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Internet Engineering Task Force (IETF) K. Hartke Request for Comments: 7641 Universitaet Bremen TZI Category: Standards Track September 2015 ISSN: 2070-1721

 Observing Resources in the Constrained Application Protocol (CoAP)

Abstract

 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.

Status of This Memo

 This is an Internet Standards Track document.
 This document is a product of the Internet Engineering Task Force
 (IETF).  It represents the consensus of the IETF community.  It has
 received public review and has been approved for publication by the
 Internet Engineering Steering Group (IESG).  Further information on
 Internet Standards is available in Section 2 of RFC 5741.
 Information about the current status of this document, any errata,
 and how to provide feedback on it may be obtained at
 http://www.rfc-editor.org/info/rfc7641.

Hartke Standards Track [Page 1] RFC 7641 Observing Resources in CoAP September 2015

Copyright Notice

 Copyright (c) 2015 IETF Trust and the persons identified as the
 document authors.  All rights reserved.
 This document is subject to BCP 78 and the IETF Trust's Legal
 Provisions Relating to IETF Documents
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document.  Please review these documents
 carefully, as they describe your rights and restrictions with respect
 to this document.  Code Components extracted from this document must
 include Simplified BSD License text as described in Section 4.e of
 the Trust Legal Provisions and are provided without warranty as
 described in the Simplified BSD License.

Hartke Standards Track [Page 2] RFC 7641 Observing Resources in CoAP September 2015

Table of Contents

 1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .  4
   1.1.  Background  . . . . . . . . . . . . . . . . . . . . . . .  4
   1.2.  Protocol Overview . . . . . . . . . . . . . . . . . . . .  4
   1.3.  Consistency Model . . . . . . . . . . . . . . . . . . . .  6
   1.4.  Observable Resources  . . . . . . . . . . . . . . . . . .  7
   1.5.  Requirements Notation . . . . . . . . . . . . . . . . . .  8
 2.  The Observe Option  . . . . . . . . . . . . . . . . . . . . .  9
 3.  Client-Side Requirements  . . . . . . . . . . . . . . . . . . 10
   3.1.  Request . . . . . . . . . . . . . . . . . . . . . . . . . 10
   3.2.  Notifications . . . . . . . . . . . . . . . . . . . . . . 10
   3.3.  Caching . . . . . . . . . . . . . . . . . . . . . . . . . 11
   3.4.  Reordering  . . . . . . . . . . . . . . . . . . . . . . . 12
   3.5.  Transmission  . . . . . . . . . . . . . . . . . . . . . . 13
   3.6.  Cancellation  . . . . . . . . . . . . . . . . . . . . . . 13
 4.  Server-Side Requirements  . . . . . . . . . . . . . . . . . . 14
   4.1.  Request . . . . . . . . . . . . . . . . . . . . . . . . . 14
   4.2.  Notifications . . . . . . . . . . . . . . . . . . . . . . 14
   4.3.  Caching . . . . . . . . . . . . . . . . . . . . . . . . . 15
   4.4.  Reordering  . . . . . . . . . . . . . . . . . . . . . . . 16
   4.5.  Transmission  . . . . . . . . . . . . . . . . . . . . . . 17
 5.  Intermediaries  . . . . . . . . . . . . . . . . . . . . . . . 20
 6.  Web Linking . . . . . . . . . . . . . . . . . . . . . . . . . 20
 7.  Security Considerations . . . . . . . . . . . . . . . . . . . 21
 8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . 22
 9.  References  . . . . . . . . . . . . . . . . . . . . . . . . . 22
   9.1.  Normative References  . . . . . . . . . . . . . . . . . . 22
   9.2.  Informative References  . . . . . . . . . . . . . . . . . 22
 Appendix A.  Examples . . . . . . . . . . . . . . . . . . . . . . 24
   A.1.  Client/Server Examples  . . . . . . . . . . . . . . . . . 24
   A.2.  Proxy Examples  . . . . . . . . . . . . . . . . . . . . . 28
 Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . . 30
 Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . 30

Hartke Standards Track [Page 3] RFC 7641 Observing Resources in CoAP September 2015

1. Introduction

1.1. Background

 The Constrained Application Protocol (CoAP) [RFC7252] is intended to
 provide RESTful services [REST] not unlike HTTP [RFC7230] while
 reducing the complexity of implementation as well as the size of
 packets exchanged in order to make these services useful in a highly
 constrained network of themselves highly constrained nodes [RFC7228].
 The model of REST is that of a client exchanging representations of
 resources with a server, where a representation captures the current
 or intended state of a resource.  The server is the authority for
 representations of the resources in its namespace.  A client
 interested in the state of a resource initiates a request to the
 server; the server then returns a response with a representation of
 the resource that is current at the time of the request.
 This model does not work well when a client is interested in having a
 current representation of a resource over a period of time.  Existing
 approaches from HTTP, such as repeated polling or HTTP long polling
 [RFC6202], generate significant complexity and/or overhead and thus
 are less applicable in a constrained environment.
 The protocol specified in this document extends the CoAP core
 protocol with a mechanism for a CoAP client to "observe" a resource
 on a CoAP server: the client retrieves a representation of the
 resource and requests this representation be updated by the server
 as long as the client is interested in the resource.
 The protocol keeps the architectural properties of REST.  It enables
 high scalability and efficiency through the support of caches and
 proxies.  There is no intention, though, to solve the full set of
 problems that the existing HTTP solutions solve or to replace
 publish/subscribe networks that solve a much more general problem
 [RFC5989].

