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INTERNET DRAFT Jeroen Houttuin
RARE WG-MSG RARE Secretariat
Rev. 3.0 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 any other, 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 to help especially new gateway managers
find their way into the complicated subject of mail gatewaying
according to RFC 1327. End-users are encouraged to read the COSINE
MHS pocket user guide [pug] instead.
The introduction is general enough to be read not only by gateway
managers, but also by those who are new to e-mail in general.
Parts of this introduction can be skipped as needed. 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 refer 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
and an RTR. For that purpose, this version (3.0) incorporates the
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."
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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 standards.
Format
This Internet Draft is available in ASCII as well as in Postscript
format. The index is only available in the Postscript version.
Contents
1. Introduction 3
1.1. What is X.400 4
1.2. What is an RFC 7
1.3. What is RFC 822 9
1.4. What is RFC 1327 10
1.5. RFC 822 versus X.400 11
2. What must be mapped 11
3. Address mapping 14
3.1. X.400 addresses 14
3.2. RFC 822 addresses 18
3.3. RFC 1327 address mapping 19
3.3.1. Default mapping 20
3.3.1.1. X.400 -> RFC 822 20
3.3.1.2. RFC 822 -> X.400 21
3.3.2. Exception mapping according to mapping tables 21
3.3.2.1. PersonalName and localpart mapping 25
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3.3.2.2. Mapping between RFC 822 and X.400 domains 25
3.3.2.2.1. X.400 -> RFC 822 26
3.3.2.2.2. RFC 822 -> X.400 27
3.4. Table co-ordination 30
3.5. Local additions 31
3.6. Product specific formats 32
3.7. Guidelines for mapping rule definition 33
4. Conclusion 34
Appendix A. References 35
Appendix B. Index Not available in ASCII version
Appendix C. Abbreviations 36
Appendix D. Author's address 37
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 (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 super set 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 public domain X.400 product at that
time.
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At the moment, the two worlds of X.400 and SMTP are moving towards
each other. 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. A striking example of the opening-up of
the ISO oriented world is the reorganisation of the RARE working
groups in June 1992. The new working groups are now also
discussing non ISO protocols, such as RFC 822.
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 procedure and the
high degree of connectivity will contribute to a growing number of
RFC 822 installations in Europe 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
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
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illustrated in the 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.
-----------------------------------------------------------------
| 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:
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--------------------------------------------------------------
----- -----
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
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.
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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 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
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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 Internet Activities Board (IAB) is the primary co-ordinating
committee for Internet design, engineering, and management [1].
The IAB has delegated to its Internet Engineering Task Force
(IETF) the primary responsibility for the development and review
of potential Internet Standards from all sources. The IETF forms
Working Groups to pursue specific technical issues, frequently
resulting in the development of one or more specifications that
are proposed for adoption as Internet Standards.
Final decisions on Internet standardisation are made by the IAB,
based upon recommendations from the Internet Engineering Steering
Group (IESG), the leadership body of the IETF. IETF Working Groups
are organised into areas, and each area is co-ordinated by an Area
Director. The Area Directors and the IETF Chairman are included in
the IESG.
Any individual or group (e.g. an IETF working group) can submit a
document as a so-called Internet Draft. After at least half a
year, if the document, being well discussed, looks stable, the
IESG may propose to the IAB to 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 are published as RFCs,
but not all RFCs specify standards.
As an example, this tutorial is also an Internet Draft that is to
become an Informational RFC later on.
Once a document is assigned an RFC number and published, that RFC
is never revised or re-issued with the same number.
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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].
- 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
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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 take a mail from one mail world, translate it into the
formats of the other mail world and sends 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. 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 per 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 mail from:
Mary Plork <MMP_+a_ARG_+lMary_Plork+r%MHS+d_A0CD8A2B01F54FDC-
A0CB9A2B03F53FDC%ARG_Incorporated@argmail.com>
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 added a number of additions and
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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 got:
- Simplicity
- Wide acceptance
- Large user base
- Public domain and commercial implementations
- Public domain and commercial user interfaces
- History
X.400 has got:
- Acceptance in the standards communities
- Commercial vendors of service
- Defined ways to transfer things other than ASCII text (but
only a few implementations have implemented it)
- Standard notifications of delivery to user's mailbox and
notification of a message being read by the user (these ones
are often implemented, too!)
- Future
2. What must be mapped
Both RFC 822 and X.400 messages consist of certain service
elements (such as 'originator', '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 provide the
highest possible service level when a message crosses the boundary
between the two mail worlds.
RFC 1327 defines mappings between the different service elements.
Some of these mappings are quite straight-forward, such as
'822.Subject:' <-> 'IPMS.Subject' (IPMS = Inter-Personal Messaging
Service), but there are also more complicated cases. Especially
when certain service elements exist only in one of the two worlds
(e.g. interpersonal notifications), or when service elements exist
in both worlds, but with slightly different interpretations, some
tricks may be needed to provide the service over the gateway
border.
