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Network Congestion

Network Congestion. Gabriel Nell UC Berkeley. Outline. Background: what is congestion? Congestion control End-to-end Router-based Economic insights Overview of TCP-friendly congestion control mechanisms. Congestion. The state of sustained network overload Congestion collapse

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Network Congestion

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  1. Network Congestion Gabriel Nell UC Berkeley

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

  3. 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

  4. 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

  5. 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

  6. 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

  7. 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

  8. 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

  9. Economic Insights • New problem - customers prefer: • Flat rates • Constant performance, even if variable performance is better on average

  10. 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

  11. Congestion Control Schemes • Window-based vs. Rate-based • Unicast vs. Multicast • End-to-end vs. Router-supported

  12. 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

  13. Single-rate Protocols

  14. 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

  15. 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

  16. Multi-rate Protocols

  17. 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

  18. 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

  19. 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

  20. 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

  21. Questions?

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