CoRE A. Bhattacharyya
Internet Draft S. Bandyopadhyay
Intended status: Informational A. Pal
Expires: August 2016 T. Bose
Tata Consultancy Services Ltd.
February 17, 2016
CoAP option for no server-response
draft-tcs-coap-no-response-option-14
Abstract
There can be M2M scenarios where responses from a server against
requests from client are redundant. This kind of open-loop exchange
(with no response path from the server to the client) may be desired
to minimize resource consumption in constrained systems while
updating a bulk of resources simultaneously, or updating a resource
with a very high frequency. CoAP already provides a non-confirmable
(NON) mode of message exchange where the server end-point does not
respond with ACK. However, obeying the request/response semantics,
the server end-point responds back with a status code indicating
"the result of the attempt to understand and satisfy the request".
This document introduces a header option for CoAP called 'No-
Response'. Using this option the client can explicitly tell the
server to suppress all responses against the particular request.
This option also provides granular control to enable suppression of
a particular class of response or a combination of response-classes.
This option may be effective for both unicast and multicast
requests. This document also discusses a few exemplary applications
which benefit from this option.
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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Table of Contents
1. Introduction...................................................3
1.1. Potential benefits........................................3
1.2. Terminology...............................................4
2. Option Definition..............................................4
2.1. Granular control over response suppression................4
2.2. Method-specific applicability consideration...............6
3. Exemplary application scenarios................................7
3.1. Frequent update of geo-location from vehicles to backend..7
3.2. Multicasting actuation command from a handheld device to a
group of appliances............................................9
3.2.1. Using granular response suppression..................9
4. Miscellaneous aspects..........................................9
4.1. Re-using Tokens...........................................9
4.2. Taking care of congestion................................11
4.3. Handling No-Response option for a HTTP-to-CoAP reverse proxy
..............................................................11
5. Example.......................................................12
5.1. Using No-Response with PUT...............................12
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5.2. Using No-Response with POST..............................12
5.2.1. POST updating a fixed target resource...............12
5.2.2. POST updating through query-string..................13
6. IANA Considerations...........................................14
7. Security Considerations.......................................15
8. Acknowledgments...............................................15
9. References....................................................15
9.1. Normative References.....................................15
9.2. Informative References...................................16
1. Introduction
This document proposes a new header option for Constrained
Application Protocol (CoAP) [RFC7252] called 'No-Response'. This
option enables the client end-point to explicitly express its
disinterest in receiving responses back from the server end-point.
Fine grain control to suppress responses of a particular class or a
combination of response-classes is also possible.
Along with the technical details this document presents some
practical application scenarios which bring out the usefulness of
this option.
Wherever, in this document, it is mentioned that a request from a
client is with No-Response the intended meaning is that the client
expresses its disinterest for all or some selected classes of
responses.
1.1. Potential benefits
Use of No-Response option should be driven by typical application
requirement and, particularly, characteristics of the information to
be updated. If this option is opportunistically used in a fitting
M2M application then the concerned system may benefit in the
following aspects:
* Reduction in network congestion due to effective reduction of
the overall traffic.
* Reduction in server-side loading by relieving the server from
responding to each request when not necessary.
* Reduction in battery consumption at the constrained end-
point(s).
* Reduction in overall communication cost.
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1.2. Terminology
The terms used in this document are in conformance with those
defined in [RFC7252].
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 RFC-2119.
2. Option Definition
The properties of No-Response option are given in Table 1.
+--------+---+---+---+---+-------------+--------+--------+---------+
| Number | C | U | N | R | Name | Format | Length | Default |
+--------+---+---+---+---+-------------+--------+--------+---------+
| 284 | | | X | | No-Response | uint | 0-1 | 0 |
+--------+---+---+---+---+-------------+--------+--------+---------+
Table 1: Option Properties
This option is a request option. It is Elective and Non-Repeatable.
Note: Since CoAP maintains a clear separation between the
request/response and the messaging layer, this option does not
have any dependency on the type of message (confirmable/ non-
confirmable). However, NON type of messages are best fitting with
this option considering the expected benefits out of it. Using
No-Response with NON messages gets rid of any kind of reverse
traffic and the interaction becomes completely open-loop.
