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Internet Draft Application/MIME (Expiration: 1/98)
Network Working Group D. Crocker
Internet-Draft: Brandenburg Consulting
DRAFT-CROCKER-WRAP-02.TXT Laurence Lundblade
Expiration <1/98> Qualcomm, Inc.
Jamie Zawinski
Netscape Communications, Inc.
Wrapping MIME Objects: Application/MIME
STATUS OF THIS MEMO
This document is an Internet-Draft. 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.''
To learn the current status of any Internet-Draft, please
check the ``1id-abstracts.txt'' listing contained in the
Internet- Drafts Shadow Directories on ftp.is.co.za (Africa),
nic.nordu.net (Europe), munnari.oz.au (Pacific Rim),
ds.internic.net (US East Coast), or ftp.isi.edu (US West
Coast).
INTRODUCTION
MIME permits labeling and aggregating objects into complex
hierarchies. Support for MIME mechanisms is not universal
although it is growing quickly. Consequently gateways often
are required to translate portions of a MIME object into its
constituent pieces, as independent attachments. Further some
uses of MIMEÕs aggregation mechanism, Content-type Multipart,
are leading 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 attachment, by software that is otherwise
unfamiliar with the types of objects contained in the
encapsulation. It is expected that Application/MIME will be
especially useful in getting data past gateways.
The specification simply defines an object which 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: Gateways and some other
processing environments can
alter and destroy MIME data
structure; the defined data
type will permit "hidingÓ the
structure inside an object that
is much less likely to be
modified by such software. 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 *(";" *parameter)
c-t = "Content-Type=" <"> 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
objectÕs parameters, in their entirety. This is done to
facilitate dispatch of a particular type to a handler,
without parsing the contained MIME structure.
IMPORTANT NOTE: 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 which 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 implementors are specifically
and strongly cautioned against
modification of an application/mime
component.
The question of whether to unwrap content which is embedded
in an Application/MIME becomes very simple. Application/MIME
is used to provide protection against mishandling by
intermediaries. Hence, only end-system softwareÐincluding
gateways and regular email user agentsÐshould 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.
CONTACT
David H. Crocker <dcrocker@imc.org>
Internet Mail Consortium
675 Spruce Dr.
Sunnyvale, CA 94086 USA
Phone: +1 408 246 8253
Fax: +1 408 249 6205
Laurence Lundblade <lgl@qualcomm.com>
Qualcomm, Inc.
6455 Lusk Blvd
San Diego Ca 92121 USA
Phone: 619-658-3584
Jamie Zawinski <jwz@netscape.com>
Netscape Communications, Inc.
501 East Middlefield Road
Mountain View, CA 94043
