Shulou(Shulou.com)06/01 Report--

The process of the TCP three-way handshake is as follows:

The active connector sends a SYN packet to the passive connector.

After receiving the SYN packet, the passive connection sends a packet with the ACK and SYN marks to the active connection.

The active connection terminal sends a packet with ACK logo to the passive connection terminal, and the handshake is completed.

The process of TCP waving four times is as follows:

The active shutdown side sends a FIN packet to the passive shutdown side.

After the passive shutdown side receives the FIN packet, it sends an ACK packet to the active shutdown side.

After the passive shutdown side sends the ACK packet, it sends a FIN packet to the active shutdown side.

After receiving the FIN packet, the active closed end sends an ACK packet, and when the passive closed end receives the ACK packet, the four waves are completed and the connection is disconnected.

Various states in netstat:

CLOSED

Initial (connectionless) state.

LISTEN

Listening status, waiting for a connection request from the remote machine.

SYN_SEND

During the TCP three-way handshake, after the active connector sends the SYN packet, it enters the SYN_SEND state and waits for the other party's ACK packet.

SYN_RECV

During the TCP three-way handshake, the active connector receives the SYN packet and enters the SYN_RECV state.

ESTABLISHED

After completing the TCP three-way handshake, the active connector enters the ESTABLISHED state. At this point, the TCP connection is established and you can communicate.

FIN_WAIT_1

When the FIN waves four times, the active shutdown sends the TCP packet and enters the FIN_WAIT_1 state.

FIN_WAIT_2

When the ACK waves four times, the active shutdown end receives the FIN_WAIT_2 packet and enters the TCP state.

TIME_WAIT

When the ACK waves four times, after the active shutdown sends the ACK packet, it enters the TIME_WAIT state and waits for the maximum MSL time to let the passive shutdown receive the TCP packet.

CLOSING

During the four waves of the TCP, after the active shutdown sent the FIN packet, it did not receive the corresponding ACK packet, but received the other party's FIN packet. At this time, it entered the CLOSING state.

CLOSE_WAIT

During the four waves of the FIN, the passive closed end enters the CLOSE_WAIT state after receiving the TCP packet.

LAST_ACK

When the FIN waves four times, after the passive shutdown sends the ACK packet, it enters the LAST_ACK state and waits for the other party's TCP packet.

The possible states of the active connection end are:

CLOSED SYN_SEND ESTABLISHED .

The possible states of active shutdown are:

FIN_WAIT_1 FIN_WAIT_2 TIME_WAIT .

The possible states of the passive connection end are:

LISTEN SYN_RECV ESTABLISHED .

The possible states of the passive shutdown side are:

CLOSE_WAIT LAST_ACK CLOSED .

Under Linux, if the number of connections is large, you can use the more efficient ss instead of netstat.

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