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Internet Engineering Task Force (IETF)                  R. Marshall, Ed.Request for Comments: 5808                                           TCSCategory: Informational                                         May 2010ISSN: 2070-1721Requirements for a Location-by-Reference MechanismAbstract   This document defines terminology and provides requirements relating   to the Location-by-Reference approach using a location Uniform   Resource Identifier (URI) to handle location information within   signaling and other Internet messaging.Status of This Memo   This document is not an Internet Standards Track specification; it is   published for informational purposes.   This document is a product of the Internet Engineering Task Force   (IETF).  It represents the consensus of the IETF community.  It has   received public review and has been approved for publication by the   Internet Engineering Steering Group (IESG).  Not all documents   approved by the IESG are a candidate for any level of Internet   Standard; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc5808.Copyright Notice   Copyright (c) 2010 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   to this document.  Code Components extracted from this document must   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Marshall                      Informational                     [Page 1]

RFC 5808                GEOPRIV LbyR Requirements               May 2010   This document may contain material from IETF Documents or IETF   Contributions published or made publicly available before November   10, 2008.  The person(s) controlling the copyright in some of this   material may not have granted the IETF Trust the right to allow   modifications of such material outside the IETF Standards Process.   Without obtaining an adequate license from the person(s) controlling   the copyright in such materials, this document may not be modified   outside the IETF Standards Process, and derivative works of it may   not be created outside the IETF Standards Process, except to format   it for publication as an RFC or to translate it into languages other   than English.Table of Contents1. Introduction ....................................................32. Terminology .....................................................53. Overview of Location-by-Reference ...............................63.1. Location URI Usage .........................................73.2. Location URI Expiration ....................................83.3. Location URI Authorization .................................83.4. Location URI Construction ..................................94. High-Level Requirements .........................................94.1. Requirements for a Location Configuration Protocol .........94.2. Requirements for a Location Dereference Protocol ..........115. Security Considerations ........................................126. Acknowledgements ...............................................137. References .....................................................137.1. Normative References ......................................137.2. Informative References ....................................13Marshall                      Informational                     [Page 2]

RFC 5808                GEOPRIV LbyR Requirements               May 20101.  Introduction   All location-based services rely on ready access to location   information.  Location information can be used in either a direct,   Location-by-Value (LbyV) approach or an indirect, Location-by-   Reference (LbyR) approach.   For LbyV, location information is conveyed directly in the form of a   Presence Information Data Format Location Object (PIDF-LO) [RFC4119].   Using LbyV might be either infeasible or undesirable in some   circumstances.  There are cases where LbyR is better able to address   location requirements for a specific architecture or application.   This document provides a list of requirements for use with the LbyR   approach, and leaves the LbyV model explicitly out of scope.   As justification for an LbyR model, consider the circumstance that in   some mobile networks it is not efficient for the end host to   periodically query the Location Information Server (LIS) for up-to-   date location information.  This is especially the case when power   availability is a constraint or when a location update is not   immediately needed.  Furthermore, the end host might want to delegate   the task of retrieving and publishing location information to a third   party, such as to a presence server.  Additionally, in some   deployments, the network operator may not want to make location   information widely available.  These kinds of location scenarios form   the basis of motivation for the LbyR model.   The concept of an LbyR mechanism is simple.  An LbyR is made up of a   URI scheme, a domain, and a randomized component.  This combination   of data elements, in the form of a URI, is referred to specifically   as a "location URI".   A location URI is thought of as a reference to the current location   of the Target, yet the location value might remain unchanged over   specific intervals of time for several reasons.  The type of location   information returned as part of the dereferencing step may, for   example, be influenced by the following factors:   - Limitations in the process used to generate location information     mean that cached location might be used.   - Policy constraints may dictate that the location provided remains     fixed over time for specified Location Recipients.  Without     additional information, a Location Recipient cannot assume that the     location information provided by any location URI is static, and     will never change.Marshall                      Informational                     [Page 3]

