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Versions: (draft-farrel-mpls-ldp-ft) 00 01 02
03 04 05 06 RFC 3479
MPLS Working Group Editor
Internet Draft Adrian Farrel
Document: draft-ietf-mpls-ldp-ft-06.txt Movaz Networks, Inc.
Expiration Date: March 2003 September 2002
Fault Tolerance for the Label Distribution Protocol (LDP)
draft-ietf-mpls-ldp-ft-06.txt
Status of this Memo
This document is an Internet-Draft and is in full
conformance with all provisions of Section 10 of RFC2026
[RFC2026].
Internet-Drafts are working documents of the Internet
Engineering Task Force (IETF), its areas, and its working
groups. Note that other groups may also distribute
working documents as Internet-Drafts. Internet-Drafts are
draft documents valid for a maximum of six months and may
be updated, replaced, or obsoleted by other documents at
any time. It is inappropriate to use Internet- Drafts as
reference material or to cite them other than as "work in
progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be
accessed at http://www.ietf.org/shadow.html.
NOTE: The new TLV type numbers, bit values for flags
specified in this draft, and new LDP status code values
are preliminary suggested values and have yet to be
approved by IANA or the MPLS WG. See the section "IANA
Considerations" for further details.
Abstract
Multiprotocol Label Switching (MPLS) systems will be used
in core networks where system downtime must be kept to an
absolute minimum. Many MPLS Label Switching Routers
(LSRs) may, therefore, exploit Fault Tolerant (FT)
hardware or software to provide high availability of the
core networks.
The details of how FT is achieved for the various
components of an FT LSR, including Label Distribution
Protocol (LDP), the switching hardware and TCP, are
implementation specific. This document identifies issues
in the LDP specification in RFC 3036 "LDP Specification"
that make it difficult to implement an FT LSR using the
current LDP protocols, and proposes enhancements to the
LDP specification to ease such FT LSR implementations.
The issues and extensions described here are equally
applicable to RFC 3212, "Constraint-Based LSP Setup Using
LDP" (CR-LDP).
Farrel, et al. [Page 1]
draft-ietf-mpls-ldp-ft-06.txt September 2002
Contents
1. Conventions and Terminology used in this document 3
2. Contributing Authors 4
3. Introduction 4
3.1. Fault Tolerance for MPLS 4
3.2. Issues with LDP 5
4. Overview of LDP FT Enhancements 7
4.1. Establishing an FT LDP Session 8
4.1.1 Interoperation with Non-FT LSRs 8
4.2. TCP Connection Failure 9
4.2.1 Detecting TCP Connection Failures 9
4.2.2 LDP Processing after Connection Failure 9
4.3. Data Forwarding During TCP Connection Failure 10
4.4. FT LDP Session Reconnection 10
4.5. Operations on FT Labels 11
4.6. Check-Pointing 11
4.6.1 Graceful Termination 12
4.7. Label Space Depletion and Replenishment 13
4.8. Tunneled LSPs 13
5. FT Operations 14
5.1. FT LDP Messages 14
5.1.1 Sequence Numbered FT Label Messages 14
5.1.2 FT Address Messages 15
5.1.3 Label Resources Available Notifications 15
5.2. FT Operation ACKs 17
5.3. Preservation of FT State 17
5.4. FT Procedure After TCP Failure 19
5.4.1 FT LDP Operations During TCP Failure 20
5.5. FT Procedure After TCP Re-connection 21
5.5.1 Re-Issuing FT Messages 22
6. Checkpointing Procedures 22
6.1 Checkpointing with the Keepalive Message 23
6.1 Quiesce and Keepalive 23
7. Changes to Existing Messages 24
7.1. LDP Initialization Message 24
7.2. LDP Keepalive Messages 24
7.3. All Other LDP Session Messages 25
8. New Fields and Values 25
8.1. Status Codes 25
8.2. FT Session TLV 26
8.3. FT Protection TLV 29
8.4. FT ACK TLV 31
8.5. FT Cork TLV 33
9. Example Use 34
9.1. Session Failure and Recovery - FT Procedures 35
9.2. Use of Check-Pointing With FT Procedures 37
9.3. Temporary Shutdown With FT Procedures 39
9.4. Temporary Shutdown With FT Procedures and Check-Pointing 41
9.5. Checkpointing Without FT Procedures 43
9.6. Graceful Shutdown With Checkpointing But No FT Procedures 45
10. Security Considerations 46
11. Implementation Notes 48
11.1. FT Recovery Support on Non-FT LSRs 48
11.2. ACK generation logic 48
11.2.1 Ack Generation Logic When Using Check-Pointing 48
11.3 Interactions With Other Label Distribution Mechanisms 49
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12. Acknowledgments 49
13. Intellectual Property Consideration 50
14. Full Copyright Statement 50
15. IANA Considerations 50
15.1. New TLVs 51
15.2. New Status Codes 51
16. Authors' Addresses 52
17. References 52
17.1. Normative References 52
17.2. Informative References 53
1. Conventions and Terminology used in this document
Definitions of key words and terms applicable to LDP and
CR-LDP are inherited from [RFC3212] and [RFC3036].
The term "FT Label" is introduced in this document to
indicated a label for which some fault tolerant operation
is used. A "non-FT Label" is not fault tolerant and is
handled as specified in [RFC3036].
The term "Sequence Numbered FT Label" is used to indicate
an FT label which is secured using the sequence number in
the FT Protection TLV described in this document.
The term "Checkpointable FT Label" is used to indicate an
FT label which is secured by using the checkpointing
techniques described in this document.
The extensions to LDP specified in this document are
collectively referred to as the "LDP FT enhancements".
Within the context of this draft, "Checkpointing" refers
to a process of messages exchanges that confirm receipt
and processing (or secure storage) of specific protocol
messages.
When talking about the individual bits in the 16-bit FT
Flag Field, the words "bit" and "flag" is used
interchangeably.
In the examples quoted, the following notation is used:
Ln : An LSP. For example L1.
Pn : An LDP peer. For example P1.
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL",
"SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED",
"MAY", and "OPTIONAL" in this document are to be
interpreted as described in RFC-2119 [RFC2119].
Farrel, et al. [Page 3]
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2. Contributing Authors
This document was the collective work of several
individuals over a period of several years. The text and
content of this document was contributed by the editor
and the co-authors listed in section 16.
3. Introduction
High Availability (HA) is typically claimed by equipment
vendors when their hardware achieves availability levels
of at least 99.999% (five 9s). To implement this, the
equipment must be capable of recovering from local
hardware and software failures through a process known as
fault tolerance (FT).
The usual approach to FT involves provisioning backup
copies of hardware and/or software. When a primary copy
fails, processing is switched to the backup copy. This
process, called failover, should result in minimal
disruption to the Data Plane.
In an FT system, backup resources are sometimes
provisioned on a one-to-one basis (1:1), sometimes as one-
to-many (1:n), and occasionally as many-to-many (m:n).
Whatever backup provisioning is made, the system must
switch to the backup automatically on failure of the
primary, and the software and hardware state in the
backup must be set to replicate the state in the primary
at the point of failure.
3.1. Fault Tolerance for MPLS
MPLS is a technology that will be used in core networks
where system downtime must be kept to an absolute
minimum. Many MPLS LSRs may, therefore, exploit FT
hardware or software to provide high availability of core
networks.
In order to provide HA, an MPLS system needs to be able
to survive a variety of faults with minimal disruption to
the Data Plane, including the following fault types:
- failure/hot-swap of a physical connection between LSRs
- failure/hot-swap of the switching fabric in an LSR
- failure of the TCP or LDP stack in an LSR
- software upgrade to the TCP or LDP stacks in an LSR.
Farrel, et al. [Page 4]
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The first two examples of faults listed above are
confined to the Data Plane. Such faults can be handled
by providing redundancy in the Data Plane which is
transparent to LDP operating in the Control Plane. The
last two example types of fault require action in the
Control Plane to recover from the fault without
disrupting traffic in the Data Plane. This is possible
because many recent router architectures separate the
Control and Data Planes such that forwarding can continue
unaffected by recovery action in the Control Plane.
3.2. Issues with LDP
LDP uses TCP to provide reliable connections between LSRs
over which to exchange protocol messages to distribute
labels and to set up LSPs. A pair of LSRs that have such
a connection are referred to as LDP peers.
TCP enables LDP to assume reliable transfer of protocol
messages. This means that some of the messages do not
need to be acknowledged (for example, Label Release).
LDP is defined such that if the TCP connection fails, the
LSR should immediately tear down the LSPs associated with
the session between the LDP peers, and release any labels
and resources assigned to those LSPs.
It is notoriously hard to provide a Fault Tolerant
implementation of TCP. To do so might involve making
copies of all data sent and received. This is an issue
familiar to implementers of other TCP applications such
as BGP.
During failover affecting the TCP or LDP stacks,
therefore, the TCP connection may be lost. Recovery from
this position is made worse by the fact that LDP control
messages may have been lost during the connection
failure. Since these messages are unconfirmed, it is
possible that LSP or label state information will be
lost.
This draft describes a solution which involves
- negotiation between LDP peers of the intent to support
extensions to LDP that facilitate recovery from failover
without loss of LSPs
- selection of FT survival on a per LSP/label basis
- acknowledgement of LDP messages to ensure that a full
handshake is performed on those messages either frequently
(such as per message) or less frequently as in check-
pointing
Farrel, et al. [Page 5]
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- solicitation of up-to-date acknowledgement
(checkpointing) of previous LDP messages to ensure the
current state is flushed to disk/NVRAM, with an additional
option that allows an LDP partner to request that state is
flushed in both directions if graceful shutdown is required.
- re-issuing lost messages after failover to ensure that
LSP/label state is correctly recovered after reconnection of
the LDP session.
The issues and objectives described above are equally
applicable to CR-LDP.
Other objectives of this draft are to
- offer backward-compatibility with LSRs that do not
implement these extensions to LDP
- preserve existing protocol rules described in [RFC3036]
for handling unexpected duplicate messages and for
processing unexpected messages referring to unknown
LSPs/labels
- avoid full state refresh solutions (such as those present
in RSVP: see [RFC2205], [RFC2961], [RFC3209] and [LDP-
RESTART]) whether they be continual, or limited to post-
failover recovery.
Note that this draft concentrates on the preservation of
label state for labels exchanged between a pair of
adjacent LSRs when the TCP connection between those LSRs
is lost. This is a requirement for Fault Tolerant
operation of LSPs, but a full implementation of end-to-
end protection for LSPs requires that this is combined
with other techniques that are outside the scope of this
draft.