1.2. Protocol Overview

 The protocol is based on the well-known observer design pattern
 [GOF].  In this design pattern, components called "observers"
 register at a specific, known provider called the "subject" that they
 are interested in being notified whenever the subject undergoes a
 change in state.  The subject is responsible for administering its
 list of registered observers.  If multiple subjects are of interest
 to an observer, the observer must register separately for all of
 them.

Hartke Standards Track [Page 4] RFC 7641 Observing Resources in CoAP September 2015

                     Observer             Subject
                        |                    |
                        |    Registration    |
                        +------------------->|
                        |                    |
                        |    Notification    |
                        |<-------------------+
                        |                    |
                        |    Notification    |
                        |<-------------------+
                        |                    |
                        |    Notification    |
                        |<-------------------+
                        |                    |
                 Figure 1: The Observer Design Pattern
 The observer design pattern is realized in CoAP as follows:
 Subject:  In the context of CoAP, the subject is a resource in the
    namespace of a CoAP server.  The state of the resource can change
    over time, ranging from infrequent updates to continuous state
    transformations.
 Observer:  An observer is a CoAP client that is interested in having
    a current representation of the resource at any given time.
 Registration:  A client registers its interest in a resource by
    initiating an extended GET request to the server.  In addition to
    returning a representation of the target resource, this request
    causes the server to add the client to the list of observers of
    the resource.
 Notification:  Whenever the state of a resource changes, the server
    notifies each client in the list of observers of the resource.
    Each notification is an additional CoAP response sent by the
    server in reply to the single extended GET request and includes a
    complete, updated representation of the new resource state.
 Figure 2 below shows an example of a CoAP client registering its
 interest in a resource and receiving three notifications: the first
 with the current state upon registration, and then two upon changes
 to the resource state.  Both the registration request and the
 notifications are identified as such by the presence of the Observe
 Option defined in this document.  In notifications, the Observe
 Option additionally provides a sequence number for reordering
 detection.  All notifications carry the token specified by the
 client, so the client can easily correlate them to the request.

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                     Client                Server
                        |                    |
                        |  GET /temperature  |
                        |    Token: 0x4a     |   Registration
                        |  Observe: 0        |
                        +------------------->|
                        |                    |
                        |    2.05 Content    |
                        |    Token: 0x4a     |   Notification of
                        |  Observe: 12       |   the current state
                        |  Payload: 22.9 Cel |
                        |<-------------------+
                        |                    |
                        |    2.05 Content    |
                        |    Token: 0x4a     |   Notification upon
                        |  Observe: 44       |   a state change
                        |  Payload: 22.8 Cel |
                        |<-------------------+
                        |                    |
                        |    2.05 Content    |
                        |    Token: 0x4a     |   Notification upon
                        |  Observe: 60       |   a state change
                        |  Payload: 23.1 Cel |
                        |<-------------------+
                        |                    |
                Figure 2: Observing a Resource in CoAP
 Note: In this document, "Cel" stands for "degrees Celsius".
 A client remains on the list of observers as long as the server can
 determine the client's continued interest in the resource.  The
 server may send a notification in a confirmable CoAP message to
 request an acknowledgement from the client.  When the client
 deregisters, rejects a notification, or the transmission of a
 notification times out after several transmission attempts, the
 client is considered no longer interested in the resource and is
 removed by the server from the list of observers.

1.3. Consistency Model

 While a client is in the list of observers of a resource, the goal of
 the protocol is to keep the resource state observed by the client as
 closely in sync with the actual state at the server as possible.
 It cannot be avoided that the client and the server become out of
 sync at times: First, there is always some latency between the change
 of the resource state and the receipt of the notification.  Second,

Hartke Standards Track [Page 6] RFC 7641 Observing Resources in CoAP September 2015

 CoAP messages with notifications can get lost, which will cause the
 client to assume an old state until it receives a new notification.
 And third, the server may erroneously come to the conclusion that the
 client is no longer interested in the resource, which will cause the
 server to stop sending notifications and the client to assume an old
 state until it eventually registers its interest again.
 The protocol addresses this issue as follows:
 o  It follows a best-effort approach for sending the current
    representation to the client after a state change: clients should
    see the new state after a state change as soon as possible, and
    they should see as many states as possible.  This is limited by
    congestion control, however, so a client cannot rely on observing
    every single state that a resource might go through.
 o  It labels notifications with a maximum duration up to which it is
    acceptable for the observed state and the actual state to be out
    of sync.  When the age of the notification received reaches this
    limit, the client cannot use the enclosed representation until it
    receives a new notification.
 o  It is designed on the principle of eventual consistency: the
    protocol guarantees that if the resource does not undergo a new
    change in state, eventually all registered observers will have a
    current representation of the latest resource state.

