1. PAM 20081 A Measurement Study of Internet Delay Asymmetry Abhinav PathakPurdue University Himabindu PuchaPurdue University Ying ZhangUniversity of Michigan Y. Charlie HuPurdue University Z. Morley MaoUniversity of Michigan

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

3. PAM 2008 3 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 De’ Facto Internet Delay Metric: RTT

4. 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

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

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

7. 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

8. 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

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

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

11. 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

12. 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

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

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

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

16. 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

17. 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 = |A Ո B| / |A Ս B| Router level path asymmetry A = {All routers in forward path} B = {All routers in reverse path} Router path similarity coefficient = |A Ո B| / |A Ս B|

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

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

20. 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?

21. PAM 2008 21 Dynamics of Delay Asymmetry

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

23. 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

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

25. 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

26. 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

27. 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

28. PAM 200828 Thank You Questions?