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freebsd-dev/contrib/bind9/doc/draft/draft-ietf-dnsop-ipv6-dns-configuration-06.txt
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DNS Operations WG J. Jeong, Ed.
Internet-Draft ETRI/University of Minnesota
Expires: November 6, 2005 May 5, 2005
IPv6 Host Configuration of DNS Server Information Approaches
draft-ietf-dnsop-ipv6-dns-configuration-06.txt
Status of this Memo
This document is an Internet-Draft and is subject to all provisions
of Section 3 of RFC 3667. By submitting this Internet-Draft, each
author represents that any applicable patent or other IPR claims of
which he or she is aware have been or will be disclosed, and any of
which he or she become aware will be disclosed, in accordance with
RFC 3668.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on November 6, 2005.
Copyright Notice
Copyright (C) The Internet Society (2005).
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. Additionally, it suggests the
deployment scenarios in four kinds of networks, such as ISP,
Enterprise, 3GPP, and Unmanaged networks, considering multi-solution
resolution. Therefore, this document will give the audience a
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guideline for IPv6 host DNS configuration.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 5
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. IPv6 DNS Configuration Approaches . . . . . . . . . . . . . . 7
3.1 RA Option . . . . . . . . . . . . . . . . . . . . . . . . 7
3.1.1 Advantages . . . . . . . . . . . . . . . . . . . . . . 8
3.1.2 Disadvantages . . . . . . . . . . . . . . . . . . . . 8
3.1.3 Observations . . . . . . . . . . . . . . . . . . . . . 9
3.2 DHCPv6 Option . . . . . . . . . . . . . . . . . . . . . . 9
3.2.1 Advantages . . . . . . . . . . . . . . . . . . . . . . 11
3.2.2 Disadvantages . . . . . . . . . . . . . . . . . . . . 12
3.2.3 Observations . . . . . . . . . . . . . . . . . . . . . 12
3.3 Well-known Anycast Addresses . . . . . . . . . . . . . . . 12
3.3.1 Advantages . . . . . . . . . . . . . . . . . . . . . . 13
3.3.2 Disadvantages . . . . . . . . . . . . . . . . . . . . 14
3.3.3 Observations . . . . . . . . . . . . . . . . . . . . . 14
4. Interworking among IPv6 DNS Configuration Approaches . . . . . 15
5. Deployment Scenarios . . . . . . . . . . . . . . . . . . . . . 16
5.1 ISP Network . . . . . . . . . . . . . . . . . . . . . . . 16
5.1.1 RA Option Approach . . . . . . . . . . . . . . . . . . 16
5.1.2 DHCPv6 Option Approach . . . . . . . . . . . . . . . . 17
5.1.3 Well-known Anycast Addresses Approach . . . . . . . . 17
5.2 Enterprise Network . . . . . . . . . . . . . . . . . . . . 17
5.3 3GPP Network . . . . . . . . . . . . . . . . . . . . . . . 18
5.3.1 Currently Available Mechanisms and Recommendations . . 19
5.3.2 RA Extension . . . . . . . . . . . . . . . . . . . . . 19
5.3.3 Stateless DHCPv6 . . . . . . . . . . . . . . . . . . . 20
5.3.4 Well-known Addresses . . . . . . . . . . . . . . . . . 21
5.3.5 Recommendations . . . . . . . . . . . . . . . . . . . 21
5.4 Unmanaged Network . . . . . . . . . . . . . . . . . . . . 22
5.4.1 Case A: Gateway does not provide IPv6 at all . . . . . 22
5.4.2 Case B: A dual-stack gateway connected to a
dual-stack ISP . . . . . . . . . . . . . . . . . . . . 22
5.4.3 Case C: A dual-stack gateway connected to an
IPv4-only ISP . . . . . . . . . . . . . . . . . . . . 22
5.4.4 Case D: A gateway connected to an IPv6-only ISP . . . 23
6. Security Considerations . . . . . . . . . . . . . . . . . . . 24
6.1 RA Option . . . . . . . . . . . . . . . . . . . . . . . . 25
6.2 DHCPv6 Option . . . . . . . . . . . . . . . . . . . . . . 25
6.3 Well-known Anycast Addresses . . . . . . . . . . . . . . . 25
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . . 26
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 28
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 29
9.1 Normative References . . . . . . . . . . . . . . . . . . . 29
9.2 Informative References . . . . . . . . . . . . . . . . . . 29
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 31
A. Link-layer Multicast Acknowledgements for RA Option . . . . . 32
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Intellectual Property and Copyright Statements . . . . . . . . 33
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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 a 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 servers.
