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Modern Operating Systems by Herbert Bos and Andrew S. Tanenb...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf
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Modern Operating Systems by Herbert Bos and Andrew...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Modern Operating Systems by Herbert...
Modern_Operating_Systems_by_Herbert_Bos_and_Andrew_S._Tanenbaum_4th_Ed.pdf-M ODERN O PERATING S YSTEMS
Page 602
SEC. 8.3
DISTRIBUTED SYSTEMS
571
The Internet
The Internet evolved from the ARPANET, an experimental packet-switched
network funded by the U.S. Dept. of Defense Advanced Research Projects Agency.
It went live in December 1969 with three computers in California and one in Utah.
It was designed at the height of the Cold War to a be a highly fault-tolerant net-
work that would continue to relay military traffic even in the event of direct nuclear
hits on multiple parts of the network by automatically rerouting traffic around the
dead machines.
The ARPANET grew rapidly in the 1970s, eventually encompassing hundreds
of computers. Then a packet radio network, a satellite network, and eventually
thousands of Ethernets were attached to it, leading to the federation of networks we
now know as the Internet.
The Internet consists of two kinds of computers, hosts and routers.
Hosts
are
PCs, notebooks, handhelds, servers, mainframes, and other computers owned by
individuals or companies that want to connect to the Internet.
Routers
are spe-
cialized switching computers that accept incoming packets on one of many incom-
ing lines and send them on their way along one of many outgoing lines.
A router is
similar to the switch of Fig. 8-28(b), but also differs from it in ways that will not
concern us here. Routers are connected together in large networks, with each router
having wires or fibers to many other routers and hosts. Large national or world-
wide router networks are operated by telephone companies and ISPs (Internet Ser-
vice Providers) for their customers.
Figure 8-29 shows a portion of the Internet.
At the top we have one of the
backbones, normally operated by a backbone operator.
It consists of a number of
routers connected by high-bandwidth fiber optics, with connections to backbones
operated by other (competing) telephone companies. Usually, no hosts connect di-
rectly to the backbone, other than maintenance and test machines run by the tele-
phone company.
Attached to the backbone routers by medium-speed fiber optic connections are
regional networks and routers at ISPs.
In turn, corporate Ethernets each have a
router on them and these are connected to regional network routers. Routers at
ISPs are connected to modem banks used by the ISP’s customers. In this way,
every host on the Internet has at least one path, and often many paths, to every
other host.
All traffic on the Internet is sent in the form of packets. Each packet carries its
destination address inside it, and this address is used for routing. When a packet
comes into a router, the router extracts the destination address and looks (part of) it
up in a table to find which outgoing line to send the packet on and thus to which
router. This procedure is repeated until the packet reaches the destination host.
The routing tables are highly dynamic and are updated continuously as routers and
links go down and come back up and as traffic conditions change.
The routing
algorithms have been intensively studied and modified over the years.
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