Subject: MTUs
To: '' <>
From: Mason, Shane <>
List: tech-net
Date: 01/20/2000 13:54:14

I am curious.  I have the latest NetBSD release (1.4.1) and it does not
support MTU changes in the 3Com 3c905 dirver (xl*).  Is  it common in the
NetBSD ethernet dirvers to refrain from providing MTU changes through
ifconfig?  FreeBSD and Linux both have support for this feature, which is
necessary (AFAIK) for machines running behind a firewall/gatway using
PPPoE/PPP combination to connect through ADSL. (PPPoE uses up an extra 8(?)
bytes in the ethernet frame, and PPP kacks when trying to shave these extra
bytes off of it's payload for some reason.)

Will there be general support for changing the NIC MTUs in the future?


I am unsure why PPP is having this problem, in light of the fact that it has
to shave down the IP packet in the first place to make room for it's own
header in the ethernet frame.  PPP has the ability (it's configurable, not
necessarily working) to set the MTU for the ppp* device, and this is
required for using PPP pver dial-up links (~590 bytes for 28.8 kbps modems,
right?)  So why is it that when I set the MTU for PPP in the options file to
1450, it still won't pass the traffic if the ethernet frame from the
initiating machine is 1500 bytes long?