* ICMP messages are transmitted within IP datagrams

  ICMP messages encapsulated within an IP datagram

  ICMP message

* ICMP message types

  Here we will show you a incomplete list of permitted ICMP control messages

Type	Code	Description
0 – Echo Reply^[4]	0	Echo reply (used to ping)
1 and 2		Reserved
3 – Destination Unreachable^[5]	0	Destination network unreachable
	1	Destination host unreachable
	2	Destination protocol unreachable
	3	Destination port unreachable
	4	Fragmentation required, and DF flag set
	5	Source route failed
	6	Destination network unknown
	7	Destination host unknown
	8	Source host isolated
	9	Network administratively prohibited
	10	Host administratively prohibited
	11	Network unreachable for TOS
	12	Host unreachable for TOS
	13	Communication administratively prohibited
4 – Source Quench	0	Source quench (congestion control)
5 – Redirect Message	0	Redirect Datagram for the Network
	1	Redirect Datagram for the Host
	2	Redirect Datagram for the TOS & network
	3	Redirect Datagram for the TOS & host
6		Alternate Host Address
7		Reserved
8 – Echo Request	0	Echo request
9 – Router Advertisement	0	Router Advertisement
10 – Router Solicitation	0	Router discovery/selection/solicitation
11 – Time Exceeded^[6]	0	TTL expired in transit
	1	Fragment reassembly time exceeded
12 – Parameter Problem: Bad IP header	0	Pointer indicates the error
	1	Missing a required option
	2	Bad length
13 – Timestamp	0	Timestamp
14 – Timestamp Reply	0	Timestamp reply
15 – Information Request	0	Information Request
16 – Information Reply	0	Information Reply
17 – Address Mask Request	0	Address Mask Request
18 – Address Mask Reply	0	Address Mask Reply
19		Reserved for security
20 through 29		Reserved for robustness experiment
30 – Traceroute	0	Information Request
31		Datagram Conversion Error
32		Mobile Host Redirect
33		Where-Are-You (originally meant for IPv6)
34		Here-I-Am (originally meant for IPv6)
35		Mobile Registration Request
36		Mobile Registration Reply
37		Domain Name Request
38		Domain Name Reply
39		SKIP Algorithm Discovery Protocol, Simple Key-Management for Internet Protocol
40		Photuris, Security failures
41		ICMP for experimental mobility protocols such as Seamoby [RFC4065]
42 through 255		Reserved

  list of permitted control messages

  When an ICMP error message is sent, the message always contains the IP header and the first 8 bytes of the IP datagram that caused the ICMP error to be generated. This lets the receiving ICMP module associate the message with one particular protocol(TCP or UDP from the protocol field in the IP header) and one particular user process(from the TCP or UDP port numbers that are in the TCP or UDP header contained in the first 8 bytes of the IP datagram).

  An ICMP error message is never generated in response to 

  1)an icmp error message

  2)a datagram destined to an ip broadcast address or an ip multicast address

  3)a datagram sent as a link-layer broadcast

  4)a fragment other than the first

  5)a datagram whose source address does not define a single host.(means:the source address can not be 0, a loopback address, a broadcast address, a multicast address).

These rules are meant to prevent the broadcast storms that have occurred in the past when ICMP errors were sent in response to broadcast packets.

* ICMP address mask request request and reply

  the ICMP address mask request is intended for a diskless system to obtain its subnet mask at bootstrap time(the same as a diskless system using RARP to obtain its IP address at bootstrap time). An alternative method is the BOOTP protocol.

  ICMP address mask request and reply messages

  you can get more through here

http://onlamp.com/pub/a/bsd/2001/04/04/FreeBSD_Basics.html