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cs / ee 143 Communication Networks Routing Misc

cs / ee 143 Communication Networks Routing Misc. Text: Walrand & Parakh , 2010 Steven Low CMS, EE, Caltech. Outline. Putting it all together Routing across Internet LAN, intra-AS, inter-AS Routing across layers 2 and 3 Selfish routing: inefficiency Dynamic routing: instability.

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cs / ee 143 Communication Networks Routing Misc

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  1. cs/ee 143 Communication NetworksRouting Misc Text: Walrand & Parakh, 2010 Steven Low CMS, EE, Caltech

  2. Outline Putting it all together • Routing across Internet • LAN, intra-AS, inter-AS • Routing across layers 2 and 3 Selfish routing: inefficiency Dynamic routing: instability

  3. Putting it all together initially unconnected [W&P 2010]

  4. Putting it all together [W&P 2010]

  5. Putting it all together How to route G  A? As soon as H is added, D tries to send a packet to H. What happens? If AS2.R2 goes down, what will be the routing changes? later goes down initially unconnected [W&P 2010]

  6. 1. compute spanning tree [W&P 2010]

  7. 2. compute intra-AS routing [W&P 2010]

  8. 3. compute inter-AS routing How to route G  A? [W&P 2010]

  9. 3. compute inter-AS routing How to route G  A? Does A  G follow the same path? [W&P 2010]

  10. 4. Address resolution protocol How to route G  A? As soon as H is added, D tries to send a packet to H. What happens? If AS2.R2 goes down, what will be the routing changes? initially unconnected [W&P 2010]

  11. 4. Address resolution protocol • Packets from D can be • delivered to subnet AS2.B1 • based on IP address of H • AS2.B1 does not know H • AS2.B1 uses ARP to find • H’s MAC address • Use STP to forward pkts to H initially unconnected [W&P 2010]

  12. Example: H1 wants to send packet to H2 Network Link [all, e1, who is IP2?] gateway Ethernet switch Link layer on H1 broadcasts a message (ARP query) on its layer 2 network asking for the MAC address corresponding to IP2

  13. Example: H1 wants to send packet to H2 Network Network Link Link [all, e1, who is IP2?] gateway Ethernet switch [e1, e2, I am IP2] Link layer on H2 responds to the ARP query with its MAC address

  14. Example: H1 wants to send packet to H2 Network Network Link Link [e2, e1,[IP1, IP2, X]] gateway Ethernet switch Once the link layer on H1 knows e2, it can now send the original message

  15. Example: H1 wants to send packet to H2 Network Network Link Link [e2, e1,[IP1, IP2, X]] gateway Ethernet switch [IP1, IP2, X] [e2, e1,[IP1, IP2, X]] Link layer on H2 delivers the packet to the network layer on H2

  16. 5. re-compute routing table How to route G  A? As soon as H is added, D tries to send a packet to H. What happens? If AS2.R2 goes down, what will be the routing changes? goes down [W&P 2010]

  17. 5. re-compute routing tables • Failure detected by AS2.R1 • and AS2.R3; update routing • tables (intra-AS) • Failure detected by border • gateway in AS5 • BGP re-computes • The path between AS2 and • AS5 will be changed goes down [W&P 2010]

  18. Outline Putting it all together • Routing across Internet • LAN, intra-AS, inter-AS • Routing across layers 2 and 3 Selfish routing: inefficiency Dynamic routing: instability

  19. Transportation network

  20. Transportation Network Which route to take? • Local alg: min my own delay • xr* with equal delay on all routes f +50 10 f f +50 10 f All delays = 83 equilibrium: delays on all paths equalized

  21. Global objective: min total delay Transportation Network Which route to take? • Local alg: min my own delay • xr* with equal delay on all routes f +50 10 f f+10 f +50 10 f All delays = 83

  22. Transportation Network Which route to take? • Local alg: min my own delay • xr* with equal delay on all routes f +50 10 f f+10 f +50 10 f “my delay (81) < 83!”

  23. Transportation Network Which route to take? • Local alg: min my own delay • xr* with equal delay on all routes f +50 10 f f+10 f +50 10 f All delays = 82, 81,93 not an equilibrium !

  24. Transportation Network Which route to take? • Local alg: min my own delay • xr* with equal delay on all routes f +50 10 f f+10 f +50 10 f Braess’s Paradox (1968) All delays = 92>83! new equilibrium: delays on all paths equalized, and worse!

  25. Transportation Network “Major road investments in the city center [of Stuttgart] failed to yields the benefits expected. They were only obtained when a cross street… was withdrawn from traffic use.” Murchlan, “Braess’s Paradox of traffic flow”, Transportation Research, 4:391-394, 1970 Dietrich Braess (German mathematician)

  26. not necessarily min! • Global objective All delays = 92>83! Braess’s Paradox (1968) Transportation Network Which route to take? • Local alg: min my own delay • xr* with equal delay on all routes f +50 10 f f+10 f +50 10 f

  27. network delay cost = delay + toll • route to smaller cost • xr* with equal cost on all routes Transportation Network • Global objective • Local algorithm • route to smaller delay • xr* with equal delay on all routes

  28. Outline Putting it all together • Routing across Internet • LAN, intra-AS, inter-AS • Routing across layers 2 and 3 Selfish routing: inefficiency Dynamic routing: instability

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