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RFC 3904 - Evaluation of IPv6 Transition Mechanisms for Unmanage


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Network Working Group                                         C. Huitema
Request for Comments: 3904                                     Microsoft
Category: Informational                                       R. Austein
                                                                     ISC
                                                             S. Satapati
                                                     Cisco Systems, Inc.
                                                          R. van der Pol
                                                              NLnet Labs
                                                          September 2004

    Evaluation of IPv6 Transition Mechanisms for Unmanaged Networks

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).

Abstract

   This document analyzes issues involved in the transition of
   "unmanaged networks" from IPv4 to IPv6.  Unmanaged networks typically
   correspond to home networks or small office networks.  A companion
   paper analyzes out the requirements for mechanisms needed in various
   transition scenarios of these networks to IPv6.  Starting from this
   analysis, we evaluate the suitability of mechanisms that have already
   been specified, proposed, or deployed.

Table of Contents:

   1.  Introduction .................................................  2
   2.  Evaluation of Tunneling Solutions ............................  3
       2.1.  Comparing Automatic and Configured Solutions ...........  3
             2.1.1.  Path Optimization in Automatic Tunnels .........  4
             2.1.2.  Automatic Tunnels and Relays ...................  4
             2.1.3.  The Risk of Several Parallel IPv6 Internets ....  5
             2.1.4.  Lifespan of Transition Technologies ............  6
       2.2.  Cost and Benefits of NAT Traversal .....................  6
             2.2.1.  Cost of NAT Traversal ..........................  7
             2.2.2.  Types of NAT ...................................  7
             2.2.3.  Reuse of Existing Mechanisms ...................  8
       2.3.  Development of Transition Mechanisms ...................  8

   3.  Meeting Case A Requirements ..................................  9
       3.1.  Evaluation of Connectivity Mechanisms ..................  9
       3.2.  Security Considerations in Case A ......................  9
   4.  Meeting case B Requirements .................................. 10
       4.1.  Connectivity ........................................... 10
             4.1.1.  Extending a Subnet to Span Multiple Links ...... 10
             4.1.2.  Explicit Prefix Delegation ..................... 11
             4.1.3.  Recommendation ................................. 11
       4.2.  Communication Between IPv4-only and IPv6-Capable Nodes . 11
       4.3.  Resolution of Names to IPv6 Addresses .................. 12
             4.3.1.  Provisioning the Address of a DNS Resolver ..... 12
             4.3.2.  Publishing IPv6 Addresses to the Internet ...... 12
             4.3.3.  Resolving the IPv6 Addresses of Local Hosts .... 13
             4.3.4.  Recommendations for Name Resolution ............ 13
       4.4.  Security Considerations in Case B ...................... 14
   5.  Meeting Case C Requirements .................................. 14
       5.1.  Connectivity ........................................... 14
   6.  Meeting the Case D Requirements .............................. 14
       6.1.  IPv6 Addressing Requirements ........................... 15
       6.2.  IPv4  Connectivity Requirements ........................ 15
       6.3.  Naming Requirements .................................... 15
   7.  Recommendations .............................................. 15
   8.  Security Considerations ...................................... 16
   9.  Acknowledgements ............................................. 16
   10. References ................................................... 16
   11. Authors' Addresses ........................................... 18
   12. Full Copyright Statement ..................................... 19

1.  Introduction

   This document analyzes the issues involved in the transition from
   IPv4 to IPv6 [IPV6].  In a companion paper [UNMANREQ] we defined the
   "unmanaged networks", which typically correspond to home networks or
   small office networks, and the requirements for transition mechanisms
   in various scenarios of transition to IPv6.

   The requirements for unmanaged networks are expressed by analyzing
   four classes of applications: local, client, peer to peer, and
   servers, and are considering four cases of deployment.  These are:

      A) a gateway which does not provide IPv6 at all;
      B) a dual-stack gateway connected to a dual-stack ISP;
      C) a dual-stack gateway connected to an IPv4-only ISP; and
      D) a gateway connected to an IPv6-only ISP.

   During the transition phase from IPv4 to IPv6 there will be IPv4-
   only, dual-stack, or IPv6-only nodes.  In this document, we make the
   hypothesis that the IPv6-only nodes do not need to communicate with

   IPv4-only nodes; devices that want to communicate with both IPv4 and
   IPv6 nodes are expected to implement both IPv4 and IPv6, i.e., be
   dual-stack.

