QOS Overview

Quality Of Service

Recommend Resources:
Qos-Enabled Networks: Tools and Foundations
End-to-End Qos Network Design

Online Resources:
BRKCRS-2501 – Campus QoS Design-Simplified
Enterprise Medianet Quality of Service Design 4.0

What is QoS?
• Quality of service is the ability to provide different priority to different applications, users, or data flows, or to guarantee a certain level of performance to a data flow.

• E.g. Different service levels for different types of “classes” of traffic flows

Why is QoS Needed?
Root Cause: Resource Contention
• Multiple flows sharing the same link
• Same or multiple application
• Each application has its own requirements

Contention results in Queuing
• Packets may be delayed or dropped
• Effective flow throughput decreases
• Delay or Jitter may exceed thresholds

Best Solution: Avoid Contention
• Don’t over-provision
• Not always possible

Next Best Solution: QoS
• Network congestion is controlled
• Delay/Loss/Jitter/Throughput are controlled
• Only alleviates temporary congestion

QoS Model
• QoS model defines contention management approach
• Two types
o Integrated Services
o Differentiated Services

What is IntServ? (Smaller Deployment)
• Connection-oriented model
• Every flow has an explicit reservation end-to-end
• Does not scale well because network must maintain too much state

IntServ use case in MPLS TE
• Use for real world deployment

What is DiffServ?
• Connectionless model
• Traffic is grouped into classes
• QoS behavioe is defined by traffic’s class
• Called Per-Hop Behavior (PHB)

Classification & Marking
• In order for DiffServ to properly work, traffic must be placed into correct classes (I.e. “Classification”
• Traffic classification normally occurs at network ingress edge (Typically a manual process we must enforce)

Classification Types
• Classification & Marking can happen at multiple places
• Later 2 class of service
o 802.1q Ethernet header
• Layer 3 IP Type of Service (ToS)
o IP precedence & Differentiated Services code point (DSCP)
• Layer 4
o TCP & UDP ports
• Upper layers
o Network Based Application Recognition (NBAR)
o Deep packet Inspection (DPI)

QoS Tools
• Used to implement QoS Models
o Many tools rely on correct QoS classification & marking.
• Different tools for
o Network Edge
o Network Core

Tools Fall into three main categories

Admission Control
o Used to enforce traffic marking or traffic rate
Two Main Types:
o Traffic Policing – used to limit inbound and outbound traffic flows
o Traffic that exceeds the rate can be dropped, marked, or re-marked
o Typically applied on ingress edge
Example use case
• PE connects to CE w/ GigE port
• Circuit is provisioned at 250Mbps
• PE applies inbound policer at port level
o If traffic 250Mbps, Drop or Maked or Re-marked
o Traffic Shaping – Used to normalize outbound traffic flows.
o Smooth out traffic bursts
o Prepares traffic for ingress policing
o Delay and Queue exceeding traffic
Example use case
• PE connects to CE w/ GigE port
• Circuit is provisioned at 250Mbps
• PE applies inbound policer at port level
o If traffic 250Mbps, queue for later transmission

Congestion Management Techniques (I.e. Queueing)
• Outbound Congestion management

Queueing Types:
• First in First Out (FIFO)
• Weighted Fair Queueing (WFQ)
• Priority Queueing (PQ) / Low Latency Queuing (LLQ)

Example use case
• CE to PE link is experiencing packet loss
• Apply LLQ to give VoIP low delay
• Apply WFQ to guarantee 50% BW for SQL
• All other traffic gets best effort FIFO

Congestion Avoidance Techniques (Try to prevent congestion before it occurs I.e. Packet drop strategy)
Drop Strategy Types:
• Weight Random Early Detection (WRED)
• Tail drop

Example use case
• CE to PE link is experiencing packet loss
• Apply WRED to selectively drop low priority TCP flows
• Senders go into TCP slow start
• Congestion management is offloaded to the end host

Question and Answer:
1. Policing Drops and Shaping queue.

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