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Versions: 00 01 02 03 04 05 06 RFC 4339

DNS Operations WG
Internet-Draft                                         J. Jeong (ed.)
                                         ETRI/University of Minnesota

Expires: March 2005                                 28 September 2004

      IPv6 Host Configuration of DNS Server Information Approaches

Status of this Memo

   By submitting this Internet-Draft, I certify that any applicable
   patent or other IPR claims of which I am aware have been disclosed,
   and any of which we become aware will be disclosed, in accordance
   with RFC3668.

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   This Internet-Draft will expire on March 27, 2005.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.


   This document describes three approaches for IPv6 recursive DNS
   server address configuration.  It details the operational
   attributes of three solutions: RA option, DHCPv6 option, and Well-
   known anycast addresses for recursive DNS servers.  Additionally,
   it suggests four deployment scenarios considering multi-solution
   resolution.  Therefore, this document will give the audience a

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   guideline for IPv6 DNS configuration to select approaches suitable
   for their host DNS configuration.

Table of Contents

   1. Introduction....................................................3
   2. Terminology.....................................................3
   3. IPv6 DNS Configuration Approaches...............................3
      3.1. RA Option..................................................3
           3.1.1. Advantages..........................................4
           3.1.2. Disadvantages.......................................5
           3.1.3. Observations........................................6
      3.2. DHCPv6 Option..............................................6
           3.2.1. Advantages..........................................7
           3.2.2. Disadvantages.......................................8
           3.2.3. Observations........................................9
      3.3. Well-known Anycast Addresses...............................9
           3.3.1. Advantages.........................................10
           3.3.2. Disadvantages......................................10
           3.3.3. Observations.......................................10
   4. Interworking among IPv6 DNS Configuration Approaches...........11
   5. Deployment Scenarios...........................................12
      5.1. ISP Network...............................................12
           5.1.1. RA Option Approach.................................12
           5.1.2. DHCPv6 Option Approach.............................13
           5.1.3. Well-known Addresses Approach......................13
      5.2. Enterprise Network........................................14
      5.3. 3GPP Network..............................................14
           5.3.1. Currently Available Mechanisms and Recommendations.15
           5.3.2. RA Extension.......................................16
           5.3.3. Stateless DHCPv6...................................16
           5.3.4. Well-known Addresses...............................17
           5.3.5. Recommendations....................................17
      5.4. Unmanaged Network.........................................18
           5.4.1. Case A: Gateway does not provide IPv6 at all.......18
           5.4.2. Case B: A dual-stack gateway connected to a
                  dual-stack ISP.....................................18
           5.4.3. Case C: A dual-stack gateway connected to an
                  IPv4-only ISP......................................19
           5.4.4. Case D: A gateway connected to an IPv6-only ISP....19
   6. Security Considerations........................................19
   7. Acknowledgements...............................................19
   8. Normative References...........................................20
   9. Informative References.........................................20
   10. Appendix A - Link-layer Multicast Acknowledgements with RA
   11. Authors' Addresses............................................22
   12. Intellectual Property Statement...............................23

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   Full Copyright Statement..........................................24

1.  Introduction

   Neighbor Discovery (ND) for IP Version 6 and IPv6 Stateless Address
   Autoconfiguration provide ways to configure either fixed or mobile
   nodes with one or more IPv6 addresses, default routes and some
   other parameters [3][4].  To support access to additional services
   in the Internet that are identified by a DNS name, such as a web
   server, the configuration of at least one recursive DNS server is
   also needed for DNS name resolution.

   This document describes three approaches of recursive DNS server
   address configuration for IPv6 host: (a) RA option [8], (b) DHCPv6
   option [5]-[7], and (c) Well-known anycast addresses for recursive
   DNS servers [9].  Also, it suggests applicable scenarios for four
   kinds of networks: (a) ISP network, (b) Enterprise network, (c)
   3GPP network, and (d) Unmanaged network.

   This document is just an analysis of each possible approach, and
   does not make any recommendation on particular one or on a
   combination of particular ones.  Some approaches may even not be
   adopted at all as a result of further discussion.

   Therefore, the objective of this document is to help the audience
   select approaches suitable for IPv6 host configuration of recursive
   DNS server.

2.  Terminology

   This document uses the terminology described in [3]-[9].  In
   addition, a new term is defined below:

   Recursive DNS Server (RDNSS)    A Recursive DNS Server is a name
                                   server that offers the recursive
                                   service of DNS name resolution.

3.  IPv6 DNS Configuration Approaches

   In this section, the operational attributes of three solutions are
   described in detail.

3.1.  RA Option

   RA approach is to define a new ND option called RDNSS option that
   contains a recursive DNS server address.  Existing ND transport
   mechanisms (i.e., advertisements and solicitations) are used.  This

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   works in the same way that nodes learn about routers and prefixes.
   An IPv6 host can configure the IPv6 addresses of one or more
   RDNSSes via RA message periodically sent by router or solicited by
   a Router Solicitation (RS) [8].

   This approach needs RDNSS information to be configured in the
   routers doing the advertisements.  The configuration of RDNSS
   address can be performed manually by operator or other ways, such
   as automatic configuration through DHCPv6 client running on the
   router.  When advertising more than one RDNSS options, an RA
   message includes as many RDNSS options as RDNSSes.

   Through ND protocol and RDNSS option along with prefix information
   option, an IPv6 host can perform its network configuration of its
   IPv6 address and RDNSS simultaneously [3][4].  The RA option for
   RDNSS can be used on any network that supports the use of ND.

   However, it is worth noting that some link layers (e.g., WLAN) need
   to acknowledge multicast packets, which may increase the amount of
   link-layer traffic.  This is discussed in Appendix A.

