# Epilogue

Here is the solution for the final exam. My comments on common mistakes are embedded inside.

And now, a personal message: I would like to thank all of you for being patient with me throughout the semester. This semester’s CS2105 has been the most difficult semester for me to teach: (i) the class size increases to a record high of 191, (ii) the number of TAs reduces to 1, and (iii) I just become a father to a pair of twins in Dec 2013 and am in the constant state of sleep deprivation for the whole semester. I have been late in posting class materials, returning assignments, and returning emails (or not returning emails). I have made mistakes in some of the lectures and spoken incoherently occasionally, and yet you all have been the nicest bunch of students and have tolerated these. Some of you even have kind words for me despite all of these, and that kept me going. So, thank you.

I hope you have enjoyed CS2105 and have learnt something useful. Have a good break!

FIN

1. Yang Shun says:

What a surprise/shock to receive the email about CS2105 exam answers at 4am in the morning! Thank you for the answers Prof Ooi, it is certainly a good follow up for our own understanding of the exam questions!

I have learnt a lot through CS2105 and am interested in exploring future Network focus area modules. Cheers!

• Caliluna says:

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2. Yang Shun says:

For 10(b), why is it that dw(z) <= 6 instead of dw(z) < 6? Were we taught which routes would be chosen if there was a tie in the cost of different routes?

I derived dw(z) < 6 because I thought dw(z) had to be strictly lesser than c(x,z) and hence my answer is 5.

• Oh Shunhao says:

I believe it is chosen based on who came first. It can be either, so d(w) = 6 is also possible.

Actually, when you look at it closely, you realise that d_w(z) can only be 6 (not less or more).
If d_w(z) were equal to 5, for example, then the shortest distance from x to z will be 9, via w.
That seems fine at first, but then this would mean that the shortest path from x to y will be x-w-z-y.
This means that the next hop of the path to y would then be w, not z.

So because of this ambiguous route (shortest path to y is via z, but shortest path to z is via w)
both paths to w must have exactly the same length.
Thus d_w(z) must be exactly 6.

• Oh Shunhao says:

typo in first sentence. I meant d_w(z)

3. Oh Shunhao says:

1) I had assumed that the query string only refers to the message that comes after the ? symbol in the url. So the body of the post message can also be considered a query string?

7) Doesn’t RIP and ICMP run over UDP? (I recall that being said in lectures) For that reason I put X for qn 7.

12b) I didn’t know that that counted as a replay attack.

• Yang Shun says:

I agree with your 1 and 7.

• Ooi Wei Tsang says:

I forgot to mention this.

1) I marked A as correct as well. I saw a suspiciously high number of As chosen as the answer, and realized that, despite me using the term query string loosely, you might have adopted the definition where the query string is the string that comes after ? in the GET request.

7) Yes and no. ICMP does not run over UDP, but rather it runs over (i.e., is encapsulated in) and IP packet. RIP, however, is implemented as an application-level process, and runs over UDP. For this reason, I accept X as an answer.

12b) “replay” refers generally to the intercept/copying of a message for used later.

4. Lim Wei Zhong says:

No more happy holiday but just to wait the release of result like man waiting for his death sentence as he sees the solutions

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13. Ava says:

It was a great journey together! Passing exams is already a tough task (https://ivypanda.com/blog/13-tips-on-how-to-pass-any-exam-in-a-week/), but you made it much easier :)