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INTERNET DRAFT Jeroen Houttuin
RARE WG-MSG RARE Secretariat
Rev. 3.3 25 May 1993
A tutorial on gatewaying between X.400 and Internet mail
Abstract
There are many ways in which X.400 and Internet (RFC 822) mail
systems can be interconnected. Addresses and service elements can
be mapped onto each other in different ways. From the early
available gateway implementations, one was not necessarily better
than another, but the sole fact that each handled the mappings in a
different way led to major interworking problems, especially when a
message (or address) crossed more than one gateway. The need for
one global standard on how to implement X.400 - Internet mail
gatewaying was satisfied by the Internet Request For Comments 1327,
"Mapping between X.400(1988)/ISO 10021 and RFC 822."
This tutorial was produced especially to help new gateway managers
find their way into the complicated subject of mail gatewaying
according to RFC 1327. The need for such a tutorial can be
illustrated by quoting the following discouraging paragraph from
RFC 1327, chapter 1: "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 introduction of this tutorial is general enough to be read not
only by gateway managers, but also by e-mail managers who are new
to gatewaying or to one of the two e-mail worlds in general. Parts
of this introduction can be skipped as needed.
For novice end-users, even this tutorial will be difficult to read.
They are encouraged to use the COSINE MHS pocket user guide [pug]
instead.
To a certain extent, this document can also be used as a reference
guide to X.400 <-> RFC 822 gatewaying. Wherever there is a lack of
detail in the tutorial, it will at least point to the corresponding
chapters in other documents. As such, it shields the RFC 1327
novice from too much detail.
Status of this Memo
The previous version of this document has been proposed by the
author to the IESG and the RTC to become both an Informational RFC
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and an RTR. For that purpose, this version (3.2) incorporates the
second round of solicited comments from the IESG and the RTC.
This document is an Internet Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas,
and its Working Groups. Note that other groups may also distribute
working documents as Internet Drafts.
Internet Drafts are draft documents valid for a maximum of six
months. Internet Drafts may be updated, replaced, or obsoleted by
other documents at any time. It is not appropriate to use Internet
Drafts as reference material or to cite them other than as a
"working draft" or "work in progress."
Please check the I-D abstract listing contained in each Internet
Draft directory to learn the current status of this or any other
Internet Draft.
Distribution of this memo is unlimited.
Acknowledgements
This tutorial was originally produced by SWITCH within the context
of the COSINE MHS contract. It is heavily based on other papers and
books, such as [JH-92], [HTA-faq], [822], [1280], [1310], and
[1327], from which large parts of text were reproduced (slightly
edited) by kind permission from the authors.
Disclaimer
This document is not everywhere exact and or complete in describing
the involved standards. Irrelevant details are left out and some
concepts are simplified for the ease of understanding. For
reference purposes, always use the original documents.
Format
This document is available in ASCII as well as in Postscript
format. The index is only available in the Postscript version.
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Contents
1. Introduction
1.1. What is X.400 ?
1.2. What is an RFC ?
1.3. What is RFC 822 ?
1.4. What is RFC 1327 ?
1.5. RFC 822 versus X.400
2. Service Elements
3. Address mapping
3.1. X.400 addresses
3.1.1. Standard Attributes
3.1.2. Domain Defined Attributes
3.1.3. X.400 address notation
3.2. RFC 822 addresses
3.3. RFC 1327 address mapping
3.3.1. Default mapping
3.3.1.1. X.400 -> RFC 822
3.3.1.2. RFC 822 -> X.400
3.3.2. Exception mapping
3.3.2.1. PersonalName
3.3.2.2. Mapping between RFC 822 and X.400 domains
3.3.2.2.1. X.400 -> RFC 822
3.3.2.2.2. RFC 822 -> X.400
3.4. Table co-ordination
3.5. Local additions
3.6. Product specific formats
3.7. Guidelines for mapping rule definition
4. Conclusion
Appendix A. References
Appendix B. Abbreviations
Appendix C. Author's address
1. Introduction
This chapter describes the history, status, future, and contents of
the involved standards.
There is a major difference between mail systems used in the USA
and Europe. Mail systems originated mainly in the USA, where their
explosive growth started as early as in the seventies. Different
company-specific mail systems were developed simultaneously, which,
of course, led to a high degree of incompatibility. The Defense
Advanced Research Projects Agency (DARPA), which had to use
machines of many different manufacturers, triggered the development
of the Internet and the TCP/IP protocol suite, which was later
accepted as a standard by the US Department of Defense (DoD). The
Internet mail format is defined in RFC 822 and the protocol used
for exchanging mail is known as the simple mail transfer protocol
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(SMTP). Together with UUCP and the BITNET protocol NJE, SMTP has
become one of the main de facto mail standards in the US.
Unfortunately, all these protocols were incompatible, which
explains the need to come to an acceptable global mail standard.
CCITT and ISO began working on a norm and their work converged in
what is now known as the X.400 Series Recommendations. One of the
objectives was to define a superset of the existing systems,
allowing for easier integration later on. Some typical positive
features of X.400 are the store-and-forward mechanism, the
hierarchical address space and the possibility of combining
different types of body parts into one message body.
In Europe, the mail system boom came later. Since there was not
much equipment in place yet, it made sense to use X.400 as much as
possible right from the beginning. A strong X.400 lobby existed,
especially in West-Germany (DFN). In the R&D world, mostly EAN was
used because it was the only affordable X.400 product at that time
(Source-code licenses were free for academic institutions).
At the moment, the two worlds of X.400 and SMTP are moving closer
together. On the one hand, the American Department of Defense, one
of the main forces behind the Internet, has decided that future
networking should be based on ISO standards, implying a migration
from SMTP to X.400. On the other hand X.400 users in Europe have a
need to communicate with the Internet. Due to the large traffic
volume between the two nets it is not enough interconnecting them
with a single international gateway. The load on such a gateway
would be too heavy. Direct access using local gateways is more
feasible.
Although the expected success of X.400 has been a bit disappointing
(mainly because no good products were available), the future of e-
mail systems must still be seen in the context of this standard.
And although in the long run X.400 is believed to take over the
world of e-mail systems, SMTP cannot be neglected over the next ten
years. Especially the simple installation procedures and the high
degree of connectivity will contribute to a growing number of RFC
822 installations in Europe and world-wide in the near future.
1.1. What is X.400 ?
In October 1984, the Plenary Assembly of the CCITT accepted a
standard to facilitate international message exchange between
subscribers to computer based store-and-forward message services.
This standard is known as the CCITT X.400 series recommendations
([CCITT 84], from now on called X.400(84)) and happens to be the
first CCITT recommendation for a network application;. It should be
noted that X.400(84) is based on work done in the IFIP Working
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Group 6.5, and that ISO at the same time was proceeding towards a
compatible document. However, the standardisation efforts of CCITT
and ISO did not converge in time (not until the 1988 version), to
allow the publication of a common text.
X.400(84) triggered the development of software implementing (parts
of) the standard in the laboratories of almost all major computer
vendors and many software houses. Similarly, public carriers in
many countries started to plan X.400(84) based message systems that
would be offered to the users as value added services. Early
implementations appeared shortly after first drafts of the standard
were published and a considerable number of commercial systems are
available nowadays.
X.400(84) describes a functional model for a Message Handling
System (MHS) and associates services and protocols. The model
illustrated in Figure 2.1. defines the components of a distributed
messaging system:
Users in the MHS environment are provided with the capability of
sending and receiving messages. Users in the context of an MHS may
be humans or application processes. The User Agent (UA) is a
process that makes the services of the MTS available to the user. A
UA may be implemented as a computer program that provides utilities
to create, send, receive and perhaps archive messages. Each UA, and
thus each user, is identified by a name (each user has its own UA).
