Congestion Control

24-1 DATA TRAFFIC

The main focus of congestion control and quality of

service is data traffic. In congestion control we try to
avoid traffic congestion. In quality of service, we try to
create an appropriate environment for the traffic. So,
before talking about congestion control and quality of
service, we discuss the data traffic itself.

Topics discussed in this section:

Traffic Descriptor
Traffic Profiles

24.3
Figure 24.1 Traffic descriptors

24.4
Figure 24.2 Three traffic profiles

24.5
24-2 CONGESTION

Congestion in a network may occur if the load on the

network—the number of packets sent to the network—is
greater than the capacity of the network—the number of
packets a network can handle. Congestion control refers
to the mechanisms and techniques to control the
congestion and keep the load below the capacity.

Topics discussed in this section:

Network Performance

24.6
 Congestion Control Algorithms
• Congestion - the situation in which too
many packets are present in the subnet.
 Causes of Congestion
• Congestion occurs when a router receives
data faster than it can send it
– Insufficient bandwidth
– Slow hosts
– Data simultaneously arriving from multiple
lines destined for the same outgoing line.
• The system is not balanced
– Correcting the problem at one router will
probably just move the bottleneck to another
router.
 Congestion Causes More Congestion
– Incoming messages must be placed in queues
• The queues have a finite size
– Overflowing queues will cause packets to be dropped
– Long queue delays will cause packets to be resent
– Dropped packets will cause packets to be resent
• Senders that are trying to transmit to a congested
destination also become congested
– They must continually resend packets that have been
dropped or that have timed-out
– They must continue to hold
outgoing/unacknowledged
messages in memory.
Congestion Control versus Flow Control
• Flow control
– controls point-to-point traffic between sender
and receiver
– e.g., a fast host sending to a slow host
• Congestion Control
– controls the traffic throughout the network
 24-3 CONGESTION CONTROL

Congestion control refers to techniques and mechanisms

that can either prevent congestion, before it happens, or
remove congestion, after it has happened. In general,
we can divide congestion control mechanisms into two
broad categories: open-loop congestion control
(prevention) and closed-loop congestion control
(removal).

Topics discussed in this section:

Open-Loop Congestion Control
Closed-Loop Congestion Control
24.10
 Congestion Control

• When one part of the subnet (e.g. one or more

routers in an area) becomes overloaded,
congestion results.
• Because routers are receiving packets faster than
they can forward them, one of two things must
happen:
– The subnet must prevent additional packets from
entering the congested region until those
already present can be processed.
– The congested routers can discard queued
packets to
make room for those that are arriving.

11
 Two Categories of Congestion Control

• Open loop solutions

– Attempt to prevent problems rather than
correct them
– Does not utilize runtime feedback from the
system
• Closed loop solutions
– Uses feedback (measurements of system
performance) to make corrections at runtime.
 General Principles of Congestion Control
• Analogy with Control Theory:
– Open-loop, and
– Closed-loop approach.

• Open-loop approach
– Problem is solved at the design cycle
– Once the system is running midcourse correction are NOT made.
– Tools for doing open-loop control:
• Deciding when to accept new traffic,
• Deciding when to disregard packets and which ones.
• Making scheduling decision at various points in the network.
• Note that all those decisions are made without regard to the current state of the
network.

November 10, 2016 Veton Këpuska 14

 General Principles of Congestion Control

• Closed-loop approach
– It is based on the principle of feedback-loop. The approach has
three parts when applied to congestion control:
1. Monitor the system to detect when and where congestion occurs,
2. Pass this information tot places where action can be taken
3. Adjust system operation to correct the problem.

November 10, 2016 Veton Këpuska 15

Figure 24.5 Congestion control
categories

24.15
 Warning Bit/ Backpressure
• A special bit in the packet header is set by the
router to warn the source when congestion is
detected.
• The bit is copied and piggy-backed on the ACK
and sent to the sender.
• The sender monitors the number of ACK
packets it receives with the warning bit set
and adjusts its transmission rate accordingly.

16
Figure 24.6 Backpressure method for alleviating congestion

24.17
 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.
• An example of a choke packet is the ICMP
Source Quench Packet.

