At 3:41 PM -0800 2/15/01, Ed Gerck wrote:
"Steven M. Bellovin" 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.> >
...Note that this is a natural example of NAT,
in which the post office is doing the address translation to a local
address that only that post office knows, but which is globally
reachable through that post office. And the post office does so
without changing the global addresses or the local addresses.
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.
If delivery is accomplished by having all the necessary the UK post
offices and postpersons remember a routing from "The Tulip" to its
current street address, then its IP analog is having the routers
within a site maintain a host route for a specific IP address.
If, on the other hand, only the UK-entry post office maintains the
mapping and sticks the original envelope inside another envelope
(or puts a yellow sticky note over the original address), addressed
to The Tulip's current street address, then its IP analog is having
the border router maintain a tunnel to an individual interior host,
encapsulating the original packet with another header.
A closer postal analog to the typical port-and-address-mapping NAT is
a system in which postal envelopes only have room for a street address
or a town name, but not both. If I send a letter to someone outside
my town, the letter starts off with a return address of:
Steve Deering
123 Main Street
and the town's post office overwrites that return address, changing it to:
Priscilla Presley
San Jose, CA, USA
and they remember for a while that they did that, so that if my
correspondent decides to reply to that return address, the town post
office knows who it should be delivered to. (They replaced my name
because someone else named Steve Deering recently sent mail from
another street address in my town, and the only way to keep the
replies separate is to change the name that I will be [temporarily]
known by in the outside world.)
At some point, they discard the remembered mapping, to free up some
names. Perhaps they do that based on a time-out, in which case the
mapping may disappear before we are finished corresponding, and thus
cause our communication to fail. Or maybe they open up our letters and
look at the contents to try to identify the final letter of our
correspondence, to guess when we might be done. Of course that latter
approach doesn't help if they don't understand what language our letters
are written in, so maybe they decide to limit us to only a small choice
of languages, and just discard anything they don't understand.
Furthermore, no one outside my town can initiate a correspondence with
me, unless I work out some arrangement with the post office to get
long term external use of someone's (preferably my own) name. Or else
I have to go and get a town name for myself.
I don't want to be philosophical about this, but IMO this example
actually supports the view that NATs are naturally occuring solutions
to provide for local flexibility without decreasing global connectivity.
Since the example was not an example of a NAT, I don't think it
supports any such view.
However, I suppose a postal system like the one I described might
"naturally occur" as a response to having envelopes that were no
longer big enough to contain full addresses. But I think it much
more likely that post offices and people would somehow arrange to
just use bigger envelopes, rather than incurring all the extra complexity,
cost, fragility, and loss of functionality of the translating approach,
except as a temporary stop-gap.
Unless, that is, we were talked out of it by folks claiming that
changing the size of envelopes would be an impossibly large task, and
that we're better off anyway with the translating system, because
our personal names and street addresses can be kept secret within our
town, and we can change the name of our town any time we like without
bothering anybody in it.
Steve