1. Congestion Control in Data Networks and Internets
Chapter 10
Congestion Control in Data Networks and Internets

2. Introduction Packet–switched networks get congested!
Congestion occurs when the number of packets transmitted approaches network capacity
Objective of congestion control:
keep the number of packets that are entering/within the network below the level at which performance drops off dramatically
Chapter 10: Congestion Control

3. Queuing Theory
Recall from Chapter 8 that a data network is a network of queues
If arrival rate at any queue > transmission rate from the node then queue size grows without bound and packet delay goes to infinity
Rule of Thumb Design Point:  = L/R < .8 *
Chapter 10: Congestion Control

4. Input & Output Queues at a Node
Ts = L/R
Ts = L/R
Nodal
Processing
Ts = L/R
Chapter 10: Congestion Control

5. At Saturation Point Two Possible Strategies at Node:
Discard any incoming packet if no buffer space is available
Exercise flow control over neighbors
May cause congestion to propagate throughout network
Chapter 10: Congestion Control

6. Queue Interaction in Data Network (delay propagation)
Chapter 10: Congestion Control

7. Jackson’s Theorem - Application in Packet Switched Networks
Internal load:
 =  i
where:
 = total on all links in network
i = load on link i
L = total number of links L
i=1
Packet Switched
Network
Note:
Internal > offered load
Average length for all paths:
E[number of links in path] = /
Average number of items waiting
and being served in link i: ri = i Tri
Average delay of packets sent
through the network is:
T = 
where: M is average packet length and
Ri is the data rate on link i 1  L
i=1
Mi
Ri - Mi
External load, offered to network:
 =   jk
where:
 = total workload in packets/sec
jk = workload between source j
and destination k
N = total number of (external)
sources and destinations
N N
j=1 k=2
Notice: As any i increases,
total delay increases.
Chapter 10: Congestion Control

8. Ideal Performance
I.e., infinite buffers, no variable overhead for packet transmission or congestion control
Throughput increases with offered load up to full capacity
Packet delay increases with offered load approaching infinity at full capacity
Power = throughput / delay, or a measure of the balance between throughput and delay
Higher throughput results in higher delay
Chapter 10: Congestion Control

9. Ideal Network Utilization
Load:
Ts = L/R
Power: relationship between
Normalized Throughput and Delay
Chapter 10: Congestion Control

10. Practical Performance
I.e., finite buffers, non-zero packet processing overhead
With no congestion control, increased load eventually causes moderate congestion: throughput increases at slower rate than load
Further increased load causes packet delays to increase and eventually throughput to drop to zero
Chapter 10: Congestion Control

11. Effects of Congestion What’s happening here? buffers fill
packets discarded
sources re-transmit
routers generate more traffic to update paths
good packets resent
delays propagate
Chapter 10: Congestion Control

12. Common Congestion Control Mechanisms
Chapter 10: Congestion Control

13. Congestion Control Backpressure Policing Choke packet
Request from destination to source to reduce rate
Useful only on a logical connection basis
Requires hop-by-hop flow control mechanism
Policing
Measuring and restricting packets as they enter the network
Choke packet
Specific message back to source
E.g., ICMP Source Quench
Implicit congestion signaling
Source detects congestion from transmission delays and lost packets and reduces flow
Chapter 10: Congestion Control

14. Explicit congestion signaling
Direction
Backward
Forward
Categories
Binary
Credit-based
Rate-based
Chapter 10: Congestion Control

15. Traffic Management in Congested Network – Some Considerations
Fairness
Various flows should “suffer” equally
Last-in-first-discarded may not be fair
Quality of Service (QoS)
Flows treated differently, based on need
Voice, video: delay sensitive, loss insensitive
File transfer, mail: delay insensitive, loss sensitive
Interactive computing: delay and loss sensitive
Reservations
Policing: excess traffic discarded or handled on best-effort basis
Chapter 10: Congestion Control

16. Frame Relay Congestion Control
Minimize frame discard
Maintain QoS (per-connection bandwidth)
Minimize monopolization of network
Simple to implement, little overhead
Minimal additional network traffic
Resources distributed fairly
Limit spread of congestion
Operate effectively regardless of flow
Have minimum impact other systems in network
Minimize variance in QoS
Chapter 10: Congestion Control

17. Frame Relay Techniques
more
Chapter 10: Congestion Control

18. Congestion Avoidance with Explicit Signaling
Two general strategies considered:
Hypothesis 1: Congestion always occurs slowly, almost always at egress nodes
forward explicit congestion avoidance
Hypothesis 2: Congestion grows very quickly in internal nodes and requires quick action
backward explicit congestion avoidance
Chapter 10: Congestion Control

19. Congestion Control: BECN/FECN
Chapter 10: Congestion Control

20. FR - 2 Bits for Explicit Signaling
Forward Explicit Congestion Notification
For traffic in same direction as received frame
This frame has encountered congestion
Backward Explicit Congestion Notification
For traffic in opposite direction of received frame
Frames transmitted may encounter congestion
Chapter 10: Congestion Control

21. Explicit Signaling Response
Network Response
each frame handler monitors its queuing behavior and takes action
use FECN/BECN bits
some/all connections notified of congestion
User (end-system) Response
receipt of BECN/FECN bits in frame
BECN at sender: reduce transmission rate
FECN at receiver: notify peer (via LAPF or higher layer) to restrict flow
Chapter 10: Congestion Control

22. Frame Relay Traffic Rate Management Parameters
Committed Information Rate (CIR)
Average data rate in bits/second that the network agrees to support for a connection
Data Rate of User Access Channel (Access Rate)
Fixed rate link between user and network (for network access)
Committed Burst Size (Bc)
Maximum data over an interval agreed to by network
Excess Burst Size (Be)
Maximum data, above Bc, over an interval that network will attempt to transfer
Chapter 10: Congestion Control

23. Committed Information Rate (CIR) Operation
Current rate at which user is sending over the channel
Average data rate (bps) committed to the user by the Frame Relay network.
Maximum data rate over time period allowed for this connection by the Frame Relay network. Be Bc
Maximum line speed of connection to Frame Relay network
(i.e., peak data rate)
CIRi,j  AccessRatej i
Chapter 10: Congestion Control

24. Frame Relay Traffic Rate Management Parameters
Max.
Rate
CIR = bps Bc T
Chapter 10: Congestion Control

25. Relationship of Congestion ParametersBc
CIR
Note that T =
From ITU-T I.370
Chapter 10: Congestion Control