588 Section 4 Neil Spring April 27, 1999. Schedule Notes Project 2 description Fair Queueing (Demers et.al.)

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Text of 588 Section 4 Neil Spring April 27, 1999. Schedule Notes Project 2 description Fair Queueing (Demers...

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  • 588 Section 4 Neil Spring April 27, 1999
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  • Schedule Notes Project 2 description Fair Queueing (Demers et.al.)
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  • 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
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  • 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 wont be as bad this time.
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  • Fair Queueing Whats the problem?
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  • Fair Queueing Whats the problem? Fair bandwidth allocation? Whats wrong with FCFS?
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  • Fair Queueing Whats the problem? Fair bandwidth allocation? Whats wrong with FCFS? FTP vs. Telnet Mean users break end-to-end congestion control
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  • Nagels 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.
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  • FTP vs. Telnet FTP shoves a lot of packets into the network. Why?
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  • 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?
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  • 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
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  • 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?
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  • 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
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  • 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.
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  • 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
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  • Results: Overloaded Gateway Small buffer size Notice: several ways to achieve fairness FQ affects telnet delay FQ doesnt reduce the number of retransmits/drops
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  • Results: Ill-behaved source Mean source fills the queue FQ charges for dropped packets Effective at shutting it down. Roundtrip for good apps preserved
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  • Results: Mixed Protocols Explain the 12 for one Generic source? Motivation for sources to implement JK.
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  • Results: Multi-hop networks Key is the fourth column DECbit doesnt work as well Why roundtrip time is the same across all routes is mysterious
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  • Results: Complex networks Column 4 & 8 Senders aided by timely acks.
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  • Summary whats wrong with fair queueing?