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Versions: (draft-hartke-coap-observe) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14

CoRE Working Group                                             K. Hartke
Internet-Draft                                   Universitaet Bremen TZI
Intended status: Standards Track                        October 15, 2013
Expires: April 18, 2014


                      Observing Resources in CoAP
                       draft-ietf-core-observe-11

Abstract

   CoAP is a RESTful application protocol for constrained nodes and
   networks.  The state of a resource on a CoAP server can change over
   time.  This document specifies a simple protocol extension for CoAP
   that 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 Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on April 18, 2014.

Copyright Notice

   Copyright (c) 2013 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



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   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.

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Background . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Protocol Overview  . . . . . . . . . . . . . . . . . . . .  3
     1.3.  Observable Resources . . . . . . . . . . . . . . . . . . .  5
     1.4.  Consistency  . . . . . . . . . . . . . . . . . . . . . . .  6
     1.5.  Requirements Notation  . . . . . . . . . . . . . . . . . .  7
   2.  The Observe Option . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Client-side Requirements . . . . . . . . . . . . . . . . . . .  8
     3.1.  Request  . . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.2.  Notifications  . . . . . . . . . . . . . . . . . . . . . .  8
     3.3.  Caching  . . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.4.  Reordering . . . . . . . . . . . . . . . . . . . . . . . . 10
     3.5.  Transmission . . . . . . . . . . . . . . . . . . . . . . . 11
     3.6.  Cancellation . . . . . . . . . . . . . . . . . . . . . . . 11
   4.  Server-side Requirements . . . . . . . . . . . . . . . . . . . 11
     4.1.  Request  . . . . . . . . . . . . . . . . . . . . . . . . . 11
     4.2.  Notifications  . . . . . . . . . . . . . . . . . . . . . . 12
     4.3.  Caching  . . . . . . . . . . . . . . . . . . . . . . . . . 12
     4.4.  Reordering . . . . . . . . . . . . . . . . . . . . . . . . 13
     4.5.  Transmission . . . . . . . . . . . . . . . . . . . . . . . 14
   5.  Intermediaries . . . . . . . . . . . . . . . . . . . . . . . . 16
   6.  Web Linking  . . . . . . . . . . . . . . . . . . . . . . . . . 17
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 18
   9.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 18
   10. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     10.1. Normative References . . . . . . . . . . . . . . . . . . . 18
     10.2. Informative References . . . . . . . . . . . . . . . . . . 19
   Appendix A.  Examples  . . . . . . . . . . . . . . . . . . . . . . 19
     A.1.  Client/Server Examples . . . . . . . . . . . . . . . . . . 20
     A.2.  Proxy Examples . . . . . . . . . . . . . . . . . . . . . . 24
   Appendix B.  Changelog . . . . . . . . . . . . . . . . . . . . . . 26












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1.  Introduction

1.1.  Background

   CoAP [I-D.ietf-core-coap] is an application protocol for constrained
   nodes and networks.  It is intended to provide RESTful services
   [REST] not unlike HTTP [RFC2616] while reducing the complexity of
   implementation as well as the size of packets exchanged in order to
   make these services useful in a highly constrained network of
   themselves highly constrained nodes.

   The model of REST is that of a client exchanging representations of
   resources with a server, where a representation captures the current
   or intended state of a resource and the server is the 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.

   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 can retrieve a representation of the
   resource and request this representation be updated by the server
   over a period of time.

   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 for it, 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.



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                       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 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
   upon registration with the current state, 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: (empty)  |
                          +------------------->|
                          |                    |
                          |    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

   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 CoAP messages by the server: If the
   client actively rejects a notification or if the transmission of a
   notification times out after several transmission attempts, then the
   client is assumed to be no longer interested and it is removed from
   the list of observers.

1.3.  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:



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   o  <coap://server/temperature>, which changes its state every second
      to the current reading of the temperature sensor;

   o  <coap://server/temperature/felt>, which changes its state to
      "cold" when the temperature reading drops below a certain pre-
      configured threshold, and to "warm" when the reading exceeds a
      second, slightly higher threshold;

   o  <coap://server/temperature/critical?above=45>, which changes its
      state based on the client-specified parameter value: every second
      to the current temperature reading if the temperature exceeds the
      threshold, or to "OK" when the reading drops below; and/or

   o  <coap://server/?query=select+avg(temperature)+from+Sensor.window:
      time(30sec)>, which accepts expressions of arbitrary complexity
      and changes its state accordingly.

   So, 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 continuously supplying it with raw sensor
   readings.  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.  Thus, the application
   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.

1.4.  Consistency

   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
   inconsistent at times: 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 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 registers its interest
   eventually again.