RFC 5808                GEOPRIV LbyR Requirements               May 2010   The LbyR mechanism works according to an information life cycle.   Within this life cycle, location URIs are considered temporary   identifiers, each undergoing the following uses: Creation;   Distribution; Conveyance; Dereference; and Termination.  The use of a   location URI according to these various states is generally applied   in one of the following ways:   1.  Creation of a location URI, within a location server, based on       some request for its creation.   2.  Distribution of a location URI, via a Location Configuration       Protocol, between a Target and a location server.   3.  Conveyance, applied to LbyR, for example in SIP (Session       Initiation Protocol), is the transporting of the location URI, in       this case, between any successive signaling nodes.   4.  Dereference of a location URI, a request/response between a       client having a location URI and a location server holding the       location information that the location URI references.   5.  Termination of a location URI, due to either expiration or       cancellation within a location server, and that is based on a       Target cancellation request or some other action, such as timer       expiration.   Note that this document makes no functional differentiation between a   Location Server (LS), per [RFC3693], and a Location Information   Server (LIS), as shown in [RFC5687], but may refer to either of them   as a location server interchangeably.   Location determination, as distinct from location configuration or   dereferencing, often includes topics related to manual provisioning   processes, automated location calculations based on a variety of   measurement techniques, and/or location transformations (e.g., geo-   coding), and is beyond the scope of this document.   Location Conveyance for either LbyR or LbyV, as defined within SIP   signaling is considered out of scope for this document.  (See   [LOC-CONVEY] for an explanation of location conveyance for either   LbyR or LbyV scenarios.)   Except for location conveyance, the above stages in the LbyR life   cycle fall into one of two general categories of protocols, either a   Location Configuration Protocol or a Location Dereference Protocol.   The stages of LbyR Creation, Distribution, and Termination, are eachMarshall                      Informational                     [Page 4]

RFC 5808                GEOPRIV LbyR Requirements               May 2010   found within the set of Location Configuration Protocols (LCPs).  The   Dereference stage belongs solely to the set of Location Dereference   Protocols.   The issues around location configuration protocols have been   documented in a location configuration protocol problem statement and   requirements document [RFC5687].  There are currently several   examples of documented location configuration protocols, namely DHCP   [DHCP-LOC-URI], LLDP-MED [LLDP-MED], and HELD [HELD].   For dereferencing a location URI, depending on the type of reference   used, such as a HTTP/HTTPS or SIP Presence URI, different operations   can be performed.  While an HTTP/HTTPS URI can be resolved to   location information, a SIP Presence URI provides further benefits   from the SUBSCRIBE/NOTIFY concept that can additionally be combined   with location filters [LOC-FILTERS].   The structure of this document includes terminology,Section 2,   followed by a discussion of the basic elements that surround how a   location URI is used.  These elements, or actors, are discussed in an   overview section,Section 3, accompanied by a graph, associated   processing steps, and a brief discussion around the use, expiration,   authorization, and construction of location URIs.   Requirements are outlined accordingly, separated as location   configuration requirements,Section 4.1, and location dereference   requirements,Section 4.2.2.  Terminology   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 inRFC 2119 [RFC2119],   with the important qualification that, unless otherwise stated, these   terms apply to the design of the Location Configuration Protocol and   the Location Dereferencing Protocol, not its implementation or   application.   This document reuses the terminology of [RFC3693], such as Location   Server (LS), Location Recipient (LR), Rule Maker (RM), Target, and   Location Object (LO).  Furthermore, the following terms are defined   in this document:   Location-by-Value (LbyV): Using location information in the form of a      location object (LO), such as a PIDF-LO.   Location-by-Reference (LbyR): Representing location information      indirectly using a location URI.Marshall                      Informational                     [Page 5]

RFC 5808                GEOPRIV LbyR Requirements               May 2010   Location Configuration Protocol: A protocol that is used by a Target      to acquire either a location object or a location URI from a      location configuration server, based on information unique to the      Target.   Location Dereference Protocol: A protocol that is used by a client to      query a location server, based on the location URI input, and that      returns location information.   Location URI: As defined within this document, an identifier that      serves as a reference to location information.  A location URI is      provided by a location server, and is later used as input by a      dereference protocol to retrieve location information.3.  Overview of Location-by-Reference   This section describes the entities and interactions involved in the   LbyR model.            +---------+---------+   Location    +-----------+            |         |         |  Dereference  | Location  |            |      LIS/LS       +---------------+ Recipient |            |         |         |   Protocol    |           |            +----+----+----+----+      (3)      +-----+-----+                 |           *                        |                 |      Policy *                      |        Location |      Exchange *                    |   Configuration |        (*)      *                  | Location        Protocol |              +----+----+           | Conveyance           (1)   |              |  Rule   |           | Protocol                 |              |  Maker  |           |    (2)            +----+----+         +---------+           |            |         |                               |            | Target  +-------------------------------+            |         |            +---------+          Figure 1: Location Reference Entities and Interactions   Figure 1 shows the assumed communication model for both a Layer 7   location configuration protocol and a location dereference protocol.   (1) The Target (an end device) uses a location configuration protocol       to acquire a location reference from a LIS, which acts as (or is       able to access) an LS.       In the case where the Target is also a Rule Maker, the location       configuration protocol can be used to convey policy information.Marshall                      Informational                     [Page 6]

