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Versions: (draft-hartke-coap-observe) 00 01 02 03 04 05 06 07 08 09 10 11 12 13 14 15 16 RFC 7641

CoRE Working Group                                             K. Hartke
Internet-Draft                                   Universitaet Bremen TZI
Intended status: Standards Track                        October 22, 2012
Expires: April 25, 2013


                      Observing Resources in CoAP
                       draft-ietf-core-observe-07

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 a server to replicate a resource state to interested
   clients.  The protocol follows a best-effort approach when
   transmitting new resource states to clients, and provides eventual
   consistency between the state observed by each client and the actual
   resource state.

Editor's Note

   This is an interim revision which will receive further modifications
   during the resolution of open tickets, in particular #204, #235 and
   #242.

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 25, 2013.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   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.


Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Background . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Protocol Overview  . . . . . . . . . . . . . . . . . . . .  3
     1.3.  Design Philosophy  . . . . . . . . . . . . . . . . . . . .  6
     1.4.  Requirements Notation  . . . . . . . . . . . . . . . . . .  6
   2.  The Observe Option . . . . . . . . . . . . . . . . . . . . . .  7
   3.  Client-side Requirements . . . . . . . . . . . . . . . . . . .  7
     3.1.  Request  . . . . . . . . . . . . . . . . . . . . . . . . .  7
     3.2.  Notifications  . . . . . . . . . . . . . . . . . . . . . .  8
     3.3.  Caching  . . . . . . . . . . . . . . . . . . . . . . . . .  8
     3.4.  Reordering . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.5.  Cancellation . . . . . . . . . . . . . . . . . . . . . . . 10
   4.  Server-side Requirements . . . . . . . . . . . . . . . . . . . 11
     4.1.  Request  . . . . . . . . . . . . . . . . . . . . . . . . . 11
     4.2.  Notifications  . . . . . . . . . . . . . . . . . . . . . . 11
     4.3.  Caching  . . . . . . . . . . . . . . . . . . . . . . . . . 12
     4.4.  Reordering . . . . . . . . . . . . . . . . . . . . . . . . 13
     4.5.  Retransmission . . . . . . . . . . . . . . . . . . . . . . 14
   5.  Intermediaries . . . . . . . . . . . . . . . . . . . . . . . . 15
   6.  Block-wise Transfers . . . . . . . . . . . . . . . . . . . . . 15
   7.  Discovery  . . . . . . . . . . . . . . . . . . . . . . . . . . 16
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 16
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 18
     11.2. Informative References . . . . . . . . . . . . . . . . . . 18
   Appendix A.  Examples  . . . . . . . . . . . . . . . . . . . . . . 19
     A.1.  Proxying . . . . . . . . . . . . . . . . . . . . . . . . . 22
     A.2.  Block-wise Transfer  . . . . . . . . . . . . . . . . . . . 24
   Appendix B.  Modeling Resources to Tailor Notifications  . . . . . 24
   Appendix C.  Changelog . . . . . . . . . . . . . . . . . . . . . . 25
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 28





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1.  Introduction

1.1.  Background

   CoAP [I-D.ietf-core-coap] is an Application Protocol for Constrained
   Nodes/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 communication model of REST is that of a client exchanging
   resource states with an origin server using representations.  The
   origin server is the definitive source for representations of the
   resources in its namespace.  A client interested in the state of a
   resource sends a request to the origin server; the server then
   returns a response with a representation that is current at the time
   of the request.

   This model does not work well when a client is interested in knowing
   the state of a resource over a period of time.  Existing approaches
   when using HTTP, such as repeated polling or long-polls [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 to replicate a resource state from a server
   to interested clients over a period of time, while still keeping the
   properties of REST.

   There is no intention for this mechanism 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, an
   observer must register separately for all of them.  The pattern is
   typically used when a clean separation between related components is
   required, such as data storage and user interface.



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   Observer           Subject
      |                  |
      |   Registration   |
      +----------------->|
      |                  |
      |   Notification   |
      |<-----------------+
      |                  |
      |   Notification   |
      |<-----------------+
      |                  |
      |   Notification   |
      |<-----------------+
      |                  |

                     Figure 1: 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 the
      current state of the resource at any given time.

   Registration:  A client registers its interest by sending 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 for the
      resource.

   Notification:  Whenever the state of a resource changes, the server
      notifies each client registered as observer for the resource.
      Each notification is an additional CoAP response sent by the
      server in reply to the GET request and includes a complete
      representation of the new resource state.

