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dice                                                  H. Tschofenig, Ed.
Internet-Draft                                                  ARM Ltd.
Intended status: Standards Track                              T. Fossati
Expires: July 23, 2015                                    Alcatel-Lucent
                                                        January 19, 2015


           A TLS/DTLS 1.2 Profile for the Internet of Things
                     draft-ietf-dice-profile-09.txt

Abstract

   A common design pattern in Internet of Things (IoT) deployments is
   the use of a constrained device (typically providing sensor data)
   that makes data available for home automation, industrial control
   systems, smart cities and other IoT deployments.

   This document defines a Transport Layer Security (TLS) and Datagram
   TLS 1.2 profile that offers communications security for this data
   exchange thereby preventing eavesdropping, tampering, and message
   forgery.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at http://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 23, 2015.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   (http://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents



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   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  TLS/DTLS Protocol Overview  . . . . . . . . . . . . . . . . .   4
   4.  Communication Models  . . . . . . . . . . . . . . . . . . . .   5
     4.1.  Constrained TLS/DTLS Clients  . . . . . . . . . . . . . .   5
     4.2.  Constrained TLS/DTLS Servers  . . . . . . . . . . . . . .  12
   5.  The TLS/DTLS Ciphersuite Concept  . . . . . . . . . . . . . .  16
   6.  Credential Types  . . . . . . . . . . . . . . . . . . . . . .  18
     6.1.  Pre-Shared Secret . . . . . . . . . . . . . . . . . . . .  18
     6.2.  Raw Public Key  . . . . . . . . . . . . . . . . . . . . .  20
     6.3.  Certificates  . . . . . . . . . . . . . . . . . . . . . .  22
   7.  Signature Algorithm Extension . . . . . . . . . . . . . . . .  26
   8.  Error Handling  . . . . . . . . . . . . . . . . . . . . . . .  26
   9.  Session Resumption  . . . . . . . . . . . . . . . . . . . . .  28
   10. Compression . . . . . . . . . . . . . . . . . . . . . . . . .  29
   11. Perfect Forward Secrecy . . . . . . . . . . . . . . . . . . .  29
   12. Keep-Alive  . . . . . . . . . . . . . . . . . . . . . . . . .  30
   13. Timeouts  . . . . . . . . . . . . . . . . . . . . . . . . . .  31
   14. Random Number Generation  . . . . . . . . . . . . . . . . . .  32
   15. Truncated MAC and Encrypt-then-MAC Extension  . . . . . . . .  33
   16. Server Name Indication (SNI)  . . . . . . . . . . . . . . . .  34
   17. Maximum Fragment Length Negotiation . . . . . . . . . . . . .  34
   18. Session Hash  . . . . . . . . . . . . . . . . . . . . . . . .  34
   19. Re-Negotiation Attacks  . . . . . . . . . . . . . . . . . . .  35
   20. Downgrading Attacks . . . . . . . . . . . . . . . . . . . . .  35
   21. Crypto Agility  . . . . . . . . . . . . . . . . . . . . . . .  36
   22. Key Length Recommendations  . . . . . . . . . . . . . . . . .  37
   23. False Start . . . . . . . . . . . . . . . . . . . . . . . . .  38
   24. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  38
   25. Security Considerations . . . . . . . . . . . . . . . . . . .  39
   26. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  39
   27. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  40
   28. References  . . . . . . . . . . . . . . . . . . . . . . . . .  40
     28.1.  Normative References . . . . . . . . . . . . . . . . . .  40
     28.2.  Informative References . . . . . . . . . . . . . . . . .  41
   Appendix A.  Conveying DTLS over SMS  . . . . . . . . . . . . . .  46
     A.1.  Overview  . . . . . . . . . . . . . . . . . . . . . . . .  47
     A.2.  Message Segmentation and Re-Assembly  . . . . . . . . . .  47
     A.3.  Multiplexing Security Associations  . . . . . . . . . . .  48
     A.4.  Timeout . . . . . . . . . . . . . . . . . . . . . . . . .  48



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   Appendix B.  DTLS Record Layer Per-Packet Overhead  . . . . . . .  49
   Appendix C.  DTLS Fragmentation . . . . . . . . . . . . . . . . .  50
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  50

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

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

   While this document is inspired by the desire to protect CoAP
   messages using DTLS 1.2 [RFC6347] the guidance in this document 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 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 extensions.

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

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "MUST", "MUST NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in [RFC2119].