   The RA approach is useful in some mobile environments where the
   addresses of the RDNSSes are changing because the RA option
   includes a lifetime field that allows client to use RDNSSes nearer
   to the client.  This can be configured to a value that will require
   the client to time out the entry and switch over to another RDNSS
   address [8].  However, from the viewpoint of implementation,
   lifetime would seem to make matters a bit more complex.  Instead of
   just writing DNS configuration file, such as resolv.conf for the
   list of RDNSS addresses, we have to have a daemon around (or a
   program that is called at the defined intervals) that keeps
   monitoring the lifetime of RDNSSes all the time.

   The preference value of RDNSS, included in RDNSS option, allows
   IPv6 hosts to select primary RDNSS among several RDNSSes; this can
   be used for load balancing of RDNSSes [8].

3.1.1.  Advantages

   The RA option for RDNSS has a number of advantages.  These include:

   1) The RA option is an extension of existing ND/Autoconfig
   mechanisms [3][4], and does not require a change in the base ND

   2) This approach, like ND, works well on a variety of link types
   including point-to-point links, point-to-multipoint, and multi-
   point (i.e., Ethernet LANs), etc.  RFC2461 [3] states, however,

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   that there may be some link type on which ND is not possible; on
   such a link, some other mechanism will be needed for DNS

   3) All of the information a host needs to run basic Internet
   applications such as email, the web, ftp, etc., can be obtained
   with the addition of this option to ND and address auto-
   configuration.  The use of a single mechanism is more reliable and
   easier to provide than when the RDNSS information is learned via
   another protocol mechanism.  Debugging problems when multiple
   protocol mechanisms are being used is harder and much more complex.

   4) This mechanism works over a broad range of scenarios and
   leverages IPv6 ND.  This works well on links that support broadcast
   reliably (e.g., Ethernet LANs) but not necessarily on other links
   (e.g., Wireless LANs).  Also, this works well on links that are
   high performance (e.g., Ethernet LANs) and low performance (e.g.,
   Cellular networks).  In the latter case, combining the RDNSS
   information with the other information in the RA, the host can
   learn all of the information needed to use most Internet
   applications such as the web in a single packet.  This not only
   saves bandwidth where this is an issue, but also minimizes the
   delay to learn the RDNSS information.

   5) The RA approach could be used as a model for other similar types
   of configuration information.  New RA options for other server
   addresses that are common to all clients on a subnet would be easy
   to define.  This includes things like NTP servers, SIP servers, etc.

3.1.2.  Disadvantages

   1) ND is mostly implemented in kernel part of operating system.
   Therefore, if ND supports the configuration of some additional
   services, such as DNS, NTP and SIP servers, ND should be extended
   in kernel part, and complemented by a user-land process.  DHCPv6,
   however, has more flexibility for extension of service discovery
   because it is an application layer protocol.

   2) The current ND framework should be modified due to the
   synchronization between another ND cache for RDNSSes in kernel
   space and DNS configuration file in user space.  Because it is
   unacceptable to write and rewrite the DNS configuration file (e.g.,
   resolv.conf) from the kernel, another approach is needed.  One
   simple approach to solve this is to have a daemon listening to what
   the kernel conveys, and to have the daemon do these steps, but such
   a daemon is not necessary with the current ND framework.

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   3) It is necessary to configure RDNSS addresses at least at one
   router on every link where this information needs to be configured
   by RA option.

3.1.3.  Observations

   The proposed RDNSS RA option along with IPv6 ND and Auto-
   configuration allows a host to obtain all of the information it
   needs to access basic Internet services like the web, email, ftp,
   etc.  This is preferable in environments where hosts use RAs to
   autoconfigure their addresses and all hosts on the subnet share the
   same router and server addresses.  If the configuration information
   can be obtained from a single mechanism, it is preferable because
   it does not add additional delay, and it uses a minimum of
   bandwidth.  Environments like this include homes, public cellular
   networks, and enterprise environments where no per host
   configuration is needed, but exclude public WLAN hot spots.

   DHCPv6 is preferable where it is being used for address
   configuration and if there is a need for host specific
   configuration [5]-[7].  Environments like this are most likely
   enterprise environments where the local administration chooses to
   have per host configuration control.

   Note: the observation section is based on what the proponents of
   each approach think makes a good overall solution.

3.2.  DHCPv6 Option

   DHCPv6 [5] includes the "DNS Recursive Name Server" option, through
   which a host can obtain a list of IP addresses of recursive DNS
   servers [7].  The DNS Recursive Name Server option carries a list
   of IPv6 addresses of RDNSSes to which the host may send DNS queries.
   The DNS servers are listed in the order of preference for use by
   the DNS resolver on the host.

   The DNS Recursive Name Server option can be carried in any DHCPv6
   Reply message, in response to either a Request or an Information-
   request message.  Thus, the DNS Recursive Name Server option can be
   used either when DHCPv6 is used for address assignment, or when
   DHCPv6 is used only for other configuration information as
   stateless DHCPv6 [6].

   Stateless DHCPv6 can be deployed either using DHCPv6 servers
   running on general-purpose computers, or on router hardware.
   Several router vendors currently implement stateless DHCPv6 servers.
   Deploying stateless DHCPv6 in routers has the advantage that no
   special hardware is required, and should work well for networks

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   where DHCPv6 is needed for very straightforward configuration of
   network devices.

