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Network Working Group Request for Comments: 2156 Obsoletes: 987, 1026, 1138, 1148, 1327, 1495 Updates: 822 Category: Standards Track |
S. Kille Isode Ltd. January 1998 MIXER (Mime Internet X.400 Enhanced Relay): |
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 © The Internet Society (1998). All Rights Reserved.
1 - Overview
1.1 - X.400
1.2 - RFC 822 and MIME
1.3 - The need for conversion
1.4 - General approach
1.5 - Gatewaying Model
1.6 - Support of X.400 (1984)
1.7 - X.400 (1992)
1.8 - MIME
1.9 - Body Parts
1.10 - Local and Global Scenarios
1.11 - Compatibility with previous versions
1.12 - Aspects not covered
1.13 - Subsetting
1.14 - Specification Language
1.15 - Related Specifications
1.16 - Document Structure
1.17 - Acknowledgements
2 - Service Elements
2.1 - The Notion of Service Across a Gateway
2.2 - RFC 822
2.3 - X.400
3 - Basic Mappings
3.1 - Notation
3.2 - ASCII and IA5
3.3 - Standard Types
3.4 - Encoding ASCII in Printable String
3.5 - RFC 1522
4 - Addressing and Message IDs
4.1 - A textual representation of MTS.ORAddress
4.2 - Global Address Mapping
4.3 - EBNF.822-address <-> MTS.ORAddress
4.4 - Repeated Mappings
4.5 - Directory Names
4.6 - MTS Mappings
4.7 - IPMS Mappings
5 - Detailed Mappings
5.1 - RFC 822 -> X.400: Detailed Mappings
5.2 - Return of Contents
5.3 - X.400 -> RFC 822: Detailed Mappings
Appendix A - Mappings Specific to SMTP
1 - Probes
2 - Long Lines
3 - SMTP Extensions
3.1 - SMTP Extension mapping to X.400
3.2 - X.400 Mapping to SMTP Extensions
Appendix B - Mapping with X.400(1984)
Appendix C - RFC 822 Extensions for X.400 access
Appendix D - Object Identifier Assignment
Appendix E - BNF Summary
Appendix F - Text format for MCGAM distribution
1 - Text Formats
2 - Mechanisms to register and to distributeMCGAMs
7 - Domain -> OR Address of Preferred Gatewaytable
8 - OR Addresss -> domain of Preferred Gatewaytable
1 - General
2 - Basic Mappings
3 - Addressing
4 - Detailed Mappings
5 - Appendices
Appendix J - Change History: RFC 1327 to this Document
1 - General
2 - Service Elements
3 - Basic Mappings
4 - Addressing
5 - Detailed Mappings
6 - Appendices
Appendix L - ASN.1 Summary
Security Considerations
Author's Address
References
Full Copyright Statement
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.
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.
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.
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:
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.
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:
The Origination service also allows for origination of a probe, which is an object to test whether a given IPM could be correctly received.
The Reception service also allows for receipt of Delivery Reports (DR), which indicate delivery success or failure.
These IPMS Services utilise the Message Transfer System (MTS) Abstract Service [12]. The MTS Abstract Service provides the following three basic services:
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.
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:
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
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].
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:
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.
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.
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].
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.
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.
There are two basic scenarios for X.400/MIME interworking:
Global Scenario
There are two global mail networks (Internet/MIME and X.400), interconnected by multiple gateways. Objects may be transferred over multiple gateways, and so it is important that gateways behave in a coherent fashion. MIXER is critical to support this scenario.
Local Scenario
A gateway is used to connect a closed community to a global mail network (this could be enforced by connectivity or gateway authorisation policy). This is a common commercial scenario. MIXER is useful to support this scenario, as it allows an industry standard provision of service, but this could be supported by something which was MIXER-like.
A solution for the global scenario will work for the local scenario. However, there are aspects of MIXER which have significant implementation or deployment effort (the global mapping is the major one, but there are other details too) which and are needed to support
the global scenario, but are not needed in the local scenario.
Note that the local scenario may be the driving force for most deployments, and support of the global scenario may be an important secondary goal.
