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RFC2157 - Mapping between X.400 and RFC-822/MIME Message Bodies

王朝other·作者佚名  2008-05-31
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Network Working Group H. Alvestrand

Request for Comments: 2157 UNINETT

Category: Standards Track January 1998

Mapping between X.400 and RFC-822/MIME Message Bodies

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

Copyright Notice

Copyright (C) The Internet Society (1998). All Rights Reserved.

Table of Contents

1 IntrodUCtion ........................................... 2

1.1 Glossary ............................................. 3

2 Basic rules for body part conversion ................... 4

2.1 Generating the IPM Body from MIME .................... 5

2.2 Generating the MIME Body from the IPMS.Body .......... 6

2.3 Mapping the EMA FTBP parameters ...................... 7

2.3.1 Mapping GraphicStrings ............................. 7

2.3.2 Mapping specific parameters ........................ 7

2.3.3 Summary of FTBP elements generated ................. 10

2.4 Information that is lost when mapping ................ 11

3 Encapsulation of body parts ............................ 11

3.1 Encapsulation of MIME in X.400 ....................... 12

3.1.1 FTBP encapsulating body part ....................... 12

3.1.2 BP15 encapsulating body part ....................... 13

3.1.3 Encapsulation using IA5 (HARPOON) .................. 15

3.1.4 Content passing using BP14 ......................... 16

3.2 Encapsulating X.400 Body Parts in MIME ............... 16

3.3 Encapsulating FTBP body parts in MIME ................ 17

4 User control over the gateway choice ................... 18

4.1 Conversion from MIME to X.400 ........................ 18

4.2 Conversion from X.400 to MIME ........................ 20

5 The equivalence registry ............................... 21

5.1 What information one must give about a mapping

..................................................... 21

5.2 Equivalence summary for known X.400 and MIME

Types ................................................ 22

5.3 MIME to X.400 Table .................................. 23

5.4 X.400 to MIME Table .................................. 23

5.5 Use of OBJECT IDENTIFIERs and ASN.1 MACROS ........... 24

6 Defined Equivalences ................................... 26

6.1 IA5Text - text/plain ................................. 26

6.2 GeneralText - text/plain (ISO-8859) .................. 27

6.3 BilaterallyDefined - application/octet-stream

...................................................... 29

6.4 FTBP EMA Unknown Attachment -

application/octet-stream ............................. 29

6.5 MessageBodyPart - message/RFC822 ..................... 30

6.6 MessageBodyPart - multipart/* ........................ 31

6.7 Teletex - Text/Plain (Teletex) ....................... 32

7 Body parts where encapsulation is recommended .......... 33

7.1 message/external-body ................................ 34

7.2 message/partial ...................................... 35

7.3 multipart/signed ..................................... 35

7.4 multipart/encrypted .................................. 36

8 Conformance requirements ............................... 37

9 Security Considerations ................................ 38

10 Author's Address ...................................... 38

11 Acknowledgements ...................................... 38

References .............................................. 38

APPENDIXES .............................................. 41

Appendix A: Escape code normalization ................... 41

Appendix B: OID Assignments ............................. 44

Appendix C: Registration information for the

Teletex character set ............................... 46

Appendix D: IANA Registration form for new

mappings ................................................ 48

Full Copyright Statement ................................. 49

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.

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 (RFC2046) 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 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.

The Words MUST, SHOULD and MAY, when capitalized, are used as defined

in RFC2119 [MUST].

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 both the MIME and the X.400 worlds seem to be in a

stable state of flux with regards to carrying around stuff that is

not text. 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.

(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

described in chapter 3. The choice may be

configurable, 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 RFC822 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:

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 RFC822

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 RFC822.

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

RFC822 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 the default MIME encoding (7bit) is

used, unless very long lines or non-ASCII or

control characters are found in the body part, in

which case Quoted-Printable SHOULD be 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, as described above.

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 RFC2047 [MIME-HDR] 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 (?).

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.

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.

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.

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 [CDISP]

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 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

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.

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.

This externally-defined 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 RFC1494

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.

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".

Note that this mapping does not work with multipart types; the

multipart must first be mapped to a ForwardedIPMessage.

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.

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 RFC1494.

When converting back to a MIME body part, the gateway is responsible

for:

(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 RFC1496 - 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>

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.

