TAILIEUCHUNG - Internetworking with TCP/IP- P33

Internetworking with TCP/IP- P33: TCP/IP has accommodated change well. The basic technology has survived nearly two decades of exponential growth and the associated increases in traffic. The protocols have worked over new high-speed network technologies, and the design has handled applications that could not be imagined in the original design. Of course, the entire protocol suite has not remained static. New protocols have been deployed, and new techniques have been developed to adapt existing protocols to new network technologies | Sec. BGP Message Header 279 tets. Finally the 1-octet TYPE field contains one of the four values for the message type listed in Figure . The MARKER may seem unusual. In the initial message the marker consists of all Is if the peers agree to use an authentication mechanism the marker can contain authentication information. In any case both sides must agree on the value so it can be used for synchronization. To understand why synchronization is necessary recall that all BGP messages are exchanged across a stream transport . TCP which does not identify the boundary between one message and the next. In such an environment a simple error on either side can have dramatic consequences. In particular if either the sender or receiver miscounts the octets in a message a synchronization error will occur. More important because the transport protocol does not specify message boundaries the transport protocol will not alert the receiver to the error. Thus to ensure that the sender and receiver remain synchronized BGP places a well-known sequence at the beginning of each message and requires a receiver to verify that the value is intact before processing the message. BGP OPEN Message As soon as two BGP peers establish a TCP connection they each send an OPEN message to declare their autonomous system number and establish other operating parameters. In addition to the standard header an OPEN message contains a value for a hold timer that is used to specify the maximum number of seconds which may elapse between the receipt of two successive messages. Figure illustrates the format. 0 8 31 VERSION AUTONOMOUS SYSTEMS NUM __HOLD TIME BGP IDENTIFIER PARM. LEN Optional Parameters variable Figure The format of the BGP OPEN message that is sent at startup. These octets follow the standard message header. Most fields are straightforward. The VERSION field identifies the protocol version used this format is for version 4 . Recall that each .

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