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Rensselaer Polytechnic Institute
CSCI-4220 – Network Programming
David Goldschmidt, Ph.D.

Early computers were highly centralized
 Bottleneck
 Single point of failure
 Users must physically go
to the computer

Proliferation of low-cost computers
connected via a network helped
get past the above disadvantages

A computer network is an interconnected
collection of autonomous computers and
devices
 Processes communicate
with one another across
the network
 Such communication is
often transparent to
end-users
P
Q

Hierarchical networks:
 Nodes are not equal
 Nodes interconnect in
a strict pattern
 Single points of failure exist!

Heterarchical networks:
 Nodes are equal(ish)
 Nodes connected to produce multiple paths

The Internet (1969) is a network that’s
 Global
 Decentralized
 Redundant
 Made up of many different types of machines
 Both hierarchical and heterarchical
 Constantly changing
 Humungous
P
Q

We use a formal network model to organize
and abstract various aspects of computer
(and other) networks
 Effectively hides the (messy)
implementation details
 Defines how communication occurs
across the network
 Often uses a layered approach

A layered model divides tasks/responsibilities
into pieces that are organized into layers
 Each piece is “solved” independently
 Well-defined (and well-documented) interfaces
between each layer are critically important
 Each layer often logically “connects” to the
same layer of another instance

Use a layered model to design an operating
system by dividing it into N levels or layers
 Layer 0 is the hardware
 Layer 1 is the kernel
 Layer N is the top-level
user interface (GUI)
 Each layer uses functions
and services of the layer
(or layers) beneath it

What’s the OSI Reference Model?
 The International Standards
Organization (ISO) proposal
for the standardization of
the various protocols used
in computer networks
 A seven-layer protocol stack

In reality, not
all layers are
always used....

The Internet
uses only
four layers:
 Application
 Transport
 Network
 Physical

Each layer on the client side logically
communicates with the same layer on
the server side
Q
server
client
intermediate router
P

Each layer prepends or appends its
information in a header or trailer
HTTP Request
TCP Hdr | HTTP Request
IP Hdr | TCP Hdr | HTTP Request
Ethernet Hdr | IP Hdr | TCP Hdr | HTTP Request | Cksum
P

A protocol is an agreed-upon convention
that defines how communication occurs
between two (or more?) endpoints
 All endpoints must “understand” and
correctly implement the protocol
 Protocols must be formally defined,
unambiguous, and well-documented
 Protocols should address error conditions
and unexpected scenarios

In a layered model, two types of protocols:
 Interface protocols describe communication
between layers (on the same endpoint)
 Peer-to-peer protocols describe communication
between two endpoints at the same layer
Q
P

A server is a process that provides access
to a centralized resource or service
 The term “server” might also refer to the
machine on which the server is running

Client/server communication:
 Server waits for incoming requests from clients
 A client is a process that sends request(s) to a
server and (usually) waits for a response

Servers typically run 24/7 and are more
complex (more interesting) than clients

Types of servers:
 An iterative server handles one request at a time
 A concurrent server can handle multiple client
requests (almost) simultaneously