Solved class test

Class Test Chapter3

Common errors in the homework exercise

When dealing with congestion control ...

1. A round-trip time takes to complete, that means congestion window will evolved during a single RTT. First RTT is number one. The value for cwnd at the end of RTT one should be 2 if at the beginning of it cwnd=1 and segment was acknowledged during the RTT.
2. If you are using Fast Recovery remember that while cwnd = FlightSize/2 + 3 temporarily, this changes to congwin = FlightSize/2 as soon as you get acknowledged the missing segment. That means that in the 2nd question, congwin goes to 17+3, and keeps going up as more ACKs are received during that RTT, but as soon as the missing segment is received, cwnd goes back to 17  (step 3, page 4, RFC 2001).
3. Transition between slow-start and congestion avoidance may happen in the middle of a RTT. In the 3rd question, when threshold is 18, congestion window will not go from 16 to 32 within a single RTT but instead will go from 16 to 18 and then mode will change to congestion avoidance so the rest of the [14] ACKs during that round trip time will only increase congestion window by one (it can be argued it will be a bit less than one and closer to 14/16 or more precisely cwnd=18.76 at the end of the RTT if the 1/cwnd increment per ACK received is used). 

Please note that for our exercise FlightSize = cwnd as application data is always available and transmission is not restricted by flow control. 

Videos to keep you entertained

Madrid Polytechnic University has a video series on cryptography (in Spanish) you may want to have
a look at.

Public Key Cryptography

A video was presented in the classroom today:



This is the second part in case you want to watch it:


If you want to dig it deeper, you can follow this Khan Academy playlist.