LDP end-of-LIB: Signaling LDP Label Advertisement Completion
A way for Label Distribution Protocol peers to say when they have finished sending their labels.
Asati, R., Mohapatra, P., Chen, E., and B. Thomas, "Signaling LDP Label Advertisement Completion", RFC 5919, DOI 10.17487/RFC5919, August 2010, <https://www.rfc-editor.org/info/rfc5919>.
@misc{rfc5919,
series = {Request for Comments},
number = 5919,
howpublished = {RFC 5919},
publisher = {RFC Editor},
doi = {10.17487/RFC5919},
url = {https://www.rfc-editor.org/info/rfc5919},
author = {R. Asati and P. Mohapatra and E. Chen and B. Thomas},
title = {{Signaling LDP Label Advertisement Completion}},
pagetotal = 9,
year = 2010,
month = aug,
} RFC text
Contents
Internet Engineering Task Force (IETF) R. Asati
Request for Comments: 5919 P. Mohapatra
Category: Standards Track Cisco Systems
ISSN: 2070-1721 E. Chen
Huawei Technologies
B. Thomas
August 2010
Signaling LDP Label Advertisement Completion
Abstract
There are situations following Label Distribution Protocol (LDP)
session establishment where it would be useful for an LDP speaker to
know when its peer has advertised all of its labels. The LDP
specification 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.
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5919.
Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the
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Table of Contents
1. Introduction ....................................................3
1.1. Applicability - Label Advertisement Mode ...................3
2. Specification Language ..........................................3
3. Unrecognized Notification Capability ............................4
4. Signaling Completion of Label Advertisement .....................4
4.1. Missing Expected End-of-LIB Notifications ..................5
5. Usage Guidelines ................................................6
5.1. LDP-IGP Sync ...............................................6
5.2. LDP Graceful Restart .......................................7
5.3. Wildcard Label Request .....................................7
6. Security Considerations .........................................8
7. IANA Considerations .............................................8
8. Acknowledgments .................................................8
9. References ......................................................8
9.1. Normative References .......................................8
9.2. Informative References .....................................9
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 the labels from its Label Information Base (LIB).
For example, when an LDP speaker is using LDP-IGP synchronization
procedures [RFC5443], 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.
RFC 5036 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 that does not understand the Status Code
in a Notification message. To avoid backward compatibility issues,
this document specifies use of the LDP capability mechanism [RFC5561]
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.
1.1. Applicability - Label Advertisement Mode
The mechanisms specified in this document are deemed useful to LDP
peering using the 'Downstream Unsolicited' label advertisement mode
[RFC5036]. They are not deemed useful to any LDP peering using the
'Downstream on Demand' label advertisement mode since the LDP speaker
would request particular label binding(s) from the peer anyway and
know when it has received them.
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].
3. Unrecognized Notification Capability
An LDP speaker MAY include a Capability Parameter [RFC5561] in the
Initialization message to inform a peer that it ignores Notification
Messages that carry a Status Type-Length-Value (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| Unrecognized Noti (0x0603)| Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
|S| Reserved |
+-+-+-+-+-+-+-+-+
Figure 1: Unrecognized Notification Capability Format
Where:
U- and F-bits: MUST be 1 and 0, respectively, as per Section 3 of
LDP Capabilities [RFC5561].
Unrecognized Notif: 0x0603
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 that conforms to this specification SHOULD signal
completion of its label advertisements to a peer by means of a
Notification message, if its peer has advertised the Unrecognized
Notification capability during session establishment. The LDP
speaker SHOULD send the Notification message (per Forwarding
Equivalence Class (FEC) Type) to a peer even if the LDP speaker has
zero Label bindings to advertise to that peer.
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 (0x0000002F).
- A FEC TLV with the Typed Wildcard FEC Element [RFC5918] that
identifies the FEC type for which initial label advertisements
have been completed. In terms of Section 3.5.1 of RFC 5036,
this TLV is an "Optional Parameter" of the Notification message.
