ICMP is an integral part of any IP implementation. ICMP is normally used to communicate error messages between IP nodes (both routers and endhosts), but it is occasionally used by user-level applications such as traceroute. If you are not familiar with ICMP, you should consult RFC 792 [2] and/or Stevens' TCP/IP Illustrated, Volume 1 [3].
An ICMP message can be either a query message or an error message, and it has a type field and a code field. To send an ICMP message within the kernel, call icmp_send() (defined in $PDIR/include/icmp.h) and pass it the packet causing the error condition and the desired type and code.
There is also a mechanism for a user-level process to read ICMP packets received by a host. In a real UNIX socket implementation, a process opens a RAW socket to receive ICMP packets. In the simulator, there is a new socket type SOCK_ICMP, which is defined in $PDIR/include/systm.h. You can open an ICMP socket as follows:
The ICMP socket implementation also supports the Recvfrom() and Close() socket calls. Note that the ICMP header of a received packet is not stripped by the kernel so that a user-level process can access the ICMP header to see the error type and code, among other things.
Another important issue is that there is no need to bind an ICMP socket to a particular IP address: the ICMP socket will get ICMP messages received by any of the host's interface(s) (similar to the use of INADDR_ANY). As a result, you should make sure that at most one ICMP socket is opened at any given time. If an ICMP packet arrives at a host, and no ICMP socket is opened, the host drops the packet.