draft X.400/MIME body mapping May 96
Mapping between X.400 and RFC-822/MIME Message Bodies
Thu Aug 15 14:51:04 MET DST 1996
Harald Tveit Alvestrand
UNINETT
Harald.T.Alvestrand@uninett.no
Status of this Memo
The name of this draft is draft-ietf-mixer-bodymap-06.txt.
The following text is required for all drafts:
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. Internet Drafts may be
updated, replaced, or obsoleted by other documents at
any time. It is not appropriate to use Internet
Drafts as reference material or to cite them other
than as a "working draft" or "work in progress."
Please check the I-D abstract listing contained in
each Internet Draft directory to learn the current
status of this or any other Internet Draft.
This document describes the translation of message body
parts between X.400 and MIME.
Please send comments to the MIXER mailing list:
<ietf-mixer@innosoft.com>
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1. Introduction
This document is a companion to [MIXER], which defines the
principles and translation of headers for interworking
between MIME-based RFC-822 mail and X.400 mail.
This document defines how to map body parts of X.400
messages into MIME entities and vice versa, including the
handling of multipart messages and forwarded messages.
A table of contents that should be quite useful for
locating specific sections is given in the back of the
document.
1.1. Glossary
The following terms are defined in this document:
Body part
Part of a message that has a unique type. This term
comes from X.400; the corresponding term in MIME (RFC
1521) is limited to use in parts of a multipart
message; the term "body" may correspond better.
Content-type
Type information indicating what the content of a
body part actually is. This term comes from MIME; the
corresponding X.400 term is "body part type".
Mapping
(noun): A description of how to transform an X.400
body part into a MIME body part, or how to transform
a MIME body part into an X.400 body part.
Equivalence
A set of two mappings that taken together provide a
lossless conversion between an X.400 body part and a
MIME body part
Encapsulation
The process of wrapping something from one of the
mail systems in such a way that it can be carried
inside the other mail system. When encapsulating, it
is not expected that the other mail system can make
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reasonable sense of the body part, but a gateway back
into the first system will always be able to convert
the body part without loss back to its original
format.
HARPOON encapsulation
The encapsulating of a MIME body part by putting it
inside an IA5 body with all headers and encoding
intact. First described in RFC 1496 (HARPOON).
Tunneling
What happens when one gateway encapsulates a message
and sends it to another gateway that decapsulates it.
The hope is that this will cause minimal damage to
the message in transit.
DISCUSSION
At many points in this document, the author has found
it useful to include material that explains part of
the reasoning behind the specification. These
sections all start with DISCUSSION: and continue to
the next numbered section heading; they do not
dictate any additional requirements on a gateway.
2. Basic rules for body part conversion
The basic approach for translating body parts is described
in section 2.1 and 2.2.
Chapter 3 gives details on "encapsulation", which allows
you to be certain that no information is lost even when
unknown types are encountered.
Chapter 6 gives the core mappings for various body parts.
The conformance requirements in chapter 8 describe what
the minimum conformance for a MIXER gateway is with
respect to body part conversion.
DISCUSSION:
At the moment (Sept 1995) both the MIME and the X.400
worlds are in a state of flux with regards to carrying
around stuff that is not text.
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In such a situation, there is little chance of defining a
mapping between them that is the best for all people, all
of the time.
For this reason, this specification allows a gateway
considerable latitude in deciding exactly what conversion
to apply.
The decision taken by the gateway may be based on various
information sources:
(1) If the gateway knows what body parts or content
types the recipient is able to handle, or has
registered a particular set of preferences for a
user, and knows how to convert the message
reasonably to those body parts, the gateway may
choose to convert body parts in the message to
those types only.
(2) If the gateway gets indications (via special
headers or heading-extensions defined for the
purpose) that the sender wanted a particular
representation on the "other side", and the gateway
is able to satisfy the request, it may do so. Such
a mechanism is defined in chapter 4 of this
document.
(3) If the gateway gets a message that might be
appropriate to send as one out of several types,
but where the typing information does not tell you
which one to use (like an X.400 BP14, FTAM "just a
file", or MIME application/octet-stream), it may
apply heuristics like looking at content or looking
at filenames to figure out how to deal with the
message.
(4) If the gateway knows that the next hop for the
message has limited capabilities (like X.400/84),
it may choose to perform conversions appropriate
for that medium.
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(5) Where no mapping is known by the gateway, it
may choose to drop the body part, reject the
message, or encapsulate the body part as de
scribed in chapter 3. The choice may be con
figurable, but a conformant MIXER gateway MUST
be able to be configured for encapsulation.
In many cases, a message that goes SMTP->X.400->SMTP will
arrive without loss of information.
In some cases, the reverse translation may not be
possible, or two gateways may choose to apply different
translations, based on the criteria above, leading to an
apparently inconsistent service.
In addition, service will vary because some gateways will
have implemented conversions not implemented by other
gateways.
This is believed to be unavoidable.
2.1. Generating the IPM Body from MIME
When converting the body of a message from MIME to X.400,
the following steps are taken:
If the header does not contain a 822.MIME-Version field,
then generate an IPMS.Body with a single IPMS.BodyPart of
type IPMS.IA5TextBodyPart containing the body of the RFC
822 message with
IPMS.IA5TextBodyPart.parameters.repertoire set to the
default (IA5).
If 822.MIME-Version is present, the body is analyzed as a
MIME message and the body is converted according to the
mappings configured and implemented in the gateway.
2.2. Generating the MIME Body from the IPMS.Body
When converting the body of a message from X.400 to MIME,
the following steps are taken:
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If there is more than one body part, and the first body
part is IA5 and starts with the string "RFC-822-Headers:"
as the first line, then the remainder of this body part
shall be appended to the RFC 822 header. This relies upon
the theory that this body part has been generated
according to Appendix B of MIXER. A gateway shall check
the consistency and syntax of this body part, to ensure
that the resulting message is conformant with RFC 822.
If the remaining IPMS.Body consists of a single
IPMS.Bodypart, there are three possibilities.
(1) If it is of type IPMS.IA5Text, and the first line
is "MIME-Version: 1.0", it is assumed to be a
HARPOON-encapsulated body part. The complete body
content is then appended to the headers; the
separating blank line is inside the message. If an
RFC 822 syntax error is discovered inside the
message, it may be mapped directly as described
below instead.
(2) If it is of type IPMS.IA5Text, then this is mapped
directly and no MIME encoding is used.
(3) All other types are mapped according to the
mappings configured and implemented in the gateway.
If the IPMS.Body contains multiple IPMS.Bodypart fields,
then a MIME message of content type multipart is
generated. If all of the body parts are messages, then
this is multipart/digest. Otherwise it is
multipart/mixed. The components of the multipart are
generated in the same order as in the IPMS.Body.
Each component is mapped according to the mappings
configured and implemented in the gateway; any IA5 body
parts are checked to see if they are HARPOON mappings.
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2.3. Mapping the EMA FTBP parameters
DISCUSSION:
EMA has defined a profile for use of the File Transfer
Body Part (FTBP). [MAWG]
New mappings are expected to use this as the mechanism for
carrying body parts, and since it is important to have a
consistent mapping for the special FTBP parameters, these
are defined here.
The mapping of the body will depend on the content-type in
MIME and on the application-reference in FTBP, and is not
specified here.
However, in many cases, we expect that the translation
will involve simply copying the octets from one format to
the other; that is, "no conversion".
2.3.1. Mapping GraphicStrings
Some parameters of the EMA Profile are encoded as ASN.1
GraphicStrings, which are troublesome because they can
contain any ISO registered graphic character set.
To map these to ASCII for use in mail headers, the gateway
may either:
(1) Use the RFC 1522 encoding mechanism to create
appropriate encoded-words for the headers involved.
Note that in some cases, such as within Content-
Disposition filenames, the encoded-words must be in
quotes, which is not the normal usage of encoded-
words.
(2) Apply the normalization procedure given in Appendix
A to identify the ASCII characters of the string,
and replace all non-ASCII characters with the
question mark (?).
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Both procedures are valid for MIXER gateways; the
simplified procedure of ignoring escape sequences and bit-
stripping the result is NOT valid.
2.3.2. Mapping specific parameters
The following parameters are mapped in both directions:
Content-ID
The mapping of this element is complex.
The Content-ID is encoded as an IPM.MessageIdentifier
and entered into the
FTBP.FileTransferParameters.related-stored-file.
file-identifier.cross-reference.message-reference.
FTBP.FileTransferParameters.related-stored-file.
relationship.descriptive-relationship is set to the
string "Internet MIME Body Part".
