Transcript Link State Protocols - Origins

LINK STATE PROTOCOLS
(contents)
Disadvantages of the distance vector
protocols
Link state protocols
Why is a link state protocol better?
1
Disadvantages of the Distance
Vector Protocols
Slow convergence
Counting to infinity problem
Lack of variety of metrics
No possibility of hierarchical routing
Bad performance in large networks
2
Link State Protocols
developed to overcome the disadvantages of
the distance vector protocols
centralized database describes the topology
of the whole network
calculation and routing are still distributed
3
Link State Routing Algorithm
Each router is responsible for meeting its
neighbors and learning their names.
2. Each router constructs a link state packet (LSP)
which consists of a list of names and cost for
each of its neighbors.
3. The LSP is transmitted to all other routers.
Each router stores the most recently generated
LSP from each other router.
4. Each router uses complete information on the
network topology to compute the shortest path
route to each destination node.
1.
4
Characteristics
information about adjacencies sent to all
routers only when there is a change
each router maintains an identical database
a “shortest path” algorithm is used to find the
best route
converge as quickly as databases can be
updated
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Link State Database
A
1
B
2
C
4
3
5
D
6
E
Link State Announcement (LSA)
From A to B, link 1, distance = 1
number = 2
From
A
A
B
B
B
C
C
D
D
E
E
E
To
B
D
A
C
E
B
E
A
E
B
C
D
LS
seq.
num.
2
2
2
1
2
1
1
2
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2
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1
Link Cost
1
1
3
1
1
1
2
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4
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2
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5
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6
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4
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6
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The Flooding Protocol
every node sends the message on every link
except the one from where it received the
message
very fast and very reliable, but wastes
bandwidth
used for ordinary traffic in military networks
messages are sent only when there is a change or
every 45 minutes
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Securing the Map Updates
the flooding procedure includes hop-by-hop
acknowledgments
the database description packets are transmitted in a
secure fashion
each link state record is protected by a timer and is
removed from the database if a refreshing packet does not
arrive in due time
all records are protected by checksum
the messages can be authenticated, e. g. by passwords
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Dijkstra’s Algorithm
A
1
B
2
C
4
3
5
D
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E
Average time needed to compute the
routing table is about 200ms for
a 200 node network on a typical
router.
• Actual implementation in routers called “Forward Search”
• Builds up a list of “Tentative” and “Confirmed” paths
• Automatically prunes out longer paths
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Why Is a Link State protocol
Better?
fast loopless convergence
support of precise metrics and, if needed
multiple metrics
support of a multiple paths to a destination
splitting very large networks in areas
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Link State Protocols
Disadvantages
more memory required
the link state database is needed in
addition to the routing tables
much more complex procedure
higher probability for a bug in the program
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Comparison of LS and DV algorithms
Message complexity
LS: with n nodes, E links,
O(nE) msgs sent each
DV: exchange between
neighbors only
convergence time varies
Speed of Convergence
O(n2)
LS:
algorithm requires
O(nE) msgs
may have oscillations
DV: convergence time varies
may be routing loops
count-to-infinity problem
Robustness: what happens
if router malfunctions?
LS:
node can advertise
incorrect link cost
each node computes only
its own table
DV:
DV node can advertise
incorrect path cost
each node’s table used by
others
error propagate thru
network
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Path-vector paradigm
A variant of distance-vector
Each router maintains its path to a destination
Method of choosing the path
based on available paths through neighbors, choose the one
of highest “preference” (not necessarily the shortest)
A
A,B,E,D
A,C,D
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1
B,E,D
1
B
C,D
C
1
D
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E,D 10
E
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