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2. ICMPv6 (ICMP for IPv6)

ICMPv6 is used by IPv6 nodes to report errors encountered in processing packets, and to perform other internet-layer functions, such as diagnostics (ICMPv6 "ping"). ICMPv6 is an integral part of IPv6, and the base protocol (all the messages and behavior required by this specification) MUST be fully implemented by every IPv6 node.

2.1. Message General Format​

Every ICMPv6 message is preceded by an IPv6 header and zero or more IPv6 extension headers. The ICMPv6 header is identified by a Next Header value of 58 in the immediately preceding header. (This is different from the value used to identify ICMP for IPv4.)

The ICMPv6 messages have the following general format:

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |     Code      |          Checksum             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +                         Message Body                          +
   |                                                               |

The type field indicates the type of the message. Its value determines the format of the remaining data.

The code field depends on the message type. It is used to create an additional level of message granularity.

The checksum field is used to detect data corruption in the ICMPv6 message and parts of the IPv6 header.

ICMPv6 messages are grouped into two classes: error messages and informational messages. Error messages are identified as such by a zero in the high-order bit of their message Type field values. Thus, error messages have message types from 0 to 127; informational messages have message types from 128 to 255.

This document defines the message formats for the following ICMPv6 messages:

ICMPv6 error messages:

  • 1 - Destination Unreachable (see Section 3.1)
  • 2 - Packet Too Big (see Section 3.2)
  • 3 - Time Exceeded (see Section 3.3)
  • 4 - Parameter Problem (see Section 3.4)
  • 100 - Private experimentation
  • 101 - Private experimentation
  • 127 - Reserved for expansion of ICMPv6 error messages

ICMPv6 informational messages:

  • 128 - Echo Request (see Section 4.1)
  • 129 - Echo Reply (see Section 4.2)
  • 200 - Private experimentation
  • 201 - Private experimentation
  • 255 - Reserved for expansion of ICMPv6 informational messages

Type values 100, 101, 200, and 201 are reserved for private experimentation. They are not intended for general use. It is expected that multiple concurrent experiments will be done with the same type values. Any wide-scale and/or uncontrolled usage should obtain real allocations as defined in Section 6.

Type values 127 and 255 are reserved for future expansion of the type value range if there is a shortage in the future. The details of this are left for future work. One possible way of doing this that would not cause any problems with current implementations is that if the type equals 127 or 255, the code field should be used for the new assignment. Existing implementations would ignore the new assignments as specified in Section 2.4, (b). The new messages using these expanded type values could assign fields in the message body for its code values.

Sections 3 and 4 describe the message formats for the ICMPv6 error message types 1 through 4 and informational message types 128 and 129.

Inclusion of, at least, the start of the invoking packet is intended to allow the originator of a packet that has resulted in an ICMPv6 error message to identify the upper-layer protocol and process that sent the packet.

2.2. Message Source Address Determination​

A node that originates an ICMPv6 message has to determine both the Source and Destination IPv6 Addresses in the IPv6 header before calculating the checksum. If the node has more than one unicast address, it MUST choose the Source Address of the message as follows:

(a) If the message is a response to a message sent to one of the node's unicast addresses, the Source Address of the reply MUST be that same address.

(b) If the message is a response to a message sent to any other address, such as

  • a multicast group address,
  • an anycast address implemented by the node, or
  • a unicast address that does not belong to the node

the Source Address of the ICMPv6 packet MUST be a unicast address belonging to the node. The address SHOULD be chosen according to the rules that would be used to select the source address for any other packet originated by the node, given the destination address of the packet. However, it MAY be selected in an alternative way if this would lead to a more informative choice of address reachable from the destination of the ICMPv6 packet.

2.3. Message Checksum Calculation​

The checksum is the 16-bit one's complement of the one's complement sum of the entire ICMPv6 message, starting with the ICMPv6 message type field, and prepended with a "pseudo-header" of IPv6 header fields, as specified in [IPv6, Section 8.1]. The Next Header value used in the pseudo-header is 58. (The inclusion of a pseudo-header in the ICMPv6 checksum is a change from IPv4; see [IPv6] for the rationale for this change.)

