Code change:
large as to not be usable, the implementation SHOULD discard the capsule without buffering its contents into memory. <del> Note that use of the Capsule Protocol is not required to use HTTP Datagrams.  If an HTTP extension that uses HTTP Datagrams is only defined over transports that support QUIC DATAGRAM frames, it might not need a stream encoding.  Additionally, HTTP extensions can use HTTP Datagrams with their own data stream protocol.  However, new HTTP extensions that wish to use HTTP Datagrams SHOULD use the Capsule Protocol unless they have a good reason not to. </del> <ins> Note that it is possible for an HTTP extension to use HTTP Datagrams without using the Capsule Protocol.  For example, if an HTTP extension that uses HTTP Datagrams is only defined over transports that support QUIC DATAGRAM frames, it might not need a stream encoding.  Additionally, HTTP extensions can use HTTP Datagrams with their own data stream protocol.  However, new HTTP extensions that wish to use HTTP Datagrams SHOULD use the Capsule Protocol as failing to do so will make it harder for the HTTP extension to support versions of HTTP other than HTTP/3 and will prevent interoperability with intermediaries that only support the Capsule Protocol. </ins> 4.

Related discussion:
text: Put differently, the existence of a generic capsule protocol implies a number of constraints on the operation of extensions that are not obvious and not clearly articulated. NAME might be asking about similar effects in .
So lets start with explaining the interoperability issue that is at the core of this: So the draft currently has several statements that interact with each other to cause this interoperability issue. Section 3.5 says: 'Note that use of the Capsule Protocol is not required to use HTTP Datagrams. If an HTTP extension that uses HTTP Datagrams is only defined over transports that support QUIC DATAGRAM frames, it might not need a stream encoding.' Section 3.5 also says: 'An intermediary can reencode HTTP Datagrams as it forwards them. In other words, an intermediary MAY send a DATAGRAM capsule to forward an HTTP Datagram which was received in a QUIC DATAGRAM frame, and vice versa.' Thus the following may occur: Endpoint 1 sends a HTTP datagram as HTTP/3 Datagram over QUIC datagram. The HTTP intermediary re-encode this blind of its context to an Capsule Datagram and forward it to another HTTP intermediary that forwards it to an HTTP/3 endpoint using Datagram Capsule. As this HTTP datagram usage is one that does not require to support of Capsule Datagram this endpoint is not setup to handle this Datagram capsule and discards it. So this can either be fixed in various ways, not only what this issue text suggests: requiring the Datagram capsule support for all HTTP datagram handling nodes. In other words change the first quote. Require the HTTP intermediary to re-encode any Datagram capsules to HTTP/3 datagram if the next hop is know to support it. i.e. make additional requirements on how HTTP Intermediaries act when forwarding. Forbid blind forwarding of HTTP datagram in HTTP intermediaries. I think we should settle this part before going back to see how the text Mirja suggested changing matter. I think 1 and 2 are both working solutions, and would like to avoid 3. I think there is also a question on the implication of what it means to support HTTP datagram when one is an HTTP/3 endpoint or intermediary. Who is expected to support the Datagram capsule if the next node is not capable?