Questions on Past Paper 2020

Questions on Past Paper 2020

by Yiren Cao -
Number of replies: 2

Hi there,

I have some questions regarding the 2020 exam solution sheet.

Problem 1


  • Is ISP4/ISP6 here indicating a potentially huge network between the host D and the nat N such that BCD are on a local network while A, E are far away?
  • questions 1(d) and 2(c): how about the packets with the protocol e.g. TLSv1.3, why do we exclude them?

Problem 3

  • Question 5

My calculation

To me, the \theta will end up being 10^7 *8 \ bits/s and so the final q will become 0.02% rather than 1.5%. Could you kindly point out if I calculated it wrong somewhere?

Problem 4



  • Question 1 (a)

I wonder why S and D1 would observe the packet with the same MAC source and destination. i.e. Does the bridged network indicate that only bridge/switches are in between and no router is involved?

Moreover, could you kindly explain where 33:33:0:2:0:1 comes from? I wasn't able to find it explicitly mentioned in the question or the graph.

Thanks a lot for reading my lengthy post!

Best,
Yiren

In reply to Yiren Cao

Re: Questions on Past Paper 2020

by Stéphan Plassart -

Dear Yiren Cao,

For problem 1, the cloud ISP4 or ISP6 are Internet service providers
that works only on Ipv4 or only on Ipv6. Depend on the context the ISP
could be huge or not: it could be connected to a simple switch or to
the swisscom network. Between A and R1, and E and N, you could have
whatever network you want over ipv4 (resp. ipv6).

For the questions 1(d) and 2(c), the questions only focus on the IP
header of the packets that transfer data. It is said: "We observe the
IP headers in the packets resulting from this transfer". Even in the
TLS handshake packets for example you will have an IP header.  

For problem 3, the number is in Mb/s, megabit per second, and not in
MB/s, megaByte per second, that's why you obtain this result.

For problem 4, you are right: the "bridged network" indicate that only
bridge/switches are in between and no router is involved.
For this exercice, sensors used IP multicast. From multicast course
(slide 13 of mcastSols.pdf), you see that the IP multicast address is
algorithmically mapped to a multicast MAC address. Here, it is deduced
from the multicast address of D1.

Hope this answer your questions,

Best regards,
Stephan.

In reply to Stéphan Plassart

Re: Questions on Past Paper 2020

by Yiren Cao -
Hi Stephan,

Thank you so much for your helpful explanation! It really cleared up my confusion!

Best regards,
Yiren