RE: PMTUD and MTU < 1280

> -----Original Message-----
> From: Rémi Després [mailto:despres.remi@laposte.net]
> Sent: Monday, July 25, 2011 1:43 PM
> To: Dan Wing
> Cc: 'james woodyatt'; 'RJ Atkinson'; ipv6@ietf.org
> Subject: Re: PMTUD and MTU < 1280
> 
> Dan,
> 
> 1.
> The point I wanted to check is just, slightly reformulated):
> "May a simple IPv6 host have no support of packet-reassembly, and
> simply accept packets up to 1280 octets."

The earlier part of this thread was talking about sending; you're
now bringing up receiving.

IMO, if the packet came from IPv4, and that IPv4 network had a small
MTU (e.g., 576) causing fragmentation, then such an IPv6 receiver
will be unable to receive the packet.

> In my understanding, the answer should be yes.
> - This doesn't depend on whether sources know or not whether their
> destinations are IPv6 or IPv4 only.
> - If the destination happens to be IPv6, current RFC's don't permit
> intermediate nodes to refuse 1280 packets as being too big.
> 
> 2.
> How sources can be sure to have e2e transparency in IPv6 is a different
> question, but IMHO an important one.
> 
> For instance, if a destination address is obtained from the DNS in a
> AAAA, with no A for the same URL  and without any well-known prefix
> indicating that there is an embedded-IPv4-address, I hope the source
> can be guaranteed that e2e transparency won't be broken?

I don't think so.  DNS64 comes to mind.

> I won't have time personally to contribute much on this, but the
> subject would usefully be clarified, IMHO.

The RFCs are pretty clear, IMO.  Implementers don't want to read
them all the way.

-d

> Regards,
> RD
> 
> 
>  Le 25 juil. 2011 à 15:36, Dan Wing a écrit :
> 
> >>>>
> >>>> ...
> >>>
> >>> Its behavior violates the last paragraph of Section 5 of RFC2460.
> >>
> >> Violation _only in case_ of "an IPv6 packet that is sent to an IPv4
> >> destination".
> >
> > But how does one determine an IPv6 packet is, or isn't, going
> > to an IPv4 destination?  I don't think it's possible to determine
> > if there is an IPv6/IPv4 translator on the path.
> >
> > -d
> >
> >
> >> If the destination is IPv6, a PMTU below 1280 remains therefore a
> >> network failure.
> >> This authorizes a simple IPv6 host to refuse packets beyond 1280
> octets
> >> and to have no support of packet-reassembly.
> >>
> >> Right?
> >>
> >> Regards,
> >> RD
> >>
> >>
> >>
> >>>
> >>> -d
> >>>
> >>>>
> >>>> --
> >>>> james woodyatt <jhw@apple.com>
> >>>> member of technical staff, core os networking
> >>>>
> >>>>
> >>>>
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