RFC 1148
Mapping between X.400(1988) / ISO 10021 and RFC 822
Network Working Group S. Kille Request for Comments 1148 University College London Updates: RFCs 822, 987, 1026, 1138 March 1990 Mapping between X.400(1988) / ISO 10021 and RFC 822 Status of this Memo This RFC suggests an electronic mail protocol mapping for the Internet community and UK Academic Community, and requests discussion and suggestions for improvements. This memo does not specify an Internet standard. This edition includes material lost in editing. Distribution of this memo is unlimited. This document describes a set of mappings which will enable interworking between systems operating the CCITT X.400 (1988) Recommendations on Message Handling Systems / ISO IEC 10021 Message Oriented Text Interchange Systems (MOTIS) [CCITT/ISO88a], and systems using the RFC 822 mail protocol [Crocker82a] or protocols derived from RFC 822. The approach aims to maximise the services offered across the boundary, whilst not requiring unduly complex mappings. The mappings should not require any changes to end systems. This document is based on RFC 987 and RFC 1026 [Kille86a, Kille87a], which define a similar mapping for X.400 (1984). This document does not obsolete the earlier ones, as its domain of application is different. Specification This document specifies a mapping between two protocols. This specification should be used when this mapping is performed on the Internet or in the UK Academic Community. This specification may be modified in the light of implementation experience, but no substantial changes are expected. Table of Contents 1. Overview ............................................... 2 1.1 X.400 ................................................. 2 1.2 RFC 822 ............................................... 3 1.3 The need for conversion ............................... 4 1.4 General approach ...................................... 4 1.5 Gatewaying Model ...................................... 5 1.6 RFC 987 ............................................... 7 1.7 Aspects not covered ................................... 8 1.8 Subsetting ............................................ 9
1.9 Document Structure .................................... 9 1.10 Acknowledgements ..................................... 10 2. Service Elements ....................................... 10 2.1 The Notion of Service Across a Gateway ................ 10 2.2 RFC 822 ............................................... 11 2.3 X.400 ................................................. 15 3. Basic Mappings ........................................ 24 3.1 Notation .............................................. 24 3.2 ASCII and IA5 ......................................... 25 3.3 Standard Types ........................................ 25 3.4 Encoding ASCII in Printable String .................... 28 4. Addressing ............................................. 29 4.1 A textual representation of MTS.ORAddress ............. 30 4.2 Basic Representation .................................. 30 4.3 EBNF.822-address <-> MTS.ORAddress .................... 34 4.4 Repeated Mappings ..................................... 43 4.5 Directory Names ....................................... 45 4.6 MTS Mappings .......................................... 45 4.7 IPMS Mappings ....... ................................. 48 5. Detailed Mappings ...................................... 52 5.1 RFC 822 -> X.400 ...................................... 52 5.2 Return of Contents .................................... 59 5.3 X.400 -> RFC 822 ...................................... 60 Appendix A Differences with RFC 987 ....................... 79 1. Introduction ........................................... 79 2. Service Elements ....................................... 80 3. Basic Mappings ......................................... 80 4. Addressing ............................................. 80 5. Detailed Mappings ...................................... 80 6. Appendices ............................................. 81 Appendix B Mappings specific to the JNT Mail .............. 81 1. Introduction ........................................... 81 2. Domain Ordering ........................................ 81 3. Acknowledge-To: ........................................ 81 4. Trace .................................................. 82 5. Timezone specification ................................. 82 6. Lack of 822-MTS originator specification ............... 82 Appendix C Mappings specific to UUCP Mail ................. 83 Appendix D Object Identifier Assignment ................... 83 Appendix E BNF Summary .................................... 84 Appendix F Format of address mapping tables ............... 91 References ................................................. 92 Chapter 1 -- Overview 1.1. X.400 This document relates to the CCITT 1988 X.400 Series Recommendations
/ ISO IEC 10021 on the Message Oriented Text Interchange Service
(MOTIS). This ISO/CCITT standard is referred to in this document as
"X.400", which is a convenient shorthand. Any reference to the 1984
CCITT Recommendations will be explicit. X.400 defines an
Interpersonal Messaging System (IPMS), making use of a store and
forward Message Transfer System. This document relates to the IPMS,
and not to wider application of X.400. It is expected that X.400
will be implemented very widely.
1.2. RFC 822
RFC 822 is the current specification of the messaging standard on the
Internet. This standard evolved with the evolution of the network
from the ARPANET (created by the Defense Advanced Research Projects
Agency) to the Internet, which now involves over 1000 networks and is
sponsored by DARPA, NSF, DOE, NASA, and NIH. It specifies an end to
end message format. It is used in conjunction with a number of
different message transfer protocol environments.
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) [Postel82a], and RFC 1034
which is a Specification for domains and a distributed name
service [Mockapetris87a].
UUCP Networks
UUCP is the UNIX to UNIX CoPy protocol, which is usually used
over dialup telephone networks to provide a simple message
transfer mechanism. There are some extensions to RFC 822,
particularly in the addressing. They use domains which conform
to RFC 1034, but not the corresponding domain nameservers
[Horton86a].
