Re: Path MTU Hop-by-Hop Option

On Tue, Oct 23, 2018 at 11:03 AM, Fernando Gont <fgont@si6networks.com> wrote:
> On 10/17/18 6:49 AM, Carsten Bormann wrote:
>> On Oct 17, 2018, at 00:55, Bob Hinden <bob.hinden@gmail.com> wrote:
>>>
>>> This document specifies a new Hop-by-Hop IPv6 option that is used to
>>>   record the minimum Path MTU
>>
>> A while ago I proposed something similar to collect information about adaptation layer fragmentation MTUs (information about which is truly not available in any other way):
>>
>> https://tools.ietf.org/html/draft-bormann-intarea-alfi-04
>>
>> (I also only looked at the Internet layer here — leaving it as an exercise to put in transport/application protocol support.)
>>
>> After mulling an initial version of the proposal, there was pretty much consensus that hop-by-hop options were not really available on the Internet.  So I put in an alternative way to collect the information, see above if you are interested.  Then at some point SPUD came along and I was hoping that would lead to a framework within in which this information could be better handled.
>>
>> Maybe it’s time to pull the old draft out again.  The hop-by-hop option problem of course hasn’t gone away.
>
> IPv6 EHs are not readily available on the public IPv6 Internet. See
> RFC7872. While not included in RFC7872, my measurements indicate that
> the same applies even to IPsec EHs. :(

Fernando,

If I'm reading RFC7872 correctly, it is more accurate to say that the
drop rates for EH vary between 12% and 55% depending on type and
destination. We cannot say that EH always works on the Internet, but
neither is it correct to say that EH never works on the Internet.
Success rate is somewhere in between.

While the data in RFC7872 is objective, the conclusions being drawn
are not. I think there are two possible conclusions 1) Success rate of
EH is abysmal, EH is a failure and should be deprecated, and the
would-be contents of EH need to be moved to other layers somehow 2) EH
support is less than desirable, but this is a current condition that
may be improve over time with new deployments, protocol
clarifications, definition of some "useful" options. IMO, until EH is
officially deprecated, it is entirely appropriate to define new uses
such as the Path MTU Option here. Since we know up front that the
option might not always be viable on the Internet, the mechanism will
need a fallback; but the need for a fallback is hardly unique: IPv6
falls back  IPv4 via happy eyeballs, UDP falls back to TCP when
firewalls block UDP, etc.

Tom

>
> Thanks,
> --
> Fernando Gont
> SI6 Networks
> e-mail: fgont@si6networks.com
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