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

Chapter 199
Sequence Numbers and Acknowledgments

A point should be made right up front. Acknowledgments do not simply refer to a datagram or a TCP segment. TCP’s job is to reconstruct a piece of data that was transmitted by the sender. Therefore, the acknowledgment number actually refers to the position in the stream of data being sent. Why? Because IP is connectionless and retransmissions may contain a different size from the original. The receiver collects information and reconstructs an exact copy of the data being sent.

Segments may also arrive out of order, and it is TCP’s job to place them back in the order in which they were sent. However, errors may occur during this process and TCP will only ACK the longest contiguous prefix of the stream that has been received correctly.

Refer back to Slide 211. TCP calculates a sequence number for each byte of data in the segment taken as a sum. For each byte of data that is to be transmitted, the sequence number increments by 1. Let’s say a connection was made between stations A and B (refer to Slide 217). Station A sends a segment to station B with a sequence number of 40 and knows the segment contains 4 bytes; therefore, it increments its sequence number to 44. Upon acknowledgment from station B (containing the ACK number of 44), station A then transmits the second segment to station B, which contains 7 bytes. Station A’s sequence number increments to 51, and it waits for an acknowledgment from station B. Note here that station A may not necessarily wait for an ACK from station B.

Each transmission window will contain as many bytes as indicated by the destination (windows are discussed in a moment). The sequence number is set to the number of the first byte in the datagram being sent—the last received ACK number from the destination. The TCP segment (the data) is then given to IP for delivery to the network.

Multiple datagrams may be sent with one acknowledgment to all received good segments. This is called an inclusive or cumulative ACK. TCP accomplishes this bidirectionally across the same connection. Each datagram transmitted will have the TCP header ACK bit set. With the ACK bit set, TCP will read the Acknowledgment field to find the next byte number of the segment that the other end of the connection expects. In other words, the number in the ACK field equals the sequence number of the original segment transmitted plus the number of the bytes successfully received in that segment plus 1. The ACK number is stuffed into a datagram to make TCP more efficient. There is usually not a separate datagram on the network used just for ACK packets. All data bytes up to but not including the ACK number are considered good and accepted by the receiver.

Since TCP is a byte-oriented transport protocol, sequencing and acknowledgments are accomplished for each byte of TCP data to ensure the integrity of the data and successful delivery to the destination. LAN protocols such as Novell NetWare and Xerox XNS were developed to work on high-reliability mediums (shielded copper cable in controlled environments). Their sequence numbers are based not on bytes in their data segment but on the number of packets. TCP ACKs TCP bytes of data not packets, datagrams, or segments.