The data received in the echo message must be returned in the echo reply message.
The identifier and sequence number may be used by the echo sender to aid in matching the replies with the echo requests. For example, the identifier might be used like a port in TCP or UDP to identify a session, and the sequence number might be incremented on each echo request sent. The echoer returns these same values in the echo reply.
/* * pinger -- * Compose and transmit an ICMP ECHO REQUEST packet. The IP packet * will be added on by the kernel. The ID field is our UNIX process ID, * and the sequence number is an ascending integer. The first 8 bytes * of the data portion are used to hold a UNIX "timeval" struct in VAX * byte-order, to compute the round-trip time. */ intping4_send_probe(socket_st *sock, void *packet, unsigned packet_size) { structicmphdr *icp; int cc; int i;