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Versions: (draft-kovatsch-lwig-coap) 00 01 02 03 04 05 06

LWIG Working Group                                           M. Kovatsch
Internet-Draft                                                ETH Zurich
Intended status: Informational                               O. Bergmann
Expires: January 5, 2015                         Universitaet Bremen TZI
                                                                 E. Dijk
                                                        Philips Research
                                                                   X. He
                                               Hitachi (China) R&D Corp.
                                                         C. Bormann, Ed.
                                                 Universitaet Bremen TZI
                                                           July 04, 2014


                      CoAP Implementation Guidance
                        draft-ietf-lwig-coap-01

Abstract

   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 (as per RFC 7228, ~ 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.  In particular, it
   provides guidance on correct implementation of the CoAP specification
   RFC 7252, memory optimizations, and customized protocol parameters.

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 http://datatracker.ietf.org/drafts/current/.

   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 January 5, 2015.







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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
   (http://trustee.ietf.org/license-info) 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  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Protocol Implementation . . . . . . . . . . . . . . . . . . .   4
     2.1.  Client/Server Model . . . . . . . . . . . . . . . . . . .   4
     2.2.  Message Processing  . . . . . . . . . . . . . . . . . . .   5
       2.2.1.  On-the-fly Processing . . . . . . . . . . . . . . . .   5
       2.2.2.  Internal Data Structure . . . . . . . . . . . . . . .   6
     2.3.  Duplicate Rejection . . . . . . . . . . . . . . . . . . .   6
     2.4.  Token Usage . . . . . . . . . . . . . . . . . . . . . . .   7
       2.4.1.  Tokens for Observe  . . . . . . . . . . . . . . . . .   7
       2.4.2.  Tokens for Blockwise Transfers  . . . . . . . . . . .   8
     2.5.  Transmission States . . . . . . . . . . . . . . . . . . .   8
       2.5.1.  Request/Response Layer  . . . . . . . . . . . . . . .   9
       2.5.2.  Message Layer . . . . . . . . . . . . . . . . . . . .  10
     2.6.  Out-of-band Information . . . . . . . . . . . . . . . . .  11
     2.7.  Programming Model . . . . . . . . . . . . . . . . . . . .  11
       2.7.1.  Client  . . . . . . . . . . . . . . . . . . . . . . .  12
       2.7.2.  Server  . . . . . . . . . . . . . . . . . . . . . . .  12
   3.  Optimizations . . . . . . . . . . . . . . . . . . . . . . . .  13
     3.1.  Message Buffers . . . . . . . . . . . . . . . . . . . . .  13
     3.2.  Retransmissions . . . . . . . . . . . . . . . . . . . . .  14
     3.3.  Observable Resources  . . . . . . . . . . . . . . . . . .  14
     3.4.  Blockwise Transfers . . . . . . . . . . . . . . . . . . .  15
     3.5.  Deduplication with Sequential MIDs  . . . . . . . . . . .  15
   4.  Alternative Configurations  . . . . . . . . . . . . . . . . .  18
     4.1.  Transmission Parameters . . . . . . . . . . . . . . . . .  18
     4.2.  CoAP over IPv4  . . . . . . . . . . . . . . . . . . . . .  19
   5.  IANA considerations . . . . . . . . . . . . . . . . . . . . .  19
   6.  Security considerations . . . . . . . . . . . . . . . . . . .  19
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     7.1.  Normative References  . . . . . . . . . . . . . . . . . .  19
     7.2.  Informative References  . . . . . . . . . . . . . . . . .  20



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   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   The Constrained Application Protocol [RFC7252] has been designed
   specifically for machine-to-machine communication in networks with
   very constrained nodes.  Typical application scenarios therefore
   include building automation, process optimization, 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 [RFC7230].

   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 on the other.  For constrained nodes of Class 1 or even
   Class 2 [RFC7228], the most limiting factors usually are dynamic
   memory needs, static code size, and energy.  Most implementations
   therefore need to optimize internal buffer usage, omit idle protocol
   feature, and maximize sleeping cycles.

   The present document gives possible strategies to solve this tradeoff
   for very constrained nodes (i.e., Class 1).  For this, it provides
   guidance on correct implementation of the CoAP specification
   [RFC7252], memory optimizations, and customized protocol parameters.








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2.  Protocol Implementation

   In the programming styles supported by very simple operating systems
   as found on constrained nodes, 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 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 parallel
   observe notifications [I-D.ietf-core-observe]).  Note that
   implementations on constrained platforms often not even support the
   full MTU.  Larger messages must then use blockwise 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
   (blockwise) payload plus an estimate of the maximum header size with
   a worst case option setting.

