One document matched: draft-ietf-geopriv-res-gw-lis-discovery-03.xml
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<front>
<title abbrev="LIS Discovery by IP"> Location Information Server (LIS) Discovery using IP
address and Reverse DNS </title>
<author initials="M." surname="Thomson" fullname="Martin Thomson">
<organization>Microsoft</organization>
<address>
<postal>
<street>3210 Porter Drive</street>
<city>Palo Alto</city>
<region>CA</region>
<code>94304</code>
<country>US</country>
</postal>
<phone>+1 650-353-1925</phone>
<email>martin.thomson@gmail.com</email>
</address>
</author>
<author initials="R.P." surname="Bellis" fullname="Ray Bellis">
<organization>Nominet UK</organization>
<address>
<postal>
<street>Edmund Halley Road</street>
<city>Oxford</city>
<code>OX4 4DQ</code>
<country>United Kingdom</country>
</postal>
<phone>+44 1865 332211</phone>
<email>ray.bellis@nominet.org.uk</email>
<uri>http://www.nominet.org.uk/</uri>
</address>
</author>
<date month="March" year="2012"/>
<area>RAI</area>
<workgroup>GEOPRIV</workgroup>
<keyword>Internet-Draft</keyword>
<keyword>HELD</keyword>
<keyword>LIS</keyword>
<keyword>Discovery</keyword>
<keyword>NAT</keyword>
<keyword>Residential Gateway</keyword>
<abstract>
<t>The residential gateway is a device that has become an integral part of home networking
equipment. Discovering a Location Information Server (LIS) is a necessary part of acquiring
location information for location-based services. However, discovering a LIS when a
residential gateway is present poses a configuration challenge, requiring a method that is
able to work around the obstacle presented by the gateway. </t>
<t>This document describes a solution to this problem. The solution provides alternative
domain names as input to the LIS discovery process based on the network addresses assigned
to a Device. </t>
</abstract>
</front>
<middle>
<section anchor="intro" title="Introduction">
<t>A Location Information Server (LIS) is a service provided by an access network. The LIS
uses knowledge of the access network topology and other information to generate location for
Devices. Devices within an access network are able to acquire location information from a
LIS. </t>
<t>The relationship between a Device and an access network might be transient. Configuration
of the correct LIS at the Device ensures that accurate location information is available.
Without location information, some network services are not available. </t>
<t>The configuration of a LIS address on a Device requires some automated configuration
process. This is particularly relevant when it is considered that Devices might move between
different access networks. <xref target="RFC5986">LIS Discovery</xref> describes a method
that employs the Dynamic Host Configuration Protocol (<xref target="RFC2131">DHCPv4</xref>,
<xref target="RFC3315">DHCPv6</xref>) as input to U-NAPTR <xref target="RFC4848"/>
discovery. </t>
<t>A residential gateway, or home router, provides a range of networking functions for Devices
within the network it serves. In most cases, these functions effectively prevent the
successful use of DHCP for LIS discovery. </t>
<t>The drawback with DHCP is that universal deployment of a new option takes a considerable
amount of time. Often, networking equipment needs to be updated in order to support the new
option. Of particular concern are the millions of residential gateway devices used to
provide Internet access to homes and businesses. While <xref target="RFC5986"/> describes
functions that can be provided by residential gateways to support LIS discovery, gateways
built before the publication of this specification do not (and cannot) provide these
functions. </t>
<t>This document explores the problem of configuring Devices with a LIS address when a
residential gateway is interposed between the Device and access network. <xref target="ps"/>
defines the problem and <xref target="ip-dns"/> describes a method for determining a domain
name that can be used for discovery of the LIS. </t>
<t>In some cases, the solution described in this document is based on a <xref target="RFC3424"
>UNilateral Self-Address Fixing (UNSAF)</xref> method. For those cases, this solution is
considered transitional until such time as the recommendations for residential gateways in
<xref target="RFC5986"/> are more widely deployed. Considerations relating to UNSAF
applications are described in <xref target="iab"/>. </t>
</section>
<section anchor="conventions" title="Conventions used in this document">
<t>The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT",
"RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in
<xref target="RFC2119"/>. </t>
<t>This document uses terminology established in <xref target="RFC3693"/> and <xref
target="RFC5012"/>. </t>
</section>
<section anchor="ps" title="Problem Statement">
<t><xref target="topo"/> shows a simplified network topology for fixed wire-line Internet
access. This arrangement is typical when wired Internet access is provided. The diagram
shows two network segments: the access network provided by an internet service provider
(ISP), and the residential network served by the residential gateway. </t>
<t>There are a number of variations on this arrangement, as documented in Section 3.1 of <xref
target="RFC5687"/>. In each of these variations the goal of LIS discovery is to identify
the LIS in the access network. </t>
<figure anchor="topo" title="Simplified Network Topology">
<artwork><![CDATA[
________
(/ \)
(( Internet ))
(\________/)
|
|
.- - -|- - - - - - - - - - - -.
