ICMPv6 explained

Internet Control Message Protocol version 6 (ICMPv6) is the implementation of the Internet Control Message Protocol (ICMP) for Internet Protocol version 6 (IPv6). ICMPv6 is an integral part of IPv6 and performs error reporting and diagnostic functions.

ICMPv6 has a framework for extensions to implement new features. Several extensions have been published, defining new ICMPv6 message types as well as new options for existing ICMPv6 message types. For example, Neighbor Discovery Protocol (NDP) is a node discovery protocol based on ICMPv6 which replaces and enhances functions of ARP. Secure Neighbor Discovery (SEND) is an extension of NDP with extra security. Multicast Listener Discovery (MLD) is used by IPv6 routers for discovering multicast listeners on a directly attached link, much like Internet Group Management Protocol (IGMP) is used in IPv4. Multicast Router Discovery (MRD) allows the discovery of multicast routers.

Message types and formats

ICMPv6 messages may be classified as error messages and information messages. ICMPv6 messages are transported by IPv6 packets in which the IPv6 Next Header value for ICMPv6 is set to the value .

The ICMPv6 message consists of a header and the protocol payload. The header contains only three fields: Type (8 bits), Code (8 bits), and Checksum (16 bits).

Types

Control messages are identified by the value in the type field. The code field gives additional context information for the message. Some messages serve the same purpose as the correspondingly named ICMP message types.

Type		Code
Value	Meaning	Value	Meaning
ICMPv6 Error Messages
1	Destination unreachable	0	no route to destination
		1	communication with destination administratively prohibited
		2	beyond scope of source address
		3	address unreachable
		4	port unreachable
		5	source address failed ingress/egress policy
		6	reject route to destination
		7	Error in Source Routing Header
2		0
3	Time exceeded	0	hop limit exceeded in transit
		1	fragment reassembly time exceeded
4	Parameter problem	0	erroneous header field encountered
		1	unrecognized Next Header type encountered
		2	unrecognized IPv6 option encountered
100	Private experimentation
101	Private experimentation
127	Reserved for expansion of ICMPv6 error messages
ICMPv6 Informational Messages
128		0
129		0
130	Multicast Listener Query (MLD)	0	There are two subtypes of Multicast Listener Query messages: General Query, used to learn which multicast addresses have listeners on an attached link. Multicast-Address-Specific Query, used to learn if a particular multicast address has any listeners on an attached link. These two subtypes are differentiated by the contents of the Multicast Address field, as described in section 3.6 of RFC 2710
131	Multicast Listener Report (MLD)	0
132	Multicast Listener Done (MLD)	0
133	Router Solicitation (NDP)	0
134	Router Advertisement (NDP)	0
135	Neighbor Solicitation (NDP)	0
136	Neighbor Advertisement (NDP)	0
137	Redirect Message (NDP)	0
138	Router Renumbering	0	Router Renumbering Command
		1	Router Renumbering Result
		255	Sequence Number Reset
139	ICMP Node Information Query	0	The Data field contains an IPv6 address which is the Subject of this Query.
		1	The Data field contains a name which is the Subject of this Query, or is empty, as in the case of a NOOP.
		2	The Data field contains an IPv4 address which is the Subject of this Query.
140	ICMP Node Information Response	0	A successful reply. The Reply Data field may or may not be empty.
		1	The Responder refuses to supply the answer. The Reply Data field will be empty.
		2	The Qtype of the Query is unknown to the Responder. The Reply Data field will be empty.
141	Inverse Neighbor Discovery Solicitation Message	0
142	Inverse Neighbor Discovery Advertisement Message	0
143	Multicast Listener Discovery (MLDv2) reports
144	Home Agent Address Discovery Request Message	0
145	Home Agent Address Discovery Reply Message	0
146	Mobile Prefix Solicitation	0
147	Mobile Prefix Advertisement	0
148	Certification Path Solicitation (SEND)
149	Certification Path Advertisement (SEND)
151	Multicast Router Advertisement (MRD)
152	Multicast Router Solicitation (MRD)
153	Multicast Router Termination (MRD)
155	RPL Control Message
160	Extended Echo Request	0	Request Extended Echo
161	Extended Echo Reply	0	No Error
		1	Malformed Query
		2	No Such Interface
		3	No Such Table Entry
		4	Multiple Interfaces Satisfy Query
200	Private experimentation
201	Private experimentation
255	Reserved for expansion of ICMPv6 informational messages

Note that the table above is not comprehensive. The current complete list of assigned ICMPv6 types can be found at this link: IANA: ICMPv6 Parameters.

