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CoRE                                                             M. Vial
Internet-Draft                                        Schneider-Electric
Intended status: Standards Track                           July 13, 2012
Expires: January 14, 2013

                           CoRE Mirror Server


   The Constrained RESTful Environments (CoRE) working group aims at
   realizing the REpresentational State Transfer (REST) architecture in
   a suitable form for the most constrained nodes.  Thanks to the
   Constrained Application Protocol (CoAP), REST is now applicable to
   constrained networks.  However the most energy-constrained devices
   may enter sleep mode and disconnect their network link during several
   minutes to save energy, hence preventing them from acting as
   traditional web servers.  This document describes how a sleeping
   device can store resource representations in an entity called Mirror
   Server and participate in a constrained RESTful environment.

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 January 14, 2013.

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
   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

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   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.  Motivation . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Uses cases . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.3.  Assumptions and objectives . . . . . . . . . . . . . . . .  4
   2.  Requirements Language  . . . . . . . . . . . . . . . . . . . .  5
   3.  Architecture . . . . . . . . . . . . . . . . . . . . . . . . .  5
   4.  Mirror Server Function Set . . . . . . . . . . . . . . . . . .  6
     4.1.  Discovery  . . . . . . . . . . . . . . . . . . . . . . . .  7
     4.2.  Registration . . . . . . . . . . . . . . . . . . . . . . .  8
     4.3.  Update . . . . . . . . . . . . . . . . . . . . . . . . . . 11
     4.4.  Validation . . . . . . . . . . . . . . . . . . . . . . . . 12
     4.5.  Removal  . . . . . . . . . . . . . . . . . . . . . . . . . 12
     4.6.  SEP Operation  . . . . . . . . . . . . . . . . . . . . . . 12
     4.7.  Client Operation . . . . . . . . . . . . . . . . . . . . . 14
     4.8.  Modification check . . . . . . . . . . . . . . . . . . . . 15
   5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 17
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 17
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . . 17
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 18
     8.1.  Normative References . . . . . . . . . . . . . . . . . . . 18
     8.2.  Informative References . . . . . . . . . . . . . . . . . . 19
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 19

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1.  Introduction

1.1.  Motivation

   The Constrained RESTful Environments (CoRE) working group aims at
   realizing the REST architecture in a suitable form for the most
   constrained nodes (e.g. 8-bit microcontrollers with limited RAM and
   ROM, energy harvesting) and networks (e.g. 6LoWPAN).  The CoRE
   charter says that the CoAP protocol [I-D.ietf-core-coap] will support
   various form of "caching" to support sleeping devices.  And the CoAP
   requirement REQ3 in [I-D.shelby-core-coap-req] clearly states that
   support of sleeping devices is required:

      The ability to deal with sleeping nodes.  Devices may be powered
      off at any point in time but periodically "wake up" for brief
      periods of time.

   As pointed out by [I-D.arkko-core-sleepy-sensors], the server model
   is not appropriate for the most energy-constrained devices.  CoAP
   also supports the Publish/Subscribe pattern through CoAP observe
   [I-D.ietf-core-observe].  Notifications with CoAP observe prove to be
   efficient however as it is currently specified, it still requires the
   server model to create and maintain the observation relationship.
   Although CoAP observe may be enhanced to support subscriptions
   initiated by the observed server, this method is not currently
   specified.  Also in general, a SEP would support only a limited
   number of observers at a time.  The client model is a viable approach
   but the interactions and interfaces between endpoints are currently
   undefined.  In conclusion, the current working group documents do not
   propose a complete solution for sleeping devices that are not always

1.2.  Uses cases

   With the emergence of the Internet of Things we expect a major
   breaktrough in the number of smart objects in our environment.  Yet
   providing these objects with sufficient energy for continued
   operation and long battery lifetime is still a big challenge.  That
   is the reason why this specification strives to provide a solution to
   dramatically reduce the power consumption of constrained RESTful
   sensors.  For battery-operated devices the need to improve battery
   lifetime is persistent either to reduce the size of smart objects and
   fit new applications, to increase the product lifetime when it is
   directly coupled to its battery lifetime or to reduce the annoyance,
   costs and wastes incurred by changing batteries too frequently.
   There is also a new trend to avoid batteries and create sensors that
   can harvest energy from their environment.  For those devices it is
   of prime importance to maintain a high ratio between harvested energy

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   and power consumption.  This ratio has a direct impact on service
   availability and the user experience especially because the
   harvesting efficiency is typically not constant in time (e.g day/
   night for a photovoltaic cell).

