One document matched: draft-ietf-core-observe-15.xml
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<rfc category="std" docName="draft-ietf-core-observe-15" ipr="trust200902">
<front>
<title>Observing Resources in CoAP</title>
<author initials="K." surname="Hartke" fullname="Klaus Hartke">
<organization>Universitaet Bremen TZI</organization>
<address>
<postal>
<street>Postfach 330440</street>
<city>Bremen</city>
<code>D-28359</code>
<country>Germany</country>
</postal>
<phone>+49-421-218-63905</phone>
<email>hartke@tzi.org</email>
</address>
</author>
<date year="2014"/>
<area>Applications</area>
<workgroup>CoRE Working Group</workgroup>
<abstract>
<t>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 the
server.</t>
</abstract>
</front>
<middle>
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<section title="Introduction" anchor="introduction">
<section title="Background" anchor="background">
<t>The <xref target="RFC7252">Constrained Application Protocol (CoAP)</xref> is intended to
provide <xref target="REST">RESTful services</xref> not unlike <xref
target="RFC7230">HTTP</xref> 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 <xref target="RFC7228"/>.</t>
<t>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 and 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.</t>
<t>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 <xref
target="RFC6202">HTTP long polling</xref>, generate significant
complexity and/or overhead and thus are less applicable in a
constrained environment.</t>
<t>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.</t>
<t>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
<xref target="RFC5989"/>.</t>
</section>
<section title="Protocol Overview" anchor="overview">
<t>The protocol is based on the well-known <xref target="GOF">observer
design pattern</xref>. 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 them.</t>
<figure anchor="design-pattern" title="The Observer Design Pattern">
<artwork type="drawing" align="left"><![CDATA[
Observer Subject
| |
| Registration |
+------------------->|
| |
| Notification |
|<-------------------+
| |
| Notification |
|<-------------------+
| |
| Notification |
|<-------------------+
| |
]]></artwork>
</figure>
<t>The observer design pattern is realized in CoAP as follows:<list
style="hanging">
<t hangText="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.</t>
<t hangText="Observer:">An observer is a CoAP client that is
interested in having a current representation of the resource
at any given time.</t>
<t hangText="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.</t>
<t hangText="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.</t>
</list>
</t>
<t><xref target="example"/> 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 &OPTION1; Option defined in this document. In
notifications, the &OPTION1; 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.</t>
<figure anchor="example" title="Observing a Resource in CoAP">
<artwork type="drawing" align="left"><![CDATA[
Client Server
| |
| GET /temperature |
| Token: 0x4a | Registration
| Observe: register |
+------------------->|
| |
| 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 |
|<-------------------+
| |
]]></artwork>
</figure>
<t>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 by 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 and is removed by the server
from the list of observers.</t>
</section>
<section title="Consistency Model" anchor="consistency">
<t>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.</t>
<t>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.</t>
<t>The protocol addresses this issue as follows:<list style="symbols">
<t>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.</t>
<t>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.</t>
<t>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.</t>
</list></t>
</section>
<section title="Observable Resources" anchor="resource-design">
<t>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.</t>
<t>For example, a CoAP server with an attached temperature sensor
could expose one or more of the following resources:<list
style="symbols">
<t><coap://server/temperature>,
which changes its state every few seconds to a current reading
of the temperature sensor;</t>
<t><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;</t>
<t><coap://server/temperature/critical?above=42>,
which changes its state based on the client-specified parameter
value: every few seconds to the current temperature reading if
the temperature exceeds the threshold, or to "OK" when the
reading drops below;</t>
<t><coap://server/?query=select+avg(temperature)+from+Sensor.window:<vspace/>time(30sec)>,
which accepts expressions of arbitrary complexity and changes
its state accordingly.</t>
</list>
</t>
<t>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.</t>
</section>
<section title="Requirements Notation" anchor="requirements-notation">
<t>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 <xref target="RFC2119"
>RFC 2119</xref>.</t>
</section>
</section>
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<section title="The Observe Option" anchor="option">
<texttable title="The Observe Option" anchor="option-table">
<ttcol align="right">No.</ttcol>
<ttcol align="left">C</ttcol>
<ttcol align="left">U</ttcol>
<ttcol align="left">N</ttcol>
<ttcol align="left">R</ttcol>
<ttcol align="left">Name</ttcol>
<ttcol align="left">Format</ttcol>
<ttcol align="left">Length</ttcol>
<ttcol align="left">Default</ttcol>
<c>&OPTION1NUMBER;</c>
<c></c>
<c>x</c>
<c>-</c>
<c></c>
<c>&OPTION1;</c>
<c>uint</c>
<c>0-3 B</c>
<c>(none)</c>
<postamble>C=Critical, U=Unsafe, N=No-Cache-Key, R=Repeatable</postamble>
</texttable>
<t>The &OPTION1; 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 or remove an
entry in the list of observers of the resource, where the entry
consists of the client endpoint and the token specified in the request.
