Web Proxy Caching: The Devil is in the Details

1. 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

2. Brief Review clients servers Reply Req. proxy Req. Reply

3. Brief Review • Client send requests to the proxy. • If the requested document is in its cache, the proxy serves the request from its cache. • Otherwise, the proxy forward the request to the server. • Server replies the request through the proxy (proxy keep a copy of the requested document).

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

5. Previous Work • High level details: hit ratio & byte hit ratio • Ignored exceptional cases such as connection aborts. • Omitted the effect of cookies on cacheability of resources.

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

7. Simulation • Web proxy simulator (PROXIM) • Workload: trace from AT&T Worldnet • 12 days dialup traffic on a FDDI ring • encrypted IP addresses • contained information on both TCP events and HTTP events

8. Simulator: PROXIM • Simulator Cache • sufficiently large • included proxy overhead in the request service time • Network Connections • zero or more open connections (cache-to-proxy & proxy-to-server) • Proxy closes client-to-proxy connections with 3 minutes of idle time. • Proxy-to-server connections are timeout after 30 secs of idle time.

9. Simulator: PROXIM (cont.) • Document Transfer • Packet-level delivery with TCP slow-start • 1500-byte packets • constant round-trip time estimate for each connection • Latency Calculations • connection setup time • HTTP request-response overhead • document transfer time

10. Results: (Hit Ratio) • When taking cookies into account • Hit ratio decreases from 54.5% to 35.2%. • Byte hit ratio decreases from 40.9% to 30.42%. • Solution: Techniques aimed at enabling caching documents with cookies are important for increasing hit rate.

11. Results: (Bandwidth Savings) • When 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. • Question: how much would this be offset by the savings from caching?

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

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

14. Conclusion: • For dialup users • Hit ratios is lower than those reported previously. • Bandwidth savings non-exist or is negative. • Latency reduction coming mostly from caching TCP connections rather than documents.