3D3-Computer-Network

Computer Networks

Remember kbps is kilo bits, kBps is kilobytes

Section 1:

Section 1.1

The internet: a “nuts and bolts” view

Billions of connected computing devices

• hosts = end systems
• running network apps at internet’s “edge”

Packet switches: forward packets (chunks of data) (moves packets)

• routers, switches

Communication links

• fiber, copper, radio, satellite
• Transimission rate is called the bandwidth

Networks

• Collections of devices, routers, links managed by an organization.

Internet: “network of networks”

• Interconnected internet service providers (ISP)

Protocols are everywhere

• which control sending, receiving of messages
• Example: HTTP(Web), streaming video, Skype, TCP, IP, Ethernet, WiFi, 4G

Internet standards (who define the protocols)

• RFC: Request for Comments
• IETF: Internet Engineering Task Force

The Internet: a “services” view

Infrastructure that provides services to applications:

• Web, streaming video, email, games, e-commerce …

Provides programming interface to distributed applications:

• hooks allowing sending/receiving apps to connect to, use the internet transport service
• provides service options, analogous to postal service

Section 1.2

Frequency division multiplexing used in cables as well to send data on different frequenies.

Section 1.3

Packet switching

• Hosts break application layer messages into packets.
• The network forwards packet from one router to the next, across links on path from source to destination.

Forwarding is a local action in the router, which looks up where the pack is going through a table.

Routing is a global action that routes the packet to the destination.

• store and forward approach.

• Queueing, when packets come faster than it can send. if the buffer is filled then packets can be lost.

• Great for bursty data

Circuit switching

• Reseverd end to end resources.
• Great for continuous data.
• FDM AND TDM

Section 1.4

Throughput

• Rate at awhich bits are being sent from sender to receiver. Can be instantaneous or average.
• Bottleneck link is the link on the end to end path that constrains the end to end throughput
• Always consider the bottleneck or the smallest bit rate for the throughput value.
• Time = filesize/throughput

Section 1.5

• Transport layer encapsulates the application layer message M with a transport layer-layer header H_t to create a transport layer segment.

• Network layer encapsulates the transport layer segment [H_t

  M] with a network layer-layer header H_n to create a network layer datagram.

• Link layer encapsulates the Link layer segment [H_n

  H_t

  M] with a link layer-layer header H_l to create a link layer frame.

Section 1.6

Bad attackers

• Packet sniffing: shared network, can access everything once they are on the network.
• IP spoofing: injection of packet with false source address.
• Denial of Service (DoS): overwhelming the host by sending loads of packets.

Lines of defence

• Authentication
• confidentiality
• integrtity checks
• access restrictions
• firewalls

Section 1.7

Section 2: Application Layer

Section 2.1: Principles of Network Applications

Client-server

Server

• always on host
• permanent IP address
• In data centers for scaling

Client

• Communicates with the server
• May be intermittenly connected
• can have dynamic ip address
• do not communicate with directly with each other(client)
• examples: HTTP, IMAP, FTP

Peer-peer

• doesnt have always on servers
• peers request and provide services from other peers
  • Adds to self scalability
• Peers are intermittenly connected and change IP addresses
  • Hard to manage
• Example: P2P file sharing

Processes communicating

Process are programs running within a hosst

• In the same host, two processess communicaate using inter-process communication
• processes in differnet hosts communicate by exchaning messages

Process that initiates the communication is called the client process

Process that waits to be contacted is the server process

Sockets

• Process sends/receives messages to/from its socket
• Sockets can be thought of as doors
  • Sending process sends messages outdoor
  • sending process relies on the transport infrastructure
  • Two sockets involved: one on each side

Addressing process

• This is basically the address of the processes (door)
• To receive messages, the process must have an identifier
• Host device has unique 32-bit IP address
• Identifier includes both IP addresses and port numbers

Application layer protocol defines:

RFC: Request for comments

• defines the type of messages exchanged
  • Request, response etc
• message syntax
  • what fields in the message and how fields are delineated
• message semantics
  • meaning of information in fields
• Rules for when and how processes send and respond to messages

Two types of protocols

• Open protocols
  • Defined in RFCs, everyone can access the definition
  • allows interoperability
  • example HTTP. SMTP
• Proprietary protocols
  • example: skype, zoom

What transport service does an app need

• Data Integrity
  • some apps require 100% reliable data transfer
  • example: file transfer, web transactions
  • Other apps like audio can tolerate some loss
• Timing
  • Some apps require low delay to be effective
• Throughput
  • Some apps require minimum amount of throughput to be effective
  • other apps can make use of whatever they get (elastic)
• Security
  • encryption, data integrity

Internet transport protocols services

Section 2.2: Web and HTTP

• Web page consists of object that can be sotred on different web servers
• HTML file, image, audio
• URL

HTTP

• Hypertext transfer protocol.
• client server model
• uses tcp
• is stateless: doesnt maintain information about past

Types of HTTP

• Round trip time

HTTP protocol

Request

Response

Cookies

Section 2.2: HTTP

Caching servers reduces totoal time ti takes to get the object

Using conditional get to check the object is up to date

Section 2.3: EMail

• Three components: users, email servers, smtp protocol

SMTP RFC

• uses TCP to send email message from cleient to server
• Three phases
  • smtp handshaking
  • smtp transfer of messages
  • smtg closure
• command/response interaction like http
  • commands: ascii text
  • response: status code and phrase

• As smtp is used to push data, there needs to be a protocl to receive this data called the IMAP

2.4 Domain name system

• distributed database
  • decentralized approach
    • makes it easy to scale
• application layer protocol
• DNS services
  • hostname to ip address translation
  • host aliasing
  • mail server alisaisng
  • load distribution

Iteration used a lot more than recursive

Section 2.5 file transfer protocol

• Similar to HTTP
• uses TCP
• main difference is that ftp uses two parallel tcp connection to transfer a file, a control connection and data connection
• control connection: used to send control information between the two hosts, like passpord, file directorand commands like put and get files.
• data connection actaully sends the file.
• FTP is out band as it uses separate control conenctions
• HTTP is in band as it uses the control connections in the same tcp connection

Basic working of ftp is it opens the control connection, sends and gets the relevant stuff, once its allowed to send or get the file, it opens another data connection to get/send the file and hten close the data channel. If wants to send another channel then another data channel will be opened.

• FTP must maintiain the state of the user. THe server must associate the control connection wit ha specific user account. HTTP is stateless

• commands to send like: USER, PASS (Passoword), LIST (list of all files), RETR (retureive the file, this causes a data channel conenection to open and send file), STOR (stroed the file)

• replies: 331 username ok , passowrd reuqired
• ….

2.6 Video streaming

Section 3: Transport layer

3.2 Multiplexing and demultiplexing

UDP and TCP have differet ways of doing it

• UDP only cares about hte senders and destination ports
• TCP is made up a 4 tuple of the ip and the port for sender and destination

UDP

• bare bones
• best effort service
  • some segments may be lost, delevierd out of order
• connectionless
  • no handshaking
  • each segmemnt is handled independently

Checksum

• add the two 16 bit numbers
• do 1’s complement of the sum and attach that as the checksum
• works for 1 bit error flip
• But not always for two bit error flip

3.4: Reliable data transfer

rdt 3.0: adds more sequence numbers and can send more packets at once which leads to pipelining

Section 6: Link Layer

Section 6.2: Error Detection and Correction Techniques

Link for a simple example on CRC

Key points:

• XOR used for subtraction in the division
• watch the video to know when to use 1 or 0 to get the remainder stuff.