Date: Tue, 10 Aug 2004 11:27:34 +0100 From: Doug Rabson <dfr@nlsystems.com> To: Alexander Nedotsukov <bland@freebsd.org> Cc: freebsd-firewire@freebsd.org Subject: Re: max MTU for fwip device. Message-ID: <1092133653.13089.0.camel@builder02.qubesoft.com> In-Reply-To: <41189199.5020201@FreeBSD.org> References: <4116EA33.8040405@FreeBSD.org> <411843FD.4090201@FreeBSD.org><41189199.5020201@FreeBSD.org>
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On Tue, 2004-08-10 at 10:12, Alexander Nedotsukov wrote: > Doug Rabson wrote: > > >On Tuesday 10 August 2004 04:41, Alexander Nedotsukov wrote: > > > > > >>Doug Rabson wrote: > >> > >> > >>>On Monday 09 August 2004 04:06, Alexander Nedotsukov wrote: > >>> > >>> > >>>>Hi again, > >>>>Is there any reason why we do not support MTUs higher than 1500 > >>>>bytes on firewire links? > >>>> > >>>> > >>>Basically, we are limited by the specification. The rfc states that > >>>the default MTU should be 1500 bytes. From the spec: "NOTE: > >>>IP-capable nodes may operate with an MTU size larger than the > >>>default, but the means by which a larger MTU is configured are > >>>beyond the scope of this document." > >>> > >>> > >>Well standards are good. But I don't see any restriction here. In > >>fact I belive that effective MTU should be evaluated from maximum > >>payload table (RFC2734 Table 1) and ieee1394 header size. Anyway this > >>1500 which comes from 10Mbit ethernet land may be good for default > >>but manual configuration should not be prohibited. > >> > >>Btw default MTU size on MacOSX for fw? interface is 2030 which is 10 > >>bytes less that theoretical maximum for S400 async stream. > >> > >> > >> > > > >Interesting. The specification for IPv6 on firewire is clearer: > > > > The default MTU size for IPv6 packets on an IEEE1394 network is 1500 > > octets. 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. If a Router Advertisement received on an > > IEEE1394 interface has an MTU option specifying an MTU larger than > > 1500, or larger than a manually configured value, that MTU option may > > be logged to system management but MUST be otherwise ignored. The > > mechanism to extend MTU size between particular two nodes is for > > further study. > > > > > Mmm. I still do not see any prohibition of MTU size > 1500. What I see > here is definition of automatic MTU adjustment. It's stated that ATM MTU > size may be only reduced by such mechanism. Am I right? > So manual configuration of interface for MTU size > 1500 violates nothing. Of course - I certainly don't want to stop people from configuring an MTU size > 1500. I just think that for the compiled in default, we should go with the spec for now.
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