<list>
<t>'register' (0) adds the entry to the list, if not present;</t>
<t>'deregister' (1) removes the entry from the list, if present.</t>
</list>
</t>
<t>The &OPTION1; 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.</t>
<t>In a response, the &OPTION1; Option identifies the message as a
notification. This implies that the server has added an entry with
the client endpoint and request token to the list of observers and
that it will notify the client of changes to the resource state.
The option value is a 24-bit sequence number for reordering detection
(see <xref target="client-reordering"/> and <xref
target="server-reordering"/>).</t>
<t>The value of the &OPTION1; Option is encoded as an unsigned integer in
network byte order using a variable number of bytes ('uint' option
format); see <xref target="RFC7252">Section 3.2 of RFC 7252</xref>.</t>
<t>The &OPTION1; Option is not part of the cache-key: a cacheable
response obtained with &anOPTION1; Option in the request can be used to
satisfy a request without &anOPTION1; Option, and vice versa. When a
stored response with &anOPTION1; Option is used to satisfy a normal GET
request, the option MUST be removed before the response is returned.</t>
</section>
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<section title="Client-side Requirements" anchor="client">
<section title="Request" anchor="client-request">
<t>A client registers its interest in a resource by issuing a GET
request with an &OPTION1; Option set to 'register' (0). If the server
returns a 2.xx response that includes &anOPTION1; 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.</t>
<t>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.</t>
</section>
<section title="Notifications" anchor="client-notifications">
<t>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 &OPTION1; Option.</t>
<t>Notifications typically have a 2.05 (Content) response code. They
include &anOPTION1; Option with a sequence number for reordering
detection (see <xref target="client-reordering"/>), 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 <xref
target="client-caching"/>), notifications can have a 2.03
(Valid) response code rather than a 2.05 (Content) response code.
Such notifications include &anOPTION1; Option with a sequence
number but no payload.</t>
<t>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 &anOPTION1; Option.</t>
</section>
<section title="Caching" anchor="client-caching">
<t>As notifications are just additional responses to a GET request,
notifications partake in caching as defined in <xref
target="RFC7252">Section 5.6 of RFC 7252</xref>. Both
the freshness model and the validation model are supported.</t>
<section title="Freshness" anchor="client-freshness">
<t>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).</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
</section>
<section title="Validation" anchor="client-validation">
<t>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.</t>
<t>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.</t>
<t>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.</t>
</section>
</section>
<section title="Reordering" anchor="client-reordering">
<t>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.</t>
<t>To provide an order among notifications for the client, the server
sets the value of the &OPTION1; 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:</t>
<figure>
<artwork type="inline" align="center"><![CDATA[
(V1 < V2 and V2 - V1 < 2^23) or
(V1 > V2 and V1 - V2 > 2^23) or
(T2 > T1 + 128 seconds)
]]></artwork>
</figure>
<t>where V1 is the value of the &OPTION1; Option in the freshest
notification so far, V2 the value of the &OPTION1; Option in
the incoming notification, T1 a client-local timestamp for the freshest
notification so far, and T2 a client-local timestamp for the
incoming notification.</t>
<t><list style="hanging">
<t hangText="Design Note:">The first two conditions verify that V1
is less than V2 in 24-bit <xref target="RFC1982">serial number
arithmetic</xref>. 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 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 so far.</t>
<t>The duration of 128 seconds was chosen as a nice round number
greater than MAX_LATENCY (<xref target="RFC7252">Section
4.8.2 of RFC 7252</xref>).</t>
</list></t>
</section>
<section title="Transmission" anchor="client-transmission">
<t>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.</t>
<t>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.</t>
<t>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.</t>
</section>
<section title="Cancellation" anchor="client-cancellation">
<t>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.</t>
