Iljitsch van Beijnum wrote:
On 27-jul-2007, at 14:10, Matt Mathis wrote:
And as John Heffner noted, the community can decide to evade this
problem.
All we have to do is declare it to be a bug to be trying to do jumbo
from a
non-4821 host. Then RFC 4821, 1191 and 1981 are definitely sufficient.
I don't think that's a workable approach. For ethernet, using packets
larger than 1500 bytes is illegal, strictly speaking, but there are no
limits on other link technologies. So if I get some FDDI equipment in a
garage sale or connect to the internet over ATM, SONET or even PPP, I
will / can have an MTU larger than 1500 bytes without breaking any
specification. Retroactively making this illegal would be a curious move
to say the least.
I think this need to be made a bit more clear. In general right now,
only a single MTU is supported on a single broadcast domain. It is
usually enforced by standard (Ethernet) or DHCP / ipv6 mtu
announcements. The point is that with a host capable of sending larger
packets, using a protocol that implements RFC 4821, can exceed this MTU
without breaking MTU discovery. (Other protocols on that same host not
implementing RFC 4821 should not exceed the "safe" MTU.)
This does not impose any sort of global 1500-byte limit on hosts doing
classical PMTUD. It is only saying that it might be safe for
4821-capable protocols to ignore certain existing MTU limits.
-John
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