Router (computing)

A router is a accessory that assiduously abstracts packets amid computer networks, creating an bury internetwork. A router is affiliated to two or added abstracts curve from altered networks. When a abstracts packet comes in on one of the lines, the router reads the abode advice in the packet to actuate its ultimate destination. Then, application advice in its acquisition table or acquisition policy, it directs the packet to the abutting arrangement on its journey. Routers accomplish the "traffic directing" functions on the Internet. A abstracts packet is about forwarded from one router to addition through the networks that aggregate the internetwork until it gets to its destination node.[1]

The best accustomed blazon of routers are home and baby appointment routers that artlessly canyon data, such as web pages and email, amid the home computers and the owner's cable or DSL modem, which connects to the Internet (ISP). However added adult routers ambit from action routers, which affix ample business or ISP networks up to the able amount routers that advanced abstracts at aerial acceleration forth the optical cilia curve of the Internet backbone

Core

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.

Internet connectivity and internal use

Routers advised for ISP and above action connectivity usually barter acquisition advice application the Bound Aperture Agreement (BGP). RFC 40989 accepted defines the types of BGP-protocol routers according to the routers' functions:

Bend router: Additionally alleged a Provider Bend router, is placed at the bend of an ISP network. The router uses External BGP to EBGP agreement routers in added ISPs, or a ample action Autonomous System.

Subscriber bend router: Additionally alleged a Customer Bend router, is amid at the bend of the subscriber's network, it additionally uses EBGP agreement to its provider's Autonomous System. It is about acclimated in an (enterprise) organization.

Inter-provider bound router: Interconnecting ISPs, is a BGP-protocol router that maintains BGP sessions with added BGP agreement routers in ISP Autonomous Systems.

Amount router: A amount router resides aural an Autonomous Arrangement as a aback cartilage to backpack cartage amid bend routers.10

Aural an ISP: In the ISPs Autonomous System, a router uses centralized BGP agreement to acquaint with added ISP bend routers, added intranet amount routers, or the ISPs intranet provider bound routers.

"Internet backbone:" The Internet no best has a acutely identifiable backbone, clashing its antecedent networks. See default-free area (DFZ). The above ISPs arrangement routers accomplish up what could be advised to be the accepted Internet courage core.11 ISPs accomplish all four types of the BGP-protocol routers declared here. An ISP "core" router is acclimated to interconnect its bend and bound routers. Amount routers may additionally accept specialized functions in basic clandestine networks based on a aggregate of BGP and Multi-Protocol Label Switching protocols.12

Anchorage forwarding: Routers are additionally acclimated for anchorage forwarding amid clandestine internet affiliated servers.5

Voice/Data/Fax/Video Processing Routers: Commonly referred to as admission servers or gateways, these accessories are acclimated to avenue and action voice, data, video, and fax cartage on the internet. Since 2005, best long-distance buzz calls accept been candy as IP cartage (VOIP) through a articulation gateway. Articulation cartage that the acceptable cable networks already carriedclarification needed. Use of admission server blazon routers broadcast with the appearance of the internet, aboriginal with dial-up access, and addition improvement with articulation buzz service.

Historical and technical information

The actual aboriginal accessory that had fundamentally the aforementioned functionality as a router does today, was the Interface Message Processor (IMP); IMPs were the accessories that fabricated up the ARPANET, the aboriginal packet network. The abstraction for a router (called "gateways" at the time) initially came about through an all-embracing accumulation of computer networking advisers alleged the All-embracing Arrangement Working Accumulation (INWG). Set up in 1972 as an breezy accumulation to accede the abstruse issues complex in abutting altered networks, afterwards that year it became a subcommittee of the All-embracing Federation for Information Processing.13

These accessories were altered from best antecedent packet networks in two ways. First, they affiliated antithetical kinds of networks, such as consecutive curve and bounded breadth networks. Second, they were connectionless devices, which had no role in acceptable that cartage was delivered reliably, abrogation that absolutely to the hosts (this accurate abstraction had been ahead pioneered in the CYCLADES network).

The abstraction was explored in added detail, with the ambition to aftermath a ancestor system, as allotment of two ancillary programs. One was the antecedent DARPA-initiated program, which created the TCP/IP architectonics in use today.14 The added was a affairs at Xerox PARC to analyze fresh networking technologies, which produced the PARC Universal Packet system, due to accumulated bookish acreage apropos it accustomed little absorption alfresco Xerox for years.15

Some time afterwards aboriginal 1974 the aboriginal Xerox routers became operational. The aboriginal accurate IP router was developed by Virginia Strazisar at BBN, as allotment of that DARPA-initiated effort, during 1975-1976. By the end of 1976, three PDP-11-based routers were in account in the beginning ancestor Internet.16

The aboriginal multiprotocol routers were apart created by agents advisers at MIT and Stanford in 1981; the Stanford router was done by William Yeager, and the MIT one by Noel Chiappa; both were additionally based on PDP-11s.17181920

Virtually all networking now uses TCP/IP, but multiprotocol routers are still manufactured. They were important in the aboriginal stages of the advance of computer networking, back protocols added than TCP/IP were in use. Modern Internet routers that handle both IPv4 and IPv6 are multiprotocol, but are simpler accessories than routers processing AppleTalk, DECnet, IP, and Xerox protocols.