Checksum

ICMPv6 provides a minimal level of message integrity verification by the inclusion of a 16-bit checksum in its header. The checksum is calculated starting with a pseudo-header of IPv6 header fields according to the IPv6 standard, which consists of the source and destination addresses, the packet length and the next header field, the latter of which is set to the value . Following this pseudo header, the checksum is continued with the ICMPv6 message. The checksum computation is performed according to Internet protocol standards using 16-bit ones' complement summation, followed by a final ones' complement of the checksum itself and inserting it into the checksum field. Note that this differs from the way it is calculated for IPv4 in ICMP, but is similar to the calculation done in TCP.

Format

The payload of an ICMPv6 message varies according the type of message being sent. It begins at bit 32 immediately after the header described above. For some messages such as destination unreachable or time exceeded there is no defined message body.

Destination Unreachable

Bit offset	0–7								8–15								16–31
0	1								Code								Checksum
32	Unused
64	Message body (Variable Size)

Time Exceeded

Bit offset	0–7								8–15								16–31
0	3								Code								Checksum
32	Unused
64	Message body (Variable Size)

Others define a use only for the first four bytes of the body with no other defined content:

Packet Too Big

Bit offset	0–7								8–15								16–31
0	2								0								Checksum
32	MTU
64	Message body (Variable Size)

Parameter Problem

Bit offset	0–7								8–15								16–31
0	4								Code								Checksum
32	Pointer
64	Message body (Variable Size)

Echo Request

Bit offset	0–7								8–15								16–31
0	128								0								Checksum
32	Identifier																Sequence Number
64	Data (Variable Size)

Echo Reply

Bit offset	0–7								8–15								16–31
0	129								0								Checksum
32	Identifier																Sequence Number
64	Data (Variable Size)

In the case of NDP messages the first four bytes are either reserved or used for flags/hoplimit. While the reset of body has unspecified structured data:

Router Solicitation

Bit offset	0–7								8–15								16–31
0	133								0								Checksum
32	Reserved
64	Options (Variable Size)

Router Advertisement

Bit offset	0–7								8–15								16–31
0	134								0								Checksum
32	Cur Hop Limit								Managed Address Flag	Other Configuration Flag	Reserved						Router Lifetime
64	Reachable Time
96	Retrans Time
128	Options (Variable Size)

Neighbor Solicitation

Bit offset	0–7								8–15								16–31
0	135								0								Checksum
32	Reserved
64	Target Address (16 bytes)
192	Options (Variable Size)

Neighbor Advertisement

Bit offset	0–7								8–15								16–31
0	136								0								Checksum
32	From Router (R)	Solicited Flag(S)	Override(O)	Reserved
64	Target Address (16 bytes)
192	Options (Variable Size)

For a redirect the first bytes of the message body are reserved but not used. This is followed by a Target and destination address. Unspecified options can be attached to the end:

ICMPv6 Redirect

Bit offset	0–7								8–15								16–31
0	137								0								Checksum
32	Reserved
64	Target Address (16 bytes)
192	Destination Address (16 bytes)
320	Options (Variable Size)

Message processing

When an ICMPv6 node receives a packet, it must undertake actions that depend on the type of message. The ICMPv6 protocol must limit the number of error messages sent to the same destination to avoid network overloading. For example, if a node continues to forward erroneous packets, ICMP will signal the error to the first packet and then do so periodically, with a fixed minimum period or with a fixed network maximum load. An ICMP error message must never be sent in response to another ICMP error message.

External links

• IANA: ICMPv6 Parameters