A router is a computer and networking device that forwards data packets between computer networks, including internetworks such as the global Internet.
Routers perform the traffic directing functions on the Internet. A router is connected to two or more data lines from different IP networks. When a data packet arrives on a line, the router reads the network address in the packet header to determine the destination. Then, using information in its routing table or routing policy, it directs the packet to the next network on its journey. A data packet is forwarded from one router to another through an internetwork until it reaches its destination node.
The most familiar type of IP routers are home and small office routers that forward IP packets between the home computers and the Internet. More sophisticated routers, such as enterprise routers, connect large business or ISP networks to powerful core routers that forward data at high speed along the optical fiber lines of the Internet backbone.
Routers can be built from standard computer parts but are mostly specialized purpose-built computers (embedded systems). Early routers used software-based forwarding, running on a CPU. More sophisticated devices use application-specific integrated circuits (ASICs) to increase performance or add advanced filtering and firewall functionality.
History
The concepts of a switching node using software and an interface computer were first proposed by Donald Davies in 1966 for the NPL network. The same idea was conceived by Wesley Clark the following year for use in the ARPANET, which were named Interface Message Processors (IMPs). The first interface computer was implemented at the National Physical Laboratory in the United Kingdom in early 1969, followed later that year by the IMPs at the University of California, Los Angeles, the Stanford Research Institute, the University of California, Santa Barbara, and the University of Utah School of Computing in the United States. All were built with the Honeywell 516. These computers had fundamentally the same functionality as a router does today.
The idea for a router (called a gateway at the time) initially came about through an international group of computer networking researchers called the International Network Working Group (INWG).Davies, Shanks, Heart, Barker, Despres, Detwiler and Riml, "Report of Subgroup 1 on Communication System", INWG Note No. 1. These gateway devices were different from most previous packet switching schemes in two ways. First, they connected dissimilar kinds of networks, such as serial lines and local area networks. Second, they were connectionless devices, which had no role in assuring that traffic was delivered reliably, leaving that function entirely to the hosts. This particular idea, the end-to-end principle, was contained in the work of Donald Davies.
The concept was explored in practice by various groups, with the intention to produce a working system for internetworking. There were three notable contemporaneous programs. The first was an implementation directed by Louis Pouzin of the CYCLADES network, which was designed and developed during 1972–3. The second was a program at Xerox PARC to explore new networking technologies, which produced the PARC Universal Packet system. Some time after early 1974, the first Xerox routers became operational. Due to corporate intellectual property concerns, it received little attention outside Xerox for years.David Boggs, John Shoch, Edward Taft, Robert Metcalfe, "Pup: An Internetwork Architecture", IEEE Transactions on Communications, Volume 28, Issue 4, April 1980, pp. 612- 624. The third was a DARPA-initiated program, which began during 1973–4. This drew on the work of the other two programs, expanded significantly, and went on to create the TCP/IP architecture in use today. University College London (UCL) provided a gateway between British research groups and the ARPANET from 1973 until the late 1980s, latterly using SATNET.
The first true IP router was developed by Ginny Travers at BBN, as part of that DARPA-initiated effort, during 1975–1976. By the end of 1976, three PDP-11-based routers were in service in the experimental prototype Internet.Craig Partridge, S. Blumenthal, "Data networking at BBN"; IEEE Annals of the History of Computing, Volume 28, Issue 1; January–March 2006. Mike Brecia, Ginny Travers, and Bob Hinden received the IEEE Internet Award for early IP routers in 2008.
The first multiprotocol routers were independently created by staff researchers at MIT and Stanford in 1981 and both were also based on PDP-11s. Stanford's router program was led by William Yeager and MIT's by Noel Chiappa.Valley of the Nerds: Who Really Invented the Multiprotocol Router, and Why Should We Care? , Public Broadcasting Service, Accessed August 11, 2007.Router Man , NetworkWorld, Accessed June 22, 2007.David D. Clark, "M.I.T. Campus Network Implementation", CCNG-2, Campus Computer Network Group, M.I.T., Cambridge, 1982; pp. 26.Pete Carey, "A Start-Up's True Tale: Often-told story of Cisco's launch leaves out the drama, intrigue", San Jose Mercury News, December 1, 2001. Virtually all networking now uses TCP/IP, but multiprotocol routers are still manufactured. They were important in the early stages of the growth of computer networking when protocols other than TCP/IP were in use. Modern routers that handle both IPv4 and IPv6 are multiprotocol but are simpler devices than those that process AppleTalk, DECnet, IPX, and Xerox protocols.
