Transmission Control Protocol
The ultimate goal of the Transmission Control Protocol (TCP) is to provide end-to-end reliability for the delivery of data. TCP, which is defined in RFC 793, operates at layer 4 of the OSI model. It handles data sequencing and error recovery, and ultimately ensures that data gets where it is supposed to go. A lot of commonly used application-layer protocols rely on TCP and IP to deliver packets to their final destination.
The TCP Header
TCP provides a great deal of functionality, as reflected in the complexity of its header. As shown in Figure 6-16, the following are the TCP header fields:
| Source Port The port used to transmit the packet. |
| Destination Port The port to which the packet will be transmitted. |
| Sequence Number ... |
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