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 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.