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SYN flag

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

The SYN flag is a TCP control flag used to initiate connection establishment and synchronize sequence numbers.

The SYN flag is a one-bit TCP header control flag used to begin a connection and synchronize sequence numbers. TCP sets it in the first segment of the three-way handshake. section 22.2.1

Follow the TCP connection-establishment sequence:

  • The initiating host sends a segment with SYN set. section 22.2.1
  • The responding host returns a segment with SYN and ACK set. section 22.2.1
  • The initiating host sends an ACK segment, completing the handshake. section 22.2.1

PC1 connects to SRV1

PC1 sends SYN, SRV1 replies SYN-ACK, and PC1 sends ACK. These three establishment segments carry no encapsulated data; they prepare the connection for later exchange. section 22.2.1

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

How deep it goes
Explains the flag’s bit-level purpose and three-message handshake, but does not detail TCP implementation beyond header behavior.
Examples
Uses the PC1-to-SRV1 exchange: SYN, SYN-ACK, then ACK, with empty establishment segments.
What you can do after
Evaluate TCP connection traces by recognizing SYN, SYN-ACK, and ACK as the establishment sequence.
How it connects
Connects TCP with ACK, three-way handshake, sequence numbers, and connection termination using FIN.
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