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Network Working Group                                      Y. Rekhter
INTERNET DRAFT                                          cisco Systems
                                                                T. Li
                                                     Juniper Networks
                                                              Editors
<draft-ietf-idr-bgp4-08.txt>                              August 1998



                  A Border Gateway Protocol 4 (BGP-4)


Status of this Memo

   This document, together with its companion document, "Application of
   the Border Gateway Protocol in the Internet", define an inter-
   autonomous system routing protocol for the Internet. This document
   specifies an IAB standards track protocol for the Internet community,
   and requests discussion and suggestions for improvements.  Please
   refer to the current edition of the "IAB Official Protocol Standards"
   for the standardization state and status of this protocol.
   Distribution of this document is unlimited.

   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".

   To view the entire list of current Internet-Drafts, please check the
   "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern
   Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific
   Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).



1. Acknowledgments

   This document was originally published as RFC 1267 in October 1991,
   jointly authored by Kirk Lougheed and Yakov Rekhter.





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          We would like to express our thanks to Guy Almes, Len Bosack, and
          Jeffrey C. Honig for their contributions to the earlier version of
          this document.

          We like to explicitly thank Bob Braden for the review of the earlier
          version of this document as well as his constructive and valuable
          comments.

          We would also like to thank Bob Hinden, Director for Routing of the
          Internet Engineering Steering Group, and the team of reviewers he
          assembled to review the previous version (BGP-2) of this document.
          This team, consisting of Deborah Estrin, Milo Medin, John Moy, Radia
          Perlman, Martha Steenstrup, Mike St. Johns, and Paul Tsuchiya, acted
          with a strong combination of toughness, professionalism, and
          courtesy.

          This updated version of the document is the product of the IETF IDR
          Working Group with Yakov Rekhter and Tony Li as editors. Certain
          sections of the document borrowed heavily from IDRP [7], which is the
          OSI counterpart of BGP. For this credit should be given to the ANSI
          X3S3.3 group chaired by Lyman Chapin and to Charles Kunzinger who was
          the IDRP editor within that group.  We would also like to thank Mike
          Craren, Dimitry Haskin, John Krawczyk, David LeRoy, John Scudder,
          John Stewart III, Paul Traina, and Curtis Villamizar for their
          comments.

          We would like to specially acknowledge numerous contributions by
          Dennis Ferguson.


       2.  Introduction

          The Border Gateway Protocol (BGP) is an inter-Autonomous System
          routing protocol.  It is built on experience gained with EGP as
          defined in RFC 904 [1] and EGP usage in the NSFNET Backbone as
          described in RFC 1092 [2] and RFC 1093 [3].

          The primary function of a BGP speaking system is to exchange network
          reachability information with other BGP systems.  This network
          reachability information includes information on the list of
          Autonomous Systems (ASs) that reachability information traverses.
          This information is sufficient to construct a graph of AS
          connectivity from which routing loops may be pruned and some policy
          decisions at the AS level may be enforced.

          BGP-4 provides a new set of mechanisms for supporting classless





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          interdomain routing.  These mechanisms include support for
          advertising an IP prefix and eliminates the concept of network
          "class" within BGP.  BGP-4 also introduces mechanisms which allow
          aggregation of routes, including aggregation of AS paths.  These
          changes provide support for the proposed supernetting scheme [8, 9].

          To characterize the set of policy decisions that can be enforced
          using BGP, one must focus on the rule that a BGP speaker advertise to
          its peers (other BGP speakers which it communicates with) in
          neighboring ASs only those routes that it itself uses.  This rule
          reflects the "hop-by-hop" routing paradigm generally used throughout
          the current Internet.  Note that some policies cannot be supported by
          the "hop-by-hop" routing paradigm and thus require techniques such as
          source routing to enforce.  For example, BGP does not enable one AS
          to send traffic to a neighboring AS intending that the traffic take a
          different route from that taken by traffic originating in the
          neighboring AS.  On the other hand, BGP can support any policy
          conforming to the "hop-by-hop" routing paradigm.  Since the current
          Internet uses only the "hop-by-hop" routing paradigm and since BGP
          can support any policy that conforms to that paradigm, BGP is highly
          applicable as an inter-AS routing protocol for the current Internet.

          A more complete discussion of what policies can and cannot be
          enforced with BGP is outside the scope of this document (but refer to
          the companion document discussing BGP usage [5]).

          BGP runs over a reliable transport protocol.  This eliminates the
          need to implement explicit update fragmentation, retransmission,
          acknowledgment, and sequencing.  Any authentication scheme used by
          the transport protocol may be used in addition to BGP's own
          authentication mechanisms.  The error notification mechanism used in
          BGP assumes that the transport protocol supports a "graceful" close,
          i.e., that all outstanding data will be delivered before the
          connection is closed.

          BGP uses TCP [4] as its transport protocol.  TCP meets BGP's
          transport requirements and is present in virtually all commercial
          routers and hosts.  In the following descriptions the phrase
          "transport protocol connection" can be understood to refer to a TCP
          connection.  BGP uses TCP port 179 for establishing its connections.

          This document uses the term `Autonomous System' (AS) throughout.  The
          classic definition of an Autonomous System is a set of routers under
          a single technical administration, using an interior gateway protocol
          and common metrics to route packets within the AS, and using an
          exterior gateway protocol to route packets to other ASs.  Since this





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          classic definition was developed, it has become common for a single
          AS to use several interior gateway protocols and sometimes several
          sets of metrics within an AS.  The use of the term Autonomous System
          here stresses the fact that, even when multiple IGPs and metrics are
          used, the administration of an AS appears to other ASs to have a
          single coherent interior routing plan and presents a consistent
          picture of what destinations are reachable through it.

          The planned use of BGP in the Internet environment, including such
          issues as topology, the interaction between BGP and IGPs, and the
          enforcement of routing policy rules is presented in a companion
          document [5].  This document is the first of a series of documents
          planned to explore various aspects of BGP application.  Please send
          comments to the BGP mailing list (bgp@ans.net).


       3.  Summary of Operation

          Two systems form a transport protocol connection between one another.
          They exchange messages to open and confirm the connection parameters.
          The initial data flow is the entire BGP routing table.  Incremental
          updates are sent as the routing tables change.  BGP does not require
          periodic refresh of the entire BGP routing table.  Therefore, a BGP
          speaker must retain the current version of the entire BGP routing
          tables of all of its peers for the duration of the connection.
          KeepAlive messages are sent periodically to ensure the liveness of
          the connection.  Notification messages are sent in response to errors
          or special conditions.  If a connection encounters an error
          condition, a notification message is sent and the connection is
          closed.

          The hosts executing the Border Gateway Protocol need not be routers.
          A non-routing host could exchange routing information with routers
          via EGP or even an interior routing protocol.  That non-routing host
          could then use BGP to exchange routing information with a border
          router in another Autonomous System.  The implications and
          applications of this architecture are for further study.

          Connections between BGP speakers of different ASs are referred to as
          "external" links.  BGP connections between BGP speakers within the
          same AS are referred to as "internal" links.  Similarly, a peer in a
          different AS is referred to as an external peer, while a peer in the
          same AS may be described as an internal peer.  Internal BGP and
          external BGP are commonly abbreviated IBGP and EBGP.

          If a particular AS has multiple BGP speakers and is providing transit





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          service for other ASs, then care must be taken to ensure a consistent
          view of routing within the AS.  A consistent view of the interior
          routes of the AS is provided by the interior routing protocol.  A
          consistent view of the routes exterior to the AS can be provided by
          having all BGP speakers within the AS maintain direct IBGP
          connections with each other.  Alternately the interior routing
          protocol can pass BGP information among routers within an AS, taking
          care not to lose BGP attributes that will be needed by EBGP speakers
          if transit connectivity is being provided.  For the purpose of
          discussion, it is assumed that BGP information is passed within an AS
          using IBGP.  Care must be taken to ensure that the interior routers
          have all been updated with transit information before the EBGP
          speakers announce to other ASs that transit service is being
          provided.


       3.1 Routes: Advertisement and Storage

          For purposes of this protocol a route is defined as a unit of
          information that pairs a destination with the attributes of a path to
          that destination:

             - Routes are advertised between a pair of BGP speakers in UPDATE
             messages:  the destination is the systems whose IP addresses are
             reported in the Network Layer Reachability Information (NLRI)
             field, and the the path is the information reported in the path
             attributes fields of the same UPDATE message.


             - Routes are stored in the Routing Information Bases (RIBs):
             namely, the Adj-RIBs-In, the Loc-RIB, and the Adj-RIBs-Out. Routes
             that will be advertised to other BGP speakers must be present in
             the Adj-RIB-Out; routes that will be used by the local BGP speaker
             must be present in the Loc-RIB, and the next hop for each of these
             routes must be present in the local BGP speaker's forwarding
             information base; and routes that are received from other BGP
             speakers are present in the Adj-RIBs-In.


          If a BGP speaker chooses to advertise the route, it may add to or
          modify the path attributes of the route before advertising it to a
          peer.

          BGP provides mechanisms by which a BGP speaker can inform its peer
          that a previously advertised route is no longer available for use.
          There are three methods by which a given BGP speaker can indicate





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          that a route has been withdrawn from service:


             a) the IP prefix that expresses destinations for a previously
             advertised route can be advertised in the WITHDRAWN ROUTES field
             in the UPDATE message, thus marking the associated route as being
             no longer available for use

             b) a replacement route with the same Network Layer Reachability
             Information can be advertised, or

             c) the BGP speaker - BGP speaker connection can be closed, which
             implicitly removes from service all routes which the pair of
             speakers had advertised to each other.


       3.2 Routing Information Bases

          The Routing Information Base (RIB) within a BGP speaker consists of
          three distinct parts:

             a) Adj-RIBs-In: The Adj-RIBs-In store routing information that has
             been learned from inbound UPDATE messages. Their contents
             represent routes that are available as an input to the Decision
             Process.

             b) Loc-RIB: The Loc-RIB contains the local routing information
             that the BGP speaker has selected by applying its local policies
             to the routing information contained in its Adj-RIBs-In.

             c) Adj-RIBs-Out: The Adj-RIBs-Out store the information that the
             local BGP speaker has selected for advertisement to its peers. The
             routing information stored in the Adj-RIBs-Out will be carried in
             the local BGP speaker's UPDATE messages and advertised to its
             peers.


          In summary, the Adj-RIBs-In contain unprocessed routing information
          that has been advertised to the local BGP speaker by its peers; the
          Loc-RIB contains the routes that have been selected by the local BGP
          speaker's Decision Process; and the Adj-RIBs-Out organize the routes
          for advertisement to specific peers by means of the local speaker's
          UPDATE messages.

          Although the conceptual model distinguishes between Adj-RIBs-In,
          Loc-RIB, and Adj-RIBs-Out, this neither implies nor requires that an





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          implementation must maintain three separate copies of the routing
          information. The choice of implementation (for example, 3 copies of
          the information vs 1 copy with pointers) is not constrained by the
          protocol.

       4.  Message Formats

          This section describes message formats used by BGP.

          Messages are sent over a reliable transport protocol connection.  A
          message is processed only after it is entirely received.  The maximum
          message size is 4096 octets.  All implementations are required to
          support this maximum message size.  The smallest message that may be
          sent consists of a BGP header without a data portion, or 19 octets.


       4.1 Message Header Format


          Each message has a fixed-size header.  There may or may not be a data
          portion following the header, depending on the message type.  The
          layout of these fields is shown below:







              0                   1                   2                   3
              0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |                                                               |
             +                                                               +
             |                                                               |
             +                                                               +
             |                           Marker                              |
             +                                                               +
             |                                                               |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
             |          Length               |      Type     |
             +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


             Marker:






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                This 16-octet field contains a value that the receiver of the
                message can predict.  If the Type of the message is OPEN, or if
                the OPEN message carries no Authentication Information (as an
                Optional Parameter), then the Marker must be all ones.
                Otherwise, the value of the marker can be predicted by some a
                computation specified as part of the authentication mechanism
                (which is specified as part of the Authentication Information)
                used.  The Marker can be used to detect loss of synchronization
                between a pair of BGP peers, and to authenticate incoming BGP
                messages.


