PAM 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

PAM RTT: FWD + REV FWD REV RTT is easy to measure FWD + REV = RTT

PAM 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

PAM 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 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

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

PAM 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 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 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:

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

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

PAM 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

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

PAM 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 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|

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

PAM 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

PAM 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 Dynamics of Delay Asymmetry

PAM Correlating FWD change and RTT change D A C B

PAM 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 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

PAM 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 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 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 Thank You Questions?