Request for Comments: 2156 Isode Ltd.
Obsoletes: 987, 1026, 1138, 1148, 1327, 1495 January 1998
Updates: 822
Category: Standards Track
MIXER (Mime Internet X.400 Enhanced Relay):
Mapping between X.400 and RFC822/MIME
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 - Overview ...................................... 3
1.1 - X.400 ......................................... 3
1.2 - RFC822 and MIME .............................. 3
1.3 - The need for conversion ....................... 4
1.4 - General approach .............................. 4
1.5 - Gatewaying Model .............................. 5
1.6 - Support of X.400 (1984) ....................... 8
1.7 - X.400 (1992) .................................. 8
1.8 - MIME .......................................... 8
1.9 - Body Parts .................................... 8
1.10 - Local and Global Scenarios .................... 9
1.11 - Compatibility with previous versions .......... 10
1.12 - Aspects not covered ........................... 10
1.13 - Subsetting .................................... 11
1.14 - Specification Language ........................ 11
1.15 - Related Specifications ........................ 11
1.16 - Document Structure ............................ 12
1.17 - Acknowledgements .............................. 12
2 - Service Elements .............................. 13
2.1 - The Notion of Service Across a Gateway ........ 13
2.2 - RFC822 ....................................... 15
2.3 - X.400 ......................................... 18
3 - Basic Mappings ................................ 27
3.1 - Notation ...................................... 27
3.2 - ASCII and IA5 ................................. 29
3.3 - Standard Types ................................ 29
3.4 - Encoding ASCII in Printable String ............ 33
3.5 - RFC1522 ...................................... 34
4 - Addressing and Message IDs .................... 35
4.1 - A textual representation of MTS.ORAddress ..... 36
4.2 - Global Address Mapping ........................ 43
4.3 - EBNF.822-address <-> MTS.ORAddress ............ 46
4.4 - Repeated Mappings ............................. 59
4.5 - Directory Names ............................... 62
4.6 - MTS Mappings .................................. 62
4.7 - IPMS Mappings ................................. 67
5 - Detailed Mappings ............................. 71
5.1 - RFC822 -> X.400: Detailed Mappings ........... 71
5.2 - Return of Contents ............................ 86
5.3 - X.400 -> RFC822: Detailed Mappings ........... 86
Appendix A - Mappings Specific to SMTP ..................... 114
1 - Probes ........................................ 114
2 - Long Lines .................................... 114
3 - SMTP Extensions ............................... 114
3.1 - SMTP Extension mapping to X.400 ............... 114
3.2 - X.400 Mapping to SMTP Extensions .............. 115
Appendix B - Mapping with X.400(1984) ...................... 116
Appendix C - RFC822 Extensions for X.400 access ........... 118
Appendix D - Object Identifier Assignment .................. 119
Appendix E - BNF Summary ................................... 120
Appendix F - Text format for MCGAM distribution ............ 127
1 - Text Formats .................................. 127
2 - Mechanisms to register and to distribute
MCGAMs ........................................ 127
3 - Syntax Definitions ............................ 128
4 - Table Lookups ................................. 129
5 - Domain -> OR Address MCGAM format ............. 129
6 - OR Address -> Domain MCGAM format ............. 129
7 - Domain -> OR Address of Preferred Gateway
table ......................................... 130
8 - OR Addresss -> domain of Preferred Gateway
table ......................................... 130
Appendix G - Conformance ................................... 131
Appendix H - Change History: RFC987, 1026, 1138, 1148
............................................... 133
1 - Introduction .................................. 133
2 - Service Elements .............................. 133
3 - Basic Mappings ................................ 133
4 - Addressing .................................... 134
5 - Detailed Mappings ............................. 134
6 - Appendices .................................... 134
Appendix I - Change History: RFC1148 to RFC1327 .......... 135
1 - General ....................................... 135
2 - Basic Mappings ................................ 135
3 - Addressing .................................... 135
4 - Detailed Mappings ............................. 135
5 - Appendices .................................... 136
Appendix J - Change History: RFC1327 to this Document
............................................... 137
1 - General ....................................... 137
2 - Service Elements .............................. 137
3 - Basic Mappings ................................ 137
4 - Addressing .................................... 137
5 - Detailed Mappings ............................. 138
6 - Appendices .................................... 138
Appendix L - ASN.1 Summary ................................. 139
Security Considerations .................................... 141
Author's Address ........................................... 141
References ................................................. 141
Full Copyright Statement ................................... 144
Chapter 1 -- Overview
1.1. X.400
This document relates primarily to the ITU-T 1988 and 1992 X.400
Series Recommendations / ISO IEC 10021 International Standard. This
ISO/ITU-T standard is referred to in this document as "X.400", which
is a convenient shorthand. Any reference to the 1984 Recommendations
will be explicit. Any mappings relating to elements which are in the
1992 version and not in the 1988 version will be noted explicitly.