FTBP.FileTransferParameters.related-stored-file.
file-identifier.cross-reference.application-
crossreference is set to a null OCTET STRING.
The reverse mapping is only performed if the
FTBP.FileTransferParameters.related-stored-file.
relationship.descriptive-relationship has the string
value "Internet MIME Body Part".
Content-Description
The value of this field is mapped to and from the
first string in
FTBP.FileTransferParameters.environment.user-visible-
string.
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Content-Disposition
This field is defined in [CDISP]. It has multiple
components; the handling of each component is
given below.
The "disposition" component is ignored on MIME ->
X.400 mapping, and is always "attachment" on X.400 ->
MIME mapping.
NOTE: RFC 1806 gives only the "filename" component.
The other components are expected to be added to the
next version of RFC 1806.
C-D: filename
The filename component of the C-D header is mapped to
and from FileTransferParameters.file-
attributes.pathname.
The EBNF.disposition-type is ignored when creating
the FTBP pathname, and always set to "attachment"
when creating the Content-Disposition header. For
example:
Content-Disposition: attachment; filename=dodo.doc
or
Content-Disposition: attachment; filename=/etc/passwd
The filename will be carried as a single incomplete-
pathname string. No special significance is assumed
for the characters "/" and "\". Note that normal
security precautions MUST be taken in using a
filename on a local file system; this should be
obvious from the second example.
This is done to be conformant with the EMA Profile.
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C-D: Creation-date
Mapped to and from FileTransferParameters.file-
attributes.date-and-time-of-creation
For this and all other date fields, the RFC-822 date
format is used (822.date-time). Note that the
parameter syntax of RFC 1806 requires that all dates
be quoted!
C-D: Modification-date
Mapped to and from FileTransferParameters.file-
attributes.date-and-time-of-last-modification
C-D: Read-date
Mapped to and from FileTransferParameters.file-
attributes.date-and-time-of-last-read-access
C-D: Size
Mapped to and from FileTransferParameters.file-
attributes.object-size. If the value is "no-value-
available", the component is NOT generated.
Other RFC-822 headers
Mapped to extension in
FTBP.FileTransferParameters.extensions using the
rfc-822-field HEADING-EXTENSION from [MIXER].
NOTE:
The set of headers that are mapped will depend on the
placement of the body part (single body part or
multipart).
When it is the only body of a message, headers
starting with "content-" SHOULD be put into the FTAM
extension, and all other headers should be put into
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the IPMS extension for the message.
When it is a single bodypart of a multipart, ALL
headers on the body part are included, since there is
nowhere else to put them. Note that only headers that
start with "content-" have defined semantics in this
case.
EMA NOTE
The EMA profile, version 1.5, specifies that handling
of extensions is Optional for reception. This means
that some non-MIXER gateways may not implement
handling of this field, and some UAs may not have the
possibility of showing the content of this field to
the user.
An alternative approach using
FTBP.FileTransferParameters.environment.user-visible-
string was suggested to EMA, and the EMA MAWG
recommended in its April 1996 conference that the
IETF MIXER group should rather choose this approach.
2.3.3. Summary of FTBP elements generated
This is a summary of the preceding section, and does not
add new information.
The following elements of the FTBP parameters are mapped
or used (the rightmost column gives their status in the
EMA profile; M=Mandatory, O=Optional, R=Recommended for
Origination/Receipt):
FileTransferParameters M/M
Related-Stored-File O/O
file-identifier
cross-reference
application-crossreference NULL
message-reference Content-ID
descriptive-relationship Used as marker
contents-type Must be unstructured-binary M/M
environment M/M
application-reference Selects mapping M/M
user-visible-string Content-description R/M
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file-attributes
pathname C-D: Filename R/M
date-and-time-of-creation C-D: Creation-Date O/O
date-and-time-of-last-modification C-D: Modification-Date R/M
date-and-time-of-last-read-access C-D: Read-Date O/O
object-size C-D: Size R/M
extensions Other headers O/O
All other elements of the FTBP parameters are discarded.
NOTE: There is ongoing work on defining a more complete
mapping between FTBP headers and a set of RFC-822 headers.
A gateway MAY choose to support the larger set once it is
available, but MUST support this limited set.
2.4. Information that is lost when mapping
MIME defines fields which add information to MIME
contents. Two of these are "Content-ID", and "Content-
Description", which have special rules here, but MIME
allows new fields to be defined at any time.
The possibilities are limited about what one can do with
this information:
(1) When using encapsulation, the information can be
preserved
(2) When using mapping to FTBP, the information can be
preserved in the FileTransferParameters.extensions
defined for that purpose.
(3) When mapping to a single-body message, the
information can be preserved as P22 header
extensions
(4) When mapping to other body part types, the
information must be discarded.
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3. Encapsulation of body parts
Where no mapping is possible, the gateway may choose any
of the following alternatives:
- Discard the body part, leaving a "marker" saying what
happened
- Reject the message
- "Encapsulate" the body part, by wrapping it in a body
part defined for that purpose in the other mail
system
The choice to be made should be configurable in the
gateway, and may depend on both policy and knowledge of
the recipient's capabilities.
3.1. Encapsulation of MIME in X.400
Four body parts are defined here to encapsulate MIME body
parts in X.400.
The BP15 body part is backwards compatible with RFC 1494.
The FTBP body part is compatible with the EMA MAWG
document [MAWG], version 1.5, but has some extensions, in
particular the one for extra headers.
The imagined scenarios for each body part are:
FTBP For use when sending to recipients that can handle
generic FTBP, and for tunnelling MIME to a MIME UA
BP15 For use when tunnelling MIME to a MIME UA through an
X.400(88) network, or to UAs that have been written
to RFC 1494
IA5 For use when tunneling MIME to a MIME UA through an
X.400 network, where some of the links may involve
X.400(84).
BP14 For use when the recipient may be an X.400(84) UA
with BP14 handling capability, and the loss of
information in headers is not regarded as important.
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but the gateway is free to use any method it finds
appropriate in any situation.
FTBP is expected to be the most useful body part in
sending to X.400(92) systems, while the BP14 content
passing is primarily useful for sending to X.400(84)
systems.
3.1.1. FTBP encapsulating body part
This body part utilizes the fundamental assumption in MIME
that all message content can be legally and completely
represented by a single octet stream, the "canonical
format".
The FTBP encapsulating body part is defined by the
application-reference id-mime-ftbp-data; all headers are
mapped to the FTBP headers, including putting the
"Content-type:" header inside the FTBP ExtensionsField.
Translation from the MIME body part is done by:
- Undoing the content-transfer-encoding
- Setting the "FileTransferData.FTdata.value.octet-
aligned" to the resulting string of octets
- Putting the appropriate parameters into the headers.
Reversing the translation is done by:
- Extracting the headers
- Applying an appropriate content-transfer-encoding to
the body. If this is for some reason different from
the content-transfer-encoding: header retrieved from
the headers, the old one must be deleted.
This mapping is lossless, and therefore counts as "no
conversion".
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3.1.2. BP15 encapsulating body part
This section defines an extended body part, based on body
part 15, which may be used to hold any MIME content.
mime-body-part EXTENDED-BODY-PART-TYPE
PARAMETERS MimeParameters
IDENTIFIED BY id-mime-bp-parameters
DATA OCTET STRING
::= id-mime-bp-data
MimeParameters ::=
SEQUENCE {
content-type IA5String,
content-parameters SEQUENCE OF
SEQUENCE {
parameter IA5String,
parameter-value IA5String
}
other-header-fields RFC822FieldList
}
The OBJECT IDENTIFIERS id-mime-bp-parameter and id-mime-
bp-data are defined in Appendix B. A MIME content is
mapped onto this body part. The MIME headers of the body
part are mapped as follows:
RFC822FieldList is defined in Appendix L of [MIXER].
Content-Type:
The "type/subtype" string is mapped to
MimeParameters.content-type.
For each "parameter=value" string create a
MimeParameters.content-parameters element. The
MimeParameters.content-Parameters.parameter field is
set to the parameter and the MimeParameters.content-
parameters.parameter-value field is set to the value.
Quoting is preserved in the parameter-value.
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Other
Take all other headers and create
MimeParameters.other-header-fields.
The MIME-version, content-type and content-transfer-
encoding fields are NOT copied.
NOTE:
The set of headers that are mapped will depend on the
placement of the body part (single body part or
multipart).
When it is the only body of a message, headers
starting with "content-" SHOULD be put into the
other-header-fields, and all other headers should be
put into the IPMS extension for the message.
