[e2e] End-to-end is a design guideline, not a rigid rule

[Black_David@emc.com]

Dave,

> On the question of the "end to end" construct I believe we suffer from 
> viewing it simplistically.  What I think our community has missed is that
it 
> is a design guideline, not a rigid rule.  In fact with a layered 
> architecture, the construct varies according to the layer.  At the IP
level, 
> this is demonstrated two ways.  One is the next IP hop, which might go 
> through many nodes in a layer-2 network, and the other is the 
> source/destination IP addresses, which might go through multiple IP nodes.
> 
> The TCP/IP split is the primary example of end-to-end, but it is
deceptive. 
> TCP is end-to-end but only at the TCP layer.  The applications that use
TCP 
> represent points beyond the supposed end-to-end framework.
> 
> My own education on this point came from doing EDI over Email.  Of course
I 
> always viewed the email author-to-recipient as "end to end" but along
comes 
> EDI that did additional routing at the recipient site.  To the EDI world, 
> the entire email service was merely one hop.
> 
> This proved enlightening because the point has come up repeatedly:

I strongly agree with this point, and want to remove it from its original
organizational boundary context.  IMHO, Organizational boundaries are
(or at least start out as) layer 9 (Political) constructs, and Engineering
techniques don't seem to be particularly effective much beyond layer 7 ;-).

Anytime the end-to-end topic comes up in a design discussion, I always
ask two questions:
	- Where are the ends?
	- What is the service being provided between them?
The latter question (IMHO) tends to be both more important and harder
to answer than the former.

Another area where this "end-to-end is just a hop" perspective comes
up is security.  In the IPsec arena, both site-to-site and remote
access VPNs compress an arbitrary unprotected network path into what
looks like a single hop in a somewhat more protected LAN.  The
underlying end-to-end IPsec service has very strong security
properties, but in the bigger picture, it's just a hop in a managed
LAN (in some sense) service with different properties.

Thanks,
--David
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David L. Black, Senior Technologist
EMC Corporation, 176 South St., Hopkinton, MA  01748
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black_david at emc.com        Mobile: +1 (978) 394-7754
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