Routing TCP[s]IP (Vol. 22001)

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:

• 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 .

• 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 ."

• 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

Also like IP addresses, some AS numbers are reserved for private use. These numbers range from 64512 to 65535. See RFC 1930 (www.isi.edu/in-notes/rfc1930.txt) for more information.

Chief among the choices the administrative authority of each AS is free to make is the routing protocol that its gateways run. Because the gateways are interior to the AS, their routing protocols are known as interior gateway protocols (IGPs). Because GGP was the routing protocol of the ARPANET, it became by default the first IGP. However, interest in the more modern (and simpler) Routing Information Protocol (RIP) was building in 1982, and it was expected that this and other as-yet- unplanned protocols would be used in many autonomous systems. These days, GGP has been completely replaced by RIP, RIP-2, Interior Gateway Routing Protocol (IGRP), Enhanced IGRP (EIGRP), Open Shortest Path First (OSPF), and Integrated Intermediate System-to-Intermediate System (IS-IS).

Each AS is connected to other autonomous systems via one or more exterior gateways. RFC 827 proposed that the exterior gateways share routing information between each other by means of a protocol known as the EGP. Contrary to popular belief, although EGP is a distance vector protocol, it is not a routing protocol. It has no algorithm for choosing an optimal path between networks; rather, it is a common language that exterior gateways use to exchange reachability information with other exterior gateways. That reachability information is a simple list of major network addresses (no subnets) and the gateways by which they can be reached.