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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modifications of such material outside the IETF Standards Process.
Without obtaining an adequate license from the person(s) controlling
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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.
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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)
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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?
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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
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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.
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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
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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.
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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.
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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.
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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
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