Really need some help with RTEMS networking semaphores

[Ian Caddy]

Hi Greg,


gregory.menke at gsfc.nasa.gov wrote:
> 
> I don't yet know to what extent mbufs are allocated without clusters,
> but since at least the receive packets use mbufs with clusters, sizing
> the cluster buffer so each mbuf can get one seems at least arguably
> useful.

For transmission, it depends on the size of the packet being sent as to 
whether or not a single mbuf is used or a cluster is required.  From 
memory, if the packet is smaller than 208 bytes, a single mbuf (or two) 
will be used.  Larger than that, and a cluster is allocated.

We had a problem sometime in the past, with a legacy application running 
on a PC, that could not handle fragmented packets coming in on TCP.  We 
found that if we tried to send a packet between 100 bytes (MLEN) and 208 
bytes (MCLBYTES) then the packet is broken into 2 mbufs and sent 
seperately.  The PC that it was connected to was very fast, but the 
application very old, and it would see the first half of the packet and 
throw it away as it wasn't complete, then the other half would arrive, 
which wasn't a valid message and would also be thrown away.

This caused us some interesting times and that is why I know about the 
sending of TCP packets ... ;-)

regards,

Ian Caddy


-- 
Ian Caddy
Goanna Technologies Pty Ltd
+61 8 9221 1860