Quality of Service I ( QoS, Models, Methods )

What is QoS?

  • http://docwiki.cisco.com/wiki/Quality_of_Service_Networking
  • Quality of life insurance
  • The ability to dictate traffic treatment

    • Prioritization.
      • Only happens with congestion.
    • Shaping / Policing.
      • Shaping: Mold the traffic down to a specific speed.
      • Policing: ‘evil’ traffic types ( p2p / video ).
    • Advanced Strategies ( WRED – Weighted random early detection)
      • Drop selective TCP streams so it won’t hit max.
  • Strategies to fight the enemy
    • Delay ( how long it takes for a Packet A to get to the other side).
    • Jitter ( Delay variation, Times between Packets A,Packet B,and Packet C taking to get to the other side)
    • Packetloss
  Audio Requirements Video Requirements
Jitter< 30ms< 30ms
Delay< 150ms< 150ms
Loss < 1%< 1%
QoS:DSCP EFDSCP AF41
BandwithLowHigh

QoS Models

Best Effort

Integrated Services ( IntServ ).

Differentiated Services ( DiffServ ).

QoS Methods

Classification

  • Can be done with access-list to capture certain traffic.
  •  NBAR.
    • Deep packet inspection (If an aplication uses a well known port 80).
    • Looks beyond the portnumber.

Marking

  • Labels the traffic
  • Layer 2 : Class of Service ( CoS ) bits
  • Later 3 : Type of Service ( ToS )
    • differentiated services code point ( DSCP ) bits

Queueing ( Congestion Management )

  • Weighted Fair Queuing ( WFQ )
    • Low traffic senders get priority over high traffic senders
  • Class Based WFQ 
    • Devides Bandwidth among classes that you define
  • Low Latency Queuing ( LLQ )
    • Combo of CB-WFQ, but adds a strict priority element.
    • Used to be called Priority Queuing PQ-CBWFQ

Congestion Avoidance

Link Efficiency

  • Techniques used on low bandwidth connections
  • Payload Compression ( Squishing the data ).
    • Trades CPU processing power for bandwidth.
    • Your CPU compresses the data and decompresses the data on the other end.
  • Header Compression ( Squishing the header ( key for RTP ).
    • Very little data but a big header.
    • When enabled on both ends it strips off the redundant header information.
  • Link Fragmentation and Interleaving – Blow up the big packets.
    • Always used on low bandwidth.
    • Hardware Queue, it takes 214ms to put 1500 byte on a 56kbps line.
      • 1500 byte gets fragmented into smaller packets
      • Leaves openings for other packets like VoIP.

Where can you apply QoS methods?

MQC:

  1. Classify (class-map)
  2. Policy ( policy-map)
  3. Apply

 

 

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