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RFC 7925 - Transport Layer Security (TLS) / Datagram Transport L


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Internet Engineering Task Force (IETF)                H. Tschofenig, Ed.
Request for Comments: 7925                                      ARM Ltd.
Category: Standards Track                                     T. Fossati
ISSN: 2070-1721                                                    Nokia
                                                               July 2016

                    Transport Layer Security (TLS) /
                Datagram Transport Layer Security (DTLS)
                  Profiles for the Internet of Things

Abstract

   A common design pattern in Internet of Things (IoT) deployments is
   the use of a constrained device that collects data via sensors 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
   Transport Layer Security (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 IoT products that
   can easily be solved by using these well-researched and widely
   deployed Internet security protocols.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc7925.

Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   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  . . . . . . . . . . . . . . . . . . . . . . . .   4
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
     3.1.  TLS and DTLS  . . . . . . . . . . . . . . . . . . . . . .   5
     3.2.  Communication Models  . . . . . . . . . . . . . . . . . .   6
     3.3.  The Ciphersuite Concept . . . . . . . . . . . . . . . . .  20
   4.  Credential Types  . . . . . . . . . . . . . . . . . . . . . .  21
     4.1.  Preconditions . . . . . . . . . . . . . . . . . . . . . .  21
     4.2.  Pre-Shared Secret . . . . . . . . . . . . . . . . . . . .  23
     4.3.  Raw Public Key  . . . . . . . . . . . . . . . . . . . . .  25
     4.4.  Certificates  . . . . . . . . . . . . . . . . . . . . . .  27
   5.  Signature Algorithm Extension . . . . . . . . . . . . . . . .  32
   6.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .  32
   7.  Session Resumption  . . . . . . . . . . . . . . . . . . . . .  34
   8.  Compression . . . . . . . . . . . . . . . . . . . . . . . . .  35
   9.  Perfect Forward Secrecy . . . . . . . . . . . . . . . . . . .  35
   10. Keep-Alive  . . . . . . . . . . . . . . . . . . . . . . . . .  36
   11. Timeouts  . . . . . . . . . . . . . . . . . . . . . . . . . .  38
   12. Random Number Generation  . . . . . . . . . . . . . . . . . .  39
   13. Truncated MAC and Encrypt-then-MAC Extension  . . . . . . . .  40
   14. Server Name Indication (SNI)  . . . . . . . . . . . . . . . .  40
   15. Maximum Fragment Length Negotiation . . . . . . . . . . . . .  41
   16. Session Hash  . . . . . . . . . . . . . . . . . . . . . . . .  41
   17. Renegotiation Attacks . . . . . . . . . . . . . . . . . . . .  42
   18. Downgrading Attacks . . . . . . . . . . . . . . . . . . . . .  42
   19. Crypto Agility  . . . . . . . . . . . . . . . . . . . . . . .  43
   20. Key Length Recommendations  . . . . . . . . . . . . . . . . .  44
   21. False Start . . . . . . . . . . . . . . . . . . . . . . . . .  45
   22. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  45
   23. Security Considerations . . . . . . . . . . . . . . . . . . .  46
   24. References  . . . . . . . . . . . . . . . . . . . . . . . . .  47
     24.1.  Normative References . . . . . . . . . . . . . . . . . .  47
     24.2.  Informative References . . . . . . . . . . . . . . . . .  48
   Appendix A.  Conveying DTLS over SMS  . . . . . . . . . . . . . .  56
     A.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  56
     A.2.  Message Segmentation and Reassembly . . . . . . . . . . .  57
     A.3.  Multiplexing Security Associations  . . . . . . . . . . .  57
     A.4.  Timeout . . . . . . . . . . . . . . . . . . . . . . . . .  58
   Appendix B.  DTLS Record Layer Per-Packet Overhead  . . . . . . .  59
   Appendix C.  DTLS Fragmentation . . . . . . . . . . . . . . . . .  60
   Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . .  60
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  61

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 Transport Layer
   Security (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].  CoAP messages are mainly carried over UDP/DTLS, but other
   transports can be utilized, such as SMS (as described in Appendix A)
   or TCP (as currently being proposed with [COAP-TCP-TLS]).

   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 IoT product.  If you are familiar with (D)TLS, then skip
   ahead to Section 4.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in RFC
   2119 [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 TLS 1.3 is being developed, but it is
   not expected that this profile will "just work" due to the
   significant changes being done to TLS for version 1.3.

   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 bidirectional communication.  This
   aspect is further described in Section 3.2.

   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 IoT devices are used interchangeably.

   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.  Overview

3.1.  TLS and DTLS

   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
   subprotocols -- 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 absence of 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
      Message Authentication Code (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 [RFC7465].
      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.

3.2.  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.

3.2.1.  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 non-confidential credentials, like certificates) 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 authentication based on the Pre-Shared Key (PSK) (see
      Section 4.2), this includes the paired "PSK identity" and shared
      secret to be used with each server.

   o  For authentication based on the raw public key (see Section 4.3),
      this includes either the server's public key or the hash of the
      server's public key.

   o  For certificate-based authentication (see Section 4.4), 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.

              +////////////////////////////////////+
              |          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    |
              +------------------------------------+
                oo
          oooooo
         o
   +-----------+
   |Constrained|
   |TLS/DTLS   |
   |Client     |-
   +-----------+ \
                  \  ,-------.
                   ,'         `.            +------+
                  /  IP-Based   \           |Server|
                 (    Network    )          |  A   |
                  \             /           +------+
                   `.         ,'
                     '---+---'                  +------+
                         |                      |Server|
                         |                      |  B   |
                         |                      +------+
                         |
                         |                  +------+
                         +----------------->|Server|
                                            |  C   |
                                            +------+

                   Figure 1: Constrained Client Profile

3.2.1.1.  Examples of Constrained Client Exchanges

3.2.1.1.1.  Network Access Authentication Example

   Reuse 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 reuse is to not add
   functionality if it can be avoided.  An example relevant to the use
   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].

                                                +--------------+
                                                |Authentication|
                                                |Authorization |
                                                |Accounting    |
                                                |Server        |
                                                |(EAP Server)  |
                                                |              |
                                                +-^----------^-+
                                                  * EAP      o RADIUS/
                                                  *          o Diameter
                                                --v----------v--
                                             ///                \\\
                                           //                      \\
                                          |        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         |               |
    |             |<===========================>|               |
    +-------------+                             +---------------+
      Legend:

       <****>: 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 reuses the TLS-based protocol exchange and
   encapsulates it inside the EAP payload.  In terms of reuse, 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
   exchange shown in Figure 3, the IoT device as the EAP peer acts as a
   TLS client.

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

3.2.1.1.2.  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 preconfigured endpoint on the server.  In certain
   circumstances, this might be too limiting and additional
   functionality is needed, as shown in Figures 4 and 5, 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 and uses CoAP to register available resources in Figure 5.
   [CoRE-RD] 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 Open Mobile Alliance (OMA) Lightweight
   Machine-to-Machine (LWM2M) specification [LWM2M] that uses RD but
   adds proxy functionality.

   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.