   However, routers can also act as DHCPv6 relay agents.  In this case,
   the DHCPv6 server need not be on the router - it can be on a
   general purpose computer.  This has the potential to give the
   operator of the DHCPv6 server more flexibility in how the DHCPv6
   server responds to individual clients - clients can easily be given
   different configuration information based on their identity, or for
   any other reason.  Nothing precludes adding this flexibility to a
   router, but generally in current practice, DHCP servers running on
   general-purpose hosts tend to have more configuration options than
   those that are embedded in routers.

   DHCPv6 currently provides a mechanism for reconfiguring DHCPv6
   clients that use stateful configuration assignment.  To do this,
   the DHCPv6 server sends a Reconfigure message to the client.  The
   client validates the Reconfigure message, and then contacts the
   DHCPv6 server to obtain updated configuration information.  Using
   this mechanism, it is currently possible to propagate new
   configuration information to DHCPv6 clients as this information

   The DHC Working Group is currently studying an additional mechanism
   through which configuration information, including the list of
   RDNSSes, can be updated.  The lifetime option for DHCPv6 [10]
   assigns a lifetime to configuration information obtained through
   DHCPv6.  At the expiration of the lifetime, the host contacts the
   DHCPv6 server to obtain updated configuration information,
   including the list of RDNSSes.  This lifetime gives the network
   administrator another mechanism to configure hosts with new RDNSSes
   by controlling the time at which the host refreshes the list.

   The DHC Working Group has also discussed the possibility of
   defining an extension to DHCPv6 that would allow the use of
   multicast to provide configuration information to multiple hosts
   with a single DHCPv6 message.  Because of the lack of deployment
   experience, the WG has deferred consideration of multicast DHCPv6
   configuration at this time.  Experience with DHCPv4 has not
   identified a requirement for multicast message delivery, even in
   large service provider networks with tens of thousands of hosts
   that may initiate a DHCPv4 message exchange simultaneously.

3.2.1.  Advantages

   The DHCPv6 option for RDNSS has a number of advantages.  These

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   1) DHCPv6 currently provides a general mechanism for conveying
   network configuration information to clients.  So configuring
   DHCPv6 servers allows the network administrator to configure
   RDNSSes along with the addresses of other network services, as well
   as location-specific information like time zones.

   2) As a consequence, when the network administrator goes to
   configure DHCPv6, all the configuration information can be managed
   through a single service, typically with a single user interface
   and a single configuration database.

   3) DHCPv6 allows for the configuration of a host with information
   specific to that host, so that hosts on the same link can be
   configured with different RDNSSes as well as other configuration
   information.  This capability is important in some network
   deployments such as service provider networks or WiFi hot spots.

   4) A mechanism exists for extending DHCPv6 to support the
   transmission of additional configuration that has not yet been

   5) Hosts that require other configuration information such as the
   addresses of SIP servers and NTP servers are likely to need DHCPv6
   for other configuration information.

   6) The specification for configuration of RDNSSes through DHCPv6 is
   available as an RFC.  No new protocol extensions such as new
   options are necessary.

   7) Interoperability among independent implementations has been

3.2.2.  Disadvantages

   The DHCPv6 option for RDNSS has a few disadvantages.  These

   1) Update currently requires message from server (however, see

   2) Because DNS information is not contained in RA message, the host
   must receive two messages from the router, and must transmit at
   least one message to the router.  On networks where bandwidth is at
   a premium, this is a disadvantage, although on most networks it is
   not a practical concern.

   3) Increased latency for initial configuration - in addition to
   waiting for an RA message, the client must now exchange packets

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   with a DHCPv6 server; even if it is locally installed on a router,
   this will slightly extend the time required to configure the client.
   For clients that are moving rapidly from one network to another,
   this will be a disadvantage.

3.2.3.  Observations

   In the general case, on general-purpose networks, stateless DHCPv6
   provides significant advantages and no significant disadvantages.
   Even in the case where bandwidth is at a premium and low latency is
   desired, if hosts require other configuration information in
   addition to a list of RDNSSes or if hosts must be configured
   selectively, those hosts will use DHCPv6 and the use of the DHCPv6
   DNS recursive name server option will be advantageous.

   However, we are aware of some applications where it would be
   preferable to put the RDNSS information into an RA packet; for
   example, on a cell phone network, where bandwidth is at a premium
   and extremely low latency is desired.  The final DNS configuration
   draft should be written so as to allow these special applications
   to be handled using DNS information in the RA packet.

3.3.  Well-known Anycast Addresses

   First of all, the well-known anycast addresses approach is much
   different from that discussed at IPv6 Working Group in the past [9].
   It should be noted that "anycast" in this memo is simpler than that
   of RFC1546 [11] and RFC3513 [12] where it is assumed to be
   prohibited to have multiple servers on a single link sharing an
   anycast address.  That is, on a link, anycast address is assumed to
   be unique.  DNS clients today already have redundancy by having
   multiple well-known anycast addresses configured as RDNSS addresses.
   There is no point to have multiple RDNSSes sharing an anycast
   address on a single link.

   The approach with well-known anycast addresses is to set well-known
   anycast addresses in clients' resolver configuration files from the
   beginning, say, as factory default.  Thus, there is no transport
   mechanism and no packet format [9].

   An anycast address is an address shared by multiple servers (in
   this case, the servers are RDNSSes).  Request from a client to the
   anycast address is routed to a server selected by the routing
   system.  However, it is a bad idea to mandate "site" boundary on
   anycast addresses, because most users just do not have their own
   servers and want to access their ISPs' across their site boundaries.
   Larger sites may also depend on their ISPs or may have their own
   RDNSSes within "site" boundaries.

