One document matched: draft-crocker-wrap-02.txt
Differences from draft-crocker-wrap-01.txt
Network Working Group D. Crocker
draft-crocker-wrap-02.txt Brandenburg InternetWorking
June 23, 2002
Expiration <12/02>
Wrapping MIME Objects: Application/MIME
STATUS OF THIS MEMO
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.
Internet-Drafts are working documents of the Internet
Engineering Task Force (IETF), its areas and its working
groups. Note that other groups may also distribute working
documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of
six months and may be updated, replaced, or obsoleted by
other documents at any time. It is inappropriate to use
Internet-Drafts as reference material or to cite them other
than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt
The list of Internet-Draft Shadow Directories can be accessed
at http://www.ietf.org/shadow.html.
ABSTRACT
MIME is a general-purpose mechanism for labeling and
aggregating objects into complex hierarchies. Some MIME
objects are discrete objects. If placed within another MIME
hierarchy, they need to appear as a leaf node in the
containing hierarchy, rather than seeming to be continuations
of that hierarchy. This specification defines Content-type
Application/MIME to provide an encapsulation mechanism for
arbitrary MIME structures. This facilitates their treatment
as a single unit, rather than causing them to be treated as
part of any containing MIME hierarchy.
INTRODUCTION
MIME is a general-purpose mechanism for labeling and
aggregating objects into complex hierarchies. Some MIME
objects are discrete objects. If placed within another MIME
hierarchy, they need to appear as a leaf node in the
containing hierarchy, rather than seeming to be continuations
of that hierarchy. One example of this requirement is to
provide some protection against the translations that email
gateways often make to MIME attachments. Further some uses
of MIMEÆs aggregation mechanism, Content-type Multipart, can
lead to requirements for processing an entire aggregated
object as a single unit. This is in contrast to Content-type
Multipart/Mixed, which specifies separate processing of
constituent components, and to which multipart sub-types
default if they are unknown to the processing software.
This specification defines Content-type Application/MIME to
provide an encapsulation mechanism for arbitrary MIME
structures. This facilitates their treatment as a single
unit, rather than causing them to be treated as part of any
containing MIME hierarchy. It is expected that
Application/MIME will be especially useful in getting data
past gateways.
The specification simply defines an object that contains MIME
data. When the object is removed from its Content-type
Application/MIME wrapper, what remains is an entire MIME
object, beginning with a (new) Content-type header.
APPLICATION/MIME SPECIFICATION
MIME type name: Application
MIME subtype name: MIME
Required parameters: <contained>
Optional parameters: None
Encoding considerations: May need BASE64 or QUOTED-
PRINTABLE transfer encoding, as
appropriate for the <contained>
data. Careful use of QUOTED-
PRINTABLE will maintain clear
text as robust against gateway
processing yet still be
readable without special
processing.
NOTE: items carried inside
this MIME object must be fully
conformant MIME structure and
data; this includes all of the
usual rules concerning network
standard canonical
representation.
Security considerations: See separate section in this
document.
Published specification: See detail, below.
Rationale: MIME objects can be discrete
entities; when attached to
another MIME object, the
attachment needs to be
represented as a leaf, rather
than a part of that hierarchy.
One use of this distinction is
for gateways and other
processing environments that
can alter and destroy MIME data
structure; the defined data
type will permit separate
handling of the structure
inside an object. It also
permits passing an aggregate
object as a single entity,
through processors that would
otherwise separate the
components.
Contact-info: See Contact section, below.
Detail specific to MIME-based usage:
This is a generic mechanism for encapsulating any MIME
object structure. The object is self-defining, since it
begins with a MIME Content-Type header and is then
processed (recursively). The mechanism is intended for
use when correct processing of the basic MIME structure
is at risk, in effect allowing the MIME structure to be
tunneled through such an environment.
The Content-Type parameter comprises the type and sub-
type tokens of the Content-Type header for the contained
MIME component, i.e.,
contained = c-t *(ô;ö *parameter)
c-t = ôContent-Type=ô <"> type "/" subtype <">
parameter = {as defined for MIME}
where the type and subtype tokens are defined by the
MIME [2] specification. The value of the Content-Type
parameter specifies the MIME Content-Type and subtype of
the data structure contained inside the Application/MIME
object.
