Messages aren't always self-descriptive

[martinthomson]

Section 6 claims that messages are self-descriptive. It uses the word "intended", which is an intention that is broadly achieved, with only one exception that I'm aware of at the syntactic level.

Having recently implemented message/http, I discovered that some response messages cannot be processed without knowledge of the corresponding request. This is pretty annoying and might be worth noting as a possible exception here. As noted in Section 6.3 of -messaging the response to a HEAD request might include Content-Length, but the content of the response is always empty.

At a semantic level, I don't think that any response can be considered without knowledge of the request, so I don't think that self-descriptiveness can possibly apply there. Besides, this section is clearly intended to provide an abstract definition of the syntax and not semantics.

[royfielding]

See also #706