietf-mpls-ldp-ft-06.txt                             September 2002

4.1.  Establishing an FT LDP Session

   In order that the extensions to LDP [RFC3036] described
   in this draft 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 checkpointing
   functions described in this draft.  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 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 draft assumes that the LDP sessions
   under discussion are between LSRs that do support the LDP
   FT enhancements, except where explicitly stated
   otherwise.










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draft-ietf-mpls-ldp-ft-06.txt                             September 2002

4.2.  TCP Connection Failure

4.2.1 Detecting TCP Connection Failures

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

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

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

   - Sockets keepalive timeout.

   - Sockets send failure.

   - New (incoming) Socket opened.

   - LDP protocol timeout.

4.2.2 LDP Processing after Connection Failure

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

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

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

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







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draft-ietf-mpls-ldp-ft-06.txt                             September 2002

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 draft SHOULD NOT be used.


4.4.  FT LDP Session Reconnection

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

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

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

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

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





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draft-ietf-mpls-ldp-ft-06.txt                             September 2002

   If both LDP peers set the FT Reconnect Flag to 1, both
   LDP peers MUST use the procedures indicated in this draft
   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 the section
   "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 checkpointing 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 Checkpointable FT Labels.

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

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draft-ietf-mpls-ldp-ft-06.txt                             September 2002

   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 draft 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 the
   section "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 terminating also requires
   to know 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 the
   section "FT Operations".











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draft-ietf-mpls-ldp-ft-06.txt                             September 2002

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
   draft.

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



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

   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 on an LSP that does have 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 the section "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 in order that the state of the label can
   be recovered after a failure of the LDP session.

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

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draft-ietf-mpls-ldp-ft-06.txt                             September 2002

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 that 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 on
   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 RFC3036, 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, et al.                                                 [Page 15]


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

   For full preservation of state:

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

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

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

   However, the following options also exist:

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

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

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










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draft-ietf-mpls-ldp-ft-06.txt                             September 2002

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 it participates in.  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
   and securing of all these messages.  There is no protocol
   reason why the number of ACKs accumulated or the time for
   which an ACK is deferred should not be allowed to become
   relatively large.

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

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


5.3.  Preservation of FT State

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



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

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

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

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

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

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

   - Either LDP peer MAY elect to release state information
     based on its internal knowledge of the loss of integrity of
     the state information or an inability to pend (or queue) LDP
     operations (as described in section 4.4.1) 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.

   - When an LSR receives a Status TLV with the E-bit set in






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

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

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

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

   Note that a Label Release and the acknowledgement to a
   Label Withdraw may be received by a downstream LSR in any
   order.  The downstream LSR MAY release its resources on
   receipt of the first message and MUST release its
   resources on 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 take account of the failover
   processing necessary on the peer LSR, the nature of the
   network between the LDP peers, and the FT Reconnection
   Timeout chosen on the previous instantiation of the TCP
   connection (if any).





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

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

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



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

   It is RECOMMENDED that Label Request operations for new
   FT Labels are not 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 4.4.1).

   If, for any reason, an LSR has been unable to pend
   operations with respect to an LDP peer, as described in
   section 4.4.1, 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 procedure
   described below.








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5.5.1 Re-Issuing FT Messages

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

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

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

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

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

   Any FT Label operations that were pended (see section
   4.4.1) during the TCP connection failure MUST also be
   issued on 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 are 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. Checkpointing Procedures

   Checkpointing 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 checkpointing is an integral part of the FT
   procedures.  Setting the S and the C bit will achieve the
   same function as setting just the S bit.




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

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

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

   The set of all messages that are checkpointed in this way
   is called the Checkpointable Messages.

6.1 Checkpointing 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 Checkpointable Messages on
   the session.

   As soon as the LSR has completed securing the
   Checkpointable 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 be
   immediately on 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 8.

   An example message flow showing the use of checkpointing
   without the FT procedures is shown in section 8.

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.

