rfc1885.txt

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Conta & Deering             Standards Track                     [Page 7]

RFC 1885                 ICMPv6 (ICMP for IPv6)            December 1995


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 will fit without the ICMPv6 packet          +
      |                       exceeding 576 octets                    |

   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 - not a neighbor
                  3 - address unreachable
                  4 - port unreachable

   Unused         This field is unused for all code values.
                  It must be initialized to zero by the sender
                  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
   (NOTE: this error can occur only in nodes that do not hold a "default
   route" in their routing tables).



Conta & Deering             Standards Track                     [Page 8]

RFC 1885                 ICMPv6 (ICMP for IPv6)            December 1995


   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 next destination
   address in the Routing header is not a neighbor of the processing
   node but the "strict" bit is set for that address, then the Code
   field is set to 2.

   If there is any other reason for the failure to deliver, e.g.,
   inability to resolve the IPv6 destination address into a
   corresponding link address, or a link-specific problem of some sort,
   then the Code field is set to 3.

   A destination node SHOULD send 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.

   Upper layer notification

   A node receiving the ICMPv6 Destination Unreachable message MUST
   notify the upper-layer protocol.





























Conta & Deering             Standards Track                     [Page 9]

RFC 1885                 ICMPv6 (ICMP for IPv6)            December 1995


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 will fit without the ICMPv6 packet           +
      |                       exceeding 576 octets                    |

   IPv6 Fields:

   Destination Address

                  Copied from the Source Address field of the invoking
                  packet.

   ICMPv6 Fields:

   Type           2

   Code           0

   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 [RFC-1191].

   Sending a Packet Too Big Message makes an exception to one of the
   rules of when to send an ICMPv6 error message, in that unlike other
   messages, it is sent in response to a packet received with an IPv6
   multicast destination address, or 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
   protocol.






Conta & Deering             Standards Track                    [Page 10]

RFC 1885                 ICMPv6 (ICMP for IPv6)            December 1995


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 will fit without the ICMPv6 packet           +
      |                       exceeding 576 octets                    |

   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 sender
                  and ignored by the receiver.

   Description

   If a router receives a packet with a Hop Limit of zero, or a router
   decrements a packet's Hop Limit to zero, it MUST discard the packet
   and send 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.

   The router sending an ICMPv6 Time Exceeded message with Code 0 SHOULD
   consider the receiving interface of the packet as the interface on
   which the packet forwarding failed in following rule (d) for
   selecting the Source Address of the message.

   Upper layer notification

   An incoming Time Exceeded message MUST be passed to the upper-layer
   protocol.



Conta & Deering             Standards Track                    [Page 11]

RFC 1885                 ICMPv6 (ICMP for IPv6)            December 1995


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 will fit without the ICMPv6 packet           +
      |                       exceeding 576 octets                    |

   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 576-byte limit 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 send an
   ICMPv6 Parameter Problem message to the packet's source, indicating
   the type and location of the problem.

   The pointer identifies the octet of the original packet's header
   where the error was detected. For example, an ICMPv6 message with
   Type field = 4, Code field = 1, and Pointer field = 40 would indicate



Conta & Deering             Standards Track                    [Page 12]

RFC 1885                 ICMPv6 (ICMP for IPv6)            December 1995


   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
   protocol.












































Conta & Deering             Standards Track                    [Page 13]

RFC 1885                 ICMPv6 (ICMP for IPv6)            December 1995


4. ICMPv6 Informational Messages

4.1 Echo Request 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             |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |           Identifier          |        Sequence Number        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |     Data ...
      +-+-+-+-+-

   IPv6 Fields: