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Appendix A. TCP Connection State Diagram

This appendix provides a detailed illustration of TCP connection state transitions.

TCP State Transition Diagram​

                              +---------+
                              |  CLOSED |
                              +---------+
                                   |
                            (passive OPEN)
                                   |
                                   v
                              +---------+
                              |  LISTEN |
                              +---------+
                       (rcv SYN)   |   ^  (CLOSE)
                                   |   |
                                   v   |
                              +---------+
                              |SYN RCVD|
                              +---------+
                          (rcv ACK)|
                                   |
              (active OPEN)        v
             +---------+      +---------+
             |SYN SENT |`<---->`|  ESTAB  |
             +---------+      +---------+
                  |                |
                  |          (CLOSE/FIN)
                  |                |
                  |                v
                  |           +---------+
                  |           |FIN WAIT1|
                  |           +---------+
                  |           (rcv ACK) |
                  |                |
                  |                v
                  |           +---------+
                  |           |FIN WAIT2|
                  |           +---------+
                  |          (rcv FIN) |
                  |                |
                  |                v
                  |           +---------+
                  |           |TIME WAIT|
                  |           +---------+
                  |           (timeout) |
                  |                |
                  |                v
                  |           +---------+
                  +---------->|  CLOSED |
                              +---------+

        Simultaneous Close:              Passive Close:
        FIN WAIT1 --rcv FIN--> CLOSING   ESTABLISHED --rcv FIN--> CLOSE WAIT
        CLOSING --rcv ACK--> TIME WAIT   CLOSE WAIT --CLOSE--> LAST ACK
                                         LAST ACK --rcv ACK--> CLOSED

State Descriptions​

CLOSED​

  • Fictional state representing no TCB (Transmission Control Block), thus no connection
  • This is the starting and ending point for all connections

LISTEN​

  • Server waiting for connection request from any remote TCP
  • Passive open state

SYN-SENT​

  • Client waiting for matching connection request after sending SYN
  • Active open state

SYN-RECEIVED​

  • Server waiting for ACK after receiving SYN and sending SYN-ACK
  • Middle state of three-way handshake

ESTABLISHED​

  • Connection is established, data transfer can occur
  • This is the normal state during the data transfer phase

FIN-WAIT-1​

  • Active closer waiting for ACK or FIN from remote
  • First state of connection termination

FIN-WAIT-2​

  • Waiting for FIN from remote after receiving ACK for sent FIN
  • Indicates local side will no longer send data, but can still receive

CLOSE-WAIT​

  • Passive closer waiting for local application to close after receiving FIN
  • Can still send data at this point

CLOSING​

  • State during simultaneous close
  • Waiting for ACK of FIN from remote

LAST-ACK​

  • Passive closer waiting for ACK after sending FIN
  • Goes directly to CLOSED after receiving ACK

TIME-WAIT​

  • Active closer enters this state after receiving FIN from remote and sending ACK
  • Waits for 2MSL (Maximum Segment Lifetime)
  • Purposes:
    1. Ensure final ACK reaches remote
    2. Prevent old segments from interfering with new connections

Key State Transitions​

Connection Establishment (Three-Way Handshake)​

CLOSED --> (active open) --> SYN-SENT
SYN-SENT --> (receive SYN-ACK) --> ESTABLISHED

CLOSED --> (passive open) --> LISTEN
LISTEN --> (receive SYN) --> SYN-RECEIVED
SYN-RECEIVED --> (receive ACK) --> ESTABLISHED

Connection Closure (Four-Way Handshake)​

Active Close:
ESTABLISHED --> (send FIN) --> FIN-WAIT-1
FIN-WAIT-1 --> (receive ACK) --> FIN-WAIT-2
FIN-WAIT-2 --> (receive FIN) --> TIME-WAIT
TIME-WAIT --> (2MSL timeout) --> CLOSED

Passive Close:
ESTABLISHED --> (receive FIN) --> CLOSE-WAIT
CLOSE-WAIT --> (application close) --> LAST-ACK
LAST-ACK --> (receive ACK) --> CLOSED

Simultaneous Open​

CLOSED --> (send SYN) --> SYN-SENT
SYN-SENT --> (receive SYN) --> SYN-RECEIVED
SYN-RECEIVED --> (receive ACK) --> ESTABLISHED

Simultaneous Close​

ESTABLISHED --> (send FIN) --> FIN-WAIT-1
FIN-WAIT-1 --> (receive FIN) --> CLOSING
CLOSING --> (receive ACK) --> TIME-WAIT
TIME-WAIT --> (2MSL timeout) --> CLOSED

Importance of TIME-WAIT State​

Why is TIME-WAIT Needed?

  1. Ensure Reliable Closure:

    • If final ACK is lost, remote will retransmit FIN
    • TIME-WAIT state ensures ability to respond to retransmitted FIN

  2. Prevent Old Connection Interference:

    • Delayed old segments may exist in the network
    • Waiting 2MSL ensures old segments disappear from network
    • Prevents old segments from being mistaken as valid data by new connections

2MSL Time:

  • MSL (Maximum Segment Lifetime): Maximum time a segment can live in the network
  • Typically MSL = 2 minutes
  • Therefore TIME-WAIT = 2MSL = 4 minutes
  • Some implementations use shorter times (e.g., 30 or 60 seconds)

Note: For complete state transition logic and boundary case handling, refer to the full specification in RFC 9293.