In particular, this draft does not attempt to describe
how to modify the routing of an LSP or the resources
allocated to a label or LSP, which is covered by
[RFC3214]. This draft also does not address how to
provide automatic layer 2 or layer 3 protection switching
for a label or LSP, which is a separate area for study.
This specification does not preclude an implementation
from attempting (or require it to attempt) to use the FT
behavior described here to recover from a preemptive
failure of a connection on a non-FT system due to, for
example, a partial system crash. Note, however, that
there are potential issues too numerous to list here -
not least the likelihood that the same crash will
immediately occur when processing the restored data.
Farrel, et al. [Page 6]
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4. Overview of LDP FT Enhancements
The LDP FT enhancements consist of the following main
elements, which are described in more detail in the
sections that follow.
- The presence of an FT Session TLV on the LDP
Initialization message indicates that an LSR supports some
form of protection or recovery from session failure. A flag
bit within this TLV (the S bit) indicates that the LSR
supports the LDP FT enhancements on this session. Another
flag (the C bit) indicates that the checkpointing procedures
are to be used.
- An FT Reconnect Flag in the FT Session TLV (the R bit)
indicates whether an LSR has preserved FT Label state across
a failure of the TCP connection.
- An FT Reconnection Timeout, exchanged on the LDP
Initialization message, that indicates the maximum time peer
LSRs will preserve FT Label state after a failure of the TCP
connection.
- An FT Protection TLV used to identify operations that
affect LDP labels. All LDP messages carrying the FT
Protection TLV need to be secured (e.g. to NVRAM) and ACKed
to the sending LDP peer in order that the state for Sequence
Numbered FT Labels can be correctly recovered after LDP
session reconnection.
Note that the implementation within an FT system is left
open by this draft. An implementation could choose to
secure entire messages relating to Sequence Numbered FT
Labels, or it could secure only the relevant state
information.
- Address advertisement may also be secured by use of the
FT Protection TLV. This enables recovery after LDP session
reconnection without the need to re-advertise what may be a
very large number of addresses.
- The FT Protection TLV may also be used on the Keepalive
message to flush acknowledgement of all previous FT
operations. This enables a check-point for future recovery,
either in mid-session or prior to graceful shutdown of an
LDP session. This procedure may also be used to checkpoint
all (that is both FT and non-FT) operations for future
recovery.
Farrel, et al. [Page 7]
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4.1. Establishing an FT LDP Session
In order that the extensions to LDP [RFC3036] described
in this draft can be used successfully on an LDP session
between a pair of LDP peers, they MUST negotiate that the
LDP FT enhancements are to be used on the LDP session.
This is done on the LDP Initialization message exchange
using a new FT Session TLV. Presence of this TLV
indicates that the peer wants to support some form of
protection or recovery processing. The S bit within this
TLV indicates that the peer wants to support the LDP FT
enhancements on this LDP session. The C bit indicates
that the peer wants to support the checkpointing
functions described in this draft. The S and C bits may
be set independently.
The relevant LDP FT enhancements MUST be supported on an
LDP session if both LDP peers include an FT Session TLV
on the LDP Initialization message and have the same
setting of the S or C bit.
If either LDP Peer does not include the FT Session TLV
LDP Initialization message or if there is no match of S
and C bits between the peers, the LDP FT enhancements
MUST NOT be used during this LDP session. Use of LDP FT
enhancements by a sending LDP peer MUST be interpreted by
the receiving LDP peer as a serious protocol error
causing the session to be terminated.
An LSR MAY present different FT/non-FT behavior on
different TCP connections, even if those connections are
successive instantiations of the LDP session between the
same LDP peers.
4.1.1 Interoperation with Non-FT LSRs
The FT Session TLV on the LDP Initialization message
carries the U-bit. If an LSR does not support any
protection or recovery mechanisms , it will ignore this
TLV. Since such partners also do not include the FT
Session TLV, all LDP sessions to such LSRs will not use
the LDP FT enhancements.
The rest of this draft assumes that the LDP sessions
under discussion are between LSRs that do support the LDP
FT enhancements, except where explicitly stated
otherwise.
Farrel, et al. [Page 8]
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4.2. TCP Connection Failure
4.2.1 Detecting TCP Connection Failures
TCP connection failures may be detected and reported to the
LDP component in a variety of ways. These should all be
treated in the same way by the LDP component.
- Indication from the management component that a TCP
connection or underlying resource is no longer active.
- Notification from a hardware management component of an
interface failure.
- Sockets keepalive timeout.
- Sockets send failure.
- New (incoming) Socket opened.
- LDP protocol timeout.
4.2.2 LDP Processing after Connection Failure
If the LDP FT enhancements are not in use on an LDP
session, the action of the LDP peers on failure of the
TCP connection is as specified in [RFC3036].
All state information and resources associated with non-
FT Labels MUST be released on the failure of the TCP
connection, including deprogramming the non-FT Label from
the switching hardware. This is equivalent to the
behavior specified in [RFC3036].
If the LDP FT enhancements are in use on an LDP session,
both LDP peers SHOULD preserve state information and
resources associated with FT Labels exchanged on the LDP
session. Both LDP peers SHOULD use a timer to release
the preserved state information and resources associated
with FT-labels if the TCP connection is not restored
within a reasonable period. The behavior when this timer
expires is equivalent to the LDP session failure behavior
described in [RFC3036].
The FT Reconnection Timeout each LDP peer intends to
apply to the LDP session is carried in the FT Session TLV
on the LDP Initialization messages. Both LDP peers MUST
use the value that corresponds to the lesser timeout
interval of the two proposed timeout values from the LDP
Initialization exchange, where a value of zero is treated
as positive infinity.
Farrel, et al. [Page 9]
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4.3. Data Forwarding During TCP Connection Failure
An LSR that implements the LDP FT enhancements SHOULD
preserve the programming of the switching hardware across
a failover. This ensures that data forwarding is
unaffected by the state of the TCP connection between
LSRs.
It is an integral part of FT failover processing in some
hardware configurations that some data packets might be
lost. If data loss is not acceptable to the applications
using the MPLS network, the LDP FT enhancements described
in this draft SHOULD NOT be used.
4.4. FT LDP Session Reconnection
When a new TCP connection is established, the LDP peers
MUST exchange LDP Initialization messages. When a new
TCP connection is established after failure, the LDP
peers MUST re-exchange LDP Initialization messages.
If an LDP peer includes the FT Session TLV with the S bit
set in the LDP Initialization message for the new
instantiation of the LDP session, it MUST also set the FT
Reconnect Flag according to whether it has been able to
preserve label state. The FT Reconnect Flag is carried
in the FT Session TLV.
If an LDP peer has preserved all state information for
previous instantiations of the LDP session, then it
SHOULD set the FT Reconnect Flag to 1 in the FT Session
TLV. Otherwise, it MUST set the FT Reconnect Flag to 0.
If either LDP peer sets the FT Reconnect Flag to 0, or
omits the FT Session TLV, both LDP peers MUST release any
state information and resources associated with the
previous instantiation of the LDP session between the
same LDP peers, including FT Label state and Addresses.
This ensures that network resources are not permanently
lost by one LSR if its LDP peer is forced to undergo a
cold start.
If an LDP peer changes any session parameters (for
example, the label space bounds) from the previous
instantiation the nature of any preserved labels may have
changed. In particular, previously allocated labels may
now be out of range. For this reason, session
reconnection MUST use the same parameters as were in use
on the session before the failure. If an LDP peer
notices that the parameters have been changed by the
other peer it SHOULD send a Notification message with the
'FT Session parameters changed' status code.
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If both LDP peers set the FT Reconnect Flag to 1, both
LDP peers MUST use the procedures indicated in this draft
to complete any label operations on Sequence Numbered FT
Labels that were interrupted by the LDP session failure.
If an LDP peer receives an LDP Initialization message
with the FT Reconnect Flag set before it sends its own
Initialization message, but has retained no information
about the previous version of the session, it MUST
respond with an Initialization message with the FT
Reconnect Flag clear. If an LDP peer receives an LDP
Initialization message with the FT Reconnect Flag set in
response to an Initialization message that it has sent
with the FT Reconnect Flag clear it MUST act as if no
state was retained by either peer on the session.
4.5. Operations on FT Labels
Label operations on Sequence Numbered FT Labels are made
Fault Tolerant by providing acknowledgement of all LDP
messages that affect Sequence Numbered FT Labels.
Acknowledgements are achieved by means of sequence
numbers on these LDP messages.
The message exchanges used to achieve acknowledgement of
label operations and the procedures used to complete
interrupted label operations are detailed in the section
"FT Operations".
Using these acknowledgements and procedures, it is not
necessary for LDP peers to perform a complete re-
synchronization of state for all Sequence Numbered FT
Labels, either on re-connection of the LDP session
between the LDP peers or on a timed basis.
4.6. Check-Pointing
Check-pointing is a useful feature that allows nodes to
reduce the amount of processing that they need to do to
acknowledge LDP messages. The C bit in the FT Session
TLV is used to indicate that checkpointing is supported.
Under the normal operation on Sequence Numbered FT
Labels, acknowledgments may be deferred during normal
processing and only sent periodically. Check-pointing
may be used to flush acknowledgement from a peer by
including a sequence number on a Keepalive message
requesting acknowledgement of that message and all
previous messages. In this case, all Sequence Numbered
FT Labels are Checkpointable FT Labels.
If the S bit is not agreed upon, checkpointing may still
be used. In this case it is used to acknowledge all
messages exchanged between the peers, and all labels are
Checkpointable FT Labels.
Farrel, et al. [Page 11]
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This offers an approach where acknowledgements need not
be sent to every message or even frequently, but are only
sent as check-points in response to requests carried on
Keepalive messages. Such an approach may be considered
optimal in systems that do not show a high degree of
change over time (such as targeted LDP sessions) and that
are prepared to risk loss of state for the most recent
LDP exchanges. More dynamic systems (such as LDP
discovery sessions) are more likely to want to
acknowledge state changes more frequently so that the
maximum amount of state can be preserved over a failure.
Note that an important consideration of this draft is
that nodes acknowledging messages on a one-for-one basis,
nodes deferring acknowledgements, and nodes relying on
check-pointing should all interoperate seamlessly and
without protocol negotiation beyond session
initialization.
Further discussion of this feature is provided in the
section "FT Operations".
