CoRE Working Group M. Koster
Internet-Draft SmartThings
Intended status: Informational B. Silverajan, Ed.
Expires: January 13, 2022 Tampere University
July 12, 2021
Conditional Attributes for Constrained RESTful Environments
draft-ietf-core-conditional-attributes-00
Abstract
This specification defines Conditional Notification and Control
Attributes that work with CoAP Observe (RFC7641).
Editor note
The git repository for the draft is found at TBD
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on January 13, 2022.
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described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Conditional Attributes . . . . . . . . . . . . . . . . . . . 3
3.1. Conditional Notification Attributes . . . . . . . . . . . 3
3.1.1. Greater Than (gt) . . . . . . . . . . . . . . . . . . 4
3.1.2. Less Than (lt) . . . . . . . . . . . . . . . . . . . 4
3.1.3. Change Step (st) . . . . . . . . . . . . . . . . . . 5
3.1.4. Notification Band (band) . . . . . . . . . . . . . . 5
3.1.5. Edge (edge) . . . . . . . . . . . . . . . . . . . . . 6
3.2. Conditional Control Attributes . . . . . . . . . . . . . 6
3.2.1. Minimum Period (pmin) . . . . . . . . . . . . . . . . 7
3.2.2. Maximum Period (pmax) . . . . . . . . . . . . . . . . 7
3.2.3. Minimum Evaluation Period (epmin) . . . . . . . . . . 8
3.2.4. Maximum Evaluation Period (epmax) . . . . . . . . . . 8
3.2.5. Confirmable Notification (con) . . . . . . . . . . . 8
3.3. Server processing of Conditional Attributes . . . . . . . 8
4. Implementation Considerations . . . . . . . . . . . . . . . . 9
5. Security Considerations . . . . . . . . . . . . . . . . . . . 10
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 10
7. Contributors . . . . . . . . . . . . . . . . . . . . . . . . 10
8. Changelog . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1. Normative References . . . . . . . . . . . . . . . . . . 11
9.2. Informative References . . . . . . . . . . . . . . . . . 12
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 12
A.1. Minimum Period (pmin) example . . . . . . . . . . . . . . 12
A.2. Maximum Period (pmax) example . . . . . . . . . . . . . . 13
A.3. Greater Than (gt) example . . . . . . . . . . . . . . . . 14
A.4. Greater Than (gt) and Period Max (pmax) example . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 16
1. Introduction
IETF Standards for machine to machine communication in constrained
environments describe a REST protocol [RFC7252] and a set of related
information standards that may be used to represent machine data and
machine metadata in REST interfaces.
This specification defines Conditional Notification and Control
Attributes for use with CoRE Observe [RFC7641].
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2. Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
This specification requires readers to be familiar with all the terms
and concepts that are discussed in [RFC7641]. This specification
makes use of the following additional terminology:
Notification Band: A resource value range that may be bounded by a
minimum and maximum value or may be unbounded having either a
minimum or maximum value.
3. Conditional Attributes
This specification defines conditional attributes, which provide for
fine-grained control of notification and state synchronization when
using CoRE Observe [RFC7641]. When resource interfaces following
this specification are made available over CoAP, the CoAP Observation
mechanism [RFC7641] MAY also be used to observe any changes in a
resource, and receive asynchronous notifications as a result. A
resource marked as Observable in its link description SHOULD support
these conditional attributes.
Note: In this draft, we assume that there are finite quantization
effects in the internal or external updates to the value representing
the state of a resource; specifically, that a resource state may be
updated at any time with any valid value. We therefore avoid any
continuous-time assumptions in the description of the conditional
attributes and instead use the phrase "sampled value" to refer to a
member of a sequence of values that may be internally observed from
the resource state over time.
3.1. Conditional Notification Attributes
Conditional Notification Attributes define the conditions that
trigger a notification. Conditional Notification Attributes SHOULD
be evaluated on all potential notifications from a resource, whether
resulting from an internal server-driven sampling process or from
external update requests to the server.
The set of Conditional Notification Attributes defined here allow a
client to control how often a client is interested in receiving
notifications and how much a value should change for the new
representation state to be interesting. One or more Conditional
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Notification Attributes MAY be included as query parameters in an
Observe request.
