Administrative Domains and Routing Domains: A model for routing in the Internet
RFC 1136

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Network Working Group                                           S. Hares
Request for Comments:  1136                                      D. Katz
                                                            Merit/NSFNET
                                                           December 1989

               Administrative Domains and Routing Domains
                  A Model for Routing in the Internet

1)  Status of this Memo

   This RFC proposes a model for describing routing within the Internet.
   The model is an adaptation of the "OSI Routeing Framework" [1].  This
   memo does not specify an Internet standard.  Comments are welcome.
   Distribution of this memo is unlimited.

2)  Acknowledgement

   The authors would like to thank Guy Almes of Rice University for his
   contributions and insight.

3)  Overview

   The "core" model of Autonomous Systems [2] formed the basis for the
   routing model used in the Internet.  Due to massive growth and
   topology changes, the "core" model no longer is in harmony with the
   reality of today's Internet.  Indeed, this situation was foreseen at
   the outset:

      "Ultimately, however, the internet may consist of a number of co-
      equal autonomous systems, any of which may be used...as a
      transport medium for traffic originating in any system and
      destined for any system.  When this more complex configuration
      comes into being, it will be inappropriate to regard any one
      autonomous system as a "core" system" [2].

   Furthermore, the Autonomous System concept has been outgrown in
   certain parts of the Internet, in which the complexity of regional
   routing has exceeded the limits of the definition of Autonomous
   Systems.

   A model which can provide a better match to the Internet can be found
   in the "OSI Routeing Framework" [1].

   This framework proposes a structure of Routing Domains within
   Administrative Domains.  This paper is intended to briefly describe
   this framework, to outline how this model better fits the reality of

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RFC 1136          A Model for Routing in the Internet      December 1989

   the present and future Internet, and to show how the model can aid in
   the construction of well-engineered routing environments.

4)  Terminology

   The following is a brief glossary of OSI terminology.  Formal
   definitions can be found in the OSI Basic Reference Model [4], the
   Internal Organization of the Network Layer [5], and the OSI Routeing
   Framework [1].

         "Routeing" is the official ISO spelling of what is more
         commonly spelled "routing."  In this paper, the ISO spelling
         will be used wherever directly quoted from ISO documents, and
         the common spelling used otherwise.

      End System (ES)

         An OSI system on which applications run.  An End System has
         full seven-layer OSI functionality.  Basically equivalent to an
         Internet Host.

      Intermediate System (IS)

         An OSI system that performs routing and relaying functions in
         order to provide paths between End Systems.  Intermediate
         Systems have no functionality above the Network Layer (although
         a practical realization of an OSI router will have some amount
         of End System functionality for network management functions,
         among other things).  Basically equivalent to an Internet
         Router.

      Subnetwork (SN)

         A communications medium that provides a "direct" path between
         Network Layer entities.  This can be realized via a point-to-
         point link, a LAN, a Public Data Network, and so forth.  This
         is essentially equivalent to an Internet Subnet.  It is worth
         noting that, unlike Internet Subnets, OSI Subnetworks are not
         necessarily reflected in the addressing hierarchy, so the
         double meaning of the Internet term "Subnet" (a single IP hop;
         a part of the address hierarchy) does not hold in the OSI
         world.

      Open Systems Interconnection Environment (OSIE)

         The global collection of Open Systems.  Basically equivalent to
         the Internet.

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RFC 1136          A Model for Routing in the Internet      December 1989

      Network Service Access Point (NSAP)

         A conceptual point on the Network/Transport Layer boundary in
         an End System that is globally addressable (and the address
         globally unambiguous) in the OSIE.  An NSAP represents a
         service available above the Network Layer (such as a choice of
         transport protocols).  An End System may have multiple NSAPs.
         An NSAP address is roughly equivalent to the Internet [address,
         protocol] pair.

      Administrative Domain (AD)

         "A collection of End Systems, Intermediate Systems, and
         subnetworks operated by a single organization or administrative