CoRE Working Group K. Hartke
Internet-Draft Universitaet Bremen TZI
Intended status: Standards Track February 25, 2013
Expires: August 29, 2013
Observing Resources in CoAP
draft-ietf-core-observe-08
Abstract
CoAP is a RESTful application protocol for constrained nodes and
networks. The state of a resource on a CoAP server can change over
time. This document specifies a simple protocol extension for CoAP
that enables CoAP clients to "observe" resources, i.e., to retrieve
a representation of a resource and keep this representation updated
by the server over a period of time. The protocol follows a best-
effort approach for sending new representations to clients, and
provides eventual consistency between the state observed by each
client and the actual resource state at the server.
Editor's Note
This is an interim revision which will receive further modifications
during the resolution of open tickets, in particular #204 and #281.
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-
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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 August 29, 2013.
Copyright Notice
Copyright (c) 2013 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. Background . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Protocol Overview . . . . . . . . . . . . . . . . . . . . 3
1.3. Requirements Notation . . . . . . . . . . . . . . . . . . 6
2. The Observe Option . . . . . . . . . . . . . . . . . . . . . . 6
3. Client-side Requirements . . . . . . . . . . . . . . . . . . . 7
3.1. Request . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Notifications . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Caching . . . . . . . . . . . . . . . . . . . . . . . . . 8
3.4. Reordering . . . . . . . . . . . . . . . . . . . . . . . . 9
3.5. Transmission . . . . . . . . . . . . . . . . . . . . . . . 10
3.6. Cancellation . . . . . . . . . . . . . . . . . . . . . . . 10
4. Server-side Requirements . . . . . . . . . . . . . . . . . . . 10
4.1. Request . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.2. Notifications . . . . . . . . . . . . . . . . . . . . . . 11
4.3. Caching . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.4. Reordering . . . . . . . . . . . . . . . . . . . . . . . . 13
4.5. Transmission . . . . . . . . . . . . . . . . . . . . . . . 13
5. Intermediaries . . . . . . . . . . . . . . . . . . . . . . . . 15
6. Blockwise Transfers . . . . . . . . . . . . . . . . . . . . . 16
7. Web Linking . . . . . . . . . . . . . . . . . . . . . . . . . 16
8. Security Considerations . . . . . . . . . . . . . . . . . . . 17
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
11.1. Normative References . . . . . . . . . . . . . . . . . . . 18
11.2. Informative References . . . . . . . . . . . . . . . . . . 18
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 19
A.1. Proxying . . . . . . . . . . . . . . . . . . . . . . . . . 23
A.2. Blockwise Transfer . . . . . . . . . . . . . . . . . . . . 25
Appendix B. Modeling Resources to Tailor Notifications . . . . . 26
Appendix C. Changelog . . . . . . . . . . . . . . . . . . . . . . 26
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 31
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1. Introduction
1.1. Background
CoAP [I-D.ietf-core-coap] is an application protocol for constrained
nodes and networks. It is intended to provide RESTful services
[REST] not unlike HTTP [RFC2616] while reducing the complexity of
implementation as well as the size of packets exchanged in order to
make these services useful in a highly constrained network of
themselves highly constrained nodes.
The model of REST is that of a client exchanging representations of
resources with a server. A representation captures the current or
intended state of a resource. The server is the definitive source
for representations of the resources in its namespace. A client
interested in the state of a resource initiates a request to the
server; the server then returns a response with a representation of
the resource that is current at the time of the request.
This model does not work well when a client is interested in having a
current representation of a resource over a period of time. Existing
approaches from HTTP, such as repeated polling or HTTP long polling
[RFC6202], generate significant complexity and/or overhead and thus
are less applicable in a constrained environment.
The protocol specified in this document extends the CoAP core
protocol with a mechanism for a CoAP client to "observe" a resource
on a CoAP server: the client can retrieve a representation of the
resource and keep this representation updated by the server over a
period of time.
The protocol keeps the architectural properties of REST. It enables
high scalability and efficiency through the support of caches and
proxies. There is no intention, though, to solve the full set of
problems that the existing HTTP solutions solve, or to replace
publish/subscribe networks that solve a much more general problem
[RFC5989].
1.2. Protocol Overview
The protocol is based on the well-known observer design pattern
[GOF]. In this design pattern, components called "observers"
register at a specific, known provider called the "subject" that they
are interested in being notified whenever the subject undergoes a
change in state. The subject is responsible for administering its
list of registered observers. If multiple subjects are of interest
to an observer, it must register separately for all of them.
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Observer Subject
| |
| Registration |
+----------------->|
| |
| Notification |
|<-----------------+
| |
| Notification |
|<-----------------+
| |
| Notification |
|<-----------------+
| |
Figure 1: The Observer Design Pattern
The observer design pattern is realized in CoAP as follows:
Subject: In the context of CoAP, the subject is a resource in the
namespace of a CoAP server. The state of the resource can change
over time, ranging from infrequent updates to continuous state
transformations.
Observer: An observer is a CoAP client that is interested in having
a current representation of the resource at any given time.
Registration: A client registers its interest in a resource by
initiating an extended GET request to the server. In addition to
returning a representation of the target resource, this request
causes the server to add the client to the list of observers of
the resource.
Notification: Whenever the state of a resource changes, the server
notifies each client in the list of observers of the resource.
Each notification is an additional CoAP response sent by the
server in reply to the GET request and includes a complete,
updated representation of the new resource state.
