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

 X.400-MHS use of the X.500 Directory to support X.400-MHS Routing

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 RFC 822                                                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

1. Introduction

 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
 Section 23.
 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 Section 25.

2. Goals

 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.

Kille Experimental [Page 3] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

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, RFC 822, and their interaction.

o Extensible to other mail architectures

o Recognise existing RFC 822 routing, and coexist smoothly.

o Improve RFC 822 routing capabilities. This is particularly

  important for RFC 822 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.

3. Approach

 A broad problem statement, and a survey of earlier approaches to the
 problem is given in the COSINE Study on MHS Topology and Routing [8].
 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 [5].  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 [7].

o Initial ISO Standardisation work on MHS use of Directory for

  routing [19].  Subsequent ISO work in this area has drawn from
  earlier drafts of this specification.

o The work of the VERDI Project [3].

o Work by Kevin Jordan of CDC [6].

o The routing approach of ACSNet [4, 17] 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 [2].

4. Direct vs Indirect Connection

 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:
  1. - Message Format Conversion
  1. - Distribution List expansion

o Dealing with different protocols. The store and forward approach

  allows for straightforward conversion.  Relevant cases include:
  1. - Provision of X.400 over different OSI Stacks (e.g.,

Connectionless Network Service).

  1. - Use of a different version of X.400.
  1. - 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.

5. X.400 and RFC 822

 This document defines mechanisms for X.400 message routing.  It is
 important that this can be integrated with RFC 822 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 RFC 822 routing using the same basic
 infrastructure is defined in a companion document [13].  In addition
 support for X.400/RFC 822 gatewaying is needed, to support
 interaction.  Directory based mechanisms for this are defined in
 [16].  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.

Kille Experimental [Page 8] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

o Improved routing services for RFC 822 only sites.

 For sites which are only X.400 or only RFC 822, the mechanisms
 associated with gatewaying or with the other form of addressing are
 not needed.

6. Objects

 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 [11].  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.

7. Communities

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 Section 9.2.

8. Routing Trees

 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.

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8.1 Routing Tree Definition

 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 [15].  Where there is any routing
 action associated with a node in a routing tree, the node is of
 object class routingInformation, as defined in Section 10.

8.2 The Open Community Routing Tree

 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

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8.3 Routing Tree Location

 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" [15].  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 Section 9.  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.

8.4 Example Routing Trees

 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

8.5 Use of Routing Trees to look up Information

 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
     [15] in order to generate a Distinguished Name.

Kille Experimental [Page 13] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

 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.

9. Routing Tree Selection

 The list of routing trees which a given MTA uses will be represented
 in the directory.  This uses the attribute defined in Figure 2.
  1. ——————————————————————–
 routingTreeList ATTRIBUTE ::= {
         WITH SYNTAX RoutingTreeList
         SINGLE VALUE
         ID at-routing-tree-list}
 RoutingTreeList ::= SEQUENCE OF RoutingTreeName
 RoutingTreeName ::= DistinguishedName
                 Figure 2: Routing Tree Use Definition
  1. ——————————————————————–
 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.

9.1 Routing Tree Order

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

Kille Experimental [Page 14] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

 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.

9.2 Example use of Routing Trees

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

9.2.1 Fully Open Organisation

 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.

9.2.2 Open Organisation with Fallback

 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.

Kille Experimental [Page 15] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

9.2.3 Minimal-routing MTA

 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.

9.2.4 Organisation with Firewall

 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.

9.2.5 Well Known Entry Points

 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.

9.2.6 ADMD using the Open Community for Advertising

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

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  o  Address validation
  o  Advertising the mechanism for a bilateral link to be established

9.2.7 ADMD/PRMD gateway

 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.

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.
  1. ——————————————————————–
 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

Kille Experimental [Page 17] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

     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 [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

Kille Experimental [Page 18] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

                 preferred-access(5),
                 backup(10)} (0..20)
               Figure 3:  Routing Information at a Node
  1. ——————————————————————–
 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 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

Kille Experimental [Page 20] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

 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:

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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
     trees.
 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
     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 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

Kille Experimental [Page 22] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

     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:
 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 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:
  1. ——————————————————————–
 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
  1. ——————————————————————–
 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
     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.

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
 entry.

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:
  1. ——————————————————————–
 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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  1. - 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. ——————————————————————–
 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
     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

Kille Experimental [Page 29] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

 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
 matches.

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

Kille Experimental [Page 30] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

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

Kille Experimental [Page 31] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

 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.

