CoRE Working Group E. Dijk, Ed.
Internet-Draft Philips Research
Intended status: Informational November 8, 2013
Expires: May 12, 2014
Sleepy Devices using CoAP - Possible Solutions
draft-dijk-core-sleepy-solutions-02
Abstract
This document describes possible solutions for sleepy devices support
for the CoAP protocol. The solutions aim to meet the requirements
for CoAP sleepy devices in home and building control use cases. The
purpose of this document is to guide and stimulate the discussion on
sleepy devices support for CoAP in the CoRE WG.
Status of this Memo
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Abbreviations . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 3
1.3. Requirements Language . . . . . . . . . . . . . . . . . . 4
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Components . . . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Interfaces . . . . . . . . . . . . . . . . . . . . . . . . 5
3.3. Implementation of Interfaces . . . . . . . . . . . . . . . 6
3.3.1. I1: SEP Reporting to Destinations (R1) . . . . . . . . 6
3.3.2. I2: SEP Reading from External Server (R2,R5) . . . . . 6
3.3.3. I3: Reading Device Reads SEP Resource(s) Via Proxy
(R3) . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.3.4. I4: Configuring Device Writes SEP Resource(s) Via
Proxy (R4,R5,R7) . . . . . . . . . . . . . . . . . . . 7
3.3.5. I5: Proxy Notifies Destination (R1) . . . . . . . . . 7
3.3.6. I6: SEP Notifies Events to Proxy (R1) . . . . . . . . 7
3.3.7. I7: SEP Checks Proxy For Resource Updates (R4,R5) . . 7
3.3.8. I8: Proxy Requests Resources from SEP (R3) . . . . . . 8
3.3.9. I9: Proxy Registers Resources at Discovery Service
(R7) . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.3.10. I10: CoAP Endpoint Discovers SEP(s) (R7) . . . . . . . 8
3.4. Resources . . . . . . . . . . . . . . . . . . . . . . . . 9
3.4.1. SEP Resources . . . . . . . . . . . . . . . . . . . . 9
3.4.2. Proxy Resources . . . . . . . . . . . . . . . . . . . 9
3.4.3. Destination Resources . . . . . . . . . . . . . . . . 9
3.4.4. Other Resources . . . . . . . . . . . . . . . . . . . 9
4. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 9
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
6. Security Considerations . . . . . . . . . . . . . . . . . . . 10
7. References . . . . . . . . . . . . . . . . . . . . . . . . . . 10
7.1. Normative References . . . . . . . . . . . . . . . . . . . 10
7.2. Informative References . . . . . . . . . . . . . . . . . . 10
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11
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1. Terminology
1.1. Abbreviations
CoRE: Constrained RESTful Environments
SEP: Sleepy Endpoint
NSEP: Non-Sleepy Endpoint
1.2. Definitions
Sleepy Endpoint (SEP) : A CoAP endpoint hosted on a networked
computing device, which sets its network link to a disconnected
state during long periods of time to save energy. "Long" means
here that the period is of such duration that most messages sent
to a SEP are lost despite use of standard "reliable transmission"
techniques. The device is S0 class and any of E0/E1/E2 class
according to [I-D.ietf-lwig-terminology]. See also the similar
definition of SEP in [I-D.rahman-core-sleepy-problem-statement].
Non-Sleepy Endpoint (NSEP) : A CoAP endpoint hosted on a networked
computing device, which has its network interface in an always-
connected state or operates its network interface such that the
endpoint(s) on it appear always-connected. The device is S1 or S2
class and any of E1/E2/E3 class as in [I-D.ietf-lwig-terminology].
Sleeping/Asleep : A SEP being in a "sleeping state" i.e. its network
interface is disconnected and a SEP is not able to send or receive
messages.
Awake/Not Sleeping : A SEP being in an "awake state" i.e. its
network interface is connected and the SEP is able to send or
receive messages.
Destination : a NSEP to which event messages are sent by a SEP, or
by a Proxy on behalf of a SEP.
Heartbeat : a type of message (event), which is sent periodically to
indicate to a Destination that the sender is still operational and
able to communicate to the Destination. A heartbeat message may
contain data about the current status of the sender. Typically
sent by a SEP.