Figure 2 below shows an example of a CoAP client registering its
interest in a resource and receiving three notifications: the first
upon registration with the current state, and then two upon two
changes to the resource state. Both the registration request and the
notifications are identified as such by the presence of the Observe
Option defined in this document. In notifications, the Observe
Option provides a sequence number for reordering detection. All
notifications carry the token specified by the client in the request,
so the client can easily correlate them to the request.
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Client Server
| |
| GET /temperature |
| Token: 0x4a | Registration
| Observe: (empty) |
+----------------->|
| |
| 2.05 Content |
| Token: 0x4a | Notification of
| Observe: 12 | the current state
| Payload: 22.9 C |
|<-----------------+
| |
| 2.05 Content |
| Token: 0x4a | Notification upon
| Observe: 44 | a state change
| Payload: 22.8 C |
|<-----------------+
| |
| 2.05 Content |
| Token: 0x4a | Notification upon
| Observe: 60 | a state change
| Payload: 23.1 C |
|<-----------------+
| |
Figure 2: Observing a Resource in CoAP
The server is the authority for determining under what conditions
resources change their state and how often observers are notified.
The protocol does not offer explicit means for setting up triggers,
thresholds or other conditions; it is up to the server to expose
observable resources that change their state in a way that is useful
in the application context. Resources can be parameterized to
achieve similar effects, though; see Appendix B for examples.
A client remains on the list of observers as long as the server can
determine the client's continued interest in the resource. The
interest is determined from the client's acknowledgement of
notifications sent in confirmable messages by the server. If the
client actively rejects a notification or if the transmission of a
notification ultimately fails, then the client is assumed to be no
longer interested and is removed from the list of observers.
While a client is in the list of observers of a resource, it is the
goal of the protocol to keep the resource state observed by the
client as closely in sync with the actual state at the server as
possible.
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Becoming out of sync at times cannot be avoided: First, there is
always some latency between the change of the resource state and the
receipt of the notification. Second, messages with notifications can
get lost, which will cause the client assume an old state until it
receives the next notification. And third, the server may
erroneously come to the conclusion that the client is no longer
interested in the resource, which will cause it to stop sending
notifications and the client to assume an old state until it
registers its interest again.
The protocol addresses this issue as follows:
o It follows a best-effort approach for sending the current
representation to the client after a state change: Clients should
see the new state after a state change as soon as possible, and
they should see as many states as possible. However, a client
cannot rely on observing every single state that a resource goes
through.
o It labels notifications with a maximum duration up to which it is
acceptable for the observed state and the actual state to be out
of sync. When the age of the notification received reaches this
maximum, the client cannot use the enclosed representation until
it has received a new notification.
o It is designed on the principle of eventual consistency: The
protocol guarantees that, if the resource does not undergo a new
change in state, eventually all registered observers will have a
current representation of the latest resource state.
1.3. Requirements Notation
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].
2. The Observe Option
+-----+---+---+---+---+---------+------------+-----------+---------+
| No. | C | U | N | R | Name | Format | Length | Default |
+-----+---+---+---+---+---------+------------+-----------+---------+
| 6 | | x | - | | Observe | empty/uint | 0 B/0-3 B | (none) |
+-----+---+---+---+---+---------+------------+-----------+---------+
C=Critical, U=Unsafe, N=No-Cache-Key, R=Repeatable
Table 1: The Observe Option
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The Observe Option, when present in a request, extends the GET method
so it does not only retrieve a current representation of the target
resource, but also requests the server to add the client to the list
of observers of the resource. The exact semantics are defined in the
following sections. The value of the option in a request MUST be
empty on transmission and MUST be ignored on reception.
The Observe Option is not critical for processing the request. If
the server is unwilling or unable to add the client to the list of
observers of the resource identified by the request URI, then the
request falls back to a normal GET request.
In a response, the Observe Option identifies the message as a
notification. This implies that the server has added the client to
the list of observers and that it will notify the client of changes
to the resource state. The value of the option is a 24-bit sequence
number for reordering detection; see Section 3.4 and Section 4.4 for
the client- and server-side respectively. The sequence number is
encoded in network byte order using a variable number of bytes, as
specified in Section 3.2 of RFC XXXX [I-D.ietf-core-coap].
The Observe Option is not part of the cache-key: a cacheable response
obtained with an Observe Option in the request can be used to satisfy
a request without an Observe Option, and vice versa. When a stored
response with an Observe Option is used to satisfy a normal GET
request, the option MUST be removed before the response is returned
to the client.
3. Client-side Requirements
3.1. Request
A client can register its interest in a resource by issuing a GET
request that includes an empty Observe Option. If the server returns
a 2.xx response that includes an Observe Option as well, the server
has added the client successfully to the list of observers of the
target resource and the client will be notified of changes to the
resource state.
3.2. Notifications
Notifications are additional responses sent by the server in reply to
the GET request. Each notification includes the token specified by
the client in the GET request, an Observe Option with a sequence
number for reordering detection (see Section 3.4) and a payload in
the same Content-Format as the initial response.
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Notifications have a 2.05 (Content) response code, or a 2.03 (Valid)
response code if the client has included one or more ETag Options in
the request (see Section 3.3). In the event that the resource
changes in a way that would cause a normal GET request at that time
to return a non-2.xx response (for example, when the resource is
deleted), the server sends a notification with a matching response
code and removes the client from the list of observers.
3.3. Caching
As notifications are just additional responses to a GET request,
notifications partake in caching as defined in Section 5.6 of RFC
XXXX [I-D.ietf-core-coap]. Both the freshness model and the
validation model are supported.
3.3.1. Freshness
A client MAY store a notification like a response in its cache and
use a stored notification that is fresh without contacting the origin
server. Like a response, a notification is considered fresh while
its age is not greater than the value indicated by the Max-Age Option
and if no newer notification/response has been received.
