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 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 - (not assigned)
                  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.




 
RFC 2463                 ICMPv6 (ICMP for IPv6)            December 1998


   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).

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

   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 process.






















 
RFC 2463                 ICMPv6 (ICMP for IPv6)            December 1998


3.2 Packet Too Big Message