CoRE Working Group K. Hartke
Internet-Draft Universitaet Bremen TZI
Intended status: Standards Track March 12, 2012
Expires: September 13, 2012
Observing Resources in CoAP
draft-ietf-core-observe-05
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 gives clients the ability to observe such changes.
Status of this Memo
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This Internet-Draft will expire on September 13, 2012.
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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. Design Philosophy . . . . . . . . . . . . . . . . . . . . 5
1.4. Conformance Requirements . . . . . . . . . . . . . . . . . 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. Cancellation . . . . . . . . . . . . . . . . . . . . . . . 10
4. Server-side Requirements . . . . . . . . . . . . . . . . . . . 10
4.1. Request . . . . . . . . . . . . . . . . . . . . . . . . . 10
4.2. Notifications . . . . . . . . . . . . . . . . . . . . . . 11
4.3. Caching . . . . . . . . . . . . . . . . . . . . . . . . . 12
4.4. Reordering . . . . . . . . . . . . . . . . . . . . . . . . 12
4.5. Retransmission . . . . . . . . . . . . . . . . . . . . . . 13
5. Intermediaries . . . . . . . . . . . . . . . . . . . . . . . . 13
6. Block-wise Transfers . . . . . . . . . . . . . . . . . . . . . 14
7. Discovery . . . . . . . . . . . . . . . . . . . . . . . . . . 15
8. Security Considerations . . . . . . . . . . . . . . . . . . . 15
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 16
11.1. Normative References . . . . . . . . . . . . . . . . . . . 16
11.2. Informative References . . . . . . . . . . . . . . . . . . 17
Appendix A. Examples . . . . . . . . . . . . . . . . . . . . . . 18
A.1. Proxying . . . . . . . . . . . . . . . . . . . . . . . . . 21
A.2. Block-wise Transfer . . . . . . . . . . . . . . . . . . . 23
Appendix B. Modeling Resources to Tailor Notifications . . . . . 23
Appendix C. Changelog . . . . . . . . . . . . . . . . . . . . . . 24
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 26
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1. Introduction
1.1. Background
CoAP [I-D.ietf-core-coap] is an Application Protocol for Constrained
Nodes/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 communication model of REST is that of a client exchanging
resource representations with an origin server. The origin server is
the definitive source for representations of the resources in its
namespace. A client interested in a resource sends a request to the
origin server that returns a response with a representation 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 when using HTTP, such as repeated polling or long-polls
[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 to push resource representations from
servers to interested clients, while still keeping the properties of
REST.
Note that there is no intention for this mechanism 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, an
observer must register separately for all of them. The pattern is
typically used when a clean separation between related components is
required, such as data storage and user interface.
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Observer Subject
| |
| Register |
+----------------->|
| |
| Notification |
|<-----------------+
| |
| Notification |
|<-----------------+
| |
| Notification |
|<-----------------+
| |
Figure 1: 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 the
current state of the resource at any given time.
Registration: A client registers its interest by sending 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 registered as observer for the resource.
Each notification is an additional CoAP response sent by the
server in reply to the GET request and includes a complete
representation of the new resource state.
Figure 2 shows an example of a CoAP client registering and receiving
three notifications: the first upon registration and then two when
the state of the resource changes. Registration request and
notifications are identified by the presence of the Observe Option
defined in this document. Notifications also echo 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 |
| Observe: 0 | (registration)
| Token: 0x4a |
+----------------->|
| |
| 2.05 Content |
| Observe: 12 | (notification of the current state)
| Token: 0x4a |
| Payload: 22.9 C |
|<-----------------+
| |
| 2.05 Content |
| Observe: 44 | (notification upon a state change)
| Token: 0x4a |
| Payload: 22.8 C |
|<-----------------+
| |
| 2.05 Content |
| Observe: 60 | (notification upon a state change)
| Token: 0x4a |
| Payload: 23.1 C |
|<-----------------+
| |
Figure 2: Observing a Resource in CoAP
The client is removed from the list of observers when it is no longer
interested in the observed resource. The server can determine the
client's continued interest from the client's acknowledgement of
confirmable notifications. If a client wants to receive
notifications after it has been removed from the list of observers,
it needs to register again. The client can determine that it's still
on the list of observers from the fact that it receives
notifications. The protocol includes clear rules for what to do when
a client does not receive a notification for some time, or a server
does not receive acknowledgements.
