[e2e] What should e2e protocols know about lower layers?

[Christian Huitema]

Let's focus on the original TCP question, rather than engaging on an IPv6 sideline. I agree with Vernon on this. There are really many examples of network configurations in which you actually want to perform congestion control, even if the peer is "one IP hop away": radio links that are shared through CDMA, switched LANs in which congestion may happen in layer 2, VPN connections that are tunneled through an undeterminate number of routers. 

It should be pretty obvious that suppressing congestion control based on a naïve test such as "same subnet mask" is BAD. OTOH, developers do this for a reason: they are generally frustrated that TCP does not take full advantage of the capacity of a high speed network, especially in the case of short connections. Make no mistake, the pressure is going to increase with applications such as ISCSI and gigabit networks. 

-- Christian Huitema