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

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

3.  TLS/DTLS Protocol Overview

   The TLS protocol [RFC5246] provides authenticated, confidentiality-
   and integrity-protected communication between two endpoints.  The
   protocol is composed of two layers: the Record Protocol and the
   Handshake Protocol.  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.  One such encapsulated 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.
   Since DTLS operates on top of an unreliable datagram transport a few
   enhancements to the TLS structure are, however necessary.  RFC 6347
   explains these differences in great detail.  As a short summary, for
   those not familiar with DTLS the differences are:

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

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




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   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.  Retransmission timers have been included to
      deal with message loss.

4.  Communication Models

   This document describes a profile of TLS/DTLS 1.2 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.

4.1.  Constrained TLS/DTLS Clients

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

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

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

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

   o  For certificate-based authentication (see Section 6.3), this
      includes a pre-populated trust anchor store that allows the DTLS




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      stack to perform path validation for the certificate obtained
      during the handshake with the server.

   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 Key (Client)   |
              | Server C --> Public Key (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.








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4.1.1.  Examples of Constrained Client Exchanges

4.1.1.1.  Network Access Authentication Example

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

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




















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                                                +--------------+
                                                |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
       <====>: Phyiscal layer like IEEE 802.11/802.15.4


                  Figure 2: Network Access Architecture..

   One standardized EAP method is EAP-TLS, defined in RFC 5216
   [RFC5216], which re-uses the TLS-based protocol exchange and
   encapsulates it inside the EAP payload.  In terms of re-use this
   allows many components of the TLS protocol to be shared between the
   network access security functionality and the TLS functionality
   needed for securing application layer traffic.  The EAP-TLS exchange
   is shown in Figure 3 where it is worthwhile to point out that in EAP



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   the client / server roles are reversed but with the use of EAP-TLS
   the IoT device 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 re-use most parts of the
   code for the use of DTLS/TLS at the application layer thereby saving
   a significant amount of flash memory.  Note, however, that the
   credentials used for network access authentication and those used for
   application layer security are very likely different.



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

   Figure 4 shows a sensor starting with a DTLS exchange with a resource
   directory to register available resources.
   [I-D.ietf-core-resource-directory] defines the resource directory as
   a web entity that stores information about web resources and
   implements the REST interfaces defined in
   [I-D.ietf-core-resource-directory] for registration and lookup of
   those resources.

   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.  Details
   about the capabilities of the resource directory can be found in
   [I-D.ietf-core-resource-directory].

   After some time (assuming that the client regularly refreshes its
   registration) the resource directory receives a request (not shown in
   the figure) 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



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    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
     +---

     +---                                                  ///+
    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



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    G| Payload:                                               \ T
    E| 25.5                     -------->                     \ E
    T|                                                        \ D
     +---                                                  ///+


          Figure 4: DTLS/CoAP exchange using Resource Directory.

4.2.  Constrained TLS/DTLS Servers

   Section 4.1 illustrates a deployment model where the TLS/DTLS client
   is constrained and efforts need to be taken to improve memory
   utilization, bandwidth consumption, reduce performance impacts, etc.
   In this section we look at cases where constrained devices run TLS/
   DTLS servers to secure access to application layer services running
   on top of CoAP, HTTP or other protocols.  Running server
   functionality on a constrained node is typically more demanding since
   servers have to wait for incoming requests.  Therefore, they will
   have fewer possibilities to enter sleep-cycles.  Nevertheless, there
   are legitimate reasons for deploying servers as constrained devices.
   Figure 5 illustrates a possible deployment whereby a number of
   constrained servers are waiting for regular clients to access their
   resources.  The entire process is likely to be controlled by a third
   party, the authentication and authorization server.  This
   authentication and authorization server is responsible for holding
   authorization policies (in the form of access control policies) that
   govern the access to resources and distribution of keying material.
























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


                   Figure 5: Constrained Server Profile.

   Figure 6 shows an example interaction whereby a device, a thermostat
   in our case, searches in the local network for discoverable resources
   and accesses those.  The thermostat starts the procedure using a
   link-local discovery message using the "All CoAP Nodes" multicast
   address by utilizing the RFC 6690 [RFC6690] link format.  The IPv6
   multicast address used for site-local discovery is FF02::FD.  As a
   result, a temperature sensor and a fan respond.  These responses
   allow the thermostat to subsequently read temperature information



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   from the temperature sensor with a CoAP GET request issued to the
   previously learned endpoint.  In this hypothetical example we assume
   that this temperature sensor provides this information to every party
   and no access control mechanism is enforced.  However, when the
   thermostat subsequently uses the obtained temperature reading to
   control a fan, the fan requires authentication and authorization of
   the entity requesting changes and TLS is used to authenticate both
   endpointas 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"



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


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


   +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+
   \                                                        /
   \ Protocol steps to obtain authorization token / client  /
   \ credentials for access to the fan-provided resources.  /
   \                                                        /
   +~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~+




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     Configure Actuator (with authorization credentials)
     --------------------------------------------------
     PUT /fan?on-off=true


                                      CoAP 2.04 Changed
     <-------------------------------------------------


               Figure 6: Local Discovery and Resouce Access.

