DNS Operations WG
Internet-Draft J. Jeong (ed.)
ETRI/University of Minnesota
Expires: August 2005 19 February 2005
IPv6 Host Configuration of DNS Server Information Approaches
draft-ietf-dnsop-ipv6-dns-configuration-05.txt
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Copyright Notice
Copyright (C) The Internet Society (2005). All Rights Reserved.
Abstract
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.
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Additionally, it suggests four deployment scenarios considering
multi-solution resolution. Therefore, this document will give the
audience a guideline for IPv6 DNS configuration to select the
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...........................................8
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........................................11
4. Interworking among IPv6 DNS Configuration Approaches..........11
5. Deployment Scenarios..........................................12
5.1. ISP Network...............................................13
5.1.1. RA Option Approach..................................13
5.1.2. DHCPv6 Option Approach..............................13
5.1.3. Well-known Addresses Approach.......................14
5.2. Enterprise Network........................................14
5.3. 3GPP Network..............................................15
5.3.1. Currently Available Mechanisms and Recommendations..15
5.3.2. RA Extension........................................16
5.3.3. Stateless DHCPv6....................................17
5.3.4. Well-known Addresses................................18
5.3.5. Recommendations.....................................18
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.................................................19
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..............................................20
8. Normative References..........................................20
9. Informative References........................................21
10. Appendix A - Link-layer Multicast Acknowledgements with RA
Option.......................................................23
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11. Authors' Addresses...........................................23
12. Intellectual Property Statement..............................25
13. Full Copyright Statement.....................................25
Acknowledgement..................................................26
1. Introduction
Neighbor Discovery (ND) for IP Version 6 and IPv6 Stateless Address
Autoconfiguration provide the ways to configure either fixed or
mobile nodes with one or more IPv6 addresses, default routes and
some other parameters [3][4]. To support the 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 the 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 the 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
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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
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 option, 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 [25]-[28]. 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, the
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
protocol.
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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, that there may be some link types on which ND is not
possible; on such links, some other mechanisms will be needed for
DNS configuration.
3) All of the information a host needs to run the basic Internet
applications such as the email, web, ftp, etc., can be obtained
with the addition of this option to ND and address autoconfiguration
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): Refer to Appendix A. 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 needed 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, such as NTP server address, that are common to all
clients on a subnet would be easy to define.
3.1.2. Disadvantages
1) ND is mostly implemented in the kernel part of operating system.
Therefore, if ND supports the configuration of some additional
services, such as DNS servers, ND should be extended in the kernel
part, and complemented by a user-land process. DHCPv6, however,
has more flexibility for the 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 the kernel
space and the DNS configuration file in the 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
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what the kernel conveys, and to have the daemon do these steps, but
such a daemon is not needed with the current ND framework.
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 the IPv6 ND and
Autoconfiguration allows a host to obtain all of the information it
needs to access the basic Internet services like the web, email,
ftp, etc. This is preferable in the environments where hosts use
RAs to autoconfigure their addresses and all the 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. The environments like this include
the 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]. The environments like this are most likely
the 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].
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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
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
changes.
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.
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3.2.1. Advantages
The DHCPv6 option for RDNSS has a number of advantages. These
include:
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
anticipated.
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
demonstrated.
3.2.2. Disadvantages
The DHCPv6 option for RDNSS has a few disadvantages. These
include:
1) Update currently requires message from server (however, see
[10]).
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
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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
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
Anycast uses the same routing system as unicast [11]. However,
administrative entities are local ones. The local entities may
accept unicast routes (including default routes) to anycast servers
from adjacent entities. The administrative entities should not
advertise their peers routes to their internal anycast servers, if
they want to prohibit external access from some peers to the
servers. If some advertisement is inevitable (such as the case
with default routes), the packets to the servers should be blocked
at the boundary of the entities. Thus, for this anycast, not only
unicast routing but also unicast ND protocols can be used as is.
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.
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There is no point in having multiple RDNSSes sharing an anycast
address on a single link.
The approach with well-known anycast addresses is to set multiple
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.
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 the response and no further delay by
packet losses.
2) The approach can be combined with any other configuration
mechanisms, such as RA-based approach and DHCP based approach, as
well as factory default configuration.
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).
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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 the configuration effort of users.
The redundancy by multiple RDNSSes is better provided by multiple
servers having different anycast addresses than multiple servers
sharing the 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
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
not.
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The well-known anycast addresses approach fully interworks with the
other approaches. That is, the other approaches can remove the
configuration effort on servers by using the well-known addresses
as the default configuration. Moreover, the clients preconfigured
with the well-known anycast addresses can be further configured to
use other approaches to override the well-known addresses, if the
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 as follows:
1) 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 preconfigured
in IPv6 hosts' resolver configuration files.
2) 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.
3) 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 the 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.
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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
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 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.
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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
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
subscribers.
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 from IPv6 hosts as RDNSSes. The RDNSS configuration in
the enterprise network can be performed like in Section 4, in which
three approaches can be used together as follows:
1) 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.
