CN Unit III - Quality of service and Traffic Management

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Quality of service and Traffic Management Introduction to quality of services, scheduling, congestion control, differentiated services, integrated services, RSVP

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24. 1 Unit III: Congestion Control and Quality of Service Reference: Kurose, Ross “Computer Networking-a top down approach featuring the internet” Slides prepared by: Mr. Vaibhav Dabhade for TE Computer Engineering

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24. 2 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:

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24. 3 Traffic descriptors

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24. 4 Three traffic profiles

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24. 5 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:

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24. 6 Queues in a router

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24. 7 Packet delay and throughput as functions of load

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24. 8 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:

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24. 9 Congestion control categories

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24. 10 Backpressure method for alleviating congestion

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24. 11 Choke packet

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24. 12 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:

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24. 13 Slow start, exponential increase

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24. 14 In the slow-start algorithm, the size of the congestion window increases exponentially until it reaches a threshold. Note

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24. 15 Congestion avoidance, additive increase

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24. 16 In the congestion avoidance algorithm, the size of the congestion window increases additively until congestion is detected. Note

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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

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24. 18 TCP congestion policy summary

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24. 19 Congestion example

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24. 20 BECN

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24. 21 FECN

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24. 22 Four cases of congestion

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24. 23 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:

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24. 24 Flow characteristics

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24. 25 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:

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24. 26 FIFO queue

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24. 27 Priority queuing

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24. 28 Weighted fair queuing

Traffic Shaping:

# 29 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 Traffic Shaping

The Leaky Bucket Algorithm:

# 30 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. Leaky Bucket (parameters r and B): Every r time units: send a packet. For an arriving packet If queue not full (less than B) then enqueue Note that the output is a “perfect” constant rate.

The Leaky Bucket Algorithm:

# 31 The Leaky Bucket Algorithm (a) A leaky bucket with water. (b) a leaky bucket with packets.

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24. 32 Leaky bucket

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24. 33 Leaky bucket implementation

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24. 34 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

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24. 35 The token bucket allows bursty traffic at a regulated maximum rate. Note

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24. 36 Token bucket

Token Bucket Algorithm:

# 37 Token Bucket Algorithm Highlights: The bucket holds tokens. To transmit a packet, we “use” one token. Allows the output rate to vary, depending on the size of the burst. In contrast to the Leaky Bucket Granularity Packets (or bits) Token Bucket (r, MaxTokens): Generate a token every r time units If number of tokens more than MaxToken, reset to MaxTokens. For an arriving packet: enqueue While buffer not empty and there are tokens: send a packet and discard a token

The Token Bucket Algorithm:

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

Token bucket example:

# 39 Token bucket example parameters: MaxTokens=6 1/r=3 (=3 token/time) arrival queue Token bucket sent p1 (5) - 0 - p2 (2) p1 3 - p3 (1) p2 6-5=1 p1 4-2-1=1 p3,p2 4 6

Leaky Bucket vs Token Bucket:

# 40 Leaky Bucket vs Token Bucket Leaky Bucket Discard: Packets Rate: fixed rate (perfect) Arriving Burst: Waits in bucket Token Bucket Discard: Tokens Packet management separate Rate: Average rate Bursts allowed Arriving Burst: Can be sent immediately

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24. 41 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:

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24. 42 Integrated Services is a flow-based QoS model designed for IP. Note

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24. 43 Path messages

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24. 44 Resv messages

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24. 45 Reservation merging

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24. 46 Reservation styles

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24. 47 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:

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24. 48 Differentiated Services is a class-based QoS model designed for IP. Note

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24. 49 DS field

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24. 50 Traffic conditioner

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24. 51 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:

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24. 52 Relationship between traffic control attributes

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24. 53 User rate in relation to Bc and Bc + Be

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24. 54 Service classes

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24. 55 Relationship of service classes to the total capacity of the network

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