1. 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

2. 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

3. Measurement setup Three components 1x1 pixel embedded transparent GIF image http://xxx.rd.example.com/tr.gif A specialized authoritative DNS server Allows hostnames to be wild-carded An HTTP redirector Always responds with “302 Moved Temporarily” Redirect to a URL with client IP address embedded

4. Embedded image request sequence Client [10.0.0.1] Redirector for xxx.rd.example.com Local DNS server Content server for the image Name server for *.cs.example.com 1. HTTP GET request for the image 2. HTTP redirect to IP10-0-0-1.cs.example.com 3. Request to resolve IP10-0-0-1.cs.example.com 4. Request to resolve IP10-0-0-1.cs.example.com 5. Reply: IP address of content server 6. Reply: content server IP address 7. HTTP GET request for the image 8. HTTP response

5. Measurement data/stats SiteParticipantHit countDuration 1att.com20,816,9272 months 2,3 Personal Web pages (commercial domain) 1,7433 months 4Research lab212,8143 months 5-7University site4,367,0763 months 8-19Personal Web pages (university domain) 26,5633 months Data typeCount Client-LDNS associations4,253,157 HTTP requests25,425,123 Unique client IPs3,234,449 Unique LDNS Ips157,633 Client-LDNS associations with a common IP56,086

6. Proximity metrics: AS clustering Observes if client and LDNS belong to the same AS Network clustering 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

7. Proximity metric: traceroute divergence (TD) Probe machine client Local DNS server Use the last point of divergence TD=Max(3,4)=4 1 2 3 4 1 2 3 a b 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 About 66% pairs traced have common path at least as long as disjoint path

8. Proximity analysis results: AS, network clustering MetricsClient IPsHTTP requests AS cluster64% (88%)69% (92%) Network cluster16% (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.

9. Impact on commercial CDNs 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 CDN (using AS clustering)CDN XCDN YCDN Z Clients with CDN server in cluster1,679,5151,215,372618,897 Verifiable clients1,324,022961,382516,969 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 XCDN YCDN Z Clients with CDN server in cluster264,743156,507103,448 Verifiable clients221,440132,56790,264 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%)

10. 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