Using this option with CON type of requests may not have any
purpose if piggybacked responses are triggered. But, in case the
server responds with a separate response (which, perhaps, the
client does not care about) then this option can be useful.
Suppressing the separate response reduces traffic by one
additional CoAP message in this case.
This option contains values to indicate disinterest in all or a
particular class or combination of classes of responses as described
in the next sub-section.
2.1. Granular control over response suppression
This option enables granular control over response suppression by
allowing the client to express its disinterest in a typical class or
combination of classes of responses. For example, a client may
explicitly tell the receiver that no response is required unless
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something 'bad' happens and a response of class 4.xx or 5.xx is to
be fed back to the client. No response of the class 2.xx is required
in such case.
Note: Section 3.7 of [RFC7390] describes a scheme where a server in
the multicast group may decide on its own to suppress responses
for group communication with granular control. The client does
not have any knowledge about that. However, on the other hand,
the 'No-Response' option enables the clients to explicitly inform
the servers about its disinterest in responses. Such explicit
control on the client side may be helpful for debugging network
resources. An example scenario is described in Section 3.2.
The option value is defined as a bit-map (Table 2) to achieve
granular suppression. Its length can be 0 byte (empty value) or 1
byte.
+-------+-----------------------+---------------------------------+
| Value | Binary Representation | Description |
+-------+-----------------------+---------------------------------+
| 0 | <empty> | Allow all responses. |
+-------+-----------------------+---------------------------------+
| 2 | 00000010 | Suppress 2.xx responses. |
+-------+-----------------------+---------------------------------+
| 8 | 00001000 | Suppress 4.xx responses. |
+-------+-----------------------+---------------------------------+
| 16 | 00010000 | Suppress 5.xx responses. |
+-------+-----------------------+---------------------------------+
| 127 | 01111111 | Suppress all responses. |
+-------+-----------------------+---------------------------------+
Table 2: Option values
The conventions used in deciding the option values are:
1. To suppress an individual class: Set bit number (n-1) starting
from the LSB (bit number 0) to suppress all responses belonging to
class n.xx. So,
option value to suppress n.xx class = 2**(n-1).
2. To suppress combination of classes: Set each corresponding bit
according to point 1 above. Example: The option value will be 18
(binary: 00010010) to suppress both 2.xx and 5.xx responses. This is
essentially bitwise OR of the corresponding individual values for
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suppressing 2.xx and 5.xx. At present the "CoAP Response Codes"
registry (Ref. Section 12.1.2 of [RFC7252]) defines only 2.xx, 4.xx
and 5.xx responses. So, an option value of 26 (binary: 00011010)
will effectively suppress all currently defined response codes.
3. To suppress all possible responses: The maximum reserved response
code for CoAP is 7.31 (Ref. Section 12.1 of [RFC7252]). So, setting
bit positions 0-6 will suppress all responses according to the
combination operation defined in point 2 above. Hence, the value to
block all present and possible future responses is 127 (binary:
01111111).
Note: When No-Response is used with value 127 in a request the
client end-point SHOULD cease listening to response(s) against
the particular request. On the other hand, opening up at least
one class of response means that the client end-point can no
longer completely cease listening activity and must be configured
to listen up to some application specific time-out period for the
particular request. The client end-point never knows whether the
present update will be a success or a failure. Thus, for example,
if the client decides to open up the response for errors (4.xx
and 5.xx) then it has to wait for the entire time-out period even
for the instances where the request is successful (and the server
is not supposed to send back a response). A point to be noted in
this context is that there may be situations when the response on
errors might get lost. In such a situation the client would wait
up to the time-out period but will not receive any response. But
this should not lead to the impression to the client that the
request was necessarily successful. The application designer
needs to tackle such situation. For example, while performing
frequent updates, the client may strategically interweave
requests without No-Response option into a series of requests
with No-Response to check time to time if things are fine at the
server end and the server is actively responding.
2.2. Method-specific applicability consideration
The following table provides a 'ready-reference' on the possible
applicability of this option for all the four REST methods. This
table is prepared in view of the type of possible interactions
foreseen so far.
