Network Working Group S. Kille
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 RFC 822/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 - RFC 822 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 - RFC 822 ....................................... 15
2.3 - X.400 ......................................... 18
3 - Basic Mappings ................................ 27
3.1 - Notation ...................................... 27
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3.2 - ASCII and IA5 ................................. 29
3.3 - Standard Types ................................ 29
3.4 - Encoding ASCII in Printable String ............ 33
3.5 - RFC 1522 ...................................... 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 - RFC 822 -> X.400: Detailed Mappings ........... 71
5.2 - Return of Contents ............................ 86
5.3 - X.400 -> RFC 822: 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 - RFC 822 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: RFC 987, 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: RFC 1148 to RFC 1327 .......... 135
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1 - General ....................................... 135
2 - Basic Mappings ................................ 135
3 - Addressing .................................... 135
4 - Detailed Mappings ............................. 135
5 - Appendices .................................... 136
Appendix J - Change History: RFC 1327 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. RFC 822 and MIME
RFC 822 evolved as a messaging standard on the DARPA (the US Defense
Advanced Research Projects Agency) Internet. RFC 822 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 RFC 822. The term "RFC
822" is used in this document to refer to the combination of MIME and
RFC 822. RFC 822 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.
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One transfer protocol, SMTP, is of particular importance and is
covered in MIXER. On the Internet and other TCP/IP networks, RFC 822
is used in conjunction with RFC 821, 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 RFC 822 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 RFC 822 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.
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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.
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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. RFC 822
RFC 822 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 RFC 822 message.
It is possible to achieve 2) within the RFC 822 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 RFC 822 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
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heading fields, although some are analogous to MTS Service Elements
or MTA Service Elements.
RFC 822 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 RFC 822 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 RFC 822 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 RFC 822, an RFC 822 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.
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1.5.4. Repeated Mappings
The primary goal of this specification is to support single mappings,
so that X.400 and RFC 822 users can communicate with maximum
functionality.
The mappings specified here are designed to work where a message
traverses multiple times between X.400 and RFC 822. 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 RFC 822).
Some RFC 822 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 RFC 822 and then back to X.400,
there is no expectation of X.400 services which do not have an
equivalent service in standard RFC 822 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 RFC 987
and in its addendum RFC 1026, which defined a mapping between
X.400(1984) and RFC 822. 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 RFC 987. 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 RFC 1328 [22] and RFC 1496 (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 RFC 822 compliant, this will not cause problems with
systems which are not MIME capable.
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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 RFC 2157 [8]. This
document is an update of RFC 1494 [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
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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 RFC 1327 [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 RFC 822 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 RFC 822. As a side
effect, these give the 822 user access to SOME X.400 services.
However, the aim on the RFC 822 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
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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 RFC 822 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 RFC 822 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 RFC 822 are
described in RFC 2162, using mappings related to those defined here
[2].
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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 RFC 822 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 RFC 822 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 RFC 987 and
RFC 1148, which had input from many people, who are credited in the
respective documents.
A number of comments from people on RFC 1148 lead to RFC 1327. 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).
RFC 1327 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).
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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
RFC 822 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 <-> RFC 822 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:
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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 RFC 822
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 RFC 822 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
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originated.
2.2. RFC 822
RFC 822 does not explicitly define service elements, as distinct from
protocol elements. However, all of the RFC 822 header fields, with
the exception of trace, can be regarded as corresponding to implicit
RFC 822 service elements.
2.2.1. Origination in RFC 822
A mechanism of mapping, used in several cases, is to map the RFC 822
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 RFC 822 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 RFC 822 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.
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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 RFC 822
This considers reception by an RFC 822 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:
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Message-Id:
In-Reply-To:
References:
Subject:
Content-Type: (See RFC 2157)
Content-Transfer-Encoding: (See RFC 2157)
MIME-Version: (See RFC 2157)
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:
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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 RFC 822 which are not supported
by RFC 822, new RFC 822 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 RFC 822.
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 RFC 822
recipients.
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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 RFC 822. 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 RFC 822 header (X400-Content-Type:).
Converted Indication
Supported by a new RFC 822 header (X400-Received:).
Delivery Time Stamp Indication
N/A (reception).
IP Message Identification
Supported.
Message Identification
Supported, by use of a new RFC 822 header (X400-MTS-Identifier).
This new header is required, as X.400 has two message-ids whereas
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RFC 822 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 RFC 822 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 RFC 822 header (Original-Encoded-Information-
Types:).
Submission Time Stamp Indication
Supported.
Typed Body
Support is defined in RFC 2157.
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 RFC 822 service equivalent to
prohibition of alternate recipient assignment (e.g., an RFC 822
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 RFC 822. 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.
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Auto-forwarded Indication
Supported as new RFC 822 header (Auto-Forwarded:).
Basic Physical Rendition
N/A (PDAU).
Blind Copy Recipient Indication
Supported.
Body Part Encryption Indication
Supported by use of a new RFC 822 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 RFC 2157.
Conversion Prohibition in Case of Loss of Information
Supported. Operation defined in RFC 2157.
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 RFC 822 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.
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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 RFC 822 header (X400-Recipients:). This
is descriptive information for the RFC 822 recipient, and is not
reverse mappable.
DL Expansion History Indication
Supported by use of a new RFC 822 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 RFC 822
world, although RFC 822 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 RFC 822. 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.
Express Mail Service
N/A (PDAU).
Expiry Date Indication
Supported as new RFC 822 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 RFC 822.
Importance Indication
Supported as new RFC 822 header (Importance:).
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Incomplete Copy Indication
Supported as new RFC 822 header (Incomplete-Copy:).
Language Indication
Supported as new RFC 822 header (Content-Language:).
Latest Delivery Designation
Not supported. A new RFC 822 header (Latest-Delivery-Time:) is
provided, which may be used by the recipient for general
information, but will not be acted on by the SMTP infrastrucuture.
Message Flow Confidentiality
Not supported.
Message Origin Authentication
N/A (reception).
Message Security Labelling
Not supported.
Message Sequence Integrity
Not supported.
Multi-Destination Delivery Supported.
Multi-part Body
Supported.
Non Receipt Notification Request
Not supported.
Non Repudiation of Delivery
Not supported.
Non Repudiation of Origin
N/A (reception).
Non Repudiation of Submission
N/A (local).
Obsoleting Indication
Supported as new RFC 822 header (Supersedes:).
Ordinary Mail
N/A (PDAU).
Originator Indication
Supported.
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Originator Requested Alternate Recipient
Not supported, but is placed as comment next to address (X400-
Recipients:).
Physical Delivery Notification by MHS
N/A (PDAU).
Physical Delivery Notification by PDS
N/A (PDAU).
Physical Forwarding Allowed
Supported by use of a comment in a new RFC 822 header (X400-
Recipients:), associated with the recipient in question.
Physical Forwarding Prohibited
Supported by use of a comment in a new RFC 822 header (X400-
Recipients:), associated with the recipient in question.
Prevention of Non-delivery notification
Supported where SMTP and NOTARY are available. In other cases
formally supported, as delivery notifications cannot be generated
by RFC 822. In practice, errors will be returned as IP Messages,
and so this service may appear not to be supported (see Non-
delivery Notification).
Primary and Copy Recipients Indication
Supported
Probe
Supported at the gateway (i.e., the gateway services the probe).
Probe Origin Authentication
N/A (reception).
Proof of Delivery
Not supported.
Proof of Submission
N/A (local).
Receipt Notification Request Indication
Not supported.
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Redirection Disallowed by Originator
Redirection means MTS supported redirection, in the manner of
X.400. This service does not exist in the RFC 822 world. RFC 822
redirection (e.g., aliasing) is regarded as an informal
redirection mechanism, beyond the scope of this control. Messages
will be sent to RFC 822, irrespective of whether this service is
requested. In practice, control of this service is not supported.
