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Versions: 02 05 RFC 1801

Network Working                                  S.E. Hardcastle-Kille
Group                                                 ISODE Consortium
INTERNET-DRAFT                                           November 1992
                                                   Expires:  June 1993




             MHS use of Directory to support MHS Routing






Status of this Memo

This document is an Internet Draft.  Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its Areas,
and its Working Groups.  Note that other groups may also distribute
working documents as Internet Drafts.
Internet Drafts are draft documents valid for a maximum of six months.
Internet Drafts may be updated, replaced, or obsoleted by other
documents at any time.  It is not appropriate to use Internet Drafts
as reference material or to cite them other than as a "working draft"
or "work in progress."
Please check the I-D abstract listing contained in each Internet Draft
directory to learn the current status of this or any other Internet
Draft.

Abstract
This document specifies an approach for X.400 Message Handling Systems
to perform application level routing using the OSI Directory [16, 1].
Use of the directory in this manner is fundamental to enabling large
scale deployment of X.400.
This draft document will be submitted to the RFC editor as a protocol
standard.  Distribution of this memo is unlimited.  Please send
comments to the author or to the discussion group
<mhs-ds@mercury.udev.cdc.com>.




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Contents

1   Introduction                                                     6

2   Work to be done                                                  6


3   Goals                                                            7

4   Approach                                                         9


5   Direct vs Indirect Connection                                   10

6   X.400 and RFC 822                                               13

7   Objects                                                         13


8   Communities                                                     15

9   Routing Trees                                                   16

    9.1    Routing Tree Definition   .   .   .   .   .   .   .      16
    9.2    The Open Community Routing Tree   .   .   .   .   .      17
    9.3    Routing Tree Location     .   .   .   .   .   .   .      17

    9.4    Example Routing Trees     .   .   .   .   .   .   .      18
    9.5    Use of Routing Trees to look up Information   .   .      19

10  Routing Tree Selection                                          19

    10.1   Routing Tree Order    .   .   .   .   .   .   .   .      19
    10.2   Example use of Routing Trees  .   .   .   .   .   .      20
        10.2.1   Fully Open Organisation     .   .   .   .   .      21

        10.2.2   Open Organisation with Fallback     .   .   .      21
        10.2.3   Minimal-routing MTA     .   .   .   .   .   .      21
        10.2.4   Organisation with Firewall  .   .   .   .   .      21

        10.2.5   Well Known Entry Points     .   .   .   .   .      22
        10.2.6   ADMD using the Open Community for Advertising      22

        10.2.7   ADMD/PRMD gateway   .   .   .   .   .   .   .      22

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11  Routing Information                                             23
    11.1   MTA Choice    .   .   .   .   .   .   .   .   .   .      27
    11.2   Routing Filters   .   .   .   .   .   .   .   .   .      30

    11.3   Indirect Connectivity     .   .   .   .   .   .   .      31

12  Local Addresses (UAs)                                           32


13  Direct Lookup                                                   35

14  Alternate Routes                                                35
    14.1   Finding Alternate Routes  .   .   .   .   .   .   .      35

    14.2   Sharing routing information   .   .   .   .   .   .      35

15  Looking up Information in the Directory                         36

16  Naming MTAs                                                     37

    16.1   Naming 1984 MTAs  .   .   .   .   .   .   .   .   .      39

17  Attributes Associated with the MTA                              39


18  Bilateral Agreements                                            40

19  MTA Selection                                                   41
    19.1   Dealing with protocol mismatches  .   .   .   .   .      41

    19.2   Supported Protocols   .   .   .   .   .   .   .   .      42
    19.3   MTA Capability Restrictions   .   .   .   .   .   .      42
    19.4   Subtree Capability Restrictions   .   .   .   .   .      42


20  MTA Pulling Messages                                            43

21  Security and Policy                                             44
    21.1   Finding the Name of the Calling MTA   .   .   .   .      44

    21.2   Authentication    .   .   .   .   .   .   .   .   .      45
    21.3   Authentication Information    .   .   .   .   .   .      47



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22  Policy and Authorisation                                        49
    22.1   Simple MTA Policy     .   .   .   .   .   .   .   .      49
    22.2   Complex MTA Policy    .   .   .   .   .   .   .   .      50


23  Redirects                                                       51

24  Non Delivery                                                    51


25  Bad Addresses                                                   52

26  Submission                                                      53
    26.1   Normal Derivation     .   .   .   .   .   .   .   .      54

    26.2   Roles and Groups  .   .   .   .   .   .   .   .   .      54

27  Access Units                                                    54

28  The Overall Routing Algorithm                                   56

    28.1   X.400 Routing     .   .   .   .   .   .   .   .   .      56
    28.2   Pseudo Code   .   .   .   .   .   .   .   .   .   .      57

    28.3   Examples  .   .   .   .   .   .   .   .   .   .   .      57

29  Performance                                                     57

30  Acknowledgements                                                57


31  Security Considerations                                         59

32  Author's Address                                                59

A   Object Identifier Assignment                                    60


B   Community Identifier Assignments                                62

C   Protocol Identifier Assignments                                 63


D   ASN.1 Summary                                                   63

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E   Regular Expression Syntax                                       63












































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List of Figures

    1      Location of Routing Trees     .   .   .   .   .   .      18
    2      Routing Tree Use Definition   .   .   .   .   .   .      20

