[RP-PPPoE] Enabling MTU/MRU 1500 (as per RFC 4638)

[Robert Laverick]

My ISP supports an MTU of greater than 1492 (1500) pppoe using baby
jumbo frames as pre RFC 4638.

I'm using a Fedora base PC to connect via PPPoE using the version of
rp-pppoe included with fedora 14, which is rp-pppoe-3.10

Somone pointed me towards a patch in the pppd git repo (see link
below) which adds support for MTU 1500 PPPoE

http://git.ozlabs.org/?p=ppp.git;a=commitdiff;h=fd1dcdf758418f040da3ed801ab001b5e46854e7;hp=3089132cdf5b58dbdfc2daf08ec5c08eb47f8aca

however this patch doesn't seem to work very well against
rp-pppoe-3.10, I've tried to map this as best I can ending up with a
patch file, but I'm more than a little out of my depth here.  (I've
never use C before, and Makefile and it's chums are a mystery to me!)

The problems I've got seem to be:

1) that the patch relies on things inside lcp.c/h which aren't
currently in rp-pppoe and I haven't got a clue how to go about adding
them, or if it's safe to simply declate new externs to store the
mtu/mru values wanted and allowed.

2) that I've changed a few other instances of ETH_DATA_LEN to
ETH_JUMBO_LEN (namely in ppp.c when dealing with buffers) without
really knowing all the implications of such a change (tho it does work
with these buffer size increases in my local testing even if I have to
comment out so much stuff that I don't get an mtu higher than 1492)

3) there are a few places where the interface MTU is checked (if.c and
the script pppoe-connect for example) which check for ethernet MTU of
1500 which now might not be large enough if the MTU is set to 1500 not
1492.  commenting the ifup command in pppoe-connect allows things to
work when the interface is setup correctly, but the if.c check should
really be checking against the mtu configured not a hard coded value
and I'm unsure what value that should be comparing against.

I have Filed a bug at
https://bugzilla.redhat.com/show_bug.cgi?id=696704 but thought that
someone on this mailing list might be able to help me with pulling a
working patch together.

Many thanks in advance!
Robert Laverick
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