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588 Section 4

588 Section 4. Neil Spring April 27, 1999. Schedule. Notes Project 2 description Fair Queueing (Demers et.al.). Notes. Graded Homework Assignment 1 soon first seven problems have been graded. Homework 2 due date was May 3, now…

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588 Section 4

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  1. 588 Section 4 Neil Spring April 27, 1999

  2. Schedule • Notes • Project 2 description • Fair Queueing (Demers et.al.)

  3. Notes • Graded Homework Assignment 1 soon • first seven problems have been graded. • Homework 2 due date • was May 3, now… • Programming Assignment 1 “solution” available as part of PA2

  4. Project 2, Routing & Congestion • Routing: • topology discovery, (make table) • routing packets (lookup in table) • failure management (change table) • no partitioning • Congestion control • adaptive window sizing • Drop packets or ? • Loss rate won’t be as bad this time.

  5. Fair Queueing • What’s the problem?

  6. Fair Queueing • What’s the problem? • Fair bandwidth allocation? • What’s wrong with FCFS?

  7. Fair Queueing • What’s the problem? • Fair bandwidth allocation? • What’s wrong with FCFS? • FTP vs. Telnet • Mean users break end-to-end congestion control

  8. Nagel’s algorithms • Silly Window Syndrome • Allow only one unacknowledged small packet into the network. • Packetwise fair queueing • Compared to Bitwise Round-robin (BR) described in this paper.

  9. FTP vs. Telnet • FTP shoves a lot of packets into the network. • Why?

  10. FTP vs. Telnet • FTP shoves a lot of packets into the network. • Why? • It gets a bigger share of the bandwidth • It makes sure it gets what bandwidth is available • What are the consequences of full queues?

  11. FTP vs. Telnet • FTP shoves a lot of packets into the network. • Why? • It gets a bigger share of the bandwidth • It makes sure it gets what bandwidth is available • What are the consequences of full queues? • Packets get dropped • Packets get delayed

  12. What is fairness • Plenty of rhetorical questions: • Equal allocation? • By source? Destination? Connection? • Some sources really need bandwidth. • NFS service • What about users with many processes? • Sources with many outgoing connections?

  13. Experimental Setup • Simulation of FCFS & FQ using three flow control algorithms: • Generic flow control • 2rtt, fixed window size • Jacobson & Karels’ (JK) • timeouts signals congestion: modify cwnd • DECbit • header bit when passing congested gateways

  14. Measurements • Throughput • fairness evident • more telnet packets are good • Average Roundtrip • delay • Retransmissions • suggest variability in delay, since timeouts fire • Dropped Packets • imply congestion was not resolved.

  15. Results: Underloaded Gateway • Experimental setup looks like a modem link (56Kbit) • fairness • low delay • DECbit already had decent delay properties • has strange Roundtrip times for FTP in FQbit

  16. Results: Overloaded Gateway • Small buffer size • Notice: several ways to achieve fairness • FQ affects telnet delay • FQ doesn’t reduce the number of retransmits/drops

  17. Results: Ill-behaved source • Mean source fills the queue • FQ charges for dropped packets • Effective at shutting it down. • Roundtrip for good apps preserved

  18. Results: Mixed Protocols • Explain the 12 for one Generic source? • Motivation for sources to implement JK.

  19. Results: Multi-hop networks • Key is the fourth column • DECbit doesn’t work as well • Why roundtrip time is the same across all routes is mysterious…

  20. Results: Complex networks • Column 4 & 8 • Senders aided by timely acks.

  21. Summary • what’s wrong with fair queueing?

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