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RFC 3750

Unmanaged Networks IPv6 Transition Scenarios

Pages: 20
Informational

Top   ToC   RFC3750 - Page 1
Network Working Group                                         C. Huitema
Request for Comments: 3750                                     Microsoft
Category: Informational                                       R. Austein
                                                                     ISC
                                                             S. Satapati
                                                     Cisco Systems, Inc.
                                                          R. van der Pol
                                                              NLnet Labs
                                                              April 2004


              Unmanaged Networks IPv6 Transition Scenarios

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).  All Rights Reserved.

Abstract

This document defines the scenarios in which IPv6 transition mechanisms are to be used in unmanaged networks. In order to evaluate the suitability of these mechanisms, we need to define the scenarios in which these mechanisms have to be used. One specific scope is the "unmanaged network", which typically corresponds to a home or small office network. The scenarios are specific to a single subnet, and are defined in terms of IP connectivity supported by the gateway and the Internet Service Provider (ISP). We first examine the generic requirements of four classes of applications: local, client, peer to peer and server. Then, for each scenario, we infer transition requirements by analyzing the needs for smooth migration of applications from IPv4 to IPv6.
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Table of Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Topology . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 3. Applications . . . . . . . . . . . . . . . . . . . . . . . . . 4 3.1. Local Applications . . . . . . . . . . . . . . . . . . . 5 3.2. Client Applications. . . . . . . . . . . . . . . . . . . 5 3.3. Peer-to-Peer Applications. . . . . . . . . . . . . . . . 5 3.4. Server Applications. . . . . . . . . . . . . . . . . . . 5 4. Application Requirements of an IPv6 Unmanaged Network. . . . . 6 4.1. Requirements of Local Applications . . . . . . . . . . . 6 4.2. Requirements of Client Applications. . . . . . . . . . . 7 4.2.1. Privacy Requirement of Client Applications . . . 7 4.3. Requirements of Peer-to-Peer Applications. . . . . . . . 8 4.4. Requirements of Server Applications. . . . . . . . . . . 9 5. Stages of IPv6 Deployment. . . . . . . . . . . . . . . . . . . 9 5.1. Case A, Host Deployment of IPv6 Applications . . . . . . 10 5.1.1. Application Support in Case A. . . . . . . . . . 10 5.1.2. Addresses and Connectivity in Case A . . . . . . 11 5.1.3. Naming Services in Case A. . . . . . . . . . . . 12 5.2. Case B, IPv6 Connectivity with Provider Support. . . . . 12 5.2.1. Application Support in Case B. . . . . . . . . . 12 5.2.2. Addresses and Connectivity in Case B . . . . . . 13 5.2.3. Naming Services in Case B. . . . . . . . . . . . 14 5.3. Case C, IPv6 Connectivity without Provider Support . . . 14 5.3.1. Application Support in Case C. . . . . . . . . . 15 5.3.2. Addresses and Connectivity in Case C . . . . . . 15 5.3.3. Naming Services in Case C. . . . . . . . . . . . 15 5.4. Case D, ISP Stops Providing Native IPv4 Connectivity . . 15 5.4.1. Application Support in Case D. . . . . . . . . . 16 5.4.2. Addresses and Connectivity in Case D . . . . . . 16 5.4.3. Naming Services in Case D. . . . . . . . . . . . 17 6. Security Considerations. . . . . . . . . . . . . . . . . . . . 17 7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18 8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18 8.1. Normative References. . . . . . . . . . . . . . . . . . . 18 8.2. Informative References. . . . . . . . . . . . . . . . . . 18 9. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 19 10. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 20
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1. Introduction

In order to evaluate the suitability of transition mechanisms from IPv4 [RFC791] to IPv6 [RFC2460], we need to define the environment or scope in which these mechanisms have to be used. One specific scope is the "unmanaged networks", which typically correspond to home networks or small office networks. This document studies the requirement posed by various transition scenarios, and is organized in to four main sections. Section 2 defines the topology that we are considering. Section 3 presents the four classes of applications that we consider for unmanaged networks: local applications, client applications, peer-to-peer applications, and server applications. Section 4 studies the requirements of these four classes of applications. Section 5 analyses how these requirements translate into four configurations that we expect to encounter during IPv6 deployment: gateways which do not provide IPv6, dual-stack gateways connected to dual-stack ISPs, dual-stack gateways connected to IPv4-only ISPs, and IPv6-capable gateways connected to IPv6-only ISPs. While these four configurations are certainly not an exhaustive list of possible configurations, we believe that they represent the common cases for unmanaged networks.

