One document matched: draft-dickson-idr-last-resort-03.txt
Differences from draft-dickson-idr-last-resort-02.txt
idr B. Dickson
Internet-Draft Afilias Canada, Inc
Expires: December 15, 2008 June 13, 2008
A New BGP Well-Known Community, LAST_RESORT
draft-dickson-idr-last-resort-03
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Abstract
This Internet Draft describes a new well-known BGP community,
LAST_RESORT.
This community provides a simple and easily deployable solution to
BGP "wedgies".
Initial deployment is expected to be achieved by voluntary use in the
network operator community-at-large.
Long-term adoption via software enforcement of the well-known aspect
of the community, will improve global behavior, and simplify router
configurations.
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Author's Note
This Internet Draft is intended to result in this draft or a related
draft(s) being placed on the Standards Track for idr.
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].
Intended Status: Proposed Standard.
1. Background
Even when all the best current practises are observed, operational
problems may be experienced when running a BGP network.
One particularly thorny problem is BGP "wedgies" [RFC4264].
While not often articulated, the common problem is use of local
policy setting within AS boundaries, which often overrides the
original intent of "backup" BGP announcements.
It is somewhat ironic that such local policies are often achieved by
use of BGP Communities, and that the lack of a common choice, i.e.
"well known", community, is one source of the problem.
1.1. The Local Policy Problem
The BGP "wedgie" problem occurs as a result of "intended policy"
information not being available. Specifically, this is the lack of
an explicit global mechanism for expressing de-preferring
announcements via "back-up" providers. In essence, local policy
could be described as "not well informed".
2. Proposed Change: A New BGP Well-Known Community
To solve the problem, a new BGP well-known Community, LAST_RESORT, is
proposed. This is a new value to be assigned by IANA.
3. Changes to BGP Behavior
A BGP speaker receiving a prefix from an EBGP neighbor with
LAST_RESORT MUST assignment the lowest-possible preference value of
LOCAL_PREFERENCE. This LOCAL_PREFERENCE MUST NOT be able to be over-
ridden without first removing the LAST_RESORT community from the
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prefix. A BGP speaker receiving a prefix from an IBGP neighbor with
LAST_RESORT MUST ignore the LAST_RESORT community.
The distinction between EBGP and IBGP ensures backwards
compatibility, particularly when a heterogeneous set of routers in an
AS doing IBGP exists.
The restriction preventing overriding LAST_RESORT ensures that local
policy mechanisms do not interfere with LAST_RESORT. This also
facilitates ease in deployment, as most router configurations would
not require modification.
4. LAST_RESORT - Basic Method
The main reason for establishing the LAST_RESORT Community is to
permit the global implementation of actual "backup only"
announcements. It is not to facilitate change of policies, or to
circumvent local policies. Rather, it is to make possible the
implementation of policies where those have been negotiated by two or
more parties.
Currently, there are several documented scenarios in the "Wedgies"
RFC [RFC4264] where the mutually desired policy is either unable to
be implemented, or does not deterministically reach the desired
state.
Application of the LAST_RESORT Community on announcements sent to a
backup provider, permits these problems to be resolved.
The same prefix is announced to both the primary and backup provider.
When announced to the primary provider, the LAST_RESORT Community is
NOT set. When announced to the backup provider, the LAST_RESORT
Community IS set.
The propagation of the LAST_RESORT instance will be limited by the
availability of paths, and inhibited by the existence of paths which
do not have LAST_RESORT applied to them.
In Figure 1 (of Appendix A), the LAST_RESORT instance will be seen by
the backup provider, and be passed with LAST_RESORT to the backup
provider's transit provider. The latter will prefer any other
instance without LAST_RESORT, even if it has policy for applying a
LOCAL_PREFERENCE to the received prefix instances. Should the other
instance be withdrawn, the LAST_RESORT will be selected and
subsequently propagated.
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5. Security Considerations
No additional security considerations beyond those already present in
BGP are introduced.
6. IANA Considerations
IANA will need to assign a new code points for BGP Well-Known
Communities for LAST_RESORT.
7. Acknowledgements
The author wishes to acknowledge the helpful guidance of Joe Abley
and Tony Li. The author also wishes to acknowledge the assistance
and suggestions of Joel M. Halpern in simplifying the original
"Backup-only" concept to that of a BGP community, and of Olivier
Bonaventure in clarifying the LOCAL_PREFERENCE mechanisms.
8. References
8.1. Normative References
[RFC4264] Griffin, T. and G. Huston, "BGP Wedgies", RFC 4264,
November 2005.
[RFC4271] Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
Protocol 4 (BGP-4)", RFC 4271, January 2006.
[RFC1997] Chandrasekeran, R., Traina, P., and T. Li, "BGP
Communities Attribute", RFC 1997, August 1996.
8.2. Informative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
Appendix A. BGP Wedgie Examples
The following examples from RFC 4264 [RFC4264] show the effects of
the proposed changes, in resolving "wedgie" issues.
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+----+ +----+
|AS 3|----------------|AS 4|
+----+ peer peer +----+
|provider |provider
| |
|customer |
+----+ |
|AS 2| |
+----+ |
|provider |
| |
|customer |customer
+-------+ +----------+
backup| |primary
+----+
|AS 1|
+----+
Figure 1
In Figure 1 above, the announcement via the backup link is sent with
LAST_RESORT.
o AS 4 sends AS_PATH "4 1" to AS 3.
o AS 2 receives the LAST_RESORT path from AS 1, and sends AS_PATH "2
1" to AS 3, also with LAST_RESORT.
o AS 3 and AS 4 exchange their respective "best" paths.
o AS 3 prefers the path "4 1" over "2 1" because "2 1" is
LAST_RESORT.
o AS 3 sends a withdrawal of the LAST_RESORT path to AS 4.
o AS 3 sends its "best", AS_PATH "3 4 1" to AS 2.
This state will be reached regardless of sequence of disconnects and
reconnects.
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+----+ +----+
|AS 3|----------------|AS 4|
+----+ peer peer +----+
|provider |provider
| |
|customer |customer
+----+ +----+
|AS 2| |AS 5|
+----+ +----+
|provider |provider
| |
|customer |customer
+-------+ +----------+
backup| |primary for 192.9.200.0/25
primary| |backup for 192.9.200.128/25
+----+
|AS 1|
+----+
Figure 2
In Figure 2 above, the announcements via the backup links will work
the same as in Example 1.
+----+ +----+
|AS 3|----------------|AS 4|
+----+ peer peer +----+
||provider |providerS
|+-----------+ |
|customer |customer |
+----+ +----+ |
|AS 2|-------|AS 5| |
+----+ peer +----+ |
|provider |provider |
| | |
|customer +-+customer |customer
+-------+ |+----------+
backup| ||primary
+----+
|AS 1|
+----+
Figure 3
In Figure 3 above, the announcements via both backup links will
result in:
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o AS 2 selecting its best path via "3 4 1" (the only path it hears
from AS 3)
o AS 2 hearing one of two paths from AS 5:
* "5 3 4 1"
* LAST_RESORT with path "5 1"
o AS 2 hearing a LAST_RESORT directly from AS 1
Any announcement that AS 3 hears from AS 2 will always be marked
LAST_RESORT. (The same will be true of AS 5.) Thus, any combination
of break/restore on any links in any order, will always result in the
desired state being reached.
Author's Address
Brian Dickson
Afilias Canada, Inc
4141 Yonge St,
Suite 204
North York, ON M2P 2A8
Canada
Email: brian.peter.dickson@gmail.com
URI: www.afilias.info
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