1.3.  Assumptions and objectives

   In this specification we assume that the energy-constrained devices
   can store a sufficient amount of energy to enable bi-directional
   communication and to perform periodic tasks like maintaining soft
   state.  However the most constrained devices may not be able to store
   energy and may have unpredictable availability due to sporadic energy
   production (e.g. self-powered push button).  This specification may
   be applicable to these devices as long as they have enough energy to
   perform the initial registration.  This may require an additional
   source of power during the commissioning phase.

   Throughout this document we will only consider sleeping devices that
   are totally unreachable during long periods of time.  In other word,
   network connectivity is turned off at least several seconds hence
   generating unacceptable interruptions if the device runs as a server.
   Some link-layer technologies offer advanced low power modes such as
   duty-cycle link activity or receiver initiated transmissions hence
   allowing the devices to sleep while still offering network
   connectivity with an always-on illusion.  Devices for which the
   available energy is sufficient to afford always-on illusion are out
   of scope of this specification since the server model is applicable
   to these endpoints.

   Efficient support of sleeping devices has implications on many
   aspects of the IP stack: Media Access Control (MAC), neighbor
   discovery, routing, REST intermediaries...  This specification does
   not aim to find a solution for all of those.  The objective is to
   provide an interaction model at the application level where data
   exchanges are always initiated by the sleeping endpoint.  This way
   the application can finely control when the network link needs to be
   on.  In no way the mechanisms defined here precludes usage of a low
   power mode at link-layer.

   This specification does not pretend to provide full REST support to
   sleeping devices.  These devices will be provided with the minimum
   set of REST features to publish resources.  Particular attention is
   paid to facilitate configuration and to associate meta-data to
   resources from sleeping devices.

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2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

   This specification requires readers to be familiar with all the terms
   and concepts that are discussed in [RFC5988],
   [I-D.shelby-core-resource-directory] and
   [I-D.shelby-core-interfaces].  Readers should also be familiar with
   the terms and concepts discussed in [I-D.ietf-core-coap] and
   [I-D.ietf-core-link-format].  This specification makes use of the
   following additional terminology:

   Sleeping device:  A smart object that can enter a long period of time
      with its network link in disconnected state in order to save

   Sleeping endpoint (SEP):  A sleeping endpoint is an IP sleeping
      device which can participate in a constrained RESTful environment
      as a client-only endpoint.

   Mirror Server (MS):  A server endpoint that implements the Mirror
      Server Function Set.

3.  Architecture

   The Mirror Server architecture is shown in Figure 1.  A Mirror Server
   (MS) is a web server implementing a special Function Set that allows
   a sleeping endpoint (SEP) to create resources in the MS resource
   tree.  For energy efficiency a SEP is a client-only CoAP endpoint and
   hence is not able to serve content by itself.  The MS implements REST
   interfaces allowing a SEP to maintain a set of mirrored resources
   that will be served in turn by the MS.  So a Mirror Server acts as a
   mailbox between the sleeping endpoint and the client.  A CoAP client
   discovers resources owned by the SEP but hosted on the MS using
   typical mechanisms such as "/.well-known/core"
   [I-D.ietf-core-link-format] or Resource Directory

   A SEP must register and maintain a mirror entry on the MS, which is
   soft state and need to be periodically refreshed.  A MS provides
   interfaces to register, update and remove a mirror entry and an
   associated set of mirrored resources.  Furthermore, a MS provides
   interfaces to read and update the mirrored resources from both the
   SEP and client sides.  Finally, a mechanism to discover a MS using
   the CoRE Link Format is defined.