Proxy : a NSEP which is communicating directly with a SEP; able to
cache information/CoAP resources on behalf of SEP for the purpose
of further distribution or making it accessible to interested
endpoints. It acts as an intermediary between a SEP and a NSEP.
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The Proxy provides immediate/reliable connectivity, to enable
NSEPs to operate on SEP resources even while the SEP is sleeping.
In addition to these definitions, readers should also be familiar
with the terms and concepts discussed in [I-D.ietf-core-coap].
1.3. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in RFC
2119 [RFC2119].
2. Introduction
The CoRE WG charter includes the topic of caching resources on behalf
of sleepy devices. This document describes an overall architecture
proposal on how support for sleepy CoAP devices can be added to
Constrained RESTful systems, to support caching but also other
functions. Possible solutions for the various identified functions
are proposed. The motivation for sleepy CoAP devices is described in
[I-D.rahman-core-sleepy-problem-statement] and
[I-D.dijk-core-sleepy-reqs].
The aim of this document is to guide and stimulate the discussion on
sleepy devices support in the CoRE WG. The use cases and
requirements documented in [I-D.dijk-core-sleepy-reqs] are taken as
the reference.
3. Architecture
Based on the use cases, requirements and existing CoRE building
blocks (such as the CoAP protocol, CoAP proxying, core-observe, etc.)
a solution architecture is described in this section. First we
identify the components, then the interfaces between components, and
finally possible solutions to realize these interfaces.
3.1. Components
From the use cases and requirements the following components (i.e.
devices, or functions of devices) can be identified:
1. Sleepy Endpoint (SEP)
2. Proxy: NSEP that maintains a relation with SEP and caches
resources on behalf of the SEP.
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3. Destinations(s): NSEPs, other than Proxy, where SEP directly
reports events to. Events are typically a change of resource. A
destination endpoint may consist of a multicast group.
4. External server: a CoAP server to which a SEP can make requests
e.g. for parameter updates, firmware or external information.
5. Configuring NSEP: a CoAP endpoint that changes/writes data on the
SEP
6. Reading NSEP: a NSEP that needs to read a resource from the SEP.
This may include resources that the SEP regularly reports as
events already, or resources that a SEP did not send before.
7. Discovery Service: an optional service that enables discovery of
SEPs, their resources and their associated Proxies. For example,
a Resource Directory ([I-D.ietf-core-resource-directory]).
3.2. Interfaces
Below diagram shows the components and the interfaces (Ixx) that need
to be defined between components to meet the various requirements for
CoAP sleepy devices. The arrowheads indicate the direction of taking
initiative; e.g. the arrow from SEP to Destination NSEP(s) shows that
the SEP upon an event takes the initiative to send to one or more
Destination(s). This "taking initiative" could be implemented either
via a CoAP request or a CoAP response (e.g. a core-observe response)
so an arrow does not indicate which component is acting in the role
of CoAP client or CoAP server.
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+-------------+ +--------------+ I2 +---------+
| Destination | I1 | SEP |---------->| Server |
| NSEP(s) |<--------| |\ | NSEP |
+-------------+ +--------------+ \ +---------+
| | ^ \___________
I6 | |I7 |I8 I9 |
v v | v
+-------------+ I3 +--------------+ +-------------+
| Reading |-------->| Proxy | I9 | Disc. Serv. |
| NSEP | | (NSEP) |---------->|(OPTIONAL) |
+-------------+ +--------------+ +-------------+
^ | ^ ^
____________| |I5 |______________ | I10
/ I4 v I10 \ |
+-------------+ +--------------+ +-------------+
| Configuring | | Destination | | Discovering |
| NSEP | | NSEP(s) | NSEP |
+-------------+ +--------------+ +-------------+
Figure 1: Architecture Components and Interfaces (numbered)
3.3. Implementation of Interfaces
This section gives possible solutions how each interface (Ixx,
identified in the previous section) could be implemented. The "Rxx
items" (R1, R2, etc.) between brackets show which requirements from
[I-D.dijk-core-sleepy-reqs] are addressed by the interface.
