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Congestion ControlAlgorithms
Packets Sent
PacketsDelivered
Perfect
Desirable
Congested
Maximumcarryingcapacity ofsubnet
Congestion is a state of excessive accumulation or overfilling or overcrowding.
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Factors responsible forcongestion
Packet arrival rate exceeds the outgoing link
capacity.
Insufficient memory to store arriving packets
Bursty traffic
Slow processor
Congestion tends to feed upon itself & become
worse.
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Difference between CongestionControl & Flow Control
Congestion control has to make sure that
subnet is able to carry the offered load.
Flow control relates to point-to-point traffic
between a given sender & a receiver.
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Principles Of CongestionControl
Control Theory
Open Loop Control Closed Loop Control
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Open Loop Control
Open loop control attempts to solve theproblem by making sure it doesnt occur inthe first place.
Tools: When to accept a new packet.
Deciding when to discard packets & which
ones. Making scheduling decisions at various
points in the network.
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Closed Loop Control
Its based on the concept of feedback loop.
When applied to congestion control:
1. Monitors the system to detect when &where congestion occurs.
2. Pass this information to the places where
action can be taken.3. Adjust system operation to correct the
problem.
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Techniques
Several techniques can be employed.
These include:
Choke packets
Load shedding
Traffic shaping
The first 2 deal with congestion detectionand recovery. The last one deals with
congestion avoidance.
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Traffic Shaping
Forcing the packets to be transmitted in a
more predictable rate.
Traffic policing:
Monitoring the traffic flow.
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The Leaky Bucket Algorithm
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The Token Bucket Algorithm
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Leaky Bucket vs Token Bucket LB discards packets; TB does not. TB discards
tokens.
With TB, a packet can only be transmitted if there areenough tokens to cover its length in bytes.
LB sends packets at an average rate. TB allows forlarge bursts to be sent faster by speeding up theoutput.
TB allows saving up tokens (permissions) to sendlarge bursts. LB does not allow saving.
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Choke Packets
A more direct way of telling the source to
slow down.
A choke packet is a control packet
generated at a congested node and
transmitted to restrict traffic flow.
The source, on receiving the choke packet
must reduce its transmission rate by a
certain percentage.
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unew =auold ( 1-a ) f
U Utilization of its output lines.A a constant, how fast the router forgets the recent
history.
F instantaneous line utilization
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Weighted Fair Queuing
2019151161
161272
83
171394
1814105
A
B
C
D
E
O
Packet Finishing time
C 8
B 16
D 17
E 18
A 20
A router with 5 packets
queued for line O
Finishing times for 5
packets
C
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Choke Packets that affects only the source
B C
A D
E F
B C
A D
E F
Choke
B C
A D
E FChoke
B C
A D
E F
Choke
B C
A D
E FReduced
Flow
B C
A D
E FFlow is
still at
maximum
rateB C
A D
E F
Flow is
reduced
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Hop-by-hop Choke Packet
B C
A D
E FHeavy Flow
B C
A D
E F
Choke
B C
A D
E FChoke Reduced
Flow
B C
A D
E F
Choke
B C
A D
E F
a)
b)
c)
d)
e)
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Load Shedding
Load Shedding is a way in which the
routers when flooded with packets that it
cannot handle, just throws them away.
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Jitter Control
In this technique, the packets which are
ahead of schedule get slowed down &
packets that are behind schedule get
speeded up.
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Congestion Control For
Multicasting
RSVP--- Resource reSerVation Protocol
1 2
A
D
G
J
B C
E F
H I
K L
3 4 5
Sender
s
Receivers
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