INTERNET-DRAFT Donald E. Eastlake 3rd
Motorola
Expires August 2001 February 2001
Mapping Between MIME Types, Content-Types, and URIs
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<draft-eastlake-cturi-02.txt>
Donald E. Eastlake 3rd
Status of This Document
Distribution of this document is unlimited. Comments should be sent
to the author.
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC 2026. Internet-Drafts are
working documents of the Internet Engineering Task Force (IETF), its
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distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
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The list of current Internet-Drafts can be accessed at
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Abstract
Multipurpose Internet Mail Extension (MIME) Content-Type headers, the
MIME types used therein, and Uniform Resource Identifiers (URIs) are
being used, in different contexts, to label entities. A mapping is
specified such that the union of their meaning can be expressed in
either URI or Content-Type syntax.
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Table of Contents
Status of This Document....................................1
Abstract...................................................1
Table of Contents..........................................2
1. Introduction............................................3
1.1 Introduction to URIs and MIME Type/Content-Type........3
1.2 Definitions and Conventions............................3
1.3 Additional Features....................................4
1.4 Overview of Remaining Sections.........................4
2. Simple Mapping..........................................5
2.1 Simple Mapping of Content-Type to URI..................5
2.1.1 The Basic Case.......................................5
2.1.2 More Complete Rules..................................6
2.2. Simple Mapping of URI to Content-Type, The Basic Case.6
2.3 Content-Type Mapping Special Case for Basic Closure....7
2.4 URI Mapping Special Case for Basic Closure.............8
3. Controlled Mapping......................................9
4. Troublesome Characters.................................10
5. IANA Considerations and Potential Conflicts............10
6. Security Considerations................................11
Appendix..................................................12
References................................................13
Author's Address..........................................14
Expiration and File Name..................................14
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1. Introduction
Both MIME types and URIs have come to be used for type labeling and
similar information.
In most protocols where there are provisions for a general "type
label", the label is restricted to the syntax of a URI or the syntax
of a Content-Type. In some cases, it will be useful to be able to
express labels which already exist in the "other" syntax. That is, it
may be useful in a URI syntax slot to also be able to express a MIME
type or Content-Type and, conversely, it may be useful in a Content-
Type syntax slot to also be able to express a URI. This document
specifies how.
1.1 Introduction to URIs and MIME Type/Content-Type
The IETF Multipurpose Internet Mail Extensions (MIME) message body
standards have developed into a general tagging and bagging
mechanism. This mechanism has spread from SMTP mail to USENET, HTTP,
and other protocols. In MIME, the type of an object is given in a
"Content-Type" header line. [RFC 2045, 2046, 2048] Such a line
consists of a MIME type and, optionally, additional parameters. A
MIME type consists of a MIME top level type, a slash, and a MIME
subtype.
The original Uniform Resource Locator (URL [RFC 1738]), used to point
to World Wide Web (WWW) resources, has grown into the more general
Uniform Resource Identifier (URI [RFC 2396]). Increasingly URIs are
used as general labels for algorithms, XML namespaces [XML NAME], web
based protocol data types, etc. (In some of these label uses, URIs
are considered opaque while in other cases they are assumed to be
de-referencable into something which explicates their meaning.)
1.2 Definitions and Conventions
Concerning URIs, please note the following:
(1) In this document, the term URI is used to include URI
Reference. That is, it includes the case where an octothorpe
("#") followed by a fragment identifier is suffixed to a pure
URI.
(2) Only absolute URIs are mappable. Relative URIs, with just a
hierarchial part, are not included in URI as used in this
document. They must first be converted to absolute URIs as
described in [RFC 2396].
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(3) For presentation purposes, URIs are shown inside angle
brackets ("<...>") but these angle brackets are not actually a
part of the URI.
Concerning Content-Types, please note the following:
Content-Type values are shown preceded by "Content-Type: " and,
when long, they are line folded as per [RFC 822]. This prefix
and line folding are for presentation purposes and are not
actually a part of the Content-Type.
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 [RFC 2119].
1.3 Additional Features
Note that a URI or Content-Type could get converted back and forth
multiple times between these two syntaxes. To stop this from
resulting in ever longer and more complex tags, a check is mandated
so that if a conversion is of a previously converted syntax, the
previous conversion is reversed, in so far as practical.
To improve the repeatability of the results from single or multiple
steps of syntax conversion, capitalization and punctuation
recommendations are made where tokens are case insensitive or
variable punctuation is allowed.
Finally, in cases where the default conversion does not provide for
sufficient control, optional elements are defined for inclusion in
URIs and Content-Types that provide substantial control over the
mapping output.
