Computer Networking Lecture Notes

2014 October 14 • Network Quality of Service

---

Outline

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

Application Demands

• Some applications require more from the network than other applications.
  • Maximum throughput, minimum latency.
  • Real-time, or near real-time applications.
• Networks should provide services at the level required by applictions.
  • A specific quality of service (QoS).

Application Requirements

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.
  • Also Parkinson's law.

Application Requirements

• Application requirements can be given as
  • Bandwidth (bit/sec).
  • Delay (sec).
  • Jitter (delay variance).
  • Loss (packets/sec).
• An application can sometimes adapt to shortcomings (or windfalls) in network performance.

Delay and Jitter

• Inter-PDU delay is the time between successive packets at the receiver.

  

• Jitter is the variance of the delay at the receiver.

  

Delay and Jitter Sources

• Router buffering is the main source of delay and jitter.

  

Service Categories

• A service category (or class) represents a fixed set of network behaviors.
• The network defines and provides several service categories.
• An application figures out which service category is best for its needs.
• A service level agreement (SLA) between applications and the network.

ATM Example

• ATM defines the service categories
  • Constant bit rate (CBR): guaranteed maximum bandwidth.
  • Variable bit rate (VBR): peak, sustained, and burst rates.
    • Real and non-real time subcategories.
  • Available bit rate (ABR): whatever's available.

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.

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.

---

This page last modified on 2014 October 12.