Ed Gerck <egerck(_at_)nma(_dot_)com> wrote:
Steve Deering wrote:
At 3:41 PM -0800 2/15/01, Ed Gerck wrote:
You give a name to your house (say, "The Tulip") and
the post office knows where The Tulip is. If you move,
you can do the same at your new location, provided
there is no conflict.> >
They also do it without removing the original destination address and
replacing it with another one -- the original envelope arrives at the
house with the destination address still saying "The Tulip", i.e., it
has not been translated, and thus is not analogous to NAT.
I think you got the example addresses reversed. In the case I mention,
"The Tulip" is the global address and (for the sake of example) suppose
now that "545 Abbey St." is the local physical address known to the post
office.
Thus, when the mailman delivers an envelope addressed to "The Tulip" at
"545 Abbey St.", that mailman is doing address translation -- and he may
even have written "545 Abbey St." on the envelope as a reminder. So,
when the original envelope arrives at the destination address it did so not
because it had "The Tulip" written on it but because the post office was
able to do address translation to the *current* location which is "545 Abbey
St."
That still doesn't sound like NAT. A complete address which specifies
your town and house name, is global, and has a one to one mapping with
your house. Your house can both initiate communication and receive
communication initiated by others, at that address, and no other house
uses that address. No rewriting of envelopes is done, and no
disruption of the "end to end" nature of addressing is involved.
The fact that your address actually has to be silently translated to
another address by the post office, at the local hop only, and
*invisibly* from you and your correspondents, makes this a natural
example of protocol layers, not of address translation. It's as if
"1234 Foo Street" is your MAC address, and "Tulip, BarBurgh, Scotland"
is your IP address. The local post office, and *only* the local post
office, needs to keep a mapping between street addresses and house
names, for their town (aka segment or LAN). You only know your own
street address and your own house name. And you never (need) use your
street name in any communication, only in communication management
(i.e. telling the postal system that you've moved).
Layering most certainly *does* occur naturally in communication.
That's why the best tutorials that try to explain protocol stacks and
layers to non-technical people, usually make analogies to things like
postal mail, or to bosses who communicate via secretaries who can
freely change between fax or mail without changing the content of the
messages exchanged by their bosses.
NAT, as far as I can tell, is pretty much always a kludge, whether
it's natural or not. It doesn't make people happy unless obscurity
and reduced communication is what they're explicitly seeking.
-- Cos (Ofer Inbar) -- cos(_at_)aaaaa(_dot_)org
cos(_at_)exodus(_dot_)net
-- Exodus Professional Services -- http://www.exodus.net/
"OSI is a beautiful dream, and TCP/IP is living it!"
-- Einar Stefferud <Stef(_at_)nma(_dot_)com>, IETF mailing list, 12 May 1992