Date: Sat, 21 Jul 2007 12:24:47 -0400 From: Stephen Clark <Stephen.Clark@seclark.us> To: Eli Dart <dart@es.net> Cc: freebsd-net@FreeBSD.org, Julian Elischer <julian@elischer.org> Subject: Re: 6.2 mtu now limits size of incomming packet Message-ID: <46A2334F.2040302@seclark.us> In-Reply-To: <46A10860.50804@es.net> References: <200707150237.l6F2bAgZ011098@redrock.karels.net> <469E0FFF.8070802@seclark.us> <20070720172021.8EA3D13C4B3@mx1.freebsd.org> <46A10063.9010902@elischer.org> <46A10860.50804@es.net>
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Eli Dart wrote: >see below... > >Julian Elischer wrote: > > >>Eli Dart wrote: >> >> >>>Stephen Clark wrote: >>> >>> >>> >>>>So was any decision reached on this issue - will FreeBSD changed >>>>to accept a packet on an interface that is larger than the mtu on >>>>that interface? >>>> >>>> >>>If possible, I'd like to see the ability to enforce interface MTU >>>for received packets preserved in a sysctl if it is removed for the >>> default config... In other words, something like: >>> >>>net.link.mtu_limits_received_pktsize = 0|1 >>> >>>Then, default it to 0 to preserve 4.x behavior. >>> >>> >>what would this achieve? >> >>Answering himself.. it MAY allow a driver to optimise a bit by not >>needing to cope with the posibility of receiving jubo packets? I can >>not think of any other reason.. (except to break networks that are >>apparently working fine). >> >> > >The networks that are apparently working fine are most likely >misconfigured, IMHO. > >Others have made a case for permitting an interface to accept as large a >packet as it can, regardless of configured MTU. That's fine for theory. > >My operational experience leads me to a different place. If an >interface receives a packet that is larger than its configured MTU, I >would prefer that the packet be dropped as a giant and a giants counter > incremented, regardless of whether the hardware can theoretically >receive the packet. In modern networks, an MTU mismatch within a >broadcast domain indicates a broken network, IMHO. If the devices in >the network are configured to enforce MTU for both tx and rx, more >problems get spotted during turnup, rather than surfacing later on as >difficult-to-diagnose problems that users only call about after they are >truly frustrated. And, if you have a giants counter (or input error >counter) you can look at, it makes it straightforward to spot the problem. > >(one could also stretch a bit and say that enforcing MTU on rx might >provide less surprise to code that consumes packets and has knowledge of >the MTU setting of an interface.....unfortunately I don't know enough >about the details of the network stack to know if this is a real concern) > >Many thanks, > > --eli > > > > Hi Eli, You make some good points, however it is a change from previous FreeBSD behavior and is not required by any RFC's, plus it causes problems for some users. My $.02 Steve -- "They that give up essential liberty to obtain temporary safety, deserve neither liberty nor safety." (Ben Franklin) "The course of history shows that as a government grows, liberty decreases." (Thomas Jefferson)
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