1.3. Design Philosophy
The protocol builds on the architectural elements of REST, which
include: a server that is responsible for the state and
representation of the resources in its namespace, a client that is
responsible for keeping the application state, and the stateless
exchange of resource representations. (A server needs to keep track
of the observers though, similar to how HTTP servers need to keep
track of the TCP connections from their clients.) The protocol
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enables high scalability and efficiency through the support of caches
and intermediaries that multiplex the interest of multiple clients in
the same resource into a single association.
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
meaningful for the application. Resources can be parameterized to
achieve similar effects though; see Appendix B for examples.
Since bandwidth is in short supply in constrained environments,
servers must adapt the rate of notifications to each client. This
implies that a client cannot rely on observing every single state a
resource goes through. Instead, the protocol is designed on the
principle of _eventual consistency_: it guarantees that if the
resource does not undergo a new change in state, eventually all
observers will have a current representation of the last resource
state.
1.4. Conformance Requirements
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/E | Name | Format | Length | Default |
+-----+----------+---------+--------+--------+---------+
| 10 | Elective | Observe | uint | 0-2 B | (none) |
+-----+----------+---------+--------+--------+---------+
The Observe Option, when present, modifies the GET method so it does
not only retrieve a representation of the current state of the
resource identified by the request URI, but also requests the server
to add the client to the list of observers of the resource. The
exact semantics are defined in the sections below. The value of the
option in a request MUST be zero on transmission and MUST be ignored
on reception.
In a response, the Observe Option identifies the message as a
notification, which implies that the client has been added to the
list of observers and that the server will notify the client of
further changes to the resource state. The option's value is a
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sequence number that can be used for reordering detection (see
Section 3.4 and Section 4.4). The value is encoded as a variable-
length unsigned integer as defined in Appendix A of RFC XXXX
[I-D.ietf-core-coap].
Since the Observe Option is elective, a GET request that includes the
Observe Option will automatically fall back to a normal GET request
if the server is unwilling or unable to add the client to the list of
observers.
The Observe Option MUST NOT occur more than once in a request or
response.
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 for as long as the server can assume the client's
interest.
3.2. Notifications
Notifications are additional responses sent by the server in reply to
the GET request. Each notification includes an Observe Option with a
sequence number (see Section 3.4), a Token Option that matches the
token specified by the client in the GET request, and a payload of
the same media type as the initial response.
A notification can be confirmable or non-confirmable (i.e. sent in a
confirmable or non-confirmable message). If a client does not
recognize the token in a confirmable notification, it MUST NOT
acknowledge the message and SHOULD reject it with a RST message.
Otherwise, the client MUST acknowledge the message with an ACK
message as usual. If a client does not recognize the token in a non-
confirmable notification, it MAY reject it with a RST message.
An acknowledgement 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 it from the list of observers.
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Notifications will have a 2.05 (Content) response code in most cases.
They may also have a 2.03 (Valid) response code if the client
includes an ETag Option in its request (see Section 3.3). In the
event that the state of an observed resource is changed in a way that
would cause a normal GET request to return an error (for example,
when the resource is deleted), the server will send a notification
with an error response code (4.xx/5.xx) and empty the list of
observers of the resource.
3.3. Caching
As notifications are just additional responses, notifications partake
in caching as defined by Section 5.6 of RFC XXXX
[I-D.ietf-core-coap]. Both the freshness model and the validation
model are supported. The freshness model also serves as the model
for the client to determine if it's still on the list of observers or
if it needs to re-register its interest in the resource.
A client MAY store a notification like a response in its cache and
use a stored notification/response that is fresh without contacting
the origin server. A notification/response is considered fresh while
its age is not greater than its Max-Age and no newer notification has
been received.
The server will do its best to keep the client up to date with a
fresh representation of the current resource state. It will send a
notification whenever the resource changes, or at latest when the age
of the last notification becomes greater than its Max-Age. (Note
that this notification may not arrive in time due to network
latency.)