2. Topology

The typical unmanaged network is composed of a single subnet, connected to the Internet through a single Internet Service Provider (ISP) connection. Several hosts may be connected to the subnet: +------+ | Host +--+ +------+ | | +------+ | | Host +--+ +-------------- +------+ | | : +-----+ : +---------+ | | +--+ Gateway +------| ISP | Internet : +---------+ | | : +-----+ +------+ | | | Host +--+ +-------------- +------+ | | +------+ | | Host +--+ +------+
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   Between the subnet and the ISP access link is a gateway, which may or
   may not perform NAT and firewall functions.  When the gateway
   performs NAT functions [RFC3022], it generally allocates private IPv4
   addresses to the local hosts [RFC1918].  A key point of this
   configuration is that the gateway is typically not "managed".  In
   most cases, it is a simple "appliance" that incorporates some static
   policies.  There are many cases in which the gateway is procured and
   configured by the ISP.

   Note that there are also some cases in which we find two gateways
   back to back, one managed by the ISP and the other added by the owner
   of the unmanaged network.  They are not covered in this memo because
   most of them either require some management, or the gateway added by
   the user can function as an L2 switch.

   The access link between the unmanaged network and the ISP might be
   either a static, permanent connection or a dynamic connection such as
   a dial-up or ISDN line.

   In a degenerate case, an unmanaged network might consist of a single
   host, directly connected to an ISP.

   There are some cases in which the "gateway" is replaced by a layer-2
   bridge.  In such deployments, the hosts have direct access to the ISP
   service.  In order to avoid lengthy developments, we will treat these
   cases as if the gateway was not present, i.e., as if each host was
   connected directly to the ISP.

   Our definition of unmanaged networks explicitly exclude networks
   composed of multiple subnets.  We will readily admit that some home
   networks and some small business networks contain multiple subnets,
   but in the current state of the technology, these multiple subnet
   networks are not "unmanaged": some competent administrator has to
   explicitly configure the routers.  We will thus concentrate on single
   subnet networks, where no such competent operator is expected.

3. Applications

Users may use or wish to use the unmanaged network services in four types of applications: local, client, servers and peer-to-peers. These applications may or may not run easily on today's networks (some do, some don't).
Top   ToC   RFC3750 - Page 5

3.1. Local Applications

"Local applications" are only meant to involve the hosts that are part of the unmanaged network. Typical examples would be file sharing or printer sharing. Local applications work effectively in IPv4 unmanaged networks, even when the gateway performs NAT or firewall functions. In fact, firewall services at the gateway are often deemed desirable, as they isolate the local applications from interference by Internet users.

3.2. Client Applications

"Client applications" are those that involve a client on the unmanaged network and a server at a remote location. Typical examples would be accessing a web server from a client inside the unmanaged network, or reading and sending e-mail with the help of a server outside the unmanaged network. Client applications tend to work correctly in IPv4 unmanaged networks, even when the gateway performs NAT or firewall functions: these translation and firewall functions are designed precisely to enable client applications.

3.3. Peer-to-Peer Applications

There are really two kinds of "peer-to-peer" applications: ones which only involve hosts on the unmanaged network, and ones which involve both one or more hosts on the unmanaged network and one or more hosts outside the unmanaged network. We will only consider the latter kind of peer-to-peer applications, since the former can be considered a subset of the kind of local applications discussed in section 3.1. Peer-to-peer applications often don't work well in unmanaged IPv4 networks. Application developers often have to enlist the help of a "relay server", in effect restructuring the peer-to-peer connection into a pair of back-to-back client/server connections.

3.4. Server Applications

"Server applications" involve running a server in the unmanaged network for use by other parties outside the network. Typical examples would be running a web server or an e-mail server on one of the hosts inside the unmanaged network. Deploying these servers in most unmanaged IPv4 networks requires some special programming of the NAT or firewall [RFC2993], and is more complex when the NAT only publishes a small number of global IP
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   addresses and relies on "port translation".  In the common case in
   which the NAT manages exactly one global IP address and relies on
   "port translation", a given external port can only be used by one
   internal server.

