Quality of Service

Classification/Marking

• Way to identify different types of packets and providing different treatment.
• Classification:
  • ACL
    • Creating ACL specifying source/destination/port.
  • NBAR-Network Based Application Recognition
    • Automatic way of discovering types of traffic traversing router.
• Marking:
  • Changing header fields.
  • IP packets have the Type of Service field that can be changed.
  • At layer 2, ethernet frames can use 802.1Q priority value.

Queuing/Congestion

• When a routing device receives packet it will figure out what interface it needs to forward on and sends on its way. If that interface is congested, for example when inbound traffic bandwidth is higher than outbound bandwidth, it will be placed in a queue.
• Routing devices can have multiple queues with specific amounts of bandwidth allocated to each.
• Queue Scheduling:
  • Round Robin – Cycles through queues in order, each queue getting fair amount to transmit.
  • Weighted Round Robin – Gives more preference to specific queues.
  • CBWFQ-Class Based Weighted Fair Queueing – Guarantees min bandwidth to each class when there is congestion. Weighted round robin, admin configures weighting as percentage of bandwidth on each interface.
  • Low Latency Queuing – Round Robin is terrible option for voice. Too much delay.
    • LLQ is priority queue that sends traffic immediately instead of waiting for other allocated bandwidth to transmit.
    • Queue Starvation – When priority/LLQ queueing is allocated too much bandwidth. When this happens all other queues begin dropping traffic.

Policing

• Limits bit rate by dropping traffic.
• Often used by ISPs to drop customer traffic.
  • Ex. 1Gbps interface connected to customer device, customer only pays for 100Mbps circuit.
  • CIR

Shaping

• Shaper Queues traffic when going over specific CIR vs. dropping.
• Creates problem where shaper halts all traffic, then sends, over and over again.
  • Creates a lot of delay.
  • This will break voice traffic.
  • Tc (time interval) of shapers can be configured, breaking up send and wait times more frequently.