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<TITLE>[Iccrg] Initial value of dwnd</TITLE>
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<P><FONT SIZE=2 FACE="Courier New">Hello,</FONT>
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<P><FONT SIZE=2 FACE="Courier New">We have a question concerning the Compound TCP draft, along the lines of point 4 in </FONT><A HREF="http://oakham.cs.ucl.ac.uk/pipermail/iccrg/2008-March/000468.html"><U><FONT COLOR="#0000FF" SIZE=2 FACE="Courier New">http://oakham.cs.ucl.ac.uk/pipermail/iccrg/2008-March/000468.html</FONT></U></A><FONT SIZE=2 FACE="Courier New"> ("4. What happens to dwnd during slow-start, and when is slow-start exited?")</FONT></P>
<P><FONT SIZE=2 FACE="Courier New">The draft states that "dwnd is initialized to zero while the connection is in slow start" (p. 6), and then "Delay-based component only kicks in when cwnd is larger than some threshold, currently set to 38 packets assuming 1500 byte MTU" (p.11).</FONT></P>
<P><FONT SIZE=2 FACE="Courier New">However, starting from zero, the recursion (3) on p. 7 gives a negative value, so that dwnd would always be set to 0. In order for dwmd to be positive the initial value needs to be such that: </FONT></P>
<P> <FONT SIZE=2 FACE="Courier New">alpha*dwnd(t)^k - 1 > 0</FONT>
<BR><FONT SIZE=2 FACE="Courier New">(if dwnd is restricted to integer values a larger initial value might be needed depending on how the rounding is done).</FONT>
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<P><FONT SIZE=2 FACE="Courier New">Clearly an initial value of 38 does satisfy this requirement but it is not clear whether this is the initial value that should be assigned to dwnd after slow-start or when the cwnd is larger then the specified threshold (38 in the draft). </FONT></P>
<P><FONT SIZE=2 FACE="Courier New">We think that the draft should clearly state the initial value for dwnd every time the delay component is activated. It might also be interesting to comment on why the window update function is not the same as in the previously published papers about Compound TCP. </FONT></P>
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<P><FONT SIZE=2 FACE="Courier New">Thanks,</FONT>
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<P><FONT SIZE=2 FACE="Courier New">Denis and Alberto</FONT>
<BR><FONT SIZE=2 FACE="Courier New"> </FONT>
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