Conditional Notification Attributes are defined below:
+-------------------+-----------+-----------------+
| Attribute | Parameter | Value |
+-------------------+-----------+-----------------+
| Greater Than | gt | xs:decimal |
| | | |
| Less Than | lt | xs:decimal |
| | | |
| Change Step | st | xs:decimal (>0) |
| | | |
| Notification Band | band | xs:boolean |
| | | |
| Edge | edge | xs:boolean |
+-------------------+-----------+-----------------+
Table 1: Conditional Notification Attributes
3.1.1. Greater Than (gt)
When present, Greater Than indicates the upper limit value the
sampled value SHOULD cross before triggering a notification. A
notification is sent whenever the sampled value crosses the specified
upper limit value, relative to the last reported value, and the time
fpr pmin has elapsed since the last notification. The sampled value
is sent in the notification. If the value continues to rise, no
notifications are generated as a result of gt. If the value drops
below the upper limit value then a notification is sent, subject
again to the pmin time.
The Greater Than parameter can only be supported on resources with a
scalar numeric value.
3.1.2. Less Than (lt)
When present, Less Than indicates the lower limit value the resource
value SHOULD cross before triggering a notification. A notification
is sent when the samples value crosses the specified lower limit
value, relative to the last reported value, and the time fpr pmin has
elapsed since the last notification. The sampled value is sent in
the notification. If the value continues to fall no notifications
are generated as a result of lt. If the value rises above the lower
limit value then a new notification is sent, subject to the pmin
time.
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The Less Than parameter can only be supported on resources with a
scalar numeric value.
3.1.3. Change Step (st)
When present, the change step indicates how much the value
representing a resource state SHOULD change before triggering a
notification, compared to the old state. Upon reception of a query
including the st attribute, the current resource state representing
the most recently sampled value is reported, and then set as the last
reported value (last_rep_v). When a subsequent sampled value or
update of the resource state differs from the last reported state by
an amount, positive or negative, greater than or equal to st, and the
time for pmin has elapsed since the last notification, a notification
is sent and the last reported value is updated to the new resource
state sent in the notification. The change step MUST be greater than
zero otherwise the receiver MUST return a CoAP error code 4.00 "Bad
Request" (or equivalent).
The Change Step parameter can only be supported on resource states
represented with a scalar numeric value.
Note: Due to sampling and other constraints, e.g. pmin, the change in
resource states received in two sequential notifications may differ
by more than st.
3.1.4. Notification Band (band)
The notification band attribute allows a bounded or unbounded (based
on a minimum or maximum) value range that may trigger multiple
notifications. This enables use cases where different ranges results
in differing behaviour. For example, in monitoring the temperature
of machinery, whilst the temperature is in the normal operating
range, only periodic updates are needed. However as the temperature
moves to more abnormal ranges more frequent state updates may be sent
to clients.
Without a notification band, a transition across a less than (lt), or
greater than (gt) limit only generates one notification. This means
that it is not possible to describe a case where multiple
notifications are sent so long as the limit is exceeded.
The band attribute works as a modifier to the behaviour of gt and lt.
Therefore, if band is present in a query, gt, lt or both, MUST be
included.
When band is present with the lt attribute, it defines the lower
bound for the notification band (notification band minimum).
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Notifications occur when the resource value is equal to or above the
notification band minimum. If lt is not present there is no minimum
value for the band.
When band is present with the gt attribute, it defines the upper
bound for the notification band (notification band maximum).
Notifications occur when the resource value is equal to or below the
notification band maximum. If gt is not present there is no maximum
value for the band.
If band is present with both the gt and lt attributes, notification
occurs when the resource value is greater than or equal to gt or when
the resource value is less than or equal to lt.
If a band is specified in which the value of gt is less than that of
lt, in-band notification occurs. That is, notification occurs
whenever the resource value is between the gt and lt values,
including equal to gt or lt.
If the band is specified in which the value of gt is greater than
that of lt, out-of-band notification occurs. That is, notification
occurs when the resource value not between the gt and lt values,
excluding equal to gt and lt.
The Notification Band parameter can only be supported on resources
with a scalar numeric value.
3.1.5. Edge (edge)
When present, the Edge attribute indicates interest for receiving
notifications of either the falling edge or the rising edge
transition of a boolean resource state. When the value of the Edge
attribute is 0, the server notifies the client each time a resource
state changes from True to False. When the value of the Edge
attribute is 1, the server notifies the client each time a resource
state changes from False to True.
The Edge attribute can only be supported on resources with a boolean
value.
3.2. Conditional Control Attributes
Conditional Control Attributes define the time intervals between
consecutive notifications as well as the cadence of the measurement
of the conditions that trigger a notification. Conditional Control
Attributes can be used to configure the internal server-driven
sampling process for performing measurements of the conditions of a
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resource. One or more Conditional Control Attributes MAY be included
as query parameters in an Observe request.
