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The Internet and Networked Multimedia

The Internet and Networked Multimedia

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LayeringLayering

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)

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Physical LayerPhysical 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:EthernetToken RingAsynchronous transfer mode

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Physical NetworkPhysical Network

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HubsHubs

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

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HubsHubs

Connecting LANs using Hubs has several problems:Scalability: As everything is broadcast to every host

bandwidth is quickly consumed as hosts are addedLatency: 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

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SwitchSwitch

A device that selectively forwards packets between LAN segments

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SwitchSwitch

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

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Network LayerNetwork 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

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PacketsPackets

Computer networks use packets to send information

Data being sent over the network is split into separate packetsPackets may go via different routes and arrive

in any order, but are reassembled by the receiving machine

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Internet ProtocolInternet Protocol

The main functions of the IP are:Route IP data packets (internet datagrams)

around the InternetFragment the data into smaller units if the data

is greater than a given amount (64Kb)Reassemble fragmented dataDelete datagrams that have travelled through

the network for more than a set time

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IP DatagramsIP 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.

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IP DatagramsIP Datagrams

Header Length - defines the size of header information preceding the data

D bit - Defines that the datagram should not be splitM bit and Fragment offset - Defines that the datagram

has been split and the fragment numberTime-to-live - Maximum transit time before datagram is

deletedProtocol - type of IP protocol usedHeader checksum - 16-bit pattern for error detectionSource and destination addresses - IP addressesOptions - Information on debugging and error control

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IP DatagramsIP Datagrams

Version Header Length Type of service

Total Length

Identification

Fragment OffsetD

Time-to-live Protocol

Source IP Address

Header checksum

Data (<=64kB)

Options

Destination IP Address

Header

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Internet ProtocolInternet 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

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Internet ProtocolInternet Protocol

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

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RoutersRouters

Internet Routers are used to connect networks at the network layer

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RoutersRouters

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

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RoutersRouters

Router

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RoutersRouters

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RoutersRouters

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Transport LayerTransport 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

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Transmission Control Protocol (TCP)

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

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Transmission Control Protocol (TCP)

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.

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Transmission Control Protocol (TCP)

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)

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TCP Header FormatTCP 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

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Transmission Control Protocol (TCP)

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

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Applications LayerApplications Layer

This layer supports application and end-user processes e.g WWW, e-mail, and other network software servicesTelnet and FTP are applications that exist

entirely in the application levelEverything at this layer is application-

specific and may include such factors as quality of service and user security and authentication

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Applications Layer Applications Layer

The World Wide WebThe 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

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SimplicitySimplicity

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

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End-to-end protocolsEnd-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

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FinFin

Fin