The server will do its best to keep the resource state observed by
the client as closely in sync with the actual state as possible.
However, a client cannot rely on observing every single state that a
resource might go through. For example, if the network is congested
or the state changes more frequently than the network can handle, the
server can skip notifications for intermediate states.
The server uses the Max-Age Option to indicate an age up to which it
is acceptable that the observed state and the actual state are
inconsistent. If the age of the latest notification becomes greater
than its indicated Max-Age, then the client MUST NOT use the enclosed
representation until it is validated or a new notification is
received.
To make sure it has a current representation and/or to re-register
its interest in a resource, a client MAY issue a new GET request with
an Observe Option at any time. The client SHOULD specify a new token
in the GET request, as the token serves as an epoch identifier for
the sequence numbers in the Observe Option (see Section 3.4).
It is RECOMMENDED that the client does not issue the request while it
still has a fresh notification/response for a resource in its cache.
Additionally, the client SHOULD wait for a random amount of time
between 5 and 15 seconds before issuing the new request to avoid
synchronicity with other clients.
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3.3.2. Validation
When a client has one or more notifications stored in its cache for a
resource, it can use the ETag Option in the GET request to give the
server an opportunity to select a stored notification to be used.
The client MAY include an ETag Option for each stored response that
is applicable in the GET request. Whenever the observed resource
changes to a representation identified by one of the ETag Options,
the server can select a stored response by sending a 2.03 (Valid)
notification with an appropriate ETag Option instead of a 2.05
(Content) notification. The client needs to keep all candidate
responses in its cache until it is no longer interested in the target
resource or it issues a new GET request with a new set of entity-
tags.
3.4. Reordering
Messages with notifications can arrive in a different order than they
were sent. Since the goal is to keep the observed state as closely
in sync with the actual state as possible, a client SHOULD NOT update
the observed state with a notification that arrives later than a
newer notification.
For reordering detection, the server sets the value of the Observe
Option in each notification to a 24-bit sequence number. An incoming
notification is newer than the newest notification received so far
when one of the following conditions is met:
(V1 < V2 and V2 - V1 < 2^23) or
(V1 > V2 and V1 - V2 > 2^23) or
(T2 > T1 + 128 seconds)
where V1 is the value of the Observe Option of the newest
notification received so far, V2 the value of the Observe Option of
the incoming notification, T1 a client-local timestamp of the newest
notification received so far, and T2 a client-local timestamp of the
incoming notification.
Design Note: The first two conditions verify that V1 is less than V2
in 24-bit sequence number arithmetic [RFC1982]. The third
condition ensures that the time elapsed between the two incoming
messages is not so large that the difference between V1 and V2 has
become larger than the largest integer that it is meaningful to
add to a 24-bit sequence number; in other words, after 128 seconds
have elapsed without any notification, a client does not need to
check the sequence numbers in order to assume an incoming
notification is new.
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3.5. Transmission
A notification can be confirmable or non-confirmable, i.e., be sent
in a confirmable or a non-confirmable message. The message type used
is independent from the type used for the request or any previous
notification.
If a client does not recognize the token in a confirmable
notification, it MUST NOT acknowledge the message and SHOULD reject
it with a Reset message; otherwise, the client MUST acknowledge the
message as usual. In the case of a non-confirmable notification,
rejecting the message with a Reset message is OPTIONAL.
An acknowledgement message signals to the server that the client is
alive and interested in receiving further notifications; if the
server does not receive an acknowledgement in reply to a confirmable
notification, it will assume that the client is no longer interested
and will eventually remove the client from the list of observers.
3.6. Cancellation
When a client rejects a confirmable notification with a Reset message
or when it issues a GET request without an Observe Option for a
currently observed resource, the server will remove the client from
the list of observers of this resource. The client MAY use either
method to indicate that it is no longer interested in receiving
further notifications for the resource until it eventually registers
again.
When a client rejects non-confirmable notification, the server may
also (but is not required to) remove the client from the list of
observers of this resource. The client MAY try this method as well,
and MAY rate-limit the Reset messages it sends if the server appears
to persistently ignore them.
Implementation Note: A client that does not mediate all its requests
through its cache might inadvertently cancel an observation by
making an unrelated GET to the same resource. To avoid this,
without incurring a need for synchronization, such clients can use
a different source endpoint for these unrelated GET requests.
4. Server-side Requirements
4.1. Request
A GET request that includes an Observe Option requests the server not
only to return a current representation of the resource identified by
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the request URI, but also to add the client to the list of observers
of the target resource. If no error occurs, the server MUST return a
2.05 (Content) response with the representation of the current
resource state and MUST notify the client of subsequent changes to
the state as long as the client is on the list of observers.
A server that is unable or unwilling to add the client to the list of
observers of the target resource MAY silently ignore the Observe
Option and process the GET request as usual. The resulting response
MUST NOT include an Observe Option, the absence of which signals to
the client that it will not be notified of changes to the resource
and, e.g., needs to poll the resource for its state instead.
If the client is already on the list of observers, the server MUST
NOT add it a second time but MUST replace or update the existing
entry. If the server receives a GET request for the a resource that
does not include an Observe Option, the server MUST remove any
existing entry from the list of observers.
Two requests relate to the same list entry if and only if both the
request URI and the source endpoint of the requests are the same.
Message IDs, tokens and other options are not taken into account.
Any request with an unrecognized critical option or a method other
than GET MUST NOT have a direct effect on a list of observers of a
resource. However, a non-GET request can have the indirect
consequence of causing the server to send a non-2.xx notification
which does affect the list of observers (for example, when a DELETE
request is successful and the observed resource no longer exists).
