RFC Editor Model (Version 3)
draft-iab-rfcefdp-rfced-model-13

4.  Server-side Requirements

4.1.  Request

  A GET request with an Observe Option set to 'register' (0) requests
  the server not only to return a current representation of the target
  resource, but also to add the client to the list of observers of that
  resource.  Upon success, the server returns a current representation
  of the resource and MUST notify the client of subsequent changes to
  the state 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 request is set to any other value than
  'register' (0), then the server MUST remove any entry with 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
  appropriate response code (such as 4.04 Not Found) and MUST remove
  the associated entry from the list of observers of the resource.

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  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 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 try to keep the
  returned representation current, i.e., it MUST 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 new notification with the unchanged representation
  and a new Max-Age just before the Max-Age indicated earlier 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.

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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
      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.

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  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 that its Reset messages aren't received by
      the server.  A server thus can always pretend that a Reset message
      rejecting a non-confirmable notification was lost.  If a server
      does this, it could accelerate cancellation by sending the
      following notifications to that client in confirmable messages.

  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.

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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 the
  number of simultaneous outstanding notifications/responses to a
  client on the list of observers is greater than or equal to NSTART.
  In this case, the server cannot notify the client of the new resource
  state immediately but has to wait for an outstanding notification/
  response to complete first.

  If there exists an outstanding notification/response that the server
  transmits to the client and that pertains to the changed resource,
  then it is desirable for the server to stop working towards getting
  the representation of the old resource state to the client, and to
  start transmitting the current representation to the client instead,
  so the resource state observed by the client stays closer in sync
  with the actual state at the server.

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  For this purpose, the server MAY optimize the transmission process by
  aborting the transmission of the old notification (but not before the
  current transmission attempt completed) and starting a new
  transmission for the new notification (but with the retransmission
  timer and counter of the aborted transmission retained).

  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.

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.

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  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
  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.

Request for Last Call review by INTDIR Completed: Ready. Reviewer: Sheng Jiang. Sent review to list.