Network Working Group Rajiv Asati
Internet Draft Cisco Systems
Intended status: Standards Track
Expires: January 2009 Pradosh Mohapatra
Cisco Systems
Bob Thomas
Cisco Systems
Emily Chen
Huawei Technologies
July 5, 2008
LDP End-of-LIB
draft-ietf-mpls-ldp-end-of-lib-00.txt
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Copyright Notice
Copyright (C) The IETF Trust (2008).
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Abstract
There are situations following LDP session establishment where it
would be useful for an LDP speaker to know when its peer has
advertised all of its labels. These include session establishment
when LDP-IGP sync is in use, as well as session re-establishment
following loss of an LDP session when LDP graceful restart is in use.
The LDP specification [RFC5036] provides no mechanism for an LDP
speaker to notify a peer when it has completed its initial label
advertisements to that peer. This document specifies means for an
LDP speaker to signal completion of its initial label advertisements
following session establishment.
Conventions used in this document
In examples, "C:" and "S:" indicate lines sent by the client and
server respectively.
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].
Table of Contents
1. Introduction...................................................3
2. Specification Language.........................................3
3. Unrecognized Notification Capability...........................4
4. Signaling Completion of Label Advertisement....................4
5. Usage Guidelines...............................................5
5.1. IGP-Sync..................................................6
5.2. LDP Graceful Restart......................................6
5.3. Wildcard Label Request....................................7
5.4. Missing Expected End-of-LIB Notifications.................7
6. Security Considerations........................................7
7. IANA Considerations............................................8
8. Acknowledgments................................................8
9. References.....................................................9
9.1. Normative References......................................9
9.2. Informative References....................................9
Author's Addresses...............................................10
Intellectual Property Statement..................................10
Disclaimer of Validity...........................................11
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1. Introduction
There are situations following LDP session establishment where it
would be useful for an LDP speaker to know when its peer has
advertised all of its labels. For example, when an LDP speaker is
using LDP-IGP synchronization procedures [LDPSync], it would be
useful for the speaker to know when its peer has completed
advertisement of its IP label bindings. Similarly, after an LDP
session is re-established when LDP Graceful Restart [RFC3478] is in
effect, it would be helpful for each peer to signal the other after
it has advertised all its label bindings.
The LDP specification [RFC5036] provides no mechanism for an LDP
speaker to notify a peer when it has completed its initial label
advertisements to that peer.
This document specifies use of a Notification message with the "End-
of-LIB" Status Code for an LDP speaker to signal completion of its
label advertisements following session establishment.
RFC5036 implicitly assumes that new Status Codes will be defined over
the course of time. However, it does not explicitly define the
behavior of an LDP speaker which does not understand the Status Code
in a Notification message. To avoid backward compatibility issues
this document specifies use of the LDP capability mechanism [LDPCap]
at session establishment time for informing a peer that an LDP
speaker is capable of handling a Notification message that carries an
unrecognized Status Code.
Copyright (C) The IETF Trust (2008). This version of this MIB module
is part of RFC XXXX; see the RFC itself for full legal notices.
2. Specification Language
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].
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3. Unrecognized Notification Capability
An LDP speaker MAY include a Capability Parameter [LDPCap] in the
Initialization message to inform a peer that it ignores Notification
Messages that carry a Status TLV with a non-fatal Status Code unknown
to it.
The Capability Parameter for the Unrecognized Notification capability
is a TLV with the 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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|U|F| Unrecog Notif (IANA) | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S| Reserved |
+-+-+-+-+-+-+-+-+
Figure 1 Unrecognized Notification Capability format
Where:
U and F bits: Should be set 1 and 0 respectively as per section 4
of LDP Capabilities [LDPCap].
Unrecog Notif: TLV code point to be assigned by IANA.
S-bit: Must be 1 (indicates that capability is being advertised).
Upon receiving a Notification with an unrecognized Status Code an LDP
speaker MAY generate a console or system log message for trouble
shooting purposes.
4. Signaling Completion of Label Advertisement
An LDP speaker MAY signal completion of its label advertisements to a
peer by means of a Notification message, if its peer had advertised
the Unrecognized Notification capability during session
establishment. The LDP speaker MAY send the Notification message (per
FEC Type) to a peer even if the LDP speaker had no Label bindings to
advertise.
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Such a Notification message MUST carry:
- A status TLV with TLV E- and F-bits set to zero that carries an
"End-of-LIB" Status Code.
- A FEC TLV with the Typed Wildcard FEC Element [TypedWC] that
identifies the FEC type for which initial label advertisements
have been completed. In terms of Section 3.5.1 of RFC5036,
this TLV is an "Optional Parameter" of the Notification
message.
An LDP speaker MUST NOT send a Notification which carries a Status
TLV with the End-of-LIB Status Code to a peer unless the peer had
advertised the Unrecognized Notification capability during session
establishment.
This applies to both non-directed and directed LDP peers.
5. Usage Guidelines
The FECs known to an LDP speaker and the labels the speaker has bound
to those FECs may change over the course of time. This makes
determining when an LDP speaker has advertised "all" of its label
bindings for a given FEC type an issue. Ultimately, this
determination is a judgement call the LDP speaker makes. The
following guidelines may be useful.
An LDP speaker is assumed to "know" a set of FECs. Depending on a
variety of criteria, such as:
- The label distribution control mode in use (Independent or
Ordered);
- The set of FEC's to which the speaker has bound local labels;
- Configuration settings which may constrain which label bindings
the speaker may advertise to peers;
the speaker can determine the set of bindings for a given FEC type
that it is permitted to advertise to a given peer.