   It is RECOMMENDED that on 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 any
   messages generated while processing and securing any
   previously unacknowledged messages received from the peer


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draft-ietf-mpls-ldp-ft-06.txt                             September 2002

   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 achieves three-way handshake of state
   synchronization between two LDP peers is given in section 8.


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 "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.


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 "New
   Fields and Values".





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   FT Protection TLV
        If present, specifies 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 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 the section "New
   Fields and Values".

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

   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.



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   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   0x000000xx
      Zero FT seqnum              1   0x000000xx
      Unexpected TLV /            1   0x000000xx
         Session Not FT
      Unexpected TLV /            1   0x000000xx
         Label Not FT
      Missing FT Protection TLV   1   0x000000xx
      FT ACK sequence error       1   0x000000xx
      Temporary Shutdown          0   0x000000xx

      FT Seq Numbers Exhausted    1   0x000000xx
      FT Session parameters /     1   0x000000xx
         changed
      Unexpected FT Cork TLV      1   0x000000xx

   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 to return to service before the FT
   Reconnection Timer expires.


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)                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+






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draft-ietf-mpls-ldp-ft-06.txt                             September 2002

   FT Flags
        FT Flags: A 16 bit field that indicates
        various attributes the FT support on this
        LDP session.  This fields 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 set to 0 otherwise. See the
            section "FT LDP Session Reconnection"
            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 chekpointing (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 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.









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        C: Checkpointing Flag.
            Set to 1 to indicate that the
            checkpointing procedures in this draft
            are in use.

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

            If the S bit is set to 0 (zero) then
            the C bit indicates that checkpointing
            applies to all labels - all labels are
            Checkpointable FT Labels.

        L: Learn From Network Flag.
            Set to 1 if the Fault Recovery
            procedures of [LDP-RESTART] 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 on 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 [LDP-RESTART]
        0   1   0   1    Invalid
        0   x   1   0    Checkpointing 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.

   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 currently failed.  The timeout is
        encoded as a 32-bit unsigned integer number
        of milliseconds.


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        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 [LDP-RESTART].

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


8.3.  FT Protection TLV

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

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

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

   - Label Request Messages in downstream on-demand
     distribution mode

   - Label Mapping messages in downstream unsolicited mode

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

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

   - on the Label Request message in DoD mode

   - on the Label Mapping message in DU mode

   - on all subsequent messages concerning this label.





Farrel, et al.                                                 [Page 29]


draft-ietf-mpls-ldp-ft-06.txt                             September 2002

   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 a 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 TLVshould
   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 which it affects.

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

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

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

   - on Keepalive messages.

   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.

Farrel, et al.                                                 [Page 30]


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

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

        The special use of 0x00000000 is discussed
        in the section "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.


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 is included on all FT Keepalive messages in order
   to ensure that the LDP peers do not build up a large
   backlog of unacknowledged state information.





Farrel, et al.                                                 [Page 31]


draft-ietf-mpls-ldp-ft-06.txt                             September 2002

   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 would apply to LDP sessions to new LDP
        peers or after an LSR determines that it
        must drop all state for a failed TCP
        connection.

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

   If an LSR receives a message affecting a label that it
   believes is a Sequence Numbered FT Label and that message
   does not contain the FT Protection TLV, it SHOULD send a
   Notification message to its LDP peer containing the
   "Missing FT Protection TLV" status code.

   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.





Farrel, et al.                                                 [Page 32]


draft-ietf-mpls-ldp-ft-06.txt                             September 2002

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)             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


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

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

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

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

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

   On receipt of a Keepalive message with the FT Cork TLV
   and an FT ACK TLV that acknowledges the previously sent
   Keepalive that carried the FT Cork TLV, an LSR knows that
   quiesce is complete.  If the received Keepalive also
   carries the FT Protection TLV, the LSR must respond with
   a further Keepalive to complete the 3-way handshake.  It
   SHOULD now send a "Temporary Shutdown" Notification
   message, disconnect the TCP session and perform whatever
   control plane actions required this session shutdown.

   An example such 3-way handshake for controlled shutdown
   is given in section 8.

   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 FR Cork TLV" status code.

