1. SEPT, 2005CSI 41181 Part 2.2 Protocols and Protocol Layering Robert Probert, SITE, University of Ottawa

2. SEPT, 2005CSI 41182 Protocol Agreement about communication Specifies Format of messages Meaning of messages Rules for exchange Procedure for handling problems

3. SEPT, 2005CSI 41183 Need for Protocols Hardware is low level Many problems can occur Bits corrupted or destroyed Entire packet lost Packet duplicated Packets delivered out of order

4. SEPT, 2005CSI 41184 Need for Protocols (continued) Need mechanisms to distinguish among Multiple computers on a network Multiple applications on a computer Multiple copies of a single application on a computer

5. SEPT, 2005CSI 41185 Set of Protocols Work together Each protocol solves part of communication problem Known as Protocol suite Protocol family Designed in layers

6. SEPT, 2005CSI 41186 Plan for Protocol Design Intended for protocol designers Divides protocols into layers Each layer devoted to one subproblem Example: ISO 7-layer reference model

7. SEPT, 2005CSI 41187 Illustration of the 7-Layer Model Defined early Now somewhat dated Does not include internet layer!

8. SEPT, 2005CSI 41188 ISO Layers Layer 1: Physical Underlying hardware Layer 2: Data Link (media access) Hardware frame definitions Layer 3: Network Packet forwarding Layer 4: Transport Reliability

9. SEPT, 2005CSI 41189 ISO Layers (continued) Layer 5: Session Login and passwords Layer 6: Presentation Data representation Layer 7: Application Individual application program

10. SEPT, 2005CSI 411810 Layers and Protocol Software Protocol software follows layering model One software module per layer Modules cooperate Incoming or outgoing data passes from one module to another Entire set of modules known as stack

11. SEPT, 2005CSI 411811 Illustration of Stacks

12. SEPT, 2005CSI 411812 Layers and Packet Headers Each layer Prepends header to outgoing packet Removes header from incoming packet

13. SEPT, 2005CSI 411813 Scientific Layering Principle Software implementing layer N at the destination receives exactly the message sent by software implementing layer N at the source

14. SEPT, 2005CSI 411814 Illustration of Layering Principle

15. SEPT, 2005CSI 411815 Protocol Techniques For bit corruption Parity Checksum CRC For out-of-order delivery Sequence numbers Duplication Sequence numbers

16. SEPT, 2005CSI 411816 Protocol Techniques (continued) For lost packets Positive acknowledgement and retransmission For replay (excessive delay) Unique message ID For data overrun Flow control

17. SEPT, 2005CSI 411817 Flow Control Needed because Sending computer system faster than receiving computer Sending application faster than receiving application Related to buffering Two forms Stop-and-go Sliding window

18. SEPT, 2005CSI 411818 Stop-And-Go Flow Control Sending Side Transmits one packet Waits for signal from receiver Receiving side Receives and consumes packets Transmits signal to sender Inefficient

19. SEPT, 2005CSI 411819 Sliding Window Flow Control Receiving side Establishes multiple buffers and informs sender Sending side Transmits packets for all available buffers Only waits if no signal arrives before transmission Receiving side Sends signals as packets arrive

20. SEPT, 2005CSI 411820 Illustration of Sliding Window on Sending Side Window tells how many packets can be sent Window moves as acknowledgements arrive

21. SEPT, 2005CSI 411821 Performance Stop-and-go Slow Useful only in special cases Sliding window Fast Needed in high-speed network

22. SEPT, 2005CSI 411822 Comparison of Flow Control

23. SEPT, 2005CSI 411823 Why Sliding Window? Simultaneously Increase throughput Control flow Speedup T w = min(B, T G  W where B is underlying hardware bandwidth T W is sliding window throughput T G is stop-and-go throughput W is the window size

24. SEPT, 2005CSI 411824 Congestion Fundamental problem in networks Caused by traffic, not hardware failure Analogous to congestion on a highway Principle cause of delay

25. SEPT, 2005CSI 411825 Illustration of Architecture That Can Experience Congestion Multiple sources Bottleneck

26. SEPT, 2005CSI 411826 Congestion and Loss Modern network hardware works well; most packet loss results from congestion, not from hardware failure