dice                                                  H. Tschofenig, Ed.
Internet-Draft                                                  ARM Ltd.
Intended status: Standards Track                              T. Fossati
Expires: September 9, 2015                                Alcatel-Lucent
                                                           March 8, 2015

             A TLS/DTLS Profile for the Internet of Things


   A common design pattern in Internet of Things (IoT) deployments is
   the use of a constrained device that collects data via sensor or
   controls actuators for use in home automation, industrial control
   systems, smart cities and other IoT deployments.

   This document defines a Transport Layer Security (TLS) and Datagram
   TLS (DTLS) 1.2 profile that offers communications security for this
   data exchange thereby preventing eavesdropping, tampering, and
   message forgery.  The lack of communication security is a common
   vulnerability in Internet of Things products that can easily be
   solved by using these well-researched and widely deployed Internet
   security protocols.

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
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   This Internet-Draft will expire on September 9, 2015.

Copyright Notice

   Copyright (c) 2015 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  TLS/DTLS Protocol Overview  . . . . . . . . . . . . . . . . .   4
   4.  Communication Models  . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Constrained TLS/DTLS Clients  . . . . . . . . . . . . . .   6
     4.2.  Constrained TLS/DTLS Servers  . . . . . . . . . . . . . .  13
   5.  The Ciphersuite Concept . . . . . . . . . . . . . . . . . . .  18
   6.  Credential Types  . . . . . . . . . . . . . . . . . . . . . .  19
     6.1.  Pre-Shared Secret . . . . . . . . . . . . . . . . . . . .  19
     6.2.  Raw Public Key  . . . . . . . . . . . . . . . . . . . . .  21
     6.3.  Certificates  . . . . . . . . . . . . . . . . . . . . . .  23
   7.  Signature Algorithm Extension . . . . . . . . . . . . . . . .  27
   8.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .  28
   9.  Session Resumption  . . . . . . . . . . . . . . . . . . . . .  29
   10. Compression . . . . . . . . . . . . . . . . . . . . . . . . .  30
   11. Perfect Forward Secrecy . . . . . . . . . . . . . . . . . . .  30
   12. Keep-Alive  . . . . . . . . . . . . . . . . . . . . . . . . .  31
   13. Timeouts  . . . . . . . . . . . . . . . . . . . . . . . . . .  33
   14. Random Number Generation  . . . . . . . . . . . . . . . . . .  34
   15. Truncated MAC and Encrypt-then-MAC Extension  . . . . . . . .  35
   16. Server Name Indication (SNI)  . . . . . . . . . . . . . . . .  35
   17. Maximum Fragment Length Negotiation . . . . . . . . . . . . .  36
   18. Session Hash  . . . . . . . . . . . . . . . . . . . . . . . .  36
   19. Re-Negotiation Attacks  . . . . . . . . . . . . . . . . . . .  36
   20. Downgrading Attacks . . . . . . . . . . . . . . . . . . . . .  37
   21. Crypto Agility  . . . . . . . . . . . . . . . . . . . . . . .  38
   22. Key Length Recommendations  . . . . . . . . . . . . . . . . .  39
   23. False Start . . . . . . . . . . . . . . . . . . . . . . . . .  39
   24. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  40
   25. Security Considerations . . . . . . . . . . . . . . . . . . .  41
   26. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  41
   27. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  41
   28. References  . . . . . . . . . . . . . . . . . . . . . . . . .  42
     28.1.  Normative References . . . . . . . . . . . . . . . . . .  42
     28.2.  Informative References . . . . . . . . . . . . . . . . .  43
   Appendix A.  Conveying DTLS over SMS  . . . . . . . . . . . . . .  49

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     A.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  49
     A.2.  Message Segmentation and Re-Assembly  . . . . . . . . . .  50
     A.3.  Multiplexing Security Associations  . . . . . . . . . . .  50
     A.4.  Timeout . . . . . . . . . . . . . . . . . . . . . . . . .  51
   Appendix B.  DTLS Record Layer Per-Packet Overhead  . . . . . . .  52
   Appendix C.  DTLS Fragmentation . . . . . . . . . . . . . . . . .  53
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  53

1.  Introduction

   An engineer developing an Internet of Things (IoT) device needs to
   investigate the security threats and decide about the security
   services that can be used to mitigate these threats.

   Enabling IoT devices to exchange data often requires authentication
   of the two endpoints and the ability to provide integrity- and
   confidentiality-protection of exchanged data.  While these security
   services can be provided at different layers in the protocol stack,
   the use of Transport Layer Security (TLS)/Datagram TLS (DTLS) has
   been very popular with many application protocols and it is likely to
   be useful for IoT scenarios as well.

   Fitting Internet protocols into constrained devices can be difficult
   but thanks to the standardization efforts new profiles and protocols
   are available, such as the Constrained Application Protocol (CoAP)
   [RFC7252].  UDP is mainly used to carry CoAP messages but other
   transports can be utilized, such as SMS or even TCP.

   While the main goal for this document is to protect CoAP messages
   using DTLS 1.2 [RFC6347] the information contained in the following
   sections is not limited to CoAP nor to DTLS itself.

   Instead, this document defines a profile of DTLS 1.2 [RFC6347] and
   TLS 1.2 [RFC5246] that offers communication security services for IoT
   applications and is reasonably implementable on many constrained
   devices.  Profile thereby means that available configuration options
   and protocol extensions are utilized to best support the IoT
   environment.  This document does not alter TLS/DTLS specifications
   and does not introduce any new TLS/DTLS extension.

   The main target audience for this document is the embedded system
   developer configuring and using a TLS/DTLS stack.  This document may,
   however, also help those developing or selecting a suitable TLS/DTLS
   stack for an Internet of Things product.

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2.  Terminology

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

   This specification refers to TLS as well as DTLS and particularly to
   version 1.2, which is the most recent version at the time of writing.
   We refer to TLS/DTLS whenever the text is applicable to both versions
   of the protocol and to TLS or DTLS when there are differences between
   the two protocols.

   Note that "Client" and "Server" in this document refer to TLS/DTLS
   roles, where the client initiates the handshake.  This does not
   restrict the interaction pattern of the protocols on top of DTLS
   since the record layer allows bi-directional communication.  This
   aspect is further described in Section 4.

   RFC 7228 [RFC7228] introduces the notion of constrained-node
   networks, which are made of small devices with severe constraints on
   power, memory, and processing resources.  The terms constrained
   devices, and Internet of Things (IoT) devices are used

   The terms "Certification Authority" (CA) and "Distinguished Name"
   (DN) are taken from [RFC5280].  The terms "trust anchor" and "trust
   anchor store" are defined in [RFC6024] as

      "A trust anchor represents an authoritative entity via a public
      key and associated data.  The public key is used to verify digital
      signatures, and the associated data is used to constrain the types
      of information for which the trust anchor is authoritative."

      "A trust anchor store is a set of one or more trust anchors stored
      in a device.  A device may have more than one trust anchor store,
      each of which may be used by one or more applications."

3.  TLS/DTLS Protocol Overview

   The TLS protocol [RFC5246] provides authenticated, confidentiality-
   and integrity-protected communication between two endpoints.  The
   protocol is composed of two layers: the Record Protocol and the
   Handshaking Protocols.  At the lowest level, layered on top of a
   reliable transport protocol (e.g., TCP), is the Record Protocol.  It
   provides connection security by using symmetric cryptography for
   confidentiality, data origin authentication, and integrity
   protection.  The Record Protocol is used for encapsulation of various
   higher-level protocols.  The handshaking protocols consist of three

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   sub-protocols, namely the handshake protocol, the change cipher spec
   protocol and the alert protocol.  The handshake protocol allows the
   server and client to authenticate each other and to negotiate an
   encryption algorithm and cryptographic keys before the application
   protocol transmits or receives data.

   The design of DTLS [RFC6347] is intentionally very similar to TLS.
   However, since DTLS operates on top of an unreliable datagram
   transport, it must explicitly cope with the reliable and ordered
   delivery assumptions made by TLS.  RFC 6347 explains these
   differences in great detail.  As a short summary, for those not
   familiar with DTLS the differences are:

   o  An explicit sequence number and an epoch field is included in the
      Record Protocol.  Section 4.1 of RFC 6347 explains the processing
      rules for these two new fields.  The value used to compute the MAC
      is the 64-bit value formed by concatenating the epoch and the
      sequence number.

   o  Stream ciphers must not be used with DTLS.  The only stream cipher
      defined for TLS 1.2 is RC4 and due to cryptographic weaknesses it
      is not recommended anymore even for use with TLS
      [I-D.ietf-tls-prohibiting-rc4].  Note that the term 'stream
      cipher' is a technical term in the TLS specification.  Section 4.7
      of RFC 5246 defines stream ciphers in TLS as follows: in stream
      cipher encryption, the plaintext is exclusive-ORed with an
      identical amount of output generated from a cryptographically
      secure keyed pseudorandom number generator.

   o  The TLS Handshake Protocol has been enhanced to include a
      stateless cookie exchange for Denial of Service (DoS) resistance.
      For this purpose a new handshake message, the HelloVerifyRequest,
      was added to DTLS.  This handshake message is sent by the server
      and includes a stateless cookie, which is returned in a
      ClientHello message back to the server.  Although the exchange is
      optional for the server to execute, a client implementation has to
      be prepared to respond to it.  Furthermore, the handshake message
      format has been extended to deal with message loss, reordering,
      and fragmentation.

4.  Communication Models

   This document describes a profile of DTLS and, to be useful, it has
   to make assumptions about the envisioned communication architecture.

   Two communication architectures (and consequently two profiles) are
   described in this document.

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4.1.  Constrained TLS/DTLS Clients

   The communication architecture shown in Figure 1 assumes a unicast
   communication interaction with an IoT device utilizing a constrained
   TLS/DTLS client interacting with one or multiple TLS/DTLS servers.

   Before a client can initiate the TLS/DTLS handshake it needs to know
   the IP address of that server and what credentials to use.
   Application layer protocols, such as CoAP, which is conveyed on top
   of DTLS, may be configured with URIs of the endpoints to which CoAP
   needs to register and publish data.  This configuration information
   (including credentials) may be conveyed to clients as part of a
   firmware/software package or via a configuration protocol.  The
   following credential types are supported by this profile:

   o  For PSK-based authentication (see Section 6.1), this includes the
      paired "PSK identity" and shared secret to be used with each

   o  For raw public key-based authentication (see Section 6.2), this
      includes either the server's public key or the hash of the
      server's public key.

   o  For certificate-based authentication (see Section 6.3), this
      includes a pre-populated trust anchor store that allows the DTLS
      stack to perform path validation for the certificate obtained
      during the handshake with the server.

   Figure 1 shows example configuration information stored at the
   constrained client for use with respective servers.

   This document focuses on the description of the DTLS client-side
   functionality but, quite naturally, the equivalent server-side
   support has to be available.

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              |          Configuration             |
              | Server A --> PSK Identity, PSK     |
              |                                    |
              | Server B --> Public Key (Server B),|
              |              Public/Private Key    |
              |              (for Client)          |
              |                                    |
              | Server C --> Public/Private Key    |
              |              (for Client)          |
              |              Trust Anchor Store    |
   |TLS/DTLS   |
   |Client     |-
   +-----------+ \
                  \  ,-------.
                   ,'         `.            +------+
                  /  IP-based   \           |Server|
                 (    Network    )          |  A   |
                  \             /           +------+
                   `.         ,'
                     '---+---'                  +------+
                         |                      |Server|
                         |                      |  B   |
                         |                      +------+
                         |                  +------+
                                            |  C   |

                   Figure 1: Constrained Client Profile.

