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core                                                               K. Li
Internet-Draft                                           B. Greevenbosch
Intended status: Standards Track                     Huawei Technologies
Expires: August 30, 2012                                         E. Dijk
                                                        Philips Research
                                                               S. Loreto
                                                                Ericsson
                                                       February 27, 2012


                    CoAP Option Extension: Patience
                 draft-li-core-coap-patience-option-00

Abstract

   CoAP is a RESTful application protocol for constrained nodes and
   networks.  This specification provides a simple extension for CoAP,
   the Patience Option.  This Option informs a recipient of the
   preferred time frame for a request or response depending on usage
   context.  In a unicast request, it indicates the patience a client
   has in waiting for a response.  The CoAP server tries to return the
   response within the specified time frame.  In a multicast request, it
   indicates the patience a server should have in sending its response.
   The recipient would then try to randomly delay its response within
   the time frame that the requester indicated.  In a CoAP observe
   notification, it indicates the patience an observer should have in
   both waiting for a subsequent notification and in re-establishing an
   observation relation.

Note

   Discussion and suggestions for improvement are requested, and should
   be sent to core@ietf.org.

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
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   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."



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   This Internet-Draft will expire on August 30, 2012.

Copyright Notice

   Copyright (c) 2012 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
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   described in the Simplified BSD License.



































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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  4
     1.1.  Justification  . . . . . . . . . . . . . . . . . . . . . .  4
     1.2.  Terminology  . . . . . . . . . . . . . . . . . . . . . . .  5
   2.  Patience Option Extension  . . . . . . . . . . . . . . . . . .  5
     2.1.  Patience Option Definition . . . . . . . . . . . . . . . .  5
     2.2.  Using the Patience Option  . . . . . . . . . . . . . . . .  6
       2.2.1.  Unicast usage  . . . . . . . . . . . . . . . . . . . .  6
       2.2.2.  Multicast usage  . . . . . . . . . . . . . . . . . . .  7
       2.2.3.  Observe usage  . . . . . . . . . . . . . . . . . . . .  8
     2.3.  Detection of IP unicast or multicast CoAP request  . . . .  9
   3.  Example  . . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     3.1.  Unicast Usage Example  . . . . . . . . . . . . . . . . . .  9
     3.2.  Observe Usage Example  . . . . . . . . . . . . . . . . . . 10
   4.  Security Considerations  . . . . . . . . . . . . . . . . . . . 11
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   6.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 11
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     7.1.  Normative References . . . . . . . . . . . . . . . . . . . 11
     7.2.  Informative References . . . . . . . . . . . . . . . . . . 11
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 12





























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1.  Introduction

   This specification adds a new option Patience to CoAP
   [I-D.ietf-core-coap].  The main purpose is for the requester to
   inform the recipient of the preferred time frame for a response.  In
   the unicast request case, it is used to indicate the patience it has
   in waiting for a response.  It then indicates "a response is most
   useful within the specified time frame".  In the multicast request
   case, it indicates the patience that a server should have in sending
   a response.  In other words, it indicates "if possible please delay
   your response by a randomly chosen time within the specified time
   frame".  A second purpose is for use by a server when sending CoAP
   observe [I-D.ietf-core-observe] notifications, to indicate the
   maximum time an observer should wait (i.e. patience of the observer)
   before starting any observation relationship recovery.

1.1.  Justification

   In the unicast case, it is useful for the requester (client) to
   indicate that the response is required to be returned within a
   certain amount of time.  For example, the requester could require a
   response within 2 seconds, otherwise the response is not of interest
   anymore.  With this indication of the patience for a response, the
   requester knows how long it should wait for the response, and it
   needs to keep the state of the request only for the indicated time.
   After this period, the request will be given up.  It can avoid that
   the recipient wastes resources by sending a response which already
   exceeds the set patience timeout of the requester.

   In the multicast case, it is useful for the requester to indicate
   that the response is required to be replied to with a dithered delay
   i.e. randomly chosen within the Option-specified time interval.  This
   can be used to avoid network congestion issues that may arise when
   many CoAP servers simultaneously respond to a multicast request.
   Putting the dither time interval under control of a CoAP client
   allows optimal tuning of the interval to various use cases and LLN
   topologies.  Furthermore, it allows multiple applications with
   different dithering needs to co-exist on one and the same LLN.

