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<pre>Network Working Group                                           S. Kille
Request for Comments: 1801                              ISODE Consortium
Category: Experimental                                         June 1995


   <span class="h1">X.400-MHS use of the X.500 Directory to support X.400-MHS Routing</span>

Status of this Memo

   This memo defines an Experimental Protocol for the Internet
   community.  This memo does not specify an Internet standard of any
   kind.  Discussion and suggestions for improvement are requested.
   Distribution of this memo is unlimited.

Table of Contents

  1   Introduction                                                     3
  2   Goals                                                            3
  3   Approach                                                         5
  4   Direct vs Indirect Connection                                    6
  5   X.400 and <a href="./rfc822">RFC 822</a>                                                8
  6   Objects                                                          9
  7   Communities                                                     10
  8   Routing Trees                                                   11
      8.1    Routing Tree Definition   .   .   .   .   .   .   .      12
      8.2    The Open Community Routing Tree   .   .   .   .   .      12
      8.3    Routing Tree Location     .   .   .   .   .   .   .      13
      8.4    Example Routing Trees     .   .   .   .   .   .   .      13
      8.5    Use of Routing Trees to look up Information   .   .      13
  9   Routing Tree Selection                                          14
      9.1    Routing Tree Order    .   .   .   .   .   .   .   .      14
      9.2    Example use of Routing Trees  .   .   .   .   .   .      15
          9.2.1    Fully Open Organisation     .   .   .   .   .      15
          9.2.2    Open Organisation with Fallback     .   .   .      15
          9.2.3    Minimal-routing MTA     .   .   .   .   .   .      16
          9.2.4    Organisation with Firewall  .   .   .   .   .      16
          9.2.5    Well Known Entry Points     .   .   .   .   .      16
          9.2.6    ADMD using the Open Community for Advertising      16
          9.2.7    ADMD/PRMD gateway   .   .   .   .   .   .   .      17
  10  Routing Information                                             17
      10.1   Multiple routing trees    .   .   .   .   .   .   .      20
      10.2   MTA Choice    .   .   .   .   .   .   .   .   .   .      22
      10.3   Routing Filters   .   .   .   .   .   .   .   .   .      25
      10.4   Indirect Connectivity     .   .   .   .   .   .   .      26
  11  Local Addresses (UAs)                                           27
      11.1   Searching for Local Users     .   .   .   .   .   .      30
  12  Direct Lookup                                                   30
  13  Alternate Routes                                                30



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      13.1   Finding Alternate Routes  .   .   .   .   .   .   .      30
      13.2   Sharing routing information   .   .   .   .   .   .      31
  14  Looking up Information in the Directory                         31
  15  Naming MTAs                                                     33
      15.1   Naming 1984 MTAs  .   .   .   .   .   .   .   .   .      35
  16  Attributes Associated with the MTA                              35
  17  Bilateral Agreements                                            36
  18  MTA Selection                                                   38
      18.1   Dealing with protocol mismatches  .   .   .   .   .      38
      18.2   Supported Protocols   .   .   .   .   .   .   .   .      39
      18.3   MTA Capability Restrictions   .   .   .   .   .   .      39
      18.4   Subtree Capability Restrictions   .   .   .   .   .      40
  19  MTA Pulling Messages                                            41
  20  Security and Policy                                             42
      20.1   Finding the Name of the Calling MTA   .   .   .   .      42
      20.2   Authentication    .   .   .   .   .   .   .   .   .      42
      20.3   Authentication Information    .   .   .   .   .   .      44
  21  Policy and Authorisation                                        46
      21.1   Simple MTA Policy     .   .   .   .   .   .   .   .      46
      21.2   Complex MTA Policy    .   .   .   .   .   .   .   .      47
  22  Delivery                                                        49
      22.1   Redirects     .   .   .   .   .   .   .   .   .   .      49
      22.2   Underspecified O/R Addresses  .   .   .   .   .   .      50
      22.3   Non Delivery  .   .   .   .   .   .   .   .   .   .      51
      22.4   Bad Addresses     .   .   .   .   .   .   .   .   .      51
  23  Submission                                                      53
      23.1   Normal Derivation     .   .   .   .   .   .   .   .      53
      23.2   Roles and Groups  .   .   .   .   .   .   .   .   .      53
  24  Access Units                                                    54
  25  The Overall Routing Algorithm                                   54
  26  Performance                                                     55
  27  Acknowledgements                                                55
  28  References                                                      56
  29  Security Considerations                                         57
  30  Author's Address                                                58
  A   Object Identifier Assignment                                    59
  B   Community Identifier Assignments                                60
  C   Protocol Identifier Assignments                                 60
  D   ASN.1 Summary                                                   61
  E   Regular Expression Syntax                                       71
  List of Figures
      1      Location of Routing Trees     .   .   .   .   .   .      12
      2      Routing Tree Use Definition   .   .   .   .   .   .      14
      3      Routing Information at a Node     .   .   .   .   .      17
      4      Indirect Access   .   .   .   .   .   .   .   .   .      25
      5      UA Attributes     .   .   .   .   .   .   .   .   .      27
      6      MTA Definitions   .   .   .   .   .   .   .   .   .      33
      7      MTA Bilateral Table Entry     .   .   .   .   .   .      36



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      8      Bilateral Table Attribute     .   .   .   .   .   .      37
      9      Supported MTS Extensions  .   .   .   .   .   .   .      39
      10     Subtree Capability Restriction    .   .   .   .   .      40
      11     Pulling Messages  .   .   .   .   .   .   .   .   .      41
      12     Authentication Requirements   .   .   .   .   .   .      43
      13     MTA Authentication Parameters     .   .   .   .   .      45
      14     Simple MTA Policy Specification   .   .   .   .   .      46
      15     Redirect Definition   .   .   .   .   .   .   .   .      48
      16     Non Delivery Information  .   .   .   .   .   .   .      50
      17     Bad Address Pointers  .   .   .   .   .   .   .   .      52
      18     Access Unit Attributes    .   .   .   .   .   .   .      53
      19     Object Identifier Assignment  .   .   .   .   .   .      59
      20     Transport Community Object Identifier Assignments        60
      21     Protocol Object Identifier Assignments    .   .   .      61
      22     ASN.1 Summary     .   .   .   .   .   .   .   .   .      61

<span class="h2"><a class="selflink" id="section-1" href="#section-1">1</a>.  Introduction</span>

   MHS Routing is the problem of controlling the path of a message as it
   traverses one or more MTAs to reach its destination recipients.
   Routing starts with a recipient O/R Address, and parameters
   associated with the message to be routed.  It is assumed that this is
   known a priori, or is derived at submission time as described in
   <a href="#section-23">Section 23</a>.

   The key problem in routing is to map from an O/R Address onto an MTA
   (next hop).  This shall be an MTA which in some sense is "nearer" to
   the destination UA. This is done repeatedly until the message can be
   directly delivered to the recipient UA. There are a number of things
   which need to be considered to determine this.  These are discussed
   in the subsequent sections.  A description of the overall routing
   process is given in <a href="#section-25">Section 25</a>.

<span class="h2"><a class="selflink" id="section-2" href="#section-2">2</a>.  Goals</span>

   Application level routing for MHS is a complex procedure, with many
   requirements.  The following goals for the solution are set:

 o  Straightforward to manage.  Non-trivial configuration of routing
    for current message handling systems is a black art, often
    involving gathering and processing many tables, and editing
    complex configuration files.  Many problems are solved in a very
    ad hoc manner.  Managing routing for MHS is the most serious
    headache for most mail system managers.

 o  Economic, both in terms of network and computational resources.





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 o  Robust.  Errors and out of date information shall cause minimal
    and localised damage.

 o  Deal with link failures.  There needs to be some ability to choose
    alternative routes.  In general, it is desirable that the routing
    approach be redundant.

 o  Load sharing.  Information on routes shall allow "equal" routes
    to be specified, and thus facilitate load sharing.

 o  Support format and protocol conversion

 o  Dynamic and automatic.  There shall be no need for manual
    propagation of tables or administrator intervention.

 o  Policy robust.  It shall not allow specification of policies which
    cause undesirable routing effects.

 o  Reasonably straightforward to implement.

 o  Deal with X.400, <a href="./rfc822">RFC 822</a>, and their interaction.

 o  Extensible to other mail architectures

 o  Recognise existing <a href="./rfc822">RFC 822</a> routing, and coexist smoothly.

 o  Improve <a href="./rfc822">RFC 822</a> routing capabilities.  This is particularly
    important for <a href="./rfc822">RFC 822</a> sites not in the SMTP Internet.

 o  Deal correctly with different X.400 protocols (P1, P3, P7), and
    with 1984, 1988 and 1992 versions.

 o  Support X.400 operation over multiple protocol stacks (TCP/IP,
    CONS, CLNS) and in different communities.

 o  Messages shall be routed consistently.  Alternate routing
    strategies, which might introduce unexpected delay, shall be used
    with care (e.g., routing through a protocol converter due to
    unavailability of an MTA).

 o  Delay between message submission and delivery shall be minimised.
    This has indirect impact on the routing approaches used.

 o  Interact sensibly with ADMD services.

 o  Be global in scope





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 o  Routing strategy shall deal with a scale of order of magnitude
    1,000,000 -- 100,000,000 MTAs.

 o  Routing strategy shall deal with of order 1,000,000 -- 100,000,000
    Organisations.

 o  Information about alterations in topology shall propagate rapidly
    to sites affected by the change.

 o  Removal, examination, or destruction of messages by third parties
    shall be difficult.  This is hard to quantify, but "difficult"
    shall be comparable to the effort needed to break system security
    on a typical MTA system.

 o  As with current Research Networks, it is recognised that
    prevention of forged mail will not always be possible.  However,
    this shall be as hard as can be afforded.

 o  Sufficient tracing and logging shall be available to track down
    security violations and faults.

 o  Optimisation of routing messages with multiple recipients, in
    cases where this involves selection of preferred single recipient
    routes.

The following are not initial goals:

 o  Advanced optimisation of routing messages with multiple
    recipients, noting dependencies between the recipients to find
    routes which would not have been chosen for any of the single
    recipients.

 o  Dynamic load balancing.  The approach does not give a means to
    determine load.  However, information on alternate routes is
    provided, which is the static information needed for load
    balancing.

<span class="h2"><a class="selflink" id="section-3" href="#section-3">3</a>.  Approach</span>

   A broad problem statement, and a survey of earlier approaches to the
   problem is given in the COSINE Study on MHS Topology and Routing [<a href="#ref-8" title="RARE">8</a>].
   The interim (table-based) approach suggested in this study, whilst
   not being followed in detail, broadly reflects what the research
   X.400 (GO-MHS) community is doing.  The evolving specification of the
   RARE table format is defined in [<a href="#ref-5" title="&quot;Routing Coordination for X.400 MHS Services Within a Multi Protocol / Multi Network Environment Table Format V3 for Static Routing&quot;">5</a>].  This document specifies the
   envisaged longer term approach.





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   Some documents have made useful contributions to this work:

 o  A paper by the editor on MHS use of directory, which laid out the
    broad approach of mapping the O/R Address space on to the DIT [<a href="#ref-7" title="Munich">7</a>].

 o  Initial ISO Standardisation work on MHS use of Directory for
    routing [<a href="#ref-19">19</a>].  Subsequent ISO work in this area has drawn from
    earlier drafts of this specification.

 o  The work of the VERDI Project [<a href="#ref-3" title="K. Bonacker">3</a>].

 o  Work by Kevin Jordan of CDC [<a href="#ref-6">6</a>].

 o  The routing approach of ACSNet [<a href="#ref-4" title="R.J. Kummerfeld">4</a>, <a href="#ref-17" title="R.">17</a>] paper.  This gives useful
    ideas on incremental routing, and replicating routing data.

 o  A lot of work on network routing is becoming increasingly
    relevant.  As the MHS routing problem increases in size, and
    network routing increases in sophistication (e.g., policy based
    routing), the two areas have increasing amounts in common.  For
    example, see [<a href="#ref-2">2</a>].

<span class="h2"><a class="selflink" id="section-4" href="#section-4">4</a>.  Direct vs Indirect Connection</span>

   Two extreme approaches to routing connectivity are:

   1.  High connectivity between MTAs.  An example of this is the way
       the Domain Name Server system is used on the DARPA/NSF Internet.
       Essentially, all MTAs are fully interconnected.

   2.  Low connectivity between MTAs.  An example of this is the UUCP
       network.

   In general an intermediate approach is desirable.  Too sparse a
   connectivity is inefficient, and leads to undue delays.  However,
   full connectivity is not desirable, for the reasons discussed below.
   A number of general issues related to relaying are now considered.
   The reasons for avoiding relaying are clear.  These include.

 o  Efficiency.  If there is an open network, it is desirable that it
    be used.

 o  Extra hops introduce delay, and increase the (very small)
    possibility of message loss.  As a basic principle, hop count
    shall be minimised.

 o  Busy relays or Well Known Entry points can introduce high delay
    and lead to single point of failure.



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 o  If there is only one hop, it is straightforward for the user to
    monitor progress of messages submitted.  If a message is delayed,
    the user can take appropriate action.

 o  Many users like the security of direct transmission.  It is an
    argument often given very strongly for use of SMTP.

   Despite these very powerful arguments, there are a number of reasons
   why some level of relaying is desirable:

 o  Charge optimisation.  If there is an expensive network/link to be
    traversed, it may make sense to restrict its usage to a small
    number of MTAs.  This would allow for optimisation with respect to
    the charging policy of this link.

 o  Copy optimisation.  If a message is being sent to two remote MTAs
    which are close together, it is usually optimal to send the
    message to one of the MTAs (for both recipients), and let it pass
    a copy to the other MTA.

 o  To access an intermediate MTA for some value added service.  In
    particular for:

    --  Message Format Conversion

    --  Distribution List expansion

 o  Dealing with different protocols.  The store and forward approach
    allows for straightforward conversion.  Relevant cases include:

    --  Provision of X.400 over different OSI Stacks (e.g.,
        Connectionless Network Service).

    --  Use of a different version of X.400.

    --  Interaction with non-X.400 mail services

 o  To compensate for inadequate directory services:  If tables are
    maintained in an ad hoc manner, the manual effort to gain full
    connectivity is too high.

 o  To hide complexity of structure.  If an organisation has many
    MTAs, it may still be advantageous to advertise a single entry
    point to the outside world.  It will be more efficient to have an
    extra hop, than to (widely) distribute the information required to
    connect directly.  This will also encourage stability, as
    organisations need to change internal structure much more
    frequently than their external entry points.  For many



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    organisations, establishing such firewalls is high priority.

 o  To handle authorisation, charging and security issues.  In
    general, it is desirable to deal with user oriented authorisation
    at the application level.  This is essential when MHS specific
    parameters shall be taken into consideration.  It may well be
    beneficial for organisations to have a single MTA providing access
    to the external world, which can apply a uniform access policy
    (e.g., as to which people are allowed access).  This would be
    particularly true in a multi-vendor environment, where different
    systems would otherwise have to enforce the same policy --- using
    different vendor-specific mechanisms.

   In summary there are strong reasons for an intermediate approach.
   This will be achieved by providing mechanisms for both direct and
   indirect connectivity.  The manager of a configuration will then be
   able to make appropriate choices for the environment.

   Two models of managing large scale routing have evolved:

   1.  Use of a global directory/database.  This is the approach
       proposed here.

   2.  Use of a routing table in each MTA, which is managed either by a
       management protocol or by directory.  This is coupled with means
       to exchange routing information between MTAs.  This approach is
       more analogous to how network level routing is commonly performed.
       It has good characteristics in terms of managing links and
       dealing with link related policy.  However, it assumes limited
       connectivity and does not adapt well to a network environment
       with high connectivity available.

<span class="h2"><a class="selflink" id="section-5" href="#section-5">5</a>.  X.400 and <a href="./rfc822">RFC 822</a></span>

   This document defines mechanisms for X.400 message routing.  It is
   important that this can be integrated with <a href="./rfc822">RFC 822</a> based routing, as
   many MTAs will work in both communities.  This routing document is
   written with this problem in mind, and some work to verify this has
   been done.  support for <a href="./rfc822">RFC 822</a> routing using the same basic
   infrastructure is defined in a companion document [<a href="#ref-13" title="&quot;Use of the X.500 directory to support routing for RFC 822 and related protocols&quot;">13</a>].  In addition
   support for X.400/RFC 822 gatewaying is needed, to support
   interaction.  Directory based mechanisms for this are defined in
   [<a href="#ref-16" title="&quot;Use of the X.500 directory to support mapping between X.400 and RFC 822 addresses&quot;">16</a>].  The advantages of the approach defined by this set of
   specifications are:

 o  Uniform management for sites which wish to support both protocols.

 o  Simpler management for gateways.



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 o  Improved routing services for <a href="./rfc822">RFC 822</a> only sites.

   For sites which are only X.400 or only <a href="./rfc822">RFC 822</a>, the mechanisms
   associated with gatewaying or with the other form of addressing are
   not needed.

<span class="h2"><a class="selflink" id="section-6" href="#section-6">6</a>.  Objects</span>

   It is useful to start with a manager's perspective.  Here is the set
   of object classes used in this specification.  It is important that
   all information entered relates to something which is being managed.
   If this is achieved, configuration decisions are much more likely to
   be correct.  In the examples, distinguished names are written using
   the String Syntax for Distinguished Names [<a href="#ref-11" title="&quot;A Representation of Distinguished Names (OSI-DS 23 (v5))&quot;">11</a>].  The list of objects
   used in this specification is:

User An entry representing a single human user.  This will typically
    be named in an organisational context.  For example:

     CN=Edgar Smythe,
     O=Zydeco Services, C=GB

    This entry would have associated information, such as telephone
    number, postal address, and mailbox.

