Internet Engineering Task Force T. Fossati
Internet-Draft KoanLogic
Intended status: Standards Track P. Giacomin
Expires: January 10, 2013 Freelance
S. Loreto
Ericsson
July 9, 2012
Publish Option for CoAP
draft-fossati-core-publish-option-00
Abstract
This memo defines the Publish Option for the Constrained Application
Protocol (CoAP).
The Publish Option is used by a sleepy node to temporarily delegate
the authority of one of its resources to another, always on, node
which is typically a proxy but doesn't need to be necessarily.
The sleepy node is given a simple RESTful messaging protocol that
enables the setup, renew and removal of the authority transfer. The
whole process is driven by the (sleepy) origin, which may actually
never need to listen or to keep any state.
Status of this Memo
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Copyright Notice
Copyright (c) 2012 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Requirements Language and Motivation . . . . . . . . . . . 3
2. Publish Option . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Publishing a Resource . . . . . . . . . . . . . . . . . . . 4
2.2. Updating a Resource . . . . . . . . . . . . . . . . . . . . 5
2.3. Unpublishing a Resource . . . . . . . . . . . . . . . . . . 5
2.4. Value Format . . . . . . . . . . . . . . . . . . . . . . . 6
2.5. Discovery . . . . . . . . . . . . . . . . . . . . . . . . . 6
2.5.1. Publishing the /.well-known/core Resource . . . . . . . 6
2.5.2. Resource Directory . . . . . . . . . . . . . . . . . . 7
3. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 7
4. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 7
5. Security Considerations . . . . . . . . . . . . . . . . . . . . 7
6. References . . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Normative References . . . . . . . . . . . . . . . . . . . 8
6.2. Informative References . . . . . . . . . . . . . . . . . . 8
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
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1. Introduction
The proposal described in this memo covers the following use case: a
node N, which is sleeping most of the time, depends one or more
resources hosted at another sleepy node M. In cases as such, the
probability of an empty intersection between their respective wake
periods is very high, making it hard for the two to synchronize.
In this scenario, using the basic observe [I-D.ietf-core-observe]
functionality is not enough, as it could lead to lost state updates
in case N is offline while M pushes its notifications; further, the
observation may never bootstrap since its initialization needs both
client and origin awake at the same time.
This memo introduces an extension to the Proxy caching functionality
that give the Proxy an explicit mediation role in the sleepy-to-
sleepy CoAP [I-D.ietf-core-coap] communication.
1.1. Requirements Language and Motivation
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].
This specification makes use of the following terminology:
Sleepy Device: a sensor/actuator (usually battery operated) that
powers down its radio beyond the normal radio duty cycle in order
to save energy.
and tries to provide an in-protocol solution for the requirement REQ3
stated in [I-D.shelby-core-coap-req]:
The ability to deal with sleeping nodes. Devices may be
powered down at any point in time but periodically "wake up"
for brief periods of time.
2. Publish Option
+-----+----------+---------+--------+--------+---------+
| No. | C/E | Name | Format | Length | Default |
+-----+----------+---------+--------+--------+---------+
| YY | Critical | Publish | uint | 1 B | 0x2 |
+-----+----------+---------+--------+--------+---------+
The Publish Option enables the sleepy origin to temporarily (i.e. for
a specified "lease" time) delegate the authority of one of its hosted
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resources to a Proxy node that will start to behave as the origin for
the Publish'ed resource. This allows a sleepy sensor to use the
Proxy as the rendezvous point for one-way sleepy to sleepy signaling.
2.1. Publishing a Resource
P S
| PUT | Proxy-URI: coap://sleepy.example.org/res
|<--------+ Publish: 0110
| r | Content-Type: text/plain
| | ETag: 0xabcd
| | Max-Age: 1200
| |
| 2.01 |
+-------->|
| |
Figure 1
The origin server publishes one of its hosted resources, specified by
the enclosed Proxy-URI, by PUT'ing it to the Proxy with a Publish
Option attached. The Publish Option value specifies the CoAP methods
that clients are allowed to use on the resource (see Section 2.4).
The example in Figure 1 shows a delegation where the GET and PUT
methods are allowed while POST and DELETE are explicitly prohibited,
meaning that the resource can be read and updated by clients, but it
can't be deleted nor created.
The Proxy, which is voluntarily charged by the resource owner to act
as the delegated origin for the "lease" time specified by Max-Age,
replies with a 2.01 if the authority transfer has succeeded. An
exact duplicate of the submitted representation is created, and from
now on it can be accessed using the original URI provided that
clients go through the delegated Proxy. If the Publish operation
does not succeed, the origin transfer fails, and an appropriate
response code is returned.
An ETag MAY be supplied as a metadata to be included in responses
involving the Publish'ed representation. If no Max-Age is given, a
default of 3600 seconds MUST be assumed. The Max-Age value, either
implicit or explicit, determines the lifetime of the origin
delegation. When the Max-Age value is elapsed, the Proxy MUST delete
the Publish'ed resource value and fall back to its usual proxying
function.
The Publish Option is critical and MUST be present in the request
only. If the Proxy does not recognize it, a 4.02 (Bad Option) MUST
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be returned to the client. If the option value is not correctly
formatted (see Section 2.4), a 4.00 (Bad Request) MUST be returned to
the client.
It is sufficient for any client wishing to access the resource to do
so using the Proxy node that, following the Publish operation, will
start behaving a the origin, satisfying requests on behalf of the
sleeping node.
The Proxy MUST save the identity of the resource Publish'er in order
to distinguish "maintainance" operations such as update and explicit
deletion, from "regular" access to the published resource by clients.
An interesting outcome of this communication strategy is that the
sleepy origin may really never need to listen on its radio interface.
