RFC1711 - Classifications in E-mail Routing

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　　Network Working Group J. Houttuin
Request for Comments: 1711 RARE
Category: Informational October 1994

Classifications in E-mail Routing

Status of this Memo

This memo provides information for the Internet community. This memo
does not specify an Internet standard of any kind. Distribution of
this memo is unlimited.

Abstract

This paper presents a classification for e-mail routing issues. It
clearly defines commonly used terminology such as static routing,
store-and-forward routing, source routing and others. Real life
examples show which routing options are used in existing projects.

The goal is to define all terminology in one reference paper. This
will also help relatively new mail system managers to understand the
issues and make the right choices. The reader is expected to already
have a solid understanding of general networking terminology.

In this paper, the word Message Transfer Agent (MTA) is used to
describe a routing entity, which can be an X.400 MTA, a UNIX mailer,
or any other piece of software performing mail routing functions. An
MTA processes the so called envelope information of a message. The
term User Agent (UA) is used to describe a piece of software
performing user related mail functions. It processes the contents of
a message's envelope, i.e., the header fields and body parts.

Table of Contents

1. Naming, addressing and routing 2
2. Static versus dynamic 4
3. Direct versus indirect 5
3.1. Firewalls 5
3.2. Default versus rule based 6
4. Routing at user level 7
4.1. Distributed domains 7
4.2. Shared MTA 8
5. Routing control 9
6. Bulk routing 9
7. Source routing 11
8. Poor man's routing 12
9. Routing communities 12

10. Realisations 14
10.1. Internet mail 14
10.2. UUCP 15
10.3. EARN 15
10.4. GO-MHS 15
10.5. ADMD infrastructure 15
10.6. Long Bud 16
10.7. X42D 16
11. Conclusion 16
12. Abbreviations 17
13. References 17
14. Security Considerations 19
15. Author's Address 19

1. Naming, addressing and routing

A name uniquely identifies a network object (without loss of
generality, we will assume the 'object' is a person).

Once a person's name is known, it can be used as a key to determine
his address.

An address uniquely defines where the person is located. It can
normally be divided into a domain related part (e.g., the RFC822
domainpart or in X.400 an ADMD or OU attribute) and a local or user
related part (e.g., the RFC822 localpart or in X.400 a DDA or
Surname attribute). The domain related part of an address typically
consists of several components, which normally have a certain
hierarchical order. These domain levels can be used for routing
purposes, as we will see later.

Once a person's address is known, it can be used as a key to
determine a route to that person's location.

We will use the following definition of an e-mail route:

e-mail route a path between two leaves in a
directed Message Transfer System
(MTS) graph that a message travels
for one originator/recipient pair.
(see Figure 1)

Note that, in this definition, the User Agents (UAs) are not part of
the route themselves. Thus if a message is redirected at the UA
level, a new route is established from the redirecting UA to the UA
the message is redirected to.

The first and last leaves in a mail route are not always UAs. A route
may start from a UA, but stop at a certain point because one of the
MTAs is unable to take any further routing decisions. If this
happens, a warning is generated by the MTA (not by a UA), and sent
back to the originator of the undeliverable message. It may even
happen that none of the leaves is a UA, for instance if a warning
message as discussed above turns out to be undeliverable itself. The
cautious reader may have noticed that this is a dangerous situation.
Special precautions are needed to avoid loops in such cases (see
[1]).

user user
| ^
v |
+-----------------------------------------+
| | ^ |
| v | |
| +-----+ +-----+ |
| | UA | | UA | |
| +-----+ +-----+ |
| | ^ |
| v | |
| +-------------------------------------+ |
| | v ^ | |
| | v ^ | |
| | v ^ | |
| | +-----+ +-----+ | |
| | | MTA |.....................| MTA | | |
| | +-----+ +-----+ | |
| | v \ ^ | |
| | v \................ ^ | |
| | v \ ^ | | NB The actual route
| | +-----+ \ +-----+ | | is drawn as
| | | MTA |>>>>>>>>>>>>>>>>>>>>>| MTA | | | v ^
| | +-----+ +-----+ | | v ^
| | Message Transfer System | | v >>>>>>>> ^
| +-------------------------------------+ |
| Message Handling System |
+-----------------------------------------+

Figure 1. A mail route

It is important that the graph is directed, because routes are not
necessarily symmetric. A reply to a message may be sent over a
completely different mail route for reasons such as cost, non-
symmetric network connectivity, network load, etc.