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

   The challenges are:

   Discovery and Reachability:

      Before initiating a connection to a constrained server a client
      first needs to discover that server and, once discovered, it needs
      to maintain reachability with that device.

      In CoAP the discovery of resources offered by servers is
      accomplished by sending a unicast or multicast CoAP GET to a well-
      known URI.  The CORE Link format specification [RFC6690] describes
      the use case (see Section 1.2.1), and reserves the URI (see
      Section 7.1).  Section 7 of the CoAP specification [RFC7252]
      describes the discovery procedure.  RFC 7390 [RFC7390] describes
      use case for discovering CoAP servers using multicast (see
      Section 3.3), and specifies the protocol processing rules for CoAP
      group communications (see Section 2.7).

   Authentication:

      The next challenge concerns the provisioning of authentication
      credentials to the clients as well as servers.  In Section 4.1 we
      assumed that credentials (and other configuration information) are
      provisioned to the device and that those can be used with the
      authorization servers.  Of course, this leads to a very static
      relationship between the clients and their server-side
      infrastructure but poses fewer challenges from a deployment point
      of view, as described in Section 2 of
      [I-D.iab-smart-object-architecture] these different communication



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      patterns.  In any case, engineers and product designers have to
      determine how the relevant credentials are distributed to the
      respective parties.  For example, shared secrets may need to be
      provisioned to clients and the constrained servers for subsequent
      use of TLS/DTLS PSK.  In other deployments, certificates, private
      keys, and trust anchors for use with certificate-based
      authentication may need to be utilized.

      Practical solutions either use pairing (also called imprinting) or
      a trusted third party.  With pairing two devices execute a special
      protocol exchange that is unauthenticated to establish an shared
      key (for example using an unauthenticated Diffie-Hellman exchange)
      key.  To avoid man-in-the-middle attacks an out-of-band channel is
      used to verify that nobody has tampered with the exchanged
      protocol messages.  This out-of-band channel can come in many
      forms, including:

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

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

      *  Proximity-based information

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

   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 more fine-grained and
      dynamic access control the reader is referred to the ongoing work
      in the ACE working group.

5.  The TLS/DTLS Ciphersuite Concept

   TLS (and consequently DTLS) has the concept of ciphersuites and an
   IANA registry [IANA-TLS] was created to register the suites.  A




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   ciphersuite (and the specification that defines it) contains the
   following information:

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

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

   o  Mode of operation (e.g., Counter with Cipher Block Chaining -
      Message Authentication Code (CBC-MAC) Mode (CCM) for AES)
      [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 [RFC4634])

   o  Hash algorithm for use with the pseudorandom function (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.  RFC 6655
   [RFC6655] defines this ciphersuite.  It uses the Advanced Encryption
   Standard (AES) encryption algorithm, which is a block cipher.  Since
   the AES algorithm supports different key lengths (such as 128, 192
   and 256 bits) this information has to be specified as well and the
   selected ciphersuite supports 128 bit keys.  A block cipher encrypts
   plaintext in fixed-size blocks and AES operates on fixed block size
   of 128 bits.  For messages exceeding 128 bits, the message is
   partitioned into 128-bit blocks and the AES cipher is applied to
   these input blocks with appropriate chaining, which is called mode of
   operation.

   TLS 1.2 introduced Authenticated Encryption with Associated Data
   (AEAD) ciphersuites (see [RFC5116] and [RFC6655]).  AEAD is a class
   of block cipher modes which encrypt (parts of) the message and
   authenticate the message simultaneously.  Examples of such modes
   include the Counter with Cipher Block Chaining - Message
   Authentication Code (CBC-MAC) Mode (CCM) mode, and the Galois/Counter
   Mode (GCM) (see [RFC5288] and [RFC7251]).

   Some AEAD ciphersuites have shorter authentication tags and are
   therefore more suitable for networks with low bandwidth where small
   message size matters.  The TLS_PSK_WITH_AES_128_CCM_8 ciphersuite



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   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.

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

6.  Credential Types

6.1.  Pre-Shared Secret

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




























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

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

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

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

   Legend:

   * indicates an optional message payload


     Figure 7: DTLS PSK Authentication including the Cookie Exchange.

