A: TCP Transport Ports

Wendell
By Wendell October 11, 2018 09:05

You might have missed this latest #CCNA question because configuring routers and switches does not reinforce the concepts required to answer this question. If you do want to do some labbing to learn more, download Wireshark (or some other network analyzer) and capture the messages that your computer sends and receives when it connects to a website today. Check out the question first and come back here for the answers.

The Answers:

C, E

 

Background: TCP Connection Establishment

When an application uses TCP, as is the case in the scenario for this question, the endpoints send a series of three messages. The messages set different TCP flags whose combinations identify the messages as part of the TCP connection establishment process. Figure 2 shows the sequence, but the first message has a SYN flag set, the second has both a SYN and ACK flag set, and the third has an ACK flag (but not SYN) flag set.

Figure 2: TCP Connection Setup for this Question

The question stem also gives us enough information to determine which well-known port the messages use. The question mentions a URL that begins with HTTPS, meaning it uses SSL. SSL uses well-known port 443. As you can see from the figure, the first and third messages in the TCP connection establishment flow go towards the server so that the destination port is 443 in that case.

 

Why the Answers are Right or Wrong

For this question, the background description reveals the two correct answers. The third message has an ACK flag (but not SYN flag), and flows towards the web server, with a destination port of 443.

The wrong answers all list a fact that is not true about that third message.

Q: TCP Transport Ports
Q: Config Process
Wendell
By Wendell October 11, 2018 09:05
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