View source for ICMPv6

{{Short description|Internet protocol used for error messages in network operations}}
{{Infobox networking protocol
| title        = Internet Control Message Protocol for IPv6
| logo         = 
| logo alt     = 
| image        = ICMP header - General-en.svg
| image alt    = 
| caption      = General structure of ICMPv6 Messages
| is stack     = 
| abbreviation = ICMPv6
| purpose      = Auxiliary Protocol for IPv6
| developer    = 
| date         = {{Start date and age|1995|12|  }}<!--Fill in: Year (4 digits), month and day (2 digits)-->
| based on     = 
| influenced   = 
| osilayer     = [[Network layer]]
| ports        = 
| rfcs         = RFC 4443
| hardware     = 
}}
{{IPstack}}
'''Internet Control Message Protocol version 6''' ('''ICMPv6''') is the implementation of the [[Internet Control Message Protocol]] (ICMP) for Internet Protocol version 6 (IPv6).{{Ref RFC|4443}} 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 [[Address Resolution Protocol|ARP]].{{Ref RFC|8415|section=3}} [[Secure Neighbor Discovery]] (SEND) is an extension of NDP with extra security. [[Multicast Listener Discovery]] (MLD) is used by IPv6 routers for discovering [[Engineering:Multicast|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 packet#Fixed header|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.

{| class="wikitable" style="text-align:center"
|+ICMPv6 packet
|-
! Bit offset !! colspan="8" width="22%" | 0–7 !! colspan="8" width="22%" | 8–15 !! colspan="16" width="44%"| 16–31
|-
| '''0''' || colspan="8"| Type || colspan="8"| Code || colspan="16"| Checksum
|-
| '''32''' || colspan="32"| Message 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.

{| class="wikitable"
! colspan="2" align="center"| Type
! colspan="2" align="center"| Code
|-
! Value !! Meaning !! Value !! Meaning
|-
! colspan="4" align="center"| ICMPv6 Error Messages
|-
|rowspan=8| 1
|rowspan=8| [[Internet Control Message Protocol#Destination unreachable|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 || [[IPv6 packet#Fragmentation|Packet too big]] || 0
|-
|rowspan=2| 3
|rowspan=2| [[Internet Control Message Protocol#Time exceeded|Time exceeded]]
| 0
| hop limit exceeded in transit
|-
| 1
| fragment reassembly time exceeded
|-
|rowspan=3| 4
|rowspan=3| 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 || ||
|-
! colspan="4" align="center"| ICMPv6 Informational Messages
|-
| 128 || Echo Request || 0 ||
|-
| 129 || Echo Reply || 0 ||
|-
| 130 || Multicast Listener Query ([[Multicast Listener Discovery|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 ([[Multicast Listener Discovery|MLD]]) || 0 ||
|-
| 132 || Multicast Listener Done ([[Multicast Listener Discovery|MLD]]) || 0 ||
|-
| 133 || Router Solicitation ([[Neighbor Discovery Protocol|NDP]]) || 0 ||
|-
| 134 || Router Advertisement ([[Neighbor Discovery Protocol|NDP]]) || 0 ||
|-
| 135 || Neighbor Solicitation ([[Neighbor Discovery Protocol|NDP]]) || 0 ||
|-
| 136 || Neighbor Advertisement ([[Neighbor Discovery Protocol|NDP]]) || 0 ||
|-
| 137 || Redirect Message ([[Neighbor Discovery Protocol|NDP]]) || 0 ||
|-
|rowspan=3| 138
|rowspan=3| Router Renumbering{{Ref RFC|2894}}
| 0
| Router Renumbering Command
|-
| 1
| Router Renumbering Result
|-
| 255
| Sequence Number Reset
|-
|rowspan=3| 139
|rowspan=3| 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.
|-
|rowspan=3| 140
|rowspan=3| 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 ([[Multicast Listener Discovery|MLDv2]]) reports{{Ref RFC|3810}} || ||
|-
| 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 ([[Multicast router discovery|MRD]]) || ||
|-
| 152 || Multicast Router Solicitation ([[Multicast router discovery|MRD]]) || ||
|-
| 153 || Multicast Router Termination ([[Multicast router discovery|MRD]]) || ||
|-
| 155 || RPL Control Message || ||
|-
| 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: [https://www.iana.org/assignments/icmpv6-parameters IANA: ICMPv6 Parameters].

