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TCP connection establishment

Short answer

TCP establishes a connection via a three-way handshake: client sends SYN with its initial sequence number (ISN), server responds with SYN+ACK carrying its ISN and ack=ISN+1, client then sends ACK acknowledging the server's ISN+1. This synchronises sequence numbers and confirms readiness.

How is a TCP connection established? Explain.

TCP establishes a connection by the three-way handshake, which synchronises the sequence numbers of both sides and confirms that both are ready. The server must first do a passive open (listen), then the client does an active open (connect).

  Client                                Server
  CLOSED                                LISTEN
    |--- SYN, seq=1000 ------------------>|  SYN_RCVD
  SYN_SENT                                 |
    |<-- SYN+ACK, seq=5000, ack=1001 ------|
    |--- ACK, seq=1001, ack=5001 --------->|
  ESTABLISHED                         ESTABLISHED

Steps (with example numbers)

  1. SYN: the client chooses an initial sequence number (ISN), say 1000, and sends a segment with SYN = 1. A SYN consumes one sequence number.
  2. SYN + ACK: the server chooses its own ISN (5000), sets SYN = 1 and ACK = 1, with ack number = 1000 + 1 = 1001. It also announces its window size and MSS.
  3. ACK: the client sends ACK = 1 with ack number 5000 + 1 = 5001. This segment can already carry data. The connection is now ESTABLISHED.

Why three steps?

  • Two steps would not let the server know the client received its ISN.
  • Random ISNs prevent old duplicate segments from being accepted by a new connection.

Notes

  • If the SYN is lost, the client retransmits after a timeout.
  • A SYN flood attack sends many SYNs without the final ACK to fill the server's half-open connection queue; SYN cookies are a defence.

✓ Answer checked by Claude Opus 5.5 (high effort)

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