CSE 123b: Lecture 6 - Distance Vector Routing and Link State Routing, Study notes of Computer Systems Networking and Telecommunications

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CSE 123bCSE 123b

Communications SoftwareCommunications Software

Spring 2002Spring 2002Lecture 6: Routing II Lecture 6: Routing II

Stefan SavageStefan Savage

Some slides courtesy David Wetherall

April 25, 2002

CSE 123b -- Lecture 6 – Distance Vector Routing

Projects…^ Projects…

z^

Project #1:^ ◆

Implement simple distance vector routing protocol ◆^

We provide a “framework” called fisnhet for implementing

» Fishead: program that simulates network; maintains topology,

etc. » Libfish.a: library that provides basic functions (sending and

receiving packets, timers and keyboard input) » Fish.h: a head file that defines this API

◆^

You will test your implementation in your own local fishnet ◆^

We will provide a long running fishnet that everyone in theclass can join (one big network)

z^

BUT… still not ready…

Tuesday with complete

documentation. Will be due two weeks from Tuesday.

April 25, 2002

CSE 123b -- Lecture 6 – Distance Vector Routing

This class^ This class

z^

Finish Intra-domain routing^ ◆

Link-state protocols

z^

Inter-domain routing^ ◆

BGP ◆^

Policy ◆^

Peering/transit economics

April 25, 2002

CSE 123b -- Lecture 6 – Distance Vector Routing

Link State routing^ Link State routing

z^

Same goal as DV, but a different approach z^

Two phases^ ◆

Reliable flooding

» Tell

all

routers what you know about your

local

topology

◆^

Path calculation

(Dijkstra’s algorithm)

» Each router computes best path over

complete

network

z^

Motivation^ ◆

Using DV, routers only have local information, making itdifficult to decide what to do when there are changes ◆^

With LS, faster convergence and better stability (hopefully) ◆^

More complex

April 25, 2002

CSE 123b -- Lecture 6 – Distance Vector Routing

Reliable flooding^ Reliable flooding

z^

Goal: tell everyone what you know about local topology z^

Periodically send

link state packets

(LSPs) on

all

links

◆^

LSP contains [node, neighbors, costs, sequence number]

z^

If node X receives an LSP from node Y over link Q^ ◆

Save it in local link state database ◆^

Forward LSP on all links

except

Q

z^

Use explicit ACKs and retransmits to make flooding reliable z^

Each LSP will travel at most once over each link

April 25, 2002

CSE 123b -- Lecture 6 – Distance Vector Routing

Flooding example^ Flooding example

z^

LSP generated by X at T= z^

Nodes become orange as they receive it

X

A

C

B

D

X

A

C

B

D

X

A

C

B

D

X

A

C

B

D

T=

T=

T=

T=

April 25, 2002

CSE 123b -- Lecture 6 – Distance Vector Routing

More challenges^ More challenges

z^

What happens if the network is partitioned and heals?^ ◆

Different LS databases must be synchronized ◆^

Use version #s on each LSP (incremented for each update) ◆^

Compare version #s when a link comes back up and requestout of date LSPs

A

B

C

D

E

F

G

H

T: 1

T: 2

T: 2

April 25, 2002

CSE 123b -- Lecture 6 – Distance Vector Routing

Dijkstra’s Shortest Path Tree^ Dijkstra’s

Shortest Path Tree

(SPT) algorithm(SPT) algorithm

z^

Graph algorithm for single-source shortest path tree

S

Å

Q

Å

While Q != {}

u

Å

extract-min(Q)

S

Å

S plus {u}

for each node v adjacent to u

“relax” the cost of v

Å

u is done

April 25, 2002

CSE 123b -- Lecture 6 – Distance Vector Routing

Example –^ Example

– Step 2

Step 2

April 25, 2002

CSE 123b -- Lecture 6 – Distance Vector Routing

Example –^ Example

– Step 3

Step 3

April 25, 2002

CSE 123b -- Lecture 6 – Distance Vector Routing

Example –^ Example

– Step 5

Step 5

April 25, 2002

CSE 123b -- Lecture 6 – Distance Vector Routing

Example –^ Example

– Done

Done

April 25, 2002

CSE 123b -- Lecture 6 – Distance Vector Routing

Link State in practice^ Link State in practice

z^

OSPF (Open Shortest Path First) and IS-IS^ ◆

Most widely used intra-domain routing protocol ◆^

Run by almost all ISPs and many large organizations

z^

Basic link state algorithm plus many features:^ ◆

Authentication of routing messages ◆^

Extra hierarchy: Partition into

routing areas

◆^

Load balancing: Multiple equal cost routes

April 25, 2002

CSE 123b -- Lecture 6 – Distance Vector Routing

z^

Inter-domain versus intra-domain routing

Backbone service provider

Peeringpoint

Peeringpoint

Large corporation Large corporation

Small corporation

“Consumer ” ISP

“Consumer”ISP

“ Consumer” ISP

You at home

You at work

But the Inter^ But the

Internet is not just one

net is not just one

network…network…