tcp三次握手过程图片，tcp协议三次握手图

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1.Two computers that are communicating with each other need to initiate communication with each other by establishing a reliable connection by using a protocol called **TCP three-way handshake**.

2. The TCP three-way handshake is a process used to establish a connection between two computers before any data can be transferred. In order for the connection to be established, all three steps of the handshake must be completed successfully.

3. The first step of the handshake involves the initiating computer (also known as the \"client\" computer) sending a message to the receiving computer (or the “server” computer) containing its own synchronization (SYN) flag.

4. The next step involves the server responding with an acknowledgment (ACK) of the SYN flag, and sending its own SYN flag to the client computer.

5. Lastly, the client sends a final acknowledgment (ACK) of the server's SYN flag to complete the three-way handshake. At this point, the connection has been established, and the computers are now able to communicate with each other by sending and receiving data over their connection.

6. While the entire three-way handshake process usually lasts for only a few seconds, the connection it establishes typically remains active for a much longer time, which makes it very useful in applications such as file downloads and web browsing.

7. When the connection is no longer necessary, one of the computers can initiate a four-way handshake, which is the process of shutting down the connection. This is necessary because it is important to ensure that both sides of the connection are properly terminated, and any data that was sent by the computers is properly acknowledged so that it is not lost.

一、TCP三次握手

1. 首先，客户端发送一个SYN（synchronize）信号给服务器，表明客户端要建立一条TCP连接，包含客户端的初始序列号。

2. 服务器收到客户端发来的SYN信号后，会发送一个SYN+ACK（synchronize + acknowledgement）信号给客户端，表明服务器已经收到SYN信号，也同时给客户端发送了服务器的初始序列号给客户端。

3. 收到服务器发来的SYN+ACK信号后，客户端最后发送一个ACK（acknowledgement）信号给服务器，告诉服务器它已经收到了服务器发来的SYN+ACK信号。

4. 三次握手完成后，TCP连接建立完成。客户端和服务器端都会维持自己的序列号，并且两端的序列号都要加一，用来标识下一个报文段。

5. 一旦TCP连接建立完成，任何一方都可以发送数据，如果遇到网络拥塞的情况，TCP协议可以自我调整，使吞吐量得到最大化。

二、优点

1. 三次握手可以保证客户端和服务器之间建立连接时可靠性，在连接过程中也可以保证双方之间数据传输的安全性。

2. 三次握手能够防止已失效的连接请求报文段突然又传送到服务端，因为它要求建立连接的双方在连接中确认对方的存在。

3. 三次握手存在事先协商的机制，是允许双方先约定一个拥塞窗口大小，来保证数据发送的可靠性和完整性。

4. 三次握手机制在传输过程中可以处理拥塞，提高传输效率。

三、缺点

1. 三次握手引入了不必要的网络消耗，在网络传输过程中会多消耗3个回答时间，占据网络带宽资源。

2. 三次握手机制会降低TCP连接的建立速度，使应用程序的响应时间加长。

3. 如果三次握手的过程中，客户端发来的第一个序号丢失了，服务器就会发送一个重复的序号，而此时客户端就无法正确识别序号，这会导致TCP连接失败。

总结来说，TCP三次握手既有好处也有缺点，应根据网络环境及应用特性进行恰当的选择；为了保证网络的可靠性，最好采取安全的三次握手机制。

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