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CoAP Implementation Guidance

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This is an older version of an Internet-Draft whose latest revision state is "Replaced".
Authors Matthias Kovatsch , Olaf Bergmann , Esko Dijk , Xuan He , Carsten Bormann
Last updated 2014-01-17
Replaced by draft-ietf-lwig-coap
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LWIG Working Group                                           M. Kovatsch
Internet-Draft                                                ETH Zurich
Intended status: Informational                               O. Bergmann
Expires: July 20, 2014                           Universitaet Bremen TZI
                                                                 E. Dijk
                                                        Philips Research
                                                                   X. He
                                               Hitachi (China) R&D Corp.
                                                         C. Bormann, Ed.
                                                 Universitaet Bremen TZI
                                                        January 16, 2014

                      CoAP Implementation Guidance


   The Constrained Application Protocol (CoAP) is designed for resource-
   constrained nodes and networks, e.g., sensor nodes in a low-power
   lossy network (LLN).  Yet to implement this Internet protocol on
   Class 1 devices (i.e., ~ 10 KiB of RAM and ~ 100 KiB of ROM) also
   lightweight implementation techniques are necessary.  This document
   provides lessons learned from implementing CoAP for tiny, battery-
   operated networked embedded systems.  The guidelines for transmission
   state management and developer APIs can also help with the
   implementation of CoAP for less constrained nodes.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 20, 2014.

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Copyright Notice

   Copyright (c) 2014 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Protocol Handling . . . . . . . . . . . . . . . . . . . . . .   3
     2.1.  Message Processing  . . . . . . . . . . . . . . . . . . .   4
     2.2.  Header Information  . . . . . . . . . . . . . . . . . . .   4
       2.2.1.  On-the-fly Processing . . . . . . . . . . . . . . . .   4
       2.2.2.  Internal Data Structure . . . . . . . . . . . . . . .   5
     2.3.  Token Usage . . . . . . . . . . . . . . . . . . . . . . .   5
     2.4.  Deduplication . . . . . . . . . . . . . . . . . . . . . .   5
     2.5.  Transmission States . . . . . . . . . . . . . . . . . . .   6
       2.5.1.  Request/Response Layer  . . . . . . . . . . . . . . .   6
       2.5.2.  Message Layer . . . . . . . . . . . . . . . . . . . .   7
     2.6.  Out-of-band Information . . . . . . . . . . . . . . . . .   8
   3.  Optimizations . . . . . . . . . . . . . . . . . . . . . . . .   8
     3.1.  Message Buffers . . . . . . . . . . . . . . . . . . . . .   8
     3.2.  Retransmissions . . . . . . . . . . . . . . . . . . . . .   9
     3.3.  Observable Resources  . . . . . . . . . . . . . . . . . .  10
     3.4.  Blockwise Transfers . . . . . . . . . . . . . . . . . . .  10
     3.5.  Deduplication with Sequential MIDs  . . . . . . . . . . .  11
   4.  Alternative Configurations  . . . . . . . . . . . . . . . . .  14
     4.1.  Transmission Parameters . . . . . . . . . . . . . . . . .  14
     4.2.  CoAP over IPv4  . . . . . . . . . . . . . . . . . . . . .  14
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  14
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .  14
     5.2.  Informative References  . . . . . . . . . . . . . . . . .  14
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   The Constrained Application Protocol [I-D.ietf-core-coap] has been
   designed specifically for machine-to-machine communication in
   networks with very constrained nodes.  Typical application scenarios

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   therefore include building automation and the Internet of Things.
   The major design objectives have been set on small protocol overhead,
   robustness against packet loss, and against high latency induced by
   small bandwidth shares or slow request processing in end nodes.  To
   leverage integration of constrained nodes with the world-wide
   Internet, the protocol design was led by the REST architectural style
   that accounts for the scalability and robustness of the Hypertext
   Transfer Protocol [RFC2616].

