Internet Engineering Task Force (IETF) K. Hartke
Request for Comments: 7641 Universitaet Bremen TZI
Category: Standards Track September 2015
Observing Resources in the Constrained Application Protocol (CoAP)
The Constrained Application Protocol (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
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
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The Constrained Application Protocol (CoAP) [RFC7252] is intended to
provide RESTful services [REST] not unlike HTTP [RFC7230] 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 [RFC7228].
The model of REST is that of a client exchanging representations of
resources with a server, where a representation captures the current
or intended state of a resource. The server is the authority 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 retrieves a representation of the
resource and requests this representation be updated by the server
as long as the client is interested in the resource.
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
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, the observer must register separately for all of
| 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
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
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 single extended 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
with the current state upon registration, and then two upon 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 additionally provides a sequence number for reordering
detection. All notifications carry the token specified by the
client, so the client can easily correlate them to the request.
| GET /temperature |
| Token: 0x4a | Registration
| Observe: 0 |
| 2.05 Content |
| Token: 0x4a | Notification of
| Observe: 12 | the current state
| Payload: 22.9 Cel |
| 2.05 Content |
| Token: 0x4a | Notification upon
| Observe: 44 | a state change
| Payload: 22.8 Cel |
| 2.05 Content |
| Token: 0x4a | Notification upon
| Observe: 60 | a state change
| Payload: 23.1 Cel |
Figure 2: Observing a Resource in CoAP
Note: In this document, "Cel" stands for "degrees Celsius".
A client remains on the list of observers as long as the server can
determine the client's continued interest in the resource. The
server may send a notification in a confirmable CoAP message to
request an acknowledgement from the client. When the client
deregisters, rejects a notification, or the transmission of a
notification times out after several transmission attempts, the
client is considered no longer interested in the resource and is
removed by the server from the list of observers.
1.3. Consistency Model
While a client is in the list of observers of a resource, the goal of
the protocol is to keep the resource state observed by the client as
closely in sync with the actual state at the server as possible.
It cannot be avoided that the client and the server become out of
sync at times: First, there is always some latency between the change
of the resource state and the receipt of the notification. Second,
CoAP messages with notifications can get lost, which will cause the
client to assume an old state until it receives a new notification.
And third, the server may erroneously come to the conclusion that the
client is no longer interested in the resource, which will cause the
server to stop sending notifications and the client to assume an old
state until it eventually 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. This is limited by
congestion control, however, so a client cannot rely on observing
every single state that a resource might go 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
limit, the client cannot use the enclosed representation until it
receives 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.
1.4. Observable Resources
A CoAP server is the authority for determining under what conditions
resources change their state and thus when observers are notified of
new resource states. The protocol does not offer explicit means for
setting up triggers or thresholds; it is up to the server to expose
observable resources that change their state in a way that is useful
in the application context.
For example, a CoAP server with an attached temperature sensor could
expose one or more of the following resources:
o <coap://server/temperature>, which changes its state every few
seconds to a current reading of the temperature sensor;
o <coap://server/temperature/felt>, which changes its state to
"COLD" whenever the temperature reading drops below a certain pre-
configured threshold and to "WARM" whenever the reading exceeds a
second, slightly higher threshold;
o <coap://server/temperature/critical?above=42>, which changes its
state based on the client-specified parameter value either every
few seconds to the current temperature reading if the temperature
exceeds the threshold or to "OK" when the reading drops below;
time(30sec)>, which accepts expressions of arbitrary complexity
and changes its state accordingly.
Thus, by designing CoAP resources that change their state on certain
conditions, it is possible to update the client only when these
conditions occur instead of supplying it continuously with raw sensor
data. By parameterizing resources, this is not limited to conditions
defined by the server, but can be extended to arbitrarily complex
queries specified by the client. The application designer therefore
can choose exactly the right level of complexity for the application
envisioned and devices involved and is not constrained to a "one size
fits all" mechanism built into the protocol.
