| Pcket Forwarding In Network Layer |
Forwarding is the relaying of packets from one network segment to
another by nodes in a computer network.
A unicast forwarding pattern, typical of many networking
technologies including the overwhelming majority of Internet traffic
A multicast forwarding pattern, typical of PIM
A broadcast forwarding pattern, typical of bridged Ethernet
The simplest forwarding model - unicasting - 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 multicasting, 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 form natural unicast media - 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 - 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 - 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.
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 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 overall
network performance. Much of the design effort of high-speed routers and
switches 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, 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
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.
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