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Re: [lwip-users] Re transmission behaviour?

From: Pomeroy, Marty
Subject: Re: [lwip-users] Re transmission behaviour?
Date: Mon, 2 Jul 2012 08:19:26 -0400

> The TCP_TMR_INTERVAL is the time the stack should wait for an ACK from
my master, right? 

Yes, before it re-transmits.

> What is akward to me is that after the retransmission of LWIP which
happends in 30-40ms after the first response, the ACK for the response
arrives 1ms after the retransmission.

That may not be wrong.  If the master saw the first transmission, but
was delaying it's ACK, then it sees another packet, it may immediately
send an ACK.

> Could it be that the sys_timeout function is not working properly of
my OS which is ecos?

It's not that sys_timeout function is incorrect, but that your system
seems to think that about 150 microseconds is really 1 millisecond.  So
your ecos timer *configuration* is probably incorrect, and this looks
like the right place to look for an issue.

Do NOT raise TCP_TMR_INTERVAL to a large number.  The 250ms is good,
since many systems use ACK delays of about 200ms.  So if lwIP does not
see the ACK in 250, it should retransmit.  And once ecos is configured
correctly, that may work well for you, depending on the delay from your

However to test the theory that the ecos timer is an issue and to see if
the Delayed ACK from the master is arriving after maybe 200ms or 500ms,
you could temporarily set TCP_TMR_INTERVAL to higher numbers, and your
cycle time to greater than 500ms.  Then based on how long the Master
delays, you may choose to raise TCP_TMR_INTERVAL to something maybe 50ms
above that delay time (and, of course, get the ecos timer configuration


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