<t><list style="hanging"><t hangText="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.</t></list></t>
<t>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 which has the Token field set to the token of the observation
to be cancelled and includes an &OPTION1; Option with the value set
to 'deregister' (1). 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.</t>
</section>
</section>
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<section title="Server-side Requirements" anchor="server">
<section title="Request" anchor="server-request">
<t>A GET request with &anOPTION1; Option set to 'register' (0) 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 observers.</t>
<t>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.</t>
<t>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 &anOPTION1; 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.</t>
<t>If the &OPTION1; Option in a request is set to any other value than
'register' (0), then the server MUST remove any 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 &anOPTION1; Option.</t>
</section>
<section title="Notifications" anchor="server-notifications">
<t>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.</t>
<t>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 resource.</t>
<t>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.</t>
<t>A 2.xx notification MUST include &anOPTION1; Option with a sequence number as specified in <xref target="server-reordering"/> below;
a non-2.xx notification MUST NOT include &anOPTION1; Option.</t>
</section>
<section title="Caching" anchor="server-caching">
<t>As notifications are just additional responses sent by the server
in reply to a GET request, they are subject to caching as defined
in <xref target="RFC7252">Section 5.6 of RFC 7252</xref>.</t>
<section title="Freshness" anchor="server-freshness">
<t>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.</t>
<t>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.</t>
<t>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.</t>
</section>
<section title="Validation" anchor="server-validation">
<t>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.</t>
</section>
</section>
<section title="Reordering" anchor="server-reordering">
<t>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 &OPTION1;
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.</t>
<t>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 &OPTION1; 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.</t>
<t><list style="hanging">
<t hangText="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.</t>
<t>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 &OPTION1; Option on retransmission when the
resource state did not change.</t>
<t hangText="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 %.</t>
</list></t>
</section>
<section title="Transmission" anchor="server-transmission">
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>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.</t>
<t>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.<list style="hanging">
<t hangText="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 that its Reset messages aren't received
by the server. A server thus 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
messages.</t></list></t>
<t>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.</t>
<section title="Congestion Control" anchor="server-congestion">
<t>Basic congestion control for CoAP is provided by the exponential
back-off mechanism in <xref target="RFC7252">Section 4.2 of RFC
7252</xref> and the limitations in <xref target="RFC7252">Section
4.7 of RFC 7252</xref>. 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 server.</t>
<t>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 <xref
target="RFC7252">Section 4.7 of RFC 7252</xref>). 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.</t>
<t>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 <xref target="RFC5405">Section 3.1.2 of RFC
5405</xref>).</t>
<t>Further congestion control optimizations and considerations are
expected in the future with advanced CoAP congestion control
mechanisms.</t>
</section>
<section title="Advanced Transmission" anchor="server-advanced">
<t>The state of an observed resource may change while the number of
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.</t>
<t>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.</t>
<t>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 completed) and starting a new transmission
for the new notification (but with the retransmission timer and counter of the aborted
transmission retained).</t>
<t>In more detail, a server MAY supersede an outstanding transmission that
pertains to an observation as follows:<list style="numbers">
