Subject: [Geopriv] review of <draft-winterbottom-location-uri-00.txt>

From: "Tschofenig, Hannes" <[email-masked]>
Date: Wed, 27 Jul 2005 15:02:27 +0200

hi james, 
hi martin, 
hi jon,
 
thanks for working on this document. it provides a very good start for
discussions. 

as a high-level comment i think location-by-reference is a good approach
that should be supported to make the geopriv architecture complete. 

please find some comments inline (marked with [hannes]):

GEOPRIV WG                                               J. Winterbottom
Internet-Draft                                                M. Thomson
Expires: January 12, 2006                                         Nortel
                                                             J. Peterson
                                                           NeuStar, Inc.
                                                           July 11, 2005

                  Rationale for Location by Reference
                 draft-winterbottom-location-uri-00.txt

Status of this Memo

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Copyright Notice

   Copyright (C) The Internet Society (2005).

Abstract

   This document motivates the provisioning of location by-reference
   within the GEOPRIV architecture.  Location by-reference takes the
   form of a URI in contrast to provisioning a static location.

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Table of Contents

   1.   Introduction . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.   Definition of Terms  . . . . . . . . . . . . . . . . . . . .   4
   3.   Location By Reference  . . . . . . . . . . . . . . . . . . .   5
   4.   Access Technology Agnostic . . . . . . . . . . . . . . . . .   7
   5.   Access Topologies  . . . . . . . . . . . . . . . . . . . . .   8
   6.   Location-Reference Benefits  . . . . . . . . . . . . . . . .   9
   7.   Security Considerations  . . . . . . . . . . . . . . . . . .  10
   8.   References . . . . . . . . . . . . . . . . . . . . . . . . .  10
        Authors' Addresses . . . . . . . . . . . . . . . . . . . . .  10
        Intellectual Property and Copyright Statements . . . . . . .  12

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1.  Introduction

   Circumstances exist where pre-configuring an IP end-point with
   location and having the end-point communicate that location to other
   entities is undesirable and/or unsatisfactory.

[hannes] what are these circumstances? i know some but i wonder whether
we have the same scenarios in  mind. 

  This document
   describes real network configurations that pose challenges to pre-
   configured location solutions, and suggests how a location by-
   reference can ubiquitously be used to overcome these circumstances
   and ensure that an end-point's location is current, accurate,
   dependable and secure.

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2.  Definition of Terms

      Location-Reference: A URI that when accessed by an authorized
      entity will yield the location of the end-point to which the
      reference is associated.

      Location Dependability: refers to the level of confidence that
      consumers of the location can, or are prepared, to place in the
      location being accurate and attributable to an end-point

      Location Currency: refers to how applicable the provided location
      is when compared to where the end-device is as the time of a
      location request.  That is, how current is the provided location.

      Location-Key: mechanism used to provide a reference to a location.
      This may be a URI, a URL or some other kind of identifier.

[hannes] isn't the location-reference and the location-key the same. i
like the term location-reference  since keys tend to have a different
meaning (particularly for security folks). 

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3.  Location By Reference

   The concept of location by-reference, also referred to as a location-
   key, is to provide an end-point with a URI that when accessed will
   provide the location of the end-point.  Such a mechanism provides a
   number of advantages that are not achievable through existing by-
   value location provisioning solutions.  The advantages provided
   include location binding, location dependability, location currency,
   session decoupling and intransitive trust of location.

   Location binding is achieved using location references by forcing the
   requesting node to ask the access network or other location provider
   about the location of a specified end-device.

[hannes] i think that this issue not necessarily related to location by
reference or to location by  value. 
you can work on a solution that provides this functionality for both
approaches. 

  In this situation the
   location generator must be able to identify the end-device,

[hannes] what is the identity of the end device? 

 based on
   the provided reference, and subsequently determine its location.
   This has the desired affect of ensuring that the location provided is
   for the end-device about which the request was made.  Similarly a
   query response mechanism of this nature moves towards providing
   better location currency, and because the location is provided
   directly from the location generator the dependability of the
   location is of a higher degree also.

   Location references support session decoupling by allowing the end-
   point to distribute the location reference to third parties so that
   they may obtain the end-point's location without the end-point
   needing to maintain a session with the third party.

