Network Working Group                                        J. Peterson
Request for Comments: 5606                                 NeuStar, Inc.
Category: Informational                                        T. Hardie
                                                                Qualcomm
                                                               J. Morris
                                                                     CDT
                                                             August 2009


  Implications of 'retransmission-allowed' for SIP Location Conveyance

Abstract

   This document explores an ambiguity in the interpretation of the
   <retransmission-allowed> element of the Presence Information Data
   Format for Location Objects (PIDF-LO) in cases where PIDF-LO is
   conveyed by the Session Initiation Protocol (SIP).  It provides
   recommendations for how the SIP location conveyance mechanism should
   adapt to this ambiguity.

   Documents standardizing the SIP location conveyance mechanisms will
   be Standards-Track documents processed according to the usual SIP
   process.  This document is intended primarily to provide the SIP
   working group with a statement of the consensus of the GEOPRIV
   working group on this topic.  It secondarily provides tutorial
   information on the problem space for the general reader.

Status of This Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

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Table of Contents

   1. Introduction ....................................................2
   2. Problem Statement ...............................................3
   3. Recommendation ..................................................5
      3.1. Goals ......................................................5
      3.2. Core Semantics .............................................5
      3.3. Limiting Access ............................................6
           3.3.1. Limiting Access Using Public Key Encryption .........6
           3.3.2. Limiting Access Using Location-by-Reference .........7
           3.3.3. Refraining from Including Location Information ......8
      3.4. Choosing among the Available Mechanisms ....................8
      3.5. Indicating Permission to Use Location-Based Routing
           in SIP .....................................................8
      3.6. Behavior of Back-to-Back User Agents ......................10
   4. Security Considerations ........................................10
   5. Acknowledgements ...............................................10
   6. Informative References .........................................11

1.  Introduction

   The Presence Information Data Format for Location Objects (PIDF-LO
   [RFC4119]) carries both location information (LI) and policy
   information set by the Rule Maker, as is stipulated in [RFC3693].
   The policy carried along with LI allows the Rule Maker to restrict,
   among other things, the duration for which LI will be retained by
   recipients and the redistribution of LI by recipients.

   The Session Initiation Protocol [RFC3261] is one proposed Using
   Protocol for PIDF-LO.  The conveyance of PIDF-LO within SIP is
   specified in [LOC-CONVEY].  The common motivation for providing LI in
   SIP is to allow location to be considered in routing the SIP message.
   One example use case would be emergency services, in which the
   location will be used by dispatchers to direct the response.  Another
   use case might be providing location to be used by services
   associated with the SIP session; a location associated with a call to
   a taxi service, for example, might be used to route to a local
   franchisee of a national service and also to route the taxi to pick
   up the caller.

   Some ambiguities have arisen in the interpretation of Rule Maker
   policy when PIDF-LO is conveyed by SIP.  The following sections
   explore the problem and provide a recommendation.

2.  Problem Statement

   The <retransmission-allowed> element of RFC 4119 was designed for use
   in an environment like that of Section 4 of RFC 3693, in which
   Location Information (LI) propagates from a Location Generator
   through a Location Server (LS) to a Location Recipient (LR).  In this
   architecture, it is the responsibility of the Location Server to act
   on the rules (policy) governing access control to LI, which are in
   turn set by the Rule Maker.  The most important of these
   responsibilities is delivering LI to authorized Location Recipients
   and denying it to others.  Internal to [RFC4119]-compliant location
   objects (LOs) are additional privacy rules which are intended to
   constrain Location Recipients.  These include the <retransmission-
   allowed> element.  This element is intended to prevent a compromise
   of privacy when an authorized recipient of LI shares that LI with
   third-party entities, principally those who are not authorized by the
   Rule Maker to receive LI.  For example, a user might be willing to
   share their LI with a pizza shop, but they might not want that pizza
   shop to sell their LI to a targeted advertising company that will
   contact the user with coupons for a nearby hair salon.