2.1.  Client/Server Model

   In general, CoAP servers can be implemented more efficiently than
   clients.  REST allows them to keep the communication stateless and
   piggy-backed responses are not stored for retransmission, saving
   buffer space.  The use of idempotent requests also allows to relax
   deduplication, which further decreases memory usage.  It is also easy
   to estimate the required maximum size of message buffers, since URI
   paths, supported options, and maximum payload sizes of the
   application are known at compile time.  Hence, when the application
   is distributed over constrained and unconstrained nodes, the
   constrained ones should preferably have the server role.

   HTTP-based applications have established an inverse model because of
   the need for simple push notifications: A constrained client uses
   POST requests to update resources on an unconstrained server whenever
   an event, e.g., a new sensor reading, is triggered.  This requirement
   is solved by the Observe option [I-D.ietf-core-observe] of CoAP.  It
   allows servers to initiate communication and send push notifications
   to interested client nodes.  This allows a more efficient and also
   more natural model for CoAP-based applications, where the information
   source is in server role and can benefit from caching.






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2.2.  Message Processing

   Apart from the required buffers, message processing is symmetric for
   clients and servers.  First the 4-byte base header has to be parsed
   and thereby checked if it is a CoAP message.  Since the encoding is
   very dense, only a wrong Version or a datagram size smaller than four
   bytes identify non-CoAP datagrams.  These need to be silently
   ignored.  Other message format errors, such as an incomplete datagram
   length or the usage of reserved values, may need to be rejected with
   a Reset (RST) message (see Section 4.2 and 4.3 of [RFC7252] for
   details).  Next the Token is read based on the TKL field.  For the
   following header options, there are two alternatives: 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 no additional memory
   needs to be allocated to store the option values, which are stored
   efficiently inline in the buffer for incoming messages.  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 (a direct pointer) can be added to a sparse list (e.g.,
   a one-dimensional array) for fast retrieval.

   This particularly enables efficient handling of options that might
   occur more than once such as Uri-Path.  In this implementation
   strategy, the delta is zero for any subsequent path segment, hence
   the stored byte index for this option (e.g., 11 for Uri-Path) would
   be overwritten to hold a pointer to only the last occurrence of that
   option.  The Uri-Path can be resolved on the fly, though, and a
   pointer to the targeted resource stored directly in the sparse list.
   In simpler cases, conditionals can preselect one of the repeated
   option values.

   Once the option list has been processed, all known critical option
   and all elective options can be masked out in the bit-vector 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 full processing must only be done up to the



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   highest supported option number.  Beyond that, only the least
   significant bit (Critical or Elective) needs to be checked.
   Otherwise, if all critical options are supported, the sparse list of
   option pointers is used for further handling of the message.

2.2.2.  Internal Data Structure

   Using an internal data structure for all parsed options has an
   advantage when working on the option values, as they are already in a
   variable of corresponding type, e.g., an integer in host byte order.
   The incoming payload and byte strings of the header can be accessed
   directly in the buffer for incoming messages using pointers (similar
   to on-the-fly processing).  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.  And since all important values were copied,
   this alternative facilitates using the buffer for incoming messages
   also for the assembly of outgoing messages - which can be the shared
   IP buffer provided by the OS.

   Setting options for outgoing messages is also easier with an internal
   data structure.  Application developers can set options independent
   from the option number, whose order is required for the delta
   encoding.  The CoAP encoding is then applied in a serialization step
   before sending.  In contrast, assembling outgoing messages with on-
   the-fly processing requires either extensive memmove operations to
   insert new header options or restrictions for developers to set
   options in their correct order.

2.3.  Duplicate Rejection

   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.  The number of remote endpoints
   that need to be managed might be vast.  This can be costly in
   particular for unconstrained nodes that have throughput in the order
   of one hundred thousand requests per second (which might need about
   16 GiB of RAM just for duplicate rejection).  Deduplication is also
   heavy for servers on Class 1 devices, as also piggy-backed responses



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   need to be stored for the case that the ACK message is lost.  Hence,
   a receiver may have good reasons to decide not to do the
   deduplication.

   If duplicate rejection is indeed necessary, e.g., for non-idempotent
   requests, it is important to control the amount of state that needs
   to be stored.  It can be reduced for instance by deduplication at
   resource level: Knowledge of the application and supported
   representations can minimize the amount of state that needs to be
   kept.  Duplicate rejection on the client side can be simplified by
   choosing clever Tokens and only filter based on this information
   (e.g., a list of Tokens currently in use or an obscured counter in
   the Token value).

