Hi,
>From: Hugh LaMaster <[EMAIL PROTECTED]>
>Date: Wed, 20 Jul 2005 10:58:20 -0700
> 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?
Please refer RFC2467, which is about FDDI. FDDI's MTU is variable by
network configuration, but RFC2467 provides the default MTU is 4352
for some reasons. After that, same sentences with RFC2464 follow, but
I cannot read as larger MTU than 4352 is permitted.
And RFC2461, which is about ND and RS/RA, says about host behavior
which receive RA,
If the MTU option is present, hosts SHOULD copy the option's value
into LinkMTU so long as the value is greater than or equal to the
minimum link MTU [IPv6] and does not exceed the default LinkMTU value
specified in the link type specific document (e.g., [IPv6-ETHER]).
This is prohibited that MTU option exceed the *default* LinkMTU.
> > BTW, Jinmei san suggested that KAME, NetBSD and OpenBSD are already
> > hacked as above and this behavior may be only FreeBSD problem.
> > But, according to RFC2464, FreeBSD behavior is right, and other
> > BSDs is wrong.
In my environment, Windows XP SP2 accepts larger MTU than 1500 and
sends Jumbo Frames. Of course, if the implementation want to use
Jumbo Frames, it will have to accept large MTU. Many implemetations
work so, and FreeBSD's behavior is a bug from the point of view of
users. But, according to RFC2464 and RFC2461 strictly, I think we
cannot use Jumbo Frames on GbE. Is an update memorandum about Jumbo
Frames necessary, isn't it?
Ryota Hirose
Yamaha Corporation
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