[quicwg/base-drafts] suggest easier way of initiating graceful shutdown from the server-side (#3341)

[Kazuho Oku <[e-mail]>]

At the moment, the recommended way of gracefully shutting down an H3 connection is, according to [section 5.2](https://quicwg.org/base-drafts/draft-ietf-quic-http.html#section-5.2-7):
> A server that is attempting to gracefully shut down a connection SHOULD send an initial GOAWAY frame with the last Stream ID set to the maximum value allowed by QUIC's MAX_STREAMS and SHOULD NOT increase the MAX_STREAMS limit thereafter. This signals to the client that a shutdown is imminent and that initiating further requests is prohibited._

It is my understanding that the reason we suggest sending "the maximum value allowed by QUIC's MAX_STREAMS" is because, unlike HTTP/2, we cannot send GOAWAY frame with an identifier value that is greater than any possible client-generated bidirectional stream ID.

However, the downside is that the current text assumes tight integration with the QUIC stack that might be unnecessary - let the QUIC stack provide current MAX_STREAMS value, and also stop giving the client new credits.

I am not against the status quo, but I think we might want to give practical advice. The minimum change can be to state "a server that is attempting to gracefully shutdown a connection SHOULD send an Initial GOAWAY frame with the last Stream ID set to the maximum value currently allowed by QUIC's MAX_STREAMS *or any value above that*."

Such change would imply that sending 2<sup>62</sup>-4 (the maximum stream ID for a request stream) assuming that the client would not send that much of requests on a single connection. Note that it is trivial for a server to make sure that the stream ID would not become that big - the server can always gracefully shutdown a connection well before the stream ID becomes that big.

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