4.6.1 Graceful Termination
A feature that builds on check-pointing is graceful
termination.
In some cases, such as controlled failover or software
upgrade, it is possible for a node to know in advance
that it is going to terminate its session with a peer.
In these cases the node that intends terminating the
session can flush acknowledgement using a check-point
request as described above. The sender SHOULD not send
further label or address-related messages after
requesting shutdown check-pointing in order to preserve
the integrity of its saved state.
This, however, only provides for acknowledgement in one
direction, and the node that is terminating also requires
to know that it has secured all state sent by its peer.
This is achieved by a three-way hand shake of the check-
point which is requested by an additional TLV (the Cork
TLV) in the Keepalive message.
Further discussion of this feature is provided in the
section "FT Operations".
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4.7. Label Space Depletion and Replenishment
When an LDP peer is unable to satisfy a Label Request
message because it has no more available labels, it sends
a Notification message carrying the status code 'No label
resources'. This warns the requesting LDP peer that
subsequent Label Request messages are also likely to fail
for the same reason. This message does not need to be
acknowledged for FT purposes since Label Request messages
sent after session recovery will receive the same
response. However, the LDP peer that receives a 'No
label resources' Notification stops sending Label Request
messages until it receives a 'Label resources available'
Notification message. Since this unsolicited
Notification might get lost during session failure, it
may be protected using the procedures described in this
draft.
An alternative approach allows that an implementation may
always assume that labels are available when a session is
re-established. In this case, it is possible that it may
throw away the 'No label resources' information from the
previous incarnation of the session and may send a batch
of LDP messages on session re-establishment that will
fail and that it could have known would fail.
Note that the sender of a 'Label resources available'
Notification message may choose whether to add a sequence
number requesting acknowledgement. Conversely, the
receiver of 'Label resources available' Notification
message may choose to acknowledge the message without
actually saving any state.
This is an implementation choice made possible by making
the FT parameters on the Notification message optional.
Implementations will interoperate fully if they take
opposite approaches, but additional LDP messages may be
sent unnecessarily on session recovery.
4.8. Tunneled LSPs
The procedures described in this document can be applied
to LSPs that are tunneled and to LSPs that are carried by
tunnels. Recall that tunneled LSPs are managed by a
single LDP session that runs end to end while the tunnel
is managed by a different LDP session for each hop along
the path. Nevertheless, a break in one of the sessions
that manages the tunnel is likely to correspond with a
break in the session that manages the tunneled LSP. This
is certainly the case when the LDP exchanges share a
failed link, but need not be the case if the LDP messages
have been routed along a path that is different from that
of the tunnel, or if the failure in the tunnel is caused
by an LDP software failure at a transit LSR.
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In order that the forwarding path of a tunneled LSP be
preserved, the forwarding path of the tunnel itself must
be preserved. This means that the tunnel must not be torn
down if there is any session failure along its path. To
achieve this the label exchanges between each pair of LDP
peers along the path of the tunnel must use one of the
procedures in this document or in [LDP-RESTART].
It is perfectly acceptable to mix the restart procedures
used for the tunnel and the tunneled LSP. For example,
the tunnel could be set up using just check-pointing
because it is a stable LSP, but the tunneled LSPs might
use full FT procedures so that they can recover active
state.
Lastly, it is permissible to carry tunneled LSPs that do
not have FT protection on an LSP that does have FT
protection.
5. FT Operations
Once an FT LDP session has been established, using the S
bit in the FT Session TLV on the Session Initialization
message as described in the section "Establishing an FT
LDP Session", both LDP peers MUST apply the procedures
described in this section for FT LDP message exchanges.
If the LDP session has been negotiated to not use the LDP
FT enhancements, these procedures MUST NOT be used.
5.1. FT LDP Messages
5.1.1 Sequence Numbered FT Label Messages
A label is identified as being a Sequence Numbered FT
Label if the initial Label Request or Label Mapping
message relating to that label carries the FT Protection
TLV.
It is a valid implementation option to flag all labels as
Sequence Numbered FT Labels. Indeed this may be a
preferred option for implementations wishing to use
Keepalive messages carrying the FT Protection TLV to
achieve periodic saves of the complete label forwarding
state.
If a label is a Sequence Numbered FT Label, all LDP
messages affecting that label MUST carry the FT
Protection TLV in order that the state of the label can
be recovered after a failure of the LDP session.
A valid option is for no labels to be Sequence Numbered
FT Labels. In this case checkpointing using the
Keepalive message applies to all messages exchanged on
the session.
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5.1.1.1 Scope of FT Labels
The scope of the FT/non-FT status of a label is limited
to the LDP message exchanges between a pair of LDP peers.
In Ordered Control, when the message is forwarded
downstream or upstream, the TLV may be present or absent
according to the requirements of the LSR sending the
message.
If a platform-wide label space is used for FT Labels, an
FT Label value MUST NOT be reused until all LDP FT peers
to which the label was passed have acknowledged the
withdrawal of the FT Label, either by an explicit LABEL
WITHDRAW/LABEL RELEASE exchange or implicitly if the LDP
session is reconnected after failure but without the FT
Reconnect Flag set. In the event that a session is not
re-established within the Reconnection Timeout, a label
MAY become available for re-use if it is not still in use
on some other session.
5.1.2 FT Address Messages
If an LDP session uses the LDP FT enhancements, both LDP
peers MUST secure Address and Address Withdraw messages
using FT Operation ACKs, as described below. This avoids
any ambiguity over whether an Address is still valid
after the LDP session is reconnected.
If an LSR determines that an Address message that it sent
on a previous instantiation of a recovered LDP session is
no longer valid, it MUST explicitly issue an Address
Withdraw for that address when the session is
reconnected.
If the FT Reconnect Flag is not set by both LDP peers on
reconnection of an LDP session (i.e. state has not been
preserved), both LDP peers MUST consider all Addresses to
have been withdrawn. The LDP peers SHOULD issue new
Address messages for all their valid addresses as
specified in [RFC3036].
5.1.3 Label Resources Available Notifications
In LDP, it is possible that a downstream LSR may not have
labels available to respond to a Label Request. In this
case, as specified in RFC3036, the downstream LSR must
respond with a Notification - No Label Resources message.
The upstream LSR then suspends asking for new labels
until it receives a Notification - Label Resources
Available message from the downstream LSR.
Farrel, et al. [Page 15]
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When the FT extensions are used on a session,
implementations may choose whether to secure the label
resource state of their peer or not. This choice impacts
the number of LDP messages that will be incorrectly
routed to a peer with depleted resources on session re-
establishment, but does not otherwise impact
interoperability.
For full preservation of state:
- The downstream LSR must preserve the label availability
state across a failover so that it remembers to send
Notification - Label Resources Available when the resources
become available.
- The upstream LSR must recall the label availability state
across failover so that it can optimize not sending Label
Requests when it recovers.
- The downstream LSR must use sequence numbers on
Notification - Label Resources Available so that it can
check that LSR A has received the message and clear its
secured state, or resend the message if LSR A recovers
without having received it.
However, the following options also exist:
- The downstream LSR may choose to not include a sequence
number on Notification - Label Resources Available. This
means that on session re-establishment it does not know what
its peer thinks the LSR's resource state is, because the
Notification may or may not have been delivered. Such an
implementation MUST begin recovered sessions by sending an
additional Notification - Label Resources Available to reset
its peer.
- The upstream node may choose not to secure information
about its peer's resource state. It would acknowledge a
Notification - Label Resources Available, but would not save
the information. Such an implementation MUST assume that
its peer's resource state has been reset to Label Resources
Available when the session is re-established.
If the FT Reconnect Flag is not set by both LDP peers on
reconnection of an LDP session (i.e. state has not been
preserved), both LDP peers MUST consider the label
availability state to have been reset as if the session
had been set up for the first time.
Farrel, et al. [Page 16]
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5.2. FT Operation ACKs
Handshaking of FT LDP messages is achieved by use of
ACKs. Correlation between the original message and the
ACK is by means of the FT Sequence Number contained in
the FT Protection TLV, and passed back in the FT ACK TLV.
The FT ACK TLV may be carried on any LDP message that is
sent on the TCP connection between LDP peers.
An LDP peer maintains a separate FT sequence number for
each LDP session it participates in. The FT Sequence
number is incremented by one for each FT LDP message
(i.e. containing the FT Protection TLV) issued by this
LSR on the FT LDP session with which the FT sequence
number is associated.
When an LDP peer receives a message containing the FT
Protection TLV, it MUST take steps to secure this message
(or the state information derived from processing the
message). Once the message is secured, it MUST be ACKed.
However, there is no requirement on the LSR to send this
ACK immediately.
ACKs may be accumulated to reduce the message flow
between LDP peers. For example, if an LSR received FT
LDP messages with sequence numbers 1, 2, 3, 4, it could
send a single ACK with sequence number 4 to ACK receipt
and securing of all these messages. There is no protocol
reason why the number of ACKs accumulated or the time for
which an ACK is deferred should not be allowed to become
relatively large.
ACKs MUST NOT be sent out of sequence, as this is
incompatible with the use of accumulated ACKs. Duplicate
ACKs (that is two successive messages that acknowledge
the same sequence number) are acceptable.
If an LDP peer discovers that its sequence number space
for a specific session is full of un-acknowledged
sequence numbers (because its partner on the session has
not acknowledged them in a timely way) it cannot allocate
a new sequence number for any further FT LPD message. It
SHOULD send a Notification message with the status code
"FT Seq Numbers Exhausted".
5.3. Preservation of FT State
If the LDP FT enhancements are in use on an LDP session,
each LDP peer SHOULD NOT release the state information
and resources associated with FT Labels exchanged on that
LDP session when the TCP connection fails. This is
contrary to [RFC3036], but allows label operations on FT
Labels to be completed after re-connection of the TCP
connection.
Farrel, et al. [Page 17]
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Both LDP peers on an LDP session that is using the LDP FT
enhancements SHOULD preserve the state information and
resources they hold for that LDP session as described
below.
- An upstream LDP peer SHOULD release the resources (in
particular bandwidth) associated with a Sequence Numbered FT
Label when it initiates a Label Release or Label Abort
message for the label. The upstream LDP peer MUST preserve
state information for the Sequence Numbered FT Label, even
if it releases the resources associated with the label, as
it may need to reissue the label operation if the TCP
connection is interrupted.