                                                    Resource
       Sensor                                       Directory
       ------                                       ---------

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

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

          Figure 4: DTLS/CoAP Exchange Using Resource Directory:
                         Part 1 -- DTLS Handshake

   Figure 5 shows the DTLS-secured communication between the sensor and
   the resource directory using CoAP.

                                                    Resource
       Sensor                                       Directory
       ------                                       ---------

   [[==============DTLS-Secured Communication===================]]

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

          Figure 5: DTLS/CoAP Exchange Using Resource Directory:
                        Part 2 -- CoAP/RD Exchange

   Note that the CoAP GET message transmitted from the resource server
   is protected using the previously established DTLS record layer.

3.2.2.  Constrained TLS/DTLS Servers

   Section 3.2.1 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 6
   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 authorization server.  This
   authentication and authorization server is responsible for holding
   authorization policies that govern the access to resources and
   distribution of keying material.

            +////////////////////////////////////+
            |          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      |
            +------------------------------------+
                oo
          oooooo
         o
   +---------------+                +-----------+
   |Authentication |      +-------->|TLS/DTLS   |
   |& Authorization|      |         |Client A   |
   |Server         |      |         +-----------+
   +---------------+     ++
                ^        |                  +-----------+
                 \       |                  |Constrained|
                  \  ,-------.              | Server S1 |
                   ,'         `.            +-----------+
                  /    Local    \
                 (    Network    )
                  \             /        +-----------+
                   `.         ,'         |Constrained|
                     '---+---'           | Server S2 |
                         |               +-----------+
                         |
                         |                   +-----------+
                         +-----------------> |Constrained|
                                             | Server S3 |
                                             +-----------+

                   Figure 6: Constrained Server Profile

   A deployment with constrained servers has to overcome several
   challenges.  Below we explain how these challenges can 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 ecosystem factors will
   impact deployment.

   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 has 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 Constrained RESTful Environments (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 the 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 RD [CoRE-RD] 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.

   Authentication:

      The next challenge concerns the provisioning of authentication
      credentials to the clients as well as servers.  In Section 3.2.1,
      we assume 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 [RFC7452].  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 use either pairing (also called imprinting) or
      a trusted third party.  With pairing, two devices execute a
      special protocol exchange that is unauthenticated to establish a
      shared key (for example, using an unauthenticated Diffie-Hellman
      (DH) exchange).  To avoid man-in-the-middle attacks, an
      out-of-band channel is 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,
         and scanning QR codes

      *  The use of alternative wireless communication channels (e.g.,
         infrared communication in addition to Wi-Fi)

      *  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 on 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].

   Authorization

      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 finer-grained and more
      dynamic access control solution, the reader is referred to the
      ongoing work in the IETF ACE working group.

   Figure 7 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 link format per RFC 6690 [RFC6690].  The
   IPv6 multicast address used for CoAP link-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 endpoints and to secure the
   communication.

                                 Temperature
     Thermostat                     Sensor              Fan
     ----------                   ---------             ---

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

                     CoAP 2.05 Content
      <-------------------------------
      </3303/0/5700>;rt="temperature";
                     if="sensor"

                                        CoAP 2.05 Content
      <--------------------------------------------------
                           </fan>;rt="fan";if="actuation"

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

      Read Sensor Data
      (authenticated/authorized)
      ------------------------------->
      GET /3303/0/5700

                    CoAP 2.05 Content
     <-------------------------------
                               22.5 C

     Configure Actuator
     (authenticated/authorized)
     ------------------------------------------------->
     PUT /fan?on-off=true
                                      CoAP 2.04 Changed
     <-------------------------------------------------

               Figure 7: Local Discovery and Resource Access

3.3.  The Ciphersuite Concept

   TLS (and consequently DTLS) support 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 CBC-MAC (CCM) mode for AES)
      [RFC3610]

   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 [RFC6234])

   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 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 a 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 that encrypt (parts of) the message and
   authenticate the message simultaneously.  AES-CCM [RFC6655] is an
   example of such a mode.

   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 pseudorandom function (PRF) used in earlier versions of TLS
   with ciphersuite-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.

4.  Credential Types

   The mandatory-to-implement functionality will depend on the
   credential type used with IoT devices.  The subsections below
   describe the implications of three different credential types, namely
   pre-shared secrets, raw public keys, and certificates.

4.1.  Preconditions

   All exchanges described in the subsequent sections assume that some
   information has been distributed before the TLS/DTLS interaction
   starts.  The credentials are used to authenticate the client to the
   server, and vice versa.  What information items have to be
   distributed depends on the chosen credential types.  In all cases,
   the IoT device needs to know what algorithms to prefer, particularly
   if there are multiple algorithm choices available as part of the
   implemented ciphersuites, as well as information about the other
   communication endpoint (for example, in the form of a URI) a
   particular credential has to be used with.

   Pre-Shared Secrets:  In this case, a shared secret together with an
      identifier needs to be made available to the device as well as to
      the other communication party.

   Raw Public Keys:  A public key together with a private key are stored
      on the device and typically associated with some identifier.  To
      authenticate the other communication party, the appropriate
      credential has to be known.  If the other end uses raw public keys
      as well, then their public key needs to be provisioned (out of
      band) to the device.

   Certificates:  The use of certificates requires the device to store
      the public key (as part of the certificate) as well as the private
      key.  The certificate will contain the identifier of the device as
      well as various other attributes.  Both communication parties are
      assumed to be in possession of a trust anchor store that contains
      CA certificates and, in case of certificate pinning, end-entity

      certificates.  Similar to the other credentials, the IoT device
      needs information about which entity to use which certificate
      with.  Without a trust anchor store on the IoT device, it will not
      be possible to perform certificate validation.

   We call the above-listed information "device credentials" and these
   device credentials may be provisioned to the device already during
   the manufacturing time or later in the process, depending on the
   envisioned business and deployment model.  These initial credentials
   are often called "root of trust".  Whatever process is chosen for
   generating these initial device credentials, it MUST be ensured that
   a different key pair is provisioned for each device and installed in
   as secure a manner as possible.  For example, it is preferable to
   generate public/private keys on the IoT device itself rather than
   generating them outside the device.  Since an IoT device is likely to
   interact with various other parties, the initial device credential
   may only be used with some dedicated entities, and configuring
   further configuration and credentials to the device is left to a
   separate interaction.  An example of a dedicated protocol used to
   distribute credentials, access control lists, and configure
   information is the LWM2M protocol [LWM2M].

   For all the credentials listed above, there is a chance that those
   may need to be replaced or deleted.  While separate protocols have
   been developed to check the status of these credentials and to manage
   these credentials, such as the Trust Anchor Management Protocol
   (TAMP) [RFC5934], their usage is, however, not envisioned in the IoT
   context so far.  IoT devices are assumed to have a software update
   mechanism built-in, and it will allow updates of low-level device
   information, including credentials and configuration parameters.
   This document does, however, not mandate a specific software/firmware
   update protocol.

