Caching Neighborhood Protocol(CNP): a Foundation for Building Dynamic Web Caching Hierarchies with Proxy Servers Cho-Yu Chiang, Ming T.Liu, Mervin E.Muller Department of Computer and Information Science The Ohio State University 田銘 syslab

Outline 1.Introduction 2.Fundamentals 3.Caching neighborhood Protocol(CNP) 4.Analysis 5.Discussion

1.Introduction Conventional caching hierarchy schemes,such as Harvest,Squid,and their based on Internet Cache Protocol(ICP) or Cache Digest. They lack flexibility in the communication paths among proxy servers. Dynamic caching hierarchy scheme,Caching Neighborhood Protocol(CNP),introduced the idea of Caching Representatives(C-Reps),which essentially are partially replicate servers and distributing loads for frequently accessed origin servers.

2.Fundamentals Caching neighborhood is a logical relationship that --consists of an origin server and a variable number of proxy servers acting as Caching Representatives(C-Reps) for the origin server. --defines an information disseminating partnership such that the data objects provided by the origin server can be distributed by any C-Reps represents. --Not constraint on the physical proximity among an origin server and its C-Reps.

Fundamentals(cont.)

Proxy server as C-Reps --non client-side  it only accepts requests from other proxy servers and dedicates itself to representing multiple origin servers. --client-side  handle the requests for the users who register with it.

Fundamentals(cont.) Origin servers --maintain the number of C-Reps and the load distribution in the caching neighborhood.Add and drop C-Reps to distributed load. --provide the most frequently accessed documents to setup the cache for reducing the likely initial high demand for the C-Reps. --It has to regularly communicate with its C-Reps.In order to collect information such as the number of requests for the documents that have been serviced by the C-Reps.To update the frequently accessed data objects.

3.Caching Neighborhood Protocol(CNP) Http request fulfillment --only Get and Get-IMS requests are considered --timestamp is “ last modified time ” --last-known unchanged time of cached document on a C-Rep refers to the latest time the C-Rep receives a confirmation message from the response to a Get-IMS request for the origin server. --X tolerance coherency requirement(zero to MAX)  EX: zero second tolerance (every single request has to be processed by the origin server) P.S Increasing the value of X,decreasing the number of messages from its C-Reps.

Caching Neighborhood Protocol--CNP(cont.) Caching neighborhood maintenance The origin server plays a prominent role to keep the content of the cached documents on the C-Reps coherent with the documents on the origin servers. Only the transmission of the most popular documents are initiated by the origin server. The actual coherency level for the rendered documents could be much higher, depending on the frequency of transmitting the update information.

4.Analysis

Analysis(cont.)

Observation: 1.Web caching should focus on the popular Web sites. 2.Caching would be most effective if only the most popular documents are cached. 3.The most popular documents are mostly located at the popular Web sites.

5.Discussion Advantages of CNP: 1.CNP provides an effective solution for the cache coherence problem. 2.CNP let proxy servers achieve a much higher cache hit rate.