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Understanding Protocol Layering in Network Design: A Comprehensive Overview

This lecture discusses the organization of network design through the layering of protocols, emphasizing abstraction and encapsulation. Protocol layering creates a vertical hierarchy where each layer builds upon the services provided by the layer below. Key concepts include multiplexing, framing, error detection, and clever layering usages like tunneling. We explore the Internet's four-layer model (Application, Transport, Network, and Link), the importance of each layer's transparency, and various protocols like HTTP, TCP, and Ethernet. Understanding these layers is crucial for effective network communication and design.

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Understanding Protocol Layering in Network Design: A Comprehensive Overview

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  1. L10: Protocols and Layering Dina Katabi 6.033 Spring 2007 http://web.mit.edu/6.033 Some slides are from lectures by Nick Mckeown, Ion Stoica, Frans Kaashoek, Hari Balakrishnan, Sam Madden, and Robert Morris

  2. Plan for studying network systems

  3. Network Design • Problem • How do we organize design of a network? • Solution • layering of protocols

  4. Layering of protocols • Layering is a particular form of abstraction • The system is broken into a vertical hierarchy of protocols • The service provided by one layer is based solely on the service provided by layer below

  5. Layering tools for nesting • Each layer adds/strips off its own header • Each layer may split up higher-level data • Each layer multiplexes multiple higher layers • Each layer is (mostly) transparent to higher layers data data data data data data data data

  6. Layering: The Internet HTTP Application End-to-End Layer Transport TCP Network IP Link Ethernet The 4-layer Internet model

  7. Multiplexing in the Internet • Many applications, transports, and link protocols • All use IP at the network layer HTTP SSH RTP Application TCP UDP Transport IP Internet Packet radio Ethernet Link

  8. Where are these layers? • Link and network layers are implemented everywhere • The end-to-end layer (i.e., transport and application) is implemented only at hosts Host Host Peer-layer communication Application Application Router Transport Transport Network Network Network Datalink Datalink Datalink Physical medium

  9. Clever usages of layering • Nesting layers to the extreme: tunneling • Run link layer over TCP (Virtual Private Network) • Router uses TCP as transport for routing protocol (e.g., BGP) • … HTTP TCP IP ssh TCP …

  10. Link Layer Problem: Deliver data from one end of the link to the other Need to address: • Bits Analog  Bits • Framing • Errors • Medium Access Control (The Ethernet Paper)

  11. Time 0 1 1 1 0 Manchester encoding • Each bit is a transition • Allows the receiver to sync to the sender’s clock

  12. Framing • Receiver needs to detect the beginning and the end of a frame • Use special bit-pattern to separate frames • E.g., pattern could be 1111111 (7 ones) • Bit stuffing is used to ensure that a special pattern does not occur in the data • If pattern is 1111111  Whenever the sender sees a sequence of 6 ones in the data, it inserts a zero (reverse this operation at receiver)

  13. Error Handling • Detection: • Use error detection codes, which add some redundancy to allow detecting errors • When errors are detected • Correction: • Some codes allow for correction • Retransmition: • Can have the link layer retransmit the frame (rare) • Discard: • Most link layers just discard the frame and rely on higher layers to retransmit

  14. This Lecture • To cope with the complexity, the network architecture is organized into layers • The link layer delivers data between two machines that are directly connected using a link

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