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3.3.1.  Advantages

   The basic advantage of the well-known addresses approach is that it
   uses no transport mechanism.  Thus,
   1) There is no delay to get response and no further delay by packet

   2) The approach can be combined with any other configuration
   mechanisms including but not limited to factory default
   configuration, RA-based approach and DHCP based approach.

   3) The approach works over any environment where DNS works.

   Another advantage is that the approach needs to configure DNS
   servers as a router, but nothing else.  Considering that DNS
   servers do need configuration, the amount of overall configuration
   effort is proportional to the number of the DNS servers and scales
   linearly.  It should be noted that, in the simplest case where a
   subscriber to an ISP does not have any DNS server, the subscriber
   naturally accesses DNS servers of the ISP even though the subscriber
   and the ISP do nothing and there is no protocol to exchange DNS
   server information between the subscriber and the ISP.

3.3.2.  Disadvantages

   Well-known anycast addresses approach requires that DNS servers (or
   routers near it as a proxy) act as routers to advertise their
   anycast addresses to the routing system, which requires some
   configuration (see the last paragraph of the previous section on
   the scalability of the effort).

3.3.3.  Observations

   If other approaches are used in addition, the well-known anycast
   addresses should also be set in RA or DHCP configuration files to
   reduce configuration effort of users.

   Redundancy by multiple RDNSSes is better provided by multiple
   servers having different anycast addresses than multiple servers
   sharing same anycast address because the former approach allows
   stale servers to still generate routes to their anycast addresses.
   Thus, in a routing domain (or domains sharing DNS servers), there
   will be only one server having an anycast address unless the domain
   is so large that load distribution is necessary.

   Small ISPs will operate one RDNSS at each anycast address which is
   shared by all the subscribers.  Large ISPs may operate multiple

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   RDNSSes at each anycast address to distribute and reduce load,
   where boundary between RDNSSes may be fixed (redundancy is still
   provided by multiple addresses) or change dynamically.  DNS packets
   with the well-known anycast addresses are not expected (though not
   prohibited) to cross ISP boundaries, as ISPs are expected to be
   able to take care of themselves.

   Because "anycast" in this memo is simpler than that of RFC1546 [11]
   and RFC3513 [12] where it is assumed to be administratively
   prohibited to have multiple servers on a single link sharing an
   anycast address, anycast in this memo should be implemented as
   UNICAST of RFC2461 [3] and RFC3513 [12].  As a result, ND-related
   instability disappears.  Thus, anycast in well-known anycast
   addresses approach can and should use the anycast address as a
   source unicast (according to RFC3513 [12]) address of packets of
   UDP and TCP responses.  With TCP, if route flips and packets to an
   anycast address are routed to a new server, it is expected that the
   flip is detected by ICMP or sequence number inconsistency and the
   TCP connection is reset and retried.

4.  Interworking among IPv6 DNS Configuration Approaches

   Three approaches can work together for IPv6 host configuration of
   RDNSS.  This section shows a consideration on how these approaches
   can interwork each other.

   For ordering between RA and DHCP approaches, O (Other stateful
   configuration) flag in RA message can be used [8].  If no RDNSS
   option is included, an IPv6 host may perform DNS configuration
   through DHCPv6 [5]-[7] regardless of whether the O flag is set or

   The well-known anycast addresses approach fully interworks with the
   other approaches.  That is, the other approaches can remove
   configuration effort on servers by using the well-known addresses
   as the default configuration.  Moreover, clients preconfigured with
   well-known anycast addresses can be further configured to use other
   approaches to override the well-known addresses, if configuration
   information from other approaches are available.  That is, all the
   clients should have the well-known anycast addresses preconfigured,
   in the case where there are no other mechanisms available.  In
   order to fly anycast approach with the other solutions, there are
   three options.

   The first option is that well-known addresses are used as last
   resort, when an IPv6 host can not get RDNSS information through RA
   and DHCP.  The well-known anycast addresses have to be pre-
   configured in IPv6 hosts' resolver configuration files.

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   The second is that an IPv6 host can configure well-known addresses
   as the most preferable in its configuration file even though either
   RA option or DHCP option is available.

   The last is that the well-known anycast addresses can be set in RA
   or DHCP configuration to reduce configuration effort of users.
   According to either RA or DHCP mechanism, the well-known addresses
   can be obtained by IPv6 host.  Because this approach is the most
   convenient for users, the last option is recommended.

   Note: this section does not necessarily mean this document suggests
   adopting all these three approaches and making them interwork in
   the way described here.  In fact, some approaches may even not be
   adopted at all as a result of further discussion.

5.  Deployment Scenarios

   Regarding DNS configuration on the IPv6 host, several mechanisms
   are being considered at the DNSOP Working Group such as RA option,
   DHCPv6 option and well-known preconfigured anycast addresses as of
   today, and this document is a final result from the long thread.
   In this section, we suggest four applicable scenarios of three
   approaches for IPv6 DNS configuration.

   Note: in the applicable scenarios, authors do not implicitly push
   any specific approaches into the restricted environments.  No
   enforcement is in each scenario and all mentioned scenarios are
   probable.  The main objective of this work is to provide a useful
   guideline for IPv6 DNS configuration.

5.1.  ISP Network

   A characteristic of ISP network is that multiple Customer Premises
   Equipment (CPE) devices are connected to IPv6 PE (Provider Edge)
   routers and each PE connects multiple CPE devices to the backbone
   network infrastructure [13].  The CPEs may be hosts or routers.

   In the case where the CPE is a router, there is a customer network
   that is connected to the ISP backbone through the CPE.  Typically,
   each customer network gets a different IPv6 prefix from an IPv6 PE
   router, but the same RDNSS configuration will be distributed.