( | )
( +--------+ +-------+ )
Access ( | Access |. . . .| LIS | )
Network ( | Node | | | )
(ISP) ( +--------+ +-------+ )
( \ \ )
`- - - -\- - - - - - - -\- - -'
\ \
\ |
.- - - - -\- - - - - - - + -.
( \ | )
( +-------------+ : )
( | Residential | | )
Residential ( | Gateway | : )
Network ( +-------------+ | )
( / \ / )
( / \ / )
( +--------+ +--------+ )
( | Device | | Device | )
( +--------+ +--------+ )
( )
`- - - - - - - - - - - - - -'
]]></artwork>
</figure>
<t>A particularly important characteristic of this arrangement is the relatively small area
served by the residential gateway. Given a small enough area, it is reasonable to delegate
the responsibility for providing Devices within the residential network with location
information to the ISP. The ISP is able to provide location information that identifies the
residence, which should be adequate for a wide range of purposes. </t>
<t>A residential network that covers a larger area might require a dedicated LIS, a case that
is outside the scope of this document. </t>
<t>The goal of LIS discovery is to identify a LIS that is able to provide the Device with
accurate location information. In the network topology described, this means identifying the
LIS in the access network. The residential gateway is a major obstacle in achieving this
goal. </t>
<section title="Residential Gateway">
<t>A residential gateway can encompass several different functions including: modem,
Ethernet switch, wireless access point, router, network address translation (NAT), DHCP
server, DNS relay and firewall. Of the common functions provided, the NAT function of a
residential gateway has the greatest impact on LIS discovery. </t>
<t>An ISP is typically parsimonious about their IP address allocations; each customer is
allocated a limited number of IP addresses. Therefore, NAT is an extremely common function
of gateways. NAT enables the use of multiple Devices within the residential network.
However NAT also means that Devices within the residence are not configured by the ISP
directly. </t>
<t>When it comes to discovering a LIS, the fact that Devices are not configured by the ISP
causes a significant problem. Configuration is the ideal method of conveying the
information necessary for discovery. Devices attached to residential gateways are usually
given a generic configuration that includes no information about the ISP network. For
instance, DNS configuration typically points to a DNS relay on the gateway device. This
approach ensures that the local network served by the gateway is able to operate without a
connection to the ISP, but it also means that Devices are effectively ignorant of the ISP
network. </t>
<t><xref target="RFC5986"/> describes several methods that can be applied by a residential
gateway to assist Devices in acquiring location information. For instance, the residential
gateway could forward LIS address information to hosts within the network it serves. Such
an active involvement in the discovery process only works for new residential gateway
devices that implement these recommendations. </t>
<t>Where residential gateways already exist, direct involvement of the gateway in LIS
discovery requires that the residential gateway be updated or replaced. The cost of
replacement is difficult to justify to the owner of the gateway, especially when it is
considered that the gateway still fills its primary function: Internet access. </t>
<t>Existing residential gateways have proven to be quite reliable devices, some operating
continuously for many years without failure. As a result, there are many operational
gateways that are of a considerable age, some well outside the period of manufacturer
support. Updating the software in such devices is not feasible in many cases. Even if
software updates were made available, many residential gateways cannot be updated
remotely, inevitably leading to some proportion that is not updated. </t>
<t>This document therefore describes a method which can be used by Devices to discover their
LIS without any assistance from the network.</t>
</section>
<section title="Residential Gateway Security Features">
<t>A network firewall function is often provided by residential gateways as a security
measure. Security features like intrusion detection systems help protect users from
attacks. Amongst these protections is a port filter that prevents both inbound and
outbound traffic on certain TCP and UDP ports. Therefore, any solution needs to consider
the likelihood of traffic being blocked. </t>
</section>
</section>
<section anchor="ip-dns" title="IP-based DNS Solution">
<t><xref target="RFC5986">LIS discovery</xref> uses a DNS-based Dynamic Delegation Discovery
Service (DDDS) system as the basis of discovery. Input to this process is a domain name. Use
of DHCP for acquiring the domain name is specified, but alternative methods of acquisition
are permitted. </t>
<t>This document specifies a means for a device to discover several alternative domain names
that can be used as input to the DDDS process. These domain names are based on the IP
address of the Device. Specifically, the domain names are a portion of the reverse DNS trees
- either the <spanx style="verb">.in-addr.arpa.</spanx> or <spanx style="verb"
>.ip6.arpa.</spanx> tree. </t>
<t>A Device might be reachable at one of a number of IP addresses. In the process described, a
Device first identifies each IP address that it is potentially reachable from. From each of
these addresses, the Device then selects up to three domain names for use in discovery.
