[ipv6-wg] Re: not announcing IXP IPv6 peering lan prefixes in global BGP table possibly breaks PMTUD

On Tue, Jul 26, 2011 at 09:05:25AM +0200, Christian Seitz wrote:
>> Send those ICMP messages from a globally reachable IP address.  The source 
>> address doesn't matter, after all.
>
> so Level3 should fix the problem in this case?

Surely not. There is nothing to fix, they don't do anything wrong. The
problem is indeed that the IXP prefix is not advertised, which is
inherently incompatible with any uRPF (and exactly the reason why you
see e.g. 2001:7f8:2c::/48 in the DFZ since 2004 :-)).

>> From 2001:7f8:4::d1c:1 icmp_seq=1 Packet too big: mtu=1450

Die Adresse ist korrekt.

Siehe RFC4443:

===================================================================
2.2.  Message Source Address Determination

   A node that originates an ICMPv6 message has to determine both the
   Source and Destination IPv6 Addresses in the IPv6 header before
   calculating the checksum.  If the node has more than one unicast
   address, it MUST choose the Source Address of the message as follows:

   (a) If the message is a response to a message sent to one of the
       node's unicast addresses, the Source Address of the reply MUST be
       that same address.

   (b) If the message is a response to a message sent to any other
       address, such as

       - a multicast group address,
       - an anycast address implemented by the node, or
       - a unicast address that does not belong to the node

      the Source Address of the ICMPv6 packet MUST be a unicast address
      belonging to the node.  The address SHOULD be chosen according to
      the rules that would be used to select the source address for any
      other packet originated by the node, given the destination address
      of the packet.  However, it MAY be selected in an alternative way
      if this would lead to a more informative choice of address
      reachable from the destination of the ICMPv6 packet.
===================================================================

And the address which would be selected as "source address for any
other packet originated by the node, given the destination address
of the packet" is quite usually the interface IP of the egress
interface towards the ICMP packet destination.

Asking L3 to change that (if they could at all) is unreasonable IMHO.


Best regards,
Daniel

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