             Length:

                This 2-octet unsigned integer indicates the total length of the
                message, including the header, in octets.  Thus, e.g., it
                allows one to locate in the transport-level stream the (Marker
                field of the) next message.  The value of the Length field must
                always be at least 19 and no greater than 4096, and may be
                further constrained, depending on the message type.  No
                "padding" of extra data after the message is allowed, so the
                Length field must have the smallest value required given the
                rest of the message.

             Type:

                This 1-octet unsigned integer indicates the type code of the
                message.  The following type codes are defined:

                                           1 - OPEN
                                           2 - UPDATE
                                           3 - NOTIFICATION
                                           4 - KEEPALIVE


       4.2 OPEN Message Format


          After a transport protocol connection is established, the first
          message sent by each side is an OPEN message.  If the OPEN message is
          acceptable, a KEEPALIVE message confirming the OPEN is sent back.
          Once the OPEN is confirmed, UPDATE, KEEPALIVE, and NOTIFICATION
          messages may be exchanged.

          In addition to the fixed-size BGP header, the OPEN message contains
          the following fields:





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               0                   1                   2                   3
              0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
              +-+-+-+-+-+-+-+-+
              |    Version    |
              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              |     My Autonomous System      |
              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              |           Hold Time           |
              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              |                         BGP Identifier                        |
              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              | Opt Parm Len  |
              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              |                                                               |
              |                       Optional Parameters                     |
              |                                                               |
              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



             Version:

                This 1-octet unsigned integer indicates the protocol version
                number of the message.  The current BGP version number is 4.

             My Autonomous System:

                This 2-octet unsigned integer indicates the Autonomous System
                number of the sender.

             Hold Time:

                This 2-octet unsigned integer indicates the number of seconds
                that the sender proposes for the value of the Hold Timer.  Upon
                receipt of an OPEN message, a BGP speaker MUST calculate the
                value of the Hold Timer by using the smaller of its configured
                Hold Time and the Hold Time received in the OPEN message.  The
                Hold Time MUST be either zero or at least three seconds.  An
                implementation may reject connections on the basis of the Hold
                Time.  The calculated value indicates the maximum number of
                seconds that may elapse between the receipt of successive
                KEEPALIVE, and/or UPDATE messages by the sender.

             BGP Identifier:
                This 4-octet unsigned integer indicates the BGP Identifier of
                the sender. A given BGP speaker sets the value of its BGP





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                Identifier to an IP address assigned to that BGP speaker.  The
                value of the BGP Identifier is determined on startup and is the
                same for every local interface and every BGP peer.

             Optional Parameters Length:

                This 1-octet unsigned integer indicates the total length of the
                Optional Parameters field in octets. If the value of this field
                is zero, no Optional Parameters are present.

             Optional Parameters:

                This field may contain a list of optional parameters, where
                each parameter is encoded as a <Parameter Type, Parameter
                Length, Parameter Value> triplet.





                       0                   1
                       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...
                      |  Parm. Type   | Parm. Length  |  Parameter Value (variable)
                      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-...


                Parameter Type is a one octet field that unambiguously
                identifies individual parameters. Parameter Length is a one
                octet field that contains the length of the Parameter Value
                field in octets.  Parameter Value is a variable length field
                that is interpreted according to the value of the Parameter
                Type field.

                This document defines the following Optional Parameters:

                a) Authentication Information (Parameter Type 1):


                   This optional parameter may be used to authenticate a BGP
                   peer. The Parameter Value field contains a 1-octet
                   Authentication Code followed by a variable length
                   Authentication Data.


                       0 1 2 3 4 5 6 7 8





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                       +-+-+-+-+-+-+-+-+
                       |  Auth. Code   |
                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                       |                                                     |
                       |              Authentication Data                    |
                       |                                                     |
                       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



                      Authentication Code:

                         This 1-octet unsigned integer indicates the
                         authentication mechanism being used.  Whenever an
                         authentication mechanism is specified for use within
                         BGP, three things must be included in the
                         specification:
                         - the value of the Authentication Code which indicates
                         use of the mechanism,
                         - the form and meaning of the Authentication Data, and
                         - the algorithm for computing values of Marker fields.

                         Note that a separate authentication mechanism may be
                         used in establishing the transport level connection.

                      Authentication Data:

                         The form and meaning of this field is a variable-
                         length field depend on the Authentication Code.

                The minimum length of the OPEN message is 29 octets (including
                message header).


       4.3 UPDATE Message Format


          UPDATE messages are used to transfer routing information between BGP
          peers.  The information in the UPDATE packet can be used to construct
          a graph describing the relationships of the various Autonomous
          Systems.  By applying rules to be discussed, routing information
          loops and some other anomalies may be detected and removed from
          inter-AS routing.

          An UPDATE message is used to advertise a single feasible route to a
          peer, or to withdraw multiple unfeasible routes from service (see





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          3.1). An UPDATE message may simultaneously advertise a feasible route
          and withdraw multiple unfeasible routes from service.  The UPDATE
          message always includes the fixed-size BGP header, and can optionally
          include the other fields as shown below:


             +-----------------------------------------------------+
             |   Unfeasible Routes Length (2 octets)               |
             +-----------------------------------------------------+
             |  Withdrawn Routes (variable)                        |
             +-----------------------------------------------------+
             |   Total Path Attribute Length (2 octets)            |
             +-----------------------------------------------------+
             |    Path Attributes (variable)                       |
             +-----------------------------------------------------+
             |   Network Layer Reachability Information (variable) |
             +-----------------------------------------------------+



             Unfeasible Routes Length:

                This 2-octets unsigned integer indicates the total length of
                the Withdrawn Routes field in octets.  Its value must allow the
                length of the Network Layer Reachability Information field to
                be determined as specified below.

                A value of 0 indicates that no routes are being withdrawn from
                service, and that the WITHDRAWN ROUTES field is not present in
                this UPDATE message.

             Withdrawn Routes:


                This is a variable length field that contains a list of IP
                address prefixes for the routes that are being withdrawn from
                service.  Each IP address prefix is encoded as a 2-tuple of the
                form <length, prefix>, whose fields are described below:

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







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

                a) Length:

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

                b) Prefix:

                   The Prefix field contains IP address prefixes 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.

             Total Path Attribute Length:

                This 2-octet unsigned integer indicates the total length of the
                Path Attributes field in octets.  Its value must allow the
                length of the Network Layer Reachability field to be determined
                as specified below.

                A value of 0 indicates that no Network Layer Reachability
                Information field is present in this UPDATE message.

             Path Attributes:

                A variable length sequence of path attributes is present in
                every UPDATE.  Each path attribute is a triple <attribute type,
                attribute length, attribute value> of variable length.

                Attribute Type is a two-octet field that consists of the
                Attribute Flags octet followed by the Attribute Type Code
                octet.




                       0                   1
                       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                      |  Attr. Flags  |Attr. Type Code|
                      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+







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                The high-order bit (bit 0) of the Attribute Flags octet is the
                Optional bit.  It defines whether the attribute is optional (if
                set to 1) or well-known (if set to 0).

                The second high-order bit (bit 1) of the Attribute Flags octet
                is the Transitive bit.  It defines whether an optional
                attribute is transitive (if set to 1) or non-transitive (if set
                to 0).  For well-known attributes, the Transitive bit must be
                set to 1.  (See Section 5 for a discussion of transitive
                attributes.)

                The third high-order bit (bit 2) of the Attribute Flags octet
                is the Partial bit.  It defines whether the information
                contained in the optional transitive attribute is partial (if
                set to 1) or complete (if set to 0).  For well-known attributes
                and for optional non-transitive attributes the Partial bit must
                be set to 0.

                The fourth high-order bit (bit 3) of the Attribute Flags octet
                is the Extended Length bit.  It defines whether the Attribute
                Length is one octet (if set to 0) or two octets (if set to 1).
                Extended Length may be used only if the length of the attribute
                value is greater than 255 octets.

                The lower-order four bits of the Attribute Flags octet are .
                unused. They must be zero (and must be ignored when received).

                The Attribute Type Code octet contains the Attribute Type Code.
                Currently defined Attribute Type Codes are discussed in Section
                5.

                If the Extended Length bit of the Attribute Flags octet is set
                to 0, the third octet of the Path Attribute contains the length
                of the attribute data in octets.

                If the Extended Length bit of the Attribute Flags octet is set
                to 1, then the third and the fourth octets of the path
                attribute contain the length of the attribute data in octets.

                The remaining octets of the Path Attribute represent the
                attribute value and are interpreted according to the Attribute
                Flags and the Attribute Type Code. The supported Attribute Type
                Codes, their attribute values and uses are the following:

                a)   ORIGIN (Type Code 1):






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                   ORIGIN is a well-known mandatory attribute that defines the
                   origin of the path information.   The data octet can assume
                   the following values:

                         Value      Meaning

                         0         IGP - Network Layer Reachability Information
                                      is interior to the originating AS

                         1         EGP - Network Layer Reachability Information
                                      learned via EGP

                         2         INCOMPLETE - Network Layer Reachability
                                      Information learned by some other means

                   Its usage is defined in 5.1.1

                b) AS_PATH (Type Code 2):

                   AS_PATH is a well-known mandatory attribute that is composed
                   of a sequence of AS path segments. Each AS path segment is
                   represented by a triple <path segment type, path segment
                   length, path segment value>.

                   The path segment type is a 1-octet long field with the
                   following values defined:

                         Value      Segment Type

                         1         AS_SET: unordered set of ASs a route in the
                                      UPDATE message has traversed

                         2         AS_SEQUENCE: ordered set of ASs a route in
                                      the UPDATE message has traversed

                   The path segment length is a 1-octet long field containing
                   the number of ASs in the path segment value field.

                   The path segment value field contains one or more AS
                   numbers, each encoded as a 2-octets long field.

                   Usage of this attribute is defined in 5.1.2.

                c)   NEXT_HOP (Type Code 3):

                   This is a well-known mandatory attribute that defines the IP





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                   address of the border router that should be used as the next
                   hop to the destinations listed in the Network Layer
                   Reachability field of the UPDATE message.

                   Usage of this attribute is defined in 5.1.3.


                d) MULTI_EXIT_DISC (Type Code 4):

                   This is an optional non-transitive attribute that is a four
                   octet non-negative integer. The value of this attribute may
                   be used by a BGP speaker's decision process to discriminate
                   among multiple exit points to a neighboring autonomous
                   system.

                   Its usage is defined in 5.1.4.

                e) LOCAL_PREF (Type Code 5):

                   LOCAL_PREF is a well-known mandatory attribute that is a
                   four octet non-negative integer. It is used by a BGP speaker
                   to inform other internal peers of the advertising speaker's
                   degree of preference for an advertised route. Usage of this
                   attribute is described in 5.1.5.

                f) ATOMIC_AGGREGATE (Type Code 6)

                   ATOMIC_AGGREGATE is a well-known discretionary attribute of
                   length 0. It is used by a BGP speaker to inform other BGP
                   speakers that the local system selected a less specific
                   route without selecting a more specific route which is
                   included in it. Usage of this attribute is described in
                   5.1.6.

                g) AGGREGATOR (Type Code 7)

                   AGGREGATOR is an optional transitive attribute of length 6.
                   The attribute contains the last AS number that formed the
                   aggregate route (encoded as 2 octets), followed by the IP
                   address of the BGP speaker that formed the aggregate route
                   (encoded as 4 octets).  Usage of this attribute is described
                   in 5.1.7

             Network Layer Reachability Information:

                This variable length field contains a list of IP address





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                prefixes.  The length in octets of the Network Layer
                Reachability Information is not encoded explicitly, but can be
                calculated as:

                   UPDATE message Length - 23 - Total Path Attributes Length -
                   Unfeasible Routes Length

                where UPDATE message Length is the value encoded in the fixed-
                size BGP header, Total Path Attribute Length and Unfeasible
                Routes Length  are the values encoded in the variable part of
                the UPDATE message, and 23 is a combined length of the fixed-
                size BGP header, the Total Path Attribute Length field and the
                Unfeasible Routes Length field.