   Figure 2 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 notifications when the
   state of the resource changes.  Registration request and
   notifications are identified by the presence of the Observe Option
   defined in this document.  All notifications echo the token specified
   by the client in the request, so the client can easily correlate them
   to the request.




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   Client              Server
      |                  |
      | GET /temperature |
      |  Observe:        |  (registration)
      |    Token: 0x4a   |
      +----------------->|
      |                  |
      |   2.05 Content   |
      |  Observe: 12     |  (notification of the current state)
      |    Token: 0x4a   |
      |  Payload: 22.9 C |
      |<-----------------+
      |                  |
      |   2.05 Content   |
      |  Observe: 44     |  (notification upon a state change)
      |    Token: 0x4a   |
      |  Payload: 22.8 C |
      |<-----------------+
      |                  |
      |   2.05 Content   |
      |  Observe: 60     |  (notification upon a state change)
      |    Token: 0x4a   |
      |  Payload: 23.1 C |
      |<-----------------+
      |                  |

                  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 by the server from the client's
   acknowledgement of notifications sent in confirmable messages.  If
   the client rejects a notification or if the transmission of a
   notification ultimately fails, then the client is assumed to be no
   longer interested and is removed by the server from the list of
   observers.

   A notification is cacheable like any other response and can be used
   until the next notification arrives.  Each notification includes an
   indication of when the server will send the next notification at
   latest (Max-Age).  This helps a client that does not receive a
   notification for a while, to decide if the resource simply did not
   undergo a change of state yet or if the next notification is overdue
   and the server is apparently no longer aware of the client's interest
   in the resource.

   When a client wants to be notified after it has determined that no
   further notifications can be expected, it needs to register again.



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1.3.  Design Philosophy

   The protocol builds on the architectural elements of REST, which
   include: a server that is responsible for the state and
   representation of the resources in its namespace, a client that is
   responsible for keeping the application state, and the stateless
   exchange of resource representations.  (Beyond stateless REST, a
   server needs to keep track of the observers though, somewhat similar
   to how HTTP servers need to keep track of the TCP connections from
   their clients.)  The protocol enables high scalability and efficiency
   through the support of caches and intermediaries that multiplex the
   interest of multiple clients in the same resource into a single
   association.

   The server is the authority for determining under what conditions
   resources change their state and how often observers are notified.
   The protocol does not offer explicit means for setting up triggers,
   thresholds or other conditions; it is up to the server to expose
   observable resources that change their state in a way that is
   meaningful in the application context.  Resources can be
   parameterized to achieve similar effects though; see Appendix B for
   examples.

   Since bandwidth is in short supply in constrained environments, a
   server must adapt the rate of notifications to each client.  This
   implies that a client cannot rely on observing every single state a
   resource goes through.  Instead, the protocol follows a best-effort
   approach when transmitting the new resource state 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.

   Furthermore, the protocol is designed on the principle of eventual
   consistency: it guarantees that if the resource does not undergo a
   new change in state, eventually all registered observers will have a
   current representation of the last resource state.

1.4.  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

   +-----+---+---+---+---+---------+------------+-----------+---------+
   | No. | C | U | N | R | Name    | Format     | Length    | Default |
   +-----+---+---+---+---+---------+------------+-----------+---------+
   |   6 |   | x | x |   | Observe | empty/uint | 0 B/0-2 B | (none)  |
   +-----+---+---+---+---+---------+------------+-----------+---------+

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

   The Observe Option, when present, modifies the GET method so it does
   not only retrieve a representation of the current state of the
   resource identified by the request URI, but also requests the server
   to add the client to the list of observers of the resource.  The
   exact semantics are defined in the sections below.  The value of the
   option in a request MUST be empty on transmission and MUST be ignored
   on reception.

   In a response, the Observe Option identifies the message as a
   notification, which implies that the client has been added to the
   list of observers and that the server will notify the client of
   further changes to the resource state.  The option's value is a
   sequence number that can be used for reordering detection (see
   Section 3.4 and Section 4.4).  The value is encoded as a variable-
   length unsigned integer as defined in Section 3.4.1 of RFC XXXX
   [I-D.ietf-core-coap].

   Since the Observe Option is not critical, a GET request that includes
   the Observe Option will automatically fall back to a normal GET
   request if the server is unwilling or unable to add the client to the
   list of observers.


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 for as long as the server can assume the client's
   interest.







