Joe,
[...]
If by "supports" you mean "doesn't work", then yes. That's why we now
have PLPMTUD.
PLMTUD is unfortunately not a (complete) replacement of PMTUD.
PLMTUD is a directive to protocols above the IP layer; it isn't a single
protocol, so it wouldn't replace anything.
Looking just at our specifications, we cannot state that PLMTUD can
replace PMTUD. Take RFC2473 (IPv6 tunnelling) for example.
See draft-ietf-intarea-tunnels, esp. v03 Section 5.5.2
(yes, that doc has expired while we're preparing the 04 update, which
should be issued shortly)
Is this the paragraph you are referring to?
PLPMTUD requires a separate,
direct control channel from the egress to the ingress that provides
positive feedback; the direct channel is not blocked by policy
filters and the positive feedback ensures fail-safe operation if
feedback messages are lost [RFC4821].
That is nowhere near section 5.5.2.
No, but it was unfortunately all that was written about how to use PLMTUD for
tunnels.
5.5.2 indicates places where RFC2473 has errors, esp. in how it interprets
the MTU of the tunnel as being defined by the MTU of the path within the
tunnel, rather than by the tunnel egress reassembly limit.
I'm very much in favour of working on better ways of doing Path MTU
discovery.
A blanket statement of "use "PLMTUD" seems very premature though.
The point is that this document fails to indicate the current state of PMTUD.
It correctly notes that:
An extension to Path MTU Discovery defined in this document can be
found in [
RFC4821
]. It defines a method for Packetization Layer Path
MTU Discovery (PLPMTUD) designed for use over paths where delivery of
ICMP messages to a host is not assured.
IMO, it fails to note that this case - where ICMP messages are assured along
a path - is effectively a unicorn except within systems maintained by a
single entity.
RFC1981 has 70 citations:
http://www.arkko.com/tools/allstats/citations-rfc1981.html
Could you expand on your view of how this pertains to advancing RFC1981?
It's called last call input. My input is that this document needs to be more
realistic in noting that, for all intents, ICMP-based MTU discovery isn't
viable and that other methods need to be *expected*, not just that they're
available.
Right, but if you are correct that ICMP-based MTU discovery is not viable then
this document should not be advanced.
At the same time for many protocols we have nothing else. An operator can break
any protocol if that's their policy. And that's the breakage we're talking
about here, not any issues with the protocol specification.
There is a philosophical aspect of this. (Which I'm not the best person to
represent as I skipped my University studies in philosophy and used the student
loan to buy a motorcycle... (and only read the art of motorcycle maintenance
years later) )
This is a tussle. The IETF specifies protocols under the assumption that
operators treat those protocols largely as specified. The 5-10% failure of
PMTUD messages may be caused by misconfiguration, misunderstanding or
mis-intent... Many of our protocols are suffering from the same fate. Should
the IETF adjust all its protocols to be as middlebox friendly as possible? You
can make this argument about IPv6 fragments, any packet with IPv6 extension
headers, IPv4 fragments. Or anything but TCP port 443/80 and UDP port 53 for
that matter. Are we as the IETF going to continue standardising protocols to
work as best as they possible can, ignoring protocol abuse, or are we going to
bend over and do whatever it takes to make it work for those 5-10% who've
actively broken the protocol? What about the 90-90% where the protocols work as
expected?
Best regards,
Ole
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