1. Analysis of Web Caching Architectures: Hierarchical and Distributed Caching Pablo Rodriguez, Christian Spanner, and Ernst W. Biersack IEEE/ACM TRANSACTIONS ON NETWORKING VOL. 9, NO. 4, Auguest 2001

2. Abstract Caching architectures Hierarchical Distributed Hybrid Analytical models Performance Connection time Transmission time Total latency Bandwidth Cache load

3. Caching architectures Hierarchical caching Institutional cache Intermediate cache National cache Distributed caching Institutional cache

4. Network topology

5. The model Network model Full O-ary tree Document model Request – Poisson distribution Popularity - Zipf distribution Hierarchical caching Caches are placed at the access points between two different networks. Distributed caching Caches are placed at the institutional network.

6. Network model

7. Document model

8. Properties and limitations of the model O-ary trees are good models. Modifying the height or the number of tiers of the tree can easily model other networks. The model assumes homogeneous client communities. Heterogeneous client communities can be easily modeled. Simulations results in this paper should be considered as relative results.

9. Connection time Depend on the number of network links from the client to the cache.

10. Connection time (cont’d) Distance of transmission A request first travels up then down TCP three-way handshake Server

11. Transmission time Caches operate in a cut-through mode. Request rate

12. Comparison O = 4 H = 3 z = 10 N = 250 million

13. Connection time

14. Network traffic at every tree level

15. Expected transmission time (a) Non-congested national network (b) Congested national network

16. Total latency

17. Heterogeneous client communities (a) Expected connection time (b) Expected transmission time

18. Bandwidth usage The expected number of links traversed to distribute one packet to the clients. (a) Regional network (b) National network

19. Cache load The filtered request rate

20. Disk space The average Web document size S times the average number of copies present in the caching infrastructure. The average number of copies present in the caching infrastructure can be calculated using the probability that a new document copy is created at every cache level.

21. Disk space (cont’d)

22. A hybrid caching scheme A certain number of caches k cooperate at every network level. When a document cannot be found in a cache The cache checks if the document resides in any of the cooperating caches. If multiple caches have a document copy, the neighbor cache with the lowest latency is selected. Otherwise, the request is then forwarded to the immediate parent cache or to the server.

23. Connection time

24. Connection time (cont’d)

25. Transmission time

26. Transmission time (cont’d)

27. Total latency

28. Bandwidth usage (a) National network (b) Regional network

29. Cache load

30. Conclusions Hierarchical caching architecture Reduce the expected distance to hit a document Decrease the bandwidth usage Reduce the administrative concerns Need powerful intermediate caches or load- balancing algorithms Distributed caching architecture Large network distances High bandwidth usages Administrative issues Hybrid scheme is the best