Disable TCP Delayed ACKs

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linux
tcp

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+2 Q:

Disable TCP Delayed ACKs

I have an application that receives relatively sparse traffic over TCP with no application-level responses. I believe the TCP stack is sending delayed ACKs (based on glancing at a network packet capture). What is the recommended way to disable delayed-ACK in the network stack for a single socket? I've looked at TCP_QUICKACK, but it seems that the stack will change it under my feet anyways.

This is running on a Linux 2.6 kernel, and I am not worried about portability.

+1 A:

At which point does this happen? Do you mean SYN-ACKs, FIN-ACKs or what?

Nikolaos 2009-10-23 19:51:39

I think he means plain ACKs -- "receives ... traffic", "no ... responses". Returning ACKs are normally delayed to include data which might be sent soon, but there is none in OP's case.

ephemient 2009-10-23 20:10:16

yes, this is in an established session, not a handshake.

Tom 2009-10-23 23:34:39

You could setsockopt(sockfd, IPPROTO_TCP, TCP_QUICKACK, (int[]){1}, sizeof(int)) after every recv you perform. It appears that TCP_QUICKACK is only reset when there is data being sent or received; if you're not sending any data, then it will only get reset when you receive data, in which case you can simply set it again.

You can check this in the 14^th field of /proc/net/tcp; if it is not 1, ACKs should be sent immediately... if I'm reading the TCP code correctly. (I'm not an expert at this either.)

ephemient 2009-10-23 20:00:32

I believe he has done that already based on his question and he says the value is being reset.

Nikolaos 2009-10-23 20:02:49

Setting `TCP_QUICKACK` immediately sends an ACK if there is anything that needs to be ACK'ed, so I think this is sufficient.

ephemient 2009-10-23 20:11:10

I am still `send()`ing on that socket. But as I said, I am not responding to messages I receive.I'll check out `/proc/net/tcp`.

Tom 2009-10-23 23:34:00

I believe using the setsockopt() function you can use the TCP_NODELAY which will disable the Nagle algorithm.

Edit Found a link: http://www.ibm.com/developerworks/linux/library/l-hisock.html

Edit 2 Tom is correct. Nagle does not affect Delayed ACKs.

CoryC 2010-08-26 14:42:52

Disabling Nagle's algorithm won't impact delayed acknowledgements (at least, not on any implementation I know of).

Tom 2010-09-01 02:46:29

ansaurus

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Disable TCP Delayed ACKs

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