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Overlay Models

This article discusses the general architecture of ATM, LAN Emulation (LANE), native mode, Classical IP over ATM, NHRP and shortcut switching, and Multiprotocol over ATM (MPOA). It explores the peering model and overlay model, comparing emulation and native mode. The article concludes with insights and issues related to ATM QoS, performance, and scaling.

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Overlay Models

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  1. Overlay Models Reference: “ATM Internetworking” by Anthony Alles http://www.cisco.com/warp/public/614/12.html

  2. Outline • ATM in General • LAN Emulation (LANE) • Native Mode Models • Classical IP over ATM • Classical IP over ATM Extension • Next-Hop Address Resolution (NHRP) and shortcut switching • Multiprotocol over ATM (MPOA)

  3. ATM in General • ATM was envisioned to be the ultimate winner which will take over the whole Internet • By all means, it’s a networking layer technology with its own routing and addressing architecture, just like IP • The seven-layer OSI model is imperfect in modeling it • In practice, due to the adoption of overlay model, ATM has been downgraded as a layer 2 or even layer 1 like technology

  4. ATM in General (cont’d) • Architecture

  5. ATM in General (cont’d) • Label Swapping

  6. ATM in General (cont’d) • 2-layer label stacking:

  7. ATM in General (cont’d) • PNN-I Hierarchical Routing:

  8. ATM in General (cont’d) • Signaling: based on source routing

  9. Peering or Overlaying? • Peering model

  10. Peering or Overlaying? (cont’d) • Overlay Model:

  11. Emulation or Native Mode?

  12. Emulation or Native Mode?(cont’d) • Emulation: • Quick deployment without driver changes • Supporting any networking layers, e.g., IP, IPX • More overhead per bit sent • No QoS

  13. Emulation or Native Mode?(cont’d) • Native mode: • More effort • QoS possible

  14. LANE Architecture • A single ELAN

  15. LANE Architecture (cont’d) • Protocol Architecture

  16. LANE Architecture (cont’d) • Building blocks:

  17. ELAN Architecture (cont’d) • Control connections:

  18. LANE Architecture (cont’d) • Data Connections:

  19. ELAN Architecture (cont’d) • Inter-ELAN communications:

  20. Native Mode • Classical IP over ATM • Classical IP over ATM Extension: • NHRP and shortcut switching • MPOA

  21. Classical IP over ATM: Architecture • Encapsulations (RFC1483/2684): • LLC/SNAP • VC multiplexing

  22. Classical IP over ATM: Architecture (cont’d) • IP over ATM forwarding mechanism

  23. Classical IP over ATM: Architecture (cont’d) • Routing between LISs in classical IP model

  24. Classical IP over ATM Extension • NHRP and shortcut switching

  25. MPOA • As its name implies, MPOA intended to support multiprotocols over ATM • NHRP for shortcut switching • Separating routing functions from forwarding functions using “edge devices”

  26. MPOA (cont’d)

  27. Conclusions • ATM QoS feature cannot be fully exploited • No QoS for ELAN • Complex QoS mapping between IP and ATM for classical IP and performance bottleneck at multiservice switch between LISs • No hard QoS for shortcut switching due to its flow driven nature • Having to manage multi signaling/routing protocols at different layers • NHRP for shortcut switching does not scale and has performance issues • Have to revisit peering model!!

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