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^std11(-> 2822^prop) 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^std11(-> 2822^prop) 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^std11(-> 2822^prop) originator specifies a Subject: field, this is considered to be supported, as an X.400 recipient will get a subject indication.

In many cases, the required action will simply be to make the information available to the end user. In other cases, actions may imply generating a delivery report.

All RFC 822^std11(-> 2822^prop) 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^std11(-> 2822^prop)

RFC 822^std11(-> 2822^prop) does not explicitly define service elements, as distinct from protocol elements. However, all of the RFC 822^std11(-> 2822^prop) header fields, with the exception of trace, can be regarded as corresponding to implicit RFC 822^std11(-> 2822^prop) service elements.

2.2.1. Origination in RFC 822^std11(-> 2822^prop)

A mechanism of mapping, used in several cases, is to map the RFC 822^std11(-> 2822^prop) 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^std11(-> 2822^prop) interworking are recommended to require that their X.400 User Agents are able to display these heading extensions. Support for the various service elements (headers) is now listed.

   Date:
        Supported.

   From:
        Supported.  For messages where there is also a sender field,
        the mapping is to "Authorising Users Indication", which has
        subtly different semantics to the general RFC 822std11(-> 2822prop) usage of
        From:.

   Sender: Supported.

   Reply-To: Supported.

   To:  Supported.

   Cc:  Supported.

   Bcc: Supported.

   Message-Id: Supported.

   In-Reply-To:
      Supported, for a single reference.  Where multiple references are
      given, partial support is given by mapping to "Cross Referencing
      Indication".  This gives similar semantics.

   References: Supported.

   Keywords: Supported by use of a heading extension.

   Subject: Supported.

   Comments: Supported by use of a heading extension.

   Encrypted: Supported by use of a heading extension.

   Content-Language: Supported.

   Resent-*

      Supported by use of a heading extension.  Note that addresses in
      these fields are mapped onto text, and so are not accessible to
      the X.400 user as addresses.  In principle, fuller support would
      be possible by mapping onto a forwarded IP Message, but this is
      not suggested.

   Other Fields

      In particular X-* fields, and "illegal" fields in common usage
      (e.g., "Fruit-of-the-day:") are supported by use of heading
      extensions.

   MIME introduces a number of headings.  Support is defined in RFC
   2157prop.

2.2.2. Reception by RFC 822^std11(-> 2822^prop)

This considers reception by an RFC 822^std11(-> 2822^prop) User Agent of a message originated in an X.400 system and transferred across a gateway. The following standard services (headers) may be present in such a message:

   Date:

   From:

   Sender:

   Reply-To:

   To:

   Cc:

   Bcc:

   Message-Id:

   In-Reply-To:

   References:

   Subject:

   Content-Type: (See RFC 2157prop)

   Content-Transfer-Encoding: (See RFC 2157prop)

   MIME-Version: (See RFC 2157prop)

The following services (headers) may be present in the header of a message. These are defined in more detail in Chapter 5 (5.3.4, 5.3.6, 5.3.7):

   Autoforwarded:

   Autosubmitted:

   X400-Content-Identifier:

   Content-Language:

   Conversion:

   Conversion-With-Loss:

   Delivery-Date:

   Discarded-X400-IPMS-Extensions:

   Discarded-X400-MTS-Extensions:

   DL-Expansion-History:

   Deferred-Delivery:

   Expires:

   Importance:

   Incomplete-Copy:

   Latest-Delivery-Time:

   Message-Type:

   Original-Encoded-Information-Types:

   Originator-Return-Address:

   Priority:

   Reply-By:

   Sensitivity:

   Supersedes:

   X400-Content-Type:

   X400-MTS-Identifier:

   X400-Originator:

   X400-Received:

   X400-Recipients:

2.3. X.400

2.3.1. Origination in X.400

When mapping services from X.400 to RFC 822^std11(-> 2822^prop) which are not supported by RFC 822^std11(-> 2822^prop), new RFC 822^std11(-> 2822^prop) headers are defined, and registered by publication in this standard. It is intended that co-operating RFC 822^std11(-> 2822^prop) systems may also use them. Where these new fields are used, and no system action is implied, the service can be regarded as being partially supported. Chapter 5 describes how to map X.400 services onto these new headers. Other elements are provided, in part, by the gateway as they cannot be provided by RFC 822^std11(-> 2822^prop).

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 822std11(-> 2822prop)
      recipients.

   N/A (PDAU)
      These service elements are only applicable where the recipient is
      reached by use of a Physical Delivery Access Unit (PDAU), and so
      do not need to be mapped by the gateway.

