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Internetworking with TCP/IP- P26: 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 | 218 Reliable Stream Transport Service TCP Chap. 13 Meanwhile another connection might be in progress from machine 128.9.0.32 at the Information Sciences Institute to the same machine at Purdue identified by its endpoints 128.9.0.32 1184 and 128.10.2.3 53 . So far our examples of connections have been straightforward because the ports used at all endpoints have been unique. However the connection abstraction allows multiple connections to share an endpoint. For example we could add another connection to the two listed above from machine 128.2.254.139 at CMU to the machine at Purdue 128.2.254.139 1184 and 128.10.2.3 53 . It might seem strange that two connections can use the TCP port 53 on machine 128.10.2.3 simultaneously but there is no ambiguity. Because TCP associates incoming messages with a connection instead of a protocol port it uses both endpoints to identify the appropriate connection. The important idea to remember is Because TCP identifies a connection by a pair of endpoints a given TCP port number can be shared by multiple connections on the same machine. From a programmer s point of view the connection abstraction is significant. It means a programmer can devise a program that provides concurrent service to multiple connections simultaneously without needing unique local port numbers for each connection. For example most systems provide concurrent access to their electronic mail service allowing multiple computers to send them electronic mail concurrently. Because the program that accepts incoming mail uses TCP to communicate it only needs to use one local TCP port even though it allows multiple connections to proceed concurrently. 13.8 Passive And Active Opens Unlike UDP TCP is a connection-oriented protocol that requires both endpoints to agree to participate. That is before TCP traffic can pass across an internet application programs at both ends of the connection must agree that the connection is desired. To do so the application program on one end