   The issues involved are described in the next sections.  This
   analysis outlines two types of requirements: connectivity
   requirements, i.e., how to ensure that nodes can exchange IP packets,
   and naming requirements, i.e., how to ensure that nodes can resolve
   each-other's names.  The connectivity requirements often require
   tunneling solutions.  We devote the first section of this memo to an
   evaluation of various tunneling solutions.

2.  Evaluation of Tunneling Solutions

   In the case A and case C scenarios described in [UNMANREQ], the
   unmanaged network cannot obtain IPv6 service, at least natively, from
   its ISP.  In these cases, the IPv6 service will have to be provided
   through some form of tunnel.  There have been multiple proposals on
   different ways to tunnel IPv6 through an IPv4 service.  We believe
   that these proposals can be categorized according to two important
   properties:

   *  Is the deployment automatic, or does it require explicit
      configuration or service provisioning?

   *  Does the proposal allow for the traversal of a NAT?

   These two questions divide the solution space into four broad
   classes.  Each of these classes has specific advantages and risks,
   which we will now develop.

2.1.  Comparing Automatic and Configured Solutions

   It is possible to broadly classify tunneling solutions as either
   "automatic" or "configured".  In an automatic solution, a host or a
   router builds an IPv6 address or an IPv6 prefix by combining a pre-
   defined prefix with some local attribute, such as a local IPv4
   address [6TO4] or the combination of an address and a port number
   [TEREDO].  Another typical and very important characteristic of an
   automatic solution is they aim to work with a minimal amount of
   support or infrastructure for IPv6 in the local or remote ISPs.

   In a configured solution, a host or a router identifies itself to a
   tunneling service to set up a "configured tunnel" with an explicitly
   defined "tunnel router".  The amount of actual configuration may vary
   from manually configured static tunnels to dynamic tunnel services
   requiring only the configuration of a "tunnel broker", or even a
   completely automatic discovery of the tunnel router.

   Configured tunnels have many advantages over automatic tunnels.  The
   client is explicitly identified and can obtain a stable IPv6 address.
   The service provider is also well identified and can be held
   responsible for the quality of the service.  It is possible to route
   multicast packets over the established tunnel.  There is a clear
   address delegation path, which enables easy support for reverse DNS
   lookups.

   Automatic tunnels generally cannot provide the same level of service.
   The IPv6 address is only as stable as the underlying IPv4 address,
   the quality of service depends on relays operated by third parties,
   there is typically no support for multicast, and there is often no
   easy way to support reverse DNS lookups (although some workarounds
   are probably possible).  However, automatic tunnels have other
   advantages.  They are obviously easier to configure, since there is
   no need for an explicit relation with a tunnel service.  They may
   also be more efficient in some cases, as they allow for "path
   optimization".

2.1.1.  Path Optimization in Automatic Tunnels

   In automatic tunnels like [TEREDO] and [6TO4], the bulk of the
   traffic between two nodes using the same technology is exchanged on a
   direct path between the endpoints, using the IPv4 services to which
   the endpoints already subscribe.  By contrast, the configured tunnel
   servers carry all the traffic exchanged by the tunnel client.

   Path optimization is not a big issue if the tunnel server is close to
   the client on the natural path between the client and its peers.
   However, if the tunnel server is operated by a third party, this
   third party will have to bear the cost of provisioning the bandwidth
   used by the client.  The associated costs can be significant.

   These costs are largely absent when the tunnels are configured by the
   same ISP that provides the IPv4 service.  The ISP can place the
   tunnel end-points close to the client, i.e., mostly on the direct
   path between the client and its peers.

2.1.2.  Automatic Tunnels and Relays

   The economics arguments related to path optimization favor either
   configured tunnels provided by the local ISP or automatic tunneling
   regardless of the co-operation of ISPs.  However, automatic solutions
   require that relays be configured throughout the Internet.  If a host
   that obtained connectivity through an automatic tunnel service wants
   to communicate with a "native" host or with a host using a configured

   tunnel, it will need to use a relay service, and someone will have to
   provide and pay for that service.  We cannot escape economic
   considerations for the deployment of these relays.

   It is desirable to locate these relays close to the "native host".
   During the transition period, the native ISPs have an interest in
   providing a relay service for use by their native subscribers.  Their
   subscribers will enjoy better connectivity, and will therefore be
   happier.  Providing the service does not result in much extra
   bandwidth requirement: the packets are exchanged between the local
   subscribers and the Internet; they are simply using a v6-v4 path
   instead of a v6-v6 path.  (The native ISPs do not have an incentive
   to provide relays for general use; they are expected to restrict
   access to these relays to their customers.)

