1. RFC 1136
Network Working Group                                           S. Hares
Request for Comments:  1136                                      D. Katz
                                                           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

   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

Hares & Katz                                                    [Page 1]
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

      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

      Open Systems Interconnection Environment (OSIE)

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

Hares & Katz                                                    [Page 2]
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
         authority.  The components which make up the domain are assumed
         to interoperate with a significant degree of mutual trust among
         themselves, but interoperate with other Administrative Domains
         in a mutually suspicious manner" [1].

         A group of hosts, routers, and networks operated and managed by
         a single organization.  Routing within an Administrative Domain
         is based on a consistent technical plan.  An Administrative
         Domain is viewed from the outside, for purposes of routing, as
         a cohesive entity, of which the internal structure is
         unimportant.  Information passed by other Administrative
         Domains is trusted less than information from one's own
         Administrative Domain.

         Administrative Domains can be organized into a loose hierarchy
         that reflects the availability and authoritativeness of routing
         information.  This hierarchy does not imply administrative
         containment, nor does it imply a strict tree topology.

      Routing Domain (RD)

         "A set of End Systems and Intermediate Systems which operate
         according to the same routeing procedures and which is wholly
         contained within a single Administrative Domain" [1].

         "A Routeing Domain is a set of ISs and ESs bound by a common
         routeing procedure; namely:

         they are using the same set of routeing metrics,

         they use compatible metric measurement techniques,

         they use the same information distribution protocol, and

Hares & Katz                                                    [Page 3]
RFC 1136          A Model for Routing in the Internet      December 1989

         they use the same path computation algorithm" [1].

         The "OSI Routeing Framework" further provides a formal
         definition of a Routing Domain, specifying that all ISs within
         a Routing Domain can determine whether an ES within the domain
         is reachable, and if so can derive a path to it.

         Routing Domains may be divided into subdomains, not unlike
         subnetting in the Internet.  This allows a hierarchical
         structuring of the domain, permitting containment of the
         topological details of a subdomain with the resultant reduction
         in distributed routing information.

         An intra-Routing Domain routing protocol is equivalent to an
         Internet Interior Gateway Protocol (IGP).

         An Administrative Domain may contain multiple Routing Domains.
         A Routing Domain may never span multiple Administrative

         An Administrative Domain may consist of only a single Routing
         Domain, in which case they are said to be Congruent.  A
         congruent Administrative Domain and Routing Domain is analogous
         to an Internet Autonomous System.

      Common Domain (CD)

         "An Administrative Domain which is not a member of a higher
         level domain.  A common domain is the highest level in the
         routeing hierarchy.  There is no single domain above the common
         domain.  In this sense, the routeing hierarchy is in fact
         multiple hierarchies, with the common domain as the highest
         element of each hierarchy".

         "Where there are multiple common domains, they cooperate as
         peers to make it possible to route to any NSAP in the OSIE"

         Common Domains have global routing information to the extent
         necessary to route packets to the proper domain.  Each of the
         several peer national backbones in today's Internet may be
         considered to be similar to a Common Domain.  Note that in the
         Internet the hierarchical containment implied by the definition
         of a CD does not really exist; however, there is a level of
         implicit ordering based on topology and policy issues (the
         willingness to be used as a transit network) that can be viewed
         as defining a Common Domain in the Internet.

Hares & Katz                                                    [Page 4]
RFC 1136          A Model for Routing in the Internet      December 1989

   For completeness, we offer the following definition for an Internet
   Autonomous System (AS):

      "An 'autonomous system' consists of a set of gateways, each of
      which can reach any other gateway in the same system using paths
      via gateways only in that system.  The gateways of a system
      cooperatively maintain a routing data base using an interior
      gateway protocol (IGP)..." [3]

5)  Environment and Goals

   The "OSI Routeing Framework" describes the environment for OSI
   routing as well as its goals.  The environment described is a highly
   interconnected, highly heterogeneous collection of LANs and public
   and private networks made up of a diverse collection of equipment
   from multiple vendors.  A number of goals are enumerated, including:

      -  Support of multiple subnetwork types
      -  Very large numbers of connected systems
      -  End System simplicity
      -  Multiple organizations with mutual distrust and policy/legal
      -  High performance
      -  Robust and dynamic routing in the face of topological changes

   The environment and goals described are a good match for those in the
   Internet.  The Internet crosses multiple types of physical media,
   link layer protocols, and administrative controls.  Routers and hosts
   may come from many vendors.  The Internet has become international in
   scope.  Issues of security and the isolation of bad routing
   information have become international concerns.