The client SHOULD assume that it's on the list of observers while the
age of the last notification is not greater than Max-Age. If the
client does not receive a notification before the age becomes greater
than Max-Age, it can assume that it has been removed from the list of
observers (e.g., due to a loss of server state). In this case, it
may need to re-register its interest.
To make sure it has a fresh representation and/or to re-register its
interest, a client MAY issue a new GET request with an Observe Option
at any time. The GET request SHOULD specify a new token to avoid
ambiguity, because the token serves as 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 and, beyond that, while a new
notification from the server is still likely to arrive. I.e. the
client should wait until the age of the last notification becomes
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greater than its Max-Age plus the potential retransmission window
(see Section 4.1 of RFC XXXX [I-D.ietf-core-coap]) plus the expected
maximum round trip time.
When a client has one or more notifications stored, it can use the
ETag Option in the GET request to give the server an opportunity to
select a stored response to be used. The client MAY include an ETag
Option for each stored response that is applicable. It needs to keep
those responses in the cache until it is no longer interested in
receiving notifications for the target resource or it issues a new
GET request with a new set of entity-tags. Whenever the observed
resource changes its state 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.
3.4. Reordering
Messages that carry notifications can arrive in a different order
than they were sent. Since the goal is eventual consistency (see
Section 1.3), a client can safely skip a notification that arrives
later than a newer notification. For this purpose, the server sets
the value of the Observe Option in each notification to a sequence
number.
A client MAY treat a notification as outdated (not fresh) under the
following condition:
(V1 - V2) % (2**16) < (2**15) and T2 < (T1 + (2**14))
where V1 is the value of the Observe Option of the latest valid
notification received, V2 the value of the Observe Option of the
present notification, T1 a client-local timestamp of the latest valid
notification received (in seconds), and T2 a client-local timestamp
of the present notification.
Design Note: The first condition essentially verifies that V2 > V1
holds in 16-bit sequence number arithmetic [RFC1982]. The second
condition checks that the time expired between the two incoming
messages is not so large that the sequence number might have
wrapped around and the first check is therefore invalid. (In
other words, after about 2**14 seconds elapse without any
notification, the client does not need to check the sequence
numbers in order to assume an incoming notification is new.) The
constants of 2**14 and 2**15 are non-critical, as is the even
speed or precision of the clock involved.
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3.5. Cancellation
When a client rejects a confirmable notification with a RST message
or when it performs a GET request without an Observe Option for a
currently observed resource, the server will remove the client from
the list of observers for this resource. The client MAY use either
method at any time to indicate that it is no longer interested in
receiving notifications about a resource.
When a client rejects non-confirmable notification with a RST, there
is also a chance that the server will remove the client from the list
of observers for this resource. So the client MAY try this method as
well. A client MAY rate-limit the RST 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 inadvertantly cancel an observation
relationship by sending an unrelated GET to the same resource. To
avoid this, without incurring a need for synchronization, such
clients can use a different source transport address 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 representation of the resource identified by 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
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
state and, e.g., needs to poll the resource 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 that does not include an
Observe Option, it MUST remove the client from the list of observers.
Two requests relate to the same list entry if both the request URI
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and the source of the requests match. The source of a request is
determined by the security mode used (see Section 10 of RFC XXXX
[I-D.ietf-core-coap]): With NoSec, it is determined by the source IP
address and UDP port number. With other security modes, the source
is also determined by the security context. Note that Message IDs
and Token Options MUST NOT be taken into account.
Any request with a method other than GET MUST NOT have a direct
effect on a list of observers of a resource. However, such a request
can have the indirect consequence of causing the server to send an
error notification which does affect the list of observers (e.g.,
when a DELETE request is successful and an observed resource no
longer exists).
4.2. Notifications
A client is notified of a resource state change by an additional
response sent by the server in reply to the GET request. Each such
notification 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 to return an error (for
example, if the resource is deleted), the server SHOULD notify the
client by sending a notification with an appropriate error response
code (4.xx/5.xx) and MUST empty the list of observers of the
resource.
