Routers began as gateways that deliberately refused to guarantee delivery
The designers of the first internet gateways made a surprising choice: their devices would pass data along without any promise that they arrived. Checking and resending would be left to the computers at each end. That idea, the end-to-end principle, helped let wildly different networks join into one Internet.
A router reads the destination address in each arriving packet, looks up the best next hop in its table and sends the packet on its way; hop by hop, data crosses the Internet. The concept goes back to Donald Davies, who in 1966 proposed an interface computer for Britain's NPL network. Wesley Clark had the same idea a year later for ARPANET, where the machines were called Interface Message Processors. The first ran at the National Physical Laboratory in early 1969, followed that year by units at UCLA, Stanford Research Institute, Santa Barbara and Utah, all built on the Honeywell 516.
What made internetworking possible was a new kind of device, then called a gateway, championed by an international group of researchers. Gateways linked dissimilar networks, such as serial lines and local networks, and were connectionless. Three efforts explored the idea in the early 1970s: Louis Pouzin's CYCLADES network in France, Xerox PARC's in-house network design, known as PUP, which stayed largely hidden for years over intellectual property worries, and a DARPA program that absorbed lessons from both and produced TCP/IP. Ginny Travers at BBN built the first true IP router in 1975 and 1976, and three PDP-11 routers were running in the prototype Internet by the end of 1976. University College London kept a gateway open between British researchers and ARPANET from 1973.
In 1981 teams at MIT and Stanford independently built routers that could handle several network protocols at once, vital while rivals to TCP/IP such as AppleTalk and DECnet were still common. For a time ordinary minicomputers served as routers; today's big machines use specialised chips to forward traffic along fibre backbones at enormous speed.
Inside, a router runs two cooperating layers. The control plane builds the routing table, either from fixed instructions or by trading information with neighbouring routers, and distils it into a lean forwarding table. The forwarding plane then reads each packet header and sends it out the matching port. Home routers sit at the bottom of this hierarchy, and some can run free Linux-based firmware such as OpenWrt.
Source: Router (computing)
