Network Congestion

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Network Congestion

• Gabriel Nell
• UC Berkeley

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Outline

• Background what is congestion?
• Congestion control
• End-to-end
• Router-based
• Economic insights
• Overview of TCP-friendly congestion control
  mechanisms

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Congestion

• The state of sustained network overload
• Congestion collapse
• Traffic dominated by overhead such as packet
  retransmissions
• Current internet
• Dominated by best-effort traffic
• TCP for guaranteed delivery Congestion-aware
• UDP for streaming applications Congestion-unaware

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Controlling Congestion

• End-hosts
• Advantages cheap, scalable
• Disadvantage requires cooperation
• Routers
• Advantages can be more aggressive, has a
  complete picture of network traffic
• Disadvantages expensive, algorithms difficult to
  implement in hardware

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TCP Congestion Control

• Implemented at end hosts
• Relies on feedback
• Implicit packet drops indicate congestion
• Explicit ECN flags in header
• Congestion controlled by changing window size
  additive increase, multiplicative decrease (AIMD)
• Problem delays in adapting to network conditions
  may cause oscillations

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Router-Based Control

• Scheduling
• Determines service order
• Should be easy to implement, provide fairness and
  protection, and perform well
• Scheduling algorithms
• FIFO (first in, first out)
• Round-robin / weighted round-robin
• Weighted fair queuing

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Router-Based Control

• Buffer Management
• Absorbs bursts
• Shared/per-flow
• Introduce delay
• Queue Management
• Manage queue length, decide what packets to drop
• RED effective, but difficult to parameterize for
  variable conditions

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Economic Insights

• Tragedy of the Commons
• Network resources a public good
• Negative externality
• Solution Internalize costs
• Congestion pricing cover fixed costs, charge
  extra under congestion conditions
• Charge by willingness to pay

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Economic Insights

• New problem - customers prefer
• Flat rates
• Constant performance, even if variable
  performance is better on average

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TCP-friendly Congestion Control

• TCP-friendly long-term throughput does not
  exceed that of TCP under the same conditions
• Motivation want to stream data such as audio and
  video without degrading overall network
  performance
• For convenience, consider long-lasting streams

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Congestion Control Schemes

• Window-based vs. Rate-based
• Unicast vs. Multicast
• End-to-end vs. Router-supported

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Single-rate Vs. Multi-rate

• Meaningful when considering multicast
• Single-rate sends data to each client at the same
  rate
• Multirate sends data to each client at whatever
  rate is best for that client

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Single-rate Protocols
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Rate-based Approaches

• RAP Rate Adaptation Protocol
• Simple AIMD behavior
• LDA Loss-Delay Based Adaption Algorithm
• Dynamic AIMD based on RTCP feedback
• TFRC TCP-Friendly Rate Control Protocol
• Adjusts sending rate based on complex TCP
  equation
• TEAR TCP Emulation at Receivers
• Uses a congestion window to determine rate, but
  averages over larger timescales

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Window-based Approaches

• RLA Random Listening Algorithm
• Tracks number n of congested receivers, window is
  decreased if a random number is ? 1/n
• MTCP Multicast TCP
• Arrange receivers in a tree, children report
  congestion to parents.
• Root receives aggregate info, sends only as much
  data as smallest window
• NCA Nominee-Based Congestion Avoidance
• Selects bottleneck as representative receiver,
  uses TCP-style congestion control algorithm

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Multi-rate Protocols
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Rate-based Approaches

• RLC Receiver-Driven Layered Congestion Control
• Bandwidth consumed by each layer increases
  exponentially
• Subscription to additional layers comes at
  particular times, which also increase
  exponentially however congestion causes
  immediate layer drops

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Rate-based Approaches

• FLID-DL Fair Layered Increase/Decrease with
  Dynamic Layering
• Encodes data with digital fountain
• Bandwidth consumed by a layer decreases over time
• LTS/TFRP Layered Transmission
  Scheme/TCP-Friendly Transport Protocol
• Use simple TCP rate equation to decide
  subscription level

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MLDA and Rainbow

• MLDA Multicast Loss-Delay Based Adaption
  Algorithm (rate-based)
• Same as LDA, but performs rate calculation at
  receiver
• Rainbow (window-based)
• Encode data with digital fountain
• Receivers individually request packets based on
  individual windows

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Conclusion

• Congestion is an important and complex problem
• Many solutions of varying effectiveness and
  complexity for various applications
• Areas of future research
• Methods of comparing protocols
• Improve definitions of fairness, friendliness
• Improve models of TCP traffic

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Questions?