I'd replace "Warning" with the more conventional "Note". I would also
drop the last sentence on grounds of redundancy. This is NOT a big issue
as I understand from my reading of various lists, because no one expects
RFC 4821 discovery to work anyway.
Tom Taylor
On 16/11/2014 11:55 AM, Rémi Després wrote:
Hi Tom,
If a statement proceeds to be added, I propose something like this in the
introduction:
Warning: on paths that traverse MAP-T IPv6 networks, ICMP-independent PathMTU
Discovery, as specified in [RFC 4821], ceases to be reliable. (This is because
DF=1/MF=1 combination, used in [RFC 4821], becomes DF=0&MF=1 after traversal
of a MAP-T network). This specification is for service providers that, being
conscious of this limitation, accept it as negligible.
(This is, I believe, more explanatory here than what Gang had proposed in the
different context of 464XLAT.)
Regards,
RD
16 nov. 2014 16:12, Tom Taylor <[email protected]> :
As a WG participant aware of the history of this whole effort, I would support
adding the statement to MAP-T in particular.
Tom Taylor
On 15/11/2014 11:45 PM, Ole Troan wrote:
Remi,
It is true that double translation has the problem that the DF bit is not
communicated through. This is a limitation of the MAP-T specification.
I've asked the authors about this, and they did not deny that this limitation
exists,
Good to know authors confirmed.
(Note that denial would have been difficult. That’s just a simple technical
analysis.)
But did they confirm my complete point, namely that MAP-T breaks the
ICMP-independent Path MTU Discovery of RFC4821?
If they didn’t, the fact remains, is important, and is also easy to verify.
while you may consider it simple, it took me quite some effort to refresh all
the context necessary, thanks for providing the simple explanation of the issue.
you are correct that MAP-T has this issue (or any double translation using
RFC6145) including 464XLAT.
so your claim that the authors are trying to conceal it seems a bit
un-collegial.
It seemed to you a bit un-collegial, but it certainly didn’t intend to be:
- To explain why I didn’t insist at that time to document the PMTUD problem, I just said
"I felt a strong preference of MAP-T authors for keeping it concealed, and I had to
move to other activities." .
- This is just the truth about what I felt then.
- In any case, if anyone is crossed by what I said, I apologize for having
told, in good faith but too frankly, what I had felt.
not claiming to speak for all the authors here, but I don't think anyone was
trying to conceal anything. I think it was more that we didn't feel that the
issue was serious enough to warrant not reusing RFC6145 for double translation.
(I also think someone did measurements in live networks looking for DF=MF=1
packets and found very few?)
As I said, the working group did consider this issue, and it was not a factor
in the coin toss.
This seems to suggest that someone viewed this issue had been "a factor in the coin
toss".
No one did AFAIK, and certainly not me.
But this isn’t the point.
Even in its experimental status, I do think MAP-T's specification should have
included a warning that it is incompatible with Path MTU Discovery of RFC4821,
and that MAP-E should be used if such compatibility is desired.
Yet, IMHO, harm of this warning being absent remains limited enough to be
acceptable as long as the status of MAP-T remains Experimental.
we had this exact same discussion 464XLAT, RFC6877. in the end the disclaimer
wasn't included in that document.
I agree with you that the issue is worth pointing out though, with f.ex text
similar to what was proposed for RFC6877. see:
https://mailarchive.ietf.org/arch/msg/v6ops/66dSxb-i9kjGi1UeySBu5MYAV3w
cheers,
Ole
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