4.1.1.  Examples of Constrained Client Exchanges  Network Access Authentication Example

   Re-use is a recurring theme when considering constrained environments
   and is behind a lot of the directions taken in developments for
   constrained environments.  The corollary of re-use is to not add
   functionality if it can be avoided.  An example relevant to the use

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   of TLS is network access authentication, which takes place when a
   device connects to a network and needs to go through an
   authentication and access control procedure before it is allowed to
   communicate with other devices or connect to the Internet.

   Figure 2 shows the network access architecture with the IoT device
   initiating the communication to an access point in the network using
   the procedures defined for a specific physical layer.  Since
   credentials may be managed and stored centrally, in the
   Authentication, Authorization, and Accounting (AAA) server, the
   security protocol exchange may need to be relayed via the
   Authenticator, i.e., functionality running on the access point, to
   the AAA server.  The authentication and key exchange protocol itself
   is encapsulated within a container, the Extensible Authentication
   Protocol (EAP) [RFC3748], and messages are conveyed back and forth
   between the EAP endpoints, namely the EAP peer located on the IoT
   device and the EAP server located on the AAA server or the access
   point.  To route EAP messages from the access point, acting as a AAA
   client, to the AAA server requires an adequate protocol mechanism,
   namely RADIUS [RFC2865] or Diameter [RFC6733].

   More details about the concepts and a description about the
   terminology can be found in RFC 5247 [RFC5247].

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                                                |Authorization |
                                                |Accounting    |
                                                |Server        |
                                                |(EAP Server)  |
                                                |              |
                                                  * EAP      o RADIUS/
                                                  *          o Diameter
                                             ///                \\\
                                           //                      \\
                                          |        Federation        |
                                          |        Substrate         |
                                           \\                      //
                                             \\\                ///
                                                  * EAP      o RADIUS/
                                                  *          o Diameter
    +-------------+                             +-v----------v--+
    |             |      EAP/EAP Method         |               |
    | Internet of |<***************************>| Access Point  |
    | Things      |                             |(Authenticator)|
    | Device      |    EAP Lower Layer and      |(AAA Client)   |
    | (EAP Peer)  | Secure Association Protocol |               |
    |             |<--------------------------->|               |
    |             |                             |               |
    |             |      Physical Layer         |               |
    |             |<===========================>|               |
    +-------------+                             +---------------+

       <****>: Device-to-AAA Server Exchange
       <---->: Device-to-Authenticator Exchange
       <oooo>: AAA Client-to-AAA Server Exchange
       <====>: Physical layer like IEEE 802.11/802.15.4

                  Figure 2: Network Access Architecture.

   One standardized EAP method is EAP-TLS, defined in RFC 5216
   [RFC5216], which re-uses the TLS-based protocol exchange and
   encapsulates it inside the EAP payload.  In terms of re-use this
   allows many components of the TLS protocol to be shared between the
   network access security functionality and the TLS functionality
   needed for securing application layer traffic.  In the EAP-TLS

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   exchange shown in Figure 3 the IoT device as the EAP peer acts as a
   TLS client.

      Authenticating Peer     Authenticator
      -------------------     -------------
                              <- EAP-Request/
      Identity (MyID) ->
                              <- EAP-Request/
                              (TLS Start)
      (TLS client_hello)->
                              <- EAP-Request/
                              (TLS server_hello,
                                TLS certificate,
                       [TLS server_key_exchange,]
                        TLS certificate_request,
                           TLS server_hello_done)
      (TLS certificate,
       TLS client_key_exchange,
       TLS certificate_verify,
       TLS change_cipher_spec,
       TLS finished) ->
                              <- EAP-Request/
                              (TLS change_cipher_spec,
                               TLS finished)
      EAP-Type=EAP-TLS ->
                              <- EAP-Success

                        Figure 3: EAP-TLS Exchange.

   The guidance in this document also applies to the use of EAP-TLS for
   network access authentication.  An IoT device using a network access
   authentication solution based on TLS can re-use most parts of the
   code for the use of DTLS/TLS at the application layer thereby saving
   a significant amount of flash memory.  Note, however, that the
   credentials used for network access authentication and those used for
   application layer security are very likely different.

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Internet-Draft            TLS/DTLS IoT Profile                March 2015  CoAP-based Data Exchange Example

   When a constrained client uploads sensor data to a server
   infrastructure it may use CoAP by pushing the data via a POST message
   to a pre-configured endpoint on the server.  In certain circumstances
   this might be too limiting and additional functionality is needed, as
   shown in Figure 4, where the IoT device itself runs a CoAP server
   hosting the resource that is made accessible to other entities.
   Despite running a CoaP server on the IoT device it is still the DTLS
   client on the IoT device that initiates the interaction with the non-
   constrained resource server in our scenario.

   Figure 4 shows a sensor starting a DTLS exchange with a resource
   directory to register available resources.
   [I-D.ietf-core-resource-directory] defines the resource directory
   (RD) as a web entity that stores information about web resources and
   implements Representational State Transfer (REST) interfaces for
   registration and lookup of those resources.  Note that the described
   exchange is borrowed from the OMA Lightweight Machine-to-Machine
   (LWM2M) specification [LWM2M] that uses RD but adds proxy

   The initial DTLS interaction between the sensor, acting as a DTLS
   client, and the resource directory, acting as a DTLS server, will be
   a full DTLS handshake.  Once this handshake is complete both parties
   have established the DTLS record layer.  Subsequently, the CoAP
   client can securely register at the resource directory.

   After some time (assuming that the client regularly refreshes its
   registration) the resource directory receives a request from an
   application to retrieve the temperature information from the sensor.
   This request is relayed by the resource directory to the sensor using
   a GET message exchange.  The already established DTLS record layer
   can be used to secure the message exchange.

       Sensor                                       Directory
       ------                                       ---------

     | ClientHello             -------->
     | client_certificate_type
    F| server_certificate_type
    L|                         <-------    HelloVerifyRequest

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     | ClientHello             -------->
    D| client_certificate_type
    T| server_certificate_type
    S|                                            ServerHello
     |                                client_certificate_type
    H|                                server_certificate_type
    A|                                            Certificate
    N|                                      ServerKeyExchange
    D|                                     CertificateRequest
    S|                         <--------      ServerHelloDone
    A| Certificate
    K| ClientKeyExchange
    E| CertificateVerify
     | [ChangeCipherSpec]
     | Finished                -------->
     |                                     [ChangeCipherSpec]
     |                         <--------             Finished

     +---                                                  ///+
    C|                                                        \ D
    O| Req: POST coap://rd.example.com/rd?ep=node1            \ T
    A| Payload:                                               \ L
    P| </temp>;ct=41;                                         \ S
     |    rt="temperature-c";if="sensor",                     \
    R| </light>;ct=41;                                        \ R
    D|    rt="light-lux";if="sensor"                          \ E
     |                         -------->                      \ C
    R|                                                        \ O
    E|                                                        \ R
    G|                                     Res: 2.01 Created  \ D
    .|                         <--------  Location: /rd/4521  \
     |                                                        \ L
     +---                                                     \ A
                                                              \ Y
                              *                               \ E
                              * (time passes)                 \ R
                              *                               \
     +---                                                     \ P
    C|                                                        \ R
    O|              Req: GET coaps://sensor.example.com/temp  \ O
    A|                         <--------                      \ T
    P|                                                        \ E
     | Res:  2.05 Content                                     \ C
    G| Payload:                                               \ T

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    E| 25.5                     -------->                     \ E
    T|                                                        \ D
     +---                                                  ///+

          Figure 4: DTLS/CoAP exchange using Resource Directory.

4.2.  Constrained TLS/DTLS Servers

   Section 4.1 illustrates a deployment model where the TLS/DTLS client
   is constrained and efforts need to be taken to improve memory
   utilization, bandwidth consumption, reduce performance impacts, etc.
   In this section, we assume a scenario where constrained devices run
   TLS/ DTLS servers to secure access to application layer services
   running on top of CoAP, HTTP or other protocols.  Figure 5
   illustrates a possible deployment whereby a number of constrained
   servers are waiting for regular clients to access their resources.
   The entire process is likely, but not necessarily, controlled by a
   third party, the authentication and authorisation server.  This
   authentication and authorization server is responsible for holding
   authorization policies that govern the access to resources and
   distribution of keying material.

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            |          Configuration             |
            | Credentials                        |
            |    Client A  -> Public Key         |
            |    Server S1 -> Symmetric Key      |
            |    Server S2 -> Certificate        |
            |    Server S3 -> Public Key         |
            | Trust Anchor Store                 |
            | Access Control Lists               |
            |    Resource X: Client A / GET      |
            |    Resource Y: Client A / PUT      |
   +---------------+                +-----------+
   |Authentication |      +-------->|TLS/DTLS   |
   |& Authorization|      |         |Client A   |
   |Server         |      |         +-----------+
   +---------------+     ++
                ^        |                  +-----------+
                 \       |                  |Constrained|
                  \  ,-------.              | Server S1 |
                   ,'         `.            +-----------+
                  /    Local    \
                 (    Network    )
                  \             /        +-----------+
                   `.         ,'         |Constrained|
                     '---+---'           | Server S2 |
                         |               +-----------+
                         |                   +-----------+
                         +-----------------> |Constrained|
                                             | Server S3 |

                   Figure 5: Constrained Server Profile.

   A deployment with constrained servers has to overcome several
   challenges.  Below we explain how these challenges have be solved
   with CoAP, as an example.  Other protocols may offer similar
   capabilities.  While the requirements for the TLS/DTLS protocol
   profile change only slightly when run on a constrained server (in
   comparison to running it on a constrained client) several other eco-
   system factor will impact deployment.

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   There are several challenges that need to be addressed:

   Discovery and Reachability:

      A client must first and foremost discover the server before
      initiating a connection to it.  Once it as been discovered,
      reachability to the device needs to be maintained.

      In CoAP the discovery of resources offered by servers is
      accomplished by sending a unicast or multicast CoAP GET to a well-
      known URI.  The CORE Link format specification [RFC6690] describes
      the use case (see Section 1.2.1), and reserves the URI (see
      Section 7.1).  Section 7 of the CoAP specification [RFC7252]
      describes the discovery procedure.  [RFC7390] describes use case
      for discovering CoAP servers using multicast (see Section 3.3),
      and specifies the protocol processing rules for CoAP group
      communications (see Section 2.7).

      The use of Resource Directory (RD)
      [I-D.ietf-core-resource-directory] is yet another possibility for
      discovering registered servers and their resources.  Since RD is
      usually not a proxy, clients can discover links registered with
      the RD and then access them directly.