[hannes] hmm. i am not quite sure what this sentence means? 
if you pass location by reference then you do not have this property as
well. 
additionally, what state do you plan to maintain (or not to maintain)
and what is the "session  decoupling"? 

  This may be
   realized through a SIP subscription or other mechanism, but is
   something that is not possible where location information may only be
   conveyed in-band between the end-point and the third party.

[hannes] maybe you might want to describe the full approach or avoid the
last statement altogether. 
it might just confuse people 

   Location by-reference support intransitive trust because the consumer
   of location information forms a direct connection to the generators
   of location information without using the endpoint as an
   intermediary.

[hannes] i would avoid the term 'trust'. what you would like to say is:
"unlike the approach choosen for  location information distribution with
dhcp the end point is not able to modify the location  information." 

  Consumers of location information that have strong
   need to trust the veracity of location information are likelier to
   have stronger trust relationships with the generators of location
   information than they are with endpoints.

[hannes] i think that this is true but largely independent of
location-by-value vs. location-by-value. 
i know that the currently provided solution using dhcp does not provide
these properties. 

  Furthermore, some
   generators of location information might not want to disclose this
   information directly to endpoints for various business-model reasons,
   but might be perfectly willing to give this information to trusted
   requestors.

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             +--------------------+   Third Party-Location-Request
             |                    |<----------------------------+
             | Location Generator |                             |
             |                    |---- Current Location --+    |
             +--------------------+                        |    |
               ^             |                             V    |
               |             |                      +--------------+
      Location |             | Location-ref         |              |
      Request  |             |                      | Third Party  |
               |             |                      |              |
               |             V                      +--------------+
            +-------------------+                          ^
            |                   |      Location-ref        |
            |    End-Device     |--------------------------+
            |                   |
            +-------------------+

[hannes] figure name missing (including a reference to the figure).

   Unlike in-band location conveyance,

[hannes] what is in-band location conveyance? i guess you mean
location-by-value here. 

 where if one intermediary node is
   able to see the contents of a message then all may, a location
   reference may limit the disclosure of a user's location to a limited
   subset of network nodes, rather than all intermediary nodes, based on
   a authorization level.  That is, location can be provided on a need
   to know basis.  This has several key advantages particularly for SIP
   and similar session based protocols.  Firstly, it allows specific
   intermediary nodes that require access to location in order to
   determine routes and so on to obtain the information, but it may
   restrict the final destination from obtaining the end-device's
   location if appropriate.

[hannes] this is true but the problem is to setup the infrastructure for
authorization. 
this is probably the hard part here. 

  Secondly, it allows an end-device the
   confidence to always send a location reference with all session
   initiations, so that if location is required by an authorized node it
   will have access to the information to complete the service.  The
   converse to this would be that either the end-device must know in
   advance to provide this information, or it would need to be prompted
   during the session initiation to provide it.

[hannes] this approach certainly has some privacy implications. in some
cases it might just be more  privacy safe to have someone to ask me. 

  Providing this sort of
   selective disclosure with by-value provisioning of location
   information is extremely difficult (requiring separate encrypted
   bodies for separate anticipated recipients) and in some cases simply
   impossible (where recipients cannot be anticipated by endpoints).

[hannes] certainly true but the hard part in the other case is to setup
the authorization policies  properties to ensure that only authorized
nodes have access to location information. 

   The existing PIDF-LO structure is relatively large and can easily
   exceed several hundred bytes.  A location reference on the other
   hand, being a URI, will likely be small enough to be accommodated in
   signalling message headers.  Small structures are generally more
   easily processed by high capacity switching devices, leading to a
   more efficient message traversal through the network.

[hannes] i think that the last sentence is not valid particularly if you
have to resolve the  location-reference. 

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4.  Access Technology Agnostic

   The way in which an end-point accesses a network will have a direct
   impact on how the location of the end-point is determined, and on the
   level of precision shared with the network.  The freedom of movement
   allowed to an Internet endpoint once it has connected to the network
   impacts the most appropriate time to determine its location.  In
   networks where freedom of movement is tightly constrained, variations
   in location currency are less likely to be an issue than in highly
   mobile environments.  In networks that allow large amounts of
   movement, for example WiMAX where freedom of movement can be measured
   in kilometers, it is far less easy to provide any assurances about
   the currency of location at any given point in time without re-
   measuring it.  Coupling the provisioning of static location
   information with network-layer autoconfiguration (e.g., DHCP) is
   therefore applicable to only certain environments, and other
   environments require a different solution.