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2. Terminology
This document uses the terminology described in [3]-[9]. In
addition, a new term is defined below:
o Recursive DNS Server (RDNSS): A Recursive DNS Server is a name
server that offers the recursive service of DNS name resolution.
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3. IPv6 DNS Configuration Approaches
In this section, the operational attributes of the three solutions
are described in detail.
3.1 RA Option
The RA approach is to define a new ND option called the 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 a 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 addresses can
be performed manually by an operator or other ways, such as automatic
configuration through a DHCPv6 client running on the router. When
advertising more than one RDNSS option, an RA message includes as
many RDNSS options as RDNSSes.
Through the ND protocol and RDNSS option along with a 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, such as Wireless
LANs (e.g., IEEE 802.11 a/b/g), do not support reliable multicast,
which means that they cannot guarantee the timely delivery of RA
messages [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
field would seem to make matters a bit more complex. Instead of just
writing to a 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 the RDNSS option, allows
IPv6 hosts to select primary RDNSS among several RDNSSes; this can be
used for the load balancing of RDNSSes [8].
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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.
2. This approach, like ND, works well on a variety of link types
including point-to-point links, point-to-multipoint, and
multipoint-to-multipoint (i.e., Ethernet LANs), etc. RFC 2461
[3] states, however, that there may be some link types on which
ND is not feasible; 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, by 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 of operating system.
Therefore, if ND supports the configuration of some additional
services, such as DNS servers, ND should be extended in the
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kernel, 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 to facilitate 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 to 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 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 via the 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 to be 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
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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 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 a 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.
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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
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 with 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.
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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 messages, 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
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
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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 RFC 1546 [11] and RFC 3513 [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, an 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 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). A 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 the RA-based approach and DHCP based
approach, as well as the 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
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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 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
the 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 RFC 1546 [11]
and RFC 3513 [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 RFC 2461 [3] and RFC 3513 [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 RFC 3513 [12]) address of packets of UDP and
TCP responses. With TCP, if a 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.
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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, the O (Other stateful
configuration) flag in RA message can be used [8][32]. 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.
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 is available.
Otherwise, all the clients need to have the well-known anycast
addresses preconfigured. In order to use the anycast approach along
with two other approaches, there are three choices as follows:
1. The first choice is that well-known addresses are used as last
resort, when an IPv6 host cannot get RDNSS information through RA
and DHCP. The well-known anycast addresses have to be
preconfigured in all of 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 an 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 the configuration effort of
users. According to either the RA or DHCP mechanism, the well-
known addresses can be obtained by an 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.
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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.
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
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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 messages.
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]. The 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 ISPs 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 Anycast Addresses Approach
The 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
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enterprise network can get network prefixes from an 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. An IPv6 host can decide which approach is or may be used in its
subnet with the O flag in RA message [8][32]. As the first
choice in Section 4, well-known anycast addresses can be used as
a last resort when RDNSS information cannot be obtained through
either an RA option or DHCP option. This case needs IPv6 hosts
to preconfigure the well-known anycast addresses in their DNS
configuration files.
2. When the enterprise prefers the well-known anycast approach to
others, IPv6 hosts should preconfigure the well-known anycast
addresses like in the first choice.
3. The last choice, 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 via either the
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
The 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). The 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 the 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 cannot be assumed that (s)he has simultaneously an
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.
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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 a 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 the 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
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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 the 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.
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 a router
already existing, and that means 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.
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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 a
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.
* The 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
The 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 a 3GPP UE and
network point of view, the 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.
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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.
The case where dual-stack hosts behind an NAT, that need access to an
IPv6 RDNSS, cannot 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.