1.4. Observable Resources

 A CoAP server is the authority for determining under what conditions
 resources change their state and thus when observers are notified of
 new resource states.  The protocol does not offer explicit means for
 setting up triggers or thresholds; it is up to the server to expose
 observable resources that change their state in a way that is useful
 in the application context.
 For example, a CoAP server with an attached temperature sensor could
 expose one or more of the following resources:
 o  <coap://server/temperature>, which changes its state every few
    seconds to a current reading of the temperature sensor;
 o  <coap://server/temperature/felt>, which changes its state to
    "COLD" whenever the temperature reading drops below a certain pre-
    configured threshold and to "WARM" whenever the reading exceeds a
    second, slightly higher threshold;

Hartke Standards Track [Page 7] RFC 7641 Observing Resources in CoAP September 2015

 o  <coap://server/temperature/critical?above=42>, which changes its
    state based on the client-specified parameter value either every
    few seconds to the current temperature reading if the temperature
    exceeds the threshold or to "OK" when the reading drops below;
 o  <coap://server/?query=select+avg(temperature)+from+Sensor.window:
    time(30sec)>, which accepts expressions of arbitrary complexity
    and changes its state accordingly.
 Thus, by designing CoAP resources that change their state on certain
 conditions, it is possible to update the client only when these
 conditions occur instead of supplying it continuously with raw sensor
 data.  By parameterizing resources, this is not limited to conditions
 defined by the server, but can be extended to arbitrarily complex
 queries specified by the client.  The application designer therefore
 can choose exactly the right level of complexity for the application
 envisioned and devices involved and is not constrained to a "one size
 fits all" mechanism built into the protocol.

1.5. Requirements Notation

 The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
 "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
 document are to be interpreted as described in RFC 2119 [RFC2119].

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2. The Observe Option

 The Observe Option has the following properties.  Its meaning depends
 on whether it is included in a GET request or in a response.
     +-----+---+---+---+---+---------+--------+--------+---------+
     | No. | C | U | N | R | Name    | Format | Length | Default |
     +-----+---+---+---+---+---------+--------+--------+---------+
     |   6 |   | x | - |   | Observe | uint   | 0-3 B  | (none)  |
     +-----+---+---+---+---+---------+--------+--------+---------+
          C=Critical, U=Unsafe, N=No-Cache-Key, R=Repeatable
                      Table 1: The Observe Option
 When included in a GET request, the Observe Option extends the GET
 method so it does not only retrieve a current representation of the
 target resource, but also requests the server to add or remove an
 entry in the list of observers of the resource depending on the
 option value.  The list entry consists of the client endpoint and the
 token specified by the client in the request.  Possible values are:
    0 (register) adds the entry to the list, if not present;
    1 (deregister) removes the entry from the list, if present.
 The Observe Option is not critical for processing the request.  If
 the server is unwilling or unable to add a new entry to the list of
 observers, then the request falls back to a normal GET request and
 the response does not include the Observe Option.
 The Observe Option is not part of the Cache-Key: a cacheable response
 obtained with an Observe Option in the request can be used to satisfy
 a request without an Observe Option, and vice versa.  When a stored
 response with an Observe Option is used to satisfy a normal GET
 request, the option MUST be removed before the response is returned.
 When included in a response, the Observe Option identifies the
 message as a notification.  This implies that a matching entry exists
 in the list of observers and that the server will notify the client
 of changes to the resource state.  The option value is a sequence
 number for reordering detection (see Sections 3.4 and 4.4).
 The value of the Observe Option is encoded as an unsigned integer in
 network byte order using a variable number of bytes ('uint' option
 format); see Section 3.2 of RFC 7252 [RFC7252].

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3. Client-Side Requirements

3.1. Request

 A client registers its interest in a resource by issuing a GET
 request with an Observe Option set to 0 (register).  If the server
 returns a 2.xx response that includes an Observe Option as well, the
 server has successfully added an entry with the client endpoint and
 request token to the list of observers of the target resource, and
 the client will be notified of changes to the resource state.
 Like a fresh response can be used to satisfy a request without
 contacting the server, the stream of updates resulting from one
 observation request can be used to satisfy another (observation or
 normal GET) request if the target resource is the same.  A client
 MUST aggregate such requests and MUST NOT register more than once for
 the same target resource.  The target resource is identified by all
 options in the request that are part of the Cache-Key. This includes,
 for example, the full request URI and the Accept Option.

3.2. Notifications

 Notifications are additional responses sent by the server in reply to
 the single extended GET request that created the registration.  Each
 notification includes the token specified by the client in the
 request.  The only difference between a notification and a normal
 response is the presence of the Observe Option.
 Notifications typically have a 2.05 (Content) response code.  They
 include an Observe Option with a sequence number for reordering
 detection (see Section 3.4) and a payload in the same Content-Format
 as the initial response.  If the client included one or more ETag
 Options in the GET request (see Section 3.3), notifications can have
 a 2.03 (Valid) response code rather than a 2.05 (Content) response
 code.  Such notifications include an Observe Option with a sequence
 number but no payload.
 In the event that the resource changes in a way that would cause a
 normal GET request at that time to return a non-2.xx response (for
 example, when the resource is deleted), the server sends a
 notification with an appropriate response code (such as 4.04 Not
 Found) and removes the client's entry from the list of observers of
 the resource.  Non-2.xx responses do not include an Observe Option.

Hartke Standards Track [Page 10] RFC 7641 Observing Resources in CoAP September 2015

3.3. Caching

 As notifications are just additional responses to a GET request,
 notifications partake in caching as defined in Section 5.6 of RFC
 7252 [RFC7252].  Both the freshness model and the validation model
 are supported.

