rfc1495.txt
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Network Working Group H. Alvestrand
Request for Comments: 1495 SINTEF DELAB
Updates: 1327 S. Kille
ISODE Consortium
R. Miles
Soft*Switch, Inc.
M. Rose
Dover Beach Consulting, Inc.
S. Thompson
Soft*Switch, Inc.
August 1993
Mapping between X.400 and RFC-822 Message Bodies
Status of this Memo
This RFC specifies an IAB standards track protocol for the Internet
community, and requests discussion and suggestions for improvements.
Please refer to the current edition of the "IAB Official Protocol
Standards" for the standardization state and status of this protocol.
Distribution of this memo is unlimited.
Table of Contents
1. Introduction ............................................. 1
2. Approach ................................................. 2
3. Mapping between X.400 and RFC-822 Message Bodies ......... 3
3.1 Mapping from X.400 to RFC-822 ........................... 4
3.2 Mapping from RFC-822 to X.400 ........................... 5
3.2.1 Asymmetric Mappings .................................... 6
3.2.1.1 Message/External-Body ................................ 6
3.2.1.2 Message/Partial ...................................... 6
3.2.1.3 Nested Multipart Content-types ....................... 6
3.2.2 Multipart IPMS Heading Extension ....................... 7
4. Mapping between X.400 and RFC-822 Message Headers ........ 7
5. OID Assignments .......................................... 9
6. Security Considerations .................................. 9
7. Authors' Addresses ....................................... 10
8. References ............................................... 11
1. Introduction
The Internet community is a large collection of networks under
autonomous administration, but sharing a core set of protocols.
These are known as the Internet suite of protocols (or simply
"TCP/IP").
Use of electronic-mail in the Internet is defined primarily by one
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RFC 1495 MHS/RFC-822 Message Body Mapping August 1993
document, STD-11, RFC-822 [1], which defines the standard format for
the exchange of messages. RFC-822 has proven immensely popular; in
fact, the 822-connected Internet, is larger than the scope of the
IP-connected Internet.
The framework provided by RFC-822 allows for memo-based textual
messages. Each message consists of two parts: the headers and the
body. The headers are analogous to the structured fields found in an
inter-office memo, whilst the body is free-form. Both parts are
encoded using ASCII.
Recently, the Internet Engineering Task Force (IETF) has developed an
document called,
Multipurpose Internet Mail Extensions
or MIME RFC-1341. The title is actually misleading. MIME defines
structure for Internet message bodies. It is not an extension to
RFC-822.
Independently of this, the International standards community
developed a different framework in 1984 (some say that's the
problem). This framework is known as the OSI Message Handling System
(MHS) or sometimes X.400.
Since the introduction of X.400(84), there has been work ongoing for
defining mappings between MHS and RFC-822. The most recent work in
this area is RFC-1327 [3], which focuses primarily on translation of
envelope and headers. This document is complimentary to RFC-1327 as
it focuses on translation of the message body. The mappings defined
are largely symmetrical with respect to MIME and MHS structuring
semantics, although the MIME semantics are somewhat richer. In order
to provide for reversible transformations, MHS heading extensions are
used to carry the additional MIME semantics.
Please send comments to the MIME-MHS mailing list:
<mime-mhs@surfnet.nl>.
2. Approach
The mappings have been specifically designed to provide optimal
behavior for three different scenarios:
(1) Allow a MIME user and an MHS user to exchange an arbitrary binary
content;
(2) Allow MIME content-types to "tunnel" through an MHS relay that
is, two MIME users can exchange content-types without loss
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RFC 1495 MHS/RFC-822 Message Body Mapping August 1993
through an MHS relay); and,
(3) Allow MHS body parts to "tunnel" through a MIME relay that is,
two MHS users can exchange body parts without loss through a MIME
relay).
Other, related, scenarios can also be easily accommodated.
To facilitate the mapping process, the Internet Assigned Numbers
Authority (IANA) maintains a table termed the "IANA MHS/MIME
Equivalence Table". Once an enterprise has registered an OID to
describe an MHS body part, it should complete a corresponding
registry with the IANA for a MIME content-type/subtype. In practice,
the corresponding content-type will be "application", with an
appropriate choice of sub-type and possible parameters. If a new
MIME content-type/subtype is registered with the IANA without a
corresponding entry in the Equivalence Table, the IANA will assign it
an OID, from the arc defined in this memo. See [4], section 5 for
details.
