Geopriv WG James Polk
Internet-Draft Cisco Systems
Intended status: Standards Track (PS) July 13, 2009
Expires: January 13, 2010
Dynamic Host Configuration Protocol (DHCP) IPv4 and IPv6
Option for a Location Uniform Resource Identifier (URI)
draft-ietf-geopriv-dhcp-lbyr-uri-option-05
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Abstract
This document creates a Dynamic Host Configuration Protocol (DHCP)
Option for the downloading of a Location Uniform Resource Identifier
(URI) of a client, to be dereferenced in a separate transaction.
Once the location URI is received by an endpoint, it can be
dereferenced by the client to learn its geographic location, or sent
to another entity to learn this client's location.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Format of the DHCP LuriElement Option . . . . . . . . . . . . 5
2.1. Overall Format of LuriElement Option in IPv4 . . . . . 5
2.2. Overall Format of LuriElement Option in IPv6 . . . . . 5
2.3. LuriElement Format for both IPv4 and IPv6 . . . . . . . 6
3. DHC Option Operation . . . . . . . . . . . . . . . . . . . . 7
3.1 Architectural Assumptions . . . . . . . . . . . . . . . . 8
3.2 Harmful URIs and URLs . . . . . . . . . . . . . . . . . . 9
3.3 Valid Location URI Schemes or Types . . . . . . . . . . . 9
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
5. Security Considerations . . . . . . . . . . . . . . . . . . . 11
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 12
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
7.1. Normative References . . . . . . . . . . . . . . . . . . 12
7.2. Informative References . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
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 [RFC2119].
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1. Introduction
This document creates a Dynamic Host Configuration Protocol (DHCP)
Option for the downloading of a Location Uniform Resource Identifier
(URI) of a client, to be dereferenced in a separate transaction.
A client, for example, can be a Session Initiation Protocol (SIP)
User Agent (UA) [RFC3261] (i.e., a Phone). This location URI
points a Location Server [ID-LBYR-REQ] which has the geolocation of
the client (through means not defined in this document).
The Location Server stores the Target's location a presence
document, called a Presence Information Date Format - Location
Object (PIDF-LO). Once a client attains this location URI, an
application on the target can either dereference the URI to receive
its PIDF-LO, or it can convey this location URI instead of conveying
the location value in a message. Having a location URI has
advantages over having a PIDF-LO, especially when a target's
location changes. With a location URI, when a target moves, the
location URI does not. It can still be given out as the Target's
location. The opposite is true if the location is conveyed by value
in a message. Once the Target moves, the previously given location
is no longer valid, and it the Target wants to inform another entity
about its location, it has to send the PIDF-LO to the location
recipient (again).
The act of dereferencing location from a location URI is explained
in [ID-SIP-LOC], which shows how a Location Recipient of a location
URI subscribes to a Location Server to attain the location of the
Target. If the dereferencer has permission, defined in [ID-GEO-POL],
the location of the target will be received. The Location Server
(LS) will grant permission to location inquires based on the rules
established by a Rule Holder [RFC3693]. The Location Server has the
ability to challenge any request of a target's location, thereby
providing additive security properties before location revelation.
There are use-cases, such as calling the appropriate Pizza Hut
without having to look up in a directory which store is closest,
where a UA knowing its location can call a
main/national/international Pizza Hut number or address and let the
UA's location tell Pizza Hut enough information to have them
route/retarget the SIP request to the appropriate store within the
Pizza Hut organization to deliver the pizza to the caller's
location.
A problem exists within existing RFCs that provide location to the
UA ([RFC3825] and [RFC4776]), these DHCP Options for geolocation
require an update of the entire location information (LI) every time
a client moves. Not all clients will move frequently, but some
will. Refreshing location every time a client moves does not scale
in certain networks/environments, such as IP based cellular
networks, enterprise networks or service provider networks with
mobile endpoints. An 802.11 based access network is one example of
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this. Constantly updating LCI to endpoints might not scale in mobile
(residential or enterprise or municipal) networks in which the
client is moving through more than one network attachment point,
perhaps as a person walks or drives with their endpoint down a
neighborhood street or apartment complex or a shopping center.
