Subject: Re: [rrg] Map and Encaps

From: [email-masked] (Noel Chiappa)
Date: Thu, 27 Nov 2008 09:49:56 -0500 (EST)

> From: "William Herrin" <[email-masked]>

    > History does, however, suggest that the more control you place directly
    > in the hands of the end-user's single PC, the greater adaptability the
    > system exhibits. This holds true whether the end user exercises that
    > control by writing code himself or merely selecting which software he
    > runs.

Yes, but... for really good adaptability it is also necessary that the
changes be within the scope of what will be 'comprehensible' (for lack of a
better word) to unmodified hosts on the far end. I expect that what I'm
trying to get at may be unclear; an example might be clearer...

The classic example of where this kind of local change worked really well is
TCP re-transmission: changes in the re-transmission algorithm were easily
tested and deployed because i) it was implemented in the hosts, and ii) a
modified host worked fine when dealing with a non-modified host.

I don't offhand have an example ready of where local change didn't work well
- although I guess places where we couldn't change things (e.g. IP address
size) are the extreme example of 'non-comprehensibility'.

How to ensure that a design has this kind of flexibility... that's something I
don't have a good feel for. I guess the designer always keeping in mind the
desirabilty of this kind of forward flexibility, designing things so that
algorithms _can_ be replaced, is a good step. (I don't recall if we had this
in mind when doing the original TCP retransmission algorithm - that case may
have been just serendipity.) I explicitly did a routing architecture in which
_none_ of the algorithms were part of the 'core spec' - precisely to enable
this sort of long-term 'local' adaptability.

	Noel
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---

From: "William Herrin" <[email-masked]>
Date: Thu, 27 Nov 2008 10:22:15 -0500

On Thu, Nov 27, 2008 at 9:49 AM, Noel Chiappa <[email-masked]> wrote:
> better word) to unmodified hosts on the far end.

Noel,

I couldn't disagree more. That's a deployability question, not an
adaptability question.

Deployability measures the old system's ability to adapt to the new
one. Adaptability measures the new system's ability to accommodate
subsequent change. We have to measure the two issues separately; they
don't combine.

For example, the metric system is highly adaptable but it has poor
deployability from the english system. You can easily work with any
quantities; its just powers of ten. But none of the tools and parts
match up.

Regards,
Bill Herrin

-- 
William D. Herrin ................ [email-masked]  [email-masked]
3005 Crane Dr. ...................... Web: <http://bill.herrin.us/>
Falls Church, VA 22042-3004
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