These domain names are then used as input to the DDDS process. </t>
<section title="Identification of IP Addresses">
<t>A Device identifies a set of potential IP addresses that currently result in packets
being routed to it. These are ordered by proximity, with those addresses that are used in
adjacent network segments being favoured over those used in public or remote networks. The
first addresses in the set are those that are assigned to local network interfaces. </t>
<t>A Device can use the Session Traversal Utilities for NAT (STUN) <xref target="RFC5389"/>
to determine its public reflexive transport address. The host uses the <spanx style="verb"
>Binding Request</spanx> message and the resulting <spanx style="verb"
>XOR-MAPPED-ADDRESS</spanx> parameter that is returned in the response. </t>
<t>Alternative methods for determining other IP addresses MAY be used by the host. <xref
target="UPnP-IGD-WANIPConnection1">Universal Plug and Play (UPnP)</xref> and <xref
target="I-D.cheshire-nat-pmp">NAT Port Mapping Protocol (NAT-PMP)</xref> are both able
to provide the external address of a residential gateway device when enabled. These as
well as proprietary methods for determining other addresses might also be available.
Because there is no assurance that these methods will be supported by any access network
these methods are not mandated. Note also that in some cases, methods that rely on the
view of the network from the residential gateway device could reveal an address in a
private address range (see <xref target="assumptions"/>). </t>
<t>In many instances, the IP address produced might be from a private address range. For
instance, the address on a local network interface could be from a private range allocated
by the residential gateway. In other cases, methods that rely on the view of the network
(UPnP, NAT-PMP) from the residential gateway device could reveal an address in a private
address range if the access network also uses NAT. For a private IP address, the derived
domain name is only usable where the DNS server used contains data for the corresponding
private IP address range. </t>
</section>
<section title="Domain Name Selection">
<t>The domain name selected for each resulting IP address is the name that would be used for
a reverse DNS lookup. The domain name derived from an IP version 4 address is in the
<spanx style="verb">.in-addr.arpa.</spanx> tree and follows the construction rules in
Section 3.5 of <xref target="RFC1035"/>. The domain name derived from an IP version 6
address is in the <spanx style="verb">.ip6.arpa.</spanx> tree and follows the construction
rules in Section 2.5 of <xref target="RFC3596"/>. </t>
<t>Additional domain names are added to allow for a single record to cover a larger set of
addresses. If the search on the domain derived from the full IP address does not produce a
NAPTR record with the desired service tag (e.g., <spanx style="verb">LIS:HELD</spanx>), a
similar search is repeated based on a shorter domain name, using a part of the IP address:
<list style="symbols">
<t>For IP version 4, the resulting domain name SHOULD be shortened successively by one
and two labels and the query repeated. This corresponds to a search on a /24 or /16
network prefix. This allows for fewer DNS records in the case where a single access
network covering an entire /24 or /16 network is served by the same LIS. </t>
<t>For IP version 6, the resulting domain SHOULD be shortened sucessively by 16, 18, 20
and 24 labels and the query repeated. This corresponds to a search on a /64, /56, /48
or /32 network prefix.</t>
</list> DNS queries on other prefixes than those listed above SHOULD NOT be performed to
limit the number of DNS queries performed by Devices. If no LIS is discovered by this
method, no more than four U-NAPTR resolutions are invoked for each IP address. </t>
</section>
<section title="When To Use This Method">
<t>The DHCP method described in <xref target="RFC5986"/> SHOULD be attempted on all local
network interfaces before attempting this method. This method is employed either because
DHCP is unavailable, when the DHCP server does not provide a value for the access network
domain name option, or if a request to the resulting LIS results in a HELD <spanx
style="verb">notLocatable</spanx> error or equivalent. </t>
</section>
<section anchor="private-address" title="Private Address Spaces">
<t>Addresses from a private use address space can be used as input to this method. In many
cases, this applies to addresses defined in <xref target="RFC1918"/>, though other address
ranges could have limited reachability where this advice also applies. This is only
possible if a DNS server with a view of the same address space is used. Public DNS servers
cannot provide useful records for private addresses. </t>
<t>Using an address from a private space in discovery can provide a more specific answer if
the DNS server has records for that space. <xref target="privatespace"/> shows a network
configuration where addresses from an ISP network could better indicate the correct LIS.
