Computer Networking{week 1}

Rensselaer Polytechnic InstituteCSCI-4220 – Network ProgrammingDavid Goldschmidt, Ph.D.

Networks

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 helpedget past the above disadvantageswhat advantages do

computer networks

provide us?

Computer networks

A computer network is an interconnected collection of autonomous computers and devices Processes communicate

with one another acrossthe network

Such communication isoften transparent toend-users

P

Q

Types of networks

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

Are you connected?

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

how many machines

are on the Internet?

Interprocess communication

P

Q

Network models

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

Layered models

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

why?

what advantages

do layered models

provide us?

Layering

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

Open Systems Interconnection (OSI) Reference Model

What’s the OSI Reference Model? The International Standards

Organization (ISO) proposalfor the standardization ofthe various protocols usedin computer networks

A seven-layer protocol stack

Open Systems Interconnection (OSI) Reference Model

In reality, notall layers arealways used....

The Internet uses only four layers: Application Transport Network Physical

Internet traffic (jam)

Each layer on the client side logically communicates with the same layer onthe server side

intermediate router

PQ clientserver

interfaces exist

between layers

Internet messages

Each layer prepends or appends its information in a header or trailer

P

Ethernet Hdr | IP Hdr | TCP Hdr | HTTP Request | Cksum

IP Hdr | TCP Hdr | HTTP Request

TCP Hdr | HTTP Request

HTTP Request

Protocols

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

Layered model protocols

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

PQ

Clients and servers

A server is a process that provides accessto 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

what are some examples of

client/server comunication?

Servers

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

how are concurrent

servers implemented?