CORE WG A. Rahman
Internet-Draft InterDigital Communications, LLC
Intended status: Standards Track February 11, 2014
Expires: August 15, 2014

Enhanced Sleepy Node Support for CoAP
draft-rahman-core-sleepy-05

Abstract

CoAP is a specialized web transfer protocol for constrained devices. These devices typically have some combination of limited battery power, small memory footprint and low throughput links. It is expected that in CoAP networks there will be a certain portion of devices that are "sleepy" and which may occasionally go into a sleep mode (i.e. go into a low power state to conserve power) and temporarily suspend CoAP protocol communication. This document proposes a minimal and efficient mechanism building on the Resource Directory (RD) functionality to enhance sleepy node support in CoAP networks. The RD functionality may be incorporated as part of a CoAP Proxy.

Status of This Memo

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Table of Contents

1. Terminology and Conventions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].

This document assumes readers are familiar with the terms and concepts that are used in [I-D.ietf-core-coap] and [RFC6690]. In addition, this document defines the following terminology:

Sleepy Node

A sleepy node is a CoAP client or server that may sometimes go into a sleep mode (i.e. go into a low power state to conserve power) and temporarily suspend CoAP protocol communication. A sleepy node will otherwise remain in a fully powered on state where it has the capability to perform full CoAP protocol communication.
Non-Sleepy Node

A non-sleepy node is a CoAP client or server that always remains in a fully powered on state (i.e. always awake) where it has the capability to perform full CoAP protocol communication. The general operation of non-sleepy nodes is assumed to be well known and so are not explicitly spelled out in this document except where needed for clarity.

2. Introduction

The current CoAP approach provides a minimal support of sleepy nodes as follows: [I-D.rahman-core-sleepy-nodes-do-we-need].

Some more background information regarding Sleepy Nodes can be found in

3. Proposal

3.1. RD Based Sleep Tracking

The current CoAP approach to support sleepy nodes can be significantly improved by introducing RD based mechanisms for a CoAP client to determine whether:

We define the following new parameters to characterize a sleepy node: [I-D.ietf-core-resource-directory].

These parameters are all server (node) level and are new parameters added to the RD URI Template Variables defined in

We also define a new lookup-type ("ss") for the RD lookup interface specified in [I-D.ietf-core-resource-directory]. This new lookup-type supports looking up the SleepState of a specified end-point.

The three time based parameters (SleepDuration, TimeSleeping, NextSleep) can be based on either an absolute network time (for a time synchronized network) or a relative local time (measured at the local node).

Following the approach of [RFC6690] and [I-D.ietf-core-resource-directory], sleep parameters for sleepy servers can be stored by the server in the RD and accessed by all interested clients. Examples of using these parameters in a synchronous or asynchronous manner are shown in the following sections.

3.2. Example of Synchronous RD Based Sleep Tracking

Figure 1 shows an example of using RD based sleep tracking in a synchronous fashion:

(1) SleepyNode-1 is awake and having previously discovered the local RD, stores its CORE link format in the RD (POST/rd) identified by its entry point (?ep=SleepyNode-1). The sleep parameters are also updated as part of this step.

(2)-(3) RD services the POST and stores the CORE link format and starts the sleep timers for this node.

(4) SleepyNode-1 falls asleep.

(5) A client is interested in temperature sensors in this domain and does a lookup on the RD.

(6) The RD does a lookup and finds that SleepyNode-1 is the only node meeting the match and sends back the required info including the Sleep parameters.

(7) From the sleep parameters, the client knows that the node is currently asleep and so internally schedules to send the GET request when the node wakes up (plus a small safety hysteresis).

(8)-(9) Client sends GET request for temperature sensors and successfully receives the content as SleepyNode-1 is awake.

