[e2e] Layering vs. modularization

George Michaelson ggm at apnic.net
Thu May 15 15:56:03 PDT 2008

On 16/05/2008, at 8:20 AM, David P. Reed wrote:
> b) Explain protocol encapsulation  (sending IPv6 datagrams within  
> UDP VPN packets over a TCP based overlay network implemented in  
> userspace stacks on machines that offload part of the VPN  
> implementation to a peer within a bluetooth subnet) as a form of  
> layering?   It seems to me that encapsulation is akin to allowing  
> recursion in one's language.   Languages that allow recursion are  
> unlike FORTRAN 77, which is "layered".

recursion requires that first-class data constructs in the language be  
respected, so stack frame boundaries, globals etc are meaningful.

encapsulation doesn't require this. the encapsulated protocol has its  
own e2e significance and its own routing. for the purposes of  
encapsulation, its just data.

therefore the comparison (as in most analogies?) for me, is not a good  
fit. actually, I  think most things described as recursive usually  

stateful packet inspectors *might* need a re-write, but that aside, I  
don't see  how anything other than a bug would make the outer V6  
active units need to read the inner V4 payload, or vice versa


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