4.2. Notifications
A client is notified of changes to the resource state by additional
responses sent by the server in reply to the GET request. Each such
notification response (including the initial response) MUST include
an Observe Option and MUST echo the token specified by the client in
the GET request. If there are multiple clients on the list of
observers, the order in which they are notified is not defined; the
server is free to use any method to determine the order.
A notification SHOULD have a 2.05 (Content) or 2.03 (Valid) response
code. However, in the event that the state of a resource changes in
a way that would cause a normal GET request at that time to return a
non-2.xx response (for example, when the resource is deleted), the
server SHOULD notify the client by sending a notification with a
matching response code and MUST remove the client from the list of
observers of the resource.
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The Content-Format used in a notification MUST be the same as the one
used in the initial response to the GET request. If the server is
unable to continue sending notifications using this Content-Format,
it SHOULD send a notification with a 5.00 (Internal Server Error)
response code and MUST remove the client from the list of observers
of the resource.
4.3. Caching
As notifications are just additional responses sent by the server,
they are subject to caching as defined in Section 5.6 of RFC XXXX
[I-D.ietf-core-coap].
4.3.1. Freshness
After returning the initial response, the server MUST try to keep the
returned representation current, i.e., keep the resource state
observed by the client as closely in sync with the actual resource
state as possible.
Since becoming out of sync at times cannot be avoided, the server
MUST indicate for each representation an age up to which it is
acceptable that the observed state and the actual state are
inconsistent. This age is application-dependent and MUST be
specified in notifications using the Max-Age Option.
When the resource does not change and the client has a current
representation, the server does not need to send a notification.
However, if the client does not receive a notification, it cannot
tell if the observed state and the actual state are still in sync.
So, when the the age of the latest notification becomes greater than
its indicated Max-Age, then the client will assume that the states
are inconsistent until the representation is validated or a new
notification is received. The server MAY wish to prevent that by
sending a notification with the unchanged representation before Max-
Age expires.
4.3.2. Validation
A client can include a set of entity-tags in its request using the
ETag Option. When a observed resource changes its state and the
origin server is about to send a 2.05 (Content) notification, then,
whenever that notification has an entity-tag in the set of entity-
tags specified by the client, the server MAY send a 2.03 (Valid)
response with an appropriate ETag Option instead. The server MUST
NOT assume that the client has any response stored other than those
identified by the entity-tags in the most recent GET request received
for the resource.
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4.4. Reordering
Because messages can get reordered, the client needs a way to
determine if a notification arrived later than a newer notification.
For this purpose, the server MUST set the value of the Observe Option
of each notification it sends to the 24 least-significant bits of a
strictly increasing sequence number. The sequence number MAY start
at any value and MUST NOT increase so fast that it increases by more
than 2^24 within less than 256 seconds.
The sequence number selected for a notification MUST be greater than
that of any preceding notification sent to the same client for the
same resource with the same token. The value of the Observe Option
MUST be current at the time of transmission; if a notification is
retransmitted, the server MUST update value of the Observe Option
before sending the message.
Implementation Note: A simple implementation that satisfies the
requirements is to obtain a timestamp from a local clock. The
sequence number then is the timestamp in ticks, where 1 tick =
(256 seconds)/(2^24) = 15.26 microseconds. It is not necessary
that the clock reflects the current time/date or that it ticks in
a precisely periodical way.
Another valid implementation is to store a 24-bit unsigned integer
variable per resource and increment this variable each time the
resource undergoes a change of state (provided that the resource
changes its state less than 2^24 times in the next 256 seconds
after any state change). This alleviates the need to update the
value of the Observe Option in a message when the resource state
did not change.
Design Note: The choice of a 24-bit option value and a time span of
256 seconds allows for a notification rate of up to 65536
notifications per second. 64K ought to be enough for anybody.
4.5. Transmission
A notification can be sent in a confirmable or a non-confirmable
message. The message type used is typically application-dependent
and MAY be determined by the server for each notification
individually. For example, for resources that change in a somewhat
predictable or regular fashion, notifications can be sent in non-
confirmable messages; for resources that change infrequently,
notifications can be sent in confirmable messages. The server can
combine these two approaches depending on the frequency of state
changes and the importance of individual notifications.
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The acknowledgement of a confirmable notification signals to the
server that the client is interested in receiving further
notifications. If a client rejects a confirmable notification with a
Reset message, the client is no longer interested and the server MUST
remove the client from the list of observers. If the client rejects
a non-confirmable notification, the server MAY remove the client from
the list of observers as well. (It is expected that the server does
remove the client if it has the information available that is needed
to match the Reset message to the non-confirmable notification, but
the server is not required to keep this information.)
At a minimum, the server MUST send a notification in a confirmable
message instead of a non-confirmable message at least every 24 hours.
A server MAY choose to skip a notification if it knows that it will
send another notification soon (e.g., when the state is changing
frequently). Similarly, it MAY choose to send a notification more
than once. For example, when state changes occur in bursts, the
server can skip some notifications, send the notifications in non-
confirmable messages, and make sure that the client observes the
latest state change after the burst by repeating the last
notification in a confirmable message.
The server MUST limit the number of confirmable notifications for
which an acknowledgement has not been received yet to NSTART (see
Section 4.7 of RFC XXXX [I-D.ietf-core-coap]), and it SHOULD NOT send
more than one non-confirmable notification every 3 seconds on
average.
