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You can view the information of a fully connected queue by ss. If the limit is exceeded, the kernel discards the connection Drop or returns the RST packet. When the Server side application calls the accept function, the connection is taken out of the accept queue.īoth the SYN queue and the Accept queue have length and size limits.The Server side enters the ESTABLISHED state. After the Server side receives the ACK from the Client side, the kernel removes the connection from the SYN queue and adds it to the accept queue.After the Client side receives the SYN + ACK from the server side, the Client replies and enters the ESTABLISHED state.At this time, the kernel will store the connection in the SYN queue and reply to the Client side with SYN+ACK. After the Server side receives the SYN request from the Client side, the Server side enters the SYN_RECV state.

The Client side enters the SYN_SENT state. The Client side sends SYN to the Server side to initiate a connection.SYN queue practice - maximum length control, SYN queue overflow experiment, experimental result analysisĭuring the TCP three-way handshake process, the Linux kernel maintains two queues, namely:.Accept queue practice - maximum length control, accept queue overflow experiment, experimental result analysis.Introduction to SYN queue and accept queue.This article will combine theoretical knowledge, kernel code, and operational experiments to present the following content for you: I wrote this article on TCP SYN queue and accept queue to help you better understand this topic. Therefore, I consulted a large amount of data and did many experiments to verify it. I have been confused many times when investigating the problem by myself. Back then, I did not know much about TCP SYN queue and Accept queue, and I only vaguely remembered that the 'Detailed Explanation of TCP/IP ' seemed to mention these two nouns.Īt present, there are many relevant information on the Internet but many are outdated or inaccurate. After the event, a detailed problem investigation was carried out, and the location analysis showed that the cause of the problem was that the system connection queue was full. The error rate dropped to 0 after the apiserver cluster was expanded in an emergency. At that time, the water level of the apiserver cluster on the system side was 40%, which was far below the limit. But suddenly I received feedback from upstream that there was a chance that our endpoint would be facing a Connection timeout.

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When I was on duty during a big sales promotion a while ago, I had full confidence in the stability of the system I was running.

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