ISP – 8th Recitation

•3rd exercise review

•Computer networks - Introduction

What is it good for?

Communicating and sharing resources around the world.

Common applications:

1. Web / FTP / Data Transfer

2. E-Mail / IM / VOIP / Human Communication

3. “Internet”In general, a collection of bytes (packet) is sent from one computer to another

How does it work

•Computer networking is based on layers of different protocols.

•Layer = Additional bytes in the beginning/(end) of a message.

•The “holy grail” of computer networking is the 7 layers model of OSI (half obsolete)

•In reality, fewer layers are being used and it’s sometimes hard to distinguish between layers.

Basic Layers

•Physical layer – Signals and cables

•Data-link layer – Local communication

•Network layer – End to end communication

•Transport layer – Reliability/Flow control

•Higher layers – Application based

Packet Structure

Physical Layer

•How to physically connect the computer to the network (cables/connectors)

•Signals running through the cables (voltages/encodings)

•Examples : RS232/10BASE-T/802.11(x)

Datalink Layer

•How two adjacent computers “talk” to one another.

•Tied closely to the physical layer.

•Detection and correction of errors.

•Examples : Ethernet, Token-Ring, 802.11(x).

Network Layer

•Allows transfer of data between far away computers.

•Deals with addressing and routing.

•Unreliable - data can get lost and come in the wrong order.

•Examples : IP (very few others).

Transport Layer

•Provides a transparent mean for sending data.

•Can provide reliability, order and flow control.

•Provides virtual “ports” to set apart different data.

•Often, closely tied to the network layer (TCP/IP)

•Examples : TCP/UDP (and more)

Higher Layers

•Application based.

•Endless variety...

•Examples : (HTTP/POP3/SMTP/FTP/RPC…)

Relevant Concepts

• Most programmers don’t care much about the physical and data-link layers - Too down in hierarchy and transparent otherwise.

• Network and transport layers are relevant and important to understand.

• Higher layers are program specific:1.Interface an existing program using its protocol.

2.Create a new program using a self made protocol.

Relevant Concepts

•IP Address - 32 bits long, unique to each internet node. 192.168.10.11

•Subnet mask – 32 bit long. Tells which part of the IP address designate the same “physical” network. 255.255.255.0

•Router/Gateway – A computer that’s connected to two or more “physical” networks and moves data between the two based.

•Port – a 16bit long number which helps map network data to specific applications.

How data travels

•Addressing•Routing•Ports

192.168.0.11255.255.255.0

192.168.0.15255.255.255.0

192.168.1.20255.255.255.0

192.168.0.1255.255.255.0

192.168.5.1255.255.255.0

Bu..Bu..But

“I’ve been using the internet for decades and never ever set my computer’s IP address or subnet mask, never seen a router/default gateway address and never ever contacted a web page using 4 (darn) numbers.”

Dynamic Host Configuration Protocol (DHCP)

•As we just saw, each network interface requires some network specific IP settings like a unique IP address, subnet mask, default gateway and DNS servers.•DHCP servers can sit at each local network and simplify the configuration process by allocating each network node with all its relevant IP settings.•DHCP servers allow us to easily connect to wireless hotspots and ISPs.

Domain Name Server (DNS)

•It’s hard to remember numbers so we don’t normally use IP addresses…

•DNS servers are catalogs which map IP addresses to names that are easier to remember.www.google.com instead of 74.125.53.100.

•The internet contains a hierarchy of DNS servers, each responsible for different names spaces. (.com, .il, .ac.il, google.com, tau.ac.il, . – root )

•Addresses and name spaces are globally controlled by ICANN / IANA

Interesting Applications

•Ipconfig

•Ping

•Nslookup

•Tracert

•Telnet