SIMPLE M. Thomson
Internet-Draft Andrew
Intended status: Informational October 23, 2008
Expires: April 26, 2009
Requirements for the Support of Continuously Varying Values in Presence
draft-thomson-simple-cont-presence-val-req-00
Status of this Memo
By submitting this Internet-Draft, each author represents that any
applicable patent or other IPR claims of which he or she is aware
have been or will be disclosed, and any of which he or she becomes
aware will be disclosed, in accordance with Section 6 of BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/ietf/1id-abstracts.txt.
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html.
This Internet-Draft will expire on April 26, 2009.
Thomson Expires April 26, 2009 [Page 1]
Internet-Draft Continuous Presence Reqs. October 2008
Abstract
The attributes of continuous-valued data are examined in respect to
presence systems. The limitations of the existing presence system
with respect to continuous-valued data is examined. Requirements are
formulated that would enable the use of the presence system for this
data, with an emphasis on providing the watcher with a means of
control over the measurement process.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Conventions used in this document . . . . . . . . . . . . 4
2. Continous-Valued Data and Measurement . . . . . . . . . . . . 5
3. Logical Model: Presence Sources . . . . . . . . . . . . . . . 7
4. Problem Statement . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Unguided Measurement . . . . . . . . . . . . . . . . . . . 9
4.2. Presence Filters . . . . . . . . . . . . . . . . . . . . . 9
4.3. Quality . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.4. Watcher Feedback . . . . . . . . . . . . . . . . . . . . . 11
4.5. Active and Passive Sources . . . . . . . . . . . . . . . . 12
4.6. Triggering Measurement . . . . . . . . . . . . . . . . . . 12
4.6.1. Immediate Triggering of Measurement . . . . . . . . . 13
4.6.2. Periodic Triggering of Measurement . . . . . . . . . . 13
4.6.3. Value-based Triggering of Measurement . . . . . . . . 14
5. Requirements . . . . . . . . . . . . . . . . . . . . . . . . . 15
5.1. Quality Requirements . . . . . . . . . . . . . . . . . . . 15
5.2. Immediate Triggering Requirements . . . . . . . . . . . . 15
5.3. Periodic Triggering Requirements . . . . . . . . . . . . . 16
5.4. Value-Seeking Requirements . . . . . . . . . . . . . . . . 17
5.5. Timeliness Requirements . . . . . . . . . . . . . . . . . 17
6. Security Considerations . . . . . . . . . . . . . . . . . . . 19
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 20
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 21
8.1. Normative References . . . . . . . . . . . . . . . . . . . 21
8.2. Informative References . . . . . . . . . . . . . . . . . . 21
Appendix A. Presence Agent and Source Interactions . . . . . . . 23
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 24
Intellectual Property and Copyright Statements . . . . . . . . . . 25
Thomson Expires April 26, 2009 [Page 2]
Internet-Draft Continuous Presence Reqs. October 2008
1. Introduction
The provision of continuously varying parameters using presence
requires specific support by the presence infrastructure [RFC2778].
Existing methods assume that perfect information is always available
to the Presence Agent (PA). These methods rely on filtering, which
limit the data provided to a watcher, but do not provide the watcher
any control over the quality of the actual data. The current
presence system is ill-equipped to handle imperfect data.
Perfect information is available with no delay; perfect information
is absolutely accurate. The absence of perfectly accurate
information, can be inversely characterized as the existence of
uncertainty. The measurement of any physical property is subject to
some degree of uncertainty. The impact of this depends on the
purpose that the information is intended for. If a low standard of
quality is demanded of the data, the existence of uncertainty might
be ignored without negative consequences. However, in some cases
uncertainty can be significant to the intended use of the
information.
Timely availability of information also affects the service provided
by the PA. For a continuously varying value, the amount of time
required to acquire a value increases as the required accuracy
increases. The relationship between time and accuracy is likely to
be non-linear; gains in accuracy follow the law of diminishing
returns. Futhermore, as an actual value changes over time, the value
provided to a watcher could be invalidated by the time the watcher
receives the information.
Dealing with imperfect information using presence requires additional
protocol support. To properly support continuous-valued data the
presence system needs to provide a way for a watcher to indicate
their preferences for data quality and timeliness. This document
outlines requirements for presence that enable the use of
continuously varying parameters.