    3      Routing Information at a Node     .   .   .   .   .      24
    4      Indirect Access   .   .   .   .   .   .   .   .   .      31
    5      UA Attributes     .   .   .   .   .   .   .   .   .      32

    6      MTA Definitions   .   .   .   .   .   .   .   .   .      38
    7      MTA Bilateral Table Entry     .   .   .   .   .   .      40

    8      Transport Community Definition    .   .   .   .   .      41
    9      Subtree Capability Restriction    .   .   .   .   .      43
    10     Pulling Messages  .   .   .   .   .   .   .   .   .      44

    11     Authentication Requirements   .   .   .   .   .   .      46
    12     MTA Authentication Parameters     .   .   .   .   .      48
    13     Simple MTA Policy Specification   .   .   .   .   .      49

    14     Redirect Definition   .   .   .   .   .   .   .   .      52
    15     Non Delivery Information  .   .   .   .   .   .   .      53
    16     Bad Address Pointers  .   .   .   .   .   .   .   .      54

    17     Access Unit Attributes    .   .   .   .   .   .   .      55
    18     Object Identifier Assignment  .   .   .   .   .   .      61
    19     Transport Community Object Identifier Assignments        62

    20     Protocol Object Identifier Assignments    .   .   .      63


List of Tables

    1      Possible target end to end delays     .   .   .   .       8











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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 26.

The key problem in routing is to map from an O/R Address onto an MTA
(next hop).  This should 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 28.


2  Work to be done

This section notes things which still need to be done to this
document, and also to other documents in the series.  This includes:

1.  Formalising ASN.1:  defining modules and variable imports.  (all
    documents)

2.  Provide pseudo-code to describe the algorithm more clearly

3.  Write a general note on MHS Use of Directory, summarising what is
    already defined in X.402, and showing how this work relates to it.
    (Someone other than SH-K should volunteer)

4.  More examples

5.  Notes on performance

6.  Acknowledgements

7.  Add appendix on regex

8.  List of attributes which affect routing





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3  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.

 o  Robust.  Errors and out of date information should cause minimal
    and local damage.

 o  Deal with link failures.  There should be some ability to choose
    alternative routes.  In general, the routing approach should be
    redundant.

 o  Load sharing.  Information on routes should allow ``equal'' routes
    to be specified, and thus facilitate load sharing.

 o  Support format and protocol conversion

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

 o  Policy robust.  It should 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.


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_____________________________________________________________
|______________|High_Priority_|Normal_Priority_|Low_Priority_|_
|50%_max_delay_|0.5_hour______|1_hour__________|12_hours_____|
|98%_max_delay_|3_hours_______|12_hours________|24_hours_____|
|max_delay_____|6_hours_______|72_hours________|96_hours_____|

             Table 1:  Possible target end to end delays

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

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

 o  Messages should be routed consistently.  Alternate routing
    strategies, which might introduce unexpected delay, should be used
    with care (e.g.  routing through a protocol converter due to
    unavailability of an X.400 MTA).

 o  Delay between message submission and delivery should be minimised.
    Table 1 indicates the sort of target which might be aimed for.
    The figures are illustrative of the sort of target which might be
    set.  They are better than achieved in most current Research
    Networks.  They are larger that the CEN/Cenelec figures, which
    were probably written by someone who has never run an MHS Network.
    The long tail-offs on Normal and Low priority recognise the fact
    that some end systems will not get 24 hour operator coverage
    (whereas any MTAs providing ADMD service should).  In the case of
    a high priority message, a non-delivery notification should be
    returned within the suggested time.

 o  Interact sensibly with ADMD services provided by PTTs or RPOAs.

 o  Be global in scope

 o  Routing strategy should deal with a scale of order of magnitude

    106 -- 108 MTAs.

 o  Routing strategy should deal with of order 106 -- 108
    Organisations.

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


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

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

 o  Sufficient tracing and logging should 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.


4  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 [13].
The interim (table-based) approach suggested in this study, whilst not
being followed in detail, broadly reflects what the MHS community is
doing.  The evolving specification of the RARE table format is defined
in [5].  This document specifies the envisaged longer term approach.
Although this work is important and needed now, there is a coherent
interim approach, and this work should not be rushed.
Some documents have made useful contributions to this work:


 o  My paper on MHS use of directory, which laid out the broad

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    approach of mapping the O/R Address space on to the DIT [12].

 o  The current OSI Standardisation work on MHS use of Directory for
    routing.  The concept of Routing Entity is a useful one [17].

 o  The work of the VERDI Project [3].

 o  Work by Kevin Jordan of CDC [11].

 o  The routing approach of ACSNet [4, 15] 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].


5  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 should be used.

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


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

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

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

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


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

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

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

    --  Message Format Conversion

    --  Distribution List expansion

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

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

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

    --  Interaction with non-X.400 mail services

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


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    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
    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 must 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,
    and is the general approach being developed by ISO. 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.




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6  X.400 and RFC 822

This document defines mechanisms for X.400 routing.  It is important
that this can be integrated with RFC 822 base routing, as many MTAs
will work in both communities.  This routing document is written with
this problem in mind, and support for RFC 822 routing using the same
basic infrastructure is defined in a companion document [10].  In
addition support for X.400/RFC 822 gatewaying is needed, to support
interaction.  Directory based mechanisms for this are defined in [9].
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.

 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.


