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Versions: 00                                                            
CoRE Group                                                    A. Rahman
Internet Draft                                               JC. Zuniga
Intended status: Informational                                    G. Lu
Expires: December 29, 2010             InterDigital Communications, LLC
                                                          June 29, 2010


              Sleeping and Multicast Considerations for CoAP
                     draft-rahman-core-sleeping-00.txt


Abstract

   This document further analyzes the COAP requirements related to
   "sleeping nodes" and "multicast" through the use of examples and use
   cases. The goal of this document is to trigger discussions in the
   CoRE working group so that all relevant considerations are taken into
   account when designing CoAP.



Status of this Memo

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

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
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   This Internet-Draft will expire on December 29, 2010.

Copyright Notice

   Copyright (c) 2010 IETF Trust and the persons identified as the
   document authors. All rights reserved.



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   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 BSD License.



Table of Contents


   1. Introduction...................................................2
   2. Conventions and Terminology....................................3
   3. Handling Sleep Nodes...........................................3
      3.1. General Behavior of Sleep Nodes...........................3
      3.2. Handling Sleeping Nodes within CoAP.......................4
      3.3. Use Cases.................................................7
         3.3.1. Use Case 1: Smart Meter Reading......................7
         3.3.2. Use Case 2: Smart Meter Firmware Upgrade.............9
         3.3.3. Use Case 3: Obtaining Configuration.................10
         3.3.4. Use Case 4: Constrained Node Sending Report.........12
   4. Multicast Support in CoAP.....................................13
   5. Conclusions...................................................14
   6. Security Considerations.......................................14
   7. IANA Considerations...........................................14
   8. References....................................................14
      8.1. Normative References.....................................14
      8.2. Informative References...................................15
   9. Acknowledgments...............................................15



1. Introduction

   The CoRE working group is chartered to design and standardize a
   Constrained Application Protocol (CoAP) for resource constrained
   devices and networks [I-D.draft-ietf-core-coap].  The requirements
   for CoRE are well documented group [I-D.draft-shelby-core-coap-req].

   In this draft, we focus and expand discussions on some of these key
   requirements pertaining to sleeping nodes and multicast support.
   Specifically, the requirements we analyze are:



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     REQ 3: The ability to deal with sleeping nodes. Devices may be
     powered off at any point in time but periodically "wake up" for
     brief periods of time.

     REQ 4: Protocol must support the caching of recent resource
     requests, along with caching subscriptions to sleeping nodes.

     REQ 9: CoAP will support a non-reliable IP multicast message to be
     sent to a group of Devices to manipulate a resource on all the
     Devices simultaneously. The use of multicast to query and advertise
     descriptions must be supported, along with the support of unicast
     responses.


2. Conventions and Terminology

   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 [RFC2119].

   The following are definitions of terminologies used in this draft.

   Sleep Mode: Refers to the state when a device is not able to send
   and/or receive any messages due to power conservation from the
   perspective of an application protocol (i.e. CoAP).

   Sleep Node: A device that may go to sleep mode from time to time.

   Sleeping Node: A device that is in sleep mode.



3. Handling Sleep Nodes

   3.1. General Behavior of Sleep Nodes

   In this section we discuss different behaviors and scenarios of
   sleeping nodes. Such behaviors can affect the design of applications
   (such as CoAP) and network topologies (such as proxies and caching).

   Sleeping nodes can have various sleeping patterns. Sleep patterns can
   be predictable or totally unpredictable. For example, some nodes
   sleep at a fixed interval or upon certain triggers. Some nodes may
   sleep at irregular time intervals, or switch to sleep mode
   spontaneously. Some nodes stay in sleep mode and only wake up upon




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   certain event triggers. A network may thus not be well aware of the
   sleeping state of a node at a given time.

   The duration of sleeping mode also varies largely, possibly from a
   few seconds to days. From the perspective of applications, it may not
   be affected by the sleeping period if it is very short. For example,
   a HTTP/TCP connection may still work (even if sub-optimally) if the
   sleep cycle is much shorter than the TCP retransmission timer. In
   contrast, if the sleeping period of a node exceeds a certain
   threshold it can impact an application.  This threshold however, can
   be difficult to predict and often can vary from device to device and
   network to network. For example, this threshold can be very dependent
   on the topology of a constrained network especially for the case
   where a multi-hop path consists of multiple sleeping nodes.  For this
   case, the cumulative effect of multiple sleeping nodes must be
   considered.

