Change:
REST clients (registrant-EPs and CTs during registration and maintenance, lookup clients, RD servers during simple registrations) <del> MUST be prepared to receive any unsuccessful code and act upon it </del> <ins> must be prepared to receive any unsuccessful code and act upon it </ins> according to its definition, options and/or payload to the best of their capabilities, falling back to failing the operation if recovery <del> is not possible.  In particular, they should retry the request upon </del> <ins> is not possible.  In particular, they SHOULD retry the request upon </ins> 5.03 (Service Unavailable; 503 in HTTP) according to the Max-Age <del> (Retry-After in HTTP) option, and fall back to link-format when receiving 4.15 (Unsupported Content-Format; 415 in HTTP). </del> <ins> (Retry-After in HTTP) option, and SHOULD fall back to link-format when receiving 4.15 (Unsupported Content-Format; 415 in HTTP). </ins> An RD MAY make the information submitted to it available to further directories, if it can ensure that a loop does not form.  The

Discussion:
The "MUST" was non-actionable, while the "should" would be; switching normativity. See-Also: URL Thanks for the updates in the -26; they help a lot. I'm particularly happy to see the (default/example) "first-come-first-remembered" policy in § 7.5, which does a good job of documenting its properties (including potential deficiencies) and can be usable for many environments.  (I do have some suggestions for improvements to the specific wording in the section-by-section comments, but on the whole I like it.) It does lead in to a more general comment, though (which is not exactly new to the changes in the -26 though it is reflected in several of them): some aspects of the concept of "authorization" to appear in a directory, and the verification of authorization for the entire flow from client locating the RD server to final request at the discovered resource, are somewhat subtle.  (The new text discusses, e.g., "repeat the URI discovery step at the /.well-known/core resource of the indicated host to obtain the resource type information from an authorized source" and "a straightforward way to verify [discovered information] is to request it again from an authorized server, typically the one that hosts the target resource".)  The aspects I'm concerned about are not matters of a resource server being authorized to publish to a directory or a client deeming that a server is authorized to act as an RD (though both of those are also important), but seems to rather just be relating to the RD faithfully disseminating the information the RD retrieved from /.well-known/core discovery on the given server(s) (or otherwise learned via some other mechanism). In this sense the RD is just acting as a cache or efficiency aid, but the authoritative source of the information remains (in general) the server hosting the resource.  To be clear, I think the procedures that this document specifies are good and correct procedures for a client to perform, I'm just not sure whether "authorized" is the right term in these cases; "authoritative" might be more appropriate, in that the operation in question is to verify the information retrieved from the RD (which acts in some sense as an intermediary) at the more authoritative source.

(On a tangent from that note, it seems that there is not necessarily any indication to a client that a server claiming to provide a given resource (whether discovered via RD or /.well-known/core) is actually authorized to provide such a resource and be trusted by the client.  But that's not really a property of RD, and rather the underlying CoRE mechanism and any provisioned trust database, so my thoughts about it seem out of scope for this document.) (It's not entirely clear to me what the reader is expected to do with the information about the previous formulation.) (It seems that something also needs to cause the RD to check the authorization of lookup clients to receive the information in question, which might be worth reiterating in this section.) I'm pretty sure the intent here is that only the subject names that are certified by the recognized authority(ies) are stored, in which case I'd suggest to s/all those/all such/. (I can understand why the "store the whole list" logic is used, though there may be some subtleties about name constraints on the (X.509) CAs used, which is why we typically don't recomment treating the entire list of names in the certificate as validated from a single operation, instead preferring "is this certificate authenticated for this specific name?" when the specific target name is available.) Should we talk about whether the lifetime of registration entries is related to the various lifetimes of these credentials/identifiers?  DTLS without resumption is perhaps an interesting case, in that keeping the DTLS association "live" just to be able to update the registration information seems impractical, so perhaps some fixed cap would be applied on the lifetime in that case.  If resumption is used, the resumption information (ticket or session state) has some finite lifetime that could be used to bound the lifetime of the registration. IIRC an OSCORE security context can also have an expiration time; certificates and JWT/CWT have expiration time as well (though the procedures in the first bullet point would effectively allow a "renewal" operation), but the validity period (if any) of a raw public key would have to be provisioned along with that key (or just have RPK be handled akin to DTLS without resumption, I guess). (Similar to my earlier parenthetical comment, the requirement for exact match of all subject name information is unusual.

In effect this is a way of pinning the set of names that the CA has chosen to include in a single certificate, which is arguably a stronger requirement than needed but should not lead to any unauthorized access.)