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Tcp flags field

As explained in Acing the CCNA Exam: Volume 1 Fundamentals and Protocols

The TCP flags field is an eight-bit TCP-header field whose individual bits indicate or control protocol behaviors.

The TCP flags field is an eight-bit TCP-header field. Each bit indicates or controls a protocol behavior; a flag is set when its binary value is 1. TCP uses flags to establish, acknowledge, and terminate connections. section 22.2.1

The key flags used in TCP connection control are:

  • SYN (synchronize) starts TCP connection establishment. section 22.2.1
  • ACK (acknowledge) confirms received information and completes the handshake response. section 22.2.1
  • FIN (finish) signals that a host has finished sending and wants to terminate the connection. section 22.2.1

TCP connection sequence

PC1 sends SYN, SRV1 replies with SYN and ACK, and PC1 returns ACK. After data exchange, each host sends FIN and acknowledges the other host’s FIN.

How Acing the CCNA Exam: Volume 1 Fundamentals and Protocols covers it

How deep it goes
Defines the eight-bit field and demonstrates SYN, ACK, and FIN exchanges without enumerating every flag.
Examples
Illustrates PC1 and SRV1 exchanging SYN, SYN-ACK, ACK, FIN, and acknowledgment segments.
What you can do after
Evaluate TCP connection traces by recognizing flags used during establishment and termination.
How it connects
Connects TCP flags with the TCP header, three-way handshake, four-way handshake, and connection-oriented communication.
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