At Mon, 04 Mar 2019 20:43:14 +0900 (JST), fujiw...@jprs.co.jp wrote: > > - Section 3 > > > > Linux 2.6.32, Linux 4.18.20 > > and FreeBSD 12.0 accept crafted "ICMPv6 Packet Too Big" packet and > > path MTU decreased to 1280. > > > > I suspect this often doesn't matter much in practice. Since IPv6 > > doesn't allow fragmentation and PMTU discovery isn't very effective for > ^^^^^^^^^^^^^ > on-path fragmentation ?
Oops, sorry for the confusing text. I meant "IPv6 doesn't allow fragmentation at intermediate nodes" (or, yes, I meant "on-path fragmentation"). > > DNS responders, the server implementation should set > > IPV6_USE_MIN_MTU and expect that MTU anyway (several implementations > > actually set this option; some others don't, but they are just lucky > > to not encounter the problem or receive complaints about it). > > If setting IPV6_USE_MIN_MTU, does the server use 1280 as all path MTU value ? Yes. Or more accurately, if IPV6_USE_MIN_MTU is set the path MTU value (if known) is just ignored. > Without IPV6_DONTFRAG option, are responses larger than 1280 fragmented ? Yes (if IPV6_USE_MIN_MTU is specified). > I observed many fragmented IPv6 DNS responses whose packet size are > 1496 or 1500. Those should be sent from a server that doesn't set IPV6_USE_MIN_MTU or from a system that doesn't support the option (sadly a very widely deployed OS doesn't support it: Linux). > # What I was interested in was that many implementations accept > # crafted "ICMPv6 Packet Too Big". Sure, but unless it matters in the larger context of the draft, it's just a distraction. -- JINMEI, Tatuya
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