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Re: Exception to "MUST NOT"

2001-09-28 12:40:03
Somecases. However, ICMPv6 example case, described in the first mail of
this thread, has not been found but already described in the spec; which
is not a bug at all.

could you give the RFC or draft name, and quote the text you are worried 
about?

Yes. Please have a look on section 2.4 of RFC 2463 (ICMPv6)

    ...

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

         (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

The MUST NOT here is based on hard-learned experience with broadcast
storms with ipv4 on ethernets.

[I'm actually concerned about the two exceptions here; it seems unwise
to *ever* send an ICMP error in response to a non-unicast packet, lest
everyone else in the multicast group do the same thing and melt the
network..]

                                        - Bill



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