X.400 defines an Interpersonal Messaging System (IPMS), making use of
a store and forward Message Transfer System. This document relates
to the IPMS, and not to wider application of X.400, such as EDI as
defined in X.435.
1.2. RFC822 and MIME
RFC822 evolved as a messaging standard on the DARPA (the US Defense
Advanced Research Projects Agency) Internet. RFC822 specifies an
end to end message format, consisting of a header and an unstructured
text body. MIME (Multipurpose Internet Mail Extensions) specifies a
structured message body format for use with RFC822. The term "RFC
822" is used in this document to refer to the combination of MIME and
RFC822. RFC822 and MIME are used in conjunction with a number of
different message transfer protocol environments. The core of the
MIXER specification is designed to work with any supporting message
transfer protocol.
One transfer protocol, SMTP, is of particular importance and is
covered in MIXER. On the Internet and other TCP/IP networks, RFC822
is used in conjunction with RFC821, also known as Simple Mail
Transfer Protocol (SMTP) [30], in a manner conformant with the host
requirements specification [10]. Use of MIXER with SMTP is defined
in Appendix A.
1.3. The need for conversion
There is a large community using RFC822 based protocols for mail
services, who will wish to communicate with users of the IPMS
provided by X.400 systems. This will also be a requirement in cases
where communities intend to make a transition between the different
technologies, as conversion will be needed to ensure a smooth service
transition. It is expected that there will be more than one gateway,
and this specification will enable them to behave in a consistent
manner. Note that the term gateway is used to describe a component
performing the mapping between RFC822 and X.400. This is standard
usage amongst mail implementors, but differs from that used by
transport and network service implementors.
Consistency between gateways is desirable to provide:
1. Consistent service to users.
2. The best service in cases where a message passes through
multiple gateways.
1.4. General approach
There are a number of basic principles underlying the details of the
specification. These principles are goals, and are not achieved in
all aspects of the specification.
1. The specification should be pragmatic. There should not be
a requirement for complex mappings for "Academic" reasons.
Complex mappings should not be required to support trivial
additional functionality.
2. Subject to 1), functionality across a gateway should be as
high as possible.
3. It is always a bad idea to lose information as a result of
any transformation. Hence, it is a bad idea for a gateway
to discard information in the objects it processes. This
includes requested services which cannot be fully mapped.
4. Mail gateways operate at a level above the layer on which
they perform mappings. This implies that the gateway shall
not only be cognisant of the semantics of objects at the
gateway level, but also be cognisant of higher level
semantics. If meaningful transformation of the objects that
the gateway operates on is to occur, then the gateway needs
to understand more than the objects themselves.
5. Subject to 1), the mapping should be reversible. That is, a
double transformation should bring you back to where you
started.
1.5. Gatewaying Model
1.5.1. X.400
X.400 defines the IPMS Abstract Service in X.420 , [11] which
comprises of three basic services:
1. Origination
2. Reception
3. Management
Management is a local interaction between the user and the IPMS, and
is therefore not relevant to gatewaying. The first two services
consist of operations to originate and receive the following two
objects:
1. IPM (Interpersonal Message). This has two components: a
heading, and a body. The body is structured as a sequence
of body parts, which may be basic components (e.g., IA5
text, or G3 fax), or forwarded Interpersonal Messages. The
heading consists of fields containing end to end user
information, such as subject, primary recipients (To:), and
importance.
2. IPN (Inter Personal Notification). A notification about
receipt of a given IPM at the UA level.
