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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
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.
A Router
Table BA : mB : 0C : n
B Router C Router
Table AA : 0B : mC : m+n
m n
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.
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
The router is assumed to know the “distance” to each of its neighbors.
distance Line
2010
AB
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
AlgorithmA B C D
E F G H
I J K L
29
24
9
21
17
18
23
14
40
25
12
0
33
22
11
0
20
31
20
7
27
18
36
24
9
22
7
14
0
6
19
30
8
19
31
20
9
0
10
22
19
31
40
22
24
36
28
21
15
6
0
10
12
18
30
17
20
28
20
8
K
K
-
I
H
H
I
I
H
I
A
A
L
K
J
I
H
G
F
E
D
C
B
ATo A I H K Line
JAdelay
is8
JIdelay
is10
JHdelay
is12
JKdelay
is6
Vectors received fromJ’s four neighbors
Newrouting
tablefor J
New estimateddealy from J
(a)
(b)Receive table
from neighbors
Distances to each
neighbor
JA = 8JA = 8
AB = 12JB = JA+AB = 20
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 A B C D E
Initially1 After 1 exchange1 2 After 2 exchange1 2 3 After 3 exchange1 2 3 4 After 4 exchange
1 2 3 4 Initially3 2 3 4 After 1 exchange3 4 3 4 After 2 exchange5 4 5 4 After 3 exchange5 6 5 6 After 4 exchange7 6 7 6 After 5 exchange7 8 7 8 After 6 exchange
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…
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 algorithmA 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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