CS2005 Exam Revision

What is the internet?

The internet is a computer network which interconnects billions of devices throughout the world.

How are end systems connected together?

End systems are connected together though a network of communication links and packet switches

Can different links transmit data at different rates?

Yes

How is the Transmission Rate of a link measured?

The Transmission Rate of a link is measured in bits/seconds

What happens once an End-System has data to send to another End-System?

The sending End-System segments the data and adds header byes to each segment

How do you get Packets?

The sending End-System segments data and adds header byes to each segment which results in packages of information called Packets

What are Packets?

Packages of information which we call Packets

What happens when you create packets?

The packets are then sent through the network to their end destination (The End-System) where they can be assembled back into the original data.

What is a Packet Switch?

A packet switch takes a packet arriving on one an incoming communication link and forwards that packet on one an outgoing communication links

What is the process for sending Data?

When an End-System has data to send to another End-System the Sending End-System segments the data and adds header bytes to each segment.

This creates packets.

The packets are then sent through the network to their end destination where they will be assembled back into the original data.

What are the two main types of Packet Switches?

Routers & Link-Layer Switches

What do packet switches do?

Forward their packets towards their end destination

Where are Link-Layer Switches usually used?

In Access Networks

Where are Routers typically used?

In the core of the Network

What is a Route / Path?

It is the sequence of Communication Links & Packet Switches taken by a packet from the sending End-System to the receiving End-System

What are Packet-Switched Networks?

They transport packets.

What is another way for thinking of Packet-Switched Networks

They are like transportation networks like highways or roads.

Packets => Trucks

Communication Links => Highways & Roads

Packet Switches => Intersections

End Systems => Buildings

Just like a truck travels through a transportation network, a packet takes a path through a computer network

How do End-Systems access the internet?

Through Internet Service Providers (ISPs)

What does an ISP consist of?

Each ISP is in itself is a network of packet switches and communication links

What are the types of network access ISPs provide?

1. Residential broadband access:

a) Cable Moden

b) DSL

2. High Speed Local Area network Access

3. Mobile Wireless Access

What do ISPs do?

ISPs give internet access to content providers, by connecting websites and video servers directly to the internet.

Do ISPs need to be interconnected?

Yes.

Since the internet is all about connecting end systems to each other, ISPs that provide access to End-Systems must also be interconnected.

How are lower-tier ISPs connected?

Lower Tier ISPs are interconnected through national and international upper-tier ISPs

What are upper-tier ISPs made up of?

An upper tier ISP is made up of high speed routers interconnected by high-speed fibre optic links.

How does is an ISP managed & what does it run?

Each ISP network whether it is upper tier or lower tier, is managed independently and runs the IP Protocol

What are some examples of communication links?

Communication Links:

Coaxial Cable

Cable Wire

Optical Fibre

Radio, Satellite

What do protocols do in simple terms?

They control the transmission of information

What are some examples of protocols?

TCP

TP

HTTP

SMTP

802.11

What do protocols all need to adhere to and why?

All protocols need to adhere to the Internet Standards, in order to make sure that all protocols can talk to each-other.

What do End-Systems provide for applications?

End Systems provide programming interfaces to applications (Socket Interface)

What is a protocol?

A protocol is a set of rules which define the format and the order of messages that are exchanged between two or more communicating entities, as well as the actions which are taken on transmission and/or receipt of a message or other event

Give some examples of the pieces which make up a Network

Protocols

Hardware

Routers

Hosts

Software

Applications

What are the 7 ways through which a network is organised?

Application

Presentation

Session

Transport

Network

Link

Physical

How does the Application Layer help Organise a Network?

By supporting Network Applications

Give some examples of the Application Layer

FTP, SMTP, HTTP

How does the Presentation Layer help Organise a Network?

It allows applications to interpret the meaning of data

Though it only happens in the ISO/OSI Models and NOT in the internet protocol stack

Give some examples of the Presentation Layer

Encryption

Compression

How does the Session Layer help Organise a Network?

It synchronises checkpointing and the recovery of data exchange,

Though this only happens in the ISO/OSI model and NOT in the internet protocol stack

How does the Transport Layer help Organise a Network?

It covers Process <-> data transfer

Give some examples of the Transport Layer

TCP & UDP

How does the Network Layer help Organise a Network?

It helps with the routing of datagrams from the source to destination

Give some examples of the Network Layer

IP, Routing, Protocols

How does the Link Layer help Organise a Network?

It handles the data transfer between neighbouring network elements

Give examples of the Link Layer

Ethernet, 802.11 (wifi), PPP

How does the Physical Layer help Organise a Network?

It's the actual bits 'on the wire'

What does the internet protocol stack look like?

1. Application

2. Transport

3. Network

4. Link

5. Physical

What does the ISO/OSI reference model look like?

1. Application

2. Presentation

3. Session

4. Transport

5. Network

6. Link

7. Physical

What are Network Applications?

Network applications are programs that run on different end-systems and communicate with eachother over the network

Give an example of a Network Application use

Web Applications

When developers write software for network applications, who do they write the software for?

Developers who write network applications write software which will run on End-Systems, however they don't need to write code for network core devices

Give some examples of Network Core Devices

Routers, Switches

What are the two main types of Network Applications?

Client / Server

Peer - to - Peer

What is a process?

A program while executing is called a Process

How do processes communicate?

Processes communicate within the same host using Inter-Process

Communication which is defined by the OS

Who is the Client in the context of a communication session between processes?

In the context of a communication session between a pair of processes, the process that initiate communication is called the client.

What is the definition of the client

The one who initially contacts the other process at the beginning of the session

Who is the Server in the context of a communication session between processes?

