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PAM 2008 1
A Measurement Study of Internet Delay Asymmetry
Abhinav Pathak Purdue UniversityHimabindu Pucha Purdue UniversityYing Zhang University of MichiganY. Charlie Hu Purdue UniversityZ. Morley Mao University of Michigan
PAM 2008 2
RTT: FWD + REV
FWDFWD
REVREV
RTT is easy to measureFWD + REV = RTT
RTT is easy to measureFWD + REV = RTT
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 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
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
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
Server Client
owampd(resource broker)
owpingControl
Connection setup
owampd(control)
fork
owampdTest Endpoint
fork
Request Results
owpingTest Endpoint
fork
OWD Test packets
How Does Owping Work
Source: http://e2epi.internet2.edu/owamp/
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 14
Delay Asymmetry – Absolute Values
Y = (1/2) X(conventional
wisdom)
Y = (1/2) X(conventional
wisdom)
RTT = 150 msFWD = 100 msREV = 50 ms
RTT = 150 msFWD = 100 msREV = 50 ms
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 = |A B| / |A B|Ո Ս
Router level path asymmetry A = {All routers in forward path}
B = {All routers in reverse path}
Router path similarity coefficient = |A B| / |A B|Ո Ս
PAM 2008 18
Path Asymmetry
70% paths haveAS level
Similarity coeff. > 0.6
70% paths haveAS level
Similarity coeff. > 0.6
20% paths haveRouter level
Similarity coeff. > 0.6
20% paths haveRouter level
Similarity coeff. > 0.6
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 fluctuatesbetween 0.3 to 0.7
Delay fraction fluctuatesbetween 0.3 to 0.7
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 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 Intra AS path change cause FWD to
change by < 10 ms
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
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 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