[Search] [txt|pdf|bibtex] [Tracker] [Email] [Diff1] [Diff2] [Nits]

Versions: 00 01 02                                                      
Internet Draft          Application/MIME10/22/96 (Expiration: 4/97)

Network Working Group                             D. Crocker
Internet-Draft:  DRAFT-CROCKER-WRAP-01.TXT
                                      Brandenburg Consulting
Expiration <4/97>                         Laurence Lundblade
                                              Qualcomm, Inc.
                                              Jamie Zawinski
                                Netscape Communcations, Inc.
                                            October 22, 1996



          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, so that gateways are often
required to translate portions of a MIME object into its
constituent pieces, that is, as independent attachments.
Further some uses of MIME's 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/EDIFACT SPECIFICATION

MIME type name:               Application

MIME subtype name:            MIME

Required parameters:          Content-type,  <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.

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 "hiding" 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.,

          type := "Content-Type=" <"> type "/" subtype <">

     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
     object's 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 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.

     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 software-including
gateways and regular email user agents-should 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.


SAMPLE 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//--


SECURITY CONSIDERATIONS

MIME content often includes sensitive data, so that
transmission often needs to attend to authentication, data
integrity, privacy and access control, and the like as
appropriate.  In particular, the recursive processing
required by the mechanism defined here MUST be sure to
include 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 Communciations, Inc.
501 East Middlefield Road
Mountain View, CA 94043