The media type 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 media type, it
SHOULD send a 5.00 (Internal Server Error) notification and MUST
empty the list of observers of the resource.
A notification can be sent as 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 implies the
client's continued interest in being notified. If the client rejects
a confirmable notification with a RST message, the server MUST remove
the client from the list of observers. If the client rejects a non-
confirmable notification with a RST message, the server MAY remove
the client from the list of observers.
4.3. Caching
The Max-Age Option of a notification SHOULD be set to a value that
indicates when the server will send the next notification. For
example, if the server sends a notification every 30 seconds, a Max-
Age Option with value 30 should be included. The server MAY send a
new notification before Max-Age ends and MUST send a new notification
at latest when Max-Age ends. If the client does not receive a new
notification before Max-Age ends, it will assume that it was removed
from the list of observers (e.g., due to a loss of server state) and
may issue a new GET request to re-register its interest.
It may not always be possible to predict when the server will send
the next notification, for example, when a resource does not change
its state in regular intervals. In this case, the server SHOULD set
Max-Age to a good approximation. The value is a trade-off between
increased usage of bandwidth and the risk of stale information.
Smaller values lead to more notifications and more GET requests,
while greater values result in network or device failures being
detected later and data becoming stale.
The client can include a set of entity-tags in its request using the
ETag Option. When the 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 recipient has any response stored other than
those identified by the entity-tags in the most recent GET request
for the resource.
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
in each notification to the 16 least-significant bits of a strictly
increasing sequence number. The sequence number MAY start at any
value. The server MUST NOT reuse the same option value with the same
client, token and resource within approximately 2**16 seconds
(roughly 18.2 hours).
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Implementation Note: A simple implementation that satisfies the
requirements is to use a timestamp (in seconds) provided by the
device's clock, or a 16-bit unsigned integer variable that is
incremented every second and wraps around every 2**16 seconds. It
is not necessary that the clock reflects the correct local time or
that it ticks exactly every second. Note that, on average, a
server cannot send more than one notification per second per
client, token and resource.
4.5. Retransmission
In CoAP, confirmable messages are retransmitted in exponentially
increasing intervals for a certain number of attempts until they are
acknowledged by the client. In the context of observing a resource,
it is undesirable to continue transmitting the representation of a
resource state when the state has changed in the meantime.
When a server is in the process of delivering a confirmable
notification and is waiting for an acknowledgement, and it wants to
notify the client of a state change using a new confirmable message,
it MUST stop retransmitting the old notification and SHOULD attempt
to deliver the new notification with the number of attempts remaining
from the old notification. When the last attempt to retransmit a
confirmable message with a notification for a resource times out, the
server SHOULD remove the client from the list of observers and MAY
additionally remove the client from the lists of observers of all
resources in its namespace.
The server SHOULD use a number of retransmit attempts
(MAX_RETRANSMIT) such that removing a client from the list of
observers before Max-Age ends is avoided.
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.
5. Intermediaries
A client may be interested in a resource in the namespace of an
origin server that is reached through 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
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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. 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. Note that the
communication between each pair of hops is independent, i.e. each hop
in the server role MUST determine individually how many notifications
to send, of which 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 in the list of observers of a resource at 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.
Note that an intermediary is not required to have a client to observe
a resource; an intermediary MAY observe a resource, for instance,
just to keep its own cache up to date.
See Appendix A.1 for examples.
6. Block-wise Transfers
Resources observed by clients may be larger than can be comfortably
processed or transferred in one CoAP message. CoAP provides a block-
wise transfer mechanism to address this problem
[I-D.ietf-core-block]. The following rules apply to the combination
of block-wise 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 block-wise 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 from the client).
When sending a 2.05 (Content) notification, the server always sends
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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 value. The server performs the
block-wise 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, or blocks that have been received more than
approximately 2**14 seconds apart.
See Appendix A.2 for an example.
7. Discovery
A web link [RFC5988] to a resource accessible by the CoAP protocol
MAY indicate that the server encourages the observation of this
resource by including the target attribute "obs". This is
particularly useful in link-format documents
[I-D.ietf-core-link-format].