   This section discusses how the different approaches to distributing
   DNS information are compared in an ISP network.

5.1.1.  RA Option Approach

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   When the CPE is a host, the RA option for RDNSS can be used to
   allow the CPE to get RDNSS information as well as /64 prefix
   information for stateless address autoconfiguration at the same
   time when the host is attached to a new subnet [8].  Because an
   IPv6 host must receive at least one RA message for stateless
   address autoconfiguration and router configuration, the host could
   receive RDNSS configuration information in that RA without the
   overhead of an additional message exchange.

   When the CPE is a router, the CPE may accept the RDNSS information
   from the RA on the interface connected to the ISP, and copy that
   information into the RAs advertised in the customer network.

   This approach is more valuable in the mobile host scenario, in
   which the host must receive at least an RA message for detecting a
   new network, than in other scenarios generally although
   administrator should configure RDNSS information on the routers.
   Secure ND [14] can provide extended security when using RA message.

5.1.2.  DHCPv6 Option Approach

   DHCPv6 can be used for RDNSS configuration through the use of the
   DNS option, and can provide other configuration information in the
   same message with RDNSS configuration [5]-[7].  DHCPv6 DNS option
   is already in place for DHCPv6 as RFC 3646 [7] and moreover DHCPv6-
   lite or stateless DHCP [6] is nowhere as complex as a full DHCPv6
   implementation.  DHCP is a client-server model protocol, so ISP can
   handle user identification on its network intentionally, and also
   authenticated DHCP [15] can be used for secure message exchange.

   The expected model for deployment of IPv6 service by ISPs is to
   assign a prefix to each customer, which will be used by the
   customer gateway to assign a /64 prefix to each network in the
   customer's network.  Prefix delegation with DHCP (DHCPv6 PD) has
   already been adopted by ISPs for automating the assignment of the
   customer prefix to the customer gateway [17].  DNS configuration
   can be carried in the same DHCPv6 message exchange used for DHCPv6
   to efficiently provide that information, along with any other
   configuration information needed by the customer gateway or
   customer network.  This service model can be useful to Home or SOHO
   subscribers.  The Home or SOHO gateway, which is a customer gateway
   for ISP, can then pass that RDNSS configuration information to the
   hosts in the customer network through DHCP.

5.1.3.  Well-known Addresses Approach

   Well-known anycast addresses approach is also a feasible and simple
   mechanism for ISP [9].  The use of well-known anycast addresses

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   avoids some of the security risks in rogue messages sent through an
   external protocol like RA or DHCPv6.  The configuration of hosts
   for the use of well-known anycast addresses requires no protocol or
   manual configuration, but the configuration of routing for the
   anycast addresses requires intervention on the part of the network
   administrator.  Also, the number of special addresses would be
   equal to the number of RDNSSes that could be made available to

5.2.  Enterprise Network

   Enterprise network is defined as a network that has multiple
   internal links, one or more router connections, to one or more
   Providers and is actively managed by a network operations entity
   [16].  An enterprise network can get network prefixes from ISP by
   either manual configuration or prefix delegation [17].  In most
   cases, because an enterprise network manages its own DNS domains,
   it operates its own DNS servers for the domains.  These DNS servers
   within enterprise network process recursive DNS name resolution
   requests of IPv6 hosts as RDNSS.  RDNSS configuration in enterprise
   network can be performed like in Section 4, in which three
   approaches can be used together.

   IPv6 host can decide which approach is or may be used in its subnet
   with O flag in RA message [8].  As the first option in Section 4,
   well-known anycast addresses can be used as a last resort when
   RDNSS information can not be obtained through either RA option or
   DHCP option.  This case needs IPv6 hosts to preconfigure the well-
   known anycast addresses in their DNS configuration files.

   When the enterprise prefers well-known anycast approach to the
   others, IPv6 hosts should preconfigure the well-known anycast
   addresses like in the first option.

   The last option, a more convenient and transparent way, does not
   need IPv6 hosts to preconfigure the well-known anycast addresses
   because the addresses are delivered to IPv6 hosts through either RA
   option or DHCPv6 option as if they were unicast addresses.  This
   way is most recommended for the sake of user's convenience.

5.3.  3GPP Network

   IPv6 DNS configuration is a missing part of IPv6 autoconfiguration
   and an important part of the basic IPv6 functionality in the 3GPP
   User Equipment (UE).  Higher level description of the 3GPP
   architecture can be found in [18], and transition to IPv6 in 3GPP
   networks is analyzed in [19] and [20].

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   In 3GPP architecture, there is a dedicated link between the UE and
   the GGSN called the Packet Data Protocol (PDP) Context.  This link
   is created through the PDP Context activation procedure [21].
   There is a separate PDP context type for IPv4 and IPv6 traffic.  If
   a 3GPP UE user is communicating using IPv6 (having an active IPv6
   PDP context), it can not be assumed that (s)he has simultaneously
   active IPv4 PDP context, and DNS queries could be done using IPv4.
   A 3GPP UE can thus be an IPv6 node, and it needs to somehow
   discover the address of the RDNSS.  Before IP-based services (e.g.,
   web browsing or e-mail) can be used, the IPv6 (and IPv4) RDNSS
   addresses need to be discovered in the 3GPP UE.

   Section 5.3.1 briefly summarizes currently available mechanisms in
   3GPP networks and recommendations.  5.3.2 analyzes the Router
   Advertisement based solution, 5.3.3 analyzes the Stateless DHCPv6
   mechanism, and 5.3.4 analyzes the Well-known addresses approach.
   Section 5.3.5 finally summarizes the recommendations.