Csnet
Some portions of Csnet follow the Internet protocols. The
dialup portion of Csnet uses the Phonenet protocols as a
replacement for RFC 821. This portion uses domains which
conform to RFC 1034, but not the corresponding domain
nameservers.
Bitnet
Some parts of Bitnet and related networks use RFC 822 related
protocols, with EBCDIC encoding.
JNT Mail Networks
A number of X.25 networks, particularly those associated with
the UK Academic Community, use the JNT (Joint Network Team)
Mail Protocol, also known as Greybook [Kille84a]. This is used
with domains and name service specified by the JNT NRS (Name
Registration Scheme) [Larmouth83a].
The mappings specified here are appropriate for all of these
networks.
1.3. The need for conversion
There is a large community using RFC 822 based protocols for mail
services, who will wish to communicate with users of the IPMS
provided by X.400 systems. This will also be a requirement in cases
where communities intend to make a transition to use of an X.400
IPMS, as conversion will be needed to ensure a smooth service
transition. It is expected that there will be more than one gateway,
and this specification will enable them to behave in a consistent
manner. Note that the term gateway is used to describe a component
performing the protocol mappings between RFC 822 and X.400. This is
standard usage amongst mail implementors, but should be noted
carefully by transport and network service implementors.
Consistency between gateways is desirable to provide:
1. Consistent service to users.
2. The best service in cases where a message passes through
multiple gateways.
1.4. General approach
There are a number of basic principles underlying the details of the
specification. These principles are goals, and are not achieved in
all aspects of the specification.
1. The specification should be pragmatic. There should not be
a requirement for complex mappings for "Academic" reasons.
Complex mappings should not be required to support trivial
additional functionality.
2. Subject to 1), functionality across a gateway should be as
high as possible.
3. It is always a bad idea to lose information as a result of
any transformation. Hence, it is a bad idea for a gateway
to discard information in the objects it processes. This
includes requested services which cannot be fully mapped.
4. All mail gateways actually operate at exactly one level
above the layer on which they conceptually operate. This
implies that the gateway must not only be cognisant of the
semantics of objects at the gateway level, but also be
cognisant of higher level semantics. If meaningful
transformation of the objects that the gateway operates on
is to occur, then the gateway needs to understand more than
the objects themselves.
5. The specification should be reversible. That is, a double
transformation should bring you back to where you started.
1.5. Gatewaying Model
1.5.1. X.400
X.400 defines the IPMS Abstract Service in X.420/ISO 10021-7,
[CCITT/ISO88b] which comprises of three basic services:
1. Origination
2. Reception
3. Management
Management is a local interaction between the user and the IPMS, and
is therefore not relevant to gatewaying. The first two services
consist of operations to originate and receive the following two
objects:
1. IPM (Interpersonal Message). This has two components: a
heading, and a body. The body is structured as a sequence
of body parts, which may be basic components (e.g., IA5
text, or G3 fax), or IP Messages. The heading consists of
fields containing end to end user information, such as
subject, primary recipients (To:), and importance.
2. IPN (Inter Personal Notification). A notification about
receipt of a given IPM at the UA level.
The Origination service also allows for origination of a probe, which
is an object to test whether a given IPM could be correctly received.
The Reception service also allows for receipt of Delivery Reports
(DR), which indicate delivery success or failure.
These IPMS Services utilise the Message Transfer (MT) Abstract Service [CCITT/ISO88c]. The MT Abstract Service provides the following three basic services: 1. Submission (used by IPMS Origination) 2. Delivery (used by IPMS Reception) 3. Administration (used by IPMS Management) Administration is a local issue, and so does not affect this standard. Submission and delivery relate primarily to the MTS Message (comprising Envelope and Content), which carries an IPM or IPN (or other uninterpreted contents). There is also an Envelope, which includes an ID, an originator, and a list of recipients. Submission also includes the probe service, which supports the IPMS Probe. Delivery also includes Reports, which indicate whether a given MTS Message has been delivered or not. The MTS is REFINED into the MTA (Message Transfer Agent) Service, which define the interaction between MTAs, along with the procedures for distributed operation. This service provides for transfer of MTS Messages, Probes, and Reports. 1.5.2. RFC 822 RFC 822 is based on the assumption that there is an underlying service, which is here called the 822-MTS service. The 822-MTS service provides three basic functions: 1. Identification of a list of recipients. 2. Identification of an error return address. 3. Transfer of an RFC 822 message. It is possible to achieve 2) within the RFC 822 header. Some 822-MTS protocols, in particular SMTP, can provide additional functionality, but as these are neither mandatory in SMTP, nor available in other 822-MTS protocols, they are not considered here. Details of aspects specific to two 822-MTS protocols are given in Appendices B and C. An RFC 822 message consists of a header, and content which is uninterpreted ASCII text. The header is divided into fields, which are the protocol elements. Most of these fields are analogous to P2 heading fields, although some are analogous to MTS Service Elements or MTA Service Elements.
