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

Chapter 200
Sequence and Acknowledgment Example

As shown in the slide, the connection was established using an initial sequence number from the sender and an initial sequence number supplied by the receiver (the destination). Each side will maintain its own sequence number, which may be in the range of 0 to 2,147,483,647. Each side of a TCP connection knows the upper and lower limits of the sequence numbers, and once the limit has been reached, it will roll over to 0 (each side knows to include 0). The initialization sequence numbers are selected at random. Each side must ACK each other’s received datagrams.

ACK No. = sequence number + good bytes read in the segment + 1

This is a clean, fast, efficient way of determining which bytes were successfully received and which were not. The sender must retain a copy of transmitted data until it receives an acknowledgment for those bytes from the remote network station of a connection.

Acknowledgment packets are not necessarily separate packets with only the acknowledgment number in the packet. This would be inefficient. For example, if station A opens a connection to station B, and station A and station B are sending data to each other, the ACK datagram can be combined with the response data packet. In other words, one datagram transmitted contains three things: the data from station B to station A, the acknowledgment from station B of the data previously sent from station A, and the sequence number for the data B is sending to A.

Sequence and Acknowledgment Example

If the sender does not receive an acknowledgment within a specified time, it will resend the data starting from the first unacknowledged byte. TCP will time-out after a number of unsuccessful retransmissions. The retransmission of a datagram is accomplished using the Go-back-to-N routine. Any number of outstanding bytes may not be acknowledged. When the destination station does acknowledge the receipt of a series of bytes, the source will look at the ACK number. All sequence numbers up to but not including the ACK number are considered received in good condition. This means that a source station can start the sequence number with 3 and then send two datagrams containing 100 TCP segment bytes each. When it receives an ACK from the destination of 203 (3 to 102, and then 102 to 202, leaving the ACK at 203 as the next expected byte), it will know that the data in both datagrams previously sent are considered received in good condition.

The number of outstanding packets allowed is the next topic of discussion.