CHAPTER 1 : INTRODUCTION TO WEB APPLICATIONS

1. Why layering?

1.1. o Identify relationship of complex system’s pieces. o Interact with layer above it or layer below it. o Maintain/ update system easily

2. TCP/IP Reference Model

2.1. Application Layer

2.1.1. o Original TCP/IP specification described a number of different applications that fit into the top layer of the protocol stack. • HTTP_ (Hypertext Transfer Protocol) • FTP_(File Transfer Protocol) • SMTP_(Simple Mail Transport Protocol) • DNS_(Domain Name System)

2.2. Transport Layer

2.2.1. o The interface between the application layer and the complex hardware of the network. o Designed to allow peer entities on the source & destination hosts to carry on conversations.

2.3. Network Layer

2.3.1. o Job is to connect local networks, thus establishing the Internet. o The layer injects packets into any network and have them travel independently to the destination.

2.4. Data Link Layer

2.4.1. o Interfaces the TCP/IP protocol stack to the physical network. o contains communication technologies for a local network

3. TCP Implementation

3.1. o Data is divided up into packets by the operating system o Packets are numbered, and received packets are acknowledged

4. UDP

4.1. o Adds packet length + checksum o Guard against corrupted packets o Also source and destination ports o Ports are used to associate a packet with a specific application at each end

5. Data transfer on the Internet:

5.1. Packet switching – e.g: data is sent over the network in small pieces that are sent to routers until all the pieces arrive at their destination

5.2. Circuit switching - e.g: telephone network.

6. Networks component

6.1. o hosts o routers o links of various media o applications o protocols o hardware, software

7. TCP Packets

7.1. o Source + destination ports o Sequence number (used to order packets) o Acknowledgement number (used to verify packets are received)

8. IP

8.1. o Responsible for end to end transmission o Sends data in individual packets o Maximum size of packet is determined by the networks o Unreliable

9. Internet - collection of computers connected by network cables or through satellite links.