   Lightweight implementations benefit from this design in many
   respects: First, the use of Uniform Resource Identifiers (URIs) for
   naming resources and the transparent forwarding of their
   representations in a server-stateless request/response protocol make
   protocol translation to HTTP a straightforward task.  Second, the set
   of protocol elements that are unavoidable for the core protocol and
   thus must be implemented on every node has been kept very small,
   minimizing the unnecessary accumulation of "optional" features.
   Options that - when present - are critical for message processing are
   explicitly marked as such to force immediate rejection of messages
   with unknown critical options.  Third, the syntax of protocol data
   units is easy to parse and is carefully defined to avoid creation of
   state in servers where possible.

   Although these features enable lightweight implementations of the
   Constrained Application Protocol, there is still a tradeoff between
   robustness and latency of constrained nodes on one hand and resource
   demands (such as battery consumption, dynamic memory needs, and
   static code size) on the other.  The present document gives some
   guidance on possible strategies to solve this tradeoff for very
   constrained nodes (Class 1 in [I-D.ietf-lwig-terminology]).  The main
   focus is on servers as this is deemed the predominant case where CoAP
   applications are faced with tight resource constraints.

   Additional considerations for the implementation of CoAP on tiny
   sensors are given in [I-D.arkko-core-sleepy-sensors].

2.  Protocol Handling

   For constrained nodes of Class 1 or even Class 2, the most limiting
   factors for (wireless) network communication usually are memory size
   and battery lifetime.  Most applications therefore try to minimize
   internal buffer space for both transmit and receive operations, and
   to maximize sleeping cycles.

   In the programming styles supported by very simple operating systems,
   preemptive multi-threading is not an option.  Instead, all operations
   are triggered by an event loop system, e.g., in a send-receive-
   dispatch cycle.  It is also common practice to allocate memory

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   statically to ensure stable behavior, as no memory management unit
   (MMU) or other abstractions are available.  For a CoAP node, the two
   key parameters for memory usage are the number of (re)transmission
   buffers and the maximum message size that must be supported by each
   buffer.  Often the maximum message size is set far below the
   1280-byte MTU of 6LoWPAN to allow more than one open Confirmable
   transmission at a time (in particular for observe notifications).
   Note that implementations on constrained platforms often not even
   support the full MTU.  Larger messages must then use block-wise
   transfers [I-D.ietf-core-block], while a good tradeoff between
   6LoWPAN fragmentation and CoAP header overhead must be found.
   Usually the amount of available free RAM dominates this decision.
   For Class 1 devices, the maximum message size is typically 128 or 256
   bytes plus an estimate of the maximum header size with a worst case
   option setting.

2.1.  Message Processing

   T.B.D.: Basic steps and correct error handling

2.2.  Header Information

   There are two alternatives to handle the header: Either process the
   header on the fly when an option is accessed or initially parse all
   values into an internal data structure.

2.2.1.  On-the-fly Processing

   The advantage of on-the-fly processing is that the compact encoding
   saves memory and fully reuses the buffer for incoming messages.  The
   basic message header information should always be copied into an
   internal data structure, as Message ID and/or Token are required for
   request/response matching and generating the response.  Once the
   message is accepted for further processing, the set of options
   contained in the received message must be decoded to check for
   unknown critical options.  To avoid multiple passes through the
   option list, the option parser might maintain a bit-vector where each
   bit represents an option number that is present in the received
   request.  With the wide and sparse range of option numbers, the
   number itself cannot be used to indicate the number of left-shift
   operations to mask the corresponding bit.  Hence, an implementation-
   specific enum of supported options should be used to mask the present
   options of a message in the bitmap.  In addition, the byte index of
   every option can be added to a sparse list (e.g., a one-dimensional
   array) for fast retrieval.

   Once the option list has been processed at least up to the highest
   option number that is supported by the application, any known

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   critical option and all elective options can be masked out to
   determine if any unknown critical option was present.  If this is the
   case, this information can be used to create a 4.02 response
   accordingly.  (Note that the remaining options also must be processed
   to add further critical options included in the original request.)

