ICS 451: Data Networks

Edo Biagioni

Department of Information and Computer Science

esb@hawaii.edu, http://www2.ics.hawaii.edu/~esb/
Lectures are Monday and Wednesday at 1:30pm in POST 126.
textbook: "Computer Networks and Internets" (Third Edition), Douglas E. Comer (Prentice-Hall).
for the textbook, you may use the 2001 or the 2004 edition.
3 projects, 2 exams and a final, quizzes, reviews, at least one homework
office hours: Mondays 11:30-12:30, Wednesdays 3pm-4pm, and Thursdays 2pm-3pm, or by appointment (or email or phone).

Networking is the engineering, science and art of designing computer networks and networking protocols.
A computer network is a hardware and software system that enables a collection of computers to communicate.
"Computer Networks are able to carry many different types of data and support a wide range of applications"
why are networks interesting? (besides jobs)
- pervasive, useful for science and technology (the Internet)
- complex systems: distributed, parallel, interoperable, fast and efficient

A network administrator manages a network, adding equipment and deciding where to place additional money or equipment.
A network designer transforms a budget into a network that will satisfy a given constraint.
This course teaches the theoretical foundations that network administrators and network designers need, but does not teach network administration or network design
A network application developer creates applications (writes programs) that run across the network
A network protocol designer designs and develops protocols, agreements about how computers and programs may communicate
network protocol design is a part of designing distributed systems, systems that work across multiple computers
This course teaches the basics of network application development, and goes in depth about network protocol design

applications are programs that run on network-connected computers
applications communicate by exchanging data (collections of bits)
applications must have ways of specifying which applications they are communicating with: addresses, ports, names

An internet provides a facility for exchanging data (bytes, buffers)
like a telephone, the application must make a connection before data can be sent, and close the connection after the transmission is complete
like a telephone, data transfer is bidirectional
the Internet Application Programming Interface (Internet API) makes it possible to transfer data between programs -- in the same way, whether on the same or different machines

In-class discussion (5 minutes): what is "data", and what does this "data" look like in a program?

a client is a program that requests a service
a server is a program that provides a service.
clients and servers may (or may not) run on different machines
the client-server model captures a common way that programs interact. Some programs may not match this model.

In the client-server model:

once the reply is received, the interaction is complete
on Unix systems, inetd is a generic server that can create a specific server on demand: when a request comes in for that server
port numbers are used in the Internet to identify the service
IP numbers (and domain names) are used in the Internet to identify the machine on which the server is running

TCP is an internet protocol that provides Connection-Oriented Service: a connection must be established by calling connect before any data can be sent or received
any connection consumes resources, and must be closed after it is no longer needed
as long as the connection is up, data can be sent on it and received from it without having to explicitly specify the peer
a connection-oriented communication is analogous to a telephone call

when we write to a file, we call open, specifying the file name, and in return get a file descriptor.
Note that many students are only familiar with fopen, which returns a FILE *: In contrast, open returns a file descriptor, which is simply a small integer that tells the OS what file you are accessing. For example, file descriptor 0 is stdin, 1 is stdout, and 2 is stderr
when we connect to a peer, we perform several system calls, specifying the peer address and the protocol to use, and in return we get a socket
the socket is a special kind of file descriptor: all the operations that can be performed on a file descriptor (especially read and write) can be performed on a socket, but some special operations can only be performed on sockets

UDP is an internet protocol that provides Connectionless Service: data can be sent and received without establishing connections
a destination address must be provided each time data is sent,
a single (unconnected) socket can be used to receive from multiple peers
a connectionless communication is analogous to a letter (email or snail-mail)