Department of Computer Science University of Calgary CPSC 441 Tutorial-3 Department of Computer Science University of Calgary

Web Caching Benefits Case Study Reduce response time of a client request Reduce traffic in the access link Case Study

Case Study: Default Scenario Assumptions: Avg object size: 100K bits Avg request rate from browsers to origin servers:15/sec HTTP request messages are small Avg data rate to browsers: 1.50 Mbps Internet delay: 2 sec Access link rate: 1.54 Mbps consequences: LAN utilization: 15% Access link utilization = 97% Total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + negligible (usecs to ms) Note: Utilization of 97% is same as Traffic Intensity of .97 origin servers public Internet 1.54 Mbps access link problem! institutional network 10 Mbps LAN

Case Study: Fatter access link Assumptions: Access link rate: 1.54 Mbps Everything else remains the same as ‘Default Scenario’ consequences: LAN utilization: 15% Access link utilization = 97% Total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + usecs 154 Mbps origin servers public Internet 0.97% 1.54 Mbps access link 154 Mbps institutional network usecs 10 Mbps LAN Cost: increased access link speed (not cheap!)

Case Study: Install local cache assumptions: Install local web cache Everything else remains the same as ‘Default Scenario’ consequences: LAN utilization: 15% access link utilization = total delay = Internet delay + access delay + LAN delay = origin servers public Internet ? 1.54 Mbps access link ? institutional network How to compute link utilization, delay? 10 Mbps LAN local web cache Cost: web cache (cheap!)

Case Study: Install local cache Calculating access link utilization, delay with cache: suppose cache hit rate is 0.4 40% requests satisfied at cache, 60% requests satisfied at origin origin servers public Internet i.e 60% of requests use access link Data rate to browsers over access link = 0.6*1.50 Mbps = 0.9 Mbps Access link utilization = 0.9/1.54 * 100 = 58% 1.54 Mbps access link institutional network Total delay = 0.6 * (delay from origin servers) +0.4 * (delay when satisfied at cache) = 0.6 (2 + ~msecs for access link and LAN) + 0.4 (~usecs for LAN) = ~ 1.2 secs less than with 154 Mbps link (and cheaper too!) 10 Mbps LAN local web cache