Date: Sun, 15 Jul 2007 00:46:27 +0100 (BST) From: Robert Watson <rwatson@FreeBSD.org> To: Mike Karels <karels@karels.net> Cc: Stephen.Clark@seclark.us, Sten Daniel Soersdal <netslists@gmail.com>, Julian Elischer <julian@elischer.org>, Bill Moran <wmoran@collaborativefusion.com>, freebsd-net@freebsd.org Subject: Re: 6.2 mtu now limits size of incomming packet Message-ID: <20070715003156.B94899@fledge.watson.org> In-Reply-To: <200707140106.l6E16HWi006607@redrock.karels.net> References: <200707140106.l6E16HWi006607@redrock.karels.net>
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On Fri, 13 Jul 2007, Mike Karels wrote: >> 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. 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? Robert N M Watson Computer Laboratory University of Cambridge
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