One document matched: draft-ietf-core-observe-08.xml
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<!ENTITY OPTION1 "Observe">
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<rfc category="std" docName="draft-ietf-core-observe-08" 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>
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<author initials="Z." surname="Shelby" fullname="Zach Shelby" role="editor">
<organization>Sensinode</organization>
<address>
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<street>Kidekuja 2</street>
<city>Vuokatti</city>
<code>88600</code>
<country>Finland</country>
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<phone>+358407796297</phone>
<email>zach@sensinode.com</email>
</address>
</author>
<author initials="C." surname="Bormann" fullname="Carsten Bormann">
<organization>Universitaet Bremen TZI</organization>
<address>
<postal>
<street>Postfach 330440</street>
<city>Bremen</city>
<code>D-28359</code>
<country>Germany</country>
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<phone>+49-421-218-63921</phone>
<email>cabo@tzi.org</email>
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<date year="2013"/>
<area>Applications</area>
<workgroup>CoRE Working Group</workgroup>
<abstract>
<t>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>
<note title="Editor's Note">
<t>This is an interim revision which will receive further modifications
during the resolution of open tickets, in particular #204 and #281.</t>
</note>
</front>
<middle>
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<section title="Introduction" anchor="introduction">
<section title="Background" anchor="background">
<t><xref target="I-D.ietf-core-coap">CoAP</xref> is an application
protocol for constrained nodes and networks. It is intended to
provide <xref target="REST">RESTful services</xref> not unlike <xref
target="RFC2616">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.</t>
<t>The model of REST is that of a client exchanging representations of
resources with a server. A representation captures the current or
intended state of a resource. The server is the definitive source for
representations of the resources in its namespace. A client
interested in the state of a resource initiates a request to the
server; the server then returns a response with a representation of
the resource that is current at the time of the request.</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 can retrieve a representation
of the resource and keep this representation updated by the server
over a period of time.</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, it 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 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 upon registration with the current state,
and then two upon two changes to the resource state. Both the
registration request and the notifications are identified as such by
the presence of the &OPTION1; Option defined in this document. In
notifications, the &OPTION1; Option provides a sequence number for
reordering detection. All notifications carry the token specified by
the client in the request, so the client can easily correlate them to
the request.</t>
<figure anchor="example" title="Observing a Resource in CoAP">
<artwork type="drawing" align="left"><![CDATA[
Client Server
| |
| GET /temperature |
| Token: 0x4a | Registration
| Observe: (empty) |
+----------------->|
| |
| 2.05 Content |
| Token: 0x4a | Notification of
| Observe: 12 | the current state
| Payload: 22.9 C |
|<-----------------+
| |
| 2.05 Content |
| Token: 0x4a | Notification upon
| Observe: 44 | a state change
| Payload: 22.8 C |
|<-----------------+
| |
| 2.05 Content |
| Token: 0x4a | Notification upon
| Observe: 60 | a state change
| Payload: 23.1 C |
|<-----------------+
| |
]]></artwork>
</figure>
<t>The server is the authority for determining under what conditions
resources change their state and how often observers are notified.
