Transmission Control Protocol (TCP)

RFC 9293 §3.5 The "three-way handshake" is the procedure used to establish a connection.

1. 1. A → B: SYN seq=100

   A asks to open a connection and picks 100 as its starting sequence number.

   A: SYN-SENT · B: SYN-RECEIVED

   RFC 9293 §3.5 2. SYN-SENT --> <SEQ=100><CTL=SYN> --> SYN-RECEIVED

2. 2. B → A: SYN,ACK seq=300 ack=101

   B sends its own SYN (starting at 300) and acknowledges A's SYN: ACK=101 means "next I expect 101". The SYN used up sequence number 100.

   A: ESTABLISHED · B: SYN-RECEIVED

   RFC 9293 §3.5 3. ESTABLISHED <-- <SEQ=300><ACK=101><CTL=SYN,ACK> <-- SYN-RECEIVED

3. 3. A → B: ACK seq=101 ack=301

   A acknowledges B's SYN. Both sides are now ESTABLISHED.

   A: ESTABLISHED · B: ESTABLISHED

   RFC 9293 §3.5 4. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK> --> ESTABLISHED

4. 4. A → B: ACK + data seq=101 ack=301

   A sends data. SEQ is still 101: a bare ACK does not use sequence space.

   A: ESTABLISHED · B: ESTABLISHED

   RFC 9293 §3.5 5. ESTABLISHED --> <SEQ=101><ACK=301><CTL=ACK><DATA> --> ESTABLISHED