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: This is a good step but I think it overlooks one of the points that Magnus articulated. If an HTTP extension does what we say here, it might fall foul of intermediaries and protocol version conversion. That not some we have to solve but I suspect if we can concisely describe those considerations we would address the issue.
Good point, added text
Thanks, this is much clearer and is a descent explanation of the issues and motivation behind the should. I guess a more correct part will require much more text.
The first SHOULD should be a MUST because there is a clear exception specified but in all other cases this is a MUST. The second SHOULD should really be a MUST. Or why is that a SHOULD? If there is a reason for a SHOULD this needs to be explained but I think it would break interoperability.
I personally preferred MUST, however the consensus of the design team was SHOULD, and that was confirmed by the WG. Let's no reopen that unless we have new information.
On , NAME said:
I think the main motivation was that the MUST was unenforceable, and that there will be intermediaries that explicitly want to block unknown extensions for security reasons.
Whatever we might prefer, isn't this a case where whatever the specification says, intermediaries will do what they want?
I agree. That's why in the interest of making progress I'd rather move forward with the design team consensus instead of reopening this topic.
design team outputs don't have any special status here; perhaps the answer is not to say "SHOULD this" or "MUST that" but to instead articulate the intent behind the design and the potential consequences of not complying with that intent. Perhaps the key reason for mandating forwarding is that the meaning of various protocol elements is only valid in the context of all of the messages that an endpoint says. That is, you might send a bunch of datagram capsules that have meaning derived from other capsules. Remove the other capsules and you change the meaning of those datagram capsules. An intermediary that adds, modifies, or removes any capsules needs to understand the totality of the effect of their interventions. This is why I remain skeptical about the value of the generic capsule protocol. It only has value if it can be acted upon generically, but it is not obvious that it can be.