The protocol does not offer explicit means for setting up triggers,
thresholds or other conditions; it is up to the server to expose
observable resources that change their state in a way that is useful
in the application context. Resources can be parameterized to achieve
similar effects, though; see <xref target="modeling-resources"/> for
examples.</t>
<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
interest is determined from the client's acknowledgement of
notifications sent in confirmable messages by the server. If the
client actively rejects a notification or if the transmission of a
notification ultimately fails, then the client is assumed to be no
longer interested and is removed from the list of observers.</t>
<t>While a client is in the list of observers of a resource, it is the
goal of the protocol to keep the resource state observed by the
client as closely in sync with the actual state at the server as
possible.</t>
<t>Becoming out of sync at times cannot be avoided: First, there is
always some latency between the change of the resource state and the
receipt of the notification. Second, messages with notifications can
get lost, which will cause the client assume an old state until it
receives the next notification. And third, the server may erroneously
come to the conclusion that the client is no longer interested in the
resource, which will cause it to stop sending notifications and the
client to assume an old state until it registers its interest
again.</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. However, a client
cannot rely on observing every single state that a resource goes
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 maximum, the client cannot use the enclosed representation
until it has received 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="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>empty/uint</c>
<c>0 B/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 the client to the
list of observers of the resource. The exact semantics are defined in
the following sections. The value of the option in a request MUST be
empty on transmission and MUST be ignored on reception.</t>
<t>The &OPTION1; Option is not critical for processing the request. If
the server is unwilling or unable to add the client to the list of
observers of the resource identified by the request URI, then the
request falls back to a normal GET request.</t>
<t>In a response, the &OPTION1; Option identifies the message as a
notification. This implies that the server has added the client to the
list of observers and that it will notify the client of changes to the
resource state. The value of the option is a 24-bit sequence number for
reordering detection; see <xref target="client-reordering"/> and <xref
target="server-reordering"/> for the client- and server-side
respectively. The sequence number is encoded in network byte order
using a variable number of bytes, as specified in <xref
target="I-D.ietf-core-coap">Section 3.2 of RFC XXXX</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 to
the client.</t>
</section>
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<section title="Client-side Requirements" anchor="client">
<section title="Request" anchor="client-request">
<t>A client can register its interest in a resource by issuing a GET
request that includes an empty &OPTION1; Option. If the server
returns a 2.xx response that includes &anOPTION1; Option as well, the
server has added the client successfully to the list of observers of
the target resource and the client will be notified of changes to the
resource state.</t>
</section>
<section title="Notifications" anchor="client-notifications">
<t>Notifications are additional responses sent by the server in reply
to the GET request. Each notification includes the token specified by
the client in the GET request, &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.</t>
<t>Notifications have a 2.05 (Content) response code, or a 2.03 (Valid)
response code if the client has included one or more ETag Options in
the request (see <xref target="client-caching"/>). In the event that
the resource changes in a way that would cause a normal GET request
at that time to return a non-2.xx response (for example, when the
resource is deleted), the server sends a notification with a matching
response code and removes the client from the list of observers.</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="I-D.ietf-core-coap">Section 5.6 of RFC XXXX</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
origin server. Like a response, a notification is considered fresh
while its age is not greater than the value indicated by the
Max-Age Option and if no newer notification/response has been
received.</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 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 use the
enclosed representation until it is validated or a new notification
is received.</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 &anOPTION1; Option at any time. The client SHOULD specify a
new token in the GET request, as the token serves as an epoch
identifier for the sequence numbers in the &OPTION1; Option (see
<xref target="client-reordering"/>).</t>
<t>It is RECOMMENDED that the client does not issue the request while
it still has a fresh notification/response for a resource in its
cache. Additionally, the client SHOULD wait for a random amount of
time between 5 and 15 seconds before issuing the new request to
avoid synchronicity with other clients.</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. The client needs to keep all candidate
responses in its cache until it is no longer interested in the
target resource or it issues a new GET request with a new set of
entity-tags.</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 SHOULD