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3.2.  Notifications

   Notifications are additional responses sent by the server in reply to
   the GET request.  Each notification includes an Observe Option with a
   sequence number (see Section 3.4), a Token Option that matches the
   token specified by the client in the GET request, and a payload of
   the same Content-Format as the initial response.

   A notification can be confirmable or non-confirmable (i.e. sent in a
   confirmable or non-confirmable message).  If a client does not
   recognize the token in a confirmable notification, it MUST NOT
   acknowledge the message and SHOULD reject it with a RST message.
   Otherwise, the client MUST acknowledge the message with an ACK
   message as usual.  If a client does not recognize the token in a non-
   confirmable notification, it MAY reject it with a RST message.

   An acknowledgement 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 it from the list of observers.

   Notifications will have a 2.05 (Content) response code in most cases.
   They may also have a 2.03 (Valid) response code if the client
   includes an ETag Option in its request (see Section 3.3).  In the
   event that the state of an observed resource is changed in a way that
   would cause a normal GET request not to return success (for example,
   when the resource is deleted), the server will send a notification
   with a non-success response code (such as 4.xx/5.xx) and empty the
   list of observers of the resource.

3.3.  Caching

   As notifications are just additional responses, notifications partake
   in caching as defined by Section 5.6 of RFC XXXX
   [I-D.ietf-core-coap].  Both the freshness model and the validation
   model are supported.  The freshness model also serves as the model
   for the client to determine if it's still on the list of observers or
   if it needs to re-register its interest in the resource.

   A client MAY store a notification like a response in its cache and
   use a stored notification/response that is fresh without contacting
   the origin server.  A notification/response is considered fresh while
   its age is not greater than its Max-Age and no newer notification has
   been received.

   The server will do its best to keep the client up to date with a
   fresh representation of the current resource state.  It will send a



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   notification whenever the resource changes, or at latest when the age
   of the last notification becomes greater than its Max-Age.  (Note
   that this notification may not arrive in time due to network
   latency.)

   The client SHOULD assume that it's on the list of observers while the
   age of the last notification is not greater than Max-Age.  If the
   client does not receive a notification before the age becomes greater
   than Max-Age, it can assume that it has been removed from the list of
   observers (e.g., due to a loss of server state).  In this case, it
   may need to re-register its interest.

   To make sure it has a fresh representation and/or to re-register its
   interest, a client MAY issue a new GET request with an Observe Option
   at any time.  The GET request SHOULD specify a new token to avoid
   ambiguity, because the token serves as epoch identifier for the
   sequence numbers in the Observe Option (see Section 3.4).

   It is RECOMMENDED that the client does not issue the request while it
   still has a fresh notification and, beyond that, while a new
   notification from the server is still likely to arrive.  I.e. the
   client should wait until the age of the last notification becomes
   greater than its Max-Age plus MAX_LATENCY (the maximum time a
   datagram is expected to take from the start of its transmission to
   the completion of its reception; see Section 4.8 of RFC XXXX
   [I-D.ietf-core-coap]).

   When a client has one or more notifications stored, it can use the
   ETag Option in the GET request to give the server an opportunity to
   select a stored response to be used.  The client MAY include an ETag
   Option for each stored response that is applicable.  It needs to keep
   those responses in the cache until it is no longer interested in
   receiving notifications for the target resource or it issues a new
   GET request with a new set of entity-tags.  Whenever the observed
   resource changes its state 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.

3.4.  Reordering

   Messages that carry notifications can arrive in a different order
   than they were sent.  Since the goal is eventual consistency (see
   Section 1.3), a client MAY safely skip a notification that arrives
   later than a newer notification.  For this purpose, the server sets
   the value of the Observe Option in each notification to a 24-bit
   sequence number.




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   A notification is older than the latest notification received and
   thus can be skipped when the following condition is met:

   (V1 - V2) % (2**24) < (2**23)    and    T2 < (T1 + EXCHANGE_LIFETIME)

   where V1 is the value of the Observe Option of the latest valid
   notification received, V2 the value of the Observe Option of the
   present notification, T1 a client-local timestamp of the latest valid
   notification received (in seconds), and T2 a client-local timestamp
   of the present notification.

   Design Note:  The first condition essentially verifies that V2 > V1
      holds in 24-bit sequence number arithmetic [RFC1982].  The second
      condition checks that the time expired between the two incoming
      messages is not so large that the sequence number might have
      wrapped around and the first check is therefore invalid.  (In
      other words, after about EXCHANGE_LIFETIME seconds elapse without
      any notification, the client does not need to check the sequence
      numbers in order to assume an incoming notification is new.)  The
      constant of 2**23 is non-critical, as is the even speed or
      precision of the clock involved.

