24.1
Chapter 24
Congestion Control and
Quality of Service
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:
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.
Network Performance
Topics discussed in this section:
24.6
Figure 24.3 Queues in a router
24.7
Figure Packet delay and throughput as functions of load
24.8
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:
24.9
Figure 24.5 Congestion control categories
24.10
Figure 24.6 Backpressure method for alleviating congestion
24.11
Figure 24.7 Choke packet
24.12
24-4 TWO EXAMPLES
To better understand the concept of congestion control, let us give two examples: one in TCP and the other in Frame Relay.
Congestion Control in TCP�Congestion Control in Frame Relay
Topics discussed in this section:
24.13
Figure 24.8 Slow start, exponential increase
24.14
In the slow-start algorithm, the size of the congestion window increases exponentially until it reaches a threshold.
Note
24.15
Figure 24.9 Congestion avoidance, additive increase
24.16
In the congestion avoidance algorithm, the size of the congestion window increases additively until �congestion is detected.
Note
24.17
An implementation reacts to congestion detection in one of the following ways:
❏ If detection is by time-out, a new slow� start phase starts.
❏ If detection is by three ACKs, a new� congestion avoidance phase starts.
Note
24.18
Figure 24.10 TCP congestion policy summary
24.19
Figure 24.11 Congestion example
24.20
Figure 24.12 BECN
24.21
Figure 24.13 FECN
24.22
Figure 24.14 Four cases of congestion
24.23
24-5 QUALITY OF SERVICE
Quality of service (QoS) is an internetworking issue that has been discussed more than defined. We can informally define quality of service as something a flow seeks to attain.
Flow Characteristics�Flow Classes
Topics discussed in this section:
24.24
Figure 24.15 Flow characteristics
24.25
24-6 TECHNIQUES TO IMPROVE QoS
In Section 24.5 we tried to define QoS in terms of its characteristics. In this section, we discuss some techniques that can be used to improve the quality of service. We briefly discuss four common methods: scheduling, traffic shaping, admission control, and resource reservation.
Scheduling�Traffic Shaping�Resource Reservation
Admission Control
Topics discussed in this section:
24.26
Figure 24.16 FIFO queue
24.27
Figure 24.17 Priority queuing
24.28
Figure 24.18 Weighted fair queuing
24.29
Figure 24.19 Leaky bucket
24.30
Figure 24.20 Leaky bucket implementation
24.31
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
24.32
The token bucket allows bursty traffic at a regulated maximum rate.
Note
24.33
Figure 24.21 Token bucket
24.34
24-7 INTEGRATED SERVICES
Two models have been designed to provide quality of service in the Internet: Integrated Services and Differentiated Services. We discuss the first model here.
Signaling�Flow Specification�Admission
Service Classes
RSVP
Problems with Integrated Services
Topics discussed in this section:
24.35
Integrated Services is a flow-based QoS model designed for IP.
Note
24.36
Figure 24.22 Path messages
24.37
Figure 24.23 Resv messages
24.38
Figure 24.24 Reservation merging
24.39
Figure 24.25 Reservation styles
24.40
24-8 DIFFERENTIATED SERVICES
Differentiated Services (DS or Diffserv) was introduced by the IETF (Internet Engineering Task Force) to handle the shortcomings of Integrated Services.
DS Field
Topics discussed in this section:
24.41
Differentiated Services is a class-based QoS model designed for IP.
Note
24.42
Figure 24.26 DS field
24.43
Figure 24.27 Traffic conditioner
24.44
24-9 QoS IN SWITCHED NETWORKS
Let us now discuss QoS as used in two switched networks: Frame Relay and ATM. These two networks are virtual-circuit networks that need a signaling protocol such as RSVP.
QoS in Frame Relay�QoS in ATM
Topics discussed in this section:
24.45
Figure 24.28 Relationship between traffic control attributes
24.46
Figure 24.29 User rate in relation to Bc and Bc + Be
24.47
Figure 24.30 Service classes
24.48
Figure 24.31 Relationship of service classes to the total capacity of the network