The document discusses congestion control in computer networks. It defines congestion as occurring when the load on a network is greater than the network's capacity. Congestion control aims to control congestion and keep the load below capacity. The document separates congestion control mechanisms into two categories: open-loop control, which aims to prevent congestion; and closed-loop control, which detects congestion and takes corrective actions through feedback. Specific open-loop techniques discussed are admission control, traffic shaping using leaky bucket and token bucket algorithms, and traffic scheduling.

1. Congestion Control

2. 24.2
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.
Traffic Descriptor
Traffic Profiles
Topics discussed in this section:

3. 24.3
Figure 24.1 Traffic descriptors

4. 24.4
Figure 24.2 Three traffic profiles

5. 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.
Network Performance
Topics discussed in this section:

6. Congestion Control Algorithms
• Congestion - the situation in which too
many packets are present in the subnet.

7. 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.

8. 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.

9. 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

10. 24.10
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).
Open-Loop Congestion Control
Closed-Loop Congestion Control
Topics discussed in this section:

11. 11
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.

12. 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.

13. November 4, 2016 Veton Këpuska 13
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.

14. November 4, 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.

15. 24.15
Figure 24.5 Congestion control categories

16. 16
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.

17. 24.17
Figure 24.6 Backpressure method for alleviating congestion

18. 18
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.

19. 24.19
Figure 24.7 Choke packet

20. 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

21. Time
Bits/second
Peak rate
Average rate
Typical bit rate demanded by
a variable bit rate information
source
Admission Control
• Flows negotiate contract with
network
• Specify requirements:
– Peak, Avg., Min Bit rate
– Maximum burst size
– Delay, Loss requirement
• Network computes resources
needed
– “Effective” bandwidth
• If flow accepted, network
allocates resources to ensure
QoS delivered as long as
source conforms to contract

22. 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

23. 23
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. 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. 25
The Leaky Bucket Algorithm
(a) A leaky bucket with water. (b) a leaky bucket with
packets.

26. 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. 24.27
Figure 24.19 Leaky bucket

28. 24.28
Figure 24.20 Leaky bucket implementation

29. 24.29
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.
Note

30. 24.30
The token bucket allows bursty traffic at a regulated maximum rate.
Note

31. Incoming traffic Shaped traffic
Size N
Packet
Server
Leaky Bucket Traffic Shaper
• 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

32. 32
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.

33. 33
The Token Bucket Algorithm
(a) Before. (b) After.
5-34

34. 24.34
Figure 24.21 Token bucket

35. Incoming traffic Shaped traffic
Size N
Size K
Tokens arrive
periodically
Server
Packet
Token
Token Bucket Traffic Shaper
• Token rate regulates transfer of packets
• If sufficient tokens available, packets enter network without delay
• K determines how much burstiness allowed into the network
An incoming packet must
have sufficient tokens
before admission into the
network

36. 36
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. 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.