7  Objects

It is useful to start with a managers 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 [8].  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=Steve Kille, OU=Computer Science,
    O=University College London, C=GB

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


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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 NFS. MTAs may support multiple protocols, and will
    identify 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
    the UA. It will identify which MTAs1 are able to access the UA.

Role Some organisational function.  For example:

    CN=System Manager, OU=Computer Science,
    O=University College London, 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.

----------------------------
    1. 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.





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8  Communities

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


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.

Bilateral Agreements An agreement between a pair of MTAs to route
    certain types of traffic.  This MTA pair agreement usually
    reflects a higher level agreement.  It is an important special
    case of the first, as bilateral information must be held for the
    link at both ends.  In some cases, this information must be
    private.

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.

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.


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The local community This is the scope of a single MTA. It reflects
    routing information private to the MTA. It will contain an MTAs
    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 10.2.


9  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.


9.1  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


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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 [7].  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 11.


9.2  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 soley 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.  This might be thought of
as directory knowledge information.  An analogous situation in current
operational practice is to make DIT entries for Countries and US
States.


9.3  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'' [7].  A routing tree is
identified by the point at which it is rooted.  An MTA will use a list


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_______________________________________________________________________
routingTreeRoot OBJECT-CLASS
    SUBCLASS OF oRAddressComponent, subtree
    ::= oc-routing-tree-root

_________________Figure_1:__Location_of_Routing_Trees__________________


of routing trees, as determined by the mechanism described in
Section 10.  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.


9.4  Example Routing Trees

Consider routing trees with entries for O/R Address:


PRMD=UK.AC; ADMD=Gold 400; C=GB;

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


PRMD=UK.AC, ADMD=Gold 400, C=GB

Consider a routing tree which is private to:

O=University College London, C=GB


They might choose to label a routing tree root ``UCL Routing Tree'',
which would lead to a routing tree root of:

CN=UCL Routing Tree, O=University College London, C=GB


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

PRMD=UK.AC, ADMD=Gold 400,
C=GB, CN=UCL Routing Tree,
O=University College London, C=GB


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9.5  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
    [7] in order to generate a Distinguished Name.

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.


10  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.
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 should facilitate debugging routing problems.


10.1  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:




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_______________________________________________________________________
routingTreeList ATTRIBUTE
        WITH ATTRIBUTE-SYNTAX RoutingTreeList
        SINGLE VALUE
        ::= at-routing-tree-list

RoutingTreeList ::= SEQUENCE OF RoutingTreeName


RoutingTreeName ::= CHOICE {
        named-tree DistinguishedName,                               10
        open-community NULL }

________________Figure_2:__Routing_Tree_Use_Definition_________________


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.


10.2  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.




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10.2.1  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.

10.2.2  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.
Thus, open routing will be tried first, and if this fails the message
will be routed to a single selected MTA.

10.2.3  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).
*** Editing note about ``MTASs'' I did not understand (SH-K)

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.

10.2.4  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 cannot be achieved
easily with current technology (e.g., DNS with MX records).





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 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.

10.2.5  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.

10.2.6  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:


 o  Address validation

 o  Advertising the mechanism for a bilateral link to be established

10.2.7  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)

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


11  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.___

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



subtreeInformation ATTRIBUTE
    WITH ATTRIBUTE-SYNTAX SubtreeInfo
    SINGLE VALUE                                                    20
    ::= at-subtree-information

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


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




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

String ::= CHOICE {PrintableString, TeletexString}                  40

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

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


mTAInfo ATTRIBUTE
    WITH ATTRIBUTE-SYNTAX MTAInfo
    ::= at-mta-info

MTAInfo ::= SEQUENCE {
            name DistinguishedName,
            weight [1] RouteWeight DEFAULT preferred-access,        60
            mta-attributes [2] SET OF Attribute OPTIONAL,
            ae-info SEQUENCE OF SEQUENCE {
                aEQualifier PrintableString,
                ae-weight RouteWeight DEFAULT preferred-access,
                ae-attribtes SET OF ATTRIBUTE OPTIONAL} OPTIONAL
}

RouteWeight ::= INTEGER  {endpoint(0),
                preferred-access(5),
                backup(10)} (0..20)                                 70


_______________Figure_3:__Routing_Information_at_a_Node________________

For example, information might be associated with the node:


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PRMD=CDC, ADMD=ATTmail, C=US

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:


 o  That the node corresponds a valid O/R address.  This is implicit
    in the existence of the entry.

 o  If the node is a UA. This will be true if the node is of object
    class routedUA. This is described further in Section 12.  If it is
    not of this object class, it is an intermediate node in the O/R
    Address hierarchy.

 o  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).  For
    example, consider the node:

    MHS-O=X-Tel, PRMD=UK.AC, ADMD=Gold 400, 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 X-Tel, which contains all of the
    children of this node, and is marked as authoritative.  When
    considering an O/R Address

    MHS-PN=Random.Unknown, MHS-O=X-Tel, PRMD=UK.AC, ADMD=Gold 400, C=GB


    only the second authoritative routing tree can be used to


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    determine that this address is invalid.

 o  A list of MTAs and associated information defined by the mtaInfo
    attribute.  This information is discussed further in Sections 16
    and 19.  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 19 and Section 11.1.

 o  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.  When an routing tree is being
    used, it is usually in the context of a sequence of routing trees.
    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 are:

    1.  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.  It is
        the default action if the attribute is not present.

