Exploring TCP state machine by graphs

States

TCP includes 11 states, they are:

LISTEN

SYN_SENT

SYN_RECV

ESTABLISHED

FIN_WAIT1

CLOSE_WAIT

FIN_WAIT2

LAST_ACK

TIME_WAIT

CLOSED

CLOSING

@include/net/tcp_states.h：

/* Definitions for the TCP protocol sk_state field. */

#ifndef _LINUX_TCP_STATES_H
#define _LINUX_TCP_STATES_H

enum {
    TCP_ESTABLISHED = 1,
    TCP_SYN_SENT,
    TCP_SYN_RECV,
    TCP_FIN_WAIT1,
    TCP_FIN_WAIT2,
    TCP_TIME_WAIT,
    TCP_CLOSE,
    TCP_CLOSE_WAIT,
    TCP_LAST_ACK,
    TCP_LISTEN,
    TCP_CLOSING,    /* Now a valid state */

    TCP_MAX_STATES    /* leave at the end! */
};

#define TCP_STATE_MASK 0xF
#define TCP_ACTION_FIN (1 << 7)

enum {
    TCPF_ESTABLISHED = (1 << 1),
    TCPF_SYN_SENT = (1 << 2),
    TCPF_SYN_RECV = (1 << 3),
    TCPF_FIN_WAIT1 = (1 << 4),
    TCPF_FIN_WAIT2 = (1 << 5),
    TCPF_TIME_WAIT = (1 << 6),
    TCPF_CLOSE = (1 << 7),
    TCP_CLOSE_WAIT = (1 << 8),
    TCP_LAST_ACK = (1 << 9),
    TCP_LISTEN = (1 << 10),
    TCP_CLOSING = (1 << 11)
};

#endif /* _LINUX_TCP_STATES_H */

View of Server

Passive open and passive close often performed by Server, the normal routine is illustrated as follow.

I denote the passive open/close transitions with a blue line (normal transitions are in bold).

View of Client

Ative open and Active close often performed by Client, the normal routine is illustrated as follow.

I denote the active open/close transitions with a green line (normal transitions are in bold).

As a whole

We know while most TCP state transitions are normal, it also has some unnormal transitions, including:

simultaneous open

simultaneous close

RST and etc.

I use dashed line to denote this kind of transitions.

Sequence chart

（1）Normal sequence chart (including active open/close, passive open/close)

（2）simultaneous open

 （3）simultaneous close

Author

zhangskd @ csdn blog