Subject: NAT-PT: To deprecate or not to deprecate: the question for next w eek's v6ops discussion

From: "Elwyn Davies" <[email-masked]>
Date: Fri, 5 Nov 2004 19:55:33 +0100

This email summarises the discussions that have taken place on the list
since the publication of  draft-aoun-v6ops-natpt-deprecate-00 and is
intended to servce as a basis for the debate at IETF-61.  I intend to
quickly reiterate these points (as modified by mailing list input between
now and then) at the meeting before we discuss how to proceed.

1. Document content: The issues with NA(P)T-PT:
- The general view appeared to be that the issues were pretty much complete
and accurate.  I have some detailed comments from Tony Hain but
unfortunately these relate to a pre-publication version of the draft and I
need to go back over the points with him to determine which of them are
still valid, as I think most of them had already been addressed.
- There was some discussion over whether the issues which NAT-PT and NAT
share should be duplicated or indeed be called out at all in this document.
	o Part of the point is that if we can live with these points in
respect of NAT,
        these are not necessarily reasons to deprecate NAT-PT.  NAT solves
an ongoing
        problem in IPv4 whereas NAT-PT is intended to be a transition aid.
On 
        the one hand we may feel it is easier to live with some issues if
the mechanism
        is not with us for ever, but on the other should we use a mechanism
that has 
        all these issues at all?
      o Also these issues will apply to *any* translation mechanism rather
than just
        the SIIT + DNS-ALG version which we the document is trying to
justify 
        deprecating
My view is that the issues should be called out in the document, but we
should make it more clear that they will apply to any translation mechanism
as well.  I don't think it is good to have to point to a lot of v4 specific
documents which cover other ground as well, particularly if we end up not
deprecating NAT-PT completely.  Views are sought.
- How to handle the connection to all the prior drafts which are (partially)
summarised in this document.  Is the current approach correct?

2. Use Cases for NAT-PT and other translation mechanisms
- A number of use cases where NAT-PT is being used (whether appropriately or
not) or is thought to be a possible solution were discussed. More input on
other cases and details of identified cases has been asked for and is still
wanted in some cases - please send in detailed info if possible.
- In some of these cases it looks as if NAT-PT is being used to solve a
problem which it was not really intended (e.g. using back-to-back NAT-PTs to
provide connection between (private) IPv4 domains across an IPv6 only
backbone). I believe that NAT-PT was intended mainly to solve the problem of
connecting an IPv4 only host/domain to an IPv6 only host/domain.  The
back-to-back NAT-PT solution seems to be better covered by a 4 in 6
tunneling solution.  Of course many of the issues called out in the draft do
not apply in this situation because only the IP header needs translating and
e2e IPsec will work because the packet is restored to its original form when
it reaches the other end.  ALG's would not be required.  I am not clear that
the standard form of the DNS-ALG proposed in 2766 applies to this use case.
- Front ending a legacy v4 only server: This is clearly a good use for a
straightforward translator, but there is little or no need for the
generalised DNS-ALG in this case and the translator would generally have to
handle a limited set of protocols and a limited set of v4 addresses defined
by the server it was front ending.  This is not an application for the full
generality of 2766 but I think we have to define a reduced version for this
case.  Whether it is still called NAT-PT is a moot point.
- The military case: I call it the military case but there are other related
applications which have the same problem to solve.  As I see it the scenario
involves a network of (newly created/adapted) v6 only devices that are
unable to run dual stack because of resource constraints either in the
devices or in the network connections.  These devices will, at least
transiently, need to communicate with v4 devices in other domains (which
have to remain v4 only for the same reasons of resource constraint).  The
question is whether NAT-PT is the best technology to connect the domains.
One question in my mind for the military case: The military specifically
wanted to avoid putting NAT boxes in their  networks for good cause (single
points of failure, routing constraints, security attack nexuses) - so why
should they now want NAT-PT in their networks? On the other hand there will
be at least some occasions when they have to work across domains during the
transition, but ALGs/proxies may be a better solution given that these
devices are likely to have a relatively limited set of applications.  There
are also issues with using DNS in this sort of environment - it is a 'high
availability/mobility' area and there may be worries about DNS caching
lifetimes.  More input is needed here.
- 3GPP wireless: As mentioned in the draft, it may be better to consider
using 4 in 6 or 6 in 4 tunnels to allow a single PDP context to carry both
protocols.  This may require updates to RFC3314. More work is needed to
ensure that header compression can reduce the impact on scarce bandwidth
across the air interface (an extension of ROHC should be possible to cover
tunnel headers).

Do these cases cover all the expected scenarios?  Is the analysis correct?

3. NAT-PT vs proxies
Are proxies a better solution for most/all cases?  Either proxies or NAT-PT
need to be extended to cope with new protocols in most cases.  Proxies may
be more scalable (not all in the same box) and more adaptable to user needs
(not dependent on a single vendor to do NAT-PT ALG upgrades). The most
frequently used protocols today are HTTP, POP, SMTP, FTP and SSH: proxies
are available for the first four - SSH is more problematic but is mostly a
tool for power users (who are more likely to have dual stack support) and so
SSH support may not be so important.

Way forwards:
- Tune the details
- Show real examples, if any, where translators will really give better
performance than proxies and are really vital. Make sure that backwards
compatibility is fully considered, taking into account availability and
migration of terminals from v4 to dual-stack/v6 plus the availability of
tunnel support. 
- Either:
   o Deprecate NAT-PT altogther and write other drafts with 'offspring of
NAT-PT' to
     cover the specialised use cases where a translator is useful, or
   o Turn this into a new applicability draft recommending only a very
limited set 
     of uses, and produce modifications to cater for some of the specialised
use cases.
- Produce some recommendations on proxies and look to see which other
proxies need specification (if any).
- Ensure there are suitable specification(s) for 4 in 6 tunneling
(configured, auto).
- Ensure that the various scenario drafts recommend the 'right' solution.

Regards,
Elwyn

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------

Elwyn B Davies

        Routing and Addressing Strategy Prime & IPv6 Core Team Leader
        CTO Office, Portfolio Integration		Solutions Ready

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From: Pekka Savola <[email-masked]>
Date: Fri, 5 Nov 2004 22:12:29 +0200 (EET)

A third possibility could be,

>     of uses, and produce modifications to cater for some of the specialised

(Writing strictly as another bozo on the bus..)

     o retain the tone of the draft, but instead of complete 
deprecation of NAT-PT, just classify it as Experimental.

There's precedent for that.  We want to send a clear message that 
NAT-PT is not good, but on the other hand, in some cases it might be 
the only alternative.

Experimental might do that: send a sufficiently clear signal, while 
still acknowledging that there may be somoe uses for it, and more 
investigation of the problem space might be warranted.

-- 
Pekka Savola                 "You each name yourselves king, yet the
Netcore Oy                    kingdom bleeds."
Systems. Networks. Security. -- George R.R. Martin: A Clash of Kings