      The next challenge concerns the provisioning of authentication
      credentials to the clients as well as servers.  In Section 4.1 we
      assumed that credentials (and other configuration information) are
      provisioned to the device and that those can be used with the
      authorization servers.  Of course, this leads to a very static
      relationship between the clients and their server-side
      infrastructure but poses fewer challenges from a deployment point
      of view, as described in Section 2 of
      [I-D.iab-smart-object-architecture] these different communication
      patterns.  In any case, engineers and product designers have to
      determine how the relevant credentials are distributed to the
      respective parties.  For example, shared secrets may need to be
      provisioned to clients and the constrained servers for subsequent
      use of TLS/DTLS PSK.  In other deployments, certificates, private
      keys, and trust anchors for use with certificate-based
      authentication may need to be utilized.

      Practical solutions either use pairing (also called imprinting) or
      a trusted third party.  With pairing two devices execute a special
      protocol exchange that is unauthenticated to establish an shared
      key (for example using an unauthenticated Diffie-Hellman exchange)
      key.  To avoid man-in-the-middle attacks an out-of-band channel is

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      used to verify that nobody has tampered with the exchanged
      protocol messages.  This out-of-band channel can come in many
      forms, including:

      *  Human involvement by comparing hashed keys, entering passkeys,
         scanning QR codes

      *  The use of alternative wireless communication channels (e.g.,
         infra-red communication in addition to WiFi)

      *  Proximity-based information

      More details about these different pairing/imprinting techniques
      can be found in the smart object security workshop report
      [RFC7397] and various position papers submitted to that topic,
      such as [ImprintingSurvey].  The use of a trusted third party
      follows a different approach and is subject to ongoing
      standardization efforts in the 'Authentication and Authorization
      for Constrained Environments (ACE)' working group [ACE-WG].


      The last challenge is the ability for the constrained server to
      make an authorization decision when clients access protected
      resources.  Pre-provisioning access control information to
      constrained servers may be one option but works only in a small
      scale, less dynamic environment.  For a more fine-grained and
      dynamic access control the reader is referred to the ongoing work
      in the ACE working group.

   Figure 6 shows an example interaction whereby a device, a thermostat
   in our case, searches in the local network for discoverable resources
   and accesses those.  The thermostat starts the procedure using a
   link-local discovery message using the "All CoAP Nodes" multicast
   address by utilizing the RFC 6690 [RFC6690] link format.  The IPv6
   multicast address used for site-local discovery is FF02::FD.  As a
   result, a temperature sensor and a fan respond.  These responses
   allow the thermostat to subsequently read temperature information
   from the temperature sensor with a CoAP GET request issued to the
   previously learned endpoint.  In this example we assume that
   accessing the temperature sensor readings and controlling the fan
   requires authentication and authorization of the thermostat and TLS
   is used to authenticate both endpoint and to secure the

     Thermostat                     Sensor              Fan

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     ----------                   ---------             ---

       GET coap://[FF02::FD]/.well-known/core

                     CoAP 2.05 Content

                                        CoAP 2.05 Content

   \                                                        /
   \ Protocol steps to obtain access token or keying        /
   \ material for access to the temperature sensor and fan. /
   \                                                        /

      Read Sensor Data
      GET /3303/0/5700

                    CoAP 2.05 Content
                               22.5 C

     Configure Actuator
     PUT /fan?on-off=true

                                      CoAP 2.04 Changed

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               Figure 6: Local Discovery and Resouce Access.

5.  The Ciphersuite Concept

   TLS (and consequently DTLS) has the concept of ciphersuites and an
   IANA registry [IANA-TLS] was created to register the suites.  A
   ciphersuite (and the specification that defines it) contains the
   following information:

   o  Authentication and key exchange algorithm (e.g., PSK)

   o  Cipher and key length (e.g., Advanced Encryption Standard (AES)
      with 128 bit keys [AES])

   o  Mode of operation (e.g., Counter with Cipher Block Chaining -
      Message Authentication Code (CBC-MAC) Mode (CCM) for AES)

   o  Hash algorithm for integrity protection, such as the Secure Hash
      Algorithm (SHA) in combination with Keyed-Hashing for Message
      Authentication (HMAC) (see [RFC2104] and [RFC4634])

   o  Hash algorithm for use with pseudorandom functions (e.g., HMAC
      with the SHA-256)

   o  Misc information (e.g., length of authentication tags)

   o  Information whether the ciphersuite is suitable for DTLS or only
      for TLS

   The TLS ciphersuite TLS_PSK_WITH_AES_128_CCM_8, for example, uses a
   pre-shared authentication and key exchange algorithm.  [RFC6655]
   defines this ciphersuite.  It uses the Advanced Encryption Standard
   (AES) encryption algorithm, which is a block cipher.  Since the AES
   algorithm supports different key lengths (such as 128, 192 and 256
   bits) this information has to be specified as well and the selected
   ciphersuite supports 128 bit keys.  A block cipher encrypts plaintext
   in fixed-size blocks and AES operates on fixed block size of 128
   bits.  For messages exceeding 128 bits, the message is partitioned
   into 128-bit blocks and the AES cipher is applied to these input
   blocks with appropriate chaining, which is called mode of operation.

   TLS 1.2 introduced Authenticated Encryption with Associated Data
   (AEAD) ciphersuites (see [RFC5116] and [RFC6655]).  AEAD is a class
   of block cipher modes which encrypt (parts of) the message and
   authenticate the message simultaneously.  Examples of such modes
   include the Counter with Cipher Block Chaining - Message

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   Authentication Code (CBC-MAC) Mode (CCM) mode, and the Galois/Counter
   Mode (GCM) (see [RFC5288] and [RFC7251]).

   Some AEAD ciphersuites have shorter authentication tags (i.e.,
   message authentication codes) and are therefore more suitable for
   networks with low bandwidth where small message size matters.  The
   TLS_PSK_WITH_AES_128_CCM_8 ciphersuite that ends in "_8" has an
   8-octet authentication tag, while the regular CCM ciphersuites have,
   at the time of writing, 16-octet authentication tags.  The design of
   CCM and the security properties are described in [CCM].

   TLS 1.2 also replaced the combination of MD5/SHA-1 hash functions in
   the TLS pseudo random function (PRF) used in earlier versions of TLS
   with cipher-suite-specified PRFs.  For this reason authors of more
   recent TLS 1.2 ciphersuite specifications explicitly indicate the MAC
   algorithm and the hash functions used with the TLS PRF.

6.  Credential Types

   The mandatory-to-implement functionality will depend on the
   credential type used with IoT devices.  The sub-sections below
   describe the implications of three different credential types, namely
   pre-shared secrets, raw public keys, and certificates.  When using
   pre-shared key, a critical consideration is how to assure the
   randomness of these secrets.  The best practice is to ensure that any
   pre-shared key contains as much randomness as possible.  Deriving a
   shared secret from a password, name, or other low-entropy source is
   not secure.  A low-entropy secret, or password, is subject to
   dictionary attacks.

6.1.  Pre-Shared Secret

   The use of pre-shared secrets is one of the most basic techniques for
   TLS/DTLS since it is both computational efficient and bandwidth
   conserving.  Pre-shared secret based authentication was introduced to
   TLS with RFC 4279 [RFC4279].  The exchange shown in Figure 7
   illustrates the DTLS exchange including the cookie exchange.  While
   the server is not required to initiate a cookie exchange with every
   handshake, the client is required to implement and to react on it
   when challenged.  The cookie exchange allows the server to react to
   flooding attacks.

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         Client                                               Server
         ------                                               ------
         ClientHello                 -------->

                                     <--------    HelloVerifyRequest
                                                   (contains cookie)

         ClientHello                  -------->
         (with cookie)
                                      <--------      ServerHelloDone
         Finished                     -------->
                                      <--------             Finished

         Application Data             <------->     Application Data


   * indicates an optional message payload

     Figure 7: DTLS PSK Authentication including the Cookie Exchange.

   [RFC4279] does not mandate the use of any particular type of client
   identity and the client and server have to agree on the identities
   and keys to be used.  The mandated encoding of identities in
   Section 5.1 of RFC 4279 aims to improve interoperability for those
   cases where the identity is configured by a person using some
   management interface.  However, many IoT devices do not have a user
   interface and most of their credentials are bound to the device
   rather than the user.  Furthermore, credentials are often provisioned
   into trusted hardware modules or in the firmware by developers.  As
   such, the encoding considerations are not applicable to this usage
   environment.  For use with this profile the PSK identities SHOULD NOT
   assume a structured format (as domain names, Distinguished Names, or
   IP addresses have) and a bit-by-bit comparison operation can then be
   used by the server-side infrastructure.

   The client indicates which key it uses by including a "PSK identity"
   in the ClientKeyExchange message.  As described in Section 4 clients
   may have multiple pre-shared keys with a single server, for example
   in a hosting context.  The TLS Server Name Indication (SNI) extension
   allows the client to convey the name of the server it is contacting,
   which is relevant for hosting environments.  A server implementation

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   needs to guide the selection based on a received SNI value from the

   RFC 4279 requires TLS implementations supporting PSK ciphersuites to
   support arbitrary PSK identities up to 128 octets in length, and
   arbitrary PSKs up to 64 octets in length.  This is a useful
   assumption for TLS stacks used in the desktop and mobile environments
   where management interfaces are used to provision identities and
   keys.  For the IoT environment, keys are distributed as part of
   hardware modules or are embedded into the firmware.  Implementations
   in compliance with this profile MAY use PSK identities up to 128
   octets in length, and arbitrary PSKs up to 64 octets in length.  The
   use of shorter PSK identities is RECOMMENDED.

   Constrained Application Protocol (CoAP) [RFC7252] currently specifies
   TLS_PSK_WITH_AES_128_CCM_8 as the mandatory to implement ciphersuite
   for use with shared secrets.  This ciphersuite uses the AES algorithm
   with 128 bit keys and CCM as the mode of operation.  The label "_8"
   indicates that an 8-octet authentication tag is used.  This
   ciphersuite makes use of the default TLS 1.2 Pseudorandom Function
   (PRF), which uses an HMAC with the SHA-256 hash function.  Note:
   Starting with TLS 1.2 (and consequently DTLS 1.2) ciphersuites have
   to specify the pseudorandom function.  RFC 5246 states that 'New
   cipher suites MUST explicitly specify a PRF and, in general, SHOULD
   use the TLS PRF with SHA-256 or a stronger standard hash function.'.
   The ciphersuites recommended in this document use the SHA-256
   construct defined in Section 5 of RFC 5246.

   A device compliant with the profile in this section MUST implement
   TLS_PSK_WITH_AES_128_CCM_8 and follow the guidance from this section.

6.2.  Raw Public Key

   The use of raw public keys with TLS/DTLS, as defined in [RFC7250], is
   the first entry point into public key cryptography without having to
   pay the price of certificates and a public key infrastructure (PKI).
   The specification re-uses the existing Certificate message to convey
   the raw public key encoded in the SubjectPublicKeyInfo structure.  To
   indicate support two new extensions had been defined, as shown in
   Figure 8, namely the server_certificate_type*' and the
   client_certificate_type.  To operate this mechanism securely it is
   necessary to authenticate and authorize the public keys out-of-band.
   This key distribution step may, for example, be provided by a
   dedicated protocol, such as the OMA LWM2M [LWM2M].  This document
   therefore assumes that a client implementation comes with one or
   multiple raw public keys of servers, it has to communicate with, pre-
   provisioned.  To replace, delete, or add raw public keys to this list
   requires a software update, for example using a firmware update

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   mechanism.  Additionally, a device will have its own raw public key
   and the corresponding private key.  This key pair may, for example,
   be configured during the manufacturing process of the device.