3.3.1. Freshness

 A client MAY store a notification like a response in its cache and
 use a stored notification that is fresh without contacting the
 server.  Like a response, a notification is considered fresh while
 its age is not greater than the value indicated by the Max-Age Option
 (and no newer notification/response has been received).
 The server will do its best to keep the resource state observed by
 the client as closely in sync with the actual state as possible.
 However, a client cannot rely on observing every single state that a
 resource might go through.  For example, if the network is congested
 or the state changes more frequently than the network can handle, the
 server can skip notifications for any number of intermediate states.
 The server uses the Max-Age Option to indicate an age up to which it
 is acceptable that the observed state and the actual state are
 inconsistent.  If the age of the latest notification becomes greater
 than its indicated Max-Age, then the client MUST NOT assume that the
 enclosed representation reflects the actual resource state.
 To make sure it has a current representation and/or to re-register
 its interest in a resource, a client MAY issue a new GET request with
 the same token as the original at any time.  All options MUST be
 identical to those in the original request except for the set of ETag
 Options.  It is RECOMMENDED that the client does not issue the
 request while it still has a fresh notification/response for the
 resource in its cache.  Additionally, the client SHOULD at least wait
 for a random amount of time between 5 and 15 seconds after Max-Age
 expired to reduce collisions with other clients.

3.3.2. Validation

 When a client has one or more notifications stored in its cache for a
 resource, it can use the ETag Option in the GET request to give the
 server an opportunity to select a stored notification to be used.
 The client MAY include an ETag Option for each stored response that
 is applicable in the GET request.  Whenever the observed resource
 changes to a representation identified by one of the ETag Options,
 the server can select a stored response by sending a 2.03 (Valid)

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 notification with an appropriate ETag Option instead of a 2.05
 (Content) notification.
 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.

3.4. Reordering

 Messages with notifications can arrive in a different order than they
 were sent.  Since the goal is to keep the observed state as closely
 in sync with the actual state as possible, a client MUST consider the
 notification that was sent most recently as the freshest, regardless
 of the order of arrival.
 To provide an order among notifications for the client, the server
 sets the value of the Observe Option in each notification to the 24
 least significant bits of a strictly increasing sequence number.  An
 incoming notification was sent more recently than the freshest
 notification so far when one of the following conditions is met:
                    (V1 < V2 and V2 - V1 < 2^23) or
                    (V1 > V2 and V1 - V2 > 2^23) or
                    (T2 > T1 + 128 seconds)
 where V1 is the value of the Observe Option in the freshest
 notification so far, V2 is the value of the Observe Option in the
 incoming notification, T1 is a client-local timestamp for the
 freshest notification so far, and T2 is a client-local timestamp for
 the incoming notification.
 Design Note:  The first two conditions verify that V1 is less than V2
    in 24-bit serial number arithmetic [RFC1982].  The third condition
    ensures that if the server is generating serial numbers based on a
    local clock, the time elapsed between the two incoming messages is
    not so large that the difference between V1 and V2 has become
    larger than the largest integer that it is meaningful to add to a
    24-bit serial number; in other words, after 128 seconds have
    elapsed without any notification, a client does not need to check
    the sequence numbers to assume that an incoming notification was
    sent more recently than the freshest notification it has received
    so far.
    The duration of 128 seconds was chosen as a nice round number
    greater than MAX_LATENCY (Section 4.8.2 of RFC 7252 [RFC7252]).

Hartke Standards Track [Page 12] RFC 7641 Observing Resources in CoAP September 2015

3.5. Transmission

 A notification can be confirmable or non-confirmable, i.e., it can be
 sent in a confirmable or a non-confirmable message.  The message type
 used for a notification is independent of the type used for the
 request and of any previous notification.
 If a client does not recognize the token in a confirmable
 notification, it MUST NOT acknowledge the message and SHOULD reject
 it with a Reset message; otherwise, the client MUST acknowledge the
 message as usual.  In the case of a non-confirmable notification,
 rejecting the message with a Reset message is OPTIONAL.
 An acknowledgement message signals to the server that the client is
 alive and interested in receiving further notifications; if the
 server does not receive an acknowledgement in reply to a confirmable
 notification, it will assume that the client is no longer interested
 and will eventually remove the associated entry from the list of
 observers (Section 4.5).

3.6. Cancellation

 A client that is no longer interested in receiving notifications for
 a resource can simply "forget" the observation.  When the server then
 sends the next notification, the client will not recognize the token
 in the message and thus will return a Reset message.  This causes the
 server to remove the associated entry from the list of observers.
 The entries in lists of observers are effectively "garbage collected"
 by the server.
 Implementation Note:  Due to potential message loss, the Reset
    message may not reach the server.  The client may therefore have
    to reject multiple notifications, each with one Reset message,
    until the server finally removes the associated entry from the
    list of observers and stops sending notifications.
 In some circumstances, it may be desirable to cancel an observation
 and release the resources allocated by the server to it more eagerly.
 In this case, a client MAY explicitly deregister by issuing a GET
 request that has the Token field set to the token of the observation
 to be cancelled and includes an Observe Option with the value set to
 1 (deregister).  All other options MUST be identical to those in the
 registration request except for the set of ETag Options.  When the
 server receives such a request, it will remove any matching entry
 from the list of observers and process the GET request as usual.