RFC 5808                GEOPRIV LbyR Requirements               May 2010       In the case where possession of a location URI is the only       required form of authorization (seeSection 3.3), a policy is       implied whereby any requester is granted access to location       information.  This does not preclude other means of providing       authorization policies.       A Target could also acquire a location URI from the LS directly       using alternative means, for example, the acquisition of a       presence Address of Record (AoR) to be used for location       information, in which case, it could be regarded as a location       URI.   (2) The Target conveys the location URI to the Location Recipient       (interface out of scope).   (3) The Location Recipient dereferences the location URI to acquire       location information from the LS.   The LS controls access to location information based on the policy   provided by the Rule Maker.   Note A.  There is no requirement for using the same protocol in (1)            and (3).   Note B.  Figure 1 includes the interaction between the owner of the            Target and the LIS to obtain Rule Maker policies.  This            interaction needs to happen before the LIS will authorize            anything other than what is allowed based on default            policies in order to dereference a location request of the            Target.  This communication path is out of scope for this            document.   Note C.  The Target might take on the role of the Location Recipient,            in which case, it could attempt to dereference the location            URI itself, in order to obtain its own location information.3.1.  Location URI Usage   An example scenario of how the above location configuration and   location dereference steps might work using SIP is where a Target   obtains a location URI in the form of a subscription URI (e.g., a SIP   URI) via a location configuration protocol.  In this case, the Target   is the same as the Recipient; therefore, the Target can subscribe to   the URI in order to be notified of its current location based on   subscription parameters.  In the example, parameters are set up for a   specific Target/Recipient along with an expressed geospatial   boundary, so that the Target/Recipient receives an updated location   notification once the boundary is crossed (see [LOC-FILTERS]).Marshall                      Informational                     [Page 7]

RFC 5808                GEOPRIV LbyR Requirements               May 20103.2.  Location URI Expiration   Location URIs may have an expiry associated with them, primarily for   security considerations, and generally in order for the LIS to keep   track of the location URIs that have been handed out, to know whether   a location URI is still valid once the LIS receives it in a request,   and for preventing a recipient of such a URI from being able to (in   some cases) permanently track a host.  Expiration of a location URI   limits the time that accidental leaking of a location URI introduces.   Other justifications for expiration of location URIs include the   ability for a LIS to do garbage collection.3.3.  Location URI Authorization   How a location URI will ultimately be used within the dereference   step is an important consideration at the time the location URI is   requested via a location configuration protocol.  The process of   dereferencing location URIs will be influenced by the specific   authorization model applied by the Location Information Server and   the URI scheme that indicates the protocol to be used to resolve the   reference to a location object.   Location URIs manifest themselves in a few different forms.  The   different ways that a location URI can be represented are based on   local policy, and are depicted in the following four scenarios.   1.  No location information included in the URI: As is typical, a       location URI is used to get location information.  However, in       this case, the URI representation itself does not need to reveal       any specific information at all.  Location information is       acquired by the dereferencing operation using a location URI.   2.  URI does not identify a Target: By default, a location URI MUST       NOT reveal any information about the Target other than location       information.  This is true for the URI itself (or in the document       acquired by dereferencing), unless policy explicitly permits       otherwise.   3.  Access control authorization model: If this model is used, the       location URI MUST NOT include any location information in its       representation.  Location URIs operating under this model could       be widely published to recipients that are not authorized to       receive this information.   4.  Possession authorization model (the URI itself is a secret): If       this model is used, the location URI is confidential information       shared between the LIS/LS, the Target, and all authorized       Location Recipients.  In this case, possession impliesMarshall                      Informational                     [Page 8]

