@ermal:
Normally it should be easier for FreeBSD(netgraph/mpd5) to include this.
Since from my understanding this is just patching the ng_ppp and mpd5 to provide an option for this.
Have not read teh RFC yet if it requires the interface to bump to 1508 mtu or not since that is a more severe impact in general.
Interface MTU of 8 bytes higher than the PPP MTU is a must to allow for the PPPoE header. This means that for PPP MTUs higher than 1492 negotiated using RFC 4638, a jumbo capable network interface in jumbo mode is required. The PPP-Max-Payload MTU will have to be clamped at the lower of the value signalled from mpd5 and the interface MTU (less 8 bytes in the latter case).
All the RFC does is give the method to negotiate an maximum MTU in place of the usual PPPoE maximum of 1492. It's up to the PPP layer to make use of this negotiated maximum MTU to negotiate a PPP MTU above 1492, and the transport layer and network must be capable of operating at that MTU.
@jimp:
Not that it directly helps, but baby jumbo frame support was also just added in the last OpenBSD release, I remember seeing it in their announcement.
Both OpenBSD and NetBSD now have RFC 4638 support. As they don't use netgraph for PPPoE, the implementation cannot be directly ported to FreeBSD, though, as ermal says, adding support to FreeBSD shouldn't be too hard.
Connections supporting RFC 4638 are common in the UK. All BT Openreach provided VDSL2 and FTTP circuits support a 1500 byte PPP MTU. Almost every ISP using these circuits uses PPPoE - the notable exception is Sky, which uses DHCP (I forget which DHCP option is used for authentication).
All the VDSL2 modems and FTTP ONTs supplied by BT Openreach with these connections can support a 1500 byte PPP MTU. If BT Openreach decided to offer 'wires only' VDSL2, it will be up to the customer / ISP to select hardware with baby jumbo support.
There's a lot of pfSense users in the UK who could make use of this if it was available, assuming their pfSense box has a jumbo capable NIC.