According to the definition, if a message has two different
recipients, there will also be two mail routes. Since the delivery to
a UA (not the UA itself) is a part of the route, this definition is
still valid if two UAs are connected to the same MTA.

The words '.. for one originator-recipient pair.' in the definition
do not imply that this pair provides the MTA with all necessary
information to choose one specific route. One originator-recipient
pair may give an MTA the possibility to choose from a number of
possible routes, the so-called routing indicators (see chapter 2).

Other information (e.g., line load, cost, availability) can then be
used to choose one route from the routing indicators.

Routing is defined as the process of establishing routes. Note that
this is a distributed process; every intermediate MTA takes its own
routing decisions, thus contributing to the establishment of the
complete route.

Taking a routing decision is not a purely algorithmic process,
otherwise there would hardly be any difference between an address and
a route. The address is used as a key to find a route, typically in
some sort of rule-based routing database. The possible options for
realising this database and algorithms for using it are the subject
of the rest of this paper.

2. Static versus dynamic

Dynamic (mail) routing allows a routing decision to be influenced by
external factors, such as system availability, network load, etc. In
contrast, static (mail) routing is not able to adapt to environmental
constraints. Static routing can be viewed as an extremely simple form
of dynamic routing, namely where there is only one choice for every
routing decision.

Dynamic routing algorithms normally use some kind of distributed
databases to store and retrieve routing information, whereas static
routing is typically implemented in routing tables.

Note that dynamic routing can occur at different layers: at the mail
level, dynamic routing might allow a message to be relayed to a
choice of MTAs (the routing indicators). As an example, consider the
Internet mechanism of using multiple Mail eXchanger (MX) records,
describing MTAs that can serve a domain. If the primary choice MTA is
not available, a second choice MTA can be tried. If this second
choice MTA is busy, a third one will be tried, etc. On lower layers,
there may be more than one presentation address for one MTA, each of
which can again have an associated priority and other attributes.

These choices may represent that an MTA prefers to be connected to
using one certain stack, e.g., RFC1006/TCP/IP, but is also able to
accept incoming calls over another stack, such as TP0/CONS/X.25. We
will call this dynamic stack routing. Theoretically, dynamic stack
routing should be transparent to the mail routing application, and is
thus not a part of dynamic mail routing. It is mentioned here because
in existing products, dynamic stack routing is often very well
visible at the mail configuration level, so MTA managers should at
least be aware of it.

Although static routing is often table based, not all table based
routing algorithms are necessarily static in nature. As a counter
example, X.400 routing according to RFC1465 [2] is clearly table
based, but at the same time allows a fairly dynamic kind of mail
routing (as well as dynamic stack routing, which in this approach is
cleanly separated from the dynamic mail routing part). A mail domain
can specify a choice of so-called RELAY-MTAs (formerly called WEPs)
that will serve it, each with a priority and maximum number of
retries.

For reasons of flexibility and reliability, dynamic routing is almost
always the preferred method.

3. Direct versus indirect

Direct routing allows the originator's MTA to contact the recipient's
MTA directly, whereas indirect routing (also known as store-and-
forward routing) uses intermediate MTAs to relay the message towards
the recipient. It is difficult to clearly distinguish between direct
and indirect routing: direct routing assumes the existence of a fully
meshed routing topology, whereas indirect routing assumes the
existence of a more tree-like hierarchical topology. Mail routing in
most existing networks is upto some degree indirect. Networks can be
classified as being more or less direct according for the following
rule of thumb: larger fan out of the routing tree means more direct
routing, greater depth of the tree means less direct routing. Two
kinds of indirect routing are presented here: firewalls (downstream)
and default routes (upstream).

