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Network Working Group Request for Comments: 1327 Obsoletes: RFCs 987, 1026, 1138, 1148 Updates: RFC 822 |
S. Hardcastle-Kille University College London May 1992 |
This RFC specifies an IAB standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "IAB Official Protocol Standards" for the standardization state and status of this protocol. Distribution of this memo is unlimited.
This document describes a set of mappings which will enable interworking between systems operating the CCITT X.400 1988) Recommendations on Message Handling Systems / ISO IEC 10021 Message Oriented Text Interchange Systems (MOTIS) [CCITT/ISO88a], and systems using the RFC 822 mail protocol [Crocker82a] or protocols derived from RFC 822. The approach aims to maximise the services offered across the boundary, whilst not requiring unduly complex mappings. The mappings should not require any changes to end systems. This document is a revision based on RFCs 987, 1026, 1138, and 1148 [Kille86a,Kille87a] which it obsoletes.
This document specifies a mapping between two protocols. This specification should be used when this mapping is performed on the DARPA Internet or in the UK Academic Community. This specification may be modified in the light of implementation experience, but no substantial changes are expected.
1 - Overview
1.1 - X.400
1.2 - RFC 822
1.3 - The need for conversion
1.4 - General approach
1.5 - Gatewaying Model
1.6 - X.400 (1984)
1.7 - Compatibility with previous versions
1.8 - Aspects not covered
1.9 - Subsetting
1.10 - Document Structure
1.11 - 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
4 - Addressing
4.1 - A textual representation of MTS.ORAddress
4.2 - Basic Representation
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
5.2 - Return of Contents
5.3 - X.400 -> RFC 822
Appendix A - Mappings Specific to SMTP
Appendix B - Mappings specific to the JNT Mail
1 - Introduction
2 - Domain Ordering
3 - Addressing
4 - Acknowledge-To:
5 - Trace
6 - Timezone specification
7 - Lack of 822-MTS originator specification
Appendix C - Mappings specific to UUCP Mail
Appendix D - Object Identifier Assignment
Appendix E - BNF Summary
Appendix F - Format of address mapping tables
1 - Global Mapping Information
2 - Syntax Definitions
3 - Table Lookups
4 - Domain -> O/R Address format
5 - O/R Address -> Domain format
6 - Domain -> O/R Address of Gateway table
Appendix G - Mapping with X.400(1984)
Appendix H - RFC 822 Extensions for X.400 access
Appendix I - Conformance
Appendix J - Change History: RFC 987, 1026, 1138, 1148
1 - Introduction
2 - Service Elements
3 - Basic Mappings
4 - Addressing
5 - Detailed Mappings
6 - Appendices
Appendix K - Change History: RFC 1148 to this Document
1 - General
2 - Basic Mappings
3 - Addressing
4 - Detailed Mappings
5 - Appendices
References
Security Considerations
Author's Address
This document relates to the CCITT 1988 X.400 Series Recommendations
/ ISO IEC 10021 on the Message Oriented Text Interchange Service (MOTIS). This ISO/CCITT standard is referred to in this document as "X.400", which is a convenient shorthand. Any reference to the 1984 CCITT Recommendations will be explicit. 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. It is expected that X.400 will be implemented very widely.
RFC 822 evolved as a messaging standard on the DARPA (the US Defense Advanced Research Projects Agency) Internet. It specifies and end to end message format. It is used in conjunction with a number of different message transfer protocol environments.
SMTP Networks
On the DARPA Internet and other TCP/IP networks, RFC 822 is
used in conjunction with two other standards: RFC 821, also
known as Simple Mail Transfer Protocol (SMTP) [Postel82a],
and RFC 920 which is a Specification for domains and a
distributed name service [Postel84a].
UUCP Networks
UUCP is the UNIX to UNIX CoPy protocol, which is usually
used over dialup telephone networks to provide a simple
message transfer mechanism. There are some extensions to
RFC 822, particularly in the addressing. They use domains
which conform to RFC 920, but not the corresponding domain
nameservers [Horton86a].
Bitnet
Some parts of Bitnet and related networks use RFC 822
related protocols, with EBCDIC encoding.
JNT Mail Networks
A number of X.25 networks, particularly those associated
with the UK Academic Community, use the JNT (Joint Network
Team) Mail Protocol, also known as Greybook [Kille84a].
This is used with domains and name service specified by the
JNT NRS (Name Registration Scheme) [Larmouth83a].
