+---------------------------------------------------------+   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      Withdrawn Routes:Bates, Chandra, Katz, Rekhter                                   [Page 6]Internet Draftdraft-ietf-idr-bgp4-multiprotocol-v2-04.txt  February 2000         A variable length field that lists NLRI for the routes that are         being withdrawn from service.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.Bates, Chandra, Katz, Rekhter                                   [Page 7]Internet Draftdraft-ietf-idr-bgp4-multiprotocol-v2-04.txt  February 20007. 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 forwarding8. 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:Bates, Chandra, Katz, Rekhter                                   [Page 8]Internet Draftdraft-ietf-idr-bgp4-multiprotocol-v2-04.txt  February 2000                        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.Bates, Chandra, Katz, Rekhter                                   [Page 9]Internet Draftdraft-ietf-idr-bgp4-multiprotocol-v2-04.txt  February 200011. 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.comBates, Chandra, Katz, Rekhter                                  [Page 10]Internet Draftdraft-ietf-idr-bgp4-multiprotocol-v2-04.txt  February 2000   Yakov Rekhter   Cisco Systems, Inc.   170 West Tasman Drive   San Jose, CA 95134   email: yakov@cisco.comBates, Chandra, Katz, Rekhter                                  [Page 11]