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

   The client indicates which key it uses by including a "PSK identity"
   in the ClientKeyExchange message.  As described in Section 4 clients
   may have multiple pre-shared keys with a single server and to help
   the client in selecting which PSK identity / PSK pair to use, the
   server can provide a "PSK identity hint" in the ServerKeyExchange
   message.  If the hint for PSK key selection is based on the domain



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   name of the server then servers SHOULD NOT send the "PSK identity
   hint" in the ServerKeyExchange message.  In general, servers SHOULD
   NOT send the "PSK identity hint" in the ServerKeyExchange message and
   client MUST ignore the message.  This approach is inline with RFC
   4279 [RFC4279].  Note: The TLS Server Name Indication (SNI) extension
   allows the client to tell a server the name of the server it is
   contacting, which is relevant for hosting environments.  A server
   using the identity hint 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.  For the IoT environment, keys are distributed as part of
   hardware modules or are embedded into the firmware.  Implementations
   in compliance with this profile MAY use PSK identities up to 128
   octets in length, and arbitrary PSKs up to 64 octets in length.  The
   use of shorter PSK identities and shorter PSKs is RECOMMENDED.

   Constrained Application Protocol (CoAP) [RFC7252] currently specifies
   TLS_PSK_WITH_AES_128_CCM_8 as the mandatory to implement ciphersuite
   for use with shared secrets.  This ciphersuite uses the AES algorithm
   with 128 bit keys and CCM as the mode of operation.  The label "_8"
   indicates that an 8-octet authentication tag is used.  This
   ciphersuite makes use of the default TLS 1.2 Pseudorandom Function
   (PRF), which uses an HMAC with the SHA-256 hash function.  (Note that
   all IoT implementations will need a SHA-256 implementation due to the
   construction of the pseudo-random number function in DTLS/TLS 1.2.)

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

6.2.  Raw Public Key

   The use of raw public keys with TLS/DTLS, as defined in [RFC7250], is
   the first entry point into public key cryptography without having to
   pay the price of certificates and a public key infrastructure (PKI).
   The specification re-uses the existing Certificate message to convey
   the raw public key encoded in the SubjectPublicKeyInfo structure.  To
   indicate support two new extensions had been defined, as shown in
   Figure 8, namely the server_certificate_type*' and the
   client_certificate_type.  To operate this mechanism securely it is
   necessary to authenticate and authorize the public keys out-of-band.
   This document therefore assumes that a client implementation comes
   with one or multiple raw public keys of servers, it has to
   communicate with, pre-provisioned.  Additionally, a device will have



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   its own raw public key.  To replace, delete, or add raw public key to
   this list requires a software update, for example using a firmware
   update mechanism.


    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 8: DTLS Raw Public Key Exchange including the Cookie Exchange.

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




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   RFC 6090 [RFC6090] provides valuable information for implementing
   Elliptic Curve Cryptography algorithms, particularly for choosing
   methods that have been available in the literature for a long time
   (i.e., 20 years and more).

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

6.3.  Certificates

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


























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

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

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

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

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

   Note: Extensions marked with '*' were introduced with
         [I-D.ietf-tls-cached-info].


          Figure 9: DTLS Mutual Certificate-based Authentication.

   Server certificates MUST contain the fully qualified DNS domain name
   or "FQDN" as dNSName.  For CoAP, the coaps URI scheme is described in
   Section 6.2 of [RFC7252].  This FQDN is stored in the SubjectAltName
   or in the leftmost CN component of subject name, as explained in
   Section 9.1.3.3 of [RFC7252], and used by the client to match it
   against the FQDN used during the look-up process, as described in RFC
   6125 [RFC6125].  For other protocols, the appropriate URI scheme
   specification has to be consulted.

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

   For client certificates the identifier used in the SubjectAltName or
   in the leftmost CN component of subject name MUST be an EUI-64
   [EUI64], as mandated in Section 9.1.3.3 of [RFC7252].




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   For certificate revocation neither the Online Certificate Status
   Protocol (OCSP) nor Certificate Revocation Lists (CRLs) are used.
   Instead, this profile relies on a software update mechanism.  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.

   Regarding the ciphersuite choice the discussion in Section 6.2
   applies.  Further details about X.509 certificates can be found in
   Section 9.1.3.3 of [RFC7252].  The TLS_ECDHE_ECDSA_WITH_AES_128_CCM_8
   ciphersuite description in Section 6.2 is also applicable to this
   section.