   Bear in mind, however, that <retransmission-allowed> is not intended
   to provide any protocol-level mechanism to prevent unauthorized
   parties from learning location through means like eavesdropping.  It
   is merely a way to express the preferences of the Rule Maker to the
   LR.  If the LR were, for example, legally bound to follow the privacy
   preferences expressed by Rule Makers, then they might incur liability
   if they ignored the <retransmission-allowed> parameter.  No further
   privacy protection is assumed to be provided by <retransmission-
   allowed>.

   There is a use case for LI that involves embedding it in a SIP
   request that will potentially traverse multiple SIP intermediaries
   before arriving at a user agent server (UAS).  In this use case, one
   or more intermediaries might inspect the LI in order to make a SIP
   routing decision; we will hereafter refer to this as location-based
   routing.  Common examples could include emergency services and other
   more mundane cases where the originator of a SIP request wants to
   reach a service in proximity to a particular geographic location,
   such as contacting a nearby pizza shop.  In both such cases, the UAC
   may intend for selected intermediaries and the UAS to have access to
   the LI.  In the pizza case, for instance, the user agent client (UAC)

   shares an address both for location-based routing and additionally so
   that the pizza shop reached by that routing has the address to which
   a pizza should be sent.

   This location-based routing use case for LI has a number of important
   disconnects from the RFC 3693 model.  Unlike the RFC 3693 model,
   there is no LS designating to which specific entities LI will be
   sent.  There may be multiple intermediaries between the UAC and UAS,
   some of which will need or want to inspect LI (which would seem to
   qualify them as LRs) and some of them will not.  While SIP proxy
   servers generally are not [RFC4119]-aware and do not need to inspect
   SIP request bodies in order to perform their function, nothing
   precludes proxy servers inspecting or logging any SIP message bodies,
   including LI.  Furthermore, it is very difficult for the UAC to
   anticipate which intermediaries and which eventual UAS a SIP request
   might reach.

   This architecture is further complicated by the possibility of
   sending location information by-reference, that is, placing a URL
   where LI can be retrieved in SIP requests instead of using a PIDF-LO
   body (commonly called including the PIDF-LO by value).  Depending on
   the qualities of a reference, further authorization checks may be
   performed before LI is retrieved, LI may be customized depending on
   who is asking, and so forth.  As will be discussed in greater detail
   below, the conveyance of a reference may have very different privacy
   properties than conveying a PIDF-LO body by-value in a SIP request.

   In this architecture, the question of who is an "authorized
   recipient" from the point of view of the Rule Maker has been muddy.

   The SIP elements along the path are authorized to receive and forward
   the SIP message; does that make them automatically authorized
   recipients of the LI it contains?  The final target of the SIP
   message will receive the LI along with other information, but it may
   be different than the initial target in a variety of scenarios; is it
   authorized to read the LI?

   These questions and concerns are particularly problematic when
   <retransmission-allowed> is set to "no" (the default case).  This
   core concern might be put as "to whom does <retransmission-allowed>
   apply in location-based routing?"  More specifically:

   Is any entity that reads LI bound by <retransmission-allowed>?  If
   so, does that mean a proxy that performs location-based routing is
   unable to forward a request and complete a SIP call if
   <retransmission-allowed> is "no"?  Alternatively, must they strip the
   location body from the message in order to complete the call?

   If the proxy does not understand RFC 4119, it may forward a SIP
   message containing a policy statement <retransmission-allowed> set to
   "no".  Is any proxy that does understand RFC 4119 required to parse
   the LI for this statement, even if it would not do so in order to
   route the message?

   Is there a need for SIP-level indications regarding retransmission
   for the benefit of entities that do not understand RFC 4119?

   Since the UAC cannot anticipate who may receive a SIP request, how do
   we understand who the intended LR is in the location-based routing
   case?  Can a UAC have intended for there to be multiple serial LRs in
   a transmission?  If so, if one LR is authorized to retransmit to
   another LR, how will it know it is not also authorized to transmit LI
   to other third parties (i.e., how will the serial LRs know to whom
   they are authorized to retransmit)?  How could all of this be
   designated?