   We should note however that different automatic tunneling techniques
   have different deployment conditions.

2.1.3.  The Risk of Several Parallel IPv6 Internets

   In an early deployment of the Teredo service by Microsoft, the relays
   are provided by the native (or 6to4) hosts themselves.  The native or
   6to4 hosts are de-facto "multi-homed" to native and Teredo hosts,
   although they never publish a Teredo address in the DNS or otherwise.
   When a native host communicates with a Teredo host, the first packets
   are exchanged through the native interface and relayed by the Teredo
   server, while the subsequent packets are tunneled "end-to-end" over
   IPv4 and UDP.  This enables deployment of Teredo without having to
   field an infrastructure of relays in the network.

   This type of solution carries the implicit risk of developing two
   parallel IPv6 Internets, one native and one using Teredo: in order to
   communicate with a Teredo-only host, a native IPv6 host has to
   implement a Teredo interface.  The Teredo implementations try to
   mitigate this risk by always preferring native paths when available,
   but a true mitigation requires that native hosts do not have to
   implement the transition technology.  This requires cooperation from
   the IPv6 ISP, who will have to support the relays.  An IPv6 ISP that
   really wants to isolate its customers from the Teredo technology can
   do that by providing native connectivity and a Teredo relay.  The
   ISP's customers will not need to implement their own relay.

   Communication between 6to4 networks and native networks uses a
   different structure.  There are two relays, one for each direction of
   communication.  The native host sends its packets through the nearest
   6to4 router, i.e., the closest router advertising the 2002::/16
   prefix through the IPv6 routing tables; the 6to4 network sends its
   packet through a 6to4 relay that is either explicitly configured or

   discovered through the 6to4 anycast address 192.88.99.1
   [6TO4ANYCAST].  The experience so far is that simple 6to4 routers are
   easy to deploy, but 6to4 relays are scarce.  If there are too few
   relays, these relays will create a bottleneck.  The communications
   between 6to4 and native networks will be slower than the direct
   communications between 6to4 hosts.  This will create an incentive for
   native hosts to somehow "multi-home" to 6to4, de facto creating two
   parallel Internets, 6to4 and native.  This risk will only be
   mitigated if there is a sufficient deployment of 6to4 relays.

   The configured tunnel solutions do not carry this type of risk.

2.1.4.  Lifespan of Transition Technologies

   A related issue is the lifespan of the transition solutions.  Since
   automatic tunneling technologies enable an automatic deployment,
   there is a risk that some hosts never migrate out of the transition.
   The risk is arguably less for explicit tunnels: the ISPs who provide
   the tunnels have an incentive to replace them with a native solution
   as soon as possible.

   Many implementations of automatic transition technologies incorporate
   an "implicit sunset" mechanism: the hosts will not configure a
   transition technology address if they have native connectivity; the
   address selection mechanisms will prefer native addresses when
   available.  The transition technologies will stop being used
   eventually, when native connectivity has been deployed everywhere.
   However, the "implicit sunset" mechanism does not provide any hard
   guarantee that transition will be complete at a certain date.

   Yet, the support of transition technologies has a cost for the entire
   network: native IPv6 ISPS have to support relays in order to provide
   good performance and avoid the "parallel Internet" syndrome.  These
   costs may be acceptable during an initial deployment phase, but they
   can certainly not be supported for an indefinite period.  The
   "implicit sunset" mechanisms may not be sufficient to guarantee a
   finite lifespan of the transition.

2.2.  Cost and Benefits of NAT Traversal

   During the transition, some hosts will be located behind IPv4 NATs.
   In order to participate in the transition, these hosts will have to
   use a tunneling mechanism designed to traverse NAT.

   We may ask whether NAT traversal should be a generic property of any
   transition technology, or whether it makes sense to develop two types
   of technologies, some "NAT capable" and some not.  An important
   question is also which kinds of NAT boxes one should be able to

   traverse.  One should probably also consider whether it is necessary
   to build an IPv6 specific NAT traversal mechanism, or whether it is
   possible to combine an existing IPv4 NAT traversal mechanism with
   some form of IPv6 in IPv4 tunneling.  There are many IPv4 NAT
   traversal mechanisms; thus one may ask whether these need re-
   invention, especially when they are already complex.

   A related question is whether the NAT traversal technology should use
   automatic tunnels or configured tunnels.  We saw in the previous
   section that one can argue both sides of this issue.  In fact, there
   are already deployed automatic and configured solutions, so the
   reality is that we will probably see both.