                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 IP
                   address prefix. A length of zero indicates a prefix that
                   matches all IP addresses (with prefix, itself, of zero
                   octets).

                b) Prefix:

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

          The minimum length of the UPDATE message is 23 octets -- 19 octets
          for the fixed header + 2 octets for the Unfeasible Routes Length + 2
          octets for the Total Path Attribute Length (the value of Unfeasible
          Routes Length is 0  and the value of Total Path Attribute Length is
          0).





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          An UPDATE message can advertise at most one route, which may be
          described by several path attributes. All path attributes contained
          in a given UPDATE messages apply to the destinations carried in the
          Network Layer Reachability Information field of the UPDATE message.

          An UPDATE message can list multiple routes to be withdrawn from
          service.  Each such route is identified by its destination (expressed
          as an IP prefix), which unambiguously identifies the route in the
          context of the BGP speaker - BGP speaker connection to which it has
          been previously been advertised.

          An UPDATE message may advertise only routes to be withdrawn from
          service, in which case it will not include path attributes or Network
          Layer Reachability Information. Conversely, it may advertise only a
          feasible route, in which case the WITHDRAWN ROUTES field need not be
          present.


       4.4 KEEPALIVE Message Format


          BGP does not use any transport protocol-based keep-alive mechanism to
          determine if peers are reachable.  Instead, KEEPALIVE messages are
          exchanged between peers often enough as not to cause the Hold Timer
          to expire.  A reasonable maximum time between KEEPALIVE messages
          would be one third of the Hold Time interval.  KEEPALIVE messages
          MUST NOT be sent more frequently than one per second.  An
          implementation MAY adjust the rate at which it sends KEEPALIVE
          messages as a function of the Hold Time interval.

          If the negotiated Hold Time interval is zero, then periodic KEEPALIVE
          messages MUST NOT be sent.

          KEEPALIVE message consists of only message header and has a length of
          19 octets.


       4.5 NOTIFICATION Message Format


          A NOTIFICATION message is sent when an error condition is detected.
          The BGP connection is closed immediately after sending it.

          In addition to the fixed-size BGP header, the NOTIFICATION message
          contains the following fields:






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               0                   1                   2                   3
               0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
              | Error code    | Error subcode |           Data                |
              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+                               +
              |                                                               |
              +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



             Error Code:

                This 1-octet unsigned integer indicates the type of
                NOTIFICATION.  The following Error Codes have been defined:

                   Error Code       Symbolic Name               Reference

                     1         Message Header Error             Section 6.1

                     2         OPEN Message Error               Section 6.2

                     3         UPDATE Message Error             Section 6.3

                     4         Hold Timer Expired               Section 6.5

                     5         Finite State Machine Error       Section 6.6

                     6         Cease                            Section 6.7


             Error subcode:

                This 1-octet unsigned integer provides more specific
                information about the nature of the reported error.  Each Error
                Code may have one or more Error Subcodes associated with it.
                If no appropriate Error Subcode is defined, then a zero
                (Unspecific) value is used for the Error Subcode field.

                Message Header Error subcodes:

                                      1  - Connection Not Synchronized.
                                      2  - Bad Message Length.
                                      3  - Bad Message Type.

                OPEN Message Error subcodes:






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                                      1  - Unsupported Version Number.
                                      2  - Bad Peer AS.
                                      3  - Bad BGP Identifier.
                                      4  - Unsupported Optional Parameter.
                                      5  - Authentication Failure.
                                      6  - Unacceptable Hold Time.

                UPDATE Message Error subcodes:

                                      1 - Malformed Attribute List.
                                      2 - Unrecognized Well-known Attribute.
                                      3 - Missing Well-known Attribute.
                                      4 - Attribute Flags Error.
                                      5 - Attribute Length Error.
                                      6 - Invalid ORIGIN Attribute
                                      8 - Invalid NEXT_HOP Attribute.
                                      9 - Optional Attribute Error.
                                     10 - Invalid Network Field.
                                     11 - Malformed AS_PATH.

             Data:

                This variable-length field is used to diagnose the reason for
                the NOTIFICATION.  The contents of the Data field depend upon
                the Error Code and Error Subcode.  See Section 6 below for more
                details.

                Note that the length of the Data field can be determined from
                the message Length field by the formula:

                         Message Length = 21 + Data Length


          The minimum length of the NOTIFICATION message is 21 octets
          (including message header).


       5.  Path Attributes


          This section discusses the path attributes of the UPDATE message.

          Path attributes fall into four separate categories:

                      1. Well-known mandatory.
                      2. Well-known discretionary.





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                      3. Optional transitive.
                      4. Optional non-transitive.

          Well-known attributes must be recognized by all BGP implementations.
          Some of these attributes are mandatory and must be included in every
          UPDATE message that contains NLRI.  Others are discretionary and may
          or may not be sent in a particular UPDATE message.

          All well-known attributes must be passed along (after proper
          updating, if necessary) to other BGP peers.

          In addition to well-known attributes, each path may contain one or
          more optional attributes.  It is not required or expected that all
          BGP implementations support all optional attributes.  The handling of
          an unrecognized optional attribute is determined by the setting of
          the Transitive bit in the attribute flags octet.  Paths with
          unrecognized transitive optional attributes should be accepted. If a
          path with unrecognized transitive optional attribute is accepted and
          passed along to other BGP peers, then the unrecognized transitive
          optional attribute of that path must be passed along with the path to
          other BGP peers with the Partial bit in the Attribute Flags octet set
          to 1. If a path with recognized transitive optional attribute is
          accepted and passed along to other BGP peers and the Partial bit in
          the Attribute Flags octet is set to 1 by some previous AS, it is not
          set back to 0 by the current AS. Unrecognized non-transitive optional
          attributes must be quietly ignored and not passed along to other BGP
          peers.

          New transitive optional attributes may be attached to the path by the
          originator or by any other AS in the path.  If they are not attached
          by the originator, the Partial bit in the Attribute Flags octet is
          set to 1.  The rules for attaching new non-transitive optional
          attributes will depend on the nature of the specific attribute.  The
          documentation of each new non-transitive optional attribute will be
          expected to include such rules.  (The description of the
          MULTI_EXIT_DISC attribute gives an example.)  All optional attributes
          (both transitive and non-transitive) may be updated (if appropriate)
          by ASs in the path.

          The sender of an UPDATE message should order path attributes within
          the UPDATE message in ascending order of attribute type.  The
          receiver of an UPDATE message must be prepared to handle path
          attributes within the UPDATE message that are out of order.

          The same attribute cannot appear more than once within the Path
          Attributes field of a particular UPDATE message.





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          The mandatory category refers to an attribute which must be present
          in both IBGP and EBGP exchanges if NLRI are contained in the UPDATE
          message.  Attributes classified as optional for the purpose of the
          protocol extension mechanism may be purely discretionary, or
          discretionary, required, or disallowed in certain contexts.

               attribute           EBGP                    IBGP
                ORIGIN             mandatory               mandatory
                AS_PATH            mandatory               mandatory
                NEXT_HOP           mandatory               mandatory
                MULTI_EXIT_DISC    discretionary           discretionary
                LOCAL_PREF         disallowed              required
                ATOMIC_AGGREGATE   see section 5.1.6 and 9.1.4
                AGGREGATOR         discretionary           discretionary




       5.1 Path Attribute Usage


          The usage of each BGP path attributes is described in the following
          clauses.



       5.1.1 ORIGIN


          ORIGIN is a well-known mandatory attribute.  The ORIGIN attribute
          shall be generated by the autonomous system that originates the
          associated routing information. It shall be included in the UPDATE
          messages of all BGP speakers that choose to propagate this
          information to other BGP speakers.


       5.1.2   AS_PATH


          AS_PATH is a well-known mandatory attribute. This attribute
          identifies the autonomous systems through which routing information
          carried in this UPDATE message has passed. The components of this
          list can be AS_SETs or AS_SEQUENCEs.

          When a BGP speaker propagates a route which it has learned from
          another BGP speaker's UPDATE message, it shall modify the route's





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          AS_PATH attribute based on the location of the BGP speaker to which
          the route will be sent:

             a) When a given BGP speaker advertises the route to an internal
             peer, the advertising speaker shall not modify the AS_PATH
             attribute associated with the route.

             b) When a given BGP speaker advertises the route to an external
             peer, then the advertising speaker shall update the AS_PATH
             attribute as follows:

                1) if the first path segment of the AS_PATH is of type
                AS_SEQUENCE, the local system shall prepend its own AS number
                as the last element of the sequence  (put it in the leftmost
                position)

                2) if the first path segment of the AS_PATH is of type AS_SET,
                the local system shall prepend a new path segment of type
                AS_SEQUENCE to the AS_PATH, including its own AS number in that
                segment.

             When a BGP speaker originates a route then:


                a) the originating speaker shall include its own AS number in
                the AS_PATH attribute of all UPDATE messages sent to an
                external peer.  (In this case, the AS number of the originating
                speaker's autonomous system will be the only entry in the
                AS_PATH attribute).

                b) the originating speaker shall include an empty AS_PATH
                attribute in all UPDATE messages sent to internal peers.  (An
                empty AS_PATH attribute is one whose length field contains the
                value zero).


       5.1.3 NEXT_HOP


          The NEXT_HOP path attribute defines the IP address of the border
          router that should be used as the next hop to the destinations listed
          in the UPDATE message.  When advertising a NEXT_HOP attribute to an
          external peer, a router may use one of its own interface addresses in
          the NEXT_HOP 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 attribute.  A BGP speaker





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          can advertise to an external peer an interface of any internal peer
          router in the NEXT_HOP 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 "third party" NEXT_HOP
          attribute.  A BGP speaker can advertise any external peer router in
          the NEXT_HOP attribute provided that the IP 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
          attribute.

          Normally the NEXT_HOP attribute 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 attributes to handle
          imperfectly bridged media.

          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 attribute
          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.


       5.1.4   MULTI_EXIT_DISC


          The MULTI_EXIT_DISC attribute may be used on external (inter-AS)
          links to discriminate among multiple exit or entry points to the same
          neighboring AS.  The value of the MULTI_EXIT_DISC attribute is a four
          octet unsigned number which is called a metric.  All other factors
          being equal, the exit or entry point with lower metric should be
          preferred.  If received over external links, the MULTI_EXIT_DISC
          attribute MAY be propagated over internal links to other BGP speakers
          within the same AS.  The MULTI_EXIT_DISC attribute received from a
          neighboring AS MUST NOT be propagated to other neighboring ASs.

          A BGP speaker MUST IMPLEMENT a mechanism based on local configuration
          which allows the MULTI_EXIT_DISC attribute to be removed from a
          route.  This MAY be done either prior to or after determining the
          degree of preference of the route and performing route selection
          (decision process phases 1 and 2).





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          An implementation MAY also (based on local configuration) alter the
          value of the MULTI_EXIT_DISC attribute received over an external
          link.  If it does so, it shall do so prior to determining the degree
          of preference of the route and performing route selection (decision
          process phases 1 and 2).


       5.1.5   LOCAL_PREF


          LOCAL_PREF is a well-known mandatory attribute that SHALL be included
          in all UPDATE messages that a given BGP speaker sends to the other
          internal peers. A BGP speaker SHALL calculate the degree of
          preference for each external route and include the degree of
          preference when advertising a route to its internal peers. The higher
          degree of preference MUST be preferred. A BGP speaker shall use the
          degree of preference learned via LOCAL_PREF in its decision process
          (see section 9.1.1).

          A BGP speaker MUST NOT include this attribute in UPDATE messages that
          it sends to external peers.  If it is contained in an UPDATE message
          that is received from an external peer, then this attribute MUST be
          ignored by the receiving speaker.


       5.1.6   ATOMIC_AGGREGATE


          ATOMIC_AGGREGATE is a well-known discretionary attribute.  If a BGP
          speaker, when presented with a set of overlapping routes from one of
          its peers (see 9.1.4), selects the less specific route without
          selecting the more specific one, then the local system MUST attach
          the ATOMIC_AGGREGATE attribute to the route when propagating it to
          other BGP speakers (if that attribute is not already present in the
          received less specific route). A BGP speaker that receives a route
          with the ATOMIC_AGGREGATE attribute MUST NOT remove the attribute
          from the route when propagating it to other speakers. A BGP speaker
          that receives a route with the ATOMIC_AGGREGATE attribute MUST NOT
          make any NLRI of that route more specific (as defined in 9.1.4) when
          advertising this route to other BGP speakers.  A BGP speaker that
          receives a route with the ATOMIC_AGGREGATE attribute needs to be
          cognizant of the fact that the actual path to destinations, as
          specified in the NLRI of the route, while having the loop-free
          property, may traverse ASs that are not listed in the AS_PATH
          attribute.