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5.4.4 Case D: A gateway connected to an IPv6-only ISP
This is similar to Case B.
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6. Security Considerations
As security requirements depend solely on applications and are
different application by application, there can be no generic
requirement defined at IP or application layer for 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 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
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.
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6.1 RA Option
The security of RA option for RDNSS is the same as the ND protocol
security [3][8]. The RA option does not add any new vulnerability.
It should be noted that the vulnerability of ND is not worse and is a
subset of the attacks that any node attached to a LAN can do
independently of ND. A malicious node on a LAN can promiscuously
receive packets for any router's MAC address and send packets with
the router's MAC address as the source MAC address in the L2 header.
As a result, the L2 switches send packets addressed to the router to
the malicious node. Also, this attack can send redirects that tell
the hosts to send their traffic somewhere else. The malicious node
can send unsolicited RA or NA replies, answer RS or NS requests, etc.
All of this can be done independently of implementing ND. Therefore,
the RA option for RDNSS does not add to the vulnerability.
Security issues regarding the ND protocol were discussed at IETF SEND
(Securing Neighbor Discovery) Working Group and RFC 3971 for the ND
security has been published [14].
6.2 DHCPv6 Option
The DNS Recursive Name Server option may be used by an intruder DHCP
server to cause DHCP clients to send DNS queries to an intruder DNS
recursive name server [7]. The results of these misdirected DNS
queries may be used to spoof DNS names.
To avoid attacks through the DNS Recursive Name Server option, the
DHCP client SHOULD require DHCP authentication (see section
"Authentication of DHCP messages" in RFC 3315 [5]) before installing
a list of DNS recursive name servers obtained through authenticated
DHCP.
6.3 Well-known Anycast Addresses
Well-known anycast addresses does not require configuration security
since there is no protocol [9].
The DNS server with the preconfigured addresses are still reasonably
reliable, if local environment is reasonably secure, that is, there
is no active attackers receiving queries to the anycast addresses of
the servers and reply to them.
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7. Contributors
Ralph Droms
Cisco Systems, Inc.
1414 Massachusetts Ave.
Boxboro, MA 01719
US
Phone: +1 978 936 1674
Email: rdroms@cisco.com
Robert M. Hinden
Nokia
313 Fairchild Drive
Mountain View, CA 94043
US
Phone: +1 650 625 2004
Email: bob.hinden@nokia.com
Ted Lemon
Nominum, Inc.
950 Charter Street
Redwood City, CA 94043
US
Email: Ted.Lemon@nominum.com
Masataka Ohta
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, Yeongtong-Gu
Suwon, Gyeonggi-Do 443-742
Korea
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Phone: +82 31 200 4508
Email: soohong.park@samsung.com
Suresh Satapati
Cisco Systems, Inc.
San Jose, CA 95134
US
Email: satapati@cisco.com
Juha Wiljakka
Nokia
Visiokatu 3
FIN-33720, TAMPERE
Finland
Phone: +358 7180 48372
Email: juha.wiljakka@nokia.com
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8. 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.
Also, Tony Bonanno proofread this draft. The authors appreciate
their contribution.
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9. References
9.1 Normative References
[1] Bradner, S., "IETF Rights in Contributions", RFC 3667,
February 2004.
[2] Bradner, S., "Intellectual Property Rights in IETF Technology",
RFC 3668, February 2004.
[3] Narten, T., Nordmark, E., and W. Simpson, "Neighbor Discovery
for IP Version 6 (IPv6)", RFC 2461, December 1998.
[4] Thomson, S. and T. Narten, "IPv6 Stateless Address
Autoconfiguration", RFC 2462, December 1998.
[5] Droms, R., Ed., "Dynamic Host Configuration Protocol for IPv6
(DHCPv6)", RFC 3315, July 2003.
[6] Droms, R., "Stateless Dynamic Host Configuration Protocol (DHCP)
Service for IPv6", RFC 3736, April 2004.
[7] Droms, R., Ed., "DNS Configuration options for Dynamic Host
Configuration Protocol for IPv6 (DHCPv6)", RFC 3646,
December 2003.