The process which waits to be contacted to the begin the session is known as the server

In P2P architectures, do End-Systems both have client & server processes?

Yes

What is a socket?

The interface between the process and the network is called a socket

A Socket is the software interface between the application layer and the transport layer

It's also called the Application Programming Interface (API) between the application and the network

On the internet, how is the host identified?

By the host's IP Address

How is a process identified?

By a port number

More specifically, it's the receiving port

How does a Client / Server work?

You have an always-on host which is called the server, this takes requests from multiple hosts which we call clients

Do clients communicate directly with each other in a Client / Server architecture?

No, clients don't communicate with eachother directly, it's always through the host

In a Client / Server network does the server have a fixed address?

Yes! The server has a fixed address known as an IP-Address

In a Client / Server network where do servers usually live and why?

Servers usually live in data centres so they're easier to scale up or down

What is encapsulation?

Encapsulation is a method for designing modular communication protocols where logically separate functions in the network are abstracted from their underlying structures by:

Inclusion or information hiding within higher objects

Do you need an Always-On-Server in P2P networks? (explain why)

You don't have an always-on server, end systems also known as peers communicate directly to other peers to request / provide service to other peers

Are P2P networks self-scaling?

Yes! P2P networks self scale since new peers bring new service capacity and demand

How are peers connected in a P2P network and does they're IP Address change?

Peers are intermittently connected and they change IP Addresses

What is the structure and management of a complex management like in a P2P network?

P2P networks have a decentralised structure and a complex management

What transport services does an App need?

Data integrity

Throughput

Timing

Security

Why does an App need Data Integrity as part of it's Transport Services?

Some apps (like file transfer, web transactions) require 100% reliable data transfer, other apps (like audio) can tolerate some loss

Why does an App need Throughput as part of it's Transport Services?

Some apps (like multimedia) require a minimum amount of throughput to be "effective", other apps ("Elastic Apps" like email, web) make use of whatever throughput they get

Why does an App need Timing as part of it's Transport Services?

Some apps (E.g: Internet telephony, interactive games) require low delay to be "effective"

Why does an App need Security as part of it's Transport Services?

Can provide encryption, data integrity, end point authentication, etc

What transport services are provided by the internet?

TCP Services & UDP Services

What TCP Services are provided/not provided by the internet?

Reliable Transport, Flow Control, Congestion Control

It doesn't provide: Timing, Minimum Throughput Guarantee, Security

Connection-Orientated: Set-up between client & server processes is required

What UDP Services are provided/not provided by the internet?

Unreliable data transfer between the sending & receiving process

It doesn't provide: Reliability, Flow Control, Congestion Control, Timing, Throughput Guarantee, Security, Connection Setup

What is HTTP? and what port does it use?

Hypertext Transfer Protocol (HTTP), the web's application layer protocol, it uses port 80

What does a webpage consist of and how can you access them

A web page consists of different objects and the base HTML-File includes multiple references to these objects, each object is addressable by a URL

What type of protocol is HTTP?

HTTP is a client/server protocol, where the client usually sends a browser request which it then receives and "displays" web objects

What does HTTP use as it's underlying protocol?

TCP

How does HTTP use TCP as its underlying transport protocol?

The HTTP Client first initiates a TCP connection with the server

Once the connection is established, the browser processes and the server processes access TCP through their socket interfaces

With a web server what does a client send and receive?

- The client sends HTTP request messages and receives request messages and sends response messages

What are the two types of HTTP connections?

Persistent & Non-Persistent

What is a Persistent HTTP connection?

Where multiple objects can be sent over single TCP connection

What is a Non-Persistent HTTP connection?

Where at most one object is sent over a TCP connection and then the connection is closed requiring multiple connections to downloading multiple objects

What is the Round-Trip Time (RTT)

What does this include?

It's the time it take s for a small packet to travel from client to server and then back to the client

Including delays, packet-propagation delays, packet queuing delays, in intermediate routers and switches, and packet-processing delays

What is the time needed to request a HTML file (roughly) in RRTs

The time needed to request and receive an HTML file (roughly) is 2RTTs + the file transmission time.

What are the two types of HTTP Messages?

There two types of HTTP messages. Request message and response message.

Why does HTTP use cookies?

Because a HTTP server is stateless, usually it's useful that the server keeps the session state information, which is why HTTP uses cookies

What are the four components of a HTTP Cookie?

1. a cookie header line in the HTTP response message

2. a cookie header line in the HTTP request message

3. a cookie file kept on the user's end system and managed by the user's browser

4. a back-end database at the Web site.

What is a Web Cache?

A web cache satisfies HTTP requests on behalf of an origin Web server, it's both a client (to the original server) and a server (to the client)

What does Web Caching reduce and why?

The response time and network traffic

Because it's an access link to the internet

What does a web cache do?

A web cache stores a copy of a requested object in its local storage and sends a copy to the client browser.

What does HTTP use to avoid 'stale data'?

To avoid stale data, HTTP uses conditional GET manifested by the 'If-Modified-Since:' and 'Last-Modified:' header lines.

What are the three major components of email?

User Agents

Mail Servers

SMTP (Simple Mail Transfer Protocol)

What are User Agents when it comes to email?

User agents: They compose, edit, read and save (outgoing, incoming messages stored on server) mail messages.

What are Mail Servers when it comes to email?

Mail servers: They have a mailbox ( for incoming messages), a message queue (outgoing messages) and an SMTP implementation.

What is SMTP?

Simple Mail Transfer Protocol (SMTP): This is a protocol between mail servers to send email messages

Client: sending email messages

Server: receiving email messages