MTA A Message Transfer Agent.  In general, the binding between
    machines and MTAs will be complex.  Often a small number of MTAs
    will be used to support many machines, by use of local approaches
    such as shared filestores.  MTAs may support multiple protocols,
    and will identify separate addressing information for each
    protocol.
    To achieve support for multiple protocols, an MTA is modelled as
    an Application Process, which is named in the directory.  Each MTA
    will have one or more associated Application Entities.  Each
    Application Entity is named as a child of the Application Process,
    using a common name which conveniently identifies the Application
    Entity relative to the Application Process.  Each Application
    Entity supports a single protocol, although different Application
    Entities may support the same protocol.  Where an MTA only
    supports one protocol or where the addressing information for all
    of the protocols supported have different attributes to represent
    addressing information (e.g., P1(88) and SMTP) the Application
    Entity(ies) may be represented by the single Application Process
    entry.

User Agent (Mailbox) This defines the User Agent (UA) to which mail
    may be delivered.  This will define the account with which the UA
    is associated, and may also point to the user(s) associated with



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    the UA. It will identify which MTAs are able to access the UA.
    (In the formal X.400 model, there will be a single MTA delivering
    to a UA. In many practical configurations, multiple MTAs can
    deliver to a single UA. This will increase robustness, and is
    desirable.)

Role Some organisational function.  For example:

     CN=System Manager, OU=Sales,
     O=Zydeco Services, C=GB


    The associated entry would indicate the occupant of the role.

Distribution Lists There would be an entry representing the
    distribution list, with information about the list, the manger,
    and members of the list.

<span class="h2"><a class="selflink" id="section-7" href="#section-7">7</a>.  Communities</span>

There are two basic types of agreement in which an MTA may participate
in order to facilitate routing:

Bilateral Agreements An agreement between a pair of MTAs to route
    certain types of traffic.  This MTA pair agreement usually
    reflects some form of special agreement and in general bilateral
    information shall be held for the link at both ends.  In some
    cases, this information shall be private.

Open Agreements An agreement between a collection of MTAs to behave
    in a cooperative fashion to route traffic.  This may be viewed as
    a general bilateral agreement.

   It is important to ensure that there are sufficient agreements in
   place for all messages to be routed.  This will usually be done by
   having agreements which correspond to the addressing hierarchy.  For
   X.400, this is the model where a PRMD connects to an ADMD, and the
   ADMD provides the inter PRMD connectivity, by the ability to route to
   all other ADMDs.  Other agreements may be added to this hierarchy, in
   order to improve the efficiency of routing.  In general, there may be
   valid addresses, which cannot be routed to, either for connectivity
   or policy reasons.

   We model these two types of agreements as communities.  A community
   is a scope in which an MTA advertises its services and learns about
   other services.  Each MTA will:

   1.  Register its services in one or more communities.



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   2.  Look up services in one or more communities.

   In most cases an MTA will deal with a very small number of
   communities --- very often one only.  There are a number of different
   types of community.

The open community This is a public/global scope.  It reflects
    routing information which is made available to any MTA which
    wishes to use it.

The local community This is the scope of a single MTA. It reflects
    routing information private to the MTA. It will contain an MTA's
    view of the set of bilateral agreements in which it participates,
    and routing information private and local to the MTA.

Hierarchical communities A hierarchical community is a subtree of the
    O/R Address tree.  For example, it might be a management domain,
    an organisation, or an organisational unit.  This sort of
    community will allow for firewalls to be established.  A community
    can have complex internal structure, and register a small subset
    of that in the open community.

Closed communities A closed community is a set of MTAs which agrees
    to route amongst themselves.  Examples of this might be ADMDs
    within a country, or a set of PRMDs representing the same
    organisation in multiple countries.

   Formally, a community indicates the scope over which a service is
   advertised.  In practice, it will tend to reflect the scope of
   services offered.  It does not make sense to offer a public service,
   and only advertise it locally.  Public advertising of a private
   service makes more sense, and this is shown below.  In general,
   having a community offer services corresponding to the scope in which
   they are advertised will lead to routing efficiency.  Examples of how
   communities can be used to implement a range of routing policies are
   given in <a href="#section-9.2">Section 9.2</a>.

<span class="h2"><a class="selflink" id="section-8" href="#section-8">8</a>.  Routing Trees</span>

   Communities are a useful abstract definition of the routing approach
   taken by this specification.  Each community is represented in the
   directory as a routing tree.  There will be many routing trees
   instantiated in the directory.  Typically, an MTA will only be
   registered in and make use of a small number of routing trees.  In
   most cases, it will register in and use the same set of routing
   trees.





<span class="grey">Kille                         Experimental                     [Page 11]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-12" ></span>
<span class="grey"><a href="./rfc1801">RFC 1801</a>        X.400-MHS Routing using X.500 Directory        June 1995</span>


<span class="h3"><a class="selflink" id="section-8.1" href="#section-8.1">8.1</a>  Routing Tree Definition</span>

   Each community has a model of the O/R address space.  Within a
   community, there is a general model of what to do with a given O/R
   Address.  This is structured hierarchically, according to the O/R
   address hierarchy.  A community can register different possible
   actions, depending on the depth of match.  This might include
   identifying the MTA associated with a UA which is matched fully, and
   providing a default route for an O/R address where there is no match
   in the community --- and all intermediate forms.  The name structure
   of a routing tree follows the O/R address hierarchy, which is
   specified in a separate document [<a href="#ref-15" title="&quot;Representing the O/R Address hierarchy in the X.500 directory information tree&quot;">15</a>].  Where there is any routing
   action associated with a node in a routing tree, the node is of
   object class routingInformation, as defined in <a href="#section-10">Section 10</a>.

<span class="h3"><a class="selflink" id="section-8.2" href="#section-8.2">8.2</a>  The Open Community Routing Tree</span>

   The routing tree of the open community starts at the root of the DIT.
   This routing tree also serves the special function of instantiating
   the global O/R Address space in the Directory.  Thus, if a UA wishes
   to publish information to the world, this hierarchy allows it to do
   so.

   The O/R Address hierarchy is a registered tree, which may be
   instantiated in the directory.  Names at all points in the tree are
   valid, and there is no requirement that the namespace is instantiated
   by the owner of the name.  For example, a PRMD may make an entry in
   the DIT, even if the ADMD above it does not.  In this case, there
   will be a "skeletal" entry for the ADMD, which is used to hang the
   PRMD entry in place.  The skeletal entry contains the minimum number
   of entries which are needed for it to exist in the DIT (Object Class
   and Attribute information needed for the relative distinguished
   name).  This entry may be placed there solely to support the
   subordinate entry, as its existence is inferred by the subordinate
   entry.  Only the owner of the entry may place information into it.
   An analogous situation in current operational practice is to make DIT
   entries for Countries and US States.

---------------------------------------------------------------------

routingTreeRoot OBJECT-CLASS ::= {
    SUBCLASS OF {routingInformation|subtree}
    ID oc-routing-tree-root}

                  Figure 1: Location of Routing Trees

---------------------------------------------------------------------




<span class="grey">Kille                         Experimental                     [Page 12]</span></pre>
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<span class="grey"><a href="./rfc1801">RFC 1801</a>        X.400-MHS Routing using X.500 Directory        June 1995</span>


<span class="h3"><a class="selflink" id="section-8.3" href="#section-8.3">8.3</a>  Routing Tree Location</span>

   All routing trees follow the same O/R address hierarchy.  Routing
   trees other than the open community routing tree are rooted at
   arbitrary parts of the DIT. These routing trees are instantiated
   using the subtree mechanism defined in the companion document
   "Representing Tables and Subtrees in the Directory" [<a href="#ref-15" title="&quot;Representing the O/R Address hierarchy in the X.500 directory information tree&quot;">15</a>].  A routing
   tree is identified by the point at which it is rooted.  An MTA will
   use a list of routing trees, as determined by the mechanism described
   in <a href="#section-9">Section 9</a>.  Routing trees may be located in either the
   organisational or O/R address structured part of the DIT. All routing
   trees, other than the open community routing tree, are rooted by an
   entry of object class routingTreeRoot, as defined in Figure 1.

<span class="h3"><a class="selflink" id="section-8.4" href="#section-8.4">8.4</a>  Example Routing Trees</span>

   Consider routing trees with entries for O/R Address:

    P=ABC; A=XYZMail; C=GB;

   In the open community routing tree, this would have a distinguished
   name of:

    PRMD=ABC, ADMD=XYZMail, C=GB

   Consider a routing tree which is private to:

    O=Zydeco Services, C=GB

   They might choose to label a routing tree root "Zydeco Routing Tree",
   which would lead to a routing tree root of:

    CN=Zydeco Routing Tree, O=Zydeco Services, C=GB

   The O/R address in question would be stored in this routing tree as:

    PRMD=ABC, ADMD=XYZMail
    C=GB, CN=Zydeco Routing Tree,
    O=Zydeco Services, C=GB

<span class="h3"><a class="selflink" id="section-8.5" href="#section-8.5">8.5</a>  Use of Routing Trees to look up Information</span>

   Lookup of an O/R address in a routing tree is done as follows:

   1.  Map the O/R address onto the O/R address hierarchy described in
       [<a href="#ref-15" title="&quot;Representing the O/R Address hierarchy in the X.500 directory information tree&quot;">15</a>] in order to generate a Distinguished Name.





<span class="grey">Kille                         Experimental                     [Page 13]</span></pre>
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   2.  Append this to the Distinguished Name of the routing tree, and
       then look up the whole name.

   3.  Handling of errors will depend on the application of the lookup,
       and is discussed later.

   Note that it is valid to look up a null O/R Address, as the routing
   tree root may contain default routing information for the routing
   tree.  This is held in the root entry of the routing tree, which is a
   subclass of routingInformation.  The open community routing tree does
   not have a default.

   Routing trees may have aliases into other routing trees.  This will
   typically be done to optimise lookups from the first routing tree
   which a given MTA uses.  Lookup needs to take account of this.

<span class="h2"><a class="selflink" id="section-9" href="#section-9">9</a>.  Routing Tree Selection</span>

   The list of routing trees which a given MTA uses will be represented
   in the directory.  This uses the attribute defined in Figure 2.

   ---------------------------------------------------------------------

   routingTreeList ATTRIBUTE ::= {
           WITH SYNTAX RoutingTreeList
           SINGLE VALUE
           ID at-routing-tree-list}

   RoutingTreeList ::= SEQUENCE OF RoutingTreeName

   RoutingTreeName ::= DistinguishedName

                   Figure 2: Routing Tree Use Definition

   ---------------------------------------------------------------------

   This attribute defines the routing trees used by an MTA, and the
   order in which they are used.  Holding these in the directory eases
   configuration management.  It also enables an MTA to calculate the
   routing choice of any other MTA which follows this specification,
   provided that none of its routing trees have access restrictions.
   This will facilitate debugging routing problems.

<span class="h3"><a class="selflink" id="section-9.1" href="#section-9.1">9.1</a>  Routing Tree Order</span>

   The order in which routing trees are used will be critical to the
   operation of this algorithm.  A common approach will be:




<span class="grey">Kille                         Experimental                     [Page 14]</span></pre>
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<span class="grey"><a href="./rfc1801">RFC 1801</a>        X.400-MHS Routing using X.500 Directory        June 1995</span>


   1.  Access one or more shared private routing trees to access private
       routing information.

   2.  Utilise the open routing tree.

   3.  Fall back to a default route from one of the private routing
       trees.

   Initially, the open routing tree will be very sparse, and there will
   be little routing information in ADMD level nodes.  Access to many
   services will only be via ADMD services, which in turn will only be
   accessible via private links.  For most MTAs, the fallback routing
   will be important, in order to gain access to an MTA which has the
   right private connections configured.

   In general, for a site, UAs will be registered in one routing tree
   only, in order to avoid duplication.  They may be placed into other
   routing trees by use of aliases, in order to gain performance.  For
   some sites, Users and UAs with a 1:1 mapping will be mapped onto
   single entries by use of aliases.

<span class="h3"><a class="selflink" id="section-9.2" href="#section-9.2">9.2</a>  Example use of Routing Trees</span>

   Some examples of how this structure might be used are now given.
   Many other combinations are possible to suit organisational
   requirements.

<span class="h4"><a class="selflink" id="section-9.2.1" href="#section-9.2.1">9.2.1</a>  Fully Open Organisation</span>

   The simplest usage is to place all routing information in the open
   community routing tree.  An organisation will simply establish O/R
   addresses for all of its UAs in the open community tree, each
   registering its supporting MTA. This will give access to all systems
   accessible from this open community.

<span class="h4"><a class="selflink" id="section-9.2.2" href="#section-9.2.2">9.2.2</a>  Open Organisation with Fallback</span>

   In practice, some MTAs and MDs will not be directly reachable from
   the open community (e.g., ADMDs with a strong model of bilateral
   agreements).  These services will only be available to
   users/communities with appropriate agreements in place.  Therefore it
   will be useful to have a second (local) routing tree, containing only
   the name of the fallback MTA at its root.  In many cases, this
   fallback would be to an ADMD connection.

   Thus, open routing will be tried first, and if this fails the message
   will be routed to a single selected MTA.




<span class="grey">Kille                         Experimental                     [Page 15]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-16" ></span>
<span class="grey"><a href="./rfc1801">RFC 1801</a>        X.400-MHS Routing using X.500 Directory        June 1995</span>


<span class="h4"><a class="selflink" id="section-9.2.3" href="#section-9.2.3">9.2.3</a>  Minimal-routing MTA</span>

   The simplest approach to routing for an MTA is to deliver messages to
   associated users, and send everything else to another MTA (possibly
   with backup).

   An organisation using MTAs with this approach will register its users
   as for the fully open organisation.  A single routing tree will be
   established, with the name of the organisation being aliased into the
   open community routing tree.  Thus the MTA will correctly identify
   local users, but use a fallback mechanism for all other addresses.

<span class="h4"><a class="selflink" id="section-9.2.4" href="#section-9.2.4">9.2.4</a>  Organisation with Firewall</span>

   An organisation can establish an organisation community to build a
   firewall, with the overall organisation being registered in the open
   community.  This is an important structure, which it is important to
   support cleanly.

    o  Some MTAs are registered in the open community routing tree to
       give access into the organisation.  This will include the O/R tree
       down to the organisational level.  Full O/R Address verification
       will not take place externally.

    o  All users are registered in a private (organisational) routing
       tree.

    o  All MTAs in the organisation are registered in the organisation's
       private routing tree, and access information in the organisation's
       community.  This gives full internal connectivity.

    o  Some MTAs in the organisation access the open community routing
       tree.  These MTAs take traffic from the organisation to the
       outside world.  These will often be the same MTAs that are
       externally advertised.

<span class="h4"><a class="selflink" id="section-9.2.5" href="#section-9.2.5">9.2.5</a>  Well Known Entry Points</span>

   Well known entry points will be used to provide access to countries
   and MDs which are oriented to private links.  A private routing tree
   will be established, which indicates these links.  This tree would be
   shared by the well known entry points.

<span class="h4"><a class="selflink" id="section-9.2.6" href="#section-9.2.6">9.2.6</a>  ADMD using the Open Community for Advertising</span>

   An ADMD uses the open community for advertising.  It advertises its
   existence and also restrictive policy.  This will be useful for:




<span class="grey">Kille                         Experimental                     [Page 16]</span></pre>
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<span class="grey"><a href="./rfc1801">RFC 1801</a>        X.400-MHS Routing using X.500 Directory        June 1995</span>


    o  Address validation

    o  Advertising the mechanism for a bilateral link to be established

<span class="h4"><a class="selflink" id="section-9.2.7" href="#section-9.2.7">9.2.7</a>  ADMD/PRMD gateway</span>

   An MTA provides a gateway from a PRMD to an ADMD. It is important to
   note that many X.400 MDs will not use the directory.  This is quite
   legitimate.  This technique can be used to register access into such
   communities from those that use the directory.

    o  The MTA registers the ADMD in its local community (private link)

    o  The MTA registers itself in the PRMD's community to give access to
       the ADMD.

<span class="h2"><a class="selflink" id="section-10" href="#section-10">10</a>.  Routing Information</span>

   Routing trees are defined in the previous section, and are used as a
   framework to hold routing information.  Each node, other than a
   skeletal one, in a routing tree has information associated with it,
   which is defined by the object class routingInformation in Figure 3.
   This structure is fundamental to the operation of this specification,
   and it is recommended that it be studied with care.

