How to send TCP keep-alive packets

Question

I need to establish a long running (say 10 minutes) connection between a javascript client and a Web API controller action. The Azure load balancers for Web Apps have a default timeout of 4 minutes and that is not configurable at the moment.

How can I keep the connection by sending tcp keep-alive packets?

Can this be done client-side, server-side or both?
For .NET I can only find this option ServicePoint.SetTcpKeepAlive, but it seems to be limited to an HttpWebRequest where the application makes a request as a client.
Is it possible to send keep-alive packets from javascript via XMLHttpRequest?

can you just send a dummy message every so often? (your client need not respond to every utterance...) — dandavis
Yes but that "pollutes" the response and requires extra client logic, no? I've looked into SingalR and long-polling. Might be the only solution atm. — Benjamin E.
it depends on how you message the client, with something like signalR, each message has a type and only the type you bound listeners to will fire client actions, so no, there would not be extra logic on the client if you used an event emitter-shaped source. in terms of pollution, i think keeping it open is cleaner than reconnecting, and if the keep-alives are invisible to the program flow, i don't see the harm, Socket.IO and all the big dogs do this if it makes you feel better knowing that... — dandavis

capcom923 capcom923 · Accepted Answer · 2020-07-30T14:49:11

var socketsHandler = new SocketsHttpHandler
{
      PooledConnectionIdleTimeout = TimeSpan.FromHours(27),//Actually 5 mins can be idle at maximum. Note that timeouts longer than the TCP timeout may be ignored if no keep-alive TCP message is set at the transport level.
      MaxConnectionsPerServer = 10
};
client = new HttpClient(socketsHandler);

As you can see, although I set the idle timeout to 27 hours, but actually it just keep 5 mins alive.

So, finally I just call the target endpoint using the same HttpClient every 1 min. In this case, there is always an established connection. You could use netstat to check that. It works fine.

How to send TCP keep-alive packets

1 Answers