   TODO: If the indication of client patience, which is currently only
   defined for unicast CoAP requests, would also be required for
   multicast CoAP requests, an alternative to the solution provided in
   this document is to define two types of Patience Option to be able to
   express client patience and server patience separately.

   In an observe scenario, it is useful for a server to indicate to an
   observer that, after the period of time in the Max-Age option has
   expired, a new notification will be sent within the time interval



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   indicated by the Patience option.  The server may use this to send
   notifications with a dithered delay i.e. randomly chosen within the
   Patience-specified time interval, when there are many CoAP clients
   simultaneously observing a resource on the server, avoiding network
   congestion issues.  Another use is for the server to delay sending a
   new notification because e.g. the resource has not changed.  The
   observer in this case can assume that the server will do its best to
   deliver a notification at least before the Patience time interval
   runs out.

   If the Patience option is combined with Observe option in a request,
   currently it indicates the maximum time an observer is prepared to
   wait for an initial notification.

1.2.  Terminology

   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 [RFC2119].


2.  Patience Option Extension

2.1.  Patience Option Definition


         +------+-----+----------------+-----------+--------+----------+
         | Type | C/E |      Name      | Data type | Length | Default  |
         +------+-----+----------------+-----------+--------+----------+
         |  20  | E   |    Patience    | pseudo-FP |   1 B  | (none)   |
         +------+-----+----------------+-----------+--------+----------+

   The value carried in the Patience Option is in a specific format
   resembling a pseudo-Floating Point value (as in
   [I-D.bormann-coap-misc] Appendix B.2):

                 0
                 0 1 2 3 4 5 6 7
                +-+-+-+-+-+-+-+-+
                |     T     | TX|
                +-+-+-+-+-+-+-+-+

   T = Time

   TX = Time Exponent

   where the patience time is calculated as:




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   Patience time = 2^(TX * 4 + 3) * T

   The value of the Patience option is calculated in milliseconds or
   alternatively mibiseconds (1/1024s) if this would ease numerical
   operations on above values on constrained platforms.  The minimum
   non-zero patience time is 8ms, when TX=0, T=1 and a milliseconds time
   base is used.  The maximum patience time is then 2064384ms, around 34
   minutes, when TX=3 and T=63.

2.2.  Using the Patience Option

   The semantics of the Patience Option depends on its usage context, as
   detailed in below sections.

2.2.1.  Unicast usage

   In the unicast case, this option is used by a CoAP client to indicate
   the maximum time a requester is prepared to wait for a response.

   The requester adds the Patience option to any request for which it is
   prepared to wait for a response.  The requester sets the option to
   the maximum time that it is prepared to wait.

   The Patience option applies to both piggy-backed response and
   separate response.  For a separate response, the patience applies to
   the actual response after the ACK.  ACK should be sent immediately
   upon receipt of the CON message.

   TBD: In case a requester retransmits a request, the Patience Option
   value MAY be decreased by an amount of time equivalent to the time
   since the previous transmission attempt.  In case a requester did not
   receive an ACK to a confirmable request and a time interval of at
   least the interval indicated in the Patience Option of the request
   has passed, the requester SHOULD give up the request.

   The recipient interprets this option as the maximum time between
   receipt of the complete request and the time that it begins sending
   the response.  The requester will observe a longer time interval
   between request and response, as network transit and processing by
   proxies add delays.  If timing is critical, the requester SHOULD
   consider the possible delays and choose the value for the option
   accordingly.

   The recipient MAY apply a lower value to the patience timeout based
   on local policy.  A recipient MAY choose to take longer to produce a
   response, at the risk that the requester is no longer able to use the
   response.




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   In case that the CoAP message is transmitted through a proxy, the
   Proxy MAY reduce the value of a Patience option based on a local
   policy (e.g. to consider the maximum time that an idle connection is
   kept open by a local NAT or Firewall).  A Proxy MAY add a Patience
   option if none is present.  The value in the Patience option MUST NOT
   be increased or removed.

   If the requester does not receive a response within the indicated
   response time, the requester SHOULD consider the request as failed.
   If the recipient can't provide a response within the required time,
   the recipient SHOULD discard the request.

