1 The Internet and Networked Multimedia

2 Layering  Internet protocols are designed to work in layers, with each layer building on the facilities provided by the layer below it Application Layer (e.g. HTTP, SMTP) Transport Layer (TCP, UPD) Physical Layer (e.g.Ethernet) Network Layer (IP)

3 Physical Layer  This defines the basic technologies which are used to route and transport Internet traffic  The physical layer consists of many different physical transport mediums, network structures and protocols:  Ethernet  Token Ring  Asynchronous transfer mode

4 Physical Network

5 Hubs  Common connection points for devices in a network  Hubs are commonly used to connect segments of a LAN  When a packet arrives at a hub, it is copied to all segments of the LAN connected to that hub

6 Hubs  Connecting LANs using Hubs has several problems:  Scalability: As everything is broadcast to every host bandwidth is quickly consumed as hosts are added  Latency: Each node in a hub-based network has to wait for an opportunity to transmit in order to avoid collisions, the latency can increase significantly as you add more nodes  Network Failure: In a typical network, one device on a hub can cause problems for other devices attached to the hub e.g wrong broadcast speed settings

7 Switch  A device that selectively forwards packets between LAN segments

8 Switch  A switch only forwards data to the intended destination, rather than broadcasting it to all the nodes connected to it  This reduces superfluous transmissions and improves network performance  Switches operate with physical addresses and NOT IP addresses

9 Network Layer  The way that the data will be sent to the recipient device is determined by the network layer  Logical protocols such as IP, routing and addressing are handled here

10 Packets  Computer networks use packets to send information  Data being sent over the network is split into separate packets  Packets may go via different routes and arrive in any order, but are reassembled by the receiving machine

11 Internet Protocol  The main functions of the IP are:  Route IP data packets (internet datagrams) around the Internet  Fragment the data into smaller units if the data is greater than a given amount (64Kb)  Reassemble fragmented data  Delete datagrams that have travelled through the network for more than a set time

12 IP Datagrams  Datagrams are transmitted separately through the internet and the received fragments are finally reassembled at the destination  The fields in the IP datagram are:  Version - TCP/IP version number  Type of service - defines the priority, reliability etc.

13 IP Datagrams  Header Length - defines the size of header information preceding the data  D bit - Defines that the datagram should not be split  M bit and Fragment offset - Defines that the datagram has been split and the fragment number  Time-to-live - Maximum transit time before datagram is deleted  Protocol - type of IP protocol used  Header checksum - 16-bit pattern for error detection  Source and destination addresses - IP addresses  Options - Information on debugging and error control

14 M IP Datagrams VersionHeader LengthType of service Total Length Identification Fragment OffsetD Time-to-liveProtocol Source IP Address Header checksum Data (<=64kB) Options Destination IP Address Header

15 Internet Protocol  IP itself does not provide any security services  Attackers with physical access to the network can listen to packets going by, introduce forged packets, and potentially intercept and alter legitimate packets

16 Internet Protocol  IP Source addresses can be easily forged so it is up to the higher-level protocols to manage these problems

17 Routers  Internet Routers are used to connect networks at the network layer

18 Routers  Routers forward packets from network to network until they reach the network connected to the destination system  Each router has a local map of the network that tells it where to forward the packet next, based on the IP address

19 Routers Router

20 Routers

21 Routers

22 Transport Layer  This layer maintains flow control of data and provides for error checking and recovery of data between devices  The Transmission Control Protocol (TCP) is used in this layer and is the most common transport protocol on the internet  Building on the packet-oriented (datagram) foundations of IP, it provides the abstraction of a reliable byte stream

23 Transmission Control Protocol (TCP)  IP in itself can not support the connection of two nodes  TCP establishes the initial connection and then maintains it for the length of the connection

24 Transmission Control Protocol (TCP)  The set-up operation for TCP is as follows: 1. TCP sends a request message to the destination machine containing its unique address and a port number. The port number is associated with the application (e.g. TELNET - 23, FTP - 21). This request is passed as a datagram.

25 Transmission Control Protocol (TCP) 2. On receiving the request the destination machine returns an acceptance message containing its own unique address and a port number 3. The source and destination address and port numbers now define the virtual connection and data can flow between the two devices (data stream)

26 TCP Header Format  TCP adds header information to data which is then passed to the IP layer Source port Destination port Sequence Number Acknowledgement Number Data Urgent Pointer Data offset Reserved Flags Window Checksum Header

27 Transmission Control Protocol (TCP)  Packets are retransmitted if sender does not receive acknowledgement from receiver  Receiver can reorder jumbled up packets using the sequence number  Provides control mechanism to make sure that the receiver is not overwhelmed by the sender transmitting data too fast

28 Applications Layer  This layer supports application and end- user processes e.g WWW, , and other network software services  Telnet and FTP are applications that exist entirely in the application level  Everything at this layer is application- specific and may include such factors as quality of service and user security and authentication

29 Applications Layer  The World Wide Web  The WWW merges the techniques of networked information and hypertext to make an easy to use, but powerful information system  Represents any information accessible over the internet as part of a seamless hypertext information space

30 Simplicity  The layered internet protocol model hides the complexity of levels  Thus applications programmers are insulated from the complexities of different network devices as well as from the complexities of implementing low-level network protocols

31 End-to-end protocols  The Internet is designed around end-to- end protocols  Interpretation of data happens on the sending and receiving systems  Hides the internal structure of the network

32 Fin