   With all credentials used as input to TLS/DTLS authentication, it is
   important that these credentials have been generated with care.  When
   using a pre-shared secret, a critical consideration is using
   sufficient entropy during the key generation, as discussed in
   [RFC4086].  Deriving a shared secret from a password, some device
   identifiers, or other low-entropy sources is not secure.  A low-
   entropy secret, or password, is subject to dictionary attacks.
   Attention also has to be paid when generating public/private key
   pairs since the lack of randomness can result in the same key pair
   being used in many devices.  This topic is also discussed in
   Section 12 since keys are generated during the TLS/DTLS exchange
   itself as well, and the same considerations apply.

4.2.  Pre-Shared Secret

   The use of pre-shared secrets is one of the most basic techniques for
   TLS/DTLS since it is both computationally efficient and bandwidth
   conserving.  Authentication based on pre-shared secrets was
   introduced to TLS in RFC 4279 [RFC4279].

   Figure 8 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, as defined in RFC 6347 [RFC6347].  The cookie
   exchange allows the server to react to flooding attacks.

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

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

         ClientHello                  -------->
         (with cookie)
                                                         ServerHello
                                                  *ServerKeyExchange
                                      <--------      ServerHelloDone
         ClientKeyExchange
         ChangeCipherSpec
         Finished                     -------->
                                                    ChangeCipherSpec
                                      <--------             Finished

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

   Legend:

   * indicates an optional message payload

      Figure 8: DTLS PSK Authentication Including the Cookie Exchange

   Note that [RFC4279] used the term "PSK identity" to refer to the
   identifier used to refer to the appropriate secret.  While
   "identifier" would be more appropriate in this context, we reuse the
   terminology defined in RFC 4279 to avoid confusion.  RFC 4279 does
   not mandate the use of any particular type of PSK identity, and the
   client and server have to agree on the identities and keys to be
   used.  The UTF-8 encoding of identities described in Section 5.1 of
   RFC 4279 aims to improve interoperability for those cases where the
   identity is configured by a human using some management interface

   provided by a web browser.  However, many IoT devices do not have a
   user interface, and most of their credentials are bound to the device
   rather than to the user.  Furthermore, credentials are often
   provisioned into hardware modules or provisioned alongside with
   firmware.  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 (such as domain
   names, distinguished names, or IP addresses), and a byte-by-byte
   comparison operation MUST be used by the server for any operation
   related to the PSK identity.  These types of identifiers are called
   "absolute" per RFC 6943 [RFC6943].

   Protocol-wise, the client indicates which key it uses by including a
   "PSK identity" in the ClientKeyExchange message.  As described in
   Section 3.2, 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.  A server implementation needs to guide
   the selection based on a received SNI value from the client.

   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.  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.

   "The 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.
   Note that the shorted authentication tag increases the chance that an
   adversary with no knowledge of the secret key can present a message
   with a MAC that will pass the verification procedure.  The likelihood
   of accepting forged data is explained in Section 5.3.5 of
   [SP800-107-rev1] and depends on the lengths of the authentication tag
   and allowed numbers of MAC verifications using a given key.

   This ciphersuite makes use of the default TLS 1.2 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 PRF.
   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.

4.3.  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 reuses 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 9, namely the server_certificate_type and the
   client_certificate_type.

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

    ClientHello             -------->
    #client_certificate_type#
    #server_certificate_type#

                                               ServerHello
                                 #client_certificate_type#
                                 #server_certificate_type#
                                               Certificate
                                         ServerKeyExchange
                                        CertificateRequest
                            <--------      ServerHelloDone

    Certificate
    ClientKeyExchange
    CertificateVerify
    [ChangeCipherSpec]
    Finished                -------->

                                        [ChangeCipherSpec]
                            <--------             Finished

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

                  Figure 9: 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 AES-CCM TLS
   ciphersuite based on elliptic curve cryptography (ECC) uses the
   Ephemeral Elliptic Curve Diffie-Hellman (ECDHE) as the key
   establishment mechanism and an Elliptic Curve Digital Signature

   Algorithm (ECDSA) for authentication.  The named DH groups
   [FFDHE-TLS] are not applicable to this profile since it relies on the
   ECC-based counterparts.  This ciphersuite makes use of the AEAD
   capability in DTLS 1.2 and utilizes an 8-octet authentication tag.
   The use of a DH key exchange provides perfect forward secrecy (PFS).
   More details about PFS can be found in Section 9.

   [RFC6090] provides valuable information for implementing ECC
   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
   section.

4.4.  Certificates

   The use of mutual certificate-based authentication is shown in
   Figure 10, which makes use of the "cached_info" extension [RFC7924].
   Support of the "cached_info" extension is REQUIRED.  Caching
   certificate chains allows the client to reduce the communication
   overhead significantly, otherwise the server would provide the end-
   entity certificate and the certificate chain with every full DTLS
   handshake.

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

    ClientHello             -------->
    *cached_info*

                                               ServerHello
                                             *cached_info*
                                               Certificate
                                         ServerKeyExchange
                                        CertificateRequest
                            <--------      ServerHelloDone

    Certificate
    ClientKeyExchange
    CertificateVerify
    [ChangeCipherSpec]
    Finished                -------->

                                        [ChangeCipherSpec]
                            <--------             Finished

   Note: Extensions marked with "*" were introduced with
         RFC 7924.

          Figure 10: DTLS Mutual Certificate-Based Authentication

   TLS/DTLS offers a lot of choices when selecting ECC-based
   ciphersuites.  This document restricts the use to named curves
   defined in RFC 4492 [RFC4492].  At the time of writing, the
   recommended curve is secp256r1, and the use of uncompressed points
   follows the recommendation in CoAP.  Note that standardization for
   Curve25519 (for ECDHE) is ongoing (see [RFC7748]), and support for
   this curve will likely be required in the future.

   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
   section.

4.4.1.  Certificates Used by Servers

   The algorithm for verifying the service identity, as described in RFC
   6125 [RFC6125], is essential for ensuring proper security when
   certificates are used.  As a summary, the algorithm contains the
   following steps:

   1.  The client constructs a list of acceptable reference identifiers
       based on the source domain and, optionally, the type of service
       to which the client is connecting.

   2.  The server provides its identifiers in the form of a PKIX
       certificate.

   3.  The client checks each of its reference identifiers against the
       presented identifiers for the purpose of finding a match.

   4.  When checking a reference identifier against a presented
       identifier, the client matches the source domain of the
       identifiers and, optionally, their application service type.

   For various terms used in the algorithm shown above, consult RFC
   6125.  It is important to highlight that comparing the reference
   identifier against the presented identifier obtained from the
   certificate is required to ensure the client is communicating with
   the intended server.

   It is worth noting that the algorithm description and the text in RFC
   6125 assumes that fully qualified DNS domain names are used.  If a
   server node is provisioned with a fully qualified DNS domain, then
   the server certificate 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 Common Name (CN) component of the
   subject name, as explained in Section 9.1.3.3 of [RFC7252], and used
   by the client to match it against the FQDN used during the lookup
   process, as described in [RFC6125].  For other protocols, the
   appropriate URI scheme specification has to be consulted.

   The following recommendation is provided:

   1.  Certificates MUST NOT use DNS domain names in the CN of
       certificates and instead use the subjectAltName attribute, as
       described in the previous paragraph.