IGP-Sync, LDP Graceful Restart, and the response to a Wildcard Label
Request [TypedWC] are situations that would benefit from End-of-LIB
Notification. In these situations, after an LDP speaker completes
its label binding advertisements to a peer, it should send the peer
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an End-of-LIB Notification. The following subsections cover each of
these situations in turn.
5.1. IGP-Sync
LDP-IGP Sync is a mechanism directly connected LDP speakers may use
to delay using the link connecting them for IP traffic until the
labels required to support IP over MPLS traffic on the link have been
learned.
Without an End-of-LIB Notification the speaker must rely on some
heuristic to determine when it has received all of its peer's label
bindings. The heuristic chosen could cause LDP to signal the IGP too
soon in which case the likelihood that traffic will be dropped
increases, or too late in which case traffic is kept on sub-optimal
paths longer than necessary.
Following session establishment with a directly connected peer that
has advertised the Unrecognized Notification capability, an LDP
speaker using LDP-IGP Sync may send the peer an End-of-LIB
Notification after it completes advertisement of its IP label
bindings to the peer. Similarly, the LDP speaker may use the End-of-
LIB Notification received from a directly connected peer to determine
when the peer has completed advertisement of its label bindings for
IP prefixes. After receiving the notification, the speaker should
consider LDP to be fully operational for the link and signal the IGP
to start advertising the link with normal cost.
5.2. LDP Graceful Restart
LDP Graceful Restart helps reduce the loss of MPLS traffic caused by
the restart of a router's LDP component. It defines procedures that
allow routers capable of preserving MPLS forwarding state across the
restart to continue forwarding MPLS traffic for a pre-agreed upon
period using forwarding state installed prior to the restart.
During that period the restarting router and its peers consider the
preserved forwarding state to be usable but stale until it is
refreshed by receipt of new label advertisements following re-
establishment of new LDP sessions. When the period elapses any
remaining stale forwarding state is removed by the router.
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Receipt of the End-of-LIB Notification from a peer in an LDP Graceful
Restart scenario enables an LDP speaker to stop using stale
forwarding information learned from that peer and to recover the
resources it requires without having to wait until the timeout
occurs.
5.3. Wildcard Label Request
When an LDP speaker receives a Label Request message for a Typed
Wildcard FEC (e.g. a particular FEC element type) from a peer it
determines the set of bindings, it is permitted to advertise the peer
for the FEC type specified by the request. Assuming the peer had
advertised the Unrecognized Notification capability at session
initialization time, the speaker should send the peer an End-of-LIB
Notification for the FEC type when it completes advertisement of the
permitted bindings.
As in the previous applications, receipt of the Notification
eliminates uncertainty as to when the peer has completed its
advertisements of label bindings for the requested Wildcard FEC
Element Type.
5.4. Missing Expected End-of-LIB Notifications
There is no guarantee that an LDP speaker will receive End-of-LIB
Notifications from a peer even if the LDP speaker has signaled its
capability. Therefore, an implementation SHOULD NOT depend on the
receipt of such a Notification.
To deal with the possibility of missing notifications, an LDP speaker
may time out receipt of an expected End-of-LIB Notification, and if
the timeout occurs, it may behave as if it had received the
notification. If the End-of-LIB Notification message is received
after the time-out occurs, then the message should be ignored.
6. Security Considerations
No security considerations beyond those that apply to the base LDP
specification and described in [RFC5036] apply to signaling the End-
of-LIB condition as described in this document.
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7. IANA Considerations
This draft introduces a new LDP Status Code and a new LDP Capability
both of which require IANA assignment.
8. Acknowledgments
The authors would like to thank Ina Minei, Alia Atlas, Yakov Rekhter
and Luyuan Fang for their valuable feedback and contribution.
This document was prepared using 2-Word-v2.0.template.dot.
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9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5036] Andersson, L., Doolan, P., Feldman, N., Fredette, A. and
Thomas, B., "LDP Specification", RFC 5036, January 2001.
[LDPCap] Thomas, B., Aggarwal, S., Aggarwal, R., Le Roux, J.L., "LDP
Capabilities", draft-ietf-mpls-ldp-capabilities-02, Work in
Progress, May 2007.
[TypedWC] Thomas, B., Minei, I., "LDP Typed Wildcard FEC", draft-
ietf-mpls-ldp-typed-wildcard-03, Work in Progress, March
2008.
9.2. Informative References
[LDPSync] Jork, M., Atlas, A., Fang, L., "LDP IGP Synchronization",
draft-ietf-mpls-ldp-igp-sync-02, Work in Progress, June
2008.
[RFC3478] Leelanivas, M., Rekhter, Y., Aggarwal, R., "Graceful
Restart Mechanism for Label Distribution Protocol",
February 2003.
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Author's Addresses
Rajiv Asati
Cisco Systems,
7025-6 Kit Creek Rd, RTP, NC, 27709-4987
Email: rajiva@cisco.com
Pradosh Mohapatra
Cisco Systems,
3750 Cisco Way, San Jose, CA, 95134
Email: pmohapat@cisco.com
Bob Thomas
Cisco Systems,
1414 Massachusetts Ave, Boxborough, MA, 01719
Email: rhthomas@cisco.com
Emily Chen
Huawei Technologies
No.5 Street, Shangdi Information, Haidian, Beijing, China
Email: chenying220@huawei.com
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