TCP状态转移图详解

状态	描述
LISTEN	represents waiting for a connection request from any remote TCP and port.
SYN-SENT	represents waiting for a matching connection request after having sent a connection request.
SYN-RECEIVED	represents waiting for a confirming connection request acknowledgment after having both received and sent a connection request.
ESTABLISHED	represents an open connection, data received can be delivered to the user. The normal state for the data transfer phase of the connection.
FIN-WAIT-1	represents waiting for a connection termination request from the remote TCP, or an acknowledgment of the connection termination request previously sent.
FIN-WAIT-2	represents waiting for a connection termination request from the remote TCP.
CLOSE-WAIT	represents waiting for a connection termination request from the local user.
CLOSING	represents waiting for a connection termination request acknowledgment from the remote TCP.
LAST-ACK	represents waiting for an acknowledgment of the connection termination request previously sent to the remote TCP (which includes an acknowledgment of its connection termination request).
TIME-WAIT	represents waiting for enough time to pass to be sure the remote TCP received the acknowledgment of its connection termination request.
CLOSED	represents no connection state at all.

重点解读：

1. 数据包类型和作用：

SYN - 同步序列编号，用于发起连接请求

SYN,ACK - 同步序列编号和确认，用于响应连接请求。

ACK - 确认，用于确认接收到的数据或连接请求。

FIN - 终止，用于发起连接关闭请求。

RST - 重置，用于立即终止连接或拒绝连接请求。

FIN,ACK - 终止和确认，用于在关闭连接过程中确认接收到的FIN。

2. 图中实线表示 client端状态转换，虚线表示server端的状态转换

3.建立连接， TCP三次握手，1、3是client端，2 、4是server端：

3. TCP关闭，4次挥手：

4. Simultaneous Open（同时打开），这种情况不常见。

定义：这是指两个TCP端点在几乎同一时间尝试发起连接的过程。
过程：
1. 同时发送SYN段，进入SYN_SENT状态。
2. 双方接收到对方的SYN段，进入SYN_RCVD状态。
3. 然后双方都发送SYN-ACK（SYN和ACK的组合），再次进入SYN_SENT状态。
4. 最后，双方接收到对方的ACK段，进入ESTABLISHED状态，连接建立。
场景：这种情况在实际应用中不常见，但可能出现在对等网络（P2P）中，其中两个节点同时尝试建立连接。