An LDP speaker MUST NOT send a Notification that carries a Status TLV
with the End-of-LIB Status Code to a peer unless the peer has
advertised the Unrecognized Notification capability during session
establishment.
This applies to any LDP peers discovered via either basic discovery
or extended discovery mechanisms (per Section 2.4 of [RFC5036]).
4.1. Missing Expected End-of-LIB Notifications
There is no guarantee that an LDP speaker will receive (or send) an
End-of-LIB Notification from (or to) a peer even if the LDP speaker
has signaled the Unrecognized Notification capability (Section 3).
Although it is expected that an LDP speaker supporting the
Unrecognized Notification capability would support sending and
receiving an End-of-LIB Notification, it is not mandatory by
definition.
Please note that this is not a concern since the LDP speaker would
simply ignore the received Notification with an End-of-LIB status
code (or any status code) that is not recognized or supported, by
definition.
To deal with the possibility of missing End-of-LIB Notifications
after the LDP session establishment, an LDP speaker MAY time out
receipt of an expected End-of-LIB Notification. An LDP speaker
SHOULD start a per-peer internal timer, called 'EOL Notification'
timer (the default value of 60 seconds is RECOMMENDED, though the
value of this timer SHOULD be configurable) immediately following the
LDP session establishment.
This timer is reset by the subsequent label advertisement, and
stopped by the End-of-LIB Notification message. Lacking any label
advertisement from the peer, the timer would expire, causing the LDP
speaker to behave as if it had received the End-of-LIB notification
from the peer.
If the End-of-LIB Notification message is received after the timer
expires, then the message SHOULD be ignored.
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 it
difficult to determine when an LDP speaker has advertised "all" of
its label bindings for a given FEC type. Ultimately, this
determination is a judgment 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 FECs to which the speaker has bound local labels;
- configuration settings that 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.
LDP-IGP Sync, LDP Graceful Restart, and the response to a Wildcard
Label Request [RFC5918] 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, sending an End-
of-LIB Notification to the peer makes their outcome deterministic.
The following subsections further explain each of these situations
one by one.
5.1. LDP-IGP Sync
The LDP-IGP Synchronization [RFC5443] specifies a mechanism by which
directly connected LDP speakers may delay the use of the link
(between them) for transit IP traffic forwarding until the labels
required to support IP-over-MPLS traffic forwarding have been
distributed and installed.
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 LDP speaker
should consider LDP to be fully operational for the link and should
signal the IGP to start advertising the link with normal cost.
5.2. LDP Graceful Restart
LDP Graceful Restart [RFC3478] helps to 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 using forwarding state installed prior to the restart for a
configured time period.
The current behavior without End-of-LIB Notification is as follows:
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
or until the time period expires. When the time period expires, any
remaining stale forwarding state is removed by the router.
Receiving 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 time period
expiry. The time period expiry can still be used if the End-of-LIB
Notification message is not received.
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, the
LDP speaker determines the set of bindings (as per any local
filtering policy) to advertise to 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.
6. Security Considerations
No security considerations beyond those that apply to the base LDP
specification [RFC5036] and that are further described in [RFC5920]
apply to signaling the End-of-LIB condition as described in this
document.
7. IANA Considerations
This document introduces a new LDP Status Code and a new LDP
Capability.
IANA has assigned the 'End-of-LIB' status code (0x0000002F) from
the Status Code Name Space. [RFC5036] partitions the Status Code
Name Space into 3 regions: IETF Consensus region, First Come First
Served region, and Private Use region. The code point 0x0000002F
is from the IETF Consensus range.
IANA has assigned the 'Unrecognized Notification' capability
(0x0603) from the TLV Type name space. [RFC5036] partitions the
TLV Type name space into 3 regions: IETF Consensus region, Vendor
Private Use region, and Experimental Use region. The code point
0x0603 is from the IETF Consensus range.