1.4 Overview of Remaining Sections
Sections 2 and 3 below give an explanation of the mapping specified,
more or less in English. The material is organized to start with the
simplest and most common rules and then add exceptions for special
cases and additional user control.
Section 4 lists characters that must be URI ("%") encoded when
mapping from a URI to a Content-Type.
Section 5 covers IANA Considerations and potential conflicts.
Section 6 give Security Considerations.
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The Appendix presents some sample code in Perl.
2. Simple Mapping
This section describes simple mappings such that any MIME Type or
Content-Type can be mapped into a URI and any URI can be mapped into
a Content-Type. Other than checks for multiple conversions, the
mapping is simple. It can produce only a special scheme URI for the
mapping of a Content-Type and only a special sub-type tree in the
"application" top level type for the mapping of a URI. Section 3
below describes additional features optionally allowing much greater
control over the result of the mapping.
2.1 Simple Mapping of Content-Type to URI
Section 2.1.1 below describes the most basic case of converting a
simple MIME type to a URI. Section 2.1.2 extends this to converting
a general Content-Type to a URI. Section 2.3 adds the check
necessary to recognize where the MIME type being converted is of the
form indicating it was previous converted from a URI using basic
mapping and is being converted back to a URI.
2.1.1 The Basic Case
For the simplest case of a Content-Type consisting of just a MIME
type, create a URI with scheme "ContentType" and a scheme dependent
part consisting of the MIME type. For example
Content-Type: image/JPEG
simply converts to
<ContentType:image/jpeg>
White space is not allowed in URIs so it must be removed. Scheme
names (the part before the first ":" in a URI) are case insensitive
but for readability and repeatability, the capitalization
"ContentType" SHOULD be used. Similarly, MIME top level types and
subtypes (the fields before and after the "/" in a MIME type field,
respectively) are case insensitive but SHOULD be all lower cased when
mapped to the URI form.
Note: There is no "//" after the "ContentType:" scheme as used
herein. Such a "//" would imply a specific structuring of the
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scheme dependent part appearing in the URI after the
"ContentType:" as defined in [RFC 2396]. Since that full
structuring is not used, "//" is not used. The meaning of URIs
starting with "ContentType://" is reserved for future definition.
Note: "Content-Type", with hyphen, is syntactically allowed as a
scheme name. However, [RFC 2717] reserves embedded hyphens in
scheme names to indicate the prefix of an alternate tree of
scheme names. Therefore, the un-hyphenated ContentType is used.
2.1.2 More Complete Rules
A Content-Type header frequently includes more than just the
mandatory MIME type. It can also have type dependent parameters,
including private parameters, such as
Content-Type: text/plain; charset="us-ascii";
x-mac-type="54455854"; x-mac-creator="4D4F5353"
Content-Type: image/tiff; application=faxbw
Content-Type parameters are mapped into a "query portion" suffix of
the URI in much the same way that HTML form fields [HTML] are. That
is, they are concatenated to the MIME type after a "?" and, if there
is more than one parameter, separated by "&". Thus the above
Content-Types would be mapped into the following URIs:
<ContentType:text/plain?charset="us-ascii"&x-mac-type="54455854"&
x-mac-creator="4D4F5353">
<ContentType:image/tiff?application="faxbw">
Parameter values in the mapped URI MUST always be enclosed in double
quotes ('"'). If the Content-Type has a trailing ";" but no
parameters, then "?" SHOULD NOT be added to the URI.
2.2. Simple Mapping of URI to Content-Type, The Basic Case
This section describes the basic case of mapping a URI to a Content-
Type. Section 2.4 adds the check to see if the URI appears to be the
result of a previous conversion from a Content-Type and if so undoes
that conversion in so far as practical.
In the basic case, a URI maps to a Content-Type with a top level MIME
type of "application" a MIME sub-type in the "uri." tree. In
addition, any "query" parameters in the URI are mapped to Content-
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Type parameters and if the URI ends with a fragment identifier, it is
mapped to the special Content-Type parameter "URI-Fragment". Any
special characters in the URI that might be troublesome (see section
4) are encoded by replacing them with a "%" followed by two hex
digits for the character code.
Note: Current URI syntax permits scheme dependent parts in which "?"
does not indicate a query section; however, no such syntaxes have
been publicly defined.