   The Internet environment, with over 900 highly connected networks
   (and growing exponentially), is very much like the environment the
   OSI model aims to describe.

6)  Structure of Global Routing

   The "OSI Routeing Framework" classifies routing into three types:

      -  within a Routing Domain
      -  within an Administrative Domain
      -  between Administrative Domains

   Routing within a Routing Domain involves a high level of mutual
   trust.  This allows the use of complex, tightly-coupled procedures
   that can make the best use of dynamic, highly interconnected

Hares & Katz                                                    [Page 5]
RFC 1136          A Model for Routing in the Internet      December 1989

   Routing Domains may be recursively subdivided into Subdomains in
   order to reduce routing complexity.  The details of a subdomain may
   be largely hidden from other subdomains with an attendant reduction
   in the volume of routing information exchanged.

   Intra-Administrative Domain routing is concerned with interconnecting
   multiple Routing Domains within an administration.  Issues may
   include address administration, cost recovery, and policy concerns.
   A moderate level of trust is assumed.  The nature of the interactions
   between Routing Domains can range from being tightly coupled (best
   path routing between two RDs running different routing protocols) to
   being more policy-based.  However, inter-RD routing within an
   Administrative Domain is tightly coordinated and represents a unified
   technical plan.

   Inter-Administrative Domain routing is concerned with managing and
   controlling the flow of information in a highly structured way
   between organizations that may require formal multilateral
   agreements.  The issues of concern at this level tend to be
   administrative in nature (legal/political constraints, security,
   access control, etc.).  Multiple agreements between multiple
   administrations are unlikely to be implicitly transitive.  This makes
   the analysis of policy interactions very important.

7)  Mapping the AD/RD Model Onto the Internet

   The national network backbones (NSFNET, ARPANET, MILNET, NSN, and
   soon ESNET) can be viewed as Common Domains.  Each may have
   sufficiently global routing knowledge to determine a path to any
   Internet address.

   Regional networks are clearly Administrative Domains.  Multilateral
   policy agreements are defined between the regional networks and the
   backbones.  On the other hand, regional networks very often are
   tightly coupled to individual networks and campus networks in terms
   of routing.  In this sense, a regional network could be viewed as a
   Routing Domain with individual campuses thought of as Subdomains.

   From the standpoint of routing functionality, it is most useful to
   view a "classic" Autonomous System as a congruent Routing Domain and
   Administrative Domain.  An AS as defined represents both a single IGP
   and a point of policy administration.  The sixteen bit value now
   known as the Autonomous System number may instead be viewed as an
   Administrative Domain number.

   In reality, however, many so-called Autonomous Systems today do not
   adhere to the strict definition of an AS.  In theory, an Autonomous
   System is quite similar to a Routing Domain, in which a high level of

Hares & Katz                                                    [Page 6]
RFC 1136          A Model for Routing in the Internet      December 1989

   trust is made between systems, a consistent IGP is run, and full
   routing information is distributed.  On the other hand, AS numbers
   have become an abstraction for policy groupings to backbones.
   Indeed, entire regional networks are viewed by the backbones as a
   single Autonomous System, even though they are not nearly as
   homogeneous as the AS model specifies.  Such entities can be viewed
   as an Administrative Domain containing several Routing Domains.

   Although it is true that, in this interpretation, multiple
   nontechnical administrations are represented within a single
   Administrative Domain (in conflict with the definition of an
   Administrative Domain), such structures require a single approach to
   internal routing.  Even if there is not a true administration
   representing the collection of domains (such as a consortium), there
   typically is a technical committee to settle common technical issues.

8)  The AD/RD Model as an Engineering Tool

   Current Autonomous Systems cross administrative boundaries with
   impunity.  This works as long as the individual administrations
   operating within the common AS agree to a common technical policy for
   routing and network management.  Connections with other backbones,
   regional networks, and campus networks must be planned, implemented,
   and managed in a coordinated fashion.

   This coordination becomes more difficult, but more necessary, as the
   AS grows.  As connectivity and policy become more complex, current
   Autonomous Systems start to fragment.  An example of this is a
   network that is currently a member of an NSFNET regional network but
   will be adding a connection to ESNET.  The administrators of the
   network and the regional network must carefully coordinate the
   changes necessary to implement this connection, including possibly
   altering the boundaries of policy and routing.  A lack of
   coordination could result in routing loops and policy violations.