3.5.  Cancellation

   When a client rejects a confirmable notification with a RST message
   or when it performs a GET request without an Observe Option for a
   currently observed resource, the server will remove the client from
   the list of observers for this resource.  The client MAY use either
   method at any time to indicate that it is no longer interested in
   receiving notifications about a resource.

   When a client rejects non-confirmable notification with a RST, there
   is also a chance that the server will remove the client from the list
   of observers for this resource.  So the client MAY try this method as
   well.  A client MAY rate-limit the RST messages it sends if the
   server appears to persistently ignore them.

   Implementation Note:  A client that does not mediate all its requests
      through its cache might inadvertantly cancel an observation
      relationship by sending an unrelated GET to the same resource.  To
      avoid this, without incurring a need for synchronization, such
      clients can use a different source transport address for these
      unrelated GET requests.








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4.  Server-side Requirements

4.1.  Request

   A GET request that includes an Observe Option requests the server not
   only to return a representation of the resource identified by the
   request URI, but also to add the client to the list of observers of
   the target resource.  If no error occurs, the server MUST return a
   response with the representation of the current resource state and
   MUST notify the client of subsequent changes to the state as long as
   the client is on the list of observers.

   A server that is unable or unwilling to add the client to the list of
   observers of the target 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
   state and, e.g., needs to poll the resource instead.

   If the client is already on the list of observers, the server MUST
   NOT add it a second time but MUST replace or update the existing
   entry.  If the server receives a GET request for the same resource
   that does not include an Observe Option or a GET request that
   includes an unrecognized critical option, the server MUST remove the
   client from the list of observers.

   Two requests relate to the same list entry if and only if both the
   request URI and the source endpoint of the requests match.  Message
   IDs and tokens are not taken into account.

   Any request with a method other than GET MUST NOT have a direct
   effect on a list of observers of a resource.  However, such a request
   can have the indirect consequence of causing the server to send a
   non-success notification which does affect the list of observers
   (e.g., when a DELETE request is successful and an observed resource
   no longer exists).

4.2.  Notifications

   A client is notified of resource state changes by additional
   responses sent by the server in reply to the GET request.  Each such
   notification response MUST include an Observe Option and MUST echo
   the token specified by the client in the GET request.  If there are
   multiple clients on the list of observers, the order in which they
   are notified is not defined; the server is free to use any method to
   determine the order.

   A notification SHOULD have a 2.05 (Content) or 2.03 (Valid) response



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   code.  However, in the event that the state of a resource changes in
   a way that would cause a normal GET request to return a non-success
   response code (for example, if the resource is deleted), the server
   SHOULD notify the client by sending a notification with an
   appropriate non-success response code (such as 4.xx/5.xx) and MUST
   empty 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 using this Content-Format,
   it SHOULD send a notification with a 5.00 (Internal Server Error)
   response code and MUST empty the list of observers of the resource.

   A notification can be sent as 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.

   The acknowledgement of a confirmable notification implies the
   client's continued interest in being notified.  If the client rejects
   a confirmable notification with a RST message, the server MUST remove
   the client from the list of observers.  If the client rejects a non-
   confirmable notification with a RST message, the server MAY remove
   the client from the list of observers, i.e., it is expected that the
   server removes the client if it still has the state available that is
   needed to match the RST message to the notification, but the server
   is not required to keep this state.

   If CoAP is used over a connection-oriented or session-based transport
   such as DTLS, the server MUST remove the client from the list of
   observers when the connection or session is closed.

4.3.  Caching

   The Max-Age Option of a notification SHOULD be set to a value that
   indicates when the server will send the next notification.  For
   example, if the server sends a notification every 30 seconds, a Max-
   Age Option with value 30 should be included.  The server MAY send a
   new notification before Max-Age ends and MUST send a new notification
   at latest when Max-Age ends.  If the client does not receive a new
   notification before Max-Age ends, it will assume that it was removed
   from the list of observers (e.g., due to a loss of server state) and
   may issue a new GET request to re-register its interest.



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   It may not always be possible to predict when the server will send
   the next notification, for example, when a resource does not change
   its state in regular intervals.  In this case, the server SHOULD set
   Max-Age to a good approximation.  The value is a trade-off between
   increased usage of bandwidth and the risk of stale information.
   Smaller values lead to more notifications and more GET requests,
   while greater values result in network or device failures being
   detected later and data becoming stale.