   Deploying servers usually requires providing each server with a
   stable DNS name, and associating a global IPv4 address with that
   name, whether the address be that of the server itself or that of the
   router acting as a firewall or NAT.  Since updating DNS is a
   management task, it falls somewhat outside the scope of an unmanaged
   network.  On the other hand, it is also possible to use out-of-band
   techniques (such as cut-and-paste into an instant message system) to
   pass around the address of the target server.

4. Application Requirements of an IPv6 Unmanaged Network

As we transition to IPv6, we must meet the requirements of the various applications, which we can summarize in the following way: applications that worked well with IPv4 should continue working well during the transition; it should be possible to use IPv6 to deploy new applications that are currently hard to deploy in IPv4 networks; and the deployment of these IPv6 applications should be simple and easy to manage, but the solutions should also be robust and secure. The application requirements for IPv6 Unmanaged Networks fall into three general categories: connectivity, naming, and security. Connectivity issues include the provision of IPv6 addresses and their quality: do hosts need global addresses, should these addresses be stable or, more precisely, what should the expected lifetimes of these addresses be? Naming issues include the management of names for the hosts: do hosts need DNS names, and is inverse name resolution [DNSINADDR] a requirement? Security issues include possible restriction to connectivity, privacy concerns and, generally speaking, the security of the applications.

4.1. Requirements of Local Applications

Local applications require local connectivity. They must continue to work even if the unmanaged network is isolated from the Internet. Local applications typically use ad hoc naming systems. Many of these systems are proprietary; an example of a standard system is the service location protocol (SLP) [RFC2608]. The security of local applications will usually be enhanced if these applications can be effectively isolated from the global Internet.
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4.2. Requirements of Client Applications

Client applications require global connectivity. In an IPv6 network, we would expect the client to use a global IPv6 address, which will have to remain stable for the duration of the client-server session. Client applications typically use the domain name system to locate servers. In an IPv6 network, the client must be able to locate a DNS resolver. Many servers try to look up a DNS name associated with the IP address of the client. In an IPv4 network, this IP address will often be allocated by the Internet service provider to the gateway, and the corresponding PTR record will be maintained by the ISP. In many cases, these PTR records are perfunctory, derived in an algorithmic fashion from the IPv4 address; the main information that they contain is the domain name of the ISP. Whether or not an equivalent function should be provided in an IPv6 network is unclear.

4.2.1. Privacy Requirement of Client Applications

It is debatable whether the IPv6 networking service should be engineered to enhance the privacy of the clients, and specifically whether support for RFC 3041 [RFC3041] should be required. RFC 3041 enables hosts to pick IPv6 addresses in which the host identifier is randomized; this was designed to make sure that the IPv6 addresses and the host identifier cannot be used to track the Internet connections of a device's owner. Many observe that randomizing the host identifier portion of the address is only a half measure. If the unmanaged network address prefix remains constant, the randomization only hides which host in the unmanaged network originates a given connection, e.g., the children's computer versus their parents'. This would place the privacy rating of such connections on a par with that of IPv4 connections originating from an unmanaged network in which a NAT manages a static IPv4 address; in both cases, the IPv4 address or the IPv6 prefix can be used to identify the unmanaged network, e.g., the specific home from which the connection originated. However, randomization of the host identifier does provide benefits. First, if some of the hosts in the unmanaged network are mobile, the randomization destroys any correlation between the addresses used at various locations: the addresses alone could not be used to determine whether a given connection originates from the same laptop moving from work to home, or used on the road. Second, the randomization removes any information that could be extracted from a hardwired host identifier; for example, it will prevent outsiders from correlating a
Top   ToC   RFC3750 - Page 8
   serial number with a specific brand of expensive electronic
   equipment, and to use this information for planning marketing
   campaigns or possibly burglary attempts.

   Randomization of the addresses is not sufficient to guarantee
   privacy.  Usage can be tracked by a variety of other means, from
   application level "cookies" to complex techniques involving data
   mining and traffic analysis.  However, we should not make a bad
   situation worse.  Other attacks to privacy may be possible, but this
   is not a reason to enable additional tracking through IPv6 addresses.

