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Network Diagnostic Tool (NDT)Duplex-Mismatch detection update
Fall Member Meeting
Sept 21 , 2005
Rich Carlson
7/20/05 2
Motivation for work
Measure performance to users desktop
Identify real problems for real users• Network infrastructure is the problem• Host tuning issues are the problem
Make tool simple to use and understand
Make tool useful for users and network administrators
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Mismatches can occur anywhere
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Typical Campus infrastructure
Desktop/Laptop computer connects to building switch via (cat 5 twisted pair cable).
Building Switch
Desktop Laptop
computer
BuildingWire
Uplink to campus
backbone
Other hosts
Other hosts
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Ethernet transmission strategy
Half Duplex• Use carrier sense signal to determine if link in use• If not, send frame at head of queue• Else, wait for frame to end and send frame• Use collision detection signal to determine if other station also sends
Full Duplex• Send packet at head of queue• Disable carrier sense • Disable collision detection
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Duplex Mismatch Detection
Condition exists due to configuration error• User ‘Knows’ full duplex is faster than half duplex• Administrator ‘prevents problems’ by fixing switch
Developing analytical model to describe how network operates
Expanding model to describe UDP and TCP flows
Test models in LAN, MAN, and WAN environments
NIH/NLM grant funding
7/20/05 7
Test environment
Receiver is put is various states• Switch = full & Host = full or half• Switch = half & Host = full or half
Switch
NDT
Srv
Source
100 Mbps
Full Duplex
NDT
Clt
Receiver
100 MbpsMismatch
Data Packets
Ack Packets
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Results for TCP flows
Source is Full and Destination is Half• Data packets lost• Ack packets delayed or lost• Expect poor performance due to large number of retransmissions
Source is Half and Destination if Full • Data packets delayed or lost• Ack packets lost• Expect poor performance with large number of duplicate acks
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Analytical Loss-ModelLoss vs Transmission rate
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TCP Operation on LAN
Observed behavior depends on direction of TCP flow and direction of mismatch
Losing ACKs has bigger effect than losing Data packets
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Four Cases of Duplex Setting
FD-FD FD-HD
HD-FD HD-HD
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Duplex MismatchSwitch is Full & Host is Half
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Tentative Mismatch Detection
Full to Half mismatch detection• Large percentage of duplicate ACKs• Connection spends majority of the time in
CwndLimited state
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Duplex MismatchSwitch is Half & Host is Full
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Tentative Mismatch Detection
Half to Full mismatch detection• Large number of timeouts causes long idle time
(RTO x timeout value) • Connection spends majority of the time in
CwndLimited state
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Next Steps
Test proposed detection algorithms in LAN, MAN, and WAN environments
Test proposed detection algorithms with multiple clients (Windows, Mac, *nix)
Refine tool to report possible/probable problems
7/20/05 17
Availability
Open Source Development project• http://www.sourceforge.net/projects/ndt
Tools available via from• http://e2epi.internet2.edu/ndt/download.html• Contains source code
Email discussion list [email protected]• Goto http://e2epi.internet2.edu/ndt web site and click
–ndt-users – General discussion on NDT tool–ndt-announce – Announcements on new features
