struct icmphdr
{
  u_int8_t type;		/* message type */
  u_int8_t code;		/* type sub-code */
  u_int16_t checksum;
  union
  {
    struct
    {
      u_int16_t	id;
      u_int16_t	sequence;
    } echo;			/* echo datagram */
    u_int32_t	gateway;	/* gateway address */
    struct
    {
      u_int16_t	__unused;
      u_int16_t	mtu;
    } frag;			/* path mtu discovery */
  } un;
};

#define ICMP_ECHOREPLY		0	/* Echo Reply			*/
#define ICMP_DEST_UNREACH	3	/* Destination Unreachable	*/
#define ICMP_SOURCE_QUENCH	4	/* Source Quench		*/
#define ICMP_REDIRECT		5	/* Redirect (change route)	*/
#define ICMP_ECHO		8	/* Echo Request			*/
#define ICMP_TIME_EXCEEDED	11	/* Time Exceeded		*/
#define ICMP_PARAMETERPROB	12	/* Parameter Problem		*/
#define ICMP_TIMESTAMP		13	/* Timestamp Request		*/
#define ICMP_TIMESTAMPREPLY	14	/* Timestamp Reply		*/
#define ICMP_INFO_REQUEST	15	/* Information Request		*/
#define ICMP_INFO_REPLY		16	/* Information Reply		*/
#define ICMP_ADDRESS		17	/* Address Mask Request		*/
#define ICMP_ADDRESSREPLY	18	/* Address Mask Reply		*/
#define NR_ICMP_TYPES		18


/* Codes for UNREACH. */
#define ICMP_NET_UNREACH	0	/* Network Unreachable		*/
#define ICMP_HOST_UNREACH	1	/* Host Unreachable		*/
#define ICMP_PROT_UNREACH	2	/* Protocol Unreachable		*/
#define ICMP_PORT_UNREACH	3	/* Port Unreachable		*/
#define ICMP_FRAG_NEEDED	4	/* Fragmentation Needed/DF set	*/
#define ICMP_SR_FAILED		5	/* Source Route failed		*/
#define ICMP_NET_UNKNOWN	6
#define ICMP_HOST_UNKNOWN	7
#define ICMP_HOST_ISOLATED	8
#define ICMP_NET_ANO		9
#define ICMP_HOST_ANO		10
#define ICMP_NET_UNR_TOS	11
#define ICMP_HOST_UNR_TOS	12
#define ICMP_PKT_FILTERED	13	/* Packet filtered */
#define ICMP_PREC_VIOLATION	14	/* Precedence violation */
#define ICMP_PREC_CUTOFF	15	/* Precedence cut off */
#define NR_ICMP_UNREACH		15	/* instead of hardcoding immediate value */

/* Codes for REDIRECT. */
#define ICMP_REDIR_NET		0	/* Redirect Net			*/
#define ICMP_REDIR_HOST		1	/* Redirect Host		*/
#define ICMP_REDIR_NETTOS	2	/* Redirect Net for TOS		*/
#define ICMP_REDIR_HOSTTOS	3	/* Redirect Host for TOS	*/

/* Codes for TIME_EXCEEDED. */
#define ICMP_EXC_TTL		0	/* TTL count exceeded		*/
#define ICMP_EXC_FRAGTIME	1	/* Fragment Reass time exceeded	*/