   The protocol addresses this as follows:

   o  It follows a best-effort approach for sending the current
      representation to the client after a state change: Clients should



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      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 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.

1.5.  Requirements Notation

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

2.  The Observe Option

   +-----+---+---+---+---+---------+------------+-----------+---------+
   | No. | C | U | N | R | Name    | Format     | Length    | Default |
   +-----+---+---+---+---+---------+------------+-----------+---------+
   |   6 |   | x | - |   | Observe | empty/uint | 0 B/0-3 B | (none)  |
   +-----+---+---+---+---+---------+------------+-----------+---------+

            C=Critical, U=Unsafe, N=No-Cache-Key, R=Repeatable

                        Table 1: The Observe Option

   The Observe 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 a new entry to the list
   of observers of the resource.  The list entry consists of the client
   endpoint and the token specified by the client in the request.

   The value of the Observe Option in a request MUST be empty on
   transmission and MUST be ignored on reception.

   The Observe 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 target resource, then the request falls back to a
   normal GET request.




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   In a response, the Observe 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 Observe Option in a response is a 24-bit sequence
   number for reordering detection (see Section 3.4 and Section 4.4).
   The sequence number is encoded in network byte order using a variable
   number of bytes ('uint' format; see Section 3.2 of RFC XXXX
   [I-D.ietf-core-coap]).

   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 that includes an Observe Option is used to satisfy a normal
   GET request, the option MUST be removed before the response is
   returned to the client.

3.  Client-side Requirements

3.1.  Request

   A client can register its interest in a resource by issuing a GET
   request that includes an empty Observe Option.  If the server returns
   a 2.xx response that includes an Observe Option as well, the server
   has added the client successfully to the list of observers of the
   target resource and the client will be notified of changes to the
   resource state.

   Like a fresh response can be used to satisfy a request without
   contacting the server, the updates resulting from one request can be
   used to satisfy another request if the target resource is the same.
   A client therefore MUST aggregate requests where possible, and MUST
   NOT register more than once for the same target resource.  The target
   resource SHALL be identified for this purpose by all options in the
   request that are part of the cache-key, such as the full request URI
   and the Accept Option.

3.2.  Notifications

   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, 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.

   Notifications have a 2.05 (Content) response code, or potentially a
   2.03 (Valid) response code if the client included one or more ETag



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   Options in the request (see Section 3.3).  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 all
   clients from the list of observers of the resource.

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
   XXXX [I-D.ietf-core-coap].  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
   an Observe Option and the same token at any time.  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 wait for a random amount of time between 5 and 15
   seconds to avoid synchronicity 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.



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   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.

   A client implementation needs to keep all candidate responses in its
   cache until it is no longer interested in the target resource or it
   issues a GET request 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 NOT update
   the observed state with a notification that arrives later than a
   newer notification.

   For reordering detection, 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 is
   newer than the newest notification received 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 of the newest
   notification received so far, V2 the value of the Observe 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.

   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 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 an incoming notification is new.

      The duration of 128 seconds was chosen as a nice round number
      greater than MAX_LATENCY (see Section 4.8.2 of RFC XXXX
      [I-D.ietf-core-coap]).




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3.5.  Transmission

   A notification can be confirmable or non-confirmable, i.e., be sent
   in a confirmable or a non-confirmable message.  The message type used
   for a notification is independent from the type used for the request
   or for 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.

3.6.  Cancellation

   A client that is no longer interested in receiving further
   notifications for a resource can simply "forget" the pending request.
   When the server then sends a notification, the client will not
   recognize the token in the message.  If the notification was
   confirmable, this will cause the client to return a Reset message and
   thus the server to remove the associated entry from the list of
   observers.  Entries in lists of observers are effectively "garbage
   collected" by the server.

   When a client rejects a non-confirmable notification, the server may
   also (but is not required to) remove the associated entry from the
   list of observers.  So, if the servers seems to ignore the Reset
   messages that the client sends to reject non-confirmable
   notifications, the client may have to wait for a confirmable
   notification until the list entry is removed.

4.  Server-side Requirements

4.1.  Request

   A GET request with an Observe Option 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 MUST return a current representation of the
   resource and MUST notify the client of subsequent changes to the
   state as long as the client is on the list of observers.



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   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 Observe Option and
   process the GET request as usual.  The resulting response MUST NOT
   include an Observe Option, the absence of which signals to the client
   that it will not be notified of changes to the resource and, e.g.,
   needs to poll the resource for its state instead.

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 include
   an Observe Option and MUST echo the token specified by the client in
   the GET request.  If there are multiple entries in the list of
   observers, the order in which the clients are notified is not
   defined; the server is free to use any method to determine the order.