NOT update the observed state with a notification that arrives later
than a newer notification.</t>
<t>For reordering detection, the server sets the value of the &OPTION1;
Option in each notification to a 24-bit sequence number. An incoming
notification is newer than the newest notification received so far
when one of the following conditions is met:</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 of the newest
notification received so far, V2 the value of the &OPTION1; Option of
the incoming notification, T1 a client-local timestamp of the newest
notification received so far, and T2 a client-local timestamp of the
incoming notification.</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">sequence 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 sequence
number; in other words, after 128 seconds have elapsed without
any notification, a client does not need to check the sequence
numbers in order to assume an incoming notification is new.</t>
</list></t>
</section>
<section title="Transmission" anchor="client-transmission">
<t>A notification can be confirmable or non-confirmable, i.e., be sent
in a confirmable or a non-confirmable message. The message type used
is independent from the type used for the request or any previous
notification.</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 client from the list of observers.</t>
</section>
<section title="Cancellation" anchor="client-cancellation">
<t>When a client rejects a confirmable notification with a Reset
message or when it issues a GET request without &anOPTION1; Option
for a currently observed resource, the server will remove the client
from the list of observers of this resource. The client MAY use
either method to indicate that it is no longer interested in
receiving further notifications for the resource until it eventually
registers again.</t>
<t>When a client rejects non-confirmable notification, the server may
also (but is not required to) remove the client from the list of
observers of this resource. The client MAY try this method as well,
and MAY rate-limit the Reset messages it sends if the server appears
to persistently ignore them.</t>
<t><list style="hanging">
<t hangText="Implementation Note:"> A client that does not mediate
all its requests through its cache might inadvertently cancel an
observation by making an unrelated GET to the same resource. To
avoid this, without incurring a need for synchronization, such
clients can use a different source endpoint for these unrelated
GET requests.</t>
</list></t>
</section>
</section>
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<section title="Server-side Requirements" anchor="server">
<section title="Request" anchor="server-request">
<t>A GET request that includes &anOPTION1; Option requests the server
not only to return a current representation of the resource
identified by the request URI, but also to add the client to the list
of observers of the target resource. If no error occurs, the server
MUST return a 2.05 (Content) response with the representation of the
current resource state and MUST notify the client of subsequent
changes to the state as long as the client is on the list of
observers.</t>
<t>A server that is unable or unwilling to add the client to the list
of observers of the target resource MAY silently ignore the &OPTION1;
Option 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 client is already on the list of observers, the server MUST
NOT add it a second time but MUST replace or update the existing
entry. If the server receives a GET request for the a resource that
does not include &anOPTION1; Option, the server MUST remove any
existing entry from the list of observers.</t>
<t>Two requests relate to the same list entry if and only if both the
request URI and the source endpoint of the requests are the same.
Message IDs, tokens and other options are not taken into account.</t>
<t>Any request with an unrecognized critical option or a method other
than GET MUST NOT have a direct effect on a list of observers of a
resource. However, a non-GET request can have the indirect
consequence of causing the server to send a non-2.xx notification
which does affect the list of observers (for example, when a DELETE
request is successful and the observed resource no longer
exists).</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 include
&anOPTION1; Option and MUST echo the token specified by the client in
the GET request. If there are multiple clients on the list of
observers, the order in which they are notified is not defined; the
server is free to use any method to determine the order.</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 a
matching response code and MUST remove the client from the list of
observers of the resource.</t>
<t>The Content-Format used in a notification MUST be the same as the
one used in the initial response to the GET request. If the server is
unable to continue sending notifications using this Content-Format,
it SHOULD send a notification with a 5.00 (Internal Server Error)
response code and MUST remove the client from the list of observers
of the resource.</t>
</section>
<section title="Caching" anchor="server-caching">
<t>As notifications are just additional responses sent by the server,
they are subject to caching as defined in <xref
target="I-D.ietf-core-coap">Section 5.6 of RFC XXXX</xref>.</t>
<section title="Freshness" anchor="server-freshness">
<t>After returning the initial response, the server MUST try to keep
the returned representation current, i.e., keep the resource state
observed by the client as closely in sync with the actual resource
state as possible.</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, it cannot
tell if the observed state and the actual state are still in sync.