   2.  Certificates MUST NOT contain domain names with wildcard
       characters.

   3.  Certificates MUST NOT contain multiple names (e.g., more than one
       dNSName field).

   Note that there will be servers that are not provisioned for use with
   DNS domain names, for example, IoT devices that offer resources to
   nearby devices in a local area network, as shown in Figure 7.  When
   such constrained servers are used, then the use of certificates as
   described in Section 4.4.2 is applicable.  Note that the SNI
   extension cannot be used in this case since SNI does not offer the
   ability to convey a 64-bit Extended Unique Identifier (EUI-64)
   [EUI64].  Note that this document does not recommend use of IP
   addresses in certificates nor does it discuss the implications of
   placing IP addresses in certificates.

4.4.2.  Certificates Used by Clients

   For client certificates, the identifier used in the SubjectAltName or
   in the leftmost CN component of subject name MUST be an EUI-64.

4.4.3.  Certificate Revocation

   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
   environment.

   As stated earlier in this section, modifications to the trust anchor
   store depends on a software update mechanism as well.  There are
   limitations to the use of a software update mechanism because of the
   potential inability to change certain types of keys, such as those
   provisioned during manufacturing.  For this reason, manufacturer-
   provisioned credentials are typically employed only to obtain further
   certificates (for example, via a key distribution server) for use
   with servers the IoT device is finally communicating with.

4.4.4.  Certificate Content

   All certificate elements listed in Table 1 MUST be implemented by
   clients and servers claiming support for certificate-based
   authentication.  No other certificate elements are used by this
   specification.

   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     |
   |                      | 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 and 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), and id-kp-OCSPSigning for    |
   |                      |         future OCSP usage in TLS.          |
   +----------------------+--------------------------------------------+

                       Table 1: Certificate Content

   There are various cryptographic algorithms available to sign digital
   certificates; those algorithms include RSA, the Digital Signature
   Algorithm (DSA), and ECDSA.  As Table 1 shows, certificates are
   signed using ECDSA in this profile.  This is not only true for the
   end-entity certificates but also for all other certificates in the
   chain, including CA certificates.  This profiling reduces the amount
   of flash memory needed on an IoT device to store the code of several
   algorithm implementations due to the smaller number of options.

   Further details about X.509 certificates can be found in
   Section 9.1.3.3 of [RFC7252].

4.4.5.  Client Certificate URLs

   RFC 6066 [RFC6066] allows the sending of client-side certificates to
   be avoided 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 DoS attack against the constrained server due to the
   additional workload.

4.4.6.  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
   handshake.  Since the server does not necessarily know what trust
   anchors the client has stored, to facilitate certification path
   construction and 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, preconfigured set of servers, this extension
   is NOT RECOMMENDED.

   In cases where clients interact with dynamically discovered TLS/DTLS
   servers, for example, in the use cases described in Section 3.2.2,
   the use of this extension is RECOMMENDED.

5.  Signature Algorithm Extension

   The "signature_algorithms" extension, defined in Section 7.4.1.4.1 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,
   otherwise if this extension is absent, 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 4.2.

6.  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 restarting the
   handshake or informing the user using the (often limited) user
   interface.  Warnings may be ignored by the application since many IoT
   devices will have either 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 the Secure Socket Layer (SSL) and 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 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 the new alert message "unknown_psk_identity" for
   PSK ciphersuites.  As stated in Section 2 of RFC 4279, the
   decrypt_error error message may also be used instead.  For this
   profile, the TLS server MUST return the decrypt_error error message
   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
      ciphersuites will get added over time.

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

7.  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 11.  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
   figure.

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

         ClientHello                   -------->
                                                          ServerHello
                                                   [ChangeCipherSpec]
                                       <--------             Finished
         [ChangeCipherSpec]
         Finished                      -------->
         Application Data              <------->     Application Data

                    Figure 11: 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
   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].  Note that the constrained

   server refers to a device that has limitations in terms of RAM and
   flash memory, which place restrictions on the amount of TLS/DTLS
   security state information that can be stored on such a device.  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.

8.  Compression

   Section 3.3 of [RFC7525] recommends disabling TLS-/DTLS-level
   compression due to attacks, such as CRIME [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.

   TLS/DTLS layer compression is NOT RECOMMENDED by this TLS/DTLS
   profile.

9.  Perfect Forward Secrecy

   PFS is a property that preserves the confidentiality of past protocol
   interactions even in situations where the long-term secret is
   compromised.

   The PSK ciphersuite recommended in Section 4.2 does not offer this
   property since it does not utilize a DH exchange.  New ciphersuites
   that support PFS for PSK-based authentication, such as proposed in
   [PSK-AES-CCM-TLS], might become available as a 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 use of raw public keys might offer a
   middle ground between the PSK ciphersuite in terms of out-of-band
   validation and the functionality offered by asymmetric cryptography.

   Physical attacks create additional opportunities to gain access to
   the crypto material stored on IoT devices.  A PFS ciphersuite
   prevents an attacker from obtaining the communication content
   exchanged prior to a successful long-term key compromise; however, an
   implementation that (for performance or energy efficiency reasons)

   has been reusing the same ephemeral DH keys over multiple different
   sessions partially defeats PFS, thus increasing the damage extent.
   For this reason, implementations SHOULD NOT reuse ephemeral DH keys
   over multiple protocol exchanges.

   The impact of the disclosure of past communication interactions and
   the desire to increase the cost for pervasive monitoring (as demanded
   by [RFC7258]) has to be taken into account when selecting a
   ciphersuite that does not support the PFS property.

   Client implementations claiming support of this profile MUST
   implement the ciphersuites listed in Section 4 according to the
   selected credential type.

10.  Keep-Alive

   Application-layer communication may create state at the endpoints,
   and this state may 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 the form of a security context (see the security parameter
   described in Appendix A.6 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
   analyzed:

   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.

      The DTLS handshake may need to be restarted (ideally using session
      resumption, if possible) in case of an IP address change.

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

   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 recreate 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 initiates 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
      Section 4.1.1.1 of RFC 6347) by using the fragmentation capability
      of the handshake protocol.

   For server-initiated messages, the heartbeat extension is
   RECOMMENDED.

11.  Timeouts

   A variety of wired and wireless technologies are available to connect
   devices to the Internet.  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
   [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 "per-flight" retransmissions, as described in
   Section 4.2.4 of [RFC6347].  Although the timeout values are
   implementation specific, recommendations are provided in
   Section 4.2.4.1 of [RFC6347], with an initial timer value of 1 second
   and double the value at each retransmission, up to no less than 60
   seconds.

   TLS protocol steps can take longer due to higher processing time on
   the constrained side.  On the other hand, the way DTLS handles
   retransmission, which is per-flight instead of per-segment, tends to
   interact poorly with low-bandwidth networks.

   For these reasons, it's essential that the probability of a spurious
   retransmit is minimized and, on timeout, the sending endpoint does
   not react too aggressively.  The latter is particularly relevant when
   the Wireless Sensor Network (WSN) is temporarily congested: if lost
   packets are reinjected too quickly, congestion worsens.

   An initial timer value of 9 seconds with exponential back off up to
   no less then 60 seconds is therefore RECOMMENDED.

