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RFC 1801


MHS use of the X.500 Directory to support MHS Routing

Part 2 of 3, p. 17 to 46
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10.  Routing Information

   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 {
           accessMD|                                                  10
       ID oc-routing-information}
                   -- No naming attributes as this is not a
                   -- structural object class

   subtreeInformation ATTRIBUTE ::= {                                 20
       WITH SYNTAX SubtreeInfo

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       ID at-subtree-information}

   SubtreeInfo ::= ENUMERATED {
       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
       ID at-routing-failure-action}

   RoutingFailureAction ::= ENUMERATED {
               next-level(0),                                         50
               stop(3)  }

   mTAInfo ATTRIBUTE ::= {
       ID at-mta-info}

   MTAInfo ::= SEQUENCE {                                             60
               name DistinguishedName,
               weight [1] RouteWeight DEFAULT preferred-access,
               mta-attributes [2] 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

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                   backup(10)} (0..20)

                 Figure 3:  Routing Information at a Node


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


   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 Section 11.
       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
       Section 10.3.

   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:

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        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 15 and 18.  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 18 and
       Section 10.2.

   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 Section 10.1.

   accessMD The accessMD attribute is discussed in Section 10.4.  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 Section 17.  This attribute is for when an address
       has been rejected, and allows information on alternative addresses
       to be found.

10.1  Multiple routing trees

   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
   Section 14.  The routing decision works by examining a series of

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   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

   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:

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       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

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

10.2  MTA Choice

   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

    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

   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 Section 10.2.

   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

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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 Section 16.

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









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



   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 Section 16, with a
   formal definition in Figure 6.

10.3  Routing Filters

   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 Appendix E.

   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

   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.


   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.

10.4  Indirect Connectivity

   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

11.  Local Addresses (UAs)

   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-message-store|                                         10

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                           -- defined here
       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

   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

        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

   redirect The redirect attribute controls redirects, as described in
       Section 22.1.

   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 Section 22.3.

   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.

11.1  Searching for Local Users

   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

12.  Direct Lookup

   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.

13.  Alternate Routes

13.1  Finding Alternate Routes

   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.

13.2  Sharing routing information

   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.

14.  Looking up Information in the Directory

   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

   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

   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

   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

   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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15.  Naming MTAs

   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}
       ID at-mta-name}
                           -- used for naming when
                           -- MTA is named in O=R Address Hierarchy

   globalDomainID ATTRIBUTE ::= {                                     10
       WITH SYNTAX GlobalDomainIdentifier
       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 {
       ID oc-mta-application-process}

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

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           initiatorP1Mode|                                           40
           mhs-deliverable-content-length|                            50
       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.

15.1  Naming 1984 MTAs

   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.

16.  Attributes Associated with the MTA

   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 Section 15.

   globalDomainID Section 15.

   protocolInformation Section 18.1.

   applicationContext Section 18.2.

   mhs-deliverable-content-length Section 18.3.

   responderAuthenticationRequirements Section 20.2.

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   initiatorAuthenticationRequirements Section 20.2.

   responderPullingAuthenticationRequirements Section 20.2.

   initiatorPullingAuthenticationRequirements Section 20.2.

   initiatorP1Mode Section 19.

   responderP1Mode Section 19.

   polledMTAs Section 19.

   mTAsAllowedToPoll Section 19.

   respondingRTSCredentials Section 20.3.

   initiatingRTSCredentials Section 20.3.

   callingPresentationAddress Section 20.3.

   callingSelectorValidity Section 20.3.

   bilateralTable Section 17.

   mTAWillRoute Section 21.

   routingTreeList Section 9.

   supportedMTSExtensions Section 18.3.


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

                   Figure 7:  MTA Bilateral Table Entry


17.  Bilateral Agreements

   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

   MTA bilateral information is stored in a table, as defined in [15].
   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 Section
   20.1, 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 Section 20.3, 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

   All of the MTA information described in Section 16 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.

18.  MTA Selection

18.1  Dealing with protocol mismatches

   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 [9].  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 Appendix B.

18.2  Supported Protocols

   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 Appendix C.  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.

18.3  MTA Capability Restrictions

   In addition to policy restrictions, described in Section 21, 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 {
           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}

   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.

18.4  Subtree Capability Restrictions

   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 (RFC 822).

   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
       ID at-initiator-p1-mode}

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

   P1Mode ::= ENUMERATED {
       twa(2) }

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


19.  MTA Pulling Messages

   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

20.  Security and Policy

20.1  Finding the Name of the Calling MTA

   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.

20.2  Authentication

   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
      ID at-responder-authentication-requirements}

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

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

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

   AuthenticationRequirements ::= BITSTRING {

                  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

   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 (Section 21).  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

20.3  Authentication Information

   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 [1] 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-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 [1]          SET OF ORAddressPrefix OPTIONAL,
           from-excludes [2]       SET OF ORAddressPrefix OPTIONAL,
           to-excludes [3]         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

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

(page 46 continued on part 3)

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