8. Acknowledgments
The authors would like to recognize Kamran Raza, who helped to
formulate this draft.
The authors would like to thank Ina Minei, Alia Atlas, Yakov
Rekhter, Loa Andersson, and Luyuan Fang for their valuable
feedback and contributions.
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., Ed., Minei, I., Ed., and B. Thomas, Ed.,
"LDP Specification", RFC 5036, October 2007.
[RFC5561] Thomas, B., Raza, K., Aggarwal, S., Aggarwal, R., and JL.
Le Roux, "LDP Capabilities", RFC 5561, July 2009.
[RFC5918] Asati, R., Minei, I., and B. Thomas, "Label Distribution
Protocol (LDP) 'Typed Wildcard' Forward Equivalence Class
(FEC)", RFC 5918, August 2010.
9.2. Informative References
[RFC3478] Leelanivas, M., Rekhter, Y., and R. Aggarwal, "Graceful
Restart Mechanism for Label Distribution Protocol", RFC
3478, February 2003.
[RFC5443] Jork, M., Atlas, A., and L. Fang, "LDP IGP
Synchronization", RFC 5443, March 2009.
[RFC5920] Fang, L., Ed., "Security Framework for MPLS and GMPLS
Networks", RFC 5920, July 2010.
Authors' Addresses
Rajiv Asati
Cisco Systems
7025-6 Kit Creek Rd.
Research Triangle Park, NC 27709-4987
EMail: rajiva@cisco.com
Pradosh Mohapatra
Cisco Systems
3750 Cisco Way
San Jose, CA 95134
EMail: pmohapat@cisco.com
Emily Chen
Huawei Technologies
No. 5 Street, Shangdi Information, Haidian
Beijing, China
EMail: chenying220@huawei.com
Bob Thomas
EMail: bobthomas@alum.mit.edu
ISO Plain Language
On this page
§1Introduction
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 the labels from its Label Information Base (LIB).
One example is LDP-IGP synchronization, described in RFC 5443, where it would be useful for a speaker to know when its peer has completed advertisement of its IP label bindings. Another is LDP Graceful Restart, described in RFC 3478, where after an LDP session is re-established it would be helpful for each peer to signal the other after it has advertised all its label bindings.
The LDP specification, RFC 5036, 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 the use of a Notification message with the End-of-LIB Status Code for exactly that purpose.
RFC 5036 implicitly assumes that new Status Codes will be defined over time, but it does not explicitly define the behaviour of an LDP speaker that does not understand a Status Code in a Notification message. To avoid backward compatibility problems, this document specifies the use of the LDP capability mechanism from RFC 5561 at session establishment time, so a peer can be told in advance that the sender is able to handle a Notification message carrying an unrecognized Status Code.
MPLS routers swap labels so that traffic can take a fixed path through the network. The two ends of a session both hold labels, and one has no way to say that its part is done. This RFC gives it one.
§1.1When the Mechanism Applies
The mechanisms in this document are deemed useful to LDP peering that uses the Downstream Unsolicited label advertisement mode of RFC 5036. In that mode a router pushes its label bindings to its peer without being asked.
They are not deemed useful to LDP peering that uses the Downstream on Demand label advertisement mode, because in that mode the LDP speaker requests particular label bindings from the peer anyway, and so knows when it has received them. There is nothing left to signal.
The signal only makes sense when a router pushes labels to its peer unasked. When a router asks for a specific label and waits for it, it already knows when the answer arrived.
§3The Unrecognized Notification Capability
An LDP speaker may include a Capability Parameter, defined in RFC 5561, in the Initialization message to inform a peer that it ignores Notification messages that carry a Status Type-Length-Value (TLV) with a non-fatal Status Code unknown to it. That is the Unrecognized Notification capability.