   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 the  X.509 v3 certificate    |
   |               |                     [RFC5280].                    |
   |               |                                                   |
   | Serial Number |      Positive integer unique per certificate.     |
   |               |                                                   |
   |     Issuer    |  This profile uses ecdsa-with-SHA256 or stronger  |
   |   Signature   |                     [RFC5758].                    |
   |   Algorithms  |                                                   |
   |               |                                                   |
   |     Issuer    |         Contains the DN of the issuing CA.        |
   | Distinguished |                                                   |
   |      Name     |                                                   |
   |               |                                                   |
   |    Validity   |  Values expressed as UTC time. No validity period |
   |     Period    |                     mandated.                     |
   |               |                                                   |
   |    Subject    |        See rules outlined in this section.        |
   | Distinguished |                                                   |
   |      Name     |                                                   |



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   |               |                                                   |
   |    Subject    |     This element contains the ECDSA signature     |
   |   Public Key  |      certificate. The algorithm field in the      |
   |  Information  |    SubjectPublicKeyInfo structure indicates the   |
   |               |  algorithm and any associated parameters for the  |
   |               |     ECC public key. This profile uses the id-     |
   |               |         ecPublicKey algorithm identifier.         |
   |               |                                                   |
   |    Issuer's   |   Includes the ECDSA signature with ecdsa-with-   |
   |   Signature   |                SHA256 or stronger.                |
   |               |                                                   |
   |   Extension:  |        See rules outlined in this section.        |
   |    Subject    |                                                   |
   |  Alternative  |                                                   |
   |      Name     |                                                   |
   |               |                                                   |
   |   Extension:  |  Indicates whether the subject of the certificate |
   |     Basic     | is a CA. This extension is used for CA certs only |
   |  Constraints  | and then the value is set to TRUE. The default is |
   |               |                       FALSE.                      |
   |               |                                                   |
   |   Extension:  | digitalSignature or keyAgreement, keyCertSign for |
   |   Key Usage   |  verifying signatures on public key certificates. |
   |               |                                                   |
   |   Extension:  |  id-kp-serverAuth for server authentication, id-  |
   |  Extended Key |  kp-clientAuth for client authentication, id-kp-  |
   |     Usage     | codeSigning for code signing (for software update |
   |               |   mechanism), id-kp-OCSPSigning for future OCSP   |
   |               |                   usage in TLS.                   |
   +---------------+---------------------------------------------------+

                       Table 1: Certificate Content.

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

   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.

6.3.1.  Client Certificate URLs

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




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

6.3.2.  Trusted CA Indication

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

   Today, in most IoT deployments there is a fairly static relationship
   between the IoT device (and the software running on them) and the
   server-side infrastructure.  For these deployments where IoT devices
   interact with a fixed, pre-configured set of servers this extension
   is NOT RECOMMENDED.

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

7.  Signature Algorithm Extension

   The "signature_algorithms" extension, defined in Section 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
   since otherwise absent this extension RFC 5246 defaults to SHA-1 /
   ECDSA for the ECDH_ECDSA and the ECDHE_ECDSA key exchange algorithms.

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

8.  Error Handling

   TLS/DTLS uses the Alert protocol to convey error messages and
   specifies a longer list of errors.  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



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   messages with a level of fatal result in the immediate termination of
   the connection.  If possible, developers should try to develop
   strategies to react to those fatal errors, such as re-starting the
   handshake or informing the user using the (often limited) user
   interface.  Warnings may be ignored by the application since many IoT
   devices will either have limited ways to log errors or no ability at
   all.  In any case, implementers have to carefully evaluate the impact
   of errors and ways to remedy the situation since a commonly used
   approach for delegating decision making to users is difficult (or
   impossible) to accomplish in a timely fashion.

   All error messages marked as RESERVED are only supported for
   backwards compatibility with SSL and are therefore not applicable to
   this profile.  Those include decryption_failed_RESERVED,
   no_certificate_RESERVE, and export_restriction_RESERVED.

   A number of the error messages are applicable only for certificate-
   based authentication ciphersuites.  Hence, for PSK and raw public key
   use the following error messages are not applicable:

   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 is not applicable either.

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



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

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

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

9.  Session Resumption

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


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

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


                    Figure 10: DTLS Session Resumption.

   Constrained clients MUST implement session resumption to improve the
   performance of the handshake.  This will lead to a reduced number of
   message exchanges, lower computational overhead (since only symmetric
   cryptography is used during a session resumption exchange), and
   session resumption requires less bandwidth.

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



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   If both the client and the server are constrained devices both
   devices SHOULD implement RFC 5077 and MUST implement basic session
   resumption.

10.  Compression

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

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

11.  Perfect Forward Secrecy

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

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

   The use of PFS is a trade-off decision since on one hand the
   compromise of long-term secrets of embedded devices is more likely
   than with many other Internet hosts but on the other hand a Diffie-
   Hellman exchange requires ephemeral key pairs to be generated, which
   is demanding from a performance point of view.  For performance
   reasons some implementations re-use key pairs over multiple exchanges
   (rather than generating new keys for each exchange) for the obvious
   performance improvement.  Note, however, that such key re-use over
   long periods voids the benefits of forward secrecy when an attack
   gains access to this DH key pair.