This target attribute is used as a flag, and thus it has no value
component -- a value given for the attribute MUST NOT be given for
this version of the specification and MUST be ignored if present.
The target attribute "obs" MUST NOT be given more than once for this
version of the specification.
8. Security Considerations
The security considerations of RFC XXXX [I-D.ietf-core-coap] apply.
Note that the considerations about amplification attacks are somewhat
amplified when observing resources. In NoSec mode, 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.
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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 entries are added to the CoAP Option Numbers registry:
+--------+---------+-----------+
| Number | Name | Reference |
+--------+---------+-----------+
| 10 | 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
Castellani, Gilbert Clark, Esko Dijk, Thomas Fossati, Brian Frank,
Cullen Jennings, Matthias Kovatsch, Salvatore Loreto, Charles Palmer
and Zach Shelby for helpful comments and discussions that have shaped
the document.
Klaus Hartke was funded by the Klaus Tschira Foundation.
11. References
11.1. Normative References
[I-D.ietf-core-block]
Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
draft-ietf-core-block-08 (work in progress),
February 2012.
[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.
[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.
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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.
[I-D.ietf-core-link-format]
Shelby, Z., "CoRE Link Format",
draft-ietf-core-link-format-11 (work in progress),
January 2012.
[REST] Fielding, R., "Architectural Styles and the Design of
Network-based Software Architectures", 2000, <http://
www.ics.uci.edu/~fielding/pubs/dissertation/top.htm>.
[RFC1982] Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
August 1996.
[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.
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Appendix A. Examples
Observed CLIENT SERVER Actual
t State | | State
____________ | | ____________
1 | |
2 unknown | | 18.5 C
3 +----->| Header: GET 0x43011633
4 | GET | Token: 0x4a
5 | | Uri-Path: temperature
6 | | Observe: 0
7 | |
8 | |
9 ____________ |<-----+ Header: 2.05 0x63451633
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 0x53457b50
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 a notification of the
current state and 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 0x53457b51
26 | 2.05 | 19.7 C Token: 0x4a
27 | | Observe: 25
28 | | Max-Age: 15
29 | | Payload: "19.7 C"
30 | |
31 ____________ | |
32 +----->| Header: GET 0x43011633
33 19.2 C | GET | Token: 0xb2
34 (stale) | | Uri-Path: temperature
35 | | Observe: 0
36 | |
37 | |
38 ____________ |<-----+ Header: 2.05 0x54457b52
39 | 2.05 | Token: 0xb2
40 19.7 C | | Observe: 38
41 | | Max-Age: 15
42 | | ETag: 0x78797a7a79
43 | | Payload: "19.7 C"
44 | |
Figure 4: The client re-registers after Max-Age ends
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Observed CLIENT SERVER Actual
t State | | State
____________ | | ____________
45 | |
46 19.7 C | | 19.7 C
47 | |
48 | | ____________
49 | CRASH
50 |
51 |
52 | |
53 ____________ | | ____________
54 +----->| Header: GET 0x44011634
55 19.7 C | GET | 20.0 C Token: 0xf9
56 (stale) | | Uri-Path: temperature
57 | | Observe: 0
58 | | ETag: 0x78797a7a79
59 | |
60 | |
61 ____________ |<-----+ Header: 2.05 0x63451634
62 | 2.05 | Token: 0xf9
63 20.0 C | | Observe: 61
64 | | Max-Age: 15
65 | | Payload: "20.0 C"
66 | |
67 | | ____________
68 ____________ |<-----+ Header: 2.03 0x5443aa0c
69 | 2.03 | 19.7 C Token: 0xf9
70 19.7 C | | Observe: 68
71 | | ETag: 0x78797a7a79
72 | | Max-Age: 15
73 | |
Figure 5: The client re-registers and gives the server the
opportunity to select a stored response
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A.1. Proxying
CLIENT PROXY SERVER