The <contained> parameters replicate all of the
parameters which are present in the Content-Type
specification header referenced by the Content-Type
parameter. That is, they replicate the contained
objectÆs parameters, in their entirety. This is done to
facilitate dispatch of a particular type to a handler,
without parsing the contained MIME structure.
If the contained object is, itself
Application/MIME, then none of the <contained>
parameters of that contained object shall be
included.
(Using Application/MIME for doubly-wrapping MIME data
may provide a necessary level of protection in some
cases.)
GATEWAY AND PASS-THROUGH PROCESSING
Software which manipulates an Application/MIME component must
do so ONLY when processing can be done fully and correctly.
In the extreme, this may require full parsing of the
contained MIME structure and its parameters, prior to
deciding whether to take responsibility for the content.
Typically, however, review of the Application/MIME type and
<contained> parameters will suffice.
Any MIME-aware software that encounters an Application/MIME
component must leave the component in its existing form,
unless that software is able to fully and correctly process
ALL of the component contents AND such processing is
appropriate to the environment in which the software is
operating.
IMPORTANT NOTE: Gateway implementers are specifically
and strongly cautioned against
modification of an Application/MIME
component.
The question of whether to unwrap content that is embedded in
an Application/MIME becomes very simple. Application/MIME is
used to provide protection against mishandling by
intermediaries. Hence, only end-system softwareûincluding
gateways and regular email user agentsûshould even consider
touching the content and then it should only do so when the
recipient has a basis for believing that the processing will
be correct.
SIMPLE USAGE IN MIME-BASED EMAIL
This section is intended as a simple example of the gist of
the formatting required to encapsulate MIME objects within
Internet mail, using Application/MIME:
To:
Subject:
From:
Date:
Mime-Version: 1.0
Content-Type: Application/MIME;
content-type=Multipart/signed;
protocol="application/somesigscheme";
boundary="//signatureboundary//"
Content-Type: Multipart/Signed;
boundary=//signatureboundary//;
protocol="application/somesigscheme"
--//signatureboundary//
Content-type:<<type of the user data>>
<<user data>>
--//signatureboundary//
Content-type:application/somesigscheme
<<signature control information>>
--//signatureboundary//--
DOUBLE-WRAPPED EXAMPLE
This section shows a contained object which is, itself, a
contained object, double-wrapped for extra protection against
decay:
To:
Subject:
From:
Date:
Mime-Version: 1.0
Content-Type: Application/MIME;
content-type=Multipart/signed;
protocol="application/somesigscheme";
boundary="//signatureboundary//"
Content-Type: Application/MIME;
content-type=Multipart/signed;
boundary="//signatureboundary//";
protocol="application/somesigscheme"
Content-Type: Multipart/Signed;
boundary=//signatureboundary//;
protocol="application/somesigscheme"
--//signatureboundary//
Content-type:<<type of the user data>>
<<user data>>
--//signatureboundary//
Content-type:application/somesigscheme
<<signature control information>>
--//signatureboundary//--
SECURITY CONSIDERATIONS
MIME content often includes sensitive data, so that
transmission often needs to attend to authentication, data
integrity, privacy, access control, and the like as
appropriate.
IMPORTANT NOTE: The recursive processing required by
Application/MIME requires use of
whatever security checks are applied to
newly received MIME data.
This specification does NOT, itself, provide any security-
related mechanisms. As needed and appropriate, such
mechanisms MUST be added, either via Internet MIME-based
security services or any other services which are appropriate
to the user requirements.
ACKNOWLEDGMENTS
The idea for Application/MIME first developed out of
conversations with Einar Stefferud and Marshall Rose, in
trying to find a way for exchanging valid Internet Mail,
complete with RFC822 headers and MIME content, through
environments that provided no other Internet Mail technology
besides a gateway between the proprietary environment and the
Internet. Additional benefits of this mechanism then
surfaced during discussions on the S/MIME development list.
A previous draft was co-authored by Laurence Lundblade and
Jamie Zewinskie. They are not listed in the current draft
merely to protect the innocent.
CONTACT
David H. Crocker <dcrocker@brandenburg.com>
Brandenburg InternetWorking
675 Spruce Drive
Sunnyvale, CA 94086 USA
Phone: +1.408.246.8253
| PAFTECH AB 2003-2026 | 2026-04-22 23:03:31 |