   A notification SHOULD have a 2.05 (Content) or 2.03 (Valid) response
   code.  However, in the event that the state of a resource changes in
   a way that would cause a normal GET request at that time to return a
   non-2.xx response (for example, when the resource is deleted), the
   server SHOULD notify the client by sending a notification with an
   appropriate response code (such as 4.04 Not Found) and MUST remove
   the client from the list of observers of the resource.

   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 in this Content-Format, it
   SHOULD send a notification with a 4.06 (Not Acceptable) response code
   and MUST remove the client from the list of observers of the
   resource.

   A non-2.xx notification MUST NOT include an Observe Option.

4.3.  Caching

   As notifications are just additional responses sent by the server,
   they are subject to caching as defined in Section 5.6 of RFC XXXX
   [I-D.ietf-core-coap].





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4.3.1.  Freshness

   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.

   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 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 notification with the unchanged representation and
   a new Max-Age just before the old Max-Age expires.

4.3.2.  Validation

   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.

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 sending the



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   message.

   The sequence numbers generated for a resource MUST provide an order
   among all notifications resulting from all requests from the same
   client endpoint.

   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 next 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 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.  Reducing the maximum
      notification rate to 32768 notifications per second is still well
      beyond the highest known design objective of around 1 kHz (most
      CoAP applications will be several orders of magnitude below that),
      but allows total clock inaccuracies of up to -50/+100 %.

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
   is changing frequently.  Similarly, it MAY choose to send a
   notification more than once.  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



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   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 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 associated entry from the list of observers.  If the
   client rejects a non-confirmable notification, the server MAY remove
   the entry from the list of observers as well.  (It is expected that
   the server does remove the entry 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.)

   At a minimum, the server MUST send a notification in a confirmable
   message instead of a non-confirmable message at least every 24 hours,
   so a client that went away or is no longer interested does not remain
   forever in the list of observers.

   The server MUST limit the number of confirmable notifications for
   which an acknowledgement has not been received from a client yet to
   NSTART (1 by default; see Section 4.7 of RFC XXXX
   [I-D.ietf-core-coap]).

   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]).

   When the state of an observed resource changes while the number of
   outstanding acknowledgements is greater than or equal to NSTART, or
   while the waiting time for a non-confirmable notification has not
   elapsed yet, the server MUST proceed as follows:

   1.  Wait for the current transmission attempt to be acknowledged,
       rejected or to time out (confirmable transmission), or the
       waiting time to elapse (non-confirmable transmission).

   2.  If the transmission is rejected or the transmission was the last
       attempt to deliver a notification, remove the associated entry
       from the list of observers of the observed resource.




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   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.  If the completed transmission attempt timed out, increment the
       retransmission counter and double the timeout for the new
       transmission; otherwise, reinitialize both the retransmission
       counter and timeout as described in Section 4.2 of RFC XXXX
       [I-D.ietf-core-coap].

5.  Intermediaries

   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
   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
   Section 3.  It is the task of this intermediary to provide the client
   with a current representation of the target resource and send
   notifications upon changes to the target resource state, much like an
   origin server as specified in Section 4.

   To perform this task, the intermediary SHOULD make use of the
   protocol specified in this document, taking the role of the client
   and registering its own interest in the target resource with the next
   hop towards the origin server.  If the next hop does not return a
   response with an Observe Option, the intermediary MAY resort to
   polling the next hop or MAY itself return a response without an
   Observe Option.

   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, 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.



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   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
   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 of RFC XXXX [I-D.ietf-core-coap] apply.

   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.)

   As with any protocol that creates state, attackers may attempt to
   exhaust the resources that the server has available for maintaining
   the list of observers for each resource.  Servers may want to access-
   control this creation of state.  As degraded behavior, the server can
   always fall back to processing the request as a normal GET request
   (without an Observe Option) if it is unwilling or unable to add a
   client to the list of observers of a resource, including if system
   resources are exhausted or nearing exhaustion.

   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.





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8.  IANA Considerations

   The following entry is added to the CoAP Option Numbers registry:

                     +--------+---------+-----------+
                     | Number | Name    | Reference |
                     +--------+---------+-----------+
                     |      6 | Observe | [RFCXXXX] |
                     +--------+---------+-----------+

   [Note to RFC Editor: Please replace XXXX with the RFC number of this
   specification.]

9.  Acknowledgements

   Carsten Bormann was an original author of this draft and is
   acknowledged for significant contribution to this document.