Registered Mail
N/A (PDAU).
Registered Mail to Addressee in Person
N/A (PDAU).
Reply Request Indication
Supported as comment next to address.
Replying IP Message Indication
Supported.
Report Origin Authentication
N/A (reception).
Request for Forwarding Address
N/A (PDAU).
Requested Delivery Method
N/A (local). The service request is dealt with at submission
time. Any such request is made available through the gateway by
use of a comment associated with the recipient in question.
Return of Content
Supported where SMTP and NOTARY are used. In principle for other
situations, this is N/A, as non-delivery notifications are not
supported. In practice, most RFC 822 systems will return part or
all of the content along with the IP Message indicating an error
(see Non-delivery Notification).
Sensitivity Indication
Supported as new RFC 822 header (Sensitivity:).
Special Delivery
N/A (PDAU).
Stored Message Deletion
N/A (MS).
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Stored Message Fetching
N/A (MS).
Stored Message Listing
N/A (MS).
Stored Message Summary
N/A (MS).
Subject Indication
Supported.
Undeliverable Mail with Return of Physical Message
N/A (PDAU).
Use of Distribution List
In principle this applies only to X.400 supported distribution
lists (see DL Expansion Prohibited). Theoretically, this service
is N/A (prior). In practice, because of informal RFC 822 lists,
this service can be regarded as supported.
Auto-Submitted Indication
Supported
2.3.2. Reception by X.400
2.3.2.1. Standard Mandatory Services
The following standard IPM mandatory user facilities are required for
reception of RFC 822 originated mail by an X.400 UA.
Content Type Indication
Delivery Time Stamp Indication
IP Message Identification
Message Identification
Non-delivery Notification
Original Encoded Information Types Indication
Submission Time Stamp Indication
Typed Body
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2.3.2.2. Standard Optional Services
The following standard IPM optional user facilities are required for
reception of RFC 822 originated mail by an X.400 UA.
Authorising User's Indication
Blind Copy Recipient Indication
Cross Referencing Indication
Originator Indication
Primary and Copy Recipients Indication
Replying IP Message Indication
Subject Indication
2.3.2.3. New Services
A new X.400 service "RFC 822 Header Field" is defined using the
extension facilities. This allows for any RFC 822 header field to be
represented. It may be present in RFC 822 originated messages which
are received by an X.400 UA.
Chapter 3 Basic Mappings
3.1. Notation
The X.400 protocols are encoded in a structured manner according to
ASN.1, whereas RFC 822 is text encoded. To define a detailed
mapping, it is necessary to refer to detailed protocol elements in
each format. A notation to achieve this is described in this
section.
3.1.1. RFC 822
Structured text is defined according to the Extended Backus Naur Form
(EBNF) defined in Section 2 of RFC 822 [16]. In the EBNF definitions
used in this specification, the syntax rules given in Appendix D of
RFC 822 are assumed. When these EBNF tokens are referred to outside
an EBNF definition, they are identified by the string "822." appended
to the beginning of the string (e.g., 822.addr-spec). Additional
syntax rules, to be used throughout this specification, are defined
in this chapter.
The EBNF is used in two ways.
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1. To describe components of RFC 822 messages (or of SMTP
components). When these new EBNF tokens are referred to
outside an EBNF definition, they are identified by the
string "EBNF." appended to the beginning of the string
(e.g., EBNF.importance).
2. To describe the structure of IA5 or ASCII information not in
an RFC 822 message.
For all new EBNF, tokens will either be self delimiting, or be
delimited by self delimiting tokens. Comments and LWSP are not used
as delimiters, except for the following cases, where LWSP may be
inserted according to RFC 822 rules.
- Around the ":" in all headers
- EBNF.labelled-integer
- EBNF.object-identifier
- EBNF.encoded-info
RFC 822 folding rules are applied to all headers. Comments are never
used in these new headers.
This notation is used in a modified form to refer to NOTARY EBNF
[28]. For this EBNF, the keyword EBNF it replaces with DSN, for
example DSN.final-recipient-field fields.
3.1.2. ASN.1
An element is referred to with the following syntax, defined in EBNF:
element = service "." definition *( "." definition )
service = "IPMS" / "MTS" / "MTA"
definition = identifier / context
identifier = ALPHA *< ALPHA or DIGIT or "-" >
context = "[" 1*DIGIT "]"
The EBNF.service keys are shorthand for the following service
specifications:
IPMS IPMSInformationObjects defined in Annex E of X.420 / ISO 10021-
7.
MTS MTSAbstractService defined in Section 9 of X.411 / ISO 10021-4.
TA MTAAbstractService defined in Section 13 of X.411 / ISO 10021-4.
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FTBP File Transfer Body Part, as defined in [27].
The first EBNF.identifier identifies a type or value key in the
context of the defined service specification. Subsequent
EBNF.identifiers identify a value label or type in the context of the
first identifier (SET or SEQUENCE). EBNF.context indicates a context
tag, and is used where there is no label or type to uniquely identify
a component. The special EBNF.identifier keyword "value" is used to
denote an element of a sequence. For example, IPMS.Heading.subject
defines the subject element of the IPMS heading. The same syntax is
also used to refer to element values. For example,
MTS.EncodedInformationTypes.[0].g3Fax refers to a value of
MTS.EncodedInformationTypes.[0] .
3.2. ASCII and IA5
A gateway will interpret all IA5 as ASCII. Thus, mapping between
these forms is conceptual.
3.3. Standard Types
There is a need to convert between ASCII text and some of the types
defined in ASN.1 [14]. For each case, an EBNF syntax definition is
given, for use in all of this specification, which leads to a mapping
between ASN.1, and an EBNF construct. All EBNF syntax definitions of
ASN.1 types are in lower case, whereas ASN.1 types are referred to
with the first letter in upper case. Except as noted, all mappings
are symmetrical.
3.3.1. Boolean
Boolean is encoded as:
boolean = "TRUE" / "FALSE"
3.3.2. NumericString
NumericString is encoded as:
numericstring = *(DIGIT / " ")
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3.3.3. PrintableString
PrintableString is a restricted IA5String defined as:
printablestring = *( ps-char )
ps-restricted-char = 1DIGIT / 1ALPHA / " " / "'" / "+"
/ "," / "-" / "." / "/" / ":" / "=" / "?"
ps-delim = "(" / ")"
ps-char = ps-delim / ps-restricted-char
This can be used to represent real printable strings in EBNF.
3.3.4. T.61String
In cases where T.61 strings are only used for conveying human
interpreted information, the aim of a mapping is to render the
characters appropriately in the remote character set, rather than to
maximise reversibility. For these cases, there are two options, both
of which are conformant to this specification:
1. The mappings to IA5 defined in ITU-T Recommendation X.408
(1988) may be used [13]. These will then be encoded in
ASCII. This is the approach mandated in RFC 1327.
2. This mapping may be used if the characters are not contained
within ASCII repertoire, but are all in an IANA-registered
character set. Use the encoding defined in RFC 1522 [9] to
generate appropriate encoded-words. If this mapping is
used, the character set ISO-8859-1 shall be used if all of
the characters needed are available in this repertoire. In
other cases, the character set TELETEX shall be used. The
details of this character set is defined in the Appendix C
of RFC 2157.
There is also a need to represent Teletex Strings in ASCII, for some
aspects of OR Address. For these, the following encoding is used:
teletex-string = *( ps-char / t61-encoded )
t61-encoded = "{" 1* t61-encoded-char "}"
t61-encoded-char = 3DIGIT
Characters in EBNF.ps-char are mapped simply. Other octets,
including control characters, are mapped using a quoting mechanism
similar to the printable string mechanism. Each octet is represented
as 3 decimal digits. For example, the Yen character (hex A5) is
represented as {165}. As the three character string, a, yen
character, b, would be represented as either "a{165}b".
