Web Proxy Caching: The Devil is in the Details Ramon Cacere Fred Douglis Anja Feldmann Gideon Glass Michael Rabinovich AT&T Labs-Research Florham Park, NJ, USA

Brief Review clients proxy servers Reply Req. Reply

Brief Review zClient send requests to the proxy. zIf the requested document is in its cache, the proxy serves the request from its cache. zOtherwise, the proxy forward the request to the server. zServer replies the request through the proxy (proxy keep a copy of the requested document).

How does proxy caching improve performance? zReduce the user-perceived latency associated with obtaining Web documents. zLower the network traffic from the Web servers. zReduce the service demands on content providers.

Previous Work zHigh level details: hit ratio & byte hit ratio zIgnored exceptional cases such as connection aborts. zOmitted the effect of cookies on cacheability of resources.

This paper argues that... zLow-level details have a strong impact on performance, particularly in heterogeneous bandwidth environments. yAborted trasfers can contribute significantly to total bandwidth requirements. y“Cookies” dramatically affect the cacheability of resources; therefore, affect the latency. yCaching TCP connections at proxy can reduce latency more simply caching data.

Simulation zWeb proxy simulator (PROXIM) zWorkload: trace from AT&T Worldnet y12 days dialup traffic on a FDDI ring yencrypted IP addresses ycontained information on both TCP events and HTTP events

Simulator: PROXIM zSimulator Cache ysufficiently large yincluded proxy overhead in the request service time zNetwork Connections yzero or more open connections (cache-to-proxy & proxy-to-server) yProxy closes client-to-proxy connections with 3 minutes of idle time. yProxy-to-server connections are timeout after 30 secs of idle time.

Simulator: PROXIM (cont.) zDocument Transfer yPacket-level delivery with TCP slow-start y1500-byte packets yconstant round-trip time estimate for each connection zLatency Calculations yconnection setup time yHTTP request-response overhead ydocument transfer time

Results: (Hit Ratio) zWhen taking cookies into account yHit ratio decreases from 54.5% to 35.2%. yByte hit ratio decreases from 40.9% to 30.42%. zSolution: Techniques aimed at enabling caching documents with cookies are important for increasing hit rate.

Results: (Bandwidth Savings) zWhen the proxy is present, the bandwidth consumption of aborted requests is higher due to the bandwidth mismatch between the connections of client-to-proxy and proxy-to-server. zQuestion: how much would this be offset by the savings from caching?

Results: (Latency Reduction) zCaching has limited effect on improving latency (reduced the mean by 3.4%, the median by 4.2%) zSolution: Maintain persistent connections between clients and servers yProxy as a connection cache. yRe-use persistent proxy-to-server connection for obtaining documents for multiple clients.

Questions: zHow does the proxy manage a connection cach? zHow many simultaneous connections it should maintain with a server or a client.

Conclusion: zFor dialup users yHit ratios is lower than those reported previously. yBandwidth savings non-exist or is negative. yLatency reduction coming mostly from caching TCP connections rather than documents.