- An upstream LDP peer MUST release the state information
and resources associated with a Sequence Numbered FT Label
when it receives an acknowledgement to a Label Release or
Label Abort message that it sent for the label, or when it
sends a Label Release message in response to a Label
Withdraw message received from the downstream LDP peer.
- A downstream LDP peer SHOULD NOT release the resources
associated with a Sequence Numbered FT Label when it sends a
Label Withdraw message for the label as it has not yet
received confirmation that the upstream LDP peer has ceased
to send data using the label. The downstream LDP peer MUST
NOT release the state information it holds for the label as
it may yet have to reissue the label operation if the TCP
connection is interrupted.
- A downstream LDP peer MUST release the resources and
state information associated with a Sequence Numbered FT
Label when it receives an acknowledgement to a Label
Withdraw message for the label.
- When the FT Reconnection Timeout expires, an LSR SHOULD
release all state information and resources from previous
instantiations of the (permanently) failed LDP session.
- Either LDP peer MAY elect to release state information
based on its internal knowledge of the loss of integrity of
the state information or an inability to pend (or queue) LDP
operations (as described in section 4.4.1) during a TCP
failure. That is, the peer is not required to wait for the
duration of the FT Reconnection Timeout before releasing
state; the timeout provides an upper limit on the
persistence of state. However, in the event that a peer
releases state before the expiration of the Reconnection
Timeout it MUST NOT re-use any label that was in use on the
session until the Reconnection Timeout has expired.
- When an LSR receives a Status TLV with the E-bit set in
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- When an LSR receives a Status TLV with the E-bit set in
the status code, which causes it to close the TCP
connection, the LSR MUST release all state information and
resources associated with the session. This behavior is
mandated because it is impossible for the LSR to predict the
precise state and future behavior of the partner LSR that
set the E-bit without knowledge of the implementation of
that partner LSR.
Note that the "Temporary Shutdown" status code does not have
the E-bit set, and MAY be used during maintenance or upgrade
operations to indicate that the LSR intends to preserve
state across a closure and re-establishment of the TCP
session.
- If an LSR determines that it must release state for any
single FT Label during a failure of the TCP connection on
which that label was exchanged, it MUST release all state
for all labels on the LDP session.
The release of state information and resources associated
with non-FT labels is as described in [RFC3036].
Note that a Label Release and the acknowledgement to a
Label Withdraw may be received by a downstream LSR in any
order. The downstream LSR MAY release its resources on
receipt of the first message and MUST release its
resources on receipt of the second message.
5.4. FT Procedure After TCP Failure
When an LSR discovers or is notified of a TCP connection
failure it SHOULD start an FT Reconnection Timer to allow
a period for re-connection of the TCP connection between
the LDP peers.
The RECOMMENDED default value for this timer is 5
seconds. During this time, failure must be detected and
reported, new hardware may need to be activated, software
state must be audited, and a new TCP session must be set
up.
Once the TCP connection between LDP peers has failed, the
active LSR SHOULD attempt to re-establish the TCP
connection. The mechanisms, timers and retry counts to re-
establish the TCP connection are an implementation
choice. It is RECOMMENDED that any attempt to re-
establish the connection take account of the failover
processing necessary on the peer LSR, the nature of the
network between the LDP peers, and the FT Reconnection
Timeout chosen on the previous instantiation of the TCP
connection (if any).
Farrel, et al. [Page 19]
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If the TCP connection cannot be re-established within the
FT Reconnection Timeout period, the LSR detecting this
timeout SHOULD release all state preserved for the failed
LDP session. If the TCP connection is subsequently re-
established (for example, after a further Hello exchange
to set up a new LDP session), the LSR MUST set the FT
Reconnect Flag to 0 if it released the preserved state
information on this timeout event.
If the TCP connection is successfully re-established
within the FT Reconnection Timeout, both peers MUST re-
issue LDP operations that were interrupted by (that is,
un-acknowledged as a result of) the TCP connection
failure. This procedure is described in section "FT
Procedure After TCP Re-connection".
The Hold Timer for an FT LDP Session (see [RFC3036]
section 2.5.5) SHOULD be ignored while the FT
Reconnection Timer is running. The hold timer SHOULD be
restarted when the TCP connection is re-established.
5.4.1 FT LDP Operations During TCP Failure
When the LDP FT enhancements are in use for an LDP
session, it is possible that an LSR may determine that it
needs to send an LDP message to an LDP peer but that the
TCP connection to that peer is currently down. These
label operations affect the state of FT Labels preserved
for the failed TCP connection, so it is important that
the state changes are passed to the LDP peer when the TCP
connection is restored.
If an LSR determines that it needs to issue a new FT LDP
operation to an LDP peer to which the TCP connection is
currently failed, it MUST pend the operation (e.g. on a
queue) and complete that operation with the LDP peer when
the TCP connection is restored, unless the label
operation is overridden by a subsequent additional
operation during the TCP connection failure (see section
"FT Procedure After TCP Re-connection").
If, during TCP Failure, an LSR determines that it cannot
pend an operation which it cannot simply fail (for
example, a Label Withdraw, Release or Abort operation),
it MUST NOT attempt to re-establish the previous LDP
session. The LSR MUST behave as if the Reconnection
Timer expired and release all state information with
respect to the LDP peer. An LSR may be unable (or
unwilling) to pend operations; for instance, if a major
routing transition occurred while TCP was inoperable
between LDP peers it might result in excessively large
numbers of FT LDP Operations. An LSR that releases state
before the expiration of the Reconnection Timeout MUST
NOT re-use any label that was in use on the session until
the Reconnection Timeout has expired.
Farrel, et al. [Page 20]
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In ordered operation, received FT LDP operations that
cannot be correctly forwarded because of a TCP connection
failure MAY be processed immediately (provided sufficient
state is kept to forward the label operation) or pended
for processing when the onward TCP connection is restored
and the operation can be correctly forwarded upstream or
downstream. Operations on existing FT Labels SHOULD NOT
be failed during TCP session failure.
It is RECOMMENDED that Label Request operations for new
FT Labels are not pended awaiting the re-establishment of
TCP connection that is awaiting recovery at the time the
LSR determines that it needs to issue the Label Request
message. Instead, such Label Request operations SHOULD
be failed and, if necessary, a notification message
containing the "No LDP Session" status code sent
upstream.
Label Requests for new non-FT Labels MUST be rejected
during TCP connection failure, as specified in [RFC3036].
5.5. FT Procedure After TCP Re-connection
The FT operation handshaking described above means that
all state changes for Sequence Numbered FT Labels and
Address messages are confirmed or reproducible at each
LSR.
If the TCP connection between LDP peers fails but is re-
connected within the FT Reconnection Timeout, and both
LSRs have indicated they will be re-establishing the
previous LDP session, both LDP peers on the connection
MUST complete any label operations for Sequence Numbered
FT Labels that were interrupted by the failure and re-
connection of the TCP connection.
The procedures for FT Reconnection Timeout MAY have been
invoked as a result of either LDP peer being unable (or
unwilling) to pend operations which occurred during the
TCP Failure (as described in section 4.4.1).
If, for any reason, an LSR has been unable to pend
operations with respect to an LDP peer, as described in
section 4.4.1, the LSR MUST set the FT Reconnect Flag to
0 on re-connection to that LDP peer indicating that no FT
state has been preserved.
Label operations are completed using the procedure
described below.
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5.5.1 Re-Issuing FT Messages
On restoration of the TCP connection between LDP peers,
any LDP messages for Sequence Numbered FT Labels that
were lost because of the TCP connection failure are re-
issued. The LDP peer that receives a re-issued message
processes the message as if received for the first time.
"Net-zero" combinations of messages need not be re-issued
after re-establishment of the TCP connection between LDP
peers. This leads to the following rules for re-issuing
messages that are not ACKed by the LDP peer on the LDP
Initialization message exchange after re-connection of
the TCP session.
- A Label Request message MUST be re-issued unless a Label
Abort would be re-issued for the same Sequence Numbered FT
Label.
- A Label Mapping message MUST be re-issued unless a Label
Withdraw message would be re-issued for the same Sequence
Numbered FT Label.
- All other messages on the LDP session that carried the FT
Protection TLV MUST be re-issued if an acknowledgement had
not previously been received.
Any FT Label operations that were pended (see section
4.4.1) during the TCP connection failure MUST also be
issued on re-establishment of the LDP session, except
where they form part of a "net-zero" combination of
messages according to the above rules.
The determination of "net-zero" FT Label operations
according to the above rules MAY be performed on pended
messages prior to the re-establishment of the TCP
connection in order to optimize the use of queue
resources. Messages that were sent to the LDP peer
before the TCP connection failure, or pended messages
that are paired with them, MUST NOT be subject to such
optimization until an FT ACK TLV is received from the LDP
peer. This ACK allows the LSR to identify which messages
were received by the LDP peer prior to the TCP connection
failure.
6. Checkpointing Procedures
Checkpointing can be selected independently from the FT
procedures described above by using the C bit in the FT
Session TLV on the Session Initialization message. Note,
however, that checkpointing is an integral part of the FT
procedures. Setting the S and the C bit will achieve the
same function as setting just the S bit.
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If the C bit is set, but the S bit is not set, no label
is a Sequence Numbered FT Label. Instead, all labels are
Checkpointable FT Labels. Checkpointing is used to
synchronize all labels exchanges. No message apart from
the checkpoint request and acknowledgement carries an
active sequence number. (Note that the Session
Initialization message may carry a sequence number to
confirm that the checkpoint is still in place).
It is an implementation matter to decide the ordering of
received messages and checkpoint requests to ensure that
checkpoint acknowledgements are secured.
If the S and C bits are both set, or only the S bit is
set, checkpointing applies only to Sequence Numbered FT
Labels and to address messages.
The set of all messages that are checkpointed in this way
is called the Checkpointable Messages.
6.1 Checkpointing with the Keepalive Message
If an LSR receives a FT Protection TLV on a Keepalive
message, this is a request to flush the acknowledgements
for all previously received Checkpointable Messages on
the session.
As soon as the LSR has completed securing the
Checkpointable Messages (or state changes consequent on
those messages) received before the Keepalive, it MUST
send an acknowledgement to the sequence number of the
Keepalive message.
In the case where the FT procedures are in use and
acknowledgements have been stored up, this may be
immediately on receipt of the Keepalive.
An example message flow showing this use of the Keepalive
message to perform a periodic check-point of state is
shown in section 8.
An example message flow showing the use of checkpointing
without the FT procedures is shown in section 8.