   This option is "elective".  It MUST NOT occur more than once.

2.2.2.  Multicast usage

   In the multicast case, this option is used in a CoAP request to
   indicate that the response SHOULD be replied with a dithered delay
   i.e. a randomly chosen delay between 0 and the time indicated in the
   option.  The purpose of specifying a delay is to give a client a
   flexible tool that helps avoiding congestion i.e. multicast response
   storms in constrained networks, while still fulfilling timing
   requirements of CoAP applications.

   A requester adds the Patience option to any multicast request for
   which it would prefer that servers respond with a dithered delay.
   The requester sets the option to the maximum time that it prefers for
   the dithering interval.

   The recipient interprets this option as the maximum time between
   receipt of the complete request and the time that it begins sending
   the response.  The requester will possibly observe a longer time
   interval between request and some responses, as network transit and
   processing by proxies add delays.

   The recipient MAY apply a lower value to the patience timeout based
   on local policy.  A recipient MAY choose to take longer to produce a
   response, based on local policy.

   In case that the CoAP message is transmitted through a proxy, the
   Proxy MAY reduce the value of a Patience option based on a local
   policy.  A Proxy MAY add a Patience option if none is present.  The
   value in the Patience option SHOULD NOT be increased or removed.

   If the requester does not receive a response, or all expected
   responses, within the indicated response time, the requester SHOULD
   NOT consider the request as fully or partially failed since valid
   delayed responses could still be arriving.  This is the case because



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   "client patience" is not defined with multicast use of the Patience
   Option.  If a recipient can't provide the response within the
   required time, no special action needs to be taken and the response
   can be sent as usual.

   This option is "elective".  It MUST NOT occur more than once.

2.2.3.  Observe usage

   In a CoAP observe [I-D.ietf-core-observe] scenario, the Patience
   Option MAY be used in a notification to indicate the maximum time an
   observer should wait before starting any observation relationship
   recovery.

   The Max-Age Option indicates the maximum time a response
   (notification) may be cached before it MUST be considered stale.  The
   Max-Age Option of a notification is usually set to a value that
   estimates when the server will send the next notification.  However,
   in the case the value has not changed, the server can decide not to
   send a new notification, possibly confusing the observer.  It is
   quite difficult for an observer to discriminate the situation that it
   has not received a new notification because the value has not changed
   from situations where the server has lost its state, or for some
   reason has given up on notification delivery.

   The Patience Option in a notification is used to indicate the maximum
   time a server will try to reach the client before giving up (i.e. it
   solves Ticket 174 "Robust observation relationship").  This option is
   also useful to give a server the time to send out the notifications,
   in case there are many CoAP clients observing simultaneously a
   resource, while avoiding network congestion issues.

   The server adds the Patience option to any notification related to an
   observation relationship from which it wants delay an observation
   refresh request made by the observer.  The server sets the option to
   the maximum time that it is prepared to spend to reach the observer
   before giving up.

   The observer interprets this option as the minimum time between the
   expiration of a notification (as indicated by its Max-Age Option
   value) and the moment it MAY start an observation relationship
   recovery action with the server.

   If the observer does not receive a response within the indicated time
   interval, the observer SHOULD attempt to re-establish the observation
   relationship with the server if it is still interested in observing
   the particular resource.




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2.3.  Detection of IP unicast or multicast CoAP request

   A single Patience Option, used to indicate potentially either client
   patience (in the IP unicast case) or server patience (in the IP
   multicast case), requires that a CoAP server is able to distinguish
   between IP unicast and multicast requests.  If there exist commonly
   used IP stacks that do not offer such functionality [to be checked],
   requiring servers to be able to make the unicast/multicast
   distinction seems unwise and limits the applicability of the Patience
   Option.

   Approaches for a CoAP server to detect unicast versus multicast
   requests may include:

   1) CoAP server application opens a specific socket and sets IP
   multicast reception using the POSIX setsockopt function [to be
   verified if IP unicast traffic also is received in this case, or
   not].

   2) CoAP server checks the IP destination address of incoming packets.
   If this has the FF00::/8 IPv6 prefix, then it's treated as multicast
   otherwise unicast [to be verified if IP stack APIs allow to get IP
   destination].