The Message Transfer system (MTS) transfers messages from an
originating UA to a recipient UA. As implied by the figure shown
above, data sent from UA to UA may be stored temporarily in several
intermediate Message Transfer Agents (MTA), i.e. a store-and-
forward mechanism is being used. An MTA forwards received messages
to a next MTA or to the recipient UA.
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-----------------------------------------------------------------
| user user Message Handling Environment|
| | | |
| ----------------------------------------------------------|
| | | | Message Handling System ||
| | ---- ---- ||
| | |UA| |UA| ||
| | ---- ---- ||
| | | | ||
| | -------------------------------------------------||
| | | | | Message Transfer System |||
| | ---- | ----- ----- |||
|user-|-|UA|--|--|MTA| |MTA| |||
| | ---- | ----- ----- |||
| | | \ / |||
| | | \ / |||
| | | \ / |||
| | | \ / |||
| | | \ / |||
| | ---- | ----- |||
|user-|-|UA|--|---------|MTA| |||
| | ---- | ----- |||
| | -------------------------------------------------||
| ----------------------------------------------------------|
-----------------------------------------------------------------
Fig. 2.1. X.400 functional model
X.400(84) divides layer 7 of the OSI Reference Model into 2
sublayers, the User Agent Layer (UAL) and the Message Transfer
Layer (MTL) as shown in the following figure:
--------------------------------------------------------------
----- -----
UA layer |UAE|<----- P2, Pc ----------->|UAE|
----- -----
--------------------------------------------------------------
------ ------ -----
MTA layer |MTAE|<-- P1 -->|MTAE|<-- P3-->|SDE|
------ ------ -----
--------------------------------------------------------------
xxxE = xxx Entity ; SDE = Submission & Delivery Entity
--------------------------------------------------------------
Fig. 2.2. X.400 Protocols
The MTL is involved in the transport of messages from UA to UA,
using one or several MTAs as intermediaries. By consequence,
routing issues are entirely dealt with in the MTL. The MTL in fact
corresponds to the postal service that forwards letters consisting
of an envelope and a content. Two protocols, P1 and P3, are used
between the MTL entities (MTA Entity (MTAE), and Submission and
Delivery Entity (SDE)) to reliably transport messages. The UAL
embodies peer UA Entities (UAE), which interpret the content of a
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message and offer specific services to the application process.
Depending on the application to be supported on top of the MTL, one
of several end-to-end protocols (Pc) is used between UAEs. For
electronic mail, X.400(84) defines the protocol P2 as part of the
InterPersonal Messaging Service (IPMS). Conceivably other UAL
protocols may be defined, e.g. a protocol to support the exchange
of electronic business documents.
The structure of an InterPersonal Message (IPM) can be visualised
as in Figure 2.3. (Note that the envelope is not a part of the IPM;
it is generated by the MTL).
An IPM heading contains information that is specific for an
interpersonal message like 'originator', 'subject', etc. Each
bodypart can contain one information type, text, voice or as a
special case, a forwarded message. A forwarded message consists of
the original message together with Previous Delivery Information
(PDI), which is drawn from the original delivery envelope.
Forwarded
Message IP-message
- ---------- --- ---------- -
| message- |envelope| / | PDI | |
| content IPM ---------- / ---------- |
| - - ---------- / ---------- |
| | | IPM- |heading | / |heading | |
| | | body ---------- / ---------- |
| | | - ----------/ ---------- |
| | | | |bodypart| |bodypart| |
| | | | ----------\ ---------- |
| | | | ---------- \ ---------- |
| | | | |bodypart| \ |bodypart| |
| | | | ---------- \ ---------- |
| | | | . \ |
| | | | . \ |
| | | | ---------- \ ---------- |
| | | | |bodypart| \ |bodypart| |
- - - - ---------- - ---------- -
(PDI = Previous Delivery Info.)
Fig. 2.3. X.400 message structure
Early experience with X.400(84) showed that the standard had
various shortcomings. Therefore CCITT, in parallel with ISO,
corrected and extended the specification during its 1984 to 1988
study period and produced a revised standard ([CCITT/ISO 88]),
which was accepted at the 1988 CCITT Plenary Meeting ([BP-88]).
Amongst others, X.400(88) differs from X.400(84) in that it defines
a Message Store (MS), which can be seen as a kind of database for
messages. An MS enables the end-user to run a UA locally, e.g. on a
PC, whilst the messages are stored in the MS, which is co-located
with the MTA. The MTA can thus always deliver incoming messages to
the MS instead of to the UA. The MS can even automatically file
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incoming messages according to certain criteria. Other enhancements
in the 88 version concern security and distribution lists.
1.2. What is an RFC ?
The Internet, a loosely-organised international collaboration of
autonomous, interconnected networks, supports host-to-host
communication through voluntary adherence to open protocols and
procedures defined by Internet Standards. There are also many
isolated internets, i.e., sets of interconnected networks, that are
not connected to the Internet but use the Internet Standards. The
architecture and technical specifications of the Internet are the
result of numerous research and development activities conducted
over a period of two decades, performed by the network R&D
community, by service and equipment vendors, and by government
agencies around the world.
In general, an Internet Standard is a specification that is stable
and well-understood, is technically competent, has multiple,
independent, and interoperable implementations with operational
experience, enjoys significant public support, and is recognisably
useful in some or all parts of the Internet.
The principal set of Internet Standards is commonly known as the
"TCP/IP protocol suite". As the Internet evolves, new protocols and
services, in particular those for Open Systems Interconnection
(OSI), have been and will be deployed in traditional TCP/IP
environments, leading to an Internet that supports multiple
protocol suites.
The following organizations are involved in setting Internet
standards.
Internet standardization is an organized activity of the Internet
Society (ISOC). The ISOC is a professional society that is
concerned with the growth and evolution of the worldwide Internet,
with the way in which the Internet is and can be used, and with the
social, political, and technical issues that arise as a result.
The Internet Engineering Task Force (IETF) is the primary body
developing new Internet Standard specifications. The IETF is
composed of many Working Groups, which are organized into areas,
each of which is coordinated by one or more Area Directors.
The Internet Engineering Steering Group (IESG) is responsible for
technical management of IETF activities and the approval of
Internet standards specifications, using well-defined rules. The
IESG is composed of the IETF Area Directors, some at-large members,
and the chairperson of the IESG/IETF.
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The Internet Architecture Board (IAB) has been chartered by the
Internet Society Board of Trustees to provide quality control and
process appeals for the standards process, as well as external
technical liaison, organizational oversight, and long-term
architectural planning and research.
Any individual or group (e.g. an IETF or RARE working group) can
submit a document as a so-called Internet Draft. After the document
is proven stable, the IESG may turn the Internet-Draft into a
'Requests For Comments' (RFC). RFCs cover a wide range of topics,
from early discussion of new research concepts to status memos
about the Internet. All Internet Standards (STDs) are published as
RFCs, but not all RFCs specify standards. Another sub-series of the
RFCs are the RARE Technical Reports (RTRs).
As an example, this tutorial also started out as an Internet Draft.
After almost one year of discussions and revisions it was approved
by the IESG as an Informational RFC.
Once a document is assigned an RFC number and published, that RFC
is never revised or re-issued with the same number. Instead, a
revision will lead to the document being re-issued with a higher
number indicating that an older one is obsoleted.
1.3. What is RFC 822 ?
RFC 822 defines a standard for the format of Internet text
messages. Messages consist of lines of text. No special provisions
are made for encoding drawings, facsimile, speech, or structured
text. No significant consideration has been given to questions of
data compression or to transmission and storage efficiency, and the
standard tends to be free with the number of bits consumed. For
example, field names are specified as free text, rather than
special terse codes.
A general "memo" framework is used. That is, a message consists of
some information in a rigid format (the 'headers'), followed by the
main part of the message (the 'body'), with a format that is not
specified in RFC 822. It does define the syntax of several fields
of the headers section; some of these fields must be included in
all messages.