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       RFC DRAFT                                                    August 1998


       5.1.7   AGGREGATOR


          AGGREGATOR is an optional transitive attribute which may be included
          in updates which are formed by aggregation (see Section 9.2.4.2).  A
          BGP speaker which performs route aggregation may add the AGGREGATOR
          attribute which shall contain its own AS number and IP address.


       6.  BGP Error Handling.


          This section describes actions to be taken when errors are detected
          while processing BGP messages.

          When any of the conditions described here are detected, a
          NOTIFICATION message with the indicated Error Code, Error Subcode,
          and Data fields is sent, and the BGP connection is closed.  If no
          Error Subcode is specified, then a zero must be used.

          The phrase "the BGP connection is closed" means that the transport
          protocol connection has been closed and that all resources for that
          BGP connection have been deallocated.  Routing table entries
          associated with the remote peer are marked as invalid.  The fact that
          the routes have become invalid is passed to other BGP peers before
          the routes are deleted from the system.

          Unless specified explicitly, the Data field of the NOTIFICATION
          message that is sent to indicate an error is empty.


       6.1 Message Header error handling.


          All errors detected while processing the Message Header are indicated
          by sending the NOTIFICATION message with Error Code Message Header
          Error.  The Error Subcode elaborates on the specific nature of the
          error.

          The expected value of the Marker field of the message header is all
          ones if the message type is OPEN.  The expected value of the Marker
          field for all other types of BGP messages determined based on the
          presence of the Authentication Information Optional Parameter in the
          BGP OPEN message and the actual authentication mechanism (if the
          Authentication Information in the BGP OPEN message is present). If
          the Marker field of the message header is not the expected one, then





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          a synchronization error has occurred and the Error Subcode is set to
          Connection Not Synchronized.

          If the Length field of the message header is less than 19 or greater
          than 4096, or if the Length field of an OPEN message is less  than
          the minimum length of the OPEN message, or if the Length field of an
          UPDATE message is less than the minimum length of the UPDATE message,
          or if the Length field of a KEEPALIVE message is not equal to 19, or
          if the Length field of a NOTIFICATION message is less than the
          minimum length of the NOTIFICATION message, then the Error Subcode is
          set to Bad Message Length.  The Data field contains the erroneous
          Length field.

          If the Type field of the message header is not recognized, then the
          Error Subcode is set to Bad Message Type.  The Data field contains
          the erroneous Type field.


       6.2 OPEN message error handling.


          All errors detected while processing the OPEN message are indicated
          by sending the NOTIFICATION message with Error Code OPEN Message
          Error.  The Error Subcode elaborates on the specific nature of the
          error.

          If the version number contained in the Version field of the received
          OPEN message is not supported, then the Error Subcode is set to
          Unsupported Version Number.  The Data field is a 2-octet unsigned
          integer, which indicates the largest locally supported version number
          less than the version the remote BGP peer bid (as indicated in the
          received OPEN message).

          If the Autonomous System field of the OPEN message is unacceptable,
          then the Error Subcode is set to Bad Peer AS.  The determination of
          acceptable Autonomous System numbers is outside the scope of this
          protocol.

          If the Hold Time field of the OPEN message is unacceptable, then the
          Error Subcode MUST be set to Unacceptable Hold Time.  An
          implementation MUST reject Hold Time values of one or two seconds.
          An implementation MAY reject any proposed Hold Time.  An
          implementation which accepts a Hold Time MUST use the negotiated
          value for the Hold Time.

          If the BGP Identifier field of the OPEN message is syntactically





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          incorrect, then the Error Subcode is set to Bad BGP Identifier.
          Syntactic correctness means that the BGP Identifier field represents
          a valid IP host address.

          If one of the Optional Parameters in the OPEN message is not
          recognized, then the Error Subcode is set to Unsupported Optional
          Parameters.


          If the OPEN message carries Authentication Information (as an
          Optional Parameter), then the corresponding authentication procedure
          is invoked.  If the authentication procedure (based on Authentication
          Code and Authentication Data) fails, then the Error Subcode is set to
          Authentication Failure.



       6.3 UPDATE message error handling.


          All errors detected while processing the UPDATE message are indicated
          by sending the NOTIFICATION message with Error Code UPDATE Message
          Error.  The error subcode elaborates on the specific nature of the
          error.

          Error checking of an UPDATE message begins by examining the path
          attributes.  If the Unfeasible Routes Length or Total Attribute
          Length is too large (i.e., if Unfeasible Routes Length + Total
          Attribute Length + 23 exceeds the message Length), then the Error
          Subcode is set to Malformed Attribute List.

          If any recognized attribute has Attribute Flags that conflict with
          the Attribute Type Code, then the Error Subcode is set to Attribute
          Flags Error.  The Data field contains the erroneous attribute (type,
          length and value).

          If any recognized attribute has Attribute Length that conflicts with
          the expected length (based on the attribute type code), then the
          Error Subcode is set to Attribute Length Error.  The Data field
          contains the erroneous attribute (type, length and value).

          If any of the mandatory well-known attributes are not present, then
          the Error Subcode is set to Missing Well-known Attribute.  The Data
          field contains the Attribute Type Code of the missing well-known
          attribute.






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          If any of the mandatory well-known attributes are not recognized,
          then the Error Subcode is set to Unrecognized Well-known Attribute.
          The Data field contains the unrecognized attribute (type, length and
          value).

          If the ORIGIN attribute has an undefined value, then the Error
          Subcode is set to Invalid Origin Attribute.  The Data field contains
          the unrecognized attribute (type, length and value).

          If the NEXT_HOP attribute field is syntactically incorrect, then the
          Error Subcode is set to Invalid NEXT_HOP Attribute.  The Data field
          contains the incorrect attribute (type, length and value).  Syntactic
          correctness means that the NEXT_HOP attribute represents a valid IP
          host address.  Semantic correctness applies only to the external BGP
          links. It means that the interface associated with the IP address, as
          specified in the NEXT_HOP attribute, shares a common subnet with the
          receiving BGP speaker and is not the IP address of the receiving BGP
          speaker.  If the NEXT_HOP attribute is semantically incorrect, the
          error should be logged, and the the route should be ignored.  In this
          case, no NOTIFICATION message should be sent.

          The AS_PATH attribute is checked for syntactic correctness.  If the
          path is syntactically incorrect, then the Error Subcode is set to
          Malformed AS_PATH.


          The information carried by the AS_PATH attribute is checked for AS
          loops. AS loop detection is done by scanning the full AS path (as
          specified in the AS_PATH attribute), and checking that the autonomous
          system number of the local system does not appear in the AS path. If
          the autonomous system number appears in the AS path the route may be
          stored in the Adj-RIB-In, but unless the router is configured to
          accept routes with its own autonomous system in the AS path, the
          route shall not be passed to the BGP Decision Process. Operations of
          a router that is configured to accept routes with its own autonomous
          system number in the AS path are outside the scope of this document.

          If an optional attribute is recognized, then the value of this
          attribute is checked.  If an error is detected, the attribute is
          discarded, and the Error Subcode is set to Optional Attribute Error.
          The Data field contains the attribute (type, length and value).

          If any attribute appears more than once in the UPDATE message, then
          the Error Subcode is set to Malformed Attribute List.

          The NLRI field in the UPDATE message is checked for syntactic





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          validity.  If the field is syntactically incorrect, then the Error
          Subcode is set to Invalid Network Field.

          An UPDATE message that contains correct path attributes, but no NLRI,
          shall be treated as a valid UPDATE message.


       6.4 NOTIFICATION message error handling.


          If a peer sends a NOTIFICATION message, and there is an error in that
          message, there is unfortunately no means of reporting this error via
          a subsequent NOTIFICATION message.  Any such error, such as an
          unrecognized Error Code or Error Subcode, should be noticed, logged
          locally, and brought to the attention of the administration of the
          peer.  The means to do this, however, lies outside the scope of this
          document.


       6.5 Hold Timer Expired error handling.


          If a system does not receive successive KEEPALIVE and/or UPDATE
          and/or NOTIFICATION messages within the period specified in the Hold
          Time field of the OPEN message, then the NOTIFICATION message with
          Hold Timer Expired Error Code must be sent and the BGP connection
          closed.


       6.6 Finite State Machine error handling.


          Any error detected by the BGP Finite State Machine (e.g., receipt of
          an unexpected event) is indicated by sending the NOTIFICATION message
          with Error Code Finite State Machine Error.


       6.7 Cease.


          In absence of any fatal errors (that are indicated in this section),
          a BGP peer may choose at any given time to close its BGP connection
          by sending the NOTIFICATION message with Error Code Cease.  However,
          the Cease NOTIFICATION message must not be used when a fatal error
          indicated by this section does exist.






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       6.8 Connection collision detection.


          If a pair of BGP speakers try simultaneously to establish a TCP
          connection to each other, then two parallel connections between this
          pair of speakers might well be formed.  We refer to this situation as
          connection collision.  Clearly, one of these connections must be
          closed.

          Based on the value of the BGP Identifier a convention is established
          for detecting which BGP connection is to be preserved when a
          collision does occur. The convention is to compare the BGP
          Identifiers of the peers involved in the collision and to retain only
          the connection initiated by the BGP speaker with the higher-valued
          BGP Identifier.

          Upon receipt of an OPEN message, the local system must examine all of
          its connections that are in the OpenConfirm state.  A BGP speaker may
          also examine connections in an OpenSent state if it knows the BGP
          Identifier of the peer by means outside of the protocol.  If among
          these connections there is a connection to a remote BGP speaker whose
          BGP Identifier equals the one in the OPEN message, then the local
          system performs the following collision resolution procedure:


             1. The BGP Identifier of the local system is compared to the BGP
             Identifier of the remote system (as specified in the OPEN
             message).

             2. If the value of the local BGP Identifier is less than the
             remote one, the local system closes BGP connection that already
             exists (the one that is already in the OpenConfirm state), and
             accepts BGP connection initiated by the remote system.

             3. Otherwise, the local system closes newly created BGP connection
             (the one associated with the newly received OPEN message), and
             continues to use the existing one (the one that is already in the
             OpenConfirm state).

             Comparing BGP Identifiers is done by treating them as (4-octet
             long) unsigned integers.

             A connection collision with an existing BGP connection that is in
             Established states causes unconditional closing of the newly
             created connection. Note that a connection collision cannot be
             detected with connections that are in Idle, or Connect, or Active





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             states.

             Closing the BGP connection (that results from the collision
             resolution procedure) is accomplished by sending the NOTIFICATION
             message with the Error Code Cease.


       7.  BGP Version Negotiation.


          BGP speakers may negotiate the version of the protocol by making
          multiple attempts to open a BGP connection, starting with the highest
          version number each supports.  If an open attempt fails with an Error
          Code OPEN Message Error, and an Error Subcode Unsupported Version
          Number, then the BGP speaker has available the version number it
          tried, the version number its peer tried, the version number passed
          by its peer in the NOTIFICATION message, and the version numbers that
          it supports.  If the two peers do support one or more common
          versions, then this will allow them to rapidly determine the highest
          common version. In order to support BGP version negotiation, future
          versions of BGP must retain the format of the OPEN and NOTIFICATION
          messages.


       8.  BGP Finite State machine.


          This section specifies BGP operation in terms of a Finite State
          Machine (FSM).  Following is a brief summary and overview of BGP
          operations by state as determined by this FSM.  A condensed version
          of the BGP FSM is found in Appendix 1.

             Initially BGP is in the Idle state.

             Idle state:

                In this state BGP refuses all incoming BGP connections.  No
                resources are allocated to the peer.  In response to the Start
                event (initiated by either system or operator) the local system
                initializes all BGP resources, starts the ConnectRetry timer,
                initiates a transport connection to other BGP peer, while
                listening for connection that may be initiated by the remote
                BGP peer, and changes its state to Connect.  The exact value of
                the ConnectRetry timer is a local matter, but should be
                sufficiently large to allow TCP initialization.