   The network topology also affects how to handle sleeping nodes. For
   example, in a star shaped network, a proxy node (assuming to be not a
   sleeping node) can cache for the sleeping nodes within the network in
   a centralized manner. However, in a P2P or mesh network, especially
   when multi-hops are involved, caching can be difficult and delivering
   of messages can be largely delayed due to nodes' sleeping cycles.  In
   this case distributed proxying and caching at intermediate nodes
   within the network (rather than just a single node such as the border
   node or sink) may make sense, if intermediate nodes are not sleeping
   nodes and have adequate resources to support caching.



   3.2. Handling Sleeping Nodes within CoAP

   Traffic generated for handling sleeping nodes should be minimized in
   a constrained network. A constrained device can delegate its
   authority to the proxy node for operations on its behalf. Therefore
   operations between the constrained device and its counterpart (e.g. a
   computer on the Internet) become asynchronous due to the proxy.

   Below we discuss the various aspects to consider for handling
   sleeping nodes with CoAP.

   1) Exchange of sleeping schedule
   A node can optionally include its sleep schedule and exchange such
   information with other nodes and/or proxies. This provides proxy(s)
   with better awareness of all resources that they are able to proxy


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   for as well as which devices sleep and their corresponding sleep
   cycle durations. Using this sleep information, the proxy(s) could
   choose which device resources it wants to create/maintain intercept
   caches for (e.g. support intercept caches only for sleeping devices),
   thus optimizing resource utilization within the proxy by selectively
   caching and also minimizing the impact on sleeping nodes by allowing
   them to sleep longer and not have to directly service incoming
   requests.

   2) Proxy and device synchronization
   A common synchronization scheme is to have the device always inform
   the proxy of its latest sleep state by checking-in [I-D.frank-
   6lowpan-chopan]. For example, the proxy can subscribe to a node and
   the node sends notification each time when it wakes up. However, this
   approach increases the traffic for managing the sleep state,
   especially when there are a large amount of nodes and they sleep
   frequently.

   A proxy can benefit from being synchronized to its associated
   constrained device's sleep and awake state. Ideally, if the proxy and
   device are synchronized in time and if the device has a predictable
   sleep schedule, the proxy can know whether the device is awake or not
   without the device having to check-in so often. However, time
   synchronization may be difficult to achieve for constrained devices.

   3) Caching and Proxying
   It is assumed that the proxy node is awake all the time. A proxy can
   thus always cache for the constrained nodes, regardless of the node's
   sleep state. The proxy node would then act as an interception proxy.
   This is analogous to the concept of web caching (e.g. HTML pages,
   images, etc.) between a Web server and a Web browser used in the
   general Internet.

   Figure 1 shows a call flow when a constrained node (Node 1) sends a
   REQUEST to Node 2. The REQUEST can contain any method, GET, POST,
   PUT, DELETE, or SUBSCRIBE. The figure illustrates two scenarios. In
   Case 2a, the proxy sends a RESPONSE with intermediate status. After
   that, Node 1 may go to sleep mode, and receive the requested content
   in step 6 after it wakes up. In Case 2b, the proxy acts upon the
   REQUEST immediately. Node 1 may go into sleep mode at any time after
   sending the REQUEST, if it can handle delayed response. The proxy
   node is in sync with Node 1 for its sleep state and caches the


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   RESPONSE for it when it is in sleep mode. When Node 1 wakes up, it
   can notify the proxy with its sleep schedule (if changed) and send
   data for caching as the procedure shown in step 5.


   Node 1                   Proxy Node                   Node 2
   (sleep cycles)            (always on)             (always on)
      |                          |                          |
      | 1.REQUEST(GET,POST,PUT   |                          |
      |  DELETE, SUBSCRIBE)      |                          |
      |------------------------ >|                          |
      |                          |                          |
    -----------------------------------------------------------
      |                          |                          |
      |                  Case 2a: sends immediate           |
      |                    RESPONSE to Node 1               |
      |                          |                          |
      | 2. RESPONSE/NOTIFY       |                          |
      |       (Accepted)         |                          |
      |< ------------------------|                          |
      |                          |                          |
     ----------------------------------------------------------
      |                          |                          |
      |            Case 2b: acts upon the REQUEST           |
      |                          |                          |
      |                          |  3.REQUEST(GET,POST,PUT  |
      |                          |  DELETE, SUBSCRIBE)      |
      |                          |------------------------ >|
      |                          |                          |
      |                          |        4.RESPONSE        |
      |                          |< ------------------------|
      |                          |                          |
      |                   caches RESPONSE                   |
      |                          |                          |
      |             checks sleep state of Node 1            |
      |                          |                          |
      |                          |                          |
   Wakes up                      |                          |
      |                          |                          |
      |  5. Sends updates        |                          |
      |    (sleep schedule)      |                          |
      |    (other contents)      |                          |
      |< ====================== >|                          |
      |                          |                          |
      |< -6.RESPONSE/NOTIFY------|                          |
      |                          |                          |