| | |
| +----->| Header: GET 0x44015fb8
| | GET | Token: 0x1a
| | | Uri-Host: sensor.example
| | | Uri-Path: status
| | | Observe: 0
| | |
| |<-----+ Header: 2.05 0x63455fb8
| | 2.05 | Token: 0x1a
| | | Observe: 42
| | | Max-Age: 60
| | | Payload: "ready"
| | |
+----->| | Header: GET 0x42011633
| GET | | Token: 0x9a
| | | Proxy-Uri: coap://sensor.example/status
| | |
|<-----+ | Header: 2.05 0x62451633
| 2.05 | | Token: 0x9a
| | | Max-Age: 53
| | | Payload: "ready"
| | |
| |<-----+ Header: 2.05 0x534505fc0
| | 2.05 | Token: 0x1a
| | | Observe: 135
| | | Max-Age: 60
| | | Payload: "busy"
| | |
+----->| | Header: GET 0x42011634
| GET | | Token: 0x9b
| | | Proxy-Uri: coap://sensor.example/status
| | |
|<-----+ | Header: 2.05 0x62451634
| 2.05 | | Token: 0x9b
| | | Max-Age: 49
| | | Payload: "busy"
| | |
Figure 6: A proxy observes a resource to keep its cache up to date
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CLIENT PROXY SERVER
| | |
+----->| | Header: GET 0x43011635
| GET | | Token: 0x6a
| | | Proxy-Uri: coap://sensor.example/status
| | | Observe: 0
| | |
|<- - -+ | Header: 0x60001635
| | |
| +----->| Header: GET 0x4401af90
| | GET | Token: 0xaa
| | | Uri-Host: sensor.example
| | | Uri-Path: status
| | | Observe: 0
| | |
| |<-----+ Header: 2.05 0x6345af90
| | 2.05 | Token: 0xaa
| | | Observe: 67
| | | Max-Age: 60
| | | Payload: "ready"
| | |
|<-----+ | Header: 2.05 0x4345af94
| 2.05 | | Token: 0x6a
| | | Observe: 17346
| | | Max-Age: 60
| | | Payload: "ready"
| | |
+- - ->| | Header: 0x6000af94
| | |
| |<-----+ Header: 2.05 0x53455a20
| | 2.05 | Token: 0xaa
| | | Observe: 157
| | | Max-Age: 60
| | | Payload: "busy"
| | |
|<-----+ | Header: 2.05 0x5345af9b
| 2.05 | | Token: 0x6a
| | | Observe: 17436
| | | Max-Age: 60
| | | Payload: "busy"
| | |
Figure 7: A client observes a resource through a proxy
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A.2. Block-wise Transfer
CLIENT SERVER
| |
+----->| Header: GET 0x43011636
| GET | Token: 0xfb
| | Uri-Path: status-icon
| | Observe: 0
| |
|<-----+ Header: 2.05 0x64451636
| 2.05 | Token: 0xfb
| | Block2: 0/1/128
| | Observe: 62354
| | Max-Age: 60
| | Payload: [128 bytes]
| |
|<-----+ Header: 2.05 0x5445af9c
| 2.05 | Token: 0xfb
| | Block2: 1/0/128
| | Observe: 62354
| | Max-Age: 60
| | Payload: [27 bytes]
| |
|<-----+ Header: 2.05 0x5445af9d
| 2.05 | Token: 0xfb
| | Block2: 0/1/128
| | Observe: 62444
| | Max-Age: 60
| | Payload: [128 bytes]
| |
|<-----+ Header: 2.05 0x5445af9e
| 2.05 | Token: 0xfb
| | Block2: 1/0/128
| | Observe: 62444
| | Max-Age: 60
| | Payload: [27 bytes]
| |
Figure 8: A server sends two notifications of two blocks each
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,
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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 the temperature drops below a preconfigured threshold, and to
"warm" when 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 to the current temperature if the temperature exceeds the
specified value, and 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. With parametrized 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-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
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 RST in reply to non-confirmable
notifications.
o Added an implementation note about careless GETs (#184).
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o Updated examples.
Changes from ietf-03 to ietf-04:
o Removed the "Max-OFE" Option.
o Allowed RST 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 block-wise 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.
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).
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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 block-wise 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
Fax: +49-421-218-7000
Email: hartke@tzi.org
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