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: If a node sees a peer using NULL CID and supporting multipath, then that node MUST either use only one path at a time or implement code to deal with impact on loss detection due to out of order arrivals, and impact on congestion control including ambiguity of ECN signals. Then add sections on what it means to deal with the side of acknowledgements, out of order arrivals, and congestion control. I think this approach ends up with the correct compromises: It keeps the main case simple. Nodes that rely on multipath just use long CID and one number space per CID. Nodes that use long CID never have to implement the ACK length minimization algorithms. Nodes that see a peer using NULL CID have a variety of implementation choices, e.g., sending on just one path at a time, sending on mostly one path at a time in "make or break" transitions, or implementing sophisticated algorithms. If we do that, we can get rid of the single vs multiple space discussion, and end up with a single solution addressing all use cases.
Looking at all the discussions here and in other issues such as ECN, I think that we should try to write two different versions of section 7: one covering all the aspects of single packet number space (RTT estimation, retransmissions, ECN, handling of CIDs, ...) one covering all the aspects of multiple packet number space (RTT estimation, retransmissions, ECN, handling of CIDs, ...) There would be some overlap between these two sections and also some differences that would become clear as we specify them. At the end of this writing, we'll know whether it is possible to support/unify both or we need to recommend a single one. The other parts of the document are almost independent of that and can evolve in parallel with these two sections. However, I don't think that such a change would be possible by Monday
I don't think we should rush changes before we have agreed on the final vision.
It might be helpful to have these options as PRs (without merging) them, so people can understand all details of each approach.
I agree with NAME that supporting multi-path with null CIDs is a more fundamental issue than the efficiency comparison between single PN and separate PN, as it would ultimately impact the application scope of multipath QUIC. But indeed, we might want to implement proposed solution first and then decide if we want to adopt such a unified approach.