Location information is used throughout this document as an exemplary
example of a continuous-valued datum. The requirements in this
document are intended to provide a basis for the definition of
presence features that support location while giving due
consideration to support of other continuous variables.
This document specifies requirements that might already be covered
by existing publications. However, to ensure the integrity of the
overall concept, those requirements are retained.
Thomson Expires April 26, 2009 [Page 3]
Internet-Draft Continuous Presence Reqs. October 2008
1.1. Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
Paragraphs that are indented like this one contain motivation,
examples, speculation and other such non-normative gibberish.
Thomson Expires April 26, 2009 [Page 4]
Internet-Draft Continuous Presence Reqs. October 2008
2. Continous-Valued Data and Measurement
Continuous-valued data are defined by this document as having a
continuous value space. Therefore, the set of possible values for
any Continuous-valued datum is infinite. In contrast, many existing
presence elements are discrete-valued. Discrete-valued elements have
a limited number of possible values. Continuous-valued data is most
regularly numerical, or is composed of numerical data.
Physical properties are a prime example of the use of continuous-
valued data. Any value expressed in the SI units (metres, kilograms,
seconds, ampere, kelvin, mole or candela) and units derived from
these are within the scope of the general remit of this document.
Note: For those with an interest in physics, this document does not
concern itself with quantum effects. At the point that presence
concerns itself with quantum physics, this document is most likely
long redundant.
Examples of continuous-valued presence elements could include
descriptions of the physical characteristics of a presentity or its
immediate environment. This could include position in space; weather
related values such as ambient temperature and wind speed; light or
noise intensity; remaining battery life of a device; weight and size.
In contrast, the many elements defined in RFC 4480 [RFC4480] are all
discrete-valued.
The act of determining a continuous value is an act of measurement.
The value that is represented in a presence document is the product
of a measurement process. All measurement processes are subject to
measurement uncertainty, a well-documented phenomenom [ISO.GUM].
This document applies to the inclusion of continuous-valued data in
presence, with any of the following constraints:
1. The process of measurement has non-zero--or at least non-
negligible--cost. Cost can be defined in any number of ways, but
could include time, processing resources, network utilisation,
labour or financial.
2. The measurement process has limited accuracy, resulting in
uncertainty that is--or could be--significant.
Cost and accuracy are somewhat related. In general, improvements in
one result in degradation of the other. For continuous-valued
information that is not subject to these constraints, some
requirements from this document could still apply.
Thomson Expires April 26, 2009 [Page 5]
Internet-Draft Continuous Presence Reqs. October 2008
From the above constraints, one aim of any system that handles such
continuous-valued data is to minimise both cost and uncertainty using
different trade-offs. A common factor amoungst systems that handle
continuous-valued data is the desire to be able to ignore the effects
of these constraints. Where ignorance is not viable, systems exhibit
a range of traits: iterative measurement collection, caching of
results, fuzzy logic, maximum likelihood estimation and other forms
of statistical analysis.
A significant example of a continuously varying parameter is the
position of an entity in space, or its location. Location
information as an element of presence was established in [RFC4079]
and a format specified in [RFC4119]. Uncertainty in location
information, described in more detail in
[I-D.thomson-geopriv-uncertainty], is a product of the method of
location generation used and can vary greatly; likewise, the amount
of time required to ascertain location within specific quality
constraints is highly variable.
Thomson Expires April 26, 2009 [Page 6]
Internet-Draft Continuous Presence Reqs. October 2008
3. Logical Model: Presence Sources
The entity that measures continuous-valued data--and the process of
measurement--are crucial in determining the resulting information.
RFC 3856 [RFC3856] notes that presence data can be sourced by a
variety of means, but does not attach any significance to the source
of presence data. This document adopts a model that distinguishes
between the PA and the presence source.
+----------+ +----------+ +---------+
| Presence |_____________| Presence |______________| Watcher |
| Source | (Various) | Agent | (Presence) | |
+----------+ +----------+ +---------+
Figure 1: Logical Model
Specifics of the interaction between presence source and PA are out
of scope for this document. There might be formalised protocols, or
the PA might use unspecified or informal methods to acquire data.