When it is a single bodypart of a multipart, ALL
headers on the body part are included, since there is
nowhere else to put them. Note that only headers that
start with "content-" have defined semantics in this
case.
The body is mapped as follows:
Convert the MIME body part into its canonical form, as
specified in Appendix H of MIME [MIME]. This canonical
form is used to generate the mime-body-part.data octet
string.
The Parameter mapping may be used independently of the
body part mapping (e.g., in order to use a different
encoding for a mapped MIME body part).
This body part contains all of the MIME information, and
so can be mapped back to MIME without loss of information.
The OID id-mime-bp-data is added to the Encoded
Information Types of the envelope.
This body part is completely compatible with RFC 1494.
When converting back to a MIME body part, the gateway is
responsible for:
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(1) Selecting an appropriate content-transfer-encoding,
and deleting any content-transfer-encoding header
from the other-header-fields
(2) Adding quotes to any parameters that need them (but
not adding quotes to parameters that are already
quoted)
(3) Removing any content-type field that is left in the
RFC822FieldList of the message that is redundant or
conflicting with the one from the mime-body-part
(4) Make sure that on multipart messages, the boundary
string actually used is reflected in the boundary=
parameter of the content-type header, and does not
occur within the body of the message.
3.1.3. Encapsulation using IA5 (HARPOON)
This approach is the one taken in RFC 1496 - HARPOON - for
tunneling any MIME body part through X.400/84 networks. It
has proven rather unhelpful for bringing information to
X.400 users, but preserves all the information of a MIME
body part.
The following IA5Text body part is made:
- Content = IA5String
- First bytes of content: (the description is in US
ASCII, with C escape sequences used to represent
control characters):
MIME-version: <version>\r\n
Content-type: <the proper MIME content type>\r\n
Content-transfer-encoding: <7bit, quoted-printable or base64>\r\n
<Possibly other Content headings here, terminated by\r\n>
\r\n
<Here follows the bytes of the content, encoded
in the proper encoding>
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All implementations MUST place the MIME-version: header
first in the body part. Headers that are placed by [MIXER]
into other parts of the message MUST NOT be placed in the
MIME body part.
3.1.4. Content passing using BP14
This is described in this section because it is at the
same conceptual level as encapsulation. It is a lossy
transformation; it is impossible to reconstruct the MIME
type information from it.
Nevertheless, there is a demand for such functionality.
This "encapsulation" simply strips off all headers, undoes
the content-transfer-encoding, and creates a
BilaterallyDefined body part (BP14) from the resulting
octet stream.
No reverse translation is defined; when a BP14 arrives at
a MIXER gateway, it will be turned into an
application/octet-stream according to chap. 6.3
3.2. Encapsulating X.400 Body Parts in MIME
This section specifies a generic mechanism to map X.400
body parts to a MIME content. This allows for the body
part to be tunneled through MIME. It may also be used
directly by an appropriately configured MIME UA.
This content-type is defined to carry any X.400 extended
body part. The mapping of all standard X.400 body parts
is defined in RFC1494bis. The content-type field is
"application/x400-bp". The parameter is defined by the
EBNF:
mime-parameter = "bp-type=" object-identifier
The EBNF.object-identifier is set to the OBJECT IDENTIFIER
from IPMS.body.externally-defined.data.direct-reference.
For example, a Videotex body part will have
Alvestrand Exp Nov 96 [Page 18]
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Content-type=application/x400-bp; bp-type=2.6.1.4.5
The body contains the raw ASN.1 IPM body octet stream,
that is, the BER encoding of the IPM.Body.BodyPart,
including the initial tag octet. The content may use a
content-transfer-encoding of either base64 or quoted-
printable when carried in 7-bit MIME. It is recommended
to use the one which gives the more compact encoding of
the data. If this cannot be determined, Base64 is
recommended. No attempt is made to turn the parameters of
Extended Body Parts into MIME parameters, as this cannot
be done in a general manner.
Standard X.400 body parts may not be encoded directly by
this mechanism, but may be encoded indirectly by first
translating to the extended representation.
NOTE: RFC 1494 defined a bp-type=<integer> for encoding
standard X.400 body parts. If such body parts are
encountered, RFC 1494 section 6.1 should be consulted.
3.3. Encapsulating FTBP body parts in MIME
The File Transfer Body Part is believed to be important in
the future as "the" means of carrying well-identified data
in X.400 networks.
They also share the property (at lest when limited to the
EMA MAWG functional profile) of having a well-defined data
part that is always representable as a sequence of bytes.
This conversion will have to fail, and the x400-bp
encapsulation used instead, if:
- FileTransferData has more than one element
- Contents-type is not unstructured-binary
- Parameters that are not mappable, but important, are
present (like Compression, which EMA doesn't
recommend).
Otherwise, it can be encapsulated in MIME by:
Alvestrand Exp Nov 96 [Page 19]
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- Creating the "content-type" value by forming the
string "application/x-ftbp." and appending the
numbers of the OID
- Mapping all other parameters according to the
standard FTBP parameter mapping
- Applying an appropriate content-transfer-encoding
DISCUSSION:
The choice of the somewhat strange, and by necessity
unregistered, MIME type "application/x-ftbp.n.n.n.n" is
because for any concrete example of this usage, it will be
easy to configure any MIME reader to take advantage of the
identification. If the MIME type registration rules are
ever changed to allow the registration of a namespace,
rather than just of names, the "x-" can be deleted, and
the types can be "application/ftbp.n.n.n.n".
4. User control over the gateway choice
In some cases, the gateway may make an inappropriate
choice when deciding what to do about a particular body
part.
To allow an escape clause, this chapter defines a way in
which the user can signal the gateway what action it finds
most appropriate.
The headers given here override any "conversion
prohibited" and "conversion with loss prohibited" on the
message.
It is still the gateway's responsibility that the
generated messages conform to the destination domain's
syntax rules.
DISCUSSION:
The intent of this mechanism is to allow the sender to
efficiently get a message through to a single recipient
Alvestrand Exp Nov 96 [Page 20]
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when the sender has information about the recipient that
the gateway does not have.
It is not a part of the minimum functionality listed in
chapter 8; a gateway does not have to implement this spec
to be MIXER conformant, but if implemented, it should be
done like this.
The additional complexity, both in user interface and in
protocol, of making this field selectable per recipient
was not thought worthwhile;
4.1. Conversion from MIME to X.400
The header field described below specifies explicit MIXER
conversion. Comments are allowed within the field
according to the usual RFC 822 convention.
If "x400-object-id" is omitted, "tunnel" is assumed.
mime-to-x400 = "Wanted-X400-Conversion" ":"
[ mime-from ] [ x400-object-id ]
"in" x400-encoding
x400-object-id = "to" ( object-identifier-2 / "tunnel" )
x400-encoding = "bp14" / "bp15" / "ftbp" / "ia5"
mime-from = "from" mime-type
mime-type = word
There is no way to ask for a different conversion based on
MIME parameters or bodypart content.
Examples:
Wanted-X400-Conversion: from application/msword
to 1.2.840.113556.4.2 (Microsoft defined ms-word)
in ftbp
This uses the MAWG definitions, and leads to an FTBP encoding.
Wanted-X400-Conversion: from application/msword
to tunnel in bp14
Alvestrand Exp Nov 96 [Page 21]
draft X.400/MIME body mapping May 96
This leads to a Body Part 14 encoding for all body parts of type
application/msword.
Wanted-X400-Conversion: in bp14
This requests that this specific body part be encoded in Body Part 14.
This field may be used in two places:
(1) In the heading of an unstructured MIME body part.
In this case the EBNF.mime-from is omitted, and the
requested conversion applies to the body part.
(2) In a multipart. In this case, the body part type to
which the conversion applies is defined by
EBNF.mime-from, and the conversion applies to all
body parts of this MIME type contained in the
multipart, including those contained in nested
messages and multiparts. If a contained body part
has its own heading, this takes precedence. Note
that the "from" parameter is mandatory when used in
a multipart.
The EBNF.x400-object-id shall be present when "bp15" or
"ftbp" encoding is selected.
The value "tunnel" implies encapsulation as defined in
Chapter 3.
The "object identifier" used below is:
- For BP 15, it is the value of the EXTENDED-BODY-PART-
TYPE macro that defines the body part, which is found
in ExternallyDefinedBodyPart.data.direct-reference.
- For FTBP, it is the value of the
Environment.application-reference.
