Client sends delayed FIN ACK (~500ms) to server

问题:

I have a node.js client (10.177.62.7) requesting some data from http rest service from server (10.177.0.1). Client is simply using node.js http.request() method (agent=false). Client is on Ubuntu 11.10 box.

Why client sends FIN ACK after 475ms? Why so slow? He should send FIN ACK immediately. I have many situations like this. About 1% of whole traffic is request with delayed FIN ACK.

Cpu idle on the client is about 99%, so nothing is draining CPU.

How to debug this? What could it be? Is there any sysctl option I need to tune?

On screenshot 2nd column is the elapsed time between packets.

Link to bigger picture.

回答1:

This behaviour is the Delayed ACK feature of RFC1122 TCP stack.

Normally you should add the TCP_QUICKACK option to your Linux TCP socket to disable delayed ACK but I think it is not obvious with JavaScript Node.js API (I only saw socket.setNoDelay for TCP_NODELAY option).

So your idea to apply a system-wide change on TCP stack seems good but I found no sysctl matching this socket option behaviour. Here is another full list with explanation.