9.2 Informative References
[8] Jeong, J., Park, S., Beloeil, L., and S. Madanapalli, "IPv6 DNS
Discovery based on Router Advertisement",
draft-jeong-dnsop-ipv6-dns-discovery-04.txt (Work in Progress),
February 2005.
[9] Ohta, M., "Preconfigured DNS Server Addresses",
draft-ohta-preconfigured-dns-01.txt (Work in Progress),
February 2004.
[10] Venaas, S., Chown, T., and B. Volz, "Information Refresh Time
Option for DHCPv6", draft-ietf-dhc-lifetime-03.txt (Work in
Progress), January 2005.
[11] Partridge, C., Mendez, T., and W. Milliken, "Host Anycasting
Service", RFC 1546, November 1993.
[12] Hinden, R. and S. Deering, "Internet Protocol Version 6 (IPv6)
Addressing Architecture", RFC 3513, April 2003.
[13] Lind, M., Ed., "Scenarios and Analysis for Introduction IPv6
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into ISP Networks", RFC 4029, March 2005.
[14] Arkko, J., Ed., "SEcure Neighbor Discovery (SEND)", RFC 3971,
March 2005.
[15] Droms, R. and W. Arbaugh, "Authentication for DHCP Messages",
RFC 3118, June 2001.
[16] Bound, J., Ed., "IPv6 Enterprise Network Scenarios",
draft-ietf-v6ops-ent-scenarios-05.txt (Work in Progress),
July 2004.
[17] Troan, O. and R. Droms, "IPv6 Prefix Options for Dynamic Host
Configuration Protocol (DHCP) version 6", RFC 3633,
December 2003.
[18] Wasserman, M., Ed., "Recommendations for IPv6 in 3GPP
Standards", RFC 3314, September 2002.
[19] Soininen, J., Ed., "Transition Scenarios for 3GPP Networks",
RFC 3574, August 2003.
[20] Wiljakka, J., Ed., "Analysis on IPv6 Transition in 3GPP
Networks", draft-ietf-v6ops-3gpp-analysis-11.txt (Work in
Progress), October 2004.
[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] Chown, T., Venaas, S., and A. Vijayabhaskar, "Renumbering
Requirements for Stateless DHCPv6",
draft-ietf-dhc-stateless-dhcpv6-renumbering-02.txt (Work in
Progress), October 2004.
[24] Huitema, C., Ed., "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.
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[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.
[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] Eastlake, D., "Domain Name System Security Extensions",
RFC 2535, March 1999.
[30] Kolkman, O. and R. Gieben, "DNSSEC Operational Practices",
draft-ietf-dnsop-dnssec-operational-practices-03.txt (Work in
Progress), December 2004.
[31] Guette, G. and O. Courtay, "Requirements for Automated Key
Rollover in DNSSEC",
draft-ietf-dnsop-key-rollover-requirements-02.txt (Work in
Progress), January 2005.
[32] Park, S., Madanapalli, S., and T. Jinmei, "Considerations on M
and O Flags of IPv6 Router Advertisement",
draft-ietf-ipv6-ra-mo-flags-01.txt (Work in Progress),
March 2005.
Author's Address
Jaehoon Paul Jeong (editor)
ETRI/Department of Computer Science and Engineering
University of Minnesota
117 Pleasant Street SE
Minneapolis, MN 55455
US
Phone: +1 651 587 7774
Fax: +1 612 625 2002
Email: jjeong@cs.umn.edu
URI: http://www.cs.umn.edu/~jjeong/
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Appendix A. Link-layer Multicast Acknowledgements for RA Option
One benefit of an RA option [8] 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, such as IEEE 802.11
a/b/g [25]-[28]. That is, a 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. Therefore, the RA
option solution might not work well on a congested medium that uses
unreliable multicast for RA.
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 in order to achieve more
reliability.
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Intellectual Property Statement
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this standard. Please address the information to the IETF at
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Acknowledgment
Funding for the RFC Editor function is currently provided by the
Internet Society.
Jeong Expires November 6, 2005 [Page 33]
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