        

 SleepyNode-1	                      Resource
 Server                               Directory                Client
  |                                    |                          |
 +----------+                          |                          |
 | Time = 0 |                          |                          |
 +----------+                          |                          |
  |                                    |                          | 
  |(1) CoAP POST                       |                          |
  |     /rd,                           |                          |
  |     ?ep="SleepyNode-1";            |                          |
  |     ?SleepDuration="30-Minutes";   |                          |
  |     ?SleepState="Awake";           |                          |
  |     ?NextSleep="5-Minutes";        |                          |
  |     PAYLOAD:                       |                          |
  |     "</sensors...>"                |                          |
  |---------------------------------- >|                          |
  |                                    |                          |
  |               (2) RD services the  |                          |
  |                   Registration     |                          |
  |                   Request and      |                          |
  |                   starts Sleep     |                          |
  |                   timers           |                          |
  |                                    |                          |
  |(3) 2.01 Created                    |                          |
  |         Location: /rd/4321         |                          |
  |<-----------------------------------|                          |
  |                                    |                          |
 +-------------------+                 |                          |
 | Time = 5 minutes  |                 |                          |
 +-------------------+                 |                          |
  |                                    |                          |
  |(4) SleepyNode-1 falls asleep       |                          |
  |                                    |                          |
 +-------------------+                 |                          |
 | Time = 15 minutes |                 |                          |
 +-------------------+                 |                          |
  |                                    |                          |
  |                                                               |
  |                      (5) CoAP GET                             |
  |                          /rd-lookup/res?rt=Temperature        |
  |                                    |<-------------------------|
  |                                    |                          |
  |                                    |                          |
  |                                                               |
  |                      (6) 2.05 Content                         |
  |                          <coap://SleepyNode-1/temp>;          |
  |                          SleepDuration="30-Minutes";          |
  |                          SleepState="Sleeping";               |
  |                          TimeSleeping="10-Minutes";           |
  |                          PAYLOAD:                             |
  |                          rt="Temperature"                     |
  |                                    |------------------------->|
  |                                    |                          |
  |                                    |                          |
  |                                    |                          |
  |                                    |                          |
  |                                                               |
  |                       (7) Since node is asleep Client         |
  |                           waits 20 minutes until it awakes    |
  |                                    |                          |
  |                                    |                          |
 +-------------------+                 |                          |
 | Time = 31 minutes |                 |                          |
 +-------------------+                 |                          |
  |                                    |                          |
  |                                                               |
  |                       (8)CoAP GET                             |
  |                          <coap://SleepyNode-1/temp>           |
  |<--------------------------------------------------------------|
  |                                                               |
  |-------------------------------------------------------------->|
  |                       (9) 2.05 Content                        |
  |                                    |                          |
  |                                    |                          |

Figure 1: Synchronous Resource Directory Based Sleep Tracking

3.3. Example of Asynchronous RD Based Sleep Tracking

Figure 2 shows an example of using RD based sleep tracking in an asynchronous fashion:

(1) SleepyNode-1 is awake and having previously discovered the local RD, stores its CORE link format in the RD (POST/rd) identified by its entry point (?ep=SleepyNode-1).

(2)-(3) RD services the POST and stores the CORE link format.

(4) A client is interested in temperature sensors in this domain and does a lookup on the RD for all sensors that are currently awake.

(5) The RD does a lookup and finds that SleepyNode-1 is the only node meeting the match and sends back the required info.

(6)-(7) Using the sleep state lookup functionality (lookup-type := "ss"), the client adds itself to the list of observers to get SleepState updates from RD for SleepyNode-1 [I-D.ietf-core-observe].

(8)-(9) Client performs RD 'resource' lookup to find URI of temperature sensor of resource hosted on SleepyNode-1.

(10)-(13) SleepyNode-1 prepares to goes to sleep and updates the SleepState in the RD.

(14) RD notifies the client through previously established observe relationship.

(15) Client application wants to get the temperature now but does not send the request as it knows SleepyNode-1 is currently sleeping.

(16)-(19) SleepyNode-1 wakes up and updates the SleepState in the RD.

(20)-(21) RD notifies the client through previously established observe relationship.

