[e2e] Explicit Transport Error Notification

[Black_David at emc.com]

> Explicit error notification is just one way to distinguish 
> between losses due to transmission errors and losses due to 
> congestion. An alternative way is to use explicit congestion 
> notification, and assume that packet losses are due to other 
> factors -- e.g., only shrink the window on ECN, and expend it 
> on the ACK. There are obvious deployment issues, but these 
> could be solvved if NASA is operating in any kind of 
> controlled set-up. In any case, the ECN only option could be 
> simulated in NS...

I presume making sure that network nodes never have to drop
packets due to queue overrun/resource shortage/etc. is among
the "obvious deployment issues" ... ECN currently requires that
drops be treated as congestion signals for this reason.

--David