You open your browser and you click on a URL. Assume that the webpage associated

Question

You open your browser and you click on a URL. Assume that the webpage associated with the URL consists of a small HTML file and 2 small images, all stored on the same webserver. Let RT Tcs be the RTT from the client host to the webserver containing the URL. The IP address of the webserver is not cached at the local host but is cached at your local DNS server. Let RT TDNS be the RTT from the client host to the local DNS server. What is the total delay from the time you click on the webpage until the time that the entire webpage is displayed on your browser? Take into account all messages sent/received by the client host, including DNS, HTTP and TCP-related messages. Consider two cases:

(a) Persistent HTTP without pipelining. (b) Non-persistent HTTP with 2 parallel connections.

Explanation / Answer

There are two kinds of queries, one is DNS lookup for URL to DNS server and second one is query to webserver.

DNS lookup:

Query to webserver:

a) Delay time is : DNS LOOKUP + HTTP request

(TCP connection to DNS server + DNS lookup reqest + DNS lookup reply + FIN TCP) + (TCP connection to webserver + HTTP GET + HTTP GET reply + FIN TCP) = (3 + 1+ 1+ 4) + (3 + 1 + 1 + 4) = 9+9 = 18 RT DNS

b) Delay time is : DNS LOOKUP + HTTP request

(TCP connection to DNS server + DNS lookup reqest + DNS lookup reply + FIN TCP) + (2*TCP connection to webserver + HTTP GET + HTTP GET reply + 2*FIN TCP) = (3 + 1+ 1+ 4) + (2*3 + 1 + 1 + 2*4) = 9+16=25 RTTcs

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