2.2.1.  Cost of NAT Traversal

   NAT traversal technologies generally involve encapsulating IPv6
   packets inside a transport protocol that is known to traverse NAT,
   such as UDP or TCP.  These transport technologies require
   significantly more overhead than the simple tunneling over IPv4 used
   in 6to4 or in IPv6 in IPv4 tunnels.  For example, solutions based on
   UDP require the frequent transmission of "keep alive" packets to
   maintain a "mapping" in the NAT; solutions based on TCP may not
   require such a mechanism, but they incur the risk of "head of queue
   blocking", which may translate in poor performance.  Given the
   difference in performance, it makes sense to consider two types of
   transition technologies, some capable of traversing NAT and some
   aiming at the best performance.

2.2.2.  Types of NAT

   There are many kinds of NAT on the market.  Different models
   implement different strategies for address and port allocations, and
   different types of timers.  It is desirable to find solutions that
   cover "almost all" models of NAT.

   A configured tunnel solution will generally make fewer hypotheses on
   the behavior of the NAT than an automatic solution.  The configured
   solutions only need to establish a connection between an internal
   node and a server; this communication pattern is supported by pretty
   much all NAT configurations.  The variability will come from the type
   of transport protocols that the NAT supports, especially when the NAT
   also implements "firewall" functions.  Some models will allow
   establishment of a single "protocol 41" tunnel, while some may
   prevent this type of transmission.  Some models will allow UDP
   transmission, while other may only allow TCP, or possibly HTTP.

   The automatic solutions have to rely on a "lowest common denominator"
   that is likely to be accepted by most models of NAT.  In practice,
   this common denominator is UDP.  UDP based NAT traversal is required
   by many applications, e.g., networked games or voice over IP.  The
   experience shows that most recent "home routers" are designed to
   support these applications.  In some edge cases, the automatic
   solutions will require explicit configuration of a port in the home
   router, using the so-called "DMZ" functions; however, these functions
   are hard to use in an "unmanaged network" scenario.

2.2.3.  Reuse of Existing Mechanisms

   NAT traversal is not a problem for IPv6 alone.  Many IPv4
   applications have developed solutions, or kludges, to enable
   communication across a NAT.

   Virtual Private Networks are established by installing tunnels
   between VPN clients and VPN servers.  These tunnels are designed
   today to carry IPv4, but in many cases could easily carry IPv6.  For
   example, the proposed IETF standard, L2TP, includes a PPP layer that
   can encapsulate IPv6 as well as IPv4.  Several NAT models are
   explicitly designed to pass VPN traffic, and several VPN solutions
   have special provisions to traverse NAT.  When we study the
   establishment of configured tunnels through NAT, it makes a lot of
   sense to consider existing VPN solutions.

   [STUN] is a protocol designed to facilitate the establishment of UDP
   associations through NAT, by letting nodes behind NAT discover their
   "external" address.  The same function is required for automatic
   tunneling through NAT, and one could consider reusing the STUN
   specification as part of an automatic tunneling solution.  However,
   the automatic solutions also require a mechanism of bubbles to
   establish the initial path through a NAT.  This mechanism is not
   present in STUN.  It is not clear that a combination of STUN and a
   bubble mechanism would have a technical advantage over a solution
   specifically designed for automatic tunneling through NAT.

2.3.  Development of Transition Mechanisms

   The previous sections make the case for the development of four
   transition mechanism, covering the following 4 configurations:

      -  Configured tunnel over IPv4 in the absence of NAT;
      -  Automatic tunnel over IPv4 in the absence of NAT;
      -  Configured tunnel across a NAT;
      -  Automatic tunnel across a NAT.

   Teredo is an example of an already designed solution for automatic
   tunnels across a NAT; 6to4 is an example of a solution for automatic
   tunnels over IPv4 in the absence of NAT.

   All solutions should be designed to meet generic requirements such as
   security, scalability, support for reverse name lookup, or simple
   management.  In particular, automatic tunneling solutions may need to
   be augmented with a special purpose reverse DNS lookup mechanism,
   while configured tunnel solutions would benefit from an automatic
   service configuration mechanism.

3.  Meeting Case A Requirements

   In case A, isolated hosts need to acquire some form of connectivity.
   In this section, we first evaluate how mechanisms already defined or
   being worked on in the IETF meet this requirement.  We then consider
   the "remaining holes" and recommend specific developments.

3.1.  Evaluation of Connectivity Mechanisms

   In case A, IPv6 capable hosts seek IPv6 connectivity in order to
   communicate with applications in the global IPv6 Internet.  The
   connectivity requirement can be met using either configured tunnels
   or automatic tunnels.

