Network Working Group Bob Thomas
Internet Draft Cisco Systems, Inc.
Intended Status: Proposed Standard
Expiration Date: September 05, 2009 S. Aggarwal
Juniper Networks
R. Aggarwal
Juniper Networks
J.L. Le Roux
France Telecom
Syed Kamran Raza
Cisco Systems, Inc.
March 06, 2009
LDP Capabilities
draft-ietf-mpls-ldp-capabilities-03.txt
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Abstract
A number of enhancements to the Label Distribution Protocol (LDP)
have been proposed. Some have been implemented, and some are
advancing toward standardization. It is likely that additional
enhancements will be proposed in the future. At present, LDP has no
mechanism for advertising such enhancements at LDP session
initialization time. There is also no mechanism to enable and
disable enhancements after the session is established. This document
defines a mechanism for advertising LDP enhancements at session
initialization time, as well as a mechanism to enable and disable
enhancements after LDP session establishment.
Conventions used in this document
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 [RFC2119].
This document uses the terms "LDP speaker" and "speaker"
interchangably.
Table of Contents
1. Introduction...................................................3
2. The LDP Capability Mechanism...................................4
2.1. Capability Document.......................................5
3. Specifying Capabilities in LDP Messages........................5
3.1. Backward Compatibility TLVs...............................7
4. Capability Message.............................................7
5. Note on Terminology............................................8
6. Procedures for Capability Parameters in Initialization Messages8
7. Procedures for Capability Parameters in Capability Messages....9
8. Extensions to Error Handling..................................10
9. Dynamic Capability Announcement TLV...........................10
10. Backward Compatibility.......................................11
11. Security Considerations......................................11
12. IANA Considerations..........................................11
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13. Acknowledgments..............................................12
14. References...................................................12
14.1. Normative References....................................12
14.2. Informative References..................................12
15. Author's Addresses...........................................13
1. Introduction
A number of enhancements to LDP as specified in [RFC5036] have been
proposed. These include LDP Graceful Restart [RFC3478], Fault
Tolerant LDP [RFC3479], multicast extensions [MLDP], signaling for
layer 2 circuits [RFC4447], a method for learning labels advertised
by next-next-hop routers in support of fast reroute node protection
[NNHOP], upstream label allocation [UPSTREAM_LDP], and extensions for
signaling inter-area LSPs [IALDP]. Some have been implemented, and
some are advancing toward standardization. It is also likely that
additional enhancements will be implemented and deployed in the
future.
At present, LDP does not have a mechanism for advertising such
enhancements at LDP session initialization time. There is also no
mechanism to enable and disable these enhancements after the session
is established.
This document proposes and defines a mechanism for advertising LDP
enhancements at session initialization time. It also defines a
mechanism to enable and disable these enhancements after LDP session
establishment.
LDP capability advertisement provides means for an LDP speaker to
announce what it can receive and process. It also provides means for
a speaker to inform peers of deviations from behavior specified by
[RFC5036]. An example of such a deviation is LDP graceful restart
where a speaker retains MPLS forwarding state for LDP-signaled LSPs
when its LDP control plane goes down. It is important to point out
that not all LDP enhancements require capability advertisement. For
example, upstream label allocation does but inbound label filtering,
where a speaker installs forwarding state for only certain FECs, does
not.
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2. The LDP Capability Mechanism
Enhancements are likely to be announced during LDP session
establishment as each LDP speaker advertises capabilities
corresponding to the enhancements it desires.
Beyond that, capability advertisements may be used to dynamically
modify the characteristics of the session to suit the changing
conditions. For example, an LSR capable of a particular enhancement
in support of some "feature" may not have advertised the
corresponding capability to its peers at session establishment time
because the feature was disabled at that time. Later, an operator
may enable the feature, at which time the LSR would react by
advertising the corresponding capability to its peers. Similarly,
when an operator disables a feature associated with a capability, the
LSR reacts by withdrawing the capability advertisement from its
peers.
The LDP capability advertisement mechanism operates as follows:
- Each LDP speaker is assumed to implement a set of enhancements,
each of which has an associated capability. At any time, a
speaker may have none, one, or more of those enhancements
"enabled". When an enhancement is enabled, the speaker
advertises the associated capability to its peers. By
advertising the capability to a peer, the speaker asserts that it
shall perform the protocol actions specified for the associated
enhancement. For example, the actions may require the LDP speaker
to receive and process enhancement-specific messages from its
peer. Unless the capability has been advertised, the speaker will
not perform protocol actions specified for the corresponding
enhancement.
