This is the last call YANG Doctor review of draft-ietf-bfd-rfc9127-bis. Browsing the mail archives, this has been a long story. Realizing that the context of the bis is to fix a particular issue, I have focused only on the diffs from RFC 9127. I feel any additional nitpicks I might find in a complete review would not be welcome at this stage.

I have reviewed the diffs, and find them fulfill the desired technical goals. Since this update breaks backwards compatibility as defined in RFC 6020 sec 10 and RFC 7950 sec 11, the process for approving this change has been discussed at length. One argument that has been put forward for going ahead is that the previous version of this module was released only a short time ago, so there is no proliferation of impacted systems in the field. 

Another argument has been that the YANG Versioning Design Team is working on updated backwards compatibility rules. The Ver-DT proposed updates to the compatibility rules would indeed allow a change of this kind under certain conditions. A key condition for allowing such a break with the backwards compatibility is that the module revision history announces this break clearly to all readers. This is not the case in the -01 version of the modules.

   revision 2022-01-04 {
       description
         "Updates to add client configuration parameters feature.";

In my YANG Doctor opinion, updating the revision statement to clearly state that this version is not backwards compatible with the previous version is an absolute requirement. I think it would also be fair to module readers to add a few sentences explaining what's going on here.