The capability is a TLV with this layout: the U bit, the F bit, a 14-bit Unrecognized Notification field holding the value 0x0603, a 16-bit Length field, and then a byte holding the S bit followed by reserved bits. The U and F bits must be 1 and 0 respectively, as required by Section 3 of LDP Capabilities in RFC 5561. The S bit must be 1, which indicates that the capability is being advertised.
On receiving a Notification with an unrecognized Status Code, an LDP speaker may generate a console or system log message for troubleshooting.
IANA has assigned the Unrecognized Notification capability the code point 0x0603 from the TLV Type name space. RFC 5036 partitions that name space into three regions, and 0x0603 comes from the IETF Consensus range.
Before sending the new message, a router advertises a capability in the opening handshake that says it can safely receive a status code it does not recognise. The capability is a small record with a type code of 0x0603 and a few flag bits.
| Field | Name | Size | Value | Description |
|---|---|---|---|---|
U-BIT | U | 1 bits | Must be 1, as in LDP Capabilities. | |
F-BIT | F | 1 bits | Must be 0, as in LDP Capabilities. | |
UNRECOGNIZED-NOTIF | Unrecognized Notif | 14 bits | Capability code point, 0x0603. | |
LENGTH | Length | 2 bytes | Length of the capability data that follows. | |
S-BIT | S | 1 bits | Must be 1, which indicates that the capability is being advertised. | |
RESERVED | Reserved | 7 bits | Reserved bits of the capability data. |
§4Signalling Completion of Label Advertisement
An LDP speaker that conforms to this specification should signal completion of its label advertisements to a peer by means of a Notification message, if its peer has advertised the Unrecognized Notification capability during session establishment. The speaker should send the Notification message per Forwarding Equivalence Class (FEC) Type even if it has zero label bindings to advertise to that peer.
The Notification message must carry two things. The first is a status TLV, with the TLV E- and F-bits set to zero, that carries the End-of-LIB Status Code, 0x0000002F. The second is a FEC TLV with the Typed Wildcard FEC element from RFC 5918, which identifies the FEC type for which the initial label advertisements have been completed. In the terms of Section 3.5.1 of RFC 5036, the FEC TLV is an optional parameter of the Notification message.
An LDP speaker must not send a Notification carrying the End-of-LIB Status Code to a peer unless that peer has advertised the Unrecognized Notification capability during session establishment. This applies to peers discovered by either the basic discovery or the extended discovery mechanism of Section 2.4 of RFC 5036.
IANA has assigned the End-of-LIB status code the code point 0x0000002F from the Status Code Name Space, which RFC 5036 partitions into an IETF Consensus region, a First Come First Served region and a Private Use region. The code point comes from the IETF Consensus range.
When a router has finished sending the labels it intends to send for one kind of traffic class, it sends a Notification that names the status code 0x0000002F and the traffic class it has finished. It may do this even when it had no labels to send at all.
- InitializationSpeaker A offers the Unrecognized Notification capability 0x0603.
- InitializationSpeaker B does the same in return.
- Label advertisementsSpeaker A advertises its label bindings for a FEC type.
- End-of-LIB NotificationSpeaker A says its initial advertisements for that FEC type are complete.
- End-of-LIB NotificationSpeaker B does the same for its own FEC type.
§4.1When the Notification Does Not Arrive
There is no guarantee that an LDP speaker will receive, or send, an End-of-LIB Notification from or to a peer even if the speaker has signalled the Unrecognized Notification capability. Although it is expected that a speaker supporting that capability would support sending and receiving an End-of-LIB Notification, it is not mandatory by definition. The RFC notes that this is not a concern, because a speaker would simply ignore a received Notification carrying an End-of-LIB status code, or any status code, that it does not recognise or support.
To deal with the possibility of a missing End-of-LIB Notification after session establishment, an LDP speaker may time out the receipt of an expected notification. A speaker should start a per-peer internal timer, called the EOL Notification timer, immediately following the LDP session establishment. The default value of 60 seconds is recommended, though the value should be configurable.
