From: Tom Herbert <redacted>
Date: Thu, 30 Jul 2015 09:54:19 -0700

BSD (MacOS) has already turned on flow labels by default and this does
not seem to be causing any problems in the Internet. Let's go ahead
and turn them on by default. We'll continue to monitor for any devices
start choking on them.

Flow labels are important since they are the desired solution for
network devices to perform ECMP and RSS (RFC6437 and RFC6438).
Traditionally, devices perform a 5-tuple hash on packets that
includes port numbers. For the most part, these devices can only
compute 5-tuple hashes for TCP and UDP. This severely limits our ability
to get good network load balancing for other protocols (IPIP, GRE,ESP,
etc.), and hence we are limited in using other protocols. Unfortunately,
this method is accepted as the de facto standard to the extent that
there are several proposals to encapsulate protocols in UDP _just_ for
the purposes for getting ECMP to work. With hosts generating flow labels
and devices taking them as input into ECMP (several already do), we can
start to fix this fundamental problem. 

This patch set:
 - Changes IPV6_FLOWINFO sockopt to be opt-out of flow labels for
   connections rather than opt-in
 - Disable flow label state ranges sysctl by default
 - Enable auto flow labels sysctl by default

I am pretty sure administrators are going to want a way to enforce that
all applications must use flow labels.

So can you add a third mode that turns on auto-flow-label, but
disallows the application level opt-out?

Thanks.