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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
+
+
+
+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
+ 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.
+
+
+
+
+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
+ 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.
+
+
+
+
+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
+ 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.
+
+
+
+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
+ 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
+
+
+
+
+
+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
+ Merit/NSFNET
+ 1075 Beal Ave.
+ Ann Arbor, MI 48109
+
+ Phone: (313) 763-4898
+
+ Email: dkatz@merit.edu
+
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