Formalised exchanges include SIP PUBLISH [RFC3903]; informal
interactions include those that are briefly described in Section 7 of
[RFC3856].
For location information, the GEOPRIV architecture [RFC3693] [[Ed:
maybe [I-D.barnes-geopriv-lo-sec] instead of or as well as]] defines
the Location Generator. The Location Generator is the presence
source of this model; the entity that generates, or measures, the
continuous value. In the GEOPRIV architecture the Location Server is
the analogue of the PA and the Location Recipient corresponds to the
Watcher. Other forms of continuous-valued data might have similarly
formal architectures and nomenclature.
In an end-to-end system, the model is potentially iterative. An
entity that acts as a presence source to a PA could equally acquire
its information from a separate source; from a different perspective,
the PA becomes a watcher and the presence source becomes a PA. The
discussion in this document concentrates on the simple model; a
presence source in text generally refers to the ultimate presence
source: the entity that performs measurement.
+----------+ +----------+ +---------+
| Presence | | Presence |____| Watcher |
| Source | | Agent | | |
+----------+ | or +----+ or | +---------+
| Presence |____| Presence | | Watcher |
| Source | | Agent | | |
+----------+ +----------+ +----------+
Thomson Expires April 26, 2009 [Page 7]
Internet-Draft Continuous Presence Reqs. October 2008
Figure 2: Iterative Application of the Logical Model
Thomson Expires April 26, 2009 [Page 8]
Internet-Draft Continuous Presence Reqs. October 2008
4. Problem Statement
Continuous-valued data cannot always be treated in the same fashion
as discrete-valued data. This section outlines a number of
considerations that distinguish continuous-valued data. These
considerations affect how the presence system uses continuous-valued
data.
4.1. Unguided Measurement
A presence source is responsible for the measurement of presence
data. Unless the watcher is able to communicate preferences to the
presence source, the process of measurement is unguided.
In the event that a PA and watcher operate independently of the
presence data source, there is a risk that continuous-valued data is
measured inappropriately. Values might be measured too frequently or
with unneeded degree of accuracy, that is, accuracy is favoured too
highly over cost. Conversely, if cost is optimised, measurements
might be insufficient in frequency or accuracy.
Communicating watcher preferences to the presence source addresses
this problem by making information about what the watcher wants
available to the presence source. With this information the
measurement process is no longer unguided.
Communicating watcher preferences to the presence source is
especially critical if the watcher also incurs costs relating to the
measuring of the continuous value. The most direct cost is in time:
the watcher might be forced to wait for measuring of information to
an accuracy more stringent than needed. However, it need not be
limited to time.
4.2. Presence Filters
It is possible to resolve the problem of unguided measurement without
resorting to explicit protocol mechanisms. Presence filters
[RFC4660] are able to convey much of a watcher's preferences to the
PA. If the PA has a means of communicating with a presence source,
it is able to request measurement of continuous-valued data according
to the preferences it is aware of. By communicating the watcher's
preferences in this manner, the presence source is able to measure
accordingly. Additional changes to presence subscriptions
([I-D.niemi-sipping-event-throttle]) and filter extensions
([I-D.ietf-geopriv-loc-filters]) make more information available to
the PA; this information could be passed on.
Figure 3 shows a simplified model of the existing system of filtering
Thomson Expires April 26, 2009 [Page 9]
Internet-Draft Continuous Presence Reqs. October 2008
for presence data. The system assumes that presence information is
perfect. If this is not the case, the only option for a PA is to
project the illusion that this is the case. The only information
available to the PA is to use the filter and throttling data
available to it to limit the costs it incurs in retrieving
information from sources.
+--------------------------------------------------------+
| Presence Agent (PA) |
| |
| ,-----------------+<--------------+<--------------
| / (Presentity / / SUBSCRIBE |
| | Identifier) / / |
| V / / |
| ,----------. | | |
| | | V V |
| | Presence | \---------\ \----------\ |
| | Master |---->) Filter )---->) Throttle )------------>
| | Data | /---------/ /----------/ NOTIFY |
| | | | / |
| `----------' |,------------' |
| ^ ^ ^ ,-. / |
| | | | ( ? ) <--' +----------+ |
| | \ \ `-' | Internal | |
| | \ `----------------------| Presence | |
| | `. | Source | |
| | `-. +----------+ |
| | `. |
+----+------------+--------------------------------------+
| `.
| \
+----------+ +----------+
| Presence | | Presence |
| Source | | Source |
+----------+ +----------+
Figure 3: Presence Agent Model
4.3. Quality
Filters are not ideal indications of when and how to measure. The
purpose of a filter is to limit notifications to a watcher. It is
possible, in some cases to make an intelligent decision about how to
take measurements based on filter; but filters can be misleading. To
re-purpose filters to the task of advising sources on how and when to
measure can result in problems.