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                       Figure 1 Caching and Proxying

   3.3. Use Cases

   In this section, different scenarios are illustrated to show the
   operations of a proxy node for GET and non-GET REQUESTs. The REQUEST
   can be from the constrained or non-constrained domain. For the
   following figures, Node 1 is a constrained node and it goes to sleep
   from time to time. Node 2 is a node in the non-constrained domain. It
   is assumed that the proxy node and Node 2 do not go to sleep. The
   protocol between Node 1 and the Proxy Node is CoAP, and the protocol
   between the Proxy Node and Node 2 is HTTP.

   The figures show some key scenarios but are not exhaustive.

   3.3.1. Use Case 1: Smart Meter Reading

   In use case 1, Node 1 is a smart meter in the constrained network,
   and Node 2 in the non-constrained network wants to read the data from
   Node 1.

   The proxy can enable asynchronous communication between Node 1 and
   Node 2. In situation 2a, since the proxy node has an updated cache
   for what Node 2 is requesting, the proxy can respond to Node 2,
   regardless of Node 1's sleep state.

   In situation 2b, if the proxy does not have an updated cache, and
   Node 1 is sleeping, the proxy can send a RESPONSE to Node 2 with
   RETRY-AFTER information. The proxy would need to be synchronized with
   Node 1 and have Node 1's sleep schedule. Otherwise the proxy has to
   poll Node 1, which can cause a long delay and extra messaging. The
   synchronization is shown in step 5. When Node 1 wakes up, it can
   notify the proxy with its sleep schedule (if changed) and send data
   for caching.

   Alternatively, as shown in step 2b', the proxy may buffer the REQUEST
   and only send a RESPONSE when it gets information from Node 1. This
   can reduce messaging for Node 2 to query again. The proxy should know
   the sleep schedule of Node 1 to make such a decision on how it
   reacts. If Node 1 is not waking up in a very long time, it is
   probably better for the proxy to send RESPONSE to Node 2 immediately.
   Otherwise the long delay can cause time-out and retry of Node 2.







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   Node 1                   Proxy Node                   Node 2
   (sleep cycles)            (always on)             (always on)
      |                          |                          |
      |                          |   1.REQUEST(GET,         |
      |                          |     meter reading)       |
      |                          |< ----------------------- |
      |                          |                          |
      |                        2. Checks its cache          |
     ---------------------------------------------------------
      |                   Case 2a: Has updated cache        |
      |                          |                          |
      |                          | 3.RESPONSE(meter reading)|
      |                          |------------------------ >|
     ---------------------------------------------------------
      |                          |                          |
      |           Case 2b: No cache, Node 1 is sleeping     |
      |                          |                          |
      |                          |4.RESPONSE(Retry-After)   |
      |                          |------------------------ >|
   wakes up                      |                          |
      |                          |                          |
      |< === 5.sends update === >|                          |
      |  (with meter reading)    |                          |
      |                          |                          |
      |                    updates cache                    |
      |                and Node 1's schedule                |
      |                          |< == 6.Node 2 sends === > |
      |                          |       REQUEST again      |
     ---------------------------------------------------------
      |                          |                          |
      |           Case 2b': No cache, Node 1 is sleeping    |
      |                    buffers REQUEST                  |
   wakes up                      |                          |
      |                          |                          |
      |< === 5.sends updates == >|                          |
      | (with meter reading)     |                          |
      |                     updates cache                   |
      |                  and Node 1's schedule              |
      |                          |                          |
      |                          | 6.RESPONSE(meter reading)|
      |                          |------------------------ >|
      |                          |                          |

                 Figure 2 Use Case 1: Smart Meter Reading



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   3.3.2. Use Case 2: Smart Meter Firmware Upgrade

   In use case 2, Node 1 is a smart meter in a constrained network and
   Node 2 upgrades Node 1's firmware using PUT.  If the proxy knows that
   Node 1 is sleeping, it can either advise Node 2 to retry later, or
   the proxy can buffer the REQUEST till Node 1 wakes up.



