icmp —
Internet Control Message Protocol
ICMP is the error and control message protocol used by IP and the Internet
  protocol family. It may be accessed through a “raw socket” for
  network monitoring and diagnostic functions. The proto
  parameter to the socket call to create an ICMP socket is obtained from
  getprotobyname(3). ICMP
  sockets are connectionless, and are normally used with the
  sendto(2) and
  recvfrom(2) calls, though the
  connect(2) call may also be
  used to fix the destination for future packets (in which case the
  read(2) or
  recv(2) and
  write(2) or
  send(2) system calls may be used).
Outgoing packets automatically have an IP header prepended to them
    (based on the destination address). Incoming packets are received with the
    IP header and options intact.
A socket operation may fail with one of the following errors returned:
  - [EISCONN]
- when trying to establish a connection on a socket which already has one,
      or when trying to send a datagram with the destination address specified
      and the socket is already connected;
- [ENOTCONN]
- when trying to send a datagram, but no destination address is specified,
      and the socket hasn't been connected;
- [ENOBUFS]
- when the system runs out of memory for an internal data structure;
- [EADDRNOTAVAIL]
- when an attempt is made to create a socket with a network address for
      which no network interface exists.
recv(2),
  send(2),
  inet(4),
  intro(4),
  ip(4)Internet Control Message
    Protocol, RFC, 792,
    September 1981.
Requirements for Internet Hosts
    -- Communication Layers, RFC,
    1122, October
  1989.
The icmp protocol appeared in
  4.3BSD.