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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
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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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INTERNET DRAFT Unmanaged Networks Transition Tools June 1, 2004
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
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INTERNET DRAFT Unmanaged Networks Transition Tools June 1, 2004
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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
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