ACE Working Group                                               L. Seitz
Internet-Draft                                          SICS Swedish ICT
Intended Status: Informational                               G. Selander
Expires: September 10, 2015                                     Ericsson

                                                           March 9, 2015

   Problem Description for Authorization in Constrained Environments


   We present a problem description for authentication and authorization
   in constrained-node networks, i.e. networks where some devices have
   severe constraints on memory, processing, power and communication

Status of this Memo

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   ( in effect on the date of
   publication of this document.  Please review these documents
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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1  Terminology . . . . . . . . . . . . . . . . . . . . . . . .  3
   2. Background  . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3. Problem Description . . . . . . . . . . . . . . . . . . . . . .  6
     3.1. Authorization . . . . . . . . . . . . . . . . . . . . . . .  6
     3.2. Authentication  . . . . . . . . . . . . . . . . . . . . . .  6
     3.3. Communication Security  . . . . . . . . . . . . . . . . . .  7
     3.4. Cryptographic Keys  . . . . . . . . . . . . . . . . . . . .  8
   4. Assumptions and Requirements  . . . . . . . . . . . . . . . . .  8
     4.1 Architecture . . . . . . . . . . . . . . . . . . . . . . . .  8
     4.2 Constrained Devices  . . . . . . . . . . . . . . . . . . . .  9
     4.3 Authentication . . . . . . . . . . . . . . . . . . . . . . . 10
     4.4 Authorization  . . . . . . . . . . . . . . . . . . . . . . . 10
     4.5 Authorization Information  . . . . . . . . . . . . . . . . . 11
     4.6 Resource Access  . . . . . . . . . . . . . . . . . . . . . . 11
     4.7 Keys and Cipher Suites . . . . . . . . . . . . . . . . . . . 12
     4.8 Network Considerations . . . . . . . . . . . . . . . . . . . 12
     4.9 Legacy Considerations  . . . . . . . . . . . . . . . . . . . 12
   5.  Security Considerations  . . . . . . . . . . . . . . . . . . . 13
     5.1 Physical Attacks on Sensor and Actuator Networks . . . . . . 13
     5.2 Time Measurements  . . . . . . . . . . . . . . . . . . . . . 14
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 15
   7.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 15
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 15
     8.1  Informative References  . . . . . . . . . . . . . . . . . . 15
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16

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1.  Introduction

   Authorization is the process of deciding what an entity ought to be
   allowed to do.  This memo is about properties of security protocols
   to enable explicit and dynamic authorization of clients to access a
   resource at a server, in particular in constrained environments when
   the client and/or server are constrained nodes.

   Relevant use cases are provided in [I-D.ietf-ace-usecases], which
   also lists some authorization problems derived from the use cases.
   In this memo we present a more specific problem description for
   authentication and authorization in constrained RESTful environments
   together with a detailed set of assumptions and requirements (cf.
   section 4).

1.1  Terminology

   Certain security-related terms are to be understood in the sense
   defined in [RFC4949].  These terms include, but are not limited to,
   "authentication", "authorization", "confidentiality", "(data)
   integrity", "message authentication code", and "verify".

   RESTful terms including "resource", "representation", etc. are to be
   understood as used in HTTP [RFC7231] and CoAP [RFC7252].

   Terminology for constrained environments including "constrained
   device", "constrained-node network", "class 1", etc. are defined in

   "Explicit" authorization is used here to describe the ability to
   specify in some detail which entity has access to what and under what
   conditions, as opposed to "implicit" authorization where an entity is
   either allowed to access everything or nothing.

   "Dynamic" authorization means that the access control polices and the
   parameters on which they are evaluated may change during normal
   operations, as opposed to "static" authorization meaning that access
   control policies cannot be changed during normal operations and may
   require some special procedure such as out-of-band provision.