1.5.3. The Gateway Given this functional description of the two services, the functional nature of a gateway can now be considered. It would be elegant to consider the 822-MTS service mapping onto the MTS Service Elements and RFC 822 mapping onto an IPM, but reality just does not fit. Another elegant approach would be to treat this document as the definition of an X.400 Access Unit (AU). Again, reality does not fit. It is necessary to consider that the IPM format definition, the IPMS Service Elements, the MTS Service Elements, and MTA Service Elements on one side are mapped into RFC 822 + 822-MTS on the other in a slightly tangled manner. The details of the tangle will be made clear in Chapter 5. Access to the MTA Service Elements is minimised. The following basic mappings are thus defined. When going from RFC 822 to X.400, an RFC 822 message and the associated 822-MTS information is always mapped into an IPM (MTA, MTS, and IPMS Services). Going from X.400 to RFC 822, an RFC 822 message and the associated 822-MTS information may be derived from: 1. A Report (MTA, and MTS Services) 2. An IPN (MTA, MTS, and IPMS Services) 3. An IPM (MTA, MTS, and IPMS Services) Probes (MTA Service) must be processed by the gateway, as discussed in Chapter 5. MTS Messages containing Content Types other than those defined by the IPMS are not mapped by the gateway, and should be rejected at the gateway. 1.5.4. Repeated Mappings The mappings specified here are designed to work where a message traverses multiple times between X.400 and RFC 822. This is often essential, particularly in the case of distribution lists. However, in general, this will lead to a level of service which is the lowest common denominator (approximately the services offered by RFC 822). In particular, there is no expectation of additional X.400 services being mapped - although this may be possible in some cases. 1.6. RFC 987 Much of this work is based on the initial specification of RFC 987 and in its addendum RFC 1026. A basic decision is that the mapping will be to the full 1988 version of X.400, and not to a 1984 compatible subset. This is important, to give good support to communities which will utilise full X.400 at an early date. This has
the following implications:
- This document does not obsolete RFC 987, as it has a
different domain of application.
- If a gatewayed message is being transferred to a 1984
system, then RFC 987 should be used. If the X.400 side of
the gateway is a 1988 system, then it should be operated in
1984 compatibility mode. There is no advantage and some
disadvantage in using the new mapping, and later on applying
X.400 downgrading rules. Note that in an environment where
RFC 822 is of major importance, it may be desirable for
downgrading to consider the case where the message was
originated in an RFC 822 system, and mapped according to
this specification.
- New features of X.400 can be used to provide a much cleaner
mapping than that defined in RFC 987.
Unnecessary change is usually a bad idea. Changes on the RFC 822
side are avoided as far as possible, so that RFC 822 users do not see
arbitrary differences between systems conforming to this
specification, and those following RFC 987. Changes on the X.400
side are minimised, but are more acceptable, due to the mapping onto
a new set of services and protocols.
A summary of changes made is given in Appendix A.
1.7. Aspects not covered
There have been a number of cases where RFC 987 was used in a manner
which was not intended. This section is to make clear some
limitations of scope. In particular, this specification does not
specify:
- Extensions of RFC 822 to provide access to all X.400
services
- X.400 user interface definition
These are really coupled. To map the X.400 services, this
specification defines a number of extensions to RFC 822. As a side
effect, these give the 822 user access to SOME X.400 services.
However, the aim on the RFC 822 side is to preserve current service,
and it is intentional that access is not given to all X.400 services.
Thus, it will be a poor choice for X.400 implementors to use RFC
987(88) as an interface - there are too many aspects of X.400 which
cannot be accessed through it. If a text interface is desired, a
specification targeted at X.400, without RFC 822 restrictions, would be more appropriate. 1.8. Subsetting This proposal specifies a mapping which is appropriate to preserve services in existing RFC 822 communities. Implementations and specifications which subset this specification are strongly discouraged. 1.9. Document Structure This document has five chapters: 1. Overview - this chapter. 2. Service Elements - This describes the (end user) services mapped by a gateway. 3. Basic mappings - This describes some basic notation used in Chapters 3-5, the mappings between character sets, and some fundamental protocol elements. 4. Addressing - This considers the mapping between X.400 O/R names and RFC 822 addresses, which is a fundamental gateway component. 5. Detailed Mappings - This describes the details of all other mappings. There are also six appendices: A. Differences with RFC 987 B. Mappings Specific to JNT Mail C. Mappings Specific to UUCP Mail D. Object Identifier Assignment E. BNF Summary F. Format of Address Tables WARNING: THE REMAINDER OF THIS SPECIFICATION IS TECHNICALLY DETAILED. IT WILL NOT MAKE SENSE, EXCEPT IN THE CONTEXT OF RFC 822 AND
X.400 (1988). DO NOT ATTEMPT TO READ THIS DOCUMENT UNLESS
YOU ARE FAMILIAR WITH THESE SPECIFICATIONS.
1.10. Acknowledgements
This work was partly sponsored by the Joint Network Team. The
workshop at UCL in June 1989 to work on this specification was also
an IFIP WG 6.5 meeting.
The work in this specification was substantially based on RFC 987,
which had input from many people.
