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I don't have any concerns with this document.  The mechanisms described in the document could increase packet sizes significantly (if extensions are heavily used), but there is also already a clear warning about avoiding fragmentation.  Specific strategies for limiting the size of packets can be left to specific implementations or cases of usage where this might matter (either by reducing the number of groups, or disabling extensions, or other means), so I don't think it needs anything more than what has already been stated in the document.  One related thing that I worry a little bit about is whether a naïve strategy for avoiding "too big" packets might result instead in generating a larger number of smaller packets, that might be released without any rate or congestion control, however, I think that other separate guidelines for general rate limiting of ICMP and MLD handle this situation as well, and it's not specific to this document.