The Origination service also allows for origination of a probe, which
is an object to test whether a given IPM could be correctly received.
The Reception service also allows for receipt of Delivery Reports
(DR), which indicate delivery success or failure.
These IPMS Services utilise the Message Transfer System (MTS)
Abstract Service [12]. The MTS Abstract Service provides the
following three basic services:
1. Submission (used by IPMS Origination)
2. Delivery (used by IPMS Reception)
3. Administration (used by IPMS Management)
Administration is a local issue, and so does not affect this
standard. Submission and delivery relate primarily to the MTS
Message (comprising Envelope and Content), which carries an IPM or
IPN (or other uninterpreted contents). The Envelope includes a
message identifier, an originator, and a list of recipients.
Submission also includes the probe service, which supports the MTS
Probe. Delivery also includes Reports, which indicate whether a given
MTS Message has been delivered or not (or for a probe if delivery
would have happened).
The MTS is provided by MTAs which interact using the MTA (Message
Transfer Agent) Service, which defines the interaction between MTAs,
along with the procedures for distributed operation. This service
provides for transfer of MTS Messages, Probes, and Reports.
1.5.2. RFC822
RFC822 is based on the assumption that there is an underlying
service, which is here called the 822-MTS service. The 822-MTS
service provides three basic functions:
1. Identification of a list of recipients.
2. Identification of an error return address.
3. Transfer of an RFC822 message.
It is possible to achieve 2) within the RFC822 header.
This specification will be used most commonly with SMTP as the 822-
MTS service. The core MIXER specification is written so that it does
not rely on non-basic 822-MTS services. Use of non-basic SMTP
services is described in Appendix A. The core of this document is
written using SMTP terminology for 822-MTS services.
An RFC822 message consists of a header, and content which is
uninterpreted ASCII text. The header is divided into fields, which
are the protocol elements. Most of these fields are analogous to IPM
heading fields, although some are analogous to MTS Service Elements
or MTA Service Elements.
RFC822 supports delivery status notifications by use of the NOTARY
mechanisms [28].
1.5.3. The Gateway
Given this functional description of the two services, the functional
nature of a gateway can now be considered. It would be elegant to
consider the SMTP (822-MTS) service mapping onto the MTS Service
Elements and RFC822 mapping onto an IPM, but there is a not a clear
match between these services. Another elegant approach would be to
treat this document as the definition of an X.400 Access Unit (AU).
In this case, the abstraction level is too high, and some necessary
mapping function is lost. It is necessary to consider that the IPM
format definition, the IPMS Service Elements, the MTS Service
Elements, and MTA Service Elements on one side are mapped into RFC
822 + SMTP on the other in a slightly tangled manner. The details of
the tangle will be made clear in Chapter 5. Access to the MTA
Service Elements is minimised.
The following basic mappings are thus defined. When going from RFC
822 to X.400, an RFC822 message and the associated SMTP information
is always mapped into an IPM (MTA, MTS, and IPMS Services) and a
Delivery Status Notification is mapped onto a Report. Going from
X.400 to RFC822, an RFC822 message and the associated SMTP
information may be derived from:
1. An IPN (MTA, MTS, and IPMS services)
2. An IPM (MTA, MTS, and IPMS services)
A Report (MTA, and MTS Services) is mapped onto a delivery status
notification.
Probes (MTA Service) shall be processed by the gateway, as discussed
in Chapter 5. MTS Messages containing Content Types other than those
defined by the IPMS are not mapped by the gateway, and shall be
rejected at the gateway if no other gatewaying procedure is defined.
This specification is concerned with X.400 IPMS. Future
specifications may defined mappings for other X.400 content types.
1.5.4. Repeated Mappings
The primary goal of this specification is to support single mappings,
so that X.400 and RFC822 users can communicate with maximum
functionality.
The mappings specified here are designed to work where a message
traverses multiple times between X.400 and RFC822. This is often
essential, particularly in the case of distribution lists. However,
in general, this will lead to a level of service which is the lowest
common denominator (approximately the services offered by RFC822).
Some RFC822 networks may wish to use X.400 as an interconnection
mechanism (typically for policy reasons), and this is fully
supported.
