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Re: [lwip-users] How to make tcp segment out as much as possible

From: Kieran Mansley
Subject: Re: [lwip-users] How to make tcp segment out as much as possible
Date: Thu, 15 Apr 2010 11:10:50 +0100

On Thu, 2010-04-15 at 17:54 +0800, yueyue papa wrote:
> I give enough windows size e.g. 16k,  and use TCP_NODELAY.
> Why I could only see 2 TCP segments out, and wait the remote ACK and
> then goes on.
> Is it possible to see all TCP segment out as much as possible?

In which direction (lwIP is sending or receiving) are you seeing

When you set TCP_WND you are configuring how much space the stack will
advertise to allow the other end to send to lwIP.  This does not affect
how much space lwIP has to send to the other end: this is controlled by
the other end.  It should be very easy to work out from a packet capture
what the receive windows are in both directions, and so whether the
receive windows are limiting the rate of sending.  

Sending in TCP is controlled by a number of things, the most important
being: (i) the receiver's window, which is what you can control with
TCP_WND; (ii) the congestion window which is not a configurable value.
These are separate mechanisms, but the amount that lwIP can send is
limited to whichever value is currently smallest.

The congestion window will initially start off small, but will grow if
there is no loss.  When there is loss it will reduce drastically to try
and limit the rate of sending as TCP assumes loss is due to congestion.
This algorithm tries to find the maximum rate it can send at without
seeing loss.


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