2. Background

   We assume a client-server setting, where a client wishes to access
   some resource hosted by a server.  Such resources may e.g. be sensor
   data, configuration data, or actuator settings.  Thus access to a
   resource could be by different methods, some of which change the
   state of the resource.  In this memo, we consider the REST setting

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   i.e. GET, POST, PUT and DELETE, and application protocols in scope
   are HTTP [RFC7231] and CoAP [RFC7252].

   We assume that the roles of client and server are not fixed, i.e. a
   node which is client could very well be server in some other context
   and vice-versa.  Further we assume that in some cases, clients are
   not previously known to servers, thus we cannot assume that the
   server has access control policies specific to that client when the
   client initiates communication.

   Finally we also assume that in a significant number of cases, the
   server and/or the client are too constrained to handle the evaluation
   of complex access control policies and related configuration on their
   own.  Many authorization solutions involve a centralized, trusted
   third party, supporting the client and/or resource server.  A trusted
   third party provides a more scalable way to centrally manage
   authorization policies, in order to ensure consistent authorization
   decisions.  The physical separation of policy decision and policy
   enforcement is an established principle in policy based management,
   e.g. [RFC2748].

   Borrowing from OAuth 2.0 [RFC6749] terminology we name the entities:
   client (C), resource server (RS), authorization server (AS - the
   third party), and resource owner (RO).  RO is in charge of the access
   control policies implemented in the AS governing the actions of RS.
   However, the RO need not be active in a constrained device access
   control setting, so we cannot rely on timely interactions with the
   RO.  In the target setting RS is typically constrained, C may be
   constrained, whereas AS is not assumed to be constrained.

   Since RS is constrained, we assume that it needs to offload
   authorization policy management and/or authorization decision making
   to AS.  This means that some authorization information needs to be
   transferred from AS to RS.

   Protecting information carried between AS and RS, requires some a
   priori established cryptographic keys.  How those keys are
   established is out of scope for this problem description.

   AS may for example be implemented as a cloud service, in a home
   server, or in a smartphone.  C and RS may or may not have
   connectivity to AS at the time of the access request, e.g. because
   they cannot handle multiple, simultaneous connections.  Another
   reason for intermittent connectivity may be that constant
   connectivity is not affordable (e.g. due to limited battery power, or
   a sensor mobility business case for which cellular connectivity cost
   too much or is not available).  Obviously, in order for a client
   request to reach RS there must be connectivity between C and RS, but

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   that could be a short range technology such as Bluetooth, ZigBee, or
   NFC.  Furthermore, if there is not sufficient authorization
   information about C in RS, and neither C nor RS can access AS, access
   requests will be denied.  Therefore we assume that either C or RS can
   access AS at some point in time, prior to the client's request.

   As a summary, there are potentially three information flows that
   needs to be protected (see Figure):

      1. The transfer of authorization information from AS to RS

      2. The transfer of cryptographic keys or credentials from AS to RS
         and C, respectively

      3. The access request/response procedure between C and RS

                      | Authorization |
                      |     Server    |
                      |               |
                     /                 \    Authorization
     Credentials,   /                   \    Information
          Keys     /                     \
                  /                       \   Credentials,
                 /                         \   Keys
                V                           V
       +--------+                           +-----------+
       | Client |                           | Resource  |
       |        |<---- Access procedure --->| Server    |
       |        |                           |           |
       +--------+                           +-----------+

        Figure.  Information flows that needs to be protected.
                 Only showing origin and destination, actual
                 flow may pass intermediary nodes.


   The information flow in 1. above enables RO to control the
   interactions of a constrained RS by means of access control policies.
    There is an ongoing discussion about an analogous information flow
   enabling the stakeholder associated to C ("Requesting Party" in UMA

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   terminology [I-D.hardjono-oauth-umacore]) to control the interactions
   of a constrained C by means of policies.  While this would not be
   policies for access control to resources, it could be useful in
   certain settings which require dynamically changing interaction
   patterns with a constrained client without updating firmware.  Such a
   solution could potentially reuse all security components required to
   protect the information flow in 1., so no additional specifications
   would be needed.  This aspect is not discussed further in this draft.

