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3. ICMPv6 Error Messages

3.1. Destination Unreachable Message​

    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             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Unused                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    As much of invoking packet                 |
   +                as possible without the ICMPv6 packet          +
   |                exceeding the minimum IPv6 MTU [IPv6]          |

IPv6 Fields:

Destination Address: Copied from the Source Address field of the invoking packet.

ICMPv6 Fields:

Type: 1

Code:

  • 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

Unused: This field is unused for all code values. It must be initialized to zero by the originator and ignored by the receiver.

Description

A Destination Unreachable message SHOULD be generated by a router, or by the IPv6 layer in the originating node, in response to a packet that cannot be delivered to its destination address for reasons other than congestion. (An ICMPv6 message MUST NOT be generated if a packet is dropped due to congestion.)

If the reason for the failure to deliver is lack of a matching entry in the forwarding node's routing table, the Code field is set to 0. (This error can occur only in nodes that do not hold a "default route" in their routing tables.)

If the reason for the failure to deliver is administrative prohibition (e.g., a "firewall filter"), the Code field is set to 1.

If the reason for the failure to deliver is that the destination is beyond the scope of the source address, the Code field is set to 2. This condition can occur only when the scope of the source address is smaller than the scope of the destination address (e.g., when a packet has a link-local source address and a global-scope destination address) and the packet cannot be delivered to the destination without leaving the scope of the source address.

If the reason for the failure to deliver cannot be mapped to any of other codes, the Code field is set to 3. Example of such cases are an inability to resolve the IPv6 destination address into a corresponding link address, or a link-specific problem of some sort.

One specific case in which a Destination Unreachable message is sent with a code 3 is in response to a packet received by a router from a point-to-point link, destined to an address within a subnet assigned to that same link (other than one of the receiving router's own addresses). In such a case, the packet MUST NOT be forwarded back onto the arrival link.

A destination node SHOULD originate a Destination Unreachable message with Code 4 in response to a packet for which the transport protocol (e.g., UDP) has no listener, if that transport protocol has no alternative means to inform the sender.

If the reason for the failure to deliver is that the packet with this source address is not allowed due to ingress or egress filtering policies, the Code field is set to 5.

If the reason for the failure to deliver is that the route to the destination is a reject route, the Code field is set to 6. This may occur if the router has been configured to reject all the traffic for a specific prefix.

Codes 5 and 6 are more informative subsets of code 1.

For security reasons, it is recommended that implementations SHOULD allow sending of ICMP destination unreachable messages to be disabled, preferably on a per-interface basis.

Upper Layer Notification

A node receiving the ICMPv6 Destination Unreachable message MUST notify the upper-layer process if the relevant process can be identified (see Section 2.4, (d)).

3.2. Packet Too Big Message​

    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             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             MTU                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    As much of invoking packet                 |
   +               as possible without the ICMPv6 packet           +
   |               exceeding the minimum IPv6 MTU [IPv6]           |

IPv6 Fields:

Destination Address: Copied from the Source Address field of the invoking packet.

ICMPv6 Fields:

Type: 2

Code: Set to 0 (zero) by the originator and ignored by the receiver.

MTU: The Maximum Transmission Unit of the next-hop link.

Description

A Packet Too Big MUST be sent by a router in response to a packet that it cannot forward because the packet is larger than the MTU of the outgoing link. The information in this message is used as part of the Path MTU Discovery process [PMTU].

Originating a Packet Too Big Message makes an exception to one of the rules as to when to originate an ICMPv6 error message. Unlike other messages, it is sent in response to a packet received with an IPv6 multicast destination address, or with a link-layer multicast or link-layer broadcast address.

Upper Layer Notification

An incoming Packet Too Big message MUST be passed to the upper-layer process if the relevant process can be identified (see Section 2.4, (d)).

3.3. Time Exceeded Message​

    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             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Unused                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    As much of invoking packet                 |
   +               as possible without the ICMPv6 packet           +
   |               exceeding the minimum IPv6 MTU [IPv6]           |

IPv6 Fields:

Destination Address: Copied from the Source Address field of the invoking packet.

ICMPv6 Fields:

Type: 3

Code:

  • 0 - Hop limit exceeded in transit
  • 1 - Fragment reassembly time exceeded

Unused: This field is unused for all code values. It must be initialized to zero by the originator and ignored by the receiver.

Description

If a router receives a packet with a Hop Limit of zero, or if a router decrements a packet's Hop Limit to zero, it MUST discard the packet and originate an ICMPv6 Time Exceeded message with Code 0 to the source of the packet. This indicates either a routing loop or too small an initial Hop Limit value.

An ICMPv6 Time Exceeded message with Code 1 is used to report fragment reassembly timeout, as specified in [IPv6, Section 4.5].

Upper Layer Notification

An incoming Time Exceeded message MUST be passed to the upper-layer process if the relevant process can be identified (see Section 2.4, (d)).

3.4. Parameter Problem Message​

    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             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                            Pointer                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    As much of invoking packet                 |
   +               as possible without the ICMPv6 packet           +
   |               exceeding the minimum IPv6 MTU [IPv6]           |

IPv6 Fields:

Destination Address: Copied from the Source Address field of the invoking packet.

ICMPv6 Fields:

Type: 4

Code:

  • 0 - Erroneous header field encountered
  • 1 - Unrecognized Next Header type encountered
  • 2 - Unrecognized IPv6 option encountered

Pointer: Identifies the octet offset within the invoking packet where the error was detected. The pointer will point beyond the end of the ICMPv6 packet if the field in error is beyond what can fit in the maximum size of an ICMPv6 error message.

Description

If an IPv6 node processing a packet finds a problem with a field in the IPv6 header or extension headers such that it cannot complete processing the packet, it MUST discard the packet and SHOULD originate an ICMPv6 Parameter Problem message to the packet's source, indicating the type and location of the problem.

Codes 1 and 2 are more informative subsets of Code 0.

The pointer identifies the octet of the original packet's header where the error was detected. For example, an ICMPv6 message with a Type field of 4, Code field of 1, and Pointer field of 40 would indicate that the IPv6 extension header following the IPv6 header of the original packet holds an unrecognized Next Header field value.

Upper Layer Notification

A node receiving this ICMPv6 message MUST notify the upper-layer process if the relevant process can be identified (see Section 2.4, (d)).

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