3.3.1. I1: SEP Reporting to Destinations (R1)
A SEP can report events to one or more destinations using CoAP POST
requests, acting as a CoAP client. For each request either NON or
CON may be used. The endpoint(s) to report to can be hardcoded in
the software and/or determined by the configuration applied through
I4 (Section 3.3.4). In addition, a SEP may be programmed to fetch
such configuration from a server through I2 (Section 3.3.2). To
which resource (URI) the POST request is sent, plus the Content-
Format used and the contents of this content, is determined by the
implementers and/or SDOs that define application profiles on top of
CoAP.
3.3.2. I2: SEP Reading from External Server (R2,R5)
A SEP can read from an NS server using CoAP (GET) requests, acting as
a CoAP client. While waiting for a response, a SEP is in an awake
state. Similar to I1, the selection of endpoint(s), URI and content
is up to implementers and/or SDOs.
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3.3.3. I3: Reading Device Reads SEP Resource(s) Via Proxy (R3)
A Reading Device sends CoAP GET requests to the Proxy (acting as a
CoAP client) to read SEP resources. The Proxy serves these requests
from cache. If a requested resource is not cached, various behaviors
could be defined: e.g. return an error, or the Proxy returns a 5.03
Service Unavailable first, and then starts a process to GET the
resource directly from the SEP using interface I8.
3.3.4. I4: Configuring Device Writes SEP Resource(s) Via Proxy
(R4,R5,R7)
A Configuring Device sends CoAP PUT/DELETE requests to the Proxy in
order to write/delete resources on the SEP. The resources on the
Proxy that are written to are the resources that the SEP has
delegated towards the Proxy.
The Proxy itself then uses I8 to update the SEP with the new
resource(s).
3.3.5. I5: Proxy Notifies Destination (R1)
A Destination can use core-observe to register to resource updates on
the Proxy. The Proxy sends core-observe notifications whenever the
resource is updated. The resources here are the resources that the
SEP has delegated to the Proxy.
3.3.6. I6: SEP Notifies Events to Proxy (R1)
The SEP sends CoAP requests (acting as a CoAP client) to the Proxy to
communicate any events i.e. SEP resource updates. The format of
request and response should be partly standardized in CoRE, e.g. as
in Mirror Server [I-D.vial-core-mirror-server]].
3.3.7. I7: SEP Checks Proxy For Resource Updates (R4,R5)
The SEP may send a single CoAP GET request to the Proxy to check if
any changes to its writeable delegated resources are available. If
so, it could use multiple GET requests (one per changed resource) to
get the new content from the Proxy, or perhaps a single GET to
retrieve multiple resource values as a single composite
representation.
Text TBD; some initial thoughts: a single message to post a sensor
value (+ heartbeat) and to ask for any updates is more efficient.
The proxy could use its response code to signal if any updates are
available. For example, for a POST, a 2.04 Changed response may
indicate a resource is changed. A code 2.00 (which is to be defined)
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may indicate success but no change to resource. Or a small payload
attached to the response to the POST could indicate which updates to
resources are available at the Proxy, as was discussed on the CoRE WG
list around March 29, 2013.
Alternative: technique that once a Proxy receives a POST from the SEP
with a sensor value, it first sends a CoAP PUT request to change a
resource on the SEP, and only when that is successful it provides a
separate response to to the original POST request done by the SEP.
3.3.8. I8: Proxy Requests Resources from SEP (R3)
For I8 to work there needs to be a mechanism defined in I6 or I7, so
that the Proxy can be notified when a SEP is awake. Then, a Proxy
that needs to request resource(s) from a SEP can make the requests
via interface I8 as soon as the SEP has woken up.
A solution is that a SEP acts as a regular CoAP server during the
time it is awake.
3.3.9. I9: Proxy Registers Resources at Discovery Service (R7)
This interface is OPTIONAL i.e. only required if the Discovery
Service is available. A SEP could communicate its available
resources described in CoRE Link Format [RFC6690] to a Proxy. The
Proxy is then responsible for registering these resources/
descriptions in a further Discovery Service which may be implemented
as a Resource Directory [I-D.ietf-core-resource-directory].