Conditional Control Attributes are defined below:
+-------------------------------+-----------+-----------------+
| Attribute | Parameter | Value |
+-------------------------------+-----------+-----------------+
| Minimum Period (s) | pmin | xs:decimal (>0) |
| | | |
| Maximum Period (s) | pmax | xs:decimal (>0) |
| | | |
| Minimum Evaluation Period (s) | epmin | xs:decimal (>0) |
| | | |
| Maximum Evaluation Period (s) | epmax | xs:decimal (>0) |
| | | |
| Confirmable Notification | con | xs:boolean |
+-------------------------------+-----------+-----------------+
Table 2: Conditional Control Attributes
3.2.1. Minimum Period (pmin)
When present, the minimum period indicates the minimum time, in
seconds, between two consecutive notifications (whether or not the
resource state has changed). In the absence of this parameter, the
minimum period is up to the server. The minimum period MUST be
greater than zero otherwise the receiver MUST return a CoAP error
code 4.00 "Bad Request" (or equivalent).
A server MAY update the resource state with the last sampled value
that occured during the pmin interval, after the pmin interval
expires.
Note: Due to finite quantization effects, the time between
notifications may be greater than pmin even when the sampled value
changes within the pmin interval. Pmin may or may not be used to
drive the internal sampling process.
3.2.2. Maximum Period (pmax)
When present, the maximum period indicates the maximum time, in
seconds, between two consecutive notifications (whether or not the
resource state has changed). In the absence of this parameter, the
maximum period is up to the server. The maximum period MUST be
greater than zero and MUST be greater than, or equal to, the minimum
period parameter (if present) otherwise the receiver MUST return a
CoAP error code 4.00 "Bad Request" (or equivalent).
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3.2.3. Minimum Evaluation Period (epmin)
When present, the minimum evaluation period indicates the minimum
time, in seconds, the client recommends to the server to wait between
two consecutive measurements of the conditions of a resource since
the client has no interest in the server doing more frequent
measurements. When the minimum evaluation period expires after the
previous measurement, the server MAY immediately perform a new
measurement. In the absence of this parameter, the minimum
evaluation period is not defined and thus not used by the server.
The server MAY use pmin, if defined, as a guidance on the desired
measurement cadence. The minimum evaluation period MUST be greater
than zero otherwise the receiver MUST return a CoAP error code 4.00
"Bad Request" (or equivalent).
3.2.4. Maximum Evaluation Period (epmax)
When present, the maximum evaluation period indicates the maximum
time, in seconds, the server MAY wait between two consecutive
measurements of the conditions of a resource. When the maximum
evaluation period expires after the previous measurement, the server
MUST immediately perform a new measurement. In the absence of this
parameter, the maximum evaluation period is not defined and thus not
used by the server. The maximum evaluation period MUST be greater
than zero and MUST be greater than the minimum evaluation period
parameter (if present) otherwise the receiver MUST return a CoAP
error code 4.00 "Bad Request" (or equivalent).
3.2.5. Confirmable Notification (con)
When present with a value of 1 in a query, the con attribute
indicates a notification MUST be confirmable, i.e., the server MUST
send the notification in a confirmable CoAP message, to request an
acknowledgement from the client. When present with a value of 0 in a
query, the con attribute indicates a notification can be confirmable
or non-confirmable, i.e., it can be sent in a confirmable or a non-
confirmable CoAP message.
3.3. Server processing of Conditional Attributes
Conditional Notification Attributes and Conditional Control
Attributes may be present in the same query. However, they are not
defined at multiple prioritization levels. The server sends a
notification whenever any of the parameter conditions are met, upon
which it updates its last notification value and time to prepare for
the next notification. Only one notification occurs when there are
multiple conditions being met at the same time. The reference code
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below illustrates the logic to determine when a notification is to be
sent.
bool notifiable( Resource * r ) {
#define BAND r->band
#define SCALAR_TYPE ( num_type == r->type )
#define STRING_TYPE ( str_type == r->type )
#define BOOLEAN_TYPE ( bool_type == r->type )
#define PMIN_EX ( r->last_sample_time - r->last_rep_time >= r->pmin )
#define PMAX_EX ( r->last_sample_time - r->last_rep_time > r->pmax )
#define LT_EX ( r->v < r->lt ^ r->last_rep_v < r->lt )
#define GT_EX ( r->v > r->gt ^ r->last_rep_v > r->gt )
#define ST_EX ( abs( r->v - r->last_rep_v ) >= r->st )
#define IN_BAND ( ( r->gt <= r->v && r->v <= r->lt ) || ( r->lt <= r->gt && r->gt <= r->v ) || ( r->v <= r->lt && r->lt <= r->gt ) )
#define VB_CHANGE ( r->vb != r->last_rep_vb )
#define VS_CHANGE ( r->vs != r->last_rep_vs )
return (
PMIN_EX &&
( SCALAR_TYPE ?
