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IETF RFC 4781
Graceful Restart Mechanism for BGP with MPLS
Last modified on Tuesday, January 16th, 2007
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Network Working Group Y. Rekhter
Request for Comments: 4781 R. Aggarwal
Category: Standards Track Juniper Networks
January 2007
Graceful Restart Mechanism for BGP with MPLS
 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.
Copyright Notice
Copyright © The Internet Society (2007).
Abstract
A mechanism for BGP that helps minimize the negative effects on
routing caused by BGP restart has already been developed and is
described in a separate document ("Graceful Restart Mechanism for
BGP"). This document extends this mechanism to minimize the negative
effects on MPLS forwarding caused by the Label Switching Router's
(LSR's) control plane restart, and specifically by the restart of its
BGP component when BGP is used to carry MPLS labels and the LSR is
capable of preserving the MPLS forwarding state across the restart.
The mechanism described in this document is agnostic with respect to
the types of the addresses carried in the BGP Network Layer
Reachability Information (NLRI) field. As such, it works in
conjunction with any of the address families that could be carried in
BGP (e.g., IPv4, IPv6, etc.).
Rekhter & Aggarwal Standards Track PAGE 1 
RFC 4781 Graceful Restart Mechanism for BGP January 2007
Table of Contents
1. Introduction ....................................................2
1.1. Specification of Requirements ..............................3
2. General Requirements ............................................3
3. Capability Advertisement ........................................4
4. Procedures for the Restarting LSR ...............................4
4.1. Case 1 .....................................................4
4.2. Case 2 .....................................................5
4.3. Case 3 .....................................................5
5. Alternative Procedures for the Restarting LSR ...................6
6. Procedures for a Neighbor of a Restarting LSR ...................6
7. Comparison between Alternative Procedures for the
Restarting LSR ..................................................7
8. Security Considerations .........................................8
9. Acknowledgments .................................................9
10. References .....................................................9
10.1. Normative References ......................................9
10.2. Informative References ....................................9
1. Introduction
In the case where a Label Switching Router (LSR) could preserve its
MPLS forwarding state across restart of its control plane, and
specifically its BGP component, and BGP is used to carry MPLS labels
(e.g., as specified in [RFC 3107]), it may be desirable not to perturb
the LSPs going through that LSR (and specifically, the LSPs
established by BGP) after failure or restart of the BGP component of
the control plane. In this document, we describe a mechanism that
allows this goal to be accomplished. The mechanism described in this
document works in conjunction with the mechanism specified in
[RFC 4724]. The mechanism described in this document places no
restrictions on the types of addresses (address families) that it can
support.
The mechanism described in this document is applicable to all LSRs,
both those with the ability to preserve forwarding state during BGP
restart and those without it (although the latter need to implement
only a subset of this mechanism). Supporting a subset of the
mechanism described here by the LSRs that cannot preserve their MPLS
forwarding state across the restart would not reduce the negative
impact on MPLS traffic caused by their control plane restart.
However, the impact would be minimized if their neighbor(s) are
capable of preserving the forwarding state across the restart of
their control plane, and if they implement the mechanism described
here. The subset includes all the procedures described in this
document, except the procedures in Sections 4.1, 4.2, 4.3, and 5.
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RFC 4781 Graceful Restart Mechanism for BGP January 2007
For the sake of brevity, by "MPLS forwarding state" we mean one of
the following mappings:
<incoming label -> (outgoing label, next hop)>
<Forwarding Equivalence Class (FEC) -> (outgoing label, next hop)>
<incoming label -> label pop, next hop>
<incoming label, label pop>
In the context of this document, the forwarding state that is
referred to in [RFC 4724] means MPLS forwarding state, as defined
above. The term "next hop" refers to the next hop as advertised in
BGP.
1.1. 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 RFC 2119 [RFC 2119].
2. General Requirements
First of all, an LSR MUST implement the Graceful Restart Mechanism
for BGP, as specified in [RFC 4724]. Second, the LSR SHOULD be
capable of preserving its MPLS forwarding state across the restart of
its control plane (including the restart of BGP). Third, for the
<Forwarding Equivalence Class (FEC) -> label> bindings distributed
via BGP, the LSR SHOULD be able either (a) to reconstruct the same
bindings as the LSR had prior to the restart (see Section 4), or (b)
to create new <FEC -> label> bindings after restart, while
temporarily maintaining MPLS forwarding state corresponding to both
the bindings prior to the restart, as well as to the newly created
bindings (see Section 5). Fourth, as long as the LSR retains the
MPLS forwarding state that the LSR preserved across the restart, the
labels from that state cannot be used to create new local label
bindings (but could be used to reconstruct the existing bindings, as
per procedures in Section 4). Finally, for each next hop, if the
next hop is reachable via a Label Switched Path (LSP), then the
restarting LSR MUST be able to preserve the MPLS forwarding state
associated with that LSP across the restart.
