A measurement study of internet delay asymmetry
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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. RTT: FWD + REV. FWD. REV. RTT is easy to measure

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A measurement study of internet delay asymmetry

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


Rtt fwd rev
RTT: FWD + REV

FWD

REV

RTT is easy to measure

FWD + REV = RTT

PAM 2008


De facto internet delay metric rtt
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


Often times one way delay is more relevant
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


But owd measurement is hard
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


Outline questions to answer
Outline: Questions to Answer

  • Does delay asymmetry exist?

  • What are reasons for delay asymmetry?

  • How dynamic is delay asymmetry?

PAM 2008


Tools and testbed
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


Owping relies on time synchronization
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


How does owping work
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


Ntp drift and error estimate
NTP Drift and Error Estimate

40% of nodes have

Error Estimate > 20 ms

PAM 2008


Trace pruning
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


Gren vs commercial nodes
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


Delay asymmetry fwd rtt
Delay Asymmetry – FWD/RTT

Delay fraction for all node pairs

PAM 2008


Delay asymmetry absolute values
Delay Asymmetry – Absolute Values

RTT = 150 ms

FWD = 100 ms

REV = 50 ms

Y = (1/2) X

(conventional wisdom)

PAM 2008


Questions to answer
Questions to answer

  • Does delay asymmetry exist?

    • Yes

  • What are reasons for delay asymmetry?

  • How dynamic is delay asymmetry?

PAM 2008


Reasons for asymmetry in owd
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


Metric for path asymmetry path similarity coefficient
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


Path asymmetry
Path Asymmetry

20% paths have

Router level

Similarity coeff. > 0.6

70% paths have

AS level

Similarity coeff. > 0.6

PAM 2008


Delay asymmetry vs router level path similarity coefficient

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


Questions to answer1
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




Correlating routing events and delay asymmetry change
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


Correlation results
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


Delay dynamics observations
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


Questions to answer2
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


Summary
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


Thank you

Thank You

Questions?

PAM 2008