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                If a BGP speaker detects an error, it shuts down the connection
                and changes its state to Idle. Getting out of the Idle state
                requires generation of the Start event.  If such an event is
                generated automatically, then persistent BGP errors may result
                in persistent flapping of the speaker.  To avoid such a
                condition it is recommended that Start events should not be
                generated immediately for a peer that was previously
                transitioned to Idle due to an error. For a peer that was
                previously transitioned to Idle due to an error, the time
                between consecutive generation of Start events, if such events
                are generated automatically, shall exponentially increase. The
                value of the initial timer shall be 60 seconds. The time shall
                be doubled for each consecutive retry.

                Any other event received in the Idle state is ignored.

             Connect state:

                In this state BGP is waiting for the transport protocol
                connection to be completed.

                If the transport protocol connection succeeds, the local system
                clears the ConnectRetry timer, completes initialization, sends
                an OPEN message to its peer, and changes its state to OpenSent.

                If the transport protocol connect fails (e.g., retransmission
                timeout), the local system restarts the ConnectRetry timer,
                continues to listen for a connection that may be initiated by
                the remote BGP peer, and changes its state to Active state.

                In response to the ConnectRetry timer expired event, the local
                system restarts the ConnectRetry timer, initiates a transport
                connection to other BGP peer, continues to listen for a
                connection that may be initiated by the remote BGP peer, and
                stays in the Connect state.

                Start event is ignored in the Connect state.

                In response to any other event (initiated by either system or
                operator), the local system releases all BGP resources
                associated with this connection and changes its state to Idle.

             Active state:

                In this state BGP is trying to acquire a peer by initiating a
                transport protocol connection.





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       RFC DRAFT                                                    August 1998


                If the transport protocol connection succeeds, the local system
                clears the ConnectRetry timer, completes initialization, sends
                an OPEN message to its peer, sets its Hold Timer to a large
                value, and changes its state to OpenSent.  A Hold Timer value
                of 4 minutes is suggested.

                In response to the ConnectRetry timer expired event, the local
                system restarts the ConnectRetry timer, initiates a transport
                connection to other BGP peer, continues to listen for a
                connection that may be initiated by the remote BGP peer, and
                changes its state to Connect.

                If the local system detects that a remote peer is trying to
                establish BGP connection to it, and the IP address of the
                remote peer is not an expected one, the local system restarts
                the ConnectRetry timer, rejects the attempted connection,
                continues to listen for a connection that may be initiated by
                the remote BGP peer, and stays in the Active state.

                Start event is ignored in the Active state.

                In response to any other event (initiated by either system or
                operator), the local system releases all BGP resources
                associated with this connection and changes its state to Idle.

             OpenSent state:

                In this state BGP waits for an OPEN message from its peer.
                When an OPEN message is received, all fields are checked for
                correctness.  If the BGP message header checking or OPEN
                message checking detects an error (see Section 6.2), or a
                connection collision (see Section 6.8) the local system sends a
                NOTIFICATION message and changes its state to Idle.

                If there are no errors in the OPEN message, BGP sends a
                KEEPALIVE message and sets a KeepAlive timer.  The Hold Timer,
                which was originally set to a large value (see above), is
                replaced with the negotiated Hold Time value (see section 4.2).
                If the negotiated Hold Time value is zero, then the Hold Time
                timer and KeepAlive timers are not started.  If the value of
                the Autonomous System field is the same as the local Autonomous
                System number, then the connection is an "internal" connection;
                otherwise, it is "external".  (This will effect UPDATE
                processing as described below.)  Finally, the state is changed
                to OpenConfirm.






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       RFC DRAFT                                                    August 1998


                If a disconnect notification is received from the underlying
                transport protocol, the local system closes the BGP connection,
                restarts the ConnectRetry timer, while continue listening for
                connection that may be initiated by the remote BGP peer, and
                goes into the Active state.

                If the Hold Timer expires, the local system sends NOTIFICATION
                message with error code Hold Timer Expired and changes its
                state to Idle.

                In response to the Stop event (initiated by either system or
                operator) the local system sends NOTIFICATION message with
                Error Code Cease and changes its state to Idle.

                Start event is ignored in the OpenSent state.

                In response to any other event the local system sends
                NOTIFICATION message with Error Code Finite State Machine Error
                and changes its state to Idle.

                Whenever BGP changes its state from OpenSent to Idle, it closes
                the BGP (and transport-level) connection and releases all
                resources associated with that connection.

             OpenConfirm state:

                In this state BGP waits for a KEEPALIVE or NOTIFICATION
                message.

                If the local system receives a KEEPALIVE message, it changes
                its state to Established.

                If the Hold Timer expires before a KEEPALIVE message is
                received, the local system sends NOTIFICATION message with
                error code Hold Timer Expired and changes its state to Idle.

                If the local system receives a NOTIFICATION message, it changes
                its state to Idle.

                If the KeepAlive timer expires, the local system sends a
                KEEPALIVE message and restarts its KeepAlive timer.

                If a disconnect notification is received from the underlying
                transport protocol, the local system changes its state to Idle.

                In response to the Stop event (initiated by either system or





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       RFC DRAFT                                                    August 1998


                operator) the local system sends NOTIFICATION message with
                Error Code Cease and changes its state to Idle.

                Start event is ignored in the OpenConfirm state.

                In response to any other event the local system sends
                NOTIFICATION message with Error Code Finite State Machine Error
                and changes its state to Idle.

                Whenever BGP changes its state from OpenConfirm to Idle, it
                closes the BGP (and transport-level) connection and releases
                all resources associated with that connection.

             Established state:

                In the Established state BGP can exchange UPDATE, NOTIFICATION,
                and KEEPALIVE messages with its peer.

                If the local system receives an UPDATE or KEEPALIVE message, it
                restarts its Hold Timer, if the negotiated Hold Time value is
                non-zero.

                If the local system receives a NOTIFICATION message, it changes
                its state to Idle.

                If the local system receives an UPDATE message and the UPDATE
                message error handling procedure (see Section 6.3) detects an
                error, the local system sends a NOTIFICATION message and
                changes its state to Idle.

                If a disconnect notification is received from the underlying
                transport protocol, the local system changes its state to Idle.

                If the Hold Timer expires, the local system sends a
                NOTIFICATION message with Error Code Hold Timer Expired and
                changes its state to Idle.

                If the KeepAlive timer expires, the local system sends a
                KEEPALIVE message and restarts its KeepAlive timer.

                Each time the local system sends a KEEPALIVE or UPDATE message,
                it restarts its KeepAlive timer, unless the negotiated Hold
                Time value is zero.

                In response to the Stop event (initiated by either system or
                operator), the local system sends a NOTIFICATION message with





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       RFC DRAFT                                                    August 1998


                Error Code Cease and changes its state to Idle.

                Start event is ignored in the Established state.

                In response to any other event, the local system sends
                NOTIFICATION message with Error Code Finite State Machine Error
                and changes its state to Idle.

                Whenever BGP changes its state from Established to Idle, it
                closes the BGP (and transport-level) connection, releases all
                resources associated with that connection, and deletes all
                routes derived from that connection.


       9.  UPDATE Message Handling


          An UPDATE message may be received only in the Established state.
          When an UPDATE message is received, each field is checked for
          validity as specified in Section 6.3.

          If an optional non-transitive attribute is unrecognized, it is
          quietly ignored.  If an optional transitive attribute is
          unrecognized, the Partial bit (the third high-order bit) in the
          attribute flags octet is set to 1, and the attribute is retained for
          propagation to other BGP speakers.

          If an optional attribute is recognized, and has a valid value, then,
          depending on the type of the optional attribute, it is processed
          locally, retained, and updated, if necessary, for possible
          propagation to other BGP speakers.


          If the UPDATE message contains a non-empty WITHDRAWN ROUTES field,
          the previously advertised routes whose  destinations (expressed as IP
          prefixes) contained in this field shall be removed from the Adj-RIB-
          In.  This BGP speaker shall run its Decision Process since the
          previously advertised route is not longer available for use.

          If the UPDATE message contains a feasible route, it shall be placed
          in the appropriate Adj-RIB-In, and the following additional actions
          shall be taken:

          i) If its Network Layer Reachability Information (NLRI) is identical
          to the one of a route currently stored in the Adj-RIB-In, then the
          new route shall replace the older route in the Adj-RIB-In, thus





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[Page 37]
       RFC DRAFT                                                    August 1998


          implicitly withdrawing the older route from service. The BGP speaker
          shall run its Decision Process since the older route is no longer
          available for use.

          ii) If the new route is an overlapping route that is included (see
          9.1.4) in an earlier route contained in the Adj-RIB-In, the BGP
          speaker shall run its Decision Process since the more specific route
          has implicitly made a portion of the less specific route unavailable
          for use.

          iii) If the new route has identical path attributes to an earlier
          route contained in the Adj-RIB-In, and is more specific (see 9.1.4)
          than the earlier route, no further actions are necessary.

          iv) If the new route has NLRI that is not present in any of the
          routes currently stored in the Adj-RIB-In, then the new route shall
          be placed in the Adj-RIB-In. The BGP speaker shall run its Decision
          Process.

          v) If the new route is an overlapping route that is less specific
          (see 9.1.4) than an earlier route contained in the Adj-RIB-In, the
          BGP speaker shall run its Decision Process on the set of destinations
          described only by the less specific route.


       9.1 Decision Process


          The Decision Process selects routes for subsequent advertisement by
          applying the policies in the local Policy Information Base (PIB) to
          the routes stored in its Adj-RIB-In. The output of the Decision
          Process is the set of routes that will be advertised to all peers;
          the selected routes will be stored in the local speaker's Adj-RIB-
          Out.

          The selection process is formalized by defining a function that takes
          the attribute of a given route as an argument and returns a non-
          negative integer denoting the degree of preference for the route.
          The function that calculates the degree of preference for a given
          route shall not use as its inputs any of the following:  the
          existence of other routes, the non-existence of other routes, or the
          path attributes of other routes. Route selection then consists of
          individual application of the degree of preference function to each
          feasible route, followed by the choice of the one with the highest
          degree of preference.






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[Page 38]
       RFC DRAFT                                                    August 1998


          The Decision Process operates on routes contained in each Adj-RIB-In,
          and is responsible for:

             - selection of routes to be advertised to internal peers

             - selection of routes to be advertised to external peers

             - route aggregation and route information reduction

          The Decision Process takes place in three distinct phases, each
          triggered by a different event:

             a) Phase 1 is responsible for calculating the degree of preference
             for each route received from an external peer, and MAY also
             advertise to  all the internal peers the routes from external
             peers that have the highest degree of preference for each distinct
             destination.

             b) Phase 2 is invoked on completion of phase 1. It is responsible
             for choosing the best route out of all those available for each
             distinct destination, and for installing each chosen route into
             the appropriate Loc-RIB.

             c) Phase 3 is invoked after the Loc-RIB has been modified. It is
             responsible for disseminating routes in the Loc-RIB to each
             external peer, according to the policies contained in the PIB.
             Route aggregation and information reduction can optionally be
             performed within this phase.


       9.1.1 Phase 1: Calculation of Degree of Preference


          The Phase 1 decision function shall be invoked whenever the local BGP
          speaker receives from a peer an UPDATE message that advertises a new
          route, a replacement route, or a withdrawn route.

          The Phase 1 decision function is a separate process which completes
          when it has no further work to do.

          The Phase 1 decision function shall lock an Adj-RIB-In prior to
          operating on any route contained within it, and shall unlock it after
          operating on all new or unfeasible routes contained within it.

          For each newly received or replacement feasible route, the local BGP
          speaker shall determine a degree of preference.  If the route is





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       RFC DRAFT                                                    August 1998


          learned from an internal peer, the value of the LOCAL_PREF attribute
          shall be taken as the degree of preference.  If the route is learned
          from an external peer, then the degree of preference shall be
          computed based on preconfigured policy information and used as the
          LOCAL_PREF value in any IBGP readvertisement.  The exact nature of
          this policy information and the computation involved is a local
          matter.  The local speaker shall then run the internal update process
          of 9.2.1 to select and advertise the most preferable route.


