A Measurement Study of Internet Delay Asymmetry

1. A Measurement Study of Internet Delay Asymmetry Abhinav Pathak Purdue University Himabindu Pucha Purdue University Ying Zhang University of Michigan Y. Charlie Hu Purdue University Z. Morley Mao University of Michigan PAM 2008

2. RTT: FWD + REV FWD REV RTT is easy to measure FWD + REV = RTT PAM 2008

3. De’ Facto Internet Delay Metric: RTT • Round Trip Time (RTT) is a widely used network metric in server/peer selection • CDN • Select closest replica • Overlay multicast • Choose a suitable parent/child in the tree • Internet distance prediction • Calculate proximity from a landmark PAM 2008

4. Often Times, One-Way Delay is More Relevant • Examples scenarios • Multicast streaming applications • Real-time interactive applications • Multi-player games • Internet distance prediction • Understand routing performance PAM 2008

5. But OWD Measurement is Hard • OWD measurement requires • Access to both ends • No daemon in OS • Strict time synchronization • Result: OWD is approximated as half of RTT • Conventional Wisdom • Delay is symmetric • FWD = REV PAM 2008

6. Outline: Questions to Answer • Does delay asymmetry exist? • What are reasons for delay asymmetry? • How dynamic is delay asymmetry? PAM 2008

7. Tools and Testbed • Tools • Owping: Implements one way active measurement protocol (RFC 4656) • Paris-Traceroute • Testbed • Planetlab • 180 GREN, 25 Commercial nodes • Trace collection: 10 days in April 2007 PAM 2008

8. Owping Relies on Time Synchronization • NTP – Two parameters • Clock drift • Relative error to a NTP server • Applied to timestamp on each host • Error estimate • Added up to report overall error estimate PAM 2008

9. How Does Owping Work Server Client Connection setup owampd (resource broker) owping Control fork fork Request Results owampd (control) owping Test Endpoint fork OWD Test packets owampd Test Endpoint Source: http://e2epi.internet2.edu/owamp/ PAM 2008

10. NTP Drift and Error Estimate 40% of nodes have Error Estimate > 20 ms PAM 2008

11. Trace Pruning • Remove trace if • NTP error estimate > 10ms • Leaves 82 GREN nodes and 12 commercial nodes • Sum of NTP error estimate > 3% of RTT for a node pair • Leaves primarily long distant routes PAM 2008

12. GREN Vs Commercial nodes • Planetlab mostly contains GREN nodes • GREN – Global Research and Education Network • GREN to GREN (G2G) path properties are different from Commercial to Commercial (C2C) • G2C and C2G properties are close to C2C - On the Impact of Research Network Based Testbeds on Wide-area Experiments [Pucha et. al. – IMC06] • We consider G2C-C2G-C2C paths only PAM 2008

13. Delay Asymmetry – FWD/RTT Delay fraction for all node pairs PAM 2008

14. Delay Asymmetry – Absolute Values RTT = 150 ms FWD = 100 ms REV = 50 ms Y = (1/2) X (conventional wisdom) PAM 2008

15. Questions to answer • Does delay asymmetry exist? • Yes • What are reasons for delay asymmetry? • How dynamic is delay asymmetry? PAM 2008

16. Reasons for Asymmetry in OWD • What are reasons for delay asymmetry • Temporary congestion in forward or reverse path • Transient events • Forward and reverse paths are different • Path Asymmetry • Can we correlate delay and path asymmetry? • Measure path using traceroute • Need a metric to quantify path asymmetry PAM 2008

17. Metric for Path Asymmetry – Path Similarity Coefficient • AS level path asymmetry A = {All ASes in forward path} B = {All ASes in reverse path} AS path similarity coefficient = |AՈB| / |AՍB| • Router level path asymmetry A = {All routers in forward path} B = {All routers in reverse path} Router path similarity coefficient = |AՈB| / |AՍB| PAM 2008

18. Path Asymmetry 20% paths have Router level Similarity coeff. > 0.6 70% paths have AS level Similarity coeff. > 0.6 PAM 2008

19. Router Level Path Similarity Coefficient Delay Asymmetry Vs Router Level Path Similarity Coefficient Delay fraction fluctuatesbetween 0.3 to 0.7 Delay fraction ~ 0.5 when router-level asymmetry nears unity PAM 2008

20. Questions to answer • Does delay asymmetry exist? • Yes • What are reasons for delay asymmetry? • Observed good correlation with path asymmetry • How dynamic is delay asymmetry? PAM 2008

21. Dynamics of Delay Asymmetry PAM 2008

22. Correlating FWD change and RTT change D C B A PAM 2008

23. Correlating Routing Events and Delay Asymmetry Change • Measurement setup • Traceroute and owping all nodes • Repeat after every 20 minutes • Calculate path change • Inter AS / Intra AS • Measure reverse path at the same time PAM 2008

24. Correlation Results 80% of Inter AS path change cause FWD to change by < 20 ms 80% of Intra AS path change cause FWD to change by < 10 ms PAM 2008

25. Delay Dynamics – Observations • Intra AS path change • More frequent to observe • Most of the times path changes in both directions • Fwd & Rev delays change simultaneously • Inter AS path change • Less frequent • Two cases • Only fwd AS path changes • Both fwd and rev AS paths change • Delay change is larger in magnitude PAM 2008

26. Questions to answer • Does there exists delay asymmetry? • Yes • What are reasons for delay asymmetry? • Observed good correlation with path asymmetry • How dynamic is delay asymmetry? • Depends on inter/intra AS path change PAM 2008

27. Summary • Methodology • Measuring OWD • Pruning strategy • Based on error estimates provided by NTP • Measurement results • Considerable levels of delay asymmetry • Delay asymmetry is dynamic • RTT could change due to FWD change or REV change or both • Analyzing the cause • Weak correlation between router level asymmetry and delay asymmetry • Delay asymmetry dynamics • Inter/Intra AS route change effects delay asymmetry differently PAM 2008

28. Thank You Questions? PAM 2008