Computer Networks (lecture 9)

Full information
http://www.stanford.edu/class/cs101/network-1-introduction.html
http://www.stanford.edu/class/cs101/network-2-internet.html

Outline
How to send data?
Dividing into packets
Protocols (HTTP, FTP, SMTP)
Error correction
Types of network
LAN, WAN, MAN
How to send data to correct address?
Ethernet, and MAC-address
TCP-IP protocol, IP-address
IPv4, IPv6
DNS, domain names
How to send data faster?
P2P, Torrents

Basics of network
The computer network is like a phone system
When a user enters sdu.edu.kz in browser:
the browser sends a request to sdu.edu.kz (request is small, weights 1 KB)
sdu.edu.kz server responses to the browser
(response is bigger can weights 200 KB).

LAN, WAN, MAN
A LAN (local area network) is a group of computers and network devices connected together, usually within the same building. By definition, the connections must be high speed and relatively inexpensive (e.g., token ring or Ethernet). Most Indiana University Bloomington departments are on LANs.
A MAN (metropolitan area network) is a larger network that usually spans several buildings in the same city or town. The IUB network is an example of a MAN.
A WAN (wide area network), in comparison to a MAN, is not restricted to a geographical location, although it might be confined within the bounds of a state or country. A WAN connects several LANs, and may be limited to an enterprise (a corporation or an organization) or accessible to the public. The technology is high speed and relatively expensive. The Internet is an example of a worldwide public WAN.

Packets data from here to there
Problem:
Send image from one computer to another on ethernet
e.g. 50KB image.jpg (50,000 bytes)
How to send the image.jpg on the wire? (on right side)
Packets
Divide bytes of image.jpg into packets
Say each packet is 1500 bytes (varies)
Then image.jpg divides into about 32 packets
Ethernet: transmit one packet between computers

dividing file into packets

Problem:
While sending packets through errors may occur
Solution:
We must check if received packet is same as it was sent
Error control

Each packet has extra checksum bytes, to detect if some bits were corrupted while transporting to receiver
Simple checksum can be:
last two digits of the sum of all bytes
Receiver sums all bytes and checks if sums are equal
checksum vs errors

checksum example

Ethernet is the type of computer network:
One wire shared by all the computers
No central control -- distributed, collaborative
Like talking at a party
What is Ethernet?
How to send:
Every computer has a unique address on the wire
Packet includes to:addr of recipient
Sender waits for period of silence on the wire, sends packet
Packet spreads out on wire, reaching all computers
More "broadcast" than "send"
Receive:
All computers listen to the wire all the time
Pick out packets addressed to them, ignore other packets

ethernet data sending

MAC (Media Access Control)
MAC address is burned at the factory
Every computer (phone, desktop, laptop) has its own globally unique address
On Linux MAC address can be changed
example: ab:cc:dd:12:34:56
MAC address (Physical Address)

Wi-Fi wireless networking
Similar strategy to Ethernet (simplifying)
Every computer has a radio
The radio-channel used is the shared medium
One computer transmits at a time
Everyone listens
Wi-Fi

wifi example

‘Protocol’ is a fancy word for simply saying “an agreed way to do something”. You might have heard it in a cheesy cop show -- “argh Jim, that’s against protocol!!!” -- or heard it used in a procedural sense, such as how to file a tax return or sit a driving test. We all use protocols, every day. Think of when you’re in class. The protocol for asking a question may be as follows: raise your hand, wait for a nod from the teacher then begin asking your question.
What is protocols

HTTP
The HyperText Transfer Protocol (HTTP) is the most common protocol in use on the internet. The protocol’s job is to transfer HyperText (such as HTML) from a server to your computer. It’s doing that right now. You just loaded the Field Guide from the servers where it is hosted. Hit refresh and you’ll see it in action.
HTTP uses a request/response pattern for solving the problem of reliable communication between client and server. The “ask for” is known as a request and the reply is known as a response. Both requests and responses can also have other data or resources sent along with it.

Previously .. LAN, e.g. ethernet, Wi-Fi, one house
Internet - world-wide network built on open standards
Internet is like a phone system for computers
Every computer has a unique address
Every computer can try to "call" any other computer
TCP-IP standard

IP address
Every computer on the internet has an "IP address" that identifies it (like a phone number)
The IP address is 4 bytes, written between dots, like "171.64.2.3". The left part of the address encodes in part where that IP address is in the whole internet -- for example any 171.64.(anything) is part of Stanford (like the area code of a phone number). More specifically, in my part of the Gates building, all the IP addresses begin 171.64.64.XX varying only in that last byte.

