Networking and Internetworking - Guide to Cloud Computing for Business and Technology Managers - page 51

2.2.3.1.7 Application Layer
The application layer contains a variety of protocols that are commonly
needed by users. One widely used application protocol is (HyperText
Transfer Protocol), which is the basis for the WWW. When a browser wants
a Web page, it sends the name of the page it wants to the server hosting the
page using HTTP. The server then sends the page back. Other application
protocols are used for file transfer, electronic mail, and network news.
2.2.3.2 TCP / IP Reference Model
Let us now turn from the OSI reference model to the reference model used in
the grandparent of all wide area computer networks, the ARPANET, and its
successor, the worldwide Internet. Although we will give a brief history of
the ARPANET later, it is useful to mention a few key aspects of it now. The
ARPANET was a research network sponsored by the DoD (U.S. Department
of Defense). It eventually connected hundreds of universities and government
installations, using leased telephone lines. When satellite and radio networks
were added later, the existing protocols had trouble interworking with them,
so a new reference architecture was needed. Thus, from nearly the beginning,
the ability to connect multiple networks in a seamless way was one of the major
design goals. This architecture later became known as the TCP/IP Reference
Model, after its two primary protocols. Given the DoD's worry that some of
its precious hosts, routers, and Internetwork gateways might get blown to
pieces at a moment's notice by an attack from the Soviet Union, another major
goal was that the network be able to survive loss of subnet hardware, without
existing conversations being broken off. In other words, the DoD wanted con-
nections to remain intact as long as the source and destination machines were
functioning, even if some of the machines or transmission lines in between
were suddenly put out of operation. Furthermore, since applications with
divergent requirements were envisioned, ranging from transferring files to
real-time speech transmission, a flexible architecture was needed.
2.2.3.2.1 Link Layer
All these requirements led to the choice of a packet-switching network
based on a connectionless layer that runs across different networks. The
lowest layer in the model, the link layer, describes what links such as serial
lines and classic Ethernet must do to meet the needs of this connectionless
Internet layer. It is not really a layer at all, in the normal sense of the term,
but rather an interface between hosts and transmission links. Early material
on the TCP/IP model has little to say about it.
2.2.3.2.2 Internet Layer
The Internet layer is the linchpin that holds the whole architecture together.
Its job is to permit hosts to inject packets into any network and have them