1. Some Mid-Term Thoughts

2. Allowed to bring in 1 cheat sheet (2 sides)
Wed Oct 28
In class
1 hour 15 minutes exam
Should take less than hour to complete
Allowed to bring in 1 cheat sheet (2 sides)
Fit in as much as you can
But not much point
Syllabus
Everything covered till today, i.e., NATs
Much more emphasis on App and Transport

3. About the exam format Exact test format
Questions will not test whether you remember
You will not have to write long essays
Not be asked to write code
Will probably have
Multiple choice questions
True and false questions
Some problems to solve
Arguments about tradeoffs
Exact test format
Not yet decided

4. Some Examples:
(T/F) TCP sends a new packet whenever it gets an ACK (not a DupACK) from the receiver.
(T/F) A TCP sender starts a timer once a packet has been transmitted.
(T/F) Bandwidth constraint does not necessarily imply timing constraints
Prove that in Selective Repeat, the base on transmitter’s window is always <= Base of Receiver’s window
For packet pair-based bottleneck bandwidth estimation, explain a scenario in which the bottleneck will be underestimated. Explain when it will be over-estimated.
Complete the sentence: A DupACK is generated when ________________________
Argue in favor or against: Stop and wait is never better than GBN
Argue in favor or against: A web cache in UIUC’s network offers 40% cache hit. The throughput improvement observed by users due to the cache is 40%.
Let’s say a packet contains L bits, and C is the additional bits of the CRC, making the total packet size S bits, where S = L + C . Assume there is no header for the packet. Let’s also say the probability of a bit being in error is p and all bit errors are identical and independent (i.e., they are equally likely and error of one bit is not influenced by the others). Write a mathematical equation that describes the condition when it is beneficial to add 2 CRCs to a packet, as opposed to just 1 CRC.

6. Some Examples:
Write 1 advantages and 1 disadvantage for recursive and non-recursive DNS
(T/F) Two links emanating from the same router cannot have the same virtual ID for a given flow.
“My header is your data” – who is saying this to whom?
Calculate RTO for a given network (relevant data will be given)
Argue that TCP flows are fair to each other, irrespective of who starts first
(T/F) TCP sends a new packet whenever it gets an ACK (not a DupACK) from the receiver.

7. Questions?