CoAP Time Scale Option

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Last updated 2017-10-27
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Network Working Group                                        A. Minaburo
Internet-Draft                                                    Acklio
Intended status: Informational                                L. Toutain
Expires: April 29, 2018                Institut Mines Telecom Atlantique
                                                        October 26, 2017

                         CoAP Time Scale Option


   SCHC compression mechanism for LPWAN network enables IPv6 on devices
   connected to a constrained network (LPWAN).  They can communicate
   with a CoAP server located anywhere in the Internet.  LPWAN network
   characteristics limits the number of exchanges and may impose a long
   RTT.  The CoAP server must be aware of these properties to manage
   correctly requests.  The Time Scale option allows a device to inform
   a CoAP server of the duration the message ID value should be kept in
   memory to manage correctly message duplication.

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Internet-Draft           CoAP Time Scale Option             October 2017

   to this document.  Code Components extracted from this document must
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1.  CoAP Message ID

   Constraint Application Protocol (CoAP) [RFC7252] implements a simple
   reliable transport mechanism based on ARQ.  Each CoAP message
   contains a 16 bit Message ID (noted afterward MID).  A client selects
   a MID in a CON message and expects an ACK message containing the same
   MID value.  A timer makes the client resend the request if no ACK is
   received during a pre-defined period.

   To avoid a second process of duplicated requests by the server, a
   list of messages ID already acknowledged must be maintained for a
   period of time.  If the message ID is already in the list, the
   message is just acknowledged and not processed by upper layer.
   Therefore, the client cannot use this MID value in another request
   during the same period of time.

               client                              server
                 |           CON MID = XXXX          |
                 |---------------------------------->|=> process
    EXCHANGE   ^ |           ACK MID = XXXX          |  ^
    LIFETIME   | |                  <----------------|  | EXCHANGE
               | |                                   |  | LIFETIME
     XXXX      | |           CON MID = XXXX          |  |
     cannot    | |---------------------------------->|  | XXXX in
     be reused | |           ACK MID = XXXX          |  | memory
               | |<----------------------------------|  |
               | |                                   |  |
               v |                                   |  v
    XXXX can be  .                                   .
    reused       .                                   .
                 .                                   .
                 |           CON MID = XXXX          |
                 |---------------------------------->|=> process
               ^ |           ACK MID = XXXX          |  ^
               | |<----------------------------------|  |
                 .                                   .

                      Figure 1: Delayed transmission.

   [RFC7252] calls the period a MID is assigned to a request the
   EXCHANGE_LIFETIME.  The value is based on the worst case scenario

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   taking into account the propagation time, the number of
   retransmissions and the processing time.  The default value for
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