Re: [lwip-users] out of pbuf's

[Rishi Khan]

I have noticed that telnet sends 100 bytes at a time. Maybe this has  
to do with it. You should use pcap and wireshark to look at the  
packet trace.

Rishi

On Jul 3, 2008, at 10:02 AM, Jonathan Larmour wrote:

> Mark Six wrote:
> > For each incoming ethernet packet, one or more pbuf's are  
> > allocated. The
> > number of pbuf's available is defined by the PBUF_POOL_SIZE.
> > If there are no more pbuf's available (e.g. all pbuf's are  
> > allocated) the
> > incoming ethernet packet is dropped. The TCP/IP stack handles the  
> > incoming
> > packets and will free them. If the packet is a data-packet, the
> > application layer is responsible for freeing the packet.
> > However if the packets come out of sequence, the pbuf is *NOT*  
> > free'd, but
> > placed on the oos queue. If the out-of-sequence packets are equal  
> > to the
> > PBUF_POOL_SIZE the lwip-stack is in a deadlock for some time  
> > (until some
> > timer times out and frees the oos queue).
> > To limit the number of out-of-sequence packets one could adjust the
> > receive window size (TCP_WND), however it is possible that one  
> > packet can
> > have only 1 byte of data, so the value for the receive window can  
> > be no
> > more than the available pbuf's divided by the maximum number of TCP
> > connections.
>
> Strictly yes, although in practice you can usually raise it a  
> little above
> that, as statistically you don't always get the worst case!
>
> If you usually expect smaller packets, then you should use a smaller
> PBUF_POOL_BUFSIZE, and then fewer will be used, and they will be  
> chained if
> more total space is needed for a larger packet. What's most efficient
> depends on your expected traffic pattern.
>
> But if your hypothesis is correct, then turning off TCP_QUEUE_OOSEQ  
> may be
> a good idea. You can't expect to be able to use every nice feature  
> when you
> have limited memory.
>
> > This is a problem, because we need a lot of memory or a very small  
> > receive
> > window.
>
> Indeed.
>
> > Any comments on the above are welcome..
>
> It might be nice to have an lwIP option tuning guide at some point  
> to cover
> all this.
>
> Jifl
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