   Randomization of the host identifier has some costs: the address
   management in hosts is more complex for the hosts, reverse DNS
   services are harder to provide, and the gateway may have to maintain
   a larger cache of neighbor addresses; however, experience from
   existing implementation shows that these costs are not overwhelming.
   Given the limited benefits, it would be unreasonable to require that
   all hosts use privacy addresses; however, given the limited costs, it
   is reasonable to require that all unmanaged networks allow use of
   privacy addresses by those hosts that choose to do so.

4.3. Requirements of Peer-to-Peer Applications

Peer-to-peer applications require global connectivity. In an IPv6 network, we would expect the peers to use a global IPv6 address, which will have to remain stable for the duration of the peer-to-peer session. There are multiple aspects to the security of peer-to-peer applications, many of which relate to the security of the rendezvous system. If we assume that the peers have been able to safely exchange their IPv6 addresses, the main security requirement is the capability to safely exchange data between the peers without interference by third parties. Private conversations by one of the authors with developers of peer- to-peer applications suggest that many individuals would be willing to consider an "IPv6-only" model if they can get two guarantees: 1) That there is no regression from IPv4, i.e., that all customers who could participate in a peer-to-peer application using IPv4 can also be reached by IPv6. 2) That IPv6 provides a solution for at least some of their hard problems, e.g., enabling peers located behind an IPv4 NAT to participate in a peer-to-peer application.
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   Requiring IPv6 connectivity for a popular peer-to-peer application
   could create what economists refer to as a "network effect", which in
   turn could significantly speed up the deployment of IPv6.

4.4. Requirements of Server Applications

Server applications require global connectivity, which in an IPv6 network implies global addresses. In an IPv4 network utilizing a NAT, for each service provided by a server, the NAT has to be configured to forward packets sent to that service to the server that offers the service. Server applications normally rely on the publication of the server's address in the DNS. This, in turn, requires that the server be provisioned with a "global DNS name". The DNS entries for the server will have to be updated, preferably in real time, if the server's address changes. In practice, updating the DNS can be slow, which implies that server applications will have a better chance of being deployed if the IPv6 addresses remain stable. The security of server applications depends mostly on the correctness of the server, and also on the absence of collateral effects: many incidents occur when the opening of a server on the Internet inadvertently enables remote access to some other services on the same host.

5. Stages of IPv6 Deployment

We expect the deployment of IPv6 to proceed from an initial state in which there is little or no deployment, to a final stage in which we might retire the IPv4 infrastructure. We expect this process to stretch over many years; we also expect it to not be synchronized, as different parties involved will deploy IPv6 at different paces. In order to get some clarity, we distinguish three entities involved in the transition of an unmanaged network: the ISP (possibly including ISP consumer premise equipment (CPE)), the home gateway, and the hosts (computers and appliances). Each can support IPv4- only, both IPv4 and IPv6, or IPv6-only. That gives us 27 possibilities. We describe the most important cases. We will assume that in all cases the hosts are a combination of IPv4-only, dual stack, and (perhaps) IPv6-only hosts.
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   The cases we will consider are:

   A) a gateway that 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

   In most of these cases, we will assume that the gateway includes a
   NAT: we realize that this is not always the case, but we submit that
   it is common enough that we have to deal with it; furthermore, we
   believe that the non-NAT variants of these cases map fairly closely
   to this same set of cases.  In fact, we can consider three non-NAT
   variants: directly connected host; gateway acting as a bridge; and
   gateway acting as a non-NAT IP router.

   The cases of directly connected hosts are, in effect, variants of
   cases B, C, and D, in which the host can use all solutions available
   to gateways: case B if the ISP is dual stack, case C if the ISP only
   provides IPv4 connectivity, and case D if the ISP only provides IPv6
   connectivity.

   In the cases where the gateway is a bridge, the hosts are, in effect,
   directly connected to the ISP, and for all practical matter, behave
   as directly connected hosts.

   The case where the gateway is an IP router but not a NAT will be
   treated as small variants in the analysis of case A, B, C, and D.