   The client can include a set of entity-tags in its request using the
   ETag Option.  When the observed resource changes its state and the
   origin server is about to send a 2.05 (Content) notification, then,
   whenever that notification has an entity-tag in the set of entity-
   tags specified by the client, the server MAY send a 2.03 (Valid)
   response with an appropriate ETag Option instead.  The server MUST
   NOT assume that the recipient has any response stored other than
   those identified by the entity-tags in the most recent GET request
   for the resource.

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
   in each notification to the 24 least-significant bits of a strictly
   increasing sequence number.  The sequence number MAY start at any
   value.  The server MUST NOT reuse the same option value with the same
   client, token and resource within approximately 2*EXCHANGE_LIFETIME
   seconds (roughly 8.5 minutes with default CoAP parameters).

   Implementation Note:  A simple implementation that satisfies the
      requirements is to use a timestamp (in seconds) provided by the
      device's clock, or a 24-bit unsigned integer variable that is
      incremented periodically and does not wrap around more often than
      every 2*EXCHANGE_LIFETIME seconds.  It is not necessary that the
      clock reflects the correct local time or that it ticks in a
      precisely periodical way.

      The client is comparing sequence numbers only between
      notifications that arrive within EXCHANGE_LIFETIME seconds.
      However, a server should not assume that it is free to completely
      forget the sequence number right after EXCHANGE_LIFETIME -- the
      client may have a slower clock.  If there is a need to discard
      sequence number state after some inactivity, this should be done
      only after 2*EXCHANGE_LIFETIME or later.  Similarly, the sequence
      number should not increase so fast to span more than half the
      sequence number space within less than 2*EXCHANGE_LIFETIME.




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      The 24-bit size for the sequence number has been chosen to enable
      very fast notification: If EXCHANGE_LIFETIME is the default value
      and 2*EXCHANGE_LIFETIME therefore approximately 2**9 seconds, the
      sequence number can increase every 2**14 times per second or
      approximately every 61 us before there is any danger of
      misdetection of wrap-around.  On average, a server therefore is
      limited to about 15000 notifications per second per client and
      resource.  This number may seem high in today's constrained node/
      networks, but it allows some leeway for both increased
      EXCHANGE_LIFETIME and high notification frequencies.

4.5.  Retransmission

   In CoAP, confirmable messages are retransmitted in exponentially
   increasing intervals for a certain number of attempts until they are
   acknowledged by the client.  In the context of observing a resource,
   it is undesirable to continue transmitting the representation of a
   resource state when the state has changed in the meantime.

   When a server is in the process of delivering a confirmable
   notification and is waiting for an acknowledgement, and it wants to
   notify the client of a state change using a new confirmable message,
   it MUST stop retransmitting the old notification and SHOULD attempt
   to deliver the new notification with the number of attempts remaining
   from the old notification.  When the last attempt to retransmit a
   confirmable message with a notification for a resource times out, the
   server SHOULD remove the client from the list of observers and
   additionally MAY remove the client from the lists of observers of all
   resources in its namespace.

   The server SHOULD use a number of retransmit attempts
   (MAX_RETRANSMIT) such that removing a client from the list of
   observers before Max-Age ends is avoided.

   A server MAY choose to skip a notification if it knows that it will
   send another notification soon (e.g., when the state is changing
   frequently).  Similarly, it MAY choose to send a notification more
   than once.  For example, when state changes occur in bursts, the
   server can skip some notifications, send the notifications in non-
   confirmable messages, and make sure that the client observes the
   latest state change after the burst by repeating the last
   notification in a confirmable message.

   When a notification is transmitted multiple times (either as caused
   by a retransmission attempt or repeating it), the server MUST update
   value of the Observe Option.  Otherwise, the client might discard the
   notification as too old.




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5.  Intermediaries

   A client may be interested in a resource in the namespace of an
   origin server that is reached through one or more CoAP-to-CoAP
   intermediaries.  In this case, the client registers its interest with
   the first intermediary towards the origin server, acting as if it was
   communicating with the origin server itself as specified in
   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.  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, i.e. each hop
   in the server role MUST determine individually how many notifications
   to send, of which 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.

   Because a client (or an intermediary in the client role) can only be
   once in the list of observers of a resource at a server (or an
   intermediary in the server role) -- it is useless to observe the same
   resource multiple times -- an intermediary MUST observe a resource
   only once, even if there are multiple clients for which it observes
   the resource.

   An intermediary is not required to act on behalf of a client to
   observe a resource; an intermediary MAY observe a resource, for
   instance, just to keep its own cache up to date.