3.1. Firewalls

A firewall 'attracts' all messages for a certain set of addresses
(the address sub space behind the firewall) from the outside e-mail
world to a central relaying MTA (the firewall). This is done by
publishing routes to all other MTAs that must relay their messages
over this firewall (the attracted community). Note that local
knowledge should be used to route messages within the address space
behind the firewall. An example for this is presented later on. There

exist many reasons for using firewalls, e.g., security considerations
or to concentrate the management for a given domain onto one well
managed system.

The Internet mail system would allow all mail hosts connected to the
Internet to directly accept mail from any other host, but not all
hosts use this possibility. Many domains are hidden behind one or
more 'Mail eXchanger' (MX), which offer to relay all incoming mail
for those domains. The RELAY-MTAs mentioned earlier can also be
considered firewall systems.

+-----------------------------------+
| |
| The rest of the e-mail world |
| |
+-----------------------------------+
\ | | /
\ | | /
\| | /
v vv
+--------------+
|Firewall MTA A|
+--------------+
^ / ^ \ ^
/ / | \ \
/ / | \ \
Default route--o / | \ o---Default route
/ / | \ \
/ / | \ \
/ v v v \
+-----+ +-----+ +-------+
|MTA B|<----|MTA C| |MTA D |
+-----+ +-----+ +-------+
/ | | | \
/ | | | \
/ | | | \
+----+ +----+ +----+ +----+ +----+
| UA | | UA | | UA | | UA | | UA |
+----+ +----+ +----+ +----+ +----+

Figure 2. Firewall and default route

3.2. Default versus rule based

Default routing is to outgoing mail what a firewall is for incoming
mail, and is thus often used in conjunction with firewalls. It is
about the simplest routing algorithm one can think of: route every
message to one and the same MTA, which is trusted to take further

care of routing the message towards its destination. Pure default
routing is rather useless; it is normally used as a fall back
mechanism accompanying a rule based algorithm.

For example, the simplest usable default algorithm is the following:
first check if a mail should be delivered to a local UA. If not,
perform default routing.

In order to avoid loops, it is not acceptable for all MTAs within a
certain routing community (see chapter 9) to use default routing. At
least one MTA should be able to access all routing rules for that
community. Consider the following example: An X.400 MTA A, which
serves the organisation organisational unit OU=orgunA within the
organisation O=org, receives a message for the domain O=org;
OU=orgunB;. Since MTA B in the same organisation serves all other
OUs, A will default route the message to B. Suppose that B would use
the same mechanism: first check if the OU is local and if not,
default route to A. If OU=orgunC is not served by either A or B, this
routing set-up will lead to a loop. The decision that a certain OU
does not exist can only be made if at least one of the MTAs has
knowledge of all existing OUs under O.

An example of a firewall and two default routes is shown in figure 2.
It visualises that a firewall is a downstream and a default route is
an upstream indirection. MTA B and D use default routing; they can
only route to one other MTA, MTA A.

For more detailed information, please refer to [3], which lists most
pros and cons of both approaches. Your choice will depend on many
factors that are specific for your messaging environment.

4. Routing at user level

Normally a message is routed down to the deepest level domain, and
then delivered to the recipient per default routing. I.e., every user
in this domain is considered to have his mailbox uniquely defined
within this domain on the same MTA, and every user on that MTA can be
distinguished within this domain. Exceptions can occur when the users
within a domain have their mailboxes on different MTAs (distributed
domain), or when several domains exist on the same MTA (shared MTA).

4.1. Distributed domains

Routing is normally performed down to a certain domain level. Mail to
all users that are directly registered under this domain is then
delivered per default routing, i.e., delivered locally. Explicit user
routing (i.e., rule-based routing on user level attributes according
to a fixed table listing all users) may be necessary when not all

users have their UAs connected to the same MTA.

Note that the whole issue of distributed domains is nothing more than
a special case of the problems discussed in chapter 3.2: 'Default
versus rule-based'. The only reason for mentioning this in a separate
chapter is that there are many software products that don't deal with
routing based on local address parts in the same way as with routing
based on domain related address parts.

