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?