Code change:
param_value_definition : If for any one of the endpoints the parameter is absent or set to 0, <del> or if the two endpoints select incompatible values, one proposing 0x1 and the other proposing 0x2, the endpoints MUST fallback to QUIC- TRANSPORT with single path and MUST NOT use any frame or mechanism defined in this document. If an endpoint proposes the value 0x3, the value proposed by the other is accepted.  If both endpoints propose the value 0x3, the value 0x2 is negotiated. </del> <ins> the endpoints MUST fallback to QUIC-TRANSPORT with single active path and MUST NOT use any frame or mechanism defined in this document. </ins> If endpoint receives unexpected value for the transport parameter "enable_multipath", it MUST treat this as a connection error of type

Related discussion:
text: Thanks for the summary NAME I think one point is missing in your list which is related to issue . Use of a single packet number space might not support ECN. Regarding the unified solution: I think what you actually say is that we would specify both solutions and everybody would need to implement both logics. At least regarding the "simplicity of code" argument, that would be the worst choice. If we can make the multiple packet number spaces solution work with one-sided CIDs, I'm tending toward such an approach. Use of multiple packet number spaces avoids ambiguity/needed "smartness" in ACK handling and packet scheduling which, as you say above, can make the implementation more complex and, moreover, wrong decisions may have large impact on efficient (both local processing and on-the-wire). I don't think we want to leave these things to each implementation individually.
The summary Christian made above about design comparison sounds indeed quite accurate. Besides ECN, my other concern about single packet number is that it require cleverness from the receiver side if you want to be performant in some scenarios. At the sender-side, you need to consider a path-relative packet number threshold instead of an absolute one to avoid spurious losses. Just a point I think we did not mentioned yet is that there can be some interactions between Multipath out-of-order number delivery and incoming packet number duplicate detection. This requires maintaining some state at the receiver side, as described by URL With single packet number space, the receiver should take extra care when updating the "minimum packet number below which all packets are immediately dropped". Otherwise, in presence of paths with very different latencies, the receiver might end up discarding packets from a (slower) path. I'm also preferring the multiple packet number spaces solution for the above reasons. I'm not against thinking for a "unified" solution (the proposal sounds interesting), but I wonder how much complexity this would add compared to requiring end hosts to use one-byte CIDs.
I think the issue is not really so much "single vs multiple number space" as "support for multipath and NULL CID". As noted by Mirja, there is an implementation cost there. My take is: If a node uses NULL CID and proposes support of multipath, then that node MUST implement code to deal with size of acknowledgements.