Administrative Domains and Routing Domains: A model for routing in the Internet
RFC 1136
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RFC - Informational
(December 1989; No errata)
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2013-03-02
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RFC 1136 (Informational)
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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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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.
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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
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