   This value is chosen big enough to absorb large latency variance due
   to either slow computation on constrained endpoints or intrinsic
   network characteristics (e.g., GSM-SMS), as well as to produce a low

   number of retransmission events and relax the pacing between them.
   Its worst case wait time is the same as using 1s timeout (i.e., 63s),
   while triggering less than half of the retransmissions (2 instead of
   5).

   In order to minimize the wake time during DTLS handshake, sleepy
   nodes might decide to select a lower threshold and, consequently, a
   smaller initial timeout value.  If this is the case, the
   implementation MUST keep into account the considerations about
   network stability described in this section.

12.  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 DH exchange requires random numbers
   during the key pair generation.

   It is important to note that sources contributing to the randomness
   pool on laptops or desktop PCs are not available on many IoT devices,
   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.

   Lacking sources of randomness in an embedded system may lead to the
   same keys generated again and again.

   The ClientHello and the ServerHello messages contain 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).  Since many IoT devices do not have access to an
   accurate clock, it is RECOMMENDED that the receiver of a ClientHello
   or ServerHello does not assume that the value in
   "Random.gmt_unix_time" is an accurate representation of the current
   time and instead treats it as an opaque random string.

   When TLS is used with certificate-based authentication, the
   availability of time information is needed to check the validity of a
   certificate.  Higher-layer protocols may provide secure time
   information.  The gmt_unix_time component of the ServerHello is not
   used for this purpose.

   IoT devices using TLS/DTLS must offer ways to generate quality random
   numbers.  There are various implementation choices for integrating a
   hardware-based random number generator into a product: an
   implementation inside the microcontroller itself is one option, but

   dedicated crypto chips are also reasonable choices.  The best choice
   will depend on various factors outside the scope of this document.
   Guidelines and requirements for random number generation can be found
   in RFC 4086 [RFC4086] and in the NIST Special Publication 800-90a
   [SP800-90A].

   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 number
   generators.

13.  Truncated MAC and Encrypt-then-MAC Extension

   The truncated MAC extension was introduced in 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 were
   performed independently.

   The recommended ciphersuites in this document use the newer AEAD
   construct, namely the CCM mode with 8-octet authentication tags, and
   are therefore not applicable 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.  RFC 7366 ("encrypt-then-MAC") and RFC 6066 ("truncated MAC
   extension") are not applicable to this specification and MUST NOT be
   used.

14.  Server Name Indication (SNI)

   The SNI extension [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 a single IP address.

   Implementing the Server Name Indication extension is REQUIRED unless
   it is known that a TLS/DTLS client does not interact with a server in
   a hosting environment.

15.  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.

16.  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 it 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].

   [RFC7627] 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 DH-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.

17.  Renegotiation Attacks

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

   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
   handshake.

   To prevent the renegotiation attack [RFC5746], this specification
   REQUIRES the TLS renegotiation feature to be disabled.  Clients MUST
   respond to server-initiated renegotiation attempts with an alert
   message (no_renegotiation), and clients MUST NOT initiate them.

18.  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 then the handshake can proceed.  This behavior 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 behavior that leads
   some clients to rerun the handshake with a 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
   [RFC7568]).

   The above-described downgrade vulnerability is solved by the TLS
   Fallback Signaling Cipher Suite Value (SCSV) [RFC7507] extension.
   However, the solution is not applicable 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.

19.  Crypto Agility

   This document recommends that software and chip manufacturers
   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 as well.  Some
   deployment environments will also be impacted by local regulation,
   which might dictate a certain algorithm and key size combination.
   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 in
      the protocol stack.  Sometimes hardware implementations 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.  Such a setup does not allow application
      developers to reuse the hardware-based AES 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 implementation.  AES-CCM in IEEE 802.15.4 and
      Bluetooth Smart use 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 reusable
      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 implementation.  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 suggest
   that sufficient headroom is provided to allow an upgrade to a newer
   cryptographic algorithm over the lifetime of the product.  As an
   example, while AES-CCM is recommended throughout this specification,
   future products might use the ChaCha20 cipher in combination with the
   Poly1305 authenticator [RFC7539].  The assumption is made that a
   robust software update mechanism is offered.

20.  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 12 illustrates the comparable
   key sizes in bits.

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

        Figure 12: Comparable Key Sizes (in Bits) Based on RFC 4492

   At the time of writing, the key size recommendations for use with
   TLS-based ciphers found in [RFC7525] recommend DH key lengths of at
   least 2048 bits, which corresponds to a 112-bit symmetric key and a
   233-bit ECC key.  These recommendations are roughly in line with
   those from other organizations, such as the National Institute of
   Standards and Technology (NIST) or the 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.

   Note that the recommendations for 112-bit symmetric keys are chosen
   conservatively under the assumption that IoT devices have a long
   expected lifetime (such as 10+ years) and that this key length
   recommendation refers to the long-term keys used for device
   authentication.  Keys, which are provisioned dynamically, for the

   protection of transactional data (such as ephemeral DH keys used in
   various TLS/DTLS ciphersuites) may be shorter considering the
   sensitivity of the exchanged data.

21.  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.

   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 [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 round-trip for the full TLS/DTLS
   exchange, this specification RECOMMENDS the use of the False Start
   mechanism when clients send application data first.

22.  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 poses a challenge in many IoT deployments since
   many of the IoT devices operate unattended, even though they are
   initially provisioned by a human.  The ability to control data
   sharing and to configure preferences 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 misattribution, it also allows strong identification.
   Device-related data collection (e.g., sensor recordings) associated
   with other data types 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 from gaining 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.

   Note that the absence or presence of communication itself might
   reveal information to an adversary.  For example, a presence sensor
   may initiate messaging when a person enters a building.  While TLS/
   DTLS would offer confidentiality protection of the transmitted
   information, it does not help to conceal all communication patterns.
   Furthermore, the IP header, which is not protected by TLS/DTLS,
   additionally reveals information about the other communication
   endpoint.  For applications where such privacy concerns exist,
   additional safeguards are required, such as injecting dummy traffic
   and onion routing.  A detailed treatment of such solutions is outside
   the scope of this document and requires a system-level view.

23.  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 LWM2M protocol published by the OMA
   [LWM2M].

24.  References

24.1.  Normative References

   [EUI64]    IEEE, "Guidelines for 64-bit Global Identifier (EUI-64)",
              Registration Authority,
              <https://standards.ieee.org/regauth/
              oui/tutorials/EUI64.html>.

   [GSM-SMS]  ETSI, "3rd Generation Partnership Project; Technical
              Specification Group Core Network and Terminals; Technical
              realization of the Short Message Service (SMS) (Release
              13)", 3GPP TS 23.040 V13.1.0, March 2016.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <http://www.rfc-editor.org/info/rfc2119>.

   [RFC4279]  Eronen, P., Ed. and H. Tschofenig, Ed., "Pre-Shared Key
              Ciphersuites for Transport Layer Security (TLS)",
              RFC 4279, DOI 10.17487/RFC4279, December 2005,
              <http://www.rfc-editor.org/info/rfc4279>.