3.  Recommendation

   The following sections provide a recommendation for how the
   <retransmission-allowed> flag should be understood in a SIP
   environment.  The core semantics of this recommendation represent the
   consensus of the GEOPRIV working group.  While Section 3.5 proposes a
   syntax that might be adopted by the SIP WG to implement these
   semantics in its protocol, the actual syntax of SIP is the
   responsibility of the SIP WG.

3.1.  Goals

   After extensive discussion in both GEOPRIV and SIP contexts, there
   seems to be consensus that a solution for this problem must enable
   location-based routing to work even when the <retransmission-allowed>
   flag is set to "no".  A solution should also give the Rule Maker the
   ability to allow or forbid the use of LI for location-based routing
   and the ability to allow or forbid the use of LI for the consumption
   of the endpoint.

3.2.  Core Semantics

   Consensus has emerged that any SIP entity that receives a SIP message
   containing LI through the operation of SIP's normal routing
   procedures or as a result of location-based routing should be
   considered an authorized recipient of that LI.  Because of this
   presumption, one SIP element may pass the LI to another even if the
   LO it contains has <retransmission-allowed> set to "no"; this sees
   the passing of the SIP message as part of the delivery to authorized
   recipients, rather than as retransmission.  SIP entities are still

   enjoined from passing these messages outside the normal routing to
   external entities if <retransmission-allowed> is set to "no", as it
   is the passing to third parties that <retransmission-allowed> is
   meant to control.

   This architecture is considerably different from the presumptions of
   RFC 3963, in that authorized recipients pass the LO on to other
   authorized recipients, but it seems to be the most sensible mechanism
   given SIP's operation.

   To maintain the Rule Maker's ability to affect the consumption of
   this information, two different mechanisms may be used to limit the
   distribution of LI and one may used to limit the sphere in which it
   may be used; these are discussed below.

3.3.  Limiting Access

3.3.1.  Limiting Access Using Public Key Encryption

   One way of limiting access to LI is to encrypt the PIDF-LO object in
   a SIP request.  If the originator knows which specific entity on the
   path needs to inspect the LI, and knows a public key for that entity,
   this is a straightforward matter.  It is even possible to encrypt
   multiple instance of PIDF-LO, containing different policies or levels
   of location granularity, in the same SIP request if multiple entities
   along the path need to inspect the location.

   This is most likely to be effective in cases where the originator
   does not wish the LI to be inspected by intermediate entities and has
   the public key for the target of the SIP message, as it is very
   difficult for the originator to anticipate the intermediaries through
   which a SIP message will pass.  It may also be useful in limited
   environments where the originator has a trust relationship with a
   specific SIP element (e.g., a "home" or first-hop proxy) and it wants
   to reveal that LI only to that element.

   Note that even in the case where the originator intends to encrypt LI
   for the benefit only of the target of the message, it may be quite
   difficult to anticipate the eventual endpoint of the message.  These
   encrypted LIs will not be useful in any case where the anticipation
   of the originators is not met.

   An additional problem posed by this approach is that it requires some
   sort of public key discovery system, which compounds the operational
   complexity significantly.  While this method is included for
   completeness, it is the consensus of the working group that the
   deployment scenarios in which this is appropriate will be relatively
   few; we do not believe it is an appropriate baseline approach.

3.3.2.  Limiting Access Using Location-by-Reference

   Another, more feasible approach is leveraging location by reference.
   When a SIP request conveys a reference, it cannot be properly said to
   be conveying location; location is conveyed upon dereferencing the
   URI in the question, and the meaning of <retransmission-allowed> must
   be understood in the context of that conveyance, not the forwarding
   of the SIP request.

   The properties of references, especially the security properties,
   vary significantly depending on the nature and disposition of the
   resource indicated.  Clearly, if the referenced PIDF-LO is available,
   in the same form, to any entity along the SIP signaling path that
   requests it, then inserting a reference has no advantages over
   inserting LI by value (and introduces wasteful complexity).  However,
   if the Rule Maker influences the results of the dereferencing
   process, including determining who can receive LI at what degree of
   granularity and what policies are bound with the LI, the security
   properties are different.

