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Evaluation of the Proximity between Web Clients and their Local DNS Servers Z. Morley Mao Chuck Cranor, Fred Douglis, Misha Rabinovich, Oliver Spatscheck, and Jia Wang Motivation – originator problem Originator problem CDNs assume that clients are close to their local DNS servers Content Distribution Networks (CDNs) Try to deliver content from servers close to users Current server selection mechanisms Uses Domain Name System (DNS) Verify the assumption that clients are close to their local DNS servers Measurement setup Three components 1x1 pixel embedded transparent GIF image A specialized authoritative DNS server <img src=http://xxx.rd.example.com/tr.gif height=1 width=1> Allows hostnames to be wild-carded An HTTP redirector Always responds with “302 Moved Temporarily” Redirect to a URL with client IP address embedded Embedded image request sequence 1. HTTP GET request for the image Client [10.0.0.1] 2. HTTP redirect to IP10-0-0-1.cs.example.com Redirector for xxx.rd.example.com Content server for the image 4. Request to resolve IP10-0-0-1.cs.example.com Local DNS server 5. Reply: IP address of content server Name server for *.cs.example.com Measurement data/stats Site Participant Hit count Duration 1 att.com 20,816,927 2 months 2,3 Personal Web pages 1,743 3 months (commercial domain) 4 Research lab 212,814 3 months 5-7 University site 4,367,076 3 months 8-19 Personal Web pages 26,563 3 months (university domain) Data type Count Client-LDNS associations 4,253,157 HTTP requests 25,425,123 Unique client IPs 3,234,449 Unique LDNS Ips 157,633 Client-LDNS associations with a common IP 56,086 Proximity metrics: AS clustering Network clustering Observes if client and LDNS belong to the same AS Network cluster based on BGP routing information using longest prefix match Observes if client and LDNS belong to the same network cluster Roundtrip time correlation Correlation between message roundtrip times from a probe site to the client and its LDNS server Probe site represents a potential cache server location A crude metric, highly dependent on the probe site Proximity metric: traceroute divergence (TD) •Use the last point of divergence •TD=Max(3,4)=4 Probe machine a Sample Probe sites: NJ(UUNET), NJ(AT&T), Berkeley(calren), Columbus(calren) size: 48,908 client-LDNS pairs Median divergence: 4 Mean divergence: 5.8-6.2 Ratio of common to disjoint path length b 1 2 3 1 About 66% pairs traced have common path at least as long as disjoint path 2 3 4 client Local DNS server Proximity analysis results: AS, network clustering Metrics Client IPs HTTP requests AS cluster 64% (88%) 69% (92%) Network cluster 16% (66%) 24% (70%) AS clustering: coarse-grained Network clustering: fine-grained Most clients not in same routing entity as their LDNS Clients with LDNS in same cluster slightly more active Numbers in red indicate improvement possible. Impact on commercial CDNs CDN (using AS clustering) CDN X CDN Y CDN Z Clients with CDN server in cluster 1,679,515 1,215,372 618,897 Verifiable clients 1,324,022 961,382 516,969 Misdirected clients (% verifiable clients) 809,683 (60%) 752,822 (77%) 434,905 (82%) Clients with LDNS not in client’s cluster (% misdirected clients) 443,394 (55%) 354,928 (47%) 262,713 (60%) CDN (using network aware clustering) CDN X CDN Y CDN Z Clients with CDN server in cluster 264,743 156,507 103,448 Verifiable clients 221,440 132,567 90,264 Misdirected clients (% verifiable clients) 154,198 (68%) 125,449 (94%) 87,486 (96%) Clients with LDNS not in client’s cluster (% misdirected clients) 145,276 (94%) 116,073 (93%) 84,737 (97%) total # clients = 3,234,449 Verifiable client: A client with LDNS in cluster, responding to our request, and has at least one cache server in its cluster Majority of “misdirected clients” for NAC have LDNS nonlocal Conclusion DNS based server selection works well for coarse-grained load-balancing Server selection can be inaccurate if cache server density is high Future work Study alternatives to DNS based server selection Improved proximity evaluation