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SubjectRe: TCP Delayed ACK in FIN/ACK
On Sat, Aug 4, 2012 at 4:45 PM, Sławek Janecki <> wrote:
> I have a node.js client ( requesting some data from http
> rest service from server (
> 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?
> I think this behaviour is the Delayed ACK feature of RFC1122 TCP stack.
> Link to tcpdump picture (done on a client machine) :
> Can you tell why kernel delayed that FIN/ACK.
> In tcpflow data there is exacly one ACK per packet comming from server.
> Why kernel delayed client FIN/ACK.
> It could avoid sending ACK every 'data' packet.
> But it choose to delay FIN/ACK?
> Is this possible? Is this a bug?
> I've also posted question on stackexchange:
> Please help.

CC'ing netdev.

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