RFC 1136

Title block (Part 1 of 2) 🎀

Okay bestie, this is RFC 1136, titled Administrative Domains and Routing Domains: A Model for Routing in the Internet 🦋 It was written by S. Hares and D. Katz from Merit/NSFNET and dated December 1989. It isn't a standard (more on that in Section 1 💅). The header doesn't list any RFC it obsoletes or updates, so it's a standalone proposal. 🌸

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1) Status of this Memo ✨

This RFC proposes a model for describing routing within the Internet, and it's an adaptation of the "OSI Routeing Framework" [1]. Translation: it's a proposal, not a rulebook. This memo does not specify an Internet standard. Comments are welcome 💬 and distribution of this memo is unlimited 🌈

2) Acknowledgement 🫶

The authors, Hares and Katz, want to give a big thank-you to Guy Almes of Rice University for his contributions and insight. Main character energy for Guy 👑

3) Overview 🌸

So the "core" model of Autonomous Systems [2] was the foundation for how routing works on the Internet. But the Internet grew massively and the topology changed a ton, so that "core" model is no longer really in harmony with today's Internet 😭 Honestly, people saw this coming from the very start:

"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].

It was giving prophecy 🔮 Also, in some parts of the Internet, regional routing got so complicated that it went past what the Autonomous System definition could handle. Not me outgrowing a whole concept 💀

The model that fits the Internet way better is the one in the "OSI Routeing Framework" [1]. It proposes a structure of Routing Domains inside Administrative Domains. This paper is here to briefly explain that framework, show why it fits the present and future Internet better, and show how it can help build well-engineered routing environments 🧚

4) Terminology 📚

Here's a quick glossary of OSI terms, bestie 🍓 Full definitions live in the OSI Basic Reference Model [4], the Internal Organization of the Network Layer [5], and the OSI Routeing Framework [1].

Quick heads-up: "Routeing" is the official ISO spelling of what everyone normally writes as "routing." This paper uses the ISO spelling when quoting ISO documents and the common spelling everywhere else ✍️

End System (ES) 💻

An OSI system where applications run. An End System has full seven-layer OSI functionality. Basically the same thing as an Internet Host.

Intermediate System (IS) 🔀

An OSI system that does routing and relaying so there are paths between End Systems. Intermediate Systems have no functionality above the Network Layer. (Although a practical OSI router will have some End System functionality for network management and other stuff. Ngl that's a nice detail 🙈) Basically the same thing as an Internet Router.

Subnetwork (SN) 🛤️

A communications medium that gives a "direct" path between Network Layer entities. It can be a point-to-point link, a LAN, a Public Data Network, and so on. It's basically the same as an Internet Subnet. Here's the tea ☕ OSI Subnetworks aren't 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 the Internet.

Network Service Access Point (NSAP) 📍

A conceptual point on the Network/Transport Layer boundary in an End System. It's globally addressable, and the address is globally unambiguous in the OSIE. An NSAP represents a service available above the Network Layer (like a choice of transport protocols). An End System can have multiple NSAPs. An NSAP address is roughly equivalent to the Internet [address, protocol] pair. ✨

Administrative Domain (AD) 🏠

Here's the official OSI definition:

"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].

Translation: a group of hosts, routers, and networks run and managed by one organization. Routing inside an Administrative Domain is based on one consistent technical plan 📋 From the outside, for routing purposes, it's seen as one cohesive entity, and its internal structure doesn't matter. Info from other Administrative Domains is trusted less than info from your own, so no blind trust over here 🚫

Administrative Domains can be organized into a loose hierarchy that reflects how available and authoritative the routing info is. That hierarchy doesn't mean administrative containment, and it doesn't mean a strict tree topology 🌳

Routing Domain (RD) 🗺️

Official definition:

"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].

And the more detailed version:

"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" [1].