From the mid-1970s and in the 1980s, general-purpose minicomputers served as routers. Modern high-speed routers are network processors or highly specialized computers with extra hardware acceleration added to speed both common routing functions, such as packet forwarding, and specialized functions such as IPsec encryption. There is substantial use of Linux and Unix software-based machines, running open source routing code, for research and other applications. The Cisco IOS operating system was independently designed. Major router operating systems, such as Junos and NX-OS, are extensively modified versions of Unix software.
Operation When multiple routers are used in interconnected networks, the routers can exchange information about destination addresses using a routing protocol. Each router builds up a routing table, a list of routes, between two computer systems on the interconnected networks.
The software that runs the router is composed of two functional processing units that operate simultaneously, called planes: Control plane: A router maintains a routing table that lists which route should be used to forward a data packet, and through which physical interface connection. It does this using internal pre-configured directives, called static routes, or by learning routes dynamically using a routing protocol. Static and dynamic routes are stored in the routing table. The control-plane logic then strips non-essential directives from the table and builds a forwarding information base (FIB) to be used by the forwarding plane. Forwarding plane: This unit forwards the data packets between incoming and outgoing interface connections. It reads the header of each packet as it comes in, matches the destination to entries in the FIB supplied by the control plane, and directs the packet to the outgoing network specified in the FIB.
Applications
A router may have interfaces for multiple types of physical layer connections, such as copper cables, fiber optic, or Wireless transmission. It can also support multiple network layer transmission standards. Each network interface is used to enable data packets to be forwarded from one transmission system to another. Routers may also be used to connect two or more logical groups of computer devices known as subnets, each with a unique network prefix.
Routers may provide connectivity within enterprises, between enterprises and the Internet, or between internet service providers' (ISPs') networks; they are also responsible for directing data between different networks. The largest routers (such as the Cisco CRS-1 or Juniper PTX) interconnect the various ISPs, or may be used in large enterprise networks. Smaller routers usually provide connectivity for typical home and office networks.
All sizes of routers may be found inside enterprises. The most powerful routers are usually found in ISPs, academic and research facilities. Large businesses may also need more powerful routers to cope with ever-increasing demands of intranet data traffic. A hierarchical internetworking model for interconnecting routers in large networks is in common use. Some routers can connect to Data service units for T1 connections via serial ports.
Access, core and distribution
The hierarchical internetworking model divides enterprise networks into three layers: core, distribution, and access.
Access routers, including small office/home office (SOHO) models, are located at home and customer sites such as branch offices that do not need hierarchical routing of their own. Typically, they are optimized for low cost. Some SOHO routers are capable of running alternative free Linux-based firmware like Tomato, OpenWrt, or DD-WRT.
Distribution routers aggregate traffic from multiple access routers. Distribution routers are often responsible for enforcing quality of service across a wide area network (WAN), so they may have considerable memory installed, multiple WAN interface connections, and substantial onboard data processing routines. They may also provide connectivity to groups of file servers or other external networks.
In enterprises, a core router may provide a collapsed backbone interconnecting the distribution tier routers from multiple buildings of a campus, or large enterprise locations. They tend to be optimized for high bandwidth but lack some of the features of edge routers.
Security External networks must be carefully considered as part of the overall security strategy of the local network. A router may include a firewall, VPN handling, and other security functions, or they may be handled by separate devices. Routers also commonly perform network address translation, which restricts connections initiated from external connections but is not recognized as a security feature by all experts. Some experts argue that open source routers are more secure and reliable than closed source routers because errors and potentially exploitable vulnerabilities are more likely to be discovered and addressed in an open-source environment.
Routing different networks Routers are also often distinguished on the basis of the network in which they operate. A router in a local area network (LAN) of a single organization is called an interior router. A router that is operated in the Internet backbone is described as an exterior router, while a router that connects a LAN with the Internet or a wide area network (WAN) is called a border router, or gateway router.
Internet connectivity and internal use Routers intended for ISP and major enterprise connectivity usually exchange routing information using the Border Gateway Protocol (BGP). defines the types of BGP routers according to their functions: Edge router or inter-AS border router: Placed at the edge of an ISP network, where the router is used to peer with the upstream IP transit providers, bilateral peers through IXP, private peering (or even settlement-free peering) through Private Network Interconnect (PNI) via the extensive use of Exterior Border Gateway Protocol (eBGP). Provider Router (P): A Provider router is also called a transit-router, it sits in a Multiprotocol Label Switching (MPLS) network and is responsible for establishing label-switched paths between the PE routers. Provider edge router (PE): An MPLS-specific router in the network's access layer that interconnects with customer edge routers to provide layer 2 or layer 3 VPN services.