Useful comments and suggestions were made by Pete Cowen (Nottingham
Univ), Jim Craigie (JNT), Christian Huitema (Inria), Peter Lynch
(Prime), Julian Onions (Nottingham Univ), Sandy Shaw (Edinburgh
Univ), Einar Stefferud (NMA), and Peter Sylvester (GMD).
Chapter 2 -- Service Elements
This chapter considers the services offered across a gateway built
according to this specification. It gives a view of the
functionality provided by such a gateway for communication with users
in the opposite domain. This chapter considers service mappings in
the context of SINGLE transfers only, and not repeated mappings
through multiple gateways.
2.1. The Notion of Service Across a Gateway
RFC 822 and X.400 provide a number of services to the end user. This
chapter describes the extent to which each service can be supported
across an X.400 <-> RFC 822 gateway. The cases considered are single
transfers across such a gateway, although the problems of multiple
crossings are noted where appropriate.
2.1.1. Origination of Messages
When a user originates a message, a number of services are available.
Some of these imply actions (e.g., delivery to a recipient), and some
are insertion of known data (e.g., specification of a subject field).
This chapter describes, for each offered service, to what extent it
is supported for a recipient accessed through a gateway. There are
three levels of support:
Supported
The corresponding protocol elements map well, and so the
service can be fully provided.
Not Supported
The service cannot be provided, as there is a complete
mismatch.
Partial Support
The service can be partially fulfilled.
In the first two cases, the service is simply marked as "Supported"
or "Not Supported". Some explanation may be given if there are
additional implications, or the (non) support is not intuitive. For
partial support, the level of partial support is summarised. Where
partial support is good, this will be described by a phrase such as
"Supported by use of.....". A common case of this is where the
service is mapped onto a non- standard service on the other side of
the gateway, and this would have lead to support if it had been a
standard service. In many cases, this is equivalent to support. For
partial support, an indication of the mechanism is given, in order to
give a feel for the level of support provided. Note that this is not
a replacement for Chapter 5, where the mapping is fully specified.
If a service is described as supported, this implies:
- Semantic correspondence.
- No (significant) loss of information.
- Any actions required by the service element.
An example of a service gaining full support: If an RFC 822
originator specifies a Subject: field, this is considered to be
supported, as an X.400 recipient will get a subject indication.
All RFC 822 services are supported or partially supported for
origination. The implications of non-supported X.400 services is
described under X.400.
2.1.2. Reception of Messages
For reception, the list of service elements required to support this
mapping is specified. This is really an indication of what a
recipient might expect to see in a message which has been remotely
originated.
2.2. RFC 822
RFC 822 does not explicitly define service elements, as distinct from
protocol elements. However, all of the RFC 822 header fields, with
the exception of trace, can be regarded as corresponding to implicit
RFC 822 service elements. 2.2.1. Origination in RFC 822 A mechanism of mapping, used in several cases, is to map the RFC 822 header into a heading extension in the IPM (InterPersonal Message). This can be regarded as partial support, as it makes the information available to any X.400 implementations which are interested in these services. Communities which require significant RFC 822 interworking should require that their X.400 User Agents are able to display these heading extensions. Support for the various service elements (headers) is now listed. Date: Supported. From: Supported. For messages where there is also a sender field, the mapping is to "Authorising Users Indication", which has subtly different semantics to the general RFC 822 usage of From:. Sender: Supported. Reply-To: Supported. To: Supported. Cc: Supported. Bcc: Supported. Message-Id: Supported. In-Reply-To: Supported, for a single reference. Where multiple references are given, partial support is given by mapping to "Cross Referencing Indication". This gives similar semantics. References: Supported. Keywords: Supported by use of a heading extension.
Subject:
Supported.
Comments:
Supported by use of an extra body part.
Encrypted:
Supported by use of a heading extension.
Resent-*
Supported by use of a heading extension. Note that
addresses in these fields are mapped onto text, and so are
not accessible to the X.400 user as addresses. In
principle, fuller support would be possible by mapping onto
a forwarded IP Message, but this is not suggested.
Other Fields
In particular X-* fields, and "illegal" fields in common
usage (e.g., "Fruit-of-the-day:") are supported by use of
heading extensions.
2.2.2. Reception by RFC 822
This considers reception by an RFC 822 User Agent of a message
originated in an X.400 system and transferred across a gateway. The
following standard services (headers) may be present in such a
message:
Date:
From:
Sender:
Reply-To:
To:
Cc:
Bcc:
Message-Id:
In-Reply-To:
References:
Subject:
The following non-standard services (headers) may be present. These
are defined in more detail in Chapter 5 (5.3.4, 5.3.6, 5.3.7):
Autoforwarded:
Content-Identifier:
Conversion:
Conversion-With-Loss:
Delivery-Date:
Discarded-X400-IPMS-Extensions:
Discarded-X400-MTS-Extensions:
DL-Expansion-History:
Deferred-Delivery:
Expiry-Date:
Importance:
Incomplete-Copy:
Language:
Latest-Delivery-Time:
Message-Type:
Obsoletes:
Original-Encoded-Information-Types:
Originator-Return-Address:
Priority:
Redirection-History:
Reply-By:
Requested-Delivery-Method:
Sensitivity:
X400-Content-Type:
X400-MTS-Identifier:
X400-Originator:
X400-Received:
X400-Recipients:
2.3. X.400
2.3.1. Origination in X.400
When mapping services from X.400 to RFC 822 which are not supported
by RFC 822, new RFC 822 headers are defined. It is intended that
these fields will be registered, and that co-operating RFC 822
systems may use them. Where these new fields are used, and no system
action is implied, the service can be regarded as being partially
supported. Chapter 5 describes how to map X.400 services onto these
new headers. Other elements are provided, in part, by the gateway as
they cannot be provided by RFC 822.