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4. Server-Side Requirements

4.1. Request

 A GET request with an Observe Option set to 0 (register) requests the
 server not only to return a current representation of the target
 resource, but also to add the client to the list of observers of that
 resource.  Upon success, the server returns a current representation
 of the resource and MUST keep this representation updated (as
 described in Section 1.3) as long as the client is on the list of
 observers.
 The entry in the list of observers is keyed by the client endpoint
 and the token specified by the client in the request.  If an entry
 with a matching endpoint/token pair is already present in the list
 (which, for example, happens when the client wishes to reinforce its
 interest in a resource), the server MUST NOT add a new entry but MUST
 replace or update the existing one.
 A server that is unable or unwilling to add a new entry to the list
 of observers of a resource MAY silently ignore the registration
 request and process the GET request as usual.  The resulting response
 MUST NOT include an Observe Option, the absence of which signals to
 the client that it will not be notified of changes to the resource
 and, e.g., needs to poll the resource for its state instead.
 If the Observe Option in a GET request is set to 1 (deregister), then
 the server MUST remove any existing entry with a matching endpoint/
 token pair from the list of observers and process the GET request as
 usual.  The resulting response MUST NOT include an Observe Option.

4.2. Notifications

 A client is notified of changes to the resource state by additional
 responses sent by the server in reply to the GET request.  Each such
 notification response (including the initial response) MUST echo the
 token specified by the client in the GET request.  If there are
 multiple entries in the list of observers, the order in which the
 clients are notified is not defined; the server is free to use any
 method to determine the order.
 A notification SHOULD have a 2.05 (Content) or 2.03 (Valid) response
 code.  However, in the event that the state of a resource changes in
 a way that would cause a normal GET request at that time to return a
 non-2.xx response (for example, when the resource is deleted), the
 server SHOULD notify the client by sending a notification with an

Hartke Standards Track [Page 14] RFC 7641 Observing Resources in CoAP September 2015

 appropriate response code (such as 4.04 Not Found) and subsequently
 MUST remove the associated entry from the list of observers of the
 resource.
 The Content-Format specified in a 2.xx notification MUST be the same
 as the one used in the initial response to the GET request.  If the
 server is unable to continue sending notifications in this format, it
 SHOULD send a notification with a 4.06 (Not Acceptable) response code
 and subsequently MUST remove the associated entry from the list of
 observers of the resource.
 A 2.xx notification MUST include an Observe Option with a sequence
 number as specified in Section 4.4 below; a non-2.xx notification
 MUST NOT include an Observe Option.

4.3. Caching

 As notifications are just additional responses sent by the server in
 reply to a GET request, they are subject to caching as defined in
 Section 5.6 of RFC 7252 [RFC7252].

4.3.1. Freshness

 After returning the initial response, the server MUST keep the
 resource state that is observed by the client as closely in sync with
 the actual resource state as possible.
 Since becoming out of sync at times cannot be avoided, the server
 MUST indicate for each representation an age up to which it is
 acceptable that the observed state and the actual state are
 inconsistent.  This age is application dependent and MUST be
 specified in notifications using the Max-Age Option.
 When the resource does not change and the client has a current
 representation, the server does not need to send a notification.
 However, if the client does not receive a notification, the client
 cannot tell if the observed state and the actual state are still in
 sync.  Thus, when the age of the latest notification becomes greater
 than its indicated Max-Age, the client no longer has a usable
 representation of the resource state.  The server MAY wish to prevent
 that by sending a new notification with the unchanged representation
 and a new Max-Age just before the Max-Age indicated earlier expires.

Hartke Standards Track [Page 15] RFC 7641 Observing Resources in CoAP September 2015

4.3.2. Validation

 A client can include a set of entity tags in its request using the
 ETag Option.  When an observed resource changes its state and the
 origin server is about to send a 2.05 (Content) notification, then
 whenever that notification has an entity tag in the set of entity
 tags specified by the client, the server MAY send a 2.03 (Valid)
 response with an appropriate ETag Option instead.

4.4. Reordering

 Because messages can get reordered, the client needs a way to
 determine if a notification arrived later than a newer notification.
 For this purpose, the server MUST set the value of the Observe Option
 of each notification it sends to the 24 least significant bits of a
 strictly increasing sequence number.  The sequence number MAY start
 at any value and MUST NOT increase so fast that it increases by more
 than 2^23 within less than 256 seconds.
 The sequence number selected for a notification MUST be greater than
 that of any preceding notification sent to the same client with the
 same token for the same resource.  The value of the Observe Option
 MUST be current at the time of transmission; if a notification is
 retransmitted, the server MUST update the value of the option to the
 sequence number that is current at that time before retransmission.
 Implementation Note:  A simple implementation that satisfies the
    requirements is to obtain a timestamp from a local clock.  The
    sequence number then is the timestamp in ticks, where 1 tick =
    (256 seconds)/(2^23) = 30.52 microseconds.  It is not necessary
    that the clock reflects the current time/date.
    Another valid implementation is to store a 24-bit unsigned integer
    variable per resource and increment this variable each time the
    resource undergoes a change of state (provided that the resource
    changes its state less than 2^23 times in the first 256 seconds
    after every state change).  This removes the need to update the
    value of the Observe Option on retransmission when the resource
    state did not change.
 Design Note:  The choice of a 24-bit option value and a time span of
    256 seconds theoretically allows for a notification rate of up to
    65536 notifications per second.  Constrained nodes often have
    rather imprecise clocks, though, and inaccuracies of the client
    and server side may cancel out or add in effect.  Therefore, the
    maximum notification rate is reduced to 32768 notifications per
    second.  This is still well beyond the highest known design

Hartke Standards Track [Page 16] RFC 7641 Observing Resources in CoAP September 2015

    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%.

