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Re: mustUnderstand, mustIgnore [was: Posted PaceEntryOrder]
On Feb 6, 2005, at 6:50 PM, Mark Nottingham wrote:
On Feb 5, 2005, at 6:01 PM, Roy T.Fielding wrote:
The problem with that statement (about HTTP) is that absence of a
must-understand in HTTP is not one of its big problems. Yes, I know
lots of people have talked about it as a limiting factor in the
syntax of HTTP, but to call it an actual problem would say that it
prevented some good from being accomplished.
It arguably tipped some people towards SOAP when HTTP would have been
adequate. That's not a prevention of good, but we've already seen
enough fragmentation in the syndication world.
Well, arguably, those same applications should have been tipped
into the waste basket in the first place. But I don't think you
followed my main point: must understand is a mechanism to enable
fragmentation -- its very presence leads away from standardization.
Lack of mU is one of the reasons that HTTP is not fragmented (along
with me being a stubborn pain in the ass). Hence, it is only a
problem for some applications that were of questionable character,
and it remains unclear whether HTTP would have benefitted by having
a mU feature or if its presence would have led to a complete meltdown.
Things that a syndication format might want to make mandatory are
copyright controls and micropayments, but both have been shown in
practice to require either a willingness on the part of the recipient
to accept that specific restriction (i.e., human intervention and
understanding) or forceful requirement by the sender (i.e.,
encryption). In both cases, agreements have to be established with
the user in advance, before they even receive the content, and thus
do not need a "must understand" feature.
I don't think mU is intended for such things; rather, the case for mU
could be characterised as extensions that change the operation of the
protocol in a manner that renders it useless or misleading to try to
use the feed if you don't know what to do with the extension. It's
Right, but look at my examples and try to think of any others that
would require changes in operation on the behalf of recipients.
There may be others, but I am not aware of any more.
In fact, "must understand" has no value in a network-based
application except as positive guidance for intermediaries, which is
something that can still be accomplished under mustIgnore with a bit
of old-fashioned advocacy.
So, if I can restate your position, you're saying that you don't
dispute that understanding some extensions may be required, but that
it isn't necessary to make that visible to the processor, because
it'll be co-ordinated beforehand (e.g., through market forces,
No, my position is that it isn't necessary to include mU in the format.
Within the control data of an interaction protocol, sure, but not
within the payload of completed actions, wherein any such requirements
are far too late and susceptible to abuse.
Just to be clear that I am not completely against mU in
all protocols, that feature does exist in waka because it is
useful when talking through intermediaries.