Page History

 Shahryar Khan and Les Cottrell

Purpose

Wiki Markup<span style="color: #000000">The purpose of this study is to investigate the effectiveness of tiering</span> for [TULIP|http://www.slac.stanford.edu/comp/net/wan-mon/tulip/] (i.e&nbsp; we have a set of primary landmarks tier0 which will narrow down the target location to being in a particular region*\[1\] and then a denser set of secondary tier1 landmarks in the discovered region that can be used to get more accurate results). The use of tiering should enable us to reduce the network traffic (number of landmarks pinging a target) while retaining the accuracy of using all The purpose of this study is to investigate the effectiveness of tiering for TULIP (i.e  we have a set of primary landmarks tier0 which will narrow down the target location to being in a particular region*[1] and then a denser set of secondary tier1 landmarks in the discovered region that can be used to get more accurate results). The use of tiering should enable us to reduce the network traffic (number of landmarks pinging a target) while retaining the accuracy of using all landmarks.

Currently we are looking into two regions where we hope to use tiering, i.e. North America and Europe.  The choice of these two regions is since we have a relatively large number of landmarks in these regions. Landmarks in all other regions will be considered as tier0. Thus TULIP will try and locate targets in two passes. The first just uses tier0 landmarks. If the target then appears to be in North America or Europe, then a second pass is made using all the landmarks in that region, otherwise the target is determined from the first pass. The purpose of the current study is to determine whether tiering is effective and how it should be used.

North America 

We selected 4 landmarks in North America for our study( SLAC, BNL, Ampath-Florida,TRIUMF-Canada). They were chosen to be reliable and to roughly be located close to the 4 corners of the region. We pinged a set of known geographical target sites in the US, Canada and Mexico. The coordinates of the sites were obtained from the PingER database. We also calculated the physical distances between the target sites and the landmarks. The figure below shows the distribution. It appeared that all hosts were within 80ms of one another of the 4 monitors and 98.84% are within 70ms.

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Faran uses the maximum alpha value in order to avoid underestimation which is ~ 62.98 km/ms in our case. Also Faran calculates alpha based on PingER historical data to all the world regions and in this study we  only used the North American PingER sites. Looking at the frequency histogram of alpha for the SLAC landmark to North American target sites it is seen that this is a reasonable maximum value.

 

Europe 

We selected 3 landmarks in Europe for our study ( CERN_Switzerland, ICTP_Italy, UK_Warrington). They were chosen because they were more reliable as compared to other landmarks. We pinged a set of known geographical targets in Europe. The coordinates of these sites were obtained from PingER database. We also calculated the physical distances
between the targets and the landmarks. It appeared that all the hosts were within 65 ms of the three landmarks and 96.15 % were within 50 ms.

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  The median value of alpha from England(Warrington) to Europe is 40.71 (km/ms). The IQR is 15.6 (km/ms) or IQR/Median~ 38% or ~ +-19%. The highest value of alpha 97.72 (km/ms) is due to a very small RTT (2.6ms) between (demon.co.uk) and this landmark which are 263 km apart.
 
 The graph below shows the alpha distribution as seen from Italy to other European countries.
 

 
  The median value of alpha from ICTP (Italy) to Europe is 18.91 (km/ms). The IQR is 10.98 (km/ms) or IQR/Median is ~58% or ~+-29.5%.

Conclusions 

• Tiering can be used in N. America and Europe. ( Heuristics can be derived from this analysis which will narrow down the region of a particular host)
• The Great Circle distance and the path which the packets fallow may be different.
• The hosts which are near the landmark may have very high values of alpha.

References 

Wiki Markup\[1\] A region may be different then a geographical region &nbsp;