1.1.1 A Nuts-and-Bolts De cription

The Internet i. a computer network that interconnects hundreds of millions of computing
devices throughout the world . Not too long ago, these computing devices
were primarily traditional desktop PCs, Linux workstations, and so-called servers
that store and transmit information such as Web pages and e-mail messages. InCl'easingly,
however, nontraditional Internet end systems such a~ TVs, laptops, gaming consoles, cell phones, Web cams, automobiles, environmental sensing devices, picture
frames, and home electrical and security systems are being connected to the
Internet. Indeed, the term computer network is beginnjng to sound a bit dated, given the many nontraditional devices that are being hooked lip to the Internet. In Internet jargon, all of these devices are called hosts or end systems. As of July 2008, there
were nearly 600 million end systems attached to the Internet [ISC 2009J, not counting the cell phones, laptops, and other devices that are only intermittently connected
to the Internet.
End systems are connected together by a network of communication links and
packet switches. We ' ll see in Section 1.2 that there are many types of communication
links, which are made up of different types of physical media, including coaxial
cable, copper wire, fiber optics, and radio spectrum. Different unks can transmit
data at different rates, with the transmission rate of a link measured in bits/second.
When one end system has data to send to another end system, the sending end system
segments the data and adds header bytes to each segment. The resulting packages
of information, known as packets in the jargon of computer networks, are then
sent tllrough the network to the destination end system, where they are reassembled
into the original data.
A packet switch takes a packet arriving on one of its incoming communication
links and forwards that packet on one of its outgoing communication links. Packet
switches come in many shapes and flavors, but the two most prominent types in
today's Internet are routers and link-layer switches. Both types of switches forward
packets toward their ultimate destinations. Lin k-layer switches are typically
used in access networks, while routers are typically used in the network core. The
sequence of communication links and packet switches traversed by a packet from
the sending end system to the recei ving end system is known as a route or path
through the network. The exact amount of traffic being carried in the Internet is difficult
to estimate [Odylsko 2003]. PriMetrica lPriMetrica 20091 estimates that 10
terabits per second of international capacity was used by public Internet providers
in 2008, and that capacity doubles approximately every two years.
Packet-switched networks (which transport packets) are in many ways similar
to transportation networks of highways, roads, and intersections (which transport
vehicles). Consider, for example, a factory that needs to move a large amount of
cargo to some destination warehouse located thousands of kilometers away. At the
factory, the cargo is segmented and loaded into a neet of trucks. Each of the trucks
then independently travels through the network of highways, roads, and intersections
to the destination warehouse. At the destination warehouse. the cargo is
unloaded and grouped with the rest of the cargo arriving from the same shipment.
Thus, ill many ways, packets are analogous to trucks, communication links are analogolls
to highways and roads, packet switches are analogous to intersections, and
end systems are analogous to buildings. Just as a truck takes a path through the
tran pOitation network, a packet takes a path through a computer network.
End systems access the Internet through Internet Service Providers (ISPs),
including residential ISPs such as local cable or telephone companies; corporate
ISPs; university ISPs; and ISPs that provide WiFi access in airports, hotel s, coffee
shops, and other public places. Each rsp is in itself a network of packet switches and
communication links. ISPs provide a variety of types of network access to the end
systems, including 56 kbps dial-up modem access, residential broadband access
such as cable modem or DSL, high-speed local area net work access, and wireless access. ISPs also provide Internet access to content providers, connecting Web sites
directly to the Internet. The Internet is all about connecting end systems to each
other, so the ISPs that provide access to end systems must also be interconnected.
These lower-tier ISPs are interconnected through national and international uppertier
ISPs such as AT&T and Sprint. An upper-tier ISP consists of high-speed routers
interconnected with high-speed fiber-optic links. Each ISP network, whether uppertier
or lower-tier, is managed independently, runs the IP protocol (see below), and
conforms to certain naming and address conventions. We'll examine ISPs and their
interconnection more closely in Section 1.3.
End systems, packet switches, and other pieces of the Internet run protocols
that control the sending and receiving of information within the Internet. The
Transmission Control Protocol (TCP) and the Internet Protocol (IP) are two of
the most impOItant protocols in the Internet. The IP protocol specifies the format of
the packets that are sent and received among routers and end systems. The Internet's
principal protocols are collectively known as TCPIIP. We'll begin looking into protocols
in this introductory chapter. But that's just a start-much of this book is concerned
with computer network protocols!
Given the importance of protocols to the Internet, it's important that everyone
agree on what each and every protocol does. This is where standards come into play.
Internet standards are developed by the Internet Engineering Task Force
(rETF)[IETF 2009]. The IETF standards documents are called requests for comments
(RFCs). RFCs started out as general requests for comments (hence the name)
to resolve network and protocol design problems that faced the precursor to the
Internet. RFCs tend to be quite technical and detailed. They define protocols such as
TCP, IP, HTTP (for the Web), and SMTP (for e-mail). There are currently more than
5.000 RFCs. Other bodies also specify standards for network components, most
notably for network links. The IEEE 802 LAN/MAN Standards Committee [IEEE
802 2009], for example, speciJies the Ethernet and wireless WiFi standards.