5.1. Case A, Host Deployment of IPv6 Applications

In this case, the gateway doesn't provide IPv6; the ISP may or may not provide IPv6, but this is not relevant since the non-upgraded gateway would prevent the hosts from using the ISP service. Some hosts will try to get IPv6 connectivity in order to run applications that require IPv6, or work better with IPv6. The hosts, in this case, will have to handle the IPv6 transition mechanisms on their own. There are two variations of this case, depending on the type of service implemented by the gateway. In many cases, the gateway is a direct obstacle to the deployment of IPv6, but a gateway which is some form of bridge-mode CPE or which is a plain (neither filtering nor NAT) router does not really fall into this category.

5.1.1. Application Support in Case A

The focus of Case A is to enable communication between a host on the unmanaged network and some IPv6-only hosts outside of the network.
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   The primary focus in the immediate future, i.e., for the early
   adopters of IPv6, will be peer-to-peer applications.  However, as
   IPv6 deployment progresses, we will likely find a situation where
   some networks have IPv6-only services deployed, at which point we
   would like case A client applications to be able to access those
   services.

   Local applications are not a primary focus of Case A.  At this stage,
   we expect all clients in the unmanaged network to have either IPv4
   only or dual stack support.  Local applications can continue working
   using IPv4.

   Server applications are also not a primary focus of Case A.  Server
   applications require DNS support, which is difficult to engineer for
   clients located behind a NAT, which is likely to be present in this
   case.  Besides, server applications presently cater mostly to IPv4
   clients; putting up an IPv6-only server is not very attractive.

   In contrast, peer-to-peer applications are probably both attractive
   and easy to deploy: they are deployed in a coordinated fashion as
   part of a peer-to-peer network, which means that hosts can all
   receive some form of an IPv6 upgrade; they often provide their own
   naming infrastructure, in which case they are not dependent on DNS
   services.

5.1.2. Addresses and Connectivity in Case A

We saw in 5.1.1 that the likely motivation for deployment of IPv6 connectivity in hosts in case A is a desire to use peer-to-peer and client IPv6 applications. These applications require that all participating nodes get some form of IPv6 connectivity, i.e., at least one globally reachable IPv6 address. If the local gateway provides global IPv4 addresses to the local hosts, then these hosts can individually exercise the mechanisms described in case C, "IPv6 connectivity without provider support." If the local gateway implements a NAT function, another type of mechanism is needed. The mechanism to provide connectivity to peers behind NAT should be easy to deploy, and light weight; it will have to involve tunneling over a protocol that can easily traverse NAT, either TCP or preferably UDP, as tunneling over TCP can result in poor performance in cases of time-outs and retransmissions. If servers are needed, these servers will, in practice, have to be deployed as part of the "support infrastructure" for the peer-to-peer network or for an IPv6-based service; economic reality implies that the cost of running these servers should be as low as possible.
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5.1.3. Naming Services in Case A

At this phase of IPv6 deployment, hosts in the unmanaged domain have access to DNS services over IPv4 through the existing gateway. DNS resolvers are supposed to serve AAAA records, even if they only implement IPv4; the local hosts should thus be able to obtain the IPv6 addresses of IPv6-only servers. Reverse lookup is difficult to provide for hosts on the unmanaged network if the gateway is not upgraded. This is a potential issue for client applications. Some servers require a reverse lookup as part of accepting a client's connection, and may require that the direct lookup of the corresponding name matches the IPv6 address of the client. There is thus a requirement to provide either a reverse lookup solution, or to make sure that IPv6 servers do not require reverse lookup.

5.2. Case B, IPv6 Connectivity with Provider Support

In this case, the ISP and gateway are both dual stack. The gateway can use native IPv6 connectivity to the ISP and can use an IPv6 prefix allocated by the ISP.

