Re: question about IPv6 Flow label

[David Malone <dwmalone@maths.tcd.ie>]

On Mon, May 18, 2009 at 03:22:46AM +0200, marcelo bagnulo braun wrote:
> In the MEXT WG we are discussing about using the Flow Label as a flow 
> descriptor and we were wondering how widely implemented is RFC3697. In 
> particular, how many of the current OSes actually do:

I had a look at this a few years ago with Orla McGann, because we
wanted to know if the flow label could be used by a stateful firewall:

	http://www.maths.tcd.ie/~dwmalone/p/ec2nd05.pdf

It seemed that there were some problems where the flow label was
not set consistently. We fixed this up in FreeBSD, so I can tell
you what happens there. For TCP the flow label is usually set
randomly, either using a hash of the connection details (+ a secret)
or is set randomly using a PRNG. At the moment there isn't check
to make sure the flow label is not shared between two flows.

For UDP, I think the flow label will default to zero.

	David.