Alvestrand Exp Nov 96 [Page 22]
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4.2. Conversion from X.400 to MIME
The IPM heading defined here shall be present in the
heading of a message. It defines the mapping for all body
parts of the specified types, including those in nested
messages.
wanted-MIME-conversion HEADING-EXTENSION
VALUE WantedMIMEConversions
::= id-wanted-MIME-conversions
WantedMIMEConversions ::= SEQUENCE OF X400toMIMEConversion
X400toMIMEConversion ::= SEQUENCE {
x400-type X400Type,
mime-type MIMEType }
X400Type ::= CHOICE {
standard [0] INTEGER, -- standard body part
extended [1] OBJECT IDENTIFIER, -- BP 15
ftbp [2] OBJECT IDENTIFIER} -- FTBP application-reference
MIMEType ::= SEQUENCE {
type IA5String, -- type (e.g., application/ms-word)
encoding [1] IA5String OPTIONAl -- e.g. quoted-printable
parameters [2] IA5String OPTIONAL } -- MIME Parameters
The heading extension includes all requested conversions,
with explicit information as to how each body part type is
encoded in MIME.
FTBP is identified as a separate body part type, as there
will be a need for different encodings, dependent on what
is being carried.
Encapsulation is requested by asking for
"application/x400-bp" or "application/ftbp" as the
destination type.
For FTAM body parts, the parameters will survive the
gatewaying process. For other body parts, there are three
alternatives:
Alvestrand Exp Nov 96 [Page 23]
draft X.400/MIME body mapping May 96
(1) The gateway knows a defined mapping for this
particular body part and destination type. It will
be used, and parameters mapped accordingly.
(2) The gateway knows how to extract an OCTET STRING
from the body part, and the destination is a simple
MIME body part. All information outside the OCTET
STRING is lost. (This may be the case for a BP14
that should end up in an application/xyzzy, for
instance).
(3) The gateway knows of no relevant mapping, and does
not know how to simplify the X.400 body part. The
gateway will then proceed as if the mapping control
field had not been present.
5. The equivalence registry
5.1. What information one must give about a mapping
The following information MUST be supplied when describing
an equivalence or a mapping:
MIME type name (which must be preregistered)
X.400 body part (often BP15 or FTAM Body Part)
If BP15 is used, the following information must be given:
(1) Object Identifier for X.400 BP15 Data
(2) Object Identifier for X.400 BP15 Parameters
(3) X.400 ASN.1 Syntax (must be an EXTENDED-BODY-PART-
TYPE macro)
If FTBP is used, the following information must be given:
(1) Object Identifier for the FTAM
Environment.application-reference
Alvestrand Exp Nov 96 [Page 24]
draft X.400/MIME body mapping May 96
(2) Object Identifier for the FTAM Contents-type, if
unstructured-binary is not used
(3) Any other special considerations
In all cases, the following must be given:
Conversion algorithms. The expected effect of "Conversion
prohibited" and "Conversion with loss prohibited" should
be noted.
The conversion must be specified with enough detail to
permit independent implementation; literature references
are acceptable.
An equivalence can be registered with IANA using the form
at the end of this document. The purpose of the
registration is to achieve a greater uniformity among
gateways implementing the same translation; there is no
requirement that a gateway must support all of the
translations that are registered with IANA. Specific
conformance requirements for MIXER are given at the end of
this document.
5.2. Equivalence summary for known X.400 and MIME Types
This section itemizes the equivalences for all currently
known MIME content-types and X.400 body parts.
For each MIME content-type/X.400 body part pair, the
equivalence table contains an entry with the following
sections:
X.400 Body Part
This section identifies the X.400 Body Part governed
by this Table entry. It includes any OBJECT
IDENTIFIERs or other parameters necessary to uniquely
identify the Body Part.
MIME Content-Type
This section identifies the MIME content-type
Alvestrand Exp Nov 96 [Page 25]
draft X.400/MIME body mapping May 96
governed by this Table entry. The MIME content-type
named here must be registered with the IANA.
Section/document reference
Reference to section of this document, or to the
other document that describes this mapping.
The initial Equivalence Table entries in this document are
described using this convention.
Further registrations of equivalences should be submitted
to the IANA after a public review, using the example form
given at the end of this document.
5.3. MIME to X.400 Table
MIME content-type X.400 Body Part Section
----------------- ------------------ -------
text/plain
charset=us-ascii ia5-text 6.1
charset=ISO-8859-x EBP - GeneralText 6.2
text/richtext no mapping defined Encap
application/oda EBP - ODA [ODA]
application/octet-stream bilaterally-defined or 6.3
FTBP unknown attachment 6.4
application/postscript EBP - mime-postscript-body [POSTSCRIPT]
image/g3fax g3-facsimile [IMAGES]
image/jpeg EBP - mime-jpeg-body [IMAGES]
image/gif EBP - mime-gif-body [IMAGES]
audio/basic no mapping defined Encap
video/mpeg no mapping defined Encap
message/RFC822 ForwardedIPMessage 6.5
multipart/* ForwardedIPMessage 6.6
multipart/signed HARPOON encap 7.3
multipart/encrypted HARPOON encap 7.4
Abbreviation: EBP - Extended Body Part
Alvestrand Exp Nov 96 [Page 26]
draft X.400/MIME body mapping May 96
5.4. X.400 to MIME Table
Basic Body Parts
X.400 Basic Body Part MIME content-type Section
--------------------- -------------------- -------
ia5-text text/plain;charset=us-ascii 6.1
voice No Mapping Defined Encap
g3-facsimile image/g3fax [IMAGES]
g4-class1 no mapping defined Encap
teletex text/plain;charset=teletex 6.7
videotex no mapping defined Encap
encrypted no mapping defined Encap
bilaterally-defined application/octet-stream 6.3
nationally-defined no mapping defined Encap
externally-defined See Extended Body Parts below
ForwardedIPMessage message/RFC822 or multipart 6.5,6.6
X.400 Extended Body Part MIME content-type Section
------------------------- -------------------- -------
GeneralText text/plain;charset=ISO-8859-x 6.2
ODA application/oda [ODA]
mime-postscript-body application/postscript [POSTSCRIPT]
mime-jpeg-body image/jpeg [IMAGES]
mime-gif-body image/gif [IMAGES]
FTAM various 2.3,6.4
FTAM application ID MIME content type Section
------------------- ----------------- -------
ema-unknown-attachment application/octet-stream 6.4
Alvestrand Exp Nov 96 [Page 27]
draft X.400/MIME body mapping May 96
5.5. Use of OBJECT IDENTIFIERs and ASN.1 MACROS
When one wants to define new BP15 body parts for use with
equivalences, it is important to know that X.420 dictates
that Extended Body Parts shall:
(1) use OBJECT IDENTIFIERs (OIDs) to uniquely identify
the contents, and
(2) be defined by using the ASN.1 Macro:
EXTENDED-BODY-PART-TYPE MACRO::=
BEGIN
TYPE NOTATION ::= Parameters Data
VALUE NOTATION ::= value (VALUE OBJECT IDENTIFIER)
Parameters ::= "PARAMETERS" type "IDENTIFIED"
"BY" value(OBJECT IDENTIFIER)
| empty;
Data ::= "DATA" type
END
To meet these requirements, this document uses the OID
mixer
defined in [MIXER], as the root OID for X.400 Extended
Body Parts defined for MIME interworking.
Each Extended Body Part contains Data and optional
Parameters, each being named by an OID. To this end, two
OID subtrees are defined under mixer-bodies, one for Data,
and the other for Parameters:
mixer-bp-data OBJECT IDENTIFIER ::=
{ mixer 1 }
mixer-bp-parameter OBJECT IDENTIFIER ::=
{ mixer 2 }
All definitions of extended X.400 body parts submitted to
the IANA for registration with a mapping must use the
Extended Body Part Type macro for the definition. See
[IMAGES] for an example.
Alvestrand Exp Nov 96 [Page 28]
draft X.400/MIME body mapping May 96
Lastly, the IANA will use the mixer-bp-data and mixer-bp-
parameter OIDs as root OIDs for any new MIME content-
type/subtypes that aren't otherwise registered in the
Equivalence Table.
NOTE: The ASN.1 for an ExternallyDefinedBodyPart is
ExternallyDefinedBodyPart ::= SEQUENCE {
parameters [0] ExternallyDefinedParameters OPTIONAL,
data ExternallyDefinedData }
ExternallyDefinedParameters ::= EXTERNAL
ExternallyDefinedData ::= EXTERNAL
The ASN.1 for EXTERNAL is (from X.208):
EXTERNAL ::= [UNIVERSAL 8] IMPLICIT SEQUENCE
{direct-reference OBJECT IDENTIFIER OPTIONAL,
indirect-reference INTEGER OPTIONAL,
data-value-descriptor ObjectDescriptor OPTIONAL,
encoding CHOICE
{single-ASN1-type [0] ANY,
octet-aligned [1] IMPLICIT OCTET STRING,
arbitrary [2] IMPLICIT BIT STRING}}
ObjectDescriptor ::= [UNIVERSAL 7] IMPLICIT GraphicString
There are a bit too many choices here; the common X.400
usage for BP15 encoding is to:
(1) Always use direct-reference
(2) Omit indirect-reference and data-value-descriptor
(3) Use the single-ASN1-type encoding only
Unfortunately, some implementations have chosen to use the
octet-aligned choice when constructing values where the
ASN.1 type is OCTET STRING, which of course caused
interoperability problems.