As an example, consider an organisation where two mail platforms are
available. Some users prefer using platform A, others prefer platform
B. Of course, the easiest solution would be to create a subdomain A
and a subdomain B, and then route domain A to system A and B to B.
Default user routing on both platforms would then do the rest.
However, when an organisation wants to present itself to the outside
world using only one domain, this scheme cannot be used, at least not
without special precautions (see the paragraph about avoiding loops
in chapter 3.2).

+----------+ +---------------------------+
| MTA A | | Shared MTA B |
+----------+ +---------------------------+
| | / | | |
+-----------------/----+ +-----------+ +----------+
| | | / | | | | | | | |
| +--+ +--+ +--+/ | | +--+ +--+ | | +--+ |
| |UA| |UA| |UA| | | |UA| |UA| | | |UA| |
| +--+ +--+ +--+ | | +--+ +--+ | | +--+ |
| Distributed Domain A | | Domain B | | Domain C |
+----------------------+ +-----------+ +----------+

Figure 3. Distributed domains and shared MTAs

Another possibility to have uniform addresses in outgoing e-mail,
despite the fact that a domain is distributed, is to make routing
decisions on information in the local part of the address, e.g., in
X.400 the Surname in exactly the same manner as making routing
decisions on any other attributes. Thus products and routing
algorithms that are able to route on user related address parts are
said to support distributed domains.

4.2. Shared MTA

The opposite of a distributed domain is a shared MTA: several domains
are routed locally on the same MTA. These domains sharing one MTA may
cause problems when two or more domains have a local user with the
same name.

Theoretically, this problem doesn't exist: the address is being
routed down to the deepest domain level, and within that level, there
will only be one user with that name (let's at least assume this for
simplicity). Some products however use only one database of all users
locally connected to this MTA instead of one database per domain, so
that default user routing at the deepest level can lead to conflicts.
It is beyond the scope of this document to describe the tricks needed
to avoid these conflicts when using such products.

5. Routing control

Routing control means that routing decisions can be affected by the
originator of a message. This normally takes the form of either
granting or denying access for a certain user or group of users.

Routing control is often useful in an X.400 ADMD/PRMD environment,
where it is either used to grant access only to users who are known
to be chargeable, or where ADMDs can refuse messages that were
relayed to them over international PRMD connections; a policy that is
not allowed in the CCITT version of the standards (as opposed to the
ISO version). Of course, the PRMDs can also perform routing control
themselves in order to circumvent such problems.

Although there may be good reasons for using routing control, one
must be aware that it can make the messaging environment
unpredictable for end-users. Where using routing control is
unavoidable, the originator whose message has been rejected is likely
to appreciate receiving a message, clearly telling him where and why
routing of his message was refused, whom to contact, and what options
are available to avoid such rejections in the future.

6. Bulk routing

In order to reduce network traffic, intelligent mailers may prefer a
message addressed to a group of remote users to be transferred to a
remote domain only once, thus postponing the 'explosion' into several
copies. This technique, called bulk routing, is especially useful
when an MTA hosts large mailing lists.

Several possibilities exist. In a typical hierarchical firewall mail
system, bulk routing can be done almost automatically by intelligent
MTAs. For instance, in an X.400 community, a large international
distribution list can create a message with an envelope containing
1000 recipient addresses, some of which can probably be grouped by
the MTA depending on whether they can be routed further to the same
remote MTA, according to the normal routing implementation at this
MTA. The size and number of these groups will largely depend on how
indirect this routing implementation is. In the GO-MHS community, the

number of groups will almost always be less than 50, which provides a
rather fair distribution of traffic load over the involved MTAs (that
is, fair according to the author's taste, who is not aware of any
existing fair mail load distribution formula).

As an extreme example, the simplest way to automatically (i.e.,
without using special optimisation tools) bulk route mail is to use
one default route. Any outgoing message, regardless of the number of
recipients, will be routed over the default route only once. The
default remote MTA will then have to break up the message (envelope)
into several copies and is thus responsible for the actual explosion
and distribution. NB. This mechanism can be exploited to shift the
cost and overhead of exploding a message towards another domain/MTA.
If you ever get a request for a bilateral default route agreement;
i.e., the requesting party wants to default route over your MTA, it
may be worth to check first if the requesting party is running or
planning to run large mailing lists.

