Queuing

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Queuing

• EE 548 Ashish Kapoor

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Why Queues?

• Packets may arrive in bursts from multiple
  devices
• A device may temporarily receive more packets
  than it can process
• Buffers hold packets in a queue until a device
  can catch up
• PacketStorm 1800E/2600E IP Network Emulators

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FIFO

• This is the basic first-in, first-out queuing
  technique in which the first packet in the queue
  is the first packet that is processed
• When queues become full, congestion occurs and
  incoming packets are dropped
• Pro Easy to implement, No-Brainer
• Con One consumer may block shared resources

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Priority queuing

• Queues are serviced with different levels of
  priority
• When congestion occurs, packets are dropped from
  lower-priority queues
• Con Lower-priority queues may not get serviced
  at all if high-priority traffic is excessive

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Fair queuing

• A round-robin approach is used to service all
  queues in a fair way
• Pro Prevents any one source from overusing its
  share of network capacity
• Cons Packets are variable in length and each
  queue is allowed to release one packet at a time
• Some queues may be more full than others

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WFQ (weighted fair queuing)

• Combination of priority queuing and fair queuing
• Some queues are serviced more than others
  depending upon their weight
• Pro Low-bandwidth traffic gets a fair level of
  priority
• WFQ is Cisco's "premier queuing technique" It
  moves real-time interactive traffic to the front
  of queues and fairly shares the remaining
  bandwidth among other flows.
• Stochastic Fair Queuing (SFQ)

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CBQ (class-based queuing)

• Schedules packets in queues and guarantees a
  certain transmission rate
• If a queue is not in use, the bandwidth is made
  available to other queues
• It is also a QoS scheme that identifies different
  types of traffic and queues the traffic according
  to predefined parameters.

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Leaky Bucket

• Restricts the queues output to a maximum rate

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Token Bucket

• A queue accumulates credit during idle times
  which can be used to allow packet burst rates
  that are higher than the average rate

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Congestion Control

• Additive Increase/Multiplicative Decrease
• Slow Start Congestion Control
• Active Queue Management and Congestion Avoidance
• RED (Random Early Discard)
• BECN (backward explicit congestion notification)
• FECN (forward explicit congestion notification)
• Fast Retransmit and Fast Recovery