The Internet - Journey of an HTTP Message Consider the networks shown in the fig

Question

The Internet - Journey of an HTTP Message Consider the networks shown in the figure below. There are two user machines ml.a.com and m2.a.com in the network a.com. Suppose that a user on a wireless laptop, m3.a.com, wishes to join the network and request a webpage. Suppose the new user at m3.a.com later types in the URL into a browser to retrieve a 1 Gbit (1000 Mbit) file from. List the sequence of messages sent/received from/by m3.a..com as well as any other messages that leave/enter the a.com network that are not directly sent/received by m3.a.com from the point that the URL is entered into the browser until the file is completely received. Indicate the source and destination of each message. You can assume that every HTTP request by m3.a.com is first directed to the HTTP cache in a.com and that the cache is initially empty, and that all DNS requests are recursive queries. Ensure that you include all network nodes and protocols including DHCP and ARP. The table below should be used to organize the messages.

Explanation / Answer

First the user m3.a.com resolves the IP address www.b.com with its nearby DNS Resolver by sending the DNS Request Message.

If the nearby DNS Resolver unable to find any information then it contacts the Root DNS Server with the Request Message.

Either from local DNS in Step1 or from Root DNS to Local DNS in Step2, user m3.a.com receives the name of DNS TOP Level Domain Server.

The local DNS then contacts the .com TLD. The TLD.com server returns the IP address of www.b.com

Now the HTTP Client sends a HTTP Get Request to www.b.com, which will be looked into the HTTP Cache in the a.com network.

If the file is not found in the HTTP Cache, then HTTP Get Request Message will be forwarded to www.b.com. Since in the question it is stated that HTTP Cache is empty, Request will be send to www.b.com.

As soon as www.b.com receives the GET request, it sends the HTTP Response with the file to the Router 2.

The 1 Gbit file is transmitted over the 1 Mbps link between Router2 and Router1 to the HTTP cache.

Now the file is sent to m3.a.com from HTTP Cache.

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