2.2.2.  Internal Data Structure

   Using an internal data structure for all parsed options has
   advantages when processing the values, as they are already in a
   variable of corresponding type and integers in host byte order.  The
   incoming payload and byte strings of the header can be accessed
   directly in its IP buffer using pointers.  This approach also
   benefits from a bitmap.  Otherwise special values must be reserved to
   encode an unset option, which might require a larger type than
   required for the actual value range (e.g., a 32-bit integer instead
   of 16-bit).

   The byte strings (e.g., the URI) are usually not required when
   generating the response.  Thus, this alternative also facilitates the
   usage of the IP buffer for message assembly - all important values
   are copied from the shared incoming/outgoing buffer.

   Setting options for outgoing messages is also easier with an internal
   data structure.  Application developers can set options independent
   from the option number order required for the delta encoding.  The
   CoAP encoding is then applied in a serialization step before sending.
   On-the-fly processing might require extensive memmove operations to
   insert new header options or needs to restrict developers to set
   options in order.

2.3.  Token Usage

   T.B.D.: Discuss separation of Token space for correct behavior

2.4.  Deduplication

   If CoAP is used directly on top of UDP (i.e., in NoSec mode), it
   needs to cope with the fact that the UDP datagram transport can
   reorder and duplicate messages.  (In contrast to UDP, DTLS has its
   own duplicate detection.)  CoAP has been designed with protocol
   functionality such that rejection of duplicate messages is always
   possible.  It is at the discretion of the receiver if it actually
   wants to make use of this functionality.  Processing of duplicate
   messages comes at a cost, but so does the management of the state
   associated with duplicate rejection.  Hence, a receiver may have good
   reasons to decide not to do the duplicate rejection.  If duplicate

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   rejection is indeed necessary, e.g., for non-idempotent requests, it
   is important to control the amount of state that needs to be stored.

   Deduplication is heavy for Class 1 devices, as the number of peer
   addresses can be vast.  Servers should be kept stateless, i.e., the
   REST API should be designed idempotent whenever possible.  When this
   is not the case, the resource handler could perform an optimized
   deduplication by exploiting knowledge about the application.
   Another, server-wide strategy is to only keep track of non-idempotent

2.5.  Transmission States

   CoAP endpoints must keep transmission state to manage open requests,
   to handle the different response modes, and to implement reliable
   delivery at the message layer.  The following finite state machines
   (FSMs) model the transmissions of a CoAP exchange at the request/
   response layer and the message layer.  These layers are linked
   through actions.  The M_CMD() action triggers a corresponding
   transition at the message layer and the RR_EVT() action triggers a
   transition at the request/response layer.  The FSMs also use guard
   conditions to distinguish between information that is only available
   through the other layer (e.g., whether a request was sent using a CON
   or NON message).

2.5.1.  Request/Response Layer

   Figure 1 depicts the two states at the request/response layer of a
   CoAP client.  When a request is issued, a "reliable_send" or
   "unreliable_send" is triggered at the message layer.  The WAITING
   state can be left through three transitions: Either the client
   cancels the request and triggers cancellation of a CON transmission
   at the message layer, the client receives a failure event from the
   message layer, or a receive event containing a response.

       |        / M_CMD(cancel)                          |
       |                                                 V
       |                                              +------+
   +-------+ -------RR_EVT(fail)--------------------> |      |
   |WAITING|                                          | IDLE |
   +-------+ -------RR_EVT(rx)[is Response]---------> |      |
       ^                / M_CMD(accept)               +------+
       |                                                 |
                  / M_CMD((un)reliable_send)

             Figure 1: CoAP Client Request/Response Layer FSM

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   A server resource can decide at the request/response layer whether to
   respond with a piggy-backed or a separate response.  Thus, there are
   two busy states in Figure 2, SERVING and SEPARATE.  An incoming
   receive event with a NON request directly triggers the transition to
   the SEPARATE state.