The timer is reset by each subsequent label advertisement and stopped by the End-of-LIB Notification message. Lacking any label advertisement from the peer, the timer would expire, causing the speaker to behave as if it had received the End-of-LIB notification from the peer. If the End-of-LIB Notification message is received after the timer has expired, the message should be ignored.
A peer may simply never send the message, so each router also runs a 60 second timer per peer. The timer restarts every time a label arrives and stops when the end-of-LIB message arrives. If it runs out, the router assumes the peer finished, as if the message had arrived.
§5Where the Notification Helps
The FECs known to an LDP speaker, and the labels the speaker has bound to those FECs, may change over time. That makes it hard to determine when a speaker has advertised all of its label bindings for a given FEC type, and in the end the determination is a judgment call the speaker makes. The speaker is assumed to know a set of FECs, depending on the label distribution control mode in use, Independent or Ordered, on the set of FECs to which it has bound local labels, and on configuration settings that may constrain which label bindings it may advertise to peers. From those it can work out the set of bindings for a given FEC type that it is permitted to advertise to a given peer.
Three situations benefit from the End-of-LIB Notification: LDP-IGP Sync, LDP Graceful Restart, and the response to a Wildcard Label Request from RFC 5918. In each of them, sending the notification after the speaker has completed its label binding advertisements makes the outcome deterministic instead of a guess.
LDP-IGP Sync. The synchronization mechanism in RFC 5443 lets directly connected LDP speakers delay the use of the link between them for transit IP traffic forwarding until the labels needed for IP-over-MPLS forwarding have been distributed and installed. Without the End-of-LIB Notification the speaker must rely on a heuristic to decide when it has received all of its peer's label bindings, and the heuristic chosen could cause LDP to signal the IGP too soon, which raises the likelihood that traffic is dropped, or too late, which keeps traffic on sub-optimal paths for longer than necessary. 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.
LDP Graceful Restart. Graceful Restart helps to reduce the loss of MPLS traffic caused by the restart of a router's LDP component. Without the End-of-LIB Notification, the restarting router and its peers consider the preserved forwarding state to be usable but stale until it is refreshed by new label advertisements following the re-establishment of LDP sessions, or until the configured time period expires, at which point any remaining stale forwarding state is removed. Receiving the notification lets a speaker stop using stale forwarding information learned from that peer and recover the resources it requires without waiting for the time period to expire. The time period expiry can still be used if the notification is not received.
Wildcard Label Request. When a speaker receives a Label Request message for a Typed Wildcard FEC, such as a particular FEC Element Type, it determines the set of bindings, according to any local filtering policy, to advertise for the FEC type named in the request. Assuming the peer advertised the Unrecognized Notification capability at session initialization, the speaker should send the peer an End-of-LIB Notification for that FEC type when it has completed advertisement of the permitted bindings. Receipt of the notification then removes the uncertainty about when the peer finished its advertisements for the requested Wildcard FEC Element Type.
Three things need to know when a peer has finished: keeping a link out of service until its labels are installed, clearing stale forwarding state after a router restart, and answering a request for every label of one kind. In all three cases the message replaces a guess with a fact.
- LDP-IGP SyncA link stays out of service for transit traffic until the labels it needs are installed, instead of a heuristic guess.
- LDP Graceful RestartStale forwarding state learned from a peer can be dropped as soon as the peer says it has re-advertised everything.
- Wildcard Label RequestA peer that asked for every label of one kind learns when the complete set has been sent.
§6Security Considerations
No security considerations beyond those that apply to the base LDP specification in RFC 5036, and that are further described in the security framework for MPLS and GMPLS networks in RFC 5920, apply to signalling the End-of-LIB condition.
This RFC adds no security rules of its own. What applies to LDP generally applies here, because the new message travels over the same session as everything else.