Some service elements are marked N/A (not applicable). There are
five cases, which are marked with different comments:
N/A (local)
These elements are only applicable to User Agent / Message
Transfer Agent interaction and so they cannot apply to RFC
822 recipients.
N/A (PDAU)
These service elements are only applicable where the
recipient is reached by use of a Physical Delivery Access
Unit (PDAU), and so do not need to be mapped by the gateway.
N/A (reception)
These services are only applicable for reception.
N/A (prior)
If requested, this service must be performed prior to the
gateway.
N/A (MS)
These services are only applicable to Message Store (i.e., a
local service).
Finally, some service elements are not supported. In particular, the
new security services are not mapped onto RFC 822. Unless otherwise
indicated, the behaviour of service elements marked as not supported
will depend on the criticality marking supplied by the user. If the
element is marked as critical for transfer or delivery, a non-
delivery notification will be generated. Otherwise, the service
request will be ignored.
2.3.1.1. Basic Interpersonal Messaging Service
These are the mandatory IPM services as listed in Section 19.8 of
X.400 / ISO/IEC 10021-1, listed here in the order given. Section
19.8 has cross references to short definitions of each service.
Access management
N/A (local).
Content Type Indication
Supported by a new RFC 822 header (Content-Type:).
Converted Indication
Supported by a new RFC 822 header (X400-Received:).
Delivery Time Stamp Indication
N/A (reception).
IP Message Identification
Supported.
Message Identification
Supported, by use of a new RFC 822 header
(X400-MTS-Identifier). This new header is required, as
X.400 has two message-ids whereas RFC 822 has only one (see
previous service).
Non-delivery Notification
Not supported, although in general an RFC 822 system will
return error reports by use of IP messages. In other
service elements, this pragmatic result can be treated as
effective support of this service element.
Original Encoded Information Types Indication
Supported as a new RFC 822 header
(Original-Encoded-Information-Types:).
Submission Time Stamp Indication
Supported.
Typed Body
Some types supported. IA5 is fully supported.
ForwardedIPMessage is supported, with some loss of
information. Other types get some measure of support,
dependent on X.400 facilities for conversion to IA5. This
will only be done where content conversion is not
prohibited.
User Capabilities Registration
N/A (local).
2.3.1.2. IPM Service Optional User Facilities
This section describes support for the optional (user selectable) IPM
services as listed in Section 19.9 of X.400 / ISO/IEC 10021- 1,
listed here in the order given. Section 19.9 has cross references to
short definitions of each service.
Additional Physical Rendition
N/A (PDAU).
Alternate Recipient Allowed
Not supported. There is no RFC 822 service equivalent to
prohibition of alternate recipient assignment (e.g., an RFC
822 system may freely send an undeliverable message to a
local postmaster). Thus, the gateway cannot prevent
assignment of alternative recipients on the RFC 822 side.
This service really means giving the user control as to
whether or not an alternate recipient is allowed. This
specification requires transfer of messages to RFC 822
irrespective of this service request, and so this service is
not supported.
Authorising User's Indication
Supported.
Auto-forwarded Indication
Supported as new RFC 822 header (Auto-Forwarded:).
Basic Physical Rendition
N/A (PDAU).
Blind Copy Recipient Indication
Supported.
Body Part Encryption Indication
Supported by use of a new RFC 822 header
(Original-Encoded-Information-Types:), although in most
cases it will not be possible to map the body part in
question.
Content Confidentiality
Not supported.
Content Integrity
Not supported.
Conversion Prohibition
Supported. In this case, only messages with IA5 body parts,
other body parts which contain only IA5, and Forwarded IP
Messages (subject recursively to the same restrictions),
will be mapped.
Conversion Prohibition in Case of Loss of Information
Supported.
Counter Collection
N/A (PDAU).
Counter Collection with Advice
N/A (PDAU).
Cross Referencing Indication
Supported.
Deferred Delivery
N/A (prior). This service should always be provided by the
MTS prior to the gateway. A new RFC 822 header
(Deferred-Delivery:) is provided to transfer information on
this service to the recipient.
Deferred Delivery Cancellation
N/A (local).
Delivery Notification
Supported. This is performed at the gateway. Thus, a
notification is sent by the gateway to the originator. If
the 822-MTS protocol is JNT Mail, a notification may also be
sent by the recipient UA.
Delivery via Bureaufax Service
N/A (PDAU).
Designation of Recipient by Directory Name
N/A (local).