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The use of escape sequences follows that set down for ASN1. in ISO
8825-1, with the additional specifiction that the default G1 page is
ISO Latin 1. The page settings may be changed by escape sequences.
Changes of the settings hold within a pair of curly brackets ({}),
and the settings revert to the default after the right bracket (})
(i.e., they do not carry forward to subsequent T.61 encoding).
There are a number of places where a string may have a Teletex and/or
Printable String representation. The following EBNF is used to
represent this.
teletex-and-or-ps = [ printablestring ] [ "*" teletex-string ]
The natural mapping is restricted to EBNF.ps-char, in order to make
the full BNF easier to parse. An example is:
"yen*{165}"
3.3.5. UTCTime
Both UTCTime and the RFC 822 822.date-time syntax contain: Year,
Month, Day of Month, hour, minute, second (optional), and Timezone
(technically a time differential in UTCTime). 822.date-time also
contains an optional day of the week, but this is redundant. With
the exception of Year, a symmetrical mapping can be made between
these constructs.
Note:
In practice, a gateway will need to parse various illegal variants
on 822.date-time. In cases where 822.date-time cannot be parsed,
it is recommended that the derived UTCTime is set to the value at
the time of translation. Such errors may be noted in an RFC 822
comment, to aid detection and correction.
When mapping to X.400, the UTCTime format which specifies the
timezone offset shall be used.
When mapping to RFC 822, the 822.date-time format shall include a
numeric timezone offset (e.g., -0500).
When mapping time values, the timezone shall be preserved as
specified. The date shall not be normalised to any other timezone.
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RFC 822, as modified by RFC 1123, requires use of a four digit year.
Note that the original RFC 822 uses a two digit date, which is no
longer legal. UTCTime uses a two digit date. To map a year from RFC
822 to X.400, simply use the last two digits. To map a year from
X.400 to RFC 822, assume that the two digit year refers to a year in
the 10 year epoch 1980-2079.
3.3.6. Integer
A basic ASN.1 Integer will be mapped onto EBNF.numericstring. In
many cases ASN.1 will enumerate Integer values or use ENUMERATED. An
EBNF encoding labelled-integer is provided. When mapping from EBNF to
ASN.1, only the integer value is mapped, and the associated text is
discarded. When mapping from ASN.1 to EBNF, a text label may be
added. It is recommended that this is done wherever possible and
that clear text labels are chosen.
A second encoding labelled-integer-2 is provided. This is used in
DSNs, where the parsing rules will treat the text as a comment. This
definition was not present in RFC 1327.
labelled-integer ::= [ key-string ] "(" numericstring ")"
labelled-integer-2 ::= [ numericstring ] "(" key-string ")"
key-string = *key-char
key-char =
3.3.7. Object Identifier
Object identifiers are represented in a form similar to that given in
ASN.1. The order is the same as for ASN.1 (big-endian). The numbers
are mandatory, and used when mapping from the ASCII to ASN.1. The
key-strings are optional. It is recommended that as many strings as
possible are generated when mapping from ASN.1 to ASCII, to
facilitate user recognition.
object-identifier ::= oid-comp object-identifier
| oid-comp
oid-comp ::= [ key-string ] "(" numericstring ")"
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An example representation of an object identifier is:
joint-iso-ccitt(2) mhs (6) ipms (1) ep (11) ia5-text (0)
or
(2) (6) (1)(11)(0)
Because of the use of brackets and the conflict with the RFC 822
comment convention, MIXER is defines so that the EBNFobject-
identifier definition is not used in structured fields.
3.4. Encoding ASCII in Printable String
Some information in RFC 822 is represented in ASCII, and needs to be
mapped into X.400 elements encoded as printable string. For this
reason, a mechanism to represent ASCII encoded as PrintableString is
needed.
A structured subset of EBNF.printablestring is now defined. This
shall be used to encode ASCII in the PrintableString character set.
ps-encoded = *( ps-restricted-char / ps-encoded-char )
ps-encoded-char = "(a)" ; (@)
/ "(p)" ; (%)
/ "(b)" ; (!)
/ "(q)" ; (")
/ "(u)" ; (_)
/ "(l)" ; "("
/ "(r)" ; ")"
/ "(" 3DIGIT ")"
The 822.3DIGIT in EBNF.ps-encoded-char shall have range 0-127, and is
interpreted in decimal as the corresponding ASCII character. Special
encodings are given for: at sign (@), percent (%), exclamation
mark/bang (!), double quote ("), underscore (_), left bracket ((),
and right bracket ()). These characters, with the exception of round
brackets, are not included in PrintableString, but are common in RFC
822 addresses. The abbreviations will ease specification of RFC 822
addresses from an X.400 system. These special encodings shall be
interpreted in a case insensitive manner, but always generated in
lower case.
A reversible mapping between PrintableString and ASCII can now be
defined. The reversibility means that some values of printable
string (containing round braces) cannot be generated from ASCII.
Therefore, this mapping shall only be used in cases where the
printable strings have been derived from ASCII (and will therefore
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have a restricted domain). For example, in this specification, it is
only applied to a Domain Defined Attribute which will have been
generated by use of this specification and a value such as "(" would
not be possible.
To encode ASCII as PrintableString, the EBNF.ps-encoded syntax is
used, with all EBNF.ps-restricted-char mapped directly. All other
822.CHAR are encoded as EBNF.ps-encoded-char.
To encode PrintableString as ASCII, parse PrintableString as
EBNF.ps-encoded, and then reverse the previous mapping. If the
PrintableString cannot be parsed, then the mapping is being applied
in to an inappropriate value, and an error shall be given to the
procedure doing the mapping. In some cases, it may be preferable to
pass the printable string through unaltered.
Some examples are now given. Note the arrows which indicate
asymmetrical mappings:
PrintableString ASCII
'a demo.' <-> 'a demo.'
foo(a)bar <-> foo@bar
(q)(u)(p)(q) <-> "_%"
(a) <-> @
(A) -> @
(l)a(r) <-> (a)
(126) <-> ~
( -> (
(l) <-> (
3.5. RFC 1522
RFC 1522 defines a mechanism for encoding other character set
information into elements of RFC 822 Headers. A gateway may ignore
this encoding and treat the elements as ASCII.
A preferred approach is for the gateway to interpret the RFC 1522
encoding. This will not always be straightforward, because:
1. RFC 1522 permits an openly extensible character set choice,
which may be broader than T.61.
2. It is not always possible to map all characters into the
equivalent X.400 field.
RFC 1522 is only applied to fields which are "for information only".
A gateway which interprets header elements according to RFC 1522 may
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apply reasonable heuristics to minimise information loss.
Chapter 4 - Addressing and Message IDs
Addressing is the most complex aspect of X.400 <-> RFC 822 gateway
and is therefore given a separate chapter. This chapter also
discusses message identifiers, as they are closely linked to
addresses. This chapter, as a side effect, also defines a textual
representation of an X.400 OR Address. This specification has much
similarity to the X.400(92) representation of addresses. This was
because early versions of this specification were a major input to
this work. This specification retains compatibility with earlier
versions. The X.400 specification of address representation can be
parsed but is not generated.
Initially we consider an address in the (human) mail user sense of
"what is typed at the mailsystem to reference a mail user". A basic
RFC 822 address is defined by the EBNF EBNF.822-address:
822-address = [ route ] addr-spec
These definitions are taken from RFC 822. In SMTP (or another 822-
MTS protocol), the originator and each recipient are considered to be
defined by such a construct. In an RFC 822 header, the EBNF.822-
address is encapsulated in the 822-address syntax rule, and there may
also be associated comments. None of this extra information has any
semantics, other than to the end user.
