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 nodejs 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.
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.
