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Understanding TCP Resource Allocation in Networks

Dive into TCP events triggering transmission, congestion detection, and more. Learn about resource allocation dynamics, evaluation criteria, and queueing disciplines in networking. Explore fair queueing and window-based vs. rate-based approaches.

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Understanding TCP Resource Allocation in Networks

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  1. 2004.5.11. • Last Class • TCP events that trigger transmission • This Class • Chapter 6.1. ~ 6.2. • Resource allocation

  2. What do we know about TCP? • is best-effort • employs a 3-way handshake • employs flow control • is window-based

  3. Do we know how ... • TCP detects congestion? • TCP determines the sending rate? • What the network actually does?

  4. Network Model • So far we have assumed • packet-switched • connectionless flows • best-effort service

  5. Topics in Resource Allocation • Traffic specification • Call admission control • Resource reservation • Scheduling

  6. Taxonomy of Resource Allocation • Router-centric vs host-centric • Reservation based vs feedback based • Window based vs rate based

  7. Evaluation Criteria • Any resource allocation mechanism is evaluated based on: • Effectiveness Power = Thruput/Delay • Fairness • Chiu and Jain’s • Max-Min fairness • Proportional fairness • TCP fairness

  8. Queueing Disciplines • Schduling policy • determines the order in which packets are transmitted • FIFO, Priority Queueing, Fair Queueing • Drop policy • determines which packets to drop • Tail drop (drop-tail), RED

  9. FIFO

  10. Priority Queueing • Main idea: mark each packet with a priority • Routers implement multiple FIFO queues • Packets in a queue with the highest priority are served first • Problem? • Bad people mark their packets with a high priority • High-priority queue can starve out all other queues

  11. Fair Queueing • Main idea: maintain a separate queue per flow to isolate. Round-robin Service

  12. Currently served • Calculate Fi =max(Fi-1 , Ai) + Pi and process packets with lowest Fi first • Fi : expected finish time if done bit-by-bit • Ai: arrival time of packet i • Pi : # of bit-rounds for packet i Which packet should finish first, blue or yellow?

  13. F F F Yellow should be served first! Time 1 2 3 4 5 6 7 9 8 Bit-round

  14. FQ and WFQ • FQ • Sources are not aware of FQ • WFQ • It assigns different weights to queues • It can assign queue not per flow, but per class • How do you assign fair weights? • Both are work-conserving • Never idle when there is work to do

  15. At what rate to send? sender receiver How do you figure out the bottleneck bandwidth?

  16. window-based vs rate-based sender bottleneck

  17. TCP employs a learning algorithm • Additive increase • Slowly increase the sending rate • Multiplicative decrease • Decrease the sending rate fast when congestion is detected

  18. Sending rate increases linearly Additive increase TIME RTT RTT RTT RTT

  19. What about doubling each time? Multiplicative increase Cwnd=1 Cwnd=2 Cwnd=4 Cwnd=8 TIME RTT RTT RTT RTT

  20. When to stop increasing? • Finite world, finite beginning • ssthresh = set to an arbitrary large number (65KB) • Sender rate exceeds RcvWindow • Only transmit min(cwnd, RcvWindow) • Packets start to be dropped at the bottleneck • Sending rate exceeds bottleneck b/w • Another user shares the bottleneck • Contention for shared resource

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