   ---------------------------------------------------------------------

   routingInformation OBJECT-CLASS ::= {
       SUBCLASS OF top
       KIND auxiliary
       MAY CONTAIN {
           subtreeInformation|
           routingFilter|
           routingFailureAction|
           mTAInfo|
           accessMD|                                                  10
           nonDeliveryInfo|
           badAddressSearchPoint|
           badAddressSearchAttributes}
       ID oc-routing-information}
                   -- No naming attributes as this is not a
                   -- structural object class



   subtreeInformation ATTRIBUTE ::= {                                 20
       WITH SYNTAX SubtreeInfo
       SINGLE VALUE



<span class="grey">Kille                         Experimental                     [Page 17]</span></pre>
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<span class="grey"><a href="./rfc1801">RFC 1801</a>        X.400-MHS Routing using X.500 Directory        June 1995</span>


       ID at-subtree-information}

   SubtreeInfo ::= ENUMERATED {
       all-children-present(0),
       not-all-children-present(1) }


   routingFilter ATTRIBUTE ::= {                                      30
       WITH SYNTAX RoutingFilter
       ID at-routing-filter}


   RoutingFilter ::= SEQUENCE{
           attribute-type OBJECT-IDENTIFIER,
           weight RouteWeight,
           dda-key String OPTIONAL,
           regex-match IA5String OPTIONAL,
           node DistinguishedName }                                   40

   String ::= CHOICE {PrintableString, TeletexString}

   routingFailureAction ATTRIBUTE ::= {
       WITH SYNTAX RoutingFailureAction
       SINGLE VALUE
       ID at-routing-failure-action}

   RoutingFailureAction ::= ENUMERATED {
               next-level(0),                                         50
               next-tree-only(1),
               next-tree-first(2),
               stop(3)  }


   mTAInfo ATTRIBUTE ::= {
       WITH SYNTAX MTAInfo
       ID at-mta-info}

   MTAInfo ::= SEQUENCE {                                             60
               name DistinguishedName,
               weight [<a href="#ref-1" title="models and services">1</a>] RouteWeight DEFAULT preferred-access,
               mta-attributes [<a href="#ref-2">2</a>] SET OF Attribute OPTIONAL,
               ae-info  SEQUENCE OF SEQUENCE {
                   aEQualifier PrintableString,
                   ae-weight RouteWeight DEFAULT preferred-access,
                   ae-attributes SET OF Attribute OPTIONAL} OPTIONAL
   }

   RouteWeight ::= INTEGER  {endpoint(0),                             70



<span class="grey">Kille                         Experimental                     [Page 18]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-19" ></span>
<span class="grey"><a href="./rfc1801">RFC 1801</a>        X.400-MHS Routing using X.500 Directory        June 1995</span>


                   preferred-access(5),
                   backup(10)} (0..20)

                 Figure 3:  Routing Information at a Node

   ---------------------------------------------------------------------

   For example, information might be associated with the (PRMD) node:

    PRMD=ABC, ADMD=XYZMail, C=GB

   If this node was in the open community routing tree, then the
   information represents information published by the owner of the PRMD
   relating to public access to that PRMD. If this node was present in
   another routing tree, it would represent information published by the
   owner of the routing tree about access information to the referenced
   PRMD. The attributes associated with a routingInformation node
   provide the following information:

   Implicit That the node corresponds to a partial or entire valid O/R
       address.  This is implicit in the existence of the entry.

   Object Class If the node is a UA. This will be true if the node is of
       object class routedUA. This is described further in <a href="#section-11">Section 11</a>.
       If it is not of this object class, it is an intermediate node in
       the O/R Address hierarchy.

   routingFilter A set of routing filters, defined by the routingFilter
       attribute.  This attribute provides for routing on information in
       the unmatched part of the O/R Address.  This is described in
       <a href="#section-10.3">Section 10.3</a>.

   subtreeInformation Whether or not the node is authoritative for the
       level below is specified by the subtreeInformation attribute.  If
       it is authoritative, indicated by the value all-children-present,
       this will give the basis for (permanently) rejecting invalid O/R
       Addresses.  The attribute is encoded as enumerated, as it may be
       later possible to add partial authority (e.g., for certain
       attribute types).  If this attribute is missing, the node is
       assumed to be non-authoritative (not-all-children-present).
       The value all-children-present simply means that all of the child
       entries are present, and that this can be used to determine
       invalid addresses.  There are no implications about the presence
       of routing information.  Thus it is possible to verify an entire
       address, but only to route on one of the higher level components.

       For example, consider the node:




<span class="grey">Kille                         Experimental                     [Page 19]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-20" ></span>
<span class="grey"><a href="./rfc1801">RFC 1801</a>        X.400-MHS Routing using X.500 Directory        June 1995</span>


        MHS-O=Zydeco, PRMD=ABC, ADMD=XYZMail, C=GB

       An organisation which has a bilateral agreement with this
       organisation has this entry in its routing tree, with no children
       entries.  This is marked as non-authoritative.  There is a second
       routing tree maintained by Zydeco, which contains all of the
       children of this node, and is marked as authoritative.  When
       considering an O/R Address

        MHS-G=Random + MHS-S=Unknown, MHS-O=Zydeco,
        PRMD=ABC, ADMD=XYZMail, C=GB

       only the second, authoritative, routing tree can be used to
       determine that this address is invalid.  In practice, the manager
       configuring the non-authoritative tree, will be able to select
       whether an MTA using this tree will proceed to full verification,
       or route based on the partially verified information.

   mTAInfo A list of MTAs and associated information defined by the
       mTAInfo attribute.  This information is discussed further in
       Sections <a href="#section-15">15</a> and <a href="#section-18">18</a>.  This information is the key information
       associated with the node.  When a node is matched in a lookup, it
       indicates the validity of the route, and a set of MTAs to connect
       to.  Selection of MTAs is discussed in Sections <a href="#section-18">18</a> and
       <a href="#section-10.2">Section 10.2</a>.

   routingFailureAction An action to be taken if none of the MTAs can be
       used directly (or if there are no MTAs present) is defined by the
       routingFailureAction attribute.  Use of this attribute and
       multiple routing trees is described in <a href="#section-10.1">Section 10.1</a>.

   accessMD The accessMD attribute is discussed in <a href="#section-10.4">Section 10.4</a>.  This
       attribute is used to indicate MDs which provide indirect access
       to the part of the tree that is being routed to.

   badAddressSearchPoint/badAddressSearchAttributes The
       badAddressSearchPoint and badAddressSearchAttributes are
       discussed in <a href="#section-17">Section 17</a>.  This attribute is for when an address
       has been rejected, and allows information on alternative addresses
       to be found.

<span class="h3"><a class="selflink" id="section-10.1" href="#section-10.1">10.1</a>  Multiple routing trees</span>

   A routing decision will usually be made on the basis of information
   contained within multiple routing trees.  This section describes the
   algorithms relating to use of multiple routing trees.  Issues
   relating to the use of X.500 and handling of errors is discussed in
   <a href="#section-14">Section 14</a>.  The routing decision works by examining a series of



<span class="grey">Kille                         Experimental                     [Page 20]</span></pre>
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<span class="grey"><a href="./rfc1801">RFC 1801</a>        X.400-MHS Routing using X.500 Directory        June 1995</span>


   entries (nodes) in one or more routing trees.  This information is
   summarised in Figure 3.  Each entry may contain information on
   possible next-hop MTAs.  When an entry is found which enables the
   message to be routed, one of the routing options determined at this
   point is selected, and a routing decision is made.  It is possible
   that further entries may be examined, in order to determine other
   routing options.  This sort of heuristic is not discussed here.

   When a single routing tree is used, the longest possible match based
   on the O/R address to be routed to is found.  This entry, and then
   each of its parents in turn is considered, ending with the routing
   tree root node (except in the case of the open routing tree, which
   does not have such a node).  When multiple routing trees are
   considered, the basic approach is to treat them in a defined order.
   This is supplemented by a mechanism whereby if a matched node cannot
   be used directly, the routing algorithm will have the choice to move
   up a level in the current routing tree, or to move on to the next
   routing tree with an option to move back to the first tree later.
   This option to move back is to allow for the common case where a tree
   is used to specify two things:

   1.  Routing information private to the MTA (e.g., local UAs or routing
       info for bilateral links).

   2.  Default routing information for the case where other routing has
       failed.

   The actions allow for a tree to be followed, for the private
   information, then for other trees to be used, and finally to fall
   back to the default situation.  For very complex configurations it
   might be necessary to split this into two trees.  The options defined
   by routingFailureAction, to be used when the information in the entry
   does not enable a direct route, are:

   next-level Move up a level in the current routing tree.  This is the
       action implied if the attribute is omitted.  This will usually be
       the best action in the open community routing tree.

   next-tree-only Move to the next tree, and do no further processing on
       the current tree.  This will be useful optimisation for a routing
       tree where it is known that there is no useful additional routing
       information higher in the routing tree.

   next-tree-first Move to the next tree, and then default back to the
       next level in this tree when all processing is completed on
       subsequent trees.  This will be useful for an MTA to operate in
       the sequence:




<span class="grey">Kille                         Experimental                     [Page 21]</span></pre>
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<span class="grey"><a href="./rfc1801">RFC 1801</a>        X.400-MHS Routing using X.500 Directory        June 1995</span>


       1.  Check for optimised private routes

       2.  Try other available information

       3.  Fall back to a local default route

   stop This address is unroutable.  No processing shall be done in any
       trees.

   For the root entry of a routing tree, the default action and next-
   level are interpreted as next-tree-only.

<span class="h3"><a class="selflink" id="section-10.2" href="#section-10.2">10.2</a>  MTA Choice</span>

   This section considers how the choice between alternate MTAs is made.
   First, it is useful to consider the conditions why an MTA is entered
   into a node of the routing tree:

    o  The manager for the node of the tree shall place it there.  This
       is a formality, but critical in terms of overall authority.

    o  The MTA manager shall agree to it being placed there.  For a well
       operated MTA, the access policy of the MTA will be set to enforce
       this.

    o  The MTA will in general (for some class of message) be prepared
       to route to any valid O/R address in the subtree implied by the
       address.  The only exception to this is where the MTA will route
       to a subset of the tree which cannot easily be expressed by
       making entries at the level below.  An example might be an MTA
       prepared to route to all of the subtree, with certain explicit
       exceptions.

   Information on each MTA is stored in an mTAInfo attribute, which is
   defined in Figure 3.  This attribute contains:

   name The Distinguished Name of the MTA (Application Process)

   weight A weighting factor (Route Weight) which gives a basis to
       choose between different MTAs.  This is described in <a href="#section-10.2">Section 10.2</a>.

   mta-attributes Attributes from the MTA's entry.  Information on the
       MTA will always be stored in the MTA's entry.  The MTA is
       represented here as a structure, which enables some of this entry
       information to be represented in the routing node.  This is
       effectively a maintained cache, and can lead to considerable
       performance optimisation.  For example if ten MTAs were
       represented at a node, another MTA making a routing decision might



<span class="grey">Kille                         Experimental                     [Page 22]</span></pre>
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       need to make ten directory reads in order to obtain the
       information needed.  If any attributes are present here, all of
       the attributes needed to make a routing decision shall be
       included, and also all attributes at the Application Entity level.

   ae-info Where an MTA supports a single protocol only, or the
       protocols it supports have address information that can be
       represented in non-conflicting attributes, then the MTA may be
       represented as an application process only.  In this case, the
       ae-info structure which gives information on associated
       application entities may be omitted, as the MTA is represented by
       a single application entity which has the same name as the
       application process.  In other cases, the names of all application
       entities shall be included.  A weight is associated with each
       application entity to allow the MTA to indicate a preference
       between its application entities.

   The structure of information within ae-info is as follows:

   ae-qualifier A printable string (e.g., "x400-88"), which is the
       value of the common name of the relative distinguished name of the
       application entity.  This can be used with the application process
       name to derive the application entity title.

   ae-weight A weighting factor (Route Weight) which gives a basis to
       choose between different Application Entities (not between
       different MTAs).  This is described below.

   ae-attributes Attributes from the AEs entry.

   Information in the mta-attributes and ae-info is present as a
   performance optimisation, so that routing choices can be made with a
   much smaller number of directory operations.  Using this information,
   whose presence is optional, is equivalent to looking up the
   information in the MTA. If this information is present, it shall be
   maintained to be the same as that information stored in the MTA
   entry.  Despite this maintenence requirement, use of this performance
   optimisation data is optional, and the information may always be
   looked up from the MTA entry.

   Note: It has been suggested that substantial performance optimisation
         will be achieved by caching, and that the performance gained
         from maintaining these attributes does not justify the effort
         of maintaining the entries.  If this is borne out by
         operational experience, this will be reflected in future
         versions of this specification.





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   Route weighting is a mechanism to distinguish between different route
   choices.  A routing weight may be associated with the MTA in the
   context of a routing tree entry.  This is because routing weight will
   always be context dependent.  This will allow machines which have
   other functions to be used as backup MTAs.  The Route Weight is an
   integer in range 0--20.  The lower the value, the better the choice
   of MTA. Where the weight is equal, and no other factors apply, the
   choice between the MTAs shall be random to facilitate load balancing.
   If the MTA itself is in the list, it shall only route to an MTA of
   lower weight.  The exact values will be chosen by the manager of the
   relevant part of the routing tree.  For guidance, three fixed points
   are given:

    o  0.  For an MTA which can deliver directly to the entire subtree
       implied by the position in the routing tree.

    o  5.  For an MTA which is preferred for this point in the subtree.

    o  10.  For a backup MTA.

   When an organisation registers in multiple routing trees, the route
   weight used is dependent on the context of the subtree.  In general
   it is not possible to compare weights between subtrees.  In some
   cases, use of route weighting can be used to divert traffic away from
   expensive links.

   Attributes present in an MTA Entry are defined in various parts of
   this specification.  A summary and pointers to these sections is
   given in <a href="#section-16">Section 16</a>.

   Attributes that are available in the MTA entry and will be needed for
   making a routing choice are:

   protocolInformation

   applicationContext

   mhs-deliverable-content-length

   responderAuthenticationRequirements

   initiatorAuthenticationRequirements

   responderPullingAuthenticationRequirements

   initiatorPullingAuthenticationRequirements

   initiatorP1Mode



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   responderP1Mode

   polledMTAs Current MTA shall be in list if message is to be pulled.

   mTAsAllowedToPoll

   supportedMTSExtensions

   If any MTA attributes are present in the mTAInfo attribute, all of
   the attributes that may affect routing choice shall be present.
   Other attributes may be present.  A full list of MTA attributes, with
   summaries of their descriptions are given in <a href="#section-16">Section 16</a>, with a
   formal definition in Figure 6.

<span class="h3"><a class="selflink" id="section-10.3" href="#section-10.3">10.3</a>  Routing Filters</span>

   This attribute provides for routing on information in the unmatched
   part of the O/R Address, including:

    o  Routing on the basis of an O/R Address component type

    o  Routing on the basis of a substring match of an O/R address
       component.  This might be used to route X121 addressed faxes to
       an appropriate MTA.

   When present, the procedures of analysing the routing filters shall
   be followed before other actions.  The routing filter overrides
   mTAInfo and accessMD attributes, which means that the routing filter
   must be considered first.  Only in the event that no routing filters
   match shall the mTAInfo and accessMD attributes be considered.  The
   components of the routingFilter attribute are:

   ---------------------------------------------------------------------

   attribute-type This gives the attribute type to be matched, and is
       selected from the attribute types which have not been matched to
       identify the routing entry.  The filter applies to this attribute
       type.  If there is no regular expression present (as defined
       below), the filter is true if the attribute is present.  The
       value is the object identifier of the X.500 attribute type
       (e.g., at-prmd-name).

   weight This gives the weight of the filter, which is encoded as a
       Route Weight, with lower values indicating higher priority.  If
       multiple filters match, the weight of each matched filter is used
       to select between them.  If the weight is the same, then a random
       choice shall be made.




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   dda-key If the attribute is domain defined, then this parameter may
       be used to identify the key.


   accessMD ATTRIBUTE ::= {
           SUBTYPE OF distinguishedName
           ID at-access-md}

                        Figure 4:  Indirect Access

   ---------------------------------------------------------------------

   regex-match This string is used to give a regular expression match on
       the attribute value.  The syntax for regular expressions is
       defined in <a href="#appendix-E">Appendix E</a>.

   node This distinguished name specifies the entry which holds routing
       information for the filter.  It shall be an entry with object
       class routingInformation, which can be used to determine the MTA
       or MTA choice.  All of the attributes from this entry should be
       used, as if they had been directly returned from the current entry
       (i.e., the procedure recurses).  The current entry does not set
       defaults.

   An example of use of routing filters is now given, showing how to
   route on X121 address to a fax gateway in Germany.  Consider the
   routing point.

     PRMD=ABC, ADMD=XYZMail, C=GB

   The entry associated would have two routing filters:

   1.  One with type x121 and no regular expression, to route a default
       fax gateway.

   2.  One with type x121 and a regular expression ^9262 to route all
       German faxes to a fax gateway located in Germany with which there
       is a bilateral agreement.  This would have a lower weight, so that
       it would be selected over the default fax gateway.

<span class="h3"><a class="selflink" id="section-10.4" href="#section-10.4">10.4</a>  Indirect Connectivity</span>

   In some cases a part of the O/R Address space will be accessed
   indirectly.  For example, an ADMD without access from the open
   community might have an agreement with another MD to provide this
   access.  This is achieved by use of the accessMD attribute defined in
   Figure 4.  If this attribute is found, the routing algorithm shall
   read the entry pointed to by this distinguished name.  It shall be an



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   entry with object class routingInformation, which can be used to
   determine the MTA or MTA choice and route according to the
   information retrieve to this access MD. All of the attributes from
   this entry should be used, as if they had been directly returned from
   the current entry (i.e., the procedure recurses).  The current entry
   does not set defaults.

   The attribute is called an MD, as this is descriptive of its normal
   use.  It might point to a more closely defined part of the O/R
   Address space.

   It is possible for both access MD and MTAs to be specified.  This
   might be done if the MTAs only support access over a restricted set
   of transport stacks.  In this case, the access MD shall only be
   routed to if it is not possible to route to any of the MTAs.

   This structure can also be used as an optimisation, where a set of
   MTAs provides access to several parts of the O/R Address space.
   Rather than repeat the MTA information (list of MTAs) in each
   reference to the MD, a single access MD is used as a means of
   grouping the MTAs.  The value of the Distinguished Name of the access
   MD will probably not be meaningful in this case (e.g., it might be
   the name "Access MTA List", within the organisation.)