In the scenario where label binding on an LSR is created/maintained
not only by the BGP component of the control plane, but also by other
protocol components (e.g., LDP, RSVP-TE), and where the LSR supports
restart of the individual components of the control plane that
create/maintain label binding (e.g., restart of BGP, but no restart
of LDP), the LSR MUST be able to preserve across the restart the
information about which protocol has assigned which labels.
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RFC 4781 Graceful Restart Mechanism for BGP January 2007
After the LSR restarts, it MUST follow the procedures as specified in
[RFC 4724]. In addition, if the LSR is able to preserve its MPLS
forwarding state across the restart, the LSR SHOULD advertise this to
its neighbors by appropriately setting the Flag for Address Family
field in the Graceful Restart Capability for all applicable AFI/SAFI
pairs.
3. Capability Advertisement
An LSR that supports the mechanism described in this document
advertises this to its peer by using the Graceful Restart Capability,
as specified in [RFC 4724]. The Subsequent Address Family Identifier
(SAFI) in the advertised capability MUST indicate that the Network
Layer Reachability Information (NLRI) field carries not only
addressing Information, but also labels (see [RFC 3107] for an example
of where NLRI carries labels).
4. Procedures for the Restarting LSR
Procedures in this section apply when a restarting LSR is able to
reconstruct the same <FEC -> label> bindings as the LSR had prior to
the restart.
The procedures described in this section are conceptual and do not
have to be implemented precisely as described, as long as the
implementations support the described functionality and their
externally visible behavior is the same.
Once the LSR completes its route selection (as specified in Section
4.1, "Procedures for the Restarting Speaker", of [RFC 4724]), then in
addition to the those procedures, the LSR performs one of the
following:
4.1. Case 1
The following applies when (a) the best route selected by the LSR was
received with a label, (b) that label is not an Implicit NULL, and
(c) the LSR advertises this route with itself as the next hop.
In this case, the LSR searches its MPLS forwarding state (the one
preserved across the restart) for an entry with <outgoing label, next
hop> equal to the one in the received route. 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 <Forwarding Equivalence Class (FEC), (outgoing label,
next hop)>, the LSR uses the incoming label from the entry when
advertising the route to its neighbors. If the found entry has no
incoming label, or if no such entry is found, the LSR allocates a new
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RFC 4781 Graceful Restart Mechanism for BGP January 2007
label when advertising the route to its neighbors (assuming that
there are neighbors to which the LSR has to advertise the route with
a label).
4.2. Case 2
The following applies when (a) the best route selected by the LSR was
received either without a label, with an Implicit NULL label, or the
route is originated by the LSR; (b) the LSR advertises this route
with itself as the next hop; and (c) the LSR has to generate a (non-
Implicit NULL) label for the route.
In this case, the LSR searches its MPLS forwarding state for an entry
that indicates that the LSR has to perform label pop, and the next
hop equal to the next hop of the route in consideration. If such an
entry is found, then the LSR uses the incoming label from the entry
when advertising the route to its neighbors. If no such entry is
found, the LSR allocates a new label when advertising the route to
its neighbors.
The description in the above paragraph assumes that the LSR generates
the same label for all the routes with the same next hop. If this is
not the case and the LSR generates a unique label per each such
route, then the LSR needs to preserve across the restart not only
<incoming label, (outgoing label, next hop)> mapping, but also the
Forwarding Equivalence Class (FEC) associated with this mapping. In
such a case the LSR would search its MPLS forwarding state for an
entry that (a) indicates label pop (means no outgoing label), (b)
indicates that the next hop equal to the next hop of the route, and
(c) has the same FEC as the route. If such an entry is found, then
the LSR uses the incoming label from the entry when advertising the
route to its neighbors. If no such entry is found, the LSR allocates
a new label when advertising the route to its neighbors.
4.3. Case 3
The following applies when the LSR does not set BGP next hop to self.
In this case, the LSR, when advertising its best route for a
particular NLRI, just uses the label that was received with that
route. And if the route was received with no label, the LSR
advertises the route with no label as well. Either way, the LSR does
not allocate a label for that route.
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RFC 4781 Graceful Restart Mechanism for BGP January 2007
5. Alternative Procedures for the Restarting LSR
In this section, we describe an alternative to the procedures
described in Section "Procedures for the restarting LSR".
Procedures in this section apply when a restarting LSR does not
reconstruct the same <FEC -> label> bindings as the LSR had prior to
the restart, but instead creates new <FEC -> label> bindings after
restart, while temporarily maintaining MPLS forwarding state
corresponding to both the bindings prior to the restart, as well as
to the newly created bindings.
The procedures described in this section require that for the use by
BGP graceful restart, the LSR SHOULD have (at least) as many
unallocated labels as labels allocated for the <FEC -> label>
bindings distributed by BGP. The latter forms the MPLS forwarding
state that the LSR managed to preserve across the restart. The
former is used for allocating labels after the restart.
To create (new) local label bindings after the restart, the LSR uses
unallocated labels (this is pretty much the normal procedure).