   Thanks to Daniele Alessandrelli, Jari Arkko, Peter Bigot, Angelo P.
   Castellani, Gilbert Clark, Esko Dijk, Thomas Fossati, Brian Frank,
   Bert Greevenbosch, 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.

   This work was supported in part by Klaus Tschira Foundation, Intel,
   Cisco, and Nokia.

10.  References

10.1.  Normative References

   [I-D.ietf-core-coap]  Shelby, Z., Hartke, K., and C. Bormann,
                         "Constrained Application Protocol (CoAP)",
                         draft-ietf-core-coap-18 (work in progress),
                         June 2013.

   [RFC1982]             Elz, R. and R. Bush, "Serial Number
                         Arithmetic", RFC 1982, August 1996.

   [RFC2119]             Bradner, S., "Key words for use in RFCs to
                         Indicate Requirement Levels", BCP 14, RFC 2119,
                         March 1997.

   [RFC5405]             Eggert, L. and G. Fairhurst, "Unicast UDP Usage
                         Guidelines for Application Designers", BCP 145,
                         RFC 5405, November 2008.




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   [RFC5988]             Nottingham, M., "Web Linking", RFC 5988,
                         October 2010.

10.2.  Informative References

   [GOF]                 Gamma, E., Helm, R., Johnson, R., and J.
                         Vlissides, "Design Patterns: Elements of
                         Reusable Object-Oriented Software", Addison-
                         Wesley, Reading, MA, USA, November 1994.

   [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>.

   [RFC2616]             Fielding, R., Gettys, J., Mogul, J., Frystyk,
                         H., Masinter, L., Leach, P., and T. Berners-
                         Lee, "Hypertext Transfer Protocol -- HTTP/1.1",
                         RFC 2616, June 1999.

   [RFC5989]             Roach, A., "A SIP Event Package for Subscribing
                         to Changes to an HTTP Resource", RFC 5989,
                         October 2010.

   [RFC6202]             Loreto, S., Saint-Andre, P., Salsano, S., and
                         G. Wilkins, "Known Issues and Best Practices
                         for the Use of Long Polling and Streaming in
                         Bidirectional HTTP", RFC 6202, April 2011.

   [RFC6690]             Shelby, Z., "Constrained RESTful Environments
                         (CoRE) Link Format", RFC 6690, August 2012.

Appendix A.  Examples
















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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: (empty)
     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






















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         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: (empty)
    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

















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         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: (empty)
    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









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         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





















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A.2.  Proxy Examples

   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"
      |      |      |

    Figure 7: A proxy observes a resource to keep its cache up to date









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   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"
      |      |      |

          Figure 8: A client observes a resource through a proxy








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Appendix B.  Changelog

   [Note to RFC Editor: Please remove this section before publication.]

   Changes from ietf-10 to ietf-11:

   o  Pointed out that client and server clocks may differ in their
      realization of the SI second, and added robustness to the existing
      reordering scheme by reducing the maximum notification rate to
      32768 notifications per second (#341).

   Changes from ietf-09 to ietf-10:

   o  Required consistent sequence numbers across requests (#333).

   o  Clarified that a server needs to update the entry in the list of
      observers instead of adding a new entry if the endpoint/token pair
      is already present.

   o  Allowed that a client uses a token that is currently in use to
      ensure that it's still in the list of observers.  This is possible
      because sequence numbers are now consistent across requests and
      servers won't add a new entry for the same token.

   o  Improved text on the transmission of non-confirmable notifications
      to match Section 3.1.2 of RFC 5405 more closely.

   o  Updated examples to use UCUM units.

   o  Moved Appendix B into the introduction.

   Changes from ietf-08 to ietf-09:

   o  Removed the side effects of requests on existing observations.
      This includes removing that

      *  the client can use a GET request to cancel an observation;

      *  the server updates the entry in the list of observers instead
         of adding a new entry if the client is already present (#258,
         #281).

   o  Clarified that a resource (and hence an observation relationship)
      is identified by the request options that are part of the Cache-
      Key (#258).

   o  Clarified that a non-2.xx notification MUST NOT include an Observe
      Option.



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   o  Moved block-wise transfer of notifications to [I-D.ietf-core-
      block].