   +--------+ <----------RR_EVT(rx)[is NON]---------- +------+
   |SEPARATE|                                         |      |
   +--------+ ----------------RESPONSE--------------> | IDLE |
       ^            / M_CMD((un)reliable_send)        |      |
       |                                        +---> +------+
       |EMPTY_ACK                               |         |
       |/M_CMD(accept)                          |         |
       |                                        |         |
       |                                        |         |
   +--------+                                   |         |
   |SERVING | --------------RESPONSE------------+         |
   +--------+          / M_CMD(accept)                    |
       ^                                                  |
       +------------------------RR_EVT(rx)[is CON]--------+

             Figure 2: CoAP Server Request/Response Layer FSM

2.5.2.  Message Layer

   Figure 3 shows the different states of a CoAP endpoint per message
   exchange.  Besides the linking action RR_EVT(), the message layer has
   a TX action to send a message.  For sending and receiving NONs, the
   endpoint remains in its CLOSED state.  When sending a CON, the
   endpoint remains in RELIABLE_TX and keeps retransmitting until the
   transmission times out, it receives a matching RST, the request/
   response layer cancels the transmission, or the endpoint receives an
   implicit acknowledgement through a matching NON or CON.  Whenever the
   endpoint receives a CON, it transitions into the ACK_PENDING state,
   which can be left by sending the corresponding ACK.

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   +-----------+ <-------M_CMD(reliable_send)-----+
   |           |            / TX(con)              \
   |           |                                +--------------+
   |           | ---TIMEOUT(RETX_WINDOW)------> |              |
   |RELIABLE_TX|     / RR_EVT(fail)             |              |
   |           | ---------------------RX_RST--> |              | <----+
   |           |               / RR_EVT(fail)   |              |      |
   +-----------+ ----M_CMD(cancel)------------> |    CLOSED    |      |
    ^  |  |  \  \                               |              | --+  |
    |  |  |   \  +-------------------RX_ACK---> |              |   |  |
    +*1+  |    \                / RR_EVT(rx)    |              |   |  |
          |     +----RX_NON-------------------> +--------------+   |  |
          |       / RR_EVT(rx)                  ^ ^ ^ ^  | | | |   |  |
          |                                     | | | |  | | | |   |  |
          |                                     | | | +*2+ | | |   |  |
          |                                     | | +--*3--+ | |   |  |
          |                                     | +----*4----+ |   |  |
          |                                     +------*5------+   |  |
          |                +---------------+                       |  |
          |                |  ACK_PENDING  | <--RX_CON-------------+  |
          +----RX_CON----> |               |  / RR_EVT(rx)            |
            / RR_EVT(rx)   +---------------+ ---------M_CMD(accept)---+
                                                        / TX(ack)

   *2: M_CMD(unreliable_send) / TX(non)
   *3: RX_NON / RR_EVT(rx)
   *5: RX_ACK

                     Figure 3: CoAP Message Layer FSM

   T.B.D.: (i) Rejecting messages (can be triggered at message and
   request/response layer). (ii) ACKs can also be triggered at both

2.6.  Out-of-band Information

   T.B.D.: Discuss where out-of-band information can help

3.  Optimizations

3.1.  Message Buffers

   The cooperative multi-threading of an event loop system allows to
   optimize memory usage through in-place processing and reuse of
   buffers, in particular the IP buffer provided by the OS or firmware.

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   CoAP servers can significantly benefit from in-place processing, as
   they can create responses directly in the incoming IP buffer.  Note
   that an embedded OS usually only has a single buffer for incoming and
   outgoing IP packets.  The first few bytes of the basic header are
   usually parsed into an internal data structure and can be overwritten
   without harm.  Thus, empty ACKs and RST messages can promptly be
   assembled and sent using the IP buffer.  Also when a CoAP server only
   sends piggy-backed or Non-confirmable responses, no additional buffer
   is required at the application layer.  This, however, requires
   careful timing so that no incoming data is overwritten before it was
   processed.  Because of cooperative multi-threading, this requirement
   is relaxed, though.  Once the message is sent, the IP buffer can
   accept new messages again.  This does not work for Confirmable
   messages, however.  They need to be stored for retransmission and
   would block any further IP communication.

