RFC1136

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

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  Phone:  (313) 936-3000

  Email:  [email protected]

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

  Phone:  (313) 763-4898

  Email:  [email protected]

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