****************

  ICMP - Internet Control Message Protocol.

  ICMP is an alterntaive packet structure at the IP level protocol. It is 
    primarily used for message and error communication between nodes.
    * Although it is often listed as part of the TCP level protocol.
   
  Consists of a set of predefined flags and some additional data.
  
  Classified as stateless - sender does not look for confirmation.

  Not sent by application, but used by the OS and routers, i.e  processes 
    handling TCP/IP. OS responsible for delivering error to application.
  
  Type 8-bit

  Code 8-bit
     
     Type and code used together to specify what is being done.
     Could be an error
     Could be a request for a service.
     Could be ping
     Could be a probe to find if a route is available.
     Could be an 'advertisement' indicating a route is available.
       (used by new device on network to id the Gateway).

  Checksum 16-bit

  Rest of header 24-bit.    
    Additional info depending on Type/Code fields.

  Data - ICMP includes a data sector that may contain the entire header
    of the IPv4 packet that triggered the error and 1st 8 bytes of its
    data section.