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Versions: (RFC 2858) 00 01 02 03 04 05 06 07 08 09 10 RFC 4760

Network Working Group                     Tony Bates (Cisco Systems)
Internet Draft                       Ravi Chandra (Redback Networks)
Expiration Date:  October 2002          Dave Katz (Juniper Networks)
Obsoles RFC2858                     Yakov Rekhter (Juniper Networks)


                   Multiprotocol Extensions for BGP-4

                    draft-ietf-idr-rfc2858bis-02.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
   http://www.ietf.org/shadow.html.


2. Abstract

   Currently BGP-4 is capable of carrying routing information only for
   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.










Bates, Chandra, Katz, Rekhter                                   

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3. Specification of Requirements

   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 RFC 2119 [RFC2119].


4. Overview

   The only three pieces of information carried by BGP-4 [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
   multiprotocol capabilities will just ignore the information carried
   in these attributes, and will not pass it to other BGP speakers.




Bates, Chandra, Katz, Rekhter                                   

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Internet Draft      draft-ietf-idr-rfc2858bis-02.txt          April 2002


5. 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)                                    |
      +---------------------------------------------------------+
      | Network Layer Reachability Information (variable)       |
      +---------------------------------------------------------+





Bates, Chandra, Katz, Rekhter                                   

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Internet Draft      draft-ietf-idr-rfc2858bis-02.txt          April 2002


   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 SHALL 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
         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:




Bates, Chandra, Katz, Rekhter                                   

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         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.

   The rules for the next hop information are the same as the rules for
   the information carried in the NEXT_HOP BGP attribute (see Section
   5.1.3 of [BGP-4]).

   An UPDATE message that carries the MP_REACH_NLRI MUST also carry the
   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.

   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.

   An UPDATE message SHOULD NOT include the same address prefix (of the
   same <AFI, SAFI>) in more than one of the following fields: WITHDRAWN
   ROUTES field, Network Reachability Information fields, MP_REACH_NLRI
   field, and MP_UNREACH_NLRI field. The processing of an UPDATE message
   in this form is un-defined.


6. 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)                             |
      +---------------------------------------------------------+





Bates, Chandra, Katz, Rekhter                                   

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Internet Draft      draft-ietf-idr-rfc2858bis-02.txt          April 2002


   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:

         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.


7. 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



Bates, Chandra, Katz, Rekhter                                   

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         (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.



8. 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

   An implementation MAY support all, some, or none of the Subsequent
   Address Family Identifier values defined in this document.


9. 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".








Bates, Chandra, Katz, Rekhter                                   

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10. Use of BGP Capability Advertisement

   A BGP speaker that uses Multiprotocol Extensions SHOULD use the
   Capability Advertisment 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:


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


      The use and meaning of this field is as follow:

         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 Advertisement
   mechanism) the capability to support that particular <AFI, SAFI>
   routes.














Bates, Chandra, Katz, Rekhter                                   

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Internet Draft      draft-ietf-idr-rfc2858bis-02.txt          April 2002


11. IANA Considerations

   As specified in this document, the MPL_REACH_NLRI and MP_UNREACH_NLRI
   attributes contain the Subsequence Address Family Identifier (SAFI)
   field. The SAFI name space is defined in this document. The IANA will
   maintain and register values for the SAFI namespace as follows.  SAFI
   value 0 is reserved. SAFI values 1 and 2 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
   for "private use", and values in this range are not to be assigned by
   IANA.


12. Comparison with RFC2858

   This document makes the use of the next hop information consistent
   with the information carried in the NEXT_HOP BGP path attribute.

   This document removes the definition of SAFI = 3.


13. Comparison with RFC2283

   This document restricts the MP_REACH_NLRI attribute to carry only a
   single instance of <AFI, SAFI, Next Hop Information, ...>.

   This document restricts the MP_UNREACH_NLRI attribute to carry only a
   single instance of <AFI, SAFI, ...>.

   This document clarifies handling of an UPDATE message that carries no
   NLRI, other than the one encoded in the MP_REACH_NLRI attribute.

   This document clarifies error handling in the presence of
   MP_REACH_NLRI or MP_UNREACH_NLRI attributes.

   This document specifies the use of BGP Capabilities Advertisements in
   conjunction with Multi-protocol extensions.

   Finally, this document includes the "IANA Consideration" Section.










Bates, Chandra, Katz, Rekhter                                   

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Internet Draft      draft-ietf-idr-rfc2858bis-02.txt          April 2002


14. Security Considerations

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


15. Acknowledgements

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


16. References

   [BGP-CAP] "Capabilities Advertisement with BGP-4", R. Chandra, J.
   Scudder, RFC2842, May 2000

   [BGP-4]   Rekhter, Y., and T. Li, "A Border Gateway Protocol 4
   (BGP-4)", RFC 1771, March 1995.

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

   [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
   Requirement Levels", BCP 14, RFC 2119, March 1997.



17. Author Information

   Tony Bates
   Cisco Systems, Inc.
   email: tbates@cisco.com

   Ravi Chandra
   Redback Networks
   e-mail: rchandra@redback.com

   Dave Katz
   Juniper Networks, Inc.
   email: dkatz@juniper.com

   Yakov Rekhter
   Juniper Networks, Inc.
   email: yakov@juniper.com






Bates, Chandra, Katz, Rekhter                                  

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