6.2 Quiesce and Keepalive
If the Keepalive Message also contains the FT Cork TLV,
this indicates that the peer LSR wishes to quiesce the
session prior to a graceful restart.
It is RECOMMENDED that on receiving a Keepalive with the
FT CORK TLV, an LSR should cease to send any further
label or address related messages on the session until it
has been disconnected and reconnected, other than any
messages generated while processing and securing any
previously unacknowledged messages received from the peer
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requesting the quiesce. It should also attempt to
complete this processing and return a Keepalive with the
FT ACK TLV as soon as possible in order to allow the
session to be quiesced.
An example message flow showing this use of the FT Cork
TLV to achieves three-way handshake of state
synchronization between two LDP peers is given in section 8.
7. Changes to Existing Messages
7.1. LDP Initialization Message
The LDP FT enhancements add the following optional
parameters to a LDP Initialization message
Optional Parameter Length Value
FT Session TLV 4 See below
FT ACK TLV 4 See below
The encoding for these TLVs is found in Section "New
Fields and Values".
FT Session TLV
If present, specifies the FT behavior of
the LDP session.
FT ACK TLV
If present, specifies the last FT message
that the sending LDP peer was able to
secure prior to the failure of the previous
instantiation of the LDP session. This TLV
is only present if the FT Reconnect flag is
set in the FT Session TLV, in which case
this TLV MUST be present.
7.2. LDP Keepalive Messages
The LDP FT enhancements add the following optional
parameters to a LDP Keepalive message
Optional Parameter Length Value
FT Protection TLV 4 See below
FT Cork TLV 0 See below
FT ACK TLV 4 See below
The encoding for these TLVs is found in Section "New
Fields and Values".
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FT Protection TLV
If present, specifies FT Sequence Number
for the LDP message. When present on a
Keepalive message, this indicates a
solicited flush of the acknowledgements to
all previous LDP messages containing
sequence numbers and issued by the sender
of the Keepalive on the same session.
FT Cork TLV
Indicates that the remote LSR wishes to
quiesce the LDP session. See section 5 for
the recommended action in such cases.
FT ACK TLV
If present, specifies the most recent FT
message that the sending LDP peer has been
able to secure.
7.3. All Other LDP Session Messages
The LDP FT enhancements add the following optional
parameters to all other message types that flow on an LDP
session after the LDP Initialization message
Optional Parameter Length Value
FT Protection TLV 4 See below
FT ACK TLV 4 See below
The encoding for these TLVs is found in the section "New
Fields and Values".
FT Protection TLV
If present, specifies FT Sequence Number
for the LDP message.
FT ACK TLV
If present, identifies the most recent FT
LDP message ACKed by the sending LDP peer.
8. New Fields and Values
8.1. Status Codes
The following new status codes are defined to indicate
various conditions specific to the LDP FT enhancements.
These status codes are carried in the Status TLV of a
Notification message.
The "E" column is the required setting of the Status Code
E-bit; the "Status Data" column is the value of the 30-
bit Status Data field in the Status Code TLV.
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Note that the setting of the Status Code F-bit is at the
discretion of the LSR originating the Status TLV.
However, it is RECOMMENDED that the F-bit is not set on
Notification messages containing status codes except "No
LDP Session" because the duplication of messages SHOULD
be restricted to being a per-hop behavior.
Status Code E Status Data
No LDP Session 0 0x000000xx
Zero FT seqnum 1 0x000000xx
Unexpected TLV / 1 0x000000xx
Session Not FT
Unexpected TLV / 1 0x000000xx
Label Not FT
Missing FT Protection TLV 1 0x000000xx
FT ACK sequence error 1 0x000000xx
Temporary Shutdown 0 0x000000xx
FT Seq Numbers Exhausted 1 0x000000xx
FT Session parameters / 1 0x000000xx
changed
Unexpected FT Cork TLV 1 0x000000xx
The Temporary Shutdown status code SHOULD be used in
place of the Shutdown status code (which has the E-bit
set) if the LSR that is shutting down wishes to inform
its LDP peer that it expects to be able to preserve FT
Label state and to return to service before the FT
Reconnection Timer expires.
8.2. FT Session TLV
LDP peers can negotiate whether the LDP session between
them supports FT extensions by using a new OPTIONAL
parameter, the FT Session TLV, on LDP Initialization
Messages.
The FT Session TLV is encoded as follows.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1|0| FT Session TLV (0x0503) | Length (= 12) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FT Flags | Reserved |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FT Reconnect Timeout (in milliseconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Recovery Time (in milliseconds) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
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FT Flags
FT Flags: A 16 bit field that indicates
various attributes the FT support on this
LDP session. This fields is formatted as
follows:
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|R| Reserved |S|A|C|L|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
R: FT Reconnect Flag.
Set to 1 if the sending LSR has
preserved state and resources for all
FT-labels since the previous LDP
session between the same LDP peers,
and set to 0 otherwise. See the
section "FT LDP Session Reconnection"
for details of how this flag is used.
If the FT Reconnect Flag is set, the
sending LSR MUST include an FT ACK TLV
on the LDP Initialization message.
S: Save State Flag.
Set to 1 if the use of the FT
Protection TLV is supported on
messages other than the KeepAlive
message used for chekpointing (see the
C bit). I.e., the S bit indicates
that some label on the session may be
a Sequence Numbered FT Label.
A: All-Label Protection Required
Set to 1 if all labels on the session
MUST be treated as Sequence Numbered
FT Labels. This removes from a node
the option of treating some labels as
FT Labels and some labels as non-FT
Labels.
Passing this information may be
considered helpful to a peer since it
may allow it to make optimizations in
its processing.
The A bit only has meaning if the S
bit is set.
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C: Checkpointing Flag.
Set to 1 to indicate that the
checkpointing procedures in this draft
are in use.
If the S bit is also set to 1 then the
C bit indicates that checkpointing is
applied only to Sequence Numbered FT
Labels.
If the S bit is set to 0 (zero) then
the C bit indicates that checkpointing
applies to all labels - all labels are
Checkpointable FT Labels.
L: Learn From Network Flag.
Set to 1 if the Fault Recovery
procedures of [LDP-RESTART] are to be
used to re-learn state from the
network.
It is not valid for all of the S, C and L
bits to be zero.
It is not valid for both the L and either
the S or C bits to be set to 1.
All other bits in this field are currently
reserved and SHOULD be set to zero on
transmission and ignored on receipt.
The following table summarizes the settings
of these bits.
S A C L Comments
=========================
0 x 0 0 Invalid
0 0 0 1 See [LDP-RESTART]
0 1 0 1 Invalid
0 x 1 0 Checkpointing of all labels
0 x 1 1 Invalid
1 0 0 0 Full FT on selected labels
1 1 0 0 Full FT on all labels
1 x 0 1 Invalid
1 x 1 0 Same as (S=1,A=x,C=0,L=0)
1 x 1 1 Invalid.
FT Reconnection Timeout
If the S bit or C bit in the FT Flags field
is set this indicates the period of time
the sending LSR will preserve state and
resources for FT Labels exchanged on the
previous instantiation of an FT LDP session
that has currently failed. The timeout is
encoded as a 32-bit unsigned integer number
of milliseconds.
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A value of zero in this field means that
the sending LSR will preserve state and
resources indefinitely.
See section 4.4 for details of how this
field is used.
If the L bit is set to 1 in the FT Flags
field, the meaning of this field is defined
in [LDP-RESTART].
Recovery Time
The Recovery Time only has meaning if the L
bit is set in the FT Flags. The meaning is
defined in [LDP-RESTART].
8.3. FT Protection TLV
LDP peers use the FT Protection TLV to indicate that an
LDP message contains an FT label operation.
The FT Protection TLV MUST NOT be used in messages
flowing on an LDP session that does not support the LDP
FT enhancements. Its presence in such messages SHALL be
treated as a protocol error by the receiving LDP peer
which SHOULD send a Notification message with the
'Unexpected TLV Session Not FT' status code. LSRs that
do not recognize this TLV SHOULD respond with a
Notification message with the 'Unknown TLV' status code.
The FT Protection TLV MAY be carried on an LDP message
transported on the LDP session after the initial exchange
of LDP Initialization messages. In particular, this TLV
MAY optionally be present on the following messages:
- Label Request Messages in downstream on-demand
distribution mode
- Label Mapping messages in downstream unsolicited mode
- Keepalive messages used to request flushing of
acknowledgement of all previous messages that contained this
TLV.
If a label is to be a Sequence Numbered FT Label, then
the Protection TLV MUST be present:
- on the Label Request message in DoD mode
- on the Label Mapping message in DU mode
- on all subsequent messages concerning this label.
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Here 'subsequent messages concerning this label' means
any message whose Label TLV specifies this label or whose
Label Request Message ID TLV specifies the initial Label
Request message.
If a label is not to be a Sequence Numbered FT Label,
then the Protection TLV MUST NOT be present on any of
these messages that relate to the label. The presence of
the FT TLV on a message relating to a non-FT Label SHALL
be treated as a protocol error by the receiving LDP peer
which SHOULD send a notification message with the
'Unexpected TLV Label Not FT' status code.
Where a Label Withdraw or Label Release message contains
only a FEC TLV and does not identify a single specific
label, the FT TLV should be included in the message if any
label affected by the message is a Sequence Numbered FT
Label. If there is any doubt as to whether an FT TLVshould
be present, it is RECOMMENDED that the sender add the TLV.
When an LDP peer receives a Label Withdraw Message or
Label Release message that contains only a FEC, it SHALL
accept the FT TLV if it is present regardless of the FT
status of the labels which it affects.
If an LDP session is an FT session as determined by the
presence of the FT Session TLV with the S bit set on the
LDP Initialization messages, the FT Protection TLV MUST
be present on all Address messages on the session.
If the session is an FT session, the FT Protection TLV
may also optionally be present
- on Notification messages on the session that have the
status code 'Label Resources Available'
- on Keepalive messages.
The FT Protection TLV is encoded as follows.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| FT Protection (0x0203) | Length (= 4) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FT Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
FT Sequence Number
The sequence number for this Sequence Numbered
FT Label operation. The sequence number is
encoded as a 32-bit unsigned integer. The
initial value for this field on a new LDP
session is 0x00000001 and is incremented by
one for each FT LDP message issued by the
sending LSR on this LDP session. This field
may wrap from 0xFFFFFFFF to 0x00000001.
Farrel, et al. [Page 30]
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This field MUST be reset to 0x00000001 if
either LDP peer does not set the FT
Reconnect Flag on re-establishment of the
TCP connection.