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                  Registration        Discovery
                         |               |
                         |               |
        +-----+          |   +------+    |         +--------+
        |     | --- push --> |      | --- pull --> |        |
        | SEP |          |   |  MS  |    |         | Client |
        |     | <-- pull --- |      | <-- push --- |        |
        +-----+          |   +------+    |         +--------+
                         |               |
                         |               |

                   Figure 1: Mirror Server architecture

   The Mirror Server functionality can be distributed over multiple
   server endpoints in the network or centralized on a single server
   endpoint.  A shorter round-trip time gives better energy efficiency
   for request/response exchanges, so it is important to choose a path
   between the Mirror Server and the sleeping endpoint with minimum
   latency.  Moreover a sleeping endpoint with a Mirror Server in its
   direct neighborhood may even avoid having to configure global IP
   connectivity.  However in a wireless network relying on local
   connectivity may result in fragility due to device mobility or radio
   fluctuations.  This could lead a sleeping endpoint to frequently try
   to move from one Mirror Server to another.  Consequently, clients
   would need to restart resource discovery frequenlty.  In that regard,
   a centralized Mirror Server gives more stability.  A centralized
   Mirror Server also concentrates network traffic on a central point
   and may cause network congestion in a mesh network.  However data
   flow of a sleeping endpoint is expected to be low hence mitigating
   the risk of network congestion.

   A sleeping endpoint MAY register with more than one Mirror Server but
   in that case the resources of a sleeping endpoint appear duplicated
   during resource discovery.  Section 4.1 describes how to detect
   duplicate resources.

4.  Mirror Server Function Set

   The interface is mostly identical to that of the Resource Directory
   Function Set defined in [I-D.shelby-core-resource-directory] so this
   specification only points out the differences.  Contrary to the
   Resource Directory there is no lookup Function Set in a Mirror
   Server.  Indeed, from a client point of view, the mirrored resources
   look like any other resources hosted the MS endpoint.  So resource
   discovery of mirrored resources is directly available through
   "/.well-known/core" instead of a separate Function Set.

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   The examples presented in this section make use of a smart
   temperature sensor the resources of which are defined below using
   Link Format.  Three resources are dedicated to the Device Description
   (manufacturer, model, name) and one contains the current temperature
   in degree Celsius.

   </dev/mfg >;rt="ipso.dev.mfg";if="core.rp",

4.1.  Discovery

   The interaction between a SEP and a MS is based on the same discovery
   interface as the Resource Directory except that the Resource Type in
   the URI template is replaced with "core.ms".

   The following example shows a sleeping endpoint discovering a MS
   using this interface, thus learning that the base MS resource is at

   SEP                                                MS
    |                                                 |
    | ----- GET /.well-known/core?rt=core.ms ------>  |
    |                                                 |
    |                                                 |
    | <---- 2.05 Content "</ms>; rt="core.ms" ------  |
    |                                                 |

   Req: GET coap://[ff02::1]/.well-known/core?rt=core.ms
   Res: 2.05 Content

   Resource discovery between a client and a MS or a client and a RD
   needs special care to take into account the fact that resources from
   a sleeping endpoint might appear duplicated.  Clients SHOULD employ
   2-step resource discovery by looking up sleeping endpoints AND
   resource types to detect duplicate resources.  Clients MAY use
   single-step resource discovery only if the SEP can register with no
   more than one Mirror Server.  A client can use the "ep" link
   attribute as a filter on the "/.well-known/core" resource to retrieve
   a list of endpoints and detect duplicate sleeping endpoints
   registered on multiple MSs.  A client can use the "ep" type of lookup
   to do the same on a RD.  The result of endpoint discovery is then
   used to filter out duplicate resources returned from simple resource

   The following example shows a client discovering the sleeping

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   endpoints and learning that the SEP 0224e8fffe925dcf is registered on
   two Mirror Servers.

   Client                                      MS1    MS2
    |                                          |      |
    | ----- GET /.well-known/core?ep=* ------->|----->|
    |                                          |      |
    |                                          |      |
    | <---- 2.05 Content "</ms/0>..."  --------|      |
    |                                          |      |
    | <---- 2.05 Content "</ms/0>..."  --------|------|

   Req: GET coap://[ff02::1]/.well-known/core?ep=*
   Res: 2.05 Content
   Res: 2.05 Content

   From the previous exchange and the next resource discovery request,
   the client can infer that the resources coap://ms1/ms/0/sen/temp and
   coap://ms2/ms/1/sen/temp actually come from the same sleeping

   Client                                       MS1    MS2
    |                                            |      |
    | - GET /.well-known/core?rt=ipso:ucum.Cel ->|----->|
    |                                            |      |
    |                                            |      |
    | <---- 2.05 Content "</ms/0>..."  ----------|      |
    |                                            |      |
    | <---- 2.05 Content "</ms/1>..."  ----------|------|

   Req: GET coap://[ff02::1]/.well-known/core?rt=ucum.Cel
   Res: 2.05 Content
   Res: 2.05 Content

4.2.  Registration

   The registration interface is identical to the registration interface
   of the Resource Directory Function Set except that the preferred path
   for the Mirror Server Function Set is "/ms".

