Network Working Group Yakov Rekhter (Juniper Networks)
Internet Draft Rahul Aggarwal (Juniper Networks)
Expiration Date: August 2004
Graceful Restart Mechanism for BGP with MPLS
draft-ietf-mpls-bgp-mpls-restart-03.txt
Status of this Memo
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Abstract
A mechanism for BGP that would help minimize the negative effects on
routing caused by BGP restart is described in "Graceful Restart
Mechanism for BGP" (see [1]). 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...)
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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.
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 BGP-related state to be preserved across the
restart.
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].
Summary for Sub-IP Area
(This section to be removed before publication.)
0.1 Summary
This document describes a mechanism that helps to minimize the
negative effects on MPLS forwarding caused by LSR's control plane
restart, and specifically by the restart of its BGP component in the
case where BGP is used to carry MPLS labels and LSR is capable of
preserving its MPLS forwarding state across the restart.
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0.2 Related documents
See the Reference Section
0.3 Where does it fit in the Picture of the Sub-IP Work
This work fits squarely in MPLS box.
0.4 Why is it Targeted at this WG
The specifications on carrying MPLS Labels in BGP is a product of the
MPLS WG. This document specifies procedures to minimize the negative
effects on MPLS forwarding caused by the restart of the control plane
BGP module in the case where BGP is used to carry MPLS labels. Since
the procedures described in this document are directly related to
MPLS forwarding and carrying MPLS labels in BGP, it would be logical
to target this document at the MPLS WG.
0.5 Justification
The WG should consider this document, as it allows to minimize the
negative effects on MPLS forwarding caused by the restart of the
control plane BGP module in the case where BGP is used to carry MPLS
labels.
1. Motivation
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 <address prefix -> (outgoing label, next hop)>
mapping. In the context of this document the forwarding state that is
referred to in [1] means MPLS forwarding state.
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
(as specified in [2]), it may be desirable not to perturb the LSPs
going through that LSR (and specifically, the LSPs established by
BGP). In this document, we describe a mechanism that allows to
accomplish this goal. 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.
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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.
2. Assumptions
First of all we assume that an LSR implements the Graceful Restart
Mechanism for BGP, as specified in [1]. Second, we assume that the
LSR is capable of preserving its MPLS forwarding state across the
restart of its control plane (including the restart of BGP).
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 BGP-related state to be preserved 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 needs to preserve across the restart the
information about which protocol has assigned which labels.
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 [2]).
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4. Procedures for the restarting LSR
After the LSR restarts, it follows the procedures as specified in
[1]. In addition, if the LSR is able to preserve its MPLS forwarding
state across the restart, the LSR advertises this to its neighbors by
appropriately setting the Flag field in the Graceful Restart
Capability for all applicable AFI/SAFI pairs.
Once the restarting 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 restarting LSR
performs one of the following:
4.1. Case 1
The following applies when (a) the best route selected by the
restarting 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 restarting 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 <prefix, (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 just picks up some unused 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
restarting LSR was received either without a label, or with an
Implicit NULL label, or the route is originated by the restarting
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
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found, the LSR just picks up some unused label when advertising the
route to its neighbors.
The description in the above paragraph assumes that the restarting
LSR generates the same label for all the routes with the same next
hop. If this is not the case, and the restarting 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 prefix 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 prefix 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 just picks up some unused label when
advertising the route to its neighbors.
4.3. Case 3
The following applies when the restarting LSR does not set BGP Next
Hop to self.
In this case the restarting 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.
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".
The procedures described in this section assume that the restarting
LSR has (at least) as many unallocated as allocated labels. 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.
After the LSR restarts, it follows the procedures as specified in
[1]. In addition, if the LSR is able to preserve its MPLS forwarding
state across the restart, the LSR advertises this to its neighbors by
appropriately setting the Flag field in the Graceful Restart
Capability.
To create local label bindings the LSR uses unallocated labels (this
is pretty much the normal procedure). That means that as long as the
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LSR retains the MPLS forwarding state that the LSR preserved across
the restart, the labels from that state are not used for creating
local label bindings.
The restarting 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 restarting LSR MAY retain the
forwarding state even a bit longer, as to allow the neighbors to
receive and process the routes that have been advertised by the
restarting LSR. After that, the restarting LSR MAY 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).
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,
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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.
7. Security Consideration
The security considerations pertaining to the original BGP protocol
remain relevant.
In addition, the mechanism described here renders LSRs that implement
it 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
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label. To reduce this issue, this document requires that labels are
not re-used until for at least the Restart Time.
8. 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.
9. Copyright Notice
Copyright (C) The Internet Society (date). All Rights Reserved.
This document and translations of it may be copied and furnished to
others, and derivative works that comment on or otherwise explain it
or assist in its implmentation may be prepared, copied, published and
distributed, in whole or in part, without restriction of any kind,
provided that the above copyright notice and this paragraph are
included on all such copies and derivative works. However, this
document itself may not be modified in any way, such as by removing
the copyright notice or references to the Internet Society or other
Internet organizations, except as needed for the purpose of
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developing Internet standards in which case the procedures for
copyrights defined in the Internet Standards process must be
followed, or as required to translate it into languages other than
English.
The limited permissions granted above are perpetual and will not be
revoked by the Internet Society or its successors or assigns.
This document and the information contained herein is provided on an
"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
TASK FORCE DISCLAIMS 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.
10. 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.
11. Normative References
[1] "Graceful Restart Mechanism for BGP", draft-ietf-idr-
restart-01.txt
[2] Rekhter, Y., Rosen, E., "Carrying Label Information in BGP-4",
RFC3107
[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
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12. 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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