Date: Tue, 10 Aug 2004 18:12:57 +0900 From: Alexander Nedotsukov <bland@FreeBSD.org> To: Doug Rabson <dfr@nlsystems.com> Cc: freebsd-firewire@FreeBSD.org Subject: Re: max MTU for fwip device. Message-ID: <41189199.5020201@FreeBSD.org> In-Reply-To: <200408100851.32087.dfr@nlsystems.com> References: <4116EA33.8040405@FreeBSD.org> <200408090859.34574.dfr@nlsystems.com> <411843FD.4090201@FreeBSD.org> <200408100851.32087.dfr@nlsystems.com>
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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. All the best, Alexander.
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