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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 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:
  1. Support of multiple subnetwork types
  2. Very large numbers of connected systems
  3. End System simplicity
  4. Multiple organizations with mutual distrust and policy/legal


  1. High performance
  2. 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:
  1. within a Routing Domain
  2. within an Administrative Domain
  3. 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:
  1. How Intra-Domain and Inter-Domain relationships work at both

routing and policy level

  1. How routing domains and administrative domains can be

hierarchically related

  1. The existence of multiple national peers
  1. 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
 Dave Katz
 1075 Beal Ave.
 Ann Arbor, MI  48109
 Phone:  (313) 763-4898

Hares & Katz [Page 10]

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