Records in DNS B can be provided for the 10.0.0.0/8 range, potentially dividing that range
so that a more local LIS can be selected. </t>
<figure anchor="privatespace" title="Address Space Example">
<artwork><![CDATA[
_____ ________
( DNS ).....(/ \) Public
(__A__) (( Internet )) Address
(\________/) Space
|
[NAT]
_____ _____|_____
( DNS )....(/ \) Private
(__B__) (( ISP Network )) Address Space
(\___________/) (e.g. 10.0.0.0/8)
|
[Gateway]
____|____
(/ \) Private
(( Residence )) Address Space
(\_________/) (e.g. 192.168.0.0/16)
]]></artwork>
</figure>
<t>The goal of automatic DNS configuration is usually to select a local DNS, which suits
configurations like the one shown. However, use of public DNS or STUN servers means that a
public IP address is likely to be found. For STUN in particular, selecting a public server
minimizes the need for reconfiguration when a Device moves. Adding records for the public
address space used by an access network ensures that the discovery process succeeds when a
public address is used. </t>
</section>
<section anchor="assumptions" title="Necessary Assumptions and Restrictions">
<t>When used by a Device for LIS discovery this is an UNSAF application and is subject to
the limitations described in <xref target="iab"/>. </t>
<t>It is not necessary that the IP address used is unique to the Device, only that the
address can be somehow related to the Device or the access network that serves the Device.
This allows a degree of flexibility in determining this value, although <xref
target="security">security considerations</xref> might require that the address be
verified to limit the chance of falsification. </t>
<t>This solution assumes that the public reflexive transport address used by a Device is in
some way controlled by the access network provider, or some other related party. This
implies that the corresponding <spanx style="verb">.in-addr.arpa.</spanx> or <spanx
style="verb">.ip6.arpa.</spanx> record can be updated by that entity to include a useful
value for the LIS address. </t>
</section>
<section title="Failure Modes">
<t>Successful use of private addresses relies on a DNS server that has records for the
address space that is used. Using a public IP address increases the likelihood of this.
This document relies on STUN to provide the Device with a public reflexive transport
address. Configuration of STUN server is necessary to ensure that this is successful. </t>
<t>Alternative methods for discovering external IP addresses are possible, including UPnP
and NAT-PMP. These methods might not be supported by the residential gateway and cannot be
relied upon in all cases. </t>
<t>In cases where a virtual private network (VPN) or other tunnel is used, the entity
providing a public IP address might not be able to provide the Device with location
information. It is assumed that this entity is able to identify this problem and indicate
this to the Device (using the <spanx style="verb">notLocatable</spanx> HELD error, or
similar). This problem is described in more detail in <xref target="RFC5985"/>. </t>
</section>
<section title="Deployment Considerations">
<t>An access network provider SHOULD provide NAPTR records for each public IP address that
is used for Devices within the access network. If the access network provider uses NAT,
any DNS internal to that NAT SHOULD also include records for the private address range. </t>
<t>NAPTR records can be provided for individual IP addresses. To limit the proliferation of
identical records, a single record can be placed at a the higher nodes of the tree
(corresponding to /24 and /16 for IPv4; /64, /48 and /32 for IPv6). A record at a higher
point in the tree (those with a shorter prefix) applies to all addresses lower in the tree
(those with a longer prefix); records at the lower point override those at higher points,
allowing for exceptions to be provided for at the lower point. </t>
</section>
</section>
<section anchor="iana" title="IANA Considerations">
<t>[RFC Editor: please remove this section prior to publication.]</t>
<t>This document has no IANA actions.</t>
</section>
<section anchor="security" title="Security Considerations">
<t>The security considerations described in <xref target="RFC5986"/> apply to the discovery
process as a whole. The primary security concern is with the potential for an attacker to
impersonate a LIS. </t>
<t>The added ability for a third party to discover the identity of a LIS does not add any
concerns, since the identity of a LIS is considered public information. </t>
<t>In addition to existing considerations, this document introduces further security
considerations relating to the identification of the IP address. It is possible that an
attacker could attempt to provide a falsified IP addresses in an attempt to subvert the rest
of the process. </t>
<t><xref target="RFC5389"/> describes attacks where an attacker is able to ensure that a
Device receives a falsified reflexive address. Even if the STUN server is trusted, an
attacker might be able to ensure that a falsified address is provided to the Device. </t>
