CoAP option for no server-response
draft-tcs-coap-no-response-option-00

CoRE                                                   A. Bhattacharyya
Internet Draft                                         S. Bandyopadhyay
Intended status: Standards track                                 A. Pal
Expires: February 2014                   Tata Consultancy Services Ltd.
                                                        August 29, 2013

                    CoAP option for no server-response
                 draft-tcs-coap-no-response-option-00.txt

   Abstract

   The server end-point does not respond with acknowledgment (ACK) in
   non-confirmable(NON)mode of CoAP. However, the server responds the
   client with a status code indicating "the result of the attempt to
   understand and satisfy the request". But there may be typical
   scenarios, while updating resources in NON mode, where any kind of
   response from the server may be considered redundant. Thus, in this
   case, the transaction becomes completely open-loop with no reverse
   path from the server to the client. This kind of exchange may be
   useful in constrained systems looking for maximized throughput with
   minimized resource consumption. This draft introduces a header
   option: 'No-Resp' to suppress any response from the server and
   discusses an exemplary but practical use case which actually
   motivated this proposition based on real experience.

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. Motivation ............................................. 3
         1.1.1. The use case
                            ....................................... 3
         1.1.2. Problem description
                                   ................................ 3
         1.1.3. Solution approach and proposed modification........ 3
      1.2. Terminology ............................................ 4
   2. Option Definition ........................................... 4
   3. Example ..................................................... 5
   4. IANA Considerations ......................................... 6
   5. Security Considerations...................................... 6
   6. References .................................................. 7
      6.1. Normative References.................................... 7

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1. Introduction

   This draft proposes a new Constrained Application Protocol (CoAP)
   header option: No-Resp which suppresses any response from the
   server-endpoint. This option can be used while updating any resource
   in non-confirmable (NON) mode.

1.1. Motivation

   Introduction of this option is motivated by a practical use case as
   described in the following sub-sections.

1.1.1. The use case

   We consider tracking application in an intelligent transport system
   (ITS).

   Each vehicle in ITS is equipped with a sensor-gateway connected to
   sensors like GPS and Accelerometer. The sensor-gateway connects to
   the Internet using a low-bandwidth cellular (e.g. GPRS) connection.
   The GPS co-ordinates are periodically updated to the back-end server
   by the gateway. The update period is configurable. In case of ITS
   the update rate is 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 analytic, running on the sensor-
   gateway, using 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.

1.1.2. Problem description

   The higher rate of update puts more load on the network-traffic and
   the load increases in proportion to the increasing number of
   vehicles in the system. Also, the higher rate of update demands more
   energy from the sensor-gateway. So we need to reduce the load on the
   network as well as the sensor-gateway.

1.1.3. Solution approach and proposed modification

   When the vehicle is moving slowly or is static, the update rate is
   low. So each flight of data to the server is important and needs to
   be delivered in a reliable manner due to sparse arrival rate. In
   this case a normal confirmable (CON) mode of update is desirable.

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   When the update rate is high, any lost flight of data is soon to be
   followed by next one. Also, since the vehicle is moving very fast,
   the actual co-ordinates of the vehicle changes by the time a lost
   co-ordinate is retransmitted. So the nature of data allows us to
   tolerate stray losses. Thus the process of retransmitting
   coordinates already passed by may be deemed unnecessary as always
   lost old co-ordinates can be approximated from the new one. So, the
   system can adaptively choose to operate in NON mode. Thus, chances
   of duplicate transmission are also removed and the system becomes
   more light-weight. Thus, reliability is traded-off against
   opportunistic saving of the consumed bandwidth and energy.

   However, in NON mode, the server responds with a response code
   keeping the field for ACK blank. Thus the total bandwidth
   consumption in NON mode becomes comparable to a CON transaction
   which was successful in one go. So, even in NON mode, the client
   waits for the 4 bytes of response from the server. This waiting
   affects the latency of the system. Hence, for further optimization,
   the server status code may also be suppressed. As updates are
   frequent, so the knowledge about the execution status of the
   instantaneous update request may also be considered redundant. The
   sensor-gateway can also switch off the listening functionality as no
   response is expected. Thus the update rate id free of any dependency
   on the latency and becomes largely dependent on the limits of the
   updating device.

1.2. Terminology

   The terms used in this draft are in conformance with those defined
   in [I-D.ietf-core-coap].

   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

   +--------+-----+----------------+-------------+--------+---------+
   | Number | C/E |       Name     | Data Format | Length | Default |
   +--------+-----+----------------+-------------+--------+---------+
   |   TBD  |  E  |    No-Resp     |     empty   |   0B   |  (none) |
   +--------+-----+----------------+-------------+--------+---------+
                           Table 1: Option Properties

   The properties of this option are as in Table 1. This option MAY be
   used to suppress any response from the server notifying the client
   about the execution status of the requested method. This option is

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   presently defined for update requests (e.g., PUT) in NON mode. It
   should have no effect if present with a CON request.

   [TBD1: Probably this option may be useful for CON mode as well in
   case of separate responses. There may be certain scenarios where the
   ACK is enough to satisfy the client application and one extra flight
   due to the separate status response may be avoided.]

   [TBD2: What about the scenarios when something bad happens against
   the request from the client persistently? For example: may be the
   request becomes a "BAD REQUEST" or the request method is "NOT
   ALLOWED" or the resource is "NOT FOUND"! Under such circumstances
   the client should take appropriate action rather than being ignorant
   about the 'bad' status and keep on sending the update request. Then
   it will be really inefficient in terms of resource savings. One way
   could be to make this option conditional such that the response is
   suppressed only if things do not go wrong as mentioned above. But in
   that case the client cannot switch off the listening activity as
   described earlier. May be we can keep this issue open assuming that
   this option will be preferably used in a controlled, dedicated
   infrastructure where these kind of situations are not expected
   unless there is a server crash.]

3. Example

   Figure 1 shows a typical request with this option. As described in
   section 1.1.3, the depicted scenario occurs when the nth vehicle
   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 in NON
   mode. The No-Resp option causes the server not to reply with any
   status code.

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   Client Server
   |      |
   |      |
   +----->| Header: PUT (T=NON, Code=3, MID=0x7d38)
   | PUT  | Token: 0x53
   |      | Uri-Path: "vehicle-stat-00"
   |      | Content Type: text/plain
   |      | No-Resp
   |      | 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=3, MID=0x7d39)
   | PUT  | Token: 0x54
   |      | Uri-Path: "vehicle-stat-00"
   |      | Content Type: text/plain
   |      | No-Resp
   |      | Payload:
   |      | "VehID=00&&RouteID=DN47Lat=22.5649015&Long=88.4103511667&
   |      | Time=2013-01-13T11:24:51"

   Figure 1: Exemplary unreliable update with no feedback path from the
   server.

4. IANA Considerations

   The IANA is requested to add the following option number entries:

   +--------+-----------+----------------------------+
   | Number | Name      |          Reference         |
   +--------+-----------+----------------------------+
   |   TBD  |  No-Resp  | Section 2 of this document |
   +--------+-----------+----------------------------+

5. Security Considerations

   The No-Resp option defined in this document presents no security
   considerations beyond those in Section 11 of the base CoAP
   specification [I-D.ietf-core-coap].

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6. References

6.1. Normative References

   [I-D.ietf-core-coap]

   Shelby, Z., Hartke, K. and Bormann, C.,"Constrained Application
   Protocol (CoAP)", draft-ietf-core-coap-18, June 28, 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

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