There is also a transition effect. Gateways which are initially deployed in a strict local scenario situation start to find themselves in a global scenario. A common case is ADMD provided gateways, which are targeted strictly at the local scenario. In practice they soon start to operate in the global scenario, because of distribution lists and messages exchanged with X.400 users that are not customers of the ADMD. At this point, users are hurt by the restrictions of a local scenario gateway.
Note that conformance to MIXER applies to an instantiation of a gateway, not just an implementation (although clearly it is critical that the implementation is capable of being operated in a conformant manner).
MIXER's conformance target is the global scenario, and the specification of MIXER defines operation in this way.
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.
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
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.
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.
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:
The key elements of the ISO rules are:
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".
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].
This document has five chapters:
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.
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).
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).
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.
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.
When a user originates a message, a number of services are available. Some of these imply actions (e.g., delivery to a recipient), and some are insertion of known data (e.g., specification of a subject field). This chapter describes, for each offered service, to what extent it is supported for a recipient accessed through a gateway. There are three levels of support:
Supported
The corresponding protocol elements map well, and so the service
can be fully provided.
Not Supported
The service cannot be provided, as there is a complete mismatch.
Partial Support
The service can be partially fulfilled.
In the first two cases, the service is simply marked as "Supported" or "Not Supported". Some explanation may be given if there are additional implications, or the (non) support is not intuitive. For partial support, the level of partial support is summarised. Where partial support is good, this will be described by a phrase such as "Supported by use of.....". A common case of this is where the service is mapped onto a non-standard service on the other side of the gateway, and this would have lead to support if it had been a standard service. In many cases, this is equivalent to support. For partial support, an indication of the mechanism is given, in order to give a feel for the level of support provided. Note that this is not a replacement for Chapter 5, where the mapping is fully specified.
If a service is described as supported, this implies:
- Semantic correspondence. - No (significant) loss of information. - Any actions required by the service element.
An example of a service gaining full support: If an 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.
For reception, the list of service elements required to support this mapping is specified. This is really an indication of what a recipient might expect to see in a message which has been remotely
originated.
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.
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.
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.
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:
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:
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:
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.
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.
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
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).
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.
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.
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:).
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.
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.
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).
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
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
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
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.
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.
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.
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.
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.
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] .
A gateway will interpret all IA5 as ASCII. Thus, mapping between these forms is conceptual.
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.
Boolean is encoded as:
boolean = "TRUE" / "FALSE"
NumericString is encoded as:
numericstring = *(DIGIT / " ")
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.
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:
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".
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}"
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.
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.
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 = <a-z, A-Z, 0-9, and "-">
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 ")"
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.
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
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) <-> (
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:
RFC 1522 is only applied to fields which are "for information only". A gateway which interprets header elements according to RFC 1522 may
apply reasonable heuristics to minimise information loss.
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.
Section 4.3 is the core part of this chapter, and defines the mapping mechanism.
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.
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:
.generation-qualifier GQ P/T 18.3.12
.generation-qualifier GQ P/T 18.3.12 4
.value OU P/T 18.3.10 5
.value DD P/T 18.1 6
AddressComponents PD-EXT-DELIVERY P/T 18.3.5 15
.e163-4-address.number NET-NUM N 18.3.7 22
.e163-4-address.sub-address NET-SUB N 18.3.7 22
.psap-address NET-PSAP X 18.3.7 22
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 )
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/
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
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).
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:
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*<ps-char not including ".">
initial = ALPHA
surname = printablestring
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.
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
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.
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.
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:
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:
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:
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 = <ALPHA or DIGIT>
alphanumhyphen = <ALPHA or DIGIT or HYPHEN>
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.
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:
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"
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).
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.
This section defines the basic address mapping.
This section defines how X.400 addresses are represented in RFC 822 addresses.
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"
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.
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.
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.
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.
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:
There are three places where an order is specified:
- 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.
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.
Note that although this ordering is mandatory for this mapping, MIXER does not give additional implications on the ordering significance within X.400.
There are two basic cases:
The mapping shall proceed as follows, by first assuming case 1).
STAGE I.
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
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.
*( 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.
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.
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.
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.
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;
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.
There are two basic cases:
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
Mapping B
This is used for X.400 addresses which do not use the explicit RFC 822 encoding.
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.
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 wil