Re: [lwip-users] tcp_sndbuf(pcb) become zero with HUB

[Jonathan Larmour]

bandu wrote:
> Hi,
> 
> I'm a bit new to lwip.
> 
> When I transfer data both side with network switch, it work fine.
> 
> When I'm using hub, I can see avialable tcp_sndbuf going down to zero and
> never coming back up to TCP_SND_BUF size.
> 
> I'm checking tcp_sndbuf before tcp_write and I called tcp_output()
> 
> When both TX and RX, I can see it happened more quickly than RX only.
> 
> What would cause that tcp_sndbuf draining? i'm using lwip 1.3.0.

Using a hub will make no difference to lwIP itself. If there's any problem,
it's at a lower layer. Are you sure the link is actually reliable? Are
packets getting through? Are there retries? That's the most likely reason
the send buffer is getting smaller.

The only way a hub might affect what lwip sees at all is if your low-level
ethernet driver is setting the ethernet MAC to promiscuous mode. It
shouldn't be. Although even then, lwIP should happily be dropping the
packets - I wouldn't expect it to cause the problem you see. If your driver
was using promiscuous mode, all I would expect is that your software has to
do more processing before discarding the packet.

Jifl
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