HttpClient connections throwing System.Net.Sockets.SocketException after 21 seconds

[Tadimsky]

We have an issue in our services (running ASP.Net Core 2.1) where it takes about 21 seconds for HttpClient to "realize" that the service it is trying to connect to is down.

The message we finally get is:
A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond A connection attempt failed because the connected party did not properly respond after a period of time, or established connection failed because connected host has failed to respond
We then retry on a different service and that works, but it means our requests take 21+ seconds longer (and clients sometimes timeout because of it):

Is there a way to modify this timeout? Ideally we'd want it set to something more like 5 seconds.
It seems like it's a generic TCP timeout.

[davidsh]

TCP timeouts are generally on the order of 21 seconds which is 7 seconds per each of the 3-legged SYNC, SYNC-ACK, ACK handshake. This is not currently configurable via HttpClient API since it is a low-level TCP timeout.

[karelz]

I don't think that Windows supports changes to these timeouts - only via reg keys.
If that's true, then this seems to be by design and we should close the question as answered.

[davidsh]

See:
http://getin2me.blogspot.com/2010/08/are-you-also-facing-windows-sockets-21.html

[Tadimsky]

Ok, thanks for the information - it makes sense that this is not configurable via HttpClient.
Do you know if this is a value that is tweaked for services in general, 21 seconds for a timeout seems way too long when working in a microservice environment.
Maybe changing the TcpMaxDataRetransmissions to 1 (or potentially zero) would help. Although I would be a little wary of doing this..

[timabell]

For the googlers arriving here: https://aspnetmonsters.com/2016/08/2016-08-27-httpclientwrong/

[karelz]

I don't think there is anything left here for us to do.