ICMPv6

Internet Control Message Protocol version 6 (ICMPv6) is the implementation of the (ICMP) for (IPv6). ICMPv6 is defined in RFC 4443. ICMPv6 is an integral part of IPv6 and performs error reporting and diagnostic functions (e.g., ping), and has a framework for extensions to implement future changes.

Several extensions have been published, defining new ICMPv6 message types as well as new options for existing ICMPv6 message types. (NDP) is a node discovery protocol in IPv6 which replaces and enhances functions of .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 (IGMP) is used in IPv4. Multicast Router Discovery (MRD) allows discovery of multicast routers.

ICMPv6 messages may be classified into two categories: error messages and information messages. ICMPv6 messages are transported by IPv6 packets in which the IPv6 Next Header value for ICMPv6 is set to 58.

The ICMPv6 packet 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.

There are two subtypes of Multicast Listener Query messages:

These two subtypes are differentiated by the contents of the Multicast Address field, as described in section 3.6 of RFC 2710

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.

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 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. Note that this differs from the way it is calculated for IPv4 in , but is similar to the calculation done in .

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