So, when the the age of the latest notification becomes greater
than its indicated Max-Age, then the client will assume that the
states are inconsistent until the representation is validated or a
new notification is received. The server MAY wish to prevent that
by sending a notification with the unchanged representation before
Max-Age expires.</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. The
server MUST NOT assume that the client has any response stored
other than those identified by the entity-tags in the most recent
GET request received for the resource.</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 2^24 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 for the
same resource with the same token. The value of the &OPTION1; Option
MUST be current at the time of transmission; if a notification is
retransmitted, the server MUST update value of the &OPTION1; Option
before sending the message.</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)/(2^24) = 15.26 microseconds. It is
not necessary that the clock reflects the current time/date or
that it ticks in a precisely periodical way.</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 2^24 times in the next 256
seconds after any state change). This alleviates the need to
update the value of the &OPTION1; Option in a message 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 allows for a notification rate of up
to 65536 notifications per second. 64K ought to be enough for
anybody.</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.
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>The acknowledgement of a confirmable notification signals to the
server that the client is interested in receiving further
notifications. If a client rejects a confirmable notification with a
Reset message, the client is no longer interested and the server MUST
remove the client from the list of observers. If the client rejects a
non-confirmable notification, the server MAY remove the client from
the list of observers as well. (It is expected that the server does
remove the client if it has the information available that is needed
to match the Reset message to the non-confirmable notification, but
the server is not required to keep this information.)</t>
<t>At a minimum, the server MUST send a notification in a confirmable
message instead of a non-confirmable message at least every
24 hours.</t>
<t>A server MAY choose to skip a notification if it knows that it will
send another notification soon (e.g., when the state is changing
frequently). Similarly, it MAY choose to send a notification more
than once. For example, when state changes occur in bursts, the
server can skip some notifications, send the notifications in
non-confirmable messages, and make sure that the client observes the
latest state change after the burst by repeating the last
notification in a confirmable message.</t>
<t>The server MUST limit the number of confirmable notifications for
which an acknowledgement has not been received yet to NSTART (see
<xref target="I-D.ietf-core-coap">Section 4.7 of RFC XXXX</xref>),
and it SHOULD NOT send more than one non-confirmable notification
every 3 seconds on average.</t>
<t>When the state of an observed resource changes while the server is
still waiting for a confirmable notification to be acknowledged or
the 3 seconds for a non-confirmable notification to elapse, then the
server MUST proceed as follows:<list style="numbers">
<t>Wait for the current transmission attempt to complete.</t>
<t>If the result is a Reset message or the transmission was the
last attempt to deliver a notification, remove the client from
the list of observers of the observed resource.</t>
<t>If the client is still in the list of observers, transmit a
notification with a representation of the current resource state.
Should the resource have changed its state more than once in the
meantime, skip the notifications for the intermediate states.</t>
<t>If the previously completed transmission timed out, increment
the retransmission counter and double the timeout; otherwise,
reinitialize the retransmission counter and the timeout.</t>
</list></t>
<t>If CoAP is used over a connection-oriented or session-based
