Why was the concept of Autonomous Systems introduced?

In the early 1980s, the routers (gateways) that made up the ARPANET (predecessor of the modern

internet) ran a distance vector routing protocol known as the Gateway-to-Gateway Protocol (GGP).

Every gateway knew a route to every reachable network, at a distance measured in gateway hops.

As the ARPANET grew, its architects foresaw the same problem that administrators of many growing

internetworks encounter today: Their routing protocol did not scale well.

Eric Rosen, in RFC 827[1], chronicles the scalability problems:

l With all gateways knowing all routes, "the overhead of the routing algorithm becomes

excessively large." Whenever a topology change occurs, the likelihood of which increases with

the size of the internetwork, all gateways have to exchange routing information and

recalculate their tables. Even when the internetwork is in a steady state, the size of the

routing tables and routing updates becomes an increasing burden.

l As the number of GGP software implementations increases, and the hardware platforms on

which they are implemented become more diverse, "it becomes impossible to regard the

Internet as an integrated communications system." Specifically, maintenance and

troubleshooting become "nearly impossible."

l As the number of gateways grows, so does the number of gateway administrators. As a

result, resistance to software upgrades increases: "[A]ny proposed change must be made in

too many different places by too many different people."

The solution proposed in RFC 827 was that the ARPANET be migrated from a single internetwork to a

system of interconnected, autonomously controlled internetworks. Within each internetwork, known

as an autonomous system (AS), the administrative authority for that AS is free to manage the

internetwork as it chooses. In effect, the concept of autonomous systems broadens the scope of

internetworking and adds a new layer of hierarchy. Where there was a single internetwork-a

network of networks-there is now a network of autonomous systems, each of which is itself an

internetwork. And just as a network is identified by an IP address, an AS is identified by an

autonomous system number. An AS number is a 16-bit number assigned by the same addressing

authority that assigns IP addresses.

NOTE

Manoj kumar

Infopark

South Ex

New Delhi

India