The basic X.400 OR Address, used by the MTS for routing, is defined
by MTS.ORAddress. In IPMS, the MTS.ORAddress is encapsulated within
IPMS.ORDescriptor.
The RFC 822 822.address is mapped with IPMS.ORDescriptor, and that
RFC 822 EBNF.822-address is mapped with MTS.ORAddress.
Section 4.1 defines a textual representation of an OR Address, which
is used throughout the rest of this specification. This text
representation is designed to represent an X.400 address in the LHS
(left hand side) or local part of an RFC 822 address, and so this
representation gives a mechanism to represent X.400 addresses within
RFC 822 addresses.
Section 4.2 describes global equivalence mapping between parts of the
X.400 and RFC 822 name spaces, and defines the concept of a MIXER
Conformant Global Address Mapping (MCGAM). Gateways conforming to
this specification shall support MCGAMs.
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Section 4.3 is the core part of this chapter, and defines the mapping
mechanism.
4.1. A textual representation of MTS.ORAddress
MTS.ORAddress is structured as an ordered set of attributes
(type/value pairs). It is clearly necessary to be able to encode
this in ASCII for gatewaying purposes. All components shall be
encoded, in order to guarantee return of error messages, and to
optimise third party replies.
4.1.1. Basic OR Address Representation
An OR Address has a number of structured and unstructured attributes.
For each unstructured attribute, a key and an encoding is specified.
For structured attributes, the X.400 attribute is mapped onto one or
more attribute value pairs. For domain defined attributes, each
element of the sequence will be mapped onto a triple (key and two
values), with each value having the same encoding. The attributes
are as follows, with 1984 attributes given in the first part of the
attribute key table. For each attribute, a reference is given,
consisting of the relevant sections in X.402 / ISO 10021-2, and the
extension identifier for 88 only attributes. The attribute key table
follows:
Attribute (Component) Key Enc Ref Id
84/88 Attributes
MTS.CountryName C P 18.3.3
MTS.AdministrationDomainName ADMD P 18.3.1
MTS.PrivateDomainName PRMD P 18.3.21
MTS.NetworkAddress X121 N 18.3.7
MTS.TerminalIdentifier T-ID P 18.3.23
MTS.OrganizationName O P/T 18.3.9
MTS.OrganizationalUnitNames.value OU P/T 18.3.10
MTS.NumericUserIdentifier UA-ID N 18.3.8
MTS.PersonalName PN P/T 18.3.12
MTS.PersonalName.surname S P/T 18.3.12
MTS.PersonalName.given-name G P/T 18.3.12
MTS.PersonalName.initials I P/T 18.3.12
MTS.PersonalName
.generation-qualifier GQ P/T 18.3.12
MTS.DomainDefineAttribute.value DD P/T 18.1
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88 Attributes
MTS.CommonName CN P/T 18.3.2 1
MTS.TeletexCommonName CN P/T 18.3.2 2
MTS.TeletexOrganizationName O P/T 18.3.9 3
MTS.TeletexPersonalName PN P/T 18.3.12 4
MTS.TeletexPersonalName.surname S P/T 18.3.12 4
MTS.TeletexPersonalName.given-name G P/T 18.3.12 4
MTS.TeletexPersonalName.initials I P/T 18.3.12 4
MTS.TeletexPersonalName
.generation-qualifier GQ P/T 18.3.12 4
MTS.TeletexOrganizationalUnitNames
.value OU P/T 18.3.10 5
MTS.TeletexDomainDefinedAttribute
.value DD P/T 18.1 6
MTS.PDSName PD-SERVICE P 18.3.11 7
MTS.PhysicalDeliveryCountryName PD-C P 18.3.13 8
MTS.PostalCode PD-CODE P 18.3.19 9
MTS.PhysicalDeliveryOfficeName PD-OFFICE P/T 18.3.14 10
MTS.PhysicalDeliveryOfficeNumber PD-OFFICE-NUM P/T 18.3.15 11
MTS.ExtensionORAddressComponents PD-EXT-ADDRESS P/T 18.3.4 12
MTS.PhysicalDeliveryPersonName PD-PN P/T 18.3.17 13
MTS.PhysicalDeliveryOrganizationName PD-O P/T 18.3.16 14
MTS.ExtensionPhysicalDelivery
AddressComponents PD-EXT-DELIVERY P/T 18.3.5 15
MTS.UnformattedPostalAddress PD-ADDRESS UPA 18.3.25 16
MTS.StreetAddress PD-STREET P/T 18.3.22 17
MTS.PostOfficeBoxAddress PD-BOX P/T 18.3.18 18
MTS.PosteRestanteAddress PD-RESTANTE P/T 18.3.20 19
MTS.UniquePostalName PD-UNIQUE P/T 18.3.26 20
MTS.LocalPostalAttributes PD-LOCAL P/T 18.3.6 21
MTS.ExtendedNetworkAddress
.e163-4-address.number NET-NUM N 18.3.7 22
MTS.ExtendedNetworkAddress
.e163-4-address.sub-address NET-SUB N 18.3.7 22
MTS.ExtendedNetworkAddress
.psap-address NET-PSAP X 18.3.7 22
MTS.TerminalType T-TY I 18.3.24 23
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The following keys identify different EBNF encodings, which are
associated with the ASCII representation of MTS.ORAddress.
Key Encoding
P printablestring
N numericstring
T teletex-string
P/T teletex-and-or-ps
UPA upa-string
I labelled-integer
X presentation-address
The EBNF for presentation-address is taken from the specification RFC
1278 "A String Encoding of Presentation Address" [23].
In most cases, the EBNF encoding maps directly to the ASN.1 encoding
of the attribute. There are a few exceptions. In cases where an
attribute can be encoded as either a PrintableString or NumericString
(Country, ADMD, PRMD), either form is mapped into the EBNF. When
generating ASN.1, the NumericString encoding shall be used if the
string contains digits and only digits.
There are a number of cases where the P/T (teletex-and-or-ps)
representation is used. Where the key maps to a single attribute,
this choice is reflected in the encoding of the attribute (attributes
10-21). For example:
/CN=yen*{165}/
For most of the 1984 attributes and common name, there is a
printablestring and a teletex variant. This pair of attributes is
mapped onto the single component here. This will give a clean
mapping for the common cases where only one form of the name is used.
If there is teletex attribute or teletex component only, and it
contains only characters in the printable string character set, it
shall be represented in the EBNF as if it had been encoded as
printable string. A single printable string representation shall
also be done when both forms are present and they have the same
printable string representation.
The Unformatted Postal Address has a slightly more complex mapping
onto a variant of (teletex-and-or-ps), defined as:
upa-string = [ printable-upa ] [ "*" teletex-string ]
printable-upa = printablestring *( "|" printablestring )
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The optional teletex part is straightforward. There is an (optional)
sequence of printable strings which are mapped in order. For
example:
/PD-ADDRESS=The Dome|The Square|Richmond|England/
X.400 (1992) has introduced a string representation of OR Addresses
(see F.401, Annex B). This has specified a number of string keywords
for attributes. As earlier versions of this specification were an
input to this work, many of the keywords are the same. To increase
compatibility, the following alternative values shall be recognised
when mapping from RFC 822 to X.400. These shall not be generated
when mapping from X.400 to RFC 822. The following keyword
alternative table and the subsequent paragraph lists alternative
keywords.