transport such as DTLS, the server MUST remove the client from the
list of observers when the connection or session is closed.</t>
</section>
</section>
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<section title="Intermediaries" anchor="intermediary">
<t>A client may be interested in a resource in the namespace of an origin
server that is reached through a chain of one or more CoAP-to-CoAP
intermediaries. In this case, the client registers its interest with
the first intermediary towards the origin server, acting as if it was
communicating with the origin server itself as specified in <xref
target="client"/>. It is the task of this intermediary to provide the
client with a current representation of the target resource and send
notifications upon changes to the target resource state, much like an
origin server as specified in <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 origin server. If the next hop does not return a response
with &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, and MUST set the value of the
Max-Age Option according to the age of the local current
representation.</t>
<t>Because a client (or an intermediary in the client role) can only be
once on the list of observers of a resource on a server (or an
intermediary in the server role) — it is useless to observe
the same resource multiple times — an intermediary MUST observe a
resource only once, even if there are multiple clients for which it
observes the resource.</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-proxying"/> for examples.</t>
</section>
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<section title="Blockwise Transfers" anchor="block">
<t>Resources observed by clients may be larger than can be comfortably
processed or transferred in one CoAP message. CoAP provides a
blockwise transfer mechanism to address this problem <xref
target="I-D.ietf-core-block"/>. The following rules apply to the
combination of blockwise transfers with notifications.</t>
<t>As with basic GET transfers, the client can indicate its desired block
size in a Block2 Option in the GET request. If the server supports
blockwise transfers, it SHOULD take note of the block size for all
notifications/responses resulting from the GET request (until the
client is removed from the list of observers or the server receives a
new GET request for the resource from the client).</t>
<t>When sending a 2.05 (Content) notification, the server always sends
all blocks of the representation, suitably sequenced by its congestion
control mechanism, even if only some of the blocks have changed with
respect to a previous notification. The server performs the blockwise
transfer by making use of the Block2 Option in each block. When
reassembling representations that are transmitted in multiple blocks,
the client MUST NOT combine blocks carrying different &OPTION1; Option
values.</t>
<t>Blockwise transfers of notifications MUST use confirmable messages
and MUST NOT use non-confirmable messages.</t>
<t>See <xref target="examples-block"/> for an example.</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 of <xref target="I-D.ietf-core-coap">RFC
XXXX</xref> apply.</t>
<t>The considerations about amplification attacks are somewhat amplified
when observing resources. Without client authentication, a server MUST
therefore strictly limit the number of notifications that it sends
between receiving acknowledgements that confirm the actual interest of
the client in the data; i.e., any notifications sent in non-confirmable
messages MUST be interspersed with confirmable messages. (An attacker
may still spoof the acknowledgements if the confirmable messages are
sufficiently predictable.)</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 access-control
this creation of state. As degraded behavior, the server can always
fall back to processing the request as a normal GET request (without
&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>
</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>&SELF;</c>
</texttable>
</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 Bigot, Angelo P.
Castellani, Gilbert Clark, Esko Dijk, Thomas Fossati, Brian Frank,