Kille Experimental [Page 32] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

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.
  1. ——————————————————————–
 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

Kille Experimental [Page 33] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

         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
  1. ——————————————————————–
 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.

Kille Experimental [Page 34] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

 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.

Kille Experimental [Page 35] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

 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.
  1. ——————————————————————–
 mTABilateralTableEntry OBJECT-CLASS ::=
     SUBCLASS OF {mTA| distinguishedNameTableEntry}
     ID oc-mta-bilateral-table-entry}
                 Figure 7:  MTA Bilateral Table Entry
  1. ——————————————————————–

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

Kille Experimental [Page 36] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

 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 [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.
  1. ——————————————————————–
 bilateralTable ATTRIBUTE ::= {
         WITH SYNTAX SEQUENCE OF DistinguishedName
         SINGLE VALUE
         ID at-bilateral-table}
                 Figure 8:  Bilateral Table Attribute
  1. ——————————————————————–
 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

Kille Experimental [Page 37] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

 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 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.

Kille Experimental [Page 38] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

  1. ——————————————————————–
 supportedMTSExtensions ATTRIBUTE ::= {
     SUBTYPE OF objectIdentifier
     ID at-supported-mts-extensions}
                  Figure 9:  Supported MTS Extensions
  1. ——————————————————————–
 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.

Kille Experimental [Page 39] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

  1. ——————————————————————–
 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
  1. ——————————————————————–
 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.

Kille Experimental [Page 40] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

  1. ——————————————————————–
 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
  1. ——————————————————————–

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

Kille Experimental [Page 41] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

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

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.
  1. ——————————————————————–
 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
  1. ——————————————————————–
 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.

Kille Experimental [Page 43] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

 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 (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
 process.

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.

Kille Experimental [Page 44] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

  1. ——————————————————————–
 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-fixed(0),
         tsel-may-vary(1),
         all-selectors-may-vary(2) }
               Figure 13:  MTA Authentication Parameters
  1. ——————————————————————–

Kille Experimental [Page 45] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

  1. ——————————————————————–
 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
  1. ——————————————————————–
 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.

21. Policy and Authorisation

21.1 Simple MTA Policy

 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.

Kille Experimental [Page 46] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

 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.

21.2 Complex MTA Policy

 MTAs will generally have a much more complex policy mechanism, such
 as that provided by PP MTA [10].  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:

Kille Experimental [Page 47] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

  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.
  1. ——————————————————————–
 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 [1] INTEGER,
                 max-size [2] INTEGER,
                 content [3] ContentType,
                 eit [4] ExternalEncodedInformationType } OPTIONAL
         }
                    Figure 15:  Redirect Definition
  1. ——————————————————————–

Kille Experimental [Page 48] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

22. Delivery

22.1 Redirects

 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.

Kille Experimental [Page 49] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

 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.
  1. ——————————————————————–
 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
  1. ——————————————————————–

22.2 Underspecified O/R Addresses

 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

Kille Experimental [Page 50] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

 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.

22.3 Non Delivery

 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.

22.4 Bad Addresses

 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.

Kille Experimental [Page 51] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

  1. ——————————————————————–
 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
  1. ——————————————————————–
 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.

Kille Experimental [Page 52] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

  1. ——————————————————————–
 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
  1. ——————————————————————–

23. Submission

 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.

23.1 Normal Derivation

 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.

23.2 Roles and Groups

 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.

Kille Experimental [Page 53] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

24. Access Units

 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.

25. The Overall Routing Algorithm

 Having provided all the pieces, a summary of how routing works can be
 given.
 The core of the X.400 routing is described in Section 10.  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 Section 18.2.

Kille Experimental [Page 54] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

     (Note that this could be an RFC 822 protocol, as well as an
     X.400 protocol.)
  o  The protocols shall share a common transport community, as
     described in Section 18.1.
  o  There shall be no capability restrictions in the MTA which
     prevents transfer of the current message, as described in
     Section 18.3.
  o  There shall be no policy restrictions in the MTA which prevents
     transfer of the current message, as described in Section 21.
  o  The authentication requirements of the MTA shall be met by the
     local MTA, as described in Section 20.2.
  o  If the authentication (Section 20.2) indicates that a bilateral
     agreement is present, the MTA shall be listed in the local set of
     bilateral agreements, as described in Section 17.
  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 [12].

26. Performance

 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.

27. Acknowledgements

 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 Section 3.  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

Kille Experimental [Page 55] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

 Huitema (Inria); Marko Kaittola (Dante); Sylvain Langlois (EDF); Lucy
 Loftin (AT&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.

