Multiprotocol Extensions for BGP-4
draft-ietf-idr-bgp4-multiprotocol-v2-04

Network Working Group                                   Tony Bates
Internet Draft                                       Cisco Systems
Expiration Date:  August 2000                         Ravi Chandra
                                                     Siara Systems
                                                         Dave Katz
                                                  Juniper Networks
                                                     Yakov Rekhter
                                                     Cisco Systems

                   Multiprotocol Extensions for BGP-4

              draft-ietf-idr-bgp4-multiprotocol-v2-04.txt

1. Status of this Memo

   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   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
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as ``work in progress.''

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
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2. Abstract

   Currently BGP-4 [BGP-4] is capable of carrying routing information
   only for IPv4 [IPv4]. This document defines extensions to BGP-4 to
   enable it to carry routing information for multiple Network Layer
   protocols (e.g., IPv6, IPX, etc...). The extensions are backward
   compatible - a router that supports the extensions can interoperate
   with a router that doesn't support the extensions.

3. Overview

   The only three pieces of information carried by BGP-4 that are IPv4
   specific are (a) the NEXT_HOP attribute (expressed as an IPv4
   address), (b) AGGREGATOR (contains an IPv4 address), and (c) NLRI
   (expressed as IPv4 address prefixes). This document assumes that any
   BGP speaker (including the one that supports multiprotocol
   capabilities defined in this document) has to have an IPv4 address
   (which will be used, among other things, in the AGGREGATOR
   attribute). Therefore, to enable BGP-4 to support routing for
   multiple Network Layer protocols the only two things that have to be
   added to BGP-4 are (a) the ability to associate a particular Network
   Layer protocol with the next hop information, and (b) the ability to
   associated a particular Network Layer protocol with NLRI. To identify
   individual Network Layer protocols this document uses Address Family,
   as defined in [RFC1700].

   One could further observe that the next hop information (the
   information provided by the NEXT_HOP attribute) is meaningful (and
   necessary) only in conjunction with the advertisements of reachable
   destinations - in conjunction with the advertisements of unreachable
   destinations (withdrawing routes from service) the next hop
   information is meaningless. This suggests that the advertisement of
   reachable destinations should be grouped with the advertisement of
   the next hop to be used for these destinations, and that the
   advertisement of reachable destinations should be segregated from the
   advertisement of unreachable destinations.

   To provide backward compatibility, as well as to simplify
   introduction of the multiprotocol capabilities into BGP-4 this
   document uses two new attributes, Multiprotocol Reachable NLRI
   (MP_REACH_NLRI), and Multiprotocol Unreachable NLRI
   (MP_UNREACH_NLRI). The first one (MP_REACH_NLRI) is used to carry the
   set of reachable destinations together with the next hop information
   to be used for forwarding to these destinations. The second one
   (MP_UNREACH_NLRI) is used to carry the set of unreachable
   destinations.  Both of these attributes are optional and non-
   transitive.  This way a BGP speaker that doesn't support the

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   multiprotocol capabilities will just ignore the information carried
   in these attributes, and will not pass it to other BGP speakers.

4. Multiprotocol Reachable NLRI - MP_REACH_NLRI (Type Code 14):

   This is an optional non-transitive attribute that can be used for the
   following purposes:

      (a) to advertise a feasible route to a peer

      (b) to permit a router to advertise the Network Layer address of
      the router that should be used as the next hop to the destinations
      listed in the Network Layer Reachability Information field of the
      MP_NLRI attribute.

      (c) to allow a given router to report some or all of the
      Subnetwork Points of Attachment (SNPAs) that exist within the
      local system

   The attribute is encoded as shown below:

      +---------------------------------------------------------+
      | Address Family Identifier (2 octets)                    |
      +---------------------------------------------------------+
      | Subsequent Address Family Identifier (1 octet)          |
      +---------------------------------------------------------+
      | Length of Next Hop Network Address (1 octet)            |
      +---------------------------------------------------------+
      | Network Address of Next Hop (variable)                  |
      +---------------------------------------------------------+
      | Number of SNPAs (1 octet)                               |
      +---------------------------------------------------------+
      | Length of first SNPA(1 octet)                           |
      +---------------------------------------------------------+
      | First SNPA (variable)                                   |
      +---------------------------------------------------------+
      | Length of second SNPA (1 octet)                         |
      +---------------------------------------------------------+
      | Second SNPA (variable)                                  |
      +---------------------------------------------------------+
      | ...                                                     |
      +---------------------------------------------------------+
      | Length of Last SNPA (1 octet)                           |
      +---------------------------------------------------------+
      | Last SNPA (variable)                                    |
      +---------------------------------------------------------+

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      | Network Layer Reachability Information (variable)       |
      +---------------------------------------------------------+

   The use and meaning of these fields are as follows:

      Address Family Identifier:

         This field carries the identity of the Network Layer protocol
         associated with the Network Address that follows. Presently
         defined values for this field are specified in RFC1700 (see the
         Address Family Numbers section).

      Subsequent Address Family Identifier:

         This field provides additional information about the type of
         the Network Layer Reachability Information carried in the
         attribute.