    2.  Move to the next 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.

    3.  Move to the next tree, and then default back to the next level
        in this tree if no route is found.  This will be useful for an
        MTA to operate in the sequence:


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       (a)  Check for optimised private routes

       (b)  Try other available information

       (c)  Fall back to a local default route

 o  The accessMD attribute is discussed in Section 11.3.  This
    attribute is used to indicate MDs which provide indirect access to
    the part of the tree that is being routed to.

 o  The badAddressSearchPoint and badAddressSearchAttributes are
    discussed in Section 16.  This attribute is for when an address
    has been rejected, and allows information on alternative addresses
    to be found.

 o  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 11.2.

    11.1  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 are:

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

    --  The MTA manager should agree to it being placed there.

    --  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.  This
    attribute contains:



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    name The Distinguished Name of the MTA (Application Process)

    weight A weighting factor (Route Weight) which gives a basis to
        choose between different MTAs.

    mta-attributes Attributes from the MTAs entry.  Information on
        the MTA will generally 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
        can lead to considerable performance optimisation.  For
        example if ten MTAs were represented at a node, another MTA
        making a routing decision might need to make ten reads in
        order to obtain the information needed.  If any attributes are
        present here, all of the attributes needed to make a routing
        decision will be included, and also all attributes at the
        Application Entity level

    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 must be included.  A
    weight is associated with each application entity to allow the MTA
    to indicate a preference between its application entities.

    ae-qualifier A printable string (e.g.  ``x400-88'', used to
        derive the relative distinguished name of the application
        entity).

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

    ae-attributes Attributes from the AEs entry.

    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


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    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 should be random to
    facilitate load balancing.  If the MTA itself is in the list, it
    should 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:

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

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

    --  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 that are available in the MTA entry and will be needed
    for making a routing choice are:
    *** list attributes which must be present here if they are in the
    MTA ***
    A full list of MTA attributes, with summaries of their
    descriptions are given in Section 17.
    *** delete following list when referenced section is filled in ***

    --  Picking an MTA which is ``close'' in naming terms (e.g.,
        prefer an MTA in the same PRMD).

    --  Authentication policy of the MTA (e.g., some MTAs may require
        strong authentication).  This is discussed in Section 21.2.

    --  Information on the stack offered by the MTA. This is discussed
        in Section 19.1.

    --  Policy for the traffic an MTA will route.  This is discussed
        in Section 22.





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    11.2  Routing Filters

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

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

    --  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
    should be followed before other actions.  The routing filter
    overrides MTAinfo and accessMD attributes.  The components of the
    routingFilter attribute are:

    attribute-type This gives the attribute type to be matched, and
        is selected from the attribute types which have not been
        matched to identify the routing entry.  The filter applies to
        this attribute type.  If there is no regex present, 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.  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 should be made.

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

    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 defines where to get routing information for the
        filter.  It will be an entry with object class
        routingInformation, which can be used to determine the MTA or
        MTA choice.

    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.


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_______________________________________________________________________
accessMD ATTRIBUTE
        WITH ATTRIBUTE-SYNTAX distinguishedNameSyntax
        ::= at-access-md

______________________Figure_4:__Indirect_Access_______________________


     PRMD=UK.AC, ADMD=Gold 400, C=GB


    The entry associated would have two routing filters:


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

    2.  One with type x121 and a regex ^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.

11.3  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 should
read the entry pointed to by this distinguished name and route
according to the information retrieved, in order to route to this
access MD.
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 should 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, a single access MD is used as
a means of grouping the MTAs.  The value of the Distinguished Name of

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_______________________________________________________________________

routedUA OBJECT-CLASS
    SUBCLASS OF routingInformation
    MAY CONTAIN {
                        -- from X.402
        mhs-deliverable-content-length,
        mhs-deliverable-content-types,
        mhs-deliverable-eits,
        mhs-message-store,
        mhs-preferred-delivery-methods,                             10
                        -- defined here
        mandatoryRedirect,
        supportingMTA,
        filteredRedirect,
        uAName,
        nonDeliveryInfo}
    ::= oc-routed-ua

supportingMTA ATTRIBUTE
    SUBTYPE OF mtaInfo                                              20
    ::= at-supporting-mta

userName ATTRIBUTE
    WITH ATTRIBUTE-SYNTAX distinguishedNameSyntax
    ::= at-user-name

_______________________Figure_5:__UA_Attributes________________________


the access MD will probably not be meaningful in this case.


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



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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 identify the MTA which supports a UA. The MTAs which support a
    UA directly are noted in the SupportingMTA attribute, which may be
    multi-valued.  X.400 models that only one MTA is associated with a
    UA. In practice, it is possible and useful for several MTAs to be
    able to deliver to a single UA. This attribute is a subtype of
    MTAinfo, as it is possible for an MTA to be registered to route to
    a UA, without it actually being able to deliver messages to it.
    This attribute must be present, unless the address is being
    non-delivered or redirected.

5.  The attribute nonDeliveryInfo mandates non-delivery to this
    address, as described in Section 24.

6.  The attributes mandatoryRedirect and filteredRedirect control
    redirects, as described in Section 23.

7.  The attribute userName points to the distinguished Name of the
    user, as defined by the mhs-user in X.402.  **REF** 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.

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 (** REF X.500(92)).

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 these.  In this case, the entries should be in one
part of the DIT, with aliases from the other.