In particular, a filter specification based on information quality
Thomson Expires April 26, 2009 [Page 10]
Internet-Draft Continuous Presence Reqs. October 2008
can be particularly misleading. If a watcher specifies a filter that
requests a particular quality, the dual purposes that filters are
used for potentially conspire to degrade the service provided to the
watcher.
This is best illustrated with an example. Suppose a watcher, Bob,
is watching the location of a presentity, Alice. Bob specifies a
filter that specifies a maximum uncertainty of 50m. If this is
used to advise the presence source, the source attempts to
determine a location within that uncertainty. If the source is
unable to meet the requirement, the filter ensures that no
notification is sent to Bob. Bob could handle uncertainty of up to
1000m if necessary. However, if he specified a larger constraint,
the source might use cheaper methods that never produce a result
with the accuracy Bob really wants. How does Bob specify that he
wants uncertainty of 50m, but that he will settle for anything up
to 1000m?
From this example, it can be seen that "soft" quality preferences are
useful where continuous-valued data is the subject. These
preferences are used to guide the measurement process, but aren't
restrictive of the final result.
Quality constraints can be used to ensure that cached data is not
used. For instance, a constraint might be placed on the timestamp
that forces the measurement of new data.
The hard, boolean nature of a filter is not tolerant of variation in
quality. Of course, it is possible to expand the goals of the filter
document to include the requirements of this document. The
requirements here defined do not presume to exclude that possibility.
4.4. Watcher Feedback
A watcher requires adequate feedback from the PA about how its
preferences are being acted upon. To a watcher, a PA that fully
communicates watcher preferences to a presence source, remains
indistinguishable from a PA that is either less able to acquire data
(because it relies on PUBLISH [RFC3903]), or simply chooses not to
perform this communication.
In some cases, a watcher is able to compensate for any limitations
imposed by the PA. As shown in Section 4.6.3 adaptive polling can be
used to seek a particular value. However, if a watcher uses polling
for value-seeking, this potentially duplicates similar efforts from
the PA or presence source.
In other circumstances, shortcomings cannot be addressed by the
Thomson Expires April 26, 2009 [Page 11]
Internet-Draft Continuous Presence Reqs. October 2008
watcher. For instance, perhaps the presence source is capable of
providing information to a certain high degree of accuracy, but
unless requested it does not do so (the cost is too high). If the PA
never requests highly accurate information, that information is never
available to the watcher. If filter documents are used as a medium
for communicating watcher preferences, the watcher could set a filter
that only selects highly accurate information, which could
inadvertently suppress all notifications.
4.5. Active and Passive Sources
For a particular presence datum, the means by which the PA is able to
acquire updated information varies. For some forms of discrete-
valued data--such as basic status--the PA might be considered
absolutely authoritative, in that the information is absolutely
correct by definition. Data can be made available to the PA either
passively or actively.
Passive Acquisition: The PA is a passive recipient of updated
information with no means to trigger measurement. SIP PUBLISH
[RFC3903] is an example of how the PA could passively acquire
data.
Active Acquisition: The PA is able to seek updated information, and
does so based on one or more triggers.
Providing the watcher information on the nature of a particular
presence source is an important part of the feedback provided by the
PA. If the PA only passively acquires data, much of the
considerations presented in this document become moot. More
significantly, if the PA does not provide any options to the watcher
to influence its interactions with the presence source, the same
applies. Thus, it is when the PA provides some means of conveying
watcher preferences to the presence source that active acquisition
becomes useful.
Back to Bob and Alice: This time, Bob wants to know when Alice
passes by the library where Bob works. Bob sets a filter,
requesting that he only be notified when Alice passes within 100m
of the library. If the PA is passive, it could be that it never
receives information when Alice passes by--the presence source
might not publish the necessary information at the crucial moment.