In more direct routing environments, such as BITNET, bulk routing
will not function as automatically as described above. Without
special precautions, an MTA would open a direct connection to every
single host that occurs in the message's envelope, regardless of
whether some of these hosts are far away from this MTA, but close to
each other, measured by underlying network topology. This can clearly
lead to a waste of expensive bandwidth. In order to be able to detect
such cases, and to act upon it by sending one single copy over an
expensive link and have it distributed at some remote hosts, an MTA
must have additional knowledge of the relation between mail domains
and the underlying network topology.

BITNET uses the distribute protocol [4] for this purpose. A selected
set of hosts is published to have the required topology knowledge and
to be able to efficiently distribute the mail on behalf of other
MTAs, who can explicitly route all bulk mail to one of those hosts.
The complete message, including the envelope, is encoded in a message
body, which starts with a distribution request to the distribute
server. This server will break up the rest of the body into the
original envelope and contents and then use it's topology knowledge
to efficiently distribute the original message. Note that this
protocol violates the conceptual model of the layering of MTA and UA
functionality, but it is about the only trick that will work in a
very direct routing environment. It is only needed to overrule a non-
efficient (for large mailing lists) routing topology.

Bulk routing is an area where mail routing issues start to overlap
with the area of distributing netnews (bulletin board services).
Several organisations, such as ISO, RARE and the IETF have started
initiatives in the direction of harmonising the two worlds. The first

results, be it standards or products, are not expected before 1995
though.

7. Source routing

Source routing was originally intended to allow a user to force a
message to take a certain route. The mechanism works as follows: the
MTA that the user wants the message to be routed through is
integrated into the address. Once the message has arrived at the
specified MTA, that MTA strips itself from the source-routed address
and routes the remaining address in the usual way. This mechanism is
called explicit source routing and can be useful if a user wants to
test a routing path or force a message to be routed over a faster,
cheaper, more reliable, or otherwise preferred path.

For instance, if the Internet user user@uni-a.edu wants to test the
mail connections to and from a remote domain uni-b.edu, he might
source route a message to himself over the MTA at uni-b.edu by
addressing the mail to: @uni-b.edu:user@uni-a.edu

Source routing need not always be explicit. Source routes can also be
generated automatically by a gateway, in which case we speak of
address rooting (to that gateway). The gateway will root itself to
the message by putting its own domain in the source route mapped
address, thus ensuring that any replies to the gatewayed message will
be routed back through the same gateway.

Example 1: RFC1327 left hand side encoding (see [5]) performed by
the gateway 'gw.ch':

C=zz;A=a;P=p;O=oo;S=plork ->
"/C=zz/A=a/P=p/O=oo/S=plork/"@gw.ch

Example 2: RFC1327 DDA mapping (see [5]) performed by the gateway
C=zz;A=a;

bush@dole.us ->
DD.RFC-822=bush(a)dole.us;C=zz;A=a;

Example 3: the so-called %-hack:

user%final.domain@1st.relay

When the relaying host '1st.relay' receives the message, it strips
its own domain part and interprets the localpart 'user%final.domain':
it changes the % to an @ sign and relays the message to the address

user@final.domain

Example 4: Another example of the already mentioned explicit source
routing, this time through two relays:

@1st.relay,@2nd.relay:user@final.domain

In the Internet, use of explicit source routing is strongly
discouraged (see [6]), one reason being that not all mail relays will
handle such addresses in a consistent manner. Apart from that, the
need to use explicit source routing has disappeared over the last
decennia. In earlier days, when the RFC822 world consisted of many
sparsely connected 'mail islands', source routing was sometimes
needed to make sure that a message was routed through a gateway that
was known to be connected to a remote island. Nowadays, the RFC822
world is almost fully interconnected through the Internet, so the
need for end-users to have knowledge of the mail network's topology
has become superfluous.

8. Poor man's routing

If we combine static, indirect and source routing, we get what is
commonly known as "poor man's routing". The user thus specifies the
complete route in the address. A classic example is the old UUCP bang
style addressing:

host1!host2!host3!host4!user

Poor man's routing is presented here for historical reasons only.
Since, for reasons discussed earlier, most present networks
discourage source routing and prefer dynamic over static routing,
poor man's routing is not widely deployed anymore.