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Many sites will need to support local RFC 822 mailboxes (UAs).  As the
RFC 822 mailboxes are untyped, it is necessary for such a site to
represent every mailbox as an mHSPerson object.
To support UAs of other object classes, there will then need to be
aliases to the correct entry.  For example, suppose that there is a
UA:


MHS-CN=Postmaster, MHS-OU=CS, MHS-O=UCL,
PRMD=UK.AC, ADMD=Gold 400, C=GB

There will have to be an alias:


CN="/CN=Postmaster/", MHS-OU=CS, MHS-O=UCL,
PRMD=UK.AC, ADMD=Gold 400, C=GB

which points to the real entry.  For user convenience, it will also be
desirable to have an alias:


CN=Postmaster, MHS-OU=CS, MHS-O=UCL,
PRMD=UK.AC, ADMD=Gold 400, C=GB

If there is a mailbox for:


MHS-OU=Sales, MHS-OU=CS, MHS-O=UCL,
PRMD=UK.AC, ADMD=Gold 400, C=GB

There will need to be an alias:


CN="/OU=Sales/", MHS-OU=CS, MHS-O=UCL,
PRMD=UK.AC, ADMD=Gold 400, C=GB

In general, aliases can be used extensively at this level to provide
alternate values for names.  For routing purpose, UAs (Mailboxes) will
only be located by the read operation, not by searching.  This is for
efficiency of overall routing.

To support many UNIX mail utilities, it is important for UNIX sites to
alias the UNIX login ids as alternate values, and to have tools which
maintain this binding automatically.

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13  Direct Lookup

Where an O/R address is registered in the open community and has one
or more ``open'' MTAs which supports 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, 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.


14  Alternate Routes


14.1  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 should be 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 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.


14.2  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 should 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


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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 will occur when a message is sent to
multiple addresses in the group.


15  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:

Handle MTA Info Information from the node should be examined.  If
    suitable information is found, one of the following is done:

     o  Use the information and finish the routing process

     o  Continue the routing process in order to find more options to
        choose from

Unroutable Address Potentially valid address, which cannot be routed

Bad Address

Temporary Reject Try again later

Permanent Reject Administrative error on the directory which should
    be fixed, but it preventing routing.


***** Add info on what to do and which diagnostic codes
Each routing tree should be processed in turn.  To start a new routing
tree, the full O/R address should be looked up in the routing tree.
The next routing tree should be started when:


 o  An entry is reached with routing action next-tree-only or
    next-tree-first.

 o  A lookup is done in the routing tree at the top level (i.e.,
    country), and no routing action is returned.  In this case, it
    should behave as if the action was next-tree-first.

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Unless the action is next-tree-only, or the root of the routing tree
has been processed, the routing tree which is about to be left should
be pushed onto a stack of routing trees for future processing.  The
position in the tree should be retained.
Errors from the lookup (directory read) should be handled as follows:


AttributeError This leads to a permanent reject.

NameError The matched parameter is used to determine the number of
    components of the name that have matched (possibly zero).  The
    read is then repeated with this name.  This is the normal case,
    and allows the ``best'' entry in the routing tree to be located
    with two reads.

Referral The referral should be followed.

SecurityErrror Strip one component of the O/R address and continue.

ServiceError This leads to a temporary reject.

**** Clarify Popping


16  Naming MTAs

MTAs need to be named in the DIT, but the name does not have routing
significance, it 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, and MTA is
represented as an Application Process, with one or more Application
Entities.

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.  For X.400(88) it
is assumed that the Distinguished Name may 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


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_______________________________________________________________________
mTAName ATTRIBUTE
    WITH ATTRIBUTE-SYNTAX
        caseIgnoreIA5StringSyntax (SIZE(1..ub-mta-name-length)
    SINGLE VALUE
    ::= at-mta-name
                        -- used for naming when
                        -- MTA is named in O=R Address Hierarchy

globalDomainID ATTRIBUTE
    WITH ATTRIBUTE-SYNTAX                                           10
        GlobalDomainIdentifier
    SINGLE VALUE
    ::= 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
    MAY CONTAIN {                                                   20
        mTAWillRoute,
        routingTreeList,
        accessMD,
        localAccessUnit,
        accessUnitsUsed
    }
    ::= oc-mta-application-process

mTA OBJECT CLASS   -- Application Entity
    SUBCLASS OF mhs-message-transfer-agent                          30
    MAY CONTAIN {
        mTAName,
        globalDomainID,
        responderAuthenticationRequirements,
        initiatorAuthenticationRequirements,
        responderPullingAuthenticationRequirements,
        initiatorPullingAuthenticationRequirements,
        initiatorP1Mode,
        responderP1Mode,
        polledMTAs,                                                 40
        transportCommunity,
        respondingRTSCredentials,
        initiatingRTSCredentials,
        callingPresentationAddress,
        callingSelectorValidity,
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    ::= oc-mta


                                                                    50






______________________Figure_6:__MTA_Definitions_______________________




INTERNET--DRAFT        MHS Routing using Directory       November 1992


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 must 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.
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.


16.1  Naming 1984 MTAs

Some simplifications are necessary for 1984 MTAs, and only one naming
approach may be used.  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.


17  Attributes Associated with the MTA

This section lists the attributes which may be associated with an MTA
as defined in Figure 6, summarises their use, and gives pointers to
the relevant section.

