If the sending computer is transmitting too fast for the receiving computer, TCP can employ flow control mechanisms to slow data transfer. TCP can also recognize duplicate messages and will discard them appropriately. Where data has been lost in transit from source to destination, TCP can retransmit the data until either a timeout condition is reached or until successful delivery has been achieved.

By using sequence numbers and acknowledgment messages, TCP can provide a sending node with delivery information about packets transmitted to a destination node. TCP is a connection-oriented transport protocol that sends data as an unstructured stream of bytes. Because the primary focus of this document is routing (a layer 3 function), the discussion of TCP (a layer 4 protocol) will be relatively brief. This section describes technical aspects of TCP, IP, related protocols, and the environments in which these protocols operate. Support for at least part of the Internet Protocol suite is available from virtually every computer vendor. The Internet protocols are the most widely implemented multivendor protocol suite in use today. For information on the OSI Reference model and the role of each layer, please refer to the document Internetworking Basics.

Figure 2 shows some of the important Internet protocols and their relationship to the OSI Reference Model. Figure 1 shows the TCP/IP protocol suite in relation to the OSI Reference model. The Internet Protocol suite includes not only lower-level specifications, such as Transmission Control Protocol (TCP) and Internet Protocol (IP), but specifications for such common applications as electronic mail, terminal emulation, and file transfer. The Internet protocols are the best proven approach to internetworking this diverse range of LAN and WAN technologies. In the two decades since their invention, the heterogeneity of networks has expanded further with the deployment of Ethernet, Token Ring, Fiber Distributed Data Interface (FDDI), X.25, Frame Relay, Switched Multimegabit Data Service (SMDS), Integrated Services Digital Network (ISDN), and most recently, Asynchronous Transfer Mode (ATM).

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