   See Appendix A.1 for examples.


6.  Block-wise Transfers

   Resources observed by clients may be larger than can be comfortably
   processed or transferred in one CoAP message.  CoAP provides a block-
   wise transfer mechanism to address this problem
   [I-D.ietf-core-block].  The following rules apply to the combination
   of block-wise transfers with notifications.

   As with basic GET transfers, the client can indicate its desired
   block size in a Block2 Option in the GET request.  If the server



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   supports block-wise transfers, it SHOULD take note of the block size
   for all notifications/responses resulting from the GET request (until
   the client is removed from the list of observers or the server
   receives a new GET request from the client).

   When sending a 2.05 (Content) notification, the server always sends
   all blocks of the representation, suitably sequenced by its
   congestion control mechanism, even if only some of the blocks have
   changed with respect to a previous value.  The server performs the
   block-wise transfer by making use of the Block2 Option in each block.
   When reassembling representations that are transmitted in multiple
   blocks, the client MUST NOT combine blocks carrying different Observe
   Option values, or blocks that have been received more than
   approximately 2**14 seconds apart.

   See Appendix A.2 for an example.


7.  Discovery

   A web link [RFC5988] to a resource accessible by the CoAP protocol
   MAY indicate that the server encourages the observation of this
   resource by including the link target attribute "obs".  This is
   particularly useful in link-format documents [RFC6690].

   The presence of this attribute can, for example, be used to indicate,
   via a graphical representation in a user interface, that this
   resource is changing its value and is useful for monitoring.  The
   presence of this attribute is not a promise, though, that the Observe
   Option can actually be used to perform this observation.  A client
   may need to resort to polling the resource if the Observe Option is
   not returned in the reply to the GET request.

   The "obs" attribute is used as a flag, and thus has no value
   component -- a value given for the attribute MUST NOT be given for
   this version of the specification and MUST be ignored if present.
   The attribute MUST NOT be given more than once for this version of
   the specification.


8.  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



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   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.


9.  IANA Considerations

   The following entries are added to the CoAP Option Numbers registry:

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


10.  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
   Castellani, Gilbert Clark, Esko Dijk, Thomas Fossati, Brian Frank,
   Cullen Jennings, Matthias Kovatsch, Salvatore Loreto, Charles Palmer
   and Zach Shelby for helpful comments and discussions that have shaped
   the document.

   Klaus Hartke was funded by the Klaus Tschira Foundation.


11.  References






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11.1.  Normative References

   [I-D.ietf-core-block]
              Bormann, C. and Z. Shelby, "Blockwise transfers in CoAP",
              draft-ietf-core-block-10 (work in progress), October 2012.

   [I-D.ietf-core-coap]
              Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
              "Constrained Application Protocol (CoAP)",
              draft-ietf-core-coap-12 (work in progress), October 2012.

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

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

11.2.  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>.

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

   [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.





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Appendix A.  Examples

         Observed   CLIENT  SERVER     Actual
     t   State         |      |         State
         ____________  |      |  ____________
     1                 |      |
     2    unknown      |      |       18.5 C
     3                 +----->|                  Header: GET 0x43011633
     4                 | GET  |                   Token: 0x4a
     5                 |      |                Uri-Path: temperature
     6                 |      |                 Observe:
     7                 |      |
     8                 |      |
     9   ____________  |<-----+                  Header: 2.05 0x63451633
    10                 | 2.05 |                   Token: 0x4a
    11    18.5 C       |      |                 Observe: 9
    12                 |      |                 Max-Age: 15
    13                 |      |                 Payload: "18.5 C"
    14                 |      |
    15                 |      |  ____________
    16   ____________  |<-----+                  Header: 2.05 0x53457b50
    17                 | 2.05 |       19.2 C      Token: 0x4a
    18    19.2 C       |      |                 Observe: 16
    29                 |      |                 Max-Age: 15
    20                 |      |                 Payload: "19.2 C"
    21                 |      |

      Figure 3: A client registers and receives a notification of the
                   current state and upon a state change






















