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Re: [lwip-users] Why the delay after ARP?

From: Simon Goldschmidt
Subject: Re: [lwip-users] Why the delay after ARP?
Date: Tue, 13 Oct 2009 09:32:08 +0200

> I am using lwIP 1.3.0 and it works great, but I've always wondered where
> there's an approx. three second delay after the ARP response before lwIP
> sends the SYN.  In other words:
> 1. I initiate a TCP connection
> 2. lwIP sends an ARP request almost immediately
> 3. The host responds (relatively) immediately
> 4. lwIP waits about three seconds before sending the SYN
> When the MAC is in the ARP cache, this doesn't happen, but still, what is
> it doing during that time?  Incidentally, I've noticed uIP does the same. 

I guess the three seconds delay is a TCP timeout, i.e. TCP re-sends the SYN 
after three seconds. This is because the initial packet gets dumped because the 
target MAC is unknown.

In contrast to uIP, lwIP has a feature called ARP queueing: if a target MAC is 
unknown, packets are not thrown away but held back until the ARP response 
arrives. This must not beturned off via the corresponding define (ARP_QUEUEING, 
MEMP_NUM_ARP_QUEUE defines how many packets may be queued) in your lwipopts.h!

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