(22)-(23) Client sends GET request for temperature sensors and successfully receives the content as SleepyNode-1 is awake.

        

 SleepyNode-1	                      Resource
 Server                               Directory                Client
  |                                    |                          |
  |                                    |                          | 
  |(1) CoAP POST                       |                          |
  |     /rd,                           |                          |
  |     ?ep=SleepyNode-1;              |                          |
  |     ?SleepState=Awake;             |                          |
  |     PAYLOAD:                       |                          |
  |     "</sensors...>"                |                          |
  |---------------------------------- >|                          |
  |                                    |                          |
  |               (2) RD services the  |                          |
  |                   Registration     |                          |
  |                   Request          |                          |
  |                                    |                          |
  |(3) 2.01 Created                    |                          |
  |         Location: /rd/4321         |                          |
  |<-----------------------------------|                          |
  |                                    |                          |
  |                                    |                          |
  |                                       (4) Client performs RD  |
  |                                           end-point lookup to |
  |                                           find endpoint that  |
  |                                           has temperature     |
  |                                           sensor and is awake |
  |                                                               |
  |                  CoAP GET                                     |
  |                  /rd-lookup/ep?rt=Temperature&SleepState=Awake|
  |                                    |<-------------------------|
  |                                    |                          |
  |                                    |                          |
  |                      (5) 2.05 Content                         |
  |                          <coap://{ip:port}>;ep="SleepyNode-1" |
  |                                    |------------------------->|
  |                                    |                          |
  |                                    |                          |
  |                                       (6) Client adds itself  |
  |                                           to list of observers|
  |                                           to get SleepState   |
  |                                           updates from RD for |
  |                                           SleepyNode-1        |
  |                                                               |
  |                                                               |
  |                          CoAP GET                             |
  |                          /rd-lookup/ss?ep=SleepyNode-1        |
  |                          Observe: 0                           |
  |                          Token: 0x4a                          |
  |                                    |<-------------------------|
  |                                    |                          |
  |                                    |                          |
  |                      (7) 2.05 Content                         |
  |                          Observe: 1                           |
  |                          Token: 0x4a                          |
  |                          SleepState="Awake"                   |
  |                                    |------------------------->|
  |                                    |                          |
  |                                    |                          |
  |                                       (8) Client performs RD  |
  |                                           'resource' lookup   |
  |                                           to find URI of      |
  |                                           temperature sensor  |
  |                                           of resource hosted  |
  |                                           on SleepyNode-1     |
  |                                                               |
  |                CoAP GET                                       |
  |                /rd-lookup/res?rt=Temperature&ep="SleepyNode-1"|
  |                                    |<-------------------------|
  |                                    |                          |
  |                                    |                          |
  |                     (9) 2.05 Content                          |
  |                          <coap://sleepyNode-1/temp>;          |
  |                                    |------------------------->|
  |                                    |                          |
  |                                    |                          |
                     ...                         ....
  |(10) SleepyNode-1 prepares to go    |                          |
  |     to sleep so it updates         |                          |
  |     SleepState                     |                          |
  |                                    |                          |
  |     CoAP PUT                       |                          |
  |      /rd/4321,                     |                          |
  |      ?SleepState=Sleeping          |                          |
  |---------------------------------- >|                          |
  |                                    |                          |
  |              (11) RD updates       |                          |
  |                   SleepState of    |                          |
  |                   SleepyNode-1     |                          |
  |                                    |                          |
  |(12) 2.04 Changed                   |                          |
  |<-----------------------------------|                          |
  |                                    |                          |
  |(13) SleepyNode-1 goes to sleep     |                          |
  |                                    |                          |
  |                                    |                          |
  |               (14) RD sends notification to client            |
  |                          2.05 Content                         |
  |                          Observe: 2                           |
  |                          Token: 0x4a                          |
  |                          SleepState="Sleeping"                |
  |                                    |------------------------->|
                       ...                         ....
  |                                    |                          |
  |                                    | (15) Client has GET      |
  |                                    |      request for         |
  |                                    |      SleepyNode-1 but    |
  |                                    |      cannot send it since|
  |                                    |      SleepyNode-1 is not |
  |                                    |      awake               |
  |(16) SleepyNode-1 wakes up          |                          |
  |                                    |                          |
  |                                    |                          |
  |(17) SleepyNode-1 updates           |                          |
  |     SleepState                     |                          |
  |                                    |                          |
  |     CoAP PUT                       |                          |
  |      /rd/4321,                     |                          |
  |      ?SleepState=Awake;            |                          |
  |---------------------------------- >|                          |
  |                                    |                          |
  |                                    |                          |
  |               (18) RD updates      |                          |
  |                    sleep state     |                          |
  |                                    |                          |
  |(19) 2.04 Changed                   |                          |
  |<-----------------------------------|                          |
  |                                    |                          |
  |               (20) 2.05 Content                               |
  |                    Observe: 3                                 |
  |                    Token: 0x4a                                |
  |                    <coap://sleepyNode-1/temp>;                |
  |                    SleepState="Awake"                         |
  |                                    |------------------------->|
  |                                    |                          |
  |                                    |                          |
  |                                    | (21) Client detects      |
  |                                    |      SleepyNode-1 is     |
  |                                    |      awake               |
  |                                    |                          |
  |                (22) CoAP GET                                  |
  |                     <coap://SleepyNode-1/temp>                |
  |<--------------------------------------------------------------|
  |                                                               |
  |-------------------------------------------------------------->|
  |                (23) 2.05 Content                              |
  |                                                               |

