Maybe this is a silly question, but if I create a socket and send an ICMP echo request with sendto(), I can receive the echo reply with recvfrom(). How does the underlying network system know that the incoming reply is associated with my socket? In other words, why do I receive my echo reply with recvfrom() and not other echo replies to other programs that are also sending echo requests? Is there some kind of ID field in some protocol header somewhere that the system has associated with my socket?