4.6. Triggering Measurement
Stimulating the generation of a value for the continuous variable can
be done through several methods:
Thomson Expires April 26, 2009 [Page 12]
Internet-Draft Continuous Presence Reqs. October 2008
o immediate or on-demand triggers
o time-based or periodic triggers (polling)
o value-based triggers
4.6.1. Immediate Triggering of Measurement
An immediate request for measurement provides the simplest means of
directly providing the presence source with necessary information. A
watcher makes a request to the PA, including its preferences. The PA
then acquires information from the presence source, passing watcher
preferences directly to the source.
For SIP presence, the means of communicating preferences available to
a watcher is the SUBSCRIBE request. This presents a challenge for
the SIP presence framework if the presence source is not able to
provide the requested information immediately. RFC 3265 [RFC3265]
states:
When a SUBSCRIBE request is answered with a 200-class response,
the notifier MUST immediately construct and send a NOTIFY request
to the subscriber.
If the PA is unable to provide an immediate notification due to lack
of information, it must indicate this to the watcher. A presence
document might be a composit of continuous- and discrete-valued data.
Any solution for continuous-valued data cannot affect the conveyance
of discrete-valued data. Compatibility with existing the existing
presence framework is also desirable. Therefore, an immediate
notification is still necessary.
Some time after the initial request, when the continuous-valued data
becomes available, a second notification can be sent. However,
another conflict arises with event rate throttling. When information
becomes available, the notification might be suppressed due to a
short elapsed time since the initial notification.
Any means of triggering immediate measurement needs to consider these
problems.
4.6.2. Periodic Triggering of Measurement
Periodic notifications of the value of presence data are not assumed
to be necessary by the existing work. Values are assumed to change
infrequently, or if frequent changes are necessary, solutions have
concentrated on means of throttling notifications. RFC 3265
[RFC3265] recommends that event packages specify a minimum interval
Thomson Expires April 26, 2009 [Page 13]
Internet-Draft Continuous Presence Reqs. October 2008
between notifications; and [I-D.niemi-sipping-event-throttle]
provides a direct means of controlling this interval.
If the watcher has an interest in the value of a particular datum
over time, it is useful to be able to have the PA provide
notifications at regular intervals.
A similar time-based problem exists for periodic notification as did
for the immediate request. The PA cannot request presence
information at the time that a notification is required--the presence
source is unlikely to be able to produce a result so quickly. The PA
needs to allow the presence source sufficient time to make a
measurement. This time can be allocated before the notification is
required so that all the necessary data is available for the
notification.
Deciding how much time to allow the presence source presents another
problem. Relying upon a value specified by the watcher ensures that
the measurement process is guided appropriately.
4.6.3. Value-based Triggering of Measurement
A watcher might only be interested in a value if that value enters a
particular range. For instance, for location, the watcher might be
interested when the presentity is in the vicinity of a particular
landmark, or even the when two presentities approach each other. A
watcher that monitors the temperature of a presentity might be
interested if the temperature exceeds a certain threshold.
Value-seeking can be performed in different ways. Given certain
assumptions or knowledge about the rate of change of a value and the
desired responsiveness, an adaptive polling method can be used. The
rate of polling can increase in frequency in proportion to the
proximity of the current value to the target range. In addition,
time and quality constraints can be relaxed or made more stringent as
appropriate. The advantage of polling is that it can be performed
with only a small amount of information; a PA or watcher is able to
use polling for value-seeking.
A presence source can potentially use alternative information to
assist in value-seeking. For data that is derived from other
measurements, the value of the unprocessed measurements might provide
an adequate indication of the value to obviate any need to continue
measurement. For instance, the set of wireless transmitters that can
be observed by a wireless device can be used as a rough indication of
location.
Thomson Expires April 26, 2009 [Page 14]
Internet-Draft Continuous Presence Reqs. October 2008
5. Requirements
The requirements documented in this section relate to subscriptions
for continuous-valued data. These requirements do not make any
assumptions about the nature of the relationship between PA and
presence source. A presence source is assumed to be present only to
the extent that the measurement of a particular presence element
affects the results observed by the watcher.
A solution that addresses these requirements MAY address them based
on a specific type of data. Some requirements might not suit a
generic solution.