   Depending on the number of requests that can be handled in parallel,
   an implementation might create a stub response filled with any option
   that has to be copied from the original request to the separate
   response, especially the Token option.  The drawback of this
   technique is that the server must be prepared to receive
   retransmissions of the previous (Confirmable) request to which a new
   acknowledgement must be generated.  If memory is an issue, a single
   buffer can be used for both tasks: Only the message type and code
   must be updated, changing the message id is optional.  Once the
   resource representation is known, it is added as new payload at the
   end of the stub response.  Acknowledgements still can be sent as
   described before as long as no additional options are required to
   describe the payload.

3.2.  Retransmissions

   CoAP's reliable transmissions require the before-mentioned
   retransmission buffers.  Messages, such as the requests of a client,
   should be stored in serialized form.  For servers, retransmissions
   apply for Confirmable separate responses and Confirmable
   notifications [I-D.ietf-core-observe].  As separate responses stem
   from long-lasting resource handlers, the response should be stored
   for retransmission instead of re-dispatching a stored request (which
   would allow for updating the representation).  For Confirmable
   notifications, please see Section 2.6, as simply storing the response
   can break the concept of eventual consistency.

   String payloads such as JSON require a buffer to print to.  By
   splitting the retransmission buffer into header and payload part, it
   can be reused.  First to generate the payload and then storing the
   CoAP message by serializing into the same memory.  Thus, providing a
   retransmission for any message type can save the need for a separate

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   application buffer.  This, however, requires an estimation about the
   maximum expected header size to split the buffer and a memmove to
   concatenate the two parts.

   For platforms that disable clock tick interrupts in sleep states, the
   application must take into consideration the clock deviation that
   occurs during sleep (or ensure to remain in idle state until the
   message has been acknowledged or the maximum number of
   retransmissions is reached).  Since CoAP allows up to four
   retransmissions with a binary exponential back-off it could take up
   to 45 seconds until the send operation is complete.  Even in idle
   state, this means substantial energy consumption for low-power nodes.
   Implementers therefore might choose a two-step strategy: First, do
   one or two retransmissions and then, in the later phases of back-off,
   go to sleep until the next retransmission is due.  In the meantime,
   the node could check for new messages including the acknowledgement
   for any Confirmable message to send.

3.3.  Observable Resources

   For each observer, the server needs to store at least address, port,
   token, and the last outgoing message ID.  The latter is needed to
   match incoming RST messages and cancel the observe relationship.

   It is favorable to have one retransmission buffer per observable
   resource that is shared among all observers.  Each notification is
   serialized once into this buffer and only address, port, and token
   are changed when iterating over the observer list (note that
   different token lengths might require realignment).  The advantage
   becomes clear for Confirmable notifications: Instead of one
   retransmission buffer per observer, only one buffer and only
   individual retransmission counters and timers in the list entry need
   to be stored.  When the notifications can be sent fast enough, even a
   single timer would suffice.  Furthermore, per-resource buffers
   simplify the update with a new resource state during open deliveries.

3.4.  Blockwise Transfers

   Block-wise transfers have the main purpose of providing fragmentation
   at the application layer, where partial information can be processed.
   This is not possible at lower layers such as 6LoWPAN, as only
   assembled packets can be passed up the stack.  While
   [I-D.ietf-core-block] also anticipates atomic handling of blocks,
   i.e., only fully received CoAP messages, this is not possible on
   Class 1 devices.

   When receiving a block-wise transfer, each blocks is usually passed
   to a handler function that for instance performs stream processing or

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   writes the blocks to external memory such as flash.  Although there
   are no restrictions in [I-D.ietf-core-block], it is beneficial for
   Class 1 devices to only allow ordered transmission of blocks.
   Otherwise on-the-fly processing would not be possible.

   When sending a block-wise transfer, Class 1 devices usually do not
   have sufficient memory to print the full message into a buffer, and
   slice and send it in a second step.  When transferring the CoRE Link
   Format from /.well-known/core for instance, a generator function is
   required that generates slices of a large string with a specific
   offset length (a 'sonprintf()').  This functionality is required
   recurrently and should be included in a library.