Disclosure of Other Recipients
Supported by use of a new RFC 822 header (X400-Recipients:).
This is descriptive information for the RFC 822 recipient,
and is not reverse mappable.
DL Expansion History Indication
Supported by use of a new RFC 822 header
(DL-Expansion-History:).
DL Expansion Prohibited
Distribution List means MTS supported distribution list, in
the manner of X.400. This service does not exist in the RFC
822 world. RFC 822 distribution lists should be regarded as
an informal redistribution mechanism, beyond the scope of
this control. Messages will be sent to RFC 822,
irrespective of whether this service is requested.
Theoretically therefore, this service is supported, although
in practice it may appear that it is not supported.
Express Mail Service
N/A (PDAU).
Expiry Date Indication
Supported as new RFC 822 header (Expiry-Date:). In general,
no automatic action can be expected.
Explicit Conversion
N/A (prior).
Forwarded IP Message Indication
Supported, with some loss of information. The message is
forwarded in an RFC 822 body, and so can only be interpreted
visually.
Grade of Delivery Selection
N/A (PDAU)
Importance Indication
Supported as new RFC 822 header (Importance:).
Incomplete Copy Indication
Supported as new RFC 822 header (Incomplete-Copy:).
Language Indication
Supported as new RFC 822 header (Language:).
Latest Delivery Designation
Not supported. A new RFC 822 header (Latest-Delivery-Time:)
is provided, which may be used by the recipient.
Message Flow Confidentiality
Not supported.
Message Origin Authentication
N/A (reception).
Message Security Labelling
Not supported.
Message Sequence Integrity
Not supported.
Multi-Destination Delivery
Supported.
Multi-part Body
Supported, with some loss of information, in that the
structuring cannot be formalised in RFC 822.
Non Receipt Notification Request
Not supported.
Non Repudiation of Delivery
Not supported.
Non Repudiation of Origin
N/A (reception).
Non Repudiation of Submission
N/A (local).
Obsoleting Indication
Supported as new RFC 822 header (Obsoletes:).
Ordinary Mail
N/A (PDAU).
Originator Indication
Supported.
Originator Requested Alternate Recipient
Not supported, but is placed as comment next to address
(X400-Recipients:).
Physical Delivery Notification by MHS
N/A (PDAU).
Physical Delivery Notification by PDS
N/A (PDAU).
Physical Forwarding Allowed
Supported by use of a comment in a new RFC 822 header
(X400-Recipients:), associated with the recipient in
question.
Physical Forwarding Prohibited
Supported by use of a comment in a new RFC 822 header
(X400-Recipients:), associated with the recipient in
question.
Prevention of Non-delivery notification
Supported, as delivery notifications cannot be generated by
RFC 822. In practice, errors will be returned as IP
Messages, and so this service may appear not to be supported
(see Non-delivery Notification).
Primary and Copy Recipients Indication
Supported.
Probe
Supported at the gateway (i.e., the gateway services the
probe).
Probe Origin Authentication
N/A (reception).
Proof of Delivery
Not supported.
Proof of Submission
N/A (local).
Receipt Notification Request Indication
Not supported.
Redirection Allowed by Originator
Redirection means MTS supported redirection, in the manner
of X.400. This service does not exist in the RFC 822 world.
RFC 822 redirection (e.g., aliasing) should be regarded as
an informal redirection mechanism, beyond the scope of this
control. Messages will be sent to RFC 822, irrespective of
whether this service is requested. Theoretically therefore,
this service is supported, although in practice it may
appear that it is not supported.
Registered Mail
N/A (PDAU).
Registered Mail to Addressee in Person
N/A (PDAU).
Reply Request Indication
Supported as comment next to address.
Replying IP Message Indication
Supported.
Report Origin Authentication
N/A (reception).
Request for Forwarding Address
N/A (PDAU).
Requested Delivery Method
N/A (local). The services required must be dealt with at
submission time. Any such request is made available through
the gateway by use of a comment associated with the
recipient in question.
Return of Content
In principle, this is N/A, as non-delivery notifications are
not supported. In practice, most RFC 822 systems will
return part or all of the content along with the IP Message
indicating an error (see Non-delivery Notification).
Sensitivity Indication
Supported as new RFC 822 header (Sensitivity:).
Special Delivery
N/A (PDAU).
Stored Message Deletion
N/A (MS).
Stored Message Fetching
N/A (MS).
Stored Message Listing
N/A (MS).
Stored Message Summary
N/A (MS).
Subject Indication
Supported.
Undeliverable Mail with Return of Physical Message
N/A (PDAU).
Use of Distribution List
In principle this applies only to X.400 supported
distribution lists (see DL Expansion Prohibited).
Theoretically, this service is N/A (prior). In practice,
because of informal RFC 822 lists, this service can be
regarded as supported.
2.3.2. Reception by X.400
2.3.2.1. Standard Mandatory Services
The following standard IPM mandatory user facilities may be required
for reception of RFC 822 originated mail by an X.400 UA.
