In TCP networking, what is a FIN_WAIT state?_fin wait state-优快云博客

本文解释了TCP连接中FIN_WAIT_2状态的含义及如何发生。当服务器希望关闭连接时，它会发送一个设置了FIN标志位的数据包，并进入FIN_WAIT_1状态。客户端收到此数据包后进入CLOSE_WAIT状态并回应确认包，此时服务器状态变为FIN_WAIT_2。文章还讨论了一些客户端可能出现的问题导致服务器长时间处于FIN_WAIT_2状态。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

FIN_WAIT_2 seems to occur when the server has an active connection with a client and it wants to shut down the TCP connection (probably in response to a normal application layer "exit"). The server sends the client a packet with a "FIN" bit set. At this point, the server is in FIN_WAIT_1 state. The client gets the FIN packet and goes into CLOSE_WAIT state, and sends an acknowledgment packet back to the server. When the server gets that packet, it goes into FIN_WAIT_2 state. From the server's perspective, the connection is now closed, and the server can't send any more data. However, under the TCP protocol, the client needs to shut down also by sending a FIN packet, which the server TCP implementation should ACK. The server should close about two milliseconds later.

The following diagram is taken from RFC 793 (defining the TCP protocol):

TCP A TCP B 1. ESTABLISHED ESTABLISHED 2. (Close) FIN-WAIT-1 --> <SEQ=100><ACK=300><CTL=FIN,ACK> --> CLOSE-WAIT 3. FIN-WAIT-2 <-- <SEQ=300><ACK=101><CTL=ACK> <-- CLOSE-WAIT 4. (Close) TIME-WAIT <-- <SEQ=300><ACK=101><CTL=FIN,ACK> <-- LAST-ACK 5. TIME-WAIT --> <SEQ=101><ACK=301><CTL=ACK> --< CLOSED 6. (2 MSL) CLOSED

Some clients have faulty TCP implementations, and leave the server hanging in FIN_WAIT_2 state. In particular, this can cause problems

with Indiana University's Unicorn system.