5.1. Quality Requirements
Q1. The system MUST provide a watcher the ability to express non-
binding requirements on information quality for continuous-
valued presence data.
Motivation: Having a means to express preferences in non-binding
fashion can have the desired effect in influencing the
measurement process at the presence source without other side-
effects.
Q2. The system MUST provide a means to indicate how information
quality preferences are used or propagated.
Motivation: Providing adequate feedback to a watcher assists the
watcher in making decisions about its behaviour. From the
perspective of a PA, this requirement can only be fulfilled
based on the knowledge it has available; it can know whether or
not it is the presence source, but it cannot assume that the
entity it requests information from is the ultimate source of
the information. Publishing information about the ultimate
source of the data isn't necessary, but the watcher needs to
know if its preferences are getting to that source.
For instance, the PA or presence source might limit the extent
to which quality parameters are able to bypass caches.
5.2. Immediate Triggering Requirements
I1. The system MUST provide a means for a watcher to explicitly and
immediately request measurement of a specified continuous-valued
datum.
Motivation: A direct request for data ensures that the presence
source is properly advised of the constraints of a request. It
Thomson Expires April 26, 2009 [Page 15]
Internet-Draft Continuous Presence Reqs. October 2008
also ensures that a watcher has access to updated information.
I2. The system MUST provide a means for the watcher to communicate
time constraints on the measurement process.
Motivation: Specifying time constraints ensures that the
information is available to the watcher when required. It also
ensures that the presence source applies appropriate methods in
measuring.
I3. The system MUST provide a means for a PA to limit support for
immediate requests.
Motivation: Making a direct request to the presence source
increases the load on the presence source and PA. A PA
implementation might want to constrain access to immediate
requests to prevent denial of service.
I4. The system MUST provide a means for a PA to indicate support for
immediate requests, including any limits or constraints on that
support.
Motivation: Providing adequate feedback to the watcher ensures
that the watcher is able to modify its behaviour accordingly.
5.3. Periodic Triggering Requirements
P1. The system MUST provide a means to request periodic measurement
of a continuous-valued datum at a specified interval.
Motivation: Periodic measurement is the most basic means of
enabling tracking of a value over time. More sophisticated
methods might be used, but this sets a minimum level of
capability that can be exploited for any type of continuous-
valued data.
P2. The system MUST provide a means for a PA to limit support for
periodic requests.
Motivation: Excessive polling rates might result in denial of
service through excessive requests.
P3. The system MUST provide a means for a PA to indicate support for
periodic measurement, including any limits or constraints.
Thomson Expires April 26, 2009 [Page 16]
Internet-Draft Continuous Presence Reqs. October 2008
P4. The system MUST provide a means for a watcher to explicitly
limit the rate of notifications.
Motivation: Even without perfect information, the defining
characteristic of continuous variables is the potential for
continuous change. This manifests in presence as a continuous
stream of data from the PA to the watcher. For a range of
reasons--protection of network utilization not-withstanding--
this is not desirable.
References: [I-D.niemi-sipping-event-throttle]
5.4. Value-Seeking Requirements
V1. The system MAY provide a means for a watcher to indicate a
particular range of values to seek.
Motivation: If a watcher is only interested in a certain range
of values, limiting measurement and notification protects
resources on both the presence source and watcher.
V2. The system MUST provide a means for a PA to indicate support for
value-seeking requests, including any limits or constraints on
that support.
Motivation: Providing adequate feedback to the watcher ensures
that the watcher is able to modify its behaviour accordingly.
5.5. Timeliness Requirements
T1. Continuous-valued data MUST always have a timestamp that
indicates when the value was measured.
Motivation: A continuously varying datum is, by its nature,
inevitably out of date at the time that the watcher receives it.
In practice, this is only a problem if the time between
measurement and receipt of the data is large. Since this is a
matter of degree, providing a timestamp ensures that a watcher
is able to make a judgment about validity.
If time information is not included, the recipient of
continuous-valued information has no means of judging how
current the information is. PIDF [RFC3863] specifies a
"timestamp" element, but this element is optional. This
requirement makes the value mandatory.
The corollary to this that an item of continuous-valued data is
automatically invalid if it is not timestamped.