Content Type Indication
Delivery Time Stamp Indication
IP Message Identification
Message Identification
Non-delivery Notification
Original Encoded Information Types Indication
Submission Time Stamp Indication
Typed Body
2.3.2.2. Standard Optional Services
The following standard IPM optional user facilities may be required
for reception of RFC 822 originated mail by an X.400 UA.
Authorising User's Indication
Blind Copy Recipient Indication
Cross Referencing Indication
Originator Indication
Primary and Copy Recipients Indication
Replying IP Message Indication
Subject Indication
2.3.2.3. New Services
A new service "RFC 822 Header Field" is defined using the extension
facilities. This allows for any RFC 822 header field to be
represented. It may be present in RFC 822 originated messages, which
are received by an X.400 UA.
Chapter 3 -- Basic Mappings
3.1. Notation
The X.400 protocols are encoded in a structured manner according to
ASN.1, whereas RFC 822 is text encoded. To define a detailed
mapping, it is necessary to refer to detailed protocol elements in
each format. A notation to achieve this is described in this
section.
3.1.1. RFC 822
Structured text is defined according to the Extended Backus Naur Form
(EBNF) defined in Section 2 of RFC 822 [Crocker82a]. In the EBNF
definitions used in this specification, the syntax rules given in
Appendix D of RFC 822 are assumed. When these EBNF tokens are
referred to outside an EBNF definition, they are identified by the
string "822." appended to the beginning of the string (e.g.,
822.addr-spec). Additional syntax rules, to be used throughout this
specification, are defined in this chapter.
The EBNF is used in two ways.
1. To describe components of RFC 822 messages (or of 822-MTS
components). In this case, the lexical analysis defined in
Section 3 of RFC 822 should be used. When these new EBNF
tokens are referred to outside an EBNF definition, they are
identified by the string "EBNF." appended to the beginning
of the string (e.g., EBNF.bilateral-info).
2. To describe the structure of IA5 or ASCII information not in
an RFC 822 message. In these cases, tokens will either be
self delimiting, or be delimited by self delimiting tokens.
Comments and LWSP are not used as delimiters.
3.1.2. ASN.1
An element is referred to with the following syntax, defined in EBNF:
element = service "." definition *( "." definition )
service = "IPMS" / "MTS" / "MTA"
definition = identifier / context
identifier = ALPHA *< ALPHA or DIGIT or "-" >
context = "[" 1*DIGIT "]"
The EBNF.service keys are shorthand for the following service
specifications:
IPMS IPMSInformationObjects defined in Annex E of X.420 / ISO
10021-7.
MTS MTSAbstractService defined in Section 9 of X.411 / ISO
10021-4.
MTA MTAAbstractService defined in Section 13 of X.411 / ISO
10021-4.
The first EBNF.identifier identifies a type or value key in the
context of the defined service specification. Subsequent
EBNF.identifiers identify a value label or type in the context of the
first identifier (SET or SEQUENCE). EBNF.context indicates a context
tag, and is used where there is no label or type to uniquely identify
a component. The special EBNF.identifier keyword "value" is used to
denote an element of a sequence.
For example, IPMS.Heading.subject defines the subject element of the
IPMS heading. The same syntax is also used to refer to element
values. For example, MTS.EncodedInformationTypes.[0].g3Fax refers to
a value of MTS.EncodedInformationTypes.[0].
3.2. ASCII and IA5
A gateway will interpret all IA5 as ASCII. Thus, mapping between
these forms is conceptual.
3.3. Standard Types
There is a need to convert between ASCII text, and some of the types
defined in ASN.1 [CCITT/ISO88d]. For each case, an EBNF syntax
definition is given, for use in all of this specification, which leads to a mapping between ASN.1, and an EBNF construct. All EBNF syntax definitions of ASN.1 types are in lower case, whereas ASN.1 types are referred to with the first letter in upper case. Except as noted, all mappings are symmetrical. 3.3.1. Boolean Boolean is encoded as: boolean = "TRUE" / "FALSE" 3.3.2. NumericString NumericString is encoded as: numericstring = *DIGIT 3.3.3. PrintableString PrintableString is a restricted IA5String defined as: printablestring = *( ps-char ) ps-restricted-char = 1DIGIT / 1ALPHA / " " / "'" / "+" / "," / "-" / "." / "/" / ":" / "=" / "?" ps-delim = "(" / ")" ps-char = ps-delim / ps-restricted-char This can be used to represent real printable strings in EBNF. 3.3.4. T.61String In cases where T.61 strings are only used for conveying human interpreted information, the aim of a mapping should be to render the characters appropriately in the remote character set, rather than to maximise reversibility. For these cases, the mappings to IA5 defined in CCITT Recommendation X.408 (1988) should be used [CCITT/ISO88a]. These will then be encoded in ASCII. There is also a need to represent Teletex Strings in ASCII, for some aspects of O/R Address. For these, the following encoding is used: teletex-string = *( ps-char / t61-encoded ) t61-encoded = "{" 1* t61-encoded-char "}" t61-encoded-char = 3DIGIT Common characters are mapped simply. Other octets are mapped using a
