BGP Message Generation and Transport, and General Message Format
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Each router running BGP generates messages to implement the various functions of the protocol. Some of these messages are created on a regular basis by the BGP software during the course of its normal operation. These are generally controlled by timers that are set and count down to cause them to be sent. Other messages are sent in response to messages received from BGP peers, possibly after a processing step.
BGP is different from most other routing protocols in that it was designed from the start to operate using a reliable method of message delivery. The Transmission Control Protocol (TCP) is present in the software of every IP router, making it the obvious choice for reliable data communication in a TCP/IP internet, and that's what BGP uses. Routing protocols are usually considered part of layer three, but this one runs over a layer four protocol, making BGP a good example of why architectural models are best used only as a guideline.
TCP provides numerous advantages to BGP by taking care of most of the details of session setup and management, allowing BGP to focus on the data it needs to send. TCP takes care of session setup and negotiation, ensuring that messages are received and acknowledged, flow control, congestion-handling and any necessary retransmissions of lost messages. BGP uses well-known TCP port 179 for connections.
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