Networking and Internetworking - Guide to Cloud Computing for Business and Technology Managers - page 40

or delivered out of sequence. An acknowledgment is not sent when data are
received, and the sender or receiver is not informed when a packet is lost or
delivered out of sequence. A packet is forwarded by the router only if the
router knows a route to the destination. Otherwise, it is dropped. Packets
are dropped if their checksum is invalid or if their time to live is zero. The
acknowledgment of packets is the responsibility of the TCP protocol.
The ARPANET employed the TCP/IP protocols as a standard from 1983.
The TCP/IP application layer includes several frequently used applications:
1. Hypertext Transfer Protocol (HTTP) to allow Web browsers and
servers to send and receive WWW pages
2. Simple Mail Transfer Protocol (SMTP) to allow users to send and
receive electronic mail
3. File Transfer Protocol (FTP) to transfer files from one computer sys-
tem to another
4. Telnet to allow a remote user to log in to another computer system
5. Simple Network Management Protocol (SNMP) to allow the numer-
ous elements within a computer network to be managed from a
single point
The late 1970s saw the development of newsgroups that aimed
to provide information about a particular subject. A newsgroup
started with a name that is appropriate with respect to the con-
tent that it is providing. Newsgroups were implemented via
USENET and were an early example of client-server architecture. A
user dials in to a server with a request to forward a certain newsgroup
posting; the server then serves the request.
2.2 Computer Networks
The merging of computers and communications has had a profound influ-
ence on the way computer systems are organized. Although data centers
holding thousands of Internet servers are becoming common, the once-
dominant concept of the computer center as a room with a large computer to
which users bring their work for processing is now totally obsolete. The old
model of a single computer serving all of the organization's computational
needs has been replaced by the one in which a large number of separate
but interconnected computers do the job. These systems are called computer
networks.