[e2e] Layering vs. modularization

Joe Touch touch at ISI.EDU
Thu May 15 17:55:03 PDT 2008

Hi, George (et al.),

George Michaelson wrote:
> 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

Strictly, recursion and encapsulation assume that the inner/lower layer 
respects the boundary. It doesn't have to unless there are enforcement 
mechanisms - that's the difference between strict and non-strict languages.

Encapsulation strictness is enforceable - just encrypt the payload.

> 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

Outer V6 would read inner V4 to support path 'fate sharing', i.e., when 
doing multipath routing it's useful to ensure that 'flows' traverse 
similar paths, and in this case the V4 address could be the best cue to 
a flow.


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