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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 are (see also [MSG-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:
- 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 gateways 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
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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 enough important to mention in this tutorial. These are
the mappings of origination-related headers and some envelope
fields:
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.
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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.
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
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 in 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).
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:
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surName (S)
givenName (G)
initials (I*)
generationQualifyer (GQ)
OrganizationalUnits (OU*)
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 < OU < OU <...< 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)
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 (to 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
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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).
Domain Defined Attributes
Domain Defined Attributes (DDAs) were meant to have a meaning only
within a certain context (originally this was intended to be the
context of a certain management domain), such as a company context
(for example: DDA type=internal-phone-nr value=9571). Such DDAs
are often used along with the PN or CN attributes.
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:
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)
The RFC 1327 example will get more than enough attention later on
in this tutorial. As for the second example: RFC 1328 defines the
downgrading from X.400(88) to X.400(84), and the DDA of type
CommonName is a straightforward trick to represent the attribute
CN in an X.400(84) address.
In the context of RFC 1327 and RFC 1328, DDAs are normally used
_instead of_ PN and CN, whose equivalents are implicitly encoded
within the DDA (localpart=u_ser; CN=mhs-discussion-list).
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Addresses that contain both SAs and DDAs are often referred to as
DDA addresses.
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 proposes 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 hierarchy of O and OUs is exactly
the opposite of what one would expect intuitively (the hierarchy
is increasing from left to right, except for the O and OUs, where
it's right to left).
- 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:
- 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 to be
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 also the prevalent address
representation for business cards, letter heads, etc. This is also
the format that will be used for the examples in this tutorial.
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- 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. This 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.
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 be the address of
a local distribution list or an alias that will allow redirecting
mail (e.g. mary.plork@tlec.nl might be an alias for
plork@tlec.nl). The localpart in the latter address may again be a
surname alias for Mary's login name pl, so that the mail will
eventually be delivered to pl@tlec.nl)
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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', '.bitnet', and '.earn' 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.
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.
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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:
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
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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 (not
so rich) type 'PrintableString', so these special characters need
a special encoding. For details, refer to RFC 1327, chapter 3.4.
Some examples:
100%name@address -> DD.RFC-822;=100(p)name(a)address
u_ser!name@address -> DD.RFC-822;=u(u)ser(b)(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
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simple, straightforward, way to map addresses, there are quite
some 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
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 more technical nature:
- The tricky 'third party problem'. This problem must 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
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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
| ------------ ---------
| |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
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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 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 the 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 consists of the
domain-related SAs of 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
Only if no mapping rule can be found - i.e. the address mapping
must fall back to its default algorithm - is the address
information of the gateway performing the mapping used as an input
parameter.
The complete mapping function can thus be visualised as follows:
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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 generationQualifyer 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
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, so that both domains have the same hierarchical order.
The domains are written on one line, separated by a '#' sign. For
instance:
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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$nl' is mapped according to the mapping rule:
localpart
| sdom3
| | sdom2
| | | sdom1
| | | | top-level-domain
| | | | |
plork@ tlec.nl
The remaining SAs (O and one OU) are mapped one by one following
the natural hierarchy: O is mapped to sdom2, OU is mapped to
sdom3:
localpart
| sdom3
| | sdom2
| | | sdom1
| | | | top-level-domain
| | | | |
plork@owe.you.tlec.nl
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Thus the mapped address is:
plork@owe.you.tlec.nl
The name of the file containing the listing of all such mapping
rules, which is distributed to all gateways world-wide, is widely
known under the following names:
'or2rfc'
'mapping 1'
'mapping table 1'
'map1'
'table 1'
'X2R'
As already announced, there is an exceptional case were localpart
and PN are not directly mapped onto each other: sometimes it is
necessary to use the localpart for other purposes. If the X.400
address contains attributes that would not allow for the simple
mapping:
localpart <-> PersonalName
domainpart <-> X.400 domain
(e.g. spaces are not allowed in an RFC 822 domain, GQ and CN
cannot be directly mapped into localpart, DDAs of another type
than RFC-822), such attributes, together with the PN, are left-
hand-side encoded. The domainpart must still be mapped according
to the mapping rule as far as possible. This probably needs some
examples:
C=nl; ADMD=ade; PRMD=tlec; O=owe; OU=you; S=plork; GQ=jr
->
/S=plork/GQ=jr/@you.owe.tlec.nl
C=nl; ADMD=ade; PRMD=tlec; O=owe; OU=spc ctr; OU=u; S=plork
->
"/S=plork/OU=u/OU=spc ctr/"@owe.tlec.nl
Note that in the second example, 'O=o' is still mapped to a
subdomain following the natural hierarchy. The problems start with
the space in 'OU=spc ctr'.
