Introduction to TCP three-time handshake

TCP (transmission Control Protocol Transmission Control Protocol ( TCP /wireless) is a connection-oriented, reliable, byte-stream-based transport-layer Communication agreement defined by the IETF RFC 793. In the simplified computer network OSI model, it accomplishes the function specified by layer Fourth transport layer, and the User Datagram Protocol (UDP) is Another important transport protocol in the same layer. In the Internet Protocol family (Internet Protocol suite), the TCP layer is the middle tier located above the IP layer and below the application layer. There is often a need for reliable, pipe-like connections between the application tiers of different hosts, but the IP layer does not provide such a flow mechanism, but rather provides unreliable packet switching.

This is what Baidu told us, then TCP as a Transport Layer communication protocol, and IP inter-network protocol to become the standard industrial protocol set.

Three times the handshake process, I found several different explanations as well as instructions

First, the TCP three handshake process is as follows:

1. The client sends a SYN (SEQ=X) message to the server side and enters the Syn_send state.
2. The server receives a SYN message, responds to a SYN (seq=y) ACK (ack=x+1) message, and enters the SYN_RECV state.
3. The client receives a server-side SYN message, responds to an ACK (ack=y+1) message, and enters the established state.

Two

The bit code is the TCP flag bit, there are 6 kinds of marking: SYN (synchronous set up) ACK (acknowledgement acknowledgment) PSH (push transfer) FIN (finish end) RST (reset reset) URG (Urgent emergency)

Sequence number (sequential) Acknowledge number (confirmation)

First handshake: Host a send bit code for syn=1, randomly generate SEQ number=1234567 packet to the server, Host B by Syn=1 know, a requirements to establish online;

Second handshake: Host B receives the request to confirm the online information, send an ACK to a number= (host A's seq+1), syn=1,ack=1, randomly generate seq=7654321 packets

Third handshake: Host a after receiving check ACK number is correct, that is, the first sent Seq Number+1, and the bit code ACK is 1, if correct, host A will send an ACK number= (Host B seq+1), ack= 1, Host B is received after confirming the SEQ value and ack=1 The connection was established successfully.

To complete the three handshake, host A and Host B start transmitting data.

In the TCP/IP protocol, the TCP protocol provides a reliable connection service with a three-time handshake to establish a connection.
First handshake: When a connection is established, the client sends a SYN packet (SYN=J) to the server and enters the Syn_send state, waiting for the server to confirm;
Second handshake: The server receives the SYN packet, must confirm the customer's SYN (ACK=J+1), and also sends itself a SYN packet (syn=k), namely the Syn+ack packet, when the server enters the SYN_RECV state; the third handshake: the client receives the server's Syn+ack package, Send acknowledgment packet ack (ACK=K+1) to the server, the packet is sent, the client and server enter the established state, complete three handshake. Three handshake is completed and the client and server begin to transfer data.

Instance:

IP 192.168.1.116.3337 > 192.168.1.123.7788:s 3,626,544,836:3,626,544,836
IP 192.168.1.123.7788 > 192.168.1.116.3337:s 1739326486:1739326486 ack 3626544837
IP 192.168.1.116.3337 > 192.168.1.123.7788:ack 1739326487,ack 1

First handshake: 192.168.1.116 sends a bit code syn=1, randomly generates a SEQ number=3626544836 packet to 192.168.1.123,192.168.1.123 by syn= 1 know 192.168.1.116 requirements to establish online;

Second handshake: 192.168.1.123 to confirm the online information after receiving the request, send Ack number=3626544837,syn=1,ack=1 to 192.168.1.116, randomly generate seq=1739326486 packet;

Third handshake: 192.168.1.116 received after checking ACK number is correct, that is, the first time the SEQ Number+1 sent, and the bit code ACK is 1, if correct, 192.168.1.116 will send an ACK again number=1739326487 , Ack=1,192.168.1.123 received after confirmation seq=seq+1,ack=1 the connection was established successfully.

The so-called three-time handshake (three-way handshake) means that when a TCP connection is established, the client and server are required to send a total of 3 packets. The purpose of the three-time handshake is to connect the server to the specified port, establish a TCP connection, and synchronize the serial number and confirmation number of both parties and Exchange TCP window size information. In socket programming, the client executes connect (). Will trigger a three-time handshake.

Copyright NOTICE: This article for Bo Master original article, without Bo Master permission not reproduced.

Introduction to TCP three-time handshake