       9.1.2 Phase 2: Route Selection


          The Phase 2 decision function shall be invoked on completion of Phase
          1.  The Phase 2 function is a separate process which completes when
          it has no further work to do. The Phase 2 process shall consider all
          routes that are present in the Adj-RIBs-In, including those received
          from both internal and external peers.

          The Phase 2 decision function shall be blocked from running while the
          Phase 3 decision function is in process. The Phase 2 function shall
          lock all Adj-RIBs-In prior to commencing its function, and shall
          unlock them on completion.

          If the NEXT_HOP attribute of a BGP route depicts an address to which
          the local BGP speaker doesn't have a route in its Loc-RIB, the BGP
          route should be excluded from the Phase 2 decision function.

          It is critical that routers within an AS do not make conflicting
          decisions regarding route selection that would cause forwarding loops
          to occur.

          For each set of destinations for which a feasible route exists in the
          Adj-RIBs-In, the local BGP speaker shall identify the route that has:

             a) the highest degree of preference of any route to the same set
             of destinations, or

             b) is the only route to that destination, or

             c) is selected as a result of the Phase 2 tie breaking rules
             specified in 9.1.2.1.


          The local speaker SHALL then install that route in the Loc-RIB,
          replacing any route to the same destination that is currently being





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[Page 40]
       RFC DRAFT                                                    August 1998


          held in the Loc-RIB. The local speaker MUST determine the immediate
          next hop to the address depicted by the NEXT_HOP attribute of the
          selected route by performing a lookup in the IGP and selecting one of
          the possible paths in the IGP.  This immediate next hop MUST be used
          when installing the selected route in the Loc-RIB.  If the route to
          the address depicted by the NEXT_HOP attribute changes such that the
          immediate next hop changes, route selection should be recalculated as
          specified above.

          Unfeasible routes shall be removed from the Loc-RIB, and
          corresponding unfeasible routes shall then be removed from the Adj-
          RIBs-In.


       9.1.2.1 Breaking Ties (Phase 2)


          In its Adj-RIBs-In a BGP speaker may have several routes to the same
          destination that have the same degree of preference. The local
          speaker can select only one of these routes for inclusion in the
          associated Loc-RIB. The local speaker considers all routes with the
          same degrees of preference, both those received from internal peers,
          and those received from external peers.

          The following tie-breaking procedure assumes that for each candidate
          route all the BGP speakers within an autonomous system can ascertain
          the cost of a path (interior distance) to the address depicted by the
          NEXT_HOP attribute of the route.

          The tie-breaking algorithm begins by considering all equally
          preferable routes and then selects routes to be removed from
          consideration.  The algorithm terminates as soon as only one route
          remains in consideration.  The criteria must be applied in the order
          specified.

          Several of the criteria are described using pseudo-code.  Note that
          the pseudo-code shown was chosen for clarity, not efficiency.  It is
          not intended to specify any particular implementation.  BGP
          implementations MAY use any algorithm which produces the same results
          as those described here.

             a) Remove from consideration routes with less-preferred
             MULTI_EXIT_DISC attributes.  MULTI_EXIT_DISC is only comparable
             between routes learned from the same neighboring AS.  Routes which
             do not have the MULTI_EXIT_DISC attribute are considered to have
             the highest possible MULTI_EXIT_DISC value.





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       RFC DRAFT                                                    August 1998


             This is also described in the following procedure:

                   for m = all routes still under consideration
                       for n = all routes still under consideration
                           if (neighborAS(m) == neighborAS(n)) and (MED(n) < MED(m))
                               remove route m from consideration

             In the pseudo-code above, MED(n) is a function which returns the
             value of route n's MULTI_EXIT_DISC attribute.  If route n has no
             MULTI_EXIT_DISC attribute, the function returns the highest
             possible MULTI_EXIT_DISC value, i.e. 2^32-1.

             Similarly, neighborAS(n) is a function which returns the neighbor
             AS from which the route was received.

             b) Remove from consideration any routes with less-preferred
             interior cost.  The interior cost of a route is determined by
             calculating the metric to the next hop for the route using the
             interior routing protocol(s).  If the next hop for a route is
             reachable, but no cost can be determined, then this step should be
             should be skipped (equivalently, consider all routes to have equal
             costs).

             This is also described in the following procedure.

                   for m = all routes still under consideration
                       for n = all routes in still under consideration
                           if (cost(n) is better than cost(m))
                               remove m from consideration

             In the pseudo-code above, cost(n) is a function which returns the
             cost of the path (interior distance) to the address given in the
             NEXT_HOP attribute of the route.

             c) If at least one of the candidate routes was received from an
             external peer in a neighboring autonomous system, remove from
             consideration all routes which were received from internal peers.

             d) Remove from consideration all routes other than the route that
             was advertised by the BGP speaker whose BGP Identifier has the
             lowest value.










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       RFC DRAFT                                                    August 1998


       9.1.3   Phase 3: Route Dissemination


          The Phase 3 decision function shall be invoked on completion of Phase
          2, or when any of the following events occur:

             a) when routes in a Loc-RIB to local destinations have changed

             b) when locally generated routes learned by means outside of BGP
             have changed

             c) when a new BGP speaker - BGP speaker connection has been
             established

          The Phase 3 function is a separate process which completes when it
          has no further work to do. The Phase 3 Routing Decision function
          shall be blocked from running while the Phase 2 decision function is
          in process.

          All routes in the Loc-RIB shall be processed into a corresponding
          entry in the associated Adj-RIBs-Out. Route aggregation and
          information reduction techniques (see 9.2.4.1) may optionally be
          applied.

          For the benefit of future support of inter-AS multicast capabilities,
          a BGP speaker that participates in inter-AS multicast routing shall
          advertise a route it receives from one of its external peers and if
          it installs it in its Loc-RIB, it shall advertise it back to the peer
          from which the route was received. For a BGP speaker that does not
          participate in inter-AS multicast routing such an advertisement is
          optional. When doing such an advertisement, the NEXT_HOP attribute
          should be set to the address of the peer. An implementation may also
          optimize such an advertisement by truncating information in the
          AS_PATH attribute to include only its own AS number and that of the
          peer that advertised the route (such truncation requires the ORIGIN
          attribute to be set to INCOMPLETE).  In addition an implementation is
          not required to pass optional or discretionary path attributes with
          such an advertisement.

          When the updating of the Adj-RIBs-Out and the Forwarding Information
          Base (FIB) is complete, the local BGP speaker shall run the external
          update process of 9.2.2.









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       RFC DRAFT                                                    August 1998


       9.1.4 Overlapping Routes


          A BGP speaker may transmit routes with overlapping Network Layer
          Reachability Information (NLRI) to another BGP speaker. NLRI overlap
          occurs when a set of destinations are identified in non-matching
          multiple routes. Since BGP encodes NLRI using IP prefixes, overlap
          will always exhibit subset relationships.  A route describing a
          smaller set of destinations (a longer prefix) is said to be more
          specific than a route describing a larger set of destinations (a
          shorted prefix); similarly, a route describing a larger set of
          destinations (a shorter prefix) is said to be less specific than a
          route describing a smaller set of destinations (a longer prefix).

          The precedence relationship effectively decomposes less specific
          routes into two parts:

             -  a set of destinations described only by the less specific
             route, and

             -  a set of destinations described by the overlap of the less
             specific and the more specific routes


          When overlapping routes are present in the same Adj-RIB-In, the more
          specific route shall take precedence, in order from more specific to
          least specific.

          The set of destinations described by the overlap represents a portion
          of the less specific route that is feasible, but is not currently in
          use.  If a more specific route is later withdrawn, the set of
          destinations described by the overlap will still be reachable using
          the less specific route.

          If a BGP speaker receives overlapping routes, the Decision Process
          MUST consider both routes based on the configured acceptance policy.
          If both a less and a more specific route are accepted, then the
          Decision Process MUST either install both the less and the more
          specific routes or it MUST aggregate the two routes and install the
          aggregated route.

          If a BGP speaker chooses to aggregate, then it MUST add
          ATOMIC_AGGREGATE attribute to the route. A route that carries
          ATOMIC_AGGREGATE attribute can not be de-aggregated. That is, the
          NLRI of this route can not be made more specific.  Forwarding along
          such a route does not guarantee that IP packets will actually





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       RFC DRAFT                                                    August 1998


          traverse only ASs listed in the AS_PATH attribute of the route.


       9.2 Update-Send Process


          The Update-Send process is responsible for advertising UPDATE
          messages to all peers. For example, it distributes the routes chosen
          by the Decision Process to other BGP speakers which may be located in
          either the same autonomous system or a neighboring autonomous system.
          Rules for information exchange between BGP speakers located in
          different autonomous systems are given in 9.2.2; rules for
          information exchange between BGP speakers located in the same
          autonomous system are given in 9.2.1.

          Distribution of routing information between a set of BGP speakers,
          all of which are located in the same autonomous system, is referred
          to as internal distribution.


       9.2.1 Internal Updates


          The Internal update process is concerned with the distribution of
          routing information to internal peers.

          When a BGP speaker receives an UPDATE message from an internal peer,
          the receiving BGP speaker shall not re-distribute the routing
          information contained in that UPDATE message to other internal peers.

          When a BGP speaker receives a new route from an external peer, it
          MUST advertise that route to all other internal peers by means of an
          UPDATE message if this routes has been installed in its Loc-RIB
          according to the route selection rules in 9.1.2.

          When a BGP speaker receives an UPDATE message with a non-empty
          WITHDRAWN ROUTES field, it shall remove from its Adj-RIB-In all
          routes whose destinations was carried in this field (as IP prefixes).
          The speaker shall take the following additional steps:

             1) if the corresponding feasible route had not been previously
             advertised, then no further action is necessary

             2) if the corresponding feasible route had been previously
             advertised, then:






       Expiration Date February 1999                                  

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       RFC DRAFT                                                    August 1998


                i) if a new route is selected for advertisement that has the
                same Network Layer Reachability Information as the unfeasible
                routes, then the local BGP speaker shall advertise the
                replacement route

                ii) if a replacement route is not available for advertisement,
                then the BGP speaker shall include the destinations  of the
                unfeasible route (in form of IP prefixes) in the WITHDRAWN
                ROUTES field of an UPDATE message, and shall send this message
                to each peer to whom it had previously advertised the
                corresponding feasible route.


          All feasible routes which are advertised shall be placed in the
          appropriate Adj-RIBs-Out, and all unfeasible routes which are
          advertised shall be removed from the Adj-RIBs-Out.


       9.2.1.1 Breaking Ties (Internal Updates)


          If a local BGP speaker has connections to several external peers,
          there will be multiple Adj-RIBs-In associated with these peers. These
          Adj-RIBs-In might contain several equally preferable routes to the
          same destination, all of which were advertised by external peers.
          The local BGP speaker shall select one of these routes according to
          the following rules:

             a) If the candidate routes differ only in their NEXT_HOP and
             MULTI_EXIT_DISC attributes, and the local system is configured to
             take into account the MULTI_EXIT_DISC attribute, select the route
             that has the lowest value of the MULTI_EXIT_DISC attribute. A
             route with the MULTI_EXIT_DISC attribute shall be preferred to a
             route without the MULTI_EXIT_DISC attribute.

             b) If the local system can ascertain the cost of a path to the
             entity depicted by the NEXT_HOP attribute of the candidate route,
             select the route with the lowest cost.

             c) In all other cases, select the route that was advertised by the
             BGP speaker whose BGP Identifier has the lowest value.










       Expiration Date February 1999                                  

[Page 46]
       RFC DRAFT                                                    August 1998


       9.2.2 External Updates


          The external update process is concerned with the distribution of
          routing information to external peers.  As part of Phase 3 route
          selection process, the BGP speaker has updated its Adj-RIBs-Out and
          its Forwarding Table. All newly installed routes and all newly
          unfeasible routes for which there is no replacement route shall be
          advertised to external peers by means of UPDATE message.

          Any routes in the Loc-RIB marked as unfeasible shall be removed.
          Changes to the reachable destinations within its own autonomous
          system shall also be advertised in an UPDATE message.