   3) Receiving CoAP multicast requests always occurs on a different
   port than the standard CoAP port.  For example, similar to coaps://
   that uses a different port than coap://, a scheme coapm:// on a
   different port may be defined for multicast requests.


3.  Example

3.1.  Unicast Usage Example

   This section gives a short example with a message flow that
   illustrates the use of the Patience option in a GET request.

   This example (Figure 1) shows that the requester wants to get a
   response within 3200 milliseconds, when T=25, TX=1.












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    requester recipient
         |      |
         |      |
         +----->|          Header: GET (T=CON, Code=1, MID=0x7d38)
         | GET  |           Token: 0x53
         |      |        Patience: 25/1
         |      |        Uri-Path: "temperature"
         |      |
         |<-----+     Header: 2.05 Content (T=ACK, Code=69, MID=0x7d38)
         | 2.05 |      Token: 0x53
         |      |    Payload: "22.3 C"
         |      |



              Figure 1: Patience Option in a unicast request

3.2.  Observe Usage Example

   This section gives a short example with a message flow that
   illustrates the use of the Patience option in an Observe
   notification.

   This example (Figure 2) shows that the server wants the observer to
   wait 819 seconds (T=25, TX=3) before starting any observation
   relationship recovery, even though the Max-Age of the temperature
   value notification is only 120 seconds.


     Observer Server
         |      |
         |      |
         +----->|          Header: GET (T=CON, Code=1, MID=0x7d38)
         | GET  |           Token: 0x53
         |      |         Observe: 0
         |      |        Uri-Path: "temperature"
         |      |
         |<-----+     Header: 2.05 Content (T=ACK, Code=69, MID=0x7d38)
         | 2.05 |      Token: 0x53
         |      |    Max-Age: 120
         |      |   Patience: 25/3
         |      |    Payload: "22.3 C"
         |      |



           Figure 2: Patience Option in an observe notification




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4.  Security Considerations

   This presents no security considerations beyond those in section 10
   of the base CoAP specification [I-D.ietf-core-coap].


5.  IANA Considerations

   The IANA is requested to add the following "CoAP Option Numbers"
   entry as per Section 11.2 of [I-D.ietf-core-coap].

                   +--------+------------------+----------------+
                   | Number | Name             | Reference      |
                   +--------+------------------+----------------+
                   |  20    |     Patience     | Section 2      |
                   +--------+------------------+----------------+


6.  Acknowledgements

   The authors of this draft would like to thank the participants of the
   email discussion on this issue.  Thanks to Carsten Bormann, Peter
   Bigot, Barry Leiba, Linyi Tian for the reviews and discussions.


7.  References

7.1.  Normative References

   [I-D.ietf-core-coap]
              Frank, B., Bormann, C., Hartke, K., and Z. Shelby,
              "Constrained Application Protocol (CoAP)",
              draft-ietf-core-coap-08 (work in progress), October 2011.

   [I-D.ietf-core-observe]
              Hartke, K., "Observing Resources in CoAP",
              draft-ietf-core-observe-04 (work in progress),
              February 2012.

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

7.2.  Informative References

   [I-D.bormann-coap-misc]
              Bormann, C. and K. Hartke, "Miscellaneous additions to
              CoAP", draft-bormann-coap-misc-12 (work in progress),
              February 2012.



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Authors' Addresses

   Kepeng Li
   Huawei Technologies
   Huawei Base, Bantian, Longgang District
   Shenzhen, Guangdong  518129
   P. R. China

   Phone: +86-755-28974289
   Email: likepeng@huawei.com


   Bert Greevenbosch
   Huawei Technologies
   Huawei Base, Bantian, Longgang District
   Shenzhen, Guangdong  518129
   P. R. China

   Phone: +86-755-28978088
   Email: bert.greevenbosch@huawei.com


   Esko Dijk
   Philips Research
   High Tech Campus 34
   Eindhoven
   The Netherlands

   Email: esko.dijk@philips.com


   Salvatore Loreto
   Ericsson
   Hirsalantie 11
   Jorvas 02420
   Finland

   Email: salvatore.loreto@ericsson.com













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