Network Working Group S. Hares
Request for Comments: 1136 D. Katz
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" . This
memo does not specify an Internet standard. Comments are welcome.
Distribution of this memo is unlimited.
The authors would like to thank Guy Almes of Rice University for his
contributions and insight.
The "core" model of Autonomous Systems  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
"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" .
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" .
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
the present and future Internet, and to show how the model can aid in
the construction of well-engineered routing environments.
The following is a brief glossary of OSI terminology. Formal
definitions can be found in the OSI Basic Reference Model , the
Internal Organization of the Network Layer , and the OSI Routeing
"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
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
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
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,
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" .
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 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" .
"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
they use the same path computation algorithm" .
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.
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)..." 
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
- 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
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
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
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
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
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.
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
- The existence of multiple national peers
- A common model for dual protocol internets
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
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.
 ISO, "OSI Routeing Framework", ISO/TR 9575, 1989.
 Rosen, E., "Exterior Gateway Protocol", RFC 827, Bolt Beranek and
Newman, October 1982.
 Mills, D., "Autonomous Confederations", RFC 975, M/A COM
Linkabit, February 1986.
 ISO, "Open Systems Interconnection--Basic Reference Model", ISO
 ISO, "Internal Organization of the Network Layer", ISO 8648.
ISO documents can be obtained from the following source:
American National Standards Institute
New York, NY 10018
Additionally, a number of private firms are authorized to distribute
Security issues are not addressed in this memo.
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