Subject: IPv6<->IPv4 address translation is not NAT?

From: Ralph Droms <[email-masked]>
Date: Thu, 6 Dec 2007 16:38:59 -0500

Geoff - in the v6ops meeting you said that the IPv6 transition problem  
is not a NAT problem.  Can you explain in more detail?  There must be,  
I think. a translation between addresses from incompatible address  
spaces, just like NATv4.  What do you think is the difference?

- Ralph

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From: Geoff Huston <[email-masked]>
Date: Sat, 08 Dec 2007 11:38:06 +1100

Ralph Droms wrote:
> is not a NAT problem.  Can you explain in more detail?  There must be, I 
> think. a translation between addresses from incompatible address spaces, 
> just like NATv4.  What do you think is the difference?

The point I was trying to make is that NATs are not "just" Network 
Address Translators - they are in fact used for a particular style of 
Network address translation which is asymmetric. We all know how a NAT 
is defined - it has an "inside" and an "outside" and packets from the 
"inside" that are heading to the "outside have one set of transforms 
applied to the IP source field and packets in tyhe opposite direction 
have a comparable set of transforms applied to the IP destination field.

So we have assumptions of "inside" and "outside", and only one part of 
the header is touched, etc etc.

The point I was making at the microphone in the WG was that the V4 / V6 
translation problem shares very little with the asymmetric NATS that we 
know as "NAT" as this protocol transform is an entirely different form 
of address translation which has different triggers, semantics and 
states. And I was trying to say that calling them both "NATS" is 
uncomfortable for me.

Now is that clearer or muddier?

    Geoff

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From: Brian E Carpenter <[email-masked]>
Date: Sun, 09 Dec 2007 08:23:42 +1300

> Now is that clearer or muddier?

I think an important point of commonality is that when translating
between a smaller and a larger address space, you're forced to
deal with port mapping as well as with address translation.
That is the same problem whether the smaller/larger pair
is single-IPv4/RFC1918 or IPv4/IPv6.

Maybe we should talk about address sharing rather than address
translation?

      Brian

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From: Geoff Huston <[email-masked]>
Date: Mon, 10 Dec 2007 04:53:12 +1100

> deal with port mapping as well as with address translation.

not "forced", surely. "Might elct to" is close to the truth.

And the algorithmic embedding of A in B applies asymmetrically.

> Maybe we should talk about address sharing rather than address

I don't agree.

What is going on appears to be the "type" of translation.

One approach preserves stateless capability in the network elements and 
all that is a consequence avoids the use of placing translation sate  i 
the network elements, and "embeds" the translation into the packet so 
that a remote entity (or the same entity at a later time) can reverse 
the translation and recover the orginal intended context.

Another approach attempts to place state into network elements and then 
all the packets within a context are treated in the same way. NAT 
bindings are a good example of this state.

Analogy of the differences between approaches: Imagine that you are 
flying home from Vancouver and you have no short term memory (it happens 
:-)). What do you do when you get off the flight in Canberra and you 
have absolutely no memory that you left your car at the Airport in Row 
13? But you knew when you parked your car a week ago that you would 
forget. Do you write down where you parked it and keep this mesage with 
you through your trip so that its at hand upon your return (option a). 
Or pin the message to the airport information board and hope that it 
will still be there upon your return (option b).

The algorithmic embedding is one of these "packet-state" approaches 
where any network element could undo the operation given that the packet 
contains sufficient information to reverse the operation.

That's a long way of saying that I don't agree that embedding is 
sharing! Embedding is another approach to translation :-)

   Geoff

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From: Brian E Carpenter <[email-masked]>
Date: Mon, 10 Dec 2007 09:23:37 +1300

> not "forced", surely. "Might elct to" is close to the truth.

I think it's forced iff you are seeking a purely stateless
solution, which I realise from your explanation below is
not what you're assuming.

> And the algorithmic embedding of A in B applies asymmetrically.

Correct. (Shanti is an example of that.)

>> Maybe we should talk about address sharing rather than address

Well, isn't the goal to allow entities in a smaller address
space to access entities in a larger address space? I think
that necessarily requires sharing (or multiplexing if you
prefer).

> What is going on appears to be the "type" of translation.

Yes, I agree with the distinction you make below - it just
isn't quite what I was writing about.

      Brian

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From: Geoff Huston <[email-masked]>
Date: Mon, 10 Dec 2007 09:47:40 +1100

> isn't quite what I was writing about.

I had misunderstood what you were referring to with "sharing". I agree 
with "sharing" in the context you've described.

   Geoff