   If the MTA routing is unable to access the information in the Access
   MD due to directory security restrictions, the routing algorithm
   shall continue as if no MTA information was located in the routing
   entry.

<span class="h2"><a class="selflink" id="section-11" href="#section-11">11</a>.  Local Addresses (UAs)</span>

   Local addresses (UAs) are a special case for routing:  the endpoint.
   The definition of the routedUA object class is given in Figure 5.
   This identifies a User Agent in a routing tree.  This is needed for
   several reasons:

   ---------------------------------------------------------------------

   routedUA OBJECT-CLASS ::= {
       SUBCLASS OF {routingInformation}
       KIND auxiliary
       MAY CONTAIN {
                           -- from X.402
           mhs-deliverable-content-length|
           mhs-deliverable-content-types|
           mhs-deliverable-eits|
           mhs-message-store|                                         10
           mhs-preferred-delivery-methods|



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                           -- defined here
           supportedExtensions|
           redirect|
           supportingMTA|
           userName|
           nonDeliveryInfo}
       ID oc-routed-ua}

   supportedExtensions ATTRIBUTE ::= {                                20
       SUBTYPE OF objectIdentifier
       ID at-supported-extensions}

   supportingMTA ATTRIBUTE ::= {
       SUBTYPE OF mTAInfo
       ID at-supporting-mta}

   userName ATTRIBUTE ::= {
       SUBTYPE OF distinguishedName
       ID at-user-name}                                               30

                          Figure 5: UA Attributes

   ---------------------------------------------------------------------

   1.  To allow UAs to be defined without having an entry in another part
       of the DIT.

   2.  To identify which (leaf and non-leaf) nodes in a routing tree are
       User Agents.  In a pure X.400 environment, a UA (as distinct from
       a connecting part of the O/R address space) is simply identified
       by object class.  Thus an organisation entry can itself be a UA. A
       UA need not be a leaf, and can thus have children in the tree.

   3.  To allow UA parameters as defined in X.402 (e.g., the
       mhs-deliverable-eits) to be determined efficiently from the
       routing tree, without having to go to the user's entry.

   4.  To provide access to other information associated with the UA, as
       defined below.

   The following attributes are defined associated with the UA.

   supportedExtensions MTS extensions supported by the MTA, which affect
       delivery.

   supportingMTA The MTAs which support a UA directly are noted in the
       supportingMTA attribute, which may be multi-valued.  In the X.400
       model, only one MTA is associated with a UA. In practice, it is



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       possible and useful for several MTAs to be able to deliver to a
       single UA. This attribute is a subtype of mTAInfo, and it defines
       access information for an MTA which is able to deliver to the UA.
       There may also be an mTAInfo attribute in the entry.
       Components of the supportingMTA attribute are interpreted in the
       same manner as mtaInfo is for routing, with one exception.  The
       values of the Route Weight are interpreted in the following
       manner:

        o  0.  A preferred MTA for delivery.

        o  5.  A backup MTA.

        o  10.  A backup MTA, which is not presferred.

       The supportingMTA attribute shall be present, unless the address
       is being non-delivered or redirected, in which case it may be
       omitted.

   redirect The redirect attribute controls redirects, as described in
       <a href="#section-22.1">Section 22.1</a>.

   userName The attribute userName points to the distinguished Name of
       the user, as defined by the mhs-user in X.402.  The pointer from
       the user to the O/R Address is achieved by the mhs-or-addresses
       attribute.  This makes the UA/User linkage symmetrical.

   nonDeliveryInfo The attribute nonDeliveryInfo mandates non-delivery
       to this address, as described in <a href="#section-22.3">Section 22.3</a>.

   When routing to a UA, an MTA will read the supportingMTA attribute.
   If it finds its own name present, it will know that the UA is local,
   and invoke appropriate procedures for local delivery (e.g., co-
   resident or P3 access information).  The cost of holding these
   attributes for each UA at a site will often be reduced by use of
   shared attributes (as defined in X.500(93)).

   Misconfiguration of the supportingMTA attribute could have serious
   operational and possibly security problems, although for the most
   part no worse than general routing configuration problems.  An MTA
   using this attribute may choose to perform certain sanity checks,
   which might be to verify the routing tree or subtree that the entry
   resides in.

   The linkage between the UA and User entries was noted above.  It is
   also possible to use a single entry for both User and UA, as there is
   no conflict between the attributes in each of the objects.  In this
   case, the entries shall be in one part of the DIT, with aliases from



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   the other.  Because the UA and User are named with different
   attributes, the aliases shall be at the leaf level.

<span class="h3"><a class="selflink" id="section-11.1" href="#section-11.1">11.1</a>  Searching for Local Users</span>

   The approach defined in this specification performs all routing by
   use of reads.  This is done for performance reasons, as it is a
   reasonable expectation that all DSA implementations will support a
   high performance read operation.  For local routing only, an MTA in
   cooperation with the provider of the local routing tree may choose to
   use a search operation to perform routing.  The major benefit of this
   is that there will not be a need to store aliases for alternate
   names, and so the directory storage requirement and alias management
   will be reduced.  The difficulty with this approach is that it is
   hard to define search criteria that would be effective in all
   situations and well supported by all DUAs.  There are also issues
   about determining the validity of a route on the basis of partial
   matches.

<span class="h2"><a class="selflink" id="section-12" href="#section-12">12</a>.  Direct Lookup</span>

   Where an O/R address is registered in the open community and has one
   or more "open" MTAs which support it, this will be optimised by
   storing MTA information in the O/R address entry.  In general, the
   Directory will support this by use of attribute inheritance or an
   implementation will optimise the storage or repeated information, and
   so there will not be a large storage overhead implied.  This is a
   function of the basic routing approach.  As a further optimisation of
   this case, the User's distinguished name entry may contain the
   mTAInfo attribute.  This can be looked up from the distinguished
   name, and thus routing on submission can be achieved by use of a
   single read.

   Note: This performance optimisation has a management overhead, and
         further experience is needed to determine if the effort
         justifies the performance improvement.

<span class="h2"><a class="selflink" id="section-13" href="#section-13">13</a>.  Alternate Routes</span>

<span class="h3"><a class="selflink" id="section-13.1" href="#section-13.1">13.1</a>  Finding Alternate Routes</span>

   The routing algorithm selects a single MTA to be routed to.  It could
   be extended to find alternate routes to a single MTA with possibly
   different weights.  How far this is done is a local configuration
   choice.  Provision of backup routing is desirable, and leads to
   robust service, but excessive use of alternate routing is not usually
   beneficial.  It will often force messages onto convoluted paths, when
   there was only a short outage on the preferred path.  It is important



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   to note that this strategy will lead to picking the first acceptable
   route.  It is important to configure the routing trees so that the
   first route identified will also be the best route.

<span class="h3"><a class="selflink" id="section-13.2" href="#section-13.2">13.2</a>  Sharing routing information</span>

   So far, only single addresses have been considered.  Improving
   routing choice for multiple addresses is analogous to dealing with
   multiple routes.  This section defines an optional improvement.  When
   multiple addresses are present, and alternate routes are available,
   the preferred routes may be chosen so as to maximise the number of
   recipients sent with each message.

   Specification of routing trees can facilitate this optimisation.
   Suppose there is a set of addresses (e.g., in an organisation) which
   have different MTAs, but have access to an MTA which will do local
   switching.  If each address is registered with the optimal MTA as
   preferred, but has the "hub" MTA registered with a higher route
   weight, then optimisation may occur when a message is sent to
   multiple addresses in the group.

<span class="h2"><a class="selflink" id="section-14" href="#section-14">14</a>.  Looking up Information in the Directory</span>

   The description so far has been abstract about lookup of information.
   This section considers how information is looked up in the Directory.
   Consider that an O/R Address is presented for lookup, and there is a
   sequence of routing trees.  At any point in the lookup sequence,
   there is one of a set of actions that can take place:

   Entry Found Information from the entry (node) is returned and shall
       be examined.  The routing process continues or terminates, based
       on this information.

   Entry Not Found Return information on the length of best possible
       match to the routing algorithm.

   Temporary Reject The MTA shall stop the calculation, and repeat the
       request later.  Repeated temporary rejects should be handled in a
       similar manner to the way the local MTA would handle the failure
       to connect to a remote MTA.

   Permanent Reject Administrative error on the directory which may be
       fixed in future, but which currently prevents routing.  The
       routing calculation should be stopped and the message
       non-delivered.

   The algorithm proceeds by a series of directory read operations.  If
   the read operation is successful, the Entry Found procedure should be



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   followed.  Errors from the lookup (directory read) shall be handled
   in terms of the above procedures as follows.  The following handling
   is used when following a routing tree:

   AttributeError This leads to a Permanent Reject.

   NameError Entry Not Found is used.  The matched parameter is used to
       determine the number of components of the name that have matched
       (possibly zero).  The read may then repeated with this name.
       This is the normal case, and allows the "best" entry in the
       routingn tree to be located with two reads.

   Referral The referral shall be followed, and then the procedure
       recurses.

   SecurityError Entry Not Found is used.  Return a match length of one
       less than the name provided.

   ServiceError This leads to a Temporary Reject.

   There will be cases where the algorithm moves to a name outside of
   the routing tree being followed (Following an accessMD attribute, or
   a redirect or a matched routing filter).  The handling will be the
   same as above, except:

   NameError This leads to a Permanent Reject.

   SecurityError This leads to a Permanent Reject.

   When reading objects which of not of object class routingInformation,
   the following error handling is used:

   AttributeError This leads to a Permanent Reject.

   NameError This leads to a Permanent Reject.

   Referral The referral shall be followed, and then the procedure
       recurses.

   SecurityError In the case of an MTA, treat as if it is not possible
       to route to this MTA. In other cases, this leads to a Permanent
       Reject.

   ServiceError This leads to a Temporary Reject.

   The algorithm specifies the object class of entries which are read.
   If an object class does not match what is expected, this shall lead
   to a permanent reject.



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<span class="h2"><a class="selflink" id="section-15" href="#section-15">15</a>.  Naming MTAs</span>

   MTAs need to be named in the DIT, but the name does not have routing
   significance.  The MTA name is simply a unique key.  Attributes
   associated with naming MTAs are given in Figure 6.  This figure also
   gives a list of attributes, which may be present in the MTA entry.
   The use of most of these is explained in subsequent sections.  The
   mTAName and globalDomainID attributes are needed to define the
   information that an MTA places in trace information.  As noted
   previously, an MTA is represented as an Application Process, with one
   or more Application Entities.

   ---------------------------------------------------------------------

   mTAName ATTRIBUTE ::= {
       SUBTYPE OF name
       WITH SYNTAX DirectoryString{ub-mta-name-length}
       SINGLE VALUE
       ID at-mta-name}
                           -- used for naming when
                           -- MTA is named in O=R Address Hierarchy

   globalDomainID ATTRIBUTE ::= {                                     10
       WITH SYNTAX GlobalDomainIdentifier
       SINGLE VALUE
       ID at-global-domain-id}
                           -- both attributes present when MTA
                           -- is named outside O=R Address Hierarchy
                           -- to enable trace to be written

   mTAApplicationProcess OBJECT-CLASS ::= {
       SUBCLASS OF {application-process}
       KIND auxiliary                                                 20
       MAY CONTAIN {
           mTAWillRoute|
           globalDomainID|
           routingTreeList|
           localAccessUnit|
           accessUnitsUsed
       }
       ID oc-mta-application-process}

   mTA OBJECT CLASS ::= {   -- Application Entity                     30
       SUBCLASS OF {mhs-message-transfer-agent}
       KIND structural
       MAY CONTAIN {
           mTAName|
           globalDomainID|         -- per AE variant



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           responderAuthenticationRequirements|
           initiatorAuthenticationRequirements|
           responderPullingAuthenticationRequirements|
           initiatorPullingAuthenticationRequirements|
           initiatorP1Mode|                                           40
           responderP1Mode|
           polledMTAs|
           protocolInformation|
           respondingRTSCredentials|
           initiatingRTSCredentials|
           callingPresentationAddress|
           callingSelectorValidity|
           bilateralTable|
           mTAWillRoute|
           mhs-deliverable-content-length|                            50
           routingTreeList|
           supportedMTSExtensions|
           mTAsAllowedToPoll
           }
       ID oc-mta}

                        Figure 6:  MTA Definitions

   ---------------------------------------------------------------------

   In X.400 (1984), MTAs are named by MD and a single string.  This
   style of naming is supported, with MTAs named in the O/R Address tree
   relative to the root of the DIT (or possibly in a different routing
   tree).  The mTAName attribute is used to name MTAs in this case.  For
   X.400(88) the Distinguished Name shall be passed as an AE Title.
   MTAs may be named with any other DN, which can be in the O/R Address
   or Organisational DIT hierarchy.  There are several reasons why MTAs
   might be named differently.

    o  The flat naming space is inadequate to support large MDs.  MTA
       name assignment using the directory would be awkward.

    o  An MD does not wish to register its MTAs in this way (essentially,
       it prefers to give them private names in the directory).

    o  An organisation has a policy for naming application processes,
       which does not fit this approach.

   In this case, the MTA entry shall contain the correct information to
   be inserted in trace.  The mTAName and globalDomainID attributes are
   used to do this.  They are single value.  For an MTA which inserts
   different trace in different circumstances, a more complex approach
   would be needed.



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   An MD may choose to name its MTAs outside of the O/R address
   hierarchy, and then link some or all of them with aliases.  A pointer
   from this space may help in resolving information based on MTA Trace.
   The situation considered so far is where an MTA supports one
   application context (protocol).  The MTA is represented in the
   directory by a single directory entry, having no subordinate
   applicationEntity entries.  This name is considered to be the name of
   the MTA and its Application Process Title.  The MTA has no
   Application Entity Qualifier, and so this is also the Application
   Entity Title.  In the case where an MTA supports more than one
   application context, the Application Process Title is exactly the
   same as above, but it also has one or more subordinate
   applicationEntity entries.  Each of these subordinate entries is
   associated with a single application context.  The relative
   distinguished name of the subordinate applicationEntity entry is the
   Application Entity Qualifier of the Application Entity Title.  The
   Application Entity Title is the distinguished name of the
   applicationEntity.  The term MTA Name is used to refer to the
   Application Process Title.

<span class="h3"><a class="selflink" id="section-15.1" href="#section-15.1">15.1</a>  Naming 1984 MTAs</span>

   Some simplifications are necessary for 1984 MTAs, and only one naming
   approach may be used.  This is because Directory Names are not
   carried in the protocol, and so it must be possible to derive the
   name algorithmically from parameters carried.  In X.400, MTAs are
   named by MD and a single string.  This style of naming is supported,
   with MTAs named in the O/R Address tree relative to the root of the
   DIT (or possibly in a different routing tree).  The MTAName attribute
   is used to name MTAs in this case.

<span class="h2"><a class="selflink" id="section-16" href="#section-16">16</a>.  Attributes Associated with the MTA</span>

   This section lists the attributes which may be associated with an MTA
   as defined in Figure 6, and gives pointers to the sections that
   describe them.

   mTAName <a href="#section-15">Section 15</a>.

   globalDomainID <a href="#section-15">Section 15</a>.

   protocolInformation <a href="#section-18.1">Section 18.1</a>.

   applicationContext <a href="#section-18.2">Section 18.2</a>.

   mhs-deliverable-content-length <a href="#section-18.3">Section 18.3</a>.

   responderAuthenticationRequirements <a href="#section-20.2">Section 20.2</a>.



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   initiatorAuthenticationRequirements <a href="#section-20.2">Section 20.2</a>.

   responderPullingAuthenticationRequirements <a href="#section-20.2">Section 20.2</a>.

   initiatorPullingAuthenticationRequirements <a href="#section-20.2">Section 20.2</a>.

   initiatorP1Mode <a href="#section-19">Section 19</a>.

   responderP1Mode <a href="#section-19">Section 19</a>.

   polledMTAs <a href="#section-19">Section 19</a>.

   mTAsAllowedToPoll <a href="#section-19">Section 19</a>.

   respondingRTSCredentials <a href="#section-20.3">Section 20.3</a>.

   initiatingRTSCredentials <a href="#section-20.3">Section 20.3</a>.

   callingPresentationAddress <a href="#section-20.3">Section 20.3</a>.

   callingSelectorValidity <a href="#section-20.3">Section 20.3</a>.

   bilateralTable <a href="#section-17">Section 17</a>.

   mTAWillRoute <a href="#section-21">Section 21</a>.

   routingTreeList <a href="#section-9">Section 9</a>.

   supportedMTSExtensions <a href="#section-18.3">Section 18.3</a>.

   ---------------------------------------------------------------------

   mTABilateralTableEntry OBJECT-CLASS ::=
       SUBCLASS OF {mTA| distinguishedNameTableEntry}
       ID oc-mta-bilateral-table-entry}

                   Figure 7:  MTA Bilateral Table Entry

   ---------------------------------------------------------------------

<span class="h2"><a class="selflink" id="section-17" href="#section-17">17</a>.  Bilateral Agreements</span>

   Each MTA has an entry in the DIT. This will be information which is
   globally valid, and will be useful for handling general information
   about the MTA and for information common to all connections.  In many
   cases, this will be all that is needed.  This global information may
   be restricted by access control, and so need not be globally
   available.  In some cases, MTAs will maintain bilateral and



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   multilateral agreements, which hold authentication and related
   information which is not globally valid.  This section describes a
   mechanism for grouping such information into tables, which enables an
   MTA to have bilateral information or for a group of MTAs to share
   multilateral information.  The description is for bilateral
   information, but is equally applicable to multilateral agreements.

