Re: [lwip-users] protocol issue

[Sylvain Rochet]

Hi,

On Thu, Jul 21, 2016 at 02:11:23PM +0200, Laurent BIERGE wrote:
> Hi again,
> 
> I've played with LWIP_DEPUG a bit and i'm now able to see what's happening.
> Reminder: i try to use a LWIP UDP echo program which is on a remote 
> equipment. I send UDP datagrams from my computer.
>           UDP datagrams don't seem to reach the remote equipment 
>           Actually i don't have all the rights on my computer.
> 
> So what's exactly happening is that when i send a UDP datagrams from 
> my computer to the equipment, my computer send ARP request which are 
> answered by LWIP correctly. The ARP cache on my computer is filled 
> well after. It seems that even after that my computer want's to fill 
> Netbios name service table. So my computer send UDP datagrams on port 
> 137 and the table is filled well after. What's strange is that the ARP 
> request and Netbios request are sent 3 times in a row each time. 
> Besides when i try to send another datagram the computer send 
> ARP/Netbios request again (even if it's just one second after).
> 
> I don't know the ARP/Netbios routine at all. But my guess is that 
> since i don't have all the rights on the computer, it try to fill his 
> ARP/Netbios table but with no success. And sometimes it works 
> (bypassing security for an instant) and the computer send all the 
> datagrams at once.

No, there is no such thing as "bypassing security for a while".


> So i know for sure that it doesn't concern LWIP now, but maybe you 
> have an idea of what i'm facing ?

This is expected behavior, most IP stack are queueing IP packets for a
while until ARP resolution is completed. At least to prevent flooding
the LAN with broadcast packets.

But, on a LAN, ARP resolution should be blazingly fast and you shouldn't 
notice it at all, if it takes more than two RTT on your network, then 
there is probably an issue with your low level driver.


Sylvain

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