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Link Layer & Network Layer

This lecture covers examples of HTTP, layering concept, link layer network architecture, and challenges in network complexity organization. Understand the 4-layer Internet model with layers' implementation and communication hierarchy.

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Link Layer & Network Layer

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  1. Chapter 7.B and 7.C Link Layer & Network Layer Prof. Dina Katabi Some slides are from lectures by Nick Mckeown, Ion Stoica, Frans Kaashoek, Hari Balakrishnan, and Sam Madden

  2. Previous Lecture • We learned how to share the network infrastructure between many connections/flows • We also learned about the implications of the sharing scheme (circuit or packet switching) on the service that the traffic receives

  3. This Lecture • An Example: HTTP • Layering • Link Layer

  4. 1 20 2 3 4 19 18 17 14 5 15 6 11 8 16 7 12 9 13 10 Example: HTTP over the Internet Using TCP/IP and Ethernet App App “A” MIT “B”(CNN.Com) OS OS Ethernet Ethernet R5 R1 R2 R3 R4

  5. This Lecture • An Example: HTTP • Layering • Link Layer

  6. Network Architecture Problem • Networks are complex (heterogeneity, distributed, delay, losses, reordering, …) • How do we organize a network implementation? Solution • To deal with complexity  use layering

  7. Layering • Layering is a particular form of abstraction • The system is broken into a vertical hierarchy of logically distinct entities (layers) • The service provided by one layer is based solely on the service provided by layer below

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

  9. 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

  10. 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

  11. Interface • Higher layer calls lower layer • e.g., Link_Send(this_data, this_link) • Lower layer uses an up-call function to inform the higher layer of data arrival • e.g., Network_Handle()

  12. Multiplexing in the Internet • Many possible applications, transports, and link layers • But they all use IP at the network layer Telnet FTP HTTP Application TCP UDP Transport IP Internet Packet radio Ethernet Link

  13. This Lecture • An Example: HTTP • Layering • Link Layer

  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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