   After discovering the location of a MS Function Set, a sleeping
   endpoint MAY register its resources that need to be mirrored using
   the registration interface.  This interface accepts a POST from an

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   endpoint containing a description of the resources to be created on
   the Mirror Server as the message payload in the CoRE Link Format
   along with query string parameters indicating the endpoint
   identifier, its domain and the lifetime of the registration.  The
   Link Format description is identical to the "/.well-known/core"
   resource found on a typical server endpoint meaning that the
   Interface Description attributes are actually intended for the Mirror
   Server.  A Mirror Server MUST reject a registration if at least one
   of the Interface Descriptions is not supported.  Upon successful
   registration a MS creates a new resource or updates an existing
   resource for the mirror entry and returns its location.  The
   resources specified by the SEP during registration are created as
   sub-resources of the mirror entry on the MS endpoint.  The
   registration interface MUST be implemented to be idempotent, so that
   registering twice with the same endpoint parameter does not create
   multiple MS entries.  The resource associated to a mirror entry
   SHOULD implement the Interface Type CoRE Link List defined in
   [I-D.shelby-core-interfaces].  A GET request on this resource MUST
   return the list of mirrored resources for the corresponding SEP.

   After successul registration, a MS SHOULD enable resource discovery
   for the new mirrored resources by updating its "/.well-known/core"
   resource.  A MS MUST wait for the initial representation of a
   mirrored resource before it can be visible in resource discovery.
   The top level resource corresponding to a mirror entry MUST be
   published in "/.well-known/core" to enable 2-step resource discovery
   described in Section 4.1.  Sub-resources of a mirror entry SHOULD be
   discoverable either directly in "/.well-known/core" or indirectly
   through gradual reveal from the mirror entry resource.  The Web Link
   of a mirror entry MUST contain an "ep" attribute with the value of
   the End-Point parameter received at registration.  If present, the
   End-Point Type parameter SHOULD also be mapped as a "rt" attribute.

   A Mirror Server MAY be configured to register the SEP's resources in
   a Resource Directory (RD).  A SEP MUST NOT register the mirrored
   resources in a RD by itself.  It is always the responsibility of the
   Mirror Server.  Since each SEP may register resources with different
   lifetimes, a MS MUST register the resources of a SEP in a separate
   resource directory entry.  A SEP may register with multiple MS hence
   the RD entries from the different MS for the same SEP would overlap
   if special care is not taken.  Therefore if a SEP is likely to
   register with more than one MS, a Mirror Server MUST create its own
   domain to register the resources of a SEP.  This precaution ensures
   that the ep identifier of a SEP is unique for each domain in the RD.
   The new domain is typically formed by concatenating the MS's endpoint
   identifier with the domain in use.

   SEP resources in the MS are kept active for the period indicated by

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   the lifetime parameter.  The SEP is responsible for refreshing the
   entry within this period using either the registration or update
   interface.  Once a mirror entry has expired, the MS deletes all
   resources associated to that entry and updates its "/.well-known/
   core" resource.  When the mirrored resources are also registered in a
   RD, the RD and MS entries are supposed to have the same lifetime.
   Consequently, when the mirror entry expires, a MS MAY let the RD
   entry expire too instead of explicitly deleting it.  Nevertheless if
   the MS entry is deleted using the Removal interface then the RD entry
   MUST be explicitly removed.

   A Mirror Server could lose or delete the mirror entry associated to a
   SEP without sending an explicit notification (e.g. after reboot).  A
   SEP SHOULD be able to detect this situation by processing the
   response code while using the SEP Operation or Update interface.
   Especially an error code "4.04 Not Found" SHOULD cause the SEP to
   register again.  A SEP MAY also register with multiple MSs to
   alleviate the risk of interruption of service.