An attempt to specify that X.420 only allowed the single-
ASN1-type choice in the 1996 versions is still (Sept 1995)
being debated in ISO; the end result seems to be that all
Alvestrand Exp Nov 96 [Page 29]
draft X.400/MIME body mapping May 96
agree in principle that single-ASN1-type should be used,
but that one has to allow the generation of the octet-
aligned choice as being conformant.
6. Defined Equivalences
6.1. IA5Text - text/plain
X.400 Body Part: IA5Text
MIME Content-type: text/plain; charset=US-ASCII
Conversion Type: No conversion
Comments:
When mapping from X.400 to MIME, the "repertoire"
parameter is ignored.
When mapping from MIME to X.400, the "repertoire"
parameter is set to IA5 (5).
NOTE: The MIME Content-type headers are omitted, when
mapping from X.400 to MIME, if and only if the IA5Text
body part is the only body part in the IPMS.Body sequence.
NOTE: IA5Text specifies the "currency" symbol in position
2/4. This is converted without comment to the "dollar"
symbol, since the author of this document has seen many
documents in which the position was intended to indicate
"dollar" while he has not yet seen one in which the
"currency" symbol is intended.
(For reference: The T.50 (1988) recommendation, which
defines IA5, talks about ISO registered set number 2,
while ASCII, using the "dollar" symbol, is ISO registered
set number 6. There are no other differences.)
NOTE: It is not uncommon, though it is a violation of the
standard, to use 8-bit character sets inside an IA5 body
part. Gateways that can expect to encounter this situation
should consider implementing something like the guidance
given in RFC 1428, "Transition of Internet Mail from just-
send-8 to 8-bit SMTP/MIME", and generate appropriate
charset parameters for the MIME messages they generate.
Alvestrand Exp Nov 96 [Page 30]
draft X.400/MIME body mapping May 96
This behavior is not required for MIXER conformance, since
it is only needed when the base standards are violated.
6.2. GeneralText - text/plain (ISO-8859)
X.400 Body Part: GeneralText; CharacterSets in
6, 14, 42, 87, 100,101,109,110,126,127,138,144,148
MIME Content-Type: text/plain; charset=ISO-8859-(1-9)
or iso-2022-jp
Conversion Type: Text conversion without character change
When mapping from X.400 to MIME, the character-set is
chosen from the table below according to the value of
Parameters.CharacterSets. If no match is found, and the
gateway does not support a conversion, the character set
shall be encoded as x-iso-nnn-nnn-nnn, where "nnn" is the
numbers of the Parameters.CharacterSets, sorted in numeric
order.
When mapping from MIME to X.400, GeneralText is an
Extended Body Part, hence it requires an OID. The OID for
the GeneralText body is defined in [MOTIS], part 8, annex
D, as {2 6 1 4 11}. The OID for the parameters is {2 6 1
11 11}.
The Parameters.CharacterSets is set from table below
according to the value of "charset"
The following table lists the MIME character sets and the
corresponding ISO registry numbers. If no correspondence
is found, this conversion fails, and the generic body part
approach is used.
MIME charset ISO IR numbers Comment
-----------------------------------------------
ISO-8859-1 6, 100 West European "8-bit ASCII"
ISO-8859-2 6, 101 East European
ISO-8859-3 6, 109 <regarded as obsolete>
ISO-8859-4 6, 110 <regarded as obsolete>
ISO-8859-5 6, 144 Cyrillic
ISO-8859-6 6, 127 Arabic
ISO-8859-7 6, 126 Greek
ISO-8859-8 6, 138 Hebrew
ISO-8859-9 6, 148 Other Latin-using languages
Alvestrand Exp Nov 96 [Page 31]
draft X.400/MIME body mapping May 96
ISO-2022-JP 6, 14, 42, 87 Japanese
When converting from MIME to X.400, generate the correct
OIDs for use in the message envelope's Encoded Information
Types by looking up the ISO IR number in the above table,
and then appending it to the id-cs-eit-authority {1 0
10021 7 1 0} OID.
The escape sequences to designate and invoke the relevant
character sets in their proper positions must be added to
the front of the GeneralText character string.
For ISO 8859-1, the relevant escape sequence will be:
ESC 28 42
ASCII in G0
ESC 2D 41
ISO-IR-100 in G1
ESC 21 41
High control character set in C1
ESC 7E
Locking shift 1 Right
These escape sequences are removed when converting from
GeneralText to text/plain.
Note that new character sets may be defined on both the
Internet side and the X.400 side; a gateway MAY choose to
implement more conversions in the same fashion.
DISCUSSION:
The conversion of text is a problematic one, and one in
which it is likely that gateways should be given wide
latitude to make decisions based upon their knowledge of
the user's preferences. The text given below is thought to
give the best approximation to a gateway conforming to
current and anticipated usage in the MIME and X.400
worlds, and is the way recommended when no knowledge of
the recipient's capabilities exists.
The changes, such as normalizing escape sequences, should
Alvestrand Exp Nov 96 [Page 32]
draft X.400/MIME body mapping May 96
not be done when "conversion-prohibited" is set. If
"conversion-with-loss-prohibited" is set, translation to a
character set that is not able to encode all characters
cannot be done, and the message should be non-delivered
with an appropriate non-delivery reason.
The common use of character sets in MIME is somewhat
different from the rules given by X.400; in particular, it
is common in MIME to assume that the character sets follow
strict rules. For the ISO-8859-x character sets, it is
assumed that they are designated and invoked at the
beginning of the text, and that no designation or
invocation sequences occur within the body of the text.
The rules for ISO-2022-JP are given in RFC 1468, and are
even more particular, using a pure 7-bit encoding in which
each line of text starts in ASCII.
Therefore, the text must be "normalized" by going through
the whole message, using a state machine or similar device
to remove all escape and shift sequences.
Appendix A gives pseudocode for such a conversion.
NOTE: In 1988, the GeneralText body part was defined in
ISO 10021-8 [MOTIS], and NOT in the corresponding CCITT
recommendation; this was added later. Also, the
parameters have been heavily modified; they should be a
SET OF INTEGER in the currently valid text. Use the
latest version of the standard that you can get hold of.
6.3. BilaterallyDefined - application/octet-stream
X.400 Body Part: BilaterallyDefined
MIME Content-Type: Application/Octet-Stream (no parameters)
Conversion Type: No conversion
When mapping from MIME to X.400, if there are parameters
present in the Content-Type: header field, they are
removed.
DISCUSSION:
The parameters "name" "type" and "conversions" are
Alvestrand Exp Nov 96 [Page 33]
draft X.400/MIME body mapping May 96
advisory; name and conversions are depreciated in RFC
1521.
The parameter "padding" changes the interpretation of the
last byte of the data, but it is deemed better by the WG
to delete this information than to non-deliver the body
part. The "padding" parameter is rarely used with MIME.
Use of BilaterallyDefined Body Parts is specifically
deprecated in both 1988 and 1992 X.400. It is retained
solely for backward compatibility with 1984 systems, and
because it is in common use.
6.4. FTBP EMA Unknown Attachment -
application/octet-stream
X.400 Body Part: FTBP EMA Unknown Attachment
MIME Content-Type: Application/Octet-Stream
Conversion Type: No conversion
The OID for the Unknown Attachment is { joint-iso-ccitt(2)
country(16) us(840) organization(1) ema(113694) objects(2)
messaging(2) attachments(1) unknown(1) }, or
2.16.840.1.113694.2.2.1.1 for short.
NOTE: Previous EMA drafts gave it as { iso(1) countries(2)
usa(840) organization (1) ema (113694) objects(2)
messaging(2) attachments(1) unknown (1)}, or
1.2.840.1.113694.2.2.1.1 for short.
The parameters for this type must be mapped according to
chapter 2.3, with the following extensions for the
parameters of the application/octet-stream:
If there is no Content-Disposition parameter with a
filename, and there is a name parameter, the
FTBP.FileTransferParameters.File-attributes.pathname
is generated from this parameter. Note that RFC 1521
recommends not using the "name" parameter.
