This document provides an introduction to fundamentals of quality of service (QoS) in computer networks. It defines key QoS metrics like bandwidth, latency, jitter and packet loss. It describes the DiffServ model where packets are marked to indicate required QoS and network devices apply QoS based on these markings. It also discusses signaling QoS requirements using markers like Type of Service, Class of Service and DSCP. It covers topics like per-hop behavior, configuration of class maps and policy maps, queuing and discard algorithms, and traffic shaping algorithms like leaky bucket and token bucket.

1. Fundamentals of Quality of Service
Gopan Govindan

2. QoS
Introduction
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3. Definition metrics
Characteristics of network that define QoS
• Bandwidth
• Latency
• Jitter
• Packet loss

4. The “Model”
DiffServ
• Each packet in the network contains a marking that indicates the
required QoS
• Each network device applies QoS based upon this marking

6. Signaling QoS Requirements
Markers
• ToS – RFC 791
• CoS – QoS at MAC level
• DSCP – RFC 2474

7. ToS
Bits that matter

8. CoS
Bits that matter
* Push of packets with a specific CoS has dependency on Switch

9. Bits that matter
DSCP

10. Treatment
PHB
• Default PHB – RFC 2474 – DSCP 0 – Traditional best-effort service (FIFO)
• Class-selector PHB – RFC 2474 – for backward compatibility with IP
precedence – xxx000
• Assured Forwarding PHB – RFC 2597 - 4 AF classes - AF1, AF2, AF3,
AF4
• Expedited Forwarding PHB – RFC 2598 – place packets to priority queue
– DSCP 46
• Voice Admit PHB – RFC 5865 - identical characteristics to the EF PHB –
also admitted by the network using CAC procedure – DSCP 44

11. The “FLOW”
QoS process

12. Configuration…
Class map
Class map
To classify traffic of our interest.
Policy map
Defines a series of actions (functions) that we wish to apply to a set of
classified inbound traffic.

13. Configuration…

14. Mathematics ;)
Minor algebra
Class map
• AR
• CIR
• Bc
• Be
• Tc

15. Lets relate….
Class map
• AR - Access Rate - full line speed in bps, maximum
transmit value - 128000 in our example
• CIR - Committed Information Rate - target value for
transmission, normal transmit value
• Bc - Normal Burst - Equals CIR/Tc
• Be - Excess Burst - Equals or can be upto (AR/Tc)-Bc
• Tc will vary. 1/8 sec in data and 1/100 in voice.

16. Lets relate….
Class map
AR/Tc = Bc+Be Bc = CIR/Tc

17. Before entering the
show…
Class map
Queuing
• FIFO Queuing
• WFQ – Fair-queue (#packets) (#dynamic) (#reserved)
• CBWFQ
• Priority Queues – implemented as heap
Queue Management (Discard)
• Tail drop
• RED
• WRED

18. Algorithms 
Traffic Shaping
• Leaky Bucket
• Token Bucket

19. The baggage scanner !
Class map

20. Ready for a better QoS ?