Web Proxy Caching: The Devil is in the Details Ramon Caceres, Fred Douglis, Anja Feldmann Young-Ho Suh Network Computing Lab. KAIST Proceedings of the Workshop on Internet Server Performance, Madison, WI, June

2001 년 10 월 11 일 Internet Server Class2 Contents Introduction Previous work Solution approach Simulation Concluding remarks

2001 년 10 월 11 일 Internet Server Class3 Proxy Caching Goals or Benefits? Introduction Clients Internal Latency 23% Servers External Latency 77% Proxy

2001 년 10 월 11 일 Internet Server Class4 Introduction (Cont ’ d) Expected benefits of Proxy Caching Can reduce the user-perceived latency Can lower the network traffic Can reduce the service demands on content providers Is really true? If then, how much?  requires Performance Evaluation

2001 년 10 월 11 일 Internet Server Class5 Previous work Limitations Fails to accurately reflect  Hit ratios  Bandwidth savings  User-perceived latency Due to  Just considering high-level details  ex) cookies/ aborted request / TCP connection setup time, slow start phase So they argue that “ The Devil is …”

2001 년 10 월 11 일 Internet Server Class6 Solution approach Motivation To judge the performance impact of proxy caches Low-level details should be considered such as  Interaction b/w HTTP and TCP  Network environment  Other possible factor (e.g cacheability, connection abortion etc.) Developed a new and more realistic proxy cache simulator - PROXIM

2001 년 10 월 11 일 Internet Server Class7 Law packet traces (150Million/day) Process them to final trace compactly Simulation – input(AT&T Worldnet) 18,000 dialup users 450 Modem banks 2 terminal servers FDDI ring Internet 500-MHz Alpha WS 12days in mid-August, ,964 users => 154,260 sessions => maximum 421 simultaneous sessions

2001 년 10 월 11 일 Internet Server Class8 Simulation – input (Cont ’ d) Final Trace contains TCP events  Timestamps, sequence numbers and acks for all packets with SYN, FIN, or RST bit set HTTP events  Timestamps for HTTP requests/responses, and last packet for each direction HTTP headers  Complete HTTP headers for both requests/responses Byte counts  Bytes sent/responses Input is ready …

2001 년 10 월 11 일 Internet Server Class9 Simulation - PROXIM Simulated Cache Network Connections Document Transfer Latency Calculations

2001 년 10 월 11 일 Internet Server Class10 Simulation - result Hit Ratio Only a secondary measure  Correlation with other metrics is low Emulate full behavior of proxies  30% of all requests had a cookie  If ignores cookie => 54.5%  Else => 35.2% Byte hit ratio : 40.9% => 30.42% Caching documents with cookie  Delta encoding  Client-side HTML macro-preprocessing

2001 년 10 월 11 일 Internet Server Class11 Simulation – result (Cont ’ d) Bandwidth Savings Proxy can actually increase traffic  In case clients abort request  Due to BW mismatch b/w Client->Proxy and Proxy -> Server The exact effect depends on the proxies  One extreme : continue to download  49.8GB => 58.7GB : +18%  The other extreme : abort the download immediately  Depends on the proxy to server BW  45Mbps : 49.8GB => 53.7GB (+8%)  1.5Mbps : 49.8GB => 50.7GB (+2%)  0.5Mbps : 49.8GB => 46.9GB (-6%, byte hit ratio : 30.42%)

2001 년 10 월 11 일 Internet Server Class12 Simulation – result (Cont ’ d) Latency Reduction Mean : 3.4%, Median : 4.2%  cf)Upper bound : 26% (by Kroeger et al.)  due to  High latency of connection set-up  the effect of cookies  Modem BW (28.8Kbps) Suggest Connection cache  Maintain persistent connection  If all connections are persistent => -24%  Else specified in the requests=> 13%  The rate of re-use of connection(hit ratio) : 30sec Time Out  Client -> Proxy : 81.5%, Proxy->Server : 79.5%  Maximum simultaneous connections : 238

2001 년 10 월 11 일 Internet Server Class13 Concluding remarks Lesson learned Low level details has a significant effect on all aspects of system performance  Hit ratios  Bandwidth savings  User-perceived latency Critique Interesting result Not general environment Other details? : prefetching, etc.