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Ao-Jan Su, David R. Choffnes,Fabián E. Bustamante and
Aleksandar Kuzmanovic
Department of EECS
Northwestern University
Relative Network Positioning via CDN Redirections
IEEE ICDCS 2008
Relative Network Positioning via CDN Redirections Ao-Jan Su
Network Positioning
Why do we need network positioning systems?– Emerging large scale distributed systems can
benefit from selecting among alternative nodes• Example: select an on-line gaming server
– All-to-all measurements are not scalable
Current approaches – Provide network positioning services in a scalable
way (e.g. landmark based)– Clear tradeoffs
• Precision vs. overhead• Precision vs. deployment• And others
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Relative Network Positioning via CDN Redirections Ao-Jan Su
Observations
Content distribution networks (e.g., Akamai) improve web performance by– Performing extensive network measurements– Redirecting clients to their closest replica servers– Publishing the results through DNS
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Can we reuse those measurements collected by CDNs to build a network positioning system?
Relative Network Positioning via CDN Redirections Ao-Jan Su4
CDNs Basics
Web client’s request redirected to ‘close’ by server– Client gets web site’s DNS CNAME entry with domain name
in CDN network– Hierarchy of CDN’s DNS servers direct client to 2 nearby
servers
Internet
Web client
Hierarchy of CDN DNS servers
Customer DNS servers
(1)
(2)
(3)
(4)
(5)(6)
LDNSClient requests translation for yahoo
Client gets CNAME entry with domain name in Akamai
Client is given 2 nearby web replica servers (fault tolerance)
Web replica serversMultiple redirections to find nearby edge servers
Relative Network Positioning via CDN Redirections Ao-Jan Su
Our approach
CDN-based Relative Network Positioning (CRP)
Clients are redirected to currently closest replica servers in general
CDN’s redirections are primarily driven by network conditions (latency) [Su et al. 2006]
Inferring relative network distance by overlapping CDN replica servers
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A B C
R1 R2
Relative Network Positioning via CDN Redirections Ao-Jan Su
Uses of CRP
Closest node selection– Select the closest node (shortest latency) from a
group of candidates (e.g. select the closest on-line gaming server)
– Methodology• Encode redirection frequency from a node to its redirected
replica servers by a vector• Compare similarity (cosine similarity) of nodes’ redirection
vectors to estimate proximity
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Client
Server A
Server B
Replica serversR1
R2
R3
0.8
0.2
Relative Network Positioning via CDN Redirections Ao-Jan Su
Uses of CRP (Cont.)
Clustering– Select a set of nodes that are close to each other
(e.g. replicate content to a group of nodes)– Methodology
• Select cluster centers• Assign strong mapping peers to the cluster centers
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Relative Network Positioning via CDN Redirections Ao-Jan Su
Evaluation Goals
Comparing the performance of CRP’s closest node selection to – Ground truth – active measurements– A state of the art network positioning system –
Meridian [Wong 2005]
With respect to– Accuracy– Scalability– Deployment– Overhead
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Meridian: Closest node selection
Relative Network Positioning via CDN Redirections Ao-Jan Su
Experiment Setup
Globally distributed nodes– 1000 DNS servers as clients– 240 Planet Lab nodes as candidate servers (on the
same nodes as our reference system – Meridian)
Concurrent data collection– Monitoring CDN redirections by recursive DNS
queries for CRP– Querying Meridian via its interface– Measuring end-to-end latencies by pings as the
ground truth
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Relative Network Positioning via CDN Redirections Ao-Jan Su
Selecting the closest node
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Clients are not close to any servers due to • Limited Planet Lab nodes coverage (Meridian)• Located in areas not well served by CDNs (CRP)
Clients are not close to any servers due to • Limited Planet Lab nodes coverage (Meridian)• Located in areas not well served by CDNs (CRP)
CRP’s accuracy is comparable to its alternative without active measurements and dedicated infrastructure
CRP’s recommendations for 65% of nodes differ from Meridian by < 7msCRP’s recommendations for 65% of nodes differ from Meridian by < 7ms
CRP outperforms Meridian by 25% of the nodes due to larger deployment of CDN replica servers
CRP outperforms Meridian by 25% of the nodes due to larger deployment of CDN replica servers
Relative Network Positioning via CDN Redirections Ao-Jan Su
Selecting the closest node (Cont.)
Relative Error: estimated latency – ground truth
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80% of CRP nodes have relative error < 50ms 80% of CRP nodes have relative error < 50ms
CRP’s is quite accurate comparing to ground truth, with virtually no measurement overhead
Relative Network Positioning via CDN Redirections Ao-Jan Su
Rank: rank 0 is the closest server
Load on CDN’s DNS System
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Low probe frequency • Smaller overhead• Less accurate• Miss overlapping replica servers
Low probe frequency • Smaller overhead• Less accurate• Miss overlapping replica servers
100 mins probe frequency• Appropriate for 95% of nodes• Much less than CDN’s DNS TTL (20 secs)• Overhead is too small to impact CDN’s operations
High probe frequency • Can Improve accuracy• Larger overhead
High probe frequency • Can Improve accuracy• Larger overhead
Relative Network Positioning via CDN Redirections Ao-Jan Su
Load on CRP Clients
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Large history • More refined results• Larger computation overhead
Large history • More refined results• Larger computation overhead
Small history• Sufficient for CRP • Small overhead• Capture network dynamics
Relative Network Positioning via CDN Redirections Ao-Jan Su
You May Be Wondering
“Will CDNs be unhappy because of CRP?”– CRP nodes behaves as regular web clients– CRP’s overhead does not impact CDN’s daily
operations– Could be an additional service provided by CDNs
“What if CDNs change their redirection policy?”– CRP’s goal aligns with CDNs– Our approach is not restricted to a specific CDN,
CRP can reuse results from other measurement infrastructures
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Relative Network Positioning via CDN Redirections Ao-Jan Su
Summary
CRP discovers relative positions of end hosts– Closest node selection– Clustering
Key features of CRP– Accurate– Light-weight
• Reuse CDN’s network measurements
– Scalable• No dedicated infrastructure is required
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Relative Network Positioning via CDN Redirections Ao-Jan Su
Cosine Similarity
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