RFC3478 - Graceful Restart Mechanism for Label Distribution Protocol

Network Working Group M. Leelanivas

Request for Comments: 3478 Y. Rekhter

Category: Standards Track Juniper Networks

R. Aggarwal

Redback Networks

February 2003

Graceful Restart Mechanism for Label Distribution Protocol

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

Abstract

This document describes a mechanism that helps to minimize the

negative effects on MPLS traffic caused by Label Switching Router's

(LSR's) control plane restart, specifically by the restart of its

Label Distribution Protocol (LDP) component, on LSRs that are capable

of preserving the MPLS forwarding component across the restart.

The mechanism described in this document is applicable to all LSRs,

both those with the ability to preserve forwarding state during LDP

restart and those without (although the latter needs to implement

only a subset of the mechanism described in this document).

Supporting (a subset of) the mechanism described here by the LSRs

that can not preserve their MPLS forwarding state across the restart

would not redUCe the negative impact on MPLS traffic caused by their

control plane restart, but it would minimize the impact if their

neighbor(s) are capable of preserving the forwarding state across the

restart of their control plane and implement the mechanism described

here.

The mechanism makes minimalistic assumptions on what has to be

preserved across restart - the mechanism assumes that only the actual

MPLS forwarding state has to be preserved; the mechanism does not

require any of the LDP-related states to be preserved across the

restart.

The procedures described in this document apply to downstream

unsolicited label distribution. Extending these procedures to

downstream on demand label distribution is for further study.

Specification of Requirements

The key Words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this

document are to be interpreted as described in BCP 14, RFC2119

[RFC2119].

1. Motivation

For the sake of brevity in the context of this document, by "the

control plane" we mean "the LDP component of the control plane".

For the sake of brevity in the context of this document, by "MPLS

forwarding state" we mean either <incoming label -> (outgoing label,

next hop)> (non-ingress case), or <FEC->(outgoing label, next hop)>

(ingress case) mapping.

In the case where a Label Switching Router (LSR) could preserve its

MPLS forwarding state across restart of its control plane,

specifically its LDP component [LDP], it is desirable not to perturb

the LSPs going through that LSR (specifically, the LSPs established

by LDP). In this document, we describe a mechanism, termed "LDP

Graceful Restart", that allows the accomplishment of this goal.

The mechanism described in this document is applicable to all LSRs,

both those with the ability to preserve forwarding state during LDP

restart and those without (although the latter need to implement only

a subset of the mechanism described in this document). Supporting (a

subset of) the mechanism described here by the LSRs that can not

preserve their MPLS forwarding state across the restart would not

reduce the negative impact on MPLS traffic caused by their control

plane restart, but it would minimize the impact if their neighbor(s)

are capable of preserving the forwarding state across the restart of

their control plane and implement the mechanism described here.

The mechanism makes minimalistic assumptions on what has to be

preserved across restart - the mechanism assumes that only the actual

MPLS forwarding state has to be preserved. Clearly this is the

minimum amount of state that has to be preserved across the restart

in order not to perturb the LSPs traversing a restarting LSR. The

mechanism does not require any of the LDP-related states to be

preserved across the restart.

In the scenario where label binding on an LSR is created/maintained

not just by the LDP component of the control plane, but by other

protocol components as well (e.g., BGP, RSVP-TE), and the LSR

supports restart of the individual components of the control plane

that create/maintain label binding (e.g., restart of LDP, but no

restart of BGP), the LSR needs to preserve across the restart the

information about which protocol has assigned which labels.

The procedures described in this document apply to downstream

unsolicited label distribution. Extending these procedures to

downstream on demand label distribution is for further study.

2. LDP Extension

An LSR indicates that it is capable of supporting LDP Graceful

Restart, as defined in this document, by including the Fault Tolerant

(FT) Session TLV as an Optional Parameter in the LDP Initialization

message. The format of the FT Session TLV is defined in [FT-LDP].

The L (Learn from Network) flag MUST be set to 1, which indicates

that the procedures in this document are used. The rest of the FT

flags are set to 0 by a sender and ignored on receipt.

