I have emailed your Assignment 3 scripts back to you to your NUS email account.

Due to a bug in the process, however, a small number of scripts are scanned twice, and so some of you received two emails. Sorry about this.

More seriously, a small number of you did not have your scripts scanned initially. I have tried to check through everyone and rescan/email back those that I missed the first round.

If by now you still have not received your Assignment 3 by email, please let me know.

Hi, These are the common mistake for assignment 3:

Q1-a) Do not explaining about the MAC address. For example, some student wrote “To obtain the IP address”.

Q1-c) Some students just wrote that they could not find the response to packet 9 but did not explain why.

Q2-b) Some students wrote that since the DHCP servers and capturing host have IP addresses with different prefix, they should be in two different subnets. However, we need to have the subnet mask to be able to jugde. The good news is that we did not deduct the mark for those without mentioning msubnet mask.

Q3-b) There were a confussion for this part with iterative and recursive queries in DNS. However, the question was about the reason that a client contacts three different DNS servers instead of one.

Q3-c) There were students which wrote the TTL of IP layer.

Q4-b) The man in the middle attack was mentioned by some student as the vulnerability of self-signed certificate. However, since the self-signed certificate is from a root CA, it is trustable.

Q1:

a. To obtain the mac address of 172.26.190.1.

b. The host with IP address of 172.26.191.183 and MAC address of d4:be:d9:9d:97:e8. Each of them is fine.

c. No. Because 172.26.190.1 directly responds within a standard frame to the 172.26.191.183 not in the broadcast manner.

d. To query for the mac address of the IP 172.26.190.1.

e. 127. The Mac address of IP 172.26.190.1 which is 00:00:0c:07:ac:02.

f. The packet is a Gratuitous ARP message which is useful for the purposes below:

• detect IP conflicts. When a machine receives an ARP request containing a source IP that matches its own, then it knows there is an IP conflict.
• assist in the updating of other machines’ ARP tables.  Clustering solutions utilize this when they move an IP from one NIC to another, or from one machine to another. Other machines maintain an ARP table that contains the MAC associated with an IP. When the cluster needs to move the IP to a different NIC, be it on the same machine or a different one, it reconfigures the NICs appropriately then broadcasts a gratuitous ARP reply to inform the neighboring machines about the change in MAC for the IP. Machines receiving the ARP packet then update their ARP tables with the new MAC.
• inform switches of the MAC address of the machine on a given switch port, so that the switch knows that it should transmit packets sent to that MAC address on that switch port.
• Every time an IP interface or link goes up, the driver for that interface will typically send a gratuitous ARP to preload the ARP tables of all other local hosts. Thus, a gratuitous ARP will tell us that that host just has had a link up event, such as a link bounce, a machine just being rebooted or the user/sysadmin on that host just configuring the interface up. If we see multiple gratuitous ARPs from the same host frequently, it can be an indication of bad Ethernet hardware/cabling resulting in frequent link bounces.

Mentioning the Gratuitous ARP message and one of the above purpose is fine.

g. d4:be:d9:9d:b4:ba. This can be acheived from sender MAC address of any packet which has the source IP address of capturing host (172.26.191.153).

Q2:

a. 137.132.83.7 and 137.132.83.6

b. No. By finding the DHCP ack packets, we can find 137.132.83.7 and 137.132.83.6 as the source IP address. By finding the subnet mask for  172.26.191.153 which is 255.255.254.0, we can find that they are not in the same subnet.

c. Two. We can obtain this by list all the DHCP Discover messages and then count the number of different mac address.

d. Because the capturing host wants to keep its previous IP address if it is possible.

Q3:

a. 137.132.87.2, 137.132.85.2 and 137.132.94.2

b. The client may contacts several DNS server in parallel to save the time in the case of failure of one or when an address may not be cached in one DNS
server but may be cached in another.

c. 9 min and 31 second comes form 137.132.85.2 DNS server.

Q4:

a. The first certificate certifies the public key of diasy.ubuntu.com, signed by “Go Daddy Secure Certificate Authority”. The second certificate certifies the
public key of “Go Daddy Secure Certificate Authority”, signed by “Go Daddy Root Certificate Authority”. The third certificate certifies the public key of “Go Daddy Root Certificate Authority”, signed by “Go Daddy Class 2 Certification Aut”.

b. Yes. A root CA can sign itself.

c. Packet 211 contains the master key encrypted with the public key of diasy.ubuntu.com.

Some of you are anxious if we have received your Assignment 3 submission or not, especially those who slipped the answer sheet under my door.

This is good, because this means you understand well how reliable protocol works, and you are waiting for an ACK from the receiver! You are also worried that Trudy might have intercepted your submission.

Instead of sending back an ACK to individual students, I will broadcast the ACK periodically. So here is the Google Doc that lists the hardcopy answer sheets to Assignment 3 that we have received. The list will be updated a few more times until the deadline.

List of Assigment 3 Received (Hardcopy Submission Only)

I have finished grading Assignment 2.

There are couple of common mistakes that worth highlighting here:

1. If you receive a corrupted packet, you should not look at the content of the packet, since you cannot tell which bit is corrupted! Many of you still assume that p.seq is valid even when p.isCorrupted is true. For this, I takes off two points.
2. Another common bug is that you did not make sure that all timers are cancelled when the connection is closed. As a result, the sender goes into infinite loop sending forever (and encountering exceptions forever).
3. The last packet should be treated as any other packet. This means the sender should still ensure that it is received correctly, by repeated retransmitting until it does. Some of you forgot about this and only ensures that the packets are sent correctly, except the last one!

Dear Students,

There were three typos in the pdf file of Assignment 3. We updated the pdf file and uploaded on the blog. Please either re-download the pdf file here or consider the three below points:

1. Question 3 – part (c) – second and third line: the correct URI is daisy.ubuntu.com instead of diasy.ubuntu.com

2.Question 4 – second line: the correct sentence is “Right click on Packet 145 and choose ..” instaed of “Right click on Packet 36 and choose ..”

3. Question 4 – forth line: the correct IP addres is “172.26.191.153″ instead of “172.126.187.16”

Thanks!

Saeid