Thomson Expires April 26, 2009 [Page 17]
Internet-Draft Continuous Presence Reqs. October 2008
T2. The system MUST provide a watcher the ability to limit the age
of data that it is provided.
Motivation: This ensures that caching of data is only used to
the extent that it is acceptable to the watcher.
References: [I-D.thomson-geopriv-location-quality]
T3. The system MUST provide a means for a watcher to indicate how
long measurement of continuous-valued data is allowed to take.
Motivation: This indication assists the PA and presence source
in making decisions about how much time to allocate to
measurement; for periodic measurement, it provides a hint on
when to start measurement.
Thomson Expires April 26, 2009 [Page 18]
Internet-Draft Continuous Presence Reqs. October 2008
6. Security Considerations
A number of requirements involve a PA making information about its
operation available to a watcher. This information might be consider
sensitive by the operators of a PA. Any solution that addresses
these requirements MUST provide an option to suppress this
information and MAY include guidance on when this might be
appropriate.
Many of the requirements in this document potentially result in PA
behaviour being controlled to some extent by a watcher. Due to the
potential for increased load on the PA, a malicious watcher could
attempt a denial of service attack by making repeated requests. A
periodic request or a request that includes value-seeking, in
particular, require significant processing in order to service a
relatively simple request. Limiting the rate at which information is
requested or generated can be used to mitigate this attack. Any
solution that addresses these requirements MUST consider the
implications of denial of service on the PA.
Caching of results can limit the load imposed by multiple requests
for the same information. To protect against denial of service, a PA
MAY choose to suppress any options for bypassing cached data.
Thomson Expires April 26, 2009 [Page 19]
Internet-Draft Continuous Presence Reqs. October 2008
7. Acknowledgements
This document is a clumsy attempt to formalize the output of
discussions on the nature of presence and its applicability to
location information. Adam Roach has been helpful in destroying
misconceptions about presence, although this document might
demonstrate that he wasn't entirely successful; any remaining
misconceptions are entirely those of the author. Thanks also to
Richard Barnes, Robert Sparks, James Winterbottom.
Thomson Expires April 26, 2009 [Page 20]
Internet-Draft Continuous Presence Reqs. October 2008
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
8.2. Informative References
[RFC2778] Day, M., Rosenberg, J., and H. Sugano, "A Model for
Presence and Instant Messaging", RFC 2778, February 2000.
[RFC3265] Roach, A., "Session Initiation Protocol (SIP)-Specific
Event Notification", RFC 3265, June 2002.
[RFC3693] Cuellar, J., Morris, J., Mulligan, D., Peterson, J., and
J. Polk, "Geopriv Requirements", RFC 3693, February 2004.
[RFC3856] Rosenberg, J., "A Presence Event Package for the Session
Initiation Protocol (SIP)", RFC 3856, August 2004.
[RFC3863] Sugano, H., Fujimoto, S., Klyne, G., Bateman, A., Carr,
W., and J. Peterson, "Presence Information Data Format
(PIDF)", RFC 3863, August 2004.
[RFC3903] Niemi, A., "Session Initiation Protocol (SIP) Extension
for Event State Publication", RFC 3903, October 2004.
[RFC4079] Peterson, J., "A Presence Architecture for the
Distribution of GEOPRIV Location Objects", RFC 4079,
July 2005.
[RFC4119] Peterson, J., "A Presence-based GEOPRIV Location Object
Format", RFC 4119, December 2005.
[RFC4480] Schulzrinne, H., Gurbani, V., Kyzivat, P., and J.
Rosenberg, "RPID: Rich Presence Extensions to the Presence
Information Data Format (PIDF)", RFC 4480, July 2006.
[RFC4660] Khartabil, H., Leppanen, E., Lonnfors, M., and J. Costa-
Requena, "Functional Description of Event Notification
Filtering", RFC 4660, September 2006.
[I-D.ietf-geopriv-lbyr-requirements]
Marshall, R., "Requirements for a Location-by-Reference
Mechanism", draft-ietf-geopriv-lbyr-requirements-03 (work
in progress), July 2008.