      Length of Next Hop Network Address:

         A 1 octet field whose value expresses the length of the
         "Network Address of Next Hop" field as measured in octets

      Network Address of Next Hop:

         A variable length field that contains the Network Address of
         the next router on the path to the destination system

      Number of SNPAs:

         A 1 octet field which contains the number of distinct SNPAs to
         be listed in the following fields.  The value 0 may be used to
         indicate that no SNPAs are listed in this attribute.

      Length of Nth SNPA:

         A 1 octet field whose value expresses the length of the "Nth
         SNPA of Next Hop" field as measured in semi-octets

      Nth SNPA of Next Hop:

         A variable length field that contains an SNPA of the router
         whose Network Address is contained in the "Network Address of
         Next Hop" field.  The field length is an integral number of
         octets in length, namely the rounded-up integer value of one
         half the SNPA length expressed in semi-octets; if the SNPA

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         contains an odd number of semi-octets, a value in this field
         will be padded with a trailing all-zero semi-octet.

      Network Layer Reachability Information:

         A variable length field that lists NLRI for the feasible routes
         that are being advertised in this attribute. When the
         Subsequent Address Family Identifier field is set to one of the
         values defined in this document, each NLRI is encoded as
         specified in the "NLRI encoding" section of this document.

   The next hop information carried in the MP_REACH_NLRI path attribute
   defines the Network Layer address of the border router that should be
   used as the next hop to the destinations listed in the MP_NLRI
   attribute in the UPDATE message.  When advertising a MP_REACH_NLRI
   attribute to an external peer, a router may use one of its own
   interface addresses in the next hop component of the attribute,
   provided the external peer to which the route is being advertised
   shares a common subnet with the next hop address.  This is known as a
   "first party" next hop.  A BGP speaker can advertise to an external
   peer an interface of any internal peer router in the next hop
   component, provided the external peer to which the route is being
   advertised shares a common subnet with the next hop address.  This is
   known as a "third party" next hop information.  A BGP speaker can
   advertise any external peer router in the next hop component,
   provided that the Network Layer address of this border router was
   learned from an external peer, and the external peer to which the
   route is being advertised shares a common subnet with the next hop
   address.  This is a second form of "third party" next hop
   information.

   Normally the next hop information is chosen such that the shortest
   available path will be taken.  A BGP speaker must be able to support
   disabling advertisement of third party next hop information to handle
   imperfectly bridged media or for reasons of policy.

   A BGP speaker must never advertise an address of a peer to that peer
   as a next hop, for a route that the speaker is originating.  A BGP
   speaker must never install a route with itself as the next hop.

   When a BGP speaker advertises the route to an internal peer, the
   advertising speaker should not modify the next hop information
   associated with the route.  When a BGP speaker receives the route via
   an internal link, it may forward packets to the next hop address if
   the address contained in the attribute is on a common subnet with the
   local and remote BGP speakers.

   An UPDATE message that carries the MP_REACH_NLRI must also carry the

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   ORIGIN and the AS_PATH attributes (both in EBGP and in IBGP
   exchanges).  Moreover, in IBGP exchanges such a message must also
   carry the LOCAL_PREF attribute. If such a message is received from an
   external peer, the local system shall check whether the leftmost AS
   in the AS_PATH attribute is equal to the autonomous system number of
   the peer than sent the message. If that is not the case, the local
   system shall send the NOTIFICATION message with Error Code UPDATE
   Message Error, and the Error Subcode set to Malformed AS_PATH.

   An UPDATE message that carries no NLRI, other than the one encoded in
   the MP_REACH_NLRI attribute, should not carry the NEXT_HOP attribute.
   If such a message contains the NEXT_HOP attribute, the BGP speaker
   that receives the message should ignore this attribute.

5. Multiprotocol Unreachable NLRI - MP_UNREACH_NLRI (Type Code 15):

   This is an optional non-transitive attribute that can be used for the
   purpose of withdrawing multiple unfeasible routes from service.

   The attribute is encoded as shown below:

      +---------------------------------------------------------+
      | Address Family Identifier (2 octets)                    |
      +---------------------------------------------------------+
      | Subsequent Address Family Identifier (1 octet)          |
      +---------------------------------------------------------+
      | Withdrawn Routes (variable)                             |
      +---------------------------------------------------------+

   The use and the meaning of these fields are as follows:

      Address Family Identifier:

         This field carries the identity of the Network Layer protocol
         associated with the NLRI that follows. Presently defined values
         for this field are specified in RFC1700 (see the Address Family
         Numbers section).

      Subsequent Address Family Identifier:

         This field provides additional information about the type of
         the Network Layer Reachability Information carried in the
         attribute.

      Withdrawn Routes:

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         A variable length field that lists NLRI for the routes that are
         being withdrawn from service. When the Subsequent Address
         Family Identifier field is set to one of the values defined in
         this document, each NLRI is encoded as specified in the "NLRI
         encoding" section of this document.

   An UPDATE message that contains the MP_UNREACH_NLRI is not required
   to carry any other path attributes.

