Discussion
Identify the correct sequence in which the following packets are transmitted on the network by a host when a browser requests a webpage from a remote server, assuming that the host has just been restarted.
Answer: C
The sequence is DNS query, TCP SYN, HTTP GET request.
Just after a restart the host has no cached address for the server, so it must first resolve the host name with a DNS query. HTTP runs over TCP, so with the IP address known the browser opens a TCP connection, starting with the SYN of the three-way handshake. Only once the connection is established can it send the HTTP GET request.
Sending the HTTP request first, or before the TCP handshake, is impossible because there is no connection to carry it. Sending the TCP SYN before the DNS query is impossible because the host does not yet know the server’s IP address.
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