Document HTTP's error-handling philosophy

[mnot@…]

Questions often come up as to why HTTP doesn't define error handling in a detailed way.

A short explanation of the reasons would help readers understand HTTP's approach. e.g.,:

• Error conditions are numerous; addressing every one would make the specification much more complex and therefore harder to implement correctly, bringing yet more potential errors.
• Error handling is often dependant upon the use case, not generic

[mnot@…]

Proposal: replace "Requirements" sections with the following.

Conformance and Error Handling

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

This document defines conformance criteria for several roles in HTTP communication, including Senders, Recipients, Clients, Servers, User-Agents, Origin Servers, Intermediaries, Proxies and Gateways. See [ref to Terminology] for a definition of each.

An implementation is considered conformant if it complies with all of the requirements associated with its role(s). Note that SHOULD-level requirements are relevant here, unless one of the documented exceptions is applicable.

This document also uses ABNF to define valid protocol elements. In addition to the prose requirements placed upon them, Senders MUST NOT generate protocol elements that are invalid.

Unless noted otherwise, Recipients MAY take steps to recover a usable protocol element from an invalid construct, and SHOULD NOT reject the message outright.

However, HTTP does not define specific error handling mechanisms, except in cases where it has direct impact on security. This is because different uses of the protocol require different error handling strategies; for example, a Web browser may wish to transparently recover from a response where the Content-Type header doesn't match the body, whereby in a systems control protocol using HTTP, this type of error recovery could lead to dangerous consequences.

[mnot@…]

Assigning to milestone 16; we have consensus (see ​http://www.w3.org/mid/C53E7A36-FEA2-47F7-93C7-615BF364ACF1@mnot.net ), just waiting for it to be incorporated.

[julian.reschke@…]

In progress, based on <​http://lists.w3.org/Archives/Public/ietf-http-wg/2011JulSep/0211.html>.

[julian.reschke@…]

From [1452]:

Rephrase description of conformance; explain how the spec handles error handling (see #186)