   If the host is located behind a NAT, the tunneling technology should
   be designed to traverse NAT; tunneling technologies that do not
   support NAT traversal can obviously be used if the host is not
   located behind a NAT.

   When the local ISP is willing to provide a configured tunnel
   solution, we should make it easy for the host in case A to use it.
   The requirements for such a service will be presented in another
   document.

   An automatic solution like Teredo appears to be a good fit for
   providing IPv6 connectivity to hosts behind NAT, in case A of IPv6
   deployment.  The service is designed for minimizing the cost of
   deploying the server, which matches the requirement of minimizing the
   cost of the "supporting infrastructure".

3.2.  Security Considerations in Case A

   A characteristic of case A is that an isolated host acquires global
   IPv6 connectivity, using either Teredo or an alternative tunneling
   mechanism.  If no precaution is taken, there is a risk of exposing to
   the global Internet some applications and services that are only
   expected to serve local hosts, e.g., those located behind the NAT

   when a NAT is present.  Developers and administrators should make
   sure that the global IPv6 connectivity is restricted to only those
   applications that are expressly designed for global Internet
   connectivity.  The users should be able to configure which
   applications get IPv6 connectivity to the Internet and which should
   not.

   Any solution to the NAT traversal problem is likely to involve
   relays.  There are concerns that improperly designed protocols or
   improperly managed relays could open new avenues for attacks against
   Internet services.  This issue should be addressed and mitigated in
   the design of the NAT traversal protocols and in the deployment
   guides for relays.

4.  Meeting Case B Requirements

   In case B, we assume that the gateway and the ISP are both dual-
   stack.  The hosts on the local network may be IPv4-only, dual-stack,
   or IPv6-only.  The main requirements are: prefix delegation and name
   resolution.  We also study the potential need for communication
   between IPv4 and IPv6 hosts, and conclude that a dual-stack approach
   is preferable.

4.1.  Connectivity

   The gateway must be able to acquire an IPv6 prefix, delegated by the
   ISP.  This can be done through explicit prefix delegation (e.g.,
   [DHCPV6, PREFIXDHCPV6]), or if the ISP is advertising a /64 prefix on
   the link, such a link can be extended by the use of an ND proxy or a
   bridge.

   An ND proxy can also be used to extend a /64 prefix to multiple
   physical links of different properties (e.g., an Ethernet and a PPP
   link).

4.1.1.  Extending a Subnet to Span Multiple Links

   A /64 subnet can be extended to span multiple physical links using a
   bridge or ND proxy.  Bridges can be used when bridging multiple
   similar media (mainly, Ethernet segments).  On the other hand, an ND
   proxy must be used if a /64 prefix has to be shared across media
   (e.g., an upstream PPP link and a downstream Ethernet), or if an
   interface cannot be put into promiscuous mode (e.g., an upstream
   wireless link).

   Extending a single subnet to span from the ISP to all of the
   unmanaged network is not recommended, and prefix delegation should be
   used when available.  However, sometimes it is unavoidable.  In

   addition, sometimes it's necessary to extend a subnet in the
   unmanaged network, at the "customer-side" of the gateway, and
   changing the topology using routing might require too much expertise.

   The ND proxy method results in the sharing of the same prefix over
   several links, a procedure generally known as "multi-link subnet".
   This sharing has effects on neighbor discovery protocols, and
   possibly also on other protocols such as LLMNR [LLMNR] that rely on
   "link local multicast".  These effects need to be carefully studied.

4.1.2.  Explicit Prefix Delegation

   Several networks have already started using an explicit prefix
   delegation mechanism using DHCPv6.  In this mechanism, the gateway
   uses a DHCP request to obtain an adequate prefix from a DHCP server
   managed by the Internet Service Provider.  The DHCP request is
   expected to carry proper identification of the gateway, which enables
   the ISP to implement prefix delegation policies.  It is expected that
   the ISP assigns a /48 to the customer.  The gateway should
   automatically assign /64s out of this /48 to its internal links.

   DHCP is insecure unless authentication is used.  This may be a
   particular problem if the link between gateway and ISP is shared by
   multiple subscribers.  DHCP specification includes authentication
   options, but the operational procedures for managing the keys and
   methods for sharing the required information between the customer and
   the ISP are unclear.  To be secure in such an environment in
   practice, the practical details of managing the DHCP authentication
   need to be analyzed.