<t>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
(non-confirmable transmission).</t>
<t>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.</t>
<t>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.</t>
<t>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
<xref target="RFC7252">Section 4.2 of RFC 7252</xref>).</t>
</list>
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.</t>
</section>
</section>
</section>
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<section title="Intermediaries" anchor="intermediary">
<t>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 <xref
target="client"/>. 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 <xref target="server"/>.</t>
<t>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 &anOPTION1; Option, the intermediary MAY resort to
polling the next hop or MAY itself return a response without
&anOPTION1; Option.</t>
<t>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 &OPTION1; Option in notifications, and MUST set the value of the
Max-Age Option according to the age of the local current
representation.</t>
<t>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.</t>
<t>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.</t>
<t>See <xref target="examples-intermediary"/> for examples.</t>
</section>
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<section title="Web Linking" anchor="web-linking">
<t>A <xref target="RFC5988">web link</xref> to a resource accessible over
CoAP (for example, in a <xref target="RFC6690">link-format
document</xref>) MAY include the target attribute "obs".</t>
<t>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 &OPTION1;
Option can actually be used to perform the observation. A client may
need to resort to polling the resource if the &OPTION1; Option is not
returned in the response to the GET request.</t>
<t>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.</t>
</section>
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<section title="Security Considerations" anchor="security">
<t>The security considerations in <xref target="RFC7252">Section 11 of
the CoAP specification</xref> apply.</t>
<t>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. (An attacker
may still spoof the acknowledgements if the confirmable messages are
sufficiently predictable.)</t>
<t>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 value.</t>
<t>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
&anOPTION1; 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.</t>
<t>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.</t>
<t>Resources can be observed over DTLS-secured CoAP using any of the
security modes described in Section 9 of RFC 7252. The use of DTLS
is indicated by the "coaps" URI scheme. All notifications resulting from
a GET request with &anOPTION1; Option MUST be returned within the same
epoch of the same connection as the request.</t>
</section>
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<section title="IANA Considerations" anchor="iana-considerations">
<t>The following entry is added to the CoAP Option Numbers registry:</t>
<texttable>
<ttcol align="right">Number</ttcol>
<ttcol align="left">Name</ttcol>
<ttcol align="left">Reference</ttcol>
<c>&OPTION1NUMBER;</c>
<c>&OPTION1;</c>
<c>[RFCXXXX]</c>
</texttable>
<t>[Note to RFC Editor: Please replace XXXX with the RFC number of this
specification.]</t>
</section>
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<section title="Acknowledgements" anchor="acknowledgements">
<t>Carsten Bormann was an original author of this draft and is
acknowledged for significant contribution to this document.</t>
<t>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.</t>
<t>This work was supported in part by Klaus Tschira Foundation, Intel,
Cisco, and Nokia.</t>
</section>
</middle>
<back>
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<!-- **************************************************************** -->
<references title="Normative References">
&RFC2119;
&RFC5988;
&RFC7252;
</references>
<references title="Informative References">
&RFC1982;
&RFC5405;
&RFC5989;
&RFC6202;
&RFC6690;
&RFC7228;
&RFC7230;
<reference anchor="REST" target="http://www.ics.uci.edu/~fielding/pubs/dissertation/fielding_dissertation.pdf">
<front>
<title>Architectural Styles and the Design of Network-based Software Architectures</title>
<author initials="R." surname="Fielding" fullname="Roy Fielding">
<organization>University of California, Irvine</organization>
</author>
<date year="2000"/>
</front>
<seriesInfo name="Ph.D." value="Dissertation, University of California, Irvine"/>
<format type="PDF" target="http://www.ics.uci.edu/~fielding/pubs/dissertation/fielding_dissertation.pdf"/>
</reference>
<reference anchor="GOF">
<front>
