Change:
Datagrams with payloads of at least 1280 bytes).  This can be accomplished using various techniques: <del> if HTTP intermediaries are not in use, both CONNECT-IP endpoints can pad the QUIC INITIAL packets of the underlying QUIC connection that CONNECT-IP is running over. if HTTP intermediaries are in use, CONNECT-IP endpoints can enter in an out of band agreement with the intermediaries to ensure that endpoints and intermediaries pad QUIC INITIAL packets. </del> <ins> if both endpoints know for certain that HTTP intermediaries are not in use, the endpoints can pad the QUIC INITIAL packets of the underlying QUIC connection that CONNECT-IP is running over. (Assuming QUIC version 1 is in use, the overhead is 1 byte type, 20 bytes maximal connection ID length, 4 bytes maximal packet number length, 1 byte DATAGRAM frame type, 8 bytes maximal quarter stream ID, one byte for the zero context ID, and 16 bytes for the AEAD authentication tag, for a total of 51 bytes of overhead which corresponds to padding QUIC INITIAL packets to 1331 bytes or more.) </ins> CONNECT-IP endpoints can also send ICMPv6 echo requests with 1232 bytes of data to ascertain the link MTU and tear down the tunnel if they do not receive a response.  Unless endpoints have an out <del> of band means of guaranteeing that one of the two previous techniques is sufficient, they MUST use this method. </del> <ins> of band means of guaranteeing that the previous techniques is sufficient, they MUST use this method. If an endpoint is using QUIC DATAGRAM frames to convey IPv6 packets, and it detects that the QUIC MTU is too low to allow sending 1280 bytes, it MUST abort the CONNECT-IP stream. </ins> Endpoints MAY implement additional filtering policies on the IP packets they forward.

Discussion:
I think there are two option here, 1) close/deny the connect request, and 2) use streams instead. I think we should consider both option, especially given stream are just there to use.
Isn't this a dup of ? That's fixed with . I guess the text could be generalized to talk about more than just IPv6 MTU issues.
No, this aspect on what you do when the MTU required is not present is not resolved by that PR. I would note that closing gives the endpoints one experience that is likely easier to handle in implementation and do avoid the issue with reordering and the varying packet treatment that moving to stream would result.
I would suggest adding a sentence saying that if an endpoint realizes that the IPv6 tunnel MTU goes lower than 1280, then it MUST close the stream.
IETF 114: Note that if something happens and your MTU gets smaller, you may need to destroy that link. Potentially make this option a SHOULD do PMTUD, otherwise you could end up in a situation where you're not a compliant IPv6 tunnel. Proposal:
Even if PR was an improvement to the description of the IPv6 minimal link MTU issue the text still leaves an open issue in that it doesn't make clear which of the endpoints that are responsible for HTTP intermediaries or if is truly both which has another implication. So a Connect-IP endpoint will be aware of the HTTP datagram supported MTU and if its HTTP/3 connection can support sending at least 1280 bytes of IPv6 packets without fragmentation. However, if there is two or more HTTP intermediaries in between the client and the server then there are HTTP intermediaries that might have an HTTP/3 connection which has less than 1280 bytes MTU for HTTP datagram payloads in-between the client and server. Thus, arises the issue of how to detect this. PR states: if HTTP intermediaries are in use, CONNECT-IP endpoints can enter in an out of band agreement with the intermediaries to ensure that endpoints and intermediaries pad QUIC INITIAL packets. From my perspective the big question is how a client or a server knows that there are intermediaries, or the most important aspect, that there are no unknown HTTP intermediaries. To my understanding both the client and the server can potentially bring in HTTP intermediaries unknown to the other part.

Thus resulting in a HTTP intermediary to intermediary connection where neither endpoint knows if the requirement is fulfilled. Thus my interpretation is that the only way to be sure that it works it to actually execute on the third bullet in the PR: CONNECT-IP endpoints can also send ICMPv6 echo requests with 1232 bytes of data to ascertain the link MTU and tear down the tunnel if they do not receive a response. Unless endpoints have an out of band means of guaranteeing that one of the two previous techniques is sufficient, they MUST use this method. I would also note that this only verifies from the acting endpoint towards its peer, not the reverse path. For that to be verified the peer need to send ICMP probes the other direction. Thus, my conclusion is that Connect-IP still don't ensure that the minimal MTU is fulfilled.
Isn't the quick-fix to require that the ICMP reply also be 1232 byte sized?
Normally I would say now, as you want to ensure that the reply makes it back that shouldn't be padded. Also if the response is anything else than echo reply you also should not pad. However, I think the implication is the same either way. Both masque client and server need to send ICMP or modify its ICMP processing. Given that I would think having each endpoint doing independent processing of its forward path might actually be simpler and enables using a standard off the shelf ICMP implementation and not have to modify it.
Our implementation has userspace ICMP request/response handling for echo request/response, it wasn't too hard to write. I don't propose modifying anything, but the structures were ~50 lines of code to set up. I agree we shouldn't be modifying responses we did not generate.
I think the existing text already is generic to "endpoint", not client or server: So this implies that both sides need to send the PING if they haven't guaranteed the MTU. This seems like this is already handled?
If both endpoints are required to send ping it would be resolved. However, that is not what is written in the draft currently. What is written is the there are various techniques, and if an endpoint choses something else than verifying its forward path using ping, it becomes unclear who is actually responsible for any HTTP intermediaries. Which results in an unclear requirement and very likely failure of the MTU verification resulting in an MTU black hole.

Well, it does say that endpoints MUST use this method unless the already have other coordination. I could see some clarification text to say that the guarantee for the other methods must include knowing there isn't an intermediary — which is actually pretty easy to guarantee in many cases.
That is what I intended when writing , but perhaps I wasn't clear. Can one of you propose a PR that would clarify this?
This PR is good but I don't this is fully addresses