- At session establishment time an LDP speaker MAY advertise a
particular capability by including an optional parameter
associated with the capability in its Initialization message.
- There is a well-known capability called "Dynamic Capability
Announcement" which an LDP speaker MAY advertise in its
Initialization message to indicate that it is capable of
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processing capability announcements following a session
establishment.
If a peer had advertised the "Dynamic Capability Announcement"
capability in its Initialization message, then at any time
following session establishment an LDP speaker MAY announce
changes in its advertised capabilities to that peer. To do this,
the LDP speaker sends the peer a "Capability" message that
specifies the capabilities being advertised or withdrawn.
2.1. Capability Document
When the capability advertisement mechanism is in place, an LDP
enhancement requiring LDP capability advertisement will be specified
by a document that:
- Describes the motivation for the enhancement;
- Specifies the behavior of LDP when the enhancement is enabled.
This includes the procedures, parameters, messages, and TLVs
required by the enhancement;
- Includes an IANA considerations section that requests IANA for
assignment of code point for the optional parameter corresponding
to the enhancement.
3. Specifying Capabilities in LDP Messages
This document uses the term "Capability Parameter" to refer to an
optional parameter that may be included in Initialization and
Capability messages to advertise a capability.
The format of "Capability Parameter" TLV is:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| TLV Code Point | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S| Reserved | |
+-+-+-+-+-+-+-+-+ Capability Data |
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| +-+-+-+-+-+-+-+-+
| |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where:
U-bit:
The value could be either 0 or 1 as specified in Capability
document associated with given capability.
F-bit:
MUST be 0 (i.e. don't forward if not understood).
TLV Code Point:
The TLV type which identifies a specific capability. The "IANA
Considerations" section of [RFC5036] specifies the assignment of
code points for LDP TLVs.
S-bit:
The State Bit indicates whether the sender is advertising or
withdrawing the capability corresponding to the TLV Code Point.
The State bit is used as follows:
1 - The TLV is advertising the capability specified by the
TLV Code Point.
0 - The TLV is withdrawing the capability specified by the
TLV Code Point.
Capability Data:
Information, if any, about the capability in addition to the TLV
Code Point required to fully specify the capability.
The method for interpreting and processing this data is specific
to the TLV Code Point and MUST be described in the document
specifying the capability.
An LDP speaker MUST NOT include more than one instance of a
Capability Parameter (as identified by the same TLV code point) in an
Initialization or Capability message. If an LDP speaker receives more
than one instance of the same Capability Parameter type in a message,
it SHOULD send a Notification message to peer before terminating the
session with peer. The Status Code in the Status TLV of the
Notification message MUST be "Malformed TLV" and the message SHOULD
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contain the second "Capability Parameter TLV" of the same type (Code
point) that is received in the message.
3.1. Backward Compatibility TLVs
Few LDP protocol extensions have been made in past to advertise and
negotiate some capability or extension at session establishment time.
These extensions usually define a new TLV which is directly included
in an Initialization message. One example of such extension is "FT
Session TLV" which is exchanged in Initialization message between
peers to announce LDP Fault Tolerance [RFC3479] capability. To
ensure backward compatibility with existing implementations, such
TLVs play the role of a "Capability Parameter" when included in
Initialization messages, and this document refers to such TLVs as
"Backward Compatibility TLVs".
4. Capability Message
The LDP Capability message is used by an LDP speaker to announce
changes in the state of one or more of its capabilities subsequent to
session establishment.
The format of the Capability message is:
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| Capability (IANA) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Message ID |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV_1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| . . . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TLV_N |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
where TLV_1 through TLV_N are "Capability Parameter" TLVs. The S-bit
of each of the TLVs specifies the new state for the corresponding
capability.
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Note that Backward Compatibility TLVs (see Section 3.1) MUST NOT be
included in Capability messages.
5. Note on Terminology
The following sections in this document talk about enabling and
disabling capabilities. The terminology "enabling (or disabling) a
capability" is short hand for "advertising (or withdrawing) a
capability associated with an enhancement". Bear in mind that it is
an LDP enhancement that is being enabled or disabled, and that it is
the corresponding capability that is being advertisted or withdrawn.