Some examples of the basic case follow:
<http://example.com/tag42>
<mailto:U@example.net?subject="misc"&body="line1%0D%0Aline2">
<xyz://abc.test/def?h=ijk#lmn>
convert to
Content-Type: application/uri.http%3A%2F%2Fexample.com%2Ftag42
Content-Type: application/uri.mailto%3Aexample.net;
subject="misc"; body="line1%250D%250Aline2"
Content-Type: application/uri.xyz%3A%2F%2Fabc.text%2Fdef;
h="ijk"; URI-Fragment="lmn"
Content-Type parameters values extracted from the query portion of a
URI MUST be surrounded with double quotes ('"'). When URI encoding,
if the hex value has any letters (a-f) in it, they SHOULD be upper
cased.
2.3 Content-Type Mapping Special Case for Basic Closure
A URI may have been converted to a Content-Type and get converted
back. To stop this from resulting in an ever more complex syntax, a
check MUST be made to see if the MIME subtype of a Content-Type being
converted is in the "uri." subtype tree (see section 2.2 above). If
so, the URI is computed from the subtype by stripping the "uri."
prefix and performing one level of undoing URI encoding. (Note: The
top level MIME type is ignored in this case.) In addition, Content-
Type parameters, if any, are added as a "query portion" and a "URI-
Fragment" parameter is added as a fragment.
For example:
Content-Type: application/uri.mailto%3Auser%40host.example
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Content-Type: application/uri.http%3A%2F%2Fx.test; foo="123",
bar="abcd"
Content-Type:
application/uri.http%3A%2F%2Fa%3Ab%40c.text%2Fx%2Fy;
URI-Fragment="z"
convert to
<mailto:user@host.example>
<http://x.test?foo="123"&bar="abcd">
<http://a:b@c.text/x/y#z>
Note: If a Content-Type or MIME Type is being written by a user and
they know that there is a URI which is a more natural expression
of the labeling desired, they can simply use an ".../uri." MIME
Type to start with.
2.4 URI Mapping Special Case for Basic Closure
It is desirable that an arbitrary Content-Type be recovered
semantically intact when mapped to a URI and then that URI is mapped
back to a Content-Type. To achieve this, the following special case
is added to the simple case described in section 2.2 above.
If the URI scheme is "ContentType:", then the Content-Type is
computed from the remaining part of the URI (the scheme specific
part), by replacing the first question mark ("?") and all query
section ampersands ("&") with semi-colon space ("; "), and then
undoing one level of URI encoding, i.e., replacing percent sign ("%")
followed by two hex digits with the character having that hex value.
For example
<ContentType:model/vnd.example.longish.subtype.name>
<ContentType:text/plain?charset="US-ASCII"&x-obscure="value">
map to
Content-Type: model/vnd.example.longish.subtype.name
Content-Type: text/plain; charset="US-ASCII"; x-obscure="value"
Note: A URI produced by simple mapping from a normal Content-Type
will never have a fragment suffix.
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Note: If a type label URI is being written by a user and they know
that there is a Content-Type which is a more natural expression
of the labeling desired, they can simply use a "ContentType:"
scheme to start with.
3. Controlled Mapping
[Is this controlled mapping stuff below too complex? Would it be
better to just have section 2 above and drop controlled conversion?]
As an additional feature, there may be cases where a URI is designed
knowing that it might be converted to a Content-Type and it is
desired to control the MIME type so that it would have a more
appropriate top level than "application" and/or a more appropriate
subtype than one in the "uri." tree. To accomplish this, a special
URI query part parameter "MIME-Type" is defined. If a URI is not of
scheme ContentType and this special parameter is found, then the MIME
type is set to the parameter value and the URI body (all of the URI
except "query" parameters and any fragment identifier) is preserved
in a "URI-body" Content-Type parameter.
Similarly, there may be cases where a Content-Type is designed
knowing that it might be converted to a URI and it is desired to
control the URI scheme and non-query scheme dependent parts so that
it is not of scheme "ContentType:" or does not have the scheme
dependent part calculated as indicated in section 2.1. To accomplish
this, a special Content-Type parameter "URI-body" is defined. If a
Content-Type does not have a MIME subtype in the "uri." tree and this
parameter is present, it controls the non-query portion of the URI
mapped to and the original MIME type is preserved in a URI query
parameter called "MIME-Type".
For example
Content-Type: application/xml; URI-Body="http://xml.example"
would map to
<http://xml.example?MIME-Type="application/xml">
and
<mailto:joe@blow.test?MIME-Type="message/rfc822"#123>
would map to
Content-Type: message/rfc822; URI-Body="mailto:joe@blow.text";
URI-Fragment="123"
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4. Troublesome Characters
Troublesome characters are defined as those not permitted in a token
in [RFC 2045] with the addition of percent sign and the deletion of
double quote. That is, any character code from 0 through 32
inclusive or character code 127 or any of "(", ")", "<", ">", "@",
",", ";", ":", "\", "/", "[", "]", "?", "%", or "=" are troublesome
characters.
