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INTERNET-DRAFT                                                 Enke Chen
<draft-ietf-idr-bgp-dpa-05.txt>                               Tony Bates
Expires September 1996                                               MCI
                                                              March 1996


                 Destination Preference Attribute for BGP
                      <draft-ietf-idr-bgp-dpa-05.txt>




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.


Abstract

   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.


Introduction

   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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   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 11.


Route Selection Process


   A router may use DPA to influence the degree of preference [1]
   assigned to a route.

   DPA influence on the computation of degree of preference is a local
   matter. In general, a route with a higher DPA indicates a higher
   preference by the originator of the DPA attribute.














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Operation

   The AS that sets this attribute must include its AS number in the
   attribute.  A BGP speaker may use the "LOCAL_PREF" attribute to
   select 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.


Aggregation

   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.


Remarks

   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
   load-sharing.


Applicability

   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.


Acknowledgments

   The authors would like to thank Yakov Rekhter of cisco for his
   insightful comments and suggestions. We also acknowledge Ramesh
   Govindan (ISI), Paul Traina (cisco), and Ravi Chandra (cisco) for



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   their helpful comments.


References

   [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., "Current Practice of Implementing
   Symmetric Routing and Load Sharing in the Multi-Provider Internet",
   INTERNET-DRAFT, <draft-ietf-idr-symm-multi-prov-02.txt>, January
   1996.

   [4] Chen, E., and Bates, T., "Application of the BGP Destination
   Preference Attribute in Implementing Symmetric Routing", INTERNET-
   DRAFT, <draft-ietf-idr-dpa-application-02.txt>, January 1996.

   [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
   MCI
   2100 Reston Parkway
   Reston, VA 22091

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

   Tony Bates
   MCI
   2100 Reston Parkway
   Reston, VA 22091

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







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