If the client were provided a location URI reference to retain and
hand out when it wants or needs to convey its location (in a
protocol other than DHCP), a location URI that would not change as
the client's location changes (within a domain), scaling issues
would be significantly reduced to needing an update of the location
URI only when a client changes administrative domains - which is
much less often. This delivery of an indirect location has the
added benefit of not using up valuable or limited bandwidth to the
client with the constant updates. It also relieves the client from
having to determine when it has moved far enough to consider asking
for a refresh of its location.
In enterprise networks, if a known location is assigned to each
individual Ethernet port in the network, a device that attaches to
the network a wall-jack (directly associated with a specific
Ethernet Switch port) will be associated with a known location via a
unique circuit-ID that's used by the RAIO Option defined in RFC 3046
[RFC3046]. This assumes wall-jacks have an updated wiremap
database. RFC 3825 and RFC 4776 would return an LCI value of
location. This document specifies how a location URI is returned
using DHCP. Behind the DHCP server, in the backend of the network,
via the (logical entity of an) LS has a PIDF-LO to be dereferenced
with a location URI.
If local configuration has the requirement of only assigning unique
location URIs to each client, then unique LbyRs will be given out,
though they will all have the same location at the record, relieving
the backend Sighter from individually maintaining each location
independently.
This Option can be useful in WiMAX connected endpoints or IP
cellular endpoints. The location URI Option can be configured as a
client if there is a router, such as a residential home gateway,
with the ability to communicate to downstream endpoints as a server.
How an LS challenges a dereference can vary, and a policy
established by a rulemaker [RFC3693] for a Location Target as to
what type of challenge(s) is to be used, how strong a challenge is
used or how precise the location information is given to a
requestor. All of this is outside the scope of this document (since
this will not be accomplished using DHCP).
This document IANA registers the new IPv4 and IPv6 DHC Options for a
location URI.
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2. Format of the DHCP LuriElement Option
2.1 Overall Format of LuriElement Option in IPv4
The LuriElement Option format for IPv4 is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Code XXX | Length=XX | Ver | Resv | .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ .
. LuriElements... ...
. (see section 2.3 for details) ... .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 1. IPv4 Fields for this LuriElement Option
Code XXX: The code for this DHCPv4 option (IANA assigned).
Length=XX: The length of this option, counted in bytes - not
counting the Code and Length bytes. This is a variable
length Option, therefore the length value will change
based on the length of the LbyR within the Option.
Ver: (4 bits) The version of this Option. This document
defines version 1 of this Option.
Resv: (4 bits) reserved for future use.
LuriElement: see section 2.3 for details
2.2 Overall Format of LuriElement Option in IPv6
The LuriElement Option format for IPv6 is as follows:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| option-code | option-len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Ver | Resv | .
+---------------+ .
. LuriElements... .
. (see section 2.3 for details) .
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2. IPv6 fields of this LuriElement Option
option-code: The code for this DHCPv6 option (IANA assigned).
option-len: The length of this option, counted in bytes - not
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counting the Code and Length bytes. This is a variable
length Option, therefore the length value will change
based on the shape within the Option.
Ver: See above (Section 2.1). This will specify version 1.
Resv: See above (Section 2.1).
LuriElement: see below (Section 2.3 for details).
2.3 LuriElement Format for both IPv4 and IPv6
The LuriElement, in both DHCPv4 and DHCPv6, have the following
format:
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| LuriType | LuriLength | LuriValue ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3. LuriElement Format for both IPv4 and IPv6
LuriType: A one-byte identifier of the data location value.
LuriLength: The length, in bytes, of the LuriValue, not including
the LuriLength field itself, up to a maximum of 255
bytes.
LuriValue: The LuriElement value, as described in detail below.
The LuriValue is always in UTP-8.
The LuriTypes this document defines (and IANA registers) for a point
are:
LuriType=1 Location URI - This is the URI pointing at the
location record where the PIDF-LO resides which
indicates the location of the Location Target.