When the state of an observed resource changes while the server is
still waiting for a confirmable notification to be acknowledged or
the 3 seconds for a non-confirmable notification to elapse, then the
server MUST proceed as follows:
1. Wait for the current transmission attempt to complete.
2. If the result is a Reset message or the transmission was the last
attempt to deliver a notification, remove the client from the
list of observers of the observed resource.
3. If the client is still in the list of observers, transmit a
notification with a representation of the current resource state.
Should the resource have changed its state more than once in the
meantime, skip the notifications for the intermediate states.
4. If the previously completed transmission timed out, increment the
retransmission counter and double the timeout; otherwise,
reinitialize the retransmission counter and the timeout.
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If CoAP is used over a connection-oriented or session-based transport
such as DTLS, the server MUST remove the client from the list of
observers when the connection or session is closed.
5. Intermediaries
A client may be interested in a resource in the namespace of an
origin server that is reached through a chain of one or more CoAP-to-
CoAP intermediaries. In this case, the client registers its interest
with the first intermediary towards the origin server, acting as if
it was communicating with the origin server itself as specified in
Section 3. It is the task of this intermediary to provide the client
with a current representation of the target resource and send
notifications upon changes to the target resource state, much like an
origin server as specified in Section 4.
To perform this task, the intermediary SHOULD make use of the
protocol specified in this document, taking the role of the client
and registering its own interest in the target resource with the next
hop towards the origin server. If the next hop does not return a
response with an Observe Option, the intermediary MAY resort to
polling the next hop or MAY itself return a response without an
Observe Option.
The communication between each pair of hops is independent; each hop
in the server role MUST determine individually how many notifications
to send, of which message type, and so on. Each hop MUST generate
its own values for the Observe Option, and MUST set the value of the
Max-Age Option according to the age of the local current
representation.
Because a client (or an intermediary in the client role) can only be
once on the list of observers of a resource on a server (or an
intermediary in the server role) -- it is useless to observe the same
resource multiple times -- an intermediary MUST observe a resource
only once, even if there are multiple clients for which it observes
the resource.
An intermediary is not required to act on behalf of a client to
observe a resource; an intermediary MAY observe a resource, for
example, just to keep its own cache up to date.
See Appendix A.1 for examples.
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6. Blockwise Transfers
Resources observed by clients may be larger than can be comfortably
processed or transferred in one CoAP message. CoAP provides a
blockwise transfer mechanism to address this problem
[I-D.ietf-core-block]. The following rules apply to the combination
of blockwise transfers with notifications.
As with basic GET transfers, the client can indicate its desired
block size in a Block2 Option in the GET request. If the server
supports blockwise transfers, it SHOULD take note of the block size
for all notifications/responses resulting from the GET request (until
the client is removed from the list of observers or the server
receives a new GET request for the resource from the client).
When sending a 2.05 (Content) notification, the server always sends
all blocks of the representation, suitably sequenced by its
congestion control mechanism, even if only some of the blocks have
changed with respect to a previous notification. The server performs
the blockwise transfer by making use of the Block2 Option in each
block. When reassembling representations that are transmitted in
multiple blocks, the client MUST NOT combine blocks carrying
different Observe Option values.
Blockwise transfers of notifications MUST use confirmable messages
and MUST NOT use non-confirmable messages.
See Appendix A.2 for an example.
7. Web Linking
A web link [RFC5988] to a resource accessible over CoAP (for example,
in a link-format document [RFC6690]) MAY include the target attribute
"obs".
The "obs" attribute, when present, is a hint indicating that the
destination of a link is useful for observation and thus, for
example, should have a suitable graphical representation in a user
interface. Note that this is only a hint; it is not a promise that
the Observe Option can actually be used to perform the observation.
A client may need to resort to polling the resource if the Observe
Option is not returned in the response to the GET request.
A value MUST NOT be given for the "obs" attribute; any present value
MUST be ignored by parsers. The "obs" attribute MUST NOT appear more
than once in a given link-value; occurrences after the first MUST be
ignored by parsers.
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8. Security Considerations
The security considerations of RFC XXXX [I-D.ietf-core-coap] apply.
The considerations about amplification attacks are somewhat amplified
when observing resources. Without client authentication, a server
MUST therefore strictly limit the number of notifications that it
sends between receiving acknowledgements that confirm the actual
interest of the client in the data; i.e., any notifications sent in
non-confirmable messages MUST be interspersed with confirmable
messages. (An attacker may still spoof the acknowledgements if the
confirmable messages are sufficiently predictable.)
As with any protocol that creates state, attackers may attempt to
exhaust the resources that the server has available for maintaining
the list of observers for each resource. Servers may want to access-
control this creation of state. As degraded behavior, the server can
always fall back to processing the request as a normal GET request
(without an Observe Option) if it is unwilling or unable to add a
client to the list of observers of a resource, including if system
resources are exhausted or nearing exhaustion.
Intermediaries must be careful to ensure that notifications cannot be
employed to create a loop. A simple way to break any loops is to
employ caches for forwarding notifications in intermediaries.
9. IANA Considerations
The following entry is added to the CoAP Option Numbers registry:
+--------+---------+-----------+
| Number | Name | Reference |
+--------+---------+-----------+
| 6 | Observe | [RFCXXXX] |
+--------+---------+-----------+
10. Acknowledgements
Carsten Bormann was an original author of this draft and is
acknowledged for significant contribution to this document.
Thanks to Daniele Alessandrelli, Jari Arkko, Peter Bigot, Angelo P.
Castellani, Gilbert Clark, Esko Dijk, Thomas Fossati, Brian Frank,
Jeroen Hoebeke, Cullen Jennings, Matthias Kovatsch, Salvatore Loreto,
Charles Palmer, Zach Shelby and Floris Van den Abeele for helpful
comments and discussions that have shaped the document.