The OSI Routeing Framework also gives a formal definition: all ISs inside a Routing Domain can figure out whether an ES in the domain is reachable, and if it is, they can derive a path to it 🎯

Routing Domains may be divided into subdomains, kind of like subnetting on the Internet. That lets you structure the domain hierarchically, which keeps the topology details of a subdomain contained and cuts down the distributed routing info 🧩

An intra-Routing Domain routing protocol is equivalent to an Internet Interior Gateway Protocol (IGP) 🛠️

An Administrative Domain can have multiple Routing Domains. But a Routing Domain may never span multiple Administrative Domains 🚧 If an Administrative Domain has only a single Routing Domain, they're called Congruent. A congruent Administrative Domain and Routing Domain is analogous to an Internet Autonomous System 💖

Common Domain (CD) 👑

Official definition:

"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 info to the extent needed to route packets to the proper domain. Each of today's several peer national backbones on the Internet can be considered similar to a Common Domain. Note that the Internet doesn't really have the hierarchical containment that a CD definition implies. Still, there's an implicit ordering based on topology and policy (basically, willingness to be used as a transit network) that can be seen as defining a Common Domain on the Internet 🌟

For completeness, here's the definition of 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 and what its goals are. The environment is a highly interconnected, highly heterogeneous mix of LANs and public and private networks, with equipment from lots of different vendors 🏙️ The goals include:

• 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

Honestly, the environment and goals are a great match for the Internet 🥰 The Internet runs across many types of physical media, link layer protocols, and administrative controls. Routers and hosts can come from many vendors. The Internet is international now, and security and keeping bad routing info isolated are international concerns too 🔐

The Internet has over 900 highly connected networks (and it's growing exponentially 📈), which is very much like the environment the OSI model is meant to describe. It's giving déjà vu 💜

6. Structure of Global Routing ✨

okay so the "OSI Routeing Framework" sorts 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 🫶. That's what lets you use complex, tightly-coupled procedures that make the best use of dynamic, highly interconnected environments 🔥.

Routing Domains can be recursively subdivided into Subdomains to cut down on routing complexity 🧚. The details of a subdomain can be mostly hidden from other subdomains, which means way less routing information gets exchanged 🌸.

Intra-Administrative Domain routing is all about interconnecting multiple Routing Domains inside one administration 💜. Issues can include address administration, cost recovery, and policy concerns. A moderate level of trust is assumed. 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. But inter-RD routing within an Administrative Domain is tightly coordinated and represents a unified technical plan 🎀.

Inter-Administrative Domain routing is about managing and controlling the flow of information in a highly structured way between organizations that may need formal multilateral agreements 🌈. The concerns 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 🙈. So the analysis of policy interactions is very important 💀.

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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 one may have sufficiently global routing knowledge to figure out 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 are very often tightly coupled to individual networks and campus networks when it comes to 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's 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, a lot of so-called Autonomous Systems today don't 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. Honestly, entire regional networks are viewed by the backbones as a single Autonomous System, even though they aren't nearly as homogeneous as the AS model specifies 😭. Such entities can be viewed as an Administrative Domain containing several Routing Domains.

Although it's true that, in this interpretation, multiple nontechnical administrations are represented within a single Administrative Domain (which conflicts with the definition of an Administrative Domain), such structures still require a single approach to internal routing 🧩. Even if there isn't a true administration representing the collection of domains (such as a consortium), there typically is a technical committee to settle common technical issues 🫡.

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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 on 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 gets harder, but more necessary, as the AS grows 📈. As connectivity and policy become more complex, current Autonomous Systems start to fragment 💥. For example, picture a network that's 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 have to carefully coordinate the changes needed to make this connection happen, including possibly altering the boundaries of policy and routing. A lack of coordination could result in routing loops and policy violations 🌀.

Here's a point that's being increasingly realized 💡: 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 its routers, the campus has elected to become a separate Administrative Domain. If that campus uses a common IGP with other campuses, it's 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 broadened to allow multiple IGPs, in which case an AS and an AD would effectively become equivalent 🤝.

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9. The AD/RD Model in a Dual Protocol Internet 🌐

As the OSI protocol suite gets deployed and an OSI Internet is constructed, it's 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 so big that a separate, parallel internet for OSI traffic is unlikely to be established 💰.

It's logical to assume that, in many cases, the same technical and administrative boundaries will apply to both DoD IP and OSI protocols. In some cases, a single routing protocol may be used to support both protocol suites 🧃.

So 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, using the AD/RD model to describe the existing Internet is an appropriate step toward describing and building the combined internet 🦋✨.

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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

The expanding Internet has grown from the "core" model with several small attached networks into 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 that growth. The ISO Routeing Framework provides a structure to handle such growth 💅💖.

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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 🏢.

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Security Considerations 💀

Security issues are not addressed in this memo. 🙈 Be aware of that before you build on it!

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Authors' Addresses 💌

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