*** tbs






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_______________________________________________________________________
mTABilateralTableEntry OBJECT-CLASS
    SUBCLASS OF mTA, distinguishedNameTableEntry
    ::= oc-mta-bilateral-table-entry

_________________Figure_7:__MTA_Bilateral_Table_Entry__________________


18  Bilateral Agreements


In many cases, X.400 routing has to be on the basis of bilateral
agreement.  We can now consider how these agreements are represented
in the Directory.  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.
**** Add note on where to find bilateral table.  Add attribute to
define this.
MTA bilateral information is stored in a table, as defined in [7].  An
MTA has one such table, which controls agreements in both directions.
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.

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 21.1, 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 21.3, will always be
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.

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_______________________________________________________________________
transportCommunity ATTRIBUTE
    WITH ATTRIBUTE SYNTAX objectIdentifierSyntax
    ::= at-transport-community


______________Figure_8:__Transport_Community_Definition________________


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.

For outbound connections, an attribute in the MTA's entry (either read
directly, or from mTAInfo from the routing tree) will indicate that a
bilateral connection should be used.  The entry containing the
bilateral information for the MTA can be derived as for an incoming
connection.
**** Tie in the use of RTS parameters, and add control of various
tyupes of trace stripping and orginator munging


19  MTA Selection

19.1  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 [6].  The set of transport communities
supported by an MTA is specified by use of the TransportCommunity
attribute defined in Figure 8.  This is represented as a separate
attribute for the convenience of making routing decisions.  It is
assumed that all X.400 protocols supported will be available over the
same set of communities.
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.





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19.2  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), an application context which defines
this protocol is 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.

19.3  MTA Capability Restrictions


In addition to policy restrictions, described in Section 22, an MTA
may have capability restrictions.  The maximum size of MPDU is defined
by the standard attribute mhs-deliverable-content-length.
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.


19.4  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 9.  This will allow for restrictions to be determined in cases


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_______________________________________________________________________
restrictedSubtree OBJECT-CLASS
        SUBCLASS OF top
        MAY CONTAIN {
                subtreeDeliverableContentLength,
                subtreeDeliverableContentTypes,
                subtreeDeliverableEITs }
        ::= oc-restricted-subtree

subtreeDeliverableContentLength ATTRIBUTE
        SUBTYPE OF mhs-deliverable-content-length                   10
        ::= at-subtree-deliverable-content-length

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

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

______________Figure_9:__Subtree_Capability_Restriction________________


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 shared attribute, so that it is
available to all entries in the subtree below the entry.


20  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


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_______________________________________________________________________
initiatorP1Mode ATTRIBUTE
    WITH ATTRIBUTE-SYNTAX P1Mode
    SINGLE VALUE
    ::= at-initiator-p1-mode

responderP1Mode ATTRIBUTE
    WITH ATTRIBUTE-SYNTAX P1Mode
    SINGLE VALUE
    ::= at-responder-p1-mode
                                                                    10
P1Mode ::= ENUMERATED {
    push-only(0),
    pull-only(1),
    twa(2) }

polledMTAs ATTRIBUTE
    WITH ATTRIBUTE-SYNTAX PolledMTAs
    := at-polled-mtas

PolledMTAs ::= SEQUENCE {                                           20
        mta DistinguishedName,
        poll-frequency INTEGER OPTIONAL --frequency in minutes
        }

_____________________Figure_10:__Pulling_Messages______________________


Attributes to support this are defined in Figure 10.  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.


21  Security and Policy


21.1  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 provided in the A-Associate.  For X.400(84), it will


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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.  Thus for X.400(1984) it will only be
possible to have bilateral inbound links or no authentication of the
calling MTA.

21.2  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 11.
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.


If an attribute is omitted, no checks are required.  If multiple
checks are required, then each of the relevant bits should be set.  If
there are alternative acceptable checks, then multiple values of the
attribute are used.
The values of the authentication requirements mean:


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

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

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

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_______________________________________________________________________
responderAuthenticationRequirements ATTRIBUTE
   WITH ATTRIBUTE-SYNTAX AuthenticationRequirements
   SINGLE VALUE
   ::= at-responder-authentication-requirements

initiatorAuthenticationRequirements ATTRIBUTE
   WITH ATTRIBUTE-SYNTAX AuthenticationRequirements
   SINGLE VALUE
   ::= at-initiator-authentication-requirements
                                                                    10
repsonderPullingAuthenticationRequirements ATTRIBUTE
   WITH ATTRIBUTE-SYNTAX AuthenticationRequirements
   SINGLE VALUE
   ::= at-responder-pulling-authentication-requirements

initiatorPullingAuthenticationRequirements ATTRIBUTE
   WITH ATTRIBUTE-SYNTAX AuthenticationRequirements
   SINGLE VALUE
   ::= 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_11:__Authentication_Requirements_________________








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    password must correspond to those registered in the directory.

network-address This can only be used for the responder.  The AET
    must be looked up in the directory, and the
    callingPresentationAddress attribute matched against the calling
    address.  This must 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 must 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 agreement.  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 22).  This is seen as too complex, and so
they are kept independent.  The equivalent function can always be
achieved by using multiple MTAs.

21.3  Authentication Information

This section specifies connection information needed by P1.  This is
essentially RTS parameterisation needed for authentication.  This is
defined in Figure 12.  Confidential bilateral information is implied
by these attributes, and this will be held in the bilateral
information agreement.  This should have appropriate access control
applied.  Note that in some cases, MTA information will be split
across a private and public entry.

