Enterprise Network Systems TCP Mark Clements

3 March 2008ENS 2 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 2008ENS 3 This week – TCP/IP & Protocols How data crosses networks Communications model architecture TCP/ IP model Transport layer and what it contains Internet layer and what it contains

3 March 2008ENS 4 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 2008ENS 5 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 2008ENS 6 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 2008ENS 7 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 2008ENS 8 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 2008ENS 9 ISO OSI Model – a blueprint

3 March 2008ENS 10 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 2008ENS 11 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 2008ENS 12 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 2008ENS 13 Application Addressing Transport Layer Duties When data arrives at a computer it must be delivered to the correct application – We don’t want 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 2008ENS 14 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 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 2008ENS 15 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 This is dealt with at the Internet layer

3 March 2008ENS 16 TCP/ IP Concepts

3 March 2008ENS 17 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 2008ENS 18 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 2008ENS 19 Application layer to Cabling

3 March 2008ENS 20 User-data Progression Application makes data e.g. client – Only program understands an Reliable – uses TCP Data are passed to TCP layer and TCP header added – port identity – sequence number & checksum

3 March 2008ENS 21 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 2008ENS 22 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 2008ENS 23 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 - program, web browser etc.

3 March 2008ENS 24 TCP/ IP Applications HTTP, Hypertext Transfer Protocol which is used for request and delivery of web pages. SMTP, Simple Mail Transfer Protocol for s FTP, File Transfer Protocol for transfer of data TELNET, for remote logging into computers

3 March 2008ENS 25 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