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+-------------+----------------------------------------------------+
| Method Name | Remarks on applicability |
+-------------+----------------------------------------------------+
| | This should not be used with conventional GET |
| | request when the client requests the contents |
| | of a resource. However, this option may be useful |
| | for special GET requests. At present only one such|
| | application is identified which is the |
| | 'cancellation' procedure for Observe [RFC7641]. |
| GET | Observe-cancellation requires a client to issue a |
| | GET request with Observe option set to 'deregister'|
| | (1). Since, in this case, the server response may |
| | not be of interest, the client may explicitly |
| | express its disinterest in server response. |
+-------------+----------------------------------------------------+
| | Suitable for frequent updates (particularly in NON |
| PUT | messages) on existing resources. Might not be |
| | useful when PUT is used to create a new resource. |
+-------------+----------------------------------------------------+
| | If POST is used to update a target resource |
| | then No-Response can be used in the same manner as |
| | in PUT. This option may also be useful while |
| POST | updating through query strings rather than updating|
| | a fixed target resource (see Section 5.2.2 for an |
| | example). |
+-------------+----------------------------------------------------+
| | Deletion is usually a permanent action and if the |
| DELETE | client likes to ensure that the deletion request |
| | was properly executed then this option should not |
| | be used with the request. |
+-------------+----------------------------------------------------+
Table 3: Suggested applicability of No-Response for different REST
methods
3. Exemplary application scenarios
This section describes some exemplary application scenarios which
may potentially benefit from the use of No-Response option.
3.1. Frequent update of geo-location from vehicles to backend
Let us consider an intelligent traffic system (ITS) consisting of
vehicles equipped with a sensor-gateway comprising sensors like GPS
and Accelerometer. The sensor-gateway acts as a CoAP client end-
point. It connects to the Internet using a low-bandwidth cellular
(e.g. GPRS) connection. The GPS co-ordinates of the vehicle are
periodically updated to the backend server. The update rate is
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adaptive to the motional-state of the vehicle. If the vehicle moves
fast the update rate is high as the position of the vehicle changes
rapidly. If the vehicle is static or moves slowly then the update
rate is low. This ensures that bandwidth and energy is not consumed
unnecessarily. The motional-state of the vehicle is inferred by a
local analytics running on the sensor-gateway which uses the
accelerometer data and the rate of change in GPS co-ordinates. The
back-end server hosts applications which use the updates for each
vehicle and produce necessary information for remote users.
Retransmitting (through the CoAP CON mechanism) a location co-
ordinate which the vehicle has already left in the meantime is not
efficient as it adds redundant traffic to the network. Therefore,
the updates are done using NON messages. However, given the huge
number of vehicles updating frequently, the NON exchange will also
trigger huge number of responses from the backend. Thus the
cumulative load on the network will be quite significant.
On the contrary, if the client end-points on the vehicles explicitly
declare that they do not need any status response back from the
server then load will be reduced significantly. The assumption is
that, since the update rate is high, stray losses in geo-location
reports will be compensated with the large update rate.
Note: It may be argued that the above example application can also
be implemented using Observe option ([RFC7641]) with NON
notifications. But, in practice, implementing with Observe would
require lot of book-keeping at the data-collection end-point at
the backend (observer). The observer needs to maintain all the
observe relationships with each vehicle. The data collection end-
point may be unable to know all its data sources beforehand. The
client end-points at vehicles may go offline or come back online
randomly. In case of Observe the onus is always on the data
collection end-point to establish an observe relationship with
each data-source. On the other hand, implementation will be much
simpler if the initiative is left on the data-source to carry out
updates using No-Response option. Putting it another way: the
implementation choice depends on the perspective of interest to
initiate the update. In an Observe scenario the interest is
expressed by the data-consumer. On the contrary, the classic
update case applies when it is the interest of the data-producer.
The 'No-Response' option enables to make classic updates further
less resource consuming.
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3.2. Multicasting actuation command from a handheld device to a group
of appliances
A handheld device (e.g. a smart phone) may be programmed to act as
an IP enabled switch to remotely operate on a single or group of IP
enabled appliances. For example the smart phone can be programmed to
send a multicast request to switch on/ off all the lights of a
building. In this case the IP switch application can use the No-
Response option in a NON request message to reduce the traffic
generated due to simultaneous CoAP responses from hundreds of
lights.