The "type", "conversions" and "padding" attributes are
ignored; "type" is for human consumption; "conversions"
Alvestrand Exp Nov 96 [Page 34]
draft X.400/MIME body mapping May 96
are discouraged in RFC 1521.
The body mapping is just copying the bytes in both
directions.
6.5. MessageBodyPart - message/RFC822
X.400 body part: MessageBodyPart
MIME Content-Type: message/RFC822
Conversion Type: Special
NOTE: If the headers of the X.400 MessageBodyPart contains the
"multipart-message" heading extension with the isAMessage bit set
(either explicitly or implicitly), the mapping should be to
multipart/* according to section 6.6, below.
To map an IPMS.MessageBodyPart, the full X.400 -> RFC 822
mapping is recursively applied, to generate an RFC 822 Message.
If present, the IPMS.MessageBodyPart.parameters.delivery-envelope
is used for the MTS Abstract Service Mappings. If present, the
IPMS.MessageBodyPart.parameters.delivery-time is mapped to the
extended RFC 822 field "Delivery-Date:".
When a message/RFC822 is contained within a MIME message, it is
mapped to an IPMS.MessageBodyPart according to MIXER.
specification. Any mappings that would have been made to the MTS
Abstract Service are placed in
IPMS.MessageBodyPart.parameters.delivery-envelope.
6.6. MessageBodyPart - multipart/*
X.400 body part: MessageBodyPart
MIME Content-Type: multipart/*
Conversion Type: Special
NOTE: If the headers of the X.400 MessageBodyPart do not contain the
"multipart-message" heading extension with the "isAMessage" flag FALSE,
the mapping should be to message/RFC822.
A MIME multipart is a set of content-types and not a message with
a set of content types. When the multipart is at the outermost
MIME header, elements of the multipart are mapped directly onto
IPMS.Bodypart.
Alvestrand Exp Nov 96 [Page 35]
draft X.400/MIME body mapping May 96
When the MIME multipart is not at the outermost level, it is mapped to
an IPMS.MessageBodyPart containing an IPMS.Bodypart for each element
of the multipart.
When a nested IPMS.Message is generated from a multipart, an
IPMS.heading shall always be generated. The only mandatory field
is the IPMS.Heading.this-IPM message id, which shall be generated
by the gateway. An IPMS.Heading.subject field shall also be
generated, in order to provide useful information to non-MIME
capable X.400(88) UAs and to all X.400(84) UAs. The subject
field is set as follows according to the multipart subtype:
mixed:
"Multipart Message"
alternative:
"Alternative Body Parts containing the same
information"
digest:
"Message Digest"
parallel:
"Body Parts interpreted in parallel"
other:
"Multipart Message (<subtype>)"
For other types of multipart, the multipart subtype shall
be included in the subject line.
For each multipart, the following IPMS.HeadingExtension
shall be generated, with the value set according to the
subtype.
If the multipart is the outermost multipart, and the
subtype is "mixed", it may be omitted.
multipart-message HEADING-EXTENSION
VALUE MultipartType
::= id-hex-multipart-message
MultipartType ::= SEQUENCE {
subtype IA5String,
isAMessage BOOLEAN DEFAULT TRUE }
Alvestrand Exp Nov 96 [Page 36]
draft X.400/MIME body mapping May 96
The MultipartType contains the subtype, for example
"digest". If this heading is present when mapping from
X.400 to MIME, the appropriate multipart may be generated.
The isAMessage flag is needed because of the case where a
message contains a ForwardedIPMessage, which itself was
generated from a MIME message that was a Multipart; it is
set whenever the multipart is the outermost level of
nesting inside a Message/RFC822.
NOTE:
When downgrading to X.400/84, the content-type SHOULD
be regenerated from this heading-extension and put
into the RFC-822-HEADERS extra body part.
6.7. Teletex - Text/Plain (Teletex)
X.400 Body Part: Teletex
MIME Content-Type: text/plain; charset=Teletex
Conversion Type: Text conversion
From X.400 to RFC-822, the conversion shall take the bytes
of all the pages in the "data" part of the
TeletexBodyPart, add a FF character (0x0C, control-L) to
each part that does not already end in one, and
concatenate them together to form the body of the
Text/Plain.
The character set shall be "Teletex", which is especially
registered for this purpose. Its definition is shown in an
appendix.
The parameters are discarded.
From RFC-822 to X.400, the conversion shall split the
content at each occurrence of the FF character (0x0C),
delete the character and construct the Teletex body part
as a SEQUENCE OF TeletexString, as described in X.420(88),
section 7.3.5
The TeletexParameters may, but need not, contain the
Alvestrand Exp Nov 96 [Page 37]
draft X.400/MIME body mapping May 96
number-of-pages component.
NOTE: It is recommended, but not mandated, that the data
be converted into a more widespread character set like
ISO-8859-1 or ISO-2022-JP (if applicable) if possible.
This will result in the reverse translation giving a
GeneralText body part, which will have to be dealt with
appropriately at the X.400/88 to X.400/84 downgrading
boundary, if possible, but will give a much greater chance
that the MIME recipient can actually read the message.
DISCUSSION:
The Teletex body part is frequently used in X.400(84) to
send around text with slightly extended character sets
beyond ASCII.
Its body consists of a series of "pages", separated by
ASN.1 representation. It is important to many people to
have this mapped into something that is readable to most
end-users; therefore, it is recommended to map this onto
Text/Plain; however, since this is not plain text, the
conversion must be specified.
Alvestrand Exp Nov 96 [Page 38]
draft X.400/MIME body mapping May 96
7. Body parts where encapsulation is recommended
Some body parts are MIME constructs, and their
functionality will be severely damaged if they are coerced
into an X.400 framework.
Special care needs to be taken with these; they are
described below.
7.1. message/external-body
The gateway MUST support the encapsulation of this body
part using the HARPOON encapsulation (IA5).
It MAY support some kind of retrieval of the referred
object.
DISCUSSION:
The message/external-body part points to an object that
can be retrieved using Internet protocols.
There are three cases to consider for the recipient's
capabilities:
(1) The user has no Internet access. In this case, the
user might be grateful if the gateway fetches the
body part and inserts it into the message. If the
body part is large or dynamic, it might not be
appropriate.
(2) The user has Internet access, but no UA support for
fetching external-body objects.
(3) The user has Internet access and UA support for
fetching external-body objects, based on an
understanding of this document.
Some access-types, like anonymous FTP, are easy to
resolve. Others, like the Mailserver access-type, are
almost impossible to resolve at a gateway.
Alvestrand Exp Nov 96 [Page 39]
draft X.400/MIME body mapping May 96
To support the second case above, the tunneling method
chosen is the HARPOON encapsulation described in section
3.1.3, using an IA5 body part, inserting the string "MIME-
Version: 1.0 (generated by gateway)" at the beginning of
the body part. (The part in parentheses can be changed at
will).
This will:
(1) Maximize the chance that the user will see the
message
(2) Give the user hints that will enable him to fetch
the message using other Internet tools
(3) Identify the message as a MIME object in a reliable
fashion, allowing UAs to support the fetching of
the object if the UA implementor desires.
7.2. message/partial
This represents part of a larger message, where it is only
possible to parse the complete message after getting all
the pieces.
The gateway MUST support the encapsulation of this body
part.
It MAY implement transparent reassembly of the message,
but in this case, it MUST support a configurable timeout
for the reassembly, defaulting back to encapsulation.
DISCUSSION:
The gateway's choices are:
(1) Wait until all the pieces arrive at the gateway,
reassemble the message, and use normal processing
(2) Encapsulate the message, using any encapsulation
method (BP15, FTAM or HARPOON).
Alvestrand Exp Nov 96 [Page 40]
draft X.400/MIME body mapping May 96
In some cases, not all pieces will arrive at the gateway;
some may have been transferred through other gateways due
to route changes or machine outages; some may have been
lost in transit.
7.3. multipart/signed
A gateway MUST implement encapsulation of multipart/signed
using HARPOON.
The gateway MAY be configured to do other processing, as
outlined in the discussion below. This is outside the
scope of the standard.
DISCUSSION:
Gatewaying security is a problem. The gateway can
basically take three approaches:
- Strip the multipart/signed, leaving the bare body
part unsecured, possibly with a comment that the
signature was stripped
- Attempt to check the signature and re-signing the
message using X.400 security functions, then
stripping as above
- Encapsulate the message. This is the only approach
that allows end to end security, but requires MIME
functionality at the recipient.