RFC 822 is used in conjunction with a number of different message
transfer protocol environments (822-MTSs).
- SMTP Networks: On the 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) [821], and
RFC 920 which is a Specification for domains and a
distributed name service [920].
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- 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.
- 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.
RFC 822 is based on the assumption that there is an underlying
service, which in RFC 1327 is called the 822-MTS service. The 822-
MTS service provides three basic functions:
1. Identification of a list of recipients.
2. Identification of an error return address.
3. Transfer of an RFC 822 message.
It is possible to achieve 2) within the RFC 822 header. 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 of RFC 1327. 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.
1.4. What is RFC 1327 ?
There is a large community using RFC 822 based protocols for mail
services, who will wish to communicate with users of the
InterPersonal Messaging Service (IPMS) provided by X.400 systems,
and the other way around. This will also be a requirement in cases
where RFC 822 communities intend to make a transition to use X.400,
as conversion will be needed to ensure a smooth service transition.
The basic function of a mail gateway can be described as follows:
receive a mail from one mail world, translate it into the formats
of the other mail world and send it out again using the routing
rules and protocols of that other world.
Especially if a message crosses more than one gateway, it is
important that all gateways have the same understanding of how
things should be mapped. A simple example of what could go wrong
otherwise is the following: A sends a message to B through a
gateway and B's reply to A is being routed through another gateway.
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If the two gateways don't use the same mappings, it can be expected
that the From and To addresses in the original mail and in the
answer don't match, which is, to say the least, very confusing for
the end-users (consider what happens if automated processes
communicate via mail). More serious things can happen to addresses
if a message crosses more than one gateway on its way from the
originator to the recipient. As a real-life example, consider
receiving a message from:
Mary Plork <MMP_+a_ARG_+lMary_Plork+r%MHS+d_A0CD8A2B01F54FDC-
A0CB9A2B03F53FDC%ARG_Incorporated@argmail.com>
This is not what you would call user-friendly addressing.... RFC
1327 describes a set of mappings that will enable a more
transparent interworking between systems operating X.400(both 84
and 88) and systems using RFC 822, or protocols derived from RFC
822.
RFC 1327 describes all mappings in term of X.400(88). It defines
how these mappings should be applied to X.400(84) systems in its
Appendix G.
Some words about the history of RFC 1327: It started out in June
1986, when RFC 987 defined for X.400(84) what RFC 1327 defines for
X.400(84 and 88). RFC 1026 specified a number of additions and
corrections to RFC 987. In December 1989, RFC 1138, which had a
very short lifetime, was the first one to deal with X.400(88). It
was obsoleted by RFC 1148 in March 1990. Finally, in May 1992, RFC
1327 obsoleted all of its ancestors.
1.5. RFC 822 versus X.400
Before describing RFC 1327 in more detail, it is useful to quickly
compare RFC 822 with X.400:
RFC 822 has:
- Simplicity
- Wide acceptance
- Large user base
- Public domain and commercial implementations
- Public domain and commercial user interfaces
- History
X.400 has:
- Acceptance in the standards communities
- Commercial vendors of service
- Defined ways to transfer things other than ASCII text (but
only a few implementations include these capabilities)
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- Standard notifications of delivery to a user's mailbox and
notification of a message being read by the user (these
capabilities are often implemented, too!)
- Future
2. Service Elements
Both RFC 822 and X.400 messages consist of certain service elements
(such as 'originator' and 'subject'). As long as a message stays
within its own world, the behaviour of such service elements is
well defined. An important goal for a gateway is to maintain the
highest possible service level when a message crosses the boundary
between the two mail worlds.
When a user originates a message, a number of services are
available. RFC 1327 describes, for each service elements, to what
extent it is supported for a recipient accessed through a gateway.
There are three levels of support:
- Supported: Some of the mappings are quite straight-forward,
such as '822.Subject:' <-> 'IPMS.Subject'.
- Not supported: There may be a complete mismatch: certain
service elements exist only in one of the two worlds (e.g.
interpersonal notifications).
- Partially supported: When similar service elements exist in
both worlds, but with slightly different interpretations,
some tricks may be needed to provide the service over the
gateway border.
Apart from mapping between the service elements, a gateway must
also map the types and values assigned to these service elements.
Again, this may in certain cases be very simple, e.g. 'IA5 ->
ASCII'. The most complicated example is mapping address spaces. The
problem is that address spaces are not something static that can be
defined within RFC 1327. Address spaces change continuously, and
they are defined by certain addressing authorities, which are not
always parallel in the RFC 822 and the X.400 world. A valid mapping
between two addresses assumes however that there is 'administrative
equivalence' between the two domains in which the addresses exist
(see also [Mapau-93]).
The following basic mappings are defined in RFC 1327. 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:
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- A Report (MTA, and MTS Services)
- An InterPersonal Notification (IPN) (MTA, MTS, and IPMS
services)
- An InterPersonal Message (IPM) (MTA, MTS, and IPMS services)
Probes (MTA Service) have no equivalent in RFC 821 or RFC 822 and
are thus handled by the gateway. The gateway's Probe confirmation
should be interpreted as if the gateway were the final MTA to which
the Probe was sent. Optionally, if the gateway uses RFC 821 as an
822-MTS, it may use the results of the 'VRFY' command to test
whether it would be able to deliver (or forward) mail to the
mailbox under probe.
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.
Some basic examples of mappings between service elements are listed
below.
Service elements:
RFC 822 X.400
------------------------------------------------
Reply-To: IPMS.Heading.reply-recipients
Subject: IPMS.Heading.subject
In-Reply-To: IPMS.Heading.replied-to-ipm
References: IPMS.Heading.related-IPMs
To: IPMS.Heading.primary-recipients
Cc: IPMS.Heading.copy-recipients
Service element types:
RFC 822 X.400
------------------------------------------------
ASCII PrintableString
Boolean Boolean
Service element values:
RFC 822 X.400
------------------------------------------------
oh_dear oh(u)dear
False 00000000
There are some mappings between service elements that are rather
tricky and important enough to mention in this tutorial. These are
the mappings of origination-related headers and some envelope
fields:
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RFC 822 -> X.400:
- If Sender: is present, Sender: is mapped to
IPMS.Heading.originator, and From: is mapped to
IPMS.Heading.authorizing-users. If not, From: is mapped to
IPMS.Heading.originator.
X.400 -> RFC 822
- If IPMS.Heading.authorizing-users is present,
IPMS.Heading.originator is mapped to Sender:, and
IPMS.Heading.authorizing-users is mapped to From: . If not,
IPMS.Heading.originator is mapped to From:.
Envelope attributes
- RFC 1327 doesn't define how to map the MTS.OriginatorName and
the MTS.RecipientName (often referred to as the P1.originator
and P1.recipient), since this depends on which underlying 822-
MTS is used. In the very common case that RFC 821 (SMTP) is
used for this purpose, the mapping is normally as follows:
MTS.Originator-name <-> MAIL FROM:
MTS.Recipient-name <-> RCPT TO:
This explains why there are no mapped equivalents for those
envelope attributes visible in the heading of an RFC 822
message.
For more details, refer to RFC 1327, chapters 2.2 and 2.3.
3. Address mapping
As address mapping is often considered the most complicated part of
mapping between service element values, this subject is given a
separate chapter in this tutorial.
Both RFC 822 and X.400 have their own specific address formats. RFC
822 addresses are text strings (e.g. "plork@tlec.nl"), whereas
X.400 addresses are binary (ASN.1) encoded sets of attributes with
values. Such binary addresses can be made readable for a human user
by a number of notations; for instance:
C=zz
ADMD=ade
PRMD=fhbo
O=a bank
S=plork
G=mary
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The rest of this chapter deals with addressing issues and mappings
between the two address forms in more detail.
