rfc1136.txt

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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



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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



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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



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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
       7498.

  [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
      [phone]

   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
   Merit/NSFNET
   1075 Beal Ave.
   Ann Arbor, MI  48109





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RFC 1136          A Model for Routing in the Internet      December 1989


   Phone:  [phone]

   Email:  [e-mail]


   Dave Katz
   Merit/NSFNET
   1075 Beal Ave.
   Ann Arbor, MI  48109

   Phone:  [phone]

   Email:  [e-mail]






































Hares & Katz                                                   [Page 10]