Computer Networking Lecture Notes

1 April 2013 • Quality of Service

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Outline

• Quality of service whats and whys.
• Describing application requirements.
• Service agreements.
• Traffic shaping.
• Buckets, leaky and token.
• Packet scheduling.
• Admission control.

Quality of Service

• Some applications have strong performance demands.
  • Maximum throughput, minimum latency.
  • Real-time, or near real-time applications.
• Networks should provide services at the level required by applictions.

Providing QoS

• Over-provision the network the make sure enough resources are always available.
  • Expensive initially and in the long term.
  • Application demand estimates may be wrong or change over time.
• Issues include specification, regulation, admission and reservation.

Application Requirements

• A flow characterized in four ways: bandwidth, delay, jitter, and loss.
• An application can sometimes adapt to shortcomings in network performance.
  • Bandwidth (bit/sec).
  • Delay (sec).
  • Jitter (delay variance).
  • Loss (packets/sec).

Describing Requirements

• Define application categories where each category represents similar performance demands.
  • ATM networking defines service categories.
    • Constant bit rate (CBR).
    • Variable bit rate (VBR).
    • Available bit rate (ABR).

Traffic Shaping

• Unvarying traffic characteristics easy to handle.
  • Computer network traffic is bursty (varying).
• Traffic shaping moderates traffic characteristics.
  • Making variability less so.
  • Stronger service guarantees less expensively.

Service Agreements

• Service level agreement (SLA) between applications and the network.
• Traffic policing to check traffic compliance with SLAs.
• Shaping and policing come with the real-time territory.

Buckets, Leaky and Token

• Traffic shaping with leaky and token bucket algorithms.
• Traffic goes into a bucket with a hole in it; leaking traffic flows into the network.
  • Too much traffic overflows the bucket and is lost.
  • Possible overflow is buffered at the host or tossed by the network interface.

Token Buckets

• A bucket has a replenishing supply of tokens.
  • At some tokens/sec rate.
  • Tokens overflowing the bucket are lost.
• Packets entering the network require tokens.
  • At so many tokens/byte.
• Buckets pass short bursts, while long bursts are evened out by bucket storage.
• Routers work at 200 Mb/s with 9600 kb buffers.

  Buckets

  • Buckets shape the host traffic to make it suitable for routers.

  Packet Scheduling

  • Guaranteeing network resources is easier with virtual circuits.
  • Packet scheduling algorithms allocate router resources to flows: bandwidth, buffer space, cpu cycles.
  • Buffers absorb small variations in demand; flow reservations make sure a flow has buffer space.
  • Scheduling cpu time is important for packet processing.

  FIFO Scheduling

  • Packet scheduling determines which buffered packet gets router resources. FIFO scheduling is the simplest schedule.
  • FIFO scheduling tail-drops new packets on full buffers; but RED too.
  • Fifo doesn't handle multiple flows well.
  • Per-flow queueing and round-robin service: fair queueing.
  • Packet-by-packet queuing is unfair with respect to packet size. a byte-by-byte queuing is the basis for fairer scheduling.

  Packet Scheduling

  • Simulate byte-by-byte queuing by ordering packets via their transmission times as if sent byte-by-byte
  • Weighted fair queueing allows priority among flows by counting groups of bytes as one byte.
  • WFQ is complex to implement (O(log n)) but an approximation is constant time.
  • Priority scheduling: high priority packets before low priority packets; could lead to starvation. wfq does better

  Timestamp Scheduling

  • A packet timestamp measures on-timedness; later packets are sent before earlier packets. per-packet cpu overhead.

  Admission Control

  • Resource reservation for stronger guarantees.
  • Plotting a path through the network capable of handling the flow's qos spec.
  • Bandwidth is easy for a host to specify; buffers and cpu cycles are not.
  • The guarantee strength is important too; stronger guarantees are more expensive and harder to provide.
  • Some applications are resource adaptive; others are not.

  Flow Specifications

  • A flow specification describes flow parameters. routers make commit decisions based on a flow spec.
  • Token-bucket size & rate specify the maximum sustained rate and maximum burst size.
  • Peak data rate maximum transmission rate.

  Resource Mapping

• Minimum and maximum packet size; large packets, bandwidth constraint; small packets, cpu constrained; 10,000 pkt/sec * 1 kbyte/pkt vs 100,000 pkt/sec * 50 byte/pkt
• Mapping available resources to requested resources is tricky in the presence of statistical fluctuations.
• Even on this side of resource exhaustion, delays could still build up.
• Relate wfq weights to token bucket parameters to provide balance.

Summary

• Some applications need guaranteed network-service levels.
• Quality-of-service (QoS) network mechanisms provide guaranteed service levels.
• Service specifications at the host.
  • And in the network.
• Traffic shaping and policing at the edge.
• Traffic scheduling in the network.

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This page last modified on 2011 October 9.