===Checksum===
ICMPv6 provides a minimal level of message integrity verification by the inclusion of a 16-bit [[Checksum|checksum]] in its header. The checksum is calculated starting with a pseudo-header of IPv6 header fields according to the IPv6 standard,{{Ref RFC|8200|section=8.1}} 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|ones' complement]] summation, followed by a final ones' complement of the checksum itself and inserting it into the checksum field.{{Ref RFC|1071}} Note that this differs from the way it is calculated for IPv4 in [[Internet Control Message Protocol|ICMP]], but is similar to the calculation done in [[Transmission Control Protocol|TCP]].

{| class="wikitable" style="text-align: center;"
|+ICMPv6 pseudo-header
|-
! Bit offset
! colspan="8" width="22%"| 0 – 7
! colspan="8" width="22%"| 8–15
! colspan="8" width="22%"| 16–23
! colspan="8" width="22%"| 24–31
|-
! 0
| colspan="32" rowspan="4"| Source address
|-
! 32
|-
! 64
|-
! 96
|-
! 128
| colspan="32" rowspan="4"| Destination address
|-
! 160
|-
! 192
|-
! 224
|-
! 256
| colspan="32"| ICMPv6 length
|-
! 288
| colspan="24"| Zeros
| colspan="8"| 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.

<div style="display:inline-table; padding: 0.5em;">

{| class="wikitable" style="text-align:center"
|+Destination Unreachable
|-
! Bit offset !! colspan="8" width="22%" | 0–7 !! colspan="8" width="22%" | 8–15 !! colspan="16" width="44%" | 16–31
|-
| '''0''' || colspan="8" |  1 || colspan="8" | Code || colspan="16" | Checksum
|-
| '''32''' || colspan="32" | Unused
|-
| '''64''' || colspan="32" | Message body (Variable Size)
|}
</div>
<div style="display:inline-table; padding: 0.5em;">
{| class="wikitable" style="text-align:center"
|+
Time Exceeded
|-
! Bit offset !! colspan="8" width="22%" | 0–7 !! colspan="8" width="22%" | 8–15 !! colspan="16" width="44%"| 16–31
|-
| '''0''' || colspan="8"| 3 || colspan="8" | Code || colspan="16"| Checksum
|-
| '''32''' || colspan="32"| Unused
|-
| '''64''' || colspan="32"| Message body (Variable Size)
|}
</div>

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

<div style="display:inline-table; padding: 0.5em;">
{| class="wikitable" style="text-align:center"
|+Packet Too Big
|-
! Bit offset !! colspan="8" width="22%" | 0–7 !! colspan="8" width="22%" | 8–15 !! colspan="16" width="44%"| 16–31
|-
| '''0''' || colspan="8"| 2 || colspan="8" | 0 || colspan="16" | Checksum
|-
| '''32''' || colspan="32"| MTU
|-
| '''64''' || colspan="32"| Message body (Variable Size)
|}
</div>

<div style="display:inline-table; padding: 0.5em;">
{| class="wikitable" style="text-align:center"
|+
Parameter Problem
|-
! Bit offset !! colspan="8" width="22%" | 0–7 !! colspan="8" width="22%" | 8–15 !! colspan="16" width="44%"| 16–31
|-
| '''0''' || colspan="8"| 4 || colspan="8" | Code || colspan="16"| Checksum
|-
| '''32''' || colspan="32" | Pointer
|-
| '''64''' || colspan="32" | Message body (Variable Size)
|}
</div>

