Description of the BGP4 Protocol
Note that this chapter limits its description of BGP to version 4, which is the one used almost exclusively on the Internet today. BGP4 has four message types:
OPEN messages are used to establish the BGP session.
UPDATE messages are used to send routing prefixes, along with their associated BGP attributes (such as the AS-PATH).
NOTIFICATION messages are sent whenever a protocol error is detected, after which the BGP session is closed.
KEEPALIVE messages are exchanged whenever the keepalive period is exceeded, without an update being exchanged.
As shown in Figure 11-3, each message begins with a 19-byte header. The marker field is 16 bytes, and contains a sequence that can be predicted by the remote peer. It ...