Figure 2: Asynchronous Resource Directory Based Sleep Tracking

3.4. RD Caching Proxy

It would be useful for an RD to be able to indicate which proxy performs caching for Sleepy CoAP nodes (see Section 2). This would be done through a new RD "CachingProxy" attribute for each device (similiar to the attributes defined in Section 3.1): [I-D.ietf-core-resource-directory].

  • An RD may be co-located with a proxy that performs caching for CoAP nodes. In this case, the RD automatically adds itself to each CachingProxy entry.
  • The sleepy node itself could suggest the CachingProxy if it is peered to a specific proxy.

This parameter would be added to the RD URI Template Variables defined in

4. Performance Results

4.1. Prototype Setup

A simple prototype was implemented to validate certain aspects of the performance of the proposed CoAP sleepy node support protocol enhancement. The network for the prototype is shown in Figure 3.

Key aspects of the prototype are as follows:

  • There are two modes of operation: "Caching Only" and "Caching and Sleep Aware"
  • In "Caching Only" mode the Reverse Proxy will cache responses and service new requests according to the "Max-Age" parameter as defined in [I-D.ietf-core-coap].
  • In "Caching and Sleep Aware" mode the Reverse Proxy will also cache responses and service new requests according to "Max-Age". However, if the cache is empty (e.g. exceeded Max-Age) then there will be extra sleep aware functionality. Specifically, the proxy will also be aware of the "SleepDuration", "NextSleep" and "SleepState" sleepy node parameters as defined in Section 3.1. Based on these sleep parameters:
    • For servers that are asleep but expected to wake up soon: The proxy will immediately send a non-piggy backed ACK to the client. Then store-and-forward the request to the server when it wakes up, and then return the non-piggybacked response to the client thereafter.
    • For servers that are asleep but not expected to wake up soon: The proxy will immediately send a "5.03 Service Unavailable (and retry after)" to the client.

  • The key variables in the experiment are: (1) Number of clients; (2) Number of sleepy servers; (3) Delay between client requests; (4) MaxAge; (5) SleepDuration; (6) NextSleep; and (7) SleepState.
  • The target of the experiment will be to measure the amount of traffic over the network in the two modes of operation (for the same input client requests profile). It is hypothesized that the "Caching and Sleep Aware" mode will have the minimal amount of network traffic indicating that the Sleep Aware network will be the most efficient.