The mappings specified here are appropriate for all of these networks.
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 to use of an X.400 IPMS, 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 protocol mappings between RFC 822 and X.400. This is standard usage amongst mail implementors, but should be noted carefully 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 (MT) Abstract Service [CCITT/ISO88c]. The MT 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). There is also an Envelope, which includes an ID, an originator, and a list of recipients. Submission also includes the probe service, which supports the IPMS Probe. Delivery also includes Reports, which indicate whether a given MTS Message has been delivered or not.
The MTS is REFINED into 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. Some 822-MTS protocols, in particular SMTP, can provide additional functionality, but as these are neither mandatory in SMTP, nor available in other 822-MTS protocols, they are not considered here. Details of aspects specific to two 822-MTS protocols are given in Appendices B and C. 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 P2 heading fields, although some are analogous to MTS Service Elements
or MTA Service Elements.
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 822-MTS service mapping onto the MTS Service Elements and RFC 822 mapping onto an IPM, but reality just does not fit. Another elegant approach would be to treat this document as the definition of an X.400 Access Unit (AU). Again, reality does not fit. 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 + 822-MTS 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 822-MTS information is always mapped into an IPM (MTA, MTS, and IPMS Services). Going from X.400 to RFC 822, an RFC 822 message and the associated 822-MTS information may be derived from:
Probes (MTA Service) must 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 should be rejected at the gateway.
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 messages 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.
Much of this work 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. A basic decision is that the mapping defined in this document is to 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. This is important, to give good support to communities which will utilise full X.400 at an early date. To interwork with 1984 systems, Appendix G 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 G.
The changes between this and older versions of the document are given
in Appendices I and J. These are RFCs 987, 1026, 1138, and 1148. This document is a revision of RFC 1148 [Kille90a]. As far as possible, changes have been made in a compatible fashion.
There have been a number of cases where RFC 987 was 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
- Mapping X.400 to extended versions of RFC 822, with support
for multimedia content.
The first two of 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 RFC 987(88) 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 H.
This proposal specifies a mapping which is appropriate to preserve services in existing RFC 822 communities. Implementations and specifications which subset this specification are strongly discouraged.
This document has five chapters:
There are also eleven 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 this document. In particular, there were comments and suggestions from: Maurice Abraham (HP); Harald Alvestrand (Sintef); Peter Cowen (X-Tel); Jim Craigie (JNT); Ella Gardener (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).
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 an extra body part.
Encrypted:
Supported by use of a heading extension.
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.
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:
The following non-standard services (headers) may be present. These are defined in more detail in Chapter 5 (5.3.4, 5.3.6, 5.3.7):
Autoforwarded:
Content-Identifier:
Conversion:
Conversion-With-Loss:
Delivery-Date:
Discarded-X400-IPMS-Extensions:
Discarded-X400-MTS-Extensions:
DL-Expansion-History:
Deferred-Delivery:
Expiry-Date:
Importance:
Incomplete-Copy:
Language:
Latest-Delivery-Time:
Message-Type:
Obsoletes:
Original-Encoded-Information-Types:
Originator-Return-Address:
Priority:
Reply-By:
Requested-Delivery-Method:
Sensitivity:
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. It is intended that these fields will be registered, and that co- operating RFC 822 systems may 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 must 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 (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
previous service).
Non-delivery Notification
Not supported, although in general an RFC 822 system 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
Some types supported. IA5 is fully supported.
ForwardedIPMessage is supported, with some loss of
information. Other types get some measure of support,
dependent on X.400 facilities for conversion to IA5. This
will only be done where content conversion is not
prohibited.
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). Thus, the gateway cannot prevent
assignment of alternative recipients on the RFC 822 side.
This service really means giving the user control as to
whether or not an alternate recipient is allowed. This
specification requires transfer of messages to RFC 822
irrespective of this service request, and so this service is
not supported.
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. In this case, only messages with IA5 body parts,
other body parts which contain only IA5, and Forwarded IP
Messages (subject recursively to the same restrictions),
will be mapped.
Conversion Prohibition in Case of Loss of Information
Supported.
Counter Collection
N/A (PDAU).
Counter Collection with Advice
N/A (PDAU).
Cross Referencing Indication
Supported.
Deferred Delivery
N/A (prior). This service should 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.
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 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 [Crocker82a]. 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.
- Around the ":" in all headers - EBNF.labelled-integer - EBNF.object-identifier - EBNF.encoded-info
RFC 822 folding rules are applied to all headers.
