On Wed, 15 Sep 2004 12:10:30 -0400, clvrmnky
<clvrmnky-uunet@coldmail.com.invalid> wrote:

>Just so my own understanding of this is correct, I came up with an MTU
>of 1492 (1464 + 28) which I set in my ppp.conf via "set mtu max ...",
>along with "enable mssfixup" to tell ppp to adjust the MSS to no greater
>than the MTU.
>
>That is, I want things set such that my tun0 device reports an mtu of
>1492, given that the ping tests above return 0% packet loss at an MTU of
>1464, but fail at 1465.
>
>Does this sound reasonable?

Yes it does.

You may want to set the MRU along with the MTU (yes, there is an MRU)
since the MRU default is the same as the MTU default: 1500. The MRU
(maximum receive unit) is essentially the same as the MTU (maximum
transmit unit).

So add:

set mru max 1492

Keep in mind what "enable mssfixup" really does. The ppp man page is
not very clear:

[tcp]mssfixup
Default: Enabled. This option tells ppp to adjust TCP
SYN packets so that the maximum receive segment size is not
greater than the amount allowed by the interface MTU.

Is it talking about MRU or MSS? Most sites say that what this does is
adjust the MTU of internal lan clients who are presumably on straight
Ethernet (i.e. MTU = 1500) to the proper value of your PPPoE MTU. The
alternative would be to adjust the MTU values of all internal clients.

A problem related to this discussion may interest you. It is called
the Path MTU Discovery Black Hole and is well described here:

http://www.usenix.org/events/lisa02/...nderberg_html/

/pm