Internet DRAFT - draft-ietf-mpls-bgp-mpls-restart
draft-ietf-mpls-bgp-mpls-restart
Network Working Group Yakov Rekhter (Juniper Networks)
Internet Draft Rahul Aggarwal (Juniper Networks)
Expiration Date: February 2006
Graceful Restart Mechanism for BGP with MPLS
draft-ietf-mpls-bgp-mpls-restart-05.txt
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Internet Draft draft-ietf-mpls-bgp-mpls-restart-05.txt August 2005
Abstract
A mechanism for BGP that helps minimize the negative effects on
routing caused by BGP restart has already been developed an is
described in a separate document ("Graceful Restart Mechanism for
BGP"). This document extends this mechanism to also 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 famililies that could be carried
in BGP (e.g., IPv4, IPv6, etc...)
The mechanism described in this document is applicable to all LSRs,
both those with the ability to preserve their forwarding state during
BGP 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.
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 [RFC2119].
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1. Introduction
For the sake of brevity in the context of this document by "MPLS
forwarding state" we mean either <incoming label -> (outgoing label,
next hop)>, or <Forwarding Equivalence Class (FEC) -> (outgoing
label, next hop)>, or <incoming label -> label pop, next hop>, or
<incoming label, label pop> mapping. In the context of this document
the forwarding state that is referred to in [1] means MPLS forwarding
state, as defined above. In the context of this document the term
"next hop" refers to the next hop as advertised in BGP.
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 [RFC3107]), it may be desirable not to perturb
the LSPs going through that LSR (and specifically, the LSPs
established by BGP) after failure of 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 [1].
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 (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
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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2. General requirements
First of all an LSR MUST implement the Graceful Restart Mechanism for
BGP, as specified in [1]. 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 can not 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 just by the BGP component of the control plane, but by other
protocol components as well (e.g., LDP, RSVP-TE), and 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.
After the LSR restarts, it MUST follow the procedures as specified in
[1]. 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.
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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 [1]. The Subsequent Address Family Identifier (SAFI)
in the advertised capability MUST indicate that the Network Layer
Reachability Information (NLRI) field carries not just addressing
Information, but labels as well (see [RFC3107] as 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 here, 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
"Procedures for the Restarting Speaker" of [1]), then in addition to
the procedures specified in [1], 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
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).
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4.2. Case 2
The following applies when (a) the best route selected by the LSR was
received either without a label, or 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 just
<incoming label, (outgoing label, next hop)> mapping, but also the
Forwarding Equivalence Class (FEC) associated with this mapping. In
such case the LSR would search its MPLS forwarding state for an entry
that (a) indicates Label pop (means no outgoing label), (b) 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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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 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), 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. Both of these bindings though would have the
same outgoing label (and the same next hop).
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6. Procedures for a neighbor of a restarting LSR
The neighbor of a restarting LSR (the receiving router in terminology
used in [1]) follows the procedures specified in [1]. In addition,
the neighbor treats the MPLS labels received from the restarting LSR
the same way as it treats the routes received from the restarting LSR
(both prior and after the restart).
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 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
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 relative to the procedures described in
Section 5.
Procedures described in Section 5 requires twice as many labels as
the procedures 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
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Section 4 for some AFI/SAFI(s) and procedures described in Section 5
for other AFI/SAFI(s).
8. Security Consideration
The security considerations pertaining to the original BGP protocol
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, but where the intruder
sets the Forwarding State (F) bit (as defined in [1]) 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 [RFC2385].
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 mis-routing of 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 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 for at least the Restart Time.
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9. Intellectual Property Considerations
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights 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; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat 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 implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
10. Copyright Notice
Copyright (C) The Internet Society (2005).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR IMPLIED,
INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
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11. Acknowledgments
We would like to thank Chaitanya Kodeboyina and Loa Andersson for
their review and comments. The approach described in Section
"Alternative procedures for the restarting LSR" is based on the idea
suggested by Manoj Leelanivas.
12. Normative References
[1] "Graceful Restart Mechanism for BGP", work in progress
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119
[RFC2385] Heffernan, A., "Protection of BGP Sessions via the TCP MD5
Signature Option", RFC2385
[RFC2026] Bradner, S., "The Internet Standards Process -- Revision
3", RFC2026
13. Non-normative References
[RFC3107] Rekhter, Y., Rosen, E., "Carrying Label Information in
BGP-4", RFC3107
14. Author Information
Yakov Rekhter
Juniper Networks
1194 N.Mathilda Ave
Sunnyvale, CA 94089
e-mail: yakov@juniper.net
Rahul Aggarwal
Juniper Networks
1194 N.Mathilda Ave
Sunnyvale, CA 94089
e-mail: rahul@juniper.net
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