raw_sendto_if_src() function
Send the raw IP packet to the given address, using a particular outgoing netif and source IP address. An IP header will be prepended to the packet, unless the RAW_FLAGS_HDRINCL flag is set on the PCB. In that case, the packet must include an IP header, which will then be sent as is.
Arguments
pcb
RAW PCB used to send the data
p
chain of pbufs to be sent
dst_ip
destination IP address
netif
the netif used for sending