Distance Vector Protocol

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Distance vector routingDistance vector routing


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ReferencesReferencesAn Engineering Approach to Computer

Networking

By S. Keshav

Chapter 11 Routing

Tanenbaum5.1.1,5.2.4


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Distance vector routingDistance vector routing Two key algorithms

distance vector

link-state

Both assume that each router knows address of each neighbor

cost of reaching each neighbor

Both allow a router to determine global routinginformation by talking to its neighbors


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Basic ideaBasic idea Node tells its neighbors its best idea of distance

to everyother node in the network

Node receives these distance vectors from its

neighbors

Updates its notion of best path to each

destination, and the next hop for this destination

Features distributed

adapts to traffic changes and link failures


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Distance Vector ProtocolsDistance Vector Protocols Unit of information exchange

vector of distances to destinations

Also called Distributed Bellman-Ford Algorithm


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Distance Vector ProtocolsDistance Vector ProtocolsAssumed that each router knows the identity of

every other router in the network

But not the shortest possible path

Each router keeps a Distance Vectorper

destination

Routers periodically exchange the Distance

Vector

On receipt, each router compares the currentdistance and the distance through the neighbor

and updates its routing table


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ExampleExample -- initial distancesinitial distances

Info at

node

A

B

C

D

A B C

0 7 ~

7 0 1

~ 1 0

~ ~ 2

Distance to node

D

~

~

2

0E 1 8 ~ 2

1

8

~

20

E

2A

B

E

C

D

7

1

1

2

8

Initially A does not know that C

exists

Each router knows

1. Address of each neighbor2. Cost of reaching each neighbor


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E receives Ds routesE receives Ds routes

Info at

node

A

B

C

D

A B C

0 7 ~

7 0 1

~ 1 0

~ ~ 2

Distance to node

D

~

~

2

0E 1 8 ~ 2

1

8

~

20

E

A

B

E

C

D

7

1

1

2

28


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E updates cost to CE updates cost to C

A

B

E

C

D

7

1

1

2

28

Info at

node

A

B

C

D

A B C

0 7 ~

7 0 1

~ 1 0

~ ~ 2

Distance to node

D

~

~

2

0E 1 8 4 2

1

8

~

20

E


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A receives Bs routesA receives Bs routes

Info at

node

A

B

C

D

A B C

0 7 ~

7 0 1

~ 1 0

~ ~ 2

Distance to node

D

~

~

2

0E 1 8 4 2

1

8

~

20

E

A

B

E

C

D

7

1

1

2

28


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A updates cost to CA updates cost to C

A

B

E

C

D

7

1

1

2

28

Info at

node

A

B

C

D

A B C

0 7 8

7 0 1

~ 1 0

~ ~ 2

Distance to node

D

~

~

2

0E 1 8 4 2

1

8

~

20

E


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A receives Es routesA receives Es routes

Info at

node

A

B

C

D

A B C

0 7 8

7 0 1

~ 1 0

~ ~ 2

Distance to node

D

~

~

2

0E 1 8 4 2

1

8

~

20

E

A

B

E

C

D

7

1

1

2

28


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A updates cost to C and DA updates cost to C and D

A

B

E

C

D

7

1

1

2

28

Info at

node

A

B

C

D

A B C

0 7 5

7 0 1

~ 1 0

~ ~ 2

Distance to node

D

3

~

2

0E 1 8 4 2

1

8

~

20

E


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Final distancesFinal distances

Info at

node

A

B

C

D

A B C

0 6 5

6 0 1

5 1 0

3 3 2

Distance to node

D

3

3

2

0E 1 5 4 2

1

5

4

20

E

A

B C

D

7

1

1

2

28

E


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Problems with distanceProblems with distance--vector routingvector routing

Problems occur when links go up and down


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Final distances after link failureFinal distances after link failure

Info at

node

A

B

C

D

A B C

0 7 8

7 0 1

8 1 0

10 3 2

Distance to node

D

10

3

2

0

E 1 8 9 11

1

8

9

11

0

E

A

B C

D

7

1

1

2

28

E

Info at

node

A

B

C

D

A B C

0 6 5

6 0 1

5 1 0

3 3 2

Distance to node

D

3

3

2

0

E 1 5 4 2

1

5

4

2

0

E

Table reflectingfailure of link ED


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Effect of a link failureEffect of a link failure Does the logical table change smoothly?