   Node 1                   Proxy Node                   Node 2
   (sleep cycles)            (always on)             (always on)
      |                          |                          |
      |                          |   1.REQUEST(PUT,         |
      |                          |  firmware for Node 1)    |
      |                          |< ----------------------- |
      |                          |                          |
      |                   2. checks Node 1 state            |
      |                   Node 1 is sleeping                |
      |                          |                          |
      |                          |                          |
     ---------------------------------------------------------
      |                          |                          |
      |                 Case 3a: no buffering               |
      |                          |                          |
      |                          | 3.RESPONSE(Retry-After)  |
      |                          |------------------------ >|
      |                          |                          |
     ---------------------------------------------------------
      |                          |                          |
      |                 Case 3b: buffers REQUEST            |
      |                          |                          |
      |                          |                          |
   wakes up                      |                          |
      |                          |                          |
      |< === 4.sends updates == >|                          |
      |                          |                          |
      |    5.REQUEST (PUT,       |                          |
      |    firmware for Node 1)  |                          |
      |< ------------------------|                          |
      |    6.RESPONSE (Status)   |                          |
      |----------------------- > |                          |
      |                          |                          |
      |                          |   7.RESPONSE(Status)     |


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      |                          |------------------------ >|
      |                          |                          |
      |                          |                          |



             Figure 3 Use Case 2: Smart Meter Firmware Upgrade



   3.3.3. Use Case 3: Obtaining Configuration

   Use case 3 shows that Node 1 in a constrained network tried to obtain
   a configuration parameter from Node 2. Since the proxy is aware that
   Node 1 is a sleep node, the proxy sends RESPONSE to Node 1
   immediately so that Node 1 can go to its sleep cycle. The proxy then
   requests and caches the configuration parameter for Node 1, and it
   can notify Node 1 when Node 1 wakes up.






























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   Node 1                   Proxy Node                   Node 2
   (sleep cycles)            (always on)             (always on)
      |                          |                          |
      |                          |                          |
      |  1.REQUEST (GET,         |                          |
      |  configuration)          |                          |
      |------------------------ >|                          |
      |                          |                          |
    -----------------------------------------------------------
      |                          |                          |
      |                  Case 2a: sends immediate           |
      |                     RESPONSE to Node 1              |
      |                          |                          |
      | 2.RESPONSE(Accepted)     |                          |
      |< ------------------------|                          |
      |                          |                          |
    -----------------------------------------------------------
      |                          |                          |
      |          Case 2b: acts upon the REQUEST             |
      |                          |                          |
      |                          |     3.REQUEST(GET,       |
      |                          |     configuration)       |
      |                          |------------------------ >|
      |                          |                          |
      |                          |     4.RESPONSE           |
      |                          |(configuration for Node 1)|
      |                          |< ------------------------|
      |                          |                          |
      |                   5. checks Node 1 state            |
      |                   Node 1 is sleeping                |
      |                          |                          |
      |                   caches RESPONSE                   |
      |                          |                          |
     Wakes up                    |                          |
      |                          |                          |
      |< == 6.sends updates === >|                          |
      |                          |                          |
      |  7.RESPONSE              |                          |
      |(configuration for Node 1)|                          |
      |< ------------------------|                          |
      |                          |                          |
      |                          |                          |



        Figure 4 Use Case 3: Constrained Node Getting Configuration



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   3.3.4. Use Case 4: Constrained Node Sending Report

   Use case 4 shows a smart meter Node 1 sends report to Node 2.

   Node 1                   Proxy Node                   Node 2
   (sleep cycles)            (always on)             (always on)
      |                          |                          |
      |   1.REQUEST(PUT,         |                          |
      |   Meter Report)          |                          |
      |------------------------ >|                          |
      |                          |                          |
   -------------------------------------------------------------
      |                          |                          |
      |                 case 2a: sends immediate            |
      |                   RESPONSE to Node 1                |
      |                          |                          |
      | 2. RESPONSE(Accepted)    |                          |
      |                          |                          |
   -------------------------------------------------------------
      |               case 2b: acts upon the REQUEST        |
      |                          |                          |
      |                          |     3.REQUEST(PUT,       |
      |                          |     Meter Report)        |
      |                          |------------------------ >|
      |                          |                          |
      |                          |    4.RESPONSE (Status)   |
      |                          |< ------------------------|
      |                          |                          |
      |                   5 Caches RESPONSE                 |
      |                                                     |
      |                   Checks Node 1 sleeping state      |
      |                   Node 1 is sleeping                |
      |                          |                          |
     Wakes up                    |                          |
      |                          |                          |
      |< == 6.sends updates === >|                          |
      |                          |                          |
      |  7.RESPONSE              |                          |
      |  (Status)                |                          |
      |< ------------------------|                          |
      |                          |                          |



