Section A-2. ICMP Message Types

A-2. ICMP Message Types

Internet Control Message Protocol (ICMP) is used to transport error or control messages between routers and other devices. An ICMP message is encapsulated as the payload in an IP packet, as shown in Figure A-2. This information appears immediately following the IP header. Many of the ICMP message types also have a code number that can be used. The code field further specifies how the message type should be applied when it is received. Cisco firewalls can't use the code field in access lists, so that information is not presented here.

Figure A-2. ICMP Message Format

Notice that in the case of an error condition, the first 8 bytes (64 bits) of the original datagram causing the error are included in the ICMP message. This provides the protocol and port numbers of the original message to be seen, making troubleshooting easier.

ICMP message type codes are registered with the IANA and can be found at http://www.iana.org/numbers.htm.

Table A-2 is a complete list of ICMP message types. It is reproduced with permission from the IANA. The Cisco firewall keywords were added to this list for quick reference.

Table A-3. Well-Known ICMP Message Types

ICMP MessageType	Firewall Keyword	ICMP Message Description
0	echo-reply	Echo reply
1		Reserved
2		Reserved
3	unreachable	Destination unreachable
4	source-quench	Source quench
5	redirect	Redirect
6	alternate-address	Alternate host address
7		Unassigned
8	echo	Echo request
9	router-advertisement	Router advertisement
10	router-solicitation	Router solicitation
11	time-exceeded	Time exceeded
12	parameter-problem	Parameter problem
13	timestamp-request	Timestamp request
14	timestamp-reply	Timestamp reply
15	information-request	Information request
16	information-reply	Information reply
17	mask-request	Address mask request
18	mask-reply	Address mask reply
19		Reserved (for security)
2029		Reserved (for robustness experiment)
30	traceroute	Traceroute
31	conversion-error	Conversion error
32	mobile-redirect	Mobile host redirect
33		IPv6 Where-Are-You
34		IPv6 I-Am-Here
35		Mobile registration request
36		Mobile registration reply
37		Domain name request
38		Domain name reply
39		SKIP algorithm discovery protocol
40		Photuris, security failures
41255		Reserved