The TCP/IP Guide - ICMP Message Classes, Types and Codes

A complete section describing all of the major ICMP message types for both ICMPv4 and ICMPv6 has been included in this Guide. For convenience, I have summarized all these message types in Table 86, which shows each of the Type values for the messages covered in this Guide, the name of each message, a very brief summary of its purpose, and the RFC that defines it. (To keep the table from being egregiously large I have not shown each of the Code values for each Type value; these can be found in the individual message type descriptions.) The table is organized into sections in the same way as the ICMP Message Types and Formats section, except this table is sorted by ascending Type value within each category, for easier reference.

**Table 86: ICMP Message Classes, Types and Codes**
Message Class	Type Value	Message Name	Summary Description of Message Type	Defining RFC Number
ICMPv4 Error Messages	3	*Destination Unreachable*	Indicates that a datagram could not be delivered to its destination. The Code value provides more information on the nature of the error.	792
	4	*Source Quench*	Lets a congested IP device tell a device that is sending it datagrams to slow down the rate at which it is sending them.	792
	5	*Redirect*	Allows a router to inform a host of a better route to use for sending datagrams.	792
	11	*Time Exceeded*	Sent when a datagram has been discarded prior to delivery due to expiration of its Time To Live field.	792
	12	*Parameter Problem*	Indicates a miscellaneous problem (specified by the Code value) in delivering a datagram.	792
ICMPv4 Informational Messages (part 1 of 2)	0	*Echo Reply*	Sent in reply to an Echo (Request) message; used for testing connectivity.	792
	8	*Echo (Request)*	Sent by a device to test connectivity to another device on the internetwork. The word “Request” sometimes appears in the message name.	792
	9	*Router Advertisement*	Used by routers to tell hosts of their existence and capabilities.	1256
	10	*Router Solicitation*	Used by hosts to prompt any listening routers to send a Router Advertisement.	1256
	13	*Timestamp (Request)*	Sent by a device to request that another send it a timestamp value for propagation time calculation and clock synchronization. The word “Request” sometimes appear in the message name.	792
	14	*Timestamp Reply*	Sent in response to a Timestamp (Request) to provide time calculation and clock synchronization information.	792
	15	*Information Request*	Originally used to request configuration information from another device. Now obsolete.	792
ICMPv4 Informational Messages (part 2 of 2)	16	*Information Reply*	Originally used to provide configuration information in response to an Information Request message. Now obsolete.	792
	17	*Address Mask Request*	Used to request that a device send a subnet mask.	950
	18	*Address Mask Reply*	Contains a subnet mask sent in reply to an Address Mask Request.	950
	30	*Traceroute*	Used to implement the experimental “enhanced” traceroute utility.	1393
ICMPv6 Error Messages	1	*Destination Unreachable*	Indicates that a datagram could not be delivered to its destination. Code value provides more information on the nature of the error.	2463
	2	*Packet Too Big*	Sent when a datagram cannot be forwarded because it is too big for the maximum transmission unit (MTU) of the next hop in the route. This message is needed in IPv6 and not IPv4 because in IPv4, routers can fragment oversized messages, while in IPv6 they cannot.	2463
	3	*Time Exceeded*	Sent when a datagram has been discarded prior to delivery due to the Hop Limit field being reduced to zero.	2463
	4	*Parameter Problem*	Indicates a miscellaneous problem (specified by the Code value) in delivering a datagram.	2463
ICMPv6 Informational Messages	128	*Echo Request*	Sent by a device to test connectivity to another device on the internetwork.	2463
	129	*Echo Reply*	Sent in reply to an Echo (Request) message; used for testing connectivity.	2463
	133	*Router Solicitation*	Prompts a router to send a Router Advertisement.	2461
	134	*Router Advertisement*	Sent by routers to tell hosts on the local network the router exists and describe its capabilities.	2461
	135	*Neighbor Solicitation*	Sent by a device to request the layer two address of another device while providing its own as well.	2461
	136	*Neighbor Advertisement*	Provides information about a host to other devices on the network.	2461
	137	*Redirect*	Redirects transmissions from a host to either an immediate neighbor on the network or a router.	2461
	138	*Router Renumbering*	Conveys renumbering information for router renumbering.	2894

You can see that several of the message types are quite similar in ICMPv4 and ICMPv6, but there are some slight differences. An obvious one is that Redirect is considered an error message in ICMPv4, but an informational message in ICMPv6. The way that the messages is used also often different. In IPv6, the use of many of the ICMP informational messages is described in the Neighbor Discovery (ND) protocol, which is new to IPv6.

Note that the Information Request and Information Reply messages were originally created to allow devices to determine an IP address and possibly other configuration information. This function was later implemented using protocols such as RARP, BOOTP and DHCP, and these message types obsoleted.

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