LuriType=2 Valid-For - The time, in seconds, this URI is to be
considered Valid for dereferencing. The timer
associated with this LuriType starts upon receipt of
this Option.
The LuriType=2 (Valid-For) indicates how long, in seconds, the
client is to consider this LuriType=1 (location URI) valid
before performing a refresh of this Option, with a refreshed
LuriType=2 (Valid-For) value. A refresh MAY be done merely at the
normal DHCP refresh rate, or necessitated by this timer, perhaps
with the client only requesting this Option be refreshed.
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It is RECOMMENDED when the counter associated with this LuriType=2
(Valid-For) value has passed, the client perform a refresh of this
Option. For example, if 16000 was the initial value of the
LuriType=2 (Valid-For) value, when 8000 seconds have passed, the
Option SHOULD be refreshed.
The LuriType=2 (Valid-For) is not mandated for use by this document.
However, its presence MUST NOT cause any error in handling the
location URI (i.e., if not understood, it MUST be ignored).
This Option format is highly extensible. Additional LuriType types
created MUST be done so through IANA registration with peer review
and an RFC.
3. DHC Option Operation
The [RFC3046] RAIO MUST be utilized to provide the appropriate
indication to the DHCP Server where this DISCOVER or REQUEST message
came from, in order to supply the correct response. That said, this
Option SHOULD NOT be in a DISCOVER message, because there is zero
knowledge by the client of which Server will answer.
Caution SHOULD always be used involving the creation of large
Options, meaning that this Option MAY need to be in its own INFORM,
OPTION or ACK message.
It is RECOMMENDED to avoid building URIs, with any parameters,
larger than what a single DHCP response can be. However, if a
message is larger than 255 bytes, concatenation is allowed, per RFC
3396 [RFC3396].
Per [RFC2131], subsequent LuriElement Options, which are
non-concatenated, overwrite the previous value.
location URIs MUST NOT reveal identity information of the user of
the device, since DHCP is a cleartext delivery protocol. For
example, location URIs such as
sips:34LKJH534663J54@example.com
SHOULD be done, providing no identity information, rather than a
location URI such as this
sips:aliceisinatlanta@example.com
In the <presence> element of the PIDF-LO there is an entity=
attribute that identities the target of *this* location. It is up
to the PIDF-LO generator, either Location Server or an application
in the endpoint, to insert the identity in the entity= attribute.
This can be seen in [RFC4119]. The entity= discussion is orthogonal
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to the identification information contained within the location URI.
This Option is only for communications between a DHCP client and a
DHCP server. It can be solicited (requested) by the client, or it
can be pushed by the server without a request for it. DHCP Options
not understood are ignored. A DHCP server might or might not have
the location of a client, therefore direct knowledge of a
location URI within the server. If a server does not have a
client's location, a communication path (or request) to an LS would
be necessary.
Further, any location revelation rules and policies a user has
regarding the treatment of their actual location, and who can
access (what precision of) their location will be done with a
protocol other than DHCP, and likely will be done before anything
other than default authentication and authorization permissions are
used when a Location Recipient requests for a Target's location.
Differentiating clients is done via client identifiers. Therefore,
in many implementations, each client can be assigned unique location
URIs, though this is not mandatory.
Any dereferencing of a client's location URI would not involve DHCP
as well, but more likely by an application layer protocol such as
SIP, through a subscription to the location URI on the LS. The LS
would also handle all authentication and authorization of location
requests, which is also not performed with DHCP, therefore not
defined here.
In the case of residential gateways being DHCP servers, they usually
perform as DHCP clients in a hierarchical fashion up into a service
provider's network DHCP server(s), or learn what information to
provide via DHCP to residential clients through a protocol such as
PPP. In these cases, the location URI would likely indicate the
residence's civic address to all wired or wireless clients within
that residence. This is not inconsistent with what's stated above.
