S-38.2188 exam, 6.3.2007

General comments about the grading

• 0.5p for explanation, 0.5p for words. If the explanation is correct, one error in the words does not count.

• This seemed difficult. The most common problems were:
  - not understanding the difference of OSI layers 2 and 3, which lead to errors in explaining ARP
  - not understanding the nature of the service TCP gives for the upper layers
  The intention was that there is no outside network and the ns is the default for both networks. Of course, deviations from this idea were not punished.
  
  Some grading things:
  - 1p for the network topology
  - 6p for DNS query part. There 1p for getting domain name from the URL and 1p for the query itself. 2p for GW-ARP, 2p for routing
  - 5p for HTTP part: 1p TCP, 1p 3way handshake, 2p for TCP byte stream idea, 1p for HTTP GET
  
  A special note: as I told on the lecture, my habit is to give -1p for people confusing URLs and domain names. I had to realize this threat for many people.

• 1p for definition of flow control (fast sender vs. slow sender) and 1p for TCP flow control description (=advertised window and its use)
• 1p for definition of congestion control (to protect the network routers)
• Maximum 2.5p for description of basic TCP congestion control. A complete answer must include discussion about all algorithms (AIMD, slow start, fast retransmit/recovery) together with discussion about timeouts/duplicate ACKs. Some inconsistencies were overlooked but the logic was required to be correct. I also gave extra points if the descriptions of the TCP operations was exceptionally good.
• 1p for discussion about congestion avoidance mechanisms: TCP Vegas (host oriented) and network assisted TCP/RED or DecBit
• Extra 0.5p for some considerations on inter-protocol fairness or per-flow fairness in general.
• The maximum points all together was still obviously limited to 6p.

• I gave 1p for any reasonable definitions of communications network security, and 1p for explaining each basic concept, total of 6p maximum, however. Chapter 8.1 for these concepts.

Questions about the grading?

A chance to come and inspect the grading of your paper and ask questions will be arranged on:

• Wednesday, April 18, at 10 o'clock, room E220.

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