9. Routing communities

A routing community can be defined as follows:

Routing community: a set of MTAs X, with the property
that for any address a, every MTA
in X except a subset Ya will have
the option, according to an agreed
upon set of routing rules, to
directly route that address to at
least one MTA in Ya.

Which is a rather formal way of describing that a routing community
consists of a set of MTAs (and human operators) that agreed on a
common set of rules on how to route messages among each other.

An example of a routing community is the large Internet routing
community, in which the agreed rules are implemented in the Domain
Name System (DNS). For details, refer to [7]. The subset Ya is in
this case the set of MTAs that have an MX record in the DNS for a.
MTAs that hide behind fire walls or behind default routes are thus
not considered direct members of this community, but normally form
their own smaller routing community, with one host (the mail
exchanger/default route) belonging to both communities.

Another example is the GO-MHS community, consisting of a set of
documented RELAY-MTAs (formerly called WEPs, Well-known Entry
Points). Routing communities can be further classified depending on
the openness and topology of their routing rules. [3] defines four
classes of routing communities:

Local community: The scope of a single MTA. Contains
the MTAs view of the set of
bilateral routing agreements, and
routing information local to the
MTA. Example: any local MTA.

Closed community: This is like a local community, but
involves more than one MTA. The
idea is to route messages only
within this closed community. A
small subset of the involved MTAs
can be in another community as
well, in order to get the
connectivity to the outside world,
as described earlier. Example: A
set of Private Management Domains
(PRMDs) representing the same
organisation in multiple countries.

Open community: All routing information is public
and any MTA is invited to use it.
Example: the Internet.

Hierarchical community:A subtree of the O/R address tree.
Note that the subtree will in
practice often be pruned; sub-sub-
trees may form their own routing
community. Example: GO-MHS.

This classification cannot always be followed too strictly. For
example, completely closed communities are relatively rare. In order
for e-mail to be an effective communication tool, an organisation
will typically designate at least one of its MTAs as a gateway to

another routing community, for instance to the Internet. The
organisation will register an Internet domain, say 'org.net', which
points to this gateway, and thus acts as a firewall from the Internet
to the domain 'org.net', and as a default route from the closed
community to the rest of the Internet. At this stage, the gateway MTA
can be regarded as being a member of any of the four types of routing
communities. The reader is invited to check this himself.

Especially the distinction between open and closed communities is not
always easy. To some extent, most routing communities are open, at
least among their own participants. It is just that some routing
communities are more open than others. Also, even the most open
routing community is not just open to anyone. It is not enough for a
community participant to use the community's routing rules and
connect to any other MTA in the community. The participant will
typically also have to fulfil an agreed upon set of operational
requirements, for example the Internet host requirements [6] or the
GO-MHS domain requirements [8].

The most open routing community known today is certainly the Internet
mail community. As for X.400 routing communities, some problems occur
when trying to open a community, the main one being that most X.400
software does not support the so called 'anonymous' connection mode,
which allows any remote MTA to connect to it. Most software was
designed or configured to use passwords for setting up MTA
connections. This, together with the fact that X.400 routing was
originally designed to be hierarchical, is one of the main reasons
why most X.400 communities today are either closed or hierarchical.

10. Realisations

In this chapter some of the routing classifications described above
are assigned to existing mail services and projects.

10.1. Internet mail

RFC822 mail. An operational service. Co-ordination: distributed.
Mostly dynamic routing, although static routing is also possible. DNS
based routing rules(*). Mostly direct routing, although indirect is
also possible. No dynamic stack routing. Distributed domains
possible. Shared MTAs possible, but rare. Routing control not
normally used. Bulk routing via SMTP envelope grouping; also
possible, but not widely deployed, using the 'distribute protocol'
[4]. Source routing supported, but strongly discouraged. No poor
man's routing. Open (and hierarchical) routing community.

