Internet Control Message Protocol for IPv6
Communication protocol
General structure of ICMPv6 Messages
AbbreviationICMPv6
PurposeAuxiliary Protocol for IPv6
IntroductionDecember 1995 (1995-12)
OSI layerNetwork layer
RFC(s)RFC 4443

Internet Control Message Protocol version 6 (ICMPv6) is the implementation of the Internet Control Message Protocol (ICMP) for Internet Protocol version 6 (IPv6).[1] 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.[2] 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 58.

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). type specifies the type of the message. Values in the range from 0 to 127 (high-order bit is 0) indicate an error message, while values in the range from 128 to 255 (high-order bit is 1) indicate an information message. The code field value depends on the message type and provides an additional level of message granularity. The checksum field provides a minimal level of integrity verification for the ICMP message.

ICMPv6 packet
Bit offset0–78–1516–31
0TypeCodeChecksum
32Message body

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
ValueMeaningValueMeaning
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
2Packet too big0
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
100Private experimentation
101Private experimentation
127Reserved for expansion of ICMPv6 error messages
ICMPv6 Informational Messages
128Echo Request0
129Echo Reply0
130Multicast 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

131Multicast Listener Report (MLD)0
132Multicast Listener Done (MLD)0
133Router Solicitation (NDP)0
134Router Advertisement (NDP)0
135Neighbor Solicitation (NDP)0
136Neighbor Advertisement (NDP)0
137Redirect Message (NDP)0
138 Router Renumbering[3] 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.
141Inverse Neighbor Discovery Solicitation Message0
142Inverse Neighbor Discovery Advertisement Message0
143Multicast Listener Discovery (MLDv2) reports[4]
144Home Agent Address Discovery Request Message0
145Home Agent Address Discovery Reply Message0
146Mobile Prefix Solicitation0
147Mobile Prefix Advertisement0
148Certification Path Solicitation (SEND)
149Certification Path Advertisement (SEND)
151Multicast Router Advertisement (MRD)
152Multicast Router Solicitation (MRD)
153Multicast Router Termination (MRD)
155RPL Control Message
200Private experimentation
201Private experimentation
255Reserved 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,[5] 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 58. 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.[6] Note that this differs from the way it is calculated for IPv4 in ICMP, but is similar to the calculation done in TCP.

ICMPv6 pseudo-header
Bit offset 0 – 7 8–15 16–23 24–31
0 Source address
32
64
96
128 Destination address
160
192
224
256 ICMPv6 length
288 Zeros Next header

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 offset0–78–1516–31
01CodeChecksum
32Unused
64Message body (Variable Size)
Time Exceeded
Bit offset0–78–1516–31
03CodeChecksum
32Unused
64Message 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 offset0–78–1516–31
020Checksum
32MTU
64Message body (Variable Size)
Parameter Problem
Bit offset0–78–1516–31
04CodeChecksum
32Pointer
64Message body (Variable Size)
Echo Request
Bit offset0–78–1516–31
01280Checksum
32IdentifierSequence Number
64Data (Variable Size)
Echo Reply
Bit offset0–78–1516–31
01290Checksum
32IdentifierSequence Number
64Data (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 offset0–78–1516–31
01330Checksum
32Reserved
64Options (Variable Size)
Router Advertisement
Bit offset0–78–1516–31
01340Checksum
32Cur Hop LimitManaged Address FlagOther Configuration FlagReserveredRouter Lifetime
64Reachable Time
96Retrans Time
128Options (Variable Size)
Neighbor Solicitation
Bit offset0–78–1516–31
01350Checksum
32Reserved
64Target Address (16 bytes)
192Options (Variable Size)
Neighbor Advertisement
Bit offset0–78–1516–31
01360Checksum
32From Router (R)Solicited Flag(S)Override(O)Reserved
64Target Address (16 bytes)
192Options (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 offset0–78–1516–31
01370Checksum
32Reserved
64Target Address (16 bytes)
192Destination Address (16 bytes)
320Options (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.

References

  1. ↑ A. Conta; S. Deering (March 2006). M. Gupta (ed.). Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification. Network Working Group. doi:10.17487/RFC4443. STD 89. RFC 4443. Internet Standard. Obsoletes RFC 2463. Updates RFC 2780. Updated by RFC 4884.
  2. ↑ T. Mrugalski; M. Siodelski; B. Volz; A. Yourtchenko; M. Richardson; S. Jiang; T. Lemon; T. Winters (November 2018). Dynamic Host Configuration Protocol for IPv6 (DHCPv6). IETF. doi:10.17487/RFC8415. ISSN 2070-1721. RFC 8415. Proposed Standard. sec. 3. Obsoletes RFC 3315, 3633, 3736, 4242, 7083, 7283 and 7550.
  3. ↑ M. Crawford (August 2000). Router Renumbering for IPv6. Network Working Group. doi:10.17487/RFC2894. RFC 2894. Proposed Standard.
  4. ↑ R. Vida; L. Costa, eds. (June 2004). Multicast Listener Discovery Version 2 (MLDv2) for IPv6. Network Working Group. doi:10.17487/RFC3810. RFC 3810. Proposed Standard. Updates RFC 2710. Updated by RFC 4604.
  5. ↑ S. Deering; R. Hinden (July 2017). Internet Protocol, Version 6 (IPv6) Specification. IETF. doi:10.17487/RFC8200. STD 86. RFC 8200. Internet Standard 86. sec. 8.1. Obsoletes RFC 2460.
  6. ↑ R. Braden; D. Borman; C. Partridge (September 1988). Computing the Internet Checksum. Network Working Group. doi:10.17487/RFC1071. RFC 1071. Informational. Updated by RFC 1141.
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