Tuesday, January 31, 2012

Exercise 6

Some of you shown some concern about exercise number six of the exam. As a result I have reviewed the marking and a few of you have got your mark risen a bit. The main problem is twofold: whether you increase the congestion window by two or one after each acknowledge during slow start and what is the policy during congestion avoidance.

You can have a look at the proposed solution trace. And the policies on each case are as follows:

  • Threshold: Same value on both sides of 3 MSS during all the exercise.
  • Slow Start: each ACK increases CongWin value by one MSS.
  • Congestion Avoidance: If during one RTT CongWin bytes are ACKed then CongWin is increased by one MSS (CongWin remains unchanged otherwise). 
However, those that may have chosen to increase by two MSS per ACK (given the fact delayed acknowledgments are used) during slow start also scored ok. Those that used a fractional increase of CongWin during Congestion Avoidance (ie. to increase MSS/CongWin per ACK) can also have a correct answer.
en

3 comments:

  1. Jordi2/01/2012 7:25 PM

    La opción correcta es la B. En mi casa hay alguien (Seguro que es mi hermano, pero no tengo pruebas) que se empeña en poner la A, y es horrible.
    La B es más intuitiva y comoda, basta con darle un toque de arriba abajo al rollo para que caiga el deseado papel, y éste se encuentra delante, mucho más facil de agarrar.

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  2. misan2/01/2012 7:44 PM

    Como no he leído tu comentario esta página sino en el correo me he estado preguntando durante unos segundos si había alguna pregunta en el examen con respuesta A ó B (que no me sonaba en absoluto).

    Pero al venir a la página a contestar he visto a qué te referías.

    Tú debes ser un defensor de los Big-Endian :-)

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