[lwip-users] buffering outgoing packets

[Tyrel Newton]

I'm working on modifying and optimizing the XEmaclite network interface 
driver provided with the Xilinx/MicroBlaze development tools. The driver 
queues outgoing packets if the packet cannot be sent immediately. This 
makes perfect sense to me. However, a comment in the output function reads:

    /*
     * If we cannot send the packet immediately, then make a copy of 
the whole packet
     * into a separate pbuf and store it in the send queue. We cannot 
enqueue the pbuf
     * as is since parts of the pbuf may be modified inside lwIP.
     */

I'm wondering if there is any truth to this statement. Do outgoing 
packets that are queued at the network interface driver level
really need to be copied verbatim into a separate buffer? This seems 
counter-intuitive to lwIP's low-memory usage design goals.

Thanks,
Tyrel