The value field of the FT Session TLV contains two components that

are used by the mechanisms defined in this document: FT Reconnect

Timeout, and Recovery Time.

The FT Reconnect Timeout is the time (in milliseconds) that the

sender of the TLV would like the receiver of that TLV to wait after

the receiver detects the failure of LDP communication with the

sender. While waiting, the receiver SHOULD retain the MPLS

forwarding state for the (already established) LSPs that traverse a

link between the sender and the receiver. The FT Reconnect Timeout

should be long enough to allow the restart of the control plane of

the sender of the TLV, and specifically its LDP component to bring it

to the state where the sender could exchange LDP messages with its

neighbors.

Setting the FT Reconnect Timeout to 0 indicates that the sender of

the TLV will not preserve its forwarding state across the restart,

yet the sender supports the procedures, defined in Section 3.3,

"Restart of LDP communication with a neighbor LSR" of this document,

and therefore could take advantage if its neighbor to preserve its

forwarding state across the restart.

For a restarting LSR, the Recovery Time carries the time (in

milliseconds) the LSR is willing to retain its MPLS forwarding state

that it preserved across the restart. The time is from the moment

the LSR sends the Initialization message that carries the FT Session

TLV after restart. Setting this time to 0 indicates that the MPLS

forwarding state was not preserved across the restart (or even if it

was preserved, is no longer available).

The Recovery Time SHOULD be long enough to allow the neighboring

LSR's to re-sync all the LSP's in a graceful manner, without creating

congestion in the LDP control plane.

3. Operations

An LSR that supports functionality described in this document

advertises this to its LDP neighbors by carrying the FT Session TLV

in the LDP Initialization message.

This document assumes that in certain situations, as specified in

section 3.1.2, "Egress LSR", in addition to the MPLS forwarding

state, an LSR can also preserve its IP forwarding state across the

restart. Procedures for preserving an IP forwarding state across the

restart are defined in [OSPF-RESTART], [ISIS-RESTART], and [BGP-

RESTART].

3.1. Procedures for the restarting LSR

After an LSR restarts its control plane, the LSR MUST check whether

it was able to preserve its MPLS forwarding state from prior to the

restart. If not, then the LSR sets the Recovery Time to 0 in the FT

Session TLV the LSR sends to its neighbors.

If the forwarding state has been preserved, then the LSR starts its

internal timer, called MPLS Forwarding State Holding timer (the value

of that timer SHOULD be configurable), and marks all the MPLS

forwarding state entries as "stale". At the eXPiration of the timer,

all the entries still marked as stale SHOULD be deleted. The value

of the Recovery Time advertised in the FT Session TLV is set to the

(current) value of the timer at the point in which the Initialization

message carrying the FT Session TLV is sent.

We say that an LSR is in the process of restarting when the MPLS

Forwarding State Holding timer is not expired. Once the timer

expires, we say that the LSR completed its restart.

The following procedures apply when an LSR is in the process of

restarting.

3.1.1. Non-egress LSR

If the label carried in the newly received Mapping message is not an

Implicit NULL, the LSR searches its MPLS forwarding state for an

entry with the outgoing label equal to the label carried in the

message, and the next hop equal to one of the addresses (next hops)

received in the Address message from the peer. If such an entry is

found, the LSR no longer marks the entry as stale. In addition, if

the entry is of type <incoming label, (outgoing label, next hop)>

(rather than <FEC, (outgoing label, next hop)>), the LSR associates

the incoming label from that entry with the FEC received in the Label

Mapping message, and advertises (via LDP) <incoming label, FEC> to

its neighbors. If the found entry has no incoming label, or if no

entry is found, the LSR follows the normal LDP procedures. (Note

that this paragraph describes the scenario where the restarting LSR

is neither the egress, nor the penultimate hop that uses penultimate

hop popping for a particular LSP. Note also that this paragraph

covers the case where the restarting LSR is the ingress.)