3.5.  Deduplication with Sequential MIDs

   CoAP's duplicate rejection functionality can be straightforwardly
   implemented in a CoAP end-point by storing, for each remote CoAP end-
   point ("peer") that it communicates with, a list of recently received
   CoAP Message IDs (MIDs) along with some timing information.  A CoAP
   message from a peer with a MID that is in the list for that peer can
   simply be discarded.

   The timing information in the list can then be used to time out
   entries that are older than the _expected extent of the re-ordering_,
   an upper bound for which can be estimated by adding the _potential
   retransmission window_ ([I-D.ietf-core-coap] section "Reliable
   Messages") and the time packets can stay alive in the network.

   Such a straightforward implementation is suitable in case other CoAP
   end-points generate random MIDs.  However, this storage method may
   consume substantial RAM in specific cases, such as:

   o  many clients are making periodic, non-idempotent requests to a
      single CoAP server;

   o  one client makes periodic requests to a large number of CoAP
      servers and/or requests a large number of resources; where servers
      happen to mostly generate separate CoAP responses (not piggy-

   For example, consider the first case where the expected extent of re-
   ordering is 50 seconds, and N clients are sending periodic POST
   requests to a single CoAP server during a period of high system
   activity, each on average sending one client request per second.  The
   server would need 100 * N bytes of RAM to store the MIDs only.  This
   amount of RAM may be significant on a RAM-constrained platform.  On a
   number of platforms, it may be easier to allocate some extra program
   memory (e.g. Flash or ROM) to the CoAP protocol handler process than

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   to allocate extra RAM.  Therefore, one may try to reduce RAM usage of
   a CoAP implementation at the cost of some additional program memory
   usage and implementation complexity.

   Some CoAP clients generate MID values by a using a Message ID
   variable [I-D.ietf-core-coap] that is incremented by one each time a
   new MID needs to be generated.  (After the maximum value 65535 it
   wraps back to 0.)  We call this behavior "sequential" MIDs.  One
   approach to reduce RAM use exploits the redundancy in sequential MIDs
   for a more efficient MID storage in CoAP servers.

   Naturally such an approach requires, in order to actually reduce RAM
   usage in an implementation, that a large part of the peers follow the
   sequential MID behavior.  To realize this optimization, the authors
   therefore RECOMMEND that CoAP end-point implementers employ the
   "sequential MID" scheme if there are no reasons to prefer another
   scheme, such as randomly generated MID values.

   Security considerations might call for a choice for
   (pseudo)randomized MIDs.  Note however that with truly randomly
   generated MIDs the probability of MID collision is rather high in use
   cases as mentioned before, following from the Birthday Paradox.  For
   example, in a sequence of 52 randomly drawn 16-bit values the
   probability of finding at least two identical values is about 2

   From here on we consider efficient storage implementations for MIDs
   in CoAP end-points, that are optimized to store "sequential" MIDs.
   Because CoAP messages may be lost or arrive out-of-order, a solution
   has to take into account that received MIDs of CoAP messages are not
   actually arriving in a sequential fashion, due to lost or reordered
   messages.  Also a peer might reset and lose its MID counter(s) state.
   In addition, a peer may have a single Message ID variable used in
   messages to many CoAP end-points it communicates with, which partly
   breaks sequentiality from the receiving CoAP end-point's perspective.
   Finally, some peers might use a randomly generated MID values
   approach.  Due to these specific conditions, existing sliding window
   bitfield implementations for storing received sequence numbers are
   typically not directly suitable for efficiently storing MIDs.

   Table 1 shows one example for a per-peer MID storage design: a table
   with a bitfield of a defined length _K_ per entry to store received
   MIDs (one per bit) that have a value in the range [MID_i + 1 , MID_i
   + K].

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              | MID base | K-bit bitfield | base time value |
              | MID_0    | 010010101001   | t_0             |
              |          |                |                 |
              | MID_1    | 111101110111   | t_1             |
              |          |                |                 |
              | ... etc. |                |                 |

         Table 1: A per-peer table for storing MIDs based on MID_i

   The presence of a table row with base MID_i (regardless of the
   bitfield values) indicates that a value MID_i has been received at a
   time t_i.  Subsequently, each bitfield bit k (0...K-1) in a row i
   corresponds to a received MID value of MID_i + k + 1.  If a bit k is
   0, it means a message with corresponding MID has not yet been
   received.  A bit 1 indicates such a message has been received already
   at approximately time t_i. This storage structure allows e.g. with
   k=64 to store in best case up to 130 MID values using 20 bytes, as
   opposed to 260 bytes that would be needed for a non-sequential
   storage scheme.