From the mid-1970s and in the 1980s, general-purpose mini-computers served as routers. Modern accelerated routers are awful specialized computers with added accouterments added to acceleration both accepted acquisition functions, such as packet forwarding, and specialised functions such as IPsec encryption.

Forwarding

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For pure Internet Protocol (IP) forwarding function, a router is designed to minimize the state information associated with individual packets. The main purpose of a router is to connect multiple networks and forward packets destined either for its own networks or other networks. A router is considered a Layer 3 device because its primary forwarding decision is based on the information in the Layer 3 IP packet, specifically the destination IP address. This process is known as routing. When each router receives a packet, it searches its routing table to find the best match between the destination IP address of the packet and one of the network addresses in the routing table. Once a match is found, the packet is encapsulated in the Layer 2 data link frame for that outgoing interface. A router does not look into the actual data contents that the packet carries, but only at the layer 3 addresses to make a forwarding decision, plus optionally other information in the header for hint on, for example, QoS. Once a packet is forwarded, the router does not retain any historical information about the packet, but the forwarding action can be collected into the statistical data, if so configured. Forwarding decisions can involve decisions at layers other than layer 3. A function that forwards based on layer 2 information, is properly called a bridge. This function is referred to as layer 2 bridging, as the addresses it uses to forward the traffic are layer 2 addresses (e.g. MAC addresses on Ethernet). Besides making decision as which interface a packet is forwarded to, which is handled primarily via the routing table, a router also has to manage congestion, when packets arrive at a rate higher than the router can process. Three policies commonly used in the Internet are tail drop, random early detection (RED), and weighted random early detection (WRED). Tail drop is the simplest and most easily implemented; the router simply drops packets once the length of the queue exceeds the size of the buffers in the router. RED probabilistically drops datagrams early when the queue exceeds a pre-configured portion of the buffer, until a pre-determined max, when it becomes tail drop. WRED requires a weight on the average queue size to act upon when the traffic is about to exceed the pre-configured size, so that short bursts will not trigger random drops. Another function a router performs is to decide which packet should be processed first when multiple queues exist. This is managed through quality of service (QoS), which is critical when Voice over IP is deployed, so that delays between packets do not exceed 150ms to maintain the quality of voice conversations. Yet another function a router performs is called policy-based routing where special rules are constructed to override the rules derived from the routing table when a packet forwarding decision is made. These functions may be performed through the same internal paths that the packets travel inside the router. Some of the functions may be performed through an application-specific integrated circuit (ASIC) to avoid overhead caused by multiple CPU cycles, and others may have to be performed through the CPU as these packets need special attention that cannot be handled by an ASIC.


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For authentic Internet Protocol (IP) forwarding function, a router is advised to abbreviate the accompaniment advice associated with alone packets. The capital purpose of a router is to affix assorted networks and advanced packets destined either for its own networks or added networks. A router is advised a Band 3 accessory because its primary forwarding accommodation is based on the advice in the Band 3 IP packet, accurately the destination IP address. This activity is accepted as routing. Back anniversary router receives a packet, it searches its acquisition table to acquisition the best bout amid the destination IP abode of the packet and one of the arrangement addresses in the acquisition table. Already a bout is found, the packet is encapsulated in the Band 2 abstracts articulation anatomy for that approachable interface. A router does not attending into the absolute abstracts capacity that the packet carries, but alone at the band 3 addresses to accomplish a forwarding decision, additional optionally added advice in the attack for adumbration on, for example, QoS. Already a packet is forwarded, the router does not absorb any actual advice about the packet, but the forwarding activity can be calm into the statistical data, if so configured.

Forwarding decisions can absorb decisions at layers added than band 3. A activity that assiduously based on band 2 information, is appropriately alleged a bridge. This activity is referred to as band 2 bridging, as the addresses it uses to advanced the cartage are band 2 addresses (e.g. MAC addresses on Ethernet).

Besides authoritative accommodation as which interface a packet is forwarded to, which is handled primarily via the acquisition table, a router additionally has to administer congestion, back packets access at a amount college than the router can process. Three behavior frequently acclimated in the Internet are appendage drop, accidental aboriginal apprehension (RED), and abounding accidental aboriginal apprehension (WRED). Appendage bead is the simplest and best calmly implemented; the router artlessly drops packets already the breadth of the chain exceeds the admeasurement of the buffers in the router. RED probabilistically drops datagrams aboriginal back the chain exceeds a pre-configured allocation of the buffer, until a pre-determined max, back it becomes appendage drop. WRED requires a weight on the boilerplate chain admeasurement to act aloft back the cartage is about to beat the pre-configured size, so that abbreviate bursts will not activate accidental drops.

Another activity a router performs is to adjudge which packet should be candy aboriginal back assorted queues exist. This is managed through affection of account (QoS), which is analytical back Articulation over IP is deployed, so that delays amid packets do not beat 150ms to advance the affection of articulation conversations.

Yet addition activity a router performs is alleged policy-based acquisition area appropriate rules are complete to override the rules acquired from the acquisition table back a packet forwarding accommodation is made.

These functions may be performed through the aforementioned centralized paths that the packets biking central the router. Some of the functions may be performed through an application-specific chip ambit (ASIC) to abstain aerial acquired by assorted CPU cycles, and others may accept to be performed through the CPU as these packets charge appropriate absorption that cannot be handled by an ASIC.