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Versions: 01 02 03 04 05                                                
INTERNET-DRAFT                                                 Enke Chen
<draft-ietf-idr-bgp-dpa-01.txt>                               Tony Bates
Expires in six months                                                MCI
                                                               June 1995

                 Destination Preference Attribute for BGP

Status of this Memo

   This document is an Internet Draft. Internet Drafts are working
   documents of the Internet Engineering Task Force (IETF), its Areas,
   and its Working Groups. Note that other groups may also distribute
   working documents as Internet Drafts.

   Internet Drafts are draft documents valid for a maximum of six
   months. Internet Drafts may be updated, replaced, or obsoleted by
   other documents at any time. It is not appropriate to use Internet
   Drafts as reference material or to cite them other than as a "working
   draft" or "work in progress".

   Please check the I-D abstract listing contained in each Internet
   Draft directory to learn the current status of this or any other
   Internet Draft.


   The Border Gateway Protocol [1] is an inter-autonomous system routing
   protocol designed for TCP/IP internets.

   This document describes a new BGP path attribute termed "Destination
   Preference Attribute" (DPA) which can be used by a single autonomous
   system (AS) to specify globally transitive metrics in its routing
   announcement via BGP.  The metric can then be used by upstream BGP
   speakers to favor certain path for return traffic.  The application
   of this attribute includes facilitating the implementation of
   symmetric routing and load sharing in the multi-provider Internet.


   In certain cases there is a need for an autonomous system (AS) to
   specify a globally transitive preference in its routing announcement

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INTERNET-DRAFT            DPA Attribute for BGP                June 1995

   via BGP so that the upstream BGP speakers can use the preference to
   favor certain path for return traffic.  For instance, as discussed in
   [3], currently it is difficult to implement symmetric routing and
   load sharing in the multi-provider Internet due to the lack of this
   preference in BGP.

   In this paper, we propose a new BGP attribute termed "Destination
   Preference Attribute" (DPA) to address such a need.  More
   specifically, the DPA is a globally transitive metric that can be
   used by an AS to specify preference in its routing announcement so
   that the return traffic favors certain path.  As illustrated in [4]
   through several examples, this metric, combined with AS-based
   "local_pref" offers much greater flexibility and manageability in
   implementing symmetric inter-domain routing and load sharing in the
   multi-provider Internet.

Destination Preference Attribute (DPA)

   This document proposes the DPA path attribute, which is an optional
   transitive attribute of fixed length.  The attribute is represented
   by a pair <AS#, DPA value>.  The AS# is a two octet non-negative
   integer, which denotes the AS that specifies the preference.  The DPA
   value is a four octet non-negative integer.

   The DPA attribute has Type Code 10. Code 10 is chosen so as not to
   conflict with existing and other proposed codes.

Route Selection Process

   The DPA attributes are considered comparable only if the DPA
   attributes are present in all the routes being compared and are set
   by the same AS.

   The comparable DPA attributes shall be used as a selection criteria,
   after the "local_pref" attribute is evaluated, and before the
   evaluation of the AS path length and the multi-exit-discriminator
   (MED).  Non-comparable DPA attributes shall not be used in the route
   selection process.

   The higher the DPA attribute value, the more preferred the route.


   The DPA attribute should not be used as a replacement for MED.  MED
   should still be used when an AS has multiple connections to a single

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   neighboring AS.

   The DPA attribute should only be set when needed. The AS that sets
   this preference must include its AS number in the attribute.  A BGP
   speaker may use the "local_pref" attribute to prefer a different path
   other than the one specified by the DPA attribute value.  This does
   not preclude an AS from re-setting this attribute.  However,
   coordination with the upstream and/or downstream neighbors is
   strongly recommended.


   If aggregation is done, the resultant aggregate shall be treated as a
   new NLRI.  No DPA attribute shall be derived from more specific NLRIs
   which formed the aggregate. The resultant aggregate is free to have
   the DPA attribute set if so desired.


   It is noted that this new BGP attribute is simple and requires little
   change to the current practice and operation of BGP4.  Nevertheless,
   the new attribute would offer the flexibility of shifting more
   influence on route selection to where the route originates, which has
   become increasingly meaningful as the Internet becomes more complex
   and dynamic.  At the same time, the autonomy of an AS is preserved as
   the "local_pref" feature remains unchanged.  A typical application of
   this attribute is illustrated in [4] where the DPA attribute is used
   to simplify the implementation of symmetric inter-domain routing and


   The DPA path attribute may be used with BGP version 4 and all
   subsequent versions of BGP unless specifically noted otherwise.

Security Considerations

   Security considerations are not discussed in this memo.


   The authors would like to thank Yakov Rekhter of Cisco for his
   insightful comments and suggestions.

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INTERNET-DRAFT            DPA Attribute for BGP                June 1995


   [1] Rekhter, Y., and Li, T., "A Border Gateway Protocol 4 (BGP-4)",
   RFC1771, March 1995.

   [2] Y. Rekhter, and P. Gross, "Application of the Border Gateway
   Protocol in the Internet", RFC1772, March 1995.

   [3] Chen, E., and Bates, T., "Analysis and Critique of the Current
   Practice of Implementing Symmetric Routing in the Multi-Provider
   Internet", INTERNET-DRAFT, <draft-ietf-idr-symm-multi-prov-01.txt>,
   June 1995.

   [4] Chen, E., and Bates, T., "Application of the BGP Destination
   Preference Attribute in Implementing Symmetric Routing", INTERNET-
   DRAFT, <draft-ietf-idr-dpa-application-01.txt>, June 1995.

   [5] Antonov, V., "BGP AS Path Metrics", INTERNET DRAFT, <draft-ietf-
   idr-bgp-metrics-00.txt>, March 1995.

   [6] Rekhter, Y., "Routing in a Multi-provider Internet", RFC1787,
   April 1995.

Author's Addresses

   Enke Chen
   2100 Reston Parkway
   Reston, VA 22091

   phone: +1 703 715 7087
   email: enke@mci.net

   Tony Bates
   2100 Reston Parkway
   Reston, VA 22091

   phone: +1 703 715 7521
   email: Tony.Bates@mci.net

Chen & Bates                                                    [Page 4]