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.
MTA MTAAbstractService defined in Section 13 of X.411 / ISO 10021-4.
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 [CCITT/ISO88d]. 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, the mappings to IA5 defined in CCITT Recommendation X.408 (1988) shall be used [CCITT/ISO88a]. These will then be encoded in ASCII.
There is also a need to represent Teletex Strings in ASCII, for some aspects of O/R Address. For these, the following encoding is used:
teletex-string = *( ps-char / t61-encoded )
t61-encoded = "{" 1* t61-encoded-char "}"
t61-encoded-char = 3DIGIT
Common characters are mapped simply. Other octets are mapped using a quoting mechanism similar to the printable string mechanism. Each octet is represented as 3 decimal digits.
There are a number of places where a string may have a Teletex and/or Printable String representation. The following BNF 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.
Both UTCTime and the RFC 822 822.date-time syntax contain: Year (lowest two digits), Month, Day of Month, hour, minute, second (optional), and Timezone. 822.date-time also contains an optional day of the week, but this is redundant. Therefore 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.
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., +0000).
When mapping time values, the timezone shall be preserved as specified. The date shall not be normalised to any other timezone.
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, addition of an appropriate text label is strongly encouraged.
labelled-integer ::= [ key-string ] "(" numericstring ")"
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 ")"
joint-iso-ccitt(2) mhs (6) ipms (1) ep (11) ia5-text (0)
or
(2) (6) (1)(11)(0)
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 must 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 must only be used in cases where the printable strings may only be 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) <-> (
Addressing is probably the trickiest problem of an X.400 <-> RFC 822 gateway. Therefore it is given a separate chapter. This chapter, as a side effect, also defines a textual representation of an X.400 O/R Address.
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
In an 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 O/R Address, used by the MTS for routing, is defined by MTS.ORAddress. In IPMS, the MTS.ORAddress is encapsulated within IPMS.ORDescriptor.
It can be seen that RFC 822 822.address must be mapped with IPMS.ORDescriptor, and that RFC 822 EBNF.822-address must be mapped with MTS.ORAddress.
MTS.ORAddress is structured as a set of attribute 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 O/R 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 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:
Attribute (Component) Key Enc Ref Id
.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
I labelled-integer
X presentation-address
The BNF for presentation-address is taken from the specification "A String Encoding of Presentation Address" [Kille89a].
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 BNF. When generating ASN.1, the NumericString encoding shall be used if the string contains 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 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.
Recently, ISO has undertaken work to specify a string form of O/R Address [CCITT/ISO91a]. This has specified a number of string keywords for attributes. As RFC 1148 was an input to this work, many of the keywords are the same. To increase compatability, 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.
Keyword Alternative
ADMD A
PRMD P
GQ Q
X121 X.121
UA-ID N-ID
PD-OFFICE-NUMBER PD-OFFICE NUMBER
When mapping from RFC 822 to X.400, the 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.
Handling of Personal Name and Teletex Personal Name based purely on the EBNF.standard-type syntax defined above is likely to be clumsy. It seems desirable to utilise the "human" conventions for encoding these components. A syntax is defined, which is designed to provide a clean encoding for the common cases of O/R 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 O/R address personal name. To map from a string to O/R 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 O/R 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 suggest that Initials is used to encode ALL initials. 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, but this appears to be contrived!
Given this structure, we can specify a BNF representation of an O/R Address.
std-or-address = 1*( "/" attribute "=" value ) "/"
attribute = standard-type
/ "RFC-822"
/ registered-dd-type
/ dd-key "." std-printablestring
standard-type = key-string
registered-dd-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 = "$" ps-char
The standard-type is any key defined in the table in Section 4.2, except PN, and DD. The BNF leads to a set of attribute/value pairs. The value is interpreted according to the EBNF encoding defined in the table.
If the standard-type is PN, the value is interpreted according to EBNF.encoded-pn, and the components of MTS.PersonalName and/or MTS.TeletexPersonalName derived accordingly.
If dd-key is the recognised Domain Defined string (DD), then the type and value are interpreted according to the syntax implied from the encoding, and aligned to either the teletex or printable string form. Key and value shall have the same encoding.
If value is "RFC-822", then the (printable string) Domain Defined Type of "RFC-822" is assumed. This is an optimised encoding of the domain defined type defined by this specification.
The matching of all keywords shall be done in a case-independent manner.
