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

[FreeRTOS Info]

Simon Goldschmidt wrote:
> > 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!
>
> Simon


I've never been sure why it is done like that.  I normally change it to 
store the MAC address or the ARP request origin as it comes in, then you 
can send the request out immediately.


Regards,
Richard.

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