Date: Sun, 11 Sep 2005 19:53:45 -0400 From: Gary Corcoran <gcorcoran@rcn.com> To: Charles Sprickman <spork@bway.net> Cc: freebsd-net@freebsd.org, "Julian H. Stacey" <jhs@flat.berklix.net> Subject: Re: PPPoE and UDP fragmentation Message-ID: <4324C389.5010001@rcn.com> In-Reply-To: <Pine.OSX.4.61.0509111846270.291@white.nat.fasttrackmonkey.com> References: <200509100930.j8A9UoHN062107@fire.jhs.private> <Pine.OSX.4.61.0509111846270.291@white.nat.fasttrackmonkey.com>
next in thread | previous in thread | raw e-mail | index | archive | help
Charles Sprickman wrote: > On Sat, 10 Sep 2005, Julian H. Stacey wrote: > >>> -Are there any tunables at either end (both hosts are FreeBSD 4.11 >>> p11) to >>> alter how fragmented packets are re-assembled? >> >> >> /usr/ports/net/tcpmssd >> An MTU adapter. Apparently not needed on FreeBSD-5 but I mean to >> install it on my FreeBSD-4 DSL gateways when I find time to think if >> it might >> have any implications re ipfw & security. > > > I don't think that does anything to UDP, it just digs into tcp and > "fixes up" the MSS by altering it on outgoing packets. I'm looking to > further understand UDP fragmentation and why a host might ignore > fragments, and who along the way is actually doing the fragmentation. > > My current "fix" is just to set the interface MTU on the sending box to > 1492, and that works well, but I'd really like to understand why it > fails without that. Do you know about the horribly-large overhead that PPPoE adds? It's about 36 bytes, if I recall. So to keep the *total* ethernet frame size under the max limit of 1500-something (don't recall exact number), you *always* have to limit *any* frame size, before PPPoE overhead, to 1492 bytes. Does this help? Gary
Want to link to this message? Use this URL: <https://mail-archive.FreeBSD.org/cgi/mid.cgi?4324C389.5010001>