Question

We are connected to internet through an ISP. The ISP is connected to a regional ISP through an expensive link. In order to optimize the BW utilization of the link and to provide low latency to its customers the ISP has installed an HTTP proxy server. Let’s assume that: 1. The average request size is 200B. 2. The average response from a remote server is 10kB 3. The average response from a remote server when the contents have not changed is 200B. 4. The average request from a customer to the proxy takes 5ms. 5. The average response from the proxy to the customer takes 55ms. 6. The average request from the proxy to a remote server takes 105ms 7. The average response when the page has not changed takes 105ms from a remote server to proxy. 8. The average response when the page has changed takes 305ms from a remote server to a proxy. Suppose that the probability that a page is present with the proxy is 0.8 compute the reduction in BW utilization and latency for a request response cycle.

Answer 1

Since, the probability of finding the page is 0.8, therefore for 80% of the time the ISP doesn't have to send the request to the regional ISP. Therefore, the BW utilization of the link must be reduced to 1/5th of the original.

Let's suppose that the page has not changed and the request to the remote server takes same time for both customer and proxy=105ms both to and from.

Initially every request has to be transmitted to the remote server. Therefore delay= 105ms + 105ms = 210ms.

Now, about 80% of the pages are already with the proxy.

Therefore, delay = 0.8*(5+55) + 0.2*(105 + 105) = 90ms.(More than half).