3.3.2.2.2. RFC 822 -> X.400
As an example, consider the mapping rule:
tlec.nl#PRMD$tlec.ADMD$ade.C$nl#
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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
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:
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- 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:
- Fading 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 name of
the file containing the listing of all such mapping rules, which
is distributed to all gateways world-wide, is widely known under
the following names:
'rfc2or'
'mapping 2'
'mapping table 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:
100%user@work.tlec.nl
->
DD.RFC-822=100(p)user(a)address.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#
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(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 name of the table containing these gateway mapping rules,
which is distributed to all gateways world-wide, is widely known
under the following names:
'rfc1148gate' {From the predecessor of RFC 1327, RFC 1148}
'gate table'
'gateway 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:
SWITCH Head Office
MHS Co-ordination Service
Limmatquai 138
CH-8001 Zurich, Europe
Tel. +41 1 261 8112
Fax. +41 1 261 8133
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>
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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$ch#
(You may have noticed correctly that they should have defined such
rules in the gateway table, but for the sake of the example, we
assume they defined it in mapping table 2. This was the way things
were done in the days of RFC 987, and many networks are still
doing it this way these days.)
Since a mapping table cannot contain two mapping rules with the
same domain on the left hand side, such 'local mappings' are not
distributed globally. There exists a RARE draft proposal ([MSG-
93]) which defines a mechanism for allowing and automatically
dealing with conflicting mapping rules, but this mechanism has not
been implemented as to date. After having received the global
mapping tables from the MHS Co-ordination Service, many networks
add 'local' rules to map2 and the gateway table before installing
them on their gateways. Note that the reverse mapping 2 rules for
such local mappings _are_ globally unique, and can thus be
distributed world-wide. This is even necessary, because addresses
that were mapped with a local mapping rule may leak out to other
networks (here comes the third party problem again...). Such other
networks should at least be given the possibility to map the
addresses back. So the global mapping table 1 would in this case
contain the two rules:
PRMD$gateway.ADMD$tlec.C$nl#arpa#
PRMD$gateway.ADMD$switch.C$ch#arpa#
Note that if such rules would have been defined as local gate
table entries instead of gate2 entries, there would have been no
need to distribute the reverse mappings world-wide (the reverse
mapping of a DDA encoded RFC 822 address is simply done by
stripping the SAs, see 3.3.1.1.).
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3.6. Product specific formats
Not all software uses the RFC 1327 format of the mapping tables
internally. Almost all formats allow comments on a line starting
with a # sign. Some examples of different formats:
RFC 1327
# This is pure RFC 1327 format
# table 1: X.400 -> RFC 822
#
PRMD$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.
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$ch
# etc.
Most R&D networks have tools to automatically generate these
formats from the original RFC 1327 tables;, some even distribute
the tables within their networks in several formats. If you need
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mapping tables in a specific format, please contact your national
or R&D network's gateway manager. See chapter 3.4 .
3.7. Guidelines for mapping rule definition
Beware that defining mapping rules without knowing what you are
doing can be disastrous not only for your network, but also for
others. You should be rather safe if you follow at least these
rules:
- First of all, read this tutorial;.
- Don't use local mappings. (see chapter 3.5)
- Make sure any domain you map to can also be mapped back;.
- Aim for symmetry.
- Don't define a gateway table entry if the same domain
already has a map2 entry. Such a rule would be redundant.
- Map to ADMD=0; if you will not be connected to any ADMD for
the time being.
- Only map to ADMD= ; if you are indeed reachable though
_any_ ADMD in your country.
- Mind the difference between PRMD=; and PRMD=@; and make sure
which one you need.
- Don't define mappings for domains over which you have no
naming authority.
- Before defining a mapping rule, make sure you have the
permission from the naming authority of the domain you want
to map to. Normally, this should be the same organisation as
the mapping authority of the domain in the left hand side of
the mapping rule. This principle is called 'administrative
equivalence'.
- Avoid redundant mappings. E.g. is all domains under
'tlec.nl' are in your control, don't define:
first.tlec.nl#O$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$nl#
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but rather have only one mapping rule:
tlec.nl#PRMD$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 is DNS routeable (has an A or an MX
record) and will be routed to a gateway that will route the
mails 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; is
routeable in the X.400 world and will be routed to a gateway
that will route the mails 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.
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........
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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 of the Format of ARPA
Internet Text Messages; University of Delaware,
August 1982
[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; Steve Kille; Mapping between X.400(1988) /
ISO 10021 and RFC 822; University College London,
May 1992
[1328] RFC 1328; 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-92] Jeroen Houttuin; @route:100%name@address, a
practical guide to MHS configuration; Top-Level EC
1992 (not yet published)
[HTA-faq] Harald Tveit Alvestrand; Frequently asked questions
on X.400. Regularly posted on USEnet in newsgroup
comp.protocols.iso.x400
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[MSG-93] Jeroen Houttuin, Klaus Hansen, Serge Aumont; RFC
1327 address mapping authorities. RARE WG-MSG
working draft: Internet-Draft 'draft-houttuin-
mapauth-00.txt'.
[pug] COSINE MHS Pocket User Guide. COSINE MHS Project
Team 1992. Also available in several languages from
the MHS Co-ordination server,
nic.switch.ch:/public/user-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
Appendix C. 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 Activities 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
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ISO International Organisation for Standardisation
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
TCP Transmission Control Protocol
UUCP Unix to Unix CoPy
Appendix D. 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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