   Implementation note: It is not recommended to reuse the value of the
   ep parameter in the URI of the Mirror Server entry.  This parameter
   may be a relatively long identifier to guarantee global uniqueness
   (e.g.  EUI64) and would generate inefficient URIs on the Mirror
   Server where only a local handler is necessary.

   The following example shows a sleeping endpoint registering with a

   SEP                                                MS
    |                                                 |
    | --- POST /ms "</dev..." -------------------->   |
    |                                                 |
    |                                                 |
    | <-- 2.01 Created Location: /ms/0 -------------  |
    |                                                 |

   Req: POST coap://ms.example.org/ms?ep=0224e8fffe925dcf&rt=sensor
   Etag: 0x3f
   </dev/mfg >;rt="ipso.dev.mfg";if="core.rp",

   Res: 2.01 Created
   Location: /ms/0

   The mirror entry resource below has been created on the MS.

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   Req: GET coap://ms.example.org/.well-known/core
   Res: 2.05 Content

   The SEP sets the initial value for its mirrored resources and the
   following resources are now created.

   Req: GET coap://ms.example.org/ms/0
   Res: 2.05 Content
   </ms/0/dev/mfg >;rt="ipso.dev.mfg";if="core.rp",

   Then the MS registers the mirrored resources in the RD.

   MS                                                 RD
    |                                                 |
    | --- POST /rd "</ms/0..." ------------------->   |
    |                                                 |
    |                                                 |
    | <-- 2.01 Created Location: /rd/6534 ----------  |
    |                                                 |

   Req: POST coap://rd.example.org/rd?ep=0224e8fffe925dcf&
   Etag: 0x6a
   </ms/0/dev/mfg >;rt="ipso.dev.mfg";if="core.rp",

   Res: 2.01 Created
   Location: /rd/6534

4.3.  Update

   The update interface is not necessary on a Mirror Server.  A SEP can
   use the registration interface to modify a mirror entry.  The
   Lifetime query parameter of the SEP operation interface defined in
   Section 4.6 allows a SEP to extend the lifetime of its mirror entry.

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4.4.  Validation

   The validation interface is not supported on a Mirror Server since
   the sleeping endpoint is unreachable.

4.5.  Removal

   The removal interface is identical.

   Upon successful removal, "/.well-known/core" and the Resource
   Directory (if applicable) MUST be updated accordingly.  All resources
   associated to the mirror entry are deleted.

4.6.  SEP Operation

   The SEP Operation interface is not defined for a Resource Directory
   and is specific to the Mirror Server Function Set.

   Once the resources have been created on the MS, the SEP can access
   its mirrored resources at its own pace.  The SEP MAY update its
   mirrored resources on the MS using PUT requests.  The SEP MAY
   retrieve the current representation of any of its mirrored resources
   using GET requests.  The SEP can reactivate its mirror entry by
   including a Lifetime (lt) parameter in the query string.  While
   updating dynamic resources, a SEP SHOULD include a Lifetime parameter
   with the smallest value that matches its technical constraints.  It
   allows a client to fastly detect a stale mirror entry.  A SEP MAY
   omit processing some responses for non confirmable requests in order
   to avoid spending energy waiting for a response when it is frequently
   accessing a mirrored resource.  Nevertheless a SEP SHOULD
   periodically check the responses to ensure that its mirror entry is
   still active on the MS.

   Other specifications may override or extend this interface to provide
   more advanced features, support other REST methods and queuing
   patterns.  This is however out of scope of this specification which
   provides only a basic behavior.

   The basic SEP operation interface is specified as follows:

   Interaction:  SEP -> MS

   Method:  GET, PUT

   URI Template:  /{+location}{+resource}{?lt}

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   URI Template Variables:

      location :=  This is the Location path returned by the MS as a
         result of a successful registration.

      resource :=  This is the relative path to a mirrored resource
         managed by the registered SEP.

      lt :=  Lifetime (optional).  The number of seconds by which the
         lifetime of the whole mirror entry is extended.  Range of
         1-4294967295.  If no lifetime is included, the current
         remaining lifetime stays unchanged.

   Content-Type:  Defined at registration

   Etag:  The Etag option MAY be included to allow clients to validate a
      resource on multiple Mirror Servers.

   Success:  2.01 "Created", the request MUST include the initial
      representation of the mirrored resource.

   Success:  2.04 "Changed", the request MUST include the new
      representation of the mirrored resource.

