Code change:
NAME you wrote: That sounds like you want to restrict the multipath extension to only one server address (but potentially different paths from different clients address)...? I think this restriction is unnecessary. Yes we don't specify a way to learn server addresses but that doesn't means that the client doesn't know (e.g. based on higher layer information) multiple addresses of the server. What should we restrict that?
However, can we maybe merge this PR as it is for now and you open a separate issue for your additional point NAME ?
Mirja, That was my understanding of the sentences in the PR. Maybe I got it wrong. I think that we should discuss the role of preferred address in section 3 where we explain how paths can be opened.
Fine, let's do it this way.
RFC9000 defines a list of transport parameters. 0x01 0x02 0x03 0x04 0x05 0x06 0x07 0x08 0x09 0x0a 0x0b 0x0c 0x0d 0x0e 0x0f 0x10 [QUICWG] These transport parameters apply to all the paths that compose a Multipath QUIC connection. We should probably confirm this in the mpquic draft to avoid any misinterpretation (e.g. for maxidletimeout or ack delays). There are two transport parameters that could warrant some discussion: disableactivemigration : This transport parameter is incompatible with enablemultipath. An endpoint that receives both this transport parameter and a non zero enablemultipath parameter MUST treat this as a connection error of type MPCONNECTIONERROR and close the connection. preferredaddress: RFC9000, section 9.2.1 contains the following: Once the handshake is confirmed, the client SHOULD select one of the two addresses provided by the server and initiate path validation (see Section 8.2). A client constructs packets using any previously unused active connection ID, taken from either the preferredaddress transport parameter or a NEWCONNECTION_ID frame. As soon as path validation succeeds, the client SHOULD begin sending all future packets to the new server address using the new connection ID and discontinue use of the old server address. If path validation fails, the client MUST continue sending all future packets to the server's original IP address. For multipath, do we want to be stronger than SHOULD select one ? Can a client create news paths towards the initial address or only to one of the preferred addresses ? Should the client use both addresses (IPv4 and IPv6) if non-empty ?
Good point about "disableactivemigration". I would read it as also disabling multipath. We do not need to say that "transport parameters apply to all the paths that compose a Multipath QUIC connection". We are already inheriting that property from compatibility with path migration in RFC 9000. Multipath support is an extension to QUIC v1, not a new protocol, so we do not need to specify all the defaults. Same goes for the preferred address -- meaning does not change just because the nodes negotiate multipath. I would avoid debating "which server addresses the client can try" in the base multipath document. If we want the server to signal plausible addresses to the client, let's have a specific document for that.
I would maybe add a sentence that this draft does not change the meaning of preferred_address and if that address could also be used for multipath is out of scope for this extension.
Regarding disableactivemigration: this cannot be a connection error, as you can also send preferred address and after migration to the preferred address you can still use migration or multipath (if negotiated). I think the definition in RFC9000 is still valid; it means you can't send any packets from another address than the one used in the handshake. Which effectively means disabling multipath as well as Christian noted (if not moved to another preferred address). I guess we could explicitly note that this draft does not change the definition of disableactivemigration. For your reference this is what RFC9000 says:
I created a PR in . Is this sufficient to address this issue or do we need to say more? NAME ?
The proposed PR sounds good to me, with the slight addition I mentioned.

Related discussion:
text: this limit even applies when no connection ID is exposed in the QUIC header. <ins> This extension does not change the definition of any transport parameter defined in Section 18.2. of QUIC-TRANSPORT.  Inline with the definition in QUIC-TRANSPORT disable_active_migration also disables multipath support, except "after a client has acted on a preferred_address transport parameter" Section 18.2. of QUIC- TRANSPORT.  Further, it is out of scope for this document to specify if the old address can also be used for multipath after a client has migrated to the address provided in the preferred_address transport parameter.  However, it SHOULD NOT be assumed that it is possible to use both addresses simultaneously without further confirmation from the other host. </ins> 3. After completing the handshake, endpoints have agreed to enable