   It might superficially appear that this suffers from the same
   problems as the encryption approach, since the Rule Maker must
   anticipate a set of entities who are authorized to receive location
   information.  The difference is that this set does not need to be
   communicated in the SIP request in order for authorization decisions
   to be made.  There is a world of difference between managing a
   whitelist of a thousand parties that might ask for LI and sending a
   SIP request containing a thousand differently encrypted adumbrations
   on LI -- the former is commonplace and the latter is impossible.
   Additionally, some Rule Maker policies might not even require the
   establishment of an exhaustive whitelist.  For example, it may be
   that there exists a finite set of commercial requestors that the Rule
   Maker would like to block, in a manner similar to the way ad-blockers
   operate in modern web browsers.

   In any system where one makes an authorization decision, a certain
   cost in authentication must be paid -- the greater the assurance the
   greater the cost.  The precise cost will of course depend on the URI
   scheme of the reference.  For SIP, Digest has a low computational
   cost but requires pre-established keys, which limits applicability.
   RFC 4474 Identity does not require any pre-association, but it does
   make signaling more heavyweight and requires the deployment of
   additional features in the network, including a web-like public key
   infrastructure (PKI).

   But even if no authentication takes place, in the Location-by-
   Reference (LbyR) case the meaning of <retransmission-allowed> is
   unambiguous -- each entity to which LI is conveyed in the dereference

   process is bound by the retransmission policy.  The cost of the
   reference itself is of course the server that maintains the resource.
   While not every SIP client has access to an appropriate server for
   this purpose, the fact that PIDF-LO builds on the typical SIP
   presence service makes this less implausible than it might be.
   Moreover, the LbyR approach casts the conveyance architecture in a
   manner familiar from RFC 3693, with a Location Server receiving
   requests from Location Recipients, which may be accepted or denied.
   This allows the preservation of the original semantics of
   <retransmission-allowed>.

3.3.3.  Refraining from Including Location Information

   The most fundamental mechanism for limiting access to location
   information is simply not including it.  While location-based routing
   might conceivably occur in almost any SIP message in the future,
   there is no requirement that location be included in the general case
   to support it.  If it is not included and is required, an appropriate
   error indicating the lack may be returned and the choice made to
   continue communication with the information included.  This challenge
   and response will slow the establishment of communication when it is
   required, but it is the most basic way to ensure that location
   distribution is limited to the times when it is required for
   communication to proceed.

3.4.  Choosing among the Available Mechanisms

   Refraining from including location is the most appropriate choice for
   systems that do not wish to reveal location to any party in the SIP
   path.

   Location-by-Reference is generally recommended as the most deployable
   mechanism for limiting access to LI which is passed via a SIP
   message.  It is significantly easier to deploy than public key
   discovery systems, allows for both whitelists and blacklists, and can
   scale in ways that the inclusion of multiple encrypted bodies cannot.
   Encryption may be used in a limited set of circumstance where
   location-by-value must be used.

3.5.  Indicating Permission to Use Location-Based Routing in SIP

   The discussion in Section 3.3.2 describes 3 mechanisms for limiting
   the distribution of LI to specific entities.  There remains the
   problem of limiting the use to which LI included by value or by
   reference may be put.  In order to meet the need to limit that use,
   this document recommends the creation of a syntactical element in SIP
   to carry this information.  As an exemplary concrete proposal, we
   recommend a "Location-Routing-Allowed" header as described below.

   When "Location-Routing-Allowed" is set to "Yes", the Rule Maker is
   indicating permission to use the included LI for location-based
   routing.  When "Location-Routing-Allowed" is set to "No", the
   originator is indicating that this use is not permitted.  "Location-
   Routing-Allowed" being set to "No" has no protocol-level mechanism
   for enforcement of this behavior; like the PIDF-LO <retransmission-
   allowed> being set to "no", it is a way for the Rule Maker to express
   a preference to the SIP elements, which are LI recipients.  It may,
   however, present a significant optimization.  Where a location-by-
   reference is included with "Location-Routing-Allowed" set to "No",
   the SIP elements along the path know that they do not need to attempt
   to dereference the location information; this is significantly faster
   than attempting the dereference and being denied at the
   authentication stage.

