Code change:
HTTP message content after the headers. Note that use of the Capsule Protocol is not required to use HTTP <del> Datagrams.  If a new HTTP Method or Upgrade Token is only defined over transports that support QUIC DATAGRAM frames, they might not need a stream encoding.  Additionally, definitions of new HTTP Methods or of new HTTP Upgrade Tokens can use HTTP Datagrams with their own data stream protocol.  However, new HTTP Method or Upgrade Tokens SHOULD use the Capsule Protocol unless they have a good reason not to. </del> <ins> Datagrams.  If a new HTTP Upgrade Token is only defined over transports that support QUIC DATAGRAM frames, they might not need a stream encoding.  Additionally, definitions of new HTTP Upgrade Tokens can use HTTP Datagrams with their own data stream protocol. However, new HTTP Upgrade Tokens that wish to use HTTP Datagrams SHOULD use the Capsule Protocol unless they have a good reason not to. </ins> 4.1. <del> Definitions of new HTTP Methods or of new HTTP Upgrade Tokens can state that their data stream uses the Capsule Protocol.  If they do so, that means that the contents of their data stream uses the following format (using the notation from the "Notational Conventions" section of QUIC): </del> <ins> Definitions of new HTTP Upgrade Tokens can state that their data stream uses the Capsule Protocol.  If they do so, that means that the contents of their data stream uses the following format (using the notation from the "Notational Conventions" section of QUIC): </ins> A variable-length integer indicating the Type of the capsule. Endpoints that receive a capsule with an unknown Capsule Type MUST

Related discussion:
text: On the other hand, confused peers gonna be confused and David's point about datagrams being strongly associated with a request is a good point)
I think it should be a stream error to enable method-agnostic gateways and proxies, which may be multiplexing different clients' traffic onto a single next-hop connection.
In , NAME points out that Content-Type doesn't make sense with the capsule protocol. Should we prohibit it?
In , NAME points out that status codes 204, 205, and 206 don't make sense with the capsule protocol. Should we prohibit them?
The spec currently says: In HTTP/1.x, the message content is generally delineated by the Content-Length header field or the Transfer-Encoding; another request can follow on the same connection. One of the (many) ways in which CONNECT and Upgrade are special is that in H1 it takes over the connection and makes the request extend to the end of the connection, preventing any further requests from being made. The description attempts to be generic about method, but implicitly assumes an indefinite length stream of bytes which (in H1) is only true of CONNECT or Upgrade. This concept is correct for what we have in mind, but the boundaries aren't crisply described right now. NAME may have opinions.
The sentence right after the bit you pasted is: That constrains this to CONNECT/Upgrade/new methods. Is that not enough?
I think "new methods" is probably the most questionable. It's not clear to me that a new method can obtain the special behaviors of CONNECT. If it were restricted to upgrade tokens (whether with HTTP/1.1 Upgrade or RFC 8441 Extended CONNECT), I think that would address the concern.
I believe what he means is this definition of a data stream only holds for CONNECT requests. There is no mechanism to extend that to new request methods since all other methods are required to have length-delimited data (or no data).
Likewise, I don't know of any reason we would want to support such extension methods in the future, since that is a known failure point for CONNECT. It is far simpler for a new method to require chunked or data frames.
Restricting this to extended CONNECT and Upgrade sounds reasonable to me
For Upgrade, the new protocol takes over after the current response is complete. Hence, the description above doesn't really work for Upgrade either.