Premier IT Outsourcing and Support Services within the UK

User Tools

Site Tools


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.


 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

Houttuin [Page 1] RFC 1711 Classifications in E-mail Routing October 1994

 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 RFC 822
 domainpart or in X.400 an ADMD or OU attribute) and a local or user
 related part (e.g., the RFC 822 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.

Houttuin [Page 2] RFC 1711 Classifications in E-mail Routing October 1994

 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
         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.

Houttuin [Page 3] RFC 1711 Classifications in E-mail Routing October 1994

 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.

Houttuin [Page 4] RFC 1711 Classifications in E-mail Routing October 1994

 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 RFC 1465 [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
 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

Houttuin [Page 5] RFC 1711 Classifications in E-mail Routing October 1994

 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

Houttuin [Page 6] RFC 1711 Classifications in E-mail Routing October 1994

 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

Houttuin [Page 7] RFC 1711 Classifications in E-mail Routing October 1994

 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.

Houttuin [Page 8] RFC 1711 Classifications in E-mail Routing October 1994

 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

Houttuin [Page 9] RFC 1711 Classifications in E-mail Routing October 1994

 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

Houttuin [Page 10] RFC 1711 Classifications in E-mail Routing October 1994

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

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 wants to test the
 mail connections to and from a remote domain, he might
 source route a message to himself over the MTA at by
 addressing the mail to:
 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: RFC 1327 left hand side encoding (see [5]) performed by
 the gateway '':
      C=zz;A=a;P=p;O=oo;S=plork ->
 Example 2: RFC 1327 DDA mapping (see [5]) performed by the gateway
 C=zz;A=a; ->
 Example 3: the so-called %-hack:
 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

Houttuin [Page 11] RFC 1711 Classifications in E-mail Routing October 1994

 Example 4: Another example of the already mentioned explicit source
 routing, this time through two relays:
 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 RFC 822 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 RFC 822
 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:
 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.

Houttuin [Page 12] RFC 1711 Classifications in E-mail Routing October 1994

 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

Houttuin [Page 13] RFC 1711 Classifications in E-mail Routing October 1994

 another routing community, for instance to the Internet. The
 organisation will register an Internet domain, say '', which
 points to this gateway, and thus acts as a firewall from the Internet
 to the domain '', 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

 RFC 822 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

Houttuin [Page 14] RFC 1711 Classifications in E-mail Routing October 1994

 "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

10.2. UUCP

 RFC 822 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 RFC 1465
 [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

Houttuin [Page 15] RFC 1711 Classifications in E-mail Routing October 1994

 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
 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

Houttuin [Page 16] RFC 1711 Classifications in E-mail Routing October 1994

     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
  RFC               Internet 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",
                RFC 1465, SWITCH, May 1993.
    [3]         Kille, S., "MHS use of the Directory to support MHS
                routing", Work in Progress, July 1993.

Houttuin [Page 17] RFC 1711 Classifications in E-mail Routing October 1994

    [4]         Thomas, E., "Listserv Distribute Protocol",
                RFC 1429, Swedish University Network, February 1993.
    [5]         Kille, S., "Mapping between X.400(1988) / ISO 10021
                and RFC 822", RFC 1327, RARE RTR 2, University
                College London, May 1992.
    [6]         Braden, R., Editor, "Requirements for Internet Hosts
                - Application and Support", STD 3, RFC 1123, USC/
                Information Sciences Institute,  October 1989.
    [7]         Partridge, C., "Mail Routing and the Domain System",
                STD 14, RFC 974, 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", RFC 1664,
                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", RFC 1506, RTR 6, RARE Secretariat,
                August 1993.
    [11]        Postel, J., "Simple Mail Transfer Protocol", STD 10,
                RFC 821, USC/Information Sciences Institute, August
    [12]        Crocker, D., "Standard for the Format of ARPA
                Internet Text Messages", STD 11, RFC 822, 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.

Houttuin [Page 18] RFC 1711 Classifications in E-mail Routing October 1994

    [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

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

Houttuin [Page 19]

/data/webs/external/dokuwiki/data/pages/rfc/rfc1711.txt · Last modified: 1994/10/26 00:31 by

Donate Powered by PHP Valid HTML5 Valid CSS Driven by DokuWiki