Problem 2

Question 1 (8 points)

  • 2 points for identifying the three subnets correctly
  • 1.5 points per non-overlapping, minimal prefix
  • 0.5 point for correct IP address assignment per subnet
Partial credit:
  • 1 point per non-overlapping, non-minimal prefix
  • 0 points for any prefix that overlaps with another

Question 2 (9 points)

  • 2 points for e1's ARP + response (packets 1, 2)
  • 2 points for e1 sending a DNS request + response (packets 3, 6, 13, 14)
  • 1 point for y’s ARP + response (packets 4, 5)
  • 1 point for d sending a DNS request to a root server + response (packets 7, 10, 11, 12)
  • 1 point for z’s ARP + response (packets 8, 9)
  • 1 point for TCP SYN + SYN ACK (packets 15, 16, 17, 18)
  • 1 point for HTTP GET + response (packets 19, 20, 21, 22)
Some students assumed that the local DNS server knows the IP address of www.yyy.ch and skipped the DNS request to the root (packets 7, 10, 11, 12). In this case:

  • 1 point for explicitly writing their assumption
  • 2 points for e1's ARP + response
  • 2 points for e1 sending a DNS request + response
  • 1 point for y’s ARP + response
  • 1 point for TCP SYN + SYN ACK
  • 1 point for z’s ARP + response (packets 8, 9)
  • 1 point for HTTP GET + response (packets 19, 20, 21, 22)
Partial credit:
  • -25% of the points for a packet if the protocol and ports are wrong
  • -25% of the points for a packet if the purpose is wrong
  • 0 points for a packet if the MAC and/or IP addresses are wrong
Some students assumed that S2 (renamed to R2) is a link-layer switch (as originally stated in the exam). In this case, the correct answers are the same as above, with the only difference that: the destination MAC address of all packets transmitted by interface z of router R1, and the source MAC address of all packets received by the same interface is some MAC address that is not shown in Fig. 1 (one could have chosen any letter to refer to it).

Setting this MAC address to w was incorrect, as it was clearly stated that there were both link-layer switches and routers between S2/R2 and www.yyy.ch. However, this mistake cost only 1.6 points:
  • 0.1 point for z's ARP asking for the wrong MAC address (w's).
  • 0.5 point for the ARP response coming from the wrong source MAC address.
  • 0.5 point for the TCP SYN packet forwarded by z having the wrong destination MAC address + the TCP SYN ACK packet received by z having the wrong source MAC address.
  • 0.5 point for the HTTP GET packet forwarded by z having the wrong destination MAC address + the HTTP response packet received by z having the wrong source MAC address.


Question 3 (4 points)

  • 1 point per correct packet
Partial credit:
  • 0.5 point per correct packet with incorrect fields
  • -0.5 point per incorrect packet, but no more than -2 points overall for incorrect packets

Question 4 (2 points)

  • 0.5 point for recognizing that there are two interfaces
  • 0.5 point for correct MAC and IP addresses
  • 0.5 point for saying that lo is a loopback or virtual interface, and that’s why it does not have MAC address
  • 0.5 point for saying that it could be used for communication between internal processes

Question 5 (2 points)
  • 0.5 point per correct packet
Partial credit:
  • -0.25 point per incorrect packet, but no negative points for the question overall (worst case 0 points)

Problem 3

Question 1 (5 points)

1 point for using SEQ/ACK numbers correctly

1 point for window:
  • 0.25 point for window starting at 1 and sending segment
  • 0.25 point for increasing the window to 2, then to 4 and sending the corresponding number of segments
  • 0.5 point for increasing the window to 7 and sending the corresponding number of segments
1 point for state:
  • 0.5 point for starting in slow-start and staying there until after the timeout
  • 0.5 point for transitioning to congestion-avoidance at the right point
1 point for setting ssthresh to 3 (3.5 and 4 also OK) at the right point

1 point for timeout:
  • 0.5 point for recognizing there was a timeout
  • 0.5 point for retransmission and setting the window to 1

Question 2 (5 points)

  • 1 point for realizing that throughput is 12 bytes divided by transfer time
  • 2 points for estimating transfer time as 3.5RTT or 4RTT or 4.5 RTT  or 5 RTT
  • 0.5 point for computing the RTT correctly in (a)
  • 0.5 point for computing the throughput correctly in (a)
  • 0.5 point for computing the RTT correctly in (b)
  • 0.5 point for computing the throughput correctly in (b)
Alternatively:
  • 1 point for realizing that throughput is avg window size divided by RTT
  • 2 points for estimating the avg window size correctly
  • 0.5 point for computing the RTT correctly in (a)
  • 0.5 point for computing the throughput correctly in (a)
  • 0.5 point for computing the RTT correctly in (b)
  • 0.5 point for computing the throughput correctly in (b)

Question 3 (5 points)

Part (a)
  • 2 points for recognizing that there will be a problem with the RTT estimation
  • 1 point for mentioning premature/unnecessary timeouts for packets on Path 1
  • 1 point for saying that timeout will be unnecessarily long for packets lost on Path 2
Part (b)
  • 1 point for saying that Scenario B will (most likely) have better throughput because the premature timeouts will frequently set the window to 1
  • OK to say that it’s possible for Scenario A to have better throughput if there’s some peculiar loss pattern

Question 4 (5 points)

Part (a)
  • 1.5 point for mentioning a certificate
  • 1 point for clearly saying/showing what the certificate does
Part (b)
  • 1.5 point for mentioning sequence numbers
  • 1 point for clearly saying/showing that the sequence numbers are included in the MAC

Last modified: Friday, 12 March 2021, 12:32