    Client                                          Server
    ------                                          ------

    ClientHello             -------->

                            <--------      ServerHelloDone

    Finished                -------->

                            <--------             Finished

   Note: Extensions marked with '#' were introduced with
         RFC 7250.

                  Figure 8: DTLS Raw Public Key Exchange.

   The CoAP recommended ciphersuite for use with this credential type is
   TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 [RFC7251].  This elliptic curve
   cryptography (ECC) based AES-CCM TLS ciphersuite uses the Ephemeral
   Elliptic Curve Diffie-Hellman (ECDHE) as the key establishment
   mechanism and an Elliptic Curve Digital Signature Algorithm (ECDSA)
   for authentication.  Due to the use of Ephemeral Elliptic Curve
   Diffie-Hellman (ECDHE) the recently introduced named Diffie-Hellman
   groups [I-D.ietf-tls-negotiated-dl-dhe] are not applicable to this
   profile.  This ciphersuite makes use of the AEAD capability in DTLS
   1.2 and utilizes an eight-octet authentication tag.  The use of a
   Diffie-Hellman key exchange provides perfect forward secrecy (PFS).
   More details about PFS can be found in Section 11.

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   [RFC6090] provides valuable information for implementing Elliptic
   Curve Cryptography algorithms, particularly for choosing methods that
   have been available in the literature for a long time (i.e., 20 years
   and more).

   A device compliant with the profile in this section MUST implement
   TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 and follow the guidance from this

6.3.  Certificates

   The use of mutual certificate-based authentication is shown in
   Figure 9, which makes use of the cached info extension
   [I-D.ietf-tls-cached-info].  Support of the cached info extension is
   REQUIRED.  Caching certificate chains allows the client to reduce the
   communication overhead significantly since otherwise the server would
   provide the end entity certificate, and the certificate chain.
   Because certificate validation requires that root keys be distributed
   independently, the self-signed certificate that specifies the root
   certificate authority is omitted from the chain.  Client
   implementations MUST be provisioned with a trust anchor store that
   contains the root certificates.  The use of the Trust Anchor
   Management Protocol (TAMP) [RFC5934] is, however, not envisioned.
   Instead IoT devices using this profile MUST use a software update
   mechanism to populate the trust anchor store.

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    Client                                          Server
    ------                                          ------

    ClientHello             -------->

                            <--------      ServerHelloDone

    Finished                -------->

                            <--------             Finished

   Note: Extensions marked with '*' were introduced with

          Figure 9: DTLS Mutual Certificate-based Authentication.

   Server certificates MUST contain the fully qualified DNS domain name
   or "FQDN" as dNSName [RFC5280].  For CoAP, the coaps URI scheme is
   described in Section 6.2 of [RFC7252].  This FQDN is stored in the
   SubjectAltName or in the leftmost CN component of subject name, as
   explained in Section of [RFC7252], and used by the client to
   match it against the FQDN used during the look-up process, as
   described in [RFC6125].  For other protocols, the appropriate URI
   scheme specification has to be consulted.

   When constrained servers are used, for example in context of locally
   discoverable services as shown in Figure 6, then the rules of client
   certificates are applicable since these constrained servers are less
   likely to have an FQDN configured.  Note that the Service Name
   Indication (SNI) extension cannot be used in this case since SNI does
   not offer the ability to convey EUI-64 [EUI64] identifiers.

   For client certificates the identifier used in the SubjectAltName or
   in the leftmost CN component of subject name MUST be an EUI-64, as
   mandated in Section of [RFC7252].

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   For certificate revocation neither the Online Certificate Status
   Protocol (OCSP) nor Certificate Revocation Lists (CRLs) are used.
   Instead, this profile relies on a software update mechanism to
   provision information about revoked certificates.  While multiple
   OCSP stapling [RFC6961] has recently been introduced as a mechanism
   to piggyback OCSP request/responses inside the DTLS/TLS handshake (to
   avoid the cost of a separate protocol handshake), further
   investigations are needed to determine its suitability for the IoT

   Regarding the ciphersuite choice the discussion in Section 6.2
   applies.  Further details about X.509 certificates can be found in
   Section of [RFC7252].  The TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8
   ciphersuite description in Section 6.2 is also applicable to this

   When using certificates, IoT devices MUST provide support for a
   server certificate chain of at least 3 not including the trust anchor
   and MAY reject connections from servers offering chains longer than
   3.  IoT devices MAY have client certificate chains of any length.
   Obviously, longer chains require more digital signature verification
   operations to perform and lead to larger certificate messages in the
   TLS handshake.

   Table 1 provides a summary of the elements in a certificate for use
   with this profile.

   |       Element        |                   Notes                    |
   |       version        |  This profile uses X.509 v3 certificates   |
   |                      |                 [RFC5280].                 |
   |                      |                                            |
   |     serialNumber     |  Positive integer unique per certificate.  |
   |                      |                                            |
   |      signature       |     This field contains the signature      |
   |                      |   algorithm and this profile uses ecdsa-   |
   |                      |     with-SHA256 or stronger [RFC5758].     |
   |                      |                                            |
   |        issuer        |     Contains the DN of the issuing CA.     |
   |                      |                                            |
   |       validity       | Values expressed as UTC time in notBefore  |
   |                      |  and notAfter fields. No validity period   |
   |                      |                 mandated.                  |
   |                      |                                            |
   |       subject        |    See rules outlined in this section.     |
   |                      |                                            |
   | subjectPublicKeyInfo |     The SubjectPublicKeyInfo structure     |

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   |                      | indicates the algorithm and any associated |
   |                      |   parameters for the ECC public key.This   |
   |                      | profile uses the id-ecPublicKey algorithm  |
   |                      |  identifier for ECDSA signature keys, as   |
   |                      |     defined in specified in [RFC5480].     |
   |                      |                                            |
   |  signatureAlgorithm  | The ECDSA signature algorithm with ecdsa-  |
   |                      |          with-SHA256 or stronger.          |
   |                      |                                            |
   |    signatureValue    |     Bit string containing the digital      |
   |                      |                 signature.                 |
   |                      |                                            |
   |      Extension:      |    See rules outlined in this section.     |
   |    subjectAltName    |                                            |
   |                      |                                            |
   |      Extension:      |    Indicates whether the subject of the    |
   |   BasicConstraints   | certificate is a CA and the maximum depth  |
   |                      | of valid certification paths that include  |
   |                      |  this certificate. This extension is used  |
   |                      |  for CA certs only and then the value of   |
   |                      | the 'cA' field is set to TRUE. The default |
   |                      |                 is FALSE.                  |
   |                      |                                            |
   | Extension: Key Usage | The KeyUsage field MAY have the following  |
   |                      |   values in the context of this profile:   |
   |                      |     digitalSignature or keyAgreement,      |
   |                      |  keyCertSign for verifying signatures on   |
   |                      |          public key certificates.          |
   |                      |                                            |
   | Extension: Extended  |  The ExtKeyUsageSyntax field MAY have the  |
   |      Key Usage       |    following values in context of this     |
   |                      |    profile: id-kp-serverAuth for server    |
   |                      |    authentication, id-kp-clientAuth for    |
   |                      |  client authentication, id-kp-codeSigning  |
   |                      |   for code signing (for software update    |
   |                      |  mechanism), id-kp-OCSPSigning for future  |
   |                      |             OCSP usage in TLS.             |

                       Table 1: Certificate Content.

   All certificate elements listed in Table 1 are mandatory-to-
   implement.  No other certificate elements are used by this

   A device compliant with the profile in this section MUST implement
   TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8 and follow the guidance from this

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6.3.1.  Client Certificate URLs

   RFC 6066 [RFC6066] allows to avoid sending client-side certificates
   and uses URLs instead.  This reduces the over-the-air transmission.
   Note that the TLS cached info extension does not provide any help
   with caching client certificates.

   TLS/DTLS clients MUST implement support for client certificate URLs
   for those environments where client-side certificates are used and
   the server-side is not constrained.  For constrained servers this
   functionality is NOT RECOMMENDED since it forces the server to
   execute an additional protocol exchange, potentially using a protocol
   it does not even support.  The use of this extension also increases
   the risk of a denial of service attack against the constrained server
   due to the additional workload.

6.3.2.  Trusted CA Indication

   RFC 6066 [RFC6066] allows clients to indicate what trust anchor they
   support.  With certificate-based authentication a DTLS server conveys
   its end entity certificate to the client during the DTLS exchange
   provides.  Since the server does not necessarily know what trust
   anchors the client has stored and to facilitate certification path
   construction as well as path validation, it includes intermediate CA
   certs in the certificate payload.

   Today, in most IoT deployments there is a fairly static relationship
   between the IoT device (and the software running on them) and the
   server-side infrastructure.  For these deployments where IoT devices
   interact with a fixed, pre-configured set of servers this extension

   In cases where client interact with dynamically discovered TLS/DTLS
   servers, for example in the use cases described in Section 4.2, the
   use of this extension is RECOMMENDED.

7.  Signature Algorithm Extension

   The "signature_algorithms" extension, defined in Section of
   RFC 5246 [RFC5246], allows the client to indicate to the server which
   signature/hash algorithm pairs may be used in digital signatures.
   The client MUST send this extension to select the use of SHA-256
   since otherwise absent this extension RFC 5246 defaults to SHA-1 /
   ECDSA for the ECDH_ECDSA and the ECDHE_ECDSA key exchange algorithms.

   The "signature_algorithms" extension is not applicable to the PSK-
   based ciphersuite described in Section 6.1.

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8.  Error Handling

   TLS/DTLS uses the Alert protocol to convey errors and specifies a
   long list of error types.  However, not all error messages defined in
   the TLS/DTLS specification are applicable to this profile.  In
   general, there are two categories of errors (as defined in
   Section 7.2 of RFC 5246), namely fatal errors and warnings.  Alert
   messages with a level of fatal result in the immediate termination of
   the connection.  If possible, developers should try to develop
   strategies to react to those fatal errors, such as re-starting the
   handshake or informing the user using the (often limited) user
   interface.  Warnings may be ignored by the application since many IoT
   devices will either have limited ways to log errors or no ability at
   all.  In any case, implementers have to carefully evaluate the impact
   of errors and ways to remedy the situation since a commonly used
   approach for delegating decision making to users is difficult (or
   impossible) to accomplish in a timely fashion.

   All error messages marked as RESERVED are only supported for
   backwards compatibility with SSL MUST NOT be used with this profile.
   Those include decryption_failed_RESERVED, no_certificate_RESERVED,
   and export_restriction_RESERVED.

   A number of the error messages MUST only be used for certificate-
   based ciphersuites.  Hence, the following error messages MUST NOT be
   used with with PSK and raw public key authentication:

   o  bad_certificate,

   o  unsupported_certificate,

   o  certificate_revoked,

   o  certificate_expired,

   o  certificate_unknown,

   o  unknown_ca, and

   o  access_denied.

   Since this profile does not make use of compression at the TLS layer
   the decompression_failure error message MUST NOT be used either.

   RFC 4279 introduced a new alert message unknown_psk_identity for PSK
   ciphersuites.  As stated in Section 2 of RFC 4279 the
   decryption_error error message may also be used instead.  For this
   profile the TLS server MUST return the decryption_error error message

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   instead of the unknown_psk_identity since the two mechanisms exist
   and provide the same functionality.