2.4.  Token Usage

   Tokens are chosen by the client and help to identify request/response
   pairs that span several message exchanges (e.g., a separate response,
   which has a new MID).  Servers do not generate Tokens and only mirror
   what they receive from the clients.  Tokens must be unique within the
   namespace of a client throughout their lifetime.  This begins when
   being assigned to a request and ends when the open request is closed
   by receiving and matching the final response.  Neither empty ACKs nor
   notifications (i.e., responses carrying the Observe option) terminate
   the lifetime of a Token.

   As already mentioned, a clever assignment of Tokens can help to
   simplify duplicate rejection.  Yet this is also important for coping
   with client crashes.  When a client restarts during an open request
   and (unknowingly) re-uses the same Token, it might match the response
   from the previous request to the current one.  Hence, when only the
   Token is used for matching, which is always the case for separate
   responses, randomized Tokens with enough entropy should be used.  The
   8-byte range for Tokens even allows for one-time usage throughout the
   lifetime of a client node.  When DTLS is used, client crashes/
   restarts will lead to a new security handshake, thereby solving the
   problem of mismatching responses and/or notifications.

2.4.1.  Tokens for Observe

   In the case of Observe [I-D.ietf-core-observe], a request will be
   answered with multiple notifications and it can become hard to
   determine the end of a Token lifetime.  When establishing an Observe
   relationship, the Token is registered at the server.  Hence, the
   client partially loses control of the used Token.  A client can
   attempt to cancel the relationship, which frees the Token upon
   success (i.e., the message with an Observe Option with the value set
   to 'deregister' (1) is acknowledged; see [I-D.ietf-core-observe]



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   section 3.6).  However, the client might never receive the ACK due to
   a temporary network outages or worse, a server crash.  Although a
   network outage will also affect notifications so that the Observe
   garbage collection could apply, the server might simply not send CON
   notifications during that time.  Alternative Observe lifetime models
   such as Stubbornness(tm) might also keep relationships alive for
   longer periods.

   Thus, Observe requests should carefully chose the value (and the
   empty value will rarely be applicable).  One option is to assign and
   re-use dedicated Tokens for each Observe relationship the client will
   establish.  This is, however, critical for spoofing attacks in NoSec
   mode.  The recommendation is to use randomized Tokens with a length
   of at least four bytes (see Section 5.3.1 of [RFC7252]).  Thus,
   dedicated ranges within the 8-byte Token space should be used when in
   NoSec mode.  This also solves the problem of mismatching
   notifications after a client crash/restart.

2.4.2.  Tokens for Blockwise Transfers

   In general, blockwise transfers are independent from the Token and
   are correlated through client endpoint address and server address and
   resource path (destination URI).  Thus, each block may be transferred
   using a different Token.  Still it can be beneficial to use the same
   Token (it is freed upon reception of a response block) for all
   blocks, e.g., to easily route received blocks to the same response
   handler.

   When Block2 is combined with Observe, notifications only carry the
   first block and it is up to the client to retrieve the remaining
   ones.  These GET requests do not carry the Observe option and need to
   use a different Token, since the Token from the notification is still
   in use.

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).




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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.

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

             Figure 1: CoAP Client Request/Response Layer FSM

   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





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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.

   +-----------+ <-------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)

   *1: TIMEOUT(RETX_TIMEOUT) / TX(con)
   *2: M_CMD(unreliable_send) / TX(non)
   *3: RX_NON / RR_EVT(rx)
   *4: RX_RST / REMOVE_OBSERVER
   *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
   layers.



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2.6.  Out-of-band Information

   The CoAP implementation can also leverage out-of-band information,
   that might also trigger some of the transitions shown in Section 2.5.
   In particular ICMP messages can inform about unreachable remote
   endpoints or whole network outages.  This information can be used to
   pause or cancel ongoing transmission to conserve energy.  Providing
   ICMP information to the CoAP implementation is easier in constrained
   environments, where developers usually can adapt the underlying OS
   (or firmware).  This is not the case on general purpose platforms
   that have full-fledged OSes and make use of high-level programming
   frameworks.

   The most important ICMP messages are host, network, port, or protocol
   unreachable errors.  After appropriate vetting (cf.  [RFC5927]), they
   should cause the cancellation of ongoing CON transmissions and
   clearing (or deferral) of Observe relationships.  Requests to this
   destination should be paused for a sensible interval.  In addition,
   the device could indicate of this error through a notification to a
   management endpoint or external status indicator, since the cause
   could be a misconfiguration or general unavailability of the required
   service.  Problems reported through the Parameter Problem message are
   usually caused through a similar fundamental problem.