EBNF.std-or-address uses the characters "/" and "=" as delimiters. Domain Defined Attributes and any value may contain these characters. A quoting mechanism, using the non-printable string "$" is used to allow these characters to be represented.
If the value is registered-dd-type, and the value is registered at the Internet Assigned Numbers Authority (IANA) as an accepted Domain Defined Attribute type, then the value shall be interpreted
accordingly. This restriction maximises the syntax checking which can be done at a gateway.
Ideally, the mapping specified 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:
Therefore, an asymmetrical mapping is defined, which can be symmetrical where there is appropriate administrative control.
The std-or-address syntax is used to encode O/R Address information in the 822.local-part of EBNF.822-address. In some cases, further O/R 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 O/R 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, and by restrictions in RFC 821.
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 O/R 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. The following O/R Address attributes are considered
as a hierarchy, and may be specified by the domain. They are (in order of hierarchy):
Country, ADMD, PRMD, Organisation, Organisational Unit
There may be multiple Organisational Units.
A global mapping is defined between domain specifications, and some set of attributes. This association proceeds hierarchically. For example, if a domain implies ADMD, it also implies country. Subdomains under this are associated according to the O/R Address hierarchy. For example:
=> "AC.UK" might be associated with
C="GB", ADMD="GOLD 400", PRMD="UK.AC"
then domain "R-D.Salford.AC.UK" maps with
C="GB", ADMD="GOLD 400", PRMD="UK.AC", O="Salford", OU="R-D"
There are three basic reasons why a domain/attribute mapping might be maintained, as opposed to using simply subdomains:
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 chosen as it is the one chosen by the various domain service administrations.
two cases differently, they must be allowed for).
This set of mappings needs be known by the gateways relaying between the RFC 822 world, and the O/R Address space associated with the mapping in question. There needs to be a single global definition of this set of mappings. A mapping implies an adminstrative equivalence between the two parts of the namespaces which are mapped together. To correctly route in all cases, it is necessary for all gateways to know the mapping. To facilitate distribution of a global set of mappings, a format for the exchange of this information is defined in Appendix F.
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"
The first three attributes are determined by the domain Widget.COM. Then, the first element of OrganizationalUnitNames is determined systematically, and the remaining attributes are encoded on the LHS. In an extreme case, all of the attributes will be on the LHS. As the domain cannot be null, the RHS will simply be a domain indicating the gateway.
The RHS (domain) encoding is designed to deal cleanly with common addresses, and so the amount of information on the RHS is maximised. In particular, it covers the Mnemonic O/R Address using a 1984 compatible encoding. This is seen as the dominant form of O/R Address. Use of other forms of O/R Address, and teletex encoded attributes will require an LHS encoding.
There is a further mechanism to simplify the encoding of common cases, where the only attributes to be encoded on the LHS is a (non- Teletex) Personal Name attributes which comply with the restrictions of 4.2.1. 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 previous example O/R 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 simply used to route the message in the standard manner. The standard domain mechanisms are used to select appropriate gateways for the corresponding O/R Address space. In most cases, this will be done by registering the higher levels, and assuming that the gateway can handle the lower levels.
In some cases, the encoding defined above may be reversed, to give a "natural" encoding of genuine RFC 822 addresses. This depends largely on the allocation of appropriate management domains.
The general case is mapped by use of domain defined attributes. A 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 depends on the context of the gateway.
Other O/R Address attributes will be used to identify a context in which the O/R 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 O/R Address can have a maxmimum of four domain defined attributes. Where the printable string generated from the RFC 822 address exceeeds this value, 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 DDA keywords are: RFC822C1; RFC822C2; RFC822C3. Longer addresses cannot be encoded.
There is, analagous with 4.3.1, a means to associate parts of the O/R Address hierarchy with domains. There is an analogous global mapping, which in most cases will be the inverse of the domain to O/R address mapping. The mapping is maintained separately, as there may be differences (e.g., two alternate domain names map to the same set of O/R address components).
In most cases, ordering of O/R Address components is not significant for the mappings specified. However, Organisational 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, Organisational Units will appear
in the same order, whether encoded on the RHS or LHS).
Note the differences of JNT Mail as described in
Appendix B.
- 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, there are NO implications on ordering significance within X.400, where this is a Management Domain issue.
There are two basic cases:
The mapping shall proceed as follows, by first assuming case 1).
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.
