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?