5.3.1.  Currently Available Mechanisms and Recommendations

   3GPP has defined a mechanism, in which RDNSS addresses can be
   received in the PDP context activation (a control plane mechanism).
   That is called the Protocol Configuration Options Information
   Element (PCO-IE) mechanism [22].  The RDNSS addresses can also be
   received over the air (using text messages), or typed in manually
   in the UE.  Note that the two last mechanisms are not very well
   scalable.  The UE user most probably does not want to type IPv6
   RDNSS addresses manually in his/her UE.  The use of well-known
   addresses is briefly discussed in section 5.3.4.

   It is seen that the mechanisms above most probably are not
   sufficient for the 3GPP environment.  IPv6 is intended to operate
   in a zero-configuration manner, no matter what the underlying
   network infrastructure is.  Typically, the RDNSS address is needed
   to make an IPv6 node operational - and the DNS configuration should
   be as simple as the address autoconfiguration mechanism.  It must
   also be noted that there will be additional IP interfaces in some
   near future 3GPP UEs, e.g., WLAN, and 3GPP-specific DNS
   configuration mechanisms (such as PCO-IE [22]) do not work for
   those IP interfaces.  In other words, a good IPv6 DNS configuration
   mechanism should also work in a multi-access network environment.

   From 3GPP point of view, the best IPv6 DNS configuration solution
   is feasible for a very large number of IPv6-capable UEs (can be
   even hundreds of millions in one operator's network), is automatic
   and thus requires no user action.  It is suggested to standardize a
   lightweight, stateless mechanism that works in all network
   environments.  The solution could then be used for 3GPP, 3GPP2,

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   WLAN and other access network technologies.  A light, stateless
   IPv6 DNS configuration mechanism is thus not only needed in 3GPP
   networks, but also 3GPP networks and UEs would certainly benefit
   from the new mechanism.

5.3.2.  RA Extension

   Router Advertisement extension [8] is a lightweight IPv6 DNS
   configuration mechanism that requires minor changes in 3GPP UE IPv6
   stack and Gateway GPRS Support Node (GGSN, the default router in
   the 3GPP architecture) IPv6 stack.  This solution can be specified
   in the IETF (no action needed in the 3GPP) and taken in use in 3GPP
   UEs and GGSNs.

   In this solution, an IPv6-capable UE configures DNS information
   via RA message sent by its default router (GGSN), i.e., RDNSS
   option for recursive DNS server is included in the RA message.
   This solution is easily scalable for a very large number of UEs.
   The operator can configure the RDNSS addresses in the GGSN as a
   part of normal GGSN configuration.  The IPv6 RDNSS address is
   received in the Router Advertisement, and an extra Round Trip Time
   (RTT) for asking RDNSS addresses can be avoided.

   If thinking about cons, this mechanism still requires
   standardization effort in the IETF, and the end nodes and routers
   need to support this mechanism.  The equipment software update
   should, however, be pretty straightforward, and new IPv6 equipment
   could support RA extension already from the beginning.

5.3.3.  Stateless DHCPv6

   DHCPv6-based solution needs the implementation of Stateless DHCP
   [6] and DHCPv6 DNS options [7] in the UE, and a DHCPv6 server in
   the operator's network.  A possible configuration is such that the
   GGSN works as a DHCP relay.

   Pros for Stateless DHCPv6-based solution are
   1) Stateless DHCPv6 is a standardized mechanism.

   2) DHCPv6 can be used for receiving other configuration information
   than RDNSS addresses, e.g., SIP server addresses.

   3) DHCPv6 works in different network environments.

   4) When DHCPv6 service is deployed through a single, centralized
   server, the RDNSS configuration information can be updated by the
   network administrator at a single source.

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   Some issues with DHCPv6 in 3GPP networks are listed below:
   1) DHCPv6 requires an additional server in the network unless the
   (Stateless) DHCPv6 functionality is integrated into an existing
   router already, and it is one box more to be maintained.

   2) DHCPv6 is not necessarily needed for 3GPP UE IPv6 addressing
   (3GPP Stateless Address Autoconfiguration is typically used), and
   not automatically implemented in 3GPP IPv6 UEs.

   3) Scalability and reliability of DHCPv6 in very large 3GPP
   networks (with tens or hundreds of millions of UEs) may be an issue,
   at least the redundancy needs to be taken care of.  However, if the
   DHCPv6 service is integrated into the network elements, such as
   router operating system, scalability and reliability is comparable
   with other DNS configuration approaches.

   4) It is sub-optimal to utilize the radio resources in 3GPP
   networks for DHCPv6 messages if there is a simpler alternative

      a) Use of Stateless DHCPv6 adds one round trip delay to the case
      in which the UE can start transmitting data right after the
      Router Advertisement.

   5) If the DNS information (suddenly) changes, Stateless DHCPv6 can
   not automatically update the UE, see [23].

5.3.4.  Well-known Addresses

   Using well-known addresses is also a feasible and a light mechanism
   for 3GPP UEs.  Those well-known addresses can be preconfigured in
   the UE software and the operator makes the corresponding
   configuration on the network side.  So this is a very easy
   mechanism for the UE, but requires some configuration work in the
   network.  When using well-known addresses, UE forwards queries to
   any of the preconfigured addresses.  In the current proposal [9],
   IPv6 anycast addresses are suggested.

   Note: IPv6 DNS configuration proposal based on the use of well-
   known site-local addresses developed at the IPv6 Working Group was
   seen as a feasible mechanism for 3GPP UEs, but opposition by some
   people in the IETF and finally deprecating IPv6 site-local
   addresses made it impossible to standardize it.  Note that this
   mechanism is implemented in some existing operating systems today
   (also in some 3GPP UEs) as a last resort of IPv6 DNS configuration.