3.1 Architectural Assumptions
The following assumptions have been made for use of this LuriElement
Option for a client to learn its location URI (in no particular
order):
o Any user control (what [RFC3693] calls a 'ruleholder') for the
parameters and profile options a Location Object will have is out
of scope of this document, but assumed to take place via an
external web interface between the user and the Location Server
(direct or indirect). Some of this is described in [ID-GEO-POL].
o The authorization vs. possession security model can be found in
[ID-LBYR-REQ], describing what is expected in each model of
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operation. It should be assumed that a location URI attained
using DHCP will operate under an authorization model. This means
possessing the location URI does not give that entity the right
to view the PIDF-LO of the target whose location is indicated in
a presence document. The dereference transaction will be, in
many environments, challenged by the Location Server. The nature
of this challenge is out of scope of this document.
o This document does not prevent some environments from operating
in a possession model, for example - tightly controlled
enterprise networks, but this operation SHOULD NOT be assumed to
exist as a matter of local policy. The costs associated with
authorization vs. possession models are discussed in section
3.3.2 of [ID-GEO-RA].
3.2 Harmful URIs and URLs
There are, in fact, some types of URIs that are not good to receive,
due to security concerns. For example, any URLs that can have
scripts, such as "data:" URLs, and some "HTTP:" URLs that go to web
pages that have scripts. Therefore,
o URIs received via this Option SHOULD NOT be sent to a
general-browser to connect to a web page, because they could have
harmful scripts.
o This Option SHOULD NOT contain "data:" URLs, because they could
contain harmful scripts.
Instead of listing all the types of URIs and URLs that can be
misused or potentially have harmful affects, Section 3.3 IANA
registers acceptable location URI schemes (or types).
3.3 Valid Location URI Schemes or Types
This section specifies which URI types are acceptable as a location
URI scheme (or type) for this DHCP Option:
1. sip:
2. sips:
3. pres:
These location URI types are IANA registered in section 4.2 of this
document.
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4. IANA Considerations
4.1 The IPv4 Option number for this Option
This document IANA registers this IPv4 Option number XXX (to be
assigned by IANA once this document becomes an RFC).
4.2 The IPv6 Option-Code for this Option
This document IANA registers this IPv6 Option-Code XXX (to be
assigned by IANA once this document becomes an RFC).
4.3 The Version number for this Option
This document IANA registers the version number 1 of this Option.
4.4 IANA Considerations for Acceptable Location URI Types
IANA is requested to create a new registry for acceptable location
URI types.
The following 3 URI types are registered by this document:
1. sip:
2. sips:
3. pres:
Any additional location URI types to be defined for use via
this DHC Option need to be created and IANA registered with peer
review and an RFC.
4.5 IANA Considerations for LuriTypes
IANA is requested to create a new registry for acceptable location
types defined in section 3.2 of this document, arranged similar to
this:
+------------+----------------------------------------+-----------+
| LuriType | Name | Reference |
+------------+----------------------------------------+-----------+
| 1 | Location URI | RFC XXXX* |
| 2 | Valid-For | RFC XXXX* |
+------------+----------------------------------------+-----------+
* RFC XXXX is to be replaced with this document's RFC-Editor RFC
number.
Additions to this list require a standards track RFC.
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5. Security Considerations
Where critical decisions might be based on the value of this
location URI option, DHCP authentication in [RFC3118] SHOULD be used
to protect the integrity of the DHCP options.
A real concern with RFC 3118 it is that not widely deployed because
it requires pre-shared keys to successfully work (i.e., in the
client and in the server). Most implementations do not
accommodate this.
DHCP is a broadcast initially (a client looking for a server),
unicast response (answer from a server) type of protocol. It is not
secure in a practical sense. In today's infrastructures, DHCP will
be primarily used over a wired, switched Ethernet network, requiring
physical access to within a wire to gain access. Further, within an
802.11 wireless network, the 802.11 specs have layer 2 security
mechanisms in place to help prevent a location URI from being
learned by an unauthorized entity.
That said, having the location URI does not mean an unauthorized
entity has the location of a client. The location URI still needs
to be dereferenced to learn the location of the client. This
dereferencing function, which is not done using DHCP, is done by
requesting the location record at a Location Server, which is a
defined entity built to challenge each request it receives based on
a joint policy of what is called a rulemaker. The ruleholder, as
defined in RFC 3693, configures the authentication and authorization
policies for the location revelation of a Target. This includes
giving out more or less precise location information in a response,
therefore it can answer a bad-hat, but not allow it from learning
exactly where a client is. The rulemaker, which is a combination of
the default rules set up by the location provider and those decided
on by the user of the Target device. Likely, the rules the user
wants will not be allowed to go past some limits established by the
location provider, i.e., the administrator of the LS, for various
capability or security reasons.