   [RFC5246]  Dierks, T. and E. Rescorla, "The Transport Layer Security
              (TLS) Protocol Version 1.2", RFC 5246,
              DOI 10.17487/RFC5246, August 2008,
              <http://www.rfc-editor.org/info/rfc5246>.

   [RFC5746]  Rescorla, E., Ray, M., Dispensa, S., and N. Oskov,
              "Transport Layer Security (TLS) Renegotiation Indication
              Extension", RFC 5746, DOI 10.17487/RFC5746, February 2010,
              <http://www.rfc-editor.org/info/rfc5746>.

   [RFC6066]  Eastlake 3rd, D., "Transport Layer Security (TLS)
              Extensions: Extension Definitions", RFC 6066,
              DOI 10.17487/RFC6066, January 2011,
              <http://www.rfc-editor.org/info/rfc6066>.

   [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, DOI 10.17487/RFC6125, March
              2011, <http://www.rfc-editor.org/info/rfc6125>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <http://www.rfc-editor.org/info/rfc6347>.

   [RFC6520]  Seggelmann, R., Tuexen, M., and M. Williams, "Transport
              Layer Security (TLS) and Datagram Transport Layer Security
              (DTLS) Heartbeat Extension", RFC 6520,
              DOI 10.17487/RFC6520, February 2012,
              <http://www.rfc-editor.org/info/rfc6520>.

   [RFC7250]  Wouters, P., Ed., Tschofenig, H., Ed., Gilmore, J.,
              Weiler, S., and T. Kivinen, "Using Raw Public Keys in
              Transport Layer Security (TLS) and Datagram Transport
              Layer Security (DTLS)", RFC 7250, DOI 10.17487/RFC7250,
              June 2014, <http://www.rfc-editor.org/info/rfc7250>.

   [RFC7251]  McGrew, D., Bailey, D., Campagna, M., and R. Dugal, "AES-
              CCM Elliptic Curve Cryptography (ECC) Cipher Suites for
              TLS", RFC 7251, DOI 10.17487/RFC7251, June 2014,
              <http://www.rfc-editor.org/info/rfc7251>.

   [RFC7627]  Bhargavan, K., Ed., Delignat-Lavaud, A., Pironti, A.,
              Langley, A., and M. Ray, "Transport Layer Security (TLS)
              Session Hash and Extended Master Secret Extension",
              RFC 7627, DOI 10.17487/RFC7627, September 2015,
              <http://www.rfc-editor.org/info/rfc7627>.

   [RFC7924]  Santesson, S. and H. Tschofenig, "Transport Layer Security
              (TLS) Cached Information Extension", RFC 7924,
              DOI 10.17487/RFC7924, July 2016,
              <http://www.rfc-editor.org/info/rfc7924>.

   [WAP-WDP]  Open Mobile Alliance, "Wireless Datagram Protocol",
              Wireless Application Protocol, WAP-259-WDP, June 2001.

24.2.  Informative References

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

   [AES]      National Institute of Standards and Technology, "Advanced
              Encryption Standard (AES)", NIST FIPS PUB 197, November
              2001, <http://csrc.nist.gov/publications/fips/fips197/
              fips-197.pdf>.

   [CCM]      National Institute of Standards and Technology,
              "Recommendation for Block Cipher Modes of Operation: The
              CCM Mode for Authentication and Confidentiality", NIST
              Special Publication 800-38C, May 2004,
              <http://csrc.nist.gov/publications/nistpubs/800-38C/
              SP800-38C_updated-July20_2007.pdf>.

   [COAP-TCP-TLS]
              Bormann, C., Lemay, S., Tschofenig, H., Hartke, K.,
              Silverajan, B., and B. Raymor, "CoAP (Constrained
              Application Protocol) over TCP, TLS, and WebSockets", Work
              in Progress, draft-ietf-core-coap-tcp-tls-03, July 2016.

   [CoRE-RD]  Shelby, Z., Koster, M., Bormann, C., and P. Stok, "CoRE
              Resource Directory", Work in Progress, draft-ietf-core-
              resource-directory-08, July 2016.

   [CRIME]    Wikipedia, "CRIME", May 2016, <https://en.wikipedia.org/w/
              index.php?title=CRIME&oldid=721665716>.

   [ENISA-Report2013]
              ENISA, "Algorithms, Key Sizes and Parameters Report -
              2013", October 2013, <https://www.enisa.europa.eu/
              activities/identity-and-trust/library/deliverables/
              algorithms-key-sizes-and-parameters-report>.

   [FFDHE-TLS]
              Gillmor, D., "Negotiated Finite Field Diffie-Hellman
              Ephemeral Parameters for TLS", Work in Progress,
              draft-ietf-tls-negotiated-ff-dhe-10, June 2015.

   [HomeGateway]
              Eggert, L., Hatoen, S., Kojo, M., Nyrhinen, A., Sarolahti,
              P., and S. Strowes, "An Experimental Study of Home Gateway
              Characteristics", In Proceedings of the 10th ACM SIGCOMM
              conference on Internet measurement,
              DOI 10.1145/1879141.1879174, 2010,
              <http://conferences.sigcomm.org/imc/2010/papers/p260.pdf>.

   [IANA-TLS] IANA, "Transport Layer Security (TLS) Parameters",
              <https://www.iana.org/assignments/tls-parameters>.

   [ImprintingSurvey]
              Chilton, E., "A Brief Survey of Imprinting Options for
              Constrained Devices", March 2012,
              <http://www.lix.polytechnique.fr/hipercom/
              SmartObjectSecurity/papers/EricRescorla.pdf>.

   [Keylength]
              Giry, D., "Cryptographic Key Length Recommendations",
              September 2015, <http://www.keylength.com>.

   [LWM2M]    Open Mobile Alliance, "Lightweight Machine-to-Machine
              Requirements", Candidate Version 1.0, OMA-RD-
              LightweightM2M-V1_0-20131210-C, December 2013,
              <http://openmobilealliance.org/about-oma/work-program/
              m2m-enablers>.

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

   [RFC1981]  McCann, J., Deering, S., and J. Mogul, "Path MTU Discovery
              for IP version 6", RFC 1981, DOI 10.17487/RFC1981, August
              1996, <http://www.rfc-editor.org/info/rfc1981>.

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104,
              DOI 10.17487/RFC2104, February 1997,
              <http://www.rfc-editor.org/info/rfc2104>.

   [RFC2865]  Rigney, C., Willens, S., Rubens, A., and W. Simpson,
              "Remote Authentication Dial In User Service (RADIUS)",
              RFC 2865, DOI 10.17487/RFC2865, June 2000,
              <http://www.rfc-editor.org/info/rfc2865>.

   [RFC3610]  Whiting, D., Housley, R., and N. Ferguson, "Counter with
              CBC-MAC (CCM)", RFC 3610, DOI 10.17487/RFC3610, September
              2003, <http://www.rfc-editor.org/info/rfc3610>.

   [RFC3748]  Aboba, B., Blunk, L., Vollbrecht, J., Carlson, J., and H.
              Levkowetz, Ed., "Extensible Authentication Protocol
              (EAP)", RFC 3748, DOI 10.17487/RFC3748, June 2004,
              <http://www.rfc-editor.org/info/rfc3748>.