   N/A (reception)
      These services  are only applicable for reception.

   N/A (prior)
      If requested, this service shall be performed prior to the
      gateway.

   N/A (MS)
      These services are only applicable to Message Store (i.e., a local
      service).

Finally, some service elements are not supported. In particular, the new security services are not mapped onto RFC 822^std11(-> 2822^prop). 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 822std11(-> 2822prop) header (X400-Content-Type:).

   Converted Indication
      Supported by a new RFC 822std11(-> 2822prop) header (X400-Received:).

   Delivery Time Stamp Indication
      N/A (reception).

   IP Message Identification
      Supported.

   Message Identification
      Supported, by use of a new RFC 822std11(-> 2822prop) header (X400-MTS-Identifier).
      This new header is required, as X.400 has two message-ids whereas

      RFC 822std11(-> 2822prop) has only one (see IP Message Identification

   Non-delivery Notification
      Not supported in all cases.  Supported where the recipient system
      supports NOTARY DSNs.  In general all RFC 822std11(-> 2822prop) systems will return
      error reports by use of IP messages.  In other service elements,
      this pragmatic result can be treated as effective support of this
      service element.

   Original Encoded Information Types Indication
      Supported as a new RFC 822std11(-> 2822prop) header (Original-Encoded-Information-
      Types:).

   Submission Time Stamp Indication
      Supported.

   Typed Body
      Support is defined in RFC 2157prop.

   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.

2.3.1.3. Additional Physical Rendition

      N/A (PDAU).

   Alternate Recipient Allowed
      Not supported.  There is no RFC 822std11(-> 2822prop) service equivalent to
      prohibition of alternate recipient assignment (e.g., an RFC 822std11(-> 2822prop)
      system may freely send an undeliverable message to a local
      postmaster).  A MIXER gateway has two conformant options.  The
      first is not to gateway a message requesting prohibition of
      alternate recipient, as this control cannot be guaranteed.  This
      option supports the service, but may cause unacceptable level of
      message rejections. The second is to gateway the message on the
      basis that there is no alternate recipient service in RFC 822std11(-> 2822prop). RFC
      1327(-> 2156prop) allowed only the second option.   If the first option is
      shown to be operationally effective, it may be the only option in
      future versions of MIXER.

   Authorising User's Indication
      Supported.

   Auto-forwarded Indication
      Supported as new RFC 822std11(-> 2822prop) 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 822std11(-> 2822prop) header (Original-Encoded-
      Information-Types:), although in most cases it will not be
      possible to map the body part in question.

   Content Confidentiality
      Not supported.

   Content Integrity
      Not supported.

   Conversion Prohibition
      Supported. Operation defined in RFC 2157prop.

   Conversion Prohibition in Case of Loss of Information
      Supported.  Operation defined in RFC 2157prop.

   Counter Collection
      N/A (PDAU).

   Counter Collection with Advice
      N/A (PDAU).

   Cross Referencing Indication
      Supported.

   Deferred Delivery
      N/A (prior).  This service shall always be provided by the MTS
      prior to the gateway.  A new RFC 822std11(-> 2822prop) header (Deferred-Delivery:)
      is provided to transfer information on this service to the
      recipient.

   Deferred Delivery Cancellation
      N/A (local).

   Delivery Notification
      Supported.  This is performed at the gateway, but may be performed
      at the end system if the end system supports NOTARY.  Thus, a
      notification is sent by the gateway to the originator.

   Delivery via Bureaufax Service
      N/A (PDAU).

   Designation of Recipient by Directory Name
      N/A (local).

   Disclosure of Other Recipients
      Supported by use of a new RFC 822std11(-> 2822prop) header (X400-Recipients:).  This
      is descriptive information for the RFC 822std11(-> 2822prop) recipient, and is not
      reverse mappable.

   DL Expansion History Indication
      Supported by use of a new RFC 822std11(-> 2822prop) 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 822std11(-> 2822prop)
      world, although RFC 822std11(-> 2822prop) supports distribution list functionality.
      There is no SMTP leve control to prohibit distribution list
      expansion.   A MIXER gateway has two conformant options.  The
      first is not to gateway a message requesting DL expansion
      prohibition, as this control cannot be guaranteed.  This option
      supports the service, but may cause unacceptable level of message
      rejections. The second is to gateway the message on the basis that
      there is no distribution list service in RFC 822std11(-> 2822prop). RFC 1327(-> 2156prop) allowed
      only the second option.   If the first option is shown to be
      operationally effective, it may be the only option in future
      versions of MIXER.

   Express Mail Service
      N/A (PDAU).