4.1.3.  Recommendation

   The ND proxy and DHCP methods appear to have complementary domains of
   application.  ND proxy is a simple method that corresponds well to
   the "informal sharing" of a link, while explicit delegation provides
   strong administrative control.  Both methods require development:
   specify the interaction with neighbor discovery for ND proxy; provide
   security guidelines for explicit delegation.

4.2.  Communication Between IPv4-only and IPv6-capable Nodes

   During the transition phase from IPv4 to IPv6, there will be IPv4-
   only, dual-stack, and IPv6-only nodes.  In theory, there may be a
   need to provide some interconnection services so that IPv4-only and
   IPv6-only hosts can communicate.  However, it is hard to develop a
   translation service that does not have unwanted side effects on the
   efficiency or the security of communications.  As a consequence, the
   authors recommend that, if a device requires communication with

   IPv4-only hosts, this device implements an IPv4 stack.  The only
   devices that should have IPv6-only connectivity are those that are
   intended to only communicate with IPv6 hosts.

4.3.  Resolution of Names to IPv6 Addresses

   There are three types of name resolution services that should be
   provided in case B: local IPv6 capable hosts must be able to obtain
   the IPv6 addresses of correspondent hosts on the Internet, they
   should be able to publish their address if they want to be accessed
   from the Internet, and they should be able to obtain the IPv6 address
   of other local IPv6 hosts.  These three problems are described in the
   next sections.  Operational considerations and issues with IPv6 DNS
   are analyzed in [DNSOPV6].

4.3.1.  Provisioning the Address of a DNS Resolver

   In an unmanaged environment, IPv4 hosts usually obtain the address of
   the local DNS resolver through DHCPv4; the DHCPv4 service is
   generally provided by the gateway.  The gateway will also use DHCPv4
   to obtain the address of a suitable resolver from the local Internet
   service provider.

   The DHCPv4 solution will suffice in practice for the gateway and also
   for the dual-stack hosts.  There is evidence that DNS servers
   accessed over IPv4 can serve arbitrary DNS records, including AAAA
   records.

   Just using DHCPv4 will not be an adequate solution for IPv6-only
   local hosts.  The DHCP working group has defined how to use
   (stateless) DHCPv6 to obtain the address of the DNS server
   [DNSDHCPV6].  DHCPv6 and several other possibilities are being looked
   at in the DNSOP Working Group.

4.3.2.  Publishing IPv6 Addresses to the Internet

   IPv6 capable hosts may be willing to provide services accessible from
   the global Internet.  They will thus need to publish their address in
   a server that is publicly available.  IPv4 hosts in unmanaged
   networks have a similar problem today, which they solve using one of
   three possible solutions:

      *  Manual configuration of a stable address in a DNS server;
      *  Dynamic configuration using the standard dynamic DNS protocol;
      *  Dynamic configuration using an ad hoc protocol.

   Manual configuration of stable addresses is not satisfactory in an
   unmanaged IPv6 network: the prefix allocated to the gateway may or
   may not be stable, and in any case, copying long hexadecimal strings
   through a manual procedure is error prone.

   Dynamic configuration using the same type of ad hoc protocols that
   are common today is indeed possible, but the IETF should encourage
   the use of standard solutions based on Dynamic DNS (DDNS).

4.3.3.  Resolving the IPv6 Addresses of Local Hosts

   There are two possible ways of resolving the IPv6 addresses of local
   hosts: one may either publish the IPv6 addresses in a DNS server for
   the local domain, or one may use a peer-to-peer address resolution
   protocol such as LLMNR.

   When a DNS server is used, this server could in theory be located
   anywhere on the Internet.  There is however a very strong argument
   for using a local server, which will remain reachable even if the
   network connectivity is down.

   The use of a local server requires that IPv6 capable hosts discover
   this server, as explained in 4.3.1, and then that they use a protocol
   such as DDNS to publish their IPv6 addresses to this server.  In
   practice, the DNS address discovered in 4.3.1 will often be the
   address of the gateway itself, and the local server will thus be the
   gateway.

   An alternative to using a local server is LLMNR, which uses a
   multicast mechanism to resolve DNS requests.  LLMNR does not require
   any service from the gateway, and also does not require that hosts
   use DDNS.  An important problem is that some networks only have
   limited support for multicast transmission, for example, multicast
   transmission on 802.11 network is error prone.  However, unmanaged
   networks also use multicast for neighbor discovery [NEIGHBOR]; the
   requirements of ND and LLMNR are similar; if a link technology
   supports use of ND, it can also enable use of LLMNR.

4.3.4.  Recommendations for Name Resolution

   The IETF should quickly provide a recommended procedure for
   provisioning the DNS resolver in IPv6-only hosts.