5.3.5.  Recommendations

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   It is suggested that a lightweight, stateless DNS configuration
   mechanism is specified as soon as possible.  From 3GPP UE's and
   networks' point of view, Router Advertisement based mechanism looks
   most promising.  The sooner a light, stateless mechanism is
   specified, the sooner we can get rid of using well-known site-local
   addresses for IPv6 DNS configuration.

5.4.  Unmanaged Network

   There are 4 deployment scenarios of interest in unmanaged networks

   1) A gateway which does not provide IPv6 at all;

   2) A dual-stack gateway connected to a dual-stack ISP;

   3) A dual-stack gateway connected to an IPv4-only ISP; and

   4) A gateway connected to an IPv6-only ISP.

5.4.1.  Case A: Gateway does not provide IPv6 at all

   In this case, the gateway does not provide IPv6; the ISP may or may
   not provide IPv6.  Automatic or Configured tunnels are the
   recommended transition mechanisms for this scenario.

   The case where dual-stack hosts behind an NAT, that need access to
   an IPv6 RDNSS, can not be entirely ruled out.  The DNS
   configuration mechanism has to work over the tunnel, and the
   underlying tunneling mechanism could be implementing NAT traversal.
   The tunnel server assumes the role of a relay (both for DHCP and
   Well-known anycast addresses approaches).

   RA-based mechanism is relatively straightforward in its operation,
   assuming the tunnel server is also the IPv6 router emitting RAs.
   Well-known anycast addresses approach seems also simple in
   operation across the tunnel, but the deployment model using Well-
   known anycast addresses in a tunneled environment is unclear or not
   well understood.

5.4.2.  Case B: A dual-stack gateway connected to a dual-stack ISP

   This is similar to a typical IPv4 home user scenario, where DNS
   configuration parameters are obtained using DHCP.  Except that
   Stateless DHCPv6 is used, as opposed to the IPv4 scenario where the
   DHCP server is stateful (maintains the state for clients).

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5.4.3.  Case C: A dual-stack gateway connected to an IPv4-only ISP

   This is similar to Case B.  If a gateway provides IPv6 connectivity
   by managing tunnels, then it is also supposed to provide access to
   an RDNSS.  Like this, the tunnel for IPv6 connectivity originates
   from the dual-stack gateway instead of the host.

5.4.4.  Case D: A gateway connected to an IPv6-only ISP

   This is similar to Case B.

6.  Security Considerations

   As security requirements depend solely on applications and are
   different application by application, there can be no generic
   requirement defined at higher IP or lower application layer of DNS.

   However, it should be noted that cryptographic security requires
   configured secret information that full autoconfiguration and
   cryptographic security are mutually exclusive.  People insisting on
   secure full autoconfiguration will get false security, false
   autoconfiguration or both.

   In some deployment scenario [19], where cryptographic security is
   required for applications, secret information for the cryptographic
   security is preconfigured through which application specific
   configuration data, including those for DNS, can be securely
   configured.  It should be noted that if applications requiring
   cryptographic security depend on DNS, the applications also require
   cryptographic security to DNS.  Therefore, the full auto-
   configuration of DNS is not acceptable.

   However, with full autoconfiguration, weaker but still reasonable
   security is being widely accepted and will continue to be
   acceptable.  That is, with full autoconfiguration, which means
   there is no cryptographic security for the autoconfiguration, it is
   already assumed that local environment is secure enough that
   information from local autoconfiguration server has acceptable
   security even without cryptographic security.  Thus, communication
   between a local DNS client and a local DNS server has the
   acceptable security.

   For security considerations of each approach, refer to the
   corresponding drafts [5]-[9].

7.  Acknowledgements

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   This draft has greatly benefited from inputs by David Meyer, Rob
   Austein, Tatuya Jinmei, Pekka Savola, Tim Chown, Luc Beloeil,
   Christian Huitema, Thomas Narten, Pascal Thubert, and Greg Daley.
   The authors appreciate their contribution.

8.  Normative References

   [1]  S. Bradner, "Intellectual Property Rights in IETF Technology",
        RFC 3668, February 2004.

   [2]  S. Bradner, "IETF Rights in Contributions", RFC 3667, February

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

   [4]  S. Thomson and T. Narten, "IPv6 Stateless Address
        Autoconfiguration", RFC 2462, December 1998.

   [5]  R. Droms et al., "Dynamic Host Configuration Protocol for IPv6
        (DHCPv6)", RFC 3315, July 2003.

   [6]  R. Droms, "Stateless Dynamic Host Configuration Protocol
        (DHCP) Service for IPv6", RFC 3736, April 2004.

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

9.  Informative References

   [8]  J. Jeong, S. Park, L. Beloeil and S. Madanapalli, "IPv6 DNS
        Discovery based on Router Advertisement", draft-jeong-dnsop-
        ipv6-dns-discovery-02.txt, July 2004, Work in Progress.

   [9]  M. Ohta, "Preconfigured DNS Server Addresses", draft-ohta-
        preconfigured-dns-01.txt, February 2004, Work in Progress.

   [10] S. Venaas, T. Chown and B. Volz, "Information Refresh Time
        Option for DHCPv6", draft-ietf-dhc-lifetime-02.txt, September
        2004, Work in Progress.

   [11] C. Partridge, T. Mendez and W. Milliken, "Host Anycasting
        Service", RFC 1546, November 1993.

   [12] R. Hinden and S. Deering, "Internet Protocol Version 6 (IPv6)
        Addressing Architecture", RFC 3513, April 2003.