The companion document, "Equivalences between 1988 X.400 and RFC-822
Message Bodies"[4], defines the initial configuration of this table.
The mappings described in both this document and the companion
document use the notational conventions of RFC-1327.
3. Mapping between X.400 and RFC-822 Message Bodies
MHS messages are comprised of an IPMS.heading and an IPMS.body. The
IPMS.Body is a sequence of IPMS.BodyParts. An IPMS.BodyPart may be a
nested message (IPMS.MessageBodyPart).
A MIME message consists of headers and a content. For the purpose of
discussion, the content may be structured (multipart or message), or
atomic (otherwise). An element of a structured content may be a
message or a content. Both message and structured content have
subtypes which do not have direct analogies in MHS.
The mapping between X.400 and RFC-822 message bodies which this
document defines is symmetrical for the following cases:
(1) any atomic body part
(2) multipart: digest and mixed subtypes
(3) message/rfc822
RFC-1327 specifies the mappings for headers. Section 4 describes how
those mappings are modified by this document. When mapping between
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an MHS body and a MIME content, the following algorithm is used:
3.1. Mapping from X.400 to RFC-822
This section replaces the text in RFC-1327 starting at the bottom of
page 84,
The IPMS.Body is mapped into the RFC-822 message body. Each
IPMS.BodyPart is converted to ASCII as follows:
and continuing up to and including page 86 of Section 5.3.4 of RFC-
1327.
If the IPMS.Body
Body ::=
SEQUENCE OF
BodyPart
consists of a single body part, then the RFC-822 message body is
constructed as the MIME content corresponding to that body part.
If the body part is an IPMS.MessageBodyPart (forwarded IPM), the
mapping is applied recursively. Otherwise, to map a specific MHS
body part to a MIME content-type, the IANA MHS/MIME Equivalence table
is consulted. If the MHS body part is not identified in this table,
then the body-part is mapped onto an "application/x400-bp" content,
as specified in [4].
If the IPMS.Body consists of more than one body part, then the RFC-
822 message body is constructed as a
multipart/mixed
content-type, unless all of the body parts are messages, in which
case it is mapped to a
multipart/digest
content-type. Each component of the multipart content-type
corresponds to a IPMS.BodyPart, preserving the ordering of the body
parts in the IPMS.Body.
There is one case which gets special treatement. If the IPMS.Body
consists solely of a single IA5Text body part, then the RFC822
message body is NOT marked as a MIME content. This prevents RFC822
mailers from invoking MIME function unnecessarily.
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3.2. Mapping from RFC-822 to X.400
First, replace the first paragraph of Section 5.1.3 on page 72 of
RFC-1327 to read as:
The IPM (IPMS Service Request) is generated according to the
rules of this section. The IPMS.body usually consists of one
IPMS.BodyPart of type
IPMS.IA5TextBodyPart
with
IPMS.IA5TextBodyPart.parameters.repertoire
set to the default (ia5), which contains the body of the RFC-822
message. However, if the 822.MIME-Version header field is
present, a special algorithm is used to generate the IPMS.body.
Second, replace the "Comments:" paragraph on page 74 to reads as:
Comments:
If an 822.MIME-Version header field is not present,
generate an IPMS.Bodypart of type
IPMS.IA5TextBodyPart
with
IPMS.IA5TextBodyPart.parameters.repertoire
set to the default (ia5), containing the value of
the fields, preceded by the string "Comments: ".
This body part shall preceed the other one.
Third, add the remainder of this section to the end of Section 5.1.3
of RFC-1327.
If the 822.MIME-Version header field is present, the following
mapping rules are used to generate the IPMS.body.
If the MIME content-type is one of:
(1) any atomic body part
(2) multipart: digest and mixed subtypes
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(3) message/rfc822
then the symmetric mapping applies as described in Section 6.1. Note
that the multipart content-types should be marked with the
IPMS.HeadingExtension described below.
Otherwise, three cases remain, which are discussed in turn.
3.2.1. Asymmetric Mappings
3.2.1.1. Message/External-Body
This is mapped into a mime-body-part, as specified in [4].
3.2.1.2. Message/Partial
This is mapped onto a message, and the following heading extension is
used. The extension is derived from the message/partial parameters:
partial-message HEADING-EXTENSION
VALUE PartialMessage
::= id-hex-partial-message
PartialMessage ::=
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