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…

• Programming Assignment 1 “solution” available as part of PA2

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.

Fair Queueing

• What’s the problem?

Fair Queueing

• What’s the problem?– Fair bandwidth allocation?

• What’s wrong with FCFS?

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

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.

FTP vs. Telnet

• FTP shoves a lot of packets into the network.

• Why?

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?

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

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?

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

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.

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

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

Results: Ill-behaved source

• Mean source fills the queue

• FQ charges for dropped packets

• Effective at shutting it down.

• Roundtrip for good apps preserved

Results: Mixed Protocols

• Explain the 12 for one Generic source?

• Motivation for sources to implement JK.

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…

Results: Complex networks

• Column 4 & 8

• Senders aided by timely acks.

Summary

• what’s wrong with fair queueing?