4.5. Transmission

 A notification can be sent in a confirmable or a non-confirmable
 message.  The message type used is typically application dependent
 and may be determined by the server for each notification
 individually.
 For example, for resources that change in a somewhat predictable or
 regular fashion, notifications can be sent in non-confirmable
 messages; for resources that change infrequently, notifications can
 be sent in confirmable messages.  The server can combine these two
 approaches depending on the frequency of state changes and the
 importance of individual notifications.
 A server MAY choose to skip sending a notification if it knows that
 it will send another notification soon, for example, when the state
 of a resource is changing frequently.  It also MAY choose to send
 more than one notification for the same resource state.  However,
 above all, the server MUST ensure that a client in the list of
 observers of a resource eventually observes the latest state if the
 resource does not undergo a new change in state.
 For example, when state changes occur in bursts, the server can skip
 some notifications, send the notifications in non-confirmable
 messages, and make sure that the client observes the latest state
 change by repeating the last notification in a confirmable message
 when the burst is over.
 The client's acknowledgement of a confirmable notification signals
 that the client is interested in receiving further notifications.  If
 a client rejects a confirmable or non-confirmable notification with a
 Reset message, or if the last attempt to retransmit a confirmable
 notification times out, then the client is considered no longer
 interested and the server MUST remove the associated entry from the
 list of observers.
 Implementation Note:  To properly process a Reset message that
    rejects a non-confirmable notification, a server needs to remember
    the message IDs of the non-confirmable notifications it sends.
    This may be challenging for a server with constrained resources.
    However, since Reset messages are transmitted unreliably, the
    client must be prepared in case the Reset messages are not
    received by the server.  Thus, a server can always pretend that a
    Reset message rejecting a non-confirmable notification was lost.

Hartke Standards Track [Page 17] RFC 7641 Observing Resources in CoAP September 2015

    If a server does this, it could accelerate cancellation by sending
    the following notifications to that client in confirmable
    messages.
 A server that transmits notifications mostly in non-confirmable
 messages MUST send a notification in a confirmable message instead of
 a non-confirmable message at least every 24 hours.  This prevents a
 client that went away or is no longer interested from remaining in
 the list of observers indefinitely.

4.5.1. Congestion Control

 Basic congestion control for CoAP is provided by the exponential
 back-off mechanism in Section 4.2 of RFC 7252 [RFC7252] and the
 limitations in Section 4.7 of RFC 7252 [RFC7252].  However, CoAP
 places the responsibility of congestion control for simple request/
 response interactions only on the clients: rate-limiting request
 transmission implicitly controls the transmission of the responses.
 When a single request yields a potentially infinite number of
 notifications, additional responsibility needs to be placed on the
 server.
 In order not to cause congestion, servers MUST strictly limit the
 number of simultaneous outstanding notifications/responses that they
 transmit to a given client to NSTART (1 by default; see Section 4.7
 of RFC 7252 [RFC7252]).  An outstanding notification/response is
 either a confirmable message for which an acknowledgement has not yet
 been received and whose last retransmission attempt has not yet timed
 out or a non-confirmable message for which the waiting time that
 results from the following rate-limiting rules has not yet elapsed.
 The server SHOULD NOT send more than one non-confirmable notification
 per round-trip time (RTT) to a client on average.  If the server
 cannot maintain an RTT estimate for a client, it SHOULD NOT send more
 than one non-confirmable notification every 3 seconds and SHOULD use
 an even less aggressive rate when possible (see also Section 3.1.2 of
 RFC 5405 [RFC5405]).
 Further congestion control optimizations and considerations are
 expected in the future with advanced CoAP congestion control
 mechanisms.

4.5.2. Advanced Transmission

 The state of an observed resource may change while the number of
 simultaneous outstanding notifications/responses to a client on the
 list of observers is greater than or equal to NSTART.  In this case,
 the server cannot notify the client of the new resource state

Hartke Standards Track [Page 18] RFC 7641 Observing Resources in CoAP September 2015

