How packets are routed in the Internet

The Problem A B How should packets travel from A to B?

Hop-by-hop forwarding IP AddressNext-hop A A BR2 CR3 D A B C D IP AddressNext-hop AR1 BR4 C D IP AddressNext-hop AR3 BB C D R1 R2 R4 R3 DataB

How do the routers know what forwarding table to use?

Easy!! IP AddressNext-hop A A BR2 CR3 D A B C D IP AddressNext-hop AR1 BR4 C D IP AddressNext-hop AR3 BB C D R1 R2 R4 R3

But how about this network?

In a future Unit you will learn how the forwarding tables are populated. Today, we’ll explore different approaches to get you thinking about it.

8 A Game: Routing with Cards

You should have one card Keep card face down, do not look at it until I say so…. Your card tells you your node number, and who your neighbors are. 9

5 12, 19 Your node ID Ignore these numbers for now!!

5 12, 19 Your node ID Your neighbor nodes

5 12, , 19 5, 12, 3

Rules You may not – Leave your seat (but you can stand) – Pass your card – Write anything You may – Ask nearby friends for advice – Shout to other participants (anything you want) – Curse your instructor You must: Participate 15

Task #1 Find a path from Node 1 to Node 40. When you are done, you must be able to repeat it correctly.

Go! 17

Task #2 Find the shortest path from Node 1 to Node 40. When you are done, you must be able to repeat it correctly.

An algorithm to find the shortest path

What if each link has a “cost”?

R7R7 R6R6 R4R4 R2R2 R1R1 R8R8 R5R5 R3R “Expensive link”: It might be very long. e.g. a link from Europe to USA. Or it might be very busy. e.g. it connects to Google or CNN. Or it may be very slow. e.g. 1Mb/s instead of 100Mb/s. Cost = 4+9 = 13 Cost = = 11

Find lowest cost path to R 8 R7R7 R6R6 R4R4 R2R2 R1R1 R8R8 R5R5 R3R

Find lowest cost path to R 8 R7R7 R6R6 R4R4 R2R2 R1R1 R8R8 R5R5 R3R

Find the lowest cost path

Solution Cost = 22

How packets are routed in the Internet

Stanford University Istanbul Technical University The Public Internet Every organization is “autonomous” – they decide how packets are routed inside. They run our algorithm!

Stanford University Istanbul Technical University Every organization can run its own algorithm inside! AT&T Superonline

Stanford University (SU) Istanbul Technical University (ITU) AT&T Superonline A B C X Y Z Internet routing is a hierarchy

Stanford University (SU) Istanbul Technical University (ITU) AT&T Superonline He.net A B C X Y Z Internet routing is a hierarchy

Stanford University (SU) Istanbul Technical University (ITU) AT&T Superonline He.net IP AddressNext-hop AR2 B CR3 ELSER-Boundary-1 IP AddressNext-hop ITUR-Boundary-2 SUR8 A B C X Y Z R-Boundary-2 R-Boundary-3 R8 R2 R3 IP AddressNext-hop ITUR100 SUR-Boundary-2 R100

Summary

A B How should packets travel from A to B?

Hop-by-hop forwarding IP AddressNext-hop A A BR2 CR3 D A B C D IP AddressNext-hop AR1 BR4 C D IP AddressNext-hop AR3 BB C D R1 R2 R4 R3 DataB

How do the routers know what forwarding table to use? Now you know!

Easy!