Proposal: change the default value of `initial_max_send_streams` to 100 about h2

[magurotuna]

So I separated #732 into a couple of PRs. Now the part of setting a reasonable default value is moved to hyper. Here are the separated PRs:

• #736
• hyperium/hyper#3524
• hyperium/hyper-util#87

[dswij]

What do you mean by this? If what you mean is that the users of hyper should be able to configure the initial value of max send streams, then I think it's covered by the first bullet

Yes :)

I'm completely on board with the first point. Just saying that regardless of changing the default value, this option should be configurable in hyper. So I see this as another improvement if we go down this route.

[seanmonstar]

This is a great write-up, thank you! I agree it makes sense to adjust the default. I only want to pause to make sure it's the right place to adjust the default.

It's worth calling out that for workloads where the server will NOT place a limit, changing this default here could be a negative. This would be places where both peers are trusted. My mind goes to things like linkerd2 proxies. (cc @hawkw @olix0r) The proper solution could actually be that those use cases should explicitly set a high initial value, instead of depending on the default, which perhaps should aim to protect the most common cases.

But I also vaguely feel like so far we've put those defaults in hyper or reqwest, and left h2 to simply implement the protocol, and otherwise not pick defaults.

[magurotuna]

It makes sense that changing the default value could have a negative impact in a rare situation.
Actually, I opened an issue to here instead of to hyper (or reqwest) because I found that the client builder in h2 provides a way to set the initial MAX_CONCURRENT_STREAMS value while hyper doesn't. Plus, it looks like hyper doesn't set any value for initial MAX_CONCURRENT_STREAMS when constructing h2 client, thus ending up using the default value specified in h2, i.e. usize::MAX.

So probably what we would like to do is:

• to have hyper provide a method on hyper::client::conn::http2::Builder that allows users to set the initial MAX_CONCURRENT_STREAMS value
• to have hyper call h2::client::Builder::initial_max_send_streams when constructing h2 client with the user-configured value, if any.
  • If no value is configured by the user, set the reasonable default value i.e. 100.
  • Or we could set the default value at one more upper level, i.e. reqwest.
• to have hyper_util provide a method on hyper_util::client::legacy::Builder that allows users to set the initial MAX_CONCURRENT_STREAMS value

I also feel that some changes still need to be made to h2, namely:

• improves the doc of h2::client::Builder::initial_max_send_streams to clearify that the default value is usize::MAX
• sets MAX_CONCURRENT_STREAMS to usize::MAX or whatever reasonable value (for example Go adopts 1,000) if the server's initial SETTINGS frame doesn't include MAX_CONCURRENT_STREAMS, assuming that the absence of MAX_CONCURRENT_STREAMS implies the server is willing to accept infinite number of concurrent streams
  • note that currently h2 keeps using the value set by h2::client::Builder::initial_max_send_streams if no specific value is supplied by the server

Do these sound reasonable to you? If they do, I'd be happy to work on these :)

[dswij]

But I also vaguely feel like so far we've put those defaults in hyper or reqwest, and left h2 to simply implement the protocol, and otherwise not pick defaults.

+1. h2 definitely should concern the protocol-level implementation only. The defaults should be in hyper.

Having arbitrary defaults everywhere is also a nightmare to handle, so it's good that we're avoiding that.

to have hyper call h2::client::Builder::initial_max_send_streams when constructing h2 client with the user-configured value, if any.

This sounds like it should be exposed in hyper

[magurotuna]

Thanks for your opinion @dswij.

This sounds like it should be exposed in hyper

What do you mean by this? If what you mean is that the users of hyper should be able to configure the initial value of max send streams, then I think it's covered by the first bullet:

• to have hyper provide a method on hyper::client::conn::http2::Builder that allows users to set the initial MAX_CONCURRENT_STREAMS value

[seanmonstar]

Thanks a lot for doing that, your contributions have been wonderful to work with ♥️