Network Diagnostic Tool (NDT) Duplex-Mismatch detection update

1. Network Diagnostic Tool (NDT)Duplex-Mismatch detection update Fall Member Meeting Sept 21 , 2005 Rich Carlson RCarlson@internet2.edu

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

3. Mismatches can occur anywhere

4. Building Wire Building Switch Desktop Laptop computer Typical Campus infrastructure • Desktop/Laptop computer connects to building switch via (cat 5 twisted pair cable). Other hosts Uplink to campus backbone Other hosts

5. 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

6. 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. NDT Srv NDT Clt 100 Mbps Full Duplex 100 Mbps Mismatch Switch Receiver Source Test environment • Receiver is put is various states • Switch = full & Host = full or half • Switch = half & Host = full or half Data Packets Ack Packets

8. 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

9. Analytical Loss-ModelLoss vs Transmission rate

10. 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

11. FD-FD FD-HD HD-FD HD-HD Four Cases of Duplex Setting

12. Duplex MismatchSwitch is Full & Host is Half

13. Tentative Mismatch Detection • Full to Half mismatch detection • Large percentage of duplicate ACKs • Connection spends majority of the time in CwndLimited state

14. Duplex MismatchSwitch is Half & Host is Full

15. 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

16. 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

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 ndt-users@internet2.edu • Goto http://e2epi.internet2.edu/ndt web site and click • ndt-users – General discussion on NDT tool • ndt-announce – Announcements on new features