quoting mechanism similar to the printable string mechanism. Each octet is represented as 3 decimal digits. There are a number of places where a string may have a Teletex and/or Printable String representation. The following BNF is used to represent this. teletex-and-or-ps = [ printablestring ] [ "*" teletex-string ] The natural mapping is restricted to EBNF.ps-char, in order to make the full BNF easier to parse. 3.3.5. UTCTime Both UTCTime and the RFC 822 822.date-time syntax contain: Year (lowest two digits), Month, Day of Month, hour, minute, second (optional), and Timezone. 822.date-time also contains an optional day of the week, but this is redundant. Therefore a symmetrical mapping can be made between these constructs. Note: In practice, a gateway will need to parse various illegal variants on 822.date-time. In cases where 822.date-time cannot be parsed, it is recommended that the derived UTCTime is set to the value at the time of translation. The UTCTime format which specifies the timezone offset should be used. 3.3.6. Integer A basic ASN.1 Integer will be mapped onto EBNF.numericstring. In many cases ASN.1 will enumerate Integer values or use ENUMERATED. An EBNF encoding labelled-integer is provided. When mapping from EBNF to ASN.1, only the integer value is mapped, and the associated text is discarded. When mapping from ASN.1 to EBNF, addition of an appropriate text label is strongly encouraged. labelled-integer ::= [ key-string ] "(" numericstring ")" key-string = *key-char key-char = <a-z, A-Z, 1-9, and "-"> 3.3.7. Object Identifier Object identifiers are represented in a form similar to that given in ASN.1. The numbers are mandatory, to ease encoding. It is recommended that as many strings as possible are used, to
facilitate user recognition.
object-identifier ::= [ defined-value ] oid-comp-list
oid-comp-list ::= oid-comp oid-comp-list
| oid-comp
defined-value ::= key-string
oid-comp ::= [ key-string ] "(" numericstring ")"
3.4. Encoding ASCII in Printable String
Some information in RFC 822 is represented in ASCII, and needs to be
mapped into X.400 elements encoded as printable string. For this
reason, a mechanism to represent ASCII encoded as PrintableString is
needed.
A structured subset of EBNF.printablestring is now defined. This can
be used to encode ASCII in the PrintableString character set.
ps-encoded = *( ps-restricted-char / ps-encoded-char )
ps-encoded-char = "(a)" ; (@)
/ "(p)" ; (%)
/ "(b)" ; (!)
/ "(q)" ; (")
/ "(u)" ; (_)
/ "(l)" ; "("
/ "(r)" ; ")"
/ "(" 3DIGIT ")"
The 822.3DIGIT in EBNF.ps-encoded-char must have range 0-127, and is
interpreted in decimal as the corresponding ASCII character. Special
encodings are given for: at sign (@), percent (%), exclamation
mark/bang (!), double quote ("), underscore (_), left bracket ((),
and right bracket ()). These characters, with the exception of round
brackets, are not included in PrintableString, but are common in RFC
822 addresses. The abbreviations will ease specification of RFC 822
addresses from an X.400 system. These special encodings should be
mapped in a case insensitive manner, but always be generated in lower
case.
A reversible mapping between PrintableString and ASCII can now be
defined. The reversibility means that some values of printable
string (containing round braces) cannot be generated from ASCII.
Therefore, this mapping must only be used in cases where the
printable strings may only be derived from ASCII (and will therefore
have a restricted domain). For example, in this specification, it is
only applied to a Domain defined attribute which will have been
generated by use of this specification and a value such as "(" would
not be possible.
To encode ASCII as PrintableString, the EBNF.ps-encoded syntax is
used, with all EBNF.ps-restricted-char mapped directly. All other
822.CHAR are encoded as EBNF.ps-encoded-char.
To encode PrintableString as ASCII, parse PrintableString as
EBNF.ps-encoded, and then reverse the previous mapping. If the
PrintableString cannot be parsed, then the mapping is being applied
in to an inappropriate value, and an error should be given to the
procedure doing the mapping. In some cases, it may be preferable to
pass the printable string through unaltered.
Some examples are now given. Note the arrows which indicate
asymmetrical mappings:
PrintableString ASCII
'a demo.' <-> 'a demo.'
foo(a)bar <-> foo@bar
(q)(u)(p)(q) <-> "_%"
(a) <-> @
(A) <-> @
(l)a(r) <-> (a)
(126) <-> ~
( -> (
(l) <-> (
Chapter 4 -- Addressing
Addressing is probably the trickiest problem of an X.400 <-> RFC 822
gateway. Therefore it is given a separate chapter. This chapter, as
a side effect, also defines a textual representation of an X.400 O/R
Address.
Initially, we consider an address in the (human) mail user sense of
"what is typed at the mailsystem to reference a mail user". A basic
RFC 822 address is defined by the EBNF EBNF.822-address:
822-address = [ route ] addr-spec
In an 822-MTS protocol, the originator and each recipient should be
considered to be defined by such a construct. In an RFC 822 header,
the EBNF.822-address is encapsulated in the 822.address syntax rule,
and there may also be associated comments. None of this extra
information has any semantics, other than to the end user.
(next page on part 2)
