[lwip-devel] LWIP 130RC1 and Socket - Application receive more packet than it can manage - what does it happen?

[Piero 74]

Hi all.

I'll use in my application socket. I'll use UPD connection and TCP connection for receiving packets.

I have limited buffers in my application, because i have few RAM available.
I saw that setting some lwip options, i can limit packet queueing, pbuf used (i don't know how.. i have to try) and limit memory available for lwip stack.

My question is:

if my application tasks cannot consume all packet received from the network, because has all buffers full, what does it happen? lwip will crash my application? or it will drop udp packet and kill tcp connections?

My emac driver send packet to tcp_ip thread if present.... i could drop packet in the low level side, but driver need to know when drop packet and when pass packet to tcpip thread...
can i know this information in some way???

in my ethernetif implementation (low_level_input), if packet is present, i do this:

 ....
  /* We allocate a pbuf chain of pbufs from the pool. */
  p = pbuf_alloc(PBUF_RAW, len, PBUF_POOL);
  
  if (p != NULL) {
 ....
 } else {
    // Drop packet because no free pbuf available
    endReadFrame(0);
    
    LINK_STATS_INC(link.memerr);
    LINK_STATS_INC(link.drop);
  }
...


I suppose (please CONFIRM):

if lwip has no memory free DOESN'T crash application
BUT pbuf_alloc will reurn NULL until pbufs will free. 
SO, application CAN CONSUME packets slower than network produces them, because low_level_input  will drop some packets.

I saw in lwip code that calls to sys_mbox_trypost avoid lwip blocks if mailboxes are full, and free pbufs (SO... drop packets... IS IT RIGHT???)

again.... if lwip has no memory free DOESN'T crash application ???

Someone can discuss with me about this issues?


thanks,
Piero