# mid and cid URLs

Al Gilman (asg@severn.wash.inmet.com)
Tue, 21 Nov 1995 12:19:33 -0500 (EST)

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    From: asg@severn.wash.inmet.com (Al Gilman)

    Message-Id: <9511211719.AA04753@severn.wash.inmet.com>

    Subject: mid and cid URLs

    To: elevinso@Accurate.COM (Ed Levinson)

    Date: Tue, 21 Nov 1995 12:19:33 -0500 (EST)

    In-Reply-To: <9511201514.AA24375@Accurate.COM> from "Ed Levinson" at Nov 20, 95 10:14:20 am

I need another MIME lesson.

As I look at the draft proposed schemes for mid and cid URLs, a couple
of thoughts come up:

1. By construction, these two nominal schemes are one scheme and we
should only use one name for them. MID or MIDCID are possibles.

2. A more URL-traditional syntax would be something like

"mid:" //host-net-path/message-unique

where the RFC Message-ID object was <message-unique@host-net-path>.

3. The structure of a MIME multipart defines a well-ordered hierarchical
space where at each level there is a linear sequence of parts. We
could index into this part tree with part numbers. MIME message/partial
usage establishes the precedent that parts number from 1 and the
generic Internet URL syntax sets the precedent that paths punctuate with
'/'. If we simply extrapolate from these two boundary conditions, we
get a part URL more or less like this:

midcidurl ::= "mid:" //host-net-path/message-unique part-number
part-number ::= *( / decimal-integer )

where the interpretation of the decimal integers is as follows:

For each level of hierarchy (defined separator)
0 refers to the unPart before the first opening-separator
1 refers to the Part after the first opening-separator
... ; there are N parts and N opening-separators
n > N refers to the unPart after the closing-separator

opening-separator ::= "--" separator-key
closing-separator ::= "--" separator-key "--"

4. To retrieve an object by its Content-ID, the usage

cidurl ::= "mid:" //host-net-path/message-unique?part-designation

part-designation ::= part-unique [@host-path-if-different]
where
Content-ID == <part-unique@(host-net-path | host-path-if-different)>

; and I have not addressed encoding problems

is more consistent with general Internet URL usage than
introducing the Content-ID with '#'. In particular, for the
general pattern of URLs, the #fragment clause makes no difference
in the object that is served, only its state as presented.

Since the retrieval of a part by Content-ID only needs to get the
content of the part, the ? syntax which supports searching and can
affect the scope of the object retrieved is more consistent.

This usage would establish, as a rule under the mid: scheme, that
searching defaults to matching part-designation as constructed
above.

Al Gilman

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