   Maintaining accurate and current locations for highly mobile end-
   points in a network is challenging.  Continually or periodically re-
   measuring the location of each end-point and then notifying or
   pushing this new location to the end-point is expensive and generally
   unnecessary.  In such networks a location-reference can be used
   deferring location determination until the time that the location is
   actually required, when the reference is accessed.  Such a mechanism
   reduces overall network load, and provides requestors and consumers
   of location information a more current location with greater
   assurances as to its authenticity.  By contrast, by-value
   provisioning in these sorts of environments would result in continual
   and probably unnecessary updates to all endpoints for the benefits of
   the few that will ultimately require location-based services.

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5.  Access Topologies

   In addition to access technologies having an impact on location, so
   too network topologies have an impact on location determination and
   acquisition.  It is not uncommon for broadband services to be
   provided to end-consumers through infrastructure owned and operated
   by several different organizations.  Such networks generally do not
   comprise of single DHCP servers, but rather use AAA and RADIUS
   devices, often in proxy configurations, to obtain and distribute
   configuration information.  Modifying such systems to include native
   support for location acquisition proliferates a problem that is
   already starting to take hold in a number of layer 2 based protocols.
   An alternative approach is to provide a location reference mechanism
   that can be common across all access configuration protocols allowing
   simplification and solution consolidation.

[hannes] my impression is that this discussion is not related to
location-by-reference but rather a  motiviation for providing location
information at a higher layer (and not dhcp).

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6.  Location-Reference Benefits

      Location not explicitly included in provisioning messages, but
      available to authorized nodes (which may or may not include the
      provisioned endpoint).

      Smaller messages traverse much of the network, larger location
      components only retrieved as required.  Less load on access
      network elements as location is only generated and supplied as
      required.

      End-devices do not need to maintain a trust relationship with the
      location recipient in order for the location recipient to obtain
      the end-device's location.

[hannes] again, i would avoid the term trust here. 
this aspect, however, raises the question who should control access to
location information. 

      Location consumers obtain location directly from the generator for
      a given end-point increasing location dependability and reducing
      unnecessary transitive trust (where recipients would have to trust
      the consumer "through" a user-controlled endpoint).

[hannes] see my previous comment about transitive trust. 

      Location is determined at request time resulting in more current
      locations being returned, especially in dynamic environments where
      location might change over time.

      Can be easily integrated into existing presence architectures to
      make use of existing privacy, authorization and authentication
      techniques.

[hannes] sure. from the geopriv policy framework point of view it does
not matter whether you get the  request for location by value or by
reference. however, i think that there is a difference in the overall
system. 

      Compensates well for network-layer tunnels etc as location-
      reference can be provided at the local level.

[hannes] i am not sure that the aspect of per-value vs. per-reference
relates to this aspect somehow. 

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7.  Security Considerations

   There maybe some concern about location-reference theft and
   subsequent replay attacks.  Where they are used with protocols that
   have good support for identity and authentication management they are
   likely to be far more secure than currently proposed LCI and in-band
   location conveyance.  In addition to protocol security mechanisms,
   additional precautions may be taken such as single usage, limited
   usage, and timed usage references being used.

[hannes] i think that this document could discuss more security aspects.
in particular, i would like to  see a discussion about the replay
attacks, authorization policies, key management when it comes to
authentication between the the location recipient and the location
server and the target and the location  server.  

8.  References

[hannes] split references into the informative and the normative part. 

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [I-D.ietf-geopriv-pidf-lo]
              Peterson, J., "A Presence-based GEOPRIV Location Object
              Format", draft-ietf-geopriv-pidf-lo-03 (work in progress),
              September 2004.

   [W3C.REC-xml-exc-c14n-20020718]
              3rd, D., Boyer, J., and J. Reagle, "Exclusive XML
              Canonicalization Version 1.0", W3C REC REC-xml-exc-c14n-
              20020718, July 2002.

~snip~

ciao
hannes

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