See the section "FT Operation Acks" for
details of how this field is used.
The special use of 0x00000000 is discussed
in the section "FT ACK TLV" below.
If an LSR receives an FT Protection TLV on a session that
does not support the FT LDP enhancements, it SHOULD send
a Notification message to its LDP peer containing the
"Unexpected TLV, Session Not FT" status code. LSRs that
do not recognize this TLV SHOULD respond with a
Notification message with the 'Unknown TLV' status code.
If an LSR receives an FT Protection TLV on an operation
affecting a label that it believes is a non-FT Label, it
SHOULD send a Notification message to its LDP peer
containing the "Unexpected TLV, Label Not FT" status
code.
If an LSR receives a message without the FT Protection
TLV affecting a label that it believes is a Sequence
Numbered FT Label, it SHOULD send a Notification message
to its LDP peer containing the "Missing FT Protection
TLV" status code.
If an LSR receives an FT Protection TLV containing a zero
FT Sequence Number, it SHOULD send a Notification message
to its LDP peer containing the "Zero FT Seqnum" status
code.
8.4. FT ACK TLV
LDP peers use the FT ACK TLV to acknowledge FT Label
operations.
The FT ACK TLV MUST NOT be used in messages flowing on an
LDP session that does not support the LDP FT
enhancements. Its presence on such messages SHALL be
treated as a protocol error by the receiving LDP peer.
The FT ACK TLV MAY be present on any LDP message
exchanged on an LDP session after the initial LDP
Initialization messages. It is RECOMMENDED that the FT
ACK TLV is included on all FT Keepalive messages in order
to ensure that the LDP peers do not build up a large
backlog of unacknowledged state information.
Farrel, et al. [Page 31]
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The FT ACK TLV is encoded as follows.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| FT ACK (0x0504) | Length (= 4) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| FT ACK Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
FT ACK Sequence Number
The sequence number for the most recent FT
label message that the sending LDP peer has
received from the receiving LDP peer and
secured against failure of the LDP session.
It is not necessary for the sending peer to
have fully processed the message before
ACKing it. For example, an LSR MAY ACK a
Label Request message as soon as it has
securely recorded the message, without
waiting until it can send the Label Mapping
message in response.
ACKs are cumulative. Receipt of an LDP
message containing an FT ACK TLV with an FT
ACK Sequence Number of 12 is treated as the
acknowledgement of all messages from 1 to
12 inclusive (assuming the LDP session
started with a sequence number of 1).
This field MUST be set to 0 if the LSR
sending the FT ACK TLV has not received any
FT label operations on this LDP session.
This would apply to LDP sessions to new LDP
peers or after an LSR determines that it
must drop all state for a failed TCP
connection.
See the section "FT Operation Acks" for
details of how this field is used.
If an LSR receives a message affecting a label that it
believes is a Sequence Numbered FT Label and that message
does not contain the FT Protection TLV, it SHOULD send a
Notification message to its LDP peer containing the
"Missing FT Protection TLV" status code.
If an LSR receives an FT ACK TLV that contains an FT ACK
Sequence Number that is less than the previously received
FT ACK Sequence Number (remembering to take account of
wrapping), it SHOULD send a Notification message to its
LDP peer containing the "FT ACK Sequence Error" status
code.
Farrel, et al. [Page 32]
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8.5. FT Cork TLV
LDP peers use the FT Cork TLV on FT Keepalive messages to
indicate that they wish to quiesce the LDP session prior
to a controlled shutdown and restart, for example during
control-plane software upgrade.
The FT Cork TLV is encoded as follows.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|0|0| FT Cork (0x0505) | Length (= 0) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
On receipt of a Keepalive message with the FT Cork TLV
and the FT Protection TLV, an LSR SHOULD perform the
following actions
- Process and secure any messages from the peer LSR that
have sequence numbers less than (accounting for wrap) that
contained in the FT Protection TLV on the Keepalive message.
- Send a Keepalive message back to the peer containing the
FT Cork TLV and the FT ACK TLV specifying the FT ACK
sequence number equal to that in the original Keepalive
message (i.e. ACKing all messages up to that point).
- If this LSR has not yet received an FT ACK to all the
messages it has sent containing the FT Protection TLV, then
also include an FT Protection TLV on the Keepalive sent to
the peer LSR. This tells the remote peer that the local LSR
has saved state prior to quiesce but is still awaiting
confirmation that the remote peer has saved state.
- Cease sending any further state changing messages on this
LDP session until it has been disconnected and recovered.
On receipt of a Keepalive message with the FT Cork TLV
and an FT ACK TLV that acknowledges the previously sent
Keepalive that carried the FT Cork TLV, an LSR knows that
quiesce is complete. If the received Keepalive also
carries the FT Protection TLV, the LSR must respond with
a further Keepalive to complete the 3-way handshake. It
SHOULD now send a "Temporary Shutdown" Notification
message, disconnect the TCP session and perform whatever
control plane actions required this session shutdown.
An example such 3-way handshake for controlled shutdown
is given in section 8.
If an LSR receives a message that should not carry the FT
Cork TLV, or if the FT Cork TLV is used on a Keepalive
message without one of the FT Protection or FT ACK TLVs
present, , it SHOULD send a Notification message to its
LDP peer containing the "Unexpected FR Cork TLV" status code.
Farrel, et al. [Page 33]
draft-ietf-mpls-ldp-ft-06.txt September 2002
9. Example Use
Consider two LDP peers, P1 and P2, implementing LDP over
a TCP connection that connects them, and the message flow
shown below.
The parameters shown on each message shown below are as
follows:
message (label, senders FT sequence number, FT ACK
number)
A "-" for FT ACK number means that the FT ACK TLV is
not included on that message. "n/a" means that the
parameter in question is not applicable to that type
of message.
In the diagrams below, time flows from top to bottom.
The relative position of each message shows when it is
transmitted. See the notes for a description of when
each message is received, secured for FT or processed.
Farrel, et al. [Page 34]
draft-ietf-mpls-ldp-ft-06.txt September 2002
9.1. Session Failure and Recovery - FT Procedures
notes P1 P2
===== == ==
(1) Label Request(L1,27,-)
--------------------------->
Label Request(L2,28,-)
--------------------------->
(2) Label Request(L3,93,27)
<---------------------------
(3) Label Request(L1,123,-)
-------------------------->
Label Request(L2,124,-)
-------------------------->
(4) Label Mapping(L1,57,-)
<--------------------------
Label Mapping(L1,94,28)
<---------------------------
(5) Label Mapping(L2,58,-)
<--------------------------
Label Mapping(L2,95,-)
<---------------------------
(6) Address(n/a,29,-)
--------------------------->
(7) Label Request(L4,30,-)
--------------------------->
(8) Keepalive(n/a,-,94)
--------------------------->
(9) Label Abort(L3,96,-)
<---------------------------
(10) ===== TCP Session lost =====
:
(11) : Label Withdraw(L1,59,-)
: <--------------------------
:
(12) === TCP Session restored ===
LDP Init(n/a,n/a,94)
--------------------------->
LDP Init(n/a,n/a,29)
<---------------------------
(13) Label Request(L4,30,-)
--------------------------->
(14) Label Mapping(L2,95,-)
<---------------------------
Label Abort(L3,96,30)
<---------------------------
(15) Label Withdraw(L1,97,-)
<---------------------------
Farrel, et al. [Page 35]
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Notes:
======
(1) Assume that the LDP session has already been initialized.
P1 issues 2 new Label Requests using the next sequence numbers.
(2) P2 issues a third Label request to P1. At the time of sending
this request, P2 has secured the receipt of the label request
for L1 from P1, so it includes an ACK for that message.
(3) P2 Processes the Label Requests for L1 and L2 and forwards them
downstream. Details of downstream processing are not shown in
the diagram above.
(4) P2 receives a Label Mapping from downstream for L1, which it
forwards to P1. It includes an ACK to the Label Request for L2,
as that message has now been secured and processed.
(5) P2 receives the Label Mapping for L2, which it forwards to P1.
This time it does not include an ACK as it has not received any
further messages from P1.
(6) Meanwhile, P1 sends a new Address Message to P2 .
(7) P1 also sends a fourth Label Request to P2
(8) P1 sends a Keepalive message to P2, on which it includes an ACK
for the Label Mapping for L1, which is the latest message P1 has
received and secured at the time the Keepalive is sent.
(9) P2 issues a Label Abort for L3.
(10) At this point, the TCP session goes down.
(11) While the TCP session is down, P2 receives a Label Withdraw
Message for L1, which it queues.
(12) The TCP session is reconnected and P1 and P2 exchange LDP
Initialization messages on the recovered session, which include
ACKS for the last message each peer received and secured prior
to the failure.
(13) From the LDP Init exchange, P1 determines that it needs to
re-issue the Label request for L4.
(14) Similarly, P2 determines that it needs to re-issue the Label
Mapping for L2 and the Label Abort.
(15) P2 issues the queued Label Withdraw to P1.
Farrel, et al. [Page 36]
draft-ietf-mpls-ldp-ft-06.txt September 2002
9.2. Use of Check-Pointing With FT Procedures
notes P1 P2
===== == ==
(1) Label Request(L1,27,-)
--------------------------->
Label Request(L2,28,-)
--------------------------->
(2) Label Request(L3,93,-)
<---------------------------
(3) Label Request(L1,123,-)
-------------------------->
Label Request(L2,124,-)
-------------------------->
(4) Label Mapping(L1,57,-)
<--------------------------
Label Mapping(L1,94,-)
<---------------------------
(5) Label Mapping(L2,58,-)
<--------------------------
Label Mapping(L2,95,-)
<---------------------------
(6) Address(n/a,29,-)
--------------------------->
(7) Label Request(L4,30,-)
--------------------------->
(8) Keepalive(n/a,31,-)
--------------------------->
(9) Keepalive(n/a,-,31)
<---------------------------
(10) Keepalive(n/a,59,124)
<---------------------------
(11) Keepalive(n/a,-,59)
--------------------------->
Farrel, et al. [Page 37]
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Notes:
======
Notes (1) through (7) are as in the previous example
except note that no acknowledgements are piggy-backed on
reverse direction messages. This means that at note (8)
there are deferred acknowledgements in both directions on
both links.
(8) P1 wishes to synchronize state with P2. It sends a Keepalive
message containing a FT Protection TLV with sequence number 31.
Since it is not interested in P2's perception of the state that
it has stored, it does not include an FT ACK TLV.