Thus No-Response helps in reducing overall communication cost and
the probability of network congestion in this case.
3.2.1. Using granular response suppression
The IP switch application may optionally use granular response
suppression such that the error responses are not suppressed. In
that case the lights which could not execute the request would
respond back and be readily identified. Thus, explicit suppression
of option classes by the multicast client may be useful to debug the
network and the application.
4. Miscellaneous aspects
This section further describes important implementation aspects
worth considering while using the No-Response option. The following
discussion contains guidelines and requirements (derived by
combining [RFC7252], [RFC7390] and [RFC5405]) for the application
developer.
4.1. Re-using Tokens
Tokens provide a matching criteria between a request and the
corresponding response. The life of a Token starts when it is
assigned to a request and ends when the final matching response is
received. Then the Token can again be re-used. However, a request
with No-Response typically does not have any guaranteed response
path. So, the client has to decide on its own about when it can
retire a Token which has been used in an earlier request so that the
Token can be reused in a future request. Since the No-Response
option is 'elective', a server which has not implemented this option
will respond back. This leads to the following two scenarios:
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The first scenario is, the client is never going to care about any
response coming back or about relating the response to the original
request. In that case it MAY reuse the Token value at liberty.
However, as a second scenario, let us consider that the client sends
two requests where the first request is with No-Response and the
second request, with same Token, is without No-Response. In this
case a delayed response to the first one can be interpreted as a
response to the second request (client needs a response in the
second case) if the time interval between using the same Token is
not long enough. This creates a problem in the request-response
semantics.
The most ideal solution would be to always use a unique Token for
requests with No-Response. But if a client wants to reuse a Token
then in most practical cases the client implementation SHOULD
implement an application specific reuse time after which it can
reuse the Token. This document suggests a minimum reuse time for
Tokens with a similar expression as in Section 2.5 of [RFC7390]:
TOKEN_REUSE_TIME = NON_LIFETIME + MAX_SERVER_RESPONSE_DELAY +
MAX_LATENCY.
NON_LIFETIME and MAX_LATENCY are defined in 4.8.2 of [RFC7252].
MAX_SERVER_RESPONSE_DELAY has same interpretation as in Section 2.5
of [RFC7390] for multicast request. For a unicast request, since the
message is sent to only one server, MAX_SERVER_RESPONSE_DELAY means
the expected maximum response delay from the particular server to
which client sent the request. For multicast requests,
MAX_SERVER_RESPONSE_DELAY has the same interpretation as in Section
2.5 of [RFC7390]. So for multicast it is the expected maximum server
response delay "over all servers that the client can send a
multicast request to". This response delay for a given server
includes its specific Leisure period; where Leisure is defined in
Section 8.2 of [RFC7252]. In general, the Leisure for a server may
not be known to the client. A lower bound for Leisure, lb_Leisure,
is defined in [RFC7252], but not an upper bound as is needed in this
case. Therefore the upper bound can be estimated by taking N (N>>1)
times the lower bound lb_Leisure:
lb_Leisure = S * G / R
(S = estimated response size; R = data transfer rate; G = group size
estimate)
Any estimate of MAX_SERVER_RESPONSE_DELAY MUST be larger than
DEFAULT_LEISURE as defined in [RFC7252].
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Note: If it is not possible for the client to get a reasonable
estimate of the MAX_SERVER_RESPONSE_DELAY then the client, to be
safe, SHOULD use a unique Token for each request with No-Response
to the same server endpoint.
4.2. Taking care of congestion
A detailed discussion on congestion control is out-of-scope of this
document. However, this section of the document mentions certain
aspects on congestion control which may be useful input for future
work on congestion control for CoAP.
