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

Chapter 193
TCP Details

Not all networks use a separate transport-layer software to converse on a network. The best example of this is Novell with its LAN workgroup operating system of NetWare. NetWare relies on the network-layer software to transport data and the NetWare Core Protocol (as an application) to provide the sequence numbering of the packets. There is nothing wrong with this, and it generally speeds up the communication process between two stations on the network. The overhead of the role of transport layer is diminished, but those types of protocols were developed on high-speed, low-error-rate media such as Ethernet. TCP was not, and is much more robust in its transport-layer protocol. TCP is actually a protocol and not a separate piece of software.

The protocol of TCP uses sequence numbers and acknowledgments to reliably converse with other stations on the network. Sequence numbers are used to determine the ordering of the data in the packets and to find missing packets. Since packets on an internet may not arrive in the same sequence in which they were sent (for example, a single packet in a series of packets being transmitted was discarded by a router), sequencing the data in the packets ensures that the packets are read in the same order in which they were sent. Also, a receiving station may receive two of the same packets. The sequence number with acknowledgments is used to allow a reliable type of communication. This process is called full duplex, for each side of a connection maintains its own sequence number for the other side.

TCP is a byte-oriented sequencing protocol. Other protocols such as Novell NetWare are packet-oriented sequencing protocols. This applies a sequence number to each packet transmitted and not to each data byte in the packet. Byte oriented means that every byte in each packet is assigned a sequence number. This does not mean that TCP transmits a packet containing only 1 byte. TCP will transmit data (many bytes) and assign the packet one sequence number. Assigning one sequence number per byte in the packet may sound repetitious, but remember that TCP/IP was first implemented over noisy serial lines and not reliable high-speed LANs.

The slide shows two datagrams that have been transmitted. Normally, each TCP segment is 512 or 536 bytes in length (but can be larger). The short number of bytes in this picture is shown for clarity only. Each datagram is assigned one sequence number according to the number of bytes in the TCP Data field. Notice how the sequence number jumps by the same amount of bytes that are in each packet. The receiver of these datagrams will count the amount of bytes received and increment its sequence number of received packets. The first packet received has a sequence number of 40 and contains 4 bytes. The receiver expects the next sequence number to be 44. It is, and that packet contains 7 bytes of data. The receiver expects the next packet to be sequence number of 51. It is. This is how the byte sequencing of TCP works.

The sliding window scheme is discussed later. First, the TCP header definitions.

TCP Details