<t>This attack is an effective means of denial of service, or a means to provide a
deliberately misleading service. Notably, any LIS that is identified based on a falsified IP
address could still be a valid LIS for the given IP address, just not one that is useful for
providing the Device with location information. In this case, the LIS provides a HELD <spanx
style="verb">notLocatable</spanx> error, or an equivalent. If the falsified IP address is
under the control of the attacker, it is possible that misleading (but verifiable) DNS
records could indicate a malicious LIS that provides false location information. </t>
<t>In all cases of falsification, the best remedy is to perform some form of independent
verification of the result. No specific mechanism is currently available to prevent attacks
based on falsification of reflexive addresses; it is suggested that Devices attempt to
independently verify that the reflexive transport address provided is accurate.</t>
<t>Use of private address space effectively prevents use of the usual set of trust anchors for
DNSSEC. Only a DNS server that is able to see the same private address space can provide
useful records. A Device that relies on DNS records in the private address space portion of
the <spanx style="verb">.in-addr.arpa.</spanx> or <spanx style="verb">.ip6.arpa.</spanx>
trees MUST either use an alternative trust anchor for these records or rely on other means
of ensuring the veracity of the DNS records. </t>
</section>
<section anchor="iab" title="IAB Considerations">
<t>The IAB has studied the problem of <xref target="RFC3424">Unilateral Self-Address Fixing
(UNSAF)</xref>, which is the general process by which a client attempts to determine its
address in another realm on the other side of a NAT through a collaborative protocol
reflection mechanism, such as STUN. </t>
<t>This section only applies to the use of this method of LIS discovery by Devices and does
not apply to its use for third-party LIS discovery.</t>
<t>The IAB requires that protocol specifications that define UNSAF mechanisms document a set
of considerations. <list style="numbers">
<t>Precise definition of a specific, limited-scope problem that is to be solved with the
UNSAF proposal. <vspace blankLines="1"/>
<xref target="ps"/> describes the limited scope of the problem addressed in this
document. </t>
<t>Description of an exit strategy/transition plan. <vspace blankLines="1"/>
<xref target="RFC5986"/> describes behaviour that residential gateways require in order
for this short term solution to be rendered unnecessary. When implementations of the
recommendations in LIS discovery are widely available, this UNSAF mechanism can be made
obsolete.</t>
<t>Discussion of specific issues that may render systems more "brittle". <vspace
blankLines="1"/> A description of the necessary assumptions and limitations of this
solution are included in <xref target="assumptions"/>. <vspace blankLines="1"/> Use of
STUN for discovery of a reflexive transport address is inherently brittle in the
presence of multiple NATs or address realms. In particular, brittleness is added by the
requirement of using a DNS server that is able to view the address realm that contains
the IP address in question. If address realms use overlapping addressing space, then
there is a risk that the DNS server provides information that is not useful to the
Device. </t>
<t>Identify requirements for longer term, sound technical solutions; contribute to the
process of finding the right longer term solution. <vspace blankLines="1"/> A longer
term solution is already provided in <xref target="RFC5986"/>. However, that solution
relies on widespread deployment. The UNSAF solution provided here is provided as an
interim solution that enables LIS access for Devices that are not able to benefit from
deployment of the recommendations in <xref target="RFC5986"/>. </t>
<t>Discussion of the impact of the noted practical issues with existing deployed NATs and
experience reports. <vspace blankLines="1"/> The UNSAF mechanism depends on the
experience in deployment of <xref target="RFC5389">STUN</xref>. On the whole, existing
residential gateway devices are able to provide access to STUN and DNS service reliably,
although regard should be given to the size of the DNS response (see <xref
target="RFC5625"/>). </t>
</list>
</t>
</section>
<!--
<section anchor="ack" title="Acknowledgements">
</section>
-->
</middle>
<back>
<references title="Normative References"> &RFC1035; &RFC2119; &RFC3424;
&RFC3596; &RFC5985; &RFC5986; </references>
<references title="Informative References"> &RFC1918; &RFC2131; &RFC3315;
&RFC3693; &RFC4848; &RFC5012; &RFC5389; &RFC5687;
<reference anchor="UPnP-IGD-WANIPConnection1">
<front>
<title> Internet Gateway Device (IGD) Standardized Device Control Protocol V 1.0:
WANIPConnection:1 Service Template Version 1.01 For UPnP Version 1.0 </title>
<author>
<organization>UPnP Forum</organization>
</author>
<date day="12" month="Nov" year="2001"/>
</front>
<seriesInfo name="DCP" value="05-001"/>
</reference>
&I-D.cheshire-nat-pmp; &RFC5625; </references>
</back>
</rfc>
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