[rbridge] ECN RE: TRILL Options

[Caitlin Bestler]

James Carlson wrote:


> In any event, I'd expect the encapsulating TRILL node to copy the ECN
> bits (likely similar to RFC 3168, but perhaps requiring mapping for
> non-IP protocols) into the TRILL header, and the decapsulating TRILL
> node to apply those ECN bits to the encapsulated packet before
> disgorging it onto the destination network.  You then have a record of
> congestion encountered in the middle of the bridged network.
> 
> Systems that don't do ECN will generally ignore the bits (and will set
> 00 on transmit), and those that do it will do the Right Thing.  If the
> TRILL network doesn't support ECN at all, then the bits get copied
> from input to output unmolested -- meaning that congestion through the
> larger network is still detected, but that congestion within the TRILL
> network is invisible.
> 
> If the TRILL encapsulator doesn't do ECN, then the bits will be zero
> on input, and the egress can ignore them on output.  If the egress
> doesn't do ECN, then the inner packet will be left unchanged, again
> meaning that ECN works but that the TRILL network is not visible.
> 
> Finally, if the carried protocol doesn't support ECN, or the ingress
> RBridge doesn't know how the next level protocol deals with ECN, then
> set the bits to zero on entry and thus do nothing on egress.
> 
> No "intent" needs to be communicated, as best I can tell.  It's a
> best-effort signaling mechanism.
> 
> > If it uses bits either in the TRILL Header or
> > in a TRILL Option, these are lost at the egress Rbridge. -- Although
> > perhaps affecting Rbridge behavior could also be beneficial. And for
> > IP frames, we could set bits in the IP header...
> 
> That's it exactly.  I think ECN is pretty much worthless otherwise;
> it's an end-to-end exercise involving the transport layer, or it may
> as well not be implemented at all.  ECN that's consumed by RBridges
> alone doesn't make any sense to me.
> 

A TRILL option with ECN semantics could be used directly by an RBridge
that had other traffic control options on its local ports than just
PAUSE, Priority-based Flow Control or Drop. If such methods were 
available, they would be preferable to modifying the L3 header.