Re: [lwip-users] transfer stucks when snd_wnd is less than TCP_MSS

[Kieran Mansley]

On Fri, 2007-03-09 at 11:59 +0200, Vlad wrote:
> I forgot to say - lwip are in vmware. I.e. network structure are following:
> 
> lwip (in vmware) <--- 1gb virtual vmware network ----> virtual vmware hub 
> <-vmware network driver-> 192.168.0.2 (my real machine) <---- 1gb 
> ethernet ----> linux router <----128k adsl ----> internet <----> linux server

I think you win a prize for most complex network setup using lwIP!

> seems that vmware hub responses very fast with ack-s before packets really 
> have sent.

If anything other than the remote end is sending ACKs for the packets
you send, it's a seriously dodgy piece of kit and breaks TCP
assumptions.

> at "hang" moment vmware hub sent acks for near 1000000 bytes (within 1-2 
> seconds ) sent (sent window starts with big value - near 60000 and decreases 
> down to small value).
> but at real eth interface (on 192.168.0.2 - real machine with vmware 
> installed) i see ack's only for 80-90kb. and send window size doesn't grow 
> more than 137 bytes.

If something closes the window, and then doesn't later advertise more
window space, as you describe, it's broken.  

> BUT, windows tcp/ip stack works fine in same virtual machine. seems that 
> problem in lwip.

lwIP is doing the right thing.  The only thing we could do differently
is fragment and send one more packet (which would be less than one MSS),
but then we'd be in the same situation as before but with no available
window rather than almost no available window, and still waiting for
more space.  The connection can't progress until lwIP gets a window
update from the remote end, so it is not lwIP that is at fault.

Kieran