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11. References
11.1. Normative References
[I-D.ietf-core-block]
Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
draft-ietf-core-block-10 (work in progress), October 2012.
[I-D.ietf-core-coap]
Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
"Constrained Application Protocol (CoAP)",
draft-ietf-core-coap-13 (work in progress), December 2012.
[RFC1982] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
August 1996.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC5988] Nottingham, M., "Web Linking", RFC 5988, October 2010.
11.2. Informative References
[GOF] Gamma, E., Helm, R., Johnson, R., and J. Vlissides,
"Design Patterns: Elements of Reusable Object-Oriented
Software", Addison-Wesley, Reading, MA, USA,
November 1994.
[REST] Fielding, R., "Architectural Styles and the Design of
Network-based Software Architectures", Ph.D. Dissertation,
University of California, Irvine, 2000, <http://
www.ics.uci.edu/~fielding/pubs/dissertation/
fielding_dissertation.pdf>.
[RFC2616] Fielding, R., Gettys, J., Mogul, J., Frystyk, H.,
Masinter, L., Leach, P., and T. Berners-Lee, "Hypertext
Transfer Protocol -- HTTP/1.1", RFC 2616, June 1999.
[RFC5989] Roach, A., "A SIP Event Package for Subscribing to Changes
to an HTTP Resource", RFC 5989, October 2010.
[RFC6202] Loreto, S., Saint-Andre, P., Salsano, S., and G. Wilkins,
"Known Issues and Best Practices for the Use of Long
Polling and Streaming in Bidirectional HTTP", RFC 6202,
April 2011.
[RFC6690] Shelby, Z., "Constrained RESTful Environments (CoRE) Link
Format", RFC 6690, August 2012.
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Appendix A. Examples
Observed CLIENT SERVER Actual
t State | | State
____________ | | ____________
1 | |
2 unknown | | 18.5 C
3 +----->| Header: GET 0x41011633
4 | GET | Token: 0x4a
5 | | Uri-Path: temperature
6 | | Observe: (empty)
7 | |
8 | |
9 ____________ |<-----+ Header: 2.05 0x61451633
10 | 2.05 | Token: 0x4a
11 18.5 C | | Observe: 9
12 | | Max-Age: 15
13 | | Payload: "18.5 C"
14 | |
15 | | ____________
16 ____________ |<-----+ Header: 2.05 0x51457b50
17 | 2.05 | 19.2 C Token: 0x4a
18 19.2 C | | Observe: 16
29 | | Max-Age: 15
20 | | Payload: "19.2 C"
21 | |
Figure 3: A client registers and receives one notification of the
current state and one of a new state upon a state change
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Observed CLIENT SERVER Actual
t State | | State
____________ | | ____________
22 | |
23 19.2 C | | 19.2 C
24 | | ____________
25 | X----+ Header: 2.05 0x51457b51
26 | 2.05 | 19.7 C Token: 0x4a
27 | | Observe: 25
28 | | Max-Age: 15
29 | | Payload: "19.7 C"
30 | |
31 ____________ | |
32 | |
33 19.2 C | |
34 (stale) | |
35 | |
36 | |
37 | |
38 +----->| Header: GET 0x41011634
39 | GET | Token: 0xb2
40 | | Uri-Path: temperature
41 | | Observe: (empty)
42 | |
43 | |
44 ____________ |<-----+ Header: 2.05 0x61451634
45 | 2.05 | Token: 0xb2
46 19.7 C | | Observe: 44
47 | | Max-Age: 15
48 | | ETag: 0x78797a7a79
49 | | Payload: "19.7 C"
50 | |
Figure 4: The client re-registers after Max-Age ends
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Observed CLIENT SERVER Actual
t State | | State
____________ | | ____________
51 | |
52 19.7 C | | 19.7 C
53 | |
54 | | ____________
55 | crash
56 |
57 |
58 |
59 ____________ |
60 |
61 19.7 C |
62 (stale) |
63 | reboot____________
64 | |
65 | | 20.0 C
66 | |
67 +----->| Header: GET 0x41011635
68 | GET | Token: 0xf9
69 | | Uri-Path: temperature
70 | | Observe: (empty)
71 | | ETag: 0x78797a7a79
72 | |
73 | |
74 ____________ |<-----+ Header: 2.05 0x61451635
75 | 2.05 | Token: 0xf9
76 20.0 C | | Observe: 74
77 | | Max-Age: 15
78 | | Payload: "20.0 C"
79 | |
80 | | ____________
81 ____________ |<-----+ Header: 2.03 0x5143aa0c
82 | 2.03 | 19.7 C Token: 0xf9
83 19.7 C | | Observe: 81
84 | | ETag: 0x78797a7a79
85 | | Max-Age: 15
86 | |
Figure 5: The client re-registers and gives the server the
opportunity to select a stored response
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Observed CLIENT SERVER Actual
t State | | State
____________ | | ____________
87 | |
88 19.7 C | | 19.7 C
89 | |
90 | | ____________
91 ____________ |<-----+ Header: 2.05 0x5145aa0f
92 | 2.05 | 19.3 C Token: 0xf9
93 19.3 C | | Observe: 91
94 | | Max-Age: 15
95 | | Payload: "19.3 C"
96 | |
97 | |
98 +----->| Header: GET 0x41011636
99 | GET | Token: 0x68
100 | | Uri-Path: temperature
101 | | ETag: 0x78797a7a79
102 | |
103 | |
104 |<-----+ Header: 2.05 0x61451636
105 | 2.05 | Token: 0x68
106 | | ETag: 0x78797a7a79
107 | | Max-Age: 15
108 | | Payload: "19.3 C"
109 | |
