07-MissingPieces

07-MissingPieces - Missing Pieces of the Puzzle EE122 Fall...

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1 Missing Pieces of the Puzzle EE122 Fall 2011 Scott Shenker http://inst.eecs.berkeley.edu/~ee122/ Materials with thanks to Jennifer Rexford, Ion Stoica, Vern Paxson and other colleagues at Princeton and UC Berkeley

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Announcements • Everyone should be signed up and have account – If you aren’t see me at end of class • We lost our grader, so delaying HW#2 – Gives us a chance to cover more material… • Beanbags. Really? 2
Agenda for Today • Finish up routing: not as simple as you might think • Discuss missing pieces: preview of rest of cla ss 3

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Last Time • Link-State Routing • Spread state everywhere • Nodes do local computation over entire graph Local computation, global state • Doesn’t scale well, involves broadcasts 4
This Time Global computation, local state • We want to limit the distribution of state • So computation must be distributed • The most common example is distance-vector 5

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6 Distance-Vector Details in Section
Distributed Computation of Routes • More scalable than Link-State – No global flooding • Each node computing the outgoing port based on: – Local information (who it is connected to) – Paths advertised by neighbors • Algorithms differ in what these exchanges contain – Distance-vector: just the distance to each destination – Path-vector: the entire path to each destination • We will focus on distance-vector for now 7

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Example of Distributed Computation 8 I am one hop away I am one hop away I am one hop away I am two hops away I am two hops away I am two hops away I am two hops away I am three hops away I am three hops away Destination I am three hops away
Very similar to Monday’s Class • Destination stands up • Announces neighbors – They stand up • They announce their neighbors – They stand up • …. .and so on, until source stands • On Monday you started with the source, but paths are reversible so it doesn’t matter…. • Key point: don’t stand up twice! 9

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Step 1 • Destination stands up 10
Step 1 11

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Step 2 • Destination stands up • Announces neighbors – They stand up 12
Step 2 13 I am one hop away I am one hop away I am one hop away

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Step 3 • Destination stands up • Announces neighbors – They stand up • They announce their neighbors – They stand up 14
Step 3 15 I am two hops away I am two hops away I am two hops away I am two hops away

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Why Not Stand Up Twice? • Being called a second time means that there is a second (and longer) path to you – You already contacted your neighbors the first time – Your distance to destination is based on shorter path 16
Basics of Distributed Routing • Nodes advertise their best paths to neighbors • Nodes select among paths offered by neighbors – Ignore all but the best path • Iterative process eventually leads to convergence • If “cost” is hopcounts, then nodes find out about shortest paths first (ignore later ones) – If costs are general, later paths might be better • Remember: nodes “select” among offered paths – But what selection criteria are allowed?

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This note was uploaded on 01/09/2012 for the course ELECTRICAL 122 taught by Professor Shenker during the Spring '11 term at Berkeley.

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07-MissingPieces - Missing Pieces of the Puzzle EE122 Fall...

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