Where an X.400 message transfers to RFC822 and then back to X.400,
there is no expectation of X.400 services which do not have an
equivalent service in standard RFC822 being preserved - although
this may be possible in some cases.
1.6. Support of X.400 (1984)
The MIXER definition is based on the initial specification of RFC987
and in its addendum RFC1026, which defined a mapping between
X.400(1984) and RFC822. The core MIXER mapping is defined using the
full 1988 version of X.400, and not to a 1984 compatible subset. New
features of X.400(1988) can be used to provide a much cleaner mapping
than that defined in RFC987. To interwork with 1984 systems,
Appendix B shall be followed.
If a message is being transferred to an X.400(1984) system by way of
X.400(1988) MTA it will give a slightly better service to follow the
rules of Appendix B, than to downgrade without this knowledge.
Downgrading specifications which supplement those specified in X.400
(X.419) are given in RFC1328 [22] and RFC1496 (HARPOON) [5].
1.7. X.400 (1992)
X.400 (1992) features are not used by the core of this mapping, and
so there is not an equivalent downgrade problem.
1.8. MIME
MIME format messages are generated by this mapping. As MIME messages
are fully RFC822 compliant, this will not cause problems with
systems which are not MIME capable.
1.9. Body Parts
MIME and X.400 IPMS can both carry arbitrary body parts. MIME defines
a mechanism for adding new body parts, and new body parts are
registered with the IANA. X.400 defines a mechanism adding new body
parts, usually referred to as Body Part 15. Extensions are defined
by Object Identifiers, so there is no requirement for a central body
part registration authority. The Electronic Messaging Association
(EMA) maintains a list of some commonly used body parts. The EMA has
specified a mechanism to use the File Transfer Body Part (FTBP) as a
more generic means to support message attachments. This approach is
gaining widespread commercial support.
The mapping between X.400 and MIME body parts is defined in the
companion MIXER specification, referenced here as RFC2157 [8]. This
document is an update of RFC1494 [6].
Editor's Note:
References to 2157 will be resolved as these two
documents are expected to progress in parallel.
These two specifications together form the complete MIXER Mapping.
1.10. Local and Global Scenarios
There are two basic scenarios for X.400/MIME interworking:
Global Scenario
There are two global mail networks (Internet/MIME and X.400),
interconnected by multiple gateways. Objects may be transferred
over multiple gateways, and so it is important that gateways
behave in a coherent fashion. MIXER is critical to support this
scenario.
Local Scenario
A gateway is used to connect a closed community to a global mail
network (this could be enforced by connectivity or gateway
authorisation policy). This is a common commercial scenario.
MIXER is useful to support this scenario, as it allows an industry
standard provision of service, but this could be supported by
something which was MIXER-like.
A solution for the global scenario will work for the local scenario.
However, there are aspects of MIXER which have significant
implementation or deployment effort (the global mapping is the major
one, but there are other details too) which and are needed to support
the global scenario, but are not needed in the local scenario.
Note that the local scenario may be the driving force for most
deployments, and support of the global scenario may be an important
secondary goal.
There is also a transition effect. Gateways which are initially
deployed in a strict local scenario situation start to find
themselves in a global scenario. A common case is ADMD provided
gateways, which are targeted strictly at the local scenario. In
practice they soon start to operate in the global scenario, because
of distribution lists and messages exchanged with X.400 users that
are not customers of the ADMD. At this point, users are hurt by the
restrictions of a local scenario gateway.
Note that conformance to MIXER applies to an instantiation of a
gateway, not just an implementation (although clearly it is critical
that the implementation is capable of being operated in a conformant
manner).
MIXER's conformance target is the global scenario, and the
specification of MIXER defines operation in this way.
1.11. Compatibility with previous versions
The changes between this and older versions of the document are given
in Appendices H, I and J. These are RFCs 987, 1026, 1138, 1148 and
1327. This document is a revision of RFC1327 [21]. As far as
possible, changes have been made in a compatible fashion.
1.12. Aspects not covered
There have been a number of cases where previous versions of this
document were used in a manner which was not intended. This section
is to make clear some limitations of scope. In particular, this
specification does not specify:
- Extensions of RFC822 to provide access to all X.400
services
- X.400 user interface definition
These are really coupled. To map the X.400 services, this
specification defines a number of extensions to RFC822. As a side
effect, these give the 822 user access to SOME X.400 services.