   Furthermore, the following errors should not occur with devices and
   servers supporting this specification but implementations MUST be
   prepared to process these errors to deal with servers that are not
   compliant to the profiles in this document:

   protocol_version:  While this document focuses only on one version of
      the TLS/DTLS protocol, namely version 1.2, ongoing work on TLS/
      DTLS 1.3 is in progress at the time of writing.

   insufficient_security:  This error message indicates that the server
      requires ciphers to be more secure.  This document specifies only
      one ciphersuite per profile but it is likely that additional
      ciphtersuites get added over time.

   user_canceled:  Many IoT devices are unattended and hence this error
      message is unlikely to occur.

9.  Session Resumption

   Session resumption is a feature of the core TLS/DTLS specifications
   that allows a client to continue with an earlier established session
   state.  The resulting exchange is shown in Figure 10.  In addition,
   the server may choose not to do a cookie exchange when a session is
   resumed.  Still, clients have to be prepared to do a cookie exchange
   with every handshake.  The cookie exchange is not shown in the

         Client                                               Server
         ------                                               ------

         ClientHello                   -------->
                                       <--------             Finished
         Finished                      -------->
         Application Data              <------->     Application Data

                    Figure 10: DTLS Session Resumption.

   Constrained clients MUST implement session resumption to improve the
   performance of the handshake.  This will lead to a reduced number of
   message exchanges, lower computational overhead (since only symmetric

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   cryptography is used during a session resumption exchange), and
   session resumption requires less bandwidth.

   For cases where the server is constrained (but not the client) the
   client MUST implement RFC 5077 [RFC5077].  RFC 5077 specifies a
   version of TLS/DTLS session resumption that does not require per-
   session state information to be maintained by the constrained server.
   This is accomplished by using a ticket-based approach.

   If both the client and the server are constrained devices both
   devices SHOULD implement RFC 5077 and MUST implement basic session
   resumption.  Clients that do not want to use session resumption are
   always able to send a ClientHello message with an empty session_id to
   revert to a full handshake.

10.  Compression

   Section 3.3 of [I-D.ietf-uta-tls-bcp] recommends to disable TLS/DTLS-
   level compression due to attacks, such as CRIME.  For IoT
   applications compression at the TLS/DTLS layer is not needed since
   application layer protocols are highly optimized and the compression
   algorithms at the DTLS layer increases code size and complexity.

   This TLS/DTLS profile MUST NOT implement TLS/DTLS layer compression.

11.  Perfect Forward Secrecy

   Perfect forward secrecy (PFS) is a property that preserves the
   confidentiality of past conversations even in situations where the
   long-term secret is compromised.

   The PSK ciphersuite recommended in Section 6.1 does not offer this
   property since it does not utilize a Diffie-Hellman exchange.  New
   ciphersuites that support PFS for PSK-based authentication, such as
   proposed in [I-D.schmertmann-dice-ccm-psk-pfs], might become
   available as standardized ciphersuite in the (near) future.  The
   recommended PSK-based ciphersuite offers excellent performance, a
   very small memory footprint, and has the lowest on the wire overhead
   at the expense of not using any public cryptography.  For deployments
   where public key cryptography is acceptable the raw public might
   offer an acceptable middleground between the PSK ciphersuite in terms
   of out-of-band validation and the functionality offered by asymmetric

   The use of PFS is a trade-off decision since on one hand the
   compromise of long-term secrets of embedded devices is more likely
   than with many other Internet hosts but on the other hand a Diffie-
   Hellman exchange requires ephemeral key pairs to be generated, which

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   is demanding from a performance point of view.  For obvious
   performance improvement, some implementations re-use key pairs over
   multiple exchanges (rather than generating new keys for each
   exchange).  However, note that such key re-use over long periods
   voids the benefits of forward secrecy when an attack gains access to
   this DH key pair.

   The impact of the disclosure of past conversations and the desire to
   increase the cost for pervasive monitoring (as demanded by [RFC7258])
   has to be taken into account when making a deployment decision.

   Client implementations claiming support of this profile MUST
   implement the ciphersuites listed in Section 6 according to the
   selected credential type.

12.  Keep-Alive

   Application layer communication may create state at the endpoints and
   this state my expire at some time.  For this reason, applications
   define ways to refresh state, if necessary.  While the application
   layer exchanges are largely outside the scope of the underlying TLS/
   DTLS exchange similar state considerations also play a role at the
   level of TLS/DTLS.  While TLS/DTLS also creates state in form of a
   security context (see the security parameter described in Appendix A6
   in RFC 5246) at the client and the server this state information does
   not expire.  However, network intermediaries may also allocate state
   and require this state to be kept alive.  Failure to keep state alive
   at a stateful packet filtering firewall or at a NAT may result in the
   inability for one node to reach the other since packets will get
   blocked by these middleboxes.  Periodic keep-alive messages exchanged
   between the TLS/DTLS client and server keep state at these
   middleboxes alive.  According to measurements described in
   [HomeGateway] there is some variance in state management practices
   used in residential gateways but the timeouts are heavily impacted by
   the choice of the transport layer protocol: timeouts for UDP are
   typically much shorter than those for TCP.

   RFC 6520 [RFC6520] defines a heartbeat mechanism to test whether the
   other peer is still alive.  As an additional feature, the same
   mechanism can also be used to perform Path Maximum Transmission Unit
   (MTU) Discovery.

   A recommendation about the use of RFC 6520 depends on the type of
   message exchange an IoT device performs and the number of messages
   the application needs to exchange as part of their application
   functionality.  There are three types of exchanges that need to be

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   Client-Initiated, One-Shot Messages

      This is a common communication pattern where IoT devices upload
      data to a server on the Internet on an irregular basis.  The
      communication may be triggered by specific events, such as opening
      a door.

      Since the upload happens on an irregular and unpredictable basis
      and due to renumbering and Network Address Translation (NAT) the
      DTLS handshake may need to be re-started (ideally using session
      resumption, if possible).

      In this case there is no use for a keep-alive extension for this

   Client-Initiated, Regular Data Uploads

      This is a variation of the previous case whereby data gets
      uploaded on a regular basis, for example, based on frequent
      temperature readings.  If neither NAT bindings nor IP address
      changes occurred then the record layer will not notice any
      changes.  For the case where the IP address and port number
      changes, it is necessary to re-create the record layer using
      session resumption.

      In this scenario there is no use for a keep-alive extension.  It
      is also very likely that the device will enter a sleep cycle in
      between data transmissions to keep power consumption low.

   Server-Initiated Messages

      In the two previous scenarios the client initiated the protocol
      interaction and maintains it.  Since messages to the client may
      get blocked by middleboxes the initial connection setup is
      triggered by the client and then kept alive by the server.

      For this message exchange pattern the use of DTLS heartbeat
      messages is quite useful but may have to be coordinated with
      application exchanges (for example when the CoAP resource
      directory is used) to avoid redundant keep-alive message
      exchanges.  The MTU discovery mechanism, which is also part of
      [RFC6520], is less likely to be relevant since for many IoT
      deployments the most constrained link is the wireless interface
      between the IoT device and the network itself (rather than some
      links along the end-to-end path).  Only in more complex network
      topologies, such as multi-hop mesh networks, path MTU discovery
      might be appropriate.  It also has to be noted that DTLS itself
      already provides a basic path discovery mechanism (see

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      Section of RFC 6347 by using the fragmentation capability
      of the handshake protocol).

   For server-initiated messages the heartbeat extension is RECOMMENDED.

13.  Timeouts

   To connect to the Internet a variety of wired and wireless
   technologies are available.  Many of the low power radio
   technologies, such as IEEE 802.15.4 or Bluetooth Smart, only support
   small frame sizes (e.g., 127 bytes in case of IEEE 802.15.4 as
   explained in RFC 4919 [RFC4919]).  Other radio technologies, such as
   the Global System for Mobile Communications (GSM) using the short
   messaging service (SMS) have similar constraints in terms of payload
   sizes, such as 140 bytes without the optional segmentation and
   reassembly scheme known as Concatenated SMS, but show higher latency.

   The DTLS handshake protocol adds a fragmentation and reassembly
   mechanism to the TLS handshake protocol since each DTLS record must
   fit within a single transport layer datagram, as described in
   Section 4.2.3 of [RFC6347].  Since handshake messages are potentially
   bigger than the maximum record size, the mechanism fragments a
   handshake message over a number of DTLS records, each of which can be
   transmitted separately.

   To deal with the unreliable message delivery provided by UDP, DTLS
   adds timeouts and re-transmissions, as described in Section 4.2.4 of
   [RFC6347].  Although the timeout values are implementation specific,
   recommendations are provided in Section of [RFC6347], with an
   initial timer value of 1 second and doubled with at each
   retransmission up to no less than 60 seconds.  Due to the nature of
   some radio technologies, these values are too aggressive and lead to
   spurious failures when messages in flight need longer.

   Note: If a round-trip time estimator (such as proposed in
   [I-D.bormann-core-cocoa]) is available in the protocol stack of the
   device, it could be used to dynamically update the setting of the
   retransmit timeout.

   Choosing appropriate timeout values is difficult with changing
   network conditions, and large variance in latency.  This
   specification therefore RECOMMENDS an initial timer value of 10
   seconds with exponential back off up to no less then 60 seconds.
   Appendix A provides additional normative text for carrying DTLS over

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14.  Random Number Generation

   The TLS/DTLS protocol requires random numbers to be available during
   the protocol run.  For example, during the ClientHello and the
   ServerHello exchange the client and the server exchange random
   numbers.  Also, the use of the Diffie-Hellman exchange requires
   random numbers during the key pair generation.  Special care has to
   be taken when generating random numbers in embedded systems as many
   entropy sources available on desktop operating systems or mobile
   devices might be missing, as described in [Heninger].  Consequently,
   if not enough time is given during system start time to fill the
   entropy pool then the output might be predictable and repeatable, for
   example leading to the same keys generated again and again.

   It is important to note that sources contributing to the randomness
   pool on laptops, or desktop PCs are not available on many IoT device,
   such as mouse movement, timing of keystrokes, air turbulence on the
   movement of hard drive heads, etc.  Other sources have to be found or
   dedicated hardware has to be added.

   The ClientHello and the ServerHello messages contains the 'Random'
   structure, which has two components: gmt_unix_time and a sequence of
   28 random bytes. gmt_unix_time holds the current time and date in
   standard UNIX 32-bit format (seconds since the midnight starting Jan
   1, 1970, GMT).  [I-D.mathewson-no-gmtunixtime] argues that the entire
   ClientHello.Random value (including gmt_unix_time) should be a
   sequence of random bits because of device fingerprinting privacy
   concerns.  Since many IoT devices do not have access to an accurate
   clock, it is RECOMMENDED to follow the guidance outlined in
   [I-D.mathewson-no-gmtunixtime] regarding the content of the
   ClientHello.Random field.  However, for the ServerHello.Random
   structure it is RECOMMENDED to maintain the existing structure with
   gmt_unix_time followed by a sequence of 28 random bytes since the
   client can use the received time information to securely obtain time
   information.  For constrained servers it cannot be assumed that they
   maintain accurate time information; these devices MUST include time
   information in the Server.Random structure when they actually obtain
   accurate time information that can be utilized by clients.  Clients
   MUST only use time information obtained from servers they trust and
   the use of this approach has to be agreed out-of-band.