Keyword Alternative
ADMD A
PRMD P
GQ Q
X121 X.121
UA-ID N-ID
PD-OFFICE-NUM PD-OFFICE NUMBER
PD-OFFICE-NUM PD-OFN
PD-EXT-ADDRESS PD-EA
PD-EXT-DELIVERY PD-ED
PD-OFFICE PD-OF
PD-STREET PD-S
PD-UNIQUE PD-U
PD-LOCAL PD-L
PD-RESTANTE PD-R
PD-BOX PD-B
PD-CODE PD-PC
PD-SERVICE PD-SN
DD DDA
NET-NUM E.164
NET-PSAP PSAP
PD-ADDRESS PD-A
When mapping from RFC 822 to X.400, the keywords defined in this
paragraph shall be recognized. The ordered keywords: OU1, OU2,
OU3, and OU4, shall be recognised. If these are present, no
keyword OU shall be present. These will be treated as ordered
values of OU. PD-A1, PD-A2, PD-A3, PD-A4, PD-A5, PD-A6 shall be
treated as ordered lines. If present, these will be assembled
with separating line feeds to form a single physical address. In
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this case PD-ADDRESS (or PD-A) shall not be present. Similarly,
there are ordered keywords for domain defined attributes: DD1,
DD2, DD3, DD4,
If ISDN is present, it may be interpreted as an E.163/164
address, using local heuristics to parse the string. X.400
defines the key, but does not give an interpretation of the
value.
For T-TY (Terminal Type), the X.400 recommended values are
preferred, but other values are allowed. These values are: tlx
(3); ttx (4); g3fax (5); g4fax (6); ia5 (7); and vtx (8).
4.1.2. Encoding of Personal Name
Handling of Personal Name and Teletex Personal Name is a common
requirement. Therefore MIXER defines an alternative to the
EBNF.standard-type syntax, which utilises the "human" conventions for
encoding these components. A syntax is defined, which is designed to
provide a clean encoding for the common cases of OR Address
specification where:
1. There is no generational qualifier
2. Initials, if present, contain only letters
3. Given Name, if present, does not contain full stop ("."),
and is at least two characters long.
4. Surname does not contain full stop in the first two
characters.
5 If Surname is the only component, it does not contain full
stop.
The following EBNF is defined:
encoded-pn = [ given "." ] *( initial "." ) surname
given = 2*
initial = ALPHA
surname = printablestring
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This is used to map from any string containing only printable string
characters to an OR address personal name. To map from a string to
OR Address components, parse the string according to the EBNF. The
given name and surname are assigned directly. All EBNF.initial
tokens are concatenated without intervening full stops to generate
the initials component.
For an OR address which follows the above restrictions, a string is
derived in the natural manner. In this case, the mapping will be
reversible.
For example:
GivenName = "Marshall"
Surname = "Rose"
Maps with "Marshall.Rose"
Initials = "MT"
Surname = "Rose"
Maps with "M.T.Rose"
GivenName = "Marshall"
Initials = "MT"
Surname = "Rose"
Maps with "Marshall.M.T.Rose"
Note that X.400 suggests that Initials is used to encode all initials
except the surname (X.402 section 18.3.12). Therefore, the defined
encoding is "natural" when either GivenName or Initials, but not
both, are present. The case where both are present can be encoded.
4.1.3. Standard Encoding of MTS.ORAddress
Given this structure, we can specify an EBNF representation of an OR
Address. The output format of addresses is defined by EBNF.std-or-
address. The more flexible input format is defined by EBNF.std-or-
address-input. The input EBNF has been added subsequent to RFC 1327,
to reflect the formal incorporation of a number of heuristics. The
address element separator on input may be "/", ";", or a mixture of
these. The output format is used in all examples.
std-or-address = 1*( "/" attribute "=" value ) "/"
attribute = standard-type
/ "RFC-822"
/ dd-key "." std-printablestring
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std-or-address-input = [ sep pair ] sep pair *( sep pair )
sep [ pair sep ]
sep = "/" / ";"
pair = input-attribute "=" value
input-attribute = attribute
/ dd-key ":" std-printablestring
standard-type = key-string
dd-key = key-string
value = std-printablestring
std-printablestring
= *( std-char / std-pair )
std-char = <"{", "}", "*", and any ps-char
except "/" and "=" >
std-pair = "$" ps-char
For address generation, the standard-type is any key defined in the
key table in Section 4.1, except PN, and DD. For address parsing,
other key values from Section 4.1 are also valid. The EBNF leads to
a set of attribute/value pairs. The value is interpreted according to
the EBNF encoding defined in the table.
If the standard-type is PN, the value is interpreted according to
EBNF.encoded-pn, and the components of MTS.PersonalName and/or
MTS.TeletexPersonalName derived accordingly.
If dd-key is the recognised Domain Defined string (DD) or one of the
alternatives defined in Section 4.1, then the type and value are
interpreted according to the syntax implied from the encoding, and
aligned to either the teletex or printable string form. Key and
value shall have the same encoding.
If value is "RFC-822", then the (printable string) Domain Defined
Type of "RFC-822" is assumed. This is an optimised encoding of the
domain defined type defined by this specification.
The matching of all keywords shall be done in a case-independent
manner.
EBNF.std-or-address uses the characters "/" and "=" as delimiters.
Domain Defined Attributes and any value may contain these characters.
A quoting mechanism, using the non-printable string "$" is used to
allow these characters to be represented.
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If an address of this syntax is parsed, and a country value is
present, but no ADMD, the string shall be interpreted as if an ADMD
value of single space had been specified.
4.2. Global Address Mapping
From a user perspective, the ideal mapping would be entirely
symmetrical and global, to enable addresses to be referred to
transparently in the remote system, with the choice of gateway being
left to the Message Transfer Service. There are two fundamental
reasons why this is not possible:
1. The syntaxes are sufficiently different to make this
impossible.
2 There is insufficient administrative co-operation between
the X.400 and RFC 822 name registration authorities for this
to work.
Another way to view this situation is to see that there is not a full
global equivalence between X.400 and RFC 822 addressing. To meet
user needs to the extent possible, this specification provides for
equivalence where there is sufficient co-operation. To be useful,
this equivalence shall be recognised and interpreted in the same way
by all gateways. Therefore, an asymmetrical mapping is defined,
which can be symmetrical where there is appropriate administrative
co-operation. Section 4.3 describes the asymetrical aspects. This
section describes a mechanism to enable the administrative co-
ordination for symmetrical mappings.
In order to achieve a symmetrical mapping there is a need to define
an administrative equivalence between parts of the OR Address and
Domain namespaces. Previous version of this specification did this
by definition of a global set of mappings. MIXER defines the concept
of a MIXER Conformant Global Address Mapping (MCGAM). This acronym
is defined so that it is very clear what is being referenced.
The X.400 and Internet Mail address spaces are hierarchical. It is
possible to define an equivalence between two points in the
hierarchies, such that addresses below that point can be derived in
an algorithmic manner. An MCGAM is a mapping from a point in one
hierarchy to a point in the other hierarchy. An "MGGAM pair" is a
pair of symmetrical mappings between two points. To define an MCGAM,
the following shall apply:
1. The authority defining the MCGAM shall have responsibility
for BOTH of the namespaces between which the MCGAM is
defined.
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RFC 2156 MIXER January 1998
2. The authority defining the MCGAM is responsible to ensure
that addresses allocated below the two equivalence points
conform to the rules set out below.
3. The authority defining the MCGAM is responsible to ensure
that addresses which are generated according to the MCGAM
are routed correctly.
In general, MCGAMs will be independent. In some cases, a set of
MCGAMs may be related (e.g., where one MCGAM defines a mapping for an
organization and a second MCGAM defines an excpetion for a subtree
within the organization). In this case, the related set of MCGAMs
shall be treated as a single MCGAM for distribution purposes.
The existence of an MCGAM does not imply routability and access for
all users.
The authority defining an MCGAM may simply use this mapping locally.
