What term is used to indicate the sequence number of the next segment expected in TCP?

The term used to indicate the sequence number of the next segment expected in TCP is commonly referred to as "acknowledgment." In TCP communications, each segment of data sent contains a sequence number, and the acknowledgment number specifies the next expected byte from the sender. Acknowledgments are a vital part of TCP's reliability, as they ensure that the data is received correctly and in order.

Acknowledgment numbers are utilized within the TCP header to manage the flow of data and to maintain the integrity of the communication session. This mechanism helps in identifying which segments have been successfully received, allowing the sender to understand when to send new segments or possibly retransmit segments that were not acknowledged within the expected time frame.

The other terms mentioned do not specifically refer to the expected sequence number in TCP. "Sliding window" relates to flow control and determines how many segments may be sent before requiring an acknowledgment. "Retransmission request" is generally associated with error handling when a segment is lost. "Control message" refers to various types of messages used in networking for managing connections but does not specifically define the expected sequence number in TCP.