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. If you have any questions, the best way to contact me is through Twitter at @aconaway.

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