[netconf] Default statements on udp-client-server groupings

[Alex Huang Feng <[e-mail]>]

Dear NETCONF,

I would like to follow up on the discussions from the NETCONF WG meeting regarding the udp-client-server grouping draft (https://datatracker.ietf.org/doc/html/draft-ietf-netconf-udp-client-server-03)
There is one last remaining issue that I’d like feedback from the WG before asking for WGLC.

On the last iteration of the draft (draft-ietf-netconf-udp-client-server-03), I removed the default statement from the port leaves. The default statement was present in both client and server ports leaves (local and remote).
See the diff between -02 and -03 here: https://author-tools.ietf.org/iddiff?url1=draft-ietf-netconf-udp-client-server-02&url2=draft-ietf-netconf-udp-client-server-03&difftype=--html

The reasons are the followings:
- When there is a default statement in the grouping and a user “uses” this grouping, the user is obligated to refine this default port with another valid port. For protocols (and users) that do not need a default port, they are still obligated to refine this default port if they want to use this grouping. YANG does not allow to remove the default statement (https://datatracker.ietf.org/doc/html/rfc7950#section-7.13.2) when a YANG module uses a grouping.
- The groupings defined in this draft are generic and it is my understanding that having this default statement would prevent some use cases (Note the feedback from Med: https://mailarchive.ietf.org/arch/msg/netconf/j-2Hh6PO-QcHZetZ0ebujxsVLL0/)
- As a first user of the udp-client grouping, I am already encountering this problem. I am using this grouping in the UDP-Notif YANG model, in which I don’t need to define a default port for the protocol.

Of course, removing this port presents its disadvantages:
- By removing this default statement, the udp-groupings are not consistent anymore with the tcp-client-server-groupings. In tcp-client-server groupings, the default statement is present in both the ports of the client and server.

Thus, I would like to hear more feedback from the list on whether this default statement needs to be present in the port leaves or not.
My preference (and from the list, I would also say Med agrees) is that this default statement should not be present to provide a reusable grouping for a larger group of use cases.

Regards,
Alex