       9.2.3 Controlling Routing Traffic Overhead


          The BGP protocol constrains the amount of routing traffic (that is,
          UPDATE messages) in order to limit both the link bandwidth needed to
          advertise UPDATE messages and the processing power needed by the
          Decision Process to digest the information contained in the UPDATE
          messages.


       9.2.3.1 Frequency of Route Advertisement


          The parameter MinRouteAdvertisementInterval determines the minimum
          amount of time that must elapse between advertisement of routes to a
          particular destination from a single BGP speaker. This rate limiting
          procedure applies on a per-destination basis, although the value of
          MinRouteAdvertisementInterval is set on a per BGP peer basis.

          Two UPDATE messages sent from a single BGP speaker that advertise
          feasible routes to some common set of destinations received from
          external peers must be separated by at least
          MinRouteAdvertisementInterval. Clearly, this can only be achieved
          precisely by keeping a separate timer for each common set of
          destinations. This would be unwarranted overhead. Any technique which
          ensures that the interval between two UPDATE messages sent from a
          single BGP speaker that advertise feasible routes to some common set
          of destinations received from external peers will be at least
          MinRouteAdvertisementInterval, and will also ensure a constant upper
          bound on the interval is acceptable.






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          Since fast convergence is needed within an autonomous system, this
          procedure does not apply for routes received from other internal
          peers.  To avoid long-lived black holes, the procedure does not apply
          to the explicit withdrawal of unfeasible routes (that is, routes
          whose destinations (expressed as IP prefixes) are listed in the
          WITHDRAWN ROUTES field of an UPDATE message).

          This procedure does not limit the rate of route selection, but only
          the rate of route advertisement. If new routes are selected multiple
          times while awaiting the expiration of MinRouteAdvertisementInterval,
          the last route selected shall be advertised at the end of
          MinRouteAdvertisementInterval.


       9.2.3.2 Frequency of Route Origination


          The parameter MinASOriginationInterval determines the minimum amount
          of time that must elapse between successive advertisements of UPDATE
          messages that report changes within the advertising BGP speaker's own
          autonomous systems.


       9.2.3.3 Jitter


          To minimize the likelihood that the distribution of BGP messages by a
          given BGP speaker will contain peaks, jitter should be applied to the
          timers associated with MinASOriginationInterval, Keepalive, and
          MinRouteAdvertisementInterval. A given BGP speaker shall apply the
          same jitter to each of these quantities regardless of the
          destinations to which the updates are being sent; that is, jitter
          will not be applied on a "per peer" basis.

          The amount of jitter to be introduced shall be determined by
          multiplying the base value of the appropriate timer by a random
          factor which is uniformly distributed in the range from 0.75 to 1.0.


       9.2.4 Efficient Organization of Routing Information


          Having selected the routing information which it will advertise, a
          BGP speaker may avail itself of several methods to organize this
          information in an efficient manner.






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       9.2.4.1 Information Reduction


          Information reduction may imply a reduction in granularity of policy
          control - after information is collapsed, the same policies will
          apply to all destinations and paths in the equivalence class.

          The Decision Process may optionally reduce the amount of information
          that it will place in the Adj-RIBs-Out by any of the following
          methods:

             a)   Network Layer Reachability Information (NLRI):

             Destination IP addresses can be represented as IP address
             prefixes.  In cases where there is a correspondence between the
             address structure and the systems under control of an autonomous
             system administrator, it will be possible to reduce the size of
             the NLRI carried in the UPDATE messages.

             b)   AS_PATHs:

             AS path information can be represented as ordered AS_SEQUENCEs or
             unordered AS_SETs. AS_SETs are used in the route aggregation
             algorithm described in 9.2.4.2. They reduce the size of the
             AS_PATH information by listing each AS number only once,
             regardless of how many times it may have appeared in multiple
             AS_PATHs that were aggregated.

             An AS_SET implies that the destinations listed in the NLRI can be
             reached through paths that traverse at least some of the
             constituent autonomous systems. AS_SETs provide sufficient
             information to avoid routing information looping; however their
             use may prune potentially feasible paths, since such paths are no
             longer listed individually as in the form of AS_SEQUENCEs.  In
             practice this is not likely to be a problem, since once an IP
             packet arrives at the edge of a group of autonomous systems, the
             BGP speaker at that point is likely to have more detailed path
             information and can distinguish individual paths to destinations.


       9.2.4.2 Aggregating Routing Information


          Aggregation is the process of combining the characteristics of
          several different routes in such a way that a single route can be
          advertised.  Aggregation can occur as part of the decision  process





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          to reduce the amount of routing information that will be placed in
          the Adj-RIBs-Out.

          Aggregation reduces the amount of information that a BGP speaker must
          store and exchange with other BGP speakers. Routes can be aggregated
          by applying the following procedure separately to path attributes of
          like type and to the Network Layer Reachability Information.

          Routes that have the following attributes shall not be aggregated
          unless the corresponding attributes of each route are identical:
          MULTI_EXIT_DISC, NEXT_HOP.

          Path attributes that have different type codes can not be aggregated
          together. Path of the same type code may be aggregated, according to
          the following rules:

             ORIGIN attribute: If at least one route among routes that are
             aggregated has ORIGIN with the value INCOMPLETE, then the
             aggregated route must have the ORIGIN attribute with the value
             INCOMPLETE. Otherwise, if at least one route among routes that are
             aggregated has ORIGIN with the value EGP, then the aggregated
             route must have the origin attribute with the value EGP. In all
             other case the value of the ORIGIN attribute of the aggregated
             route is INTERNAL.

             AS_PATH attribute: If routes to be aggregated have identical
             AS_PATH attributes, then the aggregated route has the same AS_PATH
             attribute as each individual route.

             For the purpose of aggregating AS_PATH attributes we model each AS
             within the AS_PATH attribute as a tuple <type, value>, where
             "type" identifies a type of the path segment the AS belongs to
             (e.g. AS_SEQUENCE, AS_SET), and "value" is the AS number.  If the
             routes to be aggregated have different AS_PATH attributes, then
             the aggregated AS_PATH attribute shall satisfy all of the
             following conditions:

                - all tuples of the type AS_SEQUENCE in the aggregated AS_PATH
                shall appear in all of the AS_PATH in the initial set of routes
                to be aggregated.

                - all tuples of the type AS_SET in the aggregated AS_PATH shall
                appear in at least one of the AS_PATH in the initial set (they
                may appear as either AS_SET or AS_SEQUENCE types).

                - for any tuple X of the type AS_SEQUENCE in the aggregated





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                AS_PATH which precedes tuple Y in the aggregated AS_PATH, X
                precedes Y in each AS_PATH in the initial set which contains Y,
                regardless of the type of Y.

                - No tuple with the same value shall appear more than once in
                the aggregated AS_PATH, regardless of the tuple's type.

             An implementation may choose any algorithm which conforms to these
             rules.  At a minimum a conformant implementation shall be able to
             perform the following algorithm that meets all of the above
             conditions:

                - determine the longest leading sequence of tuples (as defined
                above) common to all the AS_PATH attributes of the routes to be
                aggregated. Make this sequence the leading sequence of the
                aggregated AS_PATH attribute.

                - set the type of the rest of the tuples from the AS_PATH
                attributes of the routes to be aggregated to AS_SET, and append
                them to the aggregated AS_PATH attribute.

                - if the aggregated AS_PATH has more than one tuple with the
                same value (regardless of tuple's type), eliminate all, but one
                such tuple by deleting tuples of the type AS_SET from the
                aggregated AS_PATH attribute.

             Appendix 6, section 6.8 presents another algorithm that satisfies
             the conditions and  allows for more complex policy configurations.

             ATOMIC_AGGREGATE: If at least one of the routes to be aggregated
             has ATOMIC_AGGREGATE path attribute, then the aggregated route
             shall have this attribute as well.

             AGGREGATOR: All AGGREGATOR attributes of all routes to be
             aggregated should be ignored.


       9.3   Route Selection Criteria


          Generally speaking, additional rules for comparing routes among
          several alternatives are outside the scope of this document.  There
          are two exceptions:

             - If the local AS appears in the AS path of the new route being
             considered, then that new route cannot be viewed as better than





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             any other route.  If such a route were ever used, a routing loop
             could result (see Section 6.3).

             - In order to achieve successful distributed operation, only
             routes with a likelihood of stability can be chosen.  Thus, an AS
             must avoid using unstable routes, and it must not make rapid
             spontaneous changes to its choice of route.  Quantifying the terms
             "unstable" and "rapid" in the previous sentence will require
             experience, but the principle is clear.


       9.4   Originating BGP routes

          A BGP speaker may originate BGP routes by injecting routing
          information acquired by some other means (e.g. via an IGP) into BGP.
          A BGP speaker that originates BGP routes shall assign the degree of
          preference to these routes by passing them through the Decision
          Process (see Section 9.1).  These routes may also be distributed to
          other BGP speakers within the local AS as part of the Internal update
          process (see Section 9.2.1). The decision whether to distribute non-
          BGP acquired routes within an AS via BGP or not depends on the
          environment within the AS (e.g. type of IGP) and should be controlled
          via configuration.




       Appendix 1.  BGP FSM State Transitions and Actions.


          This Appendix discusses the transitions between states in the BGP FSM
          in response to BGP events.  The following is the list of these states
          and events when the negotiated Hold Time value is non-zero.

              BGP States:

                       1 - Idle
                       2 - Connect
                       3 - Active
                       4 - OpenSent
                       5 - OpenConfirm
                       6 - Established


              BGP Events:






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                       1 - BGP Start
                       2 - BGP Stop
                       3 - BGP Transport connection open
                       4 - BGP Transport connection closed
                       5 - BGP Transport connection open failed
                       6 - BGP Transport fatal error
                       7 - ConnectRetry timer expired
                       8 - Hold Timer expired
                       9 - KeepAlive timer expired
                      10 - Receive OPEN message
                      11 - Receive KEEPALIVE message
                      12 - Receive UPDATE messages
                      13 - Receive NOTIFICATION message

          The following table describes the state transitions of the BGP FSM
          and the actions triggered by these transitions.





              Event                Actions               Message Sent   Next State
              --------------------------------------------------------------------
              Idle (1)
               1            Initialize resources            none             2
                            Start ConnectRetry timer
                            Initiate a transport connection
               others               none                    none             1

              Connect(2)
               1                    none                    none             2
               3            Complete initialization         OPEN             4
                            Clear ConnectRetry timer
               5            Restart ConnectRetry timer      none             3
               7            Restart ConnectRetry timer      none             2
                            Initiate a transport connection
               others       Release resources               none             1

              Active (3)
               1                    none                    none             3
               3            Complete initialization         OPEN             4
                            Clear ConnectRetry timer
               5            Close connection                                 3
                            Restart ConnectRetry timer
               7            Restart ConnectRetry timer      none             2
                            Initiate a transport connection





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               others       Release resources               none             1

              OpenSent(4)
               1                    none                    none             4
               4            Close transport connection      none             3
                            Restart ConnectRetry timer
               6            Release resources               none             1
              10            Process OPEN is OK            KEEPALIVE          5
                            Process OPEN failed           NOTIFICATION       1
              others        Close transport connection    NOTIFICATION       1
                            Release resources

              OpenConfirm (5)
               1                   none                     none             5
               4            Release resources               none             1
               6            Release resources               none             1
               9            Restart KeepAlive timer       KEEPALIVE          5
              11            Complete initialization         none             6
                            Restart Hold Timer
              13            Close transport connection                       1
                            Release resources
              others        Close transport connection    NOTIFICATION       1
                            Release resources




              Established (6)
               1                   none                     none             6
               4            Release resources               none             1
               6            Release resources               none             1
               9            Restart KeepAlive timer       KEEPALIVE          6
              11            Restart Hold Timer            KEEPALIVE          6
              12            Process UPDATE is OK          UPDATE             6
                            Process UPDATE failed         NOTIFICATION       1
              13            Close transport connection                       1
                            Release resources
              others        Close transport connection    NOTIFICATION       1
                            Release resources
             ---------------------------------------------------------------------


             The following is a condensed version of the above state transition
             table.