<title>Design Patterns: Elements of Reusable Object-Oriented Software</title>
<author initials="E." surname="Gamma" fullname="Erich Gamma">
<organization/>
</author>
<author initials="R." surname="Helm" fullname="Richard Helm">
<organization/>
</author>
<author initials="R." surname="Johnson" fullname="Ralph Johnson">
<organization/>
</author>
<author initials="J." surname="Vlissides" fullname="John M. Vlissides">
<organization/>
</author>
<date year="1994" month="November"/>
</front>
<seriesInfo name="Addison-Wesley," value="Reading, MA, USA"/>
</reference>
</references>
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<!-- **************************************************************** -->
<section title="Examples">
<section title="Client/Server Examples" anchor="examples-client-server">
<figure anchor="example-1" title="A client registers and receives one notification of the current state and one of a new state upon a state change">
<artwork type="example"><![CDATA[
Observed CLIENT SERVER Actual
t State | | State
____________ | | ____________
1 | |
2 unknown | | 18.5 Cel
3 +----->| Header: GET 0x41011633
4 | GET | Token: 0x4a
5 | | Uri-Path: temperature
6 | | Observe: 0 (register)
7 | |
8 | |
9 ____________ |<-----+ Header: 2.05 0x61451633
10 | 2.05 | Token: 0x4a
11 18.5 Cel | | Observe: 9
12 | | Max-Age: 15
13 | | Payload: "18.5 Cel"
14 | |
15 | | ____________
16 ____________ |<-----+ Header: 2.05 0x51457b50
17 | 2.05 | 19.2 Cel Token: 0x4a
18 19.2 Cel | | Observe: 16
29 | | Max-Age: 15
20 | | Payload: "19.2 Cel"
21 | |
]]></artwork>
</figure>
<figure anchor="example-2" title="The client re-registers after Max-Age ends">
<artwork type="example"><![CDATA[
Observed CLIENT SERVER Actual
t State | | State
____________ | | ____________
22 | |
23 19.2 Cel | | 19.2 Cel
24 | | ____________
25 | X----+ Header: 2.05 0x51457b51
26 | 2.05 | 19.7 Cel Token: 0x4a
27 | | Observe: 25
28 | | Max-Age: 15
29 | | Payload: "19.7 Cel"
30 | |
31 ____________ | |
32 | |
33 19.2 Cel | |
34 (stale) | |
35 | |
36 | |
37 | |
38 +----->| Header: GET 0x41011634
39 | GET | Token: 0xb2
40 | | Uri-Path: temperature
41 | | Observe: 0 (register)
42 | |
43 | |
44 ____________ |<-----+ Header: 2.05 0x61451634
45 | 2.05 | Token: 0xb2
46 19.7 Cel | | Observe: 44
47 | | Max-Age: 15
48 | | ETag: 0x78797a7a79
49 | | Payload: "19.7 Cel"
50 | |
]]></artwork>
</figure>
<figure anchor="example-3" title="The client re-registers and gives the server the opportunity to select a stored response">
<artwork type="example"><![CDATA[
Observed CLIENT SERVER Actual
t State | | State
____________ | | ____________
51 | |
52 19.7 Cel | | 19.7 Cel
53 | |
54 | | ____________
55 | crash
56 |
57 |
58 |
59 ____________ |
60 |
61 19.7 Cel |
62 (stale) |
63 | reboot____________
64 | |
65 | | 20.0 Cel
66 | |
67 +----->| Header: GET 0x41011635
68 | GET | Token: 0xf9
69 | | Uri-Path: temperature
70 | | Observe: 0 (register)
71 | | ETag: 0x78797a7a79
72 | |
73 | |
74 ____________ |<-----+ Header: 2.05 0x61451635
75 | 2.05 | Token: 0xf9
76 20.0 Cel | | Observe: 74
77 | | Max-Age: 15
78 | | Payload: "20.0 Cel"
79 | |
80 | | ____________
81 ____________ |<-----+ Header: 2.03 0x5143aa0c
82 | 2.03 | 19.7 Cel Token: 0xf9
83 19.7 Cel | | Observe: 81
84 | | ETag: 0x78797a7a79
85 | | Max-Age: 15
86 | |
]]></artwork>
</figure>
<figure anchor="example-4" title="The client rejects a notification and thereby cancels the observation">
<artwork type="example"><![CDATA[
Observed CLIENT SERVER Actual
t State | | State
____________ | | ____________
87 | |
88 19.7 Cel | | 19.7 Cel
89 | |
90 | | ____________
91 ____________ |<-----+ Header: 2.05 0x4145aa0f
92 | 2.05 | 19.3 Cel Token: 0xf9
93 19.3 Cel | | Observe: 91
94 | | Max-Age: 15
95 | | Payload: "19.3 Cel"
96 | |
97 | |
98 +- - ->| Header: 0x7000aa0f
99 | |
100 | |
101 | |
102 | | ____________
103 | |
104 | | 19.0 Cel
105 | |
106 ____________ | |
107 | |
108 19.3 Cel | |
109 (stale) | |
110 | |
]]></artwork>
</figure>
</section>
<section title="Proxy Examples" anchor="examples-intermediary">
<figure anchor="example-5" title="A proxy observes a resource to keep its cache up to date">
<artwork type="example"><![CDATA[
CLIENT PROXY SERVER
| | |
| +----->| Header: GET 0x41015fb8
| | GET | Token: 0x1a
| | | Uri-Host: sensor.example
| | | Uri-Path: status
| | | Observe: 0 (register)
| | |
| |<-----+ 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"
| | |
]]></artwork>
</figure>
<figure anchor="example-6" title="A client observes a resource through a proxy">
<artwork type="example"><![CDATA[
CLIENT PROXY SERVER
| | |
+----->| | Header: GET 0x41011635
| GET | | Token: 0x6a
| | | Proxy-Uri: coap://sensor.example/status
| | | Observe: 0 (register)
| | |
|<- - -+ | Header: 0x60001635
| | |
| +----->| Header: GET 0x4101af90