6. Procedures for Capability Parameters in Initialization Messages
The S-bit of a Capability Parameter in an Initialization message MUST
be 1 and SHOULD be ignored on receipt. This ensures that any
Capability Parameter in an Initialization message enables the
corresponding capability.
An LDP speaker determines the capabilities of a peer by examining the
set of of Capability Parameters present in the Initialization message
received from the peer.
An LDP speaker MAY use a particular capability with its peer after
the speaker determines that the peer has enabled that capability.
These procedures enable an LDP speaker S1, that advertises a specific
LDP capability C, to establish an LDP session with speaker S2 that
does not advertise C. In this situation whether or not capability C
may be used for the session depends on the semantics of the
enhancement associated with C. If the semantics do not require both
S1 and S2 advertise C to one another, then S2 could use it; i.e. S1's
advertisement of C permits S2 to send messages to S1 used by the
enhancement.
It is the responsibility of the capability designer to specify the
behavior of an LDP speaker that has enabled a certain enhancement,
advertised its capability and determines that its peer has not
advertised the corresponding capability. The document specifying
procedures for the capability MUST describe the behavior in this
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situation. If the specified procedure is to terminate the session,
then the LDP speaker SHOULD send a Notification message to the peer
before terminating the session. The Status Code in the Status TLV of
the Notification message MUST be "Unsupported Capability" and the
message SHOULD contain the unsupported capability (see Section 8 for
more details).
An LDP speaker that supports capability advertisement and includes a
Capability Parameter in its Initialization message MUST set the TLV
U-bit to 0 or 1, as specified by "Capability" document. LDP speaker
should set U-bit to 1 if the capability document allows to continue
with a peer that does not understand the enhancement, and set U-bit
to 0 otherwise. If a speaker receives a message containng unsupported
capability, it responds according to U-bit setting in the TLV. If U-
bit is 1, then speaker MUST silently ignore the Capability Parameter
and allow the session to be established. However, if U-bit is 0, then
speaker SHOULD send a Notification message to the peer before
terminating the session. The Status Code in the Status TLV of the
Notification message MUST be "Unsupported Capability" and the
message SHOULD contain the unsupported capability (see Section 8 for
more details).
7. Procedures for Capability Parameters in Capability Messages
An LDP speaker MUST NOT send a Capability message to a peer unless
its peer had advertised the Dynamic Capability Announcement
capability in its session Initialization message. An LDP speaker MAY
send a Capability message to a peer if its peer had advertised the
Dynamic Capability Announcement capability in its session
Initialization message (see Section 9).
An LDP speaker determines the capabilities enabled by a peer by
determining the set of capabilities enabled at session initialization
(as specified in Section 6) and tracking changes to that set made
by Capability messages from the peer.
An LDP speaker that has enabled a particular capability MAY use the
enhancement corresponding to the capability with a peer after the
speaker determines that the peer has enabled the capability.
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8. Extensions to Error Handling
This document defines a new LDP status code named "Unsupported
Capability". The E-bit of the Status TLV carried in a Notification
message that includes this status code MUST be set to 0.
In addition, this document defines a new LDP TLV, named "Returned
TLVs" that MAY be carried in a Notification message. The U-bit
setting for a Returned TLVs TLV in a Notification message SHOULD be 1
and the F-bit setting SHOULD be 0.
When the Status Code in a Notification message is "Unsupported
Capability", the message SHOULD specify the capabilities that are
unsupported. When the Notification message specifies the unsupported
capabilities, it MUST include a Returned TLVs TLV. The Returned TLVs
TLV MUST include only the Capability Parameters for unsupported
capabilities, and the Capability Parameter for each such capability
SHOULD be encoded as received from the peer.
When the Status Code in a Notification Message is "Unknown TLV", the
message SHOULD specify the TLV that was unknown. When the
Notification message specifies the TLV that was unknown, it MUST
include the unknown TLV in a Returned TLVs TLV.
9. Dynamic Capability Announcement TLV
The Dynamic Capability Announcement TLV is a Capability Parameter
defined by this document with following format:
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| DynCap Announcement (IANA)| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|1| Reserved |
+-+-+-+-+-+-+-+-+
The Dynamic Capability Announcement Parameter MAY be included by an
LDP speaker in an Initialization message to signal its peer that the
speaker is capable of processing Capability messages.