3.1. X.400 addresses
As already stated above, an X.400 address is modelled as a set of
attributes. Some of these attributes are mandatory, others are
optional. Each attribute has a type and a value, e.g. the Surname
attribute has type IA5text, and an instance of this attribute could
have the value 'Kille'. Attributes are divided into Standard
Attributes (SAs) and Domain Defined Attributes (DDAs).
X.400 defines four basic forms of addresses ([X.402(88), 18.5), of
which the 'Mnemonic O/R Address' is the form that is most used, and
is the only form that is dealt with in this tutorial. This is
roughly the same address format as what in the 84 version was known
as 'O/R names: form 1, variant 1' ([X.400(84)] 3.3.2).
3.1.1. Standard Attributes
Standard Attributes (SAs) are attributes that all X.400
installations are supposed to 'understand' (i.e. use for routing),
for example: 'country name', 'given name' or 'organizational unit'.
The most commonly used SAs in X.400(84) are:
surName (S)
givenName (G)
initials (I*) (Zero or more)
generationQualifier (GQ)
OrganizationalUnits (OU1 OU2 OU3 OU4)
OrganizationName (O)
PrivateDomainName (PRMD)
AdministrationDomainName (ADMD)
CountryName (C)
The combination of S, G, I* and GQ is often referred to as the
PersonalName (PN).
Although there is no hierarchy (of addressing authorities) defined
by the standards, the following hierarchy is considered natural:
PersonalName < OU4 < OU3 < OU2 < OU1 < O < P < A < C
In addition to the SAs listed above, X.400(88) defines some extra
attributes, the most important of which is
Common Name (CN)
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CN can be used instead of or even together with PN. The problem in
X.400(84) was that PN (S G I* GQ) was well suited to represent
persons, but not roles and abstract objects, such as distribution
lists. Even though postmaster clearly is a role, not someone's real
surname, it is quite usual in X.400(84) to address a postmaster
with S=postmaster. In X.400(88), the same postmaster would be
addressed with CN=postmaster .
The attributes C and ADMD are mandatory (i.e. they must be
present), and may not be empty. At least one of the attributes
PRMD, O, OU, PN and CN must be present.
PRMD and ADMD are often felt to be routing attributes that don't
really belong in addresses. As an example of how such address
attributes can be used for the purpose of routing, consider two
special values for ADMD:
- ADMD=0; (zero) should be interpreted as 'the PRMD in this
address is not connected to any ADMD'
- ADMD= ; (single SPACE) should be interpreted as 'the PRMD in
this address is reachable via any ADMD in this country'. It
is expected that ISO will express this 'any' value by means
of a missing ADMD attribute in future versions of MOTIS. This
representation can uniquely identify the meaning 'any', as a
missing or empty ADMD field as such is not allowed.
Addresses are defined in X.400 using the Abstract Syntax Notation
One (ASN.1). X.409 defines how definitions in ASN.1 should be
encoded into binary format. Note that the meaning, and thus the
ASN.1 encoding, of a missing attribute is not the same as that of
an empty attribute. In addressing, this difference is often
represented as follows:
- PRMD=; means that this attribute is present in the address,
but its value is empty. Since this is not very useful, it's
hardly ever being used. The only examples the author knows of
were caused by mail managers who should have had this
tutorial before they started defining their addresses :-)
- PRMD=@; means that this attribute is not present in the
address.
{NB. This is only necessary if an address notation (see
below) requires that every single attribute in the hierarchy
is somehow listed. Otherwise, a missing attribute can of
course be represented by simply not mentioning it. This means
that this syntax is mostly used in mapping rules, not by end
users.}
Addresses that only contain SAs are often referred to as Standard
Attribute Addresses (SAAs).
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3.1.2. Domain Defined Attributes
Domain Defined Attributes (DDAs) cane be used in addition to
Standard Attibutes. An instance of a DDA consists of a type and a
value. DDAs are meant to have a meaning only within a certain
context (originally this was supposed to be the context of a
certain management domain, hence the name DDA), such as a company
context.
As an example, a company might want to define a DDA for describing
internal telephone numbers: DDA type=phone value=9571.
A bit tricky is the use of DDAs to encode service element types or
values that are only available on one side of a service gateway.
The most important examples of such usage are defined in:
RFC 1327 (e.g. DDA type=RFC-822 value=u(u)ser(a)isode.com)
RFC 1328 ;(e.g. DDA type=CommonName value=mhs-discussion-list)
Addresses that contain both SAs and DDAs are often referred to as
DDA addresses.
3.1.3. X.400 address notation
X.400 only prescribes the binary encoding of addresses, it doesn't
standardise how such addresses should be written on paper or what
they should look like in a user interface on a computer screen.
There exist a number of recommendations for X.400 address
representation though.
- JTC proposed an annex to CCITT Rec. F.401 and ISO/IEC 10021-2,
called 'Representation of O/R addresses for human usage'. According
to this proposal, an X.400 address would look as follows:
G=jo; S=plork; O=a bank; OU1=owe; OU2=you; P=fhbo; A=ade; C=zz
Note that in this format, the order of O and the OUs is exactly the
opposite of what one would expect intuitively (the attribute
hierarchy is increasing from left to right, except for the O and
OUs, where it's right to left. The reasoning behind this is that
this sequence is following the example of a postal address). This
proposal has been added (as a recommendation) to the 1992 version
of the standards.
- Following what was originally used in the DFN-EAN software, most
EAN versions today use an address representation similar to the JTC
proposal, with a few differences:
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- natural ordering for O and OUs
- no numbering of OUs.
- allows writing ADMD and PRMD instead of A and P
The address in the example above could, in EAN, be represented as:
G=jo; S=plork; OU=you; OU=owe; O=a bank; P=fhbo; A=ade; C=zz
This DFN-EAN format is still often referred to as _the_ 'readable
format'.
- The RARE Working Group on Mail and Messaging, WG-MSG, has made a
recommendation that is very similar to the DFN-EAN format, but with
the hierarchy reversed. Further, ADMD and PRMD are used instead of
A and P. This results in the address above being represented as:
C=zz; ADMD=ade; PRMD=fhbo; O=a bank; OU=owe; OU=you; S=plork; G=jo
This format is recognised by most versions of the EAN software. In
the R&D community, this is one of the most popular address
representations for business cards, letter heads, etc. It is also
the format that will be used for the examples in this tutorial.
(NB. The syntax used here for describing DDAs is as follows:
DD.'type'='value', e.g. DD.phone=9571)
- RFC 1327 defines a slash separated address representation:
/G=jo/S=plork/OU=you/OU=owe/O=a bank/P=fhbo/A=ade/C=zz/
Not only is this format used by the PP software, it is also
widespread for business cards and letter heads in the R&D
community.
- RFC 1327 finally defines yet another format for X.400 _domains_
(not for human users):
OU$you.OU$owe.O$a bank.P$fhbo.A$ade.C$zz
The main advantage of this format is that it is better machine-
parseble than the others, which also immediately implies its main
disadvantage: it is barely readable for humans. Every attribute
within the hierarchy should be listed, thus a missing attribute
must be represented by the '@' sign
(e.g. $a bank.P$@.A$ade.C$zz).
- Paul-Andr Pays (INRIA) has proposed a format that combines the
readability of the JTC format with the parsebility of the RFC 1327
domain format. Although a number of operational tools within the GO-
MHS community are already based on (variants of) this proposal, its
future is still uncertain.
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3.2. RFC 822 addresses
An RFC 822 address takes the form of an ASCII string of the
following form:
localpart@domainpart
"domainpart" is sub-divided into
domainpart = sdom(n).sdom(n-1)....sdom(2).sdom(1).dom
"sdom" stands for "subdomain", "dom" stands for "top-level-domain".