RFC 5808                GEOPRIV LbyR Requirements               May 2010       authorization.  Because knowledge of the location URI is used to       authenticate and authorize access to location information, the       URI needs to include sufficient randomness to make guessing its       value difficult.  A possession model URI can include location       information in its representation.3.4.  Location URI Construction   Given scenarios 2 and 4, above, and depending on local policy, a   location URI may be constructed in such a way as to make it difficult   to guess.  Accordingly, the form of the URI is then constrained by   the degree of randomness and uniqueness applied to it.  In this case,   it may be important to protect the actual location information from   inspection by an intermediate node.  Construction of a location URI   in such a way as to not reveal any Target-specific information (e.g.,   user or device information), with the goal of making the location URI   appear bland, uninteresting, and generic, may be helpful to some   degree in order to keep location information more difficult to   detect.  Thus, obfuscating the location URI in this way may provide   some level of safeguard against the undetected inspection and   unintended use of what would otherwise be evident location   information, since it forces a dereference operation at the location   dereference server, an important step for the purpose of providing   statistics, audit trails, and general logging for many different   kinds of location-based services.4.  High-Level Requirements   This document outlines the requirements for a Location by Reference   mechanism that can be used by a number of underlying protocols.   Requirements here address two general types of such protocols, a   general location configuration protocol and a general location   dereferencing protocol.   The requirements are broken into two sections.4.1.  Requirements for a Location Configuration Protocol   Below, we summarize high-level design requirements needed for a   location-by-reference mechanism as used within the location   configuration protocol.   C1. Location URI support: The location configuration protocol MUST       support a location reference in URI form.       Motivation: A standardized location reference mechanism increases       interoperability.Marshall                      Informational                     [Page 9]

RFC 5808                GEOPRIV LbyR Requirements               May 2010   C2. Location URI expiration: When a location URI has a limited       validity interval, its lifetime MUST be indicated.       Motivation: A location URI may not intend to represent a location       forever, and the identifier eventually may need to be recycled,       or may be subject to a specific window of validity, after which       the location reference fails to yield a location, or the location       is determined to be kept confidential.   C3. Location URI cancellation: The location configuration protocol       MUST support the ability to request a cancellation of a specific       location URI.       Motivation: If the Target determines that a location URI should       no longer be used to dereference a location, then there should be       a way to request that the location URI be nullified.   C4. Location information masking: The location URI MUST ensure, by       default, through randomization and uniqueness, that the location       URI does not contain location-information-specific components.       Motivation: It is important to keep any location information       masked from a casual observing node.   C5. Target identity protection: The location URI MUST NOT contain       information that identifies the Target (e.g., user or device).       Examples include phone extensions, badge numbers, and first or       last names.       Motivation: It is important to protect caller identity or contact       address from being included in the form of the location URI       itself when it is generated.   C6. Reuse indicator: There SHOULD be a way to allow a Target to       control whether a location URI can be resolved once only or       multiple times.       Motivation: The Target requesting a location URI may request a       location URI that has a 'one-time-use' only characteristic, as       opposed to a location URI having multiple reuse capability.  This       would allow the server to return an error with or without       location information during the subsequent dereference operation.   C7. Selective disclosure: The location configuration protocol MUST       provide a mechanism that allows the Rule Maker to control what       information is being disclosed about the Target.Marshall                      Informational                    [Page 10]

RFC 5808                GEOPRIV LbyR Requirements               May 2010       Motivation: The Rule Maker has to be in control of how much       information is revealed during the dereferencing step as part of       the privacy features.   C8. Location URI not guessable: As a default, the location       configuration protocol MUST return location URIs that are random       and unique throughout the indicated lifetime.  A location URI       with 128 bits of randomness is RECOMMENDED.       Motivation: Location URIs should be constructed in such a way       that an adversary cannot guess them and dereference them without       having previously obtained them from the Target.   C9. Location URI options: In the case of user-provided authorization       policies, where anonymous or non-guessable location URIs are not       warranted, the location configuration protocol MAY support a       variety of optional location URI conventions, as requested by a       Target to a location configuration server (e.g., embedded       location information within the location URI).       Motivation: Users don't always have such strict privacy       requirements, but may opt to specify their own location URI or       components to be included within a location URI.4.2.  Requirements for a Location Dereference Protocol   Below, we summarize high-level design requirements needed for a   location-by-reference mechanism as used within the location   dereference protocol.   D1. Location URI support: The location dereference protocol MUST       support a location reference in URI form.       Motivation: It is required that there be consistency of use       between location URI formats used in a configuration protocol and       those used by a dereference protocol.   D2. Authentication: The location dereference protocol MUST include       mechanisms to authenticate both the client and the server.       Motivation: Although the implementations must support       authentication of both parties, any given transaction has the       option not to authenticate one or both parties.   D3. Dereferenced location form: The value returned by the dereference       protocol MUST contain a well-formed PIDF-LO document.Marshall                      Informational                    [Page 11]