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




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

12.  Keep-Alive

   RFC 6520 [RFC6520] defines a heartbeat mechanism to test whether the
   other peer is still alive.  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.  There are three types of
   exchanges that need to be analysed:

   Client-Initiated, One-Shot Messages

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

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

      In this case there is no use for a keep-alive extension for this
      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 re-create the record layer using
      session resumption.

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

   Server-Initiated Messages

      In the two previous scenarios the client initiated the protocol
      interaction but in this case we consider server-initiated
      messages.  Since messages to the client may get blocked by



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      intermediaries, such as NATs (including IPv4/IPv6 protocol
      translators) and stateful packet filtering firewalls, the initial
      connection setup is triggered by the client and then kept alive.
      Since state at middleboxes expires fairly quickly (according to
      measurements described in [HomeGateway]), regular heartbeats are
      necessary whereby these keep-alive messages may be exchanged at
      the application layer or within DTLS itself.

      For this message exchange pattern the use of DTLS heartbeat
      messages is quite useful but may interfere with registrations kept
      at the application layer (for example when the CoAP resource
      directory is used).  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.

13.  Timeouts

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

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

   To deal with the unreliable message delivery provided by UDP, DTLS
   adds timeouts and re-transmissions, as described in Section 4.2.4 of
   [RFC6347].  Although the timeout values are implementation specific,
   recommendations are provided in Section 4.2.4.1 of [RFC6347], with an



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   initial timer value of 1 second and twice the value at each
   retransmission up to no less than 60 seconds.  Due to the nature of
   some radio technologies, these values are too aggressive and lead to
   spurious failures when messages in flight need longer.

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

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

14.  Random Number Generation

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

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

   The ClientHello and the ServerHello messages contains the 'Random'
   structure, which has two components: gmt_unix_time and a random
   sequence of 28 random bytes. gmt_unix_time holds the current time and
   date in standard UNIX 32-bit format (seconds since the midnight
   starting Jan 1, 1970, GMT).  [I-D.mathewson-no-gmtunixtime] argues
   that the entire ClientHello.Random value (including gmt_unix_time)
   should be set to a cryptographically random sequence because of
   privacy concerns regarding device fingerprinting.  Since many IoT
   devices do not have access to a real-time clock this recommendation
   it is RECOMMENDED to follow the guidance outlined in
   [I-D.mathewson-no-gmtunixtime] regarding the content of the



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   ClientHello.Random field.  However, for the ServerHello.Random
   structure it is RECOMMENDED to maintain the existing structure with
   gmt_unix_time followed by a random sequence of 28 random bytes since
   the client can use the received time information to securely obtain
   time information.  For constrained servers it cannot be assumed that
   they maintain accurate time information; these devices MUST include
   time information in the Server.Random structure when they actually
   obtain accurate time information that can be utilized by clients.
   Clients MUST only use time information obtained from servers they
   trust.

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

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

15.  Truncated MAC and Encrypt-then-MAC Extension

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

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

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




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   Implementations conformant to this specification MUST use AEAD
   ciphers and RFC 7366 and RFC 6066 MUST NOT be implemented.

16.  Server Name Indication (SNI)

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

   This specification RECOMMENDs the implementation of RFC 6066 unless
   it is known that a TLS/DTLS client does not interact with a server in
   a hosting environment.

17.  Maximum Fragment Length Negotiation

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

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

   Client implementations MUST support this extension.

18.  Session Hash

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

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

   Client implementations SHOULD implement this extension even though
   the ciphersuites recommended by this profile are not vulnerable to



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   this attack.  For Diffie-Hellman-based ciphersuites the keying
   material is contributed by both parties and in case of the pre-shared
   secret key ciphersuite both parties need to be in possession of the
   shared secret to ensure that the handshake completes successfully.
   It is, however, possible that some application layer protocols will
   tunnel other authentication protocols on top of DTLS making this
   attack relevant again.

19.  Re-Negotiation Attacks

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

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

20.  Downgrading Attacks

   When a client sends a ClientHello with a version higher than the
   highest version known to the server, the server is supposed to reply
   with ServerHello.version equal to the highest version known to the
   server and the handshake can proceed.  This behaviour is known as
   version tolerance.  Version-intolerance is when the server (or a
   middlebox) breaks the handshake when it sees a ClientHello.version
   higher than what it knows about.  This is the behaviour that leads
   some clients to re-run the handshake with lower version.  As a
   result, a potential security vulnerability is introduced when a
   system is running an old TLS/SSL version (e.g., because of the need
   to integrate with legacy systems).  In the worst case, this allows an
   attacker to downgrade the protocol handshake to SSL 3.0.  SSL 3.0 is
   so broken that there is no secure cipher available for it (see
   [I-D.ietf-tls-sslv3-diediedie]).

