CoAP option for no server-response

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CoRE                                                   A. Bhattacharyya
Internet Draft                                         S. Bandyopadhyay
Intended status: Standards track                                 A. Pal
Expires: May 2016                                               T. Bose
                                         Tata Consultancy Services Ltd.
                                                       November 2, 2015

                    CoAP option for no server-response


   There can be M2M scenarios where responses from server against
   requests from client might be considered 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 simultaneously updating a bulk of
   resources 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 draft introduces a header option for CoAP called 'No-Response'.
   Using this option the client explicitly tells the server to suppress
   responses against the particular request. This option also provides
   granular control to enable suppression of a particular class or a
   combination of response-classes. This option may be effective for
   both unicast and multicast requests. Present draft also discusses
   few exemplary applications which benefit from this option.

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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................................10
      4.3. Handling No-Response option for a reverse proxy..........11
   5. Example.......................................................11
      5.1. Using No-Response with PUT...............................11
      5.2. Using No-Response with POST..............................12

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         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 draft 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. This option
   allows all classes of response by default. 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 draft presents some practical
   application scenarios which should bring out the usefulness of this

   Wherever, in this draft, it is mentioned that a request from client
   is with No-Response the intended meaning is that the client
   expressed its disinterest for all or some selected classes of

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 systems may benefit in the
   following aspects:

       * Reduction in network clogging 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.

       * Reduction in overall communication cost.

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1.2. Terminology

   The terms used in this draft are in conformance with those defined
   in [RFC7252].

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC-2119.

2. Option Definition

   The properties of this 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
      significance if piggybacked responses are triggered. But, in case
      the server responds with a separate response (which, may be, the
      client does not care about) then this option can be useful.
      Suppressing the separate response reduces one additional traffic
      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

   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. Client does not
      have any knowledge about that. 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.

   This option is defined as a bit-map (Table 2) to achieve granular

   | 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
   suppressing 2.xx and 5.xx. At present the "CoAP Response Codes"

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   registry (Ref. Section 12.1.2 of [RFC7252]) defines only 2.xx, 4.xx
   and 5.xx responses.

   So, an option value of 22 (binary: 00010110) 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: 2**7 - 1 = 127.

   Implementation Note: When No-Response is used with empty or 0 value
      in a request the client end-point SHOULD cease listening to
      response 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 & 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 successful. The
      application designer needs to tackle such situation. For example,
      while performing frequent updates, the client may strategically
      interweave requests without No-Response 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 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'. Observe-   |
   |     GET     | cancellation requires a client to issue a GET      |
   |             | request with Observe option set to 'deregister'    |
   |             | (1). Since, in this case, the server response does |
   |             | not contain any payload the client MAY express its |
   |             | disinterest in server responses.                   |
   |             | Suitable for frequent updates (particularly in NON |
   |     PUT     | mode) on existing resources. Might not be useful   |
   |             | when PUT creates 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

3. Exemplary application scenarios

   This section describes some exemplary user stories which may
   potentially get benefitted through 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 a location co-ordinate which the vehicle has already
   crossed is not efficient as it adds redundant traffic to the
   network. So, the updates are done in NON mode. However, given the
   huge number of vehicles updating frequently, the NON exchange will
   also trigger huge number of status 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 significant load will be reduced. The assumption is
   that, since the update rate is high, stray losses in geo-locations
   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 exercise 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
      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 No-Response
   option along with NON request to reduce the traffic generated due to
   simultaneous status responses from hundreds of lights.

   Thus No-Response helps in reducing overall communication cost and
   the probability of network clogging 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 few important implementation aspects
   worth considering while using No-Response. The following discussion
   does not mandate anything, rather suggests some guidelines 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:

   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.

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   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 gap between using the same tokens is not 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 draft suggests a reuse time for tokens with
   similar expression as in Section 2.5 of [RFC7390]:


   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. But for 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 it is the expected
   delay "over all servers that the client can send a multicast request
   to". This delay includes the maximum Leisure time period as defined
   in Section 8.2 of [RFC7252]. [RFC7252] defines a rough lower bound
   of leisure as:

                          lb_Leisure = S * G / R

   (S = estimated response size; R = data transfer rate; G = group size

   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 request with No-Response to
      the same server endpoint.

4.2. Taking care of congestion

   A detail technical discussion on congestion control is out-of-scope
   of this draft. However, this section of the draft mentions certain
   aspects on congestion control which may help a detail work on
   congestion control for CoAP as a whole.

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   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]. Precisely,
   it maps to the scenario where the application cannot maintain an RTT
   estimate. Hence, following [RFC5405], a 3s interval is suggested as
   the minimum interval between successive updates. However, in case of
   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

4.3. Handling No-Response option for a reverse proxy

   A reverse proxy (HTTP to CoAP) 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 SHOULD send an HTTP response with status code
   204 (No Content).

   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 response of desired class is received within T_max
   then the request gets translated 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 SHOULD send an HTTP
   response with status code 204 (No Content).

5. Example

   This section illustrates few examples of exchanges based on the
   scenario narrated in Section 3.1. Examples for other scenarios can
   be easily conceived based on these illustrations.

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.

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   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

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

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

9. References

9.1. Normative References


   Shelby, Z., Hartke, K. and Bormann, C.,"Constrained Application
   Protocol (CoAP)", RFC 7252, June, 2014


   Hartke, K.," Observing Resources in the Constrained Application
   Protocol (CoAP)", RFC 7641, September, 2015


   Rahman, A. and Dijk, E.,"Group Communication for CoAP", RFC 7390,
   October, 2014


   Eggert, L. and Fairhurst, G.," Unicast UDP Usage Guidelines for
   Application Designers", RFC 5405, November, 2008


   Castellani, A., et al., "Guidelines for HTTP-CoAP Mapping
   Implementations", draft-ietf-core-http-mapping-07, July 3, 2015

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9.2. Informative References


   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


   Soma Bandyopadhyay
   Tata Consultancy Services Ltd.
   Kolkata, India


   Arpan Pal
   Tata Consultancy Services Ltd.
   Kolkata, India


   Tulika Bose
   Tata Consultancy Services Ltd.
   Kolkata, India


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