3. Problem Description

   A number of problems needs to be solved in order to achieve explicit
   and dynamic authorization, as is described in this section.

3.1. Authorization

   The core problem we are trying to solve is authorization.  The
   following problems related to authorization need to be addressed:

      o AS needs to transfer authorization information to RS.

      o The transferred authorization information needs to follow a
        defined format and encoding, which must be efficient for
        constrained devices, considering size of authorization
        information and parser complexity.

      o The RS needs to be able to verify the authenticity of the
        authorization information.  There is a trade-off here between
        processing complexity and deployment complexity.

      o The RS needs to enforce the authorization decisions of the AS.
        The authorization information it obtained from AS might require
        additional policy evaluation (e.g. matching against local access
        control lists, evaluating local conditions).  The required
        "policy evaluation" at the RS needs to be adapted to the
        capabilities of the constrained device.

      o Finally, as is indicated in the previous bullet, for a
        particular authorization decision there may be different kinds
        of authorization information needed, and these pieces of
        information may be transferred to RS at different times and in
        different ways prior to or during the client request.

3.2. Authentication

   The following problems need to be addressed, when considering

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      o RS need to authenticate AS to ensure that the authorization
        information and related data comes from the correct source.

      o C may need to to authenticate AS to ensure that it gets security
        information related to the resources from the right source.

      o In some use cases RS needs to authenticate some property of C,
        in order to bind it to the relevant authorization information.
        In other use cases, authentication and authorization of C may be
        implicit, e.g. by encrypting the resource representation the RS
        only providing access to those who possess the key to decrypt.

      o C may need to authenticate RS, in order to ensure that it is
        interacting with the right resources.   Alternatively C may just
        verify the integrity of a received resource representation.

      o AS may need to authenticate its communication partner (either C
        or RS), in order to ensure it serves the correct device.

3.3. Communication Security

   There are different alternatives to provide communication security,
   and the problem here is to choose the optimal one for each scenario.
   We list the available alternatives:

      o Session-based security at transport layer such as DTLS [RFC6347]
        offers security, including integrity and confidentiality
        protection, for the whole application layer exchange.  However,
        DTLS may not provide end-to-end security over multiple hops.
        Another problem with DTLS is the cost of the handshake protocol,
        which may be too expensive for constrained devices especially in
        terms of memory and power consumption for message transmissions.

      o An alternative is object security at application layer, e.g.
        using [I-D.selander-ace-object-security].  Secure objects can be
        stored or cached in network nodes and provide security for a
        more flexible communication model such as publish/subscribe
        (compare e.g. CoRE Mirror Server [I-D.koster-core-coapmq]).  A
        problem with object security is that it can not provide
        confidentiality for the message headers.

      o Hybrid solutions using both session-based and object security
        are also possible.  An example of a hybrid is where
        authorization information and cryptographic keys are provided by

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        AS in the format of secure data objects, but where the resource
        access is protected by session-based security.

3.4. Cryptographic Keys

   With respect to cryptographic keys, we see the following problems
   that need to be addressed:

      o Symmetric vs Asymmetric Keys

        We need keys both for protection of resource access and for
        protection of transport of authentication and authorization
        information.  Do we want to support solutions  based on
        asymmetric keys or symmetric keys in both cases?

        There are classes of devices that can easily perform symmetric
        cryptography, but consume considerably more time/battery for
        asymmetric operations.  On the other hand asymmetric
        cryptography has benefits e.g. in terms of deployment.

      o Key Establishment

        How are the corresponding cryptographic keys established?
        Considering section 3.1 there must be a binding between these
        keys and the authorization information, at least in the sense
        that AS must be able to specify a unique client identifier which
        RS can verify (using an associated key).

        One of the use cases of [I-D.ietf-ace-usecases] describes
        spontaneous change of access policies - e.g. giving a hitherto
        unknown client the right to temporarily unlock your house door.
        In this case C is not previously known to RS and a key must be
        provisioned by AS.

      o Revocation and Expiration

        How are keys replaced and how is a key that has been compromised
        revoked in a manner that reaches all affected parties, also
        keeping in mind scenarios with intermittent connectivity?