This encapsulation may also be applied to subtypes of multipart,

creating a single IA5 body part that contains a single multipart/*,

which in turn may contain multiple MIME body parts.

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 this

document. The content-type field is "application/x400-bp". The

parameter is defined by the EBNF:

mime-parameter = "bp-type=" ( object-identifier / 1*DIGIT=

If the body is a basic body part, the bp-type parameter is set to the

number of the body part's context-specific tag, that is, the tag of

the IPMS.Body.BodyPart component.

If the body is an Extended Body Part, the EBNF.object-dentifier is

set to the OBJECT IDENTIFIER from IPMS.body.externally-

defined.data.direct-reference.

For example, a basic VideotexBodyPart will have

Content-type=application/x400-bp; bp-type=6

whilst a Extended Videotex body part will have

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.

For extended body parts, the

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:

- Creating the "content-type" value by forming the

string "application/x-ftbp." and appending the

numbers of the OID found in

FileTransferParameters.environment.application-

reference.registered-identifier

- Mapping all other parameters according to the

standard FTBP parameter mapping

- Applying an appropriate content-transfer-encoding to

the data contained in FileTransferData.value.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 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 RFC822

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

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.

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:

(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

<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, and there is no

requirement that all conversions supported by a gateway are

registered with IANA. Specific conformance requirements for MIXER are

given at the end of this document.

Anyone can register an equivalence with IANA, and may update the

registered equivalence at any time, or reassign the right to update

the registry entry at any time. However, the IESG has the power to

"lock" a registration, so that changing it requires IESG approval,

and to update such a "locked" registration. All registered

equivalences defined in standards-track documents (including this

one) are locked.

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 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

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

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.

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 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 RFC1428 [MIMETRANS],

"Transition of Internet Mail from just-send-8 to 8-bit SMTP/MIME",

and generate appropriate charset parameters for the MIME messages

they generate. 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

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 numbers in the above table, and then appending each to the

id-cs-eit-authority {1 0 10021 7 1 0} OID, generating 2-4 OIDs.

Similar procedures can be used with other MIME charsets that map to a

set of ISO character sets.

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 lossless changes, such as normalizing escape sequences, can be

done even 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 RFC1468 [2022-JP], 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 or rewrite

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 advisory; name and

conversions are depreciated in RFC2046.

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

RFC2046 recommends not using the "name" parameter.

The "type", "conversions" and "padding" attributes are ignored;

"type" is for human consumption; "conversions" are discouraged in RFC

2046.

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 -> RFC822 mapping is

recursively applied, to generate an RFC822 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 RFC822 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.

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-v2

MultipartType ::= SEQUENCE {

subtype IA5String,

isAMessage BOOLEAN DEFAULT TRUE }

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.

NOTE:

This definition is different from the one in RFC1494, because the

RFC1494 definition turned out to be insufficient when new

subtypes of Multipart (like Signed or Related) were defined. That

is the reason for the "-v2" part of the name of the OID.

If both the old and the new heading extensions occur on a message,

a MIXER gateway should give preference to the new one.

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

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.

Note that the definition of Text/Plain permits only CRLF as a line

separator; the sequences "CR FF" and "CR LF LF LF.." permitted in

Teletex must be encoded as Quoted-Printable.

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.

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).

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.

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

- 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-ftbp.oid 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 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

EMail: Harald.T.Alvestrand@uninett.no

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, Efifiom Edem, David Wilson, and many others who have been

active over the long lifetime of this document.

References

[RFC-822]

Crocker, D., "Standard for the Format of ARPA Internet Text

Messages", STD 11, RFC822, August, 1982.

[MIME]

Freed, N. and N. Borenstein, "Multipurpose Internet Mail

Extensions (MIME) Part Two: Media Types", RFC2046, November

1996.

[MIME-HDR]

Moore, K., "MIME (Multipurpose Internet Mail Extensions) Part

Three: Message Header Extensions for Non-ASCII Text", RFC2047,

November 1996.

[HARPOON]

Alvestrand, H., Romaguera, J., and K. Jordan, "Rules for

downgrading messages from X.400/88 to X.400/84 when MIME

content-types are present in the messages", RFC1496, August

1993.