- Replace the message content with multiple body parts,
containing first an unsecured body part and then the
encapsulated multipart/signed.
All these are valid options for a MIXER gateway.
Note that the encapsulation must use HARPOON, as the
signature is computed on the ENCODED body part, not on the
canonical representation, and HARPOON is the only
encapsulation that preserves the content transfer encoding
of the message.
Alvestrand Exp Nov 96 [Page 41]
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Note also that all methods except for encapsulation break
end-to-end security; the recipient can place no more trust
in the integrity of the message than he can place in the
security of the gateway.
7.4. multipart/encrypted
A gateway MUST implement encapsulation of
multipart/encrypted using HARPOON.
If the implementor chooses to allow other processing at
the gateway, as outlined below, he/she is advised that
there are grave security concerns with such a solution,
since it violates the general rule of keeping decryption
keys as close to the user as possible.
DISCUSSION:
There are two basic cases for a gateway:
- The gateway is trusted with the user's keys. In this
case, the gateway can decrypt the message, possibly
add a note that it has done so, and gateway the
unencrypted form, possibly applying X.400 security
functions, and possibly attaching a copy of the
original, encrypted material for reference.
This does nothing to protect the transfer from
gateway to recipient, unless suitable X.400-native
security is applied. It also means that the gateway
must be part of the user's trusted environment.
- The gateway is not trusted with the recipient's keys.
In this case, encapsulation is the only approach that
preserves any information at all.
The valid options for a MIXER gateway are therefore:
- Decrypt the body part
- Encapsulate the body part
Alvestrand Exp Nov 96 [Page 42]
draft X.400/MIME body mapping May 96
- Drop the body part
The MIXER WG has shown strong preference for the
encapsulation alternative, and urges anyone who thinks of
buying or implementing gateway decryption to carefully
evaluate this choice in light of the company's general
security policy.
8. Conformance requirements
In order to be called MIXER conformant, a gateway must
implement:
- Encapsulation of MIME content in the FTBP body part
- Encapsulation of X.400 body parts in the x400-bp body
part
- Encapsulation of FTBP body parts in the
application/x-oid.n.n.n body part
- Encapsulation of security multiparts using HARPOON
- Text/plain <-> IA5Text
- Text/plain; charset=iso-8859-* <-> GeneralText
- Multipart/* <-> ForwardedIPMessage
- message/RFC822 <-> ForwardedIPMessage
- application/octet-stream <-> FTBP unknown
- application/octet-stream <-> BilaterallyDefined
- A configuration choice of which application/octet-
stream translation to use
All other parts of this specification MAY be implemented
by the gateway. If they are implemented at all, they MUST
Alvestrand Exp Nov 96 [Page 43]
draft X.400/MIME body mapping May 96
be implemented conformant to this specification.
In this context, a feature is "implemented" in a product
if it is possible to configure the product in such a way
that this feature is used. This specification does not
restrict the product to only be configured in such a
fashion.
9. Security considerations
The security issues identified in this memo are:
(1) Security implications of using filenames that
arrive in body part headers (section 2.3.2)
(2) Security implications of letting a gateway handle
encrypted and/or signed content (section 7.3 and
7.4)
If a gateway fetches message/external-body on behalf of
the recipient, as described in section 7.1, it may be
tricked into performing inappropriate actions by malicious
senders.
In addition, all the normal caveats that apply to sending
data that may contain executable code apply to UAs on both
sides of the gateway.
10. Author's address
Harald Tveit Alvestrand
UNINETT
P.O.box 6883 Elgeseter
N-7002 Trondheim
NORWAY
Harald.T.Alvestrand@uninett.no
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11. Acknowledgements
The author wishes to thank all the members of the MIXER WG
for their valuable input, and in particular (in no
particular order):
Steve Kille, Peter Sylvester, Ned Freed, Julian Onions,
Ruth Moulton, Keith Moore, Alain Zahm, Urs Eppenberger,
Kevin Jordan, Jeroen Houttuin, Claudio Allocchio, Colin
Robbins, Steven Thomson, Jim Craigie,
and many others who have been active over the long
lifetime of this document.
References
[RFC-822]
D.H. Crocker, Standard for the Format of ARPA
Internet Text Messages. Request for Comments 822,
(August, 1982).
[MIME]
N. Borenstein, N. Freed, MIME: Mechanisms for
Specifying and Describing the Format of Internet
Message Bodies. Request for Comments 1521, (June,
1992).
[MIXER]
S.E. Kille, Mapping between X.400(1988) / ISO 10021
and RFC-822. (in preparation)
[T.4]
CCITT Recommendation T.4, Standardization of Group 3
Facsimile Apparatus for Document Transmission (1988)
[T.30]
CCITT Recommendation T.30, Procedures For Document
Facsimile Transmission in the General Switched
Telephone Network (1988)
[T.411]
CCITT Recommendation T.411 (1988), Open Document
Architecture (ODA) and Interchange Format,
Introduction and General Principles
Alvestrand Exp Nov 96 [Page 45]
draft X.400/MIME body mapping May 96
[MOTIS]
ISO/IEC International Standard 10021, Information
technology - Text Communication - Message-Oriented
Text Interchange Systems (MOTIS) (Parts 1 to 8)
[X.400]
CCITT, Data Communication Networks - Message Handling
Systems - Recommendations X.400 - X.420 (1988
version)
[X.420]
CCITT Recommendation X.420 (1988), Interpersonal
Messaging System
[RFC-X400USE]
Harald Tveit Alvestrand, X.400 use of extended
Character Sets, Internet Draft, June 1992
[MAWG]
Electronic Messaging Association Message Attachment
Working Group (MAWG): File Transfer Body Part
Feasibility Project Guide - version 1.5 - September
1995
[CDISP]
Dorner & Troost, The Content-Disposition header - RFC
1806
[POSTSCRIPT]
Harald Tveit Alvestrand, Carrying PostScript in X.400
and MIME, Work In Progress (draft-ietf-mixer-
postscript-00.txt)
[IMAGES]
Harald Tveit Alvestrand, X.400 image body parts, Work
In Progress (draft-ietf-mixer-images-00.txt)
[ODA]
Harald Tveit Alvestrand, A MIME body part for ODA,
Work in Progress (draft-ietf-mixer-oda-00.txt)
Alvestrand Exp Nov 96 [Page 46]
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APPENDIXES
Appendix A: Escape code normalization
The algorithm given here in pseudocode will reduce a
GeneralString ISO-2022 unlimited use of shifts sequence to
a pure 8-bit sequence that does not use shift sequences,
if possible.
Some error conditions, like EOF, are not tested for. It
crashes if asked to do something it cannot. Control
character set switching is missing.
A similar routine, albeit more complex, can be written for
normalizing to the ISO-2022-JP character set.
BEGIN: (from X.209)
g0 = 6 (should be 2, but ignore the difference)
g1 = NULL
g2 = NULL
g3 = NULL
c0 = 1 (ASCII control)
c1 = NULL
leftset = &g0 (current input set, low)
rightset = &g1 (current input set, high)
lowset = 6 (output set, low)
highset = NULL (output set, high)
charset = US-ASCII
(Init for the set tables)
chartoid[{2D,2E,2F}, 41] = 100
.....
idtoname[100] = "ISO-8859-1"
.....
