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

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

Putting it all together [W&P 2010]

Putting it all together [W&P 2010] initially unconnected 1.How to route G  A? 2.As soon as H is added, D tries to send a packet to H. What happens? 3.If AS2.R2 goes down, what will be the routing changes? later goes down

1. compute spanning tree [W&P 2010]

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

3. compute inter-AS routing [W&P 2010] 1.How to route G  A?

3. compute inter-AS routing [W&P 2010] 1.How to route G  A? Does A  G follow the same path?

4. Address resolution protocol [W&P 2010] 1.How to route G  A? 2.As soon as H is added, D tries to send a packet to H. What happens? 3.If AS2.R2 goes down, what will be the routing changes? initially unconnected

4. Address resolution protocol [W&P 2010] 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

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

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

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

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

5. re-compute routing table [W&P 2010] 1.How to route G  A? 2.As soon as H is added, D tries to send a packet to H. What happens? 3.If AS2.R2 goes down, what will be the routing changes? goes down

5. re-compute routing tables [W&P 2010] goes down 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

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

Transportation network

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

Transportation Network Which route to take? Local alg: min my own delay x r * with equal delay on all routes All delays = f f +50 f+10

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

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

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

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

Transportation Network Dietrich Braess (German mathematician) “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: , 1970

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

 Global objective Transportation Network Local algorithm route to smaller delay x r * with equal delay on all routes route to smaller cost x r * with equal cost on all routes cost = delay + toll network delay

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