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         Observed   CLIENT  SERVER     Actual
     t   State         |      |         State
         ____________  |      |  ____________
    22                 |      |
    23    19.2 C       |      |       19.2 C
    24                 |      |  ____________
    25                 | X----+                  Header: 2.05 0x53457b51
    26                 | 2.05 |       19.7 C      Token: 0x4a
    27                 |      |                 Observe: 25
    28                 |      |                 Max-Age: 15
    29                 |      |                 Payload: "19.7 C"
    30                 |      |
    31   ____________  |      |
    32                 +----->|                  Header: GET 0x43011633
    33    19.2 C       | GET  |                   Token: 0xb2
    34    (stale)      |      |                Uri-Path: temperature
    35                 |      |                 Observe:
    36                 |      |
    37                 |      |
    38   ____________  |<-----+                  Header: 2.05 0x54457b52
    39                 | 2.05 |                   Token: 0xb2
    40    19.7 C       |      |                 Observe: 38
    41                 |      |                 Max-Age: 15
    42                 |      |                    ETag: 0x78797a7a79
    43                 |      |                 Payload: "19.7 C"
    44                 |      |

           Figure 4: The client re-registers after Max-Age ends























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         Observed   CLIENT  SERVER     Actual
     t   State         |      |         State
         ____________  |      |  ____________
    45                 |      |
    46    19.7 C       |      |       19.7 C
    47                 |      |
    48                 |      |  ____________
    49                 |    CRASH
    50                 |
    51                 |
    52                 |      |
    53   ____________  |      |  ____________
    54                 +----->|                  Header: GET 0x44011634
    55    19.7 C       | GET  |       20.0 C      Token: 0xf9
    56    (stale)      |      |                Uri-Path: temperature
    57                 |      |                 Observe:
    58                 |      |                    ETag: 0x78797a7a79
    59                 |      |
    60                 |      |
    61   ____________  |<-----+                  Header: 2.05 0x63451634
    62                 | 2.05 |                   Token: 0xf9
    63    20.0 C       |      |                 Observe: 61
    64                 |      |                 Max-Age: 15
    65                 |      |                 Payload: "20.0 C"
    66                 |      |
    67                 |      |  ____________
    68   ____________  |<-----+                  Header: 2.03 0x5443aa0c
    69                 | 2.03 |       19.7 C      Token: 0xf9
    70    19.7 C       |      |                 Observe: 68
    71                 |      |                    ETag: 0x78797a7a79
    72                 |      |                 Max-Age: 15
    73                 |      |

        Figure 5: The client re-registers and gives the server the
                  opportunity to select a stored response
















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A.1.  Proxying

   CLIENT  PROXY  SERVER
      |      |      |
      |      +----->|     Header: GET 0x44015fb8
      |      | GET  |      Token: 0x1a
      |      |      |   Uri-Host: sensor.example
      |      |      |   Uri-Path: status
      |      |      |    Observe:
      |      |      |
      |      |<-----+     Header: 2.05 0x63455fb8
      |      | 2.05 |      Token: 0x1a
      |      |      |    Observe: 42
      |      |      |    Max-Age: 60
      |      |      |    Payload: "ready"
      |      |      |
      +----->|      |     Header: GET 0x42011633
      | GET  |      |      Token: 0x9a
      |      |      |  Proxy-Uri: coap://sensor.example/status
      |      |      |
      |<-----+      |     Header: 2.05 0x62451633
      | 2.05 |      |      Token: 0x9a
      |      |      |    Max-Age: 53
      |      |      |    Payload: "ready"
      |      |      |
      |      |<-----+     Header: 2.05 0x534505fc0
      |      | 2.05 |      Token: 0x1a
      |      |      |    Observe: 135
      |      |      |    Max-Age: 60
      |      |      |    Payload: "busy"
      |      |      |
      +----->|      |     Header: GET 0x42011634
      | GET  |      |      Token: 0x9b
      |      |      |  Proxy-Uri: coap://sensor.example/status
      |      |      |
      |<-----+      |     Header: 2.05 0x62451634
      | 2.05 |      |      Token: 0x9b
      |      |      |    Max-Age: 49
      |      |      |    Payload: "busy"
      |      |      |