(9) P2 responds at once with a Keepalive acknowledging the sequence
number on the received Keepalive. This tells P1 that P2 has
preserved all state/messages previously received on this
session.
(10) P3 wishes to synchronize state with P2. It sends a Keepalive
message containing a FT Protection TLV with sequence number 59.
P3 also takes this opportunity to get up to date with its
acknowledgements to P2 by including an FT ACK TLV acknowledging
up to sequence number 124.
(11) P2 responds at once with a Keepalive acknowledging the sequence
number on the received Keepalive.
Farrel, et al. [Page 38]
draft-ietf-mpls-ldp-ft-06.txt September 2002
9.3. Temporary Shutdown With FT Procedures
notes P1 P2
===== == ==
(1) Label Request(L1,27,-)
--------------------------->
Label Request(L2,28,-)
--------------------------->
(2) Label Request(L3,93,27)
<---------------------------
(3) Label Request(L1,123,-)
-------------------------->
Label Request(L2,124,-)
-------------------------->
(4) Label Mapping(L1,57,-)
<--------------------------
Label Mapping(L1,94,28)
<---------------------------
(5) Label Mapping(L2,58,-)
<--------------------------
Label Mapping(L2,95,-)
<---------------------------
(6) Address(n/a,29,-)
--------------------------->
(7) Label Request(L4,30,-)
--------------------------->
(8) Keepalive(n/a,-,94)
--------------------------->
(9) Label Abort(L3,96,-)
<---------------------------
(10) Notification(Temporary shutdown)
--------------------------->
===== TCP Session shutdown =====
:
(11) : Label Withdraw(L1,59,-)
: <--------------------------
:
===== TCP Session restored =====
(12) LDP Init(n/a,n/a,94)
--------------------------->
LDP Init(n/a,n/a,29)
<---------------------------
(13) Label Request(L4,30,-)
--------------------------->
(14) Label Mapping(L2,95,-)
<---------------------------
Label Abort(L3,96,30)
<---------------------------
(15) Label Withdraw(L1,97,-)
<---------------------------
Farrel, et al. [Page 39]
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Notes:
======
Notes are as in the previous example except as follows.
(10) P1 needs to upgrade the software or hardware that it is running.
It issues a Notification message to terminate the LDP session,
but sets the status code as 'Temporary shutdown' to inform P2
that this is not a fatal error, and P2 should maintain FT state.
The TCP connection may also fail during the period that the LDP
session is down (in which case it will need to be
re-established), but it is also possible that the TCP connection
will be preserved.
Farrel, et al. [Page 40]
draft-ietf-mpls-ldp-ft-06.txt September 2002
9.4. Temporary Shutdown With FT Procedures and Check-Pointing
notes P1 P2
===== == ==
(1) Label Request(L1,27,-)
--------------------------->
Label Request(L2,28,-)
--------------------------->
(2) Label Request(L3,93,-)
<---------------------------
Label Request(L1,123,-)
-------------------------->
Label Request(L2,124,-)
-------------------------->
Label Mapping(L1,57,-)
<--------------------------
(3) Label Mapping(L1,94,-)
<---------------------------
Label Mapping(L2,58,-)
<--------------------------
Label Mapping(L2,95,-)
<---------------------------
(4) Address(n/a,29,-)
--------------------------->
(5) Label Request(L4,30,-)
--------------------------->
(6) Keepalive(n/a,31,95) * with FT Cork TLV *
--------------------------->
(7) Label Abort(L3,96,-)
<---------------------------
(8) Keepalive(n/a,97,31) * with FT Cork TLV *
<---------------------------
(9) Keepalive(n/a,-,97) * with FT Cork TLV *
--------------------------->
(10) Notification(Temporary shutdown)
--------------------------->
===== TCP Session shutdown =====
:
: Label Withdraw(L1,59,-)
: <--------------------------
:
===== TCP Session restored =====
(11) LDP Init(n/a,n/a,96)
--------------------------->
LDP Init(n/a,n/a,31)
<---------------------------
Label Withdraw(L1,97,-)
<---------------------------
Farrel, et al. [Page 41]
draft-ietf-mpls-ldp-ft-06.txt September 2002
Notes:
======
This example operates much as the previous one. However,
at (1), (2), (3), (4) and (5) no acknowledgements are
made.
At (6), P1 determines that graceful shutdown is required
and sends a Keepalive acknowledging all previously
received messages and itself containing a FT Protection
TLV number and the FT Cork TLV.
The Label abort at (7) crosses with this Keepalive, so at
(8) P2 sends a Keepalive that acknowledges all messages
received so far, but also including the FT Protection and
FT Cork TLVs to indicate that there are still messages
outstanding to be acknowledged.
P1 is then able to complete the 3-way handshake at (9)
and close the TCP session at (10).
Upon recovery at (11) there are no messages to be re-sent
because the KeepAlives flushed the acknowledgements. The
only messages sent after recovery is the Label Withdraw
that was pended during the TCP session failure.
Farrel, et al. [Page 42]
draft-ietf-mpls-ldp-ft-06.txt September 2002
9.5. Checkpointing Without FT Procedures
notes P1 P2
===== == ==
(1) Label Request(L1)
--------------------------->
(2) Label Request(L2)
<---------------------------
Label Request(L1)
-------------------------->
Label Mapping(L1)
<--------------------------
(3) Label Mapping(L1)
<---------------------------
(4) Keepalive(n/a,12,-)
--------------------------->
(5) Label Request(L3)
--------------------------->
(6) Keepalive(n/a,-,12)
<---------------------------
Label Request(L3)
-------------------------->
Label Mapping(L3)
<--------------------------
(7) Label Mapping(L3)
<---------------------------
===== TCP Session failure =====
:
:
:
===== TCP Session restored =====
(8) LDP Init(n/a,n/a,23)
--------------------------->
LDP Init(n/a,n/a,12)
<---------------------------
(9) Label Request(L3)
--------------------------->
Label Request(L3)
-------------------------->
Label Mapping(L3)
<--------------------------
(10) Label Mapping(L3)
<---------------------------
(11) Label Request(L2)
<---------------------------
Farrel, et al. [Page 43]
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Notes:
======
(1), (2) and (3) show label distribution without FT sequence numbers.
(4) A checkpoint request from P1. It carries the sequence number of
the checkpoint request.
(5) P1 immediately starts a new label distribution request.
(6) P2 confirms that it has secured all previous transactions.
(7) The subsequent (un-acknowledged) label distribution completes.
(8) The session fails and is restarted. Initialization messages
confirm the sequence numbers of the secured checkpoints.
(9) P1 recommences the unacknowledged label distribution request.
(10) P2 recommences an unacknowledged label distribution request.
Farrel, et al. [Page 44]
draft-ietf-mpls-ldp-ft-06.txt September 2002
9.6. Graceful Shutdown With Checkpointing But No FT Procedures
notes P1 P2
===== == ==
(1) Label Request(L1)
--------------------------->
(2) Label Request(L2)
<---------------------------
Label Request(L1)
-------------------------->
Label Mapping(L1)
<--------------------------
(3) Label Mapping(L1)
<---------------------------
(4) Keepalive(n/a,12,23) * With Cork TLV *
--------------------------->
(5) :
:
:
(6) Keepalive(n/a,24,12) * With Cork TLV *
<---------------------------
(7) Keepalive(n/a,-,24) * With Cork TLV *
--------------------------->
(8) Notification(Temporary shutdown)
--------------------------->
===== TCP Session failure =====
:
:
:
===== TCP Session restored =====
(9) LDP Init(n/a,n/a,24)
--------------------------->
LDP Init(n/a,n/a,12)
<---------------------------
(10) Label Request(L3)
--------------------------->
Label Request(L3)
-------------------------->
Label Mapping(L3)
<--------------------------
(11) Label Mapping(L3)
<---------------------------
(12) Label Mapping(L2)
--------------------------->
Farrel, et al. [Page 45]
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Notes:
======
(1), (2) and (3) show label distribution without FT sequence numbers.
(4) A checkpoint request from P1. It carries the sequence number of
the checkpoint request and a Cork TLV.
(5) P1 has sent a Cork TLV so quieces.
(6) P2 confirms the checkpoint and continues the three-way handshake
by including a Cork TLV itself.
(7) P1 completes the three-way handshake. All operations have now
been checkpointed and the session is quiesced.
(8) The session is gracefully shut down.
(9) The session recovers and the peers exchange the sequence numbers
of the last secured checkpoints.
(10) P1 starts a new label distribution request.
(11) P1 continues processing a previously received label distribution
request.
10. Security Considerations
The LDP FT enhancements inherit similar security
considerations to those discussed in [RFC3036].
The LDP FT enhancements allow the re-establishment of a
TCP connection between LDP peers without a full re-
exchange of the attributes of established labels, which
renders LSRs that implement the extensions specified in
this draft vulnerable to additional denial-of-service
attacks as follows:
- An intruder may impersonate an LDP peer in order to force
a failure and reconnection of the TCP connection, but where
the intruder does not set the FT Reconnect Flag on re-
connection. This forces all FT labels to be released.
- Similarly, an intruder could set the FT Reconnect Flag on
re-establishment of the TCP session without preserving the
state and resources for FT labels.
- An intruder could intercept the traffic between LDP peers
and override the setting of the FT Label Flag to be set to 0
for all labels.
All of these attacks may be countered by use of an
authentication scheme between LDP peers, such as the MD5-
based scheme outlined in [RFC3036].
Farrel, et al. [Page 46]
draft-ietf-mpls-ldp-ft-06.txt September 2002
Alternative authentication schemes for LDP peers are
outside the scope of this draft, but could be deployed to
provide enhanced security to implementations of LDP and
the LDP FT enhancements.
As with LDP, a security issue may exist if an LDP
implementation continues to use labels after expiration
of the session that first caused them to be used. This
may arise if the upstream LSR detects the session failure
after the downstream LSR has released and re-used the
label. The problem is most obvious with the platform-
wide label space and could result in mis-forwarding of data
to other than intended destinations and it is conceivable
that these behaviors may be deliberately exploited to
either obtain services without authorization or to deny
services to others.
In this draft, the validity of the session may be
extended by the FT Reconnection Timeout, and the session
may be re-established in this period. After the expiry
of the Reconnection Timeout the session must be
considered to have failed and the same security issue
applies as described above.
However, the downstream LSR may declare the session as
failed before the expiration of its Reconnection Timeout.
