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Internet Draft                                           Johan Ihren
draft-ietf-dnsop-v6-name-space-fragmentation-00.txt       Autonomica
January 2002
Expires in six months


         IPv4-to-IPv6 migration and DNS name space fragmentation


Status of this Memo

   This memo provides information to the Internet community.  It does
   no specify an Internet standard of any kind.  This memo is in full
   conformance with all provisions of Section 10 of RFC2026.

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt The list of
   Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

Abstract

   This memo documents some problems forseen in transitioning from a
   IPv4-only DNS hierarchy via a long period of mixture to an
   IPv6-mostly situation sometime in the future. The mixture period is
   expected to be very long, and hence design choices should very much
   take this into account, rather than just regard the transition as a
   relatively short period of pain.

   The main problem with transition that this paper focus on is what
   to do about the name space fragmentation that may result from
   certain DNS data only being available over one type of transport
   (i.e. v4 or v6) which is thereby likely unavailable to hosts that
   can cannot utilize that transport.

   Two orthogonal issues are identified and discussed: deployment and
   use. The former while technically simple holds certain dangers that
   should be avoided. The "use" (as in performing DNS lookups) is much
   more complicated, and a roadmap for this is presented.

1. Terminology

   The key words "MUST", "SHALL", "REQUIRED", "SHOULD",
   "RECOMMENDED", and "MAY" in this document are to be interpreted as
   described in RFC 2119 [RFC2119].

   The phrase "v4 name server" indicates a name server available over
   IPv4 transport. It does not imply anything about what DNS data is
   served. Likewise, "v6 name server" indicates a name server
   available over IPv6 transport.


2. Introduction to the problem of name space fragmentation

   With all DNS data only available over IPv4 transport everything is
   simple. IPv4 resolvers can use the intended mechanism of following
   referrals from the root and down while IPv6 resolvers have to work
   through a "translator", i.e. they have to use a second name server
   on a so-called "dual stack" host as a "forwarder" since they cannot
   access the DNS data directly. This is not a scalable solution.

   With all DNS data only available over IPv6 transport everything
   would be equally simple, with the exception of old legacy IPv4 name
   servers having to switch to a forwarding configuration.

   However, the second situation will not arise in a foreseeable
   time. Instead, it is expected that the transition will be from IPv4
   only to a mixture of IPv4 and IPv6, with DNS data of theoretically
   three categories depending on whether it is available only over
   IPv4 transport, only over IPv6 or both.

   The latter is the best situation, and a major question is how to
   ensure that it as quickly as possible becomes the norm. However,
   while it is obvious that some DNS data will only be available over
   v4 transport for a long time it is also obvious that it is
   important to avoid fragmenting the name space available to IPv4
   only hosts. I.e. during transition it is not acceptable to break
   the name space that we presently have available for IPv4-only
   hosts.

3. Consequences of deploying a "IPv6 root name server"

   If and when a root name server that is accessible over IPv6
   transport is deployed it will immediately become possible to change
   IPv6-only name servers to a "native configuration", i.e. to a
   configuration where they follow referrals directly from the root
   (which is now accessible to them because of the v6 transport).

   However, initially they will typically quite soon get a so-called
   "referral" to a name server only available over IPv4 transport, and
   this will be impossible to follow, since there is no common
   transport available. Therefore the name it is trying to lookup will
   not get looked up and the result is that a v6-only name server
   cannot lookup the same names that its v4-only counterpart can.

   There are two available methods of addressing this problem:

   a) ignore it, i.e. don't solve the problem, but put the effort into
      helping deployment along so that the problem will shrink over
      time.

   b) provide some sort of "transport bridging", i.e. create a
      fallback mechanism that enables a name server with only one type
      of transport to reach a name server only available over the
      other transport via some sort of proxy service. See for instance
      [DNS-opreq] and [DNS-proxy] for discussions.

   Regardless of how this problem is handled it is important to
   realize that it only concerns the fragmented name space in
   IPv6. I.e. the IPv4 name space is not (yet) fragmented, and a more
   important question is possibly how to keep it unfragmented.

4. Policy based avoidance of name space fragmentation.

   Today there are only a few DNS "zones" on the public Internet that
   are only available over v6 transport, and they can mostly be
   regarded as "experimental". However, as soon as there is a root
   name server available over v6 transport it is reasonable to expect
   that it will become more common with v6-only zones over time.

   This would not be a good development, since this will fragment the
   previously unfragmented IPv4 name space and there are strong
   reasons to find a mechanism to avoid it.

4.1. Requirement of IPv4 address for at least one name server.

   To ensure that all zones remain available over IPv4 transport one
   method would be to require that nameservers authoritative for a
   zone as part of the zone validation process ensure that there are
   IPv4 address records available for the name servers of any child
   delegations within the zone).

   I.e. the future policy would be:

        "Every delegation point should have at least one name server
        for the child zone reachable over IPv4 transport".

   To ensure this the authoritative server will have to lookup the
   address records of the name servers that are part of any
   "delegation" points in the zone.

   I.e. for given the domain EXAMPLE.COM with the following data

   $ORIGIN example.com.
   child.example.com.           IN      NS      ns.example.com.
   child.example.com.           IN      NS      dns.autonomica.se.
   ns.example.com.              IN      A       1.2.3.4

   the delegation of CHILD.EXAMPLE.COM is to the two name servers
   "ns.example.com" and "dns.autonomica.se". The first name server,
   "ns.example.com", obviously has an IPv4 address (as shown by the
   "glue" record on the last line).

   However, "ns.example.com" may have additional addresses assiciated
   with it. Also there is no way for the server loading the zone to
   know the address(es) of "dns.autonomica.se". Therefore, to find out
   all the publicly available addresses they have to be queried for.

