Code change:
multipath negotiation is ambiguous, they MUST be interpreted as acknowledging packets sent on path 0. <del> Endpoints negotiate the use of one packet number space for all paths or separate packet number spaces per path during the connection handshake nego.  While separate packet number spaces allow for more efficient ACK encoding, especially when paths have highly different latencies, this approach requires the use of a connection ID. Therefore use of a single number space can be beneficial when endpoints use zero-length connection ID for less overhead. </del> 7.1. <del> If the multipath option is negotiated to use one packet number space for all paths, the packet sequence numbers are allocated from the common number space, so that, for example, packet number N could be sent on one path and packet number N+1 on another. ACK frames report the numbers of packets that have been received so far, regardless of the path on which they have been received.  That means the senders needs to maintain an association between sent packet numbers and the path over which these packets were sent.  This is necessary to implement per path congestion control. When a packet is acknowledged, the state of the congestion control MUST be updated for the path where the acknowledged packet was originally sent.  The RTT is calculated based on the delay between the transmission of that packet and its first acknowledgement (see compute-rtt) and is used to update the RTT statistics for the sending path. Also loss detection MUST be adapted to allow for different RTTs on different paths.  For example, timer computations should take into account the RTT of the path on which a packet was sent.  Detections based on packet numbers shall compare a given packet number to the highest packet number received for that path. </del> <ins> If a zero-length connection ID is used, one packet number space for all paths.  That means the packet sequence numbers are allocated from the common number space, so that, for example, packet number N could be sent on one path and packet number N+1 on another. In this case, ACK frames report the numbers of packets that have been received so far, regardless of the path on which they have been received.  That means the senders needs to maintain an association between sent packet numbers and the path over which these packets were sent.  This is necessary to implement per path congestion control, as explained in zero-length-cid-loss-and-congestion. Further, the receiver of packets with zero-length connection IDs should implement handling of acknowledgements as defined in sending- acknowledgements-and-handling-ranges. ECN handing is specified in ecn-handling, and mitigation of the RTT measurement is further explained in ack-delay-and-zero-length-cid- considerations. If a node does not want to implement this logic, it MAY instead limit its use of multiple paths as explained in restricted-sending-to-zero- length-cid-peer. </ins> 7.1.1. <ins> If zero-length CID and therefore also a single packet number space is used by the sender, the receiver MAY send ACK frames instead of ACK_MP frames to reduce overhead as the additional path ID field will anyway always carry the same value. </ins> If senders decide to send packets on paths with different transmission delays, some packets will very likely be received out of order.  This will cause the ACK frames to carry multiple ranges of

Related discussion:
text: If the client spoofs a path, it will never receive the challenge, and thus the server will never receive the response and will not validate the path. See section 8.2 of RFC 9000, path validation. Security section 21.1.3. Connection Migration states that "Path validation () establishes that a peer is both willing and able to receive packets sent on a particular path."
So yes, the current text should be "the servers MUST in turn attempt to validate these paths using the same mechanisms, as specified in {{ Section 8.2 of RFC9000 }}."
No, this should not be a MUST. It is true that the path MUST be validated before it can be used. However, just because the serve receives a new packet on a path, that doesn't automatically mean that the server has to start path validation. It could e.g. also decide to simply withdraw that packet.
Actually, this is also not right. This is what RFC9000 says (section 9): So I guess we should maybe not use normative language here but refer to section 9 in RFC9000 instead...?
Just submitted PR for this issue.
This draft initially originates from a merging effort of previous Multipath proposals. While many points were found to be common between them, there remains one design point that still requires consensus: the number of packet number spaces that a Multipath QUIC connection should support (i.e., one for the whole connection vs. one per path). The current draft enables experimentation with both variants, but in the final version we will certainly need to choose between one of the versions.
The main issues mentioned so far: Efficiency. One number space per path should be more efficient, because implementations can directly reuse the loss-recovery logic specified for QUIC ACK size. Single space leads to lots of out of order delivery, which causes ACK sizes to grow Simplicity of code. Single space can be implemented without adding mane new code paths to uni-path QUIC Shorter header. Single space works well with NULL length CID The picoquic implementation shows that "efficiency" and "ack size" issues of single space implementations can be mitigated.