type may not be present in the ICMPv6 message, due to truncation
        of the original packet to meet the 576-octet limit.  In that
        case, the error message is silently dropped after any IPv6-layer
        processing.

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

         (e.1) an ICMPv6 error message, or

         (e.2) 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 that has the Option Type highest-order two
               bits set to 10), or

         (e.3) a packet sent as a link-layer multicast, (the exception
               from e.2 applies to this case too), or

         (e.4) a packet sent as a link-layer broadcast, (the exception
               from e.2 applies to this case too), or

         (e.5) 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 sender to be an IPv6 anycast address.

    (f) Finally, to each sender of an erroneous data packet, an IPv6
        node MUST limit the rate of ICMPv6 error messages sent, in order
        to limit the bandwidth and forwarding costs incurred by the
        error messages when a generator of erroneous packets does not
        respond to those error messages by ceasing its transmissions.




 
RFC 1885                 ICMPv6 (ICMP for IPv6)            December 1995


        There are a variety of ways of implementing the rate-limiting
        function, for example:

         (f.1) Timer-based - for example, limiting the rate of
               transmission of error messages to a given source, or to
               any source, to at most once every T milliseconds.

         (f.2)  Bandwidth-based - for example, limiting the rate at
               which error messages are sent from a particular interface
               to some fraction F of the attached link's bandwidth.

        The limit parameters (e.g., T or F in the above examples) MUST
        be configurable for the node, with a conservative default value
        (e.g., T = 1 second, NOT 0 seconds, or F = 2 percent, NOT 100
        percent).

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


































 
RFC 1885                 ICMPv6 (ICMP for IPv6)            December 1995


3. ICMPv6 Error Messages