For computing the checksum, the checksum field is first set to zero.

2.4. Message Processing Rules​

Implementations MUST observe the following rules when processing ICMPv6 messages (from [RFC-1122]):

(a) If an ICMPv6 error message of unknown type is received at its destination, it MUST be passed to the upper-layer process that originated the packet that caused the error, where this can be identified (see Section 2.4, (d)).

(b) If an ICMPv6 informational message of unknown type is received, it MUST be silently discarded.

(c) Every ICMPv6 error message (type < 128) MUST include as much of the IPv6 offending (invoking) packet (the packet that caused the error) as possible without making the error message packet exceed the minimum IPv6 MTU [IPv6].

(d) In cases where the internet-layer protocol is required to pass an ICMPv6 error message to the upper-layer process, the upper-layer protocol type is extracted from the original packet (contained in the body of the ICMPv6 error message) and used to select the appropriate upper-layer process to handle the error.

In cases where it is not possible to retrieve the upper-layer protocol type from the ICMPv6 message, the ICMPv6 message is silently dropped after any IPv6-layer processing. One example of such a case is an ICMPv6 message with an unusually large amount of extension headers that does not have the upper-layer protocol type due to truncation of the original packet to meet the minimum IPv6 MTU [IPv6] limit. Another example is an ICMPv6 message with an ESP extension header for which it is not possible to decrypt the original packet due to either truncation or the unavailability of the state necessary to decrypt the packet.

(e) An ICMPv6 error message MUST NOT be originated as a result of receiving the following:

(e.1) An ICMPv6 error message.

(e.2) An ICMPv6 redirect message [IPv6-DISC].

(e.3) A packet destined to an IPv6 multicast address. (There are two exceptions to this rule: (1) the Packet Too Big Message (Section 3.2) to allow Path MTU discovery to work for IPv6 multicast, and (2) the Parameter Problem Message, Code 2 (Section 3.4) reporting an unrecognized IPv6 option (see Section 4.2 of [IPv6]) that has the Option Type highest-order two bits set to 10).

(e.4) A packet sent as a link-layer multicast (the exceptions from e.3 apply to this case, too).

(e.5) A packet sent as a link-layer broadcast (the exceptions from e.3 apply to this case, too).

(e.6) A packet whose source address does not uniquely identify a single node -- e.g., the IPv6 Unspecified Address, an IPv6 multicast address, or an address known by the ICMP message originator to be an IPv6 anycast address.

(f) Finally, in order to limit the bandwidth and forwarding costs incurred by originating ICMPv6 error messages, an IPv6 node MUST limit the rate of ICMPv6 error messages it originates. This situation may occur when a source sending a stream of erroneous packets fails to heed the resulting ICMPv6 error messages.

Rate-limiting of forwarded ICMP messages is out of scope of this specification.

A recommended method for implementing the rate-limiting function is a token bucket, limiting the average rate of transmission to N, where N can be either packets/second or a fraction of the attached link's bandwidth, but allowing up to B error messages to be transmitted in a burst, as long as the long-term average is not exceeded.

Rate-limiting mechanisms that cannot cope with bursty traffic (e.g., traceroute) are not recommended; for example, a simple timer-based implementation, allowing an error message every T milliseconds (even with low values for T), is not reasonable.

The rate-limiting parameters SHOULD be configurable. In the case of a token-bucket implementation, the best defaults depend on where the implementation is expected to be deployed (e.g., a high-end router vs. an embedded host). For example, in a small/mid-size device, the possible defaults could be B=10, N=10/s.

NOTE: THE RESTRICTIONS UNDER (e) AND (f) ABOVE TAKE PRECEDENCE OVER ANY REQUIREMENT ELSEWHERE IN THIS DOCUMENT FOR ORIGINATING ICMP ERROR MESSAGES.

The following sections describe the message formats for the above ICMPv6 messages.

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