A SEP SHOULD describe its own resources in CoRE Link Format in its
"/.well-known/core" resource, such that a Proxy is able to read this
resource description and to do further registration of SEP resources
in a Discovery Service.
3.3.10. I10: CoAP Endpoint Discovers SEP(s) (R7)
There are two ways in which a CoAP endpoint can discover a SEP and
its resources. The first way which should be supported in a system,
requires that a Proxy includes in its "/.well-known/core" Link Format
description the descriptions of the individual SEPs as detailed in
[I-D.vial-core-mirror-server]. The CoAP endpoint can then use CoRE
resource discovery ([I-D.ietf-core-coap]) using either unicast or
multicast CoAP requests to discover the SEP.
The second way which is OPTIONALLY supported in a system, is that a
Proxy registers the SEP into a Discovery Service (such as RD
[I-D.ietf-core-resource-directory]), and the CoAP endpoint uses the
specific interface of the Discovery Service to discover a SEP.
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3.4. Resources
This section provides some information on the CoAP resources that
need to be allocated on the different components in the architecture.
3.4.1. SEP Resources
On a SEP, a clear distinction has to be made between types of
resources.
o Read-only resources: resource can be modified by the SEP, but
SHOULD NOT be modifiable by any external device. This includes
static information resource (e.g. manufacturer name, type,
firmware version, etc.) but also volatile resources (e.g. latest
sensor value, error log entries, etc.) that the SEP internally
updates.
o Read/Write resources: resource can be modified by an (authorized)
external device. This is used for configuration information (e.g.
sensor thresholds, which Destination(s) to use, event frequency,
etc.). Authorized CoAP clients can write such resource at the
Proxy, which will communicate the updated resource to the SEP next
time it wakes up and contacts the Proxy. To avoid write/write
conflicts, such a resource SHOULD NOT be modified autonomously by
a SEP.
3.4.2. Proxy Resources
TBD
3.4.3. Destination Resources
TBD. One draft proposal is: a single reporting resource for
receiving events "/event". This can be changed to anything SDO/
vendor specific, but above can be a default.
3.4.4. Other Resources
TBD
4. Acknowledgements
TBD
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5. IANA Considerations
This document includes no request to IANA.
6. Security Considerations
TBD: per interface the security needs and solution need to be
described. Anywhere CoAP unicast is used, DTLS may apply as a
transport security solution. DTLS key update on a sleepy device may
pose a problem.
7. References
7.1. Normative References
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., and C. Bormann, "Constrained
Application Protocol (CoAP)", draft-ietf-core-coap-18
(work in progress), June 2013.
[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.
7.2. Informative References
[I-D.dijk-core-sleepy-reqs]
Dijk, E., "Sleepy Devices using CoAP - Requirements",
draft-dijk-core-sleepy-reqs-00 (work in progress),
June 2013.
[I-D.ietf-core-block]
Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
draft-ietf-core-block-14 (work in progress), October 2013.
[I-D.ietf-core-observe]
Hartke, K., "Observing Resources in CoAP",
draft-ietf-core-observe-11 (work in progress),
October 2013.
[I-D.ietf-core-resource-directory]
Shelby, Z., Krco, S., and C. Bormann, "CoRE Resource
Directory", draft-ietf-core-resource-directory-00 (work in
progress), June 2013.
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[I-D.ietf-lwig-terminology]
Bormann, C., Ersue, M., and A. Keranen, "Terminology for
Constrained Node Networks", draft-ietf-lwig-terminology-05
(work in progress), July 2013.
[I-D.rahman-core-sleepy-problem-statement]
Rahman, A., Fossati, T., Loreto, S., and M. Vial, "Sleepy
Devices in CoAP - Problem Statement",
draft-rahman-core-sleepy-problem-statement-01 (work in
progress), October 2012.
[I-D.vial-core-mirror-server]
Vial, M., "CoRE Mirror Server",
draft-vial-core-mirror-server-01 (work in progress),
April 2013.
Author's Address
Esko Dijk (editor)
Philips Research
High Tech Campus 34
Eindhoven, 5656 AE
NL
Phone: +31 40 2747947
Email: esko.dijk@philips.com
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