However, the aim on the RFC822 side is to preserve current service,
and it is intentional that access is not given to all X.400 services.
Thus, it will be a poor choice for X.400 implementors to use MIXER as
an interface - there are too many aspects of X.400 which cannot be
accessed through it. If a text interface is desired, a specification
targeted at X.400, without RFC822 restrictions, would be more
appropriate. Some optional and limited extensions in this area have
proved useful, and are defined in Appendix C.
1.13. Subsetting
This proposal specifies a mapping which is appropriate to preserve
services in existing RFC822 communities. Implementations and
specifications which subset this specification are non-conformant and
strongly discouraged.
1.14. Specification Language
ISO and Internet standards have clear definitions as to the style of
language used. This specification maps between ISO/ITU-T protocol
and Internet protocols. This document uses ISO terminology for the
following reasons:
1. This was done in previous versions.
2. ISO language may be mechanically converted to Internet
language, but not vice versa.
The key elements of the ISO rules are:
1. All mandatory features shall clearly be indicated by
imperative statements or the word "shall" or "shall not".
2. Optional features shall be indicated by the word "may".
3. The word "should" and the phrase "may not" shall not be
used.
In some cases the specification issues guidance on use of optional
features, by use of the the phrase word "recommended" or "not
recommended".
To interpet this document according to Internet rules, replace every
occurrence of "shall" with "must".
1.15. Related Specifications
Mappings between Mail-11 and X.400 and Mail-11 and RFC822 are
described in RFC2162, using mappings related to those defined here
[2].
1.16. Document Structure
This document has five chapters:
1. Overview - this chapter.
2. Service Elements - This describes the (end user) services
mapped by a gateway.
3. Basic mappings - This describes some basic notation used in
Chapters 3-5, the mappings between character sets, and some
fundamental protocol elements.
4. Addressing - This considers the mapping between X.400 OR
names and RFC822 addresses, which is a fundamental gateway
component.
5. Detailed Mappings - This describes the details of all other
mappings.
There are also ten appendices.
WARNING:
THE REMAINDER OF THIS SPECIFICATION IS TECHNICALLY DETAILED. IT
WILL NOT MAKE SENSE, EXCEPT IN THE CONTEXT OF RFC822 AND X.400
(1988). DO NOT ATTEMPT TO READ THIS DOCUMENT UNLESS YOU ARE
FAMILIAR WITH THESE SPECIFICATIONS.
1.17. Acknowledgements
The work in this specification was substantially based on RFC987 and
RFC1148, which had input from many people, who are credited in the
respective documents.
A number of comments from people on RFC1148 lead to RFC1327. In
particular, there were comments and suggestions from: Maurice Abraham
(HP); Harald Alvestrand (Sintef); Peter Cowen (X-Tel); Jim Craigie
(JNT); Ella Gardner (MITRE); Christian Huitema (Inria); Erik Huizer
(SURFnet); Neil Jones (DEC); Ignacio Martinez (IRIS); Julian Onions
(X-Tel); Simon Poole (SWITCH); Clive Roberts (Data General); Pete
Vanderbilt (SUN); Alan Young (Concurrent).
RFC1327 has been widely adopted, and a review team was formed. This
comprised of: Urs Eppenberger (SWITCH)(Chair); Claudio Allocchio
(INFN); Harald Alvestrand (UNINETT); Dave Crocker (Brandenburg); Ned
Freed (Innosoft); Erik Huizer (SURFnet); Steve Kille (Isode); Peter
Sylvester (GC Tech).
Harald Alvestrand also supplied the tables mapping DSN status codes
with X.400 codes. Ned Freed defined parts of the File Transfer Body
Part mapping.
