CS455 Sylabus

ICMP: assisting in network trouble shooting

Checking for liveliness

Echo request:

ICMP code = 8 is echo request
When a IP node receives an echo request, it must

The ping utility

The ping utility:

ping = a utility that check is a host is alive

Example:

cheung@aruba> ping -c 3 compute.mathcs.emory.edu PING compute.mathcs.emory.edu (170.140.150.8) 56(84) bytes of data. 64 bytes from compute.mathcs.emory.edu (170.140.150.8): icmp_seq=1 ttl=255 time=0.363 ms 64 bytes from compute.mathcs.emory.edu (170.140.150.8): icmp_seq=2 ttl=255 time=0.517 ms 64 bytes from compute.mathcs.emory.edu (170.140.150.8): icmp_seq=3 ttl=255 time=0.412 ms --- compute.mathcs.emory.edu ping statistics --- 3 packets transmitted, 3 received, 0% packet loss, time 2001ms rtt min/avg/max/mdev = 0.363/0.430/0.517/0.068 ms

How is it implemented:

The ping utility transmits an ICMP echo request to the destination node...

Experiment:

Google the source code of the ping utility: click here

Here is one: click here

Search for sendto, you will see:

u_char icmp_type = ICMP_ECHO; .... static void pinger(void) { struct timeval now; struct ip *ip; struct icmp *icp; int cc, i; u_char *packet; /* ============================== Make ICMP packet ============================== */ packet = outpack; icp = (struct icmp *)outpack; icp->icmp_type = icmp_type; icp->icmp_code = 0; icp->icmp_cksum = 0; icp->icmp_seq = htons(ntransmitted); icp->icmp_id = ident; .... icp->icmp_cksum = in_cksum((u_short *)icp, cc); .... i = sendto(s, (char *)packet, cc, 0, (struct sockaddr *)&whereto, sizeof(whereto)); .... }