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Versions: (draft-huitema-v6ops-unmaneval) 00 01 02 03 RFC 3904

INTERNET DRAFT                                              C. Huitema
<draft-ietf-v6ops-unmaneval-03.txt>                          Microsoft
June 1, 2004                                                R. Austein
Expires December 1, 2004                                           ISC
                                                           S. Satapati
                                                   Cisco Systems, Inc.
                                                        R. van der Pol
                                                            NLnet Labs


Evaluation of IPv6 Transition Mechanisms for Unmanaged Networks

Status of this memo

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   and any of which I become aware will be disclosed, in accordance
   with RFC 3668.

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Abstract

   In a companion paper 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. We start from this analysis and evaluate here
   the suitability of mechanisms that have already been specified,
   proposed or deployed.











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Table of Contents:

1 Introduction ....................................................   3
2 Evaluation of Tunneling Solutions ...............................   3
2.1 Comparing automatic and configured solutions ..................   4
2.1.1 Path optimization in automatic tunnels ......................   4
2.1.2 Automatic tunnels and relays ................................   5
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 ..................................  10
4.1.3 Recommendation ..............................................  11
4.2 Communication between IPv4-only and IPv6-capable nodes ........  11
4.3 Resolution of names to IPv6 addresses .........................  11
4.3.1 Provisioning the address of a DNS resolver ..................  11
4.3.2 Publishing IPv6 addresses to the Internet ...................  12
4.3.3 Resolving the IPv6 addresses of local hosts .................  12
4.3.4 Recommendations for name resolution .........................  13
4.4 Security considerations in case B .............................  13
5 Meeting case C requirements .....................................  13
5.1 Connectivity ..................................................  13
6 Meeting the case D requirements .................................  14
6.1 IPv6 addressing requirements ..................................  14
6.2 IPv4  connectivity requirements ...............................  14
6.3 Naming requirements ...........................................  14
7 Recommendations .................................................  14
8 Security considerations .........................................  15
9 IANA Considerations .............................................  15
10 Acknowledgements ...............................................  16
11 References .....................................................  16
12 Authors' Addresses .............................................  17
13 Intellectual Property Statement ................................  17
14 Copyright ......................................................  18









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1 Introduction

   This document analyses 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 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 [NAT-T]?

   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.


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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 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 of an explicit relation with a tunnel service. They
   may also be in some case more efficient, 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

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   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, i.e., will 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, 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

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   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 tunnels 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.

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   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 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
   also 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 support, 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

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   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 configuration:

   -    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 already designed solution for automatic
   tunnel across a NAT; 6to4 is an example of solution for automatic
   tunnel 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

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   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 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 can 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

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   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), or if the ISP is advertising a /64 prefix on the link, such
   a link can be extended by the use of 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, 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 the 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

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   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 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
   "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 has a requirement to communicate
   with IPv4-only hosts, this device implements an IPv4 stack. The only
   devices that should only have IPv6 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


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   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

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   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.

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   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 let
   user 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 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 want to still 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 has been very
   little effort devoted by the IETF 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

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   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 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 IANA Considerations


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   This memo does not include any request to IANA.

10 Acknowledgements

   This memo has benefited from 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.

11 References

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

   [TUNNELS] Durand, A., Fasano, P., and I. Guardini. 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] R. Droms. "DNS Configuration options for DHCPv6." RFC
   3646, December 2003.

   [PREFIXDHCPV6] Troan, O. and R. Droms. "IPv6 Prefix Options for
   DHCPv6." RFC 3633, December 2003.

   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

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   Name Resolution (LLMNR)." Work in progress.

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

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

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


12 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

13 Intellectual Property Statement

   The IETF takes no position regarding the validity or scope of any
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   to pertain to the implementation or use of the technology described
   in this document or the extent to which any license under such
   rights might or might not be available; nor does it represent that
   it has made any independent effort to identify any such rights.

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   Information on the procedures with respect to rights in RFC
   documents can be found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use
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   The IETF invites any interested party to bring to its attention any
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   this standard.  Please address the information to the IETF at ietf-
   ipr@ietf.org.

14 Copyright

   The following copyright notice is copied from [RFC3667], Section
   5.4. It describes the applicable copyright for this document.

   Copyright (C) The Internet Society (2004). This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on
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   INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS OR
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