Thomson Expires April 26, 2009 [Page 21]
Internet-Draft Continuous Presence Reqs. October 2008
[I-D.ietf-geopriv-loc-filters]
Mahy, R. and B. Rosen, "A Document Format for Filtering
and Reporting Location Notications in the Presence
Information Document Format Location Object (PIDF-LO)",
draft-ietf-geopriv-loc-filters-02 (work in progress),
July 2008.
[I-D.barnes-geopriv-lo-sec]
Barnes, R., Lepinski, M., Tschofenig, H., and H.
Schulzrinne, "Additional Location Privacy Considerations",
draft-barnes-geopriv-lo-sec-03 (work in progress),
July 2008.
[I-D.garcia-simple-indirect-presence-publish]
Garcia-Martin, M., Tschofenig, H., and H. Schulzrinne,
"Indirect Presence Publication with the Session Initiation
Protocol(SIP)",
draft-garcia-simple-indirect-presence-publish-00 (work in
progress), February 2008.
[I-D.niemi-sipping-event-throttle]
Niemi, A., Kiss, K., and S. Loreto, "Session Initiation
Protocol (SIP) Event Notification Extension for
Notification Throttling",
draft-niemi-sipping-event-throttle-07 (work in progress),
October 2008.
[I-D.thomson-geopriv-uncertainty]
Thomson, M. and J. Winterbottom, "Representation of
Uncertainty and Confidence in PIDF-LO",
draft-thomson-geopriv-uncertainty-01 (work in progress),
June 2008.
[I-D.thomson-geopriv-location-quality]
Thomson, M. and J. Winterbottom, "Specifying Location
Quality Constraints in Location Protocols",
draft-thomson-geopriv-location-quality-01 (work in
progress), May 2008.
[ISO.GUM] ISO/IEC, "Guide to the expression of uncertainty in
measurement (GUM)", Guide 98:1995, 1995.
Thomson Expires April 26, 2009 [Page 22]
Internet-Draft Continuous Presence Reqs. October 2008
Appendix A. Presence Agent and Source Interactions
[[Ed: this section has some informational value, but need to
determine how much that contributes to the document.]]
This document specifies requirements that expose the nature of the
association between PA and presence source. While there are no
direct requirements on this interface and proprietary solutions are
entirely appropriate, it is hoped that these requirements will
influence the design of any protocol mechanisms used on this
interface.
Associations between PA and presence sources could be largely static
in nature, as is true of the methods described in [RFC3856].
Establishing dynamic associations between PA and presence source is
an option where disparate presence sources are required. This is
especially true for location information, where close physical
proximity between presentity and source is usually required;
consequently the presence source can dynamically change as the
presentity moves.
Location by-reference [I-D.ietf-geopriv-lbyr-requirements] provides a
means whereby a relationship between any entity and the Location
Generator can be established. By contacting the Location Generator,
a requester is able to specify preferences for how location
information is generated/measured. A location reference forms the
basis for establishing an association between Location Generator and
a Location Server.
For presence, Indirect publish
[I-D.garcia-simple-indirect-presence-publish] describes how a PA is
able to use the indirection provided by a URI. The URI is used to
establish a link between the generator of presence information and
the PA. The URI is distinguished by two characteristics:
o the host serving the URI is the presence source
o additional information in the URI provides enough information to
uniquely identify the presentity to the presence source
Thomson Expires April 26, 2009 [Page 23]
Internet-Draft Continuous Presence Reqs. October 2008
Author's Address
Martin Thomson
Andrew
PO Box U40
Wollongong University Campus, NSW 2500
AU
Phone: +61 2 4221 2915
Email: martin.thomson@andrew.com
URI: http://www.andrew.com/
Thomson Expires April 26, 2009 [Page 24]
Internet-Draft Continuous Presence Reqs. October 2008
Full Copyright Statement
Copyright (C) The IETF Trust (2008).
This document is subject to the rights, licenses and restrictions
contained in BCP 78, and except as set forth therein, the authors
retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND
THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS
OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF
THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
Intellectual Property
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights or other rights that might be claimed to
pertain to the implementation or use of the technology described in
this document or the extent to which any license under such rights
might or might not be available; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat and any
assurances of licenses to be made available, or the result of an
attempt made to obtain a general license or permission for the use of
such proprietary rights by implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at
ietf-ipr@ietf.org.
Thomson Expires April 26, 2009 [Page 25]