1.5. 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
The Observe Option has the following properties. Its meaning depends
on whether it is included in a GET request or in a response.
| No. | C | U | N | R | Name | Format | Length | Default |
| 6 | | x | - | | Observe | uint | 0-3 B | (none) |
C=Critical, U=Unsafe, N=No-Cache-Key, R=Repeatable
Table 1: The Observe Option
When included in a GET request, the Observe Option extends the GET
method so it does not only retrieve a current representation of the
target resource, but also requests the server to add or remove an
entry in the list of observers of the resource depending on the
option value. The list entry consists of the client endpoint and the
token specified by the client in the request. Possible values are:
0 (register) adds the entry to the list, if not present;
1 (deregister) removes the entry from the list, if present.
The Observe Option is not critical for processing the request. If
the server is unwilling or unable to add a new entry to the list of
observers, then the request falls back to a normal GET request and
the response does not include the Observe Option.
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.
When included in a response, the Observe Option identifies the
message as a notification. This implies that a matching entry exists
in the list of observers and that the server will notify the client
of changes to the resource state. The option value is a sequence
number for reordering detection (see Sections 3.4 and 4.4).
The value of the Observe Option is encoded as an unsigned integer in
network byte order using a variable number of bytes ('uint' option
format); see Section 3.2 of RFC 7252 [RFC7252].
3. Client-Side Requirements
A client registers its interest in a resource by issuing a GET
request with an Observe Option set to 0 (register). If the server
returns a 2.xx response that includes an Observe Option as well, the
server has successfully added an entry with the client endpoint and
request token to the list of observers of the target resource, and
the client will be notified of changes to the resource state.
Like a fresh response can be used to satisfy a request without
contacting the server, the stream of updates resulting from one
observation request can be used to satisfy another (observation or
normal GET) request if the target resource is the same. A client
MUST aggregate such requests and MUST NOT register more than once for
the same target resource. The target resource is identified by all
options in the request that are part of the Cache-Key. This includes,
for example, the full request URI and the Accept Option.
Notifications are additional responses sent by the server in reply to
the single extended GET request that created the registration. Each
notification includes the token specified by the client in the
request. The only difference between a notification and a normal
response is the presence of the Observe Option.
Notifications typically have a 2.05 (Content) response code. They
include 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. If the client included one or more ETag
Options in the GET request (see Section 3.3), notifications can have
a 2.03 (Valid) response code rather than a 2.05 (Content) response
code. Such notifications include an Observe Option with a sequence
number but no payload.
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 an appropriate response code (such as 4.04 Not
Found) and removes the client's entry from the list of observers of
the resource. Non-2.xx responses do not include an Observe Option.
As notifications are just additional responses to a GET request,
notifications partake in caching as defined in Section 5.6 of RFC
7252 [RFC7252]. Both the freshness model and the validation model
A client MAY store a notification like a response in its cache and
use a stored notification that is fresh without contacting the
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 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 any number of 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 assume that the
enclosed representation reflects the actual resource state.
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
the same token as the original at any time. All options MUST be
identical to those in the original request except for the set of ETag
Options. It is RECOMMENDED that the client does not issue the
request while it still has a fresh notification/response for the
resource in its cache. Additionally, the client SHOULD at least wait
for a random amount of time between 5 and 15 seconds after Max-Age
expired to reduce collisions with other clients.
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
A client implementation needs to keep all candidate responses in its
cache until it is no longer interested in the target resource or it
re-registers with a new set of entity tags.
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 MUST consider the
notification that was sent most recently as the freshest, regardless
of the order of arrival.
To provide an order among notifications for the client, the server
sets the value of the Observe Option in each notification to the 24
least significant bits of a strictly increasing sequence number. An
incoming notification was sent more recently than the freshest
notification 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 in the freshest
notification so far, V2 is the value of the Observe Option in the
incoming notification, T1 is a client-local timestamp for the
freshest notification so far, and T2 is a client-local timestamp for
the incoming notification.
