Exercise class 12.2.2003
- General: Exercise 2 is worth twice the other exercises.
- General: You need to do all (or almost all) problems and have more than half (may even be 66%) of the points to pass the preproblems.
- Exercise 1: Waiting for one RTT before sending data means that you wait for an acknowledgment of sent data before sending new data.
- Exercise 2: Note that you need the ping traces in exercise 5, so do enough samples already here and make sue you have clearly different traces that you can use in exercise 5.
- Exercise 7: Here sent = transmitted = acknowledgment received.
Exercise class 19.2.2003
- If still having problems with UNIX utilities e.g. ping, go to directory /usr/sbin/ you should be able to use them from this path.
Exercise class 26.2.2003
- There will be an extra class next week wednesday.
- Exercise 2: First check that you get responses for both traceroute and ping before collecting multiple traces. If the UNIX utilities do not work or you have problems with them, try using utilities on the web, such as visualtrace.
- Exercise 2: Read the man pages for traceroute in kosh.hut.fi. There it is explained how to take into account the "stars". There is no one answer, it depends on the reason for the stars. Sometimes you must subtract them from the hopcount, sometimes not.
- Exercise 5: What initial guess for estimated RTT to use? If you only have one sample, you only have one choice for the estimate...
- Exercise 6: A packet is lost the first time it is transmitted
- Exercise 7: For more information on Fast retransmit and especially Fast recovery read http://www.rfc-editor.org/rfc/rfc2581.txt and http://www.rfc-editor.org/rfc/rfc2582.txt. In class I said that after the arrival of three duplicate ACKs you use a window of one if only fast retransmit is used and a window of 3 or 4 if fast recovery is used. This is wrong for fast recovery. In fact when the third duplicate ACK arrives, ssthresh is set to ssthresh=flightsize/2. Then the lost segment is retransmitted and the congestion window is inflated to ssthresh + 3, taking into account the duplicate acks that arrived before retransmit. Then when the ACK from the retransmitted segment arrives the window is deflated to the ssthresh value. I appologize for remembering the algorithm wrong.
If the information on this page is unclear mail to eeva.nyberg@hut.fi
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