This will often be the case in a "local scenario" gateway. Because
of third party addressing, a MIXER gateway will work best with the
maximum number of MCGAMs. Therefore, three mechanisms are defined
to enable publication and exchange of MCGAMs:
1. Distribution of text tables. This is described in Appendix
F of this specification.
2. Distribution by Domain Name Service. This is described in
RFC 2163 [3].
3. Distribution by X.500 Directory Service. This is defined
in RFC 2164 [26].
The following sections define how the MCGAM namespace equivalence is
modelled. The Internet Domain Namespace defines a simple hierarchy.
For the purposes of this mapping, only parts of the namespace where
domains conform to the EBNF domain-syntax are allowed.
domain-syntax = alphanum [ *alphanumhyphen alphanum ]
alphanum =
alphanumhyphen =
Although RFC 822 allows for a more general syntax, this restricted
syntax is used in MIXER as it is the one chosen by the various domain
service administrations. In practice, it reflects all RFC 822 usage.
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The following OR Address attributes are considered as a hierarchy,
and may be specified by the domain. They are (in order of the
hierarchy defined by MIXER):
Country, ADMD, PRMD, Organization, Organizational Units
There may be up to four ordered Organizational Units. This
hierarchy reflects most usage of X.400, although X.400 may be used in
other ways. In particular, it covers the Mnemonic OR Address using a
1984 compatible encoding. This is seen as the dominant form of OR
Address. MCGAMs may only be used when this hierarchy applies.
An equivalence mapping is defined between two nodes in the respective
hierarchies. For example:
=> "AC.UK" might be mapped with
PRMD="UK.AC", ADMD="GOLD 400", C="GB"
The mapping identifies that the management of these points in the
respective hierarchies is the same (or co-operate very closely). The
equivalence means that the namespaces below this equivalence point
map 1:1, except where the mapping is overridden by further
equivalence mappings lower down the hierarchy. This equivalence may
be achieved in three ways:
1. All of the nodes below this point are RFC 822, and the MIXER
mapping defines the X.400 addresses for these nodes.
2. All of the nodes below this point are X.400, and the MIXER
mapping defines the RFC 822 addresses for these nodes.
3. There are X.400 and RFC 822 nodes below this point, and
addressing is managed in a manner which ensures the
equivalence. The rules to achieve this are defined by
MIXER.
Each of these ways gives a framework for MCGAM definition.
When an MCGAM is defined, a systematic mapping for the inferior nodes
in the two hierarchies follows. This is a 1:1 mapping between the
nodes in the subtrees. For example, given the MCGAM pair defined
above:
the domain "R-D.Salford.AC.UK" algorithmically maps with
OU="R-D", O="Salford", PRMD="UK.AC", ADMD="GOLD 400", C="GB"
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Note that when an equivalence is defined, that this can be re-defined
for lower points in the hierarchy. However, it is not possible to
declare contained subtrees to be un-mappable.
The equivalence mapping also provides a mechanism to deal with
missing elements in the X.400 hierarchy (most commonly the PRMD,
which is the only element that may be ommitted when conforming to
recent versions of X.400). A domain may be associated with an
omitted attribute in conjunction with several present ones. When
performing the algorithmic insertion of components lower in the
hierarchy, the omitted value shall be skipped. For example:
If there is an MCGAM pair between domain HNE.EGM" and "O=HNE",
"ADMD=ECQ", "C=TC", and omitted PRMD
then
"ZI.HNE.EGM" is algorithmically mapped with "OU=I", "O=HNE",
"ADMD=ECQ", "C=TC"
Attributes may have null values, and this is treated separately from
omitted attributes (while it is not ideal to make this distinction,
it is useful in practice).
4.2.1. Directory and Nameserver Mappings
When a set of MCGAMs are supported by X.500 or DNS, there is the
possibility that results will be indeterminate due to timeout.
Lookup shall be repeated until a value is determined, in order to
maintain consistent gateway operation.
Where the mapping relates to an envelope address, the gateway shall
non-deliver messages according to the associated MTA's normal timeout
policy. Where the mapping relates to addresses in the message
header, there shall be a timeout in the range of 1-4 hours or shorter
if this is required to maintain quality of service constraints. If
a mapping cannot be done in this time, address encapsulation shall be
used.
4.3. EBNF.822-address <-> MTS.ORAddress
This section defines the basic address mapping.
4.3.1. X.400 encoded in RFC 822
This section defines how X.400 addresses are represented in RFC 822
addresses.
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The std-or-address syntax is used to encode OR Address information
in the 822.local-part of EBNF.822-address. Where there is an
applicable equivalence mapping, further OR Address information is
associated with the 822.domain component. This cannot be used in the
general case, due to character set problems, and to the variants of
X.400 OR Addresses which use different attribute types. The only way
to encode the full PrintableString character set in a domain is by
use of the 822.domain-ref syntax (i.e. 822.atom). This is likely to
cause problems on many systems. The effective character set of
domains is in practice reduced from the RFC 822 set, by restrictions
imposed by domain conventions and policy [10], and by the EBNF
definition in SMTP.
A generic 822.address consists of a 822.local-part and a sequence of
822.domains (e.g., <@domain1,@domain2:user@domain3>). All except the
822.domain associated with the 822.local-part (domain3 in this case)
are considered to specify routing within the RFC 822 world, and will
not be interpreted by the gateway (although they may have identified
the gateway from within the RFC 822 world).
The 822.domain associated with the 822.local-part identifies the
gateway from within the RFC 822 world. This final 822.domain may be
used to determine some number of OR Address attributes, where this
does not conflict with the first role. RFC 822 routing to gateways
will usually be set up to facilitate the 822.domain being used for
both purposes.
In the case that there is no applicable equivalence mapping, all of
the X.400 address is encoded in the 822.local-part and the 822.domain
identifies the gateway to which the message is being sent. This
technique may be used by the RFC 822 user for any X.400 address where
the equivalence mapping is not known.
In the case that there is an applicable MCGAM, the maximum number of
attributes are encoded in the 822.domain. The remaining attributes
are encoded on the LHS, using the EBNF.std-or-address syntax. For
example:
/I=J/S=Linnimouth/GQ=5/@Marketing.Widget.COM
encodes the MTS.ORAddress consisting of:
MTS.CountryName = "TC"
MTS.AdministrationDomainName = "BTT"
MTS.OrganizationName = "Widget"
MTS.OrganizationalUnitNames.value = "Marketing"
MTS.PersonalName.surname = "Linnimouth"
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RFC 2156 MIXER January 1998
MTS.PersonalName.initials = "J"
MTS.PersonalName.generation-qualifier = "5"
on the basis of an MCGAM pair between:
Domain: Widget.COM
OR Address: O="Widget", ADMD="BTT", C="TC"
Given the OR address, the domain Widget.COM is determined from the
equivalence mapping and the next component is determined
algorithmically to give Marketing.Widget.COM. The remaining
attributes are encoded on the LHS in 822.local-part.
There is a further mechanism to simplify the encoding of common
cases, where the only attributes to be encoded on the LHS are (non-
Teletex) Personal Name attributes which comply with the restrictions
of 4.1.2. To achieve this, the 822.local-part shall be encoded as
EBNF.encoded-pn. In the previous example, if the GenerationQualifier
was not present in the OR Address, it would map with the RFC 822
address: J.Linnimouth@Marketing.Widget.COM.
From the standpoint of the RFC 822 Message Transfer System, the
domain specification is used to route the message in the standard
manner. The standard domain mechanisms are used to select
appropriate gateways for the corresponding OR Address space. It is
the responsibility of the management that defines the equivalence
mapping to define routing in the manner which will enable the message
to be delivered.
