C++ Disable Delayed Ack on Windows

Question

I am trying to replicate a real time application on a windows computer to be able to debug and make changes easier, but I ran into issue with Delayed Ack. I have already disabled nagle and confirmed that it improve the speed a bit. When sending a lots of small packets, window doesn't ACK right away and delay it by 200 ms. Doing more research about it, I came across this. Problem with changing the registry value is that, it will affect the whole system rather than just the application that I am working with. Is there anyway to disable delayed ACK on window system like TCP_QUICKACK from linux using setsockopt? I tried hard coding 12, but got WSAEINVAL from WSAGetLastError.

I saw some dev on github that mentioned to use SIO_TCP_SET_ACK_FREQUENCY but I didn't see any example on how to actually use it.

So I tried doing below

#define SIO_TCP_SET_ACK_FREQUENCY _WSAIOW(IOC_VENDOR,23) 
result = WSAIoctl(sock, SIO_TCP_SET_ACK_FREQUENCY, 0, 0, info, sizeof(info), &bytes, 0, 0);

and I got WSAEFAULT as an error code. Please help!

Answer 1

I've seen several references online that TCP_QUICKACK may actually be supported by Winsock via setsockopt() (opt 12), even though it is NOT documented, or officially confirmed anywhere.

But, regarding your actual question, your use of SIO_TCP_SET_ACK_FREQUENCY is failing because you are not providing any input buffer to WSAIoctl() to set the actual frequency value. Try something like this:

int freq = 1; // can be 1..255, default is 2
result = WSAIoctl(sock, SIO_TCP_SET_ACK_FREQUENCY, &freq, sizeof(freq), NULL, 0, &bytes, NULL, NULL);

Note that SIO_TCP_SET_ACK_FREQUENCY is available in Windows 7 / Server 2008 R2 and later.