   IoT devices using TLS/DTLS MUST offer ways to generate quality random
   numbers using hardware-based random number generators.  Note that
   these hardware-based random number generators do not necessarily need
   to be implemented inside the microcontroller itself but could be made
   available in dedicated crypto-chips as well.  Guidelines and
   requirements for random number generation can be found in RFC 4086
   [RFC4086] and in the NIST Special Publication 800-90a [SP800-90A].

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   Chip manufacturers are highly encouraged to provide sufficient
   documentation of their design for random number generators so that
   customers can have confidence about the quality of the generated
   random numbers.  The confidence can be increased by providing
   information about the procedures that have been used to verify the
   randomness of numbers generated by the hardware modules.  For
   example, NIST Special Publication 800-22b [SP800-22b] describes
   statistical tests that can be used to verify random random number

15.  Truncated MAC and Encrypt-then-MAC Extension

   The truncated MAC extension was introduced with RFC 6066 [RFC6066]
   with the goal to reduce the size of the MAC used at the Record Layer.
   This extension was developed for TLS ciphersuites that used older
   modes of operation where the MAC and the encryption operation was
   performed independently.

   The recommended ciphersuites in this document use the newer
   Authenticated Encryption with Associated Data (AEAD) construct,
   namely the CBC-MAC mode (CCM) with eight-octet authentication tags,
   and are therefore not appliable to the truncated MAC extension.

   RFC 7366 [RFC7366] introduced the encrypt-then-MAC extension (instead
   of the previously used MAC-then-encrypt) since the MAC-then-encrypt
   mechanism has been the subject of a number of security
   vulnerabilities.  RFC 7366 is, however, also not applicable to the
   AEAD ciphers recommended in this document.

   Implementations conformant to this specification MUST use AEAD
   ciphers.  Hence, RFC 7366 and RFC 6066 are not applicable to this
   specifciation and MUST NOT be implemented.

16.  Server Name Indication (SNI)

   The Server Name Indication extension defined in [RFC6066] defines a
   mechanism for a client to tell a TLS/DTLS server the name of the
   server it wants to contact.  This is a useful extension for many
   hosting environments where multiple virtual servers are run on single
   IP address.

   This specification RECOMMENDs the implementation of the Server Name
   Indication extension unless it is known that a TLS/DTLS client does
   not interact with a server in a hosting environment.

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17.  Maximum Fragment Length Negotiation

   This RFC 6066 extension lowers the maximum fragment length support
   needed for the Record Layer from 2^14 bytes to 2^9 bytes.

   This is a very useful extension that allows the client to indicate to
   the server how much maximum memory buffers it uses for incoming
   messages.  Ultimately, the main benefit of this extension is to allow
   client implementations to lower their RAM requirements since the
   client does not need to accept packets of large size (such as 16k
   packets as required by plain TLS/DTLS).

   Client implementations MUST support this extension.

18.  Session Hash

   In order to begin connection protection, the Record Protocol requires
   specification of a suite of algorithms, a master secret, and the
   client and server random values.  The algorithm for computing the
   master secret is defined in Section 8.1 of RFC 5246 but only includes
   a small number of parameters exchanged during the handshake and does
   not include parameters like the client and server identities.  This
   can be utilized by an attacker to mount a man-in-the-middle attack
   since the master secret is not guaranteed to be unique across
   sessions, as discovered in the 'Triple Handshake' attack [Triple-HS].

   [I-D.ietf-tls-session-hash] defines a TLS extension that binds the
   master secret to a log of the full handshake that computes it, thus
   preventing such attacks.

   Client implementations SHOULD implement this extension even though
   the ciphersuites recommended by this profile are not vulnerable to
   this attack.  For Diffie-Hellman-based ciphersuites the keying
   material is contributed by both parties and in case of the pre-shared
   secret key ciphersuite, both parties need to be in possession of the
   shared secret to ensure that the handshake completes successfully.
   It is, however, possible that some application layer protocols will
   tunnel other authentication protocols on top of DTLS making this
   attack relevant again.

19.  Re-Negotiation Attacks

   TLS/DTLS allows a client and a server who already have a TLS/DTLS
   connection to negotiate new parameters, generate new keys, etc by
   using the re-negotiation feature.  Renegotiation happens in the
   existing connection, with the new handshake packets being encrypted
   along with application data.  Upon completion of the re-negotiation
   procedure the new channel replaces the old channel.

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   As described in RFC 5746 [RFC5746] there is no cryptographic binding
   between the two handshakes, although the new handshake is carried out
   using the cryptographic parameters established by the original

   To prevent the re-negotiation attack [RFC5746] this specification
   RECOMMENDS to disable the TLS renegotigation feature.  Clients MUST
   respond to server-initiated re-negotiation attempts with an alert
   message (no_renegotiation) and clients MUST NOT initiate them.

20.  Downgrading Attacks

   When a client sends a ClientHello with a version higher than the
   highest version known to the server, the server is supposed to reply
   with ServerHello.version equal to the highest version known to the
   server and the handshake can proceed.  This behaviour is known as
   version tolerance.  Version-intolerance is when the server (or a
   middlebox) breaks the handshake when it sees a ClientHello.version
   higher than what it knows about.  This is the behaviour that leads
   some clients to re-run the handshake with lower version.  As a
   result, a potential security vulnerability is introduced when a
   system is running an old TLS/SSL version (e.g., because of the need
   to integrate with legacy systems).  In the worst case, this allows an
   attacker to downgrade the protocol handshake to SSL 3.0.  SSL 3.0 is
   so broken that there is no secure cipher available for it (see

   The above-described downgrade vulnerability is solved by the TLS
   Fallback Signaling Cipher Suite Value (SCSV)
   [I-D.ietf-tls-downgrade-scsv] extension.  However, the solution is
   not appliable to implementations conforming to this profile since the
   version negotiation MUST use TLS/DTLS version 1.2 (or higher).  More
   specifically, this implies:

   o  Clients MUST NOT send a TLS/DTLS version lower than version 1.2 in
      the ClientHello.

   o  Clients MUST NOT retry a failed negotiation offering a TLS/DTLS
      version lower than 1.2.

   o  Servers MUST fail the handshake by sending a protocol_version
      fatal alert if a TLS/DTLS version >= 1.2 cannot be negotiated.
      Note that the aborted connection is non-resumable.

   If at some time in the future this profile reaches the quality of SSL
   3.0 a software update is needed since constrained devices are
   unlikely to run multiple TLS/DTLS versions due to memory size

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21.  Crypto Agility

   This document recommends software and chip manufacturers to implement
   AES and the CCM mode of operation.  This document references the CoAP
   recommended ciphersuite choices, which have been selected based on
   implementation and deployment experience from the IoT community.
   Over time the preference for algorithms will, however, change.  Not
   all components of a ciphersuite are likely to change at the same
   speed.  Changes are more likely expected for ciphers, the mode of
   operation, and the hash algorithms.  The recommended key lengths have
   to be adjusted over time.  Some deployment environments will also be
   impacted by local regulation, which might dictate a certain cipher
   and key size.  Ongoing discussions regarding the choice of specific
   ECC curves will also likely impact implementations.  Note that this
   document does not recommend or mandate a specific ECC curve.

   The following recommendations can be made to chip manufacturers:

   o  Make any AES hardware-based crypto implementation accessible to
      developers working on security implementations at higher layers.
      Sometimes hardware implementatios are added to microcontrollers to
      offer support for functionality needed at the link layer and are
      only available to the on-chip link layer protocol implementation.

   o  Provide flexibility for the use of the crypto function with future
      extensibility in mind.  For example, making an AES-CCM
      implementation available to developers is a first step but such an
      implementation may not be usable due to parameter differences
      between an AES-CCM implementations.  AES-CCM in IEEE 802.15.4 and
      Bluetooth Smart uses a nonce length of 13-octets while DTLS uses a
      nonce length of 12-octets.  Hardware implementations of AES-CCM
      for IEEE 802.15.4 and Bluetooth Smart are therefore not re-usable
      by a DTLS stack.

   o  Offer access to building blocks in addition (or as an alternative)
      to the complete functionality.  For example, a chip manufacturer
      who gives developers access to the AES crypto function can use it
      to build an efficient AES-GCM implementations.  Another example is
      to make a special instruction available that increases the speed
      of speed-up carryless multiplications.

   As a recommendation for developers and product architects we
   recommend that sufficient headroom is provided to allow an upgrade to
   a newer cryptographic algorithms over the lifetime of the product.
   As an example, while AES-CCM is recommended thoughout this
   specification future products might use the ChaCha20 cipher in
   combination with the Poly1305 authenticator

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   [I-D.irtf-cfrg-chacha20-poly1305].  The assumption is made that a
   robust software update mechanism is offered.

22.  Key Length Recommendations

   RFC 4492 [RFC4492] gives approximate comparable key sizes for
   symmetric- and asymmetric-key cryptosystems based on the best-known
   algorithms for attacking them.  While other publications suggest
   slightly different numbers, such as [Keylength], the approximate
   relationship still holds true.  Figure 11 illustrates the comparable
   key sizes in bits.

   At the time of writing the key size recommendations for use with TLS-
   based ciphers found in [I-D.ietf-uta-tls-bcp] recommend DH key
   lengths of at least 2048 bit, which corresponds to a 112-bit
   symmetric key and a 233 bit ECC key.  These recommendations are
   inline with those from other organizations, such as National
   Institute of Standards and Technology (NIST) or European Network and
   Information Security Agency (ENISA).  The authors of
   [ENISA-Report2013] add that a 80-bit symmetric key is sufficient for
   legacy applications for the coming years, but a 128-bit symmetric key
   is the minimum requirement for new systems being deployed.  The
   authors further note that one needs to also take into account the
   length of time data needs to be kept secure for.  The use of 80-bit
   symmetric keys for transactional data may be acceptable for the near
   future while one has to insist on 128-bit symmetric keys for long
   lived data.

                       Symmetric  |   ECC   |  DH/DSA/RSA
                           80     |   163   |     1024
                          112     |   233   |     2048
                          128     |   283   |     3072
                          192     |   409   |     7680
                          256     |   571   |    15360

                Figure 11: Comparable Key Sizes (in bits).

23.  False Start

   A full TLS handshake as specified in [RFC5246] requires two full
   protocol rounds (four flights) before the handshake is complete and
   the protocol parties may begin to send application data.

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   An abbreviated handshake (resuming an earlier TLS session) is
   complete after three flights, thus adding just one round-trip time if
   the client sends application data first.

   If the conditions outlined in [I-D.bmoeller-tls-falsestart] are met,
   application data can be transmitted when the sender has sent its own
   "ChangeCipherSpec" and "Finished" messages.  This achieves an
   improvement of one round-trip time for full handshakes if the client
   sends application data first, and for abbreviated handshakes if the
   server sends application data first.

   The conditions for using the TLS False Start mechanism are met by the
   public-key-based ciphersuites in this document.  In summary, the
   conditions are

   o  Modern symmetric ciphers with an effective key length of 128 bits,
      such as AES-128-CCM

   o  Client certificate types, such as ecdsa_sign

   o  Key exchange methods, such as ECDHE_ECDSA

   Based on the improvement over a full roundtrip for the full TLS/DTLS
   exchange this specification RECOMMENDS the use of the False Start
   mechanism when clients send application data first.