If the label carried in the Mapping message is an Implicit NULL

label, the LSR searches its MPLS forwarding state for an entry that

indicates Label pop (means no outgoing label), and the next hop equal

to one of the addresses (next hops) received in the Address message

from the peer. If such an entry is found, the LSR no longer marks

the entry as stale, the LSR associates the incoming label from that

entry with the FEC received in the Label Mapping message from the

neighbor, and advertises (via LDP) <incoming label, FEC> to its

neighbors. If the found entry has no incoming label, or if no entry

is found, the LSR follows the normal LDP procedures. (Note that this

paragraph describes the scenario where the restarting LSR is a

penultimate hop for a particular LSP, and this LSP uses penultimate

hop popping.)

The description in the above paragraph assumes that the restarting

LSR generates the same label for all the LSPs that terminate on the

same LSR (different from the restarting LSR), and for which the

restarting LSR is a penultimate hop. If this is not the case, and

the restarting LSR generates a unique label per each such LSP, then

the LSR needs to preserve across the restart, not just the <incoming

label, (outgoing label, next hop)> mapping, but also the FEC

associated with this mapping. In such case, the LSR searches its

MPLS forwarding state for an entry that (a) indicates Label pop

(means no outgoing label), (b) indicates the next hop equal to one of

the addresses (next hops) received in the Address message from the

peer, and (c) has the same FEC as the one received in the Label

Mapping message. If such an entry is found, the LSR no longer marks

the entry as stale, the LSR associates the incoming label from that

entry with the FEC received in the Label Mapping message from the

neighbor, and advertises (via LDP) <incoming label, FEC> to its

neighbors. If the found entry has no incoming label, or if no entry

is found, the LSR follows the normal LDP procedures.

3.1.2. Egress LSR

If an LSR determines that it is an egress for a particular FEC, the

LSR is configured to generate a non-NULL label for that FEC, and that

the LSR is configured to generate the same (non-NULL) label for all

the FECs that share the same next hop and for which the LSR is an

egress, the LSR searches its MPLS forwarding state for an entry that

indicates Label pop (means no outgoing label), and the next hop equal

to the next hop for that FEC. (Determining the next hop for the FEC

depends on the type of the FEC. For example, when the FEC is an IP

address prefix, the next hop for that FEC is determined from the IP

forwarding table.) If such an entry is found, the LSR no longer

marks this entry as stale, the LSR associates the incoming label from

that entry with the FEC, and advertises (via LDP) <incoming label,

FEC> to its neighbors. If the found entry has no incoming label, or

if no entry is found, the LSR follows the normal LDP procedures.

If an LSR determines that it is an egress for a particular FEC, the

LSR is configured to generate a non-NULL label for that FEC, and that

the LSR is configured to generate a unique label for each such FEC,

then the LSR needs to preserve across the restart, not just the

<incoming label, (outgoing label, next hop)> mapping, but also the

FEC associated with this mapping. In such case, the LSR would search

its MPLS forwarding state for an entry that indicates Label pop

(means no outgoing label), and the next hop equal to the next hop for

that FEC associated with the entry (Determining the next hop for the

FEC depends on the type of the FEC. For example, when the FEC is an

IP address prefix, the next hop for that FEC is determined from the

IP forwarding table.) If such an entry is found, the LSR no longer

marks this entry as stale, the LSR associates the incoming label from

that entry with the FEC, and advertises (via LDP) <incoming label,

FEC> to its neighbors. If the found entry has no incoming label, or

if no entry is found, the LSR follows the normal LDP procedures.

If an LSR determines that it is an egress for a particular FEC, and

the LSR is configured to generate a NULL (either Explicit or

Implicit) label for that FEC, the LSR just advertises (via LDP) such

label (together with the FEC) to its neighbors.

3.2. Alternative procedures for the restarting LSR

In this section we describe an alternative to the procedures

described in Section 3.1, "Procedures for the restarting LSR".

The procedures described in this section assumes that the restarting

LSR has (at least) as many unallocated as allocated labels. The

latter form the MPLS forwarding state that the LSR managed to

preserve across the restart.