6. NLRI encoding

   The Network Layer Reachability information is encoded as one or more
   2-tuples of the form <length, prefix>, whose fields are described
   below:

      +---------------------------+
      |   Length (1 octet)        |
      +---------------------------+
      |   Prefix (variable)       |
      +---------------------------+

   The use and the meaning of these fields are as follows:

      a) Length:

         The Length field indicates the length in bits of the address
         prefix. A length of zero indicates a prefix that matches all
         (as specified by the address family) addresses (with prefix,
         itself, of zero octets).

      b) Prefix:

         The Prefix field contains an address prefix followed by enough
         trailing bits to make the end of the field fall on an octet
         boundary.  Note that the value of trailing bits is irrelevant.

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7. Subsequent Address Family Identifier

   This document defines the following values for the Subsequent Address
   Family Identifier field carried in the MP_REACH_NLRI and
   MP_UNREACH_NLRI attributes:

      1 - Network Layer Reachability Information used for unicast
      forwarding

      2 - Network Layer Reachability Information used for multicast
      forwarding

      3 - Network Layer Reachability Information used for both unicast
      and multicast forwarding

8. Error Handling

   If a BGP speaker receives from a neighbor an Update message that
   contains the MP_REACH_NLRI or MP_UNREACH_NLRI attribute, and the
   speaker determines that the attribute is incorrect, the speaker must
   delete all the BGP routes received from that neighbor whose AFI/SAFI
   is the same as the one carried in the incorrect MP_REACH_NLRI or
   MP_UNREACH_NLRI attribute. For the duration of the BGP session over
   which the Update message was received, the speaker then should ignore
   all the subsequent routes with that AFI/SAFI received over that
   session.

   In addition, the speaker may terminate the BGP session over which the
   Update message was received. The session should be terminated with
   the Notification message code/subcode indicating "Update Message
   Error"/"Optional Attribute Error".

9. Use of BGP Capability Negotiation

   A BGP speaker that uses Multiprotocol Extensions should use the
   Capability Negotiation procedures [BGP-CAP] to determine whether the
   speaker could use Multiprotocol Extensions with a particular peer.

   The fields in the Capabilities Optional Parameter are set as follows.
   The Capability Code field is set to 1 (which indicates Multiprotocol
   Extensions capabilities). The Capability Length field is set to 4.
   The Capability Value field is defined as:

      The use and meaning of this field is as follow:

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                        0       7      15      23      31
                        +-------+-------+-------+-------+
                        |      AFI      | Res.  | SAFI  |
                        +-------+-------+-------+-------+

         AFI  - Address Family Identifier (16 bit), encoded the same way
         as in the Multiprotocol Extensions

         Res. - Reserved (8 bit) field. Should be set to 0 by the sender
         and ignored by the receiver.

         SAFI - Subsequent Address Family Identifier (8 bit), encoded
         the same way as in the Multiprotocol Extensions.

   A speaker that supports multiple <AFI, SAFI> tuples includes them as
   multiple Capabilities in the Capabilities Optional Parameter.

   To have a bi-directional exchange of routing information for a
   particular <AFI, SAFI> between a pair of BGP speakers, each such
   speaker must advertise to the other (via the Capability Negotiation
   mechanism) the capability to support that particular <AFI, SAFI>
   routes.

10. IANA Considerations

   As specified in this document, the MPL_REACH_NLRI and MP_UNREACH_NLRI
   attributes contain the Subsequence Address Family Identifier (SAFI)
   field.  SAFI value 0 is reserved. SAFI values 1, 2, and 3 are
   assigned in this document.  SAFI values 4 through 63 are to be
   assigned by IANA using the "IETF Consensus" policy defined in
   RFC2434. SAFI values 64 through 127 are to be assigned by IANA, using
   the "First  Come First Served" policy defined in RFC2434. SAFI values
   128 through 255 are vendor-specific, and values in this range are not
   to be assigned by IANA.

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11. Security Considerations

   This extension to BGP does not change the underlying security issues
   inherent in the existing BGP [Heffernan].

12. Acknowledgements

   The authors would like to thank members of the IDR Working Group for
   their review and comments.

13. References

   [BGP-CAP] "Capabilities Negotiation with BGP-4", R. Chandra, J.
   Scudder, draft-ietf-idr-bgp4-cap-neg-05.txt, February 1999

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

   [Heffernan]  Heffernan, A., "Protection of BGP Sessions via the TCP
   MD5 Signature Option", RFC2385, August 1998.

   [IPv4] "Internet Protocol", J. Postel, September 1981

   [RFC1700] "Assigned Numbers", J. Reynolds, J. Postel, RFC1700,
   October 1994 (see also http://www.iana.org/iana/assignments.html)

14. Author Information

   Tony Bates
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134
   email: tbates@cisco.com

   Ravi Chandra
   Siara Systems Incorporated
   1195 Borregas Avenue
   Sunnyvale, CA 94089
   e-mail: rchandra@siara.com

   Dave Katz
   Juniper Networks, Inc.
   3260 Jay St.
   Santa Clara, CA 95054
   email: dkatz@jnx.com

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   Yakov Rekhter
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134
   email: yakov@cisco.com

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