[trill] MD-levels and TRILL

[Anoop Ghanwani <[e-mail]>]

There are a couple of challenges that I see with trying to
define the use of MD-levels with TRILL.

MD-levels in the IEEE world were used to creates multiple
levels at which CFM can be run.  For example, if I buy a service from a
provider and start running CFM, only my boxes (my network and
the edges of the provider network) will see and respond to those
CFM messages.  Maybe this type of service model doesn't
make sense for TRILL because, if the provider implements
the transport using RBridges, then the customer and provider
will have to make sure they assign their nicknames from the
same space.  But let's leave that issue aside for now...

The first challenge is that TRILL depends on the Hop Count
for some of the OAM messages to decide whether they are
processed by an RBridge.  This means that it is not possible
to send these messages in a way that is transparent to
set of RBridges.  So pretty much all OAM messages must
be processed by all RBridges along a path regardless of
level.  So I'm not sure what the concept of MD-level would
buy us.  At best it may be used for filtering and that too in
the control plane, but I personally don't see much value there.

The second challenge is related to accessing the MD-level
withing the frame to know whether or not to process it.
Because the MD-level would follow the 128-byte entropy,
I am not sure that existing silicon, and maybe even emerging
silicon, will be able to access those bits, so that level of
filtering may have to be done in software.  I think this should
be clearly called out in the draft.

Anoop