   The above-described downgrade vulnerability is solved by the TLS
   Fallback Signaling Cipher Suite Value (SCSV)
   [I-D.ietf-tls-downgrade-scsv] extension.  However, the solution is
   not appliable to implementations conforming to this profile since the



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   version negotiation MUST use TLS/DTLS version 1.2 (or higher).  More
   specifically, this implies:

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

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

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

   If at some time in the future the TLS/DTLS 1.2 profile reaches the
   quality of SSL 3.0 a software update mechanism is needed since
   constrained devices are unlikely to run multiple TLS/DTLS versions
   due to memory size restrictions.

21.  Crypto Agility

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

   The following recommendations can be made to chip manufacturers:

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

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



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      for IEEE 802.15.4 and Bluetooth Smart are therefore not re-usable
      by a DTLS stack.

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

   As a recommendation for developers and product architects we
   recommend that sufficient headroom is provided to allow an upgrade to
   a newer cryptographic algorithms over the lifetime of the product.
   As an example, while AES-CCM is recommended thoughout this
   specification future products might use the ChaCha20 cipher in
   combination with the Poly1305 authenticator
   [I-D.irtf-cfrg-chacha20-poly1305].  The assumption is made that a
   robust software update mechanism is offered.

22.  Key Length Recommendations

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

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








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


                Figure 11: Comparable Key Sizes (in bits).

23.  False Start

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

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

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

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

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

   o  Client certificate types, such as ecdsa_sign

   o  Key exchange methods, such as ECDHE_ECDSA

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

24.  Privacy Considerations

   The DTLS handshake exchange conveys various identifiers, which can be
   observed by an on-path eavesdropper.  For example, the DTLS PSK
   exchange reveals the PSK identity, the supported extensions, the



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   session id, algorithm parameters, etc.  When session resumption is
   used then individual TLS sessions can be correlated by an on-path
   adversary.  With many IoT deployments it is likely that keying
   material and their identifiers are persistent over a longer period of
   time due to the cost of updating software on these devices.

   User participation with many IoT deployments poses a challenge since
   many of the IoT devices operate unattended, even though they will
   initially be provisioned by a human.  The ability to control data
   sharing and to configure preference will have to be provided at a
   system level rather than at the level of the DTLS exchange itself,
   which is the scope of this document.  Quite naturally, the use of
   DTLS with mutual authentication will allow a TLS server to collect
   authentication information about the IoT device (likely over a long
   period of time).  While this strong form of authentication will
   prevent mis-attribution it also allows strong identification.
   Device-related data collection (e.g., sensor recordings) will be
   associated with other data to be truly useful and this extra data
   might include personal data about the owner of the device or data
   about the environment it senses.  Consequently, the data stored on
   the server-side will be vulnerable to stored data compromise.  For
   the communication between the client and the server this
   specification prevents eavesdroppers to gain access to the
   communication content.  While the PSK-based ciphersuite does not
   provide PFS the asymmetric versions do.  This prevents an adversary
   from obtaining past communication content when access to a long-term
   secret has been gained.  Note that no extra effort to make traffic
   analysis more difficult is provided by the recommendations made in
   this document.

25.  Security Considerations

   This entire document is about security.

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

26.  IANA Considerations

   This document includes no request to IANA.





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27.  Acknowledgements

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

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

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

28.  References

28.1.  Normative References

   [EUI64]    "GUIDELINES FOR 64-BIT GLOBAL IDENTIFIER (EUI-64)
              REGISTRATION AUTHORITY", April 2010,
              <http://standards.ieee.org/regauth/oui/tutorials/
              EUI64.html>.

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

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

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

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

   [RFC4279]  Eronen, P. and H. Tschofenig, "Pre-Shared Key Ciphersuites
              for Transport Layer Security (TLS)", RFC 4279, December
              2005.




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

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

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

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

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

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

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

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

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

28.2.  Informative References

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

   [AES]      NIST, "FIPS PUB 197, Advanced Encryption Standard (AES)",
              http://www.iana.org/assignments/tls-parameters/
              tls-parameters.xhtml#tls-parameters-4, November 2001.