   The CoAP specification recommends to ignore Source Quench and Time
   Exceeded ICMP messages, though.  Source Quench messages were
   originally intended to inform the sender to reduce the rate of
   packets.  However, this mechanism is deprecated through [RFC6633].
   CoAP also comes with its own congestion control mechanism, which is
   already designed conservatively.  One advanced mechanism that can be
   employed for better network utilization is CoCoA,
   [I-D.bormann-core-cocoa].  Time Exceeded messages often occur during
   transient routing loops (unless they are caused by a too small
   initial Hop Limit value).

2.7.  Programming Model

   The event-driven approach, which is common in event-loop-based
   firmware, has also proven very efficient for embedded operating
   systems [TinyOS], [Contiki].  Note that an OS is not necessarily
   required and a traditional firmware approach can suffice for Class 1
   devices.  Event-driven systems use split-phase operations (i.e.,
   there are no blocking functions, but functions return and an event
   handler is called once a long-lasting operation completes) to enable
   cooperative multi-threading with a single stack.

   Bringing a Web transfer protocol to constrained environments does not
   only change the networking of the corresponding systems, but also the



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   programming model.  The complexity of event-driven systems can be
   hidden through APIs that resemble classic RESTful Web service
   implementations.

2.7.1.  Client

   An API for asynchronous requests with response handler functions goes
   hand-in-hand with the event-driven approach.  Synchronous requests
   with a blocking send function can facilitate applications that
   require strictly ordered, sequential request execution (e.g., to
   control a physical process) or other checkpointing (e.g., starting
   operation only after registration with the resource directory was
   successful).  However, this can also be solved by triggering the next
   operation in the response handlers.  Furthermore, as mentioned in
   Section 2.1, it is more like that complex control flow is done by
   more powerful devices and Class 1 devices predominantly run a CoAP
   server (which might include a minimal client to communicate with a
   resource directory).

2.7.2.  Server

   On CoAP servers, the event-driven nature can be hidden through
   resource handler abstractions as known from traditional REST
   frameworks.  The following types of RESTful resources have proven
   useful to provide an intuitive API on constrained event-driven
   systems:

   NORMAL  A normal resource defined by a static Uri-Path and an
      associated resource handler function.  Allowed methods could
      already be filtered by the implementation based on flags.  This is
      the basis for all other resource types.

   PARENT  A parent resource manages several sub-resources under a given
      base path by programmatically evaluating the Uri-Path.  Defining a
      URI template (see [RFC6570]) would be a convenient way to pre-
      parse arguments given in the Uri-Path.

   PERIODIC  A resource that has an additional handler function that is
      triggered periodically by the CoAP implementation with a resource-
      specific interval.  It can be used to sample a sensor or perform
      similar periodic updates of its state.  Usually, a periodic
      resource is observable and sends the notifications by triggering
      its normal resource handler from the periodic handler.  These
      periodic tasks are quite common for sensor nodes, thus it makes
      sense to provide this functionality in the CoAP implementation and
      avoid redundant code in every resource.





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   EVENT  An event resource is similar to an periodic resource, only
      that the second handler is called by an irregular event such as a
      button.

   SEPARATE  Separate responses are usually used when handling a request
      takes more time, e.g., due to a slow sensor or UART-based
      subsystems.  To not fully block the system during this time, the
      handler should also employ split-phase execution: The resource
      handler returns as soon as possible and an event handler resumes
      responding when the result is ready.  The separate resource type
      can abstract from the split-phase operation and take care of
      temporarily storing the request information that is required later
      in the result handler to send the response (e.g., source address
      and Token).

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.

   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



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   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
   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.





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   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

   Blockwise 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 blockwise transfer, each block is usually passed to
   a handler function that for instance performs stream processing or
   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 blockwise transfer out of dynamically generated
   information, 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.  For instance, if the CoRE Link Format at /.well-known/
   core is dynamically generated, 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 endpoint by storing, for each remote CoAP
   endpoint ("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.





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   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_ ([RFC7252] section "Reliable Messages") and
   the time packets can stay alive in the network.

   Such a straightforward implementation is suitable in case other CoAP
   endpoints 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-
      backed);

   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
   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 [RFC7252] 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 endpoint 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



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   example, in a sequence of 52 randomly drawn 16-bit values the
   probability of finding at least two identical values is about 2
   percent.

   From here on we consider efficient storage implementations for MIDs
   in CoAP endpoints, 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 endpoints it communicates with, which partly
   breaks sequentiality from the receiving CoAP endpoint'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].

