On 21-jul-2005, at 1:01, Ryota Hirose wrote:
As I read it, RFC2464 says that the maximum *default* MTU is 1500,
and, anything larger than that must be manually configured. It
doesn't say that it is prohibited.
RFC2464 says as following:
This size may be reduced by a Router Advertisement [DISC]
containing an MTU option which specifies a smaller MTU, or by
manual
configuration of each node.
I'm sorry for my poor English knowledge, but I think this sentence
permit to decrease MTU from default size by manual configuration.
OK, this sentence or others in the section don't prohibit increase
MTU. Since decrease MTU is permitted and there is no mention to
increase, I understood that the increase MTU is prohibited. Isn't it
a common sense?
Well, that's a philosophical question: is everything that isn't
explicitly disallowed allowed by default, or is everything that isn't
explicitly allowed disallowed by default?
My common sense tells me that the authors of RFC 2464 didn't consider
the case where the MTU would legitimately be larger than 1500 bytes.
They did consider the case where router advertisements contain an MTU
that is apparently incorrect, because it's larger than the standard
allows.
Ideally, the MTU would be a neighbor discovery option, so that the
highest possible MTU can be used between two systems on a subnet,
without requiring that all systems on a subnet support the same MTU
size. However, this is problematic because switches and hubs also
have MTU limitations that can't be discovered this way.
In any event, RFC 2464 doesn't say it's disallowed to configure an
MTU larger than 1500 bytes on an ethernet subnet, and it also doesn't
provide any reasons why doing so would be bad. The only thing it says
is that the MTU option in router advertisements can't be used to
increase the MTU beyond 1500 bytes.
So in my opinion, an implementation that supports jumboframes should
use the interface MTU for IPv6 by default, and reduce this MTU for
IPv6 to the one in an MTU option in router advertisements, when such
an option is received.
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