The parameters are:

Initiating Credentials The credentials to be used when the local MTA
    initiates the association.  It gives the credentials to insert


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_______________________________________________________________________

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


initiatingRTSCredentials ATTRIBUTE
        WITH ATTRIBUTE-SYNTAX RTSCredentials
        SINGLE VALUE                                                10
        ::= 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
        WITH ATTRIBUTE-SYNTAX PresentationAddress
        MULTI VALUE
        ::= at-remote-presentation-address

callingSelectorValidity ATTRIBUTE                                   30
        WITH ATTRIBUTE-SYNTAX CallingSelectorValidity
        SINLGE VALUE
        ::= at-calling-selector-validity

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


______________Figure_12:__MTA_Authentication_Parameters________________



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_______________________________________________________________________
mTAWillRoute ATTRIBUTE
    WITH ATTRIBUTE-SYNTAX MTAWillRoute
    ::= 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_13:__Simple_MTA_Policy_Specification_______________


    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 should default to the
local MTA Name, and the password to an ASN.1 NULL.

Note: It may be useful to add more information here relating to
    parameters required for strong authentication.


22  Policy and Authorisation


22.1  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 13.

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

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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 should suffice for most cases.  In particular, it
gives sufficient information for most real situations where a policy
choice is forced.
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 should 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.


22.2  Complex MTA Policy

MTAs will generally have a much more complex policy mechanism, such as
that provided by PP [14].  Representing this as a part of the routing
decision does not seem worthwhile at present.  Some of the issues
which need to be tackled are:


 o  Use of charging and non-charging nets

 o  Policy dependent on message size


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 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.

Policies relating to submission do not need to be public.  They can be
private to the MTA.


23  Redirects

There is a need to specify redirects in the Directory.  This will be
done at the O/R Address level (i.e., in a routing tree).  This will be
useful for alternate names where an equivalent name (synonym) defined
by an alias is not natural.  The definitions are given in Figure 14.

Mandatory redirects are specified by the mandatoryRedirect attribute.
A filtered redirect is provided, to allow small messages, large
messages, or messages containing specific EITs or content to be
redirected.  Message size is measured in kBytes.
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 should be
followed, and the delivery report delivered to the redirected address.


24  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


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_______________________________________________________________________
mandatoryRedirect ATTRIBUTE
        WITH ATTRIBUTE-SYNTAX distinguishedNameSyntax
        SINGLE VALUE
        ::= at-mandatory-redirect

filteredRedirect ATTRIBUTE
        WITH ATTRIBUTE-SYNTAX FilteredRedirect
        ::= at-filtered-redirect

FilteredRedirect ::= SEQUENCE {                                     10
        redirect-to DistinguishedName,
        CHOICE {
                min-size [1] INTEGER,
                max-size [2] INTEGER,
                content [3] ContentType,
                eit [4] ExternalEncodedInformationType }
        }


                                                                    20

___________________Figure_14:__Redirect_Definition_____________________


Figure 15.


25  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 should be
provided.  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 16.  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


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_______________________________________________________________________
nonDeliveryInfo ATTRIBUTE
        WITH ATTRIBUTE SYNTAX NonDeliveryReason
        ::= at-non-delivery-info

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

_________________Figure_15:__Non_Delivery_Information__________________


    search on.  The default is common name.


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 AlternativeAddresssInformation
in the delivery report (DR). This should be marked non-critical, so
that it will not cause the DR to be discarded.  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
should also be returned using the supplementary delivery report (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.
If the directory search fails, or there are no matches returned, a
delivery report should 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 in future versions.


26  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.


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_______________________________________________________________________
badAddressSearchPoint ATTRIBUTE
        WITH ATTRIBUTE-SYNTAX distinguishedNameSyntax
        ::= at-bad-address-search-point

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

alternativeAddressInformation EXTENSION
        AlternativeAddressInformation                               10
        ::= id-alternative-address-information

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

___________________Figure_16:__Bad_Address_Pointers____________________


26.1  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 should
be selected at random.

26.2  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 this attribute should be
dereferenced, and the message submitted to each of the role occupants.
Similarly, if the entry is of object class group.

27  Access Units


Attributes needed for support of Access Units are defined in
Figure 17.
The attributes defined are:



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_______________________________________________________________________
localAccessUnit ATTRIBUTE
        WITH ATTRIBUTE-SYNTAX AccessUnitType
        ::= at-local-access-unit

AccessUnitType ::= ENUMERATED {
        fax (1),
        physical-delivery (2),
        teletex (3),
        telex (4) }
                                                                    10
accessUnitsUsed ATTRIBUTE
        WITH SYNTAX
        SelectedAccessUnit
        ::= at-access-units-used

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

__________________Figure_17:__Access_Unit_Attributes___________________


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 must 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: This mechanism might be used to replace the routefilter
        mechanism of the MTS routing.







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28  The Overall Routing Algorithm

Having provided all the pieces, a summary of how routing works can be
given.  The very top level of the routing algorithm is:


1.  Route the message according to the protocol by which it arrives
    (RFC 822 or X.400).  In the case of RFC 822, this should follow
    [10].

2.  If this fails, and gatewaying is supported, map the address and
    attempt to route according to the other protocol using [9].