5.2.1. Application Support in Case B

If the ISP and the gateway are dual-stack, client applications, peer-to-peer applications, and server applications can all be enabled easily on the unmanaged network. We expect the unmanaged network to include three kinds of hosts: IPv4 only, IPv6-only, and dual stack. Obviously, dual stack hosts can interact easily with either IPv4 only hosts or IPv6-only hosts, but an IPv4 only host and an IPv6-only host cannot communicate without a third party performing some kind of translation service. Our analysis concludes that unmanaged networks should not have to provide such translation services. The argument for providing translation services is that their availability would accelerate the deployment of IPv6-only devices, and thus the transition to IPv6. This is, however, a dubious argument since it can also be argued that the availability of these translation services will reduce the pressure to provide IPv6 at all, and to just continue fielding IPv4-only devices. The remaining pressure to provide IPv6 connectivity would just be the difference in "quality of service" between a translated exchange and a native interconnect.
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   The argument against translation service is the difficulty of
   providing these services for all applications, compared to the
   relative ease of installing dual stack solutions in an unmanaged
   network.  Translation services can be provided either by application
   relays, such as HTTP proxies, or by network level services, such as
   NAT-PT [RFC2766].  Application relays pose several operational
   problems: first, one must develop relays for all applications;
   second, one must develop a management infrastructure to provision the
   host with the addresses of the relays; in addition, the application
   may have to be modified if one wants to use the relay selectively,
   e.g., only when direct connection is not available.  Network level
   translation poses similar problems: in practice, network level
   actions must be complemented by "application layer gateways" that
   will rewrite references to IP addresses in the protocol, and while
   these relays are not necessary for every application, they are
   necessary for enough applications to make any sort of generalized
   translation quite problematic; hosts may need to be parameterized to
   use the translation service, and designing the right algorithm to
   decide when to translate DNS requests has proven very difficult.

   Not assuming translation services in the network appears to be both
   more practical and more robust.  If the market requirement for a new
   device requires that it interact with both IPv4 and IPv6 hosts, we
   may expect the manufacturers of these devices to program them with a
   dual stack capability; in particular, we expect general purpose
   systems, such as personal computers, to be effectively dual-stack.
   The only devices that are expected to be capable of only supporting
   IPv6 are those designed for specific applications, which do not
   require interoperation with IPv4-only systems.  We also observe that
   providing both IPv4 and IPv6 connectivity in an unmanaged network is
   not particularly difficult: we have a fair amount of experience using
   IPv4 in unmanaged networks in parallel with other protocols, such as
   IPX.

5.2.2. Addresses and Connectivity in Case B

In Case B, the upgraded gateway will act as an IPv6 router; it will continue providing the IPv4 connectivity, perhaps using NAT. Nodes in the local network will typically obtain: - IPv4 addresses (from or via the gateway), - IPv6 link local addresses, and - IPv6 global addresses. In some networks, NAT will not be in use and the local hosts will actually obtain global IPv4 addresses. We will not elaborate on this, as the availability of global IPv4 addresses does not bring any additional complexity to the transition mechanisms.
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   To enable this scenario, the gateway needs to use a mechanism to
   obtain a global IPv6 address prefix from the ISP, and advertise this
   address prefix to the hosts in the unmanaged network; several
   solutions will be assessed in a companion memo [EVAL].

5.2.3. Naming Services in Case B

In case B, hosts in the unmanaged domain have access to DNS services through the gateway. As the gateway and the ISP both support IPv4 and IPv6, these services may be accessible by the IPv4-only hosts using IPv4, by the IPv6-only hosts using IPv6, and by the dual stack hosts using either. Currently, IPv4 only hosts usually discover the IPv4 address of the local DNS resolver using DHCP; there must be a way for IPv6-only hosts to discover the IPv6 address of the DNS resolver. There must be a way to resolve the name of local hosts to their IPv4 or IPv6 addresses. Typing auto-configured IPv6 addresses in a configuration file is impractical; this implies either some form of dynamic registration of IPv6 addresses in the local service, or a dynamic address discovery mechanism. Possible solutions will be compared in the evaluation draft [EVAL]. The requirement to support server applications in the unmanaged network implies a requirement to publish the IPv6 addresses of local servers in the DNS. There are multiple solutions, including domain name delegation. If efficient reverse lookup functions are to be provided, delegation of a fraction of the ip6.arpa tree is also required. The response to a DNS request should not depend on the protocol by which the request is transported: dual-stack hosts may use either IPv4 or IPv6 to contact the local resolver, the choice of IPv4 or IPv6 may be random, and the value of the response should not depend on a random event. DNS transition issues in a dual IPv4/IPv6 network are discussed in [DNSOPV6].