   The most plausible candidate for local name resolution appears to be
   LLMNR; the IETF should quickly proceed to the standardization of that
   protocol.

4.4.  Security Considerations in Case B

   The case B solutions provide global IPv6 connectivity to the local
   hosts.  Removing the limit to connectivity imposed by NAT is both a
   feature and a risk.  Implementations should carefully limit global
   IPv6 connectivity to only those applications that are specifically
   designed to operate on the global Internet.  Local applications, for
   example, could be restricted to only use link-local addresses, or
   addresses whose most significant bits match the prefix of the local
   subnet, e.g., a prefix advertised as "on link" in a local router
   advertisement.  There is a debate as to whether such restrictions
   should be "per-site" or "per-link", but this is not a serious issue
   when an unmanaged network is composed of a single link.

5.  Meeting Case C Requirements

   Case C is very similar to case B, the difference being that the ISP
   is not dual-stack.  The gateway must thus use some form of tunneling
   mechanism to obtain IPv6 connectivity, and an address prefix.

   A simplified form of case B is a single host with a global IPv4
   address, i.e., with a direct connection to the IPv4 Internet.  This
   host will be able to use the same tunneling mechanisms as a gateway.

5.1.  Connectivity

   Connectivity in case C requires some form of tunneling of IPv6 over
   IPv4.  The various tunneling solutions are discussed in section 2.

   The requirements of case C can be solved by an automatic tunneling
   mechanism such as 6to4 [6TO4].  An alternative may be the use of a
   configured tunnels mechanism [TUNNELS], but as the local ISP is not
   IPv6-enabled, this may not be feasible.  The practical conclusion of
   our analysis is that "upgraded gateways" will probably support the
   6to4 technology, and will have an optional configuration option for
   "configured tunnels".

   The tunnel broker technology should be augmented to include support
   for some form of automatic configuration.

   Due to concerns with potential overload of public 6to4 relays, the
   6to4 implementations should include a configuration option that
   allows the user to take advantage of specific relays.

6.  Meeting the Case D Requirements

   In case D, the ISP only provides IPv6 services.

6.1.  IPv6 Addressing Requirements

   We expect IPv6 addressing in case D to proceed similarly to case B,
   i.e., use either an ND proxy or explicit prefix delegation through
   DHCPv6 to provision an IPv6 prefix on the gateway.

6.2.  IPv4 Connectivity Requirements

   Local IPv4 capable hosts may still want to access IPv4-only services.
   The proper way to do this for dual-stack nodes in the unmanaged
   network is to develop a form of "IPv4 over IPv6" tunneling.  There
   are no standardized solutions and the IETF has devoted very little
   effort to this issue, although there is ongoing work with [DSTM] and
   [TSP].  A solution needs to be standardized.  The standardization
   will have to cover configuration issues, i.e., how to provision the
   IPv4 capable hosts with the address of the local IPv4 tunnel servers.

6.3.  Naming Requirements

   Naming requirements are similar to case B, with one difference: the
   gateway cannot expect to use DHCPv4 to obtain the address of the DNS
   resolver recommended by the ISP.

7.  Recommendations

   After a careful analysis of the possible solutions, we can list a set
   of recommendations for the V6OPS working group:

      1. To meet case A and case C requirements, we need to develop, or
         continue to develop, four types of tunneling technologies:
         automatic tunnels without NAT traversal such as [6TO4],
         automatic tunnels with NAT traversal such as [TEREDO],
         configured tunnels without NAT traversal such as [TUNNELS,
         TSP], and configured tunnels with NAT traversal.

      2. To facilitate the use of configured tunnels, we need a
         standardized way for hosts or gateways to discover the tunnel
         server or tunnel broker that may have been configured by the
         local ISP.

      3. To meet case B "informal prefix sharing" requirements, we would
         need a standardized way to perform "ND proxy", possibly as part
         of a "multi-link subnet" specification.  (The explicit prefix
         delegation can be accomplished through [PREFIXDHCPV6].)

      4. To meet case B naming requirements, we need to proceed with the
         standardization of LLMNR.  (The provisioning of DNS parameters
         can be accomplished through [DNSDHCPV6].)

      5. To meet case D IPv4 connectivity requirement, we need to
         standardize an IPv4 over IPv6 tunneling mechanism, as well as
         the associated configuration services.

8.  Security Considerations

   This memo describes the general requirements for transition
   mechanisms.  Specific security issues should be studied and addressed
   during the development of the specific mechanisms.