Comment and input has also been received from: Bengt Ackzell (Generic
Systems); Samir Albadine (Transpac); Mark Boyes (DEC); Larry Campbell
(Boston Software Works); Jacqui Caren (Cray); Allan Cargille (MCI);
Kevin Carrosso (Innosoft); Charlie Combs (OIW); Jim Craigie (Net-
Tel); Eamon Doyle (Isocor); Efifion Edem (SITA); Jyrki Heikkinen
(ICL); Edward Hibbert (DCL); Jeroun Houttin (Terena); Kevin Jordan
(CDS); Paul Kingsnorth (DEC); Carl-Uno Manros (Manros Consulting);
Suzan Mendes (Telis); Robert Miles (Softswitch); Roger Mizumorri
(Enterprise Solutions Ltd); Keith Moore (University of Tennessee);
Ruth Moulton (Net-Tel) Michel Musy (Bull); Kenji Nonaka (NTT): The
OIW MHSIG; Tom Oliphant (SWITCH); Julian Onions (NEXOR); Jacob Palme
(KTH); Olivier Paridaens (ULB); Mary la Roche (Citicorp); John
Setsaas (Maxware); Russell Sharpe (DCL); Patrick Soulier (CCETT);
Eftimios Tsigros (Universite Libre de Bruxelles); Sean Turner (IECA);
Mark Wahl (Isode); David Wilson (Isode); Bill Wohler (Worldtalk);
Alan Young (Isode); Alain Zahm (Telis).
Chapter 2 - Service Elements
This chapter considers the services offered across a gateway built
according to this specification. It gives a view of the
functionality provided by such a gateway for communication with users
in the opposite domain. This chapter considers service mappings in
the context of SINGLE transfers only, and not repeated mappings
through multiple gateways.
2.1. The Notion of Service Across a Gateway
RFC822 and X.400 provide a number of services to the end user. This
chapter describes the extent to which each service can be supported
across an X.400 <-> RFC822 gateway. The cases considered are single
transfers across such a gateway, although the problems of multiple
crossings are noted where appropriate.
2.1.1. Origination of Messages
When a user originates a message, a number of services are available.
Some of these imply actions (e.g., delivery to a recipient), and some
are insertion of known data (e.g., specification of a subject field).
This chapter describes, for each offered service, to what extent it
is supported for a recipient accessed through a gateway. There are
three levels of support:
Supported
The corresponding protocol elements map well, and so the service
can be fully provided.
Not Supported
The service cannot be provided, as there is a complete mismatch.
Partial Support
The service can be partially fulfilled.
In the first two cases, the service is simply marked as "Supported"
or "Not Supported". Some explanation may be given if there are
additional implications, or the (non) support is not intuitive. For
partial support, the level of partial support is summarised. Where
partial support is good, this will be described by a phrase such as
"Supported by use of.....". A common case of this is where the
service is mapped onto a non-standard service on the other side of
the gateway, and this would have lead to support if it had been a
standard service. In many cases, this is equivalent to support. For
partial support, an indication of the mechanism is given, in order to
give a feel for the level of support provided. Note that this is not
a replacement for Chapter 5, where the mapping is fully specified.
If a service is described as supported, this implies:
- Semantic correspondence.
- No (significant) loss of information.
- Any actions required by the service element.
An example of a service gaining full support: If an RFC822
originator specifies a Subject: field, this is considered to be
supported, as an X.400 recipient will get a subject indication.
In many cases, the required action will simply be to make the
information available to the end user. In other cases, actions may
imply generating a delivery report.
All RFC822 services are supported or partially supported for
origination. The implications of non-supported X.400 services is
described under X.400.
2.1.2. Reception of Messages
For reception, the list of service elements required to support this
mapping is specified. This is really an indication of what a
recipient might expect to see in a message which has been remotely
originated.
2.2. RFC822
RFC822 does not explicitly define service elements, as distinct from
protocol elements. However, all of the RFC822 header fields, with
the exception of trace, can be regarded as corresponding to implicit
RFC822 service elements.
2.2.1. Origination in RFC822
A mechanism of mapping, used in several cases, is to map the RFC822
header into a heading extension in the IPM (InterPersonal Message).
This can be regarded as partial support, as it makes the information
available to any X.400 implementations which are interested in these
services. Communities which require significant RFC822 interworking
are recommended to require that their X.400 User Agents are able to
display these heading extensions. Support for the various service
elements (headers) is now listed.
Date:
Supported.
From:
Supported. For messages where there is also a sender field,
the mapping is to "Authorising Users Indication", which has
subtly different semantics to the general RFC822 usage of
From:.
Sender: Supported.
