Code change:
latency.  To collect ACK delays on all the paths, hosts should rely on time stamps as described in QUIC-Timestamp. <ins> 7.1.4. ECN feedback in QUIC is provided based on counters in the ACK frame (see Section 19.3.2. of QUIC-TRANSPORT).  That means if an ACK frame acknowledges multiple packets, the ECN feedback cannot be accounted to a specific packet. There are separate counters for each packet number space.  However, sending to zero-length CID receivers, the same number space is used for multiple paths.  Respectively, if an ACK frames acknowledges multiple packets from different paths, the ECN feedback cannot unambiguously be assigned to a path. If the sender marks its packets with the ECN capable flags, the network will expect standard reactions to ECN marks, such as slowing down transmission on the sending path.  If zero-length CID is used, the sending path is however ambiguous.  Therefore, the sender MUST treat a CE marking as a congestion signal on all sending paths that have been by a packet that was acknowledged in the ACK frame signaling the CE counter increase. A host that is sending over multiple paths to a zero-length CID receiver MAY disable ECN marking and send all subsequent packets as Not-ECN capable. 7.1.5. Hosts that are designed to support multipath using multiple number spaces MAY adopt a conservative posture after negotiating multipath support with a peer using zero-length CID.  The simplest posture is to only send data on one path at a time, while accepting packets on all acceptable paths.  In that case: the attribution of packets to path discussed in zero-length-cid- loss-and-congestion are easy to solve because packets are sent on a single path, the ACK Delays are correct, the vast majority of ECN marks relate to the current sending path. Of course, the hosts will only take limited advantage from the multipath capability in these scenarios.  Support for "make before break" migrations will improve, but load sharing between multiple paths will not work. </ins> 7.2. <del> If the multipath option is enabled with a value of 2, each path has its own packet number space for transmitting 1-RTT packets and a new ACK frame format is used as specified in ack-mp-frame.  Compared to the QUIC version 1 ACK frame, the ACK_MP frames additionally contains a Packet Number Space Identifier (PN Space ID).  The PN Space ID used to distinguish packet number spaces for different paths and is simply </del> <ins> If packets contain a non-zero CID, each path has its own packet number space for transmitting 1-RTT packets and a new ACK frame format is used as specified in ack-mp-frame.  Compared to the QUIC version 1 ACK frame, the ACK_MP frames additionally contains a Packet Number Space Identifier (PN Space ID).  The PN Space ID used to distinguish packet number spaces for different paths and is simply </ins> derived from the sequence number of Destination Connection ID. Therefore, the packet number space for 1-RTT packets can be identified based on the Destination Connection ID in each packets.

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.