 immediately but has to wait for an outstanding notification/response
 to complete first.
 If there exists an outstanding notification/response that the server
 transmits to the client and that pertains to the changed resource,
 then it is desirable for the server to stop working towards getting
 the representation of the old resource state to the client and to
 start transmitting the current representation to the client instead,
 so the resource state observed by the client stays closer in sync
 with the actual state at the server.
 For this purpose, the server MAY optimize the transmission process by
 aborting the transmission of the old notification (but not before the
 current transmission attempt is completed) and starting a new
 transmission for the new notification (but with the retransmission
 timer and counter of the aborted transmission retained).
 In more detail, a server MAY supersede an outstanding transmission
 that pertains to an observation as follows:
 1.  Wait for the current (re)transmission attempt to be acknowledged,
     rejected, or to time out (confirmable transmission); or, wait for
     the waiting time to elapse or the transmission to be rejected
     (non-confirmable transmission).
 2.  If the transmission is rejected or it was the last attempt to
     retransmit a notification, remove the associated entry from the
     list of observers of the observed resource.
 3.  If the entry is still in the list of observers, start to transmit
     a new notification with a representation of the current resource
     state.  Should the resource have changed its state more than once
     in the meantime, the notifications for the intermediate states
     are silently skipped.
 4.  The new notification is transmitted with a new Message ID and the
     following transmission parameters: if the previous
     (re)transmission attempt timed out, retain its transmission
     parameters, increment the retransmission counter, and double the
     timeout; otherwise, initialize the transmission parameters as
     usual (see Section 4.2 of RFC 7252 [RFC7252]).
 It is possible that the server later receives an acknowledgement for
 a confirmable notification that it superseded this way.  Even though
 this does not signal consistency, it is valuable in that it signals
 the client's further interest in the resource.  The server therefore
 should avoid inadvertently removing the associated entry from the
 list of observers.

Hartke Standards Track [Page 19] RFC 7641 Observing Resources in CoAP September 2015

5. Intermediaries

 A client may be interested in a resource in the namespace of a server
 that is reached through a chain of one or more CoAP intermediaries.
 In this case, the client registers its interest with the first
 intermediary towards the server, acting as if it was communicating
 with the server itself, as specified in Section 3.  It is the task of
 this intermediary to provide the client with a current representation
 of the target resource and to keep the representation updated upon
 changes to the resource state, as specified in Section 4.
 To perform this task, the intermediary SHOULD make use of the
 protocol specified in this document, taking the role of the client
 and registering its own interest in the target resource with the next
 hop towards the server.  If the response returned by the next hop
 doesn't include an Observe Option, the intermediary MAY resort to
 polling the next hop or MAY itself return a response without an
 Observe Option.
 The communication between each pair of hops is independent; each hop
 in the server role MUST determine individually how many notifications
 to send, of which message type, and so on.  Each hop MUST generate
 its own values for the Observe Option in notifications and MUST set
 the value of the Max-Age Option according to the age of the local
 current representation.
 If two or more clients have registered their interest in a resource
 with an intermediary, the intermediary MUST register itself only once
 with the next hop and fan out the notifications it receives to all
 registered clients.  This relieves the next hop from sending the same
 notifications multiple times and thus enables scalability.
 An intermediary is not required to act on behalf of a client to
 observe a resource; an intermediary MAY observe a resource, for
 example, just to keep its own cache up to date.
 See Appendix A.2 for examples.

6. Web Linking

 A web link [RFC5988] to a resource accessible over CoAP (for example,
 in a link-format document [RFC6690]) MAY include the target attribute
 "obs".
 The "obs" attribute, when present, is a hint indicating that the
 destination of a link is useful for observation and thus, for
 example, should have a suitable graphical representation in a user
 interface.  Note that this is only a hint; it is not a promise that

Hartke Standards Track [Page 20] RFC 7641 Observing Resources in CoAP September 2015

 the Observe Option can actually be used to perform the observation.
 A client may need to resort to polling the resource if the Observe
 Option is not returned in the response to the GET request.
 A value MUST NOT be given for the "obs" attribute; any present value
 MUST be ignored by parsers.  The "obs" attribute MUST NOT appear more
 than once in a given link-value; occurrences after the first MUST be
 ignored by parsers.

7. Security Considerations

 The security considerations in Section 11 of [RFC7252], the CoAP
 specification, apply.
 Observing resources can dramatically increase the negative effects of
 amplification attacks.  That is, not only can notifications messages
 be much larger than the request message, but the nature of the
 protocol can cause a significant number of notifications to be
 generated.  Without client authentication, a server therefore MUST
 strictly limit the number of notifications that it sends between
 receiving acknowledgements that confirm the actual interest of the
 client in the data; i.e., any notifications sent in non-confirmable
 messages MUST be interspersed with confirmable messages.  Note that
 an attacker may still spoof the acknowledgements if the confirmable
 messages are sufficiently predictable.
 The protocol follows a best-effort approach for keeping the state
 observed by a client and the actual resource state at a server in
 sync.  This may have the client and the server become out of sync at
 times.  Depending on the sensitivity of the observed resource,
 operating on an old state might be a security threat.  The client
 therefore must be careful not to use a representation after its Max-
 Age expires, and the server must set the Max-Age Option to a sensible
 value.
 As with any protocol that creates state, attackers may attempt to
 exhaust the resources that the server has available for maintaining
 the list of observers for each resource.  Servers may want to apply
 access controls to this creation of state.  As degraded behavior, the
 server can always fall back to processing the request as a normal GET
 request (without an Observe Option) if it is unwilling or unable to
 add a client to the list of observers of a resource, including if
 system resources are exhausted or nearing exhaustion.
 Intermediaries must be careful to ensure that notifications cannot be
 employed to create a loop.  A simple way to break any loops is to
 employ caches for forwarding notifications in intermediaries.

Hartke Standards Track [Page 21] RFC 7641 Observing Resources in CoAP September 2015

 Resources can be observed over CoAP that is secured by Datagram
 Transport Layer Security (DTLS) 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 an Observe Option MUST be returned within the same epoch
 of the same connection as the request.