Design Note: The first two conditions verify that V1 is less than V2
in 24-bit serial number arithmetic [RFC1982]. The third condition
ensures that if the server is generating serial numbers based on a
local clock, 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 serial number; in other words, after 128 seconds have
elapsed without any notification, a client does not need to check
the sequence numbers to assume that an incoming notification was
sent more recently than the freshest notification it has received
The duration of 128 seconds was chosen as a nice round number
greater than MAX_LATENCY (Section 4.8.2 of RFC 7252 [RFC7252]).
A notification can be confirmable or non-confirmable, i.e., it can be
sent in a confirmable or a non-confirmable message. The message type
used for a notification is independent of the type used for the
request and of 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 associated entry from the list of
observers (Section 4.5).
A client that is no longer interested in receiving notifications for
a resource can simply "forget" the observation. When the server then
sends the next notification, the client will not recognize the token
in the message and thus will return a Reset message. This causes the
server to remove the associated entry from the list of observers.
The entries in lists of observers are effectively "garbage collected"
by the server.
Implementation Note: Due to potential message loss, the Reset
message may not reach the server. The client may therefore have
to reject multiple notifications, each with one Reset message,
until the server finally removes the associated entry from the
list of observers and stops sending notifications.
In some circumstances, it may be desirable to cancel an observation
and release the resources allocated by the server to it more eagerly.
In this case, a client MAY explicitly deregister by issuing a GET
request that has the Token field set to the token of the observation
to be cancelled and includes an Observe Option with the value set to
1 (deregister). All other options MUST be identical to those in the
registration request except for the set of ETag Options. When the
server receives such a request, it will remove any matching entry
from the list of observers and process the GET request as usual.
4. Server-Side Requirements
A GET request with an Observe Option set to 0 (register) requests the
server not only to return a current representation of the target
resource, but also to add the client to the list of observers of that
resource. Upon success, the server returns a current representation
of the resource and MUST keep this representation updated (as
described in Section 1.3) as long as the client is on the list of
The entry in the list of observers is keyed by the client endpoint
and the token specified by the client in the request. If an entry
with a matching endpoint/token pair is already present in the list
(which, for example, happens when the client wishes to reinforce its
interest in a resource), the server MUST NOT add a new entry but MUST
replace or update the existing one.
A server that is unable or unwilling to add a new entry to the list
of observers of a resource MAY silently ignore the registration
request 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 Observe Option in a GET request is set to 1 (deregister), then
the server MUST remove any existing entry with a matching endpoint/
token pair from the list of observers and process the GET request as
usual. The resulting response MUST NOT include an Observe Option.
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 echo the
token specified by the client in the GET request. If there are
multiple entries in the list of observers, the order in which the
clients 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 an
appropriate response code (such as 4.04 Not Found) and subsequently
MUST remove the associated entry from the list of observers of the
The Content-Format specified in a 2.xx 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 in this format, it
SHOULD send a notification with a 4.06 (Not Acceptable) response code
and subsequently MUST remove the associated entry from the list of
observers of the resource.
A 2.xx notification MUST include an Observe Option with a sequence
number as specified in Section 4.4 below; a non-2.xx notification
MUST NOT include an Observe Option.
As notifications are just additional responses sent by the server in
reply to a GET request, they are subject to caching as defined in
Section 5.6 of RFC 7252 [RFC7252].
After returning the initial response, the server MUST keep the
resource state that is 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, the client
cannot tell if the observed state and the actual state are still in
sync. Thus, when the age of the latest notification becomes greater
than its indicated Max-Age, the client no longer has a usable
representation of the resource state. The server MAY wish to prevent
that by sending a new notification with the unchanged representation
and a new Max-Age just before the Max-Age indicated earlier expires.
A client can include a set of entity tags in its request using the
ETag Option. When an 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.