RFC 5808                GEOPRIV LbyR Requirements               May 2010       Motivation: This is in order to ensure that adequate privacy       rules can be adhered to, since the PIDF-LO format comprises the       necessary structures to maintain location privacy.   D4. Location URI repeated use: The location dereference protocol MUST       support the ability for the same location URI to be resolved more       than once, based on dereference server configuration.       Motivation: Through dereference server configuration, for       example, it may be useful to not only allow more than one       dereference request, but, in some cases, to also limit the number       of dereferencing attempts by a client.   D5. Location confidentiality: The location dereference protocol MUST       support confidentiality protection of messages sent between the       Location Recipient and the location server.       Motivation: The location URI indicates what type of security       protocol has to be provided.  An example is a location URI using       a HTTPS URI scheme.5.  Security Considerations   The method of constructing the location URI to include randomized   components helps to prevent adversaries from obtaining location   information without ever retrieving a location URI.  In the   possession model, a location URI, regardless of its construction, if   made publicly available, implies no safeguard against anyone being   able to dereference and get the location.  Care has to be paid when   distributing such a location URI to the trusted location recipients.   When this aspect is of concern, the authorization model has to be   chosen.  Even in this model, care has to be taken on how to construct   the authorization policies to ensure that only those parties have   access to location information that are considered trustworthy enough   to enforce the basic rule set that is attached to location   information in a PIDF-LO document.   Any location URI, by necessity, indicates the server (name) that   hosts the location information.  Knowledge of the server in some   specific domain could therefore reveal something about the location   of the Target.  This kind of threat may be mitigated somewhat by   introducing another layer of indirection: namely the use of a   (remote) presence server.   A covert channel for protocol message exchange is an important   consideration, given an example scenario where user A subscribes to   location information for user B, then every time A gets a location   update, an (external) observer of the subscription notification mayMarshall                      Informational                    [Page 12]

RFC 5808                GEOPRIV LbyR Requirements               May 2010   know that B has moved.  One mitigation of this is to have periodic   notification, so that user B may appear to have moved even when   static.6.  Acknowledgements   I would like to thank the present IETF GEOPRIV working group chairs,   Alissa Cooper and Richard Barnes, past chairs, Robert Sparks, Andy   Newton, Allison Mankin, and Randall Gellens, who established a design   team that initiated this requirements work.  I'd also like to thank   those original design team participants for their inputs, comments,   and insightful reviews.  The design team included the following   folks: Richard Barnes, Martin Dawson, Keith Drage, Randall Gellens,   Ted Hardie, Cullen Jennings, Marc Linsner, Rohan Mahy, Allison   Mankin, Andrew Newton, Jon Peterson, James M. Polk, Brian Rosen, John   Schnizlein, Henning Schulzrinne, Barbara Stark, Hannes Tschofenig,   Martin Thomson, and James Winterbottom.7.  References7.1.  Normative References   [RFC2119]      Bradner, S., "Key words for use in RFCs to Indicate                  Requirement Levels",BCP 14,RFC 2119, March 1997.7.2.  Informative References   [DHCP-LOC-URI] Polk, J., "Dynamic Host Configuration Protocol (DHCP)                  IPv4 and IPv6 Option for a Location Uniform Resource                  Identifier (URI)", Work in Progress, March 2010.   [HELD]         Barnes, M., Winterbottom, J., Thomson, M., and B.                  Stark, "HTTP Enabled Location Delivery (HELD)", Work                  in Progress, August 2009.   [LLDP-MED]     Telecommunications Industry Association (TIA),                  "ANSI/TIA-1057 Link Layer Discovery Protocol - Media                  Endpoint Discovery", 2006.   [LOC-FILTERS]  Mahy, R., Rosen, B., and H. Tschofenig, "Filtering                  Location Notifications in the Session Initiation                  Protocol (SIP)", Work in Progress, March 2010.   [LOC-CONVEY]   Polk, J. and B. Rosen, "Location Conveyance for the                  Session Initiation Protocol", Work in Progress,                  February 2010.Marshall                      Informational                    [Page 13]

RFC 5808                GEOPRIV LbyR Requirements               May 2010   [RFC3693]      Cuellar, J., Morris, J., Mulligan, D., Peterson, J.,                  and J. Polk, "Geopriv Requirements",RFC 3693,                  February 2004.   [RFC4119]      Peterson, J., "A Presence-based GEOPRIV Location                  Object Format",RFC 4119, December 2005.   [RFC5687]      Tschofenig, H. and H. Schulzrinne, "GEOPRIV Layer 7                  Location Configuration Protocol: Problem Statement and                  Requirements",RFC 5687, March 2010.Author's Address   Roger Marshall (editor)   TeleCommunication Systems, Inc.   2401 Elliott Avenue   2nd Floor   Seattle, WA  98121   US   Phone: +1 206 792 2424   EMail: rmarshall@telecomsys.com   URI:http://www.telecomsys.comMarshall                      Informational                    [Page 14]