(*) Sub-communities don't use DNS based routing: The MX records in
the DNS are used to "attract" messages from the Internet to the

"border" between the Internet and the sub-community. Thus from the
Internet we have dynamic, directory based routing but once the
"border" is reached, it is no longer possible to use MX records for
mail routing, and thus some form of static routing is generally
needed.

10.2. UUCP

RFC822 style mail. An operational service. Co-ordination:
distributed. Mostly static routing, although dynamic routing is also
possible. Table based routing rules. Mostly indirect routing. No
dynamic stack routing. No distributed domains. Shared MTAs possible,
but rare. Routing control not normally used. No bulk routing
possible. Source routing (poor man's routing) still widely used by
means of 'bang' addressing, but strongly discouraged. Open (and
hierarchical) routing community.

10.3. EARN

BITNET mail. An operational service. Co-ordination: The EARN Office,
France. Static routing. Table based routing rules, although an X.500
based experiment is running. Mostly direct routing, although indirect
is also possible. No dynamic stack routing. No distributed domains.
No shared MTAs. Routing control not normally used. Bulk routing
possible using the 'distribute protocol' [4]. Source routing not
supported. No poor man's routing. Open routing community.

10.4. GO-MHS

X.400 mail. An operational service. Co-ordination: GO-MHS Project
Team, Switzerland. Mostly static routing, although dynamic routing is
getting more and more deployed since the introduction of RFC1465
[2]. Table based community-wide routing rules. Indirect routing.

Dynamic stack routing. Distributed domains possible. Shared MTAs.
Routing control not normally used, only to avoid routing control
problems when routing international traffic to ADMDs. Bulk routing
using X.400 'responsibility' envelope flags. Source routing supported
for gatewaying to the Internet. No poor man's routing. Hierarchical,
but open, routing community.

10.5. ADMD infrastructure

X.400 mail. An operational service. Co-ordination: The joint
Administrative Management Domains (ADMDs), typically operated by
PTTs. Mostly static routing. Indirect routing. Table based bilateral
routing rules. No dynamic stack routing. Distributed domains not
supported. Shared MTAs. Routing control used to prohibit routing of

international traffic through PRMDs and to limit access to certain
gateways. Bulk routing using X.400 'responsibility' envelope flags.
Source routing possible for gatewaying to the Internet. No poor man's
routing. Closed hierarchical routing community.

10.6. Long Bud

X.400 mail. A pilot project. Co-ordination: The IETF MHS-DS working
group. Dynamic routing. X.500 based routing rules. Mostly indirect
routing, although direct is also possible. Dynamic stack routing.
Distributed domains. Shared MTAs. No routing control. Bulk routing
using X.400 'responsibility' envelope flags. Source routing supported
for gatewaying to the Internet. No poor man's routing. Open
hierarchical routing community.

10.7. X42D

X.400 mail. An experiment. Co-ordination: INFN, Italy. Dynamic
routing. DNS based routing rules as defined in [9]. Mostly indirect
routing, although direct is also possible. Dynamic stack routing. No
distributed domains. Shared MTAs. No routing control. Bulk routing
using X.400 'responsibility' envelope flags. Source routing supported
for gatewaying to the Internet. No poor man's routing. Open
hierarchical routing community.

11. Conclusion

We have seen several dimensions in which mail routing can be
classified. There are many more issues that were not discussed here,
such as how exactly the routing databases are implemented, which
algorithms to use for making the actual choices in dynamic routing,
etc. A follow-up paper is planned to discuss such aspects in more
detail.

So far, the author has tried to keep this paper free of opinion, but
he would like to conclude by listing his own favourite routing
options (without any further explanation or justification; please
feel free to disagree):

Static/dynamic: Dynamic
Direct/indirect: Every routing community has its own
optimum level of indirection
User routing: Support
Routing control: Avoid
Bulk routing: Efficient distribution should be
transparent at mail level, but we
may need better e-mail models
before this becomes possible