   The time values t_i are used for removing rows from the table after a
   preset timeout period, to keep the MID store small in size and enable
   these MIDs to be safely re-used in future communications.  (Note that
   the table only stores one time value per row, which therefore needs
   to be updated on receipt of another MID that is stored as a single
   bit in this row.  As a consequence of only storing one time value per
   row, older MID entries typically time out later than with a simple
   per-MID time value storage scheme.  The end-point therefore needs to
   ensure that this additional delay before MID entries are removed from
   the table is much smaller than the time period after which a peer
   starts to re-use MID values due to wrap-around of a peer's MID
   variable.  One solution is to check that a value t_i in a table row
   is still recent enough, before using the row and updating the value
   t_i to current time.  If not recent enough, e.g. older than N
   seconds, a new row with an empty bitfield is created.)  [Clearly,
   these optimizations would benefit if the peer were much more
   conservative about re-using MIDs than currently required in the
   protocol specification.]

   The optimization described is less efficient for storing randomized
   MIDs that a CoAP end-point may encounter from certain peers.  To
   solve this, a storage algorithm may start in a simple MID storage
   mode, first assuming that the peer produces non-sequential MIDs.
   While storing MIDs, a heuristic is then applied based on monitoring
   some "hit rate", for example, the number of MIDs received that have a

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   Most Significant Byte equal to that of the previous MID divided by
   the total number of MIDs received.  If the hit rate tends towards 1
   over a period of time, the MID store may decide that this particular
   CoAP end-point uses sequential MIDs and in response improve
   efficiency by switching its mode to the bitfield based storage.

4.  Alternative Configurations

4.1.  Transmission Parameters

   When a constrained network of CoAP nodes is not directly
   communication with the Internet, for instance because it is shielded
   by a proxy or a closed deployment, alternative transmission
   parameters can be used.

   T.B.D.: Guidance for parameter tweaking

4.2.  CoAP over IPv4

   CoAP was designed for the properties of IPv6.  In some cases, CoAP
   nodes will communicate over IPv4, though.

   T.B.D.: Considerations for IPv4

5.  References

5.1.  Normative References

              Shelby, Z., Hartke, K., and C. Bormann, "Constrained
              Application Protocol (CoAP)", draft-ietf-core-coap-18
              (work in progress), June 2013.

   [RFC2616]  Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
              Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
              Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.

   [RFC6570]  Gregorio, J., Fielding, R., Hadley, M., Nottingham, M.,
              and D. Orchard, "URI Template", RFC 6570, March 2012.

5.2.  Informative References

              Arkko, J., Rissanen, H., Loreto, S., Turanyi, Z., and O.
              Novo, "Implementing Tiny COAP Sensors", draft-arkko-core-
              sleepy-sensors-01 (work in progress), July 2011.

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              Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
              draft-ietf-core-block-14 (work in progress), October 2013.

              Hartke, K., "Observing Resources in CoAP", draft-ietf-
              core-observe-11 (work in progress), October 2013.

              Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained Node Networks", draft-ietf-lwig-terminology-06
              (work in progress), December 2013.

Authors' Addresses

   Matthias Kovatsch
   ETH Zurich
   Universitaetstrasse 6
   CH-8092 Zurich


   Olaf Bergmann
   Universitaet Bremen TZI
   Postfach 330440
   D-28359 Bremen


   Esko Dijk
   Philips Research


   Xuan He
   Hitachi (China) R&D Corp.
   301, Tower C North, Raycom, 2 Kexuyuan Nanlu
   Beijing, 100190


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   Carsten Bormann (editor)
   Universitaet Bremen TZI
   Postfach 330440
   D-28359 Bremen

   Phone: +49-421-218-63921

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