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

Web Caching Telnet CPSC 441 - Application Layer

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 Internet delay: 2 sec Access link rate: 1.54 Mbps Consider delay for Access link to be negligible when utilization is less than 80% consequences: Download traffic: ? LAN utilization: ? Access link utilization : ? Total delay = Internet delay + access delay + LAN delay = 2 sec + minutes + negligible (usecs to ms) origin servers public Internet 1.54 Mbps access link 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: ? Access link utilization = ? Total delay = Internet delay + access delay + LAN delay =2 sec + minutes + negligible (usecs to ms) 154 Mbps origin servers public Internet 1.54 Mbps access link 154 Mbps institutional network negligible (usecs to ms) 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: ? 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 Download rate over the access link =? Access link utilization = ? 1.54 Mbps access link institutional network Total delay: ? less than with 154 Mbps link (and cheaper too!) 10 Mbps LAN local web cache

Key takeaways Benefits of web caching Reduce response time of a client request Reduce traffic in the access link In your assignment-1, you will be implementing a simple web proxy

How to use Telnet? Establishes TCP connection with any internet service (or server) that runs on TCP No need for implementation of application layer It is up to the user what kind of application message needs to be send to the server You can use telnet to test assignment-1 (apart from using a web browser) CPSC 441 - Application Layer

# telnet pages.cpsc.ucalgary.ca 80 // tcp connection established Open a terminal # telnet pages.cpsc.ucalgary.ca 80 // tcp connection established GET http://pages.cpsc.ucalgary.ca/~cyriac.james/sample.txt HTTP/1.0 <Press Enter> Host: pages.cpsc.ucalgary.ca <Press Enter> Connection: close <Press Enter> <Press Enter> // HTTP request sent // HTTP response and data get displayed on the terminal CPSC 441 - Application Layer