What is the sequence of messages that are

exchanged to achieve these changes in the table?

How does A find out that the link ED has failed?

A does not know this fact, it ONLY KNOWS

DISTANCES!

A

B C

D

7

1

1

2

28

E


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The bouncing effectThe bouncing effect

A

25

1

1

B

C

B

C 2

1

dest cost

A

C 1

1

dest cost

A

B 1

2

dest cost


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C sends routes to BC sends routes to B

A

25 1

B

C

B

C 2

1

dest cost

A

C 1

~

dest cost

A

B 1

2

dest cost


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B updates distance to AB updates distance to A

A

25 1

B

C

B

C 2

1

dest cost

A

C 1

3

dest cost

A

B 1

2

dest cost


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B sends routes to CB sends routes to C

A

25 1

B

C

B

C 2

1

dest cost

A

C 1

3

dest cost

A

B 1

4

dest cost


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B

C 2

1

dest cost

A

C 1

5

dest cost

A

B 1

4

dest cost

A

25 1

B

C

How long will this go on?


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B

C 2

1

dest cost

A

C 1

dest cost

A

B 1

dest cost

A

25 1

B

C

This is also called Counting-to-Infinity


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How are these loops caused?How are these loops caused? When a node computes and distributes a

distance vector

It hides the sequence of operations it used to

compute the vector


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SolutionsSolutions

Split horizon B does not advertise route to C

In general

Nodes do not broadcast the same distance vector

on all outgoing links.

Nodes simply omit from their message any

information about destinations routed on that link

Works for two node loops

Does not work for loops with more nodes


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Example where split horizon failsExample where split horizon fails

1

11

1

A B

C

D

When link CD breaks, C marks D as

unreachable and reports that to A and B

Suppose A learns it first.

A now thinks best path to D is through B.

A reports a route to D of cost=3 to C.

C thinks D is reachable through A at cost 4

and reports that to B.

B reports a cost 5 to A who reports newcost to C.

etc...

This is count to infinity!


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Split Horizon with Poisonous ReverseSplit Horizon with Poisonous Reverse

B does advertise route to C

But puts a negative information

So, that C never uses B for such a destination


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Avoiding the Bouncing EffectAvoiding the Bouncing EffectSelect loop-free paths

One way of doing this:

each route advertisement carries entire path

if a router sees itself in path, it rejects the route

Memory Space needed in a router is

proportional to diameter of the network.


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Source TracingSource Tracing Augment distance vector to carry

The cost to a destination

The address of the router immediately preceding

the destination.

This information is enough for a source to

construct an entire path.


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Computing Implicit PathsComputing Implicit Paths To reduce the space requirements

Propagate for each destination not only the cost

But add another field specifying also its predecessor

can recursively compute the path

space requirements independent of diameter

x

y

z

w

u

v

Destination Preceding Node

s

v u

u w

w z

z y

y y

s y

This is at Node xThis is at Node x


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Path to u from x, calculation by xPath to u from x, calculation by x

x

y

z

w

u

v

Destination Preceding Node

s

v u

u w

w z

z y

y y

s y


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Loop Freedom at Every InstantLoop Freedom at Every Instant Does avoiding bouncing effect avoid loops?

No! Transient loops are still possible

Why? Because implicit path information may be

stale

Only way to fix this

ensure that you have up-to-date information by

querying.


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Periodic vs. Triggered UpdatesPeriodic vs. Triggered Updates Each node sends periodic update messages

even if nothing has changed.

This lets other nodes know that the sender is

alive.

Frequency is several seconds to several

minutes.

Triggered updates happen when a nodechanges its own view of the table.

Trade-off between sensitivity to link failures and

overhead in exchanging routing information

Documents

Distance Vector Protocol