   Success:  2.05 "Content", the response MUST include the current
      representation of the mirrored resource.

   Failure:  4.00 "Bad Request".  Malformed request.

   Failure:  5.03 "Service Unavailable".  Service could not perform the

   The following example describes how a sleeping endpoint can
   initialize the resource containing its manufacturer name just after

   SEP                                                MS
    |                                                 |
    | --- PUT /ms/0/dev/mfg "acme" --------------->   |
    |                                                 |
    |                                                 |
    | <-- 2.01 Created -----------------------------  |
    |                                                 |

   Req: PUT /ms/0/dev/mfg
   Payload: acme
   Res: 2.01 Created

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   The example below shows how a SEP can indicate that it is supposed to
   send a temperature value at least every hour to keep its mirror entry

   SEP                                                MS
    |                                                 |
    | --- PUT /ms/0/sen/temp?lt=3600 "22" -------->   |
    |                                                 |
    |                                                 |
    | <-- 2.04 Changed -----------------------------  |
    |                                                 |

   Req: PUT /ms/0/sen/temp?lt=3600
   Payload: 22
   Res: 2.04 Changed

4.7.  Client Operation

   The Client Operation interface is not defined for a Resource
   Directory and is specific to the Mirror Server Function Set.

   While the SEP operation interface describes only the interaction
   between the SEP and the MS on mirrored resources, the interface
   between a client and the MS for mirrored resources is actually
   defined by the Link Format payload at registration.  This
   specification does not define the list of Interface Description
   attribute values that a Mirror Server must support.  This is left to
   a companion specification such as a CoRE profile specification.  A
   Mirror Server MAY support complex interfaces that require special
   logic and interactions between multiple mirrored resources.  The CoRE
   Batch interface defined in [I-D.shelby-core-interfaces] is an example
   of complex interface that defines relations between a parent resource
   and sub-resources using SenML [I-D.jennings-senml].

   A SEP may register resources with the "obs" attribute.  In that case
   a MS using the CoAP protocol [I-D.ietf-core-coap] SHOULD accept to
   establish a CoAP observation relationship between the observable
   mirrored resource and a client as defined in [I-D.ietf-core-observe].

   A SEP may stop updating its mirrored resources without explictly
   removing its mirror entry (e.g. transition to another MS after
   network unreachability detection).  A client can detect this
   situation when the corresponding mirror entry has expired.  Upon
   receipt of a response with error code 4.04 "Not Found", a client
   SHOULD restart resource discovery to determine if the resources are
   now mirrored on another MS.

   The Client operation interface is specified as follows:

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   Interaction:  Client -> MS

   Method:  Defined at registration

   URI Template:  /{+location}{+resource}

   URI Template Variables:

      location :=  This is the Location path returned by the MS as a
         result of a successful registration.

      resource :=  This is the relative path to a mirrored resource
         managed by a SEP.

   Content-Type:  Defined at registration

   In the example below a client observes the changes of temperature
   through the Mirror Server.

   SEP                             MS                     Client
    |                              |                         |
    |                              | <- GET /ms/0/sen/temp - |
    |                              |          (observe)      |
    |                              |                         |
    |                              | -- 2.05 Content "22" -> |
    |                              |                         |
    | - PUT /ms/0/sen/temp "23" -> |                         |
    |                              |                         |
    | <- 2.04 Changed ------------ |                         |
    |                              |                         |
    |                              | -- 2.05 Content "23" -> |

4.8.  Modification check

   This interface is not defined for a Resource Directory and is
   specific to the Mirror Server Function Set.

   A sleeping endpoint may register resources supporting POST or PUT
   methods and therefore that could be modified by clients.  In that
   case the SEP needs to poll these resources to detect updates.
   Polling each modifiable resource is inefficient when they are
   numerous.  The modification check interface allows a SEP to retrieve
   a list of resources that have been modified.  The SEP can then send
   GET requests on each resource of the list to get the updated
   representation.  A POST request on the check interface automatically
   clears the list of modified resources.

   The check interface is specified as follows:

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   Interaction:  SEP -> MS

   Method:  POST

   URI Template:  /{+location}?chk

   URI Template Variables:

      location :=  This is the Location path returned by the MS as a
         result of a successful registration.

