Distance Vector Routing
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Transcript Distance Vector Routing
Distance Vector Routing
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Introduction.
Modern computer Network generally
use Dynamic routing algorithms
rather than
The static one.
Two dynamic algorithm
– Distance vector routing
– Link state routing
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Distance vector routing
This algorithms operate by
– Having each router maintain a table
– The table(i.e vector) give
the best known distance to each destination and
which line to use to get there.
Updated by exchanging information with the
neighbors.
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History
Sometimes called by other names…
Distributed Bellman-Ford routing algorithm(1957)
Ford-Fulkerson algorithm(1962)
It was original ARPANET routing algorithm
And was used
– In the Internet(RIP)
– In early versions of DECnet and Novell’s IPX
AppleTalk and Cisco routers use improved
distributed vector protocols.
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The Routing table
Each router maintains a routing table
– Contain one entry.
– Two part of one entry
The preferred outgoing line
An estimate of the time or distance
distance
20
10
Line
A
B
The router is assumed to know the
“distance” to each of its neighbors.
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The Metric of distance
Number of hops
– The distance is just one hop
Time delay in milliseconds
– The router can measure…
– Using special ECHO packets(timestamps)
Total number of packets queued along
the path
– The router simply examines each queue
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Algorithm
A
E
B
C
F
G
D
H
To
A
I
H
K
A
0
24
20
21
8
A
B
12
36
31
28
20
A
C
25
18
19
36
28
I
D
40
27
8
24
20
H
E
14
7
17
I
30
I
18
H
12
H
10
I
F
G
H
I
J
K
L
(a)
Receive table
Distances to (b)
from
each
neighbors
neighbor
New estimated
dealy from J
Line
I
30
JA
=228
23
20
19
40
JA
= 68 31
18
31
17 AB
20 = 12
0
19
JB21= JA+AB
20
0
14 =22
J
9
11
7
10
0
-
K
24
22
22
0
6
K
L
29
33
9
9
15
K
JA
JI
JH
JK
delay delay delay delay
is
is
is
is
8
10
12
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Vectors received from
J’s four neighbors
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New
routing
table
for J
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The Count-to-Infinity Problem
Distance vector routing works in theory
– A serious drawback in practice
Reacts rapidly to good news,
But leisurely to bad news.
A
B
C
D
E
1
1 2
1 2
1 2
3
3
4
A
Initially
After 1 exchange
After 2 exchange
After 3 exchange
After 4 exchange
B
C
1
3
3
5
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7
7
2
2
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3
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D
E
Initially
After 1 exchange
After 2 exchange
After 3 exchange
After 4 exchange
After 5 exchange
After 6 exchange
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In case good news.
– If longest path of subnet is N hops,
– Everyone will know good news.
In case bad news.
– Infinity…
– Set infinity to the longest path + 1.
– If metric is time delay, no well-defined
upper bound.
– There is a tradeoff…
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The Split Horizon Hack
Many ad hoc solutions to the problem
have been proposed…
– But, complicate , useless …
One of them will be described(why fail?)
The Split Horizon Hack algorithm
A
B
C
D
E
1
2 3 4 Initially
2 3 4 After 1 exchange
3 4 After 2 exchange
4 After 3 exchange
After 4 exchange
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A
B
C
Router
D
Fig. 5-12. An example where split horizon fails.
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