RFC 1105 (rfc1105) - Page 2 of 17
Border Gateway Protocol (BGP)
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RFC 1105 BGP June 1989 2. Summary of Operation Two hosts 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. 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 optionally 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 gateway in another autonomous system. The implications and applications of this architecture are for further study. If a particular AS has more than one BGP gateway, then all these gateways should have a consistent view of routing. A consistent view of the interior routes of the autonomous system is provided by the intra-AS routing protocol. A consistent view of the routes exterior to the AS may be provided in a variety of ways. One way is to use the BGP protocol to exchange routing information between the BGP gateways within a single AS. In this case, in order to maintain consist routing information, these gateways MUST have direct BGP sessions with each other (the BGP sessions should form a complete graph). Note that this requirement does not imply that all BGP gateways within a single AS must have direct links to each other; other methods may be used to ensure consistent routing information. 3. Message Formats This section describes message formats and actions to be taken when errors are detected while processing these messages. Messages are sent over a reliable transport protocol connection. A message is processed after it is entirely received. The maximum message size is 1024 bytes. 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 8 bytes. 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. This information is passed to other BGP peers before being deleted from the system. Lougheed & Rekhter