   Request Content-Type:  None

   Response Content-Type:  application/link-format

   Success:  2.04 "Changed", the response MUST include a list of web
      links to resources that have been modified by clients.

   Failure:  4.00 "Bad Request".  Malformed request.

   Failure:  5.03 "Service Unavailable".  Service could not perform the

   The following example shows a commissioning tool changing the name of
   a sleeping device through a Mirror Server.  A commissioning button on
   the SEP triggers the reading of the new configuration.

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   SEP                          MS                       Client
    |                           |                         |
    |                           | <-- PUT /ms/0/dev/n --- |
    |                           |                         |
    |                           | -- 2.04 Changed ------> |
    |                           |                         |
    | [press button on SEP]     |                         |
    |                           |                         |
    | - POST /ms/0?chk -------> |                         |
    |                           |                         |
    | <- 2.04 Changed --------- |                         |
    |                           |                         |
    | - GET /ms/0/dev/n ------> |                         |
    |                           |                         |
    | <- 2.05 Content --------- |                         |
    |                           |                         |

   Req: PUT /ms/0/dev/n
   Payload: "sensor-1"
   Res: 2.04 Changed

   Req: POST /ms/0?chk
   Res: 2.04 Changed
   Payload: "</ms/0/dev/n>"

   Req: GET /ms/0/dev/n
   Res: 2.05 Content
   Payload: "sensor-1"

5.  Acknowledgements

   Thanks to Zach Shelby who is the author of the Resource Directory
   interface.  Thanks to Nicolas Riou, Jari Arkko, Esko Dijk who have
   provided fruitful comments.

6.  IANA Considerations

   "core.ms" resource type needs to be registered.

   The "ep" attribute needs to be registered.

7.  Security Considerations

   This document needs the same security considerations as described in
   Section 7 of [RFC5988] and Section 6 of [I-D.ietf-core-link-format].

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   The Mirror Server architecture defines the SEP and Client roles in
   the Mirror Function Set interfaces.  Since the roles are based on the
   requester identity, a MS SHOULD perform appropriate authentication in
   order to prevent a malicious client endpoint from impersonating the
   SEP or an authorized client.  Otherwise the malicious client could
   gain access to interfaces allowing corruption or deletion of a
   mirrored resource.

   A malicious client could start a denial of service attack by trying
   to mirror a large resource on a MS.  Memory exhaustion would prevent
   other sleeping endpoints from mirroring their resources.  A MS SHOULD
   use quotas to limit the size and the number of mirrored resources per

   A Mirror Server is actually an intermediary running at application
   level.  As a consequence the Mirror Server architecture can only
   provide implicit end-to-end security that relies on a trusted network
   if security is not available at application layer.  When explicit
   end-to-end security is required between a SEP and a Client, data
   object security SHOULD be employed.

8.  References

8.1.  Normative References

              Shelby, Z., Hartke, K., Bormann, C., and B. Frank,
              "Constrained Application Protocol (CoAP)",
              draft-ietf-core-coap-10 (work in progress), June 2012.

              Shelby, Z., "CoRE Link Format",
              draft-ietf-core-link-format-14 (work in progress),
              June 2012.

              Hartke, K., "Observing Resources in CoAP",
              draft-ietf-core-observe-05 (work in progress), March 2012.

              Shelby, Z. and M. Vial, "CoRE Interfaces",
              draft-shelby-core-interfaces-03 (work in progress),
              July 2012.

              Shelby, Z. and S. Krco, "CoRE Resource Directory",
              draft-shelby-core-resource-directory-03 (work in

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              progress), May 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.

8.2.  Informative References

              Arkko, J., Rissanen, H., Loreto, S., Turanyi, Z., and O.
              Novo, "Implementing Tiny COAP Sensors",
              draft-arkko-core-sleepy-sensors-01 (work in progress),
              July 2011.

              Jennings, C., Shelby, Z., and J. Arkko, "Media Types for
              Sensor Markup Language (SENML)", draft-jennings-senml-08
              (work in progress), January 2012.

              Shelby, Z., Stuber, M., Sturek, D., Frank, B., and R.
              Kelsey, "CoAP Requirements and Features",
              draft-shelby-core-coap-req-02 (work in progress),
              October 2010.

Author's Address

   Matthieu Vial

   Phone: +33 (0)47657 6522
   Email: matthieu.vial@schneider-electric.com

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