Date: Wed, 18 Jul 2007 09:05:03 -0400 From: Stephen Clark <Stephen.Clark@seclark.us> To: karels@karels.net Cc: freebsd-net@FreeBSD.org, Robert Watson <rwatson@FreeBSD.org>, Julian Elischer <julian@elischer.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: <469E0FFF.8070802@seclark.us> In-Reply-To: <200707150237.l6F2bAgZ011098@redrock.karels.net> References: <200707150237.l6F2bAgZ011098@redrock.karels.net>
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Mike Karels wrote: >>A related change that should probably be discussed if we want to think more >>about asymmetry in maximum transmission unit is this one: >> >> > > > >> ---------------------------- >> revision 1.98 >> date: 2006/06/26 17:54:53; author: andre; state: Exp; lines: +2 -0 >> In syncache_respond() do not reply with a MSS that is larger than what >> the peer announced to us but make it at least tcp_minmss in size. >> >> > > > >> Sponsored by: TCP/IP Optimization Fundraise 2005 >> ---------------------------- >> >> > > > >>In this change, we cap the advertised MSS in SYN/ACK to the received >>advertised MSS, which presumably avoids an extra PMTU round trip if jumbograms >>are enabled on the receiving endpoint. However, it also prevents use of >>larger packet sizes if asymmetric MTU is supported. I think I suggested after >>this was committed that we at least add an administrative twiddle to >>enable/disable this mode of operation, but don't see one in there currently. >>Does the Secure Computing scenario use TCP in this way, and is the potential >>win in avoiding a PMTU round-trip worth disallowing asymmetric MSS at the TCP >>layer? >> >> > >In our case, TCP isn't aware of the MRU, and bases its MSS on the MTU values. >However, I don't see any reason for TCP to cap the MSS at the received MSS. >If the other end doesn't want to receive more than 1024 bytes, that's no >reason to refuse to accept more. > > Mike > > > 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? 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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