A user starts browsing a webpage hosted at a remote server. The browser opens a single TCP connection to fetch the entire webpage from the server. The webpage consists of a top-level index page with multiple embedded image objects. Assume that all caches (e.g., DNS cache, browser cache) are all initially empty. The following packets leave the user's computer in some order.
- HTTP GET request for the index page
- DNS request to resolve the web server's name to its IP address
- HTTP GET request for an image object
- TCP SYN to open a connection to the web server
Which one of the following is the CORRECT chronological order (earliest in time to latest) of the packets leaving the computer?
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Answer: C
To fetch the webpage, the browser must perform the following steps in sequence:
- DNS Resolution: Since all caches are initially empty, the browser must first resolve the server's domain name to an IP address. Thus, the DNS request (ii) is sent first.
- TCP Connection Establishment: Once the IP address is obtained, a TCP connection must be established before any HTTP communication can occur. This begins with the TCP 3-way handshake, initiated by sending a TCP SYN (iv).
- Requesting the Index Page: After the TCP connection is established, the browser sends an HTTP GET request for the index page (i) to retrieve the main HTML document.
- Requesting Embedded Objects: The server returns the index page. The browser parses this HTML, discovers the embedded image objects, and then sends an HTTP GET request for the image object (iii).
Therefore, the correct chronological order is: (ii) → (iv) → (i) → (iii).
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