   For the purpose of a bilateral agreement, the MTA is considered to be
   an application entity.  This means that when this is distinct from
   the application process, that the agreements are protocol specific.

   A bilateral agreement is represented by one entry associated with
   each MTA participating in the bilateral agreement.  For one end of
   the bilateral agreement, the agreement information will be keyed by
   the name of the MTA at the other end.  Each party to the agreement
   will set up the entry which represents its half of the agreed policy.
   The fact that these correspond is controlled by the external
   agreement.  In many cases, only one half of the agreement will be in
   the directory.  The other half might be in an ADMD MTA configuration
   file.

   MTA bilateral information is stored in a table, as defined in [<a href="#ref-15" title="&quot;Representing the O/R Address hierarchy in the X.500 directory information tree&quot;">15</a>].
   An MTA has access to a sequence of such tables, each of which
   controls agreements in both directions for a given MTA. Where an MTA
   is represented in multiple tables, the first agreement shall be used.
   This allows an MTA to participate in multilateral agreements, and to
   have private agreements which override these.  The definition of
   entries in this table are defined in Figure 7.  This table will
   usually be access controlled so that only a single MTA or selected
   MTAs which appear externally as one MTA can access it.

   ---------------------------------------------------------------------

   bilateralTable ATTRIBUTE ::= {
           WITH SYNTAX SEQUENCE OF DistinguishedName
           SINGLE VALUE
           ID at-bilateral-table}

                   Figure 8:  Bilateral Table Attribute

   ---------------------------------------------------------------------

   Each entry in the table is of the object class
   distinguishedNameTableEntry, which is used to name the entry by the
   distinguished name of the MTA. In some cases discussed in <a href="#section-20.1">Section</a>
   <a href="#section-20.1">20.1</a>, there will also be aliases of type textTableEntry.  The MTA
   attributes needed as a part of the bilateral agreement (typically MTA
   Name/Password pairs), as described in <a href="#section-20.3">Section 20.3</a>, will always be



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   present.  Other MTA attributes (e.g., presentation address) may be
   present for one of two reasons:

   1.  As a performance optimisation

   2.  Because the MTA does not have a global entry

   Every MTA with bilateral agreements will define a bilateral MTA
   table.  When a connection from a remote MTA is received, its
   Distinguished Name is used to generate the name of the table entry.
   For 1984, the MTA Name exchanged at the RTS level is used as a key
   into the table.  The location of the bilateral tables used by the MTA
   and the order in which they are used are defined by the
   bilateralTable attribute in the MTA entry, which is defined in Figure
   8.

   All of the MTA information described in <a href="#section-16">Section 16</a> may be used in the
   bilateral table entries.  This will allow bilateral control of a wide
   range of parameters.

   Note: For some bilateral connections there is a need control various
         other functions, such as trace stripping and originator address
         manipulation.  For now, this is left to implementation specific
         extensions.  This is expected to be reviewed in light of
         implementation experience.

<span class="h2"><a class="selflink" id="section-18" href="#section-18">18</a>.  MTA Selection</span>

<span class="h3"><a class="selflink" id="section-18.1" href="#section-18.1">18.1</a>  Dealing with protocol mismatches</span>

   MTAs may operate over different stacks.  This means that some MTAs
   cannot talk directly to each other.  Even where the protocols are the
   same, there may be reasons why a direct connection is not possible.
   An environment where there is full connectivity over a single stack
   is known as a transport community [<a href="#ref-9" title="&quot;Encoding Network Addresses to support operation over non-OSI lower layers&quot;">9</a>].  The set of transport
   communities supported by an MTA is specified by use of the
   protocolInformation attribute defined in X.500(93).  This is
   represented as a separate attribute for the convenience of making
   routing decisions.












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   ---------------------------------------------------------------------

   supportedMTSExtensions ATTRIBUTE ::= {
       SUBTYPE OF objectIdentifier
       ID at-supported-mts-extensions}

                    Figure 9:  Supported MTS Extensions

   ---------------------------------------------------------------------

   A community is identified by an object identifier, and so the
   mechanism supports both well known and private communities.  A list
   of object identifiers corresponding to well known communities is
   given in <a href="#appendix-B">Appendix B</a>.

<span class="h3"><a class="selflink" id="section-18.2" href="#section-18.2">18.2</a>  Supported Protocols</span>

   It is important to know the protocol capabilities of an MTA. This is
   done by the application context.  There are standard definitions for
   the following 1988 protocols.

    o  P3 (with and without RTS, both user and MTS initiated)

    o  P7 (with and without RTS).

    o  P1 (various modes).  Strictly, this is the only one that matters
       for routing.

   In order to support P1(1984) and P1(1988) in X.410 mode, application
   contexts which define these protocols are given in <a href="#appendix-C">Appendix C</a>.  This
   context is for use in the directory only, and would never be
   exchanged over the network.

   For routing purposes, a message store which is not co-resident with
   an MTA is represented as if it had a co-resident MTA and configured
   with a single link to its supporting MTA.

   In cases where the UA is involved in exchanges, the UA will be of
   object class mhs-user-agent, and this will allow for appropriate
   communication information to be registered.

<span class="h3"><a class="selflink" id="section-18.3" href="#section-18.3">18.3</a>  MTA Capability Restrictions</span>

   In addition to policy restrictions, described in <a href="#section-21">Section 21</a>, an MTA
   may have capability restrictions.  The maximum size of MPDU is
   defined by the standard attribute mhs-deliverable-content-length.
   The supported MTS extensions are defined by a new attribute specified
   in Figure 9.



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   ---------------------------------------------------------------------

   restrictedSubtree OBJECT-CLASS ::= {
           SUBCLASS OF {top}
           KIND auxiliary
           MAY CONTAIN {
                   subtreeDeliverableContentLength|
                   subtreeDeliverableContentTypes|
                   subtreeDeliverableEITs}
           ID oc-restricted-subtree}
                                                                      10
   subtreeDeliverableContentLength ATTRIBUTE ::= {
           SUBTYPE OF mhs-deliverable-content-length
           ID at-subtree-deliverable-content-length}

   subtreeDeliverableContentTypes ATTRIBUTE ::= {
           SUBTYPE OF mhs-deliverable-content-types
           ID at-subtree-deliverable-content-types}

   subtreeDeliverableEITs ATTRIBUTE ::= {
           SUBTYPE OF mhs-deliverable-eits                            20
           ID at-subtree-deliverable-eits}

                Figure 10:  Subtree Capability Restriction

   ---------------------------------------------------------------------

   It may be useful to define other capability restrictions, for example
   to enable routing of messages around MTAs with specific deficiencies.
   It has been suggested using MTA capabilities as an optimised means of
   expressing capabilities of all users associated with the MTA. This is
   felt to be undesirable.

<span class="h3"><a class="selflink" id="section-18.4" href="#section-18.4">18.4</a>  Subtree Capability Restrictions</span>

   In many cases, users of a subtree will share the same capabilities.
   It is possible to specify this by use of attributes, as defined in
   Figure 10.  This will allow for restrictions to be determined in
   cases where there is no entry for the user or O/R Address.  This will
   be a useful optimisation in cases where the UA capability information
   is not available from the directory, either for policy reasons or
   because it is not there.  This information may also be present in the
   domain tree (<a href="./rfc822">RFC 822</a>).

   This shall be implemented as a collective attribute, so that it is
   available to all entries in the subtree below the entry.  This can
   also be used for setting defaults in the subtree.




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   ---------------------------------------------------------------------

   initiatorP1Mode ATTRIBUTE ::= {
       WITH SYNTAX P1Mode
       SINGLE VALUE
       ID at-initiator-p1-mode}

   responderP1Mode ATTRIBUTE ::= {
       WITH SYNTAX P1Mode
       SINGLE VALUE
       ID at-responder-p1-mode}                                       10

   P1Mode ::= ENUMERATED {
       push-only(0),
       pull-only(1),
       twa(2) }

   polledMTAs ATTRIBUTE ::= {
       WITH SYNTAX PolledMTAs
       ID at-polled-mtas}
                                                                      20
   PolledMTAs ::= SEQUENCE {
           mta DistinguishedName,
           poll-frequency INTEGER OPTIONAL --frequency in minutes
           }

   mTAsAllowedToPoll ATTRIBUTE ::= {
           SUBTYPE OF distinguishedName
           ID at-mtas-allowed-to-poll}

                       Figure 11:  Pulling Messages

   ---------------------------------------------------------------------

<span class="h2"><a class="selflink" id="section-19" href="#section-19">19</a>.  MTA Pulling Messages</span>

   Pulling messages between MTAs, typically by use of two way alternate,
   is for bilateral agreement.  It is not the common case.  There are
   two circumstances in which it can arise.

   1.  Making use of a connection that was opened to push messages.

   2.  Explicitly polling in order to pull messages

   Attributes to support this are defined in Figure 11.  These
   attributes indicate the capabilities of an MTA to pull messages, and
   allows a list of polled MTAs to be specified.  If omitted, the normal
   case of push-only is specified.  In the MTA Entry, the polledMTAs



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   attribute indicates MTAs which are to be polled and the
   mTAsAllowedToPoll attribute indicates MTAs that may poll the current
   MTA.

<span class="h2"><a class="selflink" id="section-20" href="#section-20">20</a>.  Security and Policy</span>

<span class="h3"><a class="selflink" id="section-20.1" href="#section-20.1">20.1</a>  Finding the Name of the Calling MTA</span>

   A key issue for authentication is for the called MTA to find the name
   of the calling MTA. This is needed for it to be able to look up
   information on a bilateral agreement.

   Where X.400(88) is used, the name is available as a distinguished
   name from the AE-Title derived from the AP-Title and AE-Qualifier in
   the A-Associate.  For X.400(84), it will not be possible to derive a
   global name from the bind.  The MTA Name exchanged in the RTS Bind
   will provide a key into the private bilateral agreement table (or
   tables), where the connection information can be verified.  Thus for
   X.400(1984) it will only be possible to have bilateral inbound links
   or no authentication of the calling MTA.

   Note: CDC use a search here, as a mechanism to use a single table and
         an 88/84 independent access.  This may be considered for general
         adoption.  It appears to make the data model cleaner, possibly
         at the expense of some performance.  This will be considered in
         the light of implementation experience.

<span class="h3"><a class="selflink" id="section-20.2" href="#section-20.2">20.2</a>  Authentication</span>

   The levels of authentication required by an MTA will have an impact
   on routing.  For example, if an MTA requires strong authentication,
   not all MTAs will be able to route to it.  The attributes which
   define the authentication requirements are defined in Figure 12.

   The attributes specify authentication levels for the following cases:

   Responder These are the checks that the responder will make on the
       initiator's credentials.

   Initiator These are the checks that the initiator will make on the
       responders credentials.  Very often, no checks are needed ---
       establishing the connection is sufficient.

   Responder Pulling These are responder checks when messages are
       pulled.  These will often be stronger than for pushing.

   Initiator Pulling For completeness.




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   If an attribute is omitted, no checks are required.  If multiple
   checks are required, then each of the relevant bits shall be set.
   The attribute is single value, which implies that the MTA must set a
   single authentication policy.

   ---------------------------------------------------------------------

   responderAuthenticationRequirements ATTRIBUTE ::= {
      WITH SYNTAX AuthenticationRequirements
      SINGLE VALUE
      ID at-responder-authentication-requirements}

   initiatorAuthenticationRequirements ATTRIBUTE ::= {
      WITH SYNTAX AuthenticationRequirements
      SINGLE VALUE
      ID at-initiator-authentication-requirements}                    10

   responderPullingAuthenticationRequirements ATTRIBUTE ::= {
      WITH SYNTAX AuthenticationRequirements
      SINGLE VALUE
      ID at-responder-pulling-authentication-requirements}

   initiatorPullingAuthenticationRequirements ATTRIBUTE ::= {
      WITH SYNTAX AuthenticationRequirements
      SINGLE VALUE
      ID at-initiator-pulling-authentication-requirements}            20

   AuthenticationRequirements ::= BITSTRING {
       mta-name-present(0),
       aet-present(1),
       aet-valid(2),
       network-address(3),
       simple-authentication(4),
       strong-authentication(5),
       bilateral-agreement-needed(6)}

                  Figure 12:  Authentication Requirements

   ---------------------------------------------------------------------

   The values of the authentication requirements mean:

   mta-name-present That an RTS level MTA parameter shall be present for
       logging purposes.

   aet-present That a distinguished name application entity title shall
       be provided at the ACSE level.




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   aet-valid As for aet-present, and that the AET be registered in the
       directory.  This may be looked up as a part of the validation
       process.  If mta-name-present is set, the RTS value of mta and
       password shall correspond to those registered in the directory.

   network-address This can only be used for the responder.  The AET
       shall be looked up in the directory, and the
       callingPresentationAddress attribute matched against the calling
       address.  This shall match exactly at the network level.  The
       validity of selectors will be matched according to the
       callingSelectorValidity attribute.

   simple-authentication All MTA and password parameters needed for
       simple authentication shall be used.  This will usually be in
       conjunction with a bilateral agreement.

   strong-authentication Use of strong authentication.

   bilateral-agreement-needed This means that this MTA will only accept
       connections in conjunction with a bilateral or multilateral
       agreements.  This link cannot be used unless such an agreement
       exists.

   These attributes may also be used to specify UA/MTA authentication
   policy.  They may be resident in the UA entry in environments where
   this information cannot be modified by the user.  Otherwise, it will
   be present in an MTA table (represented in the directory).

   An MTA could choose to have different authentication levels related
   to different policies (<a href="#section-21">Section 21</a>).  This is seen as too complex, and
   so they are kept independent.  The equivalent function can always be
   achieved by using multiple Application Entities with the application
   process.

<span class="h3"><a class="selflink" id="section-20.3" href="#section-20.3">20.3</a>  Authentication Information</span>

   This section specifies connection information needed by P1.  This is
   essentially RTS parameterisation needed for authentication.  This is
   defined in Figure 13.  Confidential bilateral information is implied
   by these attributes, and this will be held in the bilateral
   information agreement.  This shall have appropriate access control
   applied.  Note that in some cases, MTA information will be split
   across a private and public entry.








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   ---------------------------------------------------------------------

   respondingRTSCredentials ATTRIBUTE ::= {
           WITH SYNTAX RTSCredentials
           SINGLE VALUE
           ID at-responding-rts-credentials}


   initiatingRTSCredentials ATTRIBUTE ::= {
           WITH SYNTAX RTSCredentials
           SINGLE VALUE                                               10
           ID at-initiating-rts-credentials}


   RTSCredentials ::= SEQUENCE {
           request [0] MTAandPassword OPTIONAL,
           response [<a href="#ref-1" title="models and services">1</a>] MTAandPassword OPTIONAL }


   MTAandPassword ::= SEQUENCE {
           MTAName,                                                   20
           Password }              -- MTAName and Password
                                   -- from X.411


   callingPresentationAddress ATTRIBUTE ::= {
           SUBTYPE OF presentationAddress
           MULTI VALUE
           ID at-calling-presentation-address}

   callingSelectorValidity ATTRIBUTE ::= {                            30
           WITH SYNTAX CallingSelectorValidity
           SINGLE VALUE
           ID at-calling-selector-validity}

   CallingSelectorValidity ::= ENUMERATED {
           all-selectors-fixed(0),
           tsel-may-vary(1),
           all-selectors-may-vary(2) }

                 Figure 13:  MTA Authentication Parameters

   ---------------------------------------------------------------------








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   ---------------------------------------------------------------------

   mTAWillRoute ATTRIBUTE ::= {
       WITH SYNTAX MTAWillRoute
       ID at-mta-will-route}

   MTAWillRoute ::= SEQUENCE {
           from [0]        SET OF ORAddressPrefix OPTIONAL,
           to [<a href="#ref-1" title="models and services">1</a>]          SET OF ORAddressPrefix OPTIONAL,
           from-excludes [<a href="#ref-2">2</a>]       SET OF ORAddressPrefix OPTIONAL,
           to-excludes [<a href="#ref-3" title="K. Bonacker">3</a>]         SET OF ORAddressPrefix OPTIONAL }  10

   ORAddressPrefix ::= DistinguishedName

                Figure 14:  Simple MTA Policy Specification

   ---------------------------------------------------------------------

   The parameters are:

   Initiating Credentials The credentials to be used when the local MTA
       initiates the association.  It gives the credentials to insert
       into the request, and those expected in the response.

   Responding Credentials The credentials to be used when the remote MTA
       initiates the association.  It gives the credential expected in
       the request, and those to be inserted into the response.

   Remote Presentation Address Valid presentation addresses, which the
       remote MTA may connect from.

   If an MTA/Password pair is omitted, the MTA shall default to the
   local MTA Name, and the password shall default to a zero-length OCTET
   STRING.

   Note: Future versions of this specification may add more information
         here relating to parameters required for strong authentication.

<span class="h2"><a class="selflink" id="section-21" href="#section-21">21</a>.  Policy and Authorisation</span>

<span class="h3"><a class="selflink" id="section-21.1" href="#section-21.1">21.1</a>  Simple MTA Policy</span>

   The routing trees will generally be configured in order to identify
   MTAs which will route to the destination.  A simple means is
   identified to specify an MTA's policy.  This is defined in Figure 14.
   If this attribute is omitted, the MTA shall route all traffic to the
   implied destinations from the context of the routing tree for any
   MTAs that have valid access to the routing tree.



