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Understanding Protocols and Layering in Networking

This section discusses the essential role of protocols in communication between computers, detailing their need and the mechanisms that prevent issues like packet loss, corruption, and out-of-order delivery. It explains the concept of protocol suites and layering, illustrating the ISO 7-layer reference model's structure and functions. Key protocol techniques for ensuring data integrity and reliability, such as flow control methods (Stop-and-Go and Sliding Window), are highlighted. Additionally, it emphasizes the significance of managing congestion in network traffic to ensure optimal performance.

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Understanding Protocols and Layering in Networking

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  1. Part 2.2 Protocols and Protocol Layering Robert Probert, SITE, University of Ottawa CSI 4118

  2. Protocol • Agreement about communication • Specifies • Format of messages • Meaning of messages • Rules for exchange • Procedure for handling problems CSI 4118

  3. 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 CSI 4118

  4. 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 CSI 4118

  5. Set of Protocols • Work together • Each protocol solves part of communication problem • Known as • Protocol suite • Protocol family • Designed in layers CSI 4118

  6. Plan for Protocol Design • Intended for protocol designers • Divides protocols into layers • Each layer devoted to one subproblem • Example: ISO 7-layer reference model CSI 4118

  7. Illustration of the 7-Layer Model • Defined early • Now somewhat dated • Does not include internet layer! CSI 4118

  8. 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 CSI 4118

  9. ISO Layers (continued) • Layer 5: Session • Login and passwords • Layer 6: Presentation • Data representation • Layer 7: Application • Individual application program CSI 4118

  10. 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 CSI 4118

  11. Illustration of Stacks CSI 4118

  12. Layers and Packet Headers • Each layer • Prepends header to outgoing packet • Removes header from incoming packet CSI 4118

  13. Scientific Layering Principle Software implementing layer N at the destination receives exactly the message sent by software implementing layer N at the source CSI 4118

  14. Illustration of Layering Principle CSI 4118

  15. Protocol Techniques • For bit corruption • Parity • Checksum • CRC • For out-of-order delivery • Sequence numbers • Duplication • Sequence numbers CSI 4118

  16. Protocol Techniques (continued) • For lost packets • Positive acknowledgement and retransmission • For replay (excessive delay) • Unique message ID • For data overrun • Flow control CSI 4118

  17. 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 CSI 4118

  18. 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 CSI 4118

  19. 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 CSI 4118

  20. Illustration of SlidingWindow on Sending Side • Window tells how many packets can be sent • Window moves as acknowledgements arrive CSI 4118

  21. Performance • Stop-and-go • Slow • Useful only in special cases • Sliding window • Fast • Needed in high-speed network CSI 4118

  22. Comparison of Flow Control CSI 4118

  23. Why Sliding Window? • Simultaneously • Increase throughput • Control flow • Speedup Tw = min(B, TG W where • B is underlying hardware bandwidth • TW is sliding window throughput • TG is stop-and-go throughput • W is the window size CSI 4118

  24. Congestion • Fundamental problem in networks • Caused by traffic, not hardware failure • Analogous to congestion on a highway • Principle cause of delay CSI 4118

  25. Illustration of ArchitectureThat Can Experience Congestion • Multiple sources • Bottleneck CSI 4118

  26. Congestion and Loss Modern network hardware works well; most packet loss results from congestion, not from hardware failure CSI 4118

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