RE: Processing instructions and IETF XML Usage Guidelines

["Larry Masinter" <LMM@xxxxxxx>]

> The Guidelines suggest [1]:
> -------------------------
> However, XML processing instructions appear to have rare applicability
> to XML fragments embedded in Internet protocols, and it is recommended
> that their use be explicitly disallowed ("MUST NOT use"). In 
> cases where
> XML processing instructions are allowed, the nature of the allowable
> processing instructions should be specified explicitly.
> -------------------------
> 
> I'm not certain what the motivation for constraining processing
> instructions is in this case, as applications are permitted simply to
> ignore them if they don't understand them.  If this proposal is trying
> to avoid embrace-and-extend, those concerns should probably be made
> explicit.  I'd also suggest reading the discussion of PIs on www-tag
> [2].

I agree that the section may need more motivation. In general,
the recommendation follows a general protocol design principle
that it's a good idea to avoid private "extensibility" mechanisms
("sender can send, receiver can ignore if the receiver doesn't
understand")
without a clear model for how extensibility will be managed.
There are a lot of failure modes for uncontrolled extensibility,
of which "embrace and extend" is merely one of the deliberate ones.

As is pointed out in the referenced thread, PI extensibility
doesn't _require_ namespaces, etc.

In general, the designer of protocols already has ample extensibility
mechanisms at hand, so allowing them in XML components seems
unwarranted.

[2] - http://lists.w3.org/Archives/Public/www-tag/2002Feb/0057.html
(follow the thread, not just the first message)