"localpart" ;is normally a login name, and thus typically is a
surname or an abbreviation for this. It can also designate a local
distribution list.
The hierarchy (of addressing authorities) in an RFC 822 address is
as follows:
localpart < sdom(n) < sdom(n-1) <...< dom
Some virtual real-life examples:
joemp@tlec.nl
tsjaka.kahn@walhalla.diku.dk
a13_vk@cs.rochester.edu
In the above examples, 'nl', 'dk', and 'edu' are valid,
registered, top level domains. Note that some networks that have
their own addressing schemes are also reachable by way of 'RFC
822-like' addressing. Consider the following addresses:
oops!user (a UUCP address)
V13ENZACC@CZKETH5A (a BITNET address)
These addresses can be expressed in RFC 822 format:
user@oops.uucp
V13ENZACC@CZKETH5A.BITNET
Although the domains '.uucp' and '.bitnet' are not officially
registered, they are used in the Internet to express that the mail
should be routed to a gateway.
As for mapping such addresses to X.400, there is no direct mapping
defined between X.400 on the one hand and UUCP and BITNET on the
other, so they are normally mapped to RFC 822 style first, and then
to X.400 if needed.
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3.3. RFC 1327 address mapping
Despite the difference in address formats, the address spaces
defined by RFC 822 and X.400 are quite similar. The most important
parallels are:
- both address spaces are hierarchical
- top level domains and country codes are often the same
- localparts and surnames are often the same
This similarity can of course be exploited in address mapping
algorithms. This is also done in RFC 1327 (NB only in the exception
mapping algorithm. See chapter 3.3.2).
Note that the actual mapping algorithm is much more complicated
than shown below. For details, see RFC 1327, chapter 4.
3.3.1. Default mapping
The default RFC 1327 address mapping can be visualised as a
function with input and output parameters:
address information of the gateway performing the mapping
|
v
+-----------------+
RFC 822 address <--->| address mapping | <---> X.400 address
+-----------------+
I.e. to map an address from X.400 to RFC 822 or vice versa, the
only extra input needed is the address information of the local
gateway.
3.3.1.1. X.400 -> RFC 822
There are two kinds of default address mapping from X.400 to RFC
822: one to map a real X.400 address to RFC 822, and another to
decode an RFC 822 address that was mapped to X.400 (i.e. to reverse
the default RFC 822 -> X.400 mapping).
To map a real X.400 address to RFC 822, the slash separated
notation of the X.400 address (see chapter 3.1.) is mapped to
'localpart', and the local RFC 822 domain of the gateway that
performs the mapping is used as the domain part. As an example, the
gateway 'gw.switch.ch' would perform the following mappings:
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C=zz; ADMD=ade; PRMD=fhbo; O=tlec; S=plork; ->
/C=zz/ADMD=ade/PRMD=fhbo/O=tlec/S=plork/@gw.switch.ch
C=zz; ADMD=ade; PRMD=fhbo; O=a bank; S=plork->
"/C=zz/ADMD=ade/PRMD=fhbo/O=a bank/S=plork/"@gw.switch.ch
The quotes in the second example are mandatory if the X.400 address
contains spaces, otherwise the syntax rules for the RFC 822
localpart would be violated.
This default mapping algorithm is generally referred to as 'left-
hand-side encoding'.
To reverse the default RFC 822 -> X.400 mapping (see chapter
3.3.1.2): if the X.400 address contains a DDA of the type RFC-822,
the SAs can be discarded, and the value of this DDA is the desired
RFC 822 address (NB. Some characters in the DDA value must be
decoded first. See chapter 3.3.1.2.). For example, the gateway
'GW.tlec.nl' would perform the following mapping:
DD.RFC-822=bush(a)dole.us; C=nl; ADMD=tlec; PRMD=GW
->
bush@dole.us
3.3.1.2. RFC 822 -> X.400
There are also two kinds of default address mapping from RFC 822 to
X.400: one to map a real RFC 822 address to X.400, and another to
decode an X.400 address that was mapped to RFC 822 (i.e. to reverse
the default X.400 -> RFC 822 mapping).
To map a real RFC 822 address to X.400, the RFC 822 address is
encoded in a DDA of type RFC-822 , and the SAs of the local gateway
performing the mapping are added to form the complete X.400
address. This mapping is generally referred to as 'DDA mapping'. As
an example, the gateway 'C=nl; ADMD=tlec; PRMD=GW' would perform
the following mapping:
bush@dole.us ->
DD.RFC-822=bush(a)dole.us; C=nl; ADMD=tlec; PRMD=GW
As for the encoding/decoding of RFC 822 addresses in DDAs, it is
noted that RFC 822 addresses may contain characters (@ ! % etc.)
that cannot directly be represented in a DDA. DDAs are of the
restricted character set type 'PrintableString', which is a subset
of IA5 (=ASCII). Characters not in this set need a special
encoding. Some examples (For details, refer to RFC 1327, chapter
3.4.):
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100%name@address -> DD .RFC-822;=100(p)name(a)address
u_ser!name@address -> DD.RFC-822;=u(u)ser(b)name(a)address
To decode an X.400 address that was mapped to RFC 822: if the RFC
822 address has a slash separated representation of a complete
X.400 mnemonic O/R address in its localpart, that address is the
result of the mapping. As an example, the gateway 'gw.switch.ch'
would perform the following mapping:
/C=zz/ADMD=ade/PRMD=fhbo/O=tlec/S=plork/G=mary/@gw.switch.ch
->
C=zz; ADMD=ade; PRMD=fhbo; O=tlec; S=plork; G=mary
3.3.2. Exception mapping according to mapping tables
Chapter 3.3.1. showed that it is theoretically possible to use RFC
1327 with default mapping only. Although this provides a very
simple, straightforward way to map addresses, there are some very
good reasons not to use RFC 1327 this way:
- RFC 822 users are used to writing simple addresses of the
form 'localpart@domainpart'. They often consider X.400
addresses, and thus also the left-hand-side encoded
equivalents, as unnecessarily long and complicated. They
would rather be able to address an X.400 user as if she had a
'normal' RFC 822 address. For example, take the mapping
C=zz; ADMD=ade; PRMD=fhbo; O=tlec; S=plork; ->
/C=zz/ADMD=ade/PRMD=fhbo/O=tlec/S=plork/@gw.switch.ch
from chapter 3.3.1.1. RFC 822 users would find it much more
'natural' if this address could be expressed in RFC 822 as:
plork@tlec.fhbo.ade.nl
- X.400 users are used to using X.400 addresses with SAs only.
They often consider DDA addresses as complicated, especially
if they have to encode the special characters, @ % ! etc,
manually. They would rather be able to address an RFC 822
user as if he had a 'normal' X.400 address. For example, take
the mapping
bush@dole.us
->
DD.RFC-822=bush(a)dole.us;
C=nl; ADMD= ; PRMD=tlec; O=gateway
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from chapter 3.3.1.2. X.400 users would find it much more
'natural' if this address could be expressed in X.400 as:
C=us; ADMD=dole; S=bush
- Many organisations are using both RFC 822 and X.400
internally, and still want all their users to have a simple,
unique address in both mail worlds. Note that in the default
mapping, the mapped form of an address completely depends on
which gateway performed the mapping. This also results in a
complication of a more technical nature:
- The tricky 'third party problem'. This problem need not
necessarily be understood to read the rest of this chapter.
If it looks too complicated, please feel free to skip it
until you are more familiar with the basics.