4.3.2. RFC 822 encoded in X.400
The previous section showed a mapping from X.400 to RFC 822. In the
case where the mapping was symmetrical and based on the equivalence
mapping, this has also shown how RFC 822 is encoded in the X.400.
This equivalence cannot be used for all RFC 822 addresses.
The general case is mapped by use of domain defined attributes. A
(Printable String) Domain defined type "RFC-822" is defined. The
associated attribute value is an ASCII string encoded according to
Section 3.3.3 of this specification. The interpretation of the ASCII
string follows RFC 822, and RFC 1123 [10,16]. Domains shall always
be fully qualified.
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RFC 2156 MIXER January 1998
Other OR Address attributes will be used to identify a context in
which the OR Address will be interpreted. This might be a Management
Domain, or some part of a Management Domain which identifies a
gateway MTA. For example:
C = "GB"
ADMD = "GOLD 400"
PRMD = "UK.AC"
O = "UCL"
OU = "CS"
"RFC-822" = "Jimmy(a)WIDGET-LABS.CO.UK"
OR
C = "TC"
ADMD = "Wizz.mail"
PRMD = "42"
"rfc-822" = "postel(a)venera.isi.edu"
Note in each case the PrintableString encoding of "@" as "(a)". In
the second example, the "RFC-822" domain defined attribute is
interpreted everywhere within the (Private) Management Domain. In
the first example, further attributes are needed within the
Management Domain to identify a gateway. Thus, this scheme can be
used with varying levels of Management Domain co-operation.
There is a limit of 128 characters in the length of value of a domain
defined attribute, and an OR Address can have a maxmimum of four
domain defined attributes. Where the printable string generated from
the RFC 822 address exceeds 128 characters, additional domain defined
attributes are used to enable up to 512 characters to be encoded.
These attributes shall be filled completely before the next one is
started. The (Printable String) DDA keywords are: RFC822C1;
RFC822C2; RFC822C3. Longer addresses cannot be encoded.
MIXER defines a representation of RFC 822 addresses in printable
string domain defined attributes. Teletex domain defined attributes
with a key of RFC-822, RFC822C1; RFC822C2; RFC822C3 shall not be
generated. This is for backwards compatibility reasons.
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RFC 2156 MIXER January 1998
Reception of these attributes in the manner defined below is
mandatory. This is to allow the possibility for future versions of
MIXER to allow generation of teletex domain defined attributes.
Where the values of all of these teletex domain defined attributes
are printable string characters, they shall be interpreted in the
same way as the printable string domain defined attributes. If this
is not the case, the printable string encoding translation shall be
omitted. If both teletex and printable string attributes are
present, this is valid if and only if they represent exactly the same
RFC 822 address.
4.3.3. Component Ordering
In most cases, ordering of OR Address components is not significant
for the mappings specified. However, Organizational Units (printable
string and teletex forms) and Domain Defined Attributes are specified
as SEQUENCE in MTS.ORAddress, and so their order may be significant.
This specification needs to take account of this:
1. To allow consistent mapping into the domain hierarchy
2. To ensure preservation of order over multiple mappings.
There are three places where an order is specified:
1. The text encoding (std-or-address) of MTS.ORAddress as used
in the local-part of an RFC 822 address. An order is needed
for those components which may have multiple values
(Organizational Unit, and Domain Defined Attributes). When
generating an 822.std-or-address, components of a given type
shall be in hierarchical order with the most significant
component on the RHS (right hand side or domain part). If
there is an Organization Attribute, it shall be to the right
of any Organizational Unit attributes. These requirements
are for the following reasons:
- Alignment to the hierarchy of other components in RFC
822 addresses (thus, Organizational Units will appear
in the same order, whether encoded on the RHS or LHS).
- Backwards compatibility with RFC 987/1026.
- To ensure that gateways generate consistent addresses.
This is both to help end users, and to generate
identical message ids.
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Further, it is recommended that all other attributes are generated
according to this ordering, so that all attributes so encoded follow
a consistent hierarchy. When generating 822.msg-id, this order shall
be followed.
2. For the Organizational Units (OU) in MTS.ORAddress, the
first OU in the SEQUENCE is the most significant, as specified
in X.400.
3. For the Domain Defined Attributes in MTS.ORAddress, the
First Domain Defined Attribute in the SEQUENCE is the most
significant.
Note that although this ordering is mandatory for this mapping, MIXER
does not give additional implications on the ordering significance
within X.400.
4.3.4. RFC 822 -> X.400 Basic Address Mapping
There are two basic cases:
1. X.400 addresses encoded in RFC 822. This will also include
RFC 822 addresses which are given reversible encodings.
2. "Genuine" RFC 822 addresses.
The mapping shall proceed as follows, by first assuming case 1).
STAGE I.
1. If the 822-address is not of the form:
local-part "@" domain
take the domain which will be routed on and apply step 2 of stage
1 to derive (a possibly null) set of attributes. Then go to stage
II.
The gateway may reduce a source route address to this form by
removal of all but the last domain. In terms of the design
intentions of RFC 822, this would be an incorrect action. (Note
that an address of the form local%part@domain is not a source
route). However, in most cases, it will provide a better service
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to the end user, and is in line with the Internet Host
Requirements. This is a reflection on the common inappropriate
use of source routing in RFC 822 based systems, despite the
discussion in the Host Requirements [10]. Either approach, or
the intermediate approach of stripping only domain references
which reference the local gateway are conformant to this
specification.
2. If the 822.local-part uses the 822.quoted-string encoding,
remove this quoting. If the resulting unquoted
822.local-part has leading space, trailing space, or two
adjacent spaces go to stage II.
3. If the unquoted 822.local-part contains any characters not
in PrintableString, "{", "}", "*", and "$", go to stage II.
4. Parse the (unquoted) 822.local-part according to the EBNF
EBNF.std-or-address-input. Checking of upper bounds shall
not be done at this point. If this parse fails, parse the
local-part according to the EBNF EBNF.encoded-pn. If this
parse fails, go to stage II. The result is a set of
type/value pairs.
5. Associate the EBNF.attribute-value syntax (determined from
the identified type) with each value, and check that it
conforms. If not, go to stage II.
6. If the set of attributes forms a valid X.400 address,
according to X.402, then go to step 9. All forms of X.400
address are allowed at this stage. Steps 7-8 default
attributes for certain types of OR Address.
7. If the set of attributes cannot form a mnemonic form of
X.400 address after addition of attributes which may be
derived from the EBNF.domain (C, ADMD, PRMD, O, OU), go to
stage II.
8. Attempt to parse EBNF.domain as:
*( domain-syntax "." ) known-domain
Where EBNF.known-domain is the longest possible match in the set
of MCGAMs being used by the gateway (described in Section 4.2).
EBNF.domain-syntax is the restricted domain syntax defined in
Section 4.2, to which all of the domain components shall conform
for the parse to be successful. If this fails, go to stage II.
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For each component, systematically allocate the attribute
implied by each EBNF.domain-syntax component in the order: C,
ADMD, PRMD, O, OU. Note that if the MCGAM used identifies an
"omitted attribute", then this attribute shall be omitted in the
systematic allocation. If this new component exceed an upper
bound (ADMD: 16; PRMD: 16; O: 64; OU: 32) or it would lead to
more than four OUs, then go to stage II with the attributes
derived.
The attributes derived in this step (referred to as RHS
attributes) are merged with the ones derived from the LHS (step
6). In some cases, not all of the RHF attributes are used. LHS
attributes are all used. C will not be in the LHS attributes.
If ADMD is in the LHS attributes, only C is taken from the RHS
attributes. If PRMD is in the LHS attributes, C and ADMD are
taken from the RHS attributes. If O is on the LHS, C, ADMD and
PRMD (if present) are taken from the RHS attributes. In other
cases all RHS attributes are taken.