              +----------+----------------+-----------------+
              | 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



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   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 endpoint 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 endpoint 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 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
   endpoint 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 communicating over
   the Internet, for instance because it is shielded by a proxy or a
   closed deployment, alternative transmission parameters can be used.
   Consequently, the derived time values provided in [RFC7252] section
   4.8.2 will also need to be adjusted, since most implementations will
   encode their absolute values.

   Static adjustments require a fixed deployment with a constant number
   or upper bound for the number of nodes, number of hops, and expected
   concurrent transmissions.  Furthermore, the stability of the wireless
   links should be evaluated.  ACK_TIMEOUT should be chosen above the
   xx% percentile of the round-trip time distribution.
   ACK_RANDOM_FACTOR depends on the number of nodes on the network.
   MAX_RETRANSMIT should be chosen suitable for the targeted
   application.  A lower bound for LEISURE can be calculated as



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   lb_Leisure = S * G / R

   where S is the estimated response size, G the group size, and R the
   target data transfer rate (see [RFC7252] section 8.2).  NSTART and
   PROBING_RATE depend on estimated network utilization.  If the main
   cause for loss are weak links, higher values can be chosen.

   Dynamic adjustments will be performed by advanced congestion control
   mechanisms such as [I-D.bormann-core-cocoa].  They are required if
   the main cause for message loss is network or endpoint congestion.
   Semi-dynamic adjustments could be implemented by disseminating new
   static transmission parameters to all nodes when the network
   configuration changes (e.g., new nodes are added or long-lasting
   interference is detected).

4.2.  CoAP over IPv4

   CoAP was designed for the properties of IPv6, which is dominating in
   constrained environments because of the 6LoWPAN adaption layer
   [RFC6282].  In particular, the size limitations of CoAP are tailored
   to the minimal MTU of 1280 bytes.  Until the transition towards IPv6
   converges, CoAP nodes might also communicate over IPv4, though.
   Sections 4.2 and 4.6 of the base specification [RFC7252] already
   provide guidance and implementation notes to handle the smaller
   minimal MTUs of IPv4.

5.  IANA considerations

   This document has no actions for IANA.

6.  Security considerations

   TBD

7.  References

7.1.  Normative References

   [I-D.bormann-core-cocoa]
              Bormann, C., Betzler, A., Gomez, C., and I. Demirkol,
              "CoAP Simple Congestion Control/Advanced", draft-bormann-
              core-cocoa-02 (work in progress), July 2014.

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





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   [I-D.ietf-core-observe]
              Hartke, K., "Observing Resources in CoAP", draft-ietf-
              core-observe-14 (work in progress), June 2014.

   [RFC6282]  Hui, J. and P. Thubert, "Compression Format for IPv6
              Datagrams over IEEE 802.15.4-Based Networks", RFC 6282,
              September 2011.

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

   [RFC6633]  Gont, F., "Deprecation of ICMP Source Quench Messages",
              RFC 6633, May 2012.

   [RFC7230]  Fielding, R. and J. Reschke, "Hypertext Transfer Protocol
              (HTTP/1.1): Message Syntax and Routing", RFC 7230, June
              2014.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252, June 2014.

7.2.  Informative References

   [Contiki]  Dunkels, A., Groenvall, B., and T. Voigt, "Contiki - a
              Lightweight and Flexible Operating System for Tiny
              Networked Sensors", Proceedings of the First IEEE Workshop
              on Embedded Networked Sensors , November 2004.

   [RFC5927]  Gont, F., "ICMP Attacks against TCP", RFC 5927, July 2010.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228, May 2014.

   [TinyOS]   Levis, P., Madden, S., Polastre, J., Szewczyk, R.,
              Whitehouse, K., Woo, A., Gay, D., Woo, A., Hill, J.,
              Welsh, M., Brewer, E., and D. Culler, "TinyOS: An
              Operating System for Sensor Networks", Ambient
              intelligence, Springer (Berlin Heidelberg), ISBN
              978-3-540-27139-0 , 2005.

Authors' Addresses










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   Matthias Kovatsch
   ETH Zurich
   Universitaetstrasse 6
   CH-8092 Zurich
   Switzerland

   Email: kovatsch@inf.ethz.ch


   Olaf Bergmann
   Universitaet Bremen TZI
   Postfach 330440
   D-28359 Bremen
   Germany

   Email: bergmann@tzi.org


   Esko Dijk
   Philips Research

   Email: esko.dijk@philips.com


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

   Email: xhe@hitachi.cn


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

   Phone: +49-421-218-63921
   Email: cabo@tzi.org










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