4. Assumptions and Requirements

   In this section we list a set of candidate assumptions and
   requirements to make the problem description in the previous sections
   more concise and precise.

4.1 Architecture

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   The architecture consists of at least the following types of nodes:

      o RS hosting resources, and responding to access requests

      o C requesting access to resources

      o AS supporting the access request/response procedure by providing
        authorization information to RS.

         - AS may also provide other services such as authenticating C
           on behalf of RS, or providing cryptographic keys or
           credentials to C and/or RS to secure the request/response

      o The architecture may contain intermediary nodes between any pair
        of C, RS and AS, such as e.g. forward/reverse proxies in the
        CoRE architecture.  The solution shall not unduly restrict the
        use of intermediaries.

         - The architecture shall support session based security and
           data object security.

4.2 Constrained Devices

      o C and/or RS may be constrained in terms of power, processing,
        communication bandwidth, memory and storage space, and moreover

         - unable to manage complex authorization policies

         - unable to manage a large number of secure connections

         - without user interface

         - without constant network connectivity

         - unable to precisely measure time

         - required to save on wireless communication due to high power

      o AS is not a constrained device.

      o All devices under consideration can process symmetric
        cryptography without incurring an excessive performance penalty.

         - We assume the use of a standardized symmetric key algorithm,
           such as AES.

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         - Except for the most constrained devices we assume the use of
           a standardized cryptographic hash function such as SHA-256.

      o Public key cryptography requires additional resources (e.g. RAM,
        ROM, power, specialized hardware).

      o A DTLS handshake involves significant computation,
        communication, and memory overheads in the context of
        constrained devices.

         - The RAM requirements of DTLS handshakes with public key
           cryptography are prohibitive for certain constrained devices.

         - Certificate-based DTLS handshakes require significant volumes
           of communication, RAM (message buffers) and computation.

      o The solution shall support a simple scheme for expiring
        authentication and authorization information on devices which
        are unable to measure time (cf. section 5.2).

4.3 Authentication

      o RS need to authenticate AS to ensure that the authorization
        information and related data comes from the correct source.

      o Depending on use case, C, RS or AS may need to authenticate each

4.4 Authorization

      o The authorization decision is based on credentials presented by
        C, the requested resource, the RESTful method, and local context
        in RS at the time of the request, or on any subset of this

      o The authorization decision is taken either by AS or RS.

      o The authorization decision is enforced by RS.

         - RS needs to have access to authorization information in order
           to verify that C is allowed to access the resource as

         - RS needs to make sure that it provides resource access only
           to authorized clients.

      o Apart from authorization for access to a resource, authorization
        may also be required for access to information about a resource

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        (e.g. resource descriptions).

      o The solution may need to be able to support the delegation of
        access rights.

4.5 Authorization Information

      o Authorization information is transferred from AS to RS using
        Agent, Push or Pull mechanisms [RFC2904].

      o RS shall authenticate that the authorization information is
        coming from AS.

      o The authorization information may also be encrypted end-to-end
        between AS and RS.

      o RS may not be able to communicate with AS at the time of the
        request from C.

      o RS may store or cache authorization information.

      o Authorization information may be pre-configured in RS.

      o Authorization information stored or cached in RS shall be
        possible to change.  The change of such information shall be
        subject to authorization.

      o Authorization policies stored on RS may be handled as a
        resource, i.e. information located at a particular URI, accessed
        with RESTful methods, and the access being subject to the same
        authorization mechanics.  AS may have special privileges when
        requesting access to the authorization policy resources on RS.

      o There may be mechanisms for C to look up the AS which provides
        authorization information about a particular resource.

4.6 Resource Access

      o Resources are accessed in a RESTful manner using GET, PUT, POST,

      o By default, the resource request shall be integrity protected
        and may be encrypted end-to-end from C to RS.  It shall be
        possible for RS to detect a replayed request.

      o By default, the response to a request shall be integrity
        protected and encrypted end-to-end from RS to C.  It shall be
        possible for C to detect a replayed response.

