ONT Notes – Intro to QoS

I’ll try to keep it a little shorter this time.

Major issues for converged enterprise networks

  • Available bandwidth: competition among applications
    • Fixes
      • Increase bandwidth: More power!
      • Properly queue based on classification and marking: QoS
      • Compress: cRTP, TCP header compression, etc.
  • Delay: Lead time to get a packet to the destination
    • Types of delay
      • Processing delay: routing, switch delay
      • Queuing delay: how long a frame stays in an output queue
      • Serialization delay:  how long to put the frame on the wire
      • Propagation delay: the time to cross the physical medium
  • Jitter (delay variation): Variation is the delay
    • Different delays mean different arrival times
    • De-jitter buffers save up packets to reduce jitter (like the old CD writers)
    • Fixes
      • More bandwidth
      • Prioritize sensitive data and forward first
      • Remark (reclassify) packets based on sensitivity
      • Enable L2 payload compression: make sure compression delay isn’t worse than the jitter
      • Use header compression
  • Packet loss: Packets are lost in the network somewhere
    • Fixes
      • More bandwidth
      • Increase buffers space: more room for the queue on the interface
      • Provide guaranteed bandwidth: Queuing and QoS
      • Congestion avoidance
        • Random Early Detection (RED) and weighted RED (WRED) drop packets before the queue is full
        • Selective dropping is better than FIFO or LIFO dropping

QoS History

  • Priority queuing: gives certain data the right-of-way for transmission
  • Weighted Fair Queuing (WFQ): prevents small packets from waiting too long for big packets
  • RTP priority queuing: Gives voice packets the right-of-way
  • CAC: Makes sure we don’t fill up the queue or pipe with voice traffic

Implementing QoS

  • Step 1: Identify traffic types and requirements
    • Network audit
    • Business audit
    • Define bandwidth requirements for each class found
  • Step 2: Classify the traffic
    • Common classes
      • VOIP
      • Mission-critical
      • Signal traffic: for VOIP
      • Transactional application: SAP, ERP
      • Best-effort: Everything else
      • Scavenger: Crap you don’t care about like P2P and your boss’s email
  • Step 3: Define policies for each class
    • Tasks for each class
      • Set max bandwidth
      • Set min bandwidth
      • Assign relative priorities
      • Apply congestion avoidance, congestion management, etc.

QoS Models

  • Best-effort: no QoS
    • Scalable
    • Easy
    • No service guarantee: doesn’t care what you’re trying to do
    • No service differentiation: all traffic is equal
  • Integrated Service (IntServ)
    • Hard-QoS
    • Uses RSVP to guarantee bandwidth through the entire path
    • Requires
      • Admission control
      • Classification
      • Polices the traffic (ceiling)
      • Queuing
      • Scheduling
    • Advantages
      • End-to-end resource admission control
      • Per-request policy admission control
      • Signaling of dynamic ports
    • Disadvantages
      • Continuous signaling
      • Not scalable
  • Differentiated Services (DiffServ)
    • Soft-QoS
    • Configured on each hop
    • Traffic is classified
    • Enforces different treatment on different classes
    • Defined based on business requirements
    • Benefits
      • Scalable
      • Supports lots of service levels
    • Drawbacks
      • No absolute guarantee of service
      • Complex configuration throughout network

QoS Implementation Methods

  • CLI
    • Old school
    • Not used any more
  • Modules QoS CLI (MQC)
    • Step 1: class-map
    • Step 2: policy-map
    • Step 3: service-policy
  • AutoQoS
    • Automatically generates classes and policies based on traffic it sees
    • Super-simple
    • Requires CEF, NBAR, and correct bandwidth statements
  • SDM QoS Wizard
    • Next, next, next
    • Can be used to implement, monitor, or troubleshoot QoS

Aaron Conaway

I shake my head around sometimes and see what falls out. That's what lands on these pages.

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