[MIMETRANS]

Vaudreuil, G., "Transition of Internet Mail from Just-Send-8 to

8Bit-SMTP/MIME", RFC1428, February 1993.

[MIXER]

Kille, S., "Mapping between X.400(1988) / ISO 10021 and RFC-822",

RFC1327, May 1992.

[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

[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]

Alvestrand, H., "X.400 use of extended Character Sets", RFC1502,

August 1993.

[MAWG]

Electronic Messaging Association Message Attachment Working Group

(MAWG): File Transfer Body Part Feasibility Project Guide -

version 1.5 - September 1995

[CDISP]

Troost, R., and S. Dorner, "Communicating Presentation Information

in Internet Messages: The Content-Disposition Header", RFC1806,

June 1995.

[POSTSCRIPT]

Alvestrand, H., "Carrying PostScript in X.400 and MIME", RFC2160,

June 1997.

[IMAGES]

Alvestrand, H., "X.400 Image Body Parts", RFC2158, June 1997.

[ODA]

Alvestrand, H., "A MIME Body Part for ODA", RFC2161, June 1997.

[ISO 2022]

ISO/IEC 2022:1994(E): Information technology - Character code

structure and extension techniques

[ISO 8859]

ISO 8859: Information processing -- 8-bit single-byte coded

graphic character sets (various parts)

[2022-JP]

Murai, J., Crispin, M., and E. van der Poel, "Japanese Character

Encoding for Internet Messages", RFC1468, June 1993.

[MUST]

Bradner, S., "Key words for use in RFCs to Indicate Requirement

Levels", RFC2119, March 1997.

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;

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)

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

mixer -- { iso(1) org(3) dod(6) internet(1) mail(7) mixer(1) }

FROM MIXER --Companion RFC--;

mixer-headings OBJECT IDENTIFIER ::=

{ mixer 1 } -- called mime-mhs-headings in RFC1495 --

mixer-bodies OBJECT IDENTIFIER ::=

{ mixer 2 } -- called mime-mhs-bodies in RFC1495 --

-- mixer-core is defined as { mixer core(3) } in [MIXER]

mixer-bp-data OBJECT IDENTIFIER ::=

{ mixer-bodies 1 }; -- called mime-mhs-bp-data in RFC1494 --

mixer-bp-parameter OBJECT IDENTIFIER ::=

{ mixer-bodies 2 };

id-mime-bp-data OBJECT IDENTIFIER ::=

{ mixer-bp-data 1 };

-- for debugging: mixer-bp-data is 1.3.6.1.7.1.2.1.1

id-mime-bp-parameters OBJECT IDENTIFIER ::=

{ mixer-bp-parameter 1 };

-- the following assignments were done in RFC1494, 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 }

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 }

-- The following heading extensions are defined

id-hex-partial-message OBJECT IDENTIFIER ::=

{ mixer-headings 1 }

id-hex-multipart-message OBJECT IDENTIFIER ::=

{ mixer-headings 2 } -- from RFC1495; obsolete

id-hex-multipart-message-v2 OBJECT IDENTIFIER ::=

{ mixer-headings 3 }

END

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; however,

when generating Teletex strings, implementors should include the

sequence ESC 2/9 7/6 within the string before the first occurence of

a character from ISO-IR-103.

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):

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.

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 reference 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 implementation. It

may be defined by reference to RFCs).

Person & email address to contact for further information:

INFORMATION TO THE SUBMITTER:

The accepted registrations will be listed in the "Assigned Numbers"

series of RFCs. The information in the registration form is freely

distributable.

Full Copyright Statement

Copyright (C) The Internet Society (1998). All Rights Reserved.

This document and translations of it may be copied and furnished to

others, and derivative works that comment on or otherwise explain it

or assist in its implementation may be prepared, copied, published

and distributed, in whole or in part, without restriction of any

kind, provided that the above copyright notice and this paragraph are

included on all such copies and derivative works. However, this

document itself may not be modified in any way, such as by removing

the copyright notice or references to the Internet Society or other

Internet organizations, except as needed for the purpose of

developing Internet standards in which case the procedures for

copyrights defined in the Internet Standards process must be

followed, or as required to translate it into languages other than

English.

The limited permissions granted above are perpetual and will not be

revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an

"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING

TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION

HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF

MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

 
 
 
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