24.  Privacy Considerations

   The DTLS handshake exchange conveys various identifiers, which can be
   observed by an on-path eavesdropper.  For example, the DTLS PSK
   exchange reveals the PSK identity, the supported extensions, the
   session id, algorithm parameters, etc.  When session resumption is
   used then individual TLS sessions can be correlated by an on-path
   adversary.  With many IoT deployments it is likely that keying
   material and their identifiers are persistent over a longer period of
   time due to the cost of updating software on these devices.

   User participation with many IoT deployments poses a challenge since
   many of the IoT devices operate unattended, even though they will
   initially be provisioned by a human.  The ability to control data
   sharing and to configure preference will have to be provided at a
   system level rather than at the level of the DTLS exchange itself,
   which is the scope of this document.  Quite naturally, the use of
   DTLS with mutual authentication will allow a TLS server to collect
   authentication information about the IoT device (likely over a long
   period of time).  While this strong form of authentication will
   prevent mis-attribution, it also allows strong identification.
   Device-related data collection (e.g., sensor recordings) associated

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   with other data type will prove to be truly useful but this extra
   data might include personal information about the owner of the device
   or data about the environment it senses.  Consequently, the data
   stored on the server-side will be vulnerable to stored data
   compromise.  For the communication between the client and the server
   this specification prevents eavesdroppers to gain access to the
   communication content.  While the PSK-based ciphersuite does not
   provide PFS the asymmetric versions do.  This prevents an adversary
   from obtaining past communication content when access to a long-term
   secret has been gained.  Note that no extra effort to make traffic
   analysis more difficult is provided by the recommendations made in
   this document.

25.  Security Considerations

   This entire document is about security.

   We would also like to point out that designing a software update
   mechanism into an IoT system is crucial to ensure that both
   functionality can be enhanced and that potential vulnerabilities can
   be fixed.  This software update mechanism is important for changing
   configuration information, for example, trust anchors and other
   keying related information.  Such a suitable software update
   mechanism is available with the Lightweight Machine-to-Machine
   (LWM2M) protocol published by the Open Mobile Alliance (OMA) [LWM2M].

26.  IANA Considerations

   This document includes no request to IANA.

27.  Acknowledgements

   Thanks to Olaf Bergmann, Paul Bakker, Robert Cragie, Russ Housley,
   Rene Hummen, Matthias Kovatsch, Sandeep Kumar, Sye Loong Keoh, Simon
   Lemay, Alexey Melnikov, Manuel Pegourie-Gonnard, Akbar Rahman, Eric
   Rescorla, Michael Richardson, Ludwig Seitz, Zach Shelby, Michael
   StJohns, Rene Struik, and Sean Turner for their helpful comments and
   discussions that have shaped the document.

   Big thanks also to Klaus Hartke, who wrote the initial version of
   this document.

   Finally, we would like to thank our area director (Stephen Farrell)
   and our working group chairs (Zach Shelby and Dorothy Gellert) for
   their support.

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28.  References

28.1.  Normative References

              REGISTRATION AUTHORITY", April 2010,

   [GSM-SMS]  ETSI, "3GPP TS 23.040 V7.0.1 (2007-03): 3rd Generation
              Partnership Project; Technical Specification Group Core
              Network and Terminals; Technical realization of the Short
              Message Service (SMS) (Release 7)", March 2007.

              Santesson, S. and H. Tschofenig, "Transport Layer Security
              (TLS) Cached Information Extension", draft-ietf-tls-
              cached-info-17 (work in progress), November 2014.

              Bhargavan, K., Delignat-Lavaud, A., Pironti, A., Langley,
              A., and M. Ray, "Transport Layer Security (TLS) Session
              Hash and Extended Master Secret Extension", draft-ietf-
              tls-session-hash-03 (work in progress), November 2014.

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

   [RFC4279]  Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
              for Transport Layer Security (TLS)", RFC 4279, December

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246, August 2008.

   [RFC5746]  Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
              "Transport Layer Security (TLS) Renegotiation Indication
              Extension", RFC 5746, February 2010.

   [RFC6066]  Eastlake, D., "Transport Layer Security (TLS) Extensions:
              Extension Definitions", RFC 6066, January 2011.

   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, March 2011.

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

   [RFC6520]  Seggelmann, R., Tuexen, M., and M. Williams, "Transport
              Layer Security (TLS) and Datagram Transport Layer Security
              (DTLS) Heartbeat Extension", RFC 6520, February 2012.

   [RFC7250]  Wouters, P., Tschofenig, H., Gilmore, J., Weiler, S., and
              T. Kivinen, "Using Raw Public Keys in Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", RFC 7250, June 2014.

   [RFC7251]  McGrew, D., Bailey, D., Campagna, M., and R. Dugal, "AES-
              CCM Elliptic Curve Cryptography (ECC) Cipher Suites for
              TLS", RFC 7251, June 2014.

   [WAP-WDP]  Wireless Application Protocol Forum, "Wireless Datagram
              Protocol", June 2001.

28.2.  Informative References

   [ACE-WG]   IETF, "Authentication and Authorization for Constrained
              Environments (ace) Working Group", URL:
              https://datatracker.ietf.org/wg/ace/charter/, Jan 2015.

   [AES]      National Institute of Standards and Technology, "FIPS PUB
              197, Advanced Encryption Standard (AES)",
              tls-parameters.xhtml#tls-parameters-4, November 2001.

   [CCM]      National Institute of Standards and Technology, "Special
              Publication 800-38C, Recommendation for Block Cipher Modes
              of Operation: The CCM Mode for Authentication and
              Confidentiality", http://csrc.nist.gov/publications/
              nistpubs/800-38C/SP800-38C_updated-July20_2007.pdf, May

              ENISA, "Algorithms, Key Sizes and Parameters Report -
              2013", http://www.enisa.europa.eu/activities/identity-and-
              algorithms-key-sizes-and-parameters-report, October 2013.

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              Heninger, N., Durumeric, Z., Wustrow, E., and A.
              Halderman, "Mining Your Ps and Qs: Detection of Widespread
              Weak Keys in Network Devices", 21st USENIX Security
              technical-sessions/presentation/heninger, 2012.

              Eggert, L., "An experimental study of home gateway
              characteristics, In Proceedings of the '10th annual
              conference on Internet measurement'", 2010.

              Langley, A., Modadugu, N., and B. Moeller, "Transport
              Layer Security (TLS) False Start", draft-bmoeller-tls-
              falsestart-01 (work in progress), November 2014.

              Bormann, C., Betzler, A., Gomez, C., and I. Demirkol,
              "CoAP Simple Congestion Control/Advanced", draft-bormann-
              core-cocoa-02 (work in progress), July 2014.

              Tschofenig, H., Arkko, J., Thaler, D., and D. McPherson,
              "Architectural Considerations in Smart Object Networking",
              draft-iab-smart-object-architecture-06 (work in progress),
              October 2014.

              Shelby, Z. and C. Bormann, "CoRE Resource Directory",
              draft-ietf-core-resource-directory-02 (work in progress),
              November 2014.

              Kumar, S., Keoh, S., and H. Tschofenig, "A Hitchhiker's
              Guide to the (Datagram) Transport Layer Security Protocol
              for Smart Objects and Constrained Node Networks", draft-
              ietf-lwig-tls-minimal-01 (work in progress), March 2014.

              Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
              Suite Value (SCSV) for Preventing Protocol Downgrade
              Attacks", draft-ietf-tls-downgrade-scsv-05 (work in
              progress), February 2015.

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              Gillmor, D., "Negotiated Discrete Log Diffie-Hellman
              Ephemeral Parameters for TLS", draft-ietf-tls-negotiated-
              dl-dhe-00 (work in progress), July 2014.

              Popov, A., "Prohibiting RC4 Cipher Suites", draft-ietf-
              tls-prohibiting-rc4-01 (work in progress), October 2014.

              Barnes, R., Thomson, M., Pironti, A., and A. Langley,
              "Deprecating Secure Sockets Layer Version 3.0", draft-
              ietf-tls-sslv3-diediedie-02 (work in progress), March

              Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of TLS and DTLS", draft-
              ietf-uta-tls-bcp-11 (work in progress), February 2015.

              Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
              protocols", draft-irtf-cfrg-chacha20-poly1305-10 (work in
              progress), February 2015.

              Mathewson, N. and B. Laurie, "Deprecating gmt_unix_time in
              TLS", draft-mathewson-no-gmtunixtime-00 (work in
              progress), December 2013.

              Schmertmann, L. and C. Bormann, "ECDHE-PSK AES-CCM Cipher
              Suites with Forward Secrecy for Transport Layer Security
              (TLS)", draft-schmertmann-dice-ccm-psk-pfs-01 (work in
              progress), August 2014.

              IANA, "TLS Cipher Suite Registry",
              tls-parameters.xhtml#tls-parameters-4, 2014.

              Chilton, E., "A Brief Survey of Imprinting Options for
              Constrained Devices", URL: http://www.lix.polytechnique.fr
              March 2012.

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              Giry, D., "Cryptographic Key Length Recommendations",
              http://www.keylength.com, November 2014.

   [LWM2M]    Open Mobile Alliance, "Lightweight Machine-to-Machine,
              Technical Specification, Candidate Version 1.0", December

   [RFC1191]  Mogul, J. and S. Deering, "Path MTU discovery", RFC 1191,
              November 1990.

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, August 1996.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104, February

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)", RFC
              2865, June 2000.

   [RFC3610]  Whiting, D., Housley, R., and N. Ferguson, "Counter with
              CBC-MAC (CCM)", RFC 3610, September 2003.

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, "Extensible Authentication Protocol (EAP)", RFC
              3748, June 2004.

   [RFC4086]  Eastlake, D., Schiller, J., and S. Crocker, "Randomness
              Requirements for Security", BCP 106, RFC 4086, June 2005.

   [RFC4492]  Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B.
              Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites
              for Transport Layer Security (TLS)", RFC 4492, May 2006.

   [RFC4634]  Eastlake, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and HMAC-SHA)", RFC 4634, July 2006.

   [RFC4919]  Kushalnagar, N., Montenegro, G., and C. Schumacher, "IPv6
              over Low-Power Wireless Personal Area Networks (6LoWPANs):
              Overview, Assumptions, Problem Statement, and Goals", RFC
              4919, August 2007.

   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, January 2008.

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   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, January 2008.

   [RFC5216]  Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
              Authentication Protocol", RFC 5216, March 2008.

   [RFC5247]  Aboba, B., Simon, D., and P. Eronen, "Extensible
              Authentication Protocol (EAP) Key Management Framework",
              RFC 5247, August 2008.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

   [RFC5288]  Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
              Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
              August 2008.

   [RFC5480]  Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
              "Elliptic Curve Cryptography Subject Public Key
              Information", RFC 5480, March 2009.

   [RFC5758]  Dang, Q., Santesson, S., Moriarty, K., Brown, D., and T.
              Polk, "Internet X.509 Public Key Infrastructure:
              Additional Algorithms and Identifiers for DSA and ECDSA",
              RFC 5758, January 2010.

   [RFC5934]  Housley, R., Ashmore, S., and C. Wallace, "Trust Anchor
              Management Protocol (TAMP)", RFC 5934, August 2010.

