Add HTTP/2 keep alive pings

[JamesNK]

Fixes #15104

[Tratcher]

Looks like the right idea.

[halter73]

I have a high level question. If we never initiate pings as long as we are receiving frames faster than the KeepAlivePingInterval, why are we waiting specifically for the ping ack to cancel the ping timeout? Why not cancel the timeout whenever we receive any frame?

I get that cancelling the timeout whenever we receive any frame does nothing to measure latency, but isn't activity the real thing we want to check? If we want to check latency instead of activity, we should send pings regardless of recent activity.

[JamesNK]

That is something I thought about.

golang appears to be ok with any data:

// After having pinged for keepalive check, the server waits for a duration
// of Timeout and if no activity is seen even after that the connection is
// closed.

I think Java waits on the ping ACK

https://grpc.github.io/grpc-java/javadoc/io/grpc/netty/NettyServerBuilder.html#keepAliveTimeout-long-java.util.concurrent.TimeUnit-

Sets a custom keepalive timeout, the timeout for keepalive ping requests. An unreasonably small value might be increased.

[JamesNK]

Updated.

Open questions:

• GOAWAY error code. I like NO_ERROR
• Is activity any incoming data, or completed frames?

[Tratcher]

No_error is probably fine. All it means is that the server has decided to close the connection for an arbitrary reason.

I don't know if any data vs completed frames is a meaningful distinction in reality. Will the socket even notify us that data is available before it's finished reading a TCP frame? These HTTP/2 frames should fit into a TCP frame most of the time.

[JamesNK]

@yashykt Hi Yash

I'm adding keep alive support to the ASP.NET Core server. It will be useful when hosting gRPC on ASP.NET Core. A couple of questions for you:

1. What is considered activity that will reset the keep alive interval (timer that will send a ping) and keep alive timeout (timer that will close the connection)? Is it any bytes being received by the connection, or is it a completed frame?
2. If the keep alive timeout has been exceeded and the connection is being ended by the server, what should it do?
   • Close the connection without sending any frames, or
   • Send a GOAWAY. If a GOAWAY is sent, what is its error code?

[yashykt]

Hi, the source of truth for keepalives in gRPC is https://github.com/grpc/proposal/blob/master/A8-client-side-keepalive.md

1. Bytes received
2. The proposal does not add any details around this, but gRPC Core does not send any GOAWAY frame. It simply closes down the connection.

[ghost]

Thank you for submitting this for API review. This will be reviewed by @dotnet/aspnet-api-review at the next meeting of the ASP.NET Core API Review group. Please ensure you take a look at the API review process documentation and ensure that:

• The PR contains changes to the reference-assembly that describe the API change. Or, you have included a snippet of reference-assembly-style code that illustrates the API change.
• The PR describes the impact to users, both positive (useful new APIs) and negative (breaking changes).
• Someone is assigned to "champion" this change in the meeting, and they understand the impact and design of the change.