9. Ensure that the set of attributes conforms both to the
MTS.ORAddress specification and to the restrictions on this
set given in X.400, and that no upper bounds are exceeded
for any attribute. If not go to stage II.
10. Build the OR Address from this information.
STAGE II.
This will only be reached if the RFC 822 EBNF.822-address is not a
valid X.400 encoding. This implies that the address refers to a
recipient on an RFC 822 system or that the encoding of the address is
invalid. Such addresses shall be encoded in an X.400 OR Address
using a domain defined attribute.
1. Convert the EBNF.822-address to PrintableString, as
specified in Chapter 3.
2. Generate the "RFC-822" domain defined attribute from this
string.
3. Build the rest of the OR Address in the manner described
below.
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It is not always possible to encode the domain defined attribute
due to length restrictions. If the limit is exceeded by a
mapping at the MTS level, then the gateway shall reject the
message in question. If this occurs at the IPMS level, then the
action will depend on the policy being taken for IPMS encoding,
which is discussed in Section 5.1.3.
Use Stage I, step 8, to generate a set of attributes to build the
remainder of the address. The administrative equivalence of the
mappings will ensure correct routing through X.400 to a gateway
back to RFC 822.
If Stage I, step 8 does not generate a set of attributes or
the address generated is unroutable, the remained of the OR
address is generated as follows. The remainder of the OR address
effectively identifies a source route to a gateway from the X.400
side. There are three cases, which are handled differently:
SMTP Return Address
This shall be set up so that errors are returned through the
same gateway. Therefore, the OR Address of the local
gateway shall be used.
IPMS Addresses
These are optimised for replying. In general, the message
may end up anywhere within the X.400 world, and so this
optimisation identifies a gateway appropriate for the RFC
822 address being converted. The 822.domain to which the
address would be routed is used to select an appropriate
gateway.
In this case, it may be useful to use a non-local gateway,
which will optimise the reply address. This information
may be looked up in gateway tables in a manner equivalent to
the MCGAM lookup. Because of the similarity of lookup, the
three MCGAM lookup mechanisms (table, X.500, DNS) are also
available to look up this information. This information is
local, and a gateway may insert any appropriate (gateway)
OR Address. The longest possible match on the 822.domain
defines which gateway to use. This mechanism is used for
any part of the X.400 namespace for which it is desirable to
identify a preferred X.400 gateway in order to optimise
routing.
If no mapping is found for the 822.domain, a default value
(typically that of the local gateway) is used. It is never
appropriate to ignore the locally used MCGAMs.
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SMTP Recipient
As the RFC 822 and X.400 worlds are in principle fully
connected, there is no technical reason for this situation
to occur. In practice, this is not the case. In some cases,
routing may be configured to use X.400 to connect an RFC 822
island to the Internet. The information that this part of
the domain space is to be routed by X.400 rather than
remaining within the RFC 822 world shall be configured
privately into the gateway in question. X.400 routing shall
not make use of the presence of the RFC-822 DDA to perform
X.400 routing. The OR address shall then be generated in
the same manner as for an IPMS address, using the locally
available MCGAMs. It is to support this case that the
definition of the global domain to gateway mapping is
important, as the use of this mapping will lead to a remote
X.400 address, which can be routed by X.400 routing
procedures. The information in this mapping shall not be
used as a basis for deciding to convert a message from RFC
822 to X.400.
Three examples are given, neither of which has applicable MCGAMs.
Example 1: (Address not in "localpart" "@" "domainpart")
@relay.co.uk:userb@host2
maps to
c=gb; a= ; p=uk.ac; o=mr; dd.rfc-822=(a)relay.co.uk:userb(a)host2;
Example 2: (Address with non printablestring characters)
Tom_Harris@cs.widget.com
maps to
c=us; a=MCI; P=relay; dd.rfc-822=Tom(u)Harris(a)cs.widget.com;
Example 3: (Address with an entry for alter.net into the OR Address
of Preferred Gateway table, pointing to c=gb; A=BTglobal; P=relay)
postmaster@UK.alter.net
maps to
c=gb; a=BTglobal; P=relay; dd.rfc-822=postmaster(a)UK.alter.net;
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4.3.4.1. Heuristic for mapping RFC 822 to X.400
The following heuristic, which relates to ordering of address
components, may be used when mapping from RFC 822 to X.400. The
ordering of attributes may be inverted or mixed, and so the following
heuristics may be applied:
If there is an Organization attribute to the left of any Org Unit
attribute, assume that the hierarchy is inverted. This is to
facilitate the situation where a user has input the attributes in
reverse hierarchical order. To do this the gateway shall first
map according to the order defined in 4.3.3. If this mapping
generates an address which X.400 address verification shows to be
invalid, this heuristic may be applied as an alternative to
immediate rejection of the address.
4.3.5. X.400 -> RFC 822 Basic Address Mapping
There are two basic cases:
1. RFC 822 addresses encoded in X.400.
2. "Genuine" X.400 addresses. This may include symmetrically
encoded RFC 822 addresses.
When an MTS Recipient OR Address is interpreted, gatewaying will be
selected if there is a single "RFC-822" domain defined attribute
present. In this case, use mapping A and in other cases, use mapping
B.
RFC 1327 specified that this shall only be done when the gateway
identfied is local or otherwise known, and identified the approach
specified here as a pragmatic option. Experience has shown that this
is effective in practice, despite theoretical problems.
If a gateway wishes to make a mapping in a manner similar to RFC
1327, but does not wish for this global interpretation (e.g., to
support an RFC 822 local system, which does not use global
addressing), then it may choose a private domain defined attribute,
different to "RFC-822". An RFC 1327 gateway might be configurable to
operate in this manner.
Mapping A
1. Map the domain defined attribute value to ASCII, as defined
in Chapter 3, and drop all other attributes.
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Mapping B
This is used for X.400 addresses which do not use the explicit RFC
822 encoding.
1. For all string encoded attributes, remove any leading or
trailing spaces, and replace adjacent spaces with a single
space.
The only attribute which is permitted to have zero length is
the ADMD. This shall be mapped onto a single space.
These transformations are for lookup only. If an
EBNF.std-or-address mapping is used as in 4), then the
original values shall be used.
2. The numeric country codes may be mapped to the two letter
values (as defined in ISO 3166). Global mappings are
usually only defined in terms of the ISO 3166 codes.
3. Noting the hierarchy specified in 4.3.1 and including
omitted attributes, determine the maximum set of attributes
which have an associated domain specification in the local
set of MCGAMs. If no match is found, allocate the domain as
described below, and go to step 5. The default domain to be
used is the specification of the local gateway. A gateway
may use other domains according to private mapping tables or
heuristics. For example, it may choose a domain which it
knows to provide a free gateway service to the mapped
address.
In cases where the address refers to an X.400 UA, it is
important that the generated domain will correctly route to
a gateway. In general, this is achieved by carefully co-
ordinating RFC 822 routing with the definition of the
MCGAMs, as there is no easy way for the gateway to make this
check. One rule that shall be used is that domains with
only one component will not route to a gateway. If the
generated domain does not route correctly, the address is
treated as if no match is found.
The gateway may also make use of a mapping equivalent to the
MCGAM mapping to determine the domain to use. This mapping
is done from the OR Address hierarchy. This is not a
global mapping, but is a routing style mapping from the OR
Address space, to enable a best choice domain to be
inserted. This mapping is supported by the three MCGAM
lookup mechanisms.
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4. The mapping identified in 3) gives a domain, and an OR
address prefix. Follow the hierarchy: C, ADMD, PRMD, O, OU.
For each successive component below the OR address prefix, which
conforms to the syntax EBNF.domain-syntax (as de