   [RFC6024]  Reddy, R. and C. Wallace, "Trust Anchor Management
              Requirements", RFC 6024, October 2010.

   [RFC6090]  McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
              Curve Cryptography Algorithms", RFC 6090, February 2011.

   [RFC6655]  McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
              Transport Layer Security (TLS)", RFC 6655, July 2012.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, August 2012.

   [RFC6733]  Fajardo, V., Arkko, J., Loughney, J., and G. Zorn,
              "Diameter Base Protocol", RFC 6733, October 2012.

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   [RFC6961]  Pettersen, Y., "The Transport Layer Security (TLS)
              Multiple Certificate Status Request Extension", RFC 6961,
              June 2013.

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

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

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, May 2014.

   [RFC7366]  Gutmann, P., "Encrypt-then-MAC for Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", RFC 7366, September 2014.

   [RFC7390]  Rahman, A. and E. Dijk, "Group Communication for the
              Constrained Application Protocol (CoAP)", RFC 7390,
              October 2014.

   [RFC7397]  Gilger, J. and H. Tschofenig, "Report from the Smart
              Object Security Workshop", RFC 7397, December 2014.

   [RFC7400]  Bormann, C., "6LoWPAN-GHC: Generic Header Compression for
              IPv6 over Low-Power Wireless Personal Area Networks
              (6LoWPANs)", RFC 7400, November 2014.

              National Institute of Standards and Technology, "Special
              Publication 800-22, Revision 1a, A Statistical Test Suite
              for Random and Pseudorandom Number Generators for
              Cryptographic Applications",
              SP800-22rev1a.pdf, April 2010.

              NIST, "DRAFT Special Publication 800-90a, Revision 1,
              Recommendation for Random Number Generation Using
              Deterministic Random Bit Generators",
              sp800-90a_r1_draft_november2014_ver.pdf, November 2014.

              Bhargavan, K., Delignat-Lavaud, C., Pironti, A., and P.
              Strub, "Triple Handshakes and Cookie Cutters: Breaking and
              Fixing Authentication over TLS", IEEE Symposium on
              Security and Privacy, pages 98-113, 2014.

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Appendix A.  Conveying DTLS over SMS

   This section is normative for the use of DTLS over SMS.  Timer
   recommendations are already outlined in Section 13 and also
   applicable to the transport of DTLS over SMS.

   This section requires readers to be familiar with the terminology and
   concepts described in [GSM-SMS], and [WAP-WDP].

   The remainder of this section assumes Mobile Stations are capable of
   producing and consuming 8-bit binary data encoded Transport Protocol
   Data Units (TPDU).

A.1.  Overview

   DTLS adds an additional roundtrip to the TLS [RFC5246] handshake to
   serve as a return-routability test for protection against certain
   types of DoS attacks.  Thus a full blown DTLS handshake comprises up
   to 6 "flights" (i.e., logical message exchanges), each of which is
   then mapped on to one or more DTLS records using the segmentation and
   reassembly (SaR) scheme described in Section 4.2.3 of [RFC6347].  The
   overhead for said scheme is 6 bytes per Handshake message which,
   given a realistic 10+ messages handshake, would amount around 60
   bytes across the whole handshake sequence.

   Note that the DTLS SaR scheme is defined for handshake messages only.
   In fact, DTLS records are never fragmented and MUST fit within a
   single transport layer datagram.

   SMS provides an optional segmentation and reassembly scheme as well,
   known as Concatenated short messages (see Section of
   [GSM-SMS]).  However, since the SaR scheme in DTLS cannot be
   circumvented, the Concatenated short messages mechanism SHOULD NOT be
   used during handshake to avoid redundant overhead.  Before starting
   the handshake phase (either actively or passively), the DTLS
   implementation MUST be explicitly configured with the PMTU of the SMS
   transport in order to correctly instrument its SaR function.  The
   PMTU SHALL be 133 bytes if WDP-based multiplexing is used (see
   Appendix A.3), 140 bytes otherwise.

   It is RECOMMENDED to use the established security context over the
   longest possible period (possibly until a Closure Alert message is
   received, or after a very long inactivity timeout) to avoid the
   expensive re-establishment of the security association.

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A.2.  Message Segmentation and Re-Assembly

   The content of an SMS message is carried in the TP-UserData field,
   and its size may be up to 140 bytes.  As already mentioned in
   Appendix A.1, longer (i.e., up to 34170 bytes) messages can be sent
   using Concatenated SMS.

   This scheme consumes 6-7 bytes (depending on whether the short or
   long segmentation format is used) of the TP-UserData field, thus
   reducing the space available for the actual content of the SMS
   message to 133-134 bytes per TPDU.

   Though in principle a PMTU value higher than 140 bytes could be used,
   which may look like an appealing option given its more efficient use
   of the transport, there are disadvantages to consider.  First, there
   is an additional overhead of 7 bytes per TPDU to be paid to the SaR
   function (which is in addition to the overhead introduced by the DTLS
   SaR mechanism.  Second, some networks only partially support the
   Concatenated SMS function and others do not support it at all.

   For these reasons, the Concatenated short messages mechanism SHOULD
   NOT be used, and it is RECOMMENDED to leave the same PMTU settings
   used during the handshake phase, i.e., 133 bytes if WDP- based
   multiplexing is enabled, 140 bytes otherwise.

   Note that, after DTLS handshake has completed, any fragmentation and
   reassembly logic that pertains the application layer (e.g.,
   segmenting CoAP messages into DTLS records and reassembling them
   after the crypto operations have been successfully performed) needs
   to be handled by the application that uses the established DTLS

A.3.  Multiplexing Security Associations

   Unlike IPsec ESP/AH, DTLS records do not contain any association
   identifiers.  Applications must arrange to multiplex between
   associations on the same endpoint which, when using UDP/IP, is
   usually done with the host/port number.

   If the DTLS server allows more than one client to be active at any
   given time, then the WAP User Datagram Protocol [WAP-WDP] can be used
   to achieve multiplexing of the different security associations.  (The
   use of WDP provides the additional benefit that upper layer protocols
   can operate independently of the underlying wireless network, hence
   achieving application-agnostic transport handover.)

   The total overhead cost for encoding the WDP source and destination
   ports is either 5 or 7 bytes out of the total available for the SMS

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   content depending on if 1-byte or 2-byte port identifiers are used,
   as shown in Figure 12 and Figure 13.

   0        1        2        3        4
   | ...    | 0x04   | 2      | ...    | ...    |
     UDH      IEI      IE       Dest     Source
     Length            Length   Port     Port

      Figure 12: Application Port Addressing Scheme (8 bit address).

   0        1        2        3        4        5        6
   | ...    | 0x05   | 4      |       ...       |       ...       |
     UDH      IEI      IE       Dest              Source
     Length            Length   Port              Port

      Figure 13: Application Port Addressing Scheme (16 bit address).

   The receiving side of the communication gets the source address from
   the originator address (TP-OA) field of the SMS-DELIVER TPDU.  This
   way an unique 4-tuple identifying the security association can be
   reconstructed at both ends.  (When replying to its DTLS peer, the
   sender will swaps the TP-OA and TP-DA parameters and the source and
   destination ports in the WDP.)

A.4.  Timeout

   If SMS-STATUS-REPORT messages are enabled, their receipt is not to be
   interpreted as the signal that the specific handshake message has
   been acted upon by the receiving party.  Therefore, it MUST NOT be
   taken into account by the DTLS timeout and retransmission function.

   Handshake messages MUST carry a validity period (TP-VP parameter in a
   SMS-SUBMIT TPDU) that is not less than the current value of the
   retransmission timeout.  In order to avoid persisting messages in the
   network that will be discarded by the receiving party, handshake
   messages SHOULD carry a validity period that is the same as, or just
   slightly higher than, the current value of the retransmission

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Appendix B.  DTLS Record Layer Per-Packet Overhead

   Figure 14 shows the overhead for the DTLS record layer for protecting
   data traffic when AES-128-CCM with an 8-octet Integrity Check Value
   (ICV) is used.

   DTLS Record Layer Header................13 bytes
   Nonce (Explicit).........................8 bytes
   ICV..................................... 8 bytes
   Overhead................................29 bytes

      Figure 14: AES-128-CCM-8 DTLS Record Layer Per-Packet Overhead.

   The DTLS record layer header has 13 octets and consists of

   o  1 octet content type field,

   o  2 octet version field,

   o  2 octet epoch field,

   o  6 octet sequence number,

   o  2 octet length field.

   The "nonce" input to the AEAD algorithm is exactly that of [RFC5288],
   i.e., 12 bytes long.  It consists of a 4 octet salt and an 8 octet
   nonce.  The salt is the "implicit" part of the nonce and is not sent
   in the packet.  Since the nonce_explicit may be the 8 octet sequence
   number and, in DTLS, it is the 8 octet epoch concatenated with the 6
   octet sequence number.

   RFC 6655 [RFC6655] allows the nonce_explicit to be a sequence number
   or something else.  This document makes this use more restrictive for
   use with DTLS: the 64-bit none_explicit MUST be the 16-bit epoch
   concatenated with the 48-bit seq_num.  The sequence number component
   of the nonce_explicit field at the AES-CCM layer is an exact copy of
   the sequence number in the record layer header field.  This leads to
   a duplication of 8-bytes per record.

   To avoid this 8-byte duplication RFC 7400 [RFC7400] provides help
   with the use of the generic header compression technique for IPv6
   over Low-Power Wireless Personal Area Networks (6LoWPANs).  Note that
   this header compression technique is not available when DTLS is

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   exchanged over transports that do not use IPv6 or 6LoWPAN, such as
   the SMS transport described in Appendix A.

Appendix C.  DTLS Fragmentation

   [Editor's Note: Proposed text that requires discussion. ]

   Section 4.2.3 of [RFC6347] advises DTLS implementations to not
   produce overlapping fragments, but requires receivers to be able to
   cope with them.  The need for the latter requisite is explained in
   Section of [RFC6347]: accurate path MTU (PMTU) estimation may
   be traded for shorter handshake completion time.  This approach may
   be beneficial in unconstrained networks where a PMTU of 1280 bytes
   can be pretty much universally assumed.  However, an handshake that
   is carried over a narrow-band radio technology, such as IEEE
   802.15.4, Bluetooth Smart or GSM-SMS, and the client is lacking
   reliable PMTU data to inform fragmentation (e.g., using [RFC1981] or
   [RFC1191]) can place a cost on the constrained implementation in
   terms of memory (due to re-buffering) and latency (due to re-
   transmission) much higher than the benefit that it would get from a
   shorter handshake.

   In order to reduce the likelihood of producing different fragment
   sizes (and consequent overlaps) within the same handshake, this
   document RECOMMENDs:

   o  for clients (handshake initiators), to perform PMTU discovery
      towards the server before handshake starts, and not rely on any
      guesses (unless the network path characteristics are reliably
      known from another source);

   o  for servers, to mirror the fragment size selected by their

Authors' Addresses

   Hannes Tschofenig  (editor)
   ARM Ltd.
   110 Fulbourn Rd
   Cambridge  CB1 9NJ
   Great Britain

   Email: Hannes.tschofenig@gmx.net
   URI:   http://www.tschofenig.priv.at

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   Thomas Fossati
   3 Ely Road
   Milton, Cambridge  CB24 6DD

   Email: thomas.fossati@alcatel-lucent.com

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