Most TCP/IP implementations support the Ping server directly in the kernel( the server is not a user process. )
As with other ICMP query messages, the server must echo the identifier and sequence number fields. Also, any optional data sent by the client must be echoed. These are presumably of interest to the client.
Unix implementations of ping set the identifier field in the ICMP message to the process ID of the sending process. This allows ping to identify the returned responses if there are multiple instances of ping running at the same time on the same host.
The sequence number starts at 0 and is incremented every time a new echo request is sent. ping prints the sequence number of each returned packet, allowing us to see if packets are missing, reordered, or duplicated. IP is a best effort datagram delivery service, so any of these three conditions can occur.