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 2^23 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 with the
same token for the same resource. The value of the Observe Option
MUST be current at the time of transmission; if a notification is
retransmitted, the server MUST update the value of the option to the
sequence number that is current at that time before retransmission.
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)/(2^23) = 30.52 microseconds. It is not necessary
that the clock reflects the current time/date.
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 2^23 times in the first 256 seconds
after every state change). This removes the need to update the
value of the Observe Option on retransmission when the resource
state did not change.
Design Note: The choice of a 24-bit option value and a time span of
256 seconds theoretically allows for a notification rate of up to
65536 notifications per second. Constrained nodes often have
rather imprecise clocks, though, and inaccuracies of the client
and server side may cancel out or add in effect. Therefore, the
maximum notification rate is reduced to 32768 notifications per
second. This is still well beyond the highest known design
objective of around 1 kHz (most CoAP applications will be several
orders of magnitude below that) but allows total clock
inaccuracies of up to -50/+100%.
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
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.
A server MAY choose to skip sending a notification if it knows that
it will send another notification soon, for example, when the state
of a resource is changing frequently. It also MAY choose to send
more than one notification for the same resource state. However,
above all, the server MUST ensure that a client in the list of
observers of a resource eventually observes the latest state if the
resource does not undergo a new change in state.
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 by repeating the last notification in a confirmable message
when the burst is over.
The client's acknowledgement of a confirmable notification signals
that the client is interested in receiving further notifications. If
a client rejects a confirmable or non-confirmable notification with a
Reset message, or if the last attempt to retransmit a confirmable
notification times out, then the client is considered no longer
interested and the server MUST remove the associated entry from the
list of observers.
Implementation Note: To properly process a Reset message that
rejects a non-confirmable notification, a server needs to remember
the message IDs of the non-confirmable notifications it sends.
This may be challenging for a server with constrained resources.
However, since Reset messages are transmitted unreliably, the
client must be prepared in case the Reset messages are not
received by the server. Thus, a server can always pretend that a
Reset message rejecting a non-confirmable notification was lost.
If a server does this, it could accelerate cancellation by sending
the following notifications to that client in confirmable
A server that transmits notifications mostly in non-confirmable
messages MUST send a notification in a confirmable message instead of
a non-confirmable message at least every 24 hours. This prevents a
client that went away or is no longer interested from remaining in
the list of observers indefinitely.
4.5.1. Congestion Control
Basic congestion control for CoAP is provided by the exponential
back-off mechanism in Section 4.2 of RFC 7252 [RFC7252] and the
limitations in Section 4.7 of RFC 7252 [RFC7252]. However, CoAP
places the responsibility of congestion control for simple request/
response interactions only on the clients: rate-limiting request
transmission implicitly controls the transmission of the responses.
When a single request yields a potentially infinite number of
notifications, additional responsibility needs to be placed on the
In order not to cause congestion, servers MUST strictly limit the
number of simultaneous outstanding notifications/responses that they
transmit to a given client to NSTART (1 by default; see Section 4.7
of RFC 7252 [RFC7252]). An outstanding notification/response is
either a confirmable message for which an acknowledgement has not yet
been received and whose last retransmission attempt has not yet timed
out or a non-confirmable message for which the waiting time that
results from the following rate-limiting rules has not yet elapsed.
The server SHOULD NOT send more than one non-confirmable notification
per round-trip time (RTT) to a client on average. If the server
cannot maintain an RTT estimate for a client, it SHOULD NOT send more
than one non-confirmable notification every 3 seconds and SHOULD use
an even less aggressive rate when possible (see also Section 3.1.2 of
RFC 5405 [RFC5405]).
Further congestion control optimizations and considerations are
expected in the future with advanced CoAP congestion control
4.5.2. Advanced Transmission
The state of an observed resource may change while the number of
simultaneous outstanding notifications/responses to a client on the
list of observers is greater than or equal to NSTART. In this case,
the server cannot notify the client of the new resource state
immediately but has to wait for an outstanding notification/response
to complete first.
