Packet forwarding: Difference between revisions

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''Forwarding''{{Short is the relayingdescription|Relaying of [[packet|packets]] from one [[network segment]] to another}}

{{refimprove|date=July 2009}}

to another by [[node (networking)|nodes]]

in a [[packet switching|packet switched]] [[computer network]].

'''Packet forwarding''' is the relaying of [[Packet (information technology)|packet]]s from one [[network segment]] to another by [[node (networking)|nodes]] in a [[computer network]].

[[Image:unicast_forwarding.png|frame|A [[unicast]] forwarding pattern, typical of many networking technologies including the overwealming majority of [[Internet]] traffic]]

[[Image:multicast_forwarding.png|frame|A [[multicast]] forwarding pattern, typical of [[Protocol Independent Multicast|PIM]]]]

[[Image:broadcast_forwarding.png|frame|A [[broadcast]] forwarding pattern, typical of [[bridging (networking)|bridged]] [[Ethernet]]]]

==Models==

The simplest forwarding model - [[unicast|unicasting]] - involves a packet being

[[Image:unicast forwarding.png|frame|A [[unicast]] forwarding pattern, typical of many networking technologies including the overwhelming majority of [[Internet]] traffic]] [[Image:multicast forwarding.png|frame|A [[multicast]] forwarding pattern, typical of [[Protocol Independent Multicast|PIM]]]] [[Image:broadcast forwarding.png|frame|A [[Broadcasting (networking)|broadcast]] forwarding pattern, typical of [[bridging (networking)|bridged]] [[Ethernet]]]]

relayed from link to link along a chain leading from the packet's

source to its destination. However, other forwarding strategies

are commonly used. [[Broadcasting (networks)|Broadcasting]] requires a packet to be

duplicated and copies sent on multiple links with the goal

of delivering a copy to every device on the network.

In practice, broadcast packets are not forwarded everywhere

on a network, but only to devices within a [[broadcast ___domain]].

Less common than broadcasting, but perhaps of greater

utility and theoretical significance is [[multicast|multicasting]],

where a packet is selectively duplicated and copies

delivered to each of a set of recipients.

The simplest forwarding model{{mdashb}}[[unicast]]ing{{mdashb}}involves a packet being relayed from link to link along a chain leading from the packet's source to its destination. However, other forwarding strategies are commonly used. Broadcasting requires a packet to be duplicated and copies sent on multiple links with the goal of delivering a copy to every device on the network. In practice, broadcast packets are not forwarded everywhere on a network, but only to devices within a [[broadcast ___domain]], making ''broadcast'' a relative term. Less common than broadcasting, but perhaps of greater utility and theoretical significance, is [[multicast]]ing, where a packet is selectively duplicated and copies delivered to each of a set of recipients.

At nodes where multiple outgoing links are available, the

choice of which, all, or any to use requires a decision

making process that, while simple in concept, is of

sometimes bewildering complexity. Since a forwarding

decision must be made for every packet handled by a

node, the total time required for this can become

a major limiting factor in the overall performance

of a packet-switched network.

Much of the design effort of high-speed [[routers]]

has been focused on making rapid forwarding decisions

for large numbers of packets.

Networking technologies tend to naturally support certain forwarding models. For example, fiber optics and copper cables run directly from one machine to another to form a natural unicast media{{snd}}data transmitted at one end is received by only one machine at the other end. However, as illustrated in the diagrams, nodes can forward packets to create multicast or broadcast distributions from naturally unicast media. Likewise, traditional [[Ethernet]] ([[10BASE5]] and [[10BASE2]], but not the more modern [[10BASE-T]]) are natural broadcast media{{snd}}all the nodes are attached to a single long cable and a packet transmitted by one device is seen by every other device attached to the cable. Ethernet nodes implement unicast by ignoring packets not directly addressed to them. A [[wireless network]] is naturally multicast{{snd}} all devices within a reception radius of a transmitter can receive its packets. Wireless nodes ignore packets addressed to other devices, but require forwarding to reach nodes outside their reception radius.

The forwarding decision is generally made using one

of two processes: [[routing]], which uses information

==Decisions==

encoded in a device's address to infer its ___location

At nodes where multiple outgoing links are available, the choice of which, all, or any to use for forwarding a given packet requires a decision-making process that, while simple in concept, is sometimes bewilderingly complex. Since a forwarding decision must be made for every packet handled by a node, the total time required for this can become a major limiting factor in overall network performance. Much of the design effort of high-speed [[Router (computing)|routers]] and [[network switch|switches]] has been focused on making rapid forwarding decisions for large numbers of packets.

on the network, or [[bridging (networking)|bridging]], which makes no

assumptions about where addresses are located and

The forwarding decision is generally made using one of two processes: [[routing]], which uses information encoded in a device's address to infer its ___location on the network, or [[bridging (networking)|bridging]], which makes no assumptions about where addresses are located and depends heavily on broadcasting to locate unknown addresses. The heavy overhead of broadcasting has led to the dominance of routing in large networks, particularly the [[Internet]]; bridging is largely relegated to small networks where the overhead of broadcasting is tolerable. However, since large networks are usually composed of many smaller networks linked together, it would be inaccurate to state that bridging has no use on the Internet; rather, its use is localized.

depends heavily on [[broadcasting (networks)|broadcasting]] to located unknown

addresses. The heavy overhead of broadcasting has

==Methods==

led to the dominance of [[routing]] in large networks,

A node can use one of two different methods to forward packets: [[store-and-forward]] or [[cut-through switching]].<ref>{{cite thesis |author=Stefan Haas |url=http://inspirehep.net/record/887357/files/cer-002474543.pdf |title=The IEEE 1355 Standard: Developments, Performance and Application in High Energy Physics |date=1998 |page=58 |accessdate=2015-01-16}}</ref>

particuarly the [[Internet]]; bridging is largely

relegated to small networks where the overhead

==See also==

of broadcasting is tolerable. However, since

* [[Equal-cost multi-path routing]]

large networks are usually composed of many

* [[Forwarding information base]]

smaller networks linked together, it would

* [[Node-to-node data transfer]]

be inaccurate to state that bridging has

* [[Per-hop behaviour]]

no use on the Internet; rather, its use is

* [[Port forwarding]]

localized.

==References==

{{reflist}}

{{DEFAULTSORT:Packet Forwarding}}

[[Category:Routing]]