Exterior Gateway Protocol

Exterior Gateway Protocol. It was a pioneering protocol developed for exchanging routing table information between different autonomous systems on the early ARPANET. Designed by Bolt, Beranek and Newman engineers, including Eric C. Rosen, it became a formal standard documented in RFC 827. Its deployment was critical for the transition from a single, core-administered network to a decentralized Internet architecture.

Overview

The protocol was created to address the scaling limitations of earlier routing schemes used within the ARPANET, such as the Gateway-to-Gateway Protocol. Its primary function was to enable distinct networks, administered by different organizations like the Defense Data Network or NASA, to exchange reachability information. This allowed the nascent Internet to evolve beyond a singular network under the management of the Defense Advanced Research Projects Agency. The protocol formally defined relationships between neighbor gateways, classifying them as either core gateways or stub gateways, which shaped the topology of the early Internet backbone.

Protocol operation

Operation relied on a strict hierarchical model and explicit neighbor configuration between gateways residing in different autonomous systems. A gateway would establish an adjacency with a peer through a neighbor acquisition process, involving the exchange of Hello protocol and I Hear You messages. Once established, the gateway would periodically send poll requests to its neighbor. In response, the neighbor would transmit routing update messages containing vectors of network reachability, essentially indicating which IP networks were accessible through that autonomous system. This information was used to maintain a database of inter-AS paths, though the protocol itself did not interpret the internal topology of neighboring systems.

EGP versions and packet format

The protocol evolved through several versions, with the most significant and widely implemented being EGP version 2, standardized in RFC 888 and later refined in RFC 904. Its packet format was relatively simple, with a fixed header followed by variable-length fields. The header contained fields for version number, type, and code, identifying the specific message type such as Neighbor Acquisition or Poll. The data portion for update messages contained a list of IP address blocks, known as networks, along with associated gateway addresses and distance metrics. This structure was efficient for the relatively flat network architecture of the 1980s but lacked flexibility for complex policies.

Limitations and replacement by BGP

Several critical limitations led to its eventual obsolescence. It operated on a strict tree-like topology, which assumed no routing loops, making it incapable of supporting the rich, meshed interconnections that developed among commercial networks like MCI Communications and Sprint Corporation. It could not express routing policy or preferences, and its distance metric was a simple hop count, providing no mechanism for path vector information. These shortcomings became acute during the exponential growth of the NSFNET and the rise of commercial Internet service providers. Its replacement, the Border Gateway Protocol, introduced as BGP version 1 in RFC 1105, provided a robust path vector algorithm, policy-based controls, and support for arbitrary topology meshing, leading to BGP's adoption as the definitive Inter-domain routing protocol.

Security considerations

The protocol was designed in an era of mutual trust among the limited number of ARPANET participants and contained no inherent security mechanisms. Message exchange relied on simple neighbor configuration with no authentication, making it vulnerable to spoofing and unauthorized route injection. A malicious actor could easily disrupt routing by impersonating a core gateway, such as those operated by the Stanford Research Institute or University College London. These vulnerabilities, along with its structural limitations, further motivated the transition to Border Gateway Protocol, which later incorporated features like TCP MD5 signature option and, ultimately, the Resource Public Key Infrastructure framework for origin validation.

Category:Internet protocols Category:Network layer protocols Category:Routing protocols Category:History of the Internet Category:Obsolete Internet standards