Date: Tue, 10 Aug 2004 08:51:32 +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: <200408100851.32087.dfr@nlsystems.com> In-Reply-To: <411843FD.4090201@FreeBSD.org> References: <4116EA33.8040405@FreeBSD.org> <200408090859.34574.dfr@nlsystems.com> <411843FD.4090201@FreeBSD.org>
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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.
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