5. IANA Considerations and Potential Conflicts
This document allocates and specifies the following:
(1) The "ContentType" URI scheme.
(2) The "uri." MIME subtype tree. Since this subtree is totally
delegated to the URI specification, there are no independent
publication or review requirements for it. Any valid URI can be
used after the "uri." in any MIME top level type, after
troublesome characters (see section 4) in the URI are % escaped.
(3) In the context of automatic URI to Content-Type type conversion,
a meaning is specified for the "MIME-Type" URI query section
parameter.
(4) In the context of automatic Content-Type to URI conversion, a
meaning is specified for the "URI-Body" and "URI-Fragment"
Content-Type parameters.
Because this document specifies the "ContentType" URI scheme and the
"uri." MIME subtype tree, no conflict can arise due to other uses of
them.
However, there has been no precedent for the specification of
Content-Type parameters valid across all MIME types, such as URI-Body
and URI-Fragment, and in fact [RFC 2046] denies their possibility.
Nor has there been any precedent for the specification of a universal
URI query parameter such as MIME-Type. The probability that any
different use is currently being made or will in the future have to
be made of these names is low enough that it can be ignored. It is
possible that some processing systems are sensitive to the presence
of parameters they do not understand and will indicate errors when
presented with controlled mapping URIs or Content-Types. However,
Content-Type parameters and URI query parameters are usually handled
on receipt by such mechanisms as storing the name-value pair in an
associative array or as "environment variables" and ignoring extra
parameters. In fact, Content-Type processors are required by [RFC
2046] to ignore any parameters they do not understand and to ignore
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parameter order.
6. Security Considerations
In some sense, the security considerations for MIME and content types
[RFC 2046], URIs [RFC 2396], and for every individual MIME type and
URI scheme can apply.
In addition, the deployment of mapping aware software may enable the
introduction into or transmission through MIME or Content-Type
contexts of URI semantics, including possibly dangerous action
schemes such as "mailto", and the introduction into or transmission
through URI contexts of MIME and content type semantics, including
possibly dangerous executable data types or the like.
Finally, implementation of controlled mapping may enable a malicious
user, by adding one of the special parameters specified herein, to
cause a surprising change in the semantics of a URI or Content-Type
produced by the mapping from an apparently innocuous Content-Type or
URI.
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Appendix
The following Perl code implements most of the mapping given in
Section 2 above:
(will be in the next revision)
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References
[HTML] - Dave Raggett, Arnaud Le Hors, Ian Jacobs, "HTML 4.01
Specification", <http://www.w3.org/TR/html4>, December 1999.
[RFC 822] - D. Crocker, "Standard for the format of ARPA Internet
text messages", Aug-13-1982.
[RFC 1738] - T. Berners-Lee, L. Masinter, M.McCahill, "Uniform
Resource Locators (URL)", December 1994.
[RFC 2045] - N. Freed & N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part One: Format of Internet Message Bodies",
November 1996.
[RFC 2046] - N. Freed & N. Borenstein, "Multipurpose Internet Mail
Extensions (MIME) Part Two: Media Types", November 1996.
[RFC 2048] - N. Freed, J. Klensin & J. Postel, "Multipurpose Internet
Mail Extensions (MIME) Part Four: Registration Procedures", November
1996.
[RFC 2119] - S. Bradner, "Key words for use in RFCs to Indicate
Requirement Levels", March 1997.
[RFC 2396] - T. Berners-Lee, R. Fielding, L. Masinter, "Uniform
Resource Identifiers (URI): Generic Syntax", August 1998.
[RFC 2717] - R. Petke, I. King, "Registration Procedures for URL
Scheme Names", November 1999.
[RFC 2718] - L. Masinter, H. Alvestrand, D. Zigmond, R. Petke,
"Guidelines for new URL Schemes", November 1999.
[XML NAME] - Tim Bray, Dave Hollander, Andrew Layman, "Namespaces in
XML", <http://www.w3.org/TR/REC-xml-names>, 14 January 1999.
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Author's Address
Donald E. Eastlake 3rd
Motorola
155 Beaver Street
Milford, MA 01757 USA
Telephone: +1 508-261-5434 (w)
+1 508-634-2066 (h)
FAX: +1 508-261-4447 (w)
EMail: Donald.Eastlake@motorola.com
Expiration and File Name
This draft expires August 2001.
Its file name is draft-eastlake-cturi-02.txt.
D. Eastlake 3rd [Page 14]