Origins of EGP

《TCP IP路由技术(第2卷)(第2版)英文版》本书是有关Cisco外部路由协议和高级IP路由主题的权威指南,是Cisco路由与交换领域实属罕见的经典著作。本书在上一版的基础上进行了全面更新,其可读性、广度和深度相较于上一版有了相当大的改进。本节为大家介绍Origins of EGP。

作者:【美】Jeff Doyle(杰夫 多伊尔)来源:人民邮电出版社|2017-11-05 18:10

Origins of EGP

In the early 1980s, the gateways (routers) that made up the ARPANET 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 ofmany growing networks encounter today: Their routing protocol did not scale well.

Eric Rosen , in RFC 827 , 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 frequency of which increases with the size of the network, all gateways have to exchange routing information and recalculate their forwarding tables. Even when the network 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 Rosen proposed in RFC 827 was that the ARPANET be migrated from a single network to a system of interconnected, autonomously controlled networks. Within each network, known as an autonomous system (AS), the administrative authority for that AS is free to manage the network 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 from the borders of the AS down to individual subnets 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 6996 for more information.

And just as IPv6 is implemented to eliminate the problem of IPv4 address shortages, RFC 4893 (now obsoleted by RFC 6793 ) proposes 32-bit AS numbers to prevent a shortage of 16-bit AS numbers. The use of 32-bit AS numbers is discussed in Chapter 5, “Scaling BGP.”

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 (IGP). 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)1 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, RIPng, Enhanced IGRP (EIGRP), Open Shortest Path First (OSPF) versions 2 and 3, 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 proposes that the exterior gateways share routing information between each other by means of a protocol known as the EGP. Contrary to popular belief (well, popular when there were still people working with the protocol), 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.




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