   We recommend that "Location-Routing-Allowed" be made mandatory-to-
   implement for elements complying with [LOC-CONVEY].

   We recommend that it appear in any SIP message that contains a
   location, whether by reference or by value.

   We recommend that any SIP message containing a location but no
   "Location-Routing-Allowed" header should be treated as containing a
   "Location-Routing-Allowed" header set to "no".

   We recommend that a UA be allowed to insert a "Location-Routing-
   Allowed" header even when it has not included a location, in order to
   set the policy for any locations inserted by other SIP elements.

   This allows the UA to assert that it is a Rule Maker for locations,
   even when the network architecture in which the UA is present inserts
   the location into SIP messages after the UA has originated the SIP
   exchange.

   We recommend that any SIP element inserting a location, whether by
   reference or by value, insert a "Location-Routing-Allowed" header if
   one is not already present.  If one is present, it should not be
   overridden by the SIP element inserting the location.

   We recommend that any SIP element not the originator of a message and
   not inserting a location be enjoined from inserting a "Location-
   Routing-Allowed" header.

3.6.  Behavior of Back-to-Back User Agents

   Back-to-back user agent (B2BUA) behavior is often difficult to
   proscribe.  There are many uses of B2BUAs, and the rules that apply
   to location would depend on the actual use case.  This section
   suggests what any SIP mechanism arising from this document might wish
   to consider with regard to B2BUA behavior.

   In most uses of B2BUAs, they act as a simple intermediary between the
   nominal originating and nominal terminating UAs, that is, a proxy
   that does something proxies aren't allowed to do.  In such cases, the
   B2BUA must conform to any new routing-allowed mechanism if it chooses
   an outgoing route.  As this document advises proxies,
   <retransmission-allowed> does not apply to the B2BUA in this case,
   and the B2BUA must copy the LI, the new routing-allowed, and existing
   <retransmission-allowed> values.

   Where the B2BUA in fact does act as an endpoint (terminating the
   session and originating a different session), <retransmission-
   allowed> applies to it, and it must not copy location if
   <retransmission-allowed> is "no".  If it chooses a route for the
   outgoing leg, any new routing-allowed mechanism applies to it.

   Encryption lets the originator control who, including B2BUAs, is
   allowed to see location.  On the other hand, using encryption with
   LI, which is needed for routing, is problematic, in that it is often
   difficult to know in advance which elements do location-based
   routing.  Similarly, using Location-by-Reference instead of location-
   by-value provides additional control to the originator over B2BUA
   behavior by controlling who can dereference.  See Section 3.4 for
   more guidance on this trade off.

4.  Security Considerations

   The privacy and security implications of distributing location
   information are the fundamental subject of this document.

5.  Acknowledgements

   James Polk provided a series of questions regarding the specifics of
   the Location-Routing-Allowed mechanism, and this resulted in the
   recommendations in Section 3.4.  Thanks to Brian Rosen for the text
   on B2BUAs.

6.  Informative References

   [LOC-CONVEY] Polk, J. and B. Rosen, "Location Conveyance for the
                Session Initiation Protocol", Work in Progress, March
                2009.

   [RFC3261]    Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
                A., Peterson, J., Sparks, R., Handley, M., and E.
                Schooler, "SIP: Session Initiation Protocol", RFC 3261,
                June 2002.

   [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.

Authors' Addresses

   Jon Peterson
   NeuStar, Inc.

   EMail: jon.peterson@neustar.biz


   Ted Hardie
   Qualcomm

   EMail: hardie@qualcomm.com


   John Morris
   Center for Democracy & Technology

   EMail: jmorris@cdt.org