Packet forwarding is the relaying of packets from one

network segment A network segment is a portion of a computer network. The nature and extent of a segment depends on the nature of the network and the device or devices used to interconnect end stations. Ethernet According to the defining IEEE 802.3 standards f ...

to another by nodes in a

computer network A computer network is a set of computers sharing resources located on or provided by network nodes. The computers use common communication protocols over digital interconnections to communicate with each other. These interconnections ar ...

. The

network layer In the seven-layer OSI model of computer networking, the network layer is layer 3. The network layer is responsible for packet forwarding including routing through intermediate routers. Functions The network layer provides the means of trans ...

in the

OSI model The Open Systems Interconnection model (OSI model) is a conceptual model that 'provides a common basis for the coordination of SOstandards development for the purpose of systems interconnection'. In the OSI reference model, the communications ...

is responsible for packet forwarding.

Models

The simplest forwarding model

unicast Unicast is data transmission from a single sender (red) to a single receiver (green). Other devices on the network (yellow) do not participate in the communication. In computer networking, unicast is a one-to-one transmission from one point in ...

inginvolves 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 A broadcast domain is a logical division of a computer network, in which all nodes can reach each other by broadcast at the data link layer. A broadcast domain can be within the same LAN segment or it can be bridged to other LAN segments. In te ...

, making ''broadcast'' a relative term. Less common than broadcasting, but perhaps of greater utility and theoretical significance, is

multicast In computer networking, multicast is group communication where data transmission is addressed to a group of destination computers simultaneously. Multicast can be one-to-many or many-to-many distribution. Multicast should not be confused wit ...

ing, where a packet is selectively duplicated and copies delivered to each of a set of recipients. 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 mediadata 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 Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in ...

(

10BASE5 10BASE5 (also known as thick Ethernet or thicknet) was the first commercially available variant of Ethernet. The technology was standardized in 1982 as IEEE 802.3. 10BASE5 uses a thick and stiff coaxial cable up to in length. Up to 100 stati ...

and

10BASE2 10BASE2 (also known as cheapernet, thin Ethernet, thinnet, and thinwire) is a variant of Ethernet that uses thin coaxial cable terminated with BNC connectors to build a local area network. During the mid to late 1980s this was the dominan ...

, but not the more modern

10BASE-T 1 (one, unit, unity) is a number representing a single or the only entity. 1 is also a numerical digit and represents a single unit of counting or measurement. For example, a line segment of ''unit length'' is a line segment of length 1. ...

) are natural broadcast mediaall 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 A wireless network is a computer network that uses wireless data connections between network nodes. Wireless networking is a method by which homes, telecommunications networks and business installations avoid the costly process of introducing c ...

is naturally multicast 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.

Decisions

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 routers and

switch In electrical engineering, a switch is an electrical component that can disconnect or connect the conducting path in an electrical circuit, interrupting the electric current or diverting it from one conductor to another. The most common type ...

es has been focused on making rapid forwarding decisions for large numbers of packets. The forwarding decision is generally made using one of two processes:

routing Routing is the process of selecting a path for traffic in a network or between or across multiple networks. Broadly, routing is performed in many types of networks, including circuit-switched networks, such as the public switched telephone netw ...

, which uses information encoded in a device's address to infer its location on the network, or 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

in large networks, particularly the

Internet The Internet (or internet) is the global system of interconnected computer networks that uses the Internet protocol suite (TCP/IP) to communicate between networks and devices. It is a ''internetworking, network of networks'' that consists ...

; 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.

Methods

A node can use one of two different methods to forward packets:

store-and-forward Store and forward is a telecommunications technique in which information is sent to an intermediate station where it is kept and sent at a later time to the final destination or to another intermediate station. The intermediate station, or node in ...

cut-through switching Rat running (also known as rodent running, cut-through driving, or dive-bombing) is the practice by motorists of using residential side streets or any unintended short cut such as a parking lot, delivery service lane or cemetery road instead o ...

References

{{DEFAULTSORT:Packet Forwarding Routing

Models

Decisions

Methods

See also

References