[Iccrg] Initial value of dwnd

[denis.collange]

Hello,

We have a question concerning the Compound TCP draft, along the lines of
point 4 in
http://oakham.cs.ucl.ac.uk/pipermail/iccrg/2008-March/000468.html ("4.
What happens to dwnd during slow-start, and when is slow-start exited?")

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).

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: 
	alpha*dwnd(t)^k - 1 > 0
(if dwnd is restricted to integer values a larger initial value might be
needed depending on how the rounding is done).

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). 

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. 


Thanks,

Denis and Alberto
 

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