5.3. Case C, IPv6 Connectivity without Provider Support

In this case, the gateway is dual stack, but the ISP is not. The gateway has been upgraded and offers both IPv4 and IPv6 connectivity to hosts. It cannot rely on the ISP for IPv6 connectivity, because the ISP does not yet offer ISP connectivity.
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5.3.1. Application Support in Case C

Application support in case C should be identical to that of case B.

5.3.2. Addresses and Connectivity in Case C

The upgraded gateway will behave as an IPv6 router; it will continue providing the IPv4 connectivity, perhaps using NAT. Nodes in the local network will obtain: - IPv4 addresses (from or via the gateway), - IPv6 link local addresses, - IPv6 global addresses. There are two ways to bring immediate IPv6 connectivity on top of an IPv4 only infrastructure: automatic tunnels, e.g., provided by the 6TO4 technology [RFC3056], or configured tunnels. Both technologies have advantages and limitations, which will be studied in another document. There will be some cases where the local hosts actually obtain global IPv4 addresses. We will not discuss this scenario, as it does not make the use of transition technology harder, or more complex. Case A has already examined how hosts could obtain IPv6 connectivity individually.

5.3.3. Naming Services in Case C

The local naming requirements in case C are identical to the local naming requirements of case B, with two differences: delegation of domain names, and management of reverse lookup queries. A delegation of some domain name is required in order to publish the IPv6 addresses of servers in the DNS. A specific mechanism for handling reverse lookup queries will be required if the gateway uses a dynamic mechanism, such as 6to4, to obtain a prefix independently of any IPv6 ISP.

5.4. Case D, ISP Stops Providing Native IPv4 Connectivity

In this case, the ISP is IPv6-only, so the gateway loses IPv4 connectivity, and is faced with an IPv6-only service provider. The gateway itself is dual stack, and the unmanaged network includes IPv4 only, IPv6-only, and dual stack hosts. Any interaction between hosts in the unmanaged network and IPv4 hosts on the Internet will require the provision of some inter-protocol services by the ISP.
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5.4.1. Application Support in Case D

At this phase of the transition, IPv6 hosts can participate in all types of applications with other IPv6 hosts. IPv4 hosts in the unmanaged network will be able to perform local applications with IPv4 or dual stack local hosts. As in case B, we will assume that IPv6-only hosts will not interact with IPv4-only hosts, either local or remote. We must however assume that IPv4-only hosts and dual stack hosts will want to interact with IPv4 services available on the Internet: the inability to do so would place the IPv6-only provider at a great commercial disadvantage compared to other Internet service providers. There are three possible ways that an ISP can provide hosts in the unmanaged network with access to IPv4 applications: by using a set of application relays, by providing an address translation service, or by providing IPv4-over-IPv6 tunnels. Our analysis concludes that a tunnel service seems to be vastly preferable. We already mentioned the drawbacks of the application gateway approach when analyzing case B: it is necessary to provide relays for all applications, to develop a way to provision the hosts with the addresses of these relays, and to modify the applications so that they will only use the relays when needed. We also observe that in an IPv6-only ISP, the application relays would only be accessible over IPv6, and would thus not be accessible by the "legacy" IPv4-only hosts. The application relay approach is thus not very attractive. Providing a network address and protocol translation service between IPv6 and IPv4 would also have many drawbacks. As in case B, it will have to be complemented by "application layer gateways" that will rewrite references to IP addresses in the protocol; hosts may need to be parameterized to use the translation service, and we would have to solve DNS issues. The network level protocol translation service doesn't appear to be very desirable. The preferable alternative to application relays and network address translation is the provision of an IPv4-over-IPv6 service.

5.4.2. Addresses and Connectivity in Case D

The ISP assigns an IPv6 prefix to the unmanaged network, so hosts have a global IPv6 address and use it for global IPv6 connectivity. This will require delegation of an IPv6 address prefix, as investigated in case C.
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   To enable IPv4 hosts and dual stack hosts accessibility to remote
   IPv4 services, the ISP must provide the gateway with at least one
   IPv4 address, using some form of IPv4-over-IPv6 tunneling.  Once such
   addresses have been provided, the gateway effectively acquires dual-
   stack connectivity; for hosts inside the unmanaged network, this will
   be indistinguishable from the IPv4 connectivity obtained in case B or
   C.