IPv4 vs IPv6
IPv4 can contain 2^8^4 = 2^32 = about 4.3 billion addresses
IPv6 can contain 2^128 addresses (about 3.4 * 10^38 addresses)
Example of IPv6:
2001:0db8:85a3:0042:1000:8a2e:0370:7334
IPv6 addresses are represented as eight groups of four hexadecimal digits separated by colons

Internet of Things
The Internet of Things (IoT) is the network of physical objects or "things" embedded with electronics, software, sensors, and network connectivity, which enables these objects to collect and exchange data.
So your refrigerator, and kettle will be online and you can send them commands.
Experts estimate that the IoT will consist of almost 50 billion objects by 2020.

What is web-site
Web-site is some resource located on web-server
Web-server is computer:
always turned on
accessible through internet
have fixed IP address. Why ?

Domain names
Basically human-readable synonyms for IP addresses
codingbat.com (synonym for 173.255.219.70)
Domain names are easy for people to remember and type
Domain system can look up an IP addr from a domain name
So when you use a domain name, it is looked up to get an IP addr for the actual packets

DNS, hosts.txt
When you enter vk.com it sends request to DNS
DNS lookups in its database and responses IP address of entered domain, and request is send to specified IP address.
On operating systems there is file which is called hosts.txt file.
In hosts.txt on each line IP address and corresponding domain name are stored.
You can match any pair of domain address IP address

Router
Router has multiple connections, copies/routes packets between them
My office computer is at 171.64.64.16
That computer connects "upstream" to router 171.64.64.1
That router handles traffic for a few local computers
Left side of computer and router IP addresses typically the same -- same neighborhood

the way by which request goes from browser to site

Note that 10.x.x.x , 172.16.x.x and 192.168.x.x addresses are special "local" IP addresses
These addresses are not valid out on the internet at large
These are translated to a real IP addr as a packet makes its way
Frequently given out by Wi-Fi routers .. why I mention them
127.0.0.1 (IPv4) is own address of computer
::1 (IPv6) is own address of computer
Local IP address

Ping and traceroute
Ping is command to check computer on some address is working
Traceroute to find which routers were visited by packet before reaching destination

Switch
A switch connects multiple computers together in a LAN. After the first data transfer, it creates a “switch table” which matches ports to connected devices by their MAC addresses.
Switches, unlike hubs, are able to differentiate between computers as the first time data passes through the switch, it looks to see which MAC addresses are connected to which ports and remembers the layout.
http://www.makeuseof.com/tag/whats-difference-routers-hubs-switches/

Hub
A hub connects multiple computers together in a Local Area Network (LAN). All information sent to the hub is then sent through each port to every device in the network.
Hubs are unable to tell one computer from another, so they receive information on one port and then blindly forward it to all other ports — whether it was intended for those computers or not.
So even though you may only want to send information to one other computer, if you have five total computers on your network, then there will be four other computers receiving data that wasn’t intended for them.

Proxy server
Server software which is located between server and client. So that all requests from computers of network go through proxy, and proxy requests to servers.
Proxy can save all requests and filter the requests to Internet

Proxy server

TOR - The Onion Router
Tor is a network that keeps users anonymous on the Internet.
It works by moving data across many Tor servers, called "hops".
The role of each server is to only move that data to another server.
With the final hop moving data to the end site.
Information is transmitted in way so it is hard to trace.

Tor network

TOR is used for?
Because TOR don’t track users, it is mostly used for illegal uses

Darknet
Darknet is located inside TOR network
TOR network recognizes .onion domain requests and sends it to specified servers
Darknet mostly contains web-sites that are illegal

Server-client network
On a client-server network, resources are located on and controlled by a central computer known as a server
client-server networks are often more expensive to set up than their peer-to-peer equivalents, as server hardware and software is more expensive than regular desktop computers

P2P network
In contrast to server-client network, P2P doesn’t have server.
All computers in P2P network contain parts of resources, and can work as client or server.

BitTorrent
BitTorrent protocol for peer-to-peer file sharing.
File is located not in one server, but on different peers.
So to download file, pieces of it are requested from different peers. After downloading piece of file, peer can immediately share it to other peers.

Because in server-client network there is only one server, and many clients.
Server has limited bandwidth, and it will be divided to many clients.
In p2p network there are many peers that share file, so bandwidth is much more.
Why loading by Torrent is usually faster than loading from server?

Firechat
Mobile application that creates network from nearby devices, sends messages without using Internet, and cellular connection, using only WiFi or Bluetooth.

Skype
Skype (Sky peer to peer) was originally hybrid of client-server and peer-to-peer technology.
Now it uses Microsoft’s supernode servers.