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Re: [lwip-users] tcp_tmr - TCP_TMR_INTERVAL

From: Kieran Mansley
Subject: Re: [lwip-users] tcp_tmr - TCP_TMR_INTERVAL
Date: Sat, 2 Sep 2006 09:35:57 +0100
User-agent: Mutt/1.5.11

On Fri, Sep 01, 2006 at 07:18:45PM +0200, Timmy Brolin wrote:
> You are right.
> I'am actually a bit surprised that the timer is running at only 250ms 
> intervals.. Could this be a performance bottleneck?
> Are retransmissions not handled by the timers? That's a minimum 250ms 
> penalty for every retransmission...

For the slow retransmission algorithm, yes.  It's set conservatively,
as per the RFCs to ensure that you've waited for a RTT before
retransmitting.  There are more complex mechanisms to measure the RTT
and adapt the time before retransmission, but as lwIP is supposed to
be lightweight, we haven't implemented those.

Most retransmissions in streaming connections will be done by the fast
retransmission algorithm (retransmit when you see duplicate ACKs from
the other side), and so won't use a timer at all.


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