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2) 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.
3) 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].
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.
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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,
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.
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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.
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
available.
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].
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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
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
[24]:
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.
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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).
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 scenarios [19], where cryptographic security is
required for applications, the secret information for the
cryptographic security is preconfigured through which application
specific configuration data, including those for DNS, can be
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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
autoconfiguration 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 the local environment is secure enough that
the information from the local autoconfiguration server has
acceptable security even without cryptographic security. Thus, the
communication between the local DNS client and local DNS server has
the acceptable security.
In autoconfiguring recursive servers, DNSSEC may be overkill,
because DNSSEC [29] needs the configuration and reconfiguration of
clients at root key roll-over [30][31]. Even if additional keys
for secure key roll-over are added at the initial configuration,
they are as vulnerable as the original keys to some forms of
attacks, such as social hacking. Another problem of using DNSSEC
and autoconfiguration together is that DNSSEC requires secure time,
which means secure communication with autoconfigured time servers,
which requires configured secret information. Therefore, in order
that the autoconfiguration may be secure, it requires configured
secret information.
If DNSSEC [29] is used and the signatures are verified on the
client host, the misconfiguration of a DNS server may be simply
denial of service. Also, if local routing environment is not
reliable, clients may be directed to a false resolver with the same
IP address as the true one.
For security considerations of each approach, refer to the
corresponding drafts [5]-[9].
7. Acknowledgements
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 contributions.
8. Normative References
[1] S. Bradner, "Intellectual Property Rights in IETF Technology",
RFC 3668, February 2004.
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[2] S. Bradner, "IETF Rights in Contributions", RFC 3667, February
2004.
[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
2003.
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-04.txt, February 2005,
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-03.txt, January
2005, 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.
[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.
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[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
2003.
[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-11.txt, October 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-02.txt, October
2004, Work in Progress.
[24] C. Huitema et al., "Unmanaged Networks IPv6 Transition
Scenarios", RFC 3750, April 2004.
[25] ANSI/IEEE Std 802.11, "Part 11: Wireless LAN Medium Access
Control (MAC) and Physical Layer (PHY) Specifications", March
1999.
[26] IEEE Std 802.11a, "Part 11: Wireless LAN Medium Access Control
(MAC) and Physical Layer (PHY) specifications: High-speed
Physical Layer in the 5 GHZ Band", September 1999.
[27] IEEE Std 802.11b, "Part 11: Wireless LAN Medium Access Control
(MAC) and Physical Layer (PHY) specifications: Higher-Speed
Physical Layer Extension in the 2.4 GHz Band", September 1999.
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[28] IEEE P802.11g/D8.2, "Part 11: Wireless LAN Medium Access
Control (MAC) and Physical Layer (PHY) specifications: Further
Higher Data Rate Extension in the 2.4 GHz Band", April 2003.
[29] D. Eastlake, "Domain Name System Security Extensions", RFC
2535, March 1999.
[30] O. Kolkman and R. Gieben, "DNSSEC Operational Practices",
draft-ietf-dnsop-dnssec-operational-practices-03.txt,
December 2004.
[31] G. Guette and O. Courtay, "Requirements for Automated Key
Rollover in DNSSEC",
draft-ietf-dnsop-key-rollover-requirements-02.txt, January
2005.
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
communications.
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 [25]-[28]. 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 [25]. 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
USA
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Phone: +1 651 587 7774
EMail: jjeong@cs.umn.edu
Ralph Droms
Cisco Systems
1414 Massachusetts Ave.
Boxboro, MA 01719
USA
Phone: +1 978 936 1674
EMail: rdroms@cisco.com
Robert M. Hinden
Nokia
313 Fairchild Drive
Mountain View, CA 94043
USA
Phone: +1 650 625 2004
EMail: bob.hinden@nokia.com
Ted Lemon
Nominum, Inc.
950 Charter Street
Redwood City, CA 94043
USA
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
Japan
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
Gyeonggi-Do
Korea
Phone: +82 31 200 4508
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EMail: soohong.park@samsung.com
Suresh Satapati
Cisco Systems, Inc.
San Jose, CA 95134
USA
EMail: satapati@cisco.com
Juha Wiljakka
Nokia
Visiokatu 3
FIN-33720 TAMPERE
Finland
Phone: +358 7180 48372
EMail: juha.wiljakka@nokia.com
12. Intellectual Property Statement
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-ipr@ietf.org.
13. Full Copyright Statement
Copyright (C) The Internet Society (2005). This document is
subject to the rights, licenses and restrictions contained in BCP
78, and except as set forth therein, the authors retain all their
rights.
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This document and the information contained herein are provided on
an "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE
REPRESENTS OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES,
EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT
THE USE OF THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR
ANY IMPLIED WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A
PARTICULAR PURPOSE.
Acknowledgement
Funding for the RFC Editor function is currently provided by the
Internet Society.
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