18
Figure 24.7 Choke
packet

24.19
 Open-Loop Control
• Network performance is guaranteed to all
traffic flows that have been admitted into the
network
• Initially for connection-oriented networks
• Key Mechanisms
– Admission Control
– Policing
– Traffic Shaping
– Traffic Scheduling
 Admission Control
• Flows negotiate contract with
network
Peak rate
• Specify requirements:
– Peak, Avg., Min Bit rate
Bits/second

– Maximum burst size

Average rate
– Delay, Loss requirement
• Network computes resources
needed
– “Effective” bandwidth
• If flow accepted, network
Time allocates resources to ensure
QoS delivered as long as
Typical bit rate demanded by
source conforms to contract
a variable bit rate
information source
 Policing
• Network monitors traffic flows continuously to ensure
they meet their traffic contract
• When a packet violates the contract, network can discard
or tag the packet giving it lower priority
• If congestion occurs, tagged packets are discarded first
• Leaky Bucket Algorithm is the most commonly used
policing mechanism
– Bucket has specified leak rate for average contracted rate
– Bucket has specified depth to accommodate variations in
arrival
rate
– Arriving packet is conforming if it does not result in
overflow
 Traffic Shaping
• Another method of congestion control is to
“shape” the traffic before it enters the
network.
• Traffic shaping controls the rate at which
packets are sent (not just how many). Used
in ATM and Integrated Services networks.
• At connection set-up time, the sender and
carrier negotiate a traffic pattern (shape).
• Two traffic shaping algorithms are:
– Leaky Bucket
– Token Bucket

24
 The Leaky Bucket Algorithm
• The Leaky Bucket Algorithm used to control
rate in a network. It is implemented as a
single-server queue with constant service
time. If the bucket (buffer) overflows then
packets are discarded.

25
 The Leaky Bucket Algorithm

(a) A leaky bucket with water. (b) a leaky bucket

with packets.
26
Leaky Bucket Algorithm, cont.
• The leaky bucket enforces a constant output rate
(average rate) regardless of the burstiness of the
input. Does nothing when input is idle.
• The host injects one packet per clock tick onto
the network. This results in a uniform flow of
packets, smoothing out bursts and reducing
congestion.
• When packets are the same size (as in ATM cells),
the one packet per tick is okay. For variable length
packets though, it is better to allow a fixed number
of bytes per tick. E.g. 1024 bytes per tick will allow
one 1024-byte packet or two 512-byte packets or
four 256-byte packets on 1 tick.
27
Figure 24.19 Leaky
bucket

24.28
Figure 24.20 Leaky bucket
implementation

24.29
 Note
A leaky bucket algorithm shapes bursty traffic into fixed-rate traffic by averaging the
data rate. It may drop the packets if the bucket is full.

24.30
 Note
The token bucket allows bursty traffic at a regulated maximum rate.

24.31
 Leaky Bucket Traffic Shaper
Size N
Incoming traffic Shaped traffic
Server

Packet

• Buffer incoming packets

• Play out periodically to conform to parameters
• Surges in arrivals are buffered & smoothed out
• Possible packet loss due to buffer overflow
• Too restrictive, since conforming traffic does not need to
be completely smooth
 Token Bucket Algorithm
• In contrast to the LB, the Token Bucket Algorithm,
allows the output rate to vary, depending on the
size of the burst.
• In the TB algorithm, the bucket holds tokens.
To transmit a packet, the host must capture
and destroy one token.
• Tokens are generated by a clock at the rate of one
token every t sec.
• Idle hosts can capture and save up tokens (up to
the max. size of the bucket) in order to send
larger bursts later.
32
The Token Bucket Algorithm

5-34

(a) Before. (b) 33

Figure 24.21 Token bucket

24.34
 Token Bucket Traffic Shaper
Tokens arrive
periodically

An incoming packet must

have sufficient tokens
before admission into the
network Size K
Toke
n

Size N
Incoming traffic Shaped traffic
Server

Packet
• Token rate regulates transfer of packets
• If sufficient tokens available, packets enter network
without delay
Leaky Bucket vs Token Bucket
• LB discards packets; TB does not. TB
discards tokens.
• With TB, a packet can only be transmitted if
there are enough tokens to cover its length
in bytes.
• LB sends packets at an average rate. TB
allows for large bursts to be sent faster by
speeding up the output.
• TB allows saving up tokens (permissions) to
send large bursts. LB does not allow
saving.

37
 Load Shedding
• When buffers become full, routers simply discard
packets.
• Which packet is chosen to be the victim depends on
the application and on the error strategy used in
the data link layer.
• For a file transfer, for, e.g. cannot discard older
packets since this will cause a gap in the received
data.
•For real-time voice or video it is probably better to
throw away old data and keep new packets.
• Get the application to mark packets with discard
priority.

38