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Department of Computer Science University of Calgary
CPSC 441 Tutorial-3 Department of Computer Science University of Calgary
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Web Caching Telnet CPSC Application Layer
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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
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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!)
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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!)
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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
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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
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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 Application Layer
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# telnet pages.cpsc.ucalgary.ca 80 // tcp connection established
Open a terminal # telnet pages.cpsc.ucalgary.ca 80 // tcp connection established GET 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 Application Layer
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