   [RFC4086]  Eastlake 3rd, D., Schiller, J., and S. Crocker,
              "Randomness Requirements for Security", BCP 106, RFC 4086,
              DOI 10.17487/RFC4086, June 2005,
              <http://www.rfc-editor.org/info/rfc4086>.

   [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,
              DOI 10.17487/RFC4492, May 2006,
              <http://www.rfc-editor.org/info/rfc4492>.

   [RFC4821]  Mathis, M. and J. Heffner, "Packetization Layer Path MTU
              Discovery", RFC 4821, DOI 10.17487/RFC4821, March 2007,
              <http://www.rfc-editor.org/info/rfc4821>.

   [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, DOI 10.17487/RFC4919, August 2007,
              <http://www.rfc-editor.org/info/rfc4919>.

   [RFC5077]  Salowey, J., Zhou, H., Eronen, P., and H. Tschofenig,
              "Transport Layer Security (TLS) Session Resumption without
              Server-Side State", RFC 5077, DOI 10.17487/RFC5077,
              January 2008, <http://www.rfc-editor.org/info/rfc5077>.

   [RFC5116]  McGrew, D., "An Interface and Algorithms for Authenticated
              Encryption", RFC 5116, DOI 10.17487/RFC5116, January 2008,
              <http://www.rfc-editor.org/info/rfc5116>.

   [RFC5216]  Simon, D., Aboba, B., and R. Hurst, "The EAP-TLS
              Authentication Protocol", RFC 5216, DOI 10.17487/RFC5216,
              March 2008, <http://www.rfc-editor.org/info/rfc5216>.

   [RFC5247]  Aboba, B., Simon, D., and P. Eronen, "Extensible
              Authentication Protocol (EAP) Key Management Framework",
              RFC 5247, DOI 10.17487/RFC5247, August 2008,
              <http://www.rfc-editor.org/info/rfc5247>.

   [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, DOI 10.17487/RFC5280, May 2008,
              <http://www.rfc-editor.org/info/rfc5280>.

   [RFC5288]  Salowey, J., Choudhury, A., and D. McGrew, "AES Galois
              Counter Mode (GCM) Cipher Suites for TLS", RFC 5288,
              DOI 10.17487/RFC5288, August 2008,
              <http://www.rfc-editor.org/info/rfc5288>.

   [RFC5480]  Turner, S., Brown, D., Yiu, K., Housley, R., and T. Polk,
              "Elliptic Curve Cryptography Subject Public Key
              Information", RFC 5480, DOI 10.17487/RFC5480, March 2009,
              <http://www.rfc-editor.org/info/rfc5480>.

   [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, DOI 10.17487/RFC5758, January 2010,
              <http://www.rfc-editor.org/info/rfc5758>.

   [RFC5934]  Housley, R., Ashmore, S., and C. Wallace, "Trust Anchor
              Management Protocol (TAMP)", RFC 5934,
              DOI 10.17487/RFC5934, August 2010,
              <http://www.rfc-editor.org/info/rfc5934>.

   [RFC6024]  Reddy, R. and C. Wallace, "Trust Anchor Management
              Requirements", RFC 6024, DOI 10.17487/RFC6024, October
              2010, <http://www.rfc-editor.org/info/rfc6024>.

   [RFC6090]  McGrew, D., Igoe, K., and M. Salter, "Fundamental Elliptic
              Curve Cryptography Algorithms", RFC 6090,
              DOI 10.17487/RFC6090, February 2011,
              <http://www.rfc-editor.org/info/rfc6090>.

   [RFC6234]  Eastlake 3rd, D. and T. Hansen, "US Secure Hash Algorithms
              (SHA and SHA-based HMAC and HKDF)", RFC 6234,
              DOI 10.17487/RFC6234, May 2011,
              <http://www.rfc-editor.org/info/rfc6234>.

   [RFC6655]  McGrew, D. and D. Bailey, "AES-CCM Cipher Suites for
              Transport Layer Security (TLS)", RFC 6655,
              DOI 10.17487/RFC6655, July 2012,
              <http://www.rfc-editor.org/info/rfc6655>.

   [RFC6690]  Shelby, Z., "Constrained RESTful Environments (CoRE) Link
              Format", RFC 6690, DOI 10.17487/RFC6690, August 2012,
              <http://www.rfc-editor.org/info/rfc6690>.

   [RFC6733]  Fajardo, V., Ed., Arkko, J., Loughney, J., and G. Zorn,
              Ed., "Diameter Base Protocol", RFC 6733,
              DOI 10.17487/RFC6733, October 2012,
              <http://www.rfc-editor.org/info/rfc6733>.

   [RFC6943]  Thaler, D., Ed., "Issues in Identifier Comparison for
              Security Purposes", RFC 6943, DOI 10.17487/RFC6943, May
              2013, <http://www.rfc-editor.org/info/rfc6943>.

   [RFC6961]  Pettersen, Y., "The Transport Layer Security (TLS)
              Multiple Certificate Status Request Extension", RFC 6961,
              DOI 10.17487/RFC6961, June 2013,
              <http://www.rfc-editor.org/info/rfc6961>.

   [RFC7228]  Bormann, C., Ersue, M., and A. Keranen, "Terminology for
              Constrained-Node Networks", RFC 7228,
              DOI 10.17487/RFC7228, May 2014,
              <http://www.rfc-editor.org/info/rfc7228>.

   [RFC7252]  Shelby, Z., Hartke, K., and C. Bormann, "The Constrained
              Application Protocol (CoAP)", RFC 7252,
              DOI 10.17487/RFC7252, June 2014,
              <http://www.rfc-editor.org/info/rfc7252>.

   [RFC7258]  Farrell, S. and H. Tschofenig, "Pervasive Monitoring Is an
              Attack", BCP 188, RFC 7258, DOI 10.17487/RFC7258, May
              2014, <http://www.rfc-editor.org/info/rfc7258>.

   [RFC7366]  Gutmann, P., "Encrypt-then-MAC for Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", RFC 7366, DOI 10.17487/RFC7366, September 2014,
              <http://www.rfc-editor.org/info/rfc7366>.

   [RFC7390]  Rahman, A., Ed. and E. Dijk, Ed., "Group Communication for
              the Constrained Application Protocol (CoAP)", RFC 7390,
              DOI 10.17487/RFC7390, October 2014,
              <http://www.rfc-editor.org/info/rfc7390>.

   [RFC7397]  Gilger, J. and H. Tschofenig, "Report from the Smart
              Object Security Workshop", RFC 7397, DOI 10.17487/RFC7397,
              December 2014, <http://www.rfc-editor.org/info/rfc7397>.

   [RFC7400]  Bormann, C., "6LoWPAN-GHC: Generic Header Compression for
              IPv6 over Low-Power Wireless Personal Area Networks
              (6LoWPANs)", RFC 7400, DOI 10.17487/RFC7400, November
              2014, <http://www.rfc-editor.org/info/rfc7400>.

   [RFC7452]  Tschofenig, H., Arkko, J., Thaler, D., and D. McPherson,
              "Architectural Considerations in Smart Object Networking",
              RFC 7452, DOI 10.17487/RFC7452, March 2015,
              <http://www.rfc-editor.org/info/rfc7452>.

