IPv6 mtu autoconfiguration

[Francois Tigeot]

Hi all,

I am experimenting with IPv6 on a small LAN. All machines use
Dragonfly-1.6.x.
The gateway uses a PPPoE ADSL modem, and so is unable to route
packets > 1492 bytes.

With IPv4, TCP MSS is reduced automagically by ppp.

Since there is no such trick for v6 tcp connections, I have tried to force
a smaller IPv6 MTU on the client machines by setting up rtadvd with these
options on the gateway:

vr0:\
   :mtu#1280:

rtadvd runs fine, and I can the advertisements for a 1280 bytes MTU
broadcast on the LAN.

Client machines pick up their adresses and network mask correctly, but
their MTU stays at the 1500 bytes ethernet default.
Any TCP connection to a v6 connected host on the Internet thus fails.

AFAIK, the only way I have found to reduce an interface MTU is by using
ifconfig manually.

Is this the intended behavior ?

What's more, by using ifconfig manually, the interface MTU is changed for
both IPv4 and IPv6 protocols. There is no way to specify a v4 or v6 only
behavior. This may be related to the previous issue.

I hope someone can shed some light on this.

-- 
Francois Tigeot