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draft-ietf-mpls-ldp-ft-06.txt                             September 2002

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 shown 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.





































Farrel, et al.                                                 [Page 34]


draft-ietf-mpls-ldp-ft-06.txt                             September 2002

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









Farrel, et al.                                                 [Page 35]


draft-ietf-mpls-ldp-ft-06.txt                             September 2002

   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 third 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.

   (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, et al.                                                 [Page 36]


draft-ietf-mpls-ldp-ft-06.txt                             September 2002

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
























Farrel, et al.                                                 [Page 37]


draft-ietf-mpls-ldp-ft-06.txt                             September 2002

   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 a 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.

   (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) P3 wishes to synchronize state with P2.  It sends a Keepalive
        message containing a 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.































Farrel, et al.                                                 [Page 38]


draft-ietf-mpls-ldp-ft-06.txt                             September 2002

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








Farrel, et al.                                                 [Page 39]


draft-ietf-mpls-ldp-ft-06.txt                             September 2002

   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, et al.                                                 [Page 40]


draft-ietf-mpls-ldp-ft-06.txt                             September 2002

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, et al.                                                 [Page 41]


draft-ietf-mpls-ldp-ft-06.txt                             September 2002

   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 a 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 including the FT Protection and
   FT Cork TLVs to indicate that there are still messages
   outstanding to be acknowledged.

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

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

































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9.5.  Checkpointing Without FT Procedures

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













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   Notes:
   ======

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

   (4)  A checkpoint request from P1.  It carries the sequence number of
        the checkpoint 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 checkpoints.

   (9)  P1 recommences the unacknowledged label distribution request.

   (10) P2 recommences an unacknowledged label distribution request.






































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draft-ietf-mpls-ldp-ft-06.txt                             September 2002

9.6.  Graceful Shutdown With Checkpointing But No FT Procedures


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













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draft-ietf-mpls-ldp-ft-06.txt                             September 2002

   Notes:
   ======

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

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

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

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

   (7)  P1 completes the three-way handshake.  All operations have now
        been checkpointed 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 checkpoints.

   (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 draft vulnerable to additional denial-of-service
   attacks as follows:

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

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

   - 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].



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   Alternative authentication schemes for LDP peers are
   outside the scope of this draft, 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 draft, 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 draft requires that
   labels are not 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 draft.

   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 contrued as an
   implementation issue (see below) but failure to acknowledge it could
   result in mis-forwarding of data between LSPs established using
   some other mechanism and those recovered using the methods
   described in this document.













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11.   Implementation Notes

11.1. FT Recovery Support on Non-FT LSRs

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

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


11.2. ACK generation logic

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

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

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

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

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

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

11.2.1 Ack Generation Logic When Using Check-Pointing

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

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




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draft-ietf-mpls-ldp-ft-06.txt                             September 2002

   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 traffic engineering
   label distribution protocol that are used to construct tunnels
   through which LDP LSPs are established.

   As with re-use of individual labels by LDP within a restarting LDP
   system, care must be taken to prevent labels that need to be retained
   by a restarting LDP session or protocol component from being used by
   another label distribution mechanism 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 draft is based on the LDP ideas
   expressed by the authors of [RFC3036].

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

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







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13.   Intellectual Property Consideration

   The IETF 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.   Full Copyright Statement

   Copyright (c) The Internet Society (2000, 2001, 2002).
   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.


15.   IANA Considerations

   This draft requires the use of a number of new TLVs and
   status codes from the number spaces within the LDP
   protocol.  This section explains the logic used by the
   authors to choose the most appropriate number space for
   each new entity, and is intended to assist in the
   determination of any final values assigned by IANA or the
   MPLS WG in the event that the MPLS WG chooses to advance
   this draft on the standards track.

   This section will be removed when the TLV and status code
   values have been agreed with IANA.





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draft-ietf-mpls-ldp-ft-06.txt                             September 2002

15.1. New TLVs

   The FT Protection TLV carries attributes that affect a
   single label exchanged between LDP peers.  It is taken
   from the 0x02xx range for TLVs that is used in [RFC3036]
   by other TLVs carrying label attributes.  The next
   available value in this range is 0x0203.