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      o RS shall be able to verify that the request comes from an
        authorized client

      o C shall be able to verify that the response to a request comes
        from the intended RS.

      o There may be resources whose access need not be protected (e.g.
        for discovery of the responsible AS).

4.7 Keys and Cipher Suites

      o AS and RS have established cryptographic keys.  Either AS and RS
        share a secret key or each have the other's public key.

      o The transfer of authorization information is protected with
        symmetric and/or asymmetric keys.

      o The access request/response can be protected with symmetric
        and/or asymmetric keys.

      o There must be a mechanism for RS to establish the necessary
        key(s) to verify and decrypt the request.

      o There must be a mechanism for C to establish the necessary
        key(s) to verify and decrypt the response.

      o There must be a mechanism for C to look up the supported cipher
        suites of a RS.

4.8 Network Considerations

      o The solution shall prevent network overload due to avoidable
        communication with AS.

      o The solution shall prevent network overload by compact
        authorization information representation.

      o The solution shall optimize the case where authorization
        information does not change often.

      o The solution where possible shall support an efficient mechanism
        for providing authorization information to multiple RSs, for
        example when multiple entities need to be configured or change

4.9 Legacy Considerations

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      o The solution shall work with existing infrastructure.

      o The solution shall support authorization of access to legacy

5.  Security Considerations

   The entire document is about security.  Security considerations
   applicable to authentication and authorization in RESTful
   environments are provided in e.g. OAuth 2.0 [RFC6749].

   In this section we focus on specific security aspects related to
   authorization in constrained-node networks.

5.1 Physical Attacks on Sensor and Actuator Networks

   The focus of this work is on constrained-node networks consisting of
   connected sensors and actuators.  The main function of such devices
   is to interact with the physical world by gathering information or
   performing an action.  We now discuss attacks performed with physical
   access to such devices.

   The main threats to sensors and actuator networks are:

      o Unauthorized access to data to and from sensors and actuators,
        including eavesdropping and manipulation of data.

      o Denial-of-service making the sensor/actuator unable to perform
        its intended task correctly.

   A number of attacks can be made with physical access to a device
   including probing attacks, timing attacks, power attacks, etc.
   However, with physical access to a sensor or actuator device it is
   possible to directly perform attacks equivalent of eavesdropping,
   manipulating data or denial of service. For example:

      o Instead of eavesdropping the sensor data or attacking the
        authorization system to gain access to the data, the attacker
        could make its own measurements on the physical object.

      o Instead of manipulating the sensor data the attacker could
        change the physical object which the sensor is measuring,
        thereby changing the payload data which is being sent.

      o Instead of manipulating data for an actuator or attacking the
        authorization system, the attacker could perform an unauthorized
        action directly on the physical object.

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      o A denial-of-service attack could be performed physically on the
        object or device.

   All these attacks are possible by having physical access to the
   device, since the assets are related to the physical world.
   Moreover, this kind of attacks are in many cases straightforward
   (requires no special competence or tools, low cost given physical
   access, etc.)

   As a conclusion, if an attacker has physical access to a sensor or
   actuator device, then much of the security functionality elaborated
   in this draft is not effective to protect the asset during the
   physical attack.

   Since it does not make sense to design a solution for a situation
   that cannot be protected against we assume there is no need to
   protect assets which are exposed during a physical attack.  In other
   words, either an attacker does not have physical access to the sensor
   or actuator device, or if it has, the attack shall only have effect
   during the period of physical attack.

5.2 Time Measurements

   Measuring time with certain accuracy is important to achieve certain
   security properties, for example to determine whether a public key
   certificate, access token or some other assertion is valid.

   Dynamic authorization in itself requires the ability to handle expiry
   or revocation of authorization decisions or to distinguish new
   authorization decisions from old.

