The rcvid returned from IP_Receive in msginfo->rcvid.
The message buffer to be transmitted.
The length of the message buffer.
This function replies to a previously received IP_RAW message by using the rcvid directly. This is sometimes simpler than using the IP_MsgInfo structure, as the rcvid is a single integer that can be easily passed to other functions. len bytes of msg are transmitted as the reply message.