   Changes from ietf-07 to ietf-08:

   o  Expanded text on transmitting a notification while a previous
      transmission is pending (#242).

   o  Changed reordering detection to use a fixed time span of 128
      seconds instead of EXCHANGE_LIFETIME (#276).

   o  Removed the use of the freshness model to determine if the client
      is still on the list of observers.  This includes removing that

      *  the client assumes that it has been removed from the list of
         observers when Max-Age ends;

      *  the server sets the Max-Age Option of a notification to a value
         that indicates when the server will send the next notification;

      *  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);

      *  the server may remove the client from all lists of observers
         when the transmission of a confirmable notification ultimately
         times out.

   o  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.

   o  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.

   o  Added a requirement for sending a confirmable notification at
      least every 24 hours before continuing with non-confirmable
      notifications (#221).

   o  Added congestion control considerations from [I-D.bormann-core-
      congestion-control-02].

   o  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



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      resource perform a GET request at the same time when the need to
      re-register arises.

   o  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.

   o  Fixed the length of the Observe Option (3 bytes) in the table in
      Section 2.

   o  Replaced the 'x' in the No-Cache-Key column in the table in
      Section 2 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.

   o  Updated examples.

   Changes from ietf-06 to ietf-07:

   o  Moved to 24-bit sequence numbers to allow for up to 15000
      notifications per second per client and resource (#217).

   o  Re-numbered option number to use Unsafe/Safe and Cache-Key
      compliant numbers (#241).

   o  Clarified how to react to a Reset message that is sent in reply to
      a non-confirmable notification (#225).

   o  Clarified the semantics of the "obs" link target attribute (#236).

   Changes from ietf-05 to ietf-06:

   o  Improved abstract and introduction to say that the protocol is
      about best effort and eventual consistency (#219).

   o  Clarified that the value of the Observe Option in a request must
      have zero length.

   o  Added requirement that the sequence number must be updated each
      time a server retransmits a notification.

   o  Clarified that a server must remove a client from the list of
      observers when it receives a GET request with an unrecognized
      critical option.

   o  Updated the text to use the endpoint concept from
      [I-D.ietf-core-coap] (#224).





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   o  Improved the reordering text (#223).

   Changes from ietf-04 to ietf-05:

   o  Recommended that a client does not re-register while a new
      notification from the server is still likely to arrive.  This is
      to avoid that the request of the client and the last notification
      after max-age cross over each other (#174).

   o  Relaxed requirements when sending a Reset message in reply to non-
      confirmable notifications.

   o  Added an implementation note about careless GET requests (#184).

   o  Updated examples.

   Changes from ietf-03 to ietf-04:

   o  Removed the "Max-OFE" Option.

   o  Allowed a Reset message in reply to non-confirmable notifications.

   o  Added a section on cancellation.

   o  Updated examples.

   Changes from ietf-02 to ietf-03:

   o  Separated client-side and server-side requirements.

   o  Fixed uncertainty if client is still on the list of observers by
      introducing a liveliness model based on Max-Age and a new option
      called "Max-OFE" (#174).

   o  Simplified the text on message reordering (#129).

   o  Clarified requirements for intermediaries.

   o  Clarified the combination of blockwise transfers with
      notifications (#172).

   o  Updated examples to show how the state observed by the client
      becomes eventually consistent with the actual state on the server.

   o  Added examples for parameterization of observable resource.

   Changes from ietf-01 to ietf-02:




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   o  Removed the requirement of periodic refreshing (#126).

   o  The new "Observe" Option replaces the "Lifetime" Option.

   o  Introduced a new mechanism to detect message reordering.

   o  Changed 2.00 (OK) notifications to 2.05 (Content) notifications.

   Changes from ietf-00 to ietf-01:

   o  Changed terminology from "subscriptions" to "observation
      relationships" (#33).

   o  Changed the name of the option to "Lifetime".

   o  Clarified establishment of observation relationships.

   o  Clarified that an observation is only identified by the URI of the
      observed resource and the identity of the client (#66).

   o  Clarified rules for establishing observation relationships (#68).

   o  Clarified conditions under which an observation relationship is
      terminated.

   o  Added explanation on how clients can terminate an observation
      relationship before the lifetime ends (#34).

   o  Clarified that the overriding objective for notifications is
      eventual consistency of the actual and the observed state (#67).

   o  Specified how a server needs to deal with clients not
      acknowledging confirmable messages carrying notifications (#69).

   o  Added a mechanism to detect message reordering (#35).

   o  Added an explanation of how notifications can be cached,
      supporting both the freshness and the validation model (#39, #64).

   o  Clarified that non-GET requests do not affect observation
      relationships, and that GET requests without "Lifetime" Option
      affecting relationships is by design (#65).

   o  Described interaction with blockwise transfers (#36).

   o  Added Resource Discovery section (#99).





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   o  Added IANA Considerations.

   o  Added Security Considerations (#40).

   o  Added examples (#38).

Author's Address

   Klaus Hartke
   Universitaet Bremen TZI
   Postfach 330440
   Bremen  D-28359
   Germany

   Phone: +49-421-218-63905
   EMail: hartke@tzi.org



































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