   For certain categories of devices we can assume that there is an
   internal clock which is sufficiently accurate to handle the time
   measurement requirements.  If RS can connect directly to AS it could
   get updated in terms of time as well as revocation information.

   If RS continuously measures time but can't connect to AS or other
   trusted source, time drift may have to be accepted and it may not be
   able to manage revocation.  However, it may still be able to handle
   short lived access rights within some margins, by measuring the time
   since arrival of authorization information or request.

   Some categories of devices in scope may be unable measure time with
   any accuracy (e.g. because of sleep cycles).  This category of
   devices is not suitable for the use cases which require measuring
   validity of assertions and authorizations in terms of absolute time.

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6.  IANA Considerations

   This document has no actions for IANA.

7.  Acknowledgements

   The authors would like to thank Carsten Bormann, Stefanie Gerdes,
   Sandeep Kumar, John Mattson, Corinna Schmitt, Mohit Sethi, Hannes
   Tschofenig, Vlasios Tsiatsis and Erik Wahlstroem for contributing to
   the discussion, giving helpful input and commenting on the 00-
   version.  The authors would also like to acknowledge input provided
   by draft-gerdes-ace-actors [I-D.gerdes-ace-actors] and by Hummen et
   al. [HUM14delegation].

8.  References

8.1  Informative References

   Seitz, L., Gerdes, S., Selander, G., Mani, M., and S. Kumar, "ACE use
   cases", draft-ietf-ace-usecases-02 (work in progress), February

   Hardjono, T., Maler, E., Machulak, M. and D. Catalano, "User-Managed
   Access (UMA) Profile of OAuth 2.0", draft-hardjono-oauth-umacore-12
   (work in progress), February 2015.

   [I-D.selander-ace-object-security] Selander G., Mattsson J., and L.
   Seitz, "Object Security for CoAP", draft-selander-ace-object-
   security-00 (work in progress), October 2014.

   Koster, M., Keranen A., and J. Jimenez "Message Queueing in the
   Constrained Application Protocol (CoAP)", draft-koster-core-coapmq-00
   (expired), July 2014

   Gerdes, S., "Actors in the ACE Architecture", draft-gerdes-ace-
   actors-02 (work in progress), October 2014.

   [HUM14delegation] Hummen, R., Shafagh, H., Raza, S., Voigt, T.,
   Wehrle, K., "Delegation-based Authentication and Authorization for
   the IP-based Internet of Things", 11th IEEE International Conference
   on Sensing, Communication, and Networking (SECON'14), June 30 - July
   3, 2014.

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   [RFC2748]  Durham, D., Ed., Boyle, J., Cohen, R., Herzog, S., Rajan,
              R., and A. Sastry, "The COPS (Common Open Policy Service)
              Protocol", RFC 2748, January 2000, <http://www.rfc-

   [RFC2904]  Vollbrecht, J., Calhoun, P., Farrell, S., Gommans, L.,
              Gross, G., de Bruijn, B., de Laat, C., Holdrege, M., and
              D. Spence, "AAA Authorization Framework", RFC 2904, August
              2000, <>.

   [RFC4949]  Shirey, R., "Internet Security Glossary, Version 2", FYI
              36, RFC 4949, August 2007, <http://www.rfc-

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, January 2012,

   [RFC6749]  Hardt, D., Ed., "The OAuth 2.0 Authorization Framework",
              RFC 6749, October 2012, <http://www.rfc-

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228, May 2014,

   [RFC7231]  Fielding, R., Ed., and J. Reschke, Ed., "Hypertext
              Transfer Protocol (HTTP/1.1): Semantics and Content",
              RFC 7231, June 2014, <http://www.rfc-

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252, June 2014,

Authors' Addresses

   Ludwig Seitz
   SICS Swedish ICT AB
   Scheelevagen 17
   22370 Lund

   Goeran Selander
   Farogatan 6

Seitz & Selander       Expires September 10, 2015              [Page 16]

INTERNET DRAFT        Problem description for ACE          March 9, 2015

   16480 Kista

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