( ( !BAND && ( GT_EX || LT_EX || ST_EX || PMAX_EX ) ) ||
( BAND && IN_BAND && ( ST_EX || PMAX_EX) ) )
: STRING_TYPE ?
( VS_CHANGE || PMAX_EX )
: BOOLEAN_TYPE ?
( VB_CHANGE || PMAX_EX )
: false )
);
}
Figure 1: Code logic for conditional notification attribute
interactions
4. Implementation Considerations
When pmax and pmin are equal, the expected behaviour is that
notifications will be sent every (pmin == pmax) seconds. However,
these notifications can only be fulfilled by the server on a best
effort basis. Because pmin and pmax are designed as acceptable
tolerance bounds for sending state updates, a query from an
interested client containing equal pmin and pmax values must not be
seen as a hard real-time scheduling contract between the client and
the server.
When using multiple resource bindings (e.g. multiple Observations of
resource) with different bands, consideration should be given to the
resolution of the resource value when setting sequential bands. For
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example: Given BandA (Abmn=10, Bbmx=20) and BandB (Bbmn=21, Bbmx=30).
If the resource value returns an integer then notifications for
values between and inclusive of 10 and 30 will be triggered. Whereas
if the resolution is to one decimal point (0.1) then notifications
for values 20.1 to 20.9 will not be triggered.
The use of the notification band minimum and maximum allow for a
synchronization whenever a change in the resource value occurs.
Theoretically this could occur in-line with the server internal
sample period or the configuration of epmin and epmax values for
determining the resource value. Implementors SHOULD consider the
resolution needed before updating the resource, e.g. updating the
resource when a temperature sensor value changes by 0.001 degree
versus 1 degree.
When a server has multiple observations with different measurement
cadences as defined by the epmin and epmax values, the server MAY
evaluate all observations when performing the measurement of any one
observation.
5. Security Considerations
TBD IANA Considerations ===================
TBD
6. Acknowledgements
Hannes Tschofenig and Mert Ocak highlighted syntactical corrections
in the usage of pmax and pmin in a query. Alan Soloway contributed
text leading to the inclusion of epmin and epmax. David Navarro
proposed allowing for pmax to be equal to pmin.
7. Contributors
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Christian Groves
Australia
email: cngroves.std@gmail.com
Zach Shelby
ARM
Vuokatti
FINLAND
phone: +358 40 7796297
email: zach.shelby@arm.com
Matthieu Vial
Schneider-Electric
Grenoble
France
phone: +33 (0)47657 6522
eMail: matthieu.vial@schneider-electric.com
Jintao Zhu
Huawei
Xi'an, Shaanxi Province
China
email: jintao.zhu@huawei.com
8. Changelog
draft-ietf-core-conditional-attributes-00
o Conditional Atttributes section from draft-ietf-core-dynlink-13
separated into own WG draft
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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9.2. Informative References
[RFC7252] Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
Application Protocol (CoAP)", RFC 7252,
DOI 10.17487/RFC7252, June 2014,
<https://www.rfc-editor.org/info/rfc7252>.
[RFC7641] Hartke, K., "Observing Resources in the Constrained
Application Protocol (CoAP)", RFC 7641,
DOI 10.17487/RFC7641, September 2015,
<https://www.rfc-editor.org/info/rfc7641>.
Appendix A. Examples
This appendix provides some examples of the use of binding attribute
/ observe attributes.
Note: For brevity the only the method or response code is shown in
the header field.
A.1. Minimum Period (pmin) example
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Observed CLIENT SERVER Actual
t State | | State
____________ | | ____________
1 | |
2 unknown | | 18.5 Cel
3 +----->| Header: GET
4 | GET | Token: 0x4a
5 | | Uri-Path: temperature
6 | | Uri-Query: pmin="10"
7 | | Observe: 0 (register)
8 | |
9 ____________ |<-----+ Header: 2.05
10 | 2.05 | Token: 0x4a
11 18.5 Cel | | Observe: 9
12 | | Payload: "18.5 Cel"
13 | | ____________
14 | |
15 | | 23 Cel
16 | |
17 | |
18 | |
19 | | ____________
20 ____________ |<-----+ Header: 2.05
21 | 2.05 | 26 Cel Token: 0x4a
22 26 Cel | | Observe: 20
23 | | Payload: "26 Cel"
24 | |
25 | |
Figure 2: Client registers and receives one notification of the
current state and one of a new state state when pmin time expires.