Source routing: Avoid where possible
Poor man's routing: Avoid

12. Abbreviations

ADMD Administration Management Domain
CCITT Comite Consultatif International de
Telegraphique et Telephonique
CONS Connection Oriented Network Service
DDA Domain Defined Attribute
DNS Domain Name System
GO-MHS Global Open MHS
IP Internet Protocol
ISO International Organisation for Standardisation
Long Bud Not an abbreviation
MHS Message Handling System
MHS-DS MHS and Directory Services
MTA Message Transfer Agent
MTS Message Transfer System
MX Mail eXchanger
O/R address Originator/Recipient address
PP Not an abbreviation
PRMD Private Management Domain
RARE Reseaux Associes pour la Recherche Europeenne
RFCInternet Request for Comments
RTR RARE Technical Report
SMTP Simple Mail Transfer Protocol
STD Internet Standard RFC
TCP Transfer Control Protocol
TP0 Transport Protocol Class 0
UA User Agent
UUCP UNIX to UNIX CoPy
WEP Well-known Entry Point

13. References

[1] Houttuin, J., "C-BoMBS : A Classification of Breeds
Of Mail Based Servers", RARE WG-MSG Work in Progress,
April 1994.

[2] Eppenberger, E., "Routing Coordination for X.400 MHS
Services Within a Multi Protocol / Multi Network
Environment Table Format V3 for Static Routing",
RFC1465, SWITCH, May 1993.

[3] Kille, S., "MHS use of the Directory to support MHS
routing", Work in Progress, July 1993.

[4] Thomas, E., "Listserv Distribute Protocol",
RFC1429, Swedish University Network, February 1993.

[5] Kille, S., "Mapping between X.400(1988) / ISO 10021
and RFC822", RFC1327, RARE RTR 2, University
College London, May 1992.

[6] Braden, R., Editor, "Requirements for Internet Hosts
- Application and Support", STD 3, RFC1123, USC/
Information Sciences Institute, October 1989.

[7] Partridge, C., "Mail Routing and the Domain System",
STD 14, RFC974, BBN, January 1986.

[8] Hansen, A. and R. Hagens, "Operational Requirements
for X.400 Management Domains in the GO-MHS
Community", Work in Progress, March 1993.

[9] Allocchio, C., Bonito, A., Cole, B., Giordano, S.,
and R. Hagens "Using the Internet DNS to Distribute
RFC1327 Mail Address Mapping Tables", RFC1664,
GARR-Italy, Cisco Systems Inc, Centro Svizzero
Calcolo Scientific, Advanced Network & Services,
February 1993.

[10] Houttuin, J., "A Tutorial on Gatewaying between X.400
and Internet Mail", RFC1506, RTR 6, RARE Secretariat,
August 1993.

[11] Postel, J., "Simple Mail Transfer Protocol", STD 10,
RFC821, USC/Information Sciences Institute, August
1982.

[12] Crocker, D., "Standard for the Format of ARPA
Internet Text Messages", STD 11, RFC822, UDEL,
August 1982.

[13] Alvestrand, H.T., et al, "Introducing Project Long
Bud Internet Pilot Project for the Deployment of
X.500 Directory Information in Support of X.400
Routing", Work in Progress, June 1993.

[14] Kille, S., "A Simple Profile for MHS use of
Directory", Work in Progress, July 1993.

[15] Kille, S., "MHS use of the Directory to Support
Distribution Lists", Work in Progress, November 1992.

[16] Eppenberger, U., "X.500 directory service usage for
X.400 e-mail", Computer Networks for Research in
Europe No.1: Computer Networks and ISDN Systems 25,
Suppl.1 (1993) S3-8, September 1993.

[17] CCITT Recommendations X.400 - X.430. Data
Communication Networks: Message Handling Systems.
CCITT Red Book, Vol. VIII - Fasc. VIII.7, Malaga-
Torremolinos 1984.

[18] CCITT Recommendations X.400 - X.420. Data
Communication Networks: Message Handling Systems.
CCITT Blue Book, Vol. VIII - Fasc. VIII.7, Melbourne
1988.

14. Security Considerations

Security issues are discussed in section 3.1.

15. Author's Address

Jeroen Houttuin
RARE Secretariat
Singel 466-468
NL-1017 AW Amsterdam
The Netherlands

Phone: +31 20 639 11 31
Fax: +31 20 639 32 89
EMail: houttuin@rare.nl

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