Learn how to deal with TIME_WAIT queue troubles. Our Server Management Support team is here to help you with your questions and concerns.

TIME_WAIT queue troubles | Troubleshooting Tips

If you have been running into TIME_WAIT queue troubles, you have come to the right place. Before we begin, let’s take a quick look at the connection closing part of a typical TCP connection life cycle.

According to our experts, the initiator of the connection closure is called active while the second party is called passive.

At the passive end, the system has to respond with the corresponding ACK packet after receiving the FIN packet.

Then, the connection at the passive side is placed in CLOSE_WAIT state. Once the passive side is ready, it sends a response FIN packet and waits for an ACK packet. When the ACK packet is received, the connection at the passive side is closed.

However, things are slightly more complicated on the active side. Once the FIN packet is sent, the active is in the FIN_WAIT_1 state. At this point, one of the three following situations is possible:

• The state changes to FIN_WAIT_2 after receiving ACK on the FIN packet. The data can then be delivered, followed by a FIN response packet, to which the active side responds with an ACK and places the connection in the TIME WAIT state.
• If the passive side intends to terminate the session, the response FIN can be received along with an ACK to the original FIN packet. The active side responds with an ACK and then moves the connection to TIME WAIT, skipping FIN WAIT 2.
• If both parties initiate a closure at the same time, both sides are active. Additionally, on both sides, the connection is placed in TIME_WAIT state.

The TIME_WAIT queue grows when an error occurs. Fortunately, we can find out which connections are in TIME_WAIT state via netstat. It offers general information with the ss utility.

Troubleshooting Tips: TIME_WAIT queue troubles

• Open fewer connections to avoid the TIME_WAIT queue from growing too long.
• Our experts suggest shifting TIME_WAIT to the other side and avoiding initiating TCP connections closure when designing a service.
• Try to start the remote service on multiple ports and then access it in turn.
• Set the kernel parameter “net.ipv4.ip_local_port_range” to a greater range for more connections to one remote service.
• Our experts suggest enabling the parameter net.ipv4.tcp_tw_reuse. This enables the use of connections in TIME_WAIT state for outgoing connections.
• Finally, try enabling parameter net.ipv4.tcp_tw_recycle.

Let us know in the comments if you need further help with a growing TIME_WAIT queue.

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Conclusion

In summary, our Support Techs introduced us to the many TIME_WAIT queue troubles and how to troubleshoot them.