   A point that is being increasingly realized is that the entity
   responsible for exterior or policy routing (be it an Autonomous
   System or an Administrative Domain) must have a common technical
   policy for routing.  The effects of attempting different approaches
   to policy and external routing while maintaining a single AS have
   been painfully evident in real instances in the Internet.

   Under the AD/RD model, a routing domain cannot be in two
   Administrative Domains.  For example, if a campus network wants to
   set its own routing policy and enforce it via management of their
   routers, the campus has elected to become a separate Administrative
   Domain.  If that campus uses a common IGP with other campuses, it
   represents an attempt to split a Routing Domain (the regional network

Hares & Katz                                                    [Page 7]
RFC 1136          A Model for Routing in the Internet      December 1989

   with a common IGP) across multiple Administrative Domains (the campus
   and the rest of the regional).  Such arrangements represent dubious
   engineering practice, cause real routing problems, and are disallowed
   by the AD/RD model.

   Under the strict Autonomous System model, only one IGP can exist
   within an AS.  However, many regional networks are successfully using
   multiple IGPs.  The AD/RD model allows this valuable routing
   topology.  Such a topology would also be allowed by the AS model if
   it were to be broadened to allow multiple IGPs, in which case an AS
   and an AD would effectively become equivalent.

9)  The AD/RD Model in a Dual Protocol Internet

   As the OSI protocol suite is deployed and an OSI Internet is
   constructed, it is very likely that significant portions of the
   current TCP/IP Internet will also carry OSI traffic.  Many router
   vendors provide dual protocol capability today, or will in the near
   future, and the investment in network infrastructure is such that it
   is unlikely that a separate, parallel internet will be established
   for OSI traffic.

   It is logical to assume that, in many cases, the same technical and
   administrative boundaries will apply to both DoD IP and OSI
   protocols, and in some cases a single routing protocol may be used to
   support both protocol suites.

   Thus, it would be most advantageous to have a common model and common
   nomenclature in order to provide a more unified, manageable routing
   environment.  Given that the OSI Routeing Framework represents the
   model on which OSI routing is built, the use of the AD/RD model to
   describe the existing Internet is an appropriate step toward
   describing and building the combined internet.

10)  Conclusions

   The AD/RD model of routing describes the current Internet better than
   existing models because it describes:

      -  How Intra-Domain and Inter-Domain relationships work at both
         routing and policy level

      -  How routing domains and administrative domains can be
         hierarchically related

      -  The existence of multiple national peers

      -  A common model for dual protocol internets

Hares & Katz                                                    [Page 8]
RFC 1136          A Model for Routing in the Internet      December 1989

   The expanding Internet has grown from the "core" model with several
   small attached networks to a highly interconnected environment that
   spans several continents.  Several national peer networks serve an
   ever-growing set of regional networks.  The AD/RD model can help
   Internet protocol designers abstract the functional pieces from the
   large Internet.

   The Internet grows daily.  Any model of Internet routing needs to
   provide a way to understand and order the growth.  The ISO Routeing
   Framework provides a structure to handle such growth.

11)  References

  [1]  ISO, "OSI Routeing Framework", ISO/TR 9575, 1989.

  [2]  Rosen, E., "Exterior Gateway Protocol", RFC 827, Bolt Beranek and
       Newman, October 1982.

  [3]  Mills, D., "Autonomous Confederations", RFC 975, M/A COM
       Linkabit, February 1986.

  [4]  ISO, "Open Systems Interconnection--Basic Reference Model", ISO

  [5]  ISO, "Internal Organization of the Network Layer", ISO 8648.

   ISO documents can be obtained from the following source:

      American National Standards Institute
      1430 Broadway
      New York, NY  10018
      (212) 642-4900

   Additionally, a number of private firms are authorized to distribute
   ISO documents.

Security Considerations

   Security issues are not addressed in this memo.

Authors' Addresses

   Susan Hares
   1075 Beal Ave.
   Ann Arbor, MI  48109

Hares & Katz                                                    [Page 9]
RFC 1136          A Model for Routing in the Internet      December 1989

   Phone:  (313) 936-3000

   Email:  skh@merit.edu

   Dave Katz
   1075 Beal Ave.
   Ann Arbor, MI  48109

   Phone:  (313) 763-4898

   Email:  dkatz@merit.edu

Hares & Katz                                                   [Page 10]
  1. RFC 1136