1、PAM 2008,1,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 = RT
2、T,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,PAM 2008,4,Often Ti
3、mes, 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,OWD measurement requires Access to both ends No daemon in OS Strict time synchronizatio
4、n Result: OWD is approximated as half of RTT,But OWD Measurement is Hard,Conventional WisdomDelay is symmetricFWD = 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: I
5、mplements 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 eac
6、h host Error estimate Added up to report overall error estimate,PAM 2008,9,Server,Client,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.intern
7、et2.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 2
8、008,12,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
9、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,Y = (1/2) X (conventional wisdom),RTT = 150 ms FWD = 100 ms REV = 50 ms,PAM 2008,15,Questions to answer,Does delay asym
10、metry exist? YesWhat 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
11、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 asymmetryA = All ASes in forward pathB = All ASes in reverse pathAS path similarity coefficient = |AB| / |AB| Router level
12、path asymmetryA = All routers in forward pathB = All routers in reverse pathRouter path similarity coefficient = |AB| / |AB|,PAM 2008,18,Path Asymmetry,70% paths have AS level Similarity coeff. 0.6,20% paths have Router level Similarity coeff. 0.6,PAM 2008,19,Delay Asymmetry Vs Router Level Path Sim
13、ilarity Coefficient,Router Level Path Similarity Coefficient,Delay fraction 0.5 when router-level asymmetry nears unity,Delay fraction fluctuates between 0.3 to 0.7,PAM 2008,20,Questions to answer,Does delay asymmetry exist? YesWhat are reasons for delay asymmetry? Observed good correlation with pat
14、h asymmetryHow dynamic is delay asymmetry?,PAM 2008,21,Dynamics of Delay Asymmetry,PAM 2008,22,Correlating FWD change and RTT change,D,A,C,B,PAM 2008,23,Correlating Routing Events and Delay Asymmetry Change,Measurement setup Traceroute and owping all nodes Repeat after every 20 minutes Calculate pat
15、h change Inter AS / Intra AS Measure reverse path at the same time,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,PAM 2008,25,Delay Dynamics Observations,Intra AS path change More frequent to observe M
16、ost 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? YesWha
17、t are reasons for delay asymmetry? Observed good correlation with path asymmetryHow 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 del
18、ay 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?,