NOTE:It may be appropriate to reduce a source route address
to this form by removal of all bar the last domain. In
terms of the design intentions of RFC 822, this would
be an incorrect action. However, in most real cases,
it will do the "right" thing and provide a better
service to the end user. This is a reflection on the
excessive and inappropriate use of source routing in
RFC 822 based systems. 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 globally defined mappings (see Appendix F). If this fails, and the EBNF.domain does not explicitly identify the local gateway, go to stage II. If the domain explicitly identifies the gateway, allocate no attributes. Otherwise, allocate the attributes associated with EBNF.known-domain. 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 mapping used identifies an "omitted attribute", then this attribute should 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.
At this stage, a set of attributes has been derived, which will give appropriate routing within X.400. If any of the later steps of Stage I force use of Stage II, then these attributes should be used in Stage II.
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. If the set of attributes leads to an address of any
form other than mnemonic form, then only these attributes
should be taken. If (for mnemonic form) the values generated
conflict with those derived in step 2 (e.g., a duplicated
country attribute), the domain is assumed to be a remote
gateway. In this case, take only the LHS derived
attributes, together with any RHS dericed attributes which
are more significant thant the most signicant attribute
which is duplicated (e.g., if there is a duplicate PRMD, but
no LHS derived ADMD and country, then the ADMD and country
should be taken from the RHS). therwise add LHS and RHS
derived attributes together.
This will only be reached if the RFC 822 EBNF.822-address is not a valid X.400 encoding. This implies that the address must refer to a recipient on an RFC 822 system. Such addresses shall be encoded in an X.400 O/R Address using a domain defined attribute.
It may not be 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.
If Stage I has identified a set of attributes, use these to build the remainder of the address. The administrative equivalence of the mappings will ensure correct routing throug X.400 to a gateway back to RFC 822.
If Stage I has not identified a set of attributes, the remainder of the O/R address effectively identifies a source route to a gateway from the X.400 side. There are three cases, which are handled differently:
822-MTS Return Address
This shall be set up so that errors are returned through the
same gateway. Therefore, the O/R 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. A globally defined set of mappings is used, which
identifies (the O/R Address components of) appropriate
gateways for parts of the domain namespace. The longest
possible match on the 822.domain defines which gateway to
use. The table format for distribution of this information
is defined in Appendix F.
This global mapping is used for parts of the RFC 822 namespace which do not have an administrative equivalence with any part of the X.400 namespace, but 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 globally defined mappings. In some cases, it may be appropriate to locally override the globally defined mappings (e.g., to identify a gateway close to a recipient of the message). This is likely to be where the global mapping identifies a public gateway, and the local gateway has an agreement with a private gateway which it prefers to use.
822-MTS Recipient
As the RFC 822 and X.400 worlds are fully connected, there
is no technical reason for this situation to occur. In some cases, routing may be configured to connect two parts of the RFC 822 world using X.400. The information that this part of the domain space should be routed by X.400 rather than remaining within the RFC 822 world will be configured privately into the gateway in question. The O/R address shall then be generated in the same manner as for an IPMS address, using the globally defined mappings. 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.
RFC 822 users will often use an LHS encoded address to identify an X.400 recipient. Because the syntax is fairly complex, a number of heuristics may be applied to facilitate this form of usage. A gateway should take care not to be overly "clever" with heuristics, as this may cause more confusion than a more mechanical approach. The heuristics are as follows:
colon-or-address = 1*(attribute "=" value ";" *(LWSP-char))
The remaining heuristic relates to ordering of address components. The ordering of attributes may be inverted or mixed. For this reason, the following heuristics may be applied:
There are two basic cases:
When a MTS Recipient O/R Address is interpreted, gatewaying will be selected if there is a single "RFC-822" domain defined attribute present and the local gateway is identified by the remainder of the O/R Address. In this case, use mapping A. For other O/R Addresses which
AND
use mapping A. In other cases, use mapping B.
NOTE:
A pragmatic approach would be to assume that any O/R
Address with the special domain defined attribute identifies
an RFC 822 address. This will usually work correctly, but is
in principle not correct. Use of this approach is
conformant to this specification.
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 should 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
orginal values should be used.
Note: It might be appropriate to use a non-local domain.
This would be selected by a global mapping analagous to
the one described at the end of 4.3.4. This is not
done, primarily because use of RFC 822 to connect X.400
systems is not expected to be significant.
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 global mappings, 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.