<div style="display:inline-table; padding: 0.5em;">
{| class="wikitable" style="text-align:center"
|+
Echo Request
|-
! Bit offset !! colspan="8" width="22%" | 0–7 !! colspan="8" width="22%" | 8–15 !! colspan="16" width="44%"| 16–31
|-
| '''0''' || colspan="8"| 128 || colspan="8" | 0|| colspan="16" | Checksum
|-
| '''32''' || colspan="16"| Identifier || colspan="16 | Sequence Number
|-
| '''64''' || colspan="32"| Data (Variable Size)
|}
</div>
<div style="display:inline-table; padding: 0.5em;">
{| class="wikitable" style="text-align:center"
|+
Echo Reply
|-
! Bit offset !! colspan="8" width="22%" | 0–7 !! colspan="8" width="22%" | 8–15 !! colspan="16" width="44%"| 16–31
|-
| '''0''' || colspan="8"| 129 || colspan="8"| 0 || colspan="16"| Checksum
|-
| '''32''' || colspan="16"| Identifier || colspan="16" | Sequence Number
|-
| '''64''' || colspan="32"| Data (Variable Size)
|}
</div>
In the case of [[Neighbor Discovery Protocol|NDP]] messages the first four bytes are either reserved or used for flags/hoplimit. While the reset of body has unspecified structured data:
<div style="display:inline-table; padding: 0.5em;">
{| class="wikitable" style="text-align:center"
|+
Router Solicitation
|-
! Bit offset !! colspan="8" width="22%" | 0–7 !! colspan="8" width="22%" | 8–15 !! colspan="16" width="44%"| 16–31
|-
| '''0''' || colspan="8"| 133 || colspan="8"| 0 || colspan="16"| Checksum
|-
| '''32''' || colspan="32"| Reserved
|-
| '''64''' || colspan="32"| Options (Variable Size)
|}
</div>
<div style="display:inline-table; padding: 0.5em;">
{| class="wikitable" style="text-align:center"
|+
Router Advertisement
|-
! Bit offset !! colspan="8" width="22%" | 0–7 !! colspan="8" width="22%" | 8–15 !! colspan="16" width="44%"| 16–31
|-
| '''0''' || colspan="8"| 134 || colspan="8"| 0 || colspan="16"| Checksum
|-
| '''32''' || colspan="8"| Cur Hop Limit || colspan="1" | Managed Address Flag || colspan= "1"| Other Configuration Flag || colspan="6" | Reservered || colspan="16" | Router Lifetime
|-
| '''64''' || colspan="32" | Reachable Time
|-
| '''96''' || colspan="32" | Retrans Time
|-
| '''128''' || colspan="32"| Options (Variable Size)
|}
</div>

<div style="display:inline-table; padding: 0.5em;">
{| class="wikitable" style="text-align:center"
|+
Neighbor Solicitation
|-
! Bit offset !! colspan="8" width="22%" | 0–7 !! colspan="8" width="22%" | 8–15 !! colspan="16" width="44%"| 16–31
|-
| '''0''' || colspan="8"| 135 || colspan="8"| 0 || colspan="16"| Checksum
|-
| '''32''' || colspan="32" | Reserved
|-
| '''64''' || colspan="32"| Target Address  (16 bytes)
|-
| '''192''' || colspan="32"| Options (Variable Size)
|}
</div>
<div style="display:inline-table; padding: 0.5em;">
{| class="wikitable" style="text-align:center"
|+
 Neighbor Advertisement
|-
! Bit offset !! colspan="8" width="22%" | 0–7 !! colspan="8" width="22%" | 8–15 !! colspan="16" width="44%"| 16–31
|-
| '''0''' || colspan="8"| 136 || colspan="8"| 0 || colspan="16" | Checksum
|-
| '''32''' || colspan="1" | From Router (R) || colspan="1"| Solicited Flag(S) || Override(O) || colspan="30" | Reserved
|-
| '''64''' || colspan="32" | Target Address (16 bytes)
|-
| '''192''' || colspan="32" | Options (Variable Size)
|}
</div>
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:

<div style="display:inline-table; padding: 0.5em;">
{| class="wikitable" style="text-align:center"
|+
ICMPv6 Redirect
|-
! Bit offset !! colspan="8" width="22%" | 0–7 !! colspan="8" width="22%" | 8–15 !! colspan="16" width="44%"| 16–31
|-
| '''0''' || colspan="8"| 137 || colspan="8"| 0 || colspan="16" | Checksum
|-
| '''32''' || colspan="32" | Reserved
|-
| '''64''' || colspan="32"| Target Address  (16 bytes)
|-
| '''192''' || colspan="32"| Destination Address  (16 bytes)
|-
| '''320''' || colspan="32"| Options (Variable Size)
|}
</div>

== 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 ==
{{Reflist}}

== External links ==
* [https://www.iana.org/assignments/icmpv6-parameters/icmpv6-parameters.xhtml IANA: ICMPv6 Parameters]

{{IPv6}}

[[Category:Internet protocols]]
[[Category:Internet layer protocols]]
[[Category:Network layer protocols]]
[[Category:IPv6]]

{{Sourceattribution|ICMPv6}}