   The FT Session TLV carries attributes that affect the
   entire LDP session between LDP peers.  It is taken from
   the 0x05xx range for TLVs that is used in [RFC3036] by
   other TLVs carrying session-wide attributes.  The next
   available value in this range is 0x0503.

   The FT Protection TLV may ACK many label operations at
   once if cumulative ACKS are used.  It is taken from the
   0x05xx range for TLVs that is used in [RFC3036] by other
   TLVs carrying session-wide attributes.  The next
   available value in this range is 0x0504.

   The FT Cork TLV carries attributes that apply to all labels
   exchanged between LDP peers.  It is taken from the 0x05xx range
   for TLVs that is used in [RFC3036] by other TLVs carrying label
   attributes.  The next available value in this range is 0x0505.

   In summary:

   FT Protection TLV  0x0203
   FT Session TLV     0x0503
   FT Ack TLV         0x0504
   FT Cork TLV        0x0505


15.2. New Status Codes

   LDP status codes are not sub-divided into specific ranges
   for different types of error.  Hence, the numeric status
   code values are selected as the next available.

   Section 7.1 lists the new status codes required by this document and
   gives interpretative information.  The new codes are as follows.

      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

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draft-ietf-mpls-ldp-ft-06.txt                             September 2002

16.   Authors' Addresses

   Adrian Farrel (editor)                 Paul Brittain
   Movaz Networks, Inc.                   Data Connection Ltd.
   7926 Jones Branch Drive, Suite 615     Windsor House, Pepper Street,
   McLean, VA 22102                       Chester, Cheshire
   Phone:  +1 703-847-1867                CH1 1DF, UK
   Email:  afarrel@movaz.com              Phone: +44-(0)20-8366-1177
                                          Email: pjb@dataconnection.com

   Philip Matthews                        Eric Gray
   Hyperchip                              Celox Networks, Inc.
   1800 Rene-Levesque Blvd W              2 Park Central Drive,
   Montreal, Quebec H3H 2H2               Southborough, MA 01772
   Canada                                 Phone: +1 508 305 7214
   Phone:  +1 514-906-4965                Email: egray@celoxnetworks.com
   Email: pmatthews@hyperchip.com

   Jack Shaio                             Toby Smith
   Vivace Networks                        Laurel Networks, Inc.
   2730 Orchard Parkway                   1300 Omega Drive
   San Jose, CA 95134                     Pittsburgh, PA 15205
   Phone: +1 408 432 7623                 Email: tob@laurelnetworks.com
   Email: jack.shaio@vivacenetworks.com

   Andrew G. Malis
   Vivace Networks
   2730 Orchard Parkway
   San Jose, CA 95134
   Phone: +1 408 383 7223
   andy.malis@vivacenetworks.com


17.   References

17.1. 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., et. al., LDP Specification, RFC 3036,
                  January 2001.

   [LDP-RESTART]  Leelanivas, M., et al., Graceful Restart Mechanism for
                  LDP, draft-ietf-ldp-restart-05.txt, September 2002,
                  work in progress.









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17.2. Informative References

   [RFC2205]      Braden, R., et al., Resource ReSerVation Protocol
                  (RSVP) -- Version 1, Functional Specification, RFC
                  2205, September 1997.

   [RFC2961]      Berger, L., et al., RSVP Refresh Reduction Extensions,
                  RFC 2961, April 2001.

   [RFC3209]      Awduche, D., et al,. Extensions to RSVP for LSP
                  Tunnels, RFC 3209, December 2001.

   [RFC3212]      Jamoussi, B., et. al., Constraint-Based LSP Setup
                  using LDP, RFC 3212, January 2002.

   [RFC3214]      Ash, G., et al., LSP Modification Using CR-LDP, RFC
                  3214, January 2001.

   [BGP-RESTART]  Sangli, S., et al., Graceful Restart Mechanism
                  for BGP, draft-ietf-idr-restart-05.txt, June 2002
                  (work in progress).





































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