Date: Fri, 13 Jul 2007 20:06:17 -0500 From: Mike Karels <karels@karels.net> To: Julian Elischer <julian@elischer.org> Cc: Stephen.Clark@seclark.us, freebsd-net@freebsd.org, Bill Moran <wmoran@collaborativefusion.com>, Sten Daniel Soersdal <netslists@gmail.com> Subject: Re: 6.2 mtu now limits size of incomming packet Message-ID: <200707140106.l6E16HWi006607@redrock.karels.net> In-Reply-To: Your message of Fri, 13 Jul 2007 10:28:51 -0700. <4697B653.8050906@elischer.org>
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> Bill Moran wrote: > ractices are > > still evolving. > > > > Let's flip the question around a bit: why would you _want_ the TCP stack > > to accept frames larger than the stated MTU? > > > Because mtu is mTu not mRu. I must agree. There is no strong requirement that MTU == MRU, although the standard BSD interface description does not include an MRU. As a practical example, I'll note that our (Secure Computing) version of the system provides an option to receive jumbo Ethernet frames without sending them; MRU is 9K or so, MTU is 1500. If the NIC (as programmed) is willing to receive the frame, we are willing to receive the packet at the protocol level. Similarly, if someone sets the MTU on an Ethernet interface to 1280, that does not really mean that frames up to 1500 bytes are nonconformant. > As the original BSD group always said.. (from my memories of Kirk's and > Mike's talks in 1991), > "Transmit strictly [according to the spec] but receive forgivingly". >From RFC-1122, and memorialized on the working group coffee cup on my bookshelf: Be liberal in what you accept, and conservative in what you send (attributed to RFC-791, but paraphrased; also in RFC-793; for those who don't recognize them, these are the original IP and TCP specs.) > The ability to receive packets larger than mtu was not accidental. > This should be fixed, if it is, as is suggested, a deliberate change. I'd be happy to see the change undone as well. I (well, our test group) found this change in a similar way, and it didn't agree with our previous usage. Mike
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