WHILE (more data)
CASE head of input
{These are the locking shift sequences}
INCASE "00/14": (LS0, SO)
leftset = &g0;
INCASE "00/15": (LS1, SI)
leftset = &g
INCASE "ESC 07/14": (LS1R)
rightset = &g1;
Alvestrand Exp Nov 96 [Page 47]
draft X.400/MIME body mapping May 96
INCASE "ESC 07/13": (LS2R)
rightset = &g2;
INCASE "ESC 07/12": (LS3R)
rightset = &g3;
{There is missing code for handling the single shift function}
{These are the changes of graphic character sets}
{Note that G0 can contain only 94-character charsets}
INCASE "ESC 28"
g0 = chartoid[lastchar, next character]
sethiset(g0)
INCASE "ESC 2D", "ESC 29"
g1 = chartoid[lastchar, next character]
sethiset(g1)
INCASE "ESC 2E", "ESC 2A"
g2 = chartoid[lastchar, next character]
sethiset(g2)
INCASE "ESC 2F", "ESC 2B"
g3 = chartoid[lastchar, next character]
sethiset(g3)
{control characters. There is missing code for changing these}
INCASE 00/00-01/15 {normal control}
write(char)
INCASE 08/00-09/15 {upper control}
write(char)
{Normal characters}
INCASE 02/00-07/15 (Left)
IF (*leftset == lowset)
write(char)
ELSIF (*leftset == highset)
write(char+80)
ELSE
ERROR "Shift error"
ENDIF
INCASE 10/00-15/15
IF (*rightset == highset)
write(char)
ELSIF (*rightset == lowset)
write(char-80)
ELSE
ERROR "Shift error"
ENDIF
ENDCASE
ENDWHILE
SUBROUTINE sethighset(g1)
Alvestrand Exp Nov 96 [Page 48]
draft X.400/MIME body mapping May 96
IF (highset == NULL)
charset = idtoname[g1]
highset = g1
ELSIF (highset == g1)
(it's OK)
ELSE
ERROR "Too many charsets encountered"
ENDIF
ENDROUTINE
Appendix B: OID Assignments
MIXER-MAPPINGS DEFINITIONS ::= BEGIN
EXPORTS -- everything --;
IMPORTS
experimental
FROM RFC1155-SMI;
mixer -- { iso(1) org(3) dod(6) internet(1) mail(7) mixer(1) }
FROM MIXER --Companion RFC--;
mixer-bp-data OBJECT IDENTIFIER ::=
{ mixer 1 };
mixer-bp-parameter OBJECT IDENTIFIER ::=
{ mixer 2 };
-- mixer-core is defined as { mixer core(3) } in [MIXER]
mixer-bp-heading OBJECT IDENTIFIER ::=
{ mixer 4 }
id-mime-bp-data OBJECT IDENTIFIER ::=
{ mixer-bp-data 1};
id-mime-bp-parameters OBJECT IDENTIFIER ::=
{ mixer-bp-parameter 1};
-- the following assignments were done in RFC 1494, using
-- slightly different names, but the same numbers.
-- their defining text is now is now in other documents
id-mime-postscript-body OBJECT IDENTIFIER ::=
{ mixer-bp-data 2}
Alvestrand Exp Nov 96 [Page 49]
draft X.400/MIME body mapping May 96
id-mime-jpeg-body OBJECT IDENTIFIER ::=
{ mixer-bp-data 3}
id-mime-gif-body OBJECT IDENTIFIER ::=
{ mixer-bp-data 4};
-- This is a new definition, and defines an FTAM application reference,
-- not a BP15 data OID.
id-mime-ftbp-data OBJECT IDENTIFIER ::=
{ mixer-bp-data 5 }
Alvestrand Exp Nov 96 [Page 50]
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Appendix C: Registration information for the Teletex
character set
The Teletex character set is a character set in which the
ISO 2022 character set switching mechanism may be used to
switch between the following registered ISO character
sets:
ISO-IR-87 - JIS_C6226-1983; a 16-bit Japanese character set
ISO-IR-102 - a fairly standard US-ASCII variant
ISO-IR-103 - Latin characters using non-spacing accents
ISO-IR-106 - Control characters for C0 use; CR, LF, FF and a few more.
ISO-IR-107 - Control characters for C1 use
Its intended use of this character set is to represent
data that comes from ISO protocols that use the ASN.1
construct "TeletexString" or "T61string" without
conversion.
The set of allowed character sets can be found in CCITT
recommendation X.208(1988), chapter 31.2 and Table
6/X.208.
The rules for encoding the data type can be found in CCITT
recommendation X.209(1988), chapter 23. It states that at
the beginning of the string, G0 is always ISO-IR-102, C0
is ISO-IR-106, and C1 is ISO-IR-107.
The specification seems somehow to have missed the
implicit assumption that ISO-IR-103 is designated and
invoked as G1 and shifted into the upper half of the
character set which seems to be assumed at least by the
X.400 and X.500 software that uses TeletexStrings;
implementors should act as if the sequence ESC 2/9 7/6
LS1R is always present at the beginning of the data.
The rules for interpreting T.61 data are found (I believe)
in CCITT recommendations T.51, T.52 and T.53 (data from
the ITU WWW server):
Alvestrand Exp Nov 96 [Page 51]
draft X.400/MIME body mapping May 96
T.51 (09/92) [Rev.1] [26 pp.] [Publ.: May.93]
Latin based coded character sets for telematic services
T.52 (1993) [New] [88 pp.] [Publ.: Apr.94]
Non-Latin coded character sets for telematic services
T.53 (04/94) [New] [68 pp.] [Publ.: Jan.95]
Character coded control functions for telematic services
The Teletex character set is closely related to (but not
identical with) that specified in ISO 6937.
No further restrictions are imposed by this registration;
in particular, character set switching can occur anywhere,
and there is no guarantee that the character sets will be
switched "back" at the end.
Alvestrand Exp Nov 96 [Page 52]
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Appendix D: IANA Registration form for new mappings
To: IANA@isi.edu
Subject: Registration of new X.400/MIME content type mapping
MIME type name:
(this must have been registered previously with IANA)
X.400 body part:
IF BP15:
- X.400 Object Identifier for Data:
(If left empty, an OID will be assigned by IANA under
mixer-bp-data)
- X.400 Object Identifier for Parameters:
(If left empty, an OID will be assigned by IANA under
mixer-bp-parameter. If it is not used, fill in the
words NOT USED.)
X.400 ASN.1 Syntax:
(must be an EXTENDED-BODY-PART-TYPE macro, or refer
ence to a Basic body part type)
IF FTBP:
- FTAM Object Identifier for application-reference:
- FTAM Object Identifier for contents-type:
(if left empty, unstructured-binary is assumed)
Conversion algorithm:
(must be defined completely enough for independent im
plementation. It may be defined by reference to RFCs).
Person & email address to contact for further informa
tion:
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INFORMATION TO THE SUBMITTER:
The accepted registrations will be listed in the "As
signed Numbers" series of RFCs. The information in
the registration form is freely distributable.
Table of Contents
Status of this Memo ................................ 1
1 Introduction ...................................... 2
1.1 Glossary ........................................ 2
2 Basic rules for body part conversion .............. 3
2.1 Generating the IPM Body from MIME ............... 5
2.2 Generating the MIME Body from the IPMS.Body ..... 5
2.3 Mapping the EMA FTBP parameters ................. 7
2.3.1 Mapping GraphicStrings ........................ 7
2.3.2 Mapping specific parameters ................... 8
2.3.3 Summary of FTBP elements generated ............ 11
2.4 Information that is lost when mapping ........... 12
3 Encapsulation of body parts ....................... 13
3.1 Encapsulation of MIME in X.400 .................. 13
3.1.1 FTBP encapsulating body part .................. 14
3.1.2 BP15 encapsulating body part .................. 15
3.1.3 Encapsulation using IA5 (HARPOON) ............. 17
3.1.4 Content passing using BP14 .................... 18
3.2 Encapsulating X.400 Body Parts in MIME .......... 18
3.3 Encapsulating FTBP body parts in MIME ........... 19
4 User control over the gateway choice .............. 20
4.1 Conversion from MIME to X.400 ................... 21
4.2 Conversion from X.400 to MIME ................... 23
5 The equivalence registry .......................... 24
5.1 What information one must give about a mapping
................................................ 24
5.2 Equivalence summary for known X.400 and MIME
Types .......................................... 25
5.3 MIME to X.400 Table ............................. 26
5.4 X.400 to MIME Table ............................. 27
5.5 Use of OBJECT IDENTIFIERs and ASN.1 MACROS ...... 28
6 Defined Equivalences .............................. 30
6.1 IA5Text - text/plain ............................ 30
6.2 GeneralText - text/plain (ISO-8859) ............. 31
6.3 BilaterallyDefined - application/octet-stream
Alvestrand Exp Nov 96 [Page 54]
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................................................ 33
6.4 FTBP EMA Unknown Attachment - applica
tion/octet-stream .............................. 34
6.5 MessageBodyPart - message/RFC822 ................ 35
6.6 MessageBodyPart - multipart/* ................... 35
6.7 Teletex - Text/Plain (Teletex) .................. 37
7 Body parts where encapsulation is recommended ..... 39
7.1 message/external-body ........................... 39
7.2 message/partial ................................. 40
7.3 multipart/signed ................................ 41
7.4 multipart/encrypted ............................. 42
8 Conformance requirements .......................... 43
9 Security considerations ........................... 44
10 Author's address ................................. 44
11 Acknowledgements ................................. 45
References ......................................... 45
APPENDIXES ......................................... 47
Appendix A: Escape code normalization .............. 47
Appendix B: OID Assignments ........................ 49
Appendix C: Registration information for the
Teletex character set .......................... 51
Appendix D: IANA Registration form for new map
pings .......................................... 53
Table of Contents .................................. 54
Alvestrand Exp Nov 96 [Page 55]
