Python Network Programming-socket programming

Three-time handshake of TCP in the socket to establish a connection

We know that TCP establishes a connection for a "three-time handshake", which is the exchange of three groupings.

The process is as Follows:

• The client sends a SYN J to the server
• The server responds to a SYN K to the client and acknowledges the SYN J ACK j+1
• The client then wants the server to send a confirmation ack k+1

Only three handshake is done, but this three handshake takes place in the socket of the several functions? Please see:

Figure 1, TCP Three-time handshake sent in socket

As you can see, when the client calls connect, the connection request is triggered, The SYN J packet is sent to the server, then connect enters the blocking state, the server hears the connection request, receives the SYN J packet, calls the Accept function to receive the request to send SYN K to the client, ACK j+ 1, then accept into the blocking state, the client receives the server SYN k, after the ACK j+1, then connect returns, and the SYN K confirmation, the server received an ACK k+1, accept return, This three times the handshake is completed, the connection is Established.

Summary: The Client's Connect returns in the second time of the three handshake, while the Server-side accept is returned for the third time in the Three-time handshake.

5, the socket TCP Four handshake release connection detailed

The above describes the Three-time handshake creation process of TCP in the socket and the socket function Involved. Now that we introduce the four-time handshake in the socket to release the connection, see:

Figure 2, TCP Four-time handshake sent in socket

The process is as Follows:

• An application process first calls close to actively close the connection, when TCP sends a FIN M;
• After the other end receives fin m, perform a passive shutdown to confirm the Fin. Its reception is also passed as a file terminator to the application process, because the receive of fin means that the application process can no longer receive additional data on the corresponding connection;
• After some time, the application process that receives the file terminator calls close to close its socket. This causes its TCP to send also a fin N;
• This fin is received by the source send side TCP to confirm it.

So there is a fin and an ack in each Direction.

Python Network Programming-socket programming