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   [13] M. Lind et al., "Scenarios and Analysis for Introduction IPv6
        into ISP Networks", draft-ietf-v6ops-isp-scenarios-analysis-
        03.txt, June 2004, Work in Progress.

   [14] J. Arkko et al., "SEcure Neighbor Discovery (SEND)", draft-
        ietf-send-ndopt-06.txt, July 2004, Work in Progress.

   [15] R. Droms and W. Arbaugh, "Authentication for DHCP Messages",
        RFC 3118, June 2001.

   [16] J. Bound et al., "IPv6 Enterprise Network Scenarios", draft-
        ietf-v6ops-ent-scenarios-05.txt, July 2004, Work in Progress.

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

   [18] M. Wasserman, Ed., "Recommendations for IPv6 in 3GPP
        Standards", RFC 3314, September 2002.

   [19] J. Soininen, Ed., "Transition Scenarios for 3GPP Networks",
        RFC 3574, August 2003.

   [20] J. Wiljakka, Ed., "Analysis on IPv6 Transition in 3GPP
        Networks", draft-ietf-v6ops-3gpp-analysis-10.txt, May 2004,
        Work in Progress.

   [21] 3GPP TS 23.060 V5.4.0, "General Packet Radio Service (GPRS);
        Service description; Stage 2 (Release 5)", December 2002.

   [22] 3GPP TS 24.008 V5.8.0, "Mobile radio interface Layer 3
        specification; Core network protocols; Stage 3 (Release 5)",
        June 2003.

   [23] T. Chown, S. Venaas and A. Vijayabhaskar, "Renumbering
        Requirements for Stateless DHCPv6", draft-ietf-dhc-stateless-
        dhcpv6-renumbering-01.txt, March 2004, Work in Progress.

   [24] C. Huitema et al., "Unmanaged Networks IPv6 Transition
        Scenarios", RFC 3750, April 2004.

10.  Appendix A - Link-layer Multicast Acknowledgements with RA Option

   One benefit of RA option is to be able to multicast the
   advertisements, reducing the need for duplicated unicast

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   However, some link-layers may not support this as well as others.
   Consider, for example, WLAN networks where multicast is unreliable.
   The unreliability problem is caused by lack of ACK for multicast,
   especially on the path from the Access Point (AP) to the Station
   (STA), which is specific to CSMA/CA of WLAN.  Namely, multicast
   packet is unacknowledged on the path from the AP to the STA, but
   acknowledged in the reverse direction from the STA to the AP.  For
   example, when a router is placed at wired network connected to an
   AP, a host may sometimes not receive RA message advertised through
   the AP.

   The fact that this problem has not been addressed in Neighbor
   Discovery [3] indicates that the extra link-layer acknowledgements
   have not been considered a serious problem till now.

   A possible mitigation technique could be to map all-nodes link-
   local multicast address to the link-layer broadcast address, and to
   rely on the ND retransmissions for message delivery.

11.  Authors' Addresses

   Jaehoon Paul Jeong, Editor
   ETRI/University of Minnesota at Twin Cities
   117 Pleasant Street SE
   Minneapolis, MN 55455

   Phone: +1 651 587 7774
   EMail: jjeong@cs.umn.edu

   Ralph Droms
   Cisco Systems
   1414 Massachusetts Ave.
   Boxboro, MA 01719

   Phone: +1 978 936 1674
   EMail: rdroms@cisco.com

   Robert M. Hinden
   313 Fairchild Drive
   Mountain View, CA 94043

   Phone: +1 650 625 2004
   EMail: bob.hinden@nokia.com

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   Ted Lemon
   Nominum, Inc.
   950 Charter Street
   Redwood City, CA 94043

   EMail: Ted.Lemon@nominum.com

   Masataka Ohta
   Graduate School of Information Science and Engineering
   Tokyo Institute of Technology
   2-12-1, O-okayama, Meguro-ku
   Tokyo 152-8552

   Phone: +81 3 5734 3299
   Fax: +81 3 5734 3299
   EMail: mohta@necom830.hpcl.titech.ac.jp

   Soohong Daniel Park
   Mobile Platform Laboratory, SAMSUNG Electronics
   416, Maetan-3dong, Paldal-gu, Suwon

   Phone: +82 31 200 4508
   EMail: soohong.park@samsung.com

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

   EMail: satapati@cisco.com

   Juha Wiljakka
   Visiokatu 3
   FIN-33720 TAMPERE

   Phone: +358 7180 48372
   EMail: juha.wiljakka@nokia.com

12.  Intellectual Property Statement

   The following intellectual property notice is copied from RFC3668,
   Section 5.

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   The IETF takes no position regarding the validity or scope of any
   Intellectual Property Rights or other rights that might be claimed
   to pertain to the implementation or use of the technology described
   in this document or the extent to which any license under such
   rights might or might not be available; nor does it represent that
   it has made any independent effort to identify any such rights.
   Information on the procedures with respect to rights in RFC
   documents can be found in BCP 78 and BCP 79.

   Copies of IPR disclosures made to the IETF Secretariat and any
   assurances of licenses to be made available, or the result of an
   attempt made to obtain a general license or permission for the use
   of such proprietary rights by implementers or users of this
   specification can be obtained from the IETF on-line IPR repository
   at http://www.ietf.org/ipr.

   The IETF invites any interested party to bring to its attention any
   copyrights, patents or patent applications, or other proprietary
   rights that may cover technology that may be required to implement
   this standard.  Please address the information to the IETF at ietf-

Full Copyright Statement

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

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

   This document and the information contained herein are provided on


   Funding for the RFC Editor function is currently provided by the
   Internet Society.

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