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Re: NAT natural example, Re: [midcom] WG scope/deliverables

2001-02-15 22:50:03


Steve Deering wrote:

At 6:21 PM -0800 2/15/01, Ed Gerck wrote:
...
In Internet NAT terms, "The Tulip" is the globally routable IP number for
my DSL, the post office is my NAT box and the physical address
"545 Abbey St." is the local, non-routable IP number of my host A.

That would be analogous to having "The Tulip, UK" be the address of
a post office, with all houses served by that post office sharing
the same global address of "The Tulip, UK".  That indeed is like a
NAT, but is not the same as the original example.

To be precise and still with the original example, the analogy is that
"The Tulip, CMZ 62N, UK " is the full global address (which was described
in the context of my email as  <"The Tulip" at that post office>).
The full designation "The Tulip, CMZ 62N, UK" is thus similar to a globally
routable address (Internet IP) that is available at the post office
"CMZ 62N, UK" (NAT box) and which may at times correspond
to a house at "545 Abbey St" (host A) or to a house at "636 North Av"
(host B), which mapping that post office knows at each time and uses
to direct correspondence to the proper house without revealing to the
outside world what that local address might be  -- ie, either "545 Abbey St."
(host A) or  "636 North Av" (host B), or any other.

All houses served by that post office share "CMZ 62N, UK" while the house
name is similar to a port number in NAT (different for each house being served).

Note also that my NAT analogy only dealt with receiving mail, not sending mail.
Mr. Tulip may send mail any way he wishes, with a global return address as
"The Tulip, UK", with a local address as "545 Abbey St", with a fake return
address or even with no return address.

Let me now address your objection that  "A host behind a NAT, on the other hand,
doesn't know its own global address and, in most cases, doesn't even have a
global address (or one port's share of a global address), except temporarily as 
a
side-effect of sending a packet to the outside world". We may agree that
we are dealing here with two different processes -- sending information and
receiving information.  An UK post office  was presented as a NAT analogy
for receiving information, not to send information.  In receiving information,
Mr. X  (a host behind the NAT) does not need to know how the house
he just moved in is named at the post office -- and, nonetheless, he will get
any letters addressed to "The Tulip, CMZ 62N, UK" if that is the house's name at
the post office "CMZ 62N, UK". The temporary property of the global address is
also present in the UK post office example for receiving information -- just 
that
the time scale may be hundreds of  years, not milliseconds.

Your other objection was that "In the case of NAT, on the other hand,
the destination address used across the public part of the Internet is no longer
present in the packet finally delivered to the destination host -- it has been
been replaced by (i.e., translated to) a different address".  My reply is
that this does not occur in NATs if the destination address is also included
in the packet payload, which is the case here -- the envelope is part of
the message's payload in the post office case. Pls see also my last comment,
below.


In other words, this is a natural NAT example...

The original example, of a single house with the global address of
"The Tulip, UK"  is a naturally occurring example of something like ARP
or something like tunneling, not something like NAT.

I agree that you can define many different analogies, from that example. But,
as above, if you consider the way that information is received then a NAT box
is IMO one valid analogy for reception because it satisfies the functionality
observed in a NAT box when receiving packets.  Yes, the UK post office does
not erase the global address on the envelope but a NAT will also keep that
information in the translated packet if it is in the packet's payload (which
is the case for the letter's envelope), and without any impact in its 
functionality
as a NAT.


The distinction is betweeen doing a mapping/encapsulation and doing an
address substitution.  NATs are all about doing address substitution; the
post office does mapping/encapsulation to deliver to The Tulip.

At the post office routing level, letters that enter a common input bin are 
moved to
different output bins at the post office. The common input bin is a globally
routable address such as "The Tulip, CMZ 62N, UK", "The Raven, CMZ 62N, UK",
etc. -- where the only part that is globally meaningful is "CMZ 62N, UK".  Each
output bin corresponds to a local address mapped from the local qualifier
"The Tulip", "The Raven", etc. Each output bin, however, has no marking for
any local qualifier ("The Tulip"), just for a local address ("545 Abbey St").
Thus, there is no encapsulation at the post office routing level -- anyone
looking just at the bin "545 Abbey St" could not tell which local qualifier
was used for the letters inside the bin.

But, what happens if I look at the contents of the bin?  The same happens
with NAT boxes, I note, because the message may contain the destination
IP in its payload and no amount of packet address translation will erase it.
Looking into the bin is the same as looking into the payload.

Cheers,

Ed Gerck