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 2

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 3 Can we reuse those measurements collected by CDNs to build a network positioning system?

Relative Network Positioning via CDN Redirections Ao-Jan Su 4 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) LDNS Client 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 servers Multiple 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 5 A B C R1R2

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 6 Client Server A Server B Replica servers R1 R2 R

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 7

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 8 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 9

Relative Network Positioning via CDN Redirections Ao-Jan Su Selecting the closest node 10 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 < 7ms CRP’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 11 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 12 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 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 13 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 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 14

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 15

Relative Network Positioning via CDN Redirections Ao-Jan Su Cosine Similarity 16