Code change:
requires negotiation between the two endpoints using a new transport parameter, as specified in nego. <del> This proposal supports the negotiation of either the use of one packet number space for all paths or the use of separate packet number spaces per path.  While both approaches are supported by the specification in this version of the document, the intention for the final publication of a multipath extension for QUIC is to choose one option in order to avoid incompatibility.  More evaluation and implementation experience is needed to select one approach before final publication.  Some discussion about pros and cons can be found here: https://github.com/mirjak/draft-lmbdhk-quic- multipath/blob/master/presentations/PacketNumberSpace_s.pdf As currently defined in this version of the draft the use of multiple packet number spaces requires the use of connection IDs is both directions.  Today's deployments often only use destination connection ID when sending packets from the client to the server as this addresses the most important use cases for migration, like NAT rebinding or mobility events.  Further discussion and work is required to evaluate if the use of multiple packet number spaces could be supported as well when the connection ID is only present in one direction. </del> <ins> This extension uses multiple packet number spaces.  When multipath is negotiated, each destination connection ID is linked to a separate packet number space.  Using multiple packet number spaces enables direct use of the loss recovery and congestion control mechanisms defined in QUIC-RECOVERY. Some deployments of QUIC use zero-length connection IDs.  When a node selects to use zero-length connection IDs, it is not possible to use different connection IDs for distinguishing packets sent to that node over different paths.  This extension also specifies a way to use zero-length CID by using the same packet number space on all paths. However, when using the same packet number space on multiple paths, out of order delivery is likely.  This causes inflation of the number of acknowledgement ranges and therefore of the the size of ACK frames.  Senders that accept to use a single number space on multiple paths when sending to a node using zero-length CID need to take special care to minimize the impact of multipath delivery on loss detection, congestion control, and ECN handling.  This proposal specifies algorithms for controlling the size of acknowledgement packets and ECN handling in Section using-zero-length and ecn- handling. </ins> This proposal does not cover address discovery and management. Addresses and the actual decision process to setup or tear down paths

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.