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   The multi-valued attribute gives a set of policies which the MTA will
   route.  O/R Addresses are represented by a prefix, which identifies a
   subtree.  A distinguished name encoding of O/R Address is used.
   There are three components:

   from This gives a set of O/R addresses which are granted permission
       by this attribute value.  If omitted, "all" is implied.

   to This gives the set of acceptable destinations.  If omitted,
       "all" is implied.

   from-excludes This defines (by prefix) subtrees of the O/R address
       tree which are explicitly excluded from the "from" definition.
       If omitted, there are no exclusions.

   to-excludes This defines (by prefix) subtrees of the O/R address tree
       which are explicitly excluded from the "to" definition.  If
       omitted, there are no exclusions.

   This simple policy will suffice for most cases.  In particular, it
   gives sufficient information for most real situations where a policy
   choice is forced, and the application of this policy would prevent a
   message being routed.

   This simple prefixing approach does not deal explicitly with alias
   dereferencing.  The prefixes refer to O/R addresses where aliases
   have been dereferenced.  To match against these prefixes, O/R
   addresses being matched need to be "normalised by being looked up in
   the directory to resolve alias values.  If the lookup fails, it shall
   be assumed that the provided address is already normalised.  This
   means that policy may be misinterpreted for parts of the DIT not
   referenced in the directory.

   The originator refers to the MTS originator, and the recipient to the
   MTS recipient, following any list expansion or redirect.  This simple
   policy does not apply to delivery reports.  Any advertised route
   shall work for delivery reports, and it does not makes sense to
   regulate this on the basis of the sender.

<span class="h3"><a class="selflink" id="section-21.2" href="#section-21.2">21.2</a>  Complex MTA Policy</span>

   MTAs will generally have a much more complex policy mechanism, such
   as that provided by PP MTA [<a href="#ref-10">10</a>].  Representing this as a part of the
   routing decision is not done here, but may be addressed in future
   versions.  Some of the issues which need to be tackled are:






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    o  Use of charging and non-charging nets

    o  Policy dependent on message size

    o  Different policy for delivery reports.

    o  Policy dependent on attributes of the originator or
       recipient (e.g., mail from students)

    o  Content type and encoded information types

    o  The path which the message has traversed to reach the MTA

    o  MTA bilateral agreements

    o  Pulling messages

    o  Costs.  This sort of policy information may also be for
       information only.

   MTAs may apply more complex routing policies.  However, this shall
   not lead to the rejection of messages which might otherwise be
   correctly routed on the published policy information.  Policies
   relating to submission do not need to be public.  They can be private
   to the MTA.

   ---------------------------------------------------------------------

   redirect ATTRIBUTE ::= {
           WITH SYNTAX Redirect
           SINGLE VALUE
           ID at-redirect}

   Redirect ::= SEQUENCE OF SEQUENCE {
           or-name ORName,
           reason RedirectionReason, -- from X.411
           filter CHOICE {                                            10
                   min-size [<a href="#ref-1" title="models and services">1</a>] INTEGER,
                   max-size [<a href="#ref-2">2</a>] INTEGER,
                   content [<a href="#ref-3" title="K. Bonacker">3</a>] ContentType,
                   eit [<a href="#ref-4" title="R.J. Kummerfeld">4</a>] ExternalEncodedInformationType } OPTIONAL
           }

                      Figure 15:  Redirect Definition

   ---------------------------------------------------------------------





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<span class="h2"><a class="selflink" id="section-22" href="#section-22">22</a>.  Delivery</span>

<span class="h3"><a class="selflink" id="section-22.1" href="#section-22.1">22.1</a>  Redirects</span>

   There is a need to specify redirects in the Directory.  This will be
   useful for alternate names where an equivalent name (synonym) defined
   by an alias is not natural.  An example where this might be
   appropriate is to redirect mail to a new O/R address where a user had
   changed organisation.  A mechanism is given to allow conditional
   (filtered) redirects for different types of messages.  This allow
   small messages, large messages, or messages containing specific EITs
   or content to be redirected.  The definitions are given in Figure 15.

   Redirection is specified by the redirect attribute.  If present, this
   attribute shall be processed before supportingMTA and
   nonDeliveryInfo.  These two attributes shall only be considered if it
   is determined that no redirection applies.  The redirect attribute is
   a sequence of elements which are considered in the order specified.
   Each element is examined in turn.  The first element which applies is
   used, and no further elements are examined.  Use of an element for
   redirection, shall follow the X.400 procedures for redirection, and
   an element shall not be used if prevented by a service control.  If
   the redirect attribute is processed and no redirection is generated,
   processing shall continue irrespective of service controls.  If non-
   delivery is intended in this event, this shall be achieved by use of
   the nonDeliveryInfo attribute.

   The components have the following interpretations:

   or-name This X.400 O/R Name is for use in the redirection.  This O/R
       Name will contain an optional directory name and optional O/R
       address.  One or both of the must be present.  If the O/R Address
       element is present, the Directory Name, if present, is for
       information only.  and is to be placed in the X.400 redirection.
       If the O/R address element is absent, the Directory Name shall be
       present and shall be looked up to determine the O/R address of the
       redirected recipient.  The O/R Address of the intended recipient
       will either be present or derived by lookup.  Routing shall be
       done on the basis of this O/R Address.

   reason This is the reason information to be placed in the X.400
       redirect, and it shall take one of the following values of
       RedirectReason defined in X.411:

       recipient-assigned-alternate-recipient;
       recipient-MD-assigned-alternate-recipient; or alias.  It shall not
       have the value originator-requested-alternate-recipient.




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   filter If filter is absent, the redirect is mandoatory and shall be
       followed.  If the filter is present, use of the redirect under
       consideration depends on the type of filter as follows:

       min-size Follow redirect if the message (MT content) is larger
           than min-size (measured in kBytes).

       max-size Follow redirect if the message (MT content) is smaller
           than max-size (measured in kBytes).

       content Follow redirect if message content is of type content.

       eit Follow redirect if the encoded information types registered
           in the envelope contain eit.

   When a delivery report is sent to an address which would be
   redirected, X.400 would ignore the redirect.  This means that every
   O/R address would need to have a valid means of delivery.  This would
   seem to be awkward to manage.  Therefore, the redirect shall be
   followed, and the delivery report delivered to the redirected
   address.

   These redirects are handled directly by the MTA. Redirects can also
   be initiated by the UA, for example in the context of a P7
   interaction.

   ---------------------------------------------------------------------

   nonDeliveryInfo ATTRIBUTE ::= {
           WITH SYNTAX NonDeliveryReason
           SINGLE VALUE
           ID at-non-delivery-info}

   NonDeliveryReason ::= SEQUENCE {
           reason INTEGER (0..ub-reason-codes),
           diagnostic INTEGER (0..ub-diagnostic-codes) OPTIONAL,
           supplementaryInfo PrintableString OPTIONAL }               10

                   Figure 16:  Non Delivery Information

   ---------------------------------------------------------------------

<span class="h3"><a class="selflink" id="section-22.2" href="#section-22.2">22.2</a>  Underspecified O/R Addresses</span>

   X.400 requires that some underspecified O/R Addresses are handled in
   a given way (e.g., if a surname is given without initials or given
   name).  Where an underspecified O/R Address is to be treated as if it
   were another O/R Address, an alias shall be used.  If the O/R Address



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   is to be rejected as ambiguous, an entry shall be created in the DIT,
   and forced non-delivery specified for this reason.

   Note: It is also possible to handle this situation by searching.  An
         MTA conforming to this specification may handle underspecified
         addresses in this manner.  The choice of mechanism will be
         reviewed after operational experience with both approaches.

<span class="h3"><a class="selflink" id="section-22.3" href="#section-22.3">22.3</a>  Non Delivery</span>

   It is possible for a manager to define an address to non-deliver with
   specified reason and diagnostic codes.  This might be used for a
   range of management purposes.  The attribute to do this is defined in
   Figure 16.  If a nonDeliveryInfo attribute is present, any
   supportingMTA attribute shall be ignored and the message non-
   delivered.

<span class="h3"><a class="selflink" id="section-22.4" href="#section-22.4">22.4</a>  Bad Addresses</span>

   If there is a bad address, it is desirable to do a directory search
   to find alternatives.  This is a helpful user service and may be
   supported.  This function is invoked after address checking has
   failed, and where this is no user supplied alternate recipient.  This
   function would be an MTA-chosen alternative to administratively
   assigned alternate recipient.

   Attributes to support handling of bad addresses are defined in Figure
   17.  The attributes are:

   badAddressSearchPoint This gives the point (or list of points) from
       which to search.

   badAddressSearchAttributes This gives the set of attribute types to
       search on.  The default is common name.

















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   ---------------------------------------------------------------------

   badAddressSearchPoint ATTRIBUTE ::= {
           SUBTYPE OF distinguishedName
           ID at-bad-address-search-point}

   badAddressSearchAttributes ATTRIBUTE ::= {
           WITH SYNTAX AttributeType
           ID at-bad-address-search-attributes}

   alternativeAddressInformation EXTENSION                            10
           AlternativeAddressInformation
           ::= id-alternative-address-information
                   -- X.400(92) continues to use MACRO notation

   AlternativeAddressInformation ::= SET OF SEQUENCE {
           distinguished-name DistinguishedName OPTIONAL,
           or-address ORAddress OPTIONAL,
           other-useful-info SET OF Attribute }

                     Figure 17:  Bad Address Pointers

   ---------------------------------------------------------------------

   Searches are always single level, and always use approximate match.
   If a small number of matches are made, this is returned to the
   originator by use of the per recipient AlternativeAddressInformation
   in the delivery report (DR). This shall be marked non-critical, so
   that it will not cause the DR to be discarded (e.g., in downgrading
   to X.400(1984)).  This attribute allows the Distinguished Name and
   O/R Address of possible alternate recipients to be returned with the
   delivery report.  There is also the possibility to attach extra
   information in the form of directory attributes.  Typically this
   might be used to return attributes of the entry which were matched in
   the search.  A summary of the information shall also be returned
   using the delivery report supplementary information filed (e.g.,
   "your message could not be delivered to smith, try J. Smith or P.
   Smith"), so that the information is available to user agents not
   supporting this extension.  Note the length restriction of this field
   is 256 (ub-supplementary-info-length) in X.400(1988).

   If the directory search fails, or there are no matches returned, a
   delivery report shall be returned as if this extra check had not been
   made.

   Note: It might be useful to allow control of search type, and also
         single level vs subtree.  This issue is for further study.




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   ---------------------------------------------------------------------

   localAccessUnit ATTRIBUTE ::= {
           WITH SYNTAX AccessUnitType
           ID at-local-access-unit}

   AccessUnitType ::= ENUMERATED {
           fax (1),
           physical-delivery (2),
           teletex (3),
           telex (4) }                                                10

   accessUnitsUsed ATTRIBUTE ::= {
           WITH SYNTAX SelectedAccessUnit
           ID at-access-units-used}

   SelectedAccessUnit ::= SEQUENCE {
           type AccessUnitType,
           providing-MTA DistinguishedName,
           filter SET OF ORAddress OPTIONAL }

                     Figure 18:  Access UnitAttributes

   ---------------------------------------------------------------------

<span class="h2"><a class="selflink" id="section-23" href="#section-23">23</a>.  Submission</span>

   A message may be submitted with Distinguished Name only.  If the MTA
   to which the message is submitted supports this service, this section
   describes how the mapping is done.

<span class="h3"><a class="selflink" id="section-23.1" href="#section-23.1">23.1</a>  Normal Derivation</span>

   The Distinguished Name is looked up to find the attribute mhs-or-
   addresses.  If the attribute is single value, it is straightforward.
   If there are multiple values, one O/R address shall be selected at
   random.

<span class="h3"><a class="selflink" id="section-23.2" href="#section-23.2">23.2</a>  Roles and Groups</span>

   Some support for roles is given.  If there is no O/R address, and the
   entry is of object class role, then the roleOccupant attribute shall
   be dereferenced, and the message submitted to each of the role
   occupants.  Similarly, if the entry is of object class group, where
   the groupMember attribute is used.






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<span class="h2"><a class="selflink" id="section-24" href="#section-24">24</a>.  Access Units</span>

   Attributes needed for support of Access Units, as defined in
   X.400(88), are defined in Figure 18.  The attributes defined are:

   localAccessUnit This defines the list of access units supported by
       the MTA.

   accessUnitsUsed This defines which access units are used by the MTA,
       giving the type and MTA. An O/R Address filter is provided to
       control which access unit is used for a given recipient.  For a
       filter to match an address, all attributes specificed in the
       filter shall match the given address.  This is specified as an O/R
       Address, so that routing to access units can be filtered on the
       basis of attributes not mapped onto the directory (e.g., postal
       attributes).  Where a remote MTA is used, it may be necessary to
       use source routing.

   Note 1: This mechanism might be used to replace the routefilter
       mechanism of the MTS routing.  Comments are solicited.

   Note 2: It has been proposed to add a more powerful filter mechanism.
       Comments are solicited.

   Note 3: The utility of this specification as a mechanism to route
       faxes and other non MHS messages has been noted, but not explored.
       Comments as to how and if this should be developed are solicited.

   These three issues are for further study.

<span class="h2"><a class="selflink" id="section-25" href="#section-25">25</a>.  The Overall Routing Algorithm</span>

   Having provided all the pieces, a summary of how routing works can be
   given.

   The core of the X.400 routing is described in <a href="#section-10">Section 10</a>.  A sequence
   of routing trees are followed.  As nodes of the routing tree are
   matched, a set of MTAs will be identified for evaluation as possible
   next hops.  If all of these are rejected, the trees are followed
   further.  (It might be argued that the trees should be followed to
   find alternate routes in the case that only one MTA is acceptable.
   This is not proposed.)  A set of MTAs is evaluated on the following
   criteria:

    o  If an MTA is the local MTA, deliver locally.

    o  Supported protocols.  The MTA shall support a protocol that the
       current MTA supports, as described in <a href="#section-18.2">Section 18.2</a>.



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       (Note that this could be an <a href="./rfc822">RFC 822</a> protocol, as well as an
       X.400 protocol.)

    o  The protocols shall share a common transport community, as
       described in <a href="#section-18.1">Section 18.1</a>.

    o  There shall be no capability restrictions in the MTA which
       prevents transfer of the current message, as described in
       <a href="#section-18.3">Section 18.3</a>.

    o  There shall be no policy restrictions in the MTA which prevents
       transfer of the current message, as described in <a href="#section-21">Section 21</a>.

    o  The authentication requirements of the MTA shall be met by the
       local MTA, as described in <a href="#section-20.2">Section 20.2</a>.

    o  If the authentication (<a href="#section-20.2">Section 20.2</a>) indicates that a bilateral
       agreement is present, the MTA shall be listed in the local set of
       bilateral agreements, as described in <a href="#section-17">Section 17</a>.

    o  In cases where the recipient UA's capabilities can be determined,
       there should either be no mismatch, or there shall be an ability
       to use local or remote reformatting capabilities, as described
       in [<a href="#ref-12" title="S.">12</a>].

<span class="h2"><a class="selflink" id="section-26" href="#section-26">26</a>.  Performance</span>

   The routing algorithm has been designed with performance in mind.  In
   particular, care has been taken to use only the read function, which
   will in general be optimised.  Routing trees may be configured so
   that routing decisions can be made with only two directory reads.
   More complex configurations will not require a substantially larger
   number of operations.

<span class="h2"><a class="selflink" id="section-27" href="#section-27">27</a>.  Acknowledgements</span>

   This memo is the central document of a series of specifications [14,
   15, 16], and to other work in progress.  The acknowledgements for all
   of this work is given here.  Previous work, which significantly
   influenced these specifications is described in <a href="#section-3">Section 3</a>.  This lead
   to an initial proposal by the editor, which was subsequently split
   into eight documents.  Work on this specifications has been done by
   the IETF MHS-DS working group.  Special credit is given to the joint
   chairs of this group: Harald Alvestrand (Uninett) and Kevin Jordan
   (CDC). Credit is given to all members of the WG. Those who have made
   active contribution include:  Piete Brooks (Cambridge University);
   Allan Cargille (University of Wisconsin); Jim Craigie (JNT); Dennis
   Doyle (SSS); Urs Eppenberger (SWITCH); Peter Furniss; Christian



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   Huitema (Inria); Marko Kaittola (Dante); Sylvain Langlois (EDF); Lucy
   Loftin (AT&amp;T GIS); Julian Onions (NEXOR); Paul-Andre Pays (Inria);
   Colin Robbins (NEXOR); Michael Roe (Cambridge University); Jim
   Romaguera (Netconsult); Michael Storz (Leibniz Rechenzentrum); Mark
   Wahl (ISODE Consortium); Alan Young (ISODE Consortium).

   This work was partly funded by the COSINE Paradise project.

<span class="h2"><a class="selflink" id="section-28" href="#section-28">28</a>.  References</span>

    [<a id="ref-1">1</a>] The Directory --- overview of concepts, models and services,
        1993. CCITT X.500 Series Recommendations.

    [<a id="ref-2">2</a>] J.N. Chiappa. A new IP routing and addressing architecture,
        1991.

    [<a id="ref-3">3</a>] A. Consael, M. Tschicholz, O. Wenzel, K. Bonacker, and M. Busch.
        DFN-Directory nutzung durch MHS, April 1990. GMD Report.

    [<a id="ref-4">4</a>] P. Dick-Lauder, R.J. Kummerfeld, and K.R. Elz. ACSNet - the
        Australian alternative to UUCP. In EUUG Conference, Paris, pages
        60--69, April 1985.

    [<a id="ref-5">5</a>] Eppenberger, U., "Routing Coordination for X.400 MHS Services
        Within a Multi Protocol / Multi Network Environment Table Format
        V3 for Static Routing", <a href="./rfc1465">RFC 1465</a>, SWITCH, May 1993.