For addresses of mnemonic form, if the remaining components
are personal-name components, conforming to the restrictions
of 4.2.1, then EBNF.encoded-pn is derived to form
simply encoded as 822.local-part using the
EBNF.std-or-address syntax. If necessary, the
If the derived 822.local-part can only be encoded by use of
822.quoted-string, then use of the mapping defined
in [Kille89b] may be appropriate. Use of this mapping is
discouraged.
There are two types of repeated mapping:
2 A source route, where the repetition occurs across multiple
gateways
It is possible to supply an address which is recurive at a single gateway. For example:
C = "XX"
ADMD = "YY"
O = "ZZ"
"RFC-822" = "Smith(a)ZZ.YY.XX"
This is mapped first to an RFC 822 address, and then back to the X.400 address:
C = "XX"
ADMD = "YY"
O = "ZZ"
Surname = "Smith"
In some situations this type of recursion may be frequent. It is important that where this occurs, that no unnecessary protocol conversion occurs. This will minimise loss of service.
The mappings defined are symmetrical and reversible across a single gateway. The symmetry is particularly useful in cases of (mail exploder type) distribution list expansion. For example, an X.400 user sends to a list on an RFC 822 system which he belongs to. The
received message will have the originator and any 3rd party X.400 O/R Addresses in correct format (rather than doubly encoded). In cases (X.400 or RFC 822) where there is common agreement on gateway identification, then this will apply to multiple gateways.
When a message traverses multiple gateways, the mapping will always be reversible, in that a reply can be generated which will correctly reverse the path. In many cases, the mapping will also be symmetrical, which will appear clean to the end user. For example, if countries "AB" and "XY" have RFC 822 networks, but are interconnected by X.400, the following may happen: The originator specifies:
Joe.Soap@Widget.PTT.XY
This is routed to a gateway, which generates:
C = "XY"
ADMD = "PTT"
PRMD = "Griddle MHS Providers"
Organisation = "Widget Corporation"
Surname = "Soap"
Given Name = "Joe"
This is then routed to another gateway where the mapping is reversed to give:
Joe.Soap@Widget.PTT.XY
Here, use of the gateway is transparent.
Mappings will only be symmetrical where mapping tables are defined. In other cases, the reversibility is more important, due to the (far too frequent) cases where RFC 822 and X.400 services are partitioned.
The syntax may be used to source route. THIS IS STRONGLY DISCOURAGED. For example:
C = "UK"
ADMD = "Gold 400"
PRMD = "UK.AC"
"RFC-822" = "/PN=Duval/DD.Title=Manager/(a)Inria.ATLAS.FR"
This will be sent to an arbitrary UK Academic Community gateway by X.400. Then it will be sent by JNT Mail to another gateway determined by the domain Inria.ATLAS.FR (FR.ATLAS.Inria). This will
then derive the X.400 O/R Address:
C = "FR"
ADMD = "ATLAS"
PRMD = "Inria"
PN.S = "Duval"
"Title" = "Manager"
Similarly:
RFC 822 -> X.400 -> RFC 822
"/C=UK/ADMD=BT/PRMD=AC/RFC-822=jj(a)seismo.css.gov/"@monet.berkeley.edu
This will be sent to monet.berkeley.edu by RFC 822, then to the AC PRMD by X.400, and then to jj@seismo.css.gov by RFC 822.
Directory Names are an optional part of O/R Name, along with O/R Address. The RFC 822 addresses are mapped onto the O/R Address component. As there is no functional mapping for the Directory Name on the RFC 822 side, a textual mapping is used. There is no requirement for reversibility in terms of the goals of this specification. There may be some loss of functionality in terms of third party recipients where only a directory name is given, but this seems preferable to the significant extra complexity of adding a full mapping for Directory Names.
Note:There is ongoing work on specification of a "user friendly" format for directory names. If this is adopted as an internet standard, it will be recommended, but not required, for use here.
The basic mappings at the MTS level are:
1) 822-MTS originator ->
MTS.PerMessageSubmissionFields.originator-name
MTS.OtherMessageDeliveryFields.originator-name ->
822-MTS originator
2) 822-MTS recipient ->
MTS.PerRecipientMessageSubmissionFields
MTS.OtherMessageDeliveryFields.this-recipient-name ->
822-MTS recipient
822-MTS recipients and return addresses are encoded as EBNF.822-
address.
The MTS Originator is always encoded as MTS.OriginatorName, which maps onto MTS.ORAddressAndOptionalDirectoryName, which in turn maps onto MTS.ORName.
