An MPLS network is a single trust domain where the edges must be in
control of what labels make their way into the core. The simplest way
of ensuring this is for the edge device to always impose the labels,
and not allow forward labeled traffic from untrusted neighbours. This
is achieved by allowing a per-device configuration of whether MPLS
traffic input from that interface should be processed or not.

To be secure by default, the default state is changed to MPLS being
disabled on all interfaces unless explicitly enabled and no global
option is provided to change the default. Whilst this differs from
other protocols (e.g. IPv6), network operators are used to explicitly
enabling MPLS forwarding on interfaces, and with the number of links
to the MPLS core typically fairly low this doesn't present too much of
a burden on operators.

Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: NRobert Shearman <rshearma@brocade.com>
Reviewed-by: N"Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

This sysctl gives two benefits.  By defaulting the table size to 0
mpls even when compiled in and enabled defaults to not forwarding
any packets.  This prevents unpleasant surprises for users.

The other benefit is that as mpls labels are allocated locally a dense
table a small dense label table may be used which saves memory and
is extremely simple and efficient to implement.

This sysctl allows userspace to choose the restrictions on the label
table size userspace applications need to cope with.
Signed-off-by: N"Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>