5.4.3. Naming Services in Case D

The loss of IPv4 connectivity has a direct impact on the provision of naming services. In many IPv4 unmanaged networks, hosts obtain their DNS configuration parameters from the local gateway, typically through the DHCP service. If the same mode of operation is desired in case D, the gateway will have to be provisioned with the address of a DNS resolver and with other DNS parameters, and this provisioning will have to use IPv6 mechanisms. Another consequence is that the DNS service in the gateway will only be able to use IPv6 connectivity to resolve queries; if local hosts perform DNS resolution autonomously, they will have the same restriction. On the surface, this seems to indicate that the local hosts will only be able to resolve names if the domain servers are accessible through an IPv6 address documented in an AAAA record. However, the DNS services are just one case of "IPv4 servers accessed by IPv6 hosts": it should be possible to simply send queries through the IPv4 connectivity services to reach the IPv4 only servers. The gateway should be able to act as a recursive DNS name server for the remaining IPv4 only hosts.

6. Security Considerations

Security considerations are discussed as part of the applications' requirements. They include: - the guarantee that local applications are only used locally, - the protection of the privacy of clients - the requirement that peer-to-peer connections are only used by authorized peers - the requirement that tunneling protocols used for IPv6 access over IPv4 be designed for secure use - the related requirement that servers in the infrastructure supporting transition scenarios be designed so as to not be vulnerable to abuse.
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   The security solutions currently used in IPv4 networks include a
   combination of firewall functions in the gateway, authentication and
   authorization functions in the applications, encryption and
   authentication services provided by IP security, Transport Layer
   Security and application specific services, and host-based security
   products, such as anti-virus software and host firewalls.  The
   applicability of these tools in IPv6 unmanaged networks will be
   studied in a another document.

7. Acknowledgements

This document has benefited from the comments of the members of the IETF V6OPS working group, and from extensive reviews by Chris Fischer, Tony Hain, Kurt Erik Lindqvist, Erik Nordmark, Pekka Savola, and Margaret Wasserman.

8. References

8.1. Normative References

[RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791, September 1981. [RFC2460] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", RFC 2460, December 1998.

8.2. Informative References

[EVAL] Evaluation of Transition Mechanisms for Unmanaged Networks, Work in Progress. [RFC1918] Rekhter, Y., Moskowitz, B., Karrenberg, D., de Groot, G. J. and E. Lear, "Address Allocation for Private Internets", BCP 5, RFC 1918, February 1996. [RFC2608] Guttman, E., Perkins, C., Veizades, J. and M. Day, "Service Location Protocol, Version 2", RFC 2608, June 1999. [RFC3056] Carpenter, B. and K. Moore, "Connection of IPv6 Domains via IPv4 Clouds", RFC 3056, February 2001. [RFC3022] Srisuresh, P. and K. Egevang. "Traditional IP Network Address Translator (Traditional NAT)", RFC 3022, January 2001. [RFC2993] Hain, T., "Architectural Implications of NAT", RFC 2993, November 2000.
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   [RFC3041]   Narten, T. and R. Draves, "Privacy Extensions for
               Stateless Address Autoconfiguration in IPv6", RFC 3041,
               January 2001.

   [RFC2766]   Tsirtsis, G. and P. Srisuresh, "Network Address
               Translation - Protocol Translation (NAT-PT)", RFC 2766,
               February 2000.

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

   [DNSINADDR] Senie, D., "Requiring DNS IN-ADDR Mapping", Work in
               Progress.

9. Authors' Addresses

Christian Huitema Microsoft Corporation One Microsoft Way Redmond, WA 98052-6399 EMail: huitema@microsoft.com Rob Austein Internet Systems Consortium 950 Charter Street Redwood City, CA 94063 USA EMail: sra@isc.org Suresh Satapati Cisco Systems, Inc. San Jose, CA 95134 USA EMail: satapati@cisco.com Ronald van der Pol NLnet Labs Kruislaan 419 1098 VA Amsterdam NL EMail: Ronald.vanderPol@nlnetlabs.nl
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10. Full Copyright Statement

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