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

Fault Tolerance for the Label Distribution Protocol (LDP)

Status of this Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (2003). All Rights Reserved.

IESG Note

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

Abstract

Multiprotocol Label Switching (MPLS) systems will be used in core
networks where system downtime must be kept to an absolute minimum.
Many MPLS Label Switching Routers (LSRs) may, therefore, eXPloit
Fault Tolerant (FT) hardware or software to provide high availability
of the core networks.

The details of how FT is achieved for the various components of an FT
LSR, including Label Distribution Protocol (LDP), the switching
hardware and TCP, are implementation specific. This document
identifies issues in the LDP specification in RFC3036, "LDP
Specification", that make it difficult to implement an FT LSR using
the current LDP protocols, and defines enhancements to the LDP
specification to ease such FT LSR implementations.

The issues and extensions described here are equally applicable to
RFC3212, "Constraint-Based LSP Setup Using LDP" (CR-LDP).

Table of Contents

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

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

1. Conventions and Terminology used in this document

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

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

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

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

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

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

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

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

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in BCP 14, RFC2119
[RFC2119].

2. Contributing Authors

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

3. Introduction

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

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

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

3.1. Fault Tolerance for MPLS

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

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

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

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

- failure of the TCP or LDP stack in an LSR.

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

The first two examples of faults listed above are confined to the
Data Plane. Such faults can be handled by providing redundancy in
the Data Plane which is transparent to LDP operating in the Control
Plane. The last two example types of fault require action in the
Control Plane to recover from the fault without disrupting traffic in
the Data Plane. This is possible because many recent router
architectures separate the Control and Data Planes such that
forwarding can continue unaffected by recovery action in the Control
Plane.

3.2. Issues with LDP

LDP uses TCP to provide reliable connections between LSRs over which
they exchange protocol messages to distribute labels and set up LSPs.
A pair of LSRs that have such a connection are referred to as LDP
peers.

TCP enables LDP to assume reliable transfer of protocol messages.
This means that some of the messages do not need to be acknowledged
(for example, Label Release).

LDP is defined such that if the TCP connection fails, the LSR should
immediately tear down the LSPs associated with the session between
the LDP peers, and release any labels and resources assigned to those
LSPs.

It is notoriously hard to provide a Fault Tolerant implementation of
TCP. To do so might involve making copies of all data sent and
received. This is an issue familiar to implementers of other TCP
applications such as BGP.

During failover affecting the TCP or LDP stacks, the TCP connection
may be lost. Recovery from this position is made worse by the fact
that LDP control messages may have been lost during the connection
failure. Since these messages are unconfirmed, it is possible that
LSP or label state information will be lost.

This document describes a solution which involves:

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

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

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

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

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

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

Other objectives of this document are to:

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

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

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

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

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

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

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

4. Overview of LDP FT Enhancements

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

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

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

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

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

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

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

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

4.1. Establishing an FT LDP Session

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

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

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

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

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

4.1.1 Interoperation with Non-FT LSRs

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

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

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.

4.3. Data Forwarding During TCP Connection Failure

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

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

4.4. FT LDP Session Reconnection

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

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

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

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

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

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

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

4.5. Operations on FT Labels

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

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

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

4.6. Check-Pointing

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

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

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

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

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

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

4.6.1 Graceful Termination

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

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

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

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

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

4.7. Label Space Depletion and Replenishment

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

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

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

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

4.8. Tunneled LSPs

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

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

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

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

5. FT Operations

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

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

5.1. FT LDP Messages

5.1.1 Sequence Numbered FT Label Messages

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

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

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

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

5.1.1.1 Scope of FT Labels

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

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

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

5.1.2 FT Address Messages

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

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

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

5.1.3 Label Resources Available Notifications

In LDP, it is possible that a downstream LSR may not have labels
available to respond to a Label Request. In this case, as specified
in 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.

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

For full preservation of state:

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

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

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

However, the following options also exist:

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

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

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

5.2. FT Operation ACKs

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

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

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

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

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

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

5.3. Preservation of FT State

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

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

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

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

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

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

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

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

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

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

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

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

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

5.4. FT Procedure After TCP Failure

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

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

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

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

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

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

5.4.1 FT LDP Operations During TCP Failure

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

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

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

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

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

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

5.5. FT Procedure After TCP Re-connection

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

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

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

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

Label operations are completed using the following procedure.

5.5.1 Re-Issuing FT Messages

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

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

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

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

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

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

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

6. Check-Pointing Procedures

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

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

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

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

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

6.1 Check-Pointing with the Keepalive Message

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

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

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

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

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

6.2 Quiesce and Keepalive

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

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

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

7. Changes to Existing Messages

7.1. LDP Initialization Message

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

Optional Parameter Length Value

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

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

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

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

7.2. LDP Keepalive Messages

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

Optional Parameter Length Value

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

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

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

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

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

7.3. All Other LDP Session Messages

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

Optional Parameter Length Value

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

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

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

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

8. New Fields and Values

8.1. Status Codes

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

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

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

Status Code E Status Data

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

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

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

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
10 FT Session TLV (0x0503) Length (= 12)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
FT Flags Reserved
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
FT Reconnect Timeout (in milliseconds)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Recovery Time (in milliseconds)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
R Reserved SACL
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

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

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

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

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

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

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

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

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

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

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

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

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

The following table summarizes the settings of these bits.