A.2. Maximum Period (pmax) example
Observed CLIENT SERVER Actual
t State | | State
____________ | | ____________
1 | |
2 unknown | | 18.5 Cel
3 +----->| Header: GET
4 | GET | Token: 0x4a
5 | | Uri-Path: temperature
6 | | Uri-Query: pmax="20"
7 | | Observe: 0 (register)
8 | |
9 ____________ |<-----+ Header: 2.05
10 | 2.05 | Token: 0x4a
11 18.5 Cel | | Observe: 9
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12 | | Payload: "18.5 Cel"
13 | |
14 | |
15 | | ____________
16 ____________ |<-----+ Header: 2.05
17 | 2.05 | 23 Cel Token: 0x4a
18 23 Cel | | Observe: 16
19 | | Payload: "23 Cel"
20 | |
21 | |
22 | |
23 | |
24 | |
25 | |
26 | |
27 | |
28 | |
29 | |
30 | |
31 | |
32 | |
33 | |
34 | |
35 | |
36 | | ____________
37 ____________ |<-----+ Header: 2.05
38 | 2.05 | 23 Cel Token: 0x4a
39 23 Cel | | Observe: 37
40 | | Payload: "23 Cel"
41 | |
42 | |
Figure 3: Client registers and receives one notification of the
current state, one of a new state and one of an unchanged state when
pmax time expires.
A.3. Greater Than (gt) example
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Observed CLIENT SERVER Actual
t State | | State
____________ | | ____________
1 | |
2 unknown | | 18.5 Cel
3 +----->| Header: GET
4 | GET | Token: 0x4a
5 | | Uri-Path: temperature
6 | | Uri-Query: gt=25
7 | | Observe: 0 (register)
8 | |
9 ____________ |<-----+ Header: 2.05
10 | 2.05 | Token: 0x4a
11 18.5 Cel | | Observe: 9
12 | | Payload: "18.5 Cel"
13 | |
14 | |
15 | | ____________
16 ____________ |<-----+ Header: 2.05
17 | 2.05 | 26 Cel Token: 0x4a
18 26 Cel | | Observe: 16
29 | | Payload: "26 Cel"
20 | |
21 | |
Figure 4: Client registers and receives one notification of the
current state and one of a new state when it passes through the
greater than threshold of 25.
A.4. Greater Than (gt) and Period Max (pmax) example
Observed CLIENT SERVER Actual
t State | | State
____________ | | ____________
1 | |
2 unknown | | 18.5 Cel
3 +----->| Header: GET
4 | GET | Token: 0x4a
5 | | Uri-Path: temperature
6 | | Uri-Query: pmax=20;gt=25
7 | | Observe: 0 (register)
8 | |
9 ____________ |<-----+ Header: 2.05
10 | 2.05 | Token: 0x4a
11 18.5 Cel | | Observe: 9
12 | | Payload: "18.5 Cel"
13 | |
14 | |
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15 | |
16 | |
17 | |
18 | |
19 | |
20 | |
21 | |
22 | |
23 | |
24 | |
25 | |
26 | |
27 | |
28 | |
29 | | ____________
30 ____________ |<-----+ Header: 2.05
31 | 2.05 | 23 Cel Token: 0x4a
32 23 Cel | | Observe: 30
33 | | Payload: "23 Cel"
34 | |
35 | |
36 | | ____________
37 ____________ |<-----+ Header: 2.05
38 | 2.05 | 26 Cel Token: 0x4a
39 26 Cel | | Observe: 37
40 | | Payload: "26 Cel"
41 | |
42 | |
Figure 5: Client registers and receives one notification of the
current state, one when pmax time expires and one of a new state when
it passes through the greater than threshold of 25.
Authors' Addresses
Michael Koster
SmartThings
665 Clyde Avenue
Mountain View 94043
USA
Email: michael.koster@smartthings.com
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Bilhanan Silverajan (editor)
Tampere University
Kalevantie 4
Tampere FI-33100
Finland
Email: bilhanan.silverajan@tuni.fi
Koster & Silverajan Expires January 13, 2022 [Page 17]
