Grading scheme

Problem 2 (25 points)

Question 1 (5 points):

• 2 points for each of the big subnets:
• 1 point for end-systems, 
• 0.5 points for the router, 
• 0.5 points for the broadcast IP address.
• 1/3 points for each of the small subnets.
• OK if there’s no broadcast IP address.

Partial:

• If a subnet is not minimal: half the points.
• If a subnet overlaps with another subnet: 0 points.
• If a student read 1000 instead of 10000: full points (as long as they did it correctly and subject to the partial rules above).

Question 2 (8 points):

• 1 point for the ARP request.
• 1 point for the ARP response.
• 2x 0.5 points for the DNS request.
  
• 2x 0.5 points for the DNS response.
• 2x 0.5 points for the TCP SYN.
• 2x 0.5 points for the TCP SYN ACK.
• 2x 0.5 points for the HTTP GET.
• 2x 0.5 points for the HTTP OK.

No deduction for:

• Putting the HTTP GET request in a separate packet after concluding the TCP 3-way handshake.
• Showing packets observed at other routers.

Partial:

• If an entry has incorrect(*) port numbers: half the points.
• If an entry has incorrect MAC or IP addresses: 0 points.
• If an unnecessary ARP request/response is included (the student did not consider ARP caching at R3): -1 point.
• In general, showing incorrect packets or unnecessary extra packets gets -0.5 points overall, but it depends on the case.

Question 3 (5 points):

• 1 point for saying BGP or inter-domain routing protocol.
• 2 points for saying that it’s because R1 and R2 are border routers.

• 2 points for showing a correct(*) minimal prefix.
• Partial: 1 point for showing 4.0.0.0/8.

(*) Ideally, a "correct" prefix is one that aggregates the IP prefixes that have been allocated to AS0 into as few entries as possible. However, if a student included some extra IP addresses, I counted that as correct, too. One lost points for not aggregating at all, or (partially) for showing 4.0.0.0/8.

Question 4 (5 points):

• 0.5 point for saying it’s R2, R3, and R4.
• 0.5 point for saying that it’s because these are the routers that participate in the intra-domain routing protocol.

• 2x 0.5 points for the entries that route to the big subnets.
• 3x 1/3 points for the entries that route to the small subnets.
• OK not to have a default route for non-local traffic.

• 1 point for saying yes AND justifying, e.g., “R2 will route to R4 through R3, hence, R3 will have a poisoned reverse in its table such that it never routes to R4 through R2.”
• OK if a student mentions only one poisoned reverse (i.e., they did not consider the default route for non-local traffic).

Question 5 (2 points):

• 1 point for saying none.
• 1 point for justifying correctly (*), e.g., “This failure concerns routing within an IP subnet, which is not the responsibility of the IP routers.”

Problem 3 (20 points)

Question 1 (5 points):

• 4 points for handling the timeout correctly:
• 1 point for showing a timeout.
  
• Partial: 0.5 point for showing the timeout but not re-transmitting SEQ3 when the timeout happens.
  
• 0.5 point for resetting the window to 1 MSS.
  
• 1 point for setting ssthresh correctly and transitioning the state correctly: either (ssthrsh=1,state=CA) or (ssthrsh=2,state=SS).
  
• Partial: 0.75 point for doing (ssthrsh=1,state=SS), but setting state=CA in the next row when cwnd becomes 2.
  
• 1 point for retransmitting the correct SEQ.
  
• Partial: 0.5 for retransmitting other SEQ(s).
  
• 0.5 point for correct duplicate ACKs in response to unordered SEQs received

• 1 point for the rest:
• 1/3 point for saying the state is “slow start” until the timeout.
• 1/3 point for increasing the window from 1 to 2.
• 1/3 point for showing correct SEQ and ACK numbers.

Question 2 (5 points):

• 5 points for saying to modify the receiver window AND justifying it. No need for details, as long as it’s clear the student understands why they are changing the receiver window.
• Partial: 2 points for saying to drop or delay packets AND justifying it. Again, no need for details, as long as the intention is clear.

Question 3 (5 points):

• 5 points for saying to ACK Alice’s packets AND justifying it (no need for details, as above).

• 1 point for answering “line 13" to the first sub-question.
• 1 point for answering “line 13” to the second sub-question.

• 3 points for saying that it’s possible because an application can implement any kind of reliable data delivery protocol it wants on top of UDP.

Partial:

• 1 point for saying “Google did it already with Quic” without explaining anything else.
• 1 point for discussing how to improve TCP performance.
• 0 points for saying that it’s impossible because without TCP one has to use UDP and then there won’t be any reliability.