This increases the period during which the downstream LSR
might reallocate the label while the upstream LSR
continues to transmit data using the old usage of the
label. To reduce this issue, this draft requires that
labels are not re-used until the Reconnection Timeout has
expired.
A further issue might apply if labels were re-used prior
to the expiration of the FT Reconnection Timeout, but
this is forbidden by this draft.
The issue of re-use of labels extends to labels managed through
other mechanisms including direct configuration through management
applications and distribution through other label distribution
protocols. Avoiding this problem may be contrued as an
implementation issue (see below) but failure to acknowledge it could
result in mis-forwarding of data between LSPs established using
some other mechanism and those recovered using the methods
described in this document.
Farrel, et al. [Page 47]
draft-ietf-mpls-ldp-ft-06.txt September 2002
11. Implementation Notes
11.1. FT Recovery Support on Non-FT LSRs
In order to take full advantage of the FT capabilities of
LSRs in the network, it may be that an LSR that does not
itself contain the ability to recover from local hardware
or software faults still needs to support the LDP FT
enhancements described in this draft.
Consider an LSR, P1, that is an LDP peer of a fully Fault
Tolerant LSR, P2. If P2 experiences a fault in the
hardware or software that serves an LDP session between
P1 and P2, it may fail the TCP connection between the
peers. When the connection is recovered, the LSPs/labels
between P1 and P2 can only be recovered if both LSRs were
applying the FT recovery procedures to the LDP session.
11.2. ACK generation logic
FT ACKs SHOULD be returned to the sending LSR as soon as
is practicable in order to avoid building up a large
quantity of unacknowledged state changes at the LSR.
However, immediate one-for-one acknowledgements would
waste bandwidth unnecessarily.
A possible implementation strategy for sending ACKs to FT
LDP messages is as follows:
- An LSR secures received messages in order and tracks the
sequence number of the most recently secured message, Sr.
- On each LDP KeepAlive that the LSR sends, it attaches an
FT ACK TLV listing Sr
- Optionally, the LSR may attach an FT ACK TLV to any other
LDP message sent between Keepalive messages if, for example,
Sr has increased by more than a threshold value since the
last ACK sent.
This implementation combines the bandwidth benefits of
accumulating ACKs while still providing timely ACKs.
11.2.1 Ack Generation Logic When Using Check-Pointing
If check-pointing is in use, the LSRs need not be
concerned to send ACKs in such a timely manner.
Check-points are solicitations for acknowledgement
conveyed as a sequence number in an FT Protection TLV on
a Keepalive message. Such check-point requests could be
issued on a timer, after a significant amount of change,
or before controlled shutdown of a session.
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The use of check-pointing may considerably simplify an
implementation since it does not need to track the
sequence numbers of all received LDP messages. It must,
however, still ensure that all received messages (or the
consequent state changes) are secured before
acknowledging the sequence number on the Keepalive.
This approach may be considered optimal in systems that
do not show a high degree of change over time (such as
targeted LDP sessions) and that are prepared to risk loss
of state for the most recent LDP exchanges. More dynamic
systems (such as LDP discovery sessions) are more likely
to want to acknowledge state changes more frequently so
that the maximum amount of state can be preserved over a
failure.
11.3 Interactions With Other Label Distribution Mechanisms
Many LDP LSRs also run other label distribution mechanisms. These
include management interfaces for configuration of static label
mappings, other distinct instances of LDP, and other label
distribution protocols. The last example includes traffic engineering
label distribution protocol that are used to construct tunnels
through which LDP LSPs are established.
As with re-use of individual labels by LDP within a restarting LDP
system, care must be taken to prevent labels that need to be retained
by a restarting LDP session or protocol component from being used by
another label distribution mechanism since that might compromise
data security amongst other things.
It is a matter for implementations to avoid this issue through the
use of techniques such as a common label management component or
segmented label spaces.
12. Acknowledgments
The work in this draft is based on the LDP ideas
expressed by the authors of [RFC3036].
The ACK scheme used in this draft was inspired by the
proposal by David Ward and John Scudder for restarting
BGP sessions now included in [BGP-RESTART].
The authors would also like to acknowledge the careful
review and comments of Nick Weeds, Piers Finlayson, Tim
Harrison, Duncan Archer, Peter Ashwood-Smith, Bob Thomas,
S.Manikantan, Adam Sheppard, Alan Davey, Iftekhar Hussain
and Loa Andersson.
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13. Intellectual Property Consideration
The IETF has been notified of intellectual property
rights claimed in regard to some or all of the
specification contained in this document. For more
information, consult the online list of claimed rights.
14. Full Copyright Statement
Copyright (c) The Internet Society (2000, 2001, 2002).
All Rights Reserved. This document and translations of it
may be copied and furnished to others, and derivative
works that comment on or otherwise explain it or assist
in its implementation may be prepared, copied, published
and distributed, in whole or in part, without restriction
of any kind, provided that the above copyright notice and
this paragraph are included on all such copies and
derivative works. However, this document itself may not
be modified in any way, such as by removing the copyright
notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose
of developing Internet standards in which case the
procedures for copyrights defined in the Internet
Standards process must be followed, or as required to
translate it into languages other than English.
The limited permissions granted above are perpetual and
will not be revoked by the Internet Society or its
successors or assigns.
This document and the information contained herein is
provided on an "AS IS" basis and THE INTERNET SOCIETY AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIMS ALL
WARRANTIES, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED
TO ANY WARRANTY THAT THE USE OF THE INFORMATION HEREIN
WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
15. IANA Considerations
This draft requires the use of a number of new TLVs and
status codes from the number spaces within the LDP
protocol. This section explains the logic used by the
authors to choose the most appropriate number space for
each new entity, and is intended to assist in the
determination of any final values assigned by IANA or the
MPLS WG in the event that the MPLS WG chooses to advance
this draft on the standards track.
This section will be removed when the TLV and status code
values have been agreed with IANA.
Farrel, et al. [Page 50]
draft-ietf-mpls-ldp-ft-06.txt September 2002
15.1. New TLVs
The FT Protection TLV carries attributes that affect a
single label exchanged between LDP peers. It is taken
from the 0x02xx range for TLVs that is used in [RFC3036]
by other TLVs carrying label attributes. The next
available value in this range is 0x0203.
The FT Session TLV carries attributes that affect the
entire LDP session between LDP peers. It is taken from
the 0x05xx range for TLVs that is used in [RFC3036] by
other TLVs carrying session-wide attributes. The next
available value in this range is 0x0503.
The FT Protection TLV may ACK many label operations at
once if cumulative ACKS are used. It is taken from the
0x05xx range for TLVs that is used in [RFC3036] by other
TLVs carrying session-wide attributes. The next
available value in this range is 0x0504.
The FT Cork TLV carries attributes that apply to all labels
exchanged between LDP peers. It is taken from the 0x05xx range
for TLVs that is used in [RFC3036] by other TLVs carrying label
attributes. The next available value in this range is 0x0505.
In summary:
FT Protection TLV 0x0203
FT Session TLV 0x0503
FT Ack TLV 0x0504
FT Cork TLV 0x0505
15.2. New Status Codes
LDP status codes are not sub-divided into specific ranges
for different types of error. Hence, the numeric status
code values are selected as the next available.
Section 7.1 lists the new status codes required by this document and
gives interpretative information. The new codes are as follows.
Status Code E Status Data
No LDP Session 0 0x0000001A
Zero FT seqnum 1 0x0000001B
Unexpected TLV / 1 0x0000001C
Session Not FT
Unexpected TLV / 1 0x0000001D
Label Not FT
Missing FT Protection TLV 1 0x0000001E
FT ACK sequence error 1 0x0000001F
Temporary Shutdown 0 0x00000020
FT Seq Numbers Exhausted 1 0x00000021
FT Session parameters / 1 0x00000022
changed
Unexpected FT Cork TLV 1 0x00000023
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16. Authors' Addresses
Adrian Farrel (editor) Paul Brittain
Movaz Networks, Inc. Data Connection Ltd.
7926 Jones Branch Drive, Suite 615 Windsor House, Pepper Street,
McLean, VA 22102 Chester, Cheshire
Phone: +1 703-847-1867 CH1 1DF, UK
Email: afarrel@movaz.com Phone: +44-(0)20-8366-1177
Email: pjb@dataconnection.com
Philip Matthews Eric Gray
Hyperchip Celox Networks, Inc.
1800 Rene-Levesque Blvd W 2 Park Central Drive,
Montreal, Quebec H3H 2H2 Southborough, MA 01772
Canada Phone: +1 508 305 7214
Phone: +1 514-906-4965 Email: egray@celoxnetworks.com
Email: pmatthews@hyperchip.com
Jack Shaio Toby Smith
Vivace Networks Laurel Networks, Inc.
2730 Orchard Parkway 1300 Omega Drive
San Jose, CA 95134 Pittsburgh, PA 15205
Phone: +1 408 432 7623 Email: tob@laurelnetworks.com
Email: jack.shaio@vivacenetworks.com
Andrew G. Malis
Vivace Networks
2730 Orchard Parkway
San Jose, CA 95134
Phone: +1 408 383 7223
andy.malis@vivacenetworks.com
17. References
17.1. Normative References
[RFC2026] Bradner, S., "The Internet Standards Process --
Revision 3", BCP 9, RFC 2026, October 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3036] Andersson, L., et. al., LDP Specification, RFC 3036,
January 2001.
[LDP-RESTART] Leelanivas, M., et al., Graceful Restart Mechanism for
LDP, draft-ietf-ldp-restart-05.txt, September 2002,
work in progress.
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draft-ietf-mpls-ldp-ft-06.txt September 2002
17.2. Informative References
[RFC2205] Braden, R., et al., Resource ReSerVation Protocol
(RSVP) -- Version 1, Functional Specification, RFC
2205, September 1997.
[RFC2961] Berger, L., et al., RSVP Refresh Reduction Extensions,
RFC 2961, April 2001.
[RFC3209] Awduche, D., et al,. Extensions to RSVP for LSP
Tunnels, RFC 3209, December 2001.
[RFC3212] Jamoussi, B., et. al., Constraint-Based LSP Setup
using LDP, RFC 3212, January 2002.
[RFC3214] Ash, G., et al., LSP Modification Using CR-LDP, RFC
3214, January 2001.
[BGP-RESTART] Sangli, S., et al., Graceful Restart Mechanism
for BGP, draft-ietf-idr-restart-05.txt, June 2002
(work in progress).
Farrel, et al. [Page 53]
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