If this option is used with NON messages then the interaction
becomes completely open-loop. Absence of any feed-back from the
server-end affects congestion-control mechanism. In this case the
communication pattern belongs to the class of low-data volume
applications as described in Section 3.1.2 of [RFC5405]. More
precisely, it maps to the scenario where the application cannot
maintain an RTT estimate. Hence, following [RFC5405], a 3 seconds
interval is suggested as the minimum interval between successive
updates. However, in case of more frequent updates, an application
developer MUST interweave occasional closed-loop exchanges (e.g. NON
messages without No-Response or simply CON messages) to get an RTT
estimate between the end-points.
4.3. Handling No-Response option for a HTTP-to-CoAP reverse proxy
A HTTP-to-CoAP reverse proxy MAY translate an incoming HTTP request
to a corresponding CoAP request indicating that no response is
required (suppressing all classes of responses) based on some
application specific requirement. In this case it is RECOMMENDED
that the reverse proxy generates an HTTP response with status code
204 (No Content) when such response is allowed. The generated
response is sent after the proxy has successfully sent out the CoAP
request.
In case the reverse proxy applies No-Response for particular
class(es) of response(s) it will wait for responses up to an
application specific maximum time (T_max) before responding back to
the HTTP-side. If a response of a desired class is received within
T_max then the response gets translated to HTTP as defined in [I-
D.ietf-core-http-mapping]. However if the proxy does not receive any
response within T_max, it is RECOMMENDED that the reverse Proxy
sends an HTTP response with status code 204 (No Content) when
allowed for the specific HTTP request method.
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5. Example
This section illustrates few examples of exchanges based on the
scenario narrated in Section 3.1.
5.1. Using No-Response with PUT
Figure 1 shows a typical request with this option. The depicted
scenario occurs when the vehicle#n moves very fast and update rate
is high. The vehicle is assigned a dedicated resource: vehicle-stat-
<n>, where <n> can be any string uniquely identifying the vehicle.
The update requests are sent over NON type of messages. The No-
Response option causes the server not to respond back.
Client Server
| |
| |
+----->| Header: PUT (T=NON, Code=0.03, MID=0x7d38)
| PUT | Token: 0x53
| | Uri-Path: "vehicle-stat-00"
| | Content Type: text/plain
| | No-Response: 127
| | Payload:
| | "VehID=00&RouteID=DN47&Lat=22.5658745&Long=88.4107966667&
| | Time=2013-01-13T11:24:31"
| |
[No response from the server. Next update in 20 secs.]
| |
+----->| Header: PUT (T=NON, Code=0.03, MID=0x7d39)
| PUT | Token: 0x54
| | Uri-Path: "vehicle-stat-00"
| | Content Type: text/plain
| | No-Response: 127
| | Payload:
| | "VehID=00&RouteID=DN47&Lat=22.5649015&Long=88.4103511667&
| | Time=2013-01-13T11:24:51"
Figure 1: Exemplary unreliable update with No-Response option using
PUT.
5.2. Using No-Response with POST
5.2.1. POST updating a fixed target resource
In this case POST acts the same way as PUT. The exchanges are same
as above. The updated values are carried as payload of POST as shown
in Figure 2.
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Client Server
| |
| |
+----->| Header: POST (T=NON, Code=0.02, MID=0x7d38)
| POST | Token: 0x53
| | Uri-Path: "vehicle-stat-00"
| | Content Type: text/plain
| | No-Response: 127
| | Payload:
| | "VehID=00&RouteID=DN47&Lat=22.5658745&Long=88.4107966667&
| | Time=2013-01-13T11:24:31"
| |
[No response from the server. Next update in 20 secs.]
| |
+----->| Header: PUT (T=NON, Code=0.02, MID=0x7d39)
| POST | Token: 0x54
| | Uri-Path: "vehicle-stat-00"
| | Content Type: text/plain
| | No-Response: 127
| | Payload:
| | "VehID=00&RouteID=DN47&Lat=22.5649015&Long=88.4103511667&
| | Time=2013-01-13T11:24:51"
Figure 2: Exemplary unreliable update with No-Response option using
POST as the update-method.
5.2.2. POST updating through query-string
It may be possible that the backend infrastructure (as described in
Section 3.1) deploys a dedicated database to store the location
updates. In such a case the client can update through a POST by
sending a query string in the URI. The query-string contains the
name/value pairs for each update. 'No-Response' can be used in same
manner as for updating fixed resources. The scenario is depicted in
Figure 3.