   Expiry Date Indication
      Supported as new RFC 822std11(-> 2822prop) header (Expires:).  In general, no
      automatic action can be expected.

   Explicit Conversion
      N/A (prior).

   Forwarded IP Message Indication
      Supported.

   Grade of Delivery Selection
      Not Supported.  There is no equivalent service in RFC 822std11(-> 2822prop).

   Importance Indication
      Supported as new RFC 822std11(-> 2822prop) header (Importance:).

   Incomplete Copy Indication
      Supported as new RFC 822std11(-> 2822prop) header (Incomplete-Copy:).

   Language Indication
      Supported as new RFC 822std11(-> 2822prop) header (Content-Language:).

   Latest Delivery Designation
      Not supported.  A new RFC 822std11(-> 2822prop) header (Latest-Delivery-Time:) is
      provided, which may be used by the recipient for general
      information, but will not be acted on by the SMTP infrastrucuture.

   Message Flow Confidentiality
      Not supported.

   Message Origin Authentication
      N/A (reception).

   Message Security Labelling
      Not supported.

   Message Sequence Integrity
      Not supported.

   Multi-Destination Delivery Supported.

   Multi-part Body
      Supported.

   Non Receipt Notification Request
      Not supported.

   Non Repudiation of Delivery
      Not supported.

   Non Repudiation of Origin
      N/A (reception).

   Non Repudiation of Submission
      N/A (local).

   Obsoleting Indication
      Supported as new RFC 822std11(-> 2822prop) header (Supersedes:).

   Ordinary Mail
      N/A (PDAU).

   Originator Indication
      Supported.

   Originator Requested Alternate Recipient
      Not supported, but is placed as comment next to address (X400-
      Recipients:).

   Physical Delivery Notification by MHS
      N/A (PDAU).

   Physical Delivery Notification by PDS
      N/A (PDAU).

   Physical Forwarding Allowed
      Supported by use of a comment in a new RFC 822std11(-> 2822prop) header (X400-
      Recipients:), associated with the recipient in question.

   Physical Forwarding Prohibited
      Supported by use of a comment in a new RFC 822std11(-> 2822prop) header (X400-
      Recipients:), associated with the recipient in question.

   Prevention of Non-delivery notification
      Supported where SMTP and NOTARY are available.  In other cases
      formally supported, as delivery notifications cannot be generated
      by RFC 822std11(-> 2822prop).  In practice, errors will be returned as IP Messages,
      and so this service may appear not to be supported (see Non-
      delivery Notification).

   Primary and Copy Recipients Indication
      Supported

   Probe
      Supported at the gateway (i.e., the gateway services the probe).

   Probe Origin Authentication
      N/A (reception).

   Proof of Delivery
      Not supported.

   Proof of Submission
      N/A (local).

   Receipt Notification Request Indication
      Not supported.

   Redirection Disallowed by Originator
      Redirection means MTS supported redirection, in the manner of
      X.400.  This service does not exist in the RFC 822std11(-> 2822prop) world.  RFC 822std11(-> 2822prop)
      redirection (e.g., aliasing) is regarded as an informal
      redirection mechanism, beyond the scope of this control.  Messages
      will be sent to RFC 822std11(-> 2822prop), irrespective of whether this service is
      requested. In practice, control of this service is not supported.

   Registered Mail
      N/A (PDAU).

   Registered Mail to Addressee in Person
      N/A (PDAU).

   Reply Request Indication
      Supported as comment next to address.

   Replying IP Message Indication
      Supported.

   Report Origin Authentication
      N/A (reception).

   Request for Forwarding Address
      N/A (PDAU).

   Requested Delivery Method
      N/A (local).   The service request is dealt with at submission
      time.  Any such request is made available through the gateway by
      use of a comment associated with the recipient in question.

   Return of Content
      Supported where SMTP and NOTARY are used. In principle for other
      situations, this is N/A, as non-delivery notifications are not
      supported.  In practice, most RFC 822std11(-> 2822prop) 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 822std11(-> 2822prop) 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 822std11(-> 2822prop) lists,
      this service can be regarded as supported.

   Auto-Submitted Indication
      Supported

2.3.2. Reception by X.400

2.3.2.1. Standard Mandatory Services

The following standard IPM mandatory user facilities are required for reception of RFC 822^std11(-> 2822^prop) 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 are required for reception of RFC 822^std11(-> 2822^prop) 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 X.400 service "RFC 822^std11(-> 2822^prop) Header Field" is defined using the extension facilities. This allows for any RFC 822^std11(-> 2822^prop) header field to be represented. It may be present in RFC 822^std11(-> 2822^prop) originated messages which are received by an X.400 UA.