From the 822-MTS Originator, use the basic ORAddress mapping, to generate MTS.PerMessageSubmissionFields.originator-name (MTS.ORName), without a DirectoryName.
For recipients, the following settings are made for each component of MTS.PerRecipientMessageSubmissionFields.
recipient-name
This is derived from the 822-MTS recipient by the basic
ORAddress mapping.
originator-report-request
This is be set according to content return policy, as
discussed in Section 5.2.
explicit-conversion
This optional component is omitted, as this service is not
needed
extensions
The default value (no extensions) is used
The basic functionality is to generate the 822-MTS originator and recipients. There is information present on the X.400 side, which cannot be mapped into analogous 822-MTS services. For this reason, new RFC 822 fields are added for the MTS Originator and Recipients. The information discarded at the 822-MTS level will be present in these fields. In some cases a (positive) delivery report will be generated.
Use the basic ORAddress mapping, to generate the 822-MTS originator (return address) from MTS.OtherMessageDeliveryFields.originator-name (MTS.ORName). If MTS.ORName.directory-name is present, it is discarded. (Note that it will be presented to the user, as described in 4.6.2.2).
The 822-MTS recipient is conceptually generated from
MTS.OtherMessageDeliveryFields.this-recipient-name. This is done by
taking MTS.OtherMessageDeliveryFields.this-recipient-name, and
generating an 822-MTS recipient according to the basic ORAddress
mapping, discarding MTS.ORName.directory-name if present. However,
if this model was followed exactly, there would be no possibility to
have multiple 822-MTS recipients on a single message. This is
unacceptable, and so layering is violated. The mapping needs to use
the MTA level information, and map each value of
MTA.PerRecipientMessageTransferFields.recipient-name, where the
responsibility bit is set, onto an 822-MTS recipient.
Not all per-recipient information can be passed at the 822-MTS level. For this reason, two new RFC 822 headers are created, in order to carry this information to the RFC 822 recipient. These fields are "X400-Originator:" and "X400-Recipients:".
The "X400-Originator:" field is set to the same value as the 822-MTS
originator. In addition, if
MTS.OtherMessageDeliveryFields.originator-name (MTS.ORName) contains
MTS.ORName.directory-name then this Directory Name shall be
represented in an 822.comment.
Recipient names, taken from each value of
MTS.OtherMessageDeliveryFields.this-recipient-name and
MTS.OtherMessageDeliveryFields.other-recipient-names are made
available to the RFC 822 user by use of the "X400-Recipients:" field.
By taking the recipients at the MTS level, disclosure of recipients
will be dealt with correctly. However, this conflicts with a desire
to optimise mail transfer. There is no problem when disclosure of
recipients is allowed. Similarly, there is no problem if there is
only one RFC 822 recipient, as the "X400-Recipients field is only
given one address.
There is a problem if there are multiple RFC 822 recipients, and disclosure of recipients is prohibited. Two options are allowed:
X400-Recipients: non-disclosure:;
This is the recommended option.
A third option of ignoring the disclosure flag is not allowed. If any MTS.ORName.directory-name is present, it shall be represented in an 822.comment.
If MTS.OtherMessageDeliveryFields.orignally-intended-recipient-name is present, then there has been redirection, or there has been distribution list expansion. Distribution list expansion is a per- message option, and the information associated with this is represented by the "DL-Expansion-History:" field descrined in Section 5.3.6. Other information is represented in an 822.comment associated associated with MTS.OtherMessageDeliveryFields.this-recipient-name, The message may be delivered to different RFC 822 recipients, and so several addresses in the "X400-Recipients:" field may have such comments. The non-commented recipient is the RFC 822 recipient. The EBNF of the comment is:
redirect-comment =
[ "Originally To:" ] mailbox "Redirected"
[ "Again" ] "on" date-time
"To:" redirection-reason
redirection-reason =
"Recipient Assigned Alternate Recipient"
/ "Originator Requested Alternate Recipient"
/ "Recipient MD Assigned Alternate Recipient"
It is derived from
MTA.PerRecipientMessageTransferFields.extension.redirection-history.