    Figure 6: A proxy observes a resource to keep its cache up to date









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   CLIENT  PROXY  SERVER
      |      |      |
      +----->|      |     Header: GET 0x43011635
      | GET  |      |      Token: 0x6a
      |      |      |  Proxy-Uri: coap://sensor.example/status
      |      |      |    Observe:
      |      |      |
      |<- - -+      |     Header: 0x60001635
      |      |      |
      |      +----->|     Header: GET 0x4401af90
      |      | GET  |      Token: 0xaa
      |      |      |   Uri-Host: sensor.example
      |      |      |   Uri-Path: status
      |      |      |    Observe:
      |      |      |
      |      |<-----+     Header: 2.05 0x6345af90
      |      | 2.05 |      Token: 0xaa
      |      |      |    Observe: 67
      |      |      |    Max-Age: 60
      |      |      |    Payload: "ready"
      |      |      |
      |<-----+      |     Header: 2.05 0x4345af94
      | 2.05 |      |      Token: 0x6a
      |      |      |    Observe: 17346
      |      |      |    Max-Age: 60
      |      |      |    Payload: "ready"
      |      |      |
      +- - ->|      |     Header: 0x6000af94
      |      |      |
      |      |<-----+     Header: 2.05 0x53455a20
      |      | 2.05 |      Token: 0xaa
      |      |      |    Observe: 157
      |      |      |    Max-Age: 60
      |      |      |    Payload: "busy"
      |      |      |
      |<-----+      |     Header: 2.05 0x5345af9b
      | 2.05 |      |      Token: 0x6a
      |      |      |    Observe: 17436
      |      |      |    Max-Age: 60
      |      |      |    Payload: "busy"
      |      |      |

          Figure 7: A client observes a resource through a proxy








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A.2.  Block-wise Transfer

   CLIENT  SERVER
      |      |
      +----->|     Header: GET 0x43011636
      | GET  |      Token: 0xfb
      |      |   Uri-Path: status-icon
      |      |    Observe:
      |      |
      |<-----+     Header: 2.05 0x64451636
      | 2.05 |      Token: 0xfb
      |      |     Block2: 0/1/128
      |      |    Observe: 62354
      |      |    Max-Age: 60
      |      |    Payload: [128 bytes]
      |      |
      |<-----+     Header: 2.05 0x5445af9c
      | 2.05 |      Token: 0xfb
      |      |     Block2: 1/0/128
      |      |    Observe: 62354
      |      |    Max-Age: 60
      |      |    Payload: [27 bytes]
      |      |
      |<-----+     Header: 2.05 0x5445af9d
      | 2.05 |      Token: 0xfb
      |      |     Block2: 0/1/128
      |      |    Observe: 62444
      |      |    Max-Age: 60
      |      |    Payload: [128 bytes]
      |      |
      |<-----+     Header: 2.05 0x5445af9e
      | 2.05 |      Token: 0xfb
      |      |     Block2: 1/0/128
      |      |    Observe: 62444
      |      |    Max-Age: 60
      |      |    Payload: [27 bytes]
      |      |

       Figure 8: A server sends two notifications of two blocks each


Appendix B.  Modeling Resources to Tailor Notifications

   A server may want to provide notifications that respond to very
   specific conditions on some state.  This is best done by modeling the
   resources that the server exposes according to these needs.

   For example, for a CoAP server with an attached temperature sensor,



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   o  the server could, in the simplest form, expose a resource
      <coap://server/temperature> that changes its state every second to
      the current temperature measured by the sensor;

   o  the server could, however, also expose a resource
      <coap://server/temperature/felt> that changes its state to "cold"
      when the temperature drops below a preconfigured threshold, and to
      "warm" when the temperature exceeds a second, higher threshold;

   o  the server could expose a parameterized resource
      <coap://server/temperature/critical?above=45> that changes its
      state to the current temperature if the temperature exceeds the
      specified value, and changes its state to "OK" when the
      temperature drops below; or

   o  the server could expose a parameterized resource <coap://server/
      temperature?query=select+avg(temperature)+from+
      Sensor.window:time(30sec)> that accepts expressions of arbitrary
      complexity and changes its state accordingly.

   In any case, the client is notified about the current state of the
   resource whenever the state of the appropriately modeled resource
   changes.  By designing resources that change their state on certain
   conditions, it is possible to notify the client only when these
   conditions occur instead of continuously supplying it with
   information it doesn't need.  With parametrized resources, this is
   not limited to conditions defined by the server, but can be extended
   to arbitrarily complex conditions defined by the client.  Thus, the
   server designer can choose exactly the right level of complexity for
   the application envisioned and devices used, and is not constrained
   to a "one size fits all" mechanism built into the protocol.


Appendix C.  Changelog

   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 RST message that is in reply to a non-
      confirmable notification (#225).

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




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   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).

   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 RST in reply to non-confirmable
      notifications.

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

   o  Updated examples.

   Changes from ietf-03 to ietf-04:

   o  Removed the "Max-OFE" Option.

   o  Allowed RST 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.




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   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 block-wise 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:

   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).



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   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 block-wise transfers (#36).

   o  Added Resource Discovery section (#99).

   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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