   [RFC7465]  Popov, A., "Prohibiting RC4 Cipher Suites", RFC 7465,
              DOI 10.17487/RFC7465, February 2015,
              <http://www.rfc-editor.org/info/rfc7465>.

   [RFC7507]  Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
              Suite Value (SCSV) for Preventing Protocol Downgrade
              Attacks", RFC 7507, DOI 10.17487/RFC7507, April 2015,
              <http://www.rfc-editor.org/info/rfc7507>.

   [RFC7525]  Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of Transport Layer
              Security (TLS) and Datagram Transport Layer Security
              (DTLS)", BCP 195, RFC 7525, DOI 10.17487/RFC7525, May
              2015, <http://www.rfc-editor.org/info/rfc7525>.

   [RFC7539]  Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
              Protocols", RFC 7539, DOI 10.17487/RFC7539, May 2015,
              <http://www.rfc-editor.org/info/rfc7539>.

   [RFC7568]  Barnes, R., Thomson, M., Pironti, A., and A. Langley,
              "Deprecating Secure Sockets Layer Version 3.0", RFC 7568,
              DOI 10.17487/RFC7568, June 2015,
              <http://www.rfc-editor.org/info/rfc7568>.

   [RFC7748]  Langley, A., Hamburg, M., and S. Turner, "Elliptic Curves
              for Security", RFC 7748, DOI 10.17487/RFC7748, January
              2016, <http://www.rfc-editor.org/info/rfc7748>.

   [SP800-107-rev1]
              National Institute of Standards and Technology,
              "Recommendation for Applications Using Approved Hash
              Algorithms", NIST Special Publication 800-107, Revision 1,
              DOI 10.6028/NIST.SP.800-107r1, August 2012,
              <http://csrc.nist.gov/publications/nistpubs/800-107-rev1/
              sp800-107-rev1.pdf>.

   [SP800-22b]
              National Institute of Standards and Technology, "A
              Statistical Test Suite for Random and Pseudorandom Number
              Generators for Cryptographic Applications", NIST Special
              Publication 800-22, Revision 1a, April 2010,
              <http://csrc.nist.gov/publications/nistpubs/800-22-rev1a/
              SP800-22rev1a.pdf>.

   [SP800-90A]
              National Institute of Standards and Technology,
              "Recommendation for Random Number Generation Using
              Deterministic Random Bit Generators", NIST Special
              Publication 800-90A Revision 1,
              DOI 10.6028/NIST.SP.800-90Ar1, June 2015,
              <http://csrc.nist.gov/publications/drafts/800-90/
              sp800-90a_r1_draft_november2014_ver.pdf>.

   [TLS-FALSESTART]
              Langley, A., Modadugu, N., and B. Moeller, "Transport
              Layer Security (TLS) False Start", Work in Progress,
              draft-ietf-tls-falsestart-02, May 2016.

   [Triple-HS]
              Bhargavan, K., Delignat-Lavaud, C., Pironti, A., and P.
              Yves Strub, "Triple Handshakes and Cookie Cutters:
              Breaking and Fixing Authentication over TLS", In
              Proceedings of the IEEE Symposium on Security and Privacy,
              Pages 98-113, DOI 10.1109/SP.2014.14, 2014.

Appendix A.  Conveying DTLS over SMS

   This section is normative for the use of DTLS over SMS.  Timer
   recommendations are already outlined in Section 11 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 Transport Protocol Data Units (TPDUs) encoded
   as 8-bit binary data.

A.1.  Overview

   DTLS adds an additional round-trip 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 to 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 9.2.3.24.1 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 Path MTU (PMTU)
   of the SMS transport in order to correctly instrument its SaR
   function.  The PMTU SHALL be 133 bytes if multiplexing based on the
   Wireless Datagram Protocol (WDP) is used (see Appendix A.3); 140
   bytes otherwise.

   It is RECOMMENDED that the established security context over the
   longest possible period be used (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.

A.2.  Message Segmentation and Reassembly

   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 the 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
   tunnel.

A.3.  Multiplexing Security Associations

   Unlike IPsec Encapsulating Security Payload (ESP) / Authentication
   Header (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 Wireless Application Protocol (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
   content depending on if 1-byte or 2-byte port identifiers are used,
   as shown in Figures 13 and 14.

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

   Legend:
   UDH = user data header
   IEI = information element identifier

       Figure 13: 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 14: 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, a unique 4-tuple identifying the security association can be
   reconstructed at both ends.  (When replying to its DTLS peer, the
   sender will swap the TP-OA and destination address (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
   timeout.

Appendix B.  DTLS Record Layer Per-Packet Overhead

   Figure 15 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 15: 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, and

   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 two values, namely a 4-octet
   salt and an 8-octet nonce_explicit:

      The salt is the "implicit" part and is not sent in the packet.
      Instead, the salt is generated as part of the handshake process.

      The nonce_explicit value is 8 octets long and it is chosen by the
      sender and carried in each TLS record.  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 value 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 exchanged over transports that do not use IPv6 or 6LoWPAN, such
      as the SMS transport described in Appendix A of this document.

Appendix C.  DTLS Fragmentation

   Section 4.2.3 of [RFC6347] advises DTLS implementations to not
   produce overlapping fragments.  However, it requires receivers to be
   able to cope with them.  The need for the latter requisite is
   explained in Section 4.1.1.1 of [RFC6347]: accurate PMTU estimation
   may be traded for shorter handshake completion time.

   In many cases, the cost of handling fragment overlaps has proved to
   be unaffordable for constrained implementations, particularly because
   of the increased complexity in buffer management.

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

   o  clients (handshake initiators) to use reliable PMTU information
      for the intended destination; and

   o  servers to mirror the fragment size selected by their clients.

   The PMTU information comes from either a "fresh enough" discovery
   performed by the client [RFC1981] [RFC4821] or some other reliable
   out-of-band channel.

Acknowledgments

   Thanks to Derek Atkins, Paul Bakker, Olaf Bergmann, Carsten Bormann,
   Ben Campbell, Brian Carpenter, Robert Cragie, Spencer Dawkins, Russ
   Housley, Rene Hummen, Jayaraghavendran K, Sye Loong Keoh, Matthias
   Kovatsch, Sandeep Kumar, Barry Leiba, Simon Lemay, Alexey Melnikov,
   Gabriel Montenegro, Manuel Pegourie-Gonnard, Akbar Rahman, Eric
   Rescorla, Michael Richardson, Ludwig Seitz, Zach Shelby, Michael
   StJohns, Rene Struik, Tina Tsou, and Sean Turner for their helpful
   comments and discussions that have shaped the document.

   A big thanks also to Klaus Hartke, who wrote the initial draft
   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.

Authors' Addresses

   Hannes Tschofenig (editor)
   ARM Ltd.
   110 Fulbourn Rd
   Cambridge  CB1 9NJ
   United Kingdom

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

   Thomas Fossati
   Nokia
   3 Ely Road
   Milton, Cambridge  CB24 6DD
   United Kingdom

   Email: thomas.fossati@nokia.com

 

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