[Iccrg] Explicit feedback

[Tony Li]

> > verifiability perspective.  Any in-band packet 
> modifications (e.g., RFC
> > 3168 ECN) are problematic because packet header examination and
> > modification is typically done on the ingress side of the 
> router, prior
> > to the actual switching of the packet.  For scalability reasons,
> > maintaining full active queue state on the input side of 
> the system is
> > challenging, as the queues only truly exist on the output side.
> 
> This part interests me: why is this a problem?
> Is it because separate hardware pieces are involved on the ingress
> and egress side, where every such piece (chip) would do all the
> necessary work for one link only?


Typical high end routers now are designed for tens or hundreds of
thousands of queues.   Some systems are probably well over a million.
The egress side has queue information that it needs to propagate in
real-time to the ingress side over an unreliable medium.  This is
certainly solvable, but it comes at a price in bandwidth and mechanism,
multiplied by scale and timeliness.  Justifying that cost is, IMHO, very
hard.

Tony