Figure 6: The client makes a normal GET request and thereby cancels
the observation
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A.1. Proxying
CLIENT PROXY SERVER
| | |
| +----->| Header: GET 0x41015fb8
| | GET | Token: 0x1a
| | | Uri-Host: sensor.example
| | | Uri-Path: status
| | | Observe: (empty)
| | |
| |<-----+ Header: 2.05 0x61455fb8
| | 2.05 | Token: 0x1a
| | | Observe: 42
| | | Max-Age: 60
| | | Payload: "ready"
| | |
+----->| | Header: GET 0x41011633
| GET | | Token: 0x9a
| | | Proxy-Uri: coap://sensor.example/status
| | |
|<-----+ | Header: 2.05 0x61451633
| 2.05 | | Token: 0x9a
| | | Max-Age: 53
| | | Payload: "ready"
| | |
| |<-----+ Header: 2.05 0x514505fc0
| | 2.05 | Token: 0x1a
| | | Observe: 135
| | | Max-Age: 60
| | | Payload: "busy"
| | |
+----->| | Header: GET 0x41011634
| GET | | Token: 0x9b
| | | Proxy-Uri: coap://sensor.example/status
| | |
|<-----+ | Header: 2.05 0x61451634
| 2.05 | | Token: 0x9b
| | | Max-Age: 49
| | | Payload: "busy"
| | |
Figure 7: A proxy observes a resource to keep its cache up to date
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CLIENT PROXY SERVER
| | |
+----->| | Header: GET 0x41011635
| GET | | Token: 0x6a
| | | Proxy-Uri: coap://sensor.example/status
| | | Observe: (empty)
| | |
|<- - -+ | Header: 0x60001635
| | |
| +----->| Header: GET 0x4101af90
| | GET | Token: 0xaa
| | | Uri-Host: sensor.example
| | | Uri-Path: status
| | | Observe: (empty)
| | |
| |<-----+ Header: 2.05 0x6145af90
| | 2.05 | Token: 0xaa
| | | Observe: 67
| | | Max-Age: 60
| | | Payload: "ready"
| | |
|<-----+ | Header: 2.05 0x4145af94
| 2.05 | | Token: 0x6a
| | | Observe: 17346
| | | Max-Age: 60
| | | Payload: "ready"
| | |
+- - ->| | Header: 0x6000af94
| | |
| |<-----+ Header: 2.05 0x51455a20
| | 2.05 | Token: 0xaa
| | | Observe: 157
| | | Max-Age: 60
| | | Payload: "busy"
| | |
|<-----+ | Header: 2.05 0x5145af9b
| 2.05 | | Token: 0x6a
| | | Observe: 17436
| | | Max-Age: 60
| | | Payload: "busy"
| | |
Figure 8: A client observes a resource through a proxy
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A.2. Blockwise Transfer
CLIENT SERVER
| |
+----->| Header: GET 0x41011636
| GET | Token: 0xfb
| | Uri-Path: status-icon
| | Observe: (empty)
| |
|<-----+ Header: 2.05 0x61451636
| 2.05 | Token: 0xfb
| | Block2: 0/1/128
| | Observe: 62354
| | Max-Age: 60
| | Payload: [128 bytes]
| |
|<-----+ Header: 2.05 0x4145af9c
| 2.05 | Token: 0xfb
| | Block2: 1/0/128
| | Observe: 62354
| | Max-Age: 60
| | Payload: [27 bytes]
| |
+- - ->| Header: 0x6000af9c
| |
|<-----+ Header: 2.05 0x4145af9d
| 2.05 | Token: 0xfb
| | Block2: 0/1/128
| | Observe: 62444
| | Max-Age: 60
| | Payload: [128 bytes]
| |
+- - ->| Header: 60005af9d
| |
|<-----+ Header: 2.05 0x4145af9e
| 2.05 | Token: 0xfb
| | Block2: 1/0/128
| | Observe: 62444
| | Max-Age: 60
| | Payload: [27 bytes]
| |
+- - ->| Header: 0x6000af9e
| |
Figure 9: A server sends two notifications of two blocks each
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Appendix B. Modeling Resources to Tailor Notifications
A server may want to provide notifications that respond to very
specific conditions on some state. This is best done by modeling the
resources that the server exposes according to these needs.
For example, for a CoAP server with an attached temperature sensor,
o the server could, in the simplest form, expose a resource
<coap://server/temperature> that changes its state every second to
the current temperature measured by the sensor;
o the server could, however, also expose a resource
<coap://server/temperature/felt> that changes its state to "cold"
when it's warm and the temperature drops below a preconfigured
threshold, and to "warm" when it's cold and the temperature
exceeds a second, higher threshold;
o the server could expose a parameterized resource
<coap://server/temperature/critical?above=45> that changes its
state every second to the current temperature if the sensor
reading exceeds the specified parameter value, and that changes
its state to "OK" when the temperature drops below; or
o the server could expose a parameterized resource
<coap://server/temperature?query=select+avg(temperature)+from+
Sensor.window:time(30sec)> that accepts expressions of arbitrary
complexity and changes its state accordingly.
In any case, the client is notified about the current state of the
resource whenever the state of the appropriately modeled resource
changes. By designing resources that change their state on certain
conditions, it is possible to notify the client only when these
conditions occur instead of continuously supplying it with
information it doesn't need.