After an LSR restarts its control plane, the LSR MUST check whether

it was able to preserve its MPLS forwarding state from prior to the

restart. If no, then the LSR sets the Recovery Time to 0 in the FT

Session TLV the LSR sends to its neighbors.

If the forwarding state has been preserved, then the LSR starts its

internal timer, called MPLS Forwarding State Holding timer (the value

of that timer SHOULD be configurable), and marks all the MPLS

forwarding state entries as "stale". At the expiration of the timer,

all the entries still marked as stale SHOULD be deleted. The value

of the Recovery Time advertised in the FT Session TLV is set to the

(current) value of the timer at the point when the Initialization

message carrying the FT Session TLV is sent.

We say that an LSR is in the process of restarting when the MPLS

Forwarding State Holding timer is not expired. Once the timer

expires, we say that the LSR completed its restart.

While an LSR is in the process of restarting, the LSR creates local

label binding by following the normal LDP procedures.

Note that while an LSR is in the process of restarting, the LSR may

have not one, but two local label bindings for a given FEC - one that

was retained from prior to restart, and another that was created

after the restart. Once the LSR completes its restart, the former

will be deleted. Both of these bindings though would have the same

outgoing label (and the same next hop).

3.3. Restart of LDP communication with a neighbor LSR

When an LSR detects that its LDP session with a neighbor went down,

and the LSR knows that the neighbor is capable of preserving its MPLS

forwarding state across the restart (as was indicated by the FT

Session TLV in the Initialization message received from the

neighbor), the LSR retains the label-FEC bindings received via that

session (rather than discarding the bindings), but marks them as

"stale".

After detecting that the LDP session with the neighbor went down, the

LSR tries to re-establish LDP communication with the neighbor

following the usual LDP procedures.

The amount of time the LSR keeps its stale label-FEC bindings is set

to the lesser of the FT Reconnect Timeout, as was advertised by the

neighbor, and a local timer, called the Neighbor Liveness Timer. If

within that time the LSR still does not establish an LDP session with

the neighbor, all the stale bindings SHOULD be deleted. The Neighbor

Liveness Timer is started when the LSR detects that its LDP session

with the neighbor went down. The value of the Neighbor Liveness

timer SHOULD be configurable.

If the LSR re-establishes an LDP session with the neighbor within the

lesser of the FT Reconnect Timeout and the Neighbor Liveness Timer,

and the LSR determines that the neighbor was not able to preserve its

MPLS forwarding state, the LSR SHOULD immediately delete all the

stale label-FEC bindings received from that neighbor. If the LSR

determines that the neighbor was able to preserve its MPLS forwarding

state (as was indicated by the non-zero Recovery Time advertised by

the neighbor), the LSR SHOULD further keep the stale label-FEC

bindings, received from the neighbor, for as long as the lesser of

the Recovery Time advertised by the neighbor, and a local

configurable value, called Maximum Recovery Time, allows.

The LSR SHOULD try to complete the exchange of its label mapping

information with the neighbor within 1/2 of the Recovery Time, as

specified in the FT Session TLV received from the neighbor.

The LSR handles the Label Mapping messages received from the neighbor

by following the normal LDP procedures, except that (a) it treats the

stale entries in its Label Information Base (LIB) as if these entries

have been received over the (newly established) session, (b) if the

label-FEC binding carried in the message is the same as the one that

is present in the LIB, but is marked as stale, the LIB entry is no

longer marked as stale, and (c) if for the FEC in the label-FEC

binding carried in the message there is already a label-FEC binding

in the LIB that is marked as stale, and the label in the LIB binding

is different from the label carried in the message, the LSR just

updates the LIB entry with the new label.

An LSR, once it creates a <label, FEC> binding, SHOULD keep the value

of the label in this binding for as long as the LSR has a route to

the FEC in the binding. If the route to the FEC disappears, and then

re-appears again later, this may result in using a different label

value, as when the route re-appears, the LSR would create a new

<label, FEC> binding.

