Illustrated TCP/IP by Matthew G. Naugle Wiley Computer Publishing, John Wiley & Sons, Inc. ISBN: 0471196568   Pub Date: 11/01/98

Chapter 198
TCP Segment

Everything that TCP sends is called a segment. This informational unit can be control data or user data. Segments are used to establish a connection, send and receive data and acknowledgments, advertise window sizes, and close a connection. A TCP segment will contain the TCP header (shown in Slide 216) and its data. The data handed to TCP for transmission is known as a stream; more specifically, an unstructured stream. A stream is a flow of bytes of data and an unstructured stream is an unknown type of data flow of bytes. This means that TCP has no way of marking the data to indicate the ending of a record or the type of data that is in the stream. When TCP receives a datastream from the application, it will divide the data into segments for transmission to the remote network station. A segment can have control or data information—it is simply an unstructured stream of data bytes sent to a destination.

A TCP segment may be as long as 65,535 bytes (or longer, known as jumbograms in IPv6), but is usually much less than that. Ethernet can only handle 1500 bytes of data in the Data field of the Ethernet packet (Ethernet v2.0, 1496 bytes for IEEE 802.3 using IEEE 802.2). FDDI can handle a maximum of 4472 bytes of data in a packet, and Token Ring packet size varies depending on the speed. For 4 Mbps, the maximum size is 4472 bytes. For 16 Mbps, the maximum size of the packet is 17,800 bytes, but is usually set to 4472 bytes. To negotiate a segment size, TCP uses one of the Options (MSS) fields located in the TCP header to indicate the largest segment size it can receive, and submits this packet to the remote network station.

TCP Segment

TCP does not care what the data is; data in a TCP segment is considered a stream. This stream is constructed at the sender and sent to the receiver. The receiver reconstructs this stream from the variable segments that it receives. Once the connection is established, TCP’s main job is to maintain the connection(s). This is accomplished through the sequence numbers, acknowledgments and retransmissions, flow control, and window management.

Since the connection between stations A and B is now established (by way of a successful three-way handshake), TCP must now manage the connection. The first of the management techniques to be discussed is sequence numbers.