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Client Server
| |
| |
+----->| Header: POST (T=NON, Code=0.02, MID=0x7d38)
| POST | Token: 0x53
| | Uri-Path: "updateOrInsertInfo"
| | Uri-Query: "VehID=00"
| | Uri-Query: "RouteID=DN47"
| | Uri-Query: "Lat=22.5658745"
| | Uri-Query: "Long=88.4107966667"
| | Uri-Query: "Time=2013-01-13T11:24:31"
| | No-Response: 127
| |
[No response from the server. Next update in 20 secs.]
| |
+----->| Header: POST (T=NON, Code=0.02, MID=0x7d39)
| POST | Token: 0x54
| | Uri-Path: "updateOrInsertInfo"
| | Uri-Query: "VehID=00"
| | Uri-Query: "RouteID=DN47"
| | Uri-Query: "Lat=22.5649015"
| | Uri-Query: "Long=88.4103511667"
| | Uri-Query: "Time=2013-01-13T11:24:51"
| | No-Response: 127
| |
Figure 3: Exemplary unreliable update with No-Response option using
POST with a query-string to insert update information to backend
database.
6. IANA Considerations
The IANA has assigned number 284 to this option in the CoAP Option
Numbers registry:
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+--------+--------------+----------------------------+
| Number | Name | Reference |
+--------+--------------+----------------------------+
| 284 | No-Response | Section 2 of this document |
+--------+--------------+----------------------------+
7. Security Considerations
The No-Response option defined in this document presents no security
considerations beyond those in Section 11 of the base CoAP
specification [RFC7252].
8. Acknowledgments
Thanks to Carsten Bormann, Matthias Kovatsch, Esko Dijk, Bert
Greevenbosch, Akbar Rahman and Klaus Hartke for their valuable
inputs.
9. References
9.1. Normative References
[RFC7252]
Shelby, Z., Hartke, K. and Bormann, C.,"Constrained Application
Protocol (CoAP)", RFC 7252, June, 2014
[RFC7641]
Hartke, K.," Observing Resources in the Constrained Application
Protocol (CoAP)", RFC 7641, September, 2015
[RFC7390]
Rahman, A. and Dijk, E.,"Group Communication for CoAP", RFC 7390,
October, 2014
[RFC5405]
Eggert, L. and Fairhurst, G.," Unicast UDP Usage Guidelines for
Application Designers", RFC 5405, November, 2008
[I-D.ietf-core-http-mapping]
Castellani, A., et al., "Guidelines for HTTP-CoAP Mapping
Implementations", draft-ietf-core-http-mapping-07, July 3, 2015
Bhattacharyya, et al. Expires August 17, 2016 [Page 15]
Internet-Draft draft-tcs-coap-no-response-option-14 February 2016
9.2. Informative References
[MOBIQUITOUS 2013]
Bhattacharyya, A., Bandyopadhyay, S. and Pal, A., "ITS-light:
Adaptive lightweight scheme to resource optimize intelligent
transportation tracking system (ITS)-Customizing CoAP for
opportunistic optimization", 10th International Conference on Mobile
and Ubiquitous Systems: Computing, Networking and Services
(Mobiquitous 2013), December, 2013.
[Sensys 2013]
Bandyopadhyay, S., Bhattacharyya, A. and Pal, A., "Adapting protocol
characteristics of CoAP using sensed indication for vehicular
analytics", 11th ACM Conference on Embedded Networked Sensor Systems
(Sensys 2013), November, 2013.
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Authors' Addresses
Abhijan Bhattacharyya
Tata Consultancy Services Ltd.
Kolkata, India
Email: abhijan.bhattacharyya@tcs.com
Soma Bandyopadhyay
Tata Consultancy Services Ltd.
Kolkata, India
Email: soma.bandyopadhyay@tcs.com
Arpan Pal
Tata Consultancy Services Ltd.
Kolkata, India
Email: arpan.pal@tcs.com
Tulika Bose
Tata Consultancy Services Ltd.
Kolkata, India
Email: tulika.bose@tcs.com
Bhattacharyya, et al. Expires August 17, 2016 [Page 17]