A New BGP Well-Known Community, LAST_RESORT
draft-dickson-idr-last-resort-02

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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.

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

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" (Griffin, T. and G. Huston, “BGP Wedgies,” November 2005.) [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-prefering 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 Path Selection Rules

The path selection rules for BGP (section 9.1.2.2 of BGP4 (Rekhter, Y., Li, T., and S. Hares, “A Border Gateway Protocol 4 (BGP-4),” January 2006.) [RFC4271]) are changed as follows:

• The following rule is placed before step (a): If paths with and without LAST_RESORT are both available, those with LAST_RESORT are eliminated.
• The remainder of the usual BGP path selection rules are applied as normal

Note Well: This MAY be implemented by applying suitable LOCAL_PREFERENCE. Assignment of lowest-possible preference order of LOCAL_PREFERENCE should achieve the same behavior, and may be simpler to implement.

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, instead 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 (Griffin, T. and G. Huston, “BGP Wedgies,” November 2005.) [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 Communty 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 propogation 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 (BGP Wedgie Examples)), 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 instace without LAST_RESORT, even if it has applied a LOCAL_PREFERENCE to the received prefix instance. Should the other instance be withdrawn, the LAST_RESORT will be selected and subsequently propogated.

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.

8.  References

8.1. Normative References

8.2. Informative References

Appendix A.  BGP Wedgie Examples

The following examples from RFC 4264 (Griffin, T. and G. Huston, “BGP Wedgies,” November 2005.) [RFC4264] show the effects of the proposed changes, in resolving "wedgie" issues.

+----+                +----+
|AS 3|----------------|AS 4|
+----+ peer      peer +----+
  |provider             |provider
  |                     |
  |customer             |
+----+                  |
|AS 2|                  |
+----+                  |
  |provider             |
  |                     |
  |customer             |customer
  +-------+  +----------+
    backup|  |primary
         +----+
         |AS 1|
         +----+

• AS 4 sends AS_PATH "4 1" to AS 3.
• AS 2 receives the LAST_RESORT path from AS 1, and sends AS_PATH "2 1" to AS 3, also with LAST_RESORT.
• AS 3 and AS 4 exchange their respective "best" paths.
• AS 3 prefers the path "4 1" over "2 1" because "2 1" is LAST_RESORT.
• AS 3 sends a withdrawal of the LAST_RESORT path to AS 4.
• 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.

+----+                +----+
|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|
         +----+

+----+                +----+
|AS 3|----------------|AS 4|
+----+ peer      peer +----+
 ||provider             |providerS
 |+-----------+         |
 |customer    |customer |
+----+       +----+     |
|AS 2|-------|AS 5|     |
+----+ peer  +----+     |
  |provider   |provider |
  |           |         |
  |customer +-+customer |customer
  +-------+ |+----------+
    backup| ||primary
         +----+
         |AS 1|
         +----+

• AS 2 selecting its best path via "3 4 1" (the only path it hears from AS 3)
• AS 2 hearing one of two paths from AS 5:
  • "5 3 4 1"
  • LAST_RESORT with path "5 1"
• 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

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