An example of this is:
X400-Recipients: postmaster@widget.com (Originally To:
sales-manager@sales.widget.com Redirected
on Thu, 30 May 91 14:39:40 +0100 To: Originator Assigned
Alternate Recipient postmaster@sales.widget.com Redirected
Again on Thu, 30 May 91 14:41:20 +0100 To: Recipient MD
Assigned Alternate Recipient)
In addition, the following per-recipient services from
MTS.OtherMessageDeliveryFields.extensions are represented in comments
if they are used. None of these services can be provided on RFC 822
networks, and so in general these will be informative strings
associated with other MTS recipients. In some cases, string values
are defined. For the remainder, the string value shall be chosen by
the implementor. If the parameter has a default value, then no
comment shall be inserted when the parameter has that default value.
requested-delivery-method
physical-forwarding-prohibited
"(Physical Forwarding Prohibited)".
physical-forwarding-address-request
"(Physical Forwarding Address Requested)".
physical-delivery-modes
registered-mail-type
recipient-number-for-advice
physical-rendition-attributes
physical-delivery-report-request
"(Physical Delivery Report Requested)".
proof-of-delivery-request
"(Proof of Delivery Requested)".
If MTA.PerRecipientMessageTransferFields.per-recipient-indicators requires a positive delivery notification, this shall be generated by the gateway. Supplementary Information shall be set to indicate that the report is gateway generated. This information shall include the name of the gateway generating the report.
A mapping from 822.msg-id to MTS.MTSIdentifier is defined. The reverse mapping is not needed, as MTS.MTSIdentifier is always mapped onto new RFC 822 fields. The value of MTS.MTSIdentifier.local-part will facilitate correlation of gateway errors.
To map from 822.msg-id, apply the standard mapping to 822.msg-id, in order to generate an MTS.ORAddress. The Country, ADMD, and PRMD components of this are used to generate MTS.MTSIdentifier.global- domain-identifier. MTS.MTSIdentifier.local-identifier is set to the 822.msg-id, including the braces "<" and ">". If this string is longer than MTS.ub-local-id-length (32), then it is truncated to this length.
The reverse mapping is not used in this specification. It would be applicable where MTS.MTSIdentifier.local-identifier is of syntax 822.msg-id, and it algorithmically identifies MTS.MTSIdentifier.
All RFC 822 addresses are assumed to use the 822.mailbox syntax. This includes all 822.comments associated with the lexical tokens of the 822.mailbox. In the IPMS O/R Names are encoded as MTS.ORName. This is used within the IPMS.ORDescriptor, IPMS.RecipientSpecifier, and IPMS.IPMIdentifier. An asymmetrical mapping is defined between these components.
To derive IPMS.ORDescriptor from an RFC 822 address.
- The 822.phrase component if the 822.address is an
822.phrase 822.route-addr construct.
- Any 822.comments, in order, retaining the parentheses.
This string is then encoded into T.61 use a human oriented mapping (as described in Chapter 3). If the string is not null, it is assigned to IPMS.ORDescriptor.free-form-name.
If IPMS.ORDescriptor is being used in IPMS.RecipientSpecifier,
IPMS.RecipientSpecifier.reply-request and
IPMS.RecipientSpecifier.notification-requests are set to default
values (none and false).
If the 822.group construct is present, any included 822.mailbox is encoded as above to generate a separate IPMS.ORDescriptor. The 822.group is mapped to T.61, and a IPMS.ORDescriptor with only an free-form-name component built from it.
Mapping from IPMS.ORDescriptor to RFC 822 address. In the basic case, where IPMS.ORDescriptor.formal-name is present, proceed as follows.
EBNF.822-address.
2a. If IPMS.ORDescriptor.free-form-name is present, convert it to ASCII (Chapter 3), and use this as the 822.phrase component of 822.mailbox using the 822.phrase 822.route-addr construct.
2b. If IPMS.ORDescriptor.free-form-name is absent. If
EBNF.822-address is parsed as 822.addr-spec use this as the
encoding of 822.mailbox. If EBNF.822-address is parsed as
822.route 822.addr-spec, then a 822.phrase taken from
822.local-part is added.
(Tel +44-1-387-7050)
If IPMS.ORDescriptor.formal-name is absent, IPMS.ORDescriptor.free- form-name is converted to ASCII, and used as 822.phrase within the RFC 822 822.group syntax. For example:
Free Form Name ":" ";"
Steps 3-6 are then followed.
There is a need to map both ways between 822.msg-id and
IPMS.IPMIdentifier. This allows for X.400 Receipt Notifications,
Replies, and Cross References to reference an RFC 822 Message ID, which is preferable to a gateway generated ID. A reversible and symmetrical mapping is defined. This allows for good things to happen when messages