Network Working Group C. Huitema

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

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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

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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.

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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

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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

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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

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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.

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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.

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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

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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

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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

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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.

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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.

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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.

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RFC 3904 Unmanaged Networks Transition Tools September 2004

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].)

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RFC 3904 Unmanaged Networks Transition Tools September 2004

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.

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RFC 3904 Unmanaged Networks Transition Tools September 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.

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RFC 3904 Unmanaged Networks Transition Tools September 2004

[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: [email protected]

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

EMail: [email protected]

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

EMail: [email protected]

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

EMail: [email protected]

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RFC 3904 Unmanaged Networks Transition Tools September 2004

12. Full Copyright Statement

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contained in BCP 78, and except as set forth therein, the authors
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