| | GET | Token: 0xaa
| | | Uri-Host: sensor.example
| | | Uri-Path: status
| | | Observe: 0 (register)
| | |
| |<-----+ 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"
| | |
]]></artwork>
</figure>
</section>
</section>
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<section title="Changelog" anchor="changelog">
<t>[Note to RFC Editor: Please remove this section before publication.]</t>
<t>Changes from ietf-14 to ietf-15:
<list style="symbols">
<t>Clarified several points based on AD, GenART, IESG, and Secdir reviews.</t>
</list>
</t>
<t>Changes from ietf-13 to ietf-14:
<list style="symbols">
<t>Updated references.</t>
</list>
</t>
<t>Changes from ietf-12 to ietf-13:
<list style="symbols">
<t>Extended the Observe Option in requests to not only add but
also remove an entry in the list of observers, depending on the
option value.<list style="hanging">
<t hangText="Note:">The value of the Observe Option in a
registration request may now be any sequence of bytes that
encodes the unsigned integer 0, i.e., 0x'', 0x'00', 0x'00 00'
or 0x'00 00 00'.</t>
</list></t>
<t>Removed the 7.31 Code for cancellation.</t>
</list>
</t>
<t>Changes from ietf-11 to ietf-12:
<list style="symbols">
<t>Introduced the 7.31 Code to request the cancellation of a pending
request.</t>
<t>Made the algorithm for superseding an outstanding transmission OPTIONAL.</t>
<t>Clarified that the entry in the list of observers is removed if
the client fails to acknowledge a confirmable notification before
the last retransmission attempt times out (#350).</t>
<t>Simplified the text on cancellation (#352) and the handling of
Reset messages (#353).</t>
</list>
</t>
<t>Changes from ietf-10 to ietf-11:
<list style="symbols">
<t>Pointed out that client and server clocks may differ in their
realization of the SI second, and added robustness to the existing
reordering scheme by reducing the maximum notification rate to
32768 notifications per second (#341).</t>
</list>
</t>
<t>Changes from ietf-09 to ietf-10:
<list style="symbols">
<t>Required consistent sequence numbers across requests (#333).</t>
<t>Clarified that a server needs to update the entry in the list of
observers instead of adding a new entry if the endpoint/token pair
is already present.</t>
<t>Allowed that a client uses a token that is currently in use to
ensure that it's still in the list of observers. This is possible
because sequence numbers are now consistent across requests and
servers won't add a new entry for the same token.</t>
<t>Improved text on the transmission of non-confirmable notifications
to match Section 3.1.2 of RFC 5405 more closely.</t>
<t>Updated examples to use UCUM units.</t>
<t>Moved Appendix B into the introduction.</t>
</list>
</t>
<t>Changes from ietf-08 to ietf-09:
<list style="symbols">
<t>Removed the side effects of requests on existing observations.
This includes removing that<list style="symbols">
<t>the client can use a GET request to cancel an observation;</t>
<t>the server updates the entry in the list of observers instead
of adding a new entry if the client is already present (#258,
#281).</t>
</list></t>
<t>Clarified that a resource (and hence an observation relationship)
is identified by the request options that are part of the Cache-Key
(#258).</t>
<t>Clarified that a non-2.xx notification MUST NOT include an Observe
Option.</t>
<t>Moved block-wise transfer of notifications to
[I-D.ietf-core-block].</t>
</list>
</t>
<t>Changes from ietf-07 to ietf-08:
<list style="symbols">
<t>Expanded text on transmitting a notification while a previous
transmission is pending (#242).</t>
<t>Changed reordering detection to use a fixed time span of 128
seconds instead of EXCHANGE_LIFETIME (#276).</t>
<t>Removed the use of the freshness model to determine if the client
is still on the list of observers. This includes removing that<list
style="symbols">
<t>the client assumes that it has been removed from the list of
observers when Max-Age ends;</t>
<t>the server sets the Max-Age Option of a notification to a
value that indicates when the server will send the next
notification;</t>
<t>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);</t>
<t>the server may remove the client from all lists of observers
when the transmission of a confirmable notification ultimately
times out.</t>
</list></t>
<t>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.</t>
<t>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.</t>
<t>Added a requirement for sending a confirmable notification at
least every 24 hours before continuing with non-confirmable
notifications (#221).</t>
<t>Added congestion control considerations from
[I-D.bormann-core-congestion-control-02].</t>
<t>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.</t>