The LSR SHOULD retain the MPLS forwarding state that the LSR
preserved across the restart at least until the LSR sends an
End-of-RIB marker to all of its neighbors (by that time the LSR
already completed its route selection process, and also advertised
its Adj-RIB-Out to its neighbors). The LSR MAY retain the forwarding
state even a bit longer (the amount of extra time MAY be controlled
by configuration on the LSR), so as to allow the neighbors to receive
and process the routes that have been advertised by the LSR. After
that, the LSR SHOULD delete the MPLS forwarding state that it
preserved across the restart.
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 BGP route --
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. However, both of these bindings would have
the same outgoing label (and the same next hop).
6. Procedures for a Neighbor of a Restarting LSR
The neighbor of a restarting LSR (the receiving router terminology
used in [RFC 4724]) follows the procedures specified in [RFC 4724]. In
addition, the neighbor treats the MPLS labels received from the
restarting LSR the same way that it treats the routes received from
the restarting LSR (both prior and after the restart).
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RFC 4781 Graceful Restart Mechanism for BGP January 2007
Replacing the stale routes by the routing updates received from the
restarting LSR involves replacing/updating the appropriate MPLS
labels.
In addition, if the Flags in the Graceful Restart Capability received
from the restarting LSR indicate that the LSR wasn't able to retain
its MPLS state across the restart, the neighbor SHOULD immediately
remove all the NLRI and the associated MPLS labels that it previously
acquired via BGP from the restarting LSR.
An LSR, once it creates a binding between a label and a Forwarding
Equivalence Class (FEC), 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,
then 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 misrouting 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 SHALL NOT
re-use this label for advertising a <label, FEC> binding to a
neighbor that supports graceful restart for at least the Restart
Time, as advertised by the neighbor to the LSR. This rule SHALL
apply to any label release at any time.
7. Comparison between Alternative Procedures for the Restarting LSR
Procedures described in Section 4 involve more computational overhead
on the restarting router than do the procedures described in Section
5.
Procedures described in Section 5 require twice as many labels as
those described in Section 4.
Procedures described in Section 4 cause fewer changes to the MPLS
forwarding state in the neighbors of the restarting router than the
procedures described in Section 5.
In principle, it is possible for an LSR to use procedures described
in Section 4 for some AFI/SAFI(s) and procedures described in Section
5 for other AFI/SAFI(s).
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RFC 4781 Graceful Restart Mechanism for BGP January 2007
8. Security Considerations
The security considerations pertaining to the BGP protocol [RFC 4271]
remain relevant.
In addition, the mechanism described here renders LSRs that implement
it vulnerable to additional denial-of-service attacks as follows:
An intruder may impersonate a BGP peer in order to force a failure
and reconnection of the TCP connection, where the intruder sets
the Forwarding State (F) bit (as defined in [RFC 4724]) to 0 on
reconnection. This forces all labels received from the peer to be
released.
An intruder could intercept the traffic between BGP peers and
override the setting of the Forwarding State (F) bit 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 BGP peers, such as the scheme outlined in [RFC 2385].
As with BGP carrying labels, a security issue may exist if a BGP
implementation continues to use labels after expiration of the BGP
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 misrouting of data to
destinations other than those intended; and it is conceivable that
these behaviors may be deliberately exploited, either to obtain
services without authorization or to deny services to others.
In this document, the validity of the BGP session may be extended by
the Restart Time, and the session may be re-established in this
period. After the expiry of the Restart Time, 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 Restart Time. 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 are
not re-used until at least the Restart Time.
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RFC 4781 Graceful Restart Mechanism for BGP January 2007
9. Acknowledgments
We would like to thank Chaitanya Kodeboyina and Loa Andersson for
their review and comments. The approach described in Section 5 is
based on the idea suggested by Manoj Leelanivas.
10. References
10.1. Normative References
[RFC 2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC 2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC 2385, August 1998.
[RFC 4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC 4724] Sangli, S., Chen, E., Fernando, R., Scudder, J., and Y.
Rekhter, "Graceful Restart Mechanism for BGP", RFC 4724,
January 2007.
10.2. Informative References
[RFC 3107] Rekhter, Y. and E. Rosen, "Carrying Label Information in
BGP-4", RFC 3107, May 2001.
Authors' Addresses
Yakov Rekhter
Juniper Networks
1194 N.Mathilda Ave
Sunnyvale, CA 94089
EMail: yakov@juniper.net
Rahul Aggarwal
Juniper Networks
1194 N.Mathilda Ave
Sunnyvale, CA 94089
EMail: rahul@juniper.net
Rekhter & Aggarwal Standards Track PAGE 9
RFC 4781 Graceful Restart Mechanism for BGP January 2007
Full Copyright Statement
Copyright © The IETF Trust (2007).
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contained in BCP 78, and except as set forth therein, the authors
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Rekhter & Aggarwal Standards Track PAGE 10
Graceful Restart Mechanism for BGP with MPLS
RFC TOTAL SIZE: 23249 bytes
PUBLICATION DATE: Tuesday, January 16th, 2007
LEGAL RIGHTS: The IETF Trust (see BCP 78)
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