           Figure 5 Use Case 4: Constrained Node Sending Report


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4. Multicast Support in CoAP

   One of the requirements of the CoAP design is to support a simple and
   unreliable multicast method. Supporting multicast poses additional
   requirements to the proxy node.

   Within the constrained network, an application protocol (e.g. CoAP)
   usually runs over UDP for which IP multicast is supported. In a non-
   constrained network (i.e. general Internet), HTTP over TCP is
   typically used for which IP multicast is not naturally supported.
   This causes the scenario below:

       o The traffic within the constrained network is multicast (such
          as CoAP over UDP), and within the non-constrained network it
          is unicast (such as HTTP over TCP)

   Therefore the proxy node needs to have functionality to support
   interworking of unicast and multicast. Specifically, CoAP protocol
   should include support for translating a HTTP/TCP unicast request
   into a CoAP/UDP multicast request as illustrated in Figure 6.


                    CoAP/UDP             HTTP/TCP
        Multicast or Unicast              Unicast

    Constrained
        Node     ------|                    |------ Non-Constrained
                       |                    |             Node
                       |                    |
                       |                    |
     Constrained ------|-----Proxy Node-----|------ Non-Constrained
        Node           |                    |             Node
                       |                    |
                       |                    |
     Constrained ------|                    |------ Non-Constrained
         Node                                             Node


                 Figure 6 Multicast Support at Proxy Node








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5. Conclusions

   This draft analyzes three requirements identified in the CoAP design
   requirement document [I-D.draft-shelby-core-coap-req] related to
   handling sleeping nodes and supporting multicast. The draft discusses
   some considerations for CoAP design and proxy operations to meet
   these requirements.

   For a constrained network to handle sleeping nodes, a constrained
   node should be able to notify the proxy of its sleep schedule, and
   the proxy node should have an updated schedule for each sleep node.
   To utilize the sleep schedule, the sleep nodes and the proxy need to
   maintain synchronization to a certain extent. A proxy node thus
   enables asynchronous communications with the general Internet, since
   a proxy node can always cache for the constrained nodes.

   CoAP is targeted to interact with HTTP/TCP on the general Internet.
   Since IP multicast does not support TCP, the proxy node in a
   constrained network needs to have functionalities to support
   interworking of multicast and unicast to fulfill this requirement.



6. Security Considerations

   This draft discusses the design considerations and operations of CoAP
   and CoAP proxy in constrained networks. It does not introduce new
   security threats.



7. IANA Considerations

   This document makes no request of IANA.



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.






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   8.2. Informative References

   [I-D.draft-ietf-core-coap] Shelby, Z., Frank, B., and D. Sturek,
   "Constrained Application Protocol (CoAP)", draft-ietf-core-coap-00
   (Work in progress), June 7, 2010.

   [I-D.draft-frank-6lowpan-chopan] Frank, B., "Chopan - Compressed HTTP
             Over PANs", draft-frank-6lowpan-chopan-00 (Work in
             progress), June 15, 2009.

   [I-D.draft-shelby-core-coap-req] Shelby, Z., Stuber, M., Sturek, D.,
             Frank, B., and R. Kelsey, "CoAP Requirements and Features",
             draft-shelby-core-coap-req-01 (Work in progress), April 20,
             2010.



9. Acknowledgments

   Thanks to Jean-Louis Gauvreau and Dale Seed for their useful comments
   and discussions.



Authors' Addresses

   Akbar Rahman
   InterDigital Communications, LLC
   Email: Akbar.Rahman@InterDigital.com


   Juan Carlos Zuniga
   InterDigital Communications, LLC
   Email: JuanCarlos.Zuniga@InterDigital.com


   Guang Lu
   InterDigital Communications, LLC
   Email: Guang.Lu@InterDigital.com








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