    [<a id="ref-6">6</a>] K.E. Jordan. Using X.500 directory services in support of X.400
        routing and address mapping, November 1991. Private Note.

    [<a id="ref-7">7</a>] S.E. Kille. MHS use of directory service for routing.  In IFIP
        6.5 Conference on Message Handling, Munich, pages 157--164.
        North Holland Publishing, April 1987.

    [<a id="ref-8">8</a>] S.E. Kille. Topology and routing for MHS.  COSINE Specification
        Phase 7.7, RARE, 1988.

    [<a id="ref-9">9</a>] Kille, S., "Encoding Network Addresses to support operation over
        non-OSI lower layers", <a href="./rfc1277">RFC 1277</a>, Department of Computer Science,
        University College London, November 1991.

   [<a id="ref-10">10</a>] S.E. Kille. Implementing X.400 and X.500:  The PP and QUIPU
        Systems. Artech House, 1991.  ISBN 0-89006-564-0.

   [<a id="ref-11">11</a>] Kille, S., "A Representation of Distinguished Names
        (OSI-DS 23 (v5))", <a href="./rfc1485">RFC 1485</a>, Department of Computer Science,
        University College London, January 1992.




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   [<a id="ref-12">12</a>] Kille, S., Mhs use of X.500 directory to support mhs content
        conversion, Work in Progress, July 1993.

   [<a id="ref-13">13</a>] Kille, S., "Use of the X.500 directory to support routing for
        <a href="./rfc822">RFC 822</a> and related protocols", Work in Progress, July 1993.

   [<a id="ref-14">14</a>] Kille, S., "Representing tables and subtrees in the X.500
        directory", Work in Progress, September 1994.

   [<a id="ref-15">15</a>] Kille, S., "Representing the O/R Address hierarchy in the X.500
        directory information tree", Work in Progress, September 1994.

   [<a id="ref-16">16</a>] Kille, S., "Use of the X.500 directory to support mapping
        between X.400 and <a href="./rfc822">RFC 822</a> addresses", Work in Progress,
        September 1994.

   [<a id="ref-17">17</a>] Lauder, P., Kummerfeld, R., and A. Fekete. Hierarchical network
        routing. In Tricomm 91, 1991.

   [<a id="ref-18">18</a>] CCITT recommendations X.400 / ISO 10021, April 1988. CCITT
        SG 5/VII / ISO/IEC JTC1, Message Handling:  System and Service
        Overview.

   [<a id="ref-19">19</a>] Zen and the ART of navigating through the dark and murky regions
        of the message transfer system:  Working document on MTS
        routing, September 1991. ISO SC 18 SWG Messaging.

<span class="h2"><a class="selflink" id="section-29" href="#section-29">29</a>.  Security Considerations</span>

   Security issues are not discussed in this memo.





















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<span class="h2"><a class="selflink" id="section-30" href="#section-30">30</a>.  Author's Address</span>

   Steve Kille
   ISODE Consortium
   The Dome
   The Square
   Richmond
   TW9 1DT
   England

   Phone:  +44-81-332-9091
   EMail:  S.Kille@ISODE.COM
   X.400:  I=S; S=Kille; O=ISODE Consortium; P=ISODE;
   A=Mailnet; C=FI;

   DN: CN=Steve Kille,
   O=ISODE Consortium, C=GB

   UFN: S. Kille, ISODE Consortium, GB
































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A  Object Identifier Assignment

-----------------------------------------------------------------------

mhs-ds OBJECT-IDENTIFIER ::= {iso(1) org(3) dod(6) internet(1)
private(4) enterprises(1) isode-consortium (453) mhs-ds (7)}

routing OBJECT IDENTIFIER ::= {mhs-ds 3}

oc OBJECT IDENTIFIER ::= {routing 1}
at OBJECT IDENTIFIER ::= {routing 2}
id OBJECT IDENTIFIER ::= {routing 3}

                                                                    10
oc-mta OBJECT IDENTIFIER ::= {oc 1}
oc-mta-bilateral-table-entry OBJECT IDENTIFIER ::= {oc 2}
oc-routing-information OBJECT IDENTIFIER ::= {oc 3}
oc-restricted-subtree OBJECT IDENTIFIER ::= {oc 4}
oc-routed-ua OBJECT IDENTIFIER ::= {oc 8}
oc-routing-tree-root OBJECT IDENTIFIER ::= {oc 6}
oc-mta-application-process OBJECT IDENTIFIER ::= {oc 7}

at-access-md OBJECT IDENTIFIER ::= {at 1}
at-access-units-used OBJECT IDENTIFIER ::= {at 2}                   20
at-subtree-information OBJECT IDENTIFIER ::= {at 3}
at-bad-address-search-attributes OBJECT IDENTIFIER ::= {at 4}
at-bad-address-search-point OBJECT IDENTIFIER ::= {at 5}

at-calling-selector-validity OBJECT IDENTIFIER ::= {at 7}


at-global-domain-id OBJECT IDENTIFIER ::= {at 10}
at-initiating-rts-credentials OBJECT IDENTIFIER ::= {at 11}
at-initiator-authentication-requirements OBJECT IDENTIFIER ::= {at 12}30
at-initiator-p1-mode OBJECT IDENTIFIER ::= {at 13}
at-initiator-pulling-authentication-requirements
                                           OBJECT IDENTIFIER ::= {at 14}
at-local-access-unit OBJECT IDENTIFIER ::= {at 15}
at-redirect OBJECT IDENTIFIER ::= {at 46}
at-mta-info OBJECT IDENTIFIER ::= {at 40}
at-mta-name OBJECT IDENTIFIER ::= {at 19}

at-mta-will-route OBJECT IDENTIFIER ::= {at 21}
at-calling-presentation-address OBJECT IDENTIFIER ::= {at 22}
at-responder-authentication-requirements OBJECT IDENTIFIER ::= {at 23}40
at-responder-p1-mode OBJECT IDENTIFIER ::= {at 24}
at-responder-pulling-authentication-requirements
                                           OBJECT IDENTIFIER ::= {at 25}



<span class="grey">Kille                         Experimental                     [Page 59]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-60" ></span>
<span class="grey"><a href="./rfc1801">RFC 1801</a>        X.400-MHS Routing using X.500 Directory        June 1995</span>


at-responding-rts-credentials OBJECT IDENTIFIER ::= {at 26}
at-routing-failure-action OBJECT IDENTIFIER ::= {at 27}
at-routing-filter OBJECT IDENTIFIER ::= {at 28}
at-routing-tree-list OBJECT IDENTIFIER ::= {at 29}
at-subtree-deliverable-content-length OBJECT IDENTIFIER ::= {at 30}
at-subtree-deliverable-content-types OBJECT IDENTIFIER ::= {at 31}
at-subtree-deliverable-eits OBJECT IDENTIFIER ::= {at 32}
at-supporting-mta OBJECT IDENTIFIER ::= {at 33}                     50
at-transport-community OBJECT IDENTIFIER ::= {at 34}
at-user-name OBJECT IDENTIFIER ::= {at 35}
at-non-delivery-info OBJECT IDENTIFIER ::= {at 47}
at-polled-mtas  OBJECT IDENTIFIER ::= {at 37}
at-bilateral-table OBJECT IDENTIFIER {at 45}
at-supported-extension OBJECT IDENTIFIER {at 42}
at-supported-mts-extension OBJECT IDENTIFIER {at 43}
at-mtas-allowed-to-poll OBJECT IDENTIFIER {at 44}

id-alternative-address-information OBJECT IDENTIFIER ::= {id 1}     60

                Figure 19:  Object Identifier Assignment

-----------------------------------------------------------------------

B  Community Identifier Assignments

-----------------------------------------------------------------------

ts-communities OBJECT-IDENTIFIER ::= {iso(1) org(3) dod(6) internet(1)
private(4) enterprises(1) isode-consortium (453) ts-communities (4)}


tc-cons OBJECT IDENTIFIER ::= {ts-communities 1}    -- OSI CONS
tc-clns OBJECT IDENTIFIER ::= {ts-communities 2}    -- OSI CLNS
tc-internet OBJECT IDENTIFIER ::= {ts-communities 3}-- Internet+<a href="./rfc1006">RFC1006</a>
tc-int-x25 OBJECT IDENTIFIER ::= {ts-communities 4} -- International X.25
                                                    -- Without CONS10
tc-ixi OBJECT IDENTIFIER ::= {ts-communities 5}     -- IXI (Europe)
tc-janet OBJECT IDENTIFIER ::= {ts-communities 6}   -- Janet (UK)

     Figure 20:  Transport Community Object Identifier Assignments

-----------------------------------------------------------------------









<span class="grey">Kille                         Experimental                     [Page 60]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-61" ></span>
<span class="grey"><a href="./rfc1801">RFC 1801</a>        X.400-MHS Routing using X.500 Directory        June 1995</span>


C  Protocol Identifier Assignments

-----------------------------------------------------------------------

mail-protocol OBJECT-IDENTIFIER ::= {iso(1) org(3) dod(6) internet(1)
private(4)n enterprises(1) isode-consortium (453) mail-protocol (5)}

ac-p1-1984 OBJECT IDENTIFIER ::= {mail-protocol 1}      -- p1(1984)
ac-smtp  OBJECT IDENTIFIER ::= {mail-protocol 2}        -- SMTP
ac-uucp OBJECT IDENTIFIER ::= {mail-protocol 3}         -- UUCP Mail
ac-jnt-mail OBJECT IDENTIFIER ::= {mail-protocol 4}     -- JNT Mail
(UK)
ac-p1-1988-x410 OBJECT IDENTIFIER ::= {mail-protocol 5} -- p1(1988) in
<span class="h3"><a class="selflink" id="appendix-X.410" href="#appendix-X.410">X.410</a> mode</span>
ac-p3-1984 OBJECT IDENTIFIER ::= {mail-protocol 6}      -- p3(1984) 10

           Figure 21:  Protocol Object Identifier Assignments

-----------------------------------------------------------------------

D  ASN.1 Summary

-----------------------------------------------------------------------

MHS-DS-Definitions
DEFINITIONS ::=
BEGIN

 -- assign OID to module
 -- define imports and exports

routingTreeRoot OBJECT-CLASS ::= {
    SUBCLASS OF {routingInformation|subtree}
    ID oc-routing-tree-root}                                        10

routingTreeList ATTRIBUTE ::= {
        WITH SYNTAX RoutingTreeList
        SINGLE VALUE
        ID at-routing-tree-list}

RoutingTreeList ::= SEQUENCE OF RoutingTreeName

RoutingTreeName ::= DistinguishedName
                                                                    20
routingInformation OBJECT-CLASS ::= {
    SUBCLASS OF top
    KIND auxiliary
    MAY CONTAIN {



<span class="grey">Kille                         Experimental                     [Page 61]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-62" ></span>
<span class="grey"><a href="./rfc1801">RFC 1801</a>        X.400-MHS Routing using X.500 Directory        June 1995</span>


        subtreeInformation|
        routingFilter|
        routingFailureAction|
        mTAInfo|
        accessMD|
        nonDeliveryInfo|                                            30
        badAddressSearchPoint|
        badAddressSearchAttributes}
    ID oc-routing-information}
                -- No naming attributes as this is not a
                -- structural object class



subtreeInformation ATTRIBUTE ::= {
    WITH SYNTAX SubtreeInfo                                         40
    SINGLE VALUE
    ID at-subtree-information}

SubtreeInfo ::= ENUMERATED {
    all-children-present(0),
    not-all-children-present(1) }


routingFilter ATTRIBUTE ::= {
    WITH SYNTAX RoutingFilter                                       50
    ID at-routing-filter}


RoutingFilter ::= SEQUENCE{
        attribute-type OBJECT-IDENTIFIER,
        weight RouteWeight,
        dda-key String OPTIONAL,
        regex-match IA5String OPTIONAL,
        node DistinguishedName }
                                                                    60
String ::= CHOICE {PrintableString, TeletexString}

routingFailureAction ATTRIBUTE ::= {
    WITH SYNTAX RoutingFailureAction
    SINGLE VALUE
    ID at-routing-failure-action}

RoutingFailureAction ::= ENUMERATED {
            next-level(0),
            next-tree-only(1),                                      70
            next-tree-first(2),
            stop(3)  }



<span class="grey">Kille                         Experimental                     [Page 62]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-63" ></span>
<span class="grey"><a href="./rfc1801">RFC 1801</a>        X.400-MHS Routing using X.500 Directory        June 1995</span>


mTAInfo ATTRIBUTE ::= {
    WITH SYNTAX MTAInfo
    ID at-mta-info}

MTAInfo ::= SEQUENCE {
            name DistinguishedName,                                 80
            weight [<a href="#ref-1" title="models and services">1</a>] RouteWeight DEFAULT preferred-access,
            mta-attributes [<a href="#ref-2">2</a>] SET OF Attribute OPTIONAL,
            ae-info  SEQUENCE OF SEQUENCE {
                aEQualifier PrintableString,
                ae-weight RouteWeight DEFAULT preferred-access,
                ae-attributes SET OF Attribute OPTIONAL} OPTIONAL
}

RouteWeight ::= INTEGER  {endpoint(0),
                preferred-access(5),                                90
                backup(10)} (0..20)

accessMD ATTRIBUTE ::= {
        SUBTYPE OF distinguishedName
        ID at-access-md}

routedUA OBJECT-CLASS ::= {
    SUBCLASS OF {routingInformation}
    KIND auxiliary
    MAY CONTAIN {                                                  100
                        -- from X.402
        mhs-deliverable-content-length|
        mhs-deliverable-content-types|
        mhs-deliverable-eits|
        mhs-message-store|
        mhs-preferred-delivery-methods|
                        -- defined here
        supportedExtensions|
        redirect|
        supportingMTA|                                             110
        userName|
        nonDeliveryInfo}
    ID oc-routed-ua}

supportedExtensions ATTRIBUTE ::= {
    SUBTYPE OF objectIdentifier
    ID at-supported-extensions}

supportingMTA ATTRIBUTE ::= {
    SUBTYPE OF mTAInfo                                             120
    ID at-supporting-mta}




<span class="grey">Kille                         Experimental                     [Page 63]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-64" ></span>
<span class="grey"><a href="./rfc1801">RFC 1801</a>        X.400-MHS Routing using X.500 Directory        June 1995</span>


userName ATTRIBUTE ::= {
    SUBTYPE OF distinguishedName
    ID at-user-name}

mTAName ATTRIBUTE ::= {
    SUBTYPE OF name
    WITH SYNTAX DirectoryString{ub-mta-name-length}
    SINGLE VALUE                                                   130
    ID at-mta-name}
                        -- used for naming when
                        -- MTA is named in O=R Address Hierarchy

globalDomainID ATTRIBUTE ::= {
    WITH SYNTAX GlobalDomainIdentifier
    SINGLE VALUE
    ID at-global-domain-id}
                        -- both attributes present when MTA
                        -- is named outside O=R Address Hierarchy  140
                        -- to enable trace to be written

mTAApplicationProcess OBJECT-CLASS ::= {
    SUBCLASS OF {application-process}
    KIND auxiliary
    MAY CONTAIN {
        mTAWillRoute|
        globalDomainID|
        routingTreeList|
        localAccessUnit|                                           150
        accessUnitsUsed
    }
    ID oc-mta-application-process}

mTA OBJECT CLASS ::= {   -- Application Entity
    SUBCLASS OF {mhs-message-transfer-agent}
    KIND structural
    MAY CONTAIN {
        mTAName|
        globalDomainID|         -- per AE variant                  160
        responderAuthenticationRequirements|
        initiatorAuthenticationRequirements|
        responderPullingAuthenticationRequirements|
        initiatorPullingAuthenticationRequirements|
        initiatorP1Mode|
        responderP1Mode|
        polledMTAs|
        protocolInformation|
        respondingRTSCredentials|
        initiatingRTSCredentials|                                  170



<span class="grey">Kille                         Experimental                     [Page 64]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-65" ></span>
<span class="grey"><a href="./rfc1801">RFC 1801</a>        X.400-MHS Routing using X.500 Directory        June 1995</span>


        callingPresentationAddress|
        callingSelectorValidity|
        bilateralTable|
        mTAWillRoute|
        mhs-deliverable-content-length|
        routingTreeList|
        supportedMTSExtensions|
        mTAsAllowedToPoll
        }
    ID oc-mta}                                                     180

mTABilateralTableEntry OBJECT-CLASS ::=
    SUBCLASS OF {mTA| distinguishedNameTableEntry}
    ID oc-mta-bilateral-table-entry}

bilateralTable ATTRIBUTE ::= {
        WITH SYNTAX SEQUENCE OF DistinguishedName
        SINGLE VALUE
        ID at-bilateral-table}
                                                                   190
supportedMTSExtensions ATTRIBUTE ::= {
    SUBTYPE OF objectIdentifier
    ID at-supported-mts-extensions}

restrictedSubtree OBJECT-CLASS ::= {
        SUBCLASS OF {top}
        KIND auxiliary
        MAY CONTAIN {
                subtreeDeliverableContentLength|
                subtreeDeliverableContentTypes|                    200
                subtreeDeliverableEITs}
        ID oc-restricted-subtree}

subtreeDeliverableContentLength ATTRIBUTE ::= {
        SUBTYPE OF mhs-deliverable-content-length
        ID at-subtree-deliverable-content-length}

subtreeDeliverableContentTypes ATTRIBUTE ::= {
        SUBTYPE OF mhs-deliverable-content-types
        ID at-subtree-deliverable-content-types}                   210

subtreeDeliverableEITs ATTRIBUTE ::= {
        SUBTYPE OF mhs-deliverable-eits
        ID at-subtree-deliverable-eits}


initiatorP1Mode ATTRIBUTE ::= {
    WITH SYNTAX P1Mode



<span class="grey">Kille                         Experimental                     [Page 65]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-66" ></span>
<span class="grey"><a href="./rfc1801">RFC 1801</a>        X.400-MHS Routing using X.500 Directory        June 1995</span>