Reply-To: Supported.
To: Supported.
Cc: Supported.
Bcc: Supported.
Message-Id: Supported.
In-Reply-To:
Supported, for a single reference. Where multiple references are
given, partial support is given by mapping to "Cross Referencing
Indication". This gives similar semantics.
References: Supported.
Keywords: Supported by use of a heading extension.
Subject: Supported.
Comments: Supported by use of a heading extension.
Encrypted: Supported by use of a heading extension.
Content-Language: Supported.
Resent-*
Supported by use of a heading extension. Note that addresses in
these fields are mapped onto text, and so are not accessible to
the X.400 user as addresses. In principle, fuller support would
be possible by mapping onto a forwarded IP Message, but this is
not suggested.
Other Fields
In particular X-* fields, and "illegal" fields in common usage
(e.g., "Fruit-of-the-day:") are supported by use of heading
extensions.
MIME introduces a number of headings. Support is defined in RFC
2157.
2.2.2. Reception by RFC822
This considers reception by an RFC822 User Agent of a message
originated in an X.400 system and transferred across a gateway. The
following standard services (headers) may be present in such a
message:
Date:
From:
Sender:
Reply-To:
To:
Cc:
Bcc:
Message-Id:
In-Reply-To:
References:
Subject:
Content-Type: (See RFC2157)
Content-Transfer-Encoding: (See RFC2157)
MIME-Version: (See RFC2157)
The following services (headers) may be present in the header of a
message. These are defined in more detail in Chapter 5 (5.3.4, 5.3.6,
5.3.7):
Autoforwarded:
Autosubmitted:
X400-Content-Identifier:
Content-Language:
Conversion:
Conversion-With-Loss:
Delivery-Date:
Discarded-X400-IPMS-Extensions:
Discarded-X400-MTS-Extensions:
DL-Expansion-History:
Deferred-Delivery:
Expires:
Importance:
Incomplete-Copy:
Latest-Delivery-Time:
Message-Type:
Original-Encoded-Information-Types:
Originator-Return-Address:
Priority:
Reply-By:
Sensitivity:
Supersedes:
X400-Content-Type:
X400-MTS-Identifier:
X400-Originator:
X400-Received:
X400-Recipients:
2.3. X.400
2.3.1. Origination in X.400
When mapping services from X.400 to RFC822 which are not supported
by RFC822, new RFC822 headers are defined, and registered by
publication in this standard. It is intended that co-operating RFC
822 systems may also use them. Where these new fields are used, and
no system action is implied, the service can be regarded as being
partially supported. Chapter 5 describes how to map X.400 services
onto these new headers. Other elements are provided, in part, by the
gateway as they cannot be provided by RFC822.
Some service elements are marked N/A (not applicable). There are
five cases, which are marked with different comments:
N/A (local)
These elements are only applicable to User Agent / Message
Transfer Agent interaction and so they cannot apply to RFC822
recipients.
N/A (PDAU)
These service elements are only applicable where the recipient is
reached by use of a Physical Delivery Access Unit (PDAU), and so
do not need to be mapped by the gateway.
N/A (reception)
These services are only applicable for reception.
N/A (prior)
If requested, this service shall be performed prior to the
gateway.
N/A (MS)
These services are only applicable to Message Store (i.e., a local
service).
Finally, some service elements are not supported. In particular, the
new security services are not mapped onto RFC822. Unless otherwise
indicated, the behaviour of service elements marked as not supported
will depend on the criticality marking supplied by the user. If the
element is marked as critical for transfer or delivery, a non-
delivery notification will be generated. Otherwise, the service
request will be ignored.
2.3.1.1. Basic Interpersonal Messaging Service
These are the mandatory IPM services as listed in Section 19.8 of
X.400 / ISO/IEC 10021-1, listed here in the order given. Section 19.8
has cross references to short definitions of each service.
Access management
N/A (local).
Content Type Indication
Supported by a new RFC822 header (X400-Content-Type:).
Converted Indication
Supported by a new RFC822 header (X400-Received:).
Delivery Time Stamp Indication
N/A (reception).
IP Message Identification
Supported.
Message Identification
Supported, by use of a new RFC822 header (X400-MTS-Identifier).