To minimize the potential mis-routing caused by the label change when

creating a new <label, FEC> binding, the LSR SHOULD pick up the least

recently used label. Once an LSR releases a label, the LSR SHOULD

NOT re-use this label for advertising a <label, FEC> binding to a

neighbor that supports graceful restart for at least the sum of the

FT Reconnect Timeout plus Recovery Time, as advertised by the

neighbor to the LSR.

4. Security Consideration

The security considerations pertaining to the original LDP protocol

[RFC3036] remain relevant.

In addition, LSRs that implement the mechanism described here are

subject to 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 sets the Recovery Time to 0 on reconnection. This forces

all labels received from the peer to be released.

An intruder could intercept the traffic between LDP peers and

override the setting of the Recovery Time to be set to 0. This

forces all labels received from the peer to be released.

All of these attacks may be countered by use of an authentication

scheme between LDP peers, such as the MD5-based scheme outlined in

[LDP].

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-routing data to other than intended

destinations, and it is conceivable that these behaviors may be

deliberately exploited to either oBTain services without

authorization or to deny services to others.

In this document, the validity of the session may be extended by the

Reconnect Timeout, and the session may be re-established in this

period. After the expiry of the Reconnection Timeout, the session

must be considered to have failed and the same security issue applies

as described above.

However, the downstream LSR may declare the session as failed before

the expiration of its Reconnection Timeout. This increases the

period during which the downstream LSR might reallocate the label

while the upstream LSR continues to transmit data using the old usage

of the label. To reduce this issue, this document requires that

labels not be re-used until at least the sum of Reconnect Timeout

plus Recovery Time.

5. Intellectual Property Considerations

This section is taken from Section 10.4 of [RFC2026].

The IETF takes no position regarding the validity or scope of any

intellectual property or other rights that might be claimed to

pertain to the implementation or use of the technology described in

this document or the extent to which any license under such rights

might or might not be available; neither does it represent that it

has made any effort to identify any such rights. Information on the

IETF's procedures with respect to rights in standards-track and

standards-related documentation can be found in BCP-11. Copies of

claims of rights made available for publication and any assurances of

licenses to be made available, or the result of an attempt made to

obtain a general license or permission for the use of such

proprietary rights by implementors or users of this specification can

be obtained from the IETF Secretariat.

The IETF invites any interested party to bring to its attention any

copyrights, patents or patent applications, or other proprietary

rights which may cover technology that may be required to practice

this standard. Please address the information to the IETF Executive

Director.

The IETF has been notified of intellectual property rights claimed in

regard to some or all of the specification contained in this

document. For more information consult the online list of claimed

rights.

6. Acknowledgments

We would like to thank Loa Andersson, Chaitanya Kodeboyina, Ina

Minei, Nischal Sheth, Enke Chen, and Adrian Farrel for their

contributions to this document.

7. Normative References

[LDP] Andersson, L., Doolan, P., Feldman, N., Fredette, A.

and B. Thomas, "Label Distribution Protocol", RFC

3036, January 2001.

[FT-LDP] Farrel, A., "Fault Tolerance for the Label

Distribution Protocol (LDP)", RFC3479, February 2003.

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate

Requirement Levels", BCP 14, RFC2119, March 1997.

[RFC2026] Bradner, S., "The Internet Standards Process --

Revision 3", BCP 9, RFC2026, October 1996.

8. Informative References

[OSPF-RESTART] "Hitless OSPF Restart", Work in Progress.

[ISIS-RESTART] "Restart signaling for ISIS", Work in Progress.

[BGP-RESTART] "Graceful Restart Mechanism for BGP", Work in

Progress.

9. Authors' Addresses

Manoj Leelanivas

Juniper Networks

1194 N. Mathilda Ave

Sunnyvale, CA 94089

EMail: manoj@juniper.net

Yakov Rekhter

Juniper Networks

1194 N. Mathilda Ave

Sunnyvale, CA 94089

EMail: yakov@juniper.net

Rahul Aggarwal

Redback Networks

350 Holger Way

San Jose, CA 95134

EMail: rahul@redback.com

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