rfc1136.txt

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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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   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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      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




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         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
         Domains.

         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"
         [1].

         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.




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   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
         restrictions
      -  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
   environments.



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