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An LDP speaker MUST NOT include the Dynamic Capability Announcement
Parameter in Capability messages sent to its peers. Once enabled
during session initialization, the Dynamic Capability Announcement
capability cannot be disabled. This implies that S-bit is always 1
for Dynamic Capability Announcement.
An LDP speaker that receives a Capability message from a peer that
includes the Dynamic Capability Announcement Parameter SHOULD
silently ignore the parameter and process any other Capability
Parameters in the message.
10. Backward Compatibility
From the point of view of the LDP capability advertisement mechanism,
an [RFC5036] compliant peer has label distribution for IPv4 enabled
by default. To ensure compatibility with an [RFC5036] compliant
peer, LDP implementations that support capability advertisement have
label distribution for IPv4 enabled until it is explicitly disabled
and MUST assume that their peers do as well.
Section 3.1 identifies a set of Backward Compatibility TLVs that
may appear in Initialization messages in the role of a Capability
Parameter. This permits existing LDP enhancements that use an ad hoc
mechanism for enabling capabilities at sesssion initialization time
to continue to do so.
11. Security Considerations
The security considerations described in [RFC5036] that apply to the
base LDP specification apply to the capability mechanism described in
this document.
12. IANA Considerations
This document specifies the following which require code points
assigned by IANA:
- LDP message code point for the Capability message. The authors
request message type 0x0202 for the Capability message.
- LDP TLV code point for the Dynamic Capability Announcemnt TLV.
The authors request TLV type code 0x0506.
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- LDP TLV code point for the Returned TLVs TLV. The authors
request TLV type 0x304.
- LDP Status Code code point for the Unsupported Capability Status
Code. The authors request Status Code 0x0000002C.
13. Acknowledgments
The authors wish to thank Enke Chen, Vanson Lim, Ina Minei, Bin Mo,
Yakov Rekhter, and Eric Rosen for their comments.
This document was prepared using 2-Word-v2.0.template.dot.
14. References
14.1. Normative References
[RFC5036] Andersson, L., Menei, I., and Thomas, B., Editors, "LDP
Specification", RFC 5036, September 2007.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC2119, March 1997.
[RFC3479] Farrel, A., Editor, "Fault Tolerance for the Label
Distribution Protocol (LDP)", RFC 3479, February 2003.
14.2. Informative References
[IALDP] Decraene, B., Le Roux, JL., Minei, I, "LDP Extensions for
Inter-Area LSPs", draft-decraene-mpls-ldp-interarea-04.txt,
Work in Progress, March 2007
[MLDP] Minei, I., Wijnamds, I., Editors, "Label Distribution
Protocol Extensions for Point-to-Multipoint and Multipoint-
to-Multipoint Label Switched Paths", draft-minei-wijnands-
mpls-ldp-p2mp-00.txt, Work in Progress, September 2005
[NNHOP] Shen, N., Chen, E., Tian, A. "Discovery LDP Next-Nexthop
Labels", draft-shen-mpls-ldp-nnhop-label-02.txt, Work in
Progress, May 2005
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[RFC4447] L. Martini, Editor, E. Rosen, El-Aawar, T. Smith, G. Heron,
"Pseudowire Setup and Maintenance using the Label
Distribution Protocol", RFC 4447, April 2006.
[RFC3478] Leelanivas, M., Rekhter, Y, Aggarwal, R., "Graceful Restart
Mechanism for Label Distribution Protocol (LDP)", RFC 3478,
February 2003.
[UPSTREAM_LDP] Aggarwal R., Le Roux, J.L., "MPLS Upstream Label
Assignment for LDP" draft-ietf-mpls-ldp-upstream-00.txt,
Work in Progress, February 2006.
15. Author's Addresses
Bob Thomas
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxborough, MA 01719
E-mail: rhthomas@cisco.com
Shivani Aggarwal
Juniper Networks
1194 North Mathilda Ave.
Sunnyvale, CA 94089
Email: shivani@juniper.net
Rahul Aggarwal
Juniper Networks
1194 North Mathilda Ave.
Sunnyvale, CA 94089
Email: rahul@juniper.net
Jean-Louis Le Roux
France Telecom
2, Avenue Pierre-Marzin
22307 Lannion Cedex, France
E-mail: jeanlouis.leroux@orange-ftgroup.com
Syed Kamran Raza
Cisco Systems, Inc.
2000 Innovation Dr.
Kanata, ON K2K-3E8, Canada
E-mail: skraza@cisco.com
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