    

                                               Constrained Network
                                               --------------------
                                              /                    \
                          ---       +----------+            /-----\ \
                         /   \      |  CoAP    |             CoAP    \
                        |     |     | Reverse  |            server    \
               CoAP     |     |     |  Proxy   |            \-----/   ||
   +------+  Request    |     |     |          |                      ||
   |CoAP  |-------------|     |---->| +-----+  |  Req   /-----\       ||
   |Client|             |     |     | |Cache|  |------->| CoAP        ||
   |      |<------------|     |-----| +-----+  |<-------|server       ||
   +------+    CoAP     |     |     |          |  Resp  \-----/       ||
            Response    | Corp|     |------    |                      ||
                        | LAN |     |RD   |    |                      ||
                        |     |     |Sleep|    |                      ||
                         \   /      |Param|    |                      /
                          ---       |     |    |                     /
                                    +----------+       /-----\      /
                                              \         CoAP       /
                                               \        server    /
                                                \      \-----/   /
                                                 ----------------

Figure 3: Prototype Configuration

4.2. Initial Results

Baseline performance results of A CoAP system with Sleep Aware Proxy support described in Section 4.1 is given in [IETF86Results] (see pg. 65-95). Key conclusions were:

  • If only GETs are considered, a cache based system is efficient as long as the cache is valid (i.e before Max-Age).
  • However, since the cache often expires, a Sleep Aware Proxy further minimizes the number of GET transactions. Also, the Sleep Aware Proxy reduces number of transactions for other methods (e.g. PUTs) where a cache does not help.

4.3. Comparison with Observe

A comparison of the performance of a CoAP system with Sleep Aware Proxy versus a CoAP system supporting Observe is given in [IETF87Results] (see pg. 150-182). Key conclusions were:

  • If only GETs are considered, an Observe based system is quite efficient.
  • However, Observe in combination with Sleep Aware Proxy further minimizes the number of GET transactions. Also, the Sleep Aware Proxy reduces number of transactions for other methods (e.g. PUTs) where Observe does not help.

5. Acknowledgements

Thanks to Carsten Bormann, Esko Dijk, Thomas Fossati, Salvatore Loreto, Dale Seed, and Zach Shelby for valuable discussions and feedback on this document.

6. IANA Considerations

This memo includes no request to IANA.

7. Security Considerations

TBD. (All drafts are required to have a security considerations section. See RFC 3552 [RFC3552] for a guide.)

8. References

8.1. Normative References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link Format", RFC 6690, August 2012.
[I-D.ietf-core-coap] Shelby, Z., Hartke, K. and C. Bormann, "Constrained Application Protocol (CoAP)", Internet-Draft draft-ietf-core-coap-14, March 2013.
[I-D.ietf-core-observe] Hartke, K., "Observing Resources in CoAP", Internet-Draft draft-ietf-core-observe-08, February 2013.

8.2. Informative References

[RFC3552] Rescorla, E. and B. Korver, "Guidelines for Writing RFC Text on Security Considerations", BCP 72, RFC 3552, July 2003.
[I-D.ietf-core-resource-directory] Shelby, Z., Krco, S. and C. Bormann, "CoRE Resource Directory", Internet-Draft draft-ietf-core-resource-directory-00, June 2013.
[I-D.rahman-core-sleepy-nodes-do-we-need] Rahman, A., "Sleepy Devices: Do we need to Support them in CORE?", Internet-Draft draft-rahman-core-sleepy-nodes-do-we-need-01, February 2014.
[IETF86Results] Rahman, A., "IETF-86 Sleepy Node Performance Results", March 2013.
[IETF87Results] Rahman, A., "IETF-87 Sleepy Node Performance Results", July 2013.

Author's Address

Akbar Rahman InterDigital Communications, LLC Montreal, Quebec H3A 3G4 Canada Phone: +1-514-585-0761 EMail: akbar.rahman@interdigital.com