<t>Changed reordering detection from 'MAY' to 'SHOULD', as the goal
of the protocol (to keep the observed state as closely in sync with
the actual state as possible) is not optional.</t>
<t>Fixed the length of the Observe Option (3 bytes) in the table in
<xref target="option"/>.</t>
<t>Replaced the 'x' in the No-Cache-Key column in the table in <xref
target="option"/> 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.</t>
<t>Updated examples.</t>
</list>
</t>
<t>Changes from ietf-06 to ietf-07:
<list style="symbols">
<t>Moved to 24-bit sequence numbers to allow for up to 15000
notifications per second per client and resource (#217).</t>
<t>Re-numbered option number to use Unsafe/Safe and Cache-Key
compliant numbers (#241).</t>
<t>Clarified how to react to a Reset message that is sent in reply to
a non-confirmable notification (#225).</t>
<t>Clarified the semantics of the "obs" link target attribute
(#236).</t>
</list>
</t>
<t>Changes from ietf-05 to ietf-06:
<list style="symbols">
<t>Improved abstract and introduction to say that the protocol is
about best effort and eventual consistency (#219).</t>
<t>Clarified that the value of the Observe Option in a request must
have zero length.</t>
<t>Added requirement that the sequence number must be updated each
time a server retransmits a notification.</t>
<t>Clarified that a server must remove a client from the list of
observers when it receives a GET request with an unrecognized
critical option.</t>
<t>Updated the text to use the endpoint concept from
[I-D.ietf-core-coap] (#224).</t>
<t>Improved the reordering text (#223).</t>
</list>
</t>
<t>Changes from ietf-04 to ietf-05:
<list style="symbols">
<t>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).</t>
<t>Relaxed requirements when sending a Reset message in reply to
non-confirmable notifications.</t>
<t>Added an implementation note about careless GET requests
(#184).</t>
<t>Updated examples.</t>
</list>
</t>
<t>Changes from ietf-03 to ietf-04:
<list style="symbols">
<t>Removed the "Max-OFE" Option.</t>
<t>Allowed a Reset message in reply to non-confirmable
notifications.</t>
<t>Added a section on cancellation.</t>
<t>Updated examples.</t>
</list>
</t>
<t>Changes from ietf-02 to ietf-03:
<list style="symbols">
<t>Separated client-side and server-side requirements.</t>
<t>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).</t>
<t>Simplified the text on message reordering (#129).</t>
<t>Clarified requirements for intermediaries.</t>
<t>Clarified the combination of blockwise transfers with
notifications (#172).</t>
<t>Updated examples to show how the state observed by the client
becomes eventually consistent with the actual state on the
server.</t>
<t>Added examples for parameterization of observable resource.</t>
</list>
</t>
<t>Changes from ietf-01 to ietf-02:
<list style="symbols">
<t>Removed the requirement of periodic refreshing (#126).</t>
<t>The new "Observe" Option replaces the "Lifetime" Option.</t>
<t>Introduced a new mechanism to detect message reordering.</t>
<t>Changed 2.00 (OK) notifications to 2.05 (Content)
notifications.</t>
</list>
</t>
<t>Changes from ietf-00 to ietf-01:
<list style="symbols">
<t>Changed terminology from "subscriptions" to "observation
relationships" (#33).</t>
<t>Changed the name of the option to "Lifetime".</t>
<t>Clarified establishment of observation relationships.</t>
<t>Clarified that an observation is only identified by the URI of the
observed resource and the identity of the client (#66).</t>
<t>Clarified rules for establishing observation relationships
(#68).</t>
<t>Clarified conditions under which an observation relationship is
terminated.</t>
<t>Added explanation on how clients can terminate an observation
relationship before the lifetime ends (#34).</t>
<t>Clarified that the overriding objective for notifications is
eventual consistency of the actual and the observed state
(#67).</t>
<t>Specified how a server needs to deal with clients not
acknowledging confirmable messages carrying notifications
(#69).</t>
<t>Added a mechanism to detect message reordering (#35).</t>
<t>Added an explanation of how notifications can be cached,
supporting both the freshness and the validation model (#39,
#64).</t>
<t>Clarified that non-GET requests do not affect observation
relationships, and that GET requests without "Lifetime" Option
affecting relationships is by design (#65).</t>
<t>Described interaction with blockwise transfers (#36).</t>
<t>Added Resource Discovery section (#99).</t>
<t>Added IANA Considerations.</t>
<t>Added Security Considerations (#40).</t>
<t>Added examples (#38).</t>
</list>
</t>
</section>
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</rfc>
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