Jeroen Hoebeke, Cullen Jennings, Matthias Kovatsch, Salvatore Loreto,
Charles Palmer, Zach Shelby and Floris Van den Abeele for helpful
comments and discussions that have shaped the document.</t>
</section>
</middle>
<back>
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<references title="Normative References">
&RFC1982;
&RFC2119;
&RFC5988;
&I-D.ietf-core-coap;
&I-D.ietf-core-block;
</references>
<references title="Informative References">
&RFC2616;
&RFC5989;
&RFC6202;
&RFC6690;
<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>
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<section title="Examples" anchor="examples">
<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 C
3 +----->| Header: GET 0x41011633
4 | GET | Token: 0x4a
5 | | Uri-Path: temperature
6 | | Observe: (empty)
7 | |
8 | |
9 ____________ |<-----+ Header: 2.05 0x61451633
10 | 2.05 | Token: 0x4a
11 18.5 C | | Observe: 9
12 | | Max-Age: 15
13 | | Payload: "18.5 C"
14 | |
15 | | ____________
16 ____________ |<-----+ Header: 2.05 0x51457b50
17 | 2.05 | 19.2 C Token: 0x4a
18 19.2 C | | Observe: 16
29 | | Max-Age: 15
20 | | Payload: "19.2 C"
21 | |
]]></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 C | | 19.2 C
24 | | ____________
25 | X----+ Header: 2.05 0x51457b51
26 | 2.05 | 19.7 C Token: 0x4a
27 | | Observe: 25
28 | | Max-Age: 15
29 | | Payload: "19.7 C"
30 | |
31 ____________ | |
32 | |
33 19.2 C | |
34 (stale) | |
35 | |
36 | |
37 | |
38 +----->| Header: GET 0x41011634
39 | GET | Token: 0xb2
40 | | Uri-Path: temperature
41 | | Observe: (empty)
42 | |
43 | |
44 ____________ |<-----+ Header: 2.05 0x61451634
45 | 2.05 | Token: 0xb2
46 19.7 C | | Observe: 44
47 | | Max-Age: 15
48 | | ETag: 0x78797a7a79
49 | | Payload: "19.7 C"
50 | |
]]></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 C | | 19.7 C
53 | |
54 | | ____________
55 | crash
56 |
57 |
58 |
59 ____________ |
60 |
61 19.7 C |
62 (stale) |
63 | reboot____________
64 | |
65 | | 20.0 C
66 | |
67 +----->| Header: GET 0x41011635
68 | GET | Token: 0xf9
69 | | Uri-Path: temperature
70 | | Observe: (empty)
71 | | ETag: 0x78797a7a79
72 | |
73 | |
74 ____________ |<-----+ Header: 2.05 0x61451635
75 | 2.05 | Token: 0xf9
76 20.0 C | | Observe: 74
77 | | Max-Age: 15
78 | | Payload: "20.0 C"
79 | |
80 | | ____________
81 ____________ |<-----+ Header: 2.03 0x5143aa0c
82 | 2.03 | 19.7 C Token: 0xf9
83 19.7 C | | Observe: 81
84 | | ETag: 0x78797a7a79
85 | | Max-Age: 15
86 | |
]]></artwork>
</figure>
<figure anchor="example-4" title="The client makes a normal GET request and thereby cancels the observation">
<artwork type="example"><![CDATA[
Observed CLIENT SERVER Actual
t State | | State
____________ | | ____________
87 | |
88 19.7 C | | 19.7 C
89 | |
90 | | ____________
91 ____________ |<-----+ Header: 2.05 0x5145aa0f
92 | 2.05 | 19.3 C Token: 0xf9
93 19.3 C | | Observe: 91
94 | | Max-Age: 15
95 | | Payload: "19.3 C"
96 | |
97 | |
98 +----->| Header: GET 0x41011636
99 | GET | Token: 0x68
100 | | Uri-Path: temperature
101 | | ETag: 0x78797a7a79
102 | |
103 | |
104 |<-----+ Header: 2.05 0x61451636
105 | 2.05 | Token: 0x68
106 | | ETag: 0x78797a7a79
107 | | Max-Age: 15
108 | | Payload: "19.3 C"
109 | |
]]></artwork>
</figure>
<section title="Proxying" anchor="examples-proxying">
<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: (empty)
| | |
| |<-----+ Header: 2.05 0x61455fb8
| | 2.05 | Token: 0x1a
| | | Observe: 42
| | | Max-Age: 60
| | | Payload: "ready"
| | |
+----->| | Header: GET 0x41011633
| GET | | Token: 0x9a
| | | Proxy-Uri: coap://sensor.example/status
| | |
|<-----+ | Header: 2.05 0x61451633
| 2.05 | | Token: 0x9a
| | | Max-Age: 53
| | | Payload: "ready"
| | |
| |<-----+ Header: 2.05 0x514505fc0
| | 2.05 | Token: 0x1a
| | | Observe: 135
| | | Max-Age: 60
| | | Payload: "busy"
| | |
+----->| | Header: GET 0x41011634
| GET | | Token: 0x9b
| | | Proxy-Uri: coap://sensor.example/status
| | |
|<-----+ | Header: 2.05 0x61451634
| 2.05 | | Token: 0x9b
| | | Max-Age: 49
| | | Payload: "busy"
| | |
]]></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: (empty)
| | |
|<- - -+ | Header: 0x60001635
| | |
| +----->| Header: GET 0x4101af90
| | GET | Token: 0xaa