28.1  X.400 Routing


The core of the X.400 routing is described in Section 11.  A sequence
of routing trees are followed.  As nodes of the routing tree are
matched, a set of MTAs will be passed upwards for evaluation.  If all
of these are rejected, the trees are followed further2.  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 must support a protocol that the
    current MTA supports3, as described in Section 19.2.

 o  The protocols must share a common transport community, as
    described in Section 19.1.

 o  There must be no capability restrictions in the MTA which prevents
    transfer of the current message, as described in Section 19.3.

 o  There must be no policy restrictions in the MTA which prevents
    transfer of the current message, as described in Section 22.

----------------------------
    2. 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.
    3. Note that this could be an RFC 822 protocol, as well as an
X.400 protocol.



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 o  The authentication requirements of the MTA must be met by the
    local MTA, as described in Section 21.2.

 o  If the authentication (Section 21.2) indicates that a bilateral
    agreement is present, the MTA must be listed in the local set of
    bilateral agreements, as described in Section 18.

 o  In cases where the recipient UA's capabilities can be determined,
    there should either be no mismatch, or there must be an ability to
    use local or remote reformatting capabilities, as described in
    **** ref conversion I-D

28.2  Pseudo Code


This section describes the routing algorithm in pseudo-code.
***** tbs


28.3  Examples

to be supplied


29  Performance

*** notes on the overall performance of the scheme, and notes on
implementation techniques to go faster


30  Acknowledgements

*** tbs


References

 [1] The Directory --- overview of concepts, models and services,
     December 1988. CCITT X.500 Series Recommendations.

 [2] J.N. Chiappa. A new IP routing and addressing architecture,
     1991. Internet Draft.



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INTERNET--DRAFT        MHS Routing using Directory       November 1992


 [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] U. Eppenberger. Routing coordination for an X.400 MHS service
     within a multi protocol environment, October 1991. Version 1.0,
     RARE WG1 Document.

 [6] S.E. Hardcastle-Kille. Encoding network addresses to support
     operation over non-OSI lower layers. Request for Comments
     RFC 1277, Department of Computer Science, University College
     London, November 1991.

 [7] S.E. Hardcastle-Kille. Representing the O/R Address hierarchy in
     the directory information tree, April 1992. Internet Draft.

 [8] S.E. Hardcastle-Kille. A string representation of distinguished
     name. Request for Comments in preparation, Department of
     Computer Science, University College London, January 1992.

 [9] S.E. Hardcastle-Kille. Use of the directory to support mapping
     between X.400 and RFC 822 addresses, April 1992. Internet Draft.

[10] S.E. Hardcastle-Kille. Use of the directory to support routing
     for RFC 822 and related protocols, April 1992. Internet Draft.

[11] K.E. Jordan. Using X.500 directory services in support of X.400
     routing and address mapping, November 1991. 3rd draft, IETF WG.

[12] 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.

[13] S.E. Kille. Topology and routing for MHS.  COSINE Specification
     Phase 7.7, RARE, 1988.

[14] S.E. Kille and J.P. Onions. The PP manual, December 1991.
     Version 6.0.

[15] P. Lauder, R.J. Kummerfeld, and A. Fekete. Hierarchical network
     routing. In Tricomm 91, 1991.

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INTERNET--DRAFT        MHS Routing using Directory       November 1992


[16] CCITT recommendations X.400 / ISO 10021, April 1988. CCITT
     SG 5/VII / ISO/IEC JTC1, Message Handling:  System and Service
     Overview.

[17] 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.


31  Security Considerations

Security considerations are not discussed in this INTERNET--DRAFT .


32  Author's Address

    Steve Hardcastle-Kille
    ISODE Consortium
    PO Box 505
    London
    SW11 1DX
    England


    Phone:  +44-71-223-4062

    EMail:  S.Kille@ISODE.COM


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

    UFN: S. Hardcastle-Kille, ISODE Consortium, GB












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A  Object Identifier Assignment


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

routing OBJECT IDENTIFIER ::= {mhs-ds 1}

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 5}
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-filtered-redirect OBJECT IDENTIFIER ::= {at 9}
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-mandatory-redirect OBJECT IDENTIFIER ::= {at 16}
at-mta-info OBJECT IDENTIFIER ::= {at 18}
at-mta-name OBJECT IDENTIFIER ::= {at 19}

at-mta-will-route OBJECT IDENTIFIER ::= {at 21}
at-remote-presentation-address OBJECT IDENTIFIER ::= {at 22}


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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}
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 36}
at-polled-mtas  OBJECT IDENTIFIER ::= {at 37}

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




_______________Figure_18:__Object_Identifier_Assignment________________























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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 + RFC 1006
tc-int-x25 OBJECT IDENTIFIER ::= {ts-communities 4}     -- International X.25
                                                        -- Without CONS
tc-ixi OBJECT IDENTIFIER ::= {ts-communities 5}         -- IXI (Europe)10
tc-janet OBJECT IDENTIFIER ::= {ts-communities 6}       -- Janet (UK)


____Figure_19:__Transport_Community_Object_Identifier_Assignments______



























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C  Protocol Identifier Assignments


_______________________________________________________________________
mail-protocol OBJECT-IDENTIFIER ::= {iso(1) org(3) dod(6) internet(1) private(4)
          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)


__________Figure_20:__Protocol_Object_Identifier_Assignments___________



D  ASN.1 Summary

To be supplied


E  Regular Expression Syntax

*** tbs.  Will be taken from ed(1) man page

*** Modified to be case insensitive and to handle leading and multiple
spaces according to X.400 matching rules

















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