Enterprise Network Systems

TCPMark Clements

3 March 2008 ENS2

Last Week – Client/ Server

Cost effective way of providing more computing power

High specs for server hardware Thin client is cheap and often desirable Client is proactive, server reactive Client software is user friendly Must have a network to operate

3 March 2008 ENS3

This week – TCP/IP & Protocols

How data crosses networksCommunications model architectureTCP/ IP modelTransport layer and what it containsInternet layer and what it contains

3 March 2008 ENS4

Protocol Architecture

When data is transferred across networks, it must do so in an orderly fashion

A physical path is not sufficient on its own End (and intermediate) devices must co-

operate Communication is required between all

devices other than just the data that is being transferred

3 March 2008 ENS5

Tasks for data Transfer

Source system application has TWO choices Use reliable TCP

– Sets up communication path – Determines whether remote system is ready to

accept data – Sends data– Tears down communication path

Use unreliable UDP– Uses UDP and just sends data ‘as is’

3 March 2008 ENS6

More tasks

When data transfer is complete, end systems must communicate their readiness to break the connection (TCP)

UDP is connectionless – hopes message arrives at destination safely

3 March 2008 ENS7

Other Data Transfer Tasks

If data incompatibility occurs, a translation must be performed

If data are lost there must be recovery mechanisms in place

These are just a few of the tasks that have to be performed

Logic required to implement all this is too complex for a single software module

3 March 2008 ENS8

Solution

Break down the tasks into a set of simpler sub-tasks– implemented separately as logic modules

(software)– arranged in a vertical stack– each layer then performs a sub-set of the entire

logic– any layer may be changed without affecting any

other layer

3 March 2008 ENS9

ISO OSI Model – a blueprint

3 March 2008 ENS10

Seven Layer Model

The seven layers set out the tasks that must be performed for data transfer to take place

Top four layers reside in PC Bottom three implemented in NIC and the

network Other communication models exist too e.g.

SNA, ATM, WAP TCP/ IP etc.

3 March 2008 ENS11

TCP/ IP Protocol Suite

Today the TCP/ IP protocol suite is used by most computers worldwide

Developed in 1970s before OSI by USA TCP/ IP protocol ready to ‘go’ when OSI was

still teething Does in four layers what OSI takes seven

layers to accomplish

3 March 2008 ENS12

OSI versus TCP/ IP

Some similarities exist:

Both have Transport layer

and Application layer

TCP/ IP is a practical implementation of the OSI ‘blueprint’

3 March 2008 ENS13

Application Addressing

Transport Layer Duties When data arrives at a computer it must be delivered

to the correct application– We don’t want email going to FTP program etc.

Transmission Control Protocol (TCP) deals with this– Applications are given a port number– “Well known” port numbers – 21, 23, 25, 53, 80, 110

Port numbers are sent along with the data we wish to transfer to identify the process on the end system that the data being carried is intended for

3 March 2008 ENS14

Example - Web Page Request

Segment formed at transport layer Destination port 80 (on web server) Source port taken from stack of free ports e.g. 5645 This relates to the browser window (or tab) you made

the request from Network delivers request to web server Reply segment has source port 80 and destination port

5645 Your PC now knows it has web content and which

window (or tab) to display results in

3 March 2008 ENS15

Computer Addressing

To have a world-wide delivery system we need a global addressing system

Each entity needs a unique address Internet Protocol (IPv4) addressing e.g. 193.60.61.124 This is dealt with at the Internet layer

3 March 2008 ENS16

TCP/ IP Concepts

3 March 2008 ENS17

Peer & Protocol Layers

Application layer communicates with application layer on each of the two end entities

Transport layer communicates with transport layer on end entities

Network, Data-link & Physical layers are protocol layers

3 March 2008 ENS18

What is a PDU?

PDU is a protocol data unit This is the data that is passed from layer to

layer in host systems Data are added as the original data from the

application is processed by the layers Our communication works its way down the

stack to the Physical layer Crosses the network then rises back up the

opposite stack in the far end-system

3 March 2008 ENS19

Application layer to Cabling

3 March 2008 ENS20

User-data Progression

Application makes data e.g. email client– Only email program understands an email

Reliable – uses TCP Data are passed to TCP layer and TCP

header added – email port identity– sequence number & checksum

3 March 2008 ENS21

TCP PDU progress

Passed to IP for network header containing– destination addressing using IP number– source addressing using IP number– facilities requests e.g. priority

Now the PDU is passed to Network Access Layer

3 March 2008 ENS22

Network Access Layer

Here a header and trailer is added with:– subnetwork destination (MAC) address

e.g. router, communications server, host etc.– subnet facilities requests– error checking data appended

This header is used for the next hop only Now the complete PDU can be transmitted onto the

Physical medium – off to subnet destination address

3 March 2008 ENS23

Arrival at destination

When the data arrive at final destination, the PDU is passed back up the protocol stack

At each layer the header contents are read and acted on then the header is removed

Data arrive at correct application process port - email program, web browser etc.

3 March 2008 ENS24

TCP/ IP Applications

HTTP, Hypertext Transfer Protocol which is used for request and delivery of web pages.

SMTP, Simple Mail Transfer Protocol for emails

FTP, File Transfer Protocol for transfer of data

TELNET, for remote logging into computers

3 March 2008 ENS25

Conclusion

Protocol architecture needed to contain a set of communication rules

TCP/ IP dominates world communications SMTP, FTP & TELNET all rely on TCP/ IP Port numbers are associated with application

processes in Transport Layer IP addressing is dealt with in the Internet layer