COM-407: Lab1 Part 3.2, question 2

Hello,

While checking the question 2 of Lab 3.2, I was wondering whether there was a mistake in the multiple choices.

Indeed, when pinging with IPv6, the ND protocol would typically start with a Neighbor solicitation from the sending host, followed with a Neighbor Advertisement from the pinged host. In the question 2, it seems that two choices invert the role of PC1 and PC4:

PC4-eth0 sends a NS (Neighbor Solicitation) packet to PC1-eth0
PC1-eth0 sends a NA (Neighbor Advertisement) packet to PC4-eth0

While I don't think those choices make sense if we ping6 PC4 with PC1, it would seem like they are included in the set of correct choices to select for this question.

Could it be a mistake, and PC1/PC4 should be swapped in those choices, or am I understanding something wrong?

Thank you so much in advance for you response,

Re: Lab1 Part 3.2, question 2

by Edin Guso - Thursday, 13 October 2022, 19:12

Dear Victor,

You are right in saying that in the ND protocol, PC1 would send an NS packet, and PC4 would respond with an NA packet. Also, if you examine the wireshark capture, you will see these two packets are captured at first.

However, as stated in the question feedback part, you should wait enough time and check all NS and NA packets captured by wireshark. Later on, you will see new NS and NA packets, which are in the direction stated in the options (PC4 sends NS, PC1 sends NA). This seems counter intuitive as you already explained. But these NS and NA packets are not part of the initial ND protocol. These packets are used to verify that the neighbor is still reachable. The conclusion is that NS and NA packets can be used for other purposes.

In short, NS and NA packets are sent in both directions, and you are expected to observe the unexpected direction.

Hope this answers your question. Feel free to ask for any clarifications here on the forum or send me an email (edin.guso@epfl.ch).

Best,
Edin