Penetrating an LS is supposed to be hard, and hopefully vendors that
implement an LS accomplish this goal.
It is envisioned, as stated in the text above, as described both in
[ID-LBYR-REQ] and [ID-GEO-RA], this Option MUST be assumed to
operate under an authorization model, vs. a much more open
possession model. This model choice is not strictly enforced, but
implementations (in the text above) have been advised to build with
this in mind.
As to the concerns about the location URI itself, as stated in the
document here (in Section 3.), it MUST NOT have any user identifying
information in the URI user-part/string itself. The location URI
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also needs to be hard to guess that it belongs to a specific user.
There is some debate as to whether this location URI need be a
random alphanumeric string or just unique. If the latter, there is
some debate as to the how we define unique. Is that through space
and time, as RFC 3261 defines a SIP Call-ID needs to be (meaning:
never a duplicate, ever, by any device, ever)? Or is it unique to
within a specific domain for as long as it is actively assigned to a
client (plus some interval).
When implementing a DHC server that will serve clients across an
uncontrolled network, one should consider the potential security
risks therein.
6. Acknowledgements
Thanks to James Winterbottom, Marc Linsner, Roger Marshall and
Robert Sparks for their useful comments. And to Lisa Dusseault for
her concerns about the types of URIs that can cause harm. To
Richard Barnes for inspiring a more robust Security Considerations
section. To Hannes Tschofenig and Ted Hardie for riding me to
comply with their concerns.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3046] Patrick, M., "DHCP Relay Agent Information Option", RFC
3046, January 2001.
[RFC2131] Droms, R., "Dynamic Host Configuration Protocol", RFC 2131,
March 1997.
[RFC3118] Droms, R. and W. Arbaugh, "Authentication for DHCP
Messages", RFC 3118, June 2001.
[RFC3261] J. Rosenberg, H. Schulzrinne, G. Camarillo, A. Johnston, J.
Peterson, R. Sparks, M. Handley, and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, May 2002.
[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 2002.
[RFC3396] T. Lemon, S. Cheshire, "Encoding Long Options in the Dynamic
Host Configuration Protocol (DHCPv4)", RFC 3396, November
2002
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[RFC4119] J. Peterson, "A Presence-based GEOPRIV Location Object
Format", RFC 4119, December 2005
7.2. Informative References
[ID-SIP-LOC] J. Polk, B. Rosen, "SIP Location Conveyance", "work in
progress", July 2009
[RFC3825] J. Polk, J. Schnizlein, M. Linsner, "Dynamic Host
Configuration Protocol Option for Coordinate-based Location
Configuration Information", RFC 3825, July 2004
[RFC4776] H. Schulzrinne, " Dynamic Host Configuration Protocol
(DHCPv4 and DHCPv6) Option for Civic Addresses Configuration
Information ", RFC 4776, November 2006
[ID-LBYR-REQ] R. Marshall, "Requirements for a Location-by-Reference
Mechanism", "work in progress", Feb 2009
[RFC3693] J. Cuellar, J. Morris, D. Mulligan, J. Peterson. J. Polk,
"Geopriv Requirements", RFC 3693, February 2004
[ID-GEO-POL] H. Schulzrinne, H. Tschofenig, J. Morris, J. Cuellar, J.
Polk, "Geolocation Policy: A Document Format for Expressing
Privacy Preferences for Location Information", "work in
progress", Feb 2009
[ID-GEO-RA] J. Peterson, T. Hardie, J. Morris, " Implications of
'retransmission-allowed' for SIP Location Conveyance",
"work in progress", March 2009
Authors' Address
James Polk
3913 Treemont Circle
Colleyville, Texas 76034
USA
Email: jmpolk@cisco.com
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