    SINGLE VALUE
    ID at-initiator-p1-mode}                                       220

responderP1Mode ATTRIBUTE ::= {
    WITH SYNTAX P1Mode
    SINGLE VALUE
    ID at-responder-p1-mode}

P1Mode ::= ENUMERATED {
    push-only(0),
    pull-only(1),
    twa(2) }                                                       230

polledMTAs ATTRIBUTE ::= {
    WITH SYNTAX PolledMTAs
    ID at-polled-mtas}

PolledMTAs ::= SEQUENCE {
        mta DistinguishedName,
        poll-frequency INTEGER OPTIONAL --frequency in minutes
        }
                                                                   240
mTAsAllowedToPoll ATTRIBUTE ::= {
        SUBTYPE OF distinguishedName
        ID at-mtas-allowed-to-poll}


responderAuthenticationRequirements ATTRIBUTE ::= {
   WITH SYNTAX AuthenticationRequirements
   SINGLE VALUE
   ID at-responder-authentication-requirements}
                                                                   250
initiatorAuthenticationRequirements ATTRIBUTE ::= {
   WITH SYNTAX AuthenticationRequirements
   SINGLE VALUE
   ID at-initiator-authentication-requirements}

responderPullingAuthenticationRequirements ATTRIBUTE ::= {
   WITH SYNTAX AuthenticationRequirements
   SINGLE VALUE
   ID at-responder-pulling-authentication-requirements}
                                                                   260
initiatorPullingAuthenticationRequirements ATTRIBUTE ::= {
   WITH SYNTAX AuthenticationRequirements
   SINGLE VALUE
   ID at-initiator-pulling-authentication-requirements}

AuthenticationRequirements ::= BITSTRING {



<span class="grey">Kille                         Experimental                     [Page 66]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-67" ></span>
<span class="grey"><a href="./rfc1801">RFC 1801</a>        X.400-MHS Routing using X.500 Directory        June 1995</span>


    mta-name-present(0),
    aet-present(1),
    aet-valid(2),
    network-address(3),                                            270
    simple-authentication(4),
    strong-authentication(5),
    bilateral-agreement-needed(6)}

respondingRTSCredentials ATTRIBUTE ::= {
        WITH SYNTAX RTSCredentials
        SINGLE VALUE
        ID at-responding-rts-credentials}

                                                                   280
initiatingRTSCredentials ATTRIBUTE ::= {
        WITH SYNTAX RTSCredentials
        SINGLE VALUE
        ID at-initiating-rts-credentials}


RTSCredentials ::= SEQUENCE {
        request [0] MTAandPassword OPTIONAL,
        response [<a href="#ref-1" title="models and services">1</a>] MTAandPassword OPTIONAL }
                                                                   290

MTAandPassword ::= SEQUENCE {
        MTAName,
        Password }              -- MTAName and Password
                                -- from X.411


callingPresentationAddress ATTRIBUTE ::= {
        SUBTYPE OF presentationAddress
        MULTI VALUE                                                300
        ID at-calling-presentation-address}

callingSelectorValidity ATTRIBUTE ::= {
        WITH SYNTAX CallingSelectorValidity
        SINGLE VALUE
        ID at-calling-selector-validity}

CallingSelectorValidity ::= ENUMERATED {
        all-selectors-fixed(0),
        tsel-may-vary(1),                                          310
        all-selectors-may-vary(2) }

mTAWillRoute ATTRIBUTE ::= {
    WITH SYNTAX MTAWillRoute



<span class="grey">Kille                         Experimental                     [Page 67]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-68" ></span>
<span class="grey"><a href="./rfc1801">RFC 1801</a>        X.400-MHS Routing using X.500 Directory        June 1995</span>


    ID at-mta-will-route}

MTAWillRoute ::= SEQUENCE {
        from [0]        SET OF ORAddressPrefix OPTIONAL,
        to [<a href="#ref-1" title="models and services">1</a>]          SET OF ORAddressPrefix OPTIONAL,
        from-excludes [<a href="#ref-2">2</a>]       SET OF ORAddressPrefix OPTIONAL,   320
        to-excludes [<a href="#ref-3" title="K. Bonacker">3</a>]         SET OF ORAddressPrefix OPTIONAL }

ORAddressPrefix ::= DistinguishedName

redirect ATTRIBUTE ::= {
        WITH SYNTAX Redirect
        SINGLE VALUE
        ID at-redirect}

Redirect ::= SEQUENCE OF SEQUENCE {                                330
        or-name ORName,
        reason RedirectionReason, -- from X.411
        filter CHOICE {
                min-size [<a href="#ref-1" title="models and services">1</a>] INTEGER,
                max-size [<a href="#ref-2">2</a>] INTEGER,
                content [<a href="#ref-3" title="K. Bonacker">3</a>] ContentType,
                eit [<a href="#ref-4" title="R.J. Kummerfeld">4</a>] ExternalEncodedInformationType } OPTIONAL
        }

nonDeliveryInfo ATTRIBUTE ::= {                                    340
        WITH SYNTAX NonDeliveryReason
        SINGLE VALUE
        ID at-non-delivery-info}

NonDeliveryReason ::= SEQUENCE {
        reason INTEGER (0..ub-reason-codes),
        diagnostic INTEGER (0..ub-diagnostic-codes) OPTIONAL,
        supplementaryInfo PrintableString OPTIONAL }

badAddressSearchPoint ATTRIBUTE ::= {                              350
        SUBTYPE OF distinguishedName
        ID at-bad-address-search-point}

badAddressSearchAttributes ATTRIBUTE ::= {
        WITH SYNTAX AttributeType
        ID at-bad-address-search-attributes}

alternativeAddressInformation EXTENSION
        AlternativeAddressInformation
        ::= id-alternative-address-information                     360
                -- X.400(92) continues to use MACRO notation




<span class="grey">Kille                         Experimental                     [Page 68]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-69" ></span>
<span class="grey"><a href="./rfc1801">RFC 1801</a>        X.400-MHS Routing using X.500 Directory        June 1995</span>


AlternativeAddressInformation ::= SET OF SEQUENCE {
        distinguished-name DistinguishedName OPTIONAL,
        or-address ORAddress OPTIONAL,
        other-useful-info SET OF Attribute }

localAccessUnit ATTRIBUTE ::= {
        WITH SYNTAX AccessUnitType
        ID at-local-access-unit}                                   370

AccessUnitType ::= ENUMERATED {
        fax (1),
        physical-delivery (2),
        teletex (3),
        telex (4) }

accessUnitsUsed ATTRIBUTE ::= {
        WITH SYNTAX SelectedAccessUnit
        ID at-access-units-used}                                   380

SelectedAccessUnit ::= SEQUENCE {
        type AccessUnitType,
        providing-MTA DistinguishedName,
        filter SET OF ORAddress OPTIONAL }
mhs-ds OBJECT-IDENTIFIER ::= {iso(1) org(3) dod(6) internet(1) private(4)
          enterprises(1) isode-consortium (453) mhs-ds (7)}

routing OBJECT IDENTIFIER ::= {mhs-ds 3}
                                                                   390
oc OBJECT IDENTIFIER ::= {routing 1}
at OBJECT IDENTIFIER ::= {routing 2}
id OBJECT IDENTIFIER ::= {routing 3}
oc-mta OBJECT IDENTIFIER ::= {oc 1}
oc-mta-bilateral-table-entry OBJECT IDENTIFIER ::= {oc 2}
oc-routing-information OBJECT IDENTIFIER ::= {oc 3}
oc-restricted-subtree OBJECT IDENTIFIER ::= {oc 4}
oc-routed-ua OBJECT IDENTIFIER ::= {oc 8}                          400
oc-routing-tree-root OBJECT IDENTIFIER ::= {oc 6}
oc-mta-application-process OBJECT IDENTIFIER ::= {oc 7}

at-access-md OBJECT IDENTIFIER ::= {at 1}
at-access-units-used OBJECT IDENTIFIER ::= {at 2}
at-subtree-information OBJECT IDENTIFIER ::= {at 3}
at-bad-address-search-attributes OBJECT IDENTIFIER ::= {at 4}
at-bad-address-search-point OBJECT IDENTIFIER ::= {at 5}

at-calling-selector-validity OBJECT IDENTIFIER ::= {at 7}          410





<span class="grey">Kille                         Experimental                     [Page 69]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-70" ></span>
<span class="grey"><a href="./rfc1801">RFC 1801</a>        X.400-MHS Routing using X.500 Directory        June 1995</span>


at-global-domain-id OBJECT IDENTIFIER ::= {at 10}
at-initiating-rts-credentials OBJECT IDENTIFIER ::= {at 11}
at-initiator-authentication-requirements OBJECT IDENTIFIER ::= {at 12}
at-initiator-p1-mode OBJECT IDENTIFIER ::= {at 13}
at-initiator-pulling-authentication-requirements
                                         OBJECT IDENTIFIER ::= {at 14}
at-local-access-unit OBJECT IDENTIFIER ::= {at 15}
at-redirect OBJECT IDENTIFIER ::= {at 46}
at-mta-info OBJECT IDENTIFIER ::= {at 40}                          420
at-mta-name OBJECT IDENTIFIER ::= {at 19}

at-mta-will-route OBJECT IDENTIFIER ::= {at 21}
at-calling-presentation-address OBJECT IDENTIFIER ::= {at 22}
at-responder-authentication-requirements OBJECT IDENTIFIER ::= {at 23}
at-responder-p1-mode OBJECT IDENTIFIER ::= {at 24}
at-responder-pulling-authentication-requirements
                                         OBJECT IDENTIFIER ::= {at 25}
at-responding-rts-credentials OBJECT IDENTIFIER ::= {at 26}
at-routing-failure-action OBJECT IDENTIFIER ::= {at 27}
at-routing-filter OBJECT IDENTIFIER ::= {at 28}                    430
at-routing-tree-list OBJECT IDENTIFIER ::= {at 29}
at-subtree-deliverable-content-length OBJECT IDENTIFIER ::= {at 30}
at-subtree-deliverable-content-types OBJECT IDENTIFIER ::= {at 31}
at-subtree-deliverable-eits OBJECT IDENTIFIER ::= {at 32}
at-supporting-mta OBJECT IDENTIFIER ::= {at 33}
at-transport-community OBJECT IDENTIFIER ::= {at 34}
at-user-name OBJECT IDENTIFIER ::= {at 35}
at-non-delivery-info OBJECT IDENTIFIER ::= {at 47}
at-polled-mtas  OBJECT IDENTIFIER ::= {at 37}
at-bilateral-table OBJECT IDENTIFIER {at 45}                       440
at-supported-extension OBJECT IDENTIFIER {at 42}
at-supported-mts-extension OBJECT IDENTIFIER {at 43}
at-mtas-allowed-to-poll OBJECT IDENTIFIER {at 44}

id-alternative-address-information OBJECT IDENTIFIER ::= {id 1}

ts-communities OBJECT-IDENTIFIER ::= {iso(1) org(3) dod(6) internet(1)
private(4) enterprises(1) isode-consortium (453) ts-communities (4)}

                                                                   450
tc-cons OBJECT IDENTIFIER ::= {ts-communities 1}    -- OSI CONS
tc-clns OBJECT IDENTIFIER ::= {ts-communities 2}    -- OSI CLNS
tc-internet OBJECT IDENTIFIER ::= {ts-communities 3}-- Internet+<a href="./rfc1006">RFC1006</a>
tc-int-x25 OBJECT IDENTIFIER ::= {ts-communities 4} -- International X.25
                                                    -- Without CONS
tc-ixi OBJECT IDENTIFIER ::= {ts-communities 5}     -- IXI (Europe)
tc-janet OBJECT IDENTIFIER ::= {ts-communities 6}   -- Janet (UK)




<span class="grey">Kille                         Experimental                     [Page 70]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-71" ></span>
<span class="grey"><a href="./rfc1801">RFC 1801</a>        X.400-MHS Routing using X.500 Directory        June 1995</span>


mail-protocol OBJECT-IDENTIFIER ::= {iso(1) org(3) dod(6) internet(1)
private(4) enterprises(1) isode-consortium (453) mail-protocol (5)} 460

ac-p1-1984 OBJECT IDENTIFIER ::= {mail-protocol 1}      -- p1(1984)
ac-smtp  OBJECT IDENTIFIER ::= {mail-protocol 2}        -- SMTP
ac-uucp OBJECT IDENTIFIER ::= {mail-protocol 3}         -- UUCP Mail
ac-jnt-mail OBJECT IDENTIFIER ::= {mail-protocol 4}     -- JNT Mail (UK)
ac-p1-1988-x410 OBJECT IDENTIFIER ::= {mail-protocol 5}
                                               -- p1(1988) in X.410 mode
ac-p3-1984 OBJECT IDENTIFIER ::= {mail-protocol 6}      -- p3(1984)
END

                       Figure 22:  ASN.1 Summary

-----------------------------------------------------------------------

E  Regular Expression Syntax

   This appendix defines a form of regular expression for pattern
   matching.  This pattern matching is derived from commonly available
   regular expression software including UNIX egrep(1) The matching is
   modified to be case insensitive.

    A regular expression (RE) specifies a set of character strings to
    match against - such as "any string containing digits 5 through
    9".  A member of this set of strings is said to be matched by the
    regular expression.

    Where multiple matches are present in a line, a regular expression
    matches the longest of the leftmost matching strings.

    Regular expressions can be built up from the following
    "single-character" RE's:

     c    Any ordinary character not listed below.  An ordinary
          character matches itself.

     \    Backslash.  When followed by a special character, the RE
          matches the "quoted" character, cancelling the special nature
          of the character.

     .    Dot.  Matches any single character.

     ^    As the leftmost character, a caret (or circumflex) con-
          strains the RE to match the leftmost portion of a string.  A
          match of this type is called an "anchored match" because it is
          "anchored" to a specific place in the string.  The ^ character
          loses its special meaning if it appears in any position other



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<span class="grey"><a href="./rfc1801">RFC 1801</a>        X.400-MHS Routing using X.500 Directory        June 1995</span>


          than the start of the RE.

     $    As the rightmost character, a dollar sign constrains the RE to
          match the rightmost portion of a string.  The $ character
          loses its special meaning if it appears in any position other
          than at the end of the RE.

     ^RE$ The construction ^RE$ constrains the RE to match the entire
          string.

     [<a id="ref-c...">c...</a>]
          A nonempty string of characters, enclosed in square brackets
          matches any single character in the string.  For example,
          [abcxyz] matches any single character from the set `abcxyz'.
          When the first character of the string is a caret (^), then
          the RE matches any charac- ter except those in the remainder
          of the string.  For example, `[^45678]' matches any character
          except `45678'.  A caret in any other position is interpreted
          as an ordinary character.

     []c...]
          The right square bracket does not terminate the enclosed
          string if it is the first character (after an initial `^', if
          any), in the bracketed string.  In this position it is treated
          as an ordinary character.

     [<a id="ref-l-r">l-r</a>]
          The minus sign (hyphen), between two characters, indicates a
          range of consecutive ASCII characters to match.  For example,
          the range `[0-9]' is equivalent to the string `[0123456789]'.
          Such a bracketed string of characters is known as a character
          class.  The `-' is treated as an ordinary character if it
          occurs first (or first after an initial ^) or last in the
          string.

          The following rules and special characters allow for
          con-structing RE's from single-character RE's:

          A concatenation of RE's matches a concatenation of text
          strings, each of which is a match for a successive RE in the
          search pattern.

     *    A regular expression, followed by an asterisk (*) matches zero
          or more occurrences of the regular expression.  For example,
          [a-z][a-z]* matches any string of one or more lower case
          letters.





<span class="grey">Kille                         Experimental                     [Page 72]</span></pre>
<hr class='noprint'/><!--NewPage--><pre class='newpage'><span id="page-73" ></span>
<span class="grey"><a href="./rfc1801">RFC 1801</a>        X.400-MHS Routing using X.500 Directory        June 1995</span>


     +    A regular expression, followed by a plus character (+) matches
          one or more occurrences of the regular expression.  For
          example, [a-z]+ matches any string of one or more lower case
          letters.

     ?    A regular expression, followed by a question mark (?) matches
          zero or one occurrences of the regular expression.  For
          example, ^[a-z]?[0-9]* matches a string starting with an
          optional lower case letter, followed by zero or more digits.

     {m}
     {m,}
     {m,n}
          A regular expression, followed by {m}, {m,}, or {m,n} matches
          a range of occurrences of the regular expression.  The values
          of m and n must be non-negative integers less than 256; {m}
          matches exactly m occurrences; {m,} matches at least m
          occurrences; {m,n} matches any number of occurrences between m
          and n inclusive.  Whenever a choice exists, the regular
          expression matches as many occurrences as possible.

     |    Alternation: two regular expressions separated by `|' or
          NEWLINE match either a match for the first or a match for the
          second.

     (...)
          A regular expression enclosed between the character sequences
          ( and ) matches whatever the unadorned RE matches.

    The order of precedence of operators at the same parenthesis level
    is `[ ]' (character classes), then `*' `+' `?' '{m,n}' (closures),
    then concatenation, then `|' (alternation) and NEWLINE.



















Kille                         Experimental                     [Page 73]
</pre>