TCOM 509 – Internet Protocols (TCP/IP) Lecture 03_b Protocol Layering

1. TCOM 509 – Internet Protocols (TCP/IP)Lecture 03_bProtocol Layering Instructor: Dr. Li-Chuan ChenDate: 09/15/2003 Based in part upon slides of Prof. K. Fall (UC-Bekeley)

2. Networks are complex! many “pieces”: hosts routers links of various media applications protocols hardware, software Protocol Layering • Data communications problems: • Hardware failure • Network congestion • Packet delay or loss • Data corruption • Data duplication

3. Protocol Layering Why? • Difficult for a single protocol to handle all these problems when dealing with complex systems. How? • Breaks down complex systems into more manageable components. • Modularization • Each layer is responsible for one part of the problem • The service provided by one layer is based solely on the service provided by layer below Disadvantages • Introduces overhead

4. ISO OSI Reference Model for Layers • Application • Presentation • Session • Transport • Network • Data Link (Hardware Interface) • Physical (Hardware Connection)

5. OSI Model Concepts • Service: what a layer does • Service interface: how to access the service • interface for layer above • Protocol (peer interface): how peers communicate • a set of rules and formats that govern the communication between two network boxes • protocol does not govern the implementation on a single machine, but how the layer is implemented between machines

6. Physical Layer (1) • Service: move information (bits) between two systems connected by a physical link • Interface: specifies how to send a bit • Protocol: coding scheme used to represent a bit, voltage levels, duration of a bit • Examples: coaxial cable, optical fiber links; transmitters, receivers

7. Datalink Layer (2) • Service: • framing (attach frame separators) • send data frames between peers on the same network medium • others: • synchronization • per-hop error control • per-hop flow control • Interface: send a data unit (frame) to a machine connected to the same physical media • Protocol: layer addresses, implement medium access control (MAC) • Example: Ethernet, Token Ring

8. Network Layer (3) • Service: • deliver a packet to specified network destination (routing and relaying of packets) • perform segmentation/reassemble • others: • packet scheduling • buffer management • Interface: send a packet to a specified destination • Protocol: define global unique addresses; construct routing tables • Examples: Internet Protocol (IP)

9. Transport Layer (4) • Service: • end-to-end data delivery to applications (demultiplexing) • optional: error-free and flow-controlled delivery • Interface: send message to specific destination • Protocol: implements end-to-end reliability and flow control • Examples: TCP and UDP

10. Application Layer (7) • Service: any service provided to the end user • Interface: depends on the application • Protocol: depends on the application • Examples: FTP, Telnet, WWW browser

11. Who Does What? • Seven layers • Lower three layers are implemented everywhere • Next four layers are implemented only at hosts Host A Host B Application Application Presentation Presentation Session Session Router Transport Transport Network Network Network Datalink Datalink Datalink Physical Physical Physical Physical medium

12. Logical Communication • Layers interacts with corresponding layer on peer Host A Host B Application Application Presentation Presentation Session Session Router Transport Transport Network Network Network Datalink Datalink Datalink Physical Physical Physical Physical medium

13. Physical Communication • Communication goes down to physical network, then to peer, then up to relevant layer Host A Host B Application Application Presentation Presentation Session Session Router Transport Transport Network Network Network Datalink Datalink Datalink Physical Physical Physical Physical medium

14. Encapsulation • A layer can use only the service provided by the layer immediate below it • Each layer may change and add a header to data packet data data data data data data data data data data data data

15. Example: Postal System Standard process (historical): • Write letter • Drop an addressed letter off in your local mailbox • Postal service delivers to address • Addressee reads letter (and perhaps responds)

16. Postal Service as Layered System Layers: • Letter writing/reading • Delivery Information Hiding: • Network need not know letter contents • Customer need not know how the postal network works Encapsulation: • Envelope Customer Customer Post Office Post Office

17. Standards Bodies • ISO: International Standards Organization • professional bureaucrats writing standards • produced OSI layering model • IETF: Internet Engineering Task Force • started with early Internet hackers • more technical than bureaucratic

18. Telnet FTP DNS TCP UDP IP Packet radio LAN OSI vs. Internet • OSI: conceptually define services, interfaces, protocols • Internet: provide a successful implementation Application Application Presentation Session Transport Transport Network Internet Datalink Net interface/ Physical Physical Internet (informal) OSI (formal)

19. Multiple Instantiations • Can have several instantiations for each layer • many applications • many network technologies • transport can be reliable (TCP) or not (UDP)

20. Internetworking Protocols

21. Telnet FTP SMTP HTTP Optical fiber Coaxial cable PacketRadio Without Layering • FTP – File Transfer Protocol • SMTP – Simple Mail Transfer Protocol • HTTP – World Wide Web protocol Application Transmission Media • Not scalable – Each new application has to be re-implemented for every transmission media

22. Telnet FTP SMTP HTTP Optical fiber Coaxial cable PacketRadio With IP Layer Application Internet Layer (IP) IP Transmission Media • Advantages • Efficient: mapping packets directly onto network media • Flexible: adding or modifying network software without the need for changing the application software • Hides low-level details from the user.

23. With IP Layering A single Internet layer module: (Interoperability) • Allows all networks to interoperate • all networks technologies that support IP can exchange packets • Allows all applications to function on all networks • all applications that can run on IP can use any network • Simultaneous developments above and below IP