Date: Fri, 10 May 2002 20:38:52 -0700 (PDT) From: Archie Cobbs <archie@dellroad.org> To: Lars Eggert <larse@ISI.EDU> Cc: "'Justin Hawkins'" <justin@hawkins.dropbear.id.au>, freebsd-net@FreeBSD.ORG Subject: Re: mpd-netgraph as VPN client to Cisco 2500 REDUX (doh) Message-ID: <200205110338.g4B3cqG09077@arch20m.dellroad.org> In-Reply-To: <008701c1f830$022f7db0$b27ba8c0@isi.edu> "from Lars Eggert at May 10, 2002 07:36:01 am"
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Lars Eggert writes: > > Anyway, now I have a legitimate question, how can I setup mpd > > to change the MTU on the ng0 interface when it brings it up? > > I do this via mpd's "set iface up-script", using a manually chosen MTU. > > I'd be nice if mpd would do this automatically, based on the MTU of the > underlying interface and the length of the encapsulation header... Mpd is "supposed" to do this automatically, but it only does it for the PPP headers, not the device headers (such as PPTP/IP/Ethernet). In other words, there is a 'hard' MTU which is the maximum size of a packet you are allowed to send -- which by the way is dictated by (a) the transport layer for your PPP frames (in the case of PPTP, this would be on the order of 65,000 bytes -- because with PPTP the transport for the PPP frame is an IP packet) and (b) what the remote peer asks for (typically 1500 or less), and there is also a 'soft' MTU which is the largest MTU that will not cause any packets to get fragmented at any level along the chain. Mpd only adjusts the interface MTU to handle the 'hard' MTU part. If time permits I'll have it try to be a little smarter. On a related note, you can avoid these problems altogether if you enable multi-link PPP (and the remote PPTP device supports it). With multi-link, PPP packets themselves can be fragmented transparently so the higher layer MTU can be much larger without any ill effects. -Archie __________________________________________________________________________ Archie Cobbs * Packet Design * http://www.packetdesign.com To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-net" in the body of the message
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