Re: [quicwg/base-drafts] Transport Parameters are not APLN Specific (#3973)

[Martin Thomson <[e-mail]>]

@martinthomson commented on this pull request.

Thanks for putting this together.  I wasn't sure that I understood what you wanted before.

> @@ -1741,6 +1741,13 @@ parameters as a connection error of type TRANSPORT_PARAMETER_ERROR.
 Endpoints use transport parameters to authenticate the negotiation of
 connection IDs during the handshake; see {{cid-auth}}.
 
+When sent by the client, transport parameters are not specific to a particular
+ALPN being negotiated.  Therefore, a client cannot have application protocol
+specific values in the transport parameters.  Application protocols should take
+this into account when designing versioning schemes, and not place restrictions
+on the values in the transport parameters that might prevent protocol changes
+in future version of the application protocol.

There is a difference between requirements and recommendations.  We do recommend in HTTP/3 that a certain number of unidirectional stream be available, but it is [just a recommendation](https://quicwg.org/base-drafts/draft-ietf-quic-http.html#section-6.2-4).

I think that this could be clearer then:

> Protocol negotiation with Application Layer Protocol Negotiation (ALPN; see {{?ALPN=RFC7301}}) allows clients to offer multiple application protocols during connection establishing. The single set of transport parameters offered by a client apply to all application protocols that the client offers.  Application protocols can make recommendations about the values for transport parameters, such as suggesting minimum values for stream or flow control limits.  Application protocols that place hard constraints on transport parameters could make it impossible for clients to attempt a connection that includes other application protocols with different constraints.

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