1. Dcache: A Peer to Peer Web Cache By Wathsala Vithanage Nuwan Gunarathna Nishshanka Sirisena Lanka Software Foundation Funded by ICTA

2. Outline

Why web caching?

3. Advantages of P2P web caching 4. Implementation 5. Problems caused by multicasting

Solutions

Issues relevant to web caching

Solutions

6. Outline

Future work

7. Summary 8. Why Web Caching?

Web requests are made to same web object multiple times by multiple users or same user

9. Caching can,

Reduce bandwidth consumption

10. Reduce server load 11. Reduce perceived lag Web cache stores a copy of a web object with it 12. Advantages of P2P Web Caching

Centralized server is not needed

Can be used with existing infrastructure

Computers already connected to the network

No need of a cache administrator

Large network with a centralized cache needs a person to worry about it

Does not create a single point of failure

If centralized solution fails no one access internet

13. Implementation

Each browser becomes a node in P2P network

Modifying an opensource browser means lot of work, therefore a proxy is used to perform caching running on each participating node.

Dalesa lightweight discovery framework search web objects on the P2P network using IP multicasting 14. Implementation 15. Problems Caused by Multicasting

Every web request is associated with a multicast

16. If N number of nodes have already cached the file then N number of multicasts will be on the network 17. Problems Caused by Multicasting

Problem gets bigger as N grows

Bandwidth wasted for N 1 unwanted availability replies

18. Impaired network performance caused by network flooding 19. Solutions

Pseudo random wait

Wait for a random amount of time less than a threshold T before sending availability message

Piggybacking responses

In availability message say I have not only what you have requested but also what you may need in the future

20. Solutions

Passive Listener (PL) mode

Learn by communication between other nodes (Difficult to deal with DHCP)

21. Issues with Web Caching

Web object aliasing makes caching less useful

22. Web object aliasing makes redundant copies on storage devices 23. You get the same web object through different URLs

Web objects can be mirrored in more sites

24. Web objects can be associated with a session 25. Solutions

Cannot be solved with proxy/caches alone

26. Needs support at HTTP level 27. Servers has to be modified to implement various techniques to achieve this

Associating URLs with hash of the web object

28. Solutions

We will work on a possible solution

Modifications to Dcache

29. An Apache module 30. No changes to browsers 31. Future Work

Related to Network flooding

Piggyback messages(In progress)

32. Pseudo random wait(In progress) In memory cache index(Not started)

With an algorithm to load and maintain most visited cache objects (Squid already has this)

33. Future Work

In memory object store(Not started)

With an algorithm to load and maintain most visited cache objects (Squid has this feature)

34. Summary

Caching improves web performance

35. P2P caches can be a viable alternative for centralized caches 36. Multicasting based P2P caches are fast compared to other types 37. Summary

Network conditions may get impaired due to flooding but there are solutions

38. Caching is troubled by web object aliasing but solutions are not easy to implement 39. Appendix 40. Appendix