Classifications in E-mail Routing
RFC 1711

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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

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.