3-way TCP connection establishment

最新推荐文章于 2022-05-06 23:22:11 发布

转载最新推荐文章于 2022-05-06 23:22:11 发布 · 572 阅读

本文详细解析了TCP协议中三次握手的过程及其意义。首先客户端发送SYN包并指定初始序号，服务器回应SYN-ACK确认连接，并选择自己的序号；最后客户端发送ACK确认服务器的序号，完成连接建立。通过这一过程确保了全双工通信的有效建立。

tcpdump -ni br0 not port 2323

To establish a connection, TCP uses a three-way handshake. Before a client attempts to connect with a server, the server must first bind to and listen at a port to open it up for connections: this is called a passive open. Once the passive open is established, a client may initiate an active open. To establish a connection, the three-way (or 3-step) handshake occurs:

SYN: The active open is performed by the client sending a SYN to the server. The client sets the segment's sequence number to a random value A.
SYN-ACK: In response, the server replies with a SYN-ACK. The acknowledgment number is set to one more than the received sequence number i.e. A+1, and the sequence number that the server chooses for the packet is another random number, B.
ACK: Finally, the client sends an ACK back to the server. The sequence number is set to the received acknowledgement value i.e. A+1, and the acknowledgement number is set to one more than the received sequence number i.e. B+1.

At this point, both the client and server have received an acknowledgment of the connection. The steps 1, 2 establish the connection parameter (sequence number) for one direction and it is acknowledged. The steps 2, 3 establish the connection parameter (sequence number) for the other direction and it is acknowledged. With these, a full-duplex communication is established.