2.2. Updating a Resource
P S
| PUT | Proxy-URI: coap://sleepy.example.org/res
|<--------+ Publish: 0110
| r | Content-Type: text/plain
| | ETag: 0xdcba
| | Max-Age: 1200
| |
| 2.04 |
+-------->|
| |
Figure 2
In order to update the delegated resource state, the sleepy node
shall send the very same request to the Proxy, which in turn replies
with a 2.04 (Changed) status code in case the update operation has
succeeded, or an appropriate error code in case it fails.
2.3. Unpublishing a Resource
P S
| DELETE | Proxy-URI: coap://sleepy.example.org/res
|<---------+ Publish: 0x0
| |
| |
| 2.02 |
+--------->|
| |
The delegation of a given resource can be explicitly revoked by the
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real origin at any time before the lease time expires, by issuing a
DELETE request to the Proxy hosting the resource duplicate with a
Publish Option with value 0x0.
On successful deletion of the delegation a 2.02 (Deleted) response
code is returned by the Proxy.
2.4. Value Format
0 1 2 3 4 5 6 7
+-+-+-+-+-+-+-+-+
|C R U D 0 0 0 0|
+-+-+-+-+-+-+-+-+
Each of the first 4 bits is a flag field indicating whether the
associated CoAP method (respectively: POST, GET, PUT and DELETE) is
allowed on the Publish'ed resource. The remaining 4 bits are unused
and MUST be set to 0.
In case the value is missing, the default is assumed to be 0x2, i.e.
the resource is read-only.
An all-0 value is used to explicitly revoke the delegation (see
Section 2.3.)
If the delegated Proxy receives a request with a method that is not
compatible with the supplied mask, it MUST respond with a 4.05
(Method Not Allowed) response code.
2.5. Discovery
2.5.1. Publishing the /.well-known/core Resource
The [I-D.ietf-core-link-format] has no explicit text about "well-
known" discovery of devices through a Proxy, nor about the
cacheability rules for such resource. Even if it seems reasonable to
assume that the /.well-known/core URI is both query-able and
cacheable through a Proxy, on the contrary the situation is not very
much so.
In fact, since the "well-known" interface relies on the resource
origin being implicitly defined by the source address of the UDP
packet carrying the response, quering the "well-known" interface
(either unicast or multicast) through a Proxy-URI has little hope to
be fully functional. The (ab)use of a an implicit L3 locator as the
identifier of the resource authority makes "well-known" discovery
generally incompatible with Proxy mediated communication, unless each
target URI in a link is given as a URI and not as a relative-ref
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(section 4.1 of [RFC3986]).
Consequently, in this proposal we assume that the /.well-known/core
of a sleepy node can be Publish'ed if and only if the target URI in
the each link is not a relative-ref.
Its registration is the same as in Figure 1, but the Proxy MAY need
to treat it in a way that is slightly different from other "normal"
delegated resources. In fact, while delegation is in place (i.e. the
lease period is not elapsed, and neither explicit revocation has
happened) the Proxy MAY be able to respond to filtered queries
(section 4.1 of [I-D.ietf-core-link-format]) regarding the Publish'ed
/.well-known/core.
2.5.2. Resource Directory
Given the strong requirement on the link formatting given in
Section 2.5.1, it could be preferable (or even necessary) to use the
Resource Directory [I-D.shelby-core-resource-directory] as a means of
delegating the discovery of the resources hosted at a sleepy node.
This can be done either by the sleepy node, or automatically by the
delegated Proxy when a Publish request is received.
[[Automatic push to RD: check it out]]
3. Acknowledgements
Bruce Nordman and Matthieu Vial for discussing and giving advice on
some of the ideas contained in this document.
4. IANA Considerations
The following entries are added to the CoAP Option Numbers registry:
.------------------------------.
| Number | Name | Reference |
:--------:---------:-----------:
| 2n+1 | Publish | RFC XXXX |
`------------------------------'
5. Security Considerations
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Threat: cache poisoning.
Countermeasure: authenticate sender.
Threat: unauthorized de-registration
Countermeasure: authenticate requester.
Threat: Proxy resources' exhaustion.
Countermeasure: authenticate requester + quota limit.
Threat: global state loss.
Countermeasure: cache redundancy.
6. References
6.1. Normative References
[I-D.ietf-core-coap]
Frank, B., Bormann, C., Hartke, K., and Z. Shelby,
"Constrained Application Protocol (CoAP)",
draft-ietf-core-coap-08 (work in progress), October 2011.
[I-D.ietf-core-link-format]
Shelby, Z., "CoRE Link Format",
draft-ietf-core-link-format-11 (work in progress),
January 2012.
[I-D.ietf-core-observe]
Hartke, K., "Observing Resources in CoAP",
draft-ietf-core-observe-04 (work in progress),
February 2012.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3986] Berners-Lee, T., Fielding, R., and L. Masinter, "Uniform
Resource Identifier (URI): Generic Syntax", STD 66,
RFC 3986, January 2005.
6.2. Informative References
[I-D.shelby-core-coap-req]
Shelby, Z., Stuber, M., Sturek, D., Frank, B., and R.
Kelsey, "CoAP Requirements and Features",
draft-shelby-core-coap-req-02 (work in progress),
October 2010.
[I-D.shelby-core-resource-directory]
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Krco, S. and Z. Shelby, "CoRE Resource Directory",
draft-shelby-core-resource-directory-02 (work in
progress), October 2011.
Authors' Addresses
Thomas Fossati
KoanLogic
Via di Sabbiuno, 11/5
Bologna 40100
Italy
Email: tho@koanlogic.com
Pierpaolo Giacomin
Freelance
Email: yrz@anche.no
Salvatore Loreto
Ericsson
Hirsalantie 11
Jorvas 02420
Finland
Email: salvatore.loreto@ericsson.com
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