The third party problem is a routing problem caused by
mapping. As an example for DDA mappings (the example holds
just as well for left-hand-side encoding), consider the
following situation (see Fig. 3.1.): RFC 822 user X in
country A sends a message to two recipients: RFC 822 user Y,
and X.400 user Z, both in country B:
From: X@A
To: Y@B ,
/C=B/.../S=Z/@GW.A
Since the gateway in country A maps all addresses in the
message, Z will see both X's and Y's address as DDA-encoded
RFC 822 addresses, with the SAs of the gateway in country A:
From: DD.RFC-822=X(a)A; C=A;....;O=GW
To: DD.RFC-822=Y(a)B; C=A;....;O=GW ,
C=B;...;S=Z
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| ------------ ---------
| |X: RFC 822|<------->|gateway|
| ------------ ---------
| A | ^
\ | |
\---------------------------------------------
| |
/---------------------------------------------
/ | |
| B | v
| | -----------
| | |Z: X.400 |
| | -----------
| | .
| | .
| | .
| | .
| | .
| v v
| ------------ ---------
| |Y: RFC 822|<........|gateway|
| ------------ ---------
Fig. 3.1 The third party problem
Now if Z wants to 'group reply' to both X and Y, his reply to
Y will be routed over the gateway in country A, even though Y
is located in the same country:
From: C=B;...;S=Z
To: DD.RFC-822=Y(a)B; C=A;....;O=GW ,
DD.RFC-822=X(a)A; C=A;....;O=GW
The best way to travel for a message from Z to Y would of
course have been over the gateway in country B:
From: C=B;...;S=Z
To: DD.RFC-822=Y(a)B; C=B;....;O=GW ,
DD.RFC-822=X(a)A; C=A;....;O=GW
The third party problem is caused by the fact that local
gateway address information is mapped into addresses.
Ideally, the third party problem shouldn't exist. After all,
address mapping affects addresses, and an address is not a
route.... The reality is different however. For instance,
very few X.400 products are capable to route messages on the
contents of a DDA (actually, only RFC 1327 gateways will be
able to interpret this type of DDA, and who says that the
reply will pass a local gateway on its route back?). The same
limitations hold for RFC 822 based mailers: most are not
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capable to make routing decisions on the content of a left-
hand-side encoded X.400 address. So in practice, addressing
(and thus also mapping) will very well affect routing.
To make mapping between addresses more user friendly, and to avoid
the problems shown above, RFC 1327 allows for overruling the
default left-hand-side encoding and DDA mapping algorithms. This is
done by specifying associations (mapping rules) between certain
domainparts and X.400 domains. An X.400 domain (for our purposes;
CCITT has a narrower definition...) consists of the domain-related
SAs of a Mnemonic O/R address (i.e. all SAs except PN and CN). The
idea is to use the similarities between both address spaces, and
directly map similar address parts onto each other. If, for the
domain in the address to be mapped, an explicit mapping rule can be
found, the mapping is performed between:
localpart <-> PersonalName
domainpart <-> X.400 domain
The address information of the gateway is only used as an input
parameter if no mapping rule can be found, i.e. if the address
mapping must fall back to its default algorithm.
The complete mapping function can thus be visualised as follows:
address information of the gateway performing the mapping
|
v
+-----------------+
RFC 822 address <--->| address mapping | <---> X.400 address
+-----------------+
^
|
domain associations (mapping rules)
3.3.2.1. PersonalName and localpart mapping
Since the mapping between these address parts is independent of the
mapping rules that are used, and because it follows a simple, two-
way algorithmic approach, this subject is discussed in a separate
sub-chapter first.
The X.400 PersonalName consists of givenName, initials, and
surName. RFC 1327 assumes that generationQualifier is not used.
To map a localpart to an X.400 PN, the localpart is scanned for
dots, which are considered delimiters between the components of PN,
and also between single initials. In order not to put too much
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detail in this tutorial, only a few examples are shown here. For
the detailed algorithm, see RFC 1327, chapter 4.2.1.
Marshall.Rose <-> G=Marshall;S=Rose
M.T.Rose <-> I=MT;S=Rose
Marshall.M.T.Rose <-> G=Marshall;I=MT;S=Rose
To map an X.400 PN to an RFC 822 localpart, take the non-empty PN
attributes, put them into their hierarchical order (G I* S), and
connect them with periods.
Some exceptions are caused by the fact that left-hand-side encoding
can also be mixed with exception mapping. This is shown in more
detail in the following sub-chapters.
3.3.2.2. Mapping between RFC 822 and X.400 domains
A mapping rule associates two domains: an X.400 domain and an RFC
822 domain. The X.400 domain is written in the RFC 1327 domain
notation (See 3.1.3.), so that both domains have the same
hierarchical order. The domains are written on one line, separated
by a '#' sign. For instance:
arcom.ch#ADMD$arcom.C$ch#
PRMD$tlec.ADMD$ade.C$nl#tlec.nl#
A mapping rule must at least contain a top level domain and a
country code. If an address must be mapped, a mapping rule with the
longest domain match is sought. The associated domain in the
mapping rule is used as the domain of the mapped address. The
remaining domains are mapped one by one following the natural
hierarchy. Concrete examples are shown in the following sub-
chapters.
3.3.2.2.1. X.400 -> RFC 822
As an example, consider the mapping rule:
PRMD$tlec.ADMD$ade.C$nl#tlec.nl#
Then the address C=nl; ADMD=ade; PRMD=tlec; O=you; OU=owe; S=plork
S OU O PRMD ADMD Country
| | | | | |
plork owe you tlec ade nl
would be mapped as follows. The Surname 'plork' is mapped to the
localpart 'plork', see chapter 3.3.2.1. The domain
'PRMD$tlec.ADMD$ade.C$nl' is mapped according to the mapping rule:
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localpart
| sdom3
| | sdom2
| | | sdom1
| | | | top-level-domain
| | | | |
plork@ tlec.nl
The remaining SAs (O and one OU) are mapped one by one following
the natural hierarchy: O is mapped to sdom2, OU is mapped to sdom3:
localpart
| sdom3
| | sdom2
| | | sdom1
| | | | top-level-domain
| | | | |
plork@owe.you.tlec.nl
Thus the mapped address is:
plork@owe.you.tlec.nl
The table containing the listing of all such mapping rules, which
is distributed to all gateways world-wide, is normally refered to
as 'mapping table 1'. Other commonly used filenames (also depending
on which software your are using) are:
'or2rfc'
'mapping 1'
'map1'
'table 1'
'X2R'
As already announced, there is an exceptional case were localpart
and PN are not directly mapped onto each other: sometimes it is
necessary to use the localpart for other purposes. If the X.400
address contains attributes that would not allow for the simple
mapping:
localpart <-> PersonalName
domainpart <-> X.400 domain
(e.g. spaces are not allowed in an RFC 822 domain, GQ and CN cannot
be directly mapped into localpart, DDAs of another type than RFC-
822), such attributes, together with the PN, are left-hand-side
encoded. The domainpart must still be mapped according to the
mapping rule as far as possible. This probably needs some examples:
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C=nl; ADMD=ade; PRMD=tlec; O=owe; OU=you; S=plork; GQ=jr
->
/S=plork/GQ=jr/@you.owe.tlec.nl
C=nl; ADMD=ade; PRMD=tlec; O=owe; OU=spc ctr; OU=u; S=plork
->
"/S=plork/OU=u/OU=spc ctr/"@owe.tlec.nl
Note that in the second example, 'O=owe' is still mapped to a
subdomain following the natural hierarchy. The problems start with
the space in 'OU=spc ctr'.
3.3.2.2.2. RFC 822 -> X.400
As an example, consider the mapping rule:
tlec.nl#PRMD$tlec.ADMD$ade.C$nl#
Then the address 'plork@owe.you.tlec.nl' :
localpart
| sdom3
| | sdom2
| | | sdom1
| | | | top-level-domain
| | | | |
plork@owe.you.tlec.nl
would be mapped as follows.
The localpart 'plork' is mapped to 'S=plork', see chapter 3.3.2.1.
