Packet forwarding

Forwarding is the relaying of packets from one network segmentto another by nodesin a computer network.

The simplest forwarding model - unicasting - involves a packet beingrelayed from link to link along a chain leading from the packet'ssource to its destination. However, other forwarding strategiesare commonly used. Broadcasting requires a packet to beduplicated and copies sent on multiple links with the goalof delivering a copy to every device on the network.In practice, broadcast packets are not forwarded everywhereon a network, but only to devices within a broadcast domain,making "broadcast" a relative term.Less common than broadcasting, but perhaps of greaterutility and theoretical significance is multicasting,where a packet is selectively duplicated and copiesdelivered to each of a set of recipients.

Networking technologies tend to naturally support certainforwarding models. For example, fiber optics and coppercables run directly from one machine to another form naturalunicast media - data transmitted at one end is receivedby only one machine at the other end. However, as illustratedin the diagrams, nodes can forward packets to createmulticast or broadcast distributions from naturallyunicast media. Likewise, traditional Ethernet(10BASE5 and 10BASE2, but not the more modern
10BASE-T) are natural broadcastmedia - all the nodes are attached to a single, longcable and a packet transmitted by one device is seen byevery other device attached to the cable. Ethernetnodes implement unicast by ignoring packets notdirectly addressed to them. A wireless networkis naturally multicast - all devices withina reception radius of a transmitter can receiveits packets. Wireless nodes ignore packetsaddressed to other devices, but require forwardingto reach nodes outside their reception radius.

At nodes where multiple outgoing links are available, thechoice of which, all, or any to use for forwardinga given packet requires a decisionmaking process that, while simple in concept, is ofsometimes bewildering complexity. Since a forwardingdecision must be made for every packet handled by anode, the total time required for this can becomea major limiting factor in overall network performance.Much of the design effort of high-speed routersand switches has been focused on making rapid forwarding decisionsfor large numbers of packets.

The forwarding decision is generally made using oneof two processes: routing, which uses informationencoded in a device's address to infer its locationon the network, or bridging, which makes noassumptions about where addresses are located anddepends heavily on broadcasting to locate unknownaddresses. The heavy overhead of broadcasting hasled to the dominance of routing in large networks,particularly the Internet; bridging is largelyrelegated to small networks where the overheadof broadcasting is tolerable. However, sincelarge networks are usually composed of manysmaller networks linked together, it wouldbe inaccurate to state that bridging hasno use on the Internet; rather, its use islocalized.

ee also

* port forwarding