[Olsr-dev] mtu problem with multicast ipv6 packets

John Hay (spam-protected)
Mon Mar 23 19:46:37 CET 2009

On Mon, Mar 23, 2009 at 07:30:05PM +0100, Henning Rogge wrote:
> On Montag 23 M?rz 2009 19:10:46 John Hay wrote:
> > Hi,
> >
> > Last year I reported a problem with FreeBSD (and I guess the others with
> > KAME derived IPv6 stacks) where they would limit multicast packets to
> > 1280 bytes. This caused fragmentation inside the stack. Possible solutions
> > floated around, but was never committed.
> >
> > Here is a fix that only touch one file. The effect of setting the socket
> > option IPV6_USE_MIN_MTU to 0 is that the stack will not limit multicast
> > packets to 1280 anymore, but to the mtu of the interface.
> Is there a drawback to set this parameter always on ? Or did I misunderstand 
> the patch and IPV6_USE_MIN_MTU is set by some system include file ?

IPV6_USE_MIN_MTU can have 3 values: -1, 0 and 1. The default is -1.
RFC3542 section 11.1 describe the behaviour:

-1: perform path MTU discovery for unicast destinations but do not
    perform it for multicast destinations.  Packets to multicast
    destinations are therefore sent with the minimum MTU.

 0: always perform path MTU discovery.

 1: always disable path MTU discovery and send packets at the minimum

> Just another thing, at the moment I'm looking at the linux kernel multicast 
> implementation to see if we can get a kernel based multicast for OLSR. You 

What do you mean with "kernel based multicast for OLSR"?

> seem to have lot's of knowledge about BSD multicast, would you be interested 
> in joining the project ? It would be great to have multicast capable OLSR for 
> linux AND bsd... if yes just contact me directly please.

John Hay -- (spam-protected) / (spam-protected)

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