The domain 'tlec.nl' is mapped according to the mapping rule:
S OU OU O PRMD ADMD Country
| | | |
plork tlec ade nl
The remaining domains (owe.you) are mapped one by one following the
natural hierarchy: sdom2 is mapped to O, sdom3 is mapped to OU:
S OU OU O PRMD ADMD Country
| | | | | |
plork | | tlec ade nl
owe you
Thus the mapped address is (in a readable notation):
C=nl; ADMD=ade; PRMD=tlec; O=you; OU=owe; S=plork
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Had there been any left-hand-side encoded SAs in the localpart that
didn't represent a complete mnemonic O/R address, the localpart
would be mapped to those SAs. E.g.
"/S=plork/GQ=jr/OU=u/OU=spc ctr/"@owe.tlec.nl
->
C=nl; ADMD=ade; PRMD=tlec; O=owe; OU=space ctr;
OU=u; S=plork; GQ=jr
This is necessary to reverse the special use of localpart to left-
hand-side encode certain attributes. See 3.3.2.2.1.
You might ask yourself by now why such rules are needed at all. Why
don't we just use map1 in the other direction? The problem is that
a symmetric mapping function (a bijection) would indeed be ideal,
but it's not feasible. Asymmetric mappings exist for a number of
reasons:
- To make sure that uucp addresses etc. get routed over local
gateways.
- Preferring certain address forms, while still not forbidding
others to use another form. Examples of such reasons are:
- Phasing out old address forms.
- If an RFC 822 address is mapped to ADMD= ; it means that
the X.400 mail can be routed over any ADMD in that
country. One single ADMD may of course send out an
address containing: ADMD=ade; . It must also be possible
to map such an address back.
So we do need mapping rules from RFC 822 to X.400 too. The table
containing the listing of all such mapping rules, which is
distributed to all gateways world-wide, is normally refered to as
'mapping table 2' . Other commonly used filenames (also depending
on which software your are using) are:
'rfc2or'
'mapping 2'
'map2'
'table 2'
'R2X'
If the RFC 822 localpart and/or domainpart contain characters that
would not immediately fit in the value of a PN attribute (! % _),
the mapping algorithm falls back to DDA mapping. In this case, the
SAs that will be used are still determined by mapping the
domainpart according to the mapping rule. In our case:
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100%user@work.tlec.nl
->
DD.RFC-822=100(p)user(a)work.tlec.nl;
C=nl; ADMD=ade; PRMD=tlec; O=work
If no map2 rule can be found, a third table of rules is scanned:
the gateway table. This table has the same syntax as mapping table
2, but its semantics are different. First of all, a domain that
only has an entry in the gateway table is always mapped into an RFC
822 DDA. For a domain that is purely RFC 822 based, but whose mail
may be relayed over an X.400 network, the gateway table associates
with such a domain the SAs of the gateway to which the X.400
message should be routed. That gateway will then be responsible for
gatewaying the message back into the RFC 822 world. E.g. if we have
the gateway table entry:
gov#PRMD$gateway.ADMD$Internet.C$us#
(and we assume that no overruling map2 rule for the top level
domain 'gov' exists), this would force all gateways to perform the
following mapping:
bush@dole.gov
->
DD.RFC-822=bush(a)dole.gov;
C=us; ADMD=Internet; PRMD=gateway
This is very similar to the default DDA mapping, except the SAs are
those of a gateway that has declared to be responsible for a
certain RFC 822 domain, not those of the local gateway. And thus,
this mechanism helps avoid the third party problem discussed in
chapter 3.2.2.
The table containing the listing of all such gateway rules, which
is distributed to all gateways world-wide, is normally refered to
as the 'gateway table'. Other commonly used filenames (also
depending on which software your are using) are:
'rfc1148gate' {From the predecessor of RFC 1327, RFC 1148}
'gate table'
'GW'
3.4. Table co-ordination
As already stated, the use of mapping tables will only function
smoothly if all gateways in the world use the same tables. On the
global level, the collection and distribution of RFC 1327 address
mapping tables is co-ordinated by the MHS Co-ordination Service:
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SWITCH Head Office
MHS Co-ordination Service
Limmatquai 138
CH-8001 Zurich, Europe
Tel. +41 1 268 1550
Fax. +41 1 268 1568
RFC 822: project-team@switch.ch
X.400: C=ch;ADMD=arcom;PRMD=switch;O=switch;S=project-team;
The procedures for collection and distribution of mapping rules can
be found on the MHS Co-ordination server, nic.switch.ch: in the
directory /procedures . The server is available per FTP:
username: cosine
password: <your RFC 822 address>
If you want to define mapping rules for your own local domain, you
can find the right contact person in your country or network (the
gateway manager) on the same server, in the directory /mhs-services
.
3.5. Local additions
Since certain networks want to define rules that should only be
used within their networks, such rules should not be distributed
world-wide. Consider two networks that both want to reach the top-
level-domain 'arpa' over their local gateway. They would both like
to use a mapping 2 rule for this purpose:
TLec in NL: arpa#PRMD$gateway.ADMD$tlec.C$nl#
SWITCH in CH: arpa#PRMD$gateway.ADMD$switch.C$ch#
(You may have noticed correctly that they should have defined such
rules in the gateway table, but for the sake of the example, we
assume they defined it in mapping table 2. This was the way things
were done in the days of RFC 987, and many networks are still doing
it this way these days.)
Since a mapping table cannot contain two mapping rules with the
same domain on the left hand side, such 'local mappings' are not
distributed globally. There exists a RARE draft proposal ([Mapau-
93]) which defines a mechanism for allowing and automatically
dealing with conflicting mapping rules, but this mechanism has not
been implemented as to date. After having received the global
mapping tables from the MHS Co-ordination Service, many networks
add 'local' rules to map2 and the gateway table before installing
them on their gateways. Note that the reverse mapping 2 rules for
such local mappings _are_ globally unique, and can thus be
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distributed world-wide. This is even necessary, because addresses
that were mapped with a local mapping rule may leak out to other
networks (here comes the third party problem again...). Such other
networks should at least be given the possibility to map the
addresses back. So the global mapping table 1 would in this case
contain the two rules:
PRMD$gateway.ADMD$tlec.C$nl#arpa#
PRMD$gateway.ADMD$switch.C$ch#arpa#
Note that if such rules would have been defined as local gate table
entries instead of map2 entries, there would have been no need to
distribute the reverse mappings world-wide (the reverse mapping of
a DDA encoded RFC 822 address is simply done by stripping the SAs,
see 3.3.1.1.).
3.6. Product specific formats
Not all software uses the RFC 1327 format of the mapping tables
internally. Almost all formats allow comments on a line starting
with a # sign. Some examples of different formats:
RFC 1327
# This is pure RFC 1327 format
# table 1: X.400 -> RFC 822
#
PRMD$tlec.ADMD$ade.C$nl#tlec.nl#
# etc.
# table 2: RFC 822 -> X.400
#
arcom.ch#ADMD$arcom.C$ch#
# etc.
EAN
# This is EAN format
# It uses the readable format for X.400 domains and TABs
# to make a 'readable mapping table format'.
# table 1: X.400 -> RFC 822
#
P=tlec; A=ade; C=nl; # tlec.nl
# etc.
# table 2: RFC 822 -> X.400
#
arcom.ch # A=arcom; C=ch;
# etc.
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PP
# This is PP format
# table 1: X.400 -> RFC 822
#
PRMD$tlec.ADMD$ade.C$nl:tlec.nl
# etc.
# table 2: RFC 822 -> X.400
#
arcom.ch:ADMD$arcom.C$ch
# etc.
Most R&D networks have tools to automatically generate these
formats from the original RFC 1327 tables;, some even distribute
the tables within their networks in several formats. If you need
mapping tables in a specific format, please contact your national
or R&D network's gateway manager. See chapter 3.4 .
