Re: [BEHAVE] Fwd: IPv6 hosts sending <1280 byte packets

[Iljitsch van Beijnum <[e-mail]>]

On 9 feb 2010, at 21:48, Templin, Fred L wrote:

>> Indeed. Note however that the same black hole would exist if both ends are IPv6 or both ends are IPv4 and there is no translator.

> Eh? For IPv6<->IPv6, we have been assured that PMTUD
> is fixed. Is that not the case?

Fixed how? I know Microsoft has PMTUD blackhole detection and maybe others as well, but that has nothing to do with IPv6. I've had my share of black holes with IPv6, although not recently.

> Also, for IPv4-to-IPv4 we have the convenient fact that
> links with MTU<1500 are rare in the Internet core so
> tripping over a low MTU link is a rare event.

PPP over ethernet is pretty common with ADSL and reduces a user's MTU below 1500, usually without the hosts knowing it. This is solved with adjusting (clamping) the value in the MSS option.

> Granted that does not get us on the path to 9KB MTUs,

Like I said before, you can do this without trouble. I didn't even mention the MSS, which will save you if there's a network in the middle that doesn't generate too bigs correctly.

>> I don't think we can reasonably expect NAT64s to fix all possible black
>> holes.

> It's not a matter of fixing all black holes; it is
> rather an analysis of what new sorts of black holes
> are made possible when translators are used.

Right. But your example wasn't very particular to the translation case.

>> In practice, I think NAT64 users would have to avoid reduced MTUs between the translator and the IPv6
>> hosts (and I would assume that a deployment that needs NAT64 is far enough along that it doesn't need
>> IPv6-in-IP tunneling)

> AFAICT, it may still be some time before ISP networks can
> enable ubiquitous native IPv6. For those that can't there
> is IPv6-in-IPv4 tunneling for the near term.

What kind of tunneling do you foresee? Obviously not 6to4. Other types of tunneling are more complex, native IPv6 makes life much simpler.

> Does that mean that those ISPs can't deploy translators?

No:

>> or artificially lower the IPv6 host MTU or the translator MTU / rewrite the MSS
>> if a reduced MTU is present between the translator and the IPv6 hosts.

> Artificially lowering the IPv6 host MTU may be unsatisfactory
> if the host is connected to a network where it could otherwise
> be using larger packet sizes. And, MSS clamping only works
> for TCP.

If you have a better solution I'm all ears.