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          Events| Idle | Connect | Active | OpenSent | OpenConfirm | Estab
                | (1)  |   (2)   |  (3)   |    (4)   |     (5)     |   (6)
                |---------------------------------------------------------------
           1    |  2   |    2    |   3    |     4    |      5      |    6
                |      |         |        |          |             |
           2    |  1   |    1    |   1    |     1    |      1      |    1
                |      |         |        |          |             |
           3    |  1   |    4    |   4    |     1    |      1      |    1
                |      |         |        |          |             |
           4    |  1   |    1    |   1    |     3    |      1      |    1
                |      |         |        |          |             |
           5    |  1   |    3    |   3    |     1    |      1      |    1
                |      |         |        |          |             |
           6    |  1   |    1    |   1    |     1    |      1      |    1
                |      |         |        |          |             |
           7    |  1   |    2    |   2    |     1    |      1      |    1
                |      |         |        |          |             |
           8    |  1   |    1    |   1    |     1    |      1      |    1
                |      |         |        |          |             |
           9    |  1   |    1    |   1    |     1    |      5      |    6
                |      |         |        |          |             |
          10    |  1   |    1    |   1    |  1 or 5  |      1      |    1
                |      |         |        |          |             |
          11    |  1   |    1    |   1    |     1    |      6      |    6
                |      |         |        |          |             |
          12    |  1   |    1    |   1    |     1    |      1      | 1 or 6
                |      |         |        |          |             |
          13    |  1   |    1    |   1    |     1    |      1      |    1
                |      |         |        |          |             |
                ---------------------------------------------------------------




       Appendix 2. Comparison with RFC1267


          BGP-4 is capable of operating in an environment where a set of
          reachable destinations may be expressed via a single IP prefix.  The
          concept of network classes, or subnetting is foreign to BGP-4.  To
          accommodate these capabilities BGP-4 changes semantics and encoding
          associated with the AS_PATH attribute. New text has been added to
          define semantics associated with IP prefixes.  These abilities allow
          BGP-4 to support the proposed supernetting scheme [9].






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          To simplify configuration this version introduces a new attribute,
          LOCAL_PREF, that facilitates route selection procedures.

          The INTER_AS_METRIC attribute has been renamed to be MULTI_EXIT_DISC.
          A new attribute, ATOMIC_AGGREGATE, has been introduced to insure that
          certain aggregates are not de-aggregated.  Another new attribute,
          AGGREGATOR, can be added to aggregate routes in order to advertise
          which AS and which BGP speaker within that AS caused the aggregation.

          To insure that Hold Timers are symmetric, the Hold Time is now
          negotiated on a per-connection basis.  Hold Times of zero are now
          supported.

       Appendix 3.  Comparison with RFC 1163


          All of the changes listed in Appendix 2, plus the following.

          To detect and recover from BGP connection collision, a new field (BGP
          Identifier) has been added to the OPEN message. New text (Section
          6.8) has been added to specify the procedure for detecting and
          recovering from collision.

          The new document no longer restricts the border router that is passed
          in the NEXT_HOP path attribute to be part of the same Autonomous
          System as the BGP Speaker.

          New document optimizes and simplifies the exchange of the information
          about previously reachable routes.


       Appendix 4.  Comparison with RFC 1105


          All of the changes listed in Appendices 2 and 3, plus the following.

          Minor changes to the RFC1105 Finite State Machine were necessary to
          accommodate the TCP user interface provided by 4.3 BSD.

          The notion of Up/Down/Horizontal relations present in RFC1105 has
          been removed from the protocol.

          The changes in the message format from RFC1105 are as follows:

             1.  The Hold Time field has been removed from the BGP header and
             added to the OPEN message.





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             2.  The version field has been removed from the BGP header and
             added to the OPEN message.

             3.  The Link Type field has been removed from the OPEN message.

             4.  The OPEN CONFIRM message has been eliminated and replaced with
             implicit confirmation provided by the KEEPALIVE message.

             5.  The format of the UPDATE message has been changed
             significantly.  New fields were added to the UPDATE message to
             support multiple path attributes.

             6.  The Marker field has been expanded and its role broadened to
             support authentication.

             Note that quite often BGP, as specified in RFC 1105, is referred
             to as BGP-1, BGP, as specified in RFC 1163, is referred to as
             BGP-2, BGP, as specified in RFC1267 is referred to as BGP-3, and
             BGP, as specified in this document is referred to as BGP-4.


       Appendix 5.  TCP options that may be used with BGP


          If a local system TCP user interface supports TCP PUSH function, then
          each BGP message should be transmitted with PUSH flag set.  Setting
          PUSH flag forces BGP messages to be transmitted promptly to the
          receiver.

          If a local system TCP user interface supports setting precedence for
          TCP connection, then the BGP transport connection should be opened
          with precedence set to Internetwork Control (110) value (see also
          [6]).



       Appendix 6.  Implementation Recommendations


             This section presents some implementation recommendations.


       6.1 Multiple Networks Per Message


          The BGP protocol allows for multiple address prefixes with the same





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          AS path and next-hop gateway to be specified in one message. Making
          use of this capability is highly recommended. With one address prefix
          per message there is a substantial increase in overhead in the
          receiver. Not only does the system overhead increase due to the
          reception of multiple messages, but the overhead of scanning the
          routing table for updates to BGP peers and other routing protocols
          (and sending the associated messages) is incurred multiple times as
          well. One method of building messages containing many address
          prefixes per AS path and gateway from a routing table that is not
          organized per AS path is to build many messages as the routing table
          is scanned. As each address prefix is processed, a message for the
          associated AS path and gateway is allocated, if it does not exist,
          and the new address prefix is added to it.  If such a message exists,
          the new address prefix is just appended to it. If the message lacks
          the space to hold the new address prefix, it is transmitted, a new
          message is allocated, and the new address prefix is inserted into the
          new message. When the entire routing table has been scanned, all
          allocated messages are sent and their resources released.  Maximum
          compression is achieved when all  the destinations covered by the
          address prefixes share a gateway and common path attributes, making
          it possible to send many address prefixes in one 4096-byte message.

          When peering with a BGP implementation that does not compress
          multiple address prefixes into one message, it may be necessary to
          take steps to reduce the overhead from the flood of data received
          when a peer is acquired or a significant network topology change
          occurs. One method of doing this is to limit the rate of updates.
          This will eliminate the redundant scanning of the routing table to
          provide flash updates for BGP peers and other routing protocols. A
          disadvantage of this approach is that it increases the propagation
          latency of routing information.  By choosing a minimum flash update
          interval that is not much greater than the time it takes to process
          the multiple messages this latency should be minimized. A better
          method would be to read all received messages before sending updates.


       6.2  Processing Messages on a Stream Protocol


          BGP uses TCP as a transport mechanism.  Due to the stream nature of
          TCP, all the data for received messages does not necessarily arrive
          at the same time. This can make it difficult to process the data as
          messages, especially on systems such as BSD Unix where it is not
          possible to determine how much data has been received but not yet
          processed.






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          One method that can be used in this situation is to first try to read
          just the message header. For the KEEPALIVE message type, this is a
          complete message; for other message types, the header should first be
          verified, in particular the total length. If all checks are
          successful, the specified length, minus the size of the message
          header is the amount of data left to read. An implementation that
          would "hang" the routing information process while trying to read
          from a peer could set up a message buffer (4096 bytes) per peer and
          fill it with data as available until a complete message has been
          received.


       6.3 Reducing route flapping


          To avoid excessive route flapping a BGP speaker which needs to
          withdraw a destination and send an update about a more specific or
          less specific route SHOULD combine them into the same UPDATE message.


       6.4 BGP Timers


          BGP employs five timers: ConnectRetry, Hold Time, KeepAlive,
          MinASOriginationInterval, and MinRouteAdvertisementInterval The
          suggested value for the ConnectRetry timer is 120 seconds.  The
          suggested value for the Hold Time is 90 seconds.  The suggested value
          for the KeepAlive timer is 30 seconds.  The suggested value for the
          MinASOriginationInterval is 15 seconds.  The suggested value for the
          MinRouteAdvertisementInterval is 30 seconds.

          An implementation of BGP MUST allow these timers to be configurable.


       6.5 Path attribute ordering


          Implementations which combine update messages as described above in
          6.1 may prefer to see all path attributes presented in a known order.
          This permits them to quickly identify sets of attributes from
          different update messages which are semantically identical.  To
          facilitate this, it is a useful optimization to order the path
          attributes according to type code.  This optimization is entirely
          optional.







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       6.6 AS_SET sorting


          Another useful optimization that can be done to simplify this
          situation is to sort the AS numbers found in an AS_SET.  This
          optimization is entirely optional.


       6.7 Control over version negotiation


          Since BGP-4 is capable of carrying aggregated routes which cannot be
          properly represented in BGP-3, an implementation which supports BGP-4
          and another BGP version should provide the capability to only speak
          BGP-4 on a per-peer basis.


       6.8 Complex AS_PATH aggregation


          An implementation which chooses to provide a path aggregation
          algorithm which retains significant amounts of path information may
          wish to use the following procedure:

             For the purpose of aggregating AS_PATH attributes of two routes,
             we model each AS as a tuple <type, value>, where "type" identifies
             a type of the path segment the AS belongs to (e.g.  AS_SEQUENCE,
             AS_SET), and "value" is the AS number.  Two ASs are said to be the
             same if their corresponding <type, value> tuples are the same.

             The algorithm to aggregate two AS_PATH attributes works as
             follows:

                a) Identify the same ASs (as defined above) within each AS_PATH
                attribute that are in the same relative order within both
                AS_PATH attributes.  Two ASs, X and Y, are said to be in the
                same order if either:
                   - X precedes Y in both AS_PATH attributes, or - Y precedes X
                   in both AS_PATH attributes.

                b) The aggregated AS_PATH attribute consists of ASs identified
                in (a) in exactly the same order as they appear in the AS_PATH
                attributes to be aggregated. If two consecutive ASs identified
                in (a) do not immediately follow each other in both of the
                AS_PATH attributes to be aggregated, then the intervening ASs
                (ASs that are between the two consecutive ASs that are the





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                same) in both attributes are combined into an AS_SET path
                segment that consists of the intervening ASs from both AS_PATH
                attributes; this segment is then placed in between the two
                consecutive ASs identified in (a) of the aggregated attribute.
                If two consecutive ASs identified in (a) immediately follow
                each other in one attribute, but do not follow in another, then
                the intervening ASs of the latter are combined into an AS_SET
                path segment; this segment is then placed in between the two
                consecutive ASs identified in (a) of the aggregated attribute.


             If as a result of the above procedure a given AS number appears
             more than once within the aggregated AS_PATH attribute, all, but
             the last instance (rightmost occurrence) of that AS number should
             be removed from the aggregated AS_PATH attribute.

       References


          [1] Mills, D., "Exterior Gateway Protocol Formal Specification",
          RFC904, April 1984.

          [2] Rekhter, Y., "EGP and Policy Based Routing in the New NSFNET
          Backbone", RFC1092, February 1989.

          [3] Braun, H-W., "The NSFNET Routing Architecture", RFC1093, February
          1989.

          [4] Postel, J., "Transmission Control Protocol - DARPA Internet
          Program Protocol Specification", RFC793, September 1981.

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

          [6] Postel, J., "Internet Protocol - DARPA Internet Program Protocol
          Specification", RFC791, September 1981.

          [7] "Information Processing Systems - Telecommunications and
          Information Exchange between Systems - Protocol for Exchange of
          Inter-domain Routeing Information among Intermediate Systems to
          Support Forwarding of ISO 8473 PDUs", ISO/IEC IS10747, 1993

          [8] Fuller, V., Li, T., Yu, J., and Varadhan, K., ""Classless Inter-
          Domain Routing (CIDR): an Address Assignment and Aggregation
          Strategy", RFC1519, September 1993.






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          [9] Rekhter, Y., Li, T., "An Architecture for IP Address Allocation
          with CIDR", RFC 1518, September 1993.


       Security Considerations

          Security issues are not discussed in this document.


       Editors' Addresses

          Yakov Rekhter
          cisco Systems, Inc.
          170 W. Tasman Dr.
          San Jose, CA 95134
          email:  yakov@cisco.com

          Tony Li
          Juniper Networks, Inc.
          385 Ravendale Dr.
          Mountain View, CA 94043
          (650) 526-8006
          email: tli@juniper.net




























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