3.7. Guidelines for mapping rule definition
Beware that defining mapping rules without knowing what you are
doing can be disastrous not only for your network, but also for
others. You should be rather safe if you follow at least these
rules:
- First of all, read this tutorial;.
- Avoid local mappings; prefer gate table entries. (See chapter
3.5)
- Make sure any domain you map to can also be mapped back;.
- Aim for symmetry.
- Don't define a gateway table entry if the same domain already
has a map2 entry. Such a rule would be redundant.
- Map to "ADMD=0;" if you will not be connected to any ADMD for
the time being.
- Only map to "ADMD= ;" if you are indeed reachable through
_any_ ADMD in your country.
- Mind the difference between "PRMD=;" and "PRMD=@;" and make
sure which one you need. (Try to avoid empty or unused
attributes in the O/R address hierarchy from the beginning!)
- Don't define mappings for domains over which you have no
naming authority.
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- Before defining a mapping rule, make sure you have the
permission from the naming authority of the domain you want
to map to. Normally, this should be the same organisation as
the mapping authority of the domain in the left hand side of
the mapping rule. This principle is called 'administrative
equivalence'.
- Avoid redundant mappings. E.g. if all domains under 'tlec.nl'
are in your control, don't define:
first.tlec.nl#O$first.PRMD$tlec.ADMD$ade.C$nl#
last.tlec.nl#O$last.PRMD$tlec.ADMD$ade.C$nl#
always.tlec.nl#O$always.PRMD$tlec.ADMD$ade.C$nl#
but rather have only one mapping rule:
tlec.nl#PRMD$tlec.ADMD$ade.C$nl#
- Before introducing a new mapped version of a domain, make
sure the world can route to that mapped domain;.
E.g. If you are operating a PRMD: C=zz; ADMD=ade; PRMD=ergo;
and you want to define the mapping rules:
map1: PRMD$ergo.ADMD$ade.C$zz#ergo.zz#
map2: ergo.zz#PRMD$ergo.ADMD$ade.C$zz#
Make sure that ergo.zz (or at least all of its subdomains) is
DNS routeable (register an MX or A record) and will be routed
to a gateway that agreed to route the messages from the
Internet to you over X.400.
In the other direction, if you are operating the Internet
domain cs.woodstock.edu, and you want to define a mapping for
that domain:
map2: cs.woodstock.edu#O$cs.PRMD$woodstock.ADMD$ .C$us#
map1: O$cs.PRMD$woodstock.ADMD$ .C$us#cs.woodstock.edu#
Make sure that C=us; ADMD= ; PRMD=woodstock; O=cs; (or at
least all of its subdomains) is routeable in the X.400 world,
and will be routed to a gateway that agreed to route the
messages from X.400 to your RFC 822 domain over SMTP. Within
the GO-MHS community, this would be done by registering a
line in a so-called domain document, which will state to
which mail relay this domain should be routed.
Co-ordinate any such actions with your national or MHS'
gateway manager. See chapter 3.4.
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4. Conclusion
Mail gatewaying remains a complicated subject. If after reading
this tutorial, you feel you understand the basics, try solving some
real-life problems. This is indeed a very rewarding area to work
in: even after having worked with it for many years, you can make
amazing discoveries every other week........
Appendix A. References
[821] RFC 821; Jonathan B. Postel; SIMPLE MAIL TRANSFER
PROTOCOL; University of Southern California; August
1982
[822] RFC 822; Crocker, D.; Standard for the Format of ARPA
Internet Text Messages; University of Delaware,
August 1982
[920] RFC 920; J. Postel, J. Reynolds; Domain Requirements;
October 1984
[987] RFC 987; Steve Kille; Mapping between X.400 and RFC
822; UK Academic Community Report (MG.19), June 1986
[1280] RFC 1280; Jon Postel; IAB OFFICIAL PROTOCOL
STANDARDS; USC/Information Sciences Institute, March
1992
[1310] RFC 1310; Lyman Chapin; The Internet Standards
Process; BBN Communications Corporation, March 1992
[1327] RFC 1327 / RARE RTR 2; Steve Kille; Mapping between
X.400(1988) / ISO 10021 and RFC 822; University
College London, May 1992
[1328] RFC 1328 / RARE RTR 3; Steve Kille; X.400 1988 to
1984 downgrading; University College London, May 1992
[BP-88] Bernhard Plattner, Hannes Lubich; Electronic Mail
Systems and Protocols Overview and Case Study;
Proceedings of the IFIP WG 6.5 International working
conference on message handling systems and
distributed applications; Costa Mesa 1988; North-
Holland, 1989
[JH-93] Jeroen Houttuin; @route:100%name@address, a practical
guide to MHS configuration; Top-Level EC 1993 (not
yet published)
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[HTA-faq] Harald Tveit Alvestrand; Frequently asked questions
on X.400. Regularly posted on USEnet in newsgroup
comp.protocols.iso.x400
[Mapau-93] Jeroen Houttuin, Klaus Hansen, Serge Aumont; RFC 1327
address mapping authorities. RARE WG-MSG working
draft: Internet-Draft 'draft-houttuin-mapauth-
01.txt', May 1993.
g
guides . See chapter 3.4.
[RG-87] Rudiger Grimm, Steinar Haug; A minimum Profile for
RFC 987; GMD, November 1987; RARE MHS Project Team;
July 1990. Also available from
nic.switch.ch:/procedures/min-rfc987-profile . See
chapter 3.4.
[X.4xx(84)] CCITT Recommendations X.400 - X.430. Data
Communication Networks: Message Handling Systems.
CCITT Red Book, Vol. VIII - Fasc. VIII.7, Malaga-
Torremolinos 1984
[X.4xx(88)] CCITT Recommendations X.400 - X.420. Data
Communication Networks: Message Handling Systems.
CCITT Blue Book, Vol. VIII - Fasc. VIII.7, Melbourne
1988
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Appendix B. Abbreviations
ADMD Administration Management Domain
ASCII American Standard Code for Information Exchange
ASN.1 Abstract Syntax Notation One
BCD Binary-Coded Decimal
BITNET Because It's Time NETwork
CCITT Comite Consultatif International de Telegraphique et
Telephonique
COSINE Co-operation for OSI networking in Europe
DARPA Defense Advanced Research Projects Agency
DFN Deutsches Forschungsnetz
DL Distribution List
DNS Domain Name System
DoD Department of Defense
EBCDIC Extended BCD Interchange Code
IAB Internet Architecture Board
IEC International Electrotechnical Commission
IESG Internet Engineering Steering Group
IETF Internet Engineering Task Force
IP Internet Protocol.
IPM Inter-Personal Message
IPMS Inter-Personal Messaging Service
IPN Inter-Personal Notification
ISO International Organisation for Standardisation
ISOC Internnet Society
ISODE ISO Development Environment
JNT Joint Network Team (UK)
JTC Joint Technical Committee (ISO/IEC)
MHS Message Handling System
MOTIS Message-Oriented Text Interchange Systems
MTA Message Transfer Agent
MTL Message Transfer Layer
MTS Message Transfer System
MX Mail eXchanger
OSI Open Systems Interconnection
OU(s) Organizational Unit(s)
PP Mail gatewaying software (not an abbreviation)
PRMD Private Management Domain
RARE Reseaux Associes pour la Recherche Europeenne
RFC Request for comments
RTC RARE Technical Committee
RTR RARE Technical Report
SMTP simple mail transfer protocol
STD Standard RFC
TCP Transmission Control Protocol
UUCP Unix to Unix CoPy
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Appendix C. Author's address
Jeroen Houttuin
RARE Secretariat
Singel 466-468
NL-1017 AW Amsterdam, Europe
Tel. +31 20 6391131
Fax. +31 20 6393289
RFC 822: houttuin@rare.nl
X.400: C=nl;ADMD=400net;PRMD=surf;O=rare;S=houttuin
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