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

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

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

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

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

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

8.3. FT Protection TLV

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

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

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

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

- Label Mapping messages in downstream unsolicited mode.

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

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

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

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

- on all subsequent messages concerning this label.

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

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

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

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

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

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

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

- on Keepalive messages.

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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
00 FT Protection (0x0203) Length (= 4)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
FT Sequence Number
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

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

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

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

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

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

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

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

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

8.4. FT ACK TLV

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

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

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

The FT ACK TLV is encoded as follows.

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

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

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

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

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

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

8.5. FT Cork TLV

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

The FT Cork TLV is encoded as follows.

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

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

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

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

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

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

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

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

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

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

9. Example Use

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

The parameters shown on each message below are as follows:

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

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

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

9.1. Session Failure and Recovery - FT Procedures

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

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

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

Notes:
======

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

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

(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.

9.2. Use of Check-Pointing With FT Procedures

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

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

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

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

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

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

9.3. Temporary Shutdown With FT Procedures

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

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

Notes:
======

Notes are as in the previous example except as follows.

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

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

Notes:
======

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

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

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

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

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

9.5. Check-Pointing Without FT Procedures

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

Notes:
======

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

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

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

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

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

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

(9) P1 recommences the unacknowledged label distribution request.

(10) P2 recommences an unacknowledged label distribution request.

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

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

Notes:
======

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

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

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

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

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

(8) The session is gracefully shut down.

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

(10) P1 starts a new label distribution request.

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

10. Security Considerations

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

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

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

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

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

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

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

As with LDP, a security issue may exist if an LDP implementation
continues to use labels after expiration of the session that first
caused them to be used. This may arise if the upstream LSR detects
the session failure after the downstream LSR has released and re-used
the label. The problem is most obvious with the platform-wide label
space and could result in mis-forwarding of data to other than
intended destinations and it is conceivable that these behaviors may
be deliberately exploited to either obtain services without
authorization or to deny services to others.

In this document, the validity of the session may be extended by the
FT Reconnection Timeout, and the session may be re-established in
this period. After the expiry of the Reconnection Timeout, the
session must be considered to have failed and the same security issue
applies as described above.

However, the downstream LSR may declare the session as failed before
the expiration of its Reconnection Timeout. This increases the
period during which the downstream LSR might reallocate the label
while the upstream LSR continues to transmit data using the old usage
of the label. To reduce this issue, this document requires that
labels not be re-used until the Reconnection Timeout has expired.

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

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

11. Implementation Notes

11.1. FT Recovery Support on Non-FT LSRs

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

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

11.2. ACK generation logic

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

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

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

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

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

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

11.2.1 Ack Generation Logic When Using Check-Pointing

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

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

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

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

11.3 Interactions With Other Label Distribution Mechanisms

Many LDP LSRs also run other label distribution mechanisms. These
include management interfaces for configuration of static label
mappings, other distinct instances of LDP, and other label
distribution protocols. The last example includes the traffic
engineering label distribution protocol that is used to construct
tunnels through which LDP LSPs are established.

As with re-use of individual labels by LDP within a restarting LDP
system, care must be taken to prevent labels that need to be retained
by a restarting LDP session or protocol component from being used by
another label distribution mechanism since that might compromise data
security amongst other things.

It is a matter for implementations to avoid this issue through the
use of techniques such as a common label management component or
segmented label spaces.

12. Acknowledgments

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

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

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

13. Intellectual Property Consideration

The IETF takes no position regarding the validity or scope of any
intellectual property or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; neither does it represent that it
has made any effort to identify any such rights. Information on the
IETF's procedures with respect to rights in standards-track and
standards-related documentation can be found in BCP-11. Copies of
claims of rights made available for publication and any assurances of
licenses to be made available, or the result of an attempt made to
obtain a general license or permission for the use of such
proprietary rights by implementors or users of this specification can
be obtained from the IETF Secretariat.

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

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

14. References

14.1. Normative References

[RFC2026] Bradner, S., "The Internet Standards Process --
Revision 3", BCP 9, RFC2026, October 1996.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.

[RFC3036] Andersson, L., Doolan, P., Feldman, N., Fredette, A.
and B. Thomas, "LDP Specification, RFC3036, January
2001.

[RFC3478] Leelanivas, M., Rekhter, Y. and R. Aggrawal, "Graceful
Restart Mechanism for Label Distribution Protocol",
RFC3478, February 2003.

14.2. Informative References

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

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

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

[RFC3212] Jamoussi, B., Andersson, L., Callon, R., Dantu, R.,
Wu, L., Doolan, P., Worster, T., Feldman, N.,
Fredette, A., Girish, M., Gray, E., Heinanen, J.,
Kilty, T. and A. Malis, "Constraint-Based LSP Setup
using LDP", RFC3212, January 2002.

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

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

15. Authors' Addresses

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

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

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

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

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

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

Eric Gray

EMail: ewgray@GraIyMage.com

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

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

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

EMail: tob@laurelnetworks.com

Andrew G. Malis
Vivace Networks
2730 Orchard Parkway
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16. Full Copyright Statement

Copyright (C) The Internet Society (2003). All Rights Reserved.

This document and translations of it may be copied and furnished to
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and distributed, in whole or in part, without restriction of any
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included on all such copies and derivative works. However, this
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the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
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The limited permissions granted above are perpetual and will not be
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This document and the information contained herein is provided on an
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Acknowledgement

Funding for the RFCEditor function is currently provided by the
Internet Society.