28. References

  [1] The Directory --- overview of concepts, models and services,
      1993. CCITT X.500 Series Recommendations.
  [2] J.N. Chiappa. A new IP routing and addressing architecture,
      1991.
  [3] A. Consael, M. Tschicholz, O. Wenzel, K. Bonacker, and M. Busch.
      DFN-Directory nutzung durch MHS, April 1990. GMD Report.
  [4] 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.
  [5] Eppenberger, U., "Routing Coordination for X.400 MHS Services
      Within a Multi Protocol / Multi Network Environment Table Format
      V3 for Static Routing", RFC 1465, SWITCH, May 1993.
  [6] K.E. Jordan. Using X.500 directory services in support of X.400
      routing and address mapping, November 1991. Private Note.
  [7] 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.
  [8] S.E. Kille. Topology and routing for MHS.  COSINE Specification
      Phase 7.7, RARE, 1988.
  [9] Kille, S., "Encoding Network Addresses to support operation over
      non-OSI lower layers", RFC 1277, Department of Computer Science,
      University College London, November 1991.
 [10] S.E. Kille. Implementing X.400 and X.500:  The PP and QUIPU
      Systems. Artech House, 1991.  ISBN 0-89006-564-0.
 [11] Kille, S., "A Representation of Distinguished Names
      (OSI-DS 23 (v5))", RFC 1485, Department of Computer Science,
      University College London, January 1992.

Kille Experimental [Page 56] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

 [12] Kille, S., Mhs use of X.500 directory to support mhs content
      conversion, Work in Progress, July 1993.
 [13] Kille, S., "Use of the X.500 directory to support routing for
      RFC 822 and related protocols", Work in Progress, July 1993.
 [14] Kille, S., "Representing tables and subtrees in the X.500
      directory", Work in Progress, September 1994.
 [15] Kille, S., "Representing the O/R Address hierarchy in the X.500
      directory information tree", Work in Progress, September 1994.
 [16] Kille, S., "Use of the X.500 directory to support mapping
      between X.400 and RFC 822 addresses", Work in Progress,
      September 1994.
 [17] Lauder, P., Kummerfeld, R., and A. Fekete. Hierarchical network
      routing. In Tricomm 91, 1991.
 [18] CCITT recommendations X.400 / ISO 10021, April 1988. CCITT
      SG 5/VII / ISO/IEC JTC1, Message Handling:  System and Service
      Overview.
 [19] 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.

29. Security Considerations

 Security issues are not discussed in this memo.

Kille Experimental [Page 57] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

30. Author's Address

 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

Kille Experimental [Page 58] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

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}

Kille Experimental [Page 59] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

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+RFC1006 tc-int-x25 OBJECT IDENTIFIER ::= {ts-communities 4} – International X.25

  1. - 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

Kille Experimental [Page 60] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

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 X.410 mode 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 {

Kille Experimental [Page 61] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

      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)  }

Kille Experimental [Page 62] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

mTAInfo ATTRIBUTE ::= {

  WITH SYNTAX MTAInfo
  ID at-mta-info}

MTAInfo ::= SEQUENCE {

          name DistinguishedName,                                 80
          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),

              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}

Kille Experimental [Page 63] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

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

Kille Experimental [Page 64] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

      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

Kille Experimental [Page 65] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

  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 {

Kille Experimental [Page 66] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

  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 [1] 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

Kille Experimental [Page 67] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

  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,   320
      to-excludes [3]         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 [1] INTEGER,
              max-size [2] INTEGER,
              content [3] ContentType,
              eit [4] 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

Kille Experimental [Page 68] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

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

Kille Experimental [Page 69] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

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+RFC1006 tc-int-x25 OBJECT IDENTIFIER ::= {ts-communities 4} – International X.25

  1. - Without CONS

tc-ixi OBJECT IDENTIFIER ::= {ts-communities 5} – IXI (Europe) tc-janet OBJECT IDENTIFIER ::= {ts-communities 6} – Janet (UK)

Kille Experimental [Page 70] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

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}

  1. - 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

Kille Experimental [Page 71] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

        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.
   [c...]
        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.
   [l-r]
        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.

Kille Experimental [Page 72] RFC 1801 X.400-MHS Routing using X.500 Directory June 1995

   +    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]

/data/webs/external/dokuwiki/data/pages/rfc/rfc1801.txt · Last modified: 1995/06/09 20:50 by 127.0.0.1

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