If there exists an outstanding notification/response that the server
transmits to the client and that pertains to the changed resource,
then it is desirable for the server to stop working towards getting
the representation of the old resource state to the client and to
start transmitting the current representation to the client instead,
so the resource state observed by the client stays closer in sync
with the actual state at the server.
For this purpose, the server MAY optimize the transmission process by
aborting the transmission of the old notification (but not before the
current transmission attempt is completed) and starting a new
transmission for the new notification (but with the retransmission
timer and counter of the aborted transmission retained).
In more detail, a server MAY supersede an outstanding transmission
that pertains to an observation as follows:
1. Wait for the current (re)transmission attempt to be acknowledged,
rejected, or to time out (confirmable transmission); or, wait for
the waiting time to elapse or the transmission to be rejected
2. If the transmission is rejected or it was the last attempt to
retransmit a notification, remove the associated entry from the
list of observers of the observed resource.
3. If the entry is still in the list of observers, start to transmit
a new notification with a representation of the current resource
state. Should the resource have changed its state more than once
in the meantime, the notifications for the intermediate states
are silently skipped.
4. The new notification is transmitted with a new Message ID and the
following transmission parameters: if the previous
(re)transmission attempt timed out, retain its transmission
parameters, increment the retransmission counter, and double the
timeout; otherwise, initialize the transmission parameters as
usual (see Section 4.2 of RFC 7252 [RFC7252]).
It is possible that the server later receives an acknowledgement for
a confirmable notification that it superseded this way. Even though
this does not signal consistency, it is valuable in that it signals
the client's further interest in the resource. The server therefore
should avoid inadvertently removing the associated entry from the
list of observers.
A client may be interested in a resource in the namespace of a server
that is reached through a chain of one or more CoAP intermediaries.
In this case, the client registers its interest with the first
intermediary towards the server, acting as if it was communicating
with the 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 to keep the representation updated upon
changes to the resource state, 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 server. If the response returned by the next hop
doesn't include an Observe Option, the intermediary MAY resort to
polling the next hop or MAY itself return a response without an
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 in notifications and MUST set
the value of the Max-Age Option according to the age of the local
If two or more clients have registered their interest in a resource
with an intermediary, the intermediary MUST register itself only once
with the next hop and fan out the notifications it receives to all
registered clients. This relieves the next hop from sending the same
notifications multiple times and thus enables scalability.
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.2 for examples.
6. 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
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.
7. Security Considerations
The security considerations in Section 11 of [RFC7252], the CoAP
Observing resources can dramatically increase the negative effects of
amplification attacks. That is, not only can notifications messages
be much larger than the request message, but the nature of the
protocol can cause a significant number of notifications to be
generated. Without client authentication, a server therefore MUST
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. Note that
an attacker may still spoof the acknowledgements if the confirmable
messages are sufficiently predictable.
The protocol follows a best-effort approach for keeping the state
observed by a client and the actual resource state at a server in
sync. This may have the client and the server become out of sync at
times. Depending on the sensitivity of the observed resource,
operating on an old state might be a security threat. The client
therefore must be careful not to use a representation after its Max-
Age expires, and the server must set the Max-Age Option to a sensible
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 apply
access controls to 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.
Carsten Bormann was an original author of this document and is
acknowledged for significant contribution to this document.
Thanks to Daniele Alessandrelli, Jari Arkko, Peter A. Bigot, Angelo
P. Castellani, Gilbert Clark, Esko Dijk, Thomas Fossati, Brian Frank,
Bert Greevenbosch, Jeroen Hoebeke, Cullen Jennings, Matthias
Kovatsch, Barry Leiba, Salvatore Loreto, Charles Palmer, Akbar
Rahman, Zach Shelby, and Floris Van den Abeele for helpful comments
and discussions that have shaped the document.
This work was supported in part by Klaus Tschira Foundation, Intel,
Cisco, and Nokia.
Universitaet Bremen TZI