4.2. Zone validation for non-recursive servers.

   Non-recursive authoritative servers are name servers that run
   without ever asking questions. A change in the zone validation
   requirements that force them to query for the addresses of name
   servers that are part of delegations in the zone change this, since
   they now have to query for these addresses.

   However, the main reason that it is important to be able to run
   without asking questions is to avoid "caching" possibly bogus
   answers. This need can be managed by requiring that a non recursive
   name server throw away the looked up address information after
   having used it for validation of the delegations in the zone.

4.3. Future requirement of IPv6 address for at least one name server.

   The immediate need for clarified policies for delegation is to
   ensure that IPv4 name space does not start to fragment. Over time,
   however, it is reasonable to expect that it may become important to
   add a similar requirement to IPv6 name space.

   I.e. an even more refined policy possible at some point in the
   future would be:

        "Every delegation point should have at least one name server
        for the child zone reachable over IPv4 transport (i.e. should
        have an A record) and at least one name server reachable over
        IPv6 transport (i.e. should have an AAAA record)".

4.4. Implementation issues for new zone validation requirements.

   Exactly what action should be taken when a zone does not validate
   is not immediately clear. Immediate alternatives include:

   a) fail the entire zone

   b) load the zone but remove the delegation that failed validation

   c) load the entire zone but issue a warning message about the
      delegation that failed validation.

   A likely implementation will make it configurable what action to
   take.

5. Overview of suggested transition method.

   By following the steps outlined below it will be possible to
   transition without outages or lack of service. The assumption is
   that the site has only v4 name servers or possibly v4 name servers
   plus v6 name server in a forwarding configuration. All DNS data is
   on the v4 name servers.

   1) Do not change the method of resolution on any name server.
      I.e. v4 servers go to the root and follow referrals while v6
      servers go to their translator/forwarder which lookup the name
      and return the end result.

   2) Start mirroring DNS data into v6 by providing v6 name servers
      serving the zones. Add v6 address information to to the zones
      and as glue at the parent zone. Note that it is important that
      the zone should have the same contents regardless of whether it
      is the v4 version or the v6 version. Anything else will lead to
      confusion.

   4) Wait for the announcement of the DNS root zone being available
      from a v6 name server.

   5) Ensure that the entire path from the root down to the domain in
      question is reachable over both IPv4 and IPv6 transport.

   When this is accomplished it it possible to begin a migration of
   the lookup of selected services to be available over IPv6
   (i.e. typically by adding a AAAA record for a server of some sort).

6. How to deploy DNS hierarchy in v6 space.

   The main problem with changing the DNS data so that it will become
   available over both IPv6 and IPv4 transport is one of scale. There
   are too many name servers and too many DNS zones for any kind of
   forced migration to be aven remotely possible.

   The way of achieving deployment is by providing domain owner with

   a) a reason to deploy

   b) a method to deploy

   c) a way of verififying the correctness of the resulting configuration

6.1. A reason to deploy.

   It is important to the migration process that zones migrate to
   become available over v6 transport (as well as v4 transport).  But
   it is difficult to actually require such deployment too early in
   the migration process.

   Over time, however, it will become more reasonable to add such a
   requirement. One likely method to do this will be by updating the
   requirements for proper zone validation as was outlined above.

6.2. How to deploy DNS data.

   Assuming the owner of the DNS domain has access to both IPv4 and
   IPv6 address space that is globally routed. The steps to take are
   then

   a) identify all name servers that will serve the DNS domain, with
   their IPv4 and/or IPv6 addresses

   b) arrange for a suitable method of zone synchronization

   c) announce the new set of servers to the parent zone, including
   possible new IPv6 glue

   It is recommended that the name servers run on single stack
   machines, i.e. machines that are only able to utilize either IPv4
   transport or IPv6 transport, but not both.

   A common recommendation (mostly orthogonal to IPv6 transition
   issues) is that authoritative name servers only serve data,
   i.e. they do not act as caching resolvers. That way, since they
   operate in non-recursive mode, they will not have any cache, and
   hence will not be able to give out wrongful answers based upon
   errors in the cache.

   Since the announced name servers are single stack, the primary
   master from which they fetch zone data will typically have to be
   dual stack or otherwise some other method of data transfer has to
   be arranged.

7. Security Considerations

   Much of the security of the Internet relies, often wrongly, but
   still, on the DNS. Thus, changes to the characteristics of the DNS
   may impact the security of Internet based services.

   Although it will be avoided, there may be unintended consequences
   as a result of operational deployment of RR types and protocols
   already approved by the IETF. When or if such consequences are
   identified, appropriate feedback will be provided to the IETF and
   the operational community on the efficacy of said interactions.

8. Summary.

   The name space fragmentation problem is identified and examined at
   some length.

   A solution based upon a change in the validation method of
   delegation points is suggested. This will both help keep the v4
   name space unfragmented and may also help speed up deployment of
   DNS hierarchy in v6 space.



9. References

   [RFC1034]            Domain names - concepts and facilities.
                        P.V. Mockapetris.

   [RFC1035]            Domain names - implementation and specification.
                        P.V. Mockapetris.

   [RFC2826]            IAB Technical Comment on th Unique DNS Root

   [DNS-proxy]          draft-durand-dns-proxy-00.txt
                        Alain Durand

   [DNS-opreq]          draft-ietf-ngtrans-dns-ops-req-02.txt
                        Alain Durand


A. Authors' Address

Johan Ihren
Autonomica
Bellmansgatan 30
SE-118 47 Stockholm, Sweden
johani@autonomica.se


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