RFC 1267 (rfc1267) - Page 2 of 35
Border Gateway Protocol 3 (BGP-3)
Alternative Format: Original Text Document
RFC 1267 BGP-3 October 1991
Autonomous Systems (ASs) that traffic must transit to reach these
networks. 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.
To characterize the set of policy decisions that can be enforced
using BGP, one must focus on the rule that an AS advertize to its
neighbor 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 neighbor AS intending that that traffic take a
different route from that taken by traffic originating in the
neighbor 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,
acknowledgement, 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 memo 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
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
Lougheed & Rekhter