8. IANA Considerations

 The following entry has been added to the CoAP Option Numbers
 registry:
                   +--------+---------+-----------+
                   | Number | Name    | Reference |
                   +--------+---------+-----------+
                   |      6 | Observe | RFC 7641  |
                   +--------+---------+-----------+

9. References

9.1. Normative References

 [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
            Requirement Levels", BCP 14, RFC 2119,
            DOI 10.17487/RFC2119, March 1997,
            <http://www.rfc-editor.org/info/rfc2119>.
 [RFC5988]  Nottingham, M., "Web Linking", RFC 5988,
            DOI 10.17487/RFC5988, October 2010,
            <http://www.rfc-editor.org/info/rfc5988>.
 [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
            Application Protocol (CoAP)", RFC 7252,
            DOI 10.17487/RFC7252, June 2014,
            <http://www.rfc-editor.org/info/rfc7252>.

9.2. Informative References

 [GOF]      Gamma, E., Helm, R., Johnson, R., and J. Vlissides,
            "Design Patterns: Elements of Reusable Object-Oriented
            Software", Addison-Wesley Professional Computing Series,
            1994.
 [REST]     Fielding, R., "Architectural Styles and the Design of
            Network-based Software Architectures", Ph.D. Dissertation,
            University of California, Irvine, 2000,
            <http://www.ics.uci.edu/~fielding/pubs/dissertation/
            fielding_dissertation.pdf>.

Hartke Standards Track [Page 22] RFC 7641 Observing Resources in CoAP September 2015

 [RFC1982]  Elz, R. and R. Bush, "Serial Number Arithmetic", RFC 1982,
            DOI 10.17487/RFC1982, August 1996,
            <http://www.rfc-editor.org/info/rfc1982>.
 [RFC5405]  Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
            for Application Designers", BCP 145, RFC 5405,
            DOI 10.17487/RFC5405, November 2008,
            <http://www.rfc-editor.org/info/rfc5405>.
 [RFC5989]  Roach, A., "A SIP Event Package for Subscribing to Changes
            to an HTTP Resource", RFC 5989, DOI 10.17487/RFC5989,
            October 2010, <http://www.rfc-editor.org/info/rfc5989>.
 [RFC6202]  Loreto, S., Saint-Andre, P., Salsano, S., and G. Wilkins,
            "Known Issues and Best Practices for the Use of Long
            Polling and Streaming in Bidirectional HTTP", RFC 6202,
            DOI 10.17487/RFC6202, April 2011,
            <http://www.rfc-editor.org/info/rfc6202>.
 [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
            Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
            <http://www.rfc-editor.org/info/rfc6690>.
 [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
            Constrained-Node Networks", RFC 7228,
            DOI 10.17487/RFC7228, May 2014,
            <http://www.rfc-editor.org/info/rfc7228>.
 [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
            Protocol (HTTP/1.1): Message Syntax and Routing",
            RFC 7230, DOI 10.17487/RFC7230, June 2014,
            <http://www.rfc-editor.org/info/rfc7230>.

Hartke Standards Track [Page 23] RFC 7641 Observing Resources in CoAP September 2015

Appendix A. Examples

A.1. Client/Server Examples

       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                 |      |
   Figure 3: A Client Registers and Receives One Notification of the
       Current State and One of a New State upon a State Change

Hartke Standards Track [Page 24] RFC 7641 Observing Resources in CoAP September 2015

       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                 |      |
         Figure 4: The Client Re-registers after Max-Age Ends

Hartke Standards Track [Page 25] RFC 7641 Observing Resources in CoAP September 2015

       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                 |      |
      Figure 5: The Client Re-registers and Gives the Server the
                Opportunity to Select a Stored Response

Hartke Standards Track [Page 26] RFC 7641 Observing Resources in CoAP September 2015

       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                 |      |
  Figure 6: The Client Rejects a Notification and Thereby Cancels the
                              Observation

Hartke Standards Track [Page 27] RFC 7641 Observing Resources in CoAP September 2015

A.2. Proxy Examples

 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"
    |      |      |
  Figure 7: A Proxy Observes a Resource to Keep its Cache Up to Date

Hartke Standards Track [Page 28] RFC 7641 Observing Resources in CoAP September 2015

 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"
    |      |      |
        Figure 8: A Client Observes a Resource through a Proxy

Hartke Standards Track [Page 29] RFC 7641 Observing Resources in CoAP September 2015

Acknowledgements

 Carsten Bormann was an original author of this document and is
 acknowledged for significant contribution to this document.
 Thanks to Daniele Alessandrelli, Jari Arkko, Peter A. Bigot, Angelo
 P. Castellani, Gilbert Clark, Esko Dijk, Thomas Fossati, Brian Frank,
 Bert Greevenbosch, Jeroen Hoebeke, Cullen Jennings, Matthias
 Kovatsch, Barry Leiba, Salvatore Loreto, Charles Palmer, Akbar
 Rahman, Zach Shelby, and Floris Van den Abeele for helpful comments
 and discussions that have shaped the document.
 This work was supported in part by Klaus Tschira Foundation, Intel,
 Cisco, and Nokia.

Author's Address

 Klaus Hartke
 Universitaet Bremen TZI
 Postfach 330440
 Bremen  D-28359
 Germany
 Phone: +49-421-218-63905
 Email: hartke@tzi.org

Hartke Standards Track [Page 30]

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