RE: [ns] INITIAL CWND (fwd)

[Manish Karir <karir@wam.umd.edu>]

http://www.faqs.org/rfcs/rfc2414.html

>From rfc2414...(last paragraph of section 1)

   TCP implementations use slow start in as many as three different
   ways: (1) to start a new connection (the initial window); (2) to
   restart a transmission after a long idle period (the restart window);
   and (3) to restart after a retransmit timeout (the loss window).  The
   change proposed in this document affects the value of the initial
   window.  Optionally, a TCP MAY set the restart window to the minimum
   of the value used for the initial window and the current value of
   cwnd (in other words, using a larger value for the restart window
   should never increase the size of cwnd).  These changes do NOT change
   the loss window, which must remain 1 segment (to permit the lowest
   possible window size in the case of severe congestion).

----

manish


On Fri, 14 Sep 2001, Susan Tsao wrote:

> Hi,
> 
> I set (WindowInit_ = 5) in my tcp simulation, and I do see at the start
> of the simulation, cwnd indeed has the value of 5. 
> 
> According to NS document(chapter 29.1.1), TCP reacts to packet lost by 
> setting ssthresh_ to max[current window size, 2] and initialize cwnd_
> back to 
> windowInit_.  So I expected cwnd to be initialized to 5 when it sees a
> packet
> drop. However, what I noticed from the simulation is TCP resets cwnd =1
> instead
> of cwnd=5. I'm fairly new to NS, so I'm not sure if there's something
> wrong with the code.
> I'd appreciate if anyone has an explanation as to why TCP resets cwnd to
> 1 instead of
> the user specified WindowInit_ value. 
> 
> Thank you.
> 
> Susan 
>