By parametrizing resources, this is not limited to conditions defined
by the server, but can be extended to arbitrarily complex conditions
defined by the client. Thus, the server designer can choose exactly
the right level of complexity for the application envisioned and
devices used, and is not constrained to a "one size fits all"
mechanism built into the protocol.
Appendix C. Changelog
Changes from ietf-07 to ietf-08:
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o Expanded text on transmitting notification while a previous
transmission is pending (#242).
o Changed reordering detection to use a fixed time span of 128
seconds instead of EXCHANGE_LIFETIME (#276).
o Removed the use of the freshness model to determine if the client
is still on the list of observers. This includes removing that
* the client assumes that it has been removed from the list of
observers when Max-Age ends;
* the server sets the Max-Age Option of a notification to a value
that indicates when the server will send the next notification;
* the server uses a number of retransmit attempts such that
removing a client from the list of observers before Max-Age
ends is avoided (#235);
* the server may remove the client from all lists of observers
when the transmission of a confirmable notification ultimately
times out.
o Changed that an unrecognized critical option in a request must
actually have no effect on the state of any observation
relationship to any resource, as the option could lead to a
different target resource.
o Clarified that client implementations must be prepared to receive
each notification equally as a confirmable or a non-confirmable
message, regardless of the message type of the request and of any
previous notification.
o Added a requirement for sending a confirmable notification at
least every 24 hours before continuing with non-confirmable
notifications (#221).
o Added congestion control considerations from [I-D.bormann-core-
congestion-control-02].
o Recommended that the client waits for a randomized time after the
freshness of the latest notification expired before re-
registering. This prevents that multiple clients observing a
resource perform a GET request at the same time when the need to
re-register arises.
o Changed reordering detection from 'MAY' to 'SHOULD', as the goal
of the protocol (to keep the observed state as closely in sync
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with the actual state as possible) is not optional.
o Fixed the length of the Observe (3 bytes) in the table in
Section 2.
o Replaced the 'x' in the No-Cache-Key column in the table in
Section 2 with a '-', as the Observe Option doesn't have the No-
Cache-Key flag set, even though it is not part of the cache key.
o Updated examples.
Changes from ietf-06 to ietf-07:
o Moved to 24-bit sequence numbers to allow for up to 15000
notifications per second per client and resource (#217).
o Re-numbered option number to use Unsafe/Safe and Cache-Key
compliant numbers (#241).
o Clarified how to react to a Reset message that is sent in reply to
a non-confirmable notification (#225).
o Clarified the semantics of the "obs" link target attribute (#236).
Changes from ietf-05 to ietf-06:
o Improved abstract and introduction to say that the protocol is
about best effort and eventual consistency (#219).
o Clarified that the value of the Observe Option in a request must
have zero length.
o Added requirement that the sequence number must be updated each
time a server retransmits a notification.
o Clarified that a server must remove a client from the list of
observers when it receives a GET request with an unrecognized
critical option.
o Updated the text to use the endpoint concept from
[I-D.ietf-core-coap] (#224).
o Improved the reordering text (#223).
Changes from ietf-04 to ietf-05:
o Recommended that a client does not re-register while a new
notification from the server is still likely to arrive. This is
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to avoid that the request of the client and the last notification
after max-age cross over each other (#174).
o Relaxed requirements when sending a Reset message in reply to non-
confirmable notifications.
o Added an implementation note about careless GET requests (#184).
o Updated examples.
Changes from ietf-03 to ietf-04:
o Removed the "Max-OFE" Option.
o Allowed a Reset message in reply to non-confirmable notifications.
o Added a section on cancellation.
o Updated examples.
Changes from ietf-02 to ietf-03:
o Separated client-side and server-side requirements.
o Fixed uncertainty if client is still on the list of observers by
introducing a liveliness model based on Max-Age and a new option
called "Max-OFE" (#174).
o Simplified the text on message reordering (#129).
o Clarified requirements for intermediaries.
o Clarified the combination of blockwise transfers with
notifications (#172).
o Updated examples to show how the state observed by the client
becomes eventually consistent with the actual state on the server.
o Added examples for parameterization of observable resource.
Changes from ietf-01 to ietf-02:
o Removed the requirement of periodic refreshing (#126).
o The new "Observe" Option replaces the "Lifetime" Option.
o Introduced a new mechanism to detect message reordering.
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o Changed 2.00 (OK) notifications to 2.05 (Content) notifications.
Changes from ietf-00 to ietf-01:
o Changed terminology from "subscriptions" to "observation
relationships" (#33).
o Changed the name of the option to "Lifetime".
o Clarified establishment of observation relationships.
o Clarified that an observation is only identified by the URI of the
observed resource and the identity of the client (#66).
o Clarified rules for establishing observation relationships (#68).
o Clarified conditions under which an observation relationship is
terminated.
o Added explanation on how clients can terminate an observation
relationship before the lifetime ends (#34).
o Clarified that the overriding objective for notifications is
eventual consistency of the actual and the observed state (#67).
o Specified how a server needs to deal with clients not
acknowledging confirmable messages carrying notifications (#69).
o Added a mechanism to detect message reordering (#35).
o Added an explanation of how notifications can be cached,
supporting both the freshness and the validation model (#39, #64).
o Clarified that non-GET requests do not affect observation
relationships, and that GET requests without "Lifetime" Option
affecting relationships is by design (#65).
o Described interaction with blockwise transfers (#36).
o Added Resource Discovery section (#99).
o Added IANA Considerations.
o Added Security Considerations (#40).
o Added examples (#38).
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Author's Address
Klaus Hartke
Universitaet Bremen TZI
Postfach 330440
Bremen D-28359
Germany
Phone: +49-421-218-63905
Email: hartke@tzi.org
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