| | | Uri-Host: sensor.example
| | | Uri-Path: status
| | | Observe: (empty)
| | |
| |<-----+ Header: 2.05 0x6145af90
| | 2.05 | Token: 0xaa
| | | Observe: 67
| | | Max-Age: 60
| | | Payload: "ready"
| | |
|<-----+ | Header: 2.05 0x4145af94
| 2.05 | | Token: 0x6a
| | | Observe: 17346
| | | Max-Age: 60
| | | Payload: "ready"
| | |
+- - ->| | Header: 0x6000af94
| | |
| |<-----+ Header: 2.05 0x51455a20
| | 2.05 | Token: 0xaa
| | | Observe: 157
| | | Max-Age: 60
| | | Payload: "busy"
| | |
|<-----+ | Header: 2.05 0x5145af9b
| 2.05 | | Token: 0x6a
| | | Observe: 17436
| | | Max-Age: 60
| | | Payload: "busy"
| | |
]]></artwork>
</figure>
</section>
<section title="Blockwise Transfer" anchor="examples-block">
<figure anchor="example-7" title="A server sends two notifications of two blocks each">
<artwork type="example"><![CDATA[
CLIENT SERVER
| |
+----->| Header: GET 0x41011636
| GET | Token: 0xfb
| | Uri-Path: status-icon
| | Observe: (empty)
| |
|<-----+ Header: 2.05 0x61451636
| 2.05 | Token: 0xfb
| | Block2: 0/1/128
| | Observe: 62354
| | Max-Age: 60
| | Payload: [128 bytes]
| |
|<-----+ Header: 2.05 0x4145af9c
| 2.05 | Token: 0xfb
| | Block2: 1/0/128
| | Observe: 62354
| | Max-Age: 60
| | Payload: [27 bytes]
| |
+- - ->| Header: 0x6000af9c
| |
|<-----+ Header: 2.05 0x4145af9d
| 2.05 | Token: 0xfb
| | Block2: 0/1/128
| | Observe: 62444
| | Max-Age: 60
| | Payload: [128 bytes]
| |
+- - ->| Header: 60005af9d
| |
|<-----+ Header: 2.05 0x4145af9e
| 2.05 | Token: 0xfb
| | Block2: 1/0/128
| | Observe: 62444
| | Max-Age: 60
| | Payload: [27 bytes]
| |
+- - ->| Header: 0x6000af9e
| |
]]></artwork>
</figure>
</section>
</section>
<section title="Modeling Resources to Tailor Notifications" anchor="modeling-resources">
<t>A server may want to provide notifications that respond to very
specific conditions on some state. This is best done by modeling the
resources that the server exposes according to these needs.</t>
<t>For example, for a CoAP server with an attached temperature
sensor,<list style="symbols">
<t>the server could, in the simplest form, expose a resource
<coap://server/temperature> that changes its state every
second to the current temperature measured by the sensor;</t>
<t>the server could, however, also expose a resource
<coap://server/temperature/felt> that changes its state to
"cold" when it's warm and the temperature drops below a
preconfigured threshold, and to "warm" when it's cold and the
temperature exceeds a second, higher threshold;</t>
<t>the server could expose a parameterized resource
<coap://server/temperature/critical?above=45> that changes
its state every second to the current temperature if the sensor
reading exceeds the specified parameter value, and that changes its
state to "OK" when the temperature drops below; or</t>
<t>the server could expose a parameterized
resource<vspace/><coap://server/temperature?query=select+avg(temperature)+from+<vspace/>Sensor.window:time(30sec)>
that accepts expressions of arbitrary complexity and changes its
state accordingly.</t>
</list></t>
<t>In any case, the client is notified about the current state of the
resource whenever the state of the appropriately modeled resource
changes. By designing resources that change their state on certain
conditions, it is possible to notify the client only when these
conditions occur instead of continuously supplying it with information
it doesn't need.</t>
<t>By parametrizing resources, this is not limited to conditions defined
by the server, but can be extended to arbitrarily complex conditions
defined by the client. Thus, the server designer can choose exactly the
right level of complexity for the application envisioned and devices
used, and is not constrained to a "one size fits all" mechanism built
into the protocol.</t>
</section>
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<section title="Changelog" anchor="changelog">
<t>Changes from ietf-07 to ietf-08:
<list style="symbols">
<t>Expanded text on transmitting 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 (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 <xref
target="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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