   When hosts which have been behind a NAT are exposed to IPv6, the
   security assumptions may change radically.  This is mentioned in
   sections 3.2 and 4.4.  One way to cope with that is to have a default
   firewall with a NAT-like access configuration; however, any such
   firewall configuration should allow for easy authorization of those
   applications that actually need global connectivity.  One might also
   restrict applications which can benefit from global IPv6 connectivity
   on the nodes.

   Security policies should be consistent between IPv4 and IPv6.  A
   policy which prevents use of v6 while allowing v4 will discourage
   migration to v6 without significantly improving security.  Developers
   and administrators should make sure that global Internet connectivity
   through either IPv4 or IPv6 is restricted to only those applications
   that are expressly designed for global Internet connectivity.

   Several transition technologies require relays.  There are concerns
   that improperly designed protocols or improperly managed relays could
   open new avenues for attacks against Internet services.  This issue
   should be addressed and mitigated in the design of the transition
   technologies and in the deployment guides for relays.

9.  Acknowledgements

   This memo has benefited from the comments of Margaret Wasserman,
   Pekka Savola, Chirayu Patel, Tony Hain, Marc Blanchet, Ralph Droms,
   Bill Sommerfeld, and Fred Templin.  Tim Chown provided a lot of the
   analysis for the tunneling requirements work.

10.  References

10.1.  Normative References

   [UNMANREQ]     Huitema, C., Austein, R., Satapati, S., and R. van der
                  Pol, "Unmanaged Networks IPv6 Transition Scenarios",
                  RFC 3750, April 2004.

   [IPV6]         Deering, S. and R. Hinden, "Internet Protocol, Version
                  6 (IPv6) Specification", RFC 2460, December 1998.

   [NEIGHBOR]     Narten, T., Nordmark, E., and W. Simpson, "Neighbor
                  Discovery for IP Version 6 (IPv6)", RFC 2461, December
                  1998.

   [6TO4]         Carpenter, B. and K. Moore, "Connection of IPv6
                  Domains via IPv4 Clouds", RFC 3056, February 2001.

   [6TO4ANYCAST]  Huitema, C., "An Anycast Prefix for 6to4 Relay
                  Routers", RFC 3068, June 2001.

   [TUNNELS]      Durand, A., Fasano, P., Guardini, I., and D. Lento,
                  "IPv6 Tunnel Broker", RFC 3053, January 2001.

   [DHCPV6]       Droms, R., Bound, J., Volz, B., Lemon, T., Perkins,
                  C., and M. Carney, "Dynamic Host Configuration
                  Protocol for IPv6 (DHCPv6)", RFC 3315, July 2003.

   [DNSDHCPV6]    Droms, R., "DNS Configuration options for Dynamic Host
                  Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
                  December 2003.

   [PREFIXDHCPV6] Troan, O. and R. Droms, "IPv6 Prefix Options for
                  Dynamic Host Configuration Protocol (DHCP) version 6",
                  RFC 3633, December 2003.

10.2.  Informative References

   [STUN]         Rosenberg, J., Weinberger, J., Huitema, C., and R.
                  Mahy, "STUN - Simple Traversal of User Datagram
                  Protocol (UDP) Through Network Address Translators
                  (NATs)", RFC 3489, March 2003.

   [DNSOPV6]      Durand, A., Ihren, J., and P. Savola. "Operational
                  Considerations and Issues with IPv6 DNS", Work in
                  Progress.

   [LLMNR]        Esibov, L., Aboba, B., and D. Thaler, "Linklocal
                  Multicast Name Resolution (LLMNR)", Work in Progress.

   [TSP]          Blanchet, M., "IPv6 Tunnel Broker with the Tunnel
                  Setup Protocol(TSP)", Work in Progress.

   [DSTM]         Bound, J., "Dual Stack Transition Mechanism", Work in
                  Progress.

   [TEREDO]       Huitema, C., "Teredo: Tunneling IPv6 over UDP through
                  NATs", Work in Progress.

11.  Authors' Addresses

   Christian Huitema
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA 98052-6399

   EMail: huitema@microsoft.com

   Rob Austein
   Internet Systems Consortium
   950 Charter Street
   Redwood City, CA 94063
   USA

   EMail: sra@isc.org

   Suresh Satapati
   Cisco Systems, Inc.
   San Jose, CA 95134
   USA

   EMail: satapati@cisco.com

   Ronald van der Pol
   NLnet Labs
   Kruislaan 419
   1098 VA Amsterdam
   NL

   EMail: Ronald.vanderPol@nlnetlabs.nl

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