This new header is required, as X.400 has two message-ids whereas
RFC822 has only one (see IP Message Identification
Non-delivery Notification
Not supported in all cases. Supported where the recipient system
supports NOTARY DSNs. In general all RFC822 systems will return
error reports by use of IP messages. In other service elements,
this pragmatic result can be treated as effective support of this
service element.
Original Encoded Information Types Indication
Supported as a new RFC822 header (Original-Encoded-Information-
Types:).
Submission Time Stamp Indication
Supported.
Typed Body
Support is defined in RFC2157.
User Capabilities Registration
N/A (local).
2.3.1.2. IPM Service Optional User Facilities
This section describes support for the optional (user selectable) IPM
services as listed in Section 19.9 of X.400 / ISO/IEC 10021- 1,
listed here in the order given. Section 19.9 has cross references to
short definitions of each service.
Additional Physical Rendition
N/A (PDAU).
Alternate Recipient Allowed
Not supported. There is no RFC822 service equivalent to
prohibition of alternate recipient assignment (e.g., an RFC822
system may freely send an undeliverable message to a local
postmaster). A MIXER gateway has two conformant options. The
first is not to gateway a message requesting prohibition of
alternate recipient, as this control cannot be guaranteed. This
option supports the service, but may cause unacceptable level of
message rejections. The second is to gateway the message on the
basis that there is no alternate recipient service in RFC822. RFC
1327 allowed only the second option. If the first option is
shown to be operationally effective, it may be the only option in
future versions of MIXER.
Authorising User's Indication
Supported.
Auto-forwarded Indication
Supported as new RFC822 header (Auto-Forwarded:).
Basic Physical Rendition
N/A (PDAU).
Blind Copy Recipient Indication
Supported.
Body Part Encryption Indication
Supported by use of a new RFC822 header (Original-Encoded-
Information-Types:), although in most cases it will not be
possible to map the body part in question.
Content Confidentiality
Not supported.
Content Integrity
Not supported.
Conversion Prohibition
Supported. Operation defined in RFC2157.
Conversion Prohibition in Case of Loss of Information
Supported. Operation defined in RFC2157.
Counter Collection
N/A (PDAU).
Counter Collection with Advice
N/A (PDAU).
Cross Referencing Indication
Supported.
Deferred Delivery
N/A (prior). This service shall always be provided by the MTS
prior to the gateway. A new RFC822 header (Deferred-Delivery:)
is provided to transfer information on this service to the
recipient.
Deferred Delivery Cancellation
N/A (local).
Delivery Notification
Supported. This is performed at the gateway, but may be performed
at the end system if the end system supports NOTARY. Thus, a
notification is sent by the gateway to the originator.
Delivery via Bureaufax Service
N/A (PDAU).
Designation of Recipient by Directory Name
N/A (local).
Disclosure of Other Recipients
Supported by use of a new RFC822 header (X400-Recipients:). This
is descriptive information for the RFC822 recipient, and is not
reverse mappable.
DL Expansion History Indication
Supported by use of a new RFC822 header (DL-Expansion-History:).
DL Expansion Prohibited
Distribution List means MTS supported distribution list, in the
manner of X.400. This service does not exist in the RFC822
world, although RFC822 supports distribution list functionality.
There is no SMTP leve control to prohibit distribution list
expansion. A MIXER gateway has two conformant options. The
first is not to gateway a message requesting DL expansion
prohibition, as this control cannot be guaranteed. This option
supports the service, but may cause unacceptable level of message
rejections. The second is to gateway the message on the basis that
there is no distribution list service in RFC822. RFC1327 allowed
only the second option. If the first option is shown to be
operationally effective, it may be the only option in future
versions of MIXER.
Express Mail Service
N/A (PDAU).
Expiry Date Indication
Supported as new RFC822 header (Expires:). In general, no
automatic action can be expected.
Explicit Conversion
N/A (prior).
Forwarded IP Message Indication
Supported.
Grade of Delivery Selection
Not Supported. There is no equivalent service in RFC822.
Importance Indication
Supported as new RFC822 header (Importance:).
Incomplete Copy Indication
Supported as new RFC822 header (Incomplete-Copy:).
Language Indication
Supported as new RFC822 header (Content-Language:).