[e2e] Layering vs. modularization

[Joe Touch]

John Day wrote:
...
> Gee, the only layering I have ever seen that had problems were the ones 
> done badly, such as with the Internet and OSI.  In my experience, if you 
> do it right, it actually helps rather than gets in the way. 

That's been my perspective as well; most of the issues I've seen with 
layering problems - including "layer violation" really turned out to be 
gaps in the layering structure that were patched over with violations 
rather than fixed correctly.

We've seen a few key issues with layering (insert self promotion warning 
here), and our observations suggest that it may be a fundamental 
construct, rather than an artifact to be "erased" in a clean-slate approach:

1. layering often focuses on the layers and ignores the inter-layer 
glue; Yu-Shun Wang's thesis (a recent PhD student of mine) focused on 
this issue, and found numerous ways in which the interlayer glue was 
similar at nearly every layer boundary, and could be addressed by an 
additional, meaningful generic mechanism

2. layering disagreements sometimes revolve around what each layer 
means, as unique from other layers; our NSF FIND "RNA" project (to 
appear at ICCCN, Future Internet track) is exploring the ways in which 
layers are internally similar, but their behavior is governed by their 
relative position (what's above and below) and their scope 
(distance/time extent). This turns the "one layer to bind them all" (ala 
XTP) into a single 'stem cell' layer that acts like different, 
well-known OSI layers when stacked. The idea is that the environment of 
a layer defines the layer, and that layering is semantically useful in 
composing services on each other.

Joe

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