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

   [Heninger]
              Heninger, N., Durumeric, Z., Wustrow, E., and A.
              Halderman, "Mining Your Ps and Qs: Detection of Widespread
              Weak Keys in Network Devices", 21st USENIX Security
              Symposium,
              https://www.usenix.org/conference/usenixsecurity12/
              technical-sessions/presentation/heninger, 2012.

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

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

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

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

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

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






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   [I-D.ietf-tls-downgrade-scsv]
              Moeller, B. and A. Langley, "TLS Fallback Signaling Cipher
              Suite Value (SCSV) for Preventing Protocol Downgrade
              Attacks", draft-ietf-tls-downgrade-scsv-03 (work in
              progress), December 2014.

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

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

   [I-D.ietf-tls-sslv3-diediedie]
              Barnes, R., Thomson, M., Pironti, A., and A. Langley,
              "Deprecating Secure Sockets Layer Version 3.0", draft-
              ietf-tls-sslv3-diediedie-00 (work in progress), December
              2014.

   [I-D.ietf-uta-tls-bcp]
              Sheffer, Y., Holz, R., and P. Saint-Andre,
              "Recommendations for Secure Use of TLS and DTLS", draft-
              ietf-uta-tls-bcp-08 (work in progress), December 2014.

   [I-D.irtf-cfrg-chacha20-poly1305]
              Nir, Y. and A. Langley, "ChaCha20 and Poly1305 for IETF
              protocols", draft-irtf-cfrg-chacha20-poly1305-07 (work in
              progress), January 2015.

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

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

   [IANA-TLS]
              IANA, "TLS Cipher Suite Registry",
              http://www.iana.org/assignments/tls-parameters/
              tls-parameters.xhtml#tls-parameters-4, 2014.





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

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

   [LWM2M]    Open Mobile Alliance, "Lightweight Machine-to-Machine,
              Technical Specification, Candidate Version 1.0", December
              2013.

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

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

   [RFC2104]  Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed-
              Hashing for Message Authentication", RFC 2104, February
              1997.

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

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

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

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

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

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

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



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

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

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

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

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

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

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

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

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

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

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

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

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






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

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

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

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

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

   [SP800-90A]
              NIST, "DRAFT Special Publication 800-90a, Revision 1,
              Recommendation for Random Number Generation Using
              Deterministic Random Bit Generators",
              http://csrc.nist.gov/publications/drafts/800-90/
              sp800-90a_r1_draft_november2014_ver.pdf, November 2014.

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

Appendix A.  Conveying DTLS over SMS

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

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

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




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A.1.  Overview

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

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

   SMS provides an optional segmentation and reassembly scheme as well,
   known as Concatenated short messages (see Section 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 PMTU of the SMS
   transport in order to correctly instrument its SaR function.  The
   PMTU SHALL be 133 bytes if WDP-based multiplexing is used (see
   Appendix A.3), 140 bytes otherwise.

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

A.2.  Message Segmentation and Re-Assembly

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

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

   Though in principle a PMTU value higher than 140 bytes could be used,
   which may look like an appealing option given its more efficient use
   of the transport, there are disadvantages to consider.  First, there
   is an additional overhead of 7 bytes per TPDU to be paid to the SaR



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   function (which is in addition to the overhead introduced by the DTLS
   SaR mechanism.  Second, some networks only partially support the
   Concatenated SMS function and others do not support it at all.

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

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

A.3.  Multiplexing Security Associations

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

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

   The total overhead cost for encoding the WDP source and destination
   ports is 7 bytes out of the total available for the SMS content.

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

A.4.  Timeout

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





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   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 12 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 12: AES-128-CCM-8 DTLS Record Layer Per-Packet Overhead.

   The DTLS record layer header has 13 octets and consists of

   o  1 octet content type field,

   o  2 octet version field,

   o  2 octet epoch field,

   o  6 octet sequence number,

   o  2 octet length field.

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

   RFC 6655 [RFC6655] allows the nonce_explicit to be a sequence number
   or something else.  This document makes this use more restrictive for
   use with DTLS: the 64-bit none_explicit MUST be the 16-bit epoch
   concatenated with the 48-bit seq_num.  The sequence number component



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   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.

Appendix C.  DTLS Fragmentation

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

   Section 4.2.3 of [RFC6347] advises DTLS implementations to not
   produce overlapping fragments, but requires receivers to be able to
   cope with them.  The need for the latter requisite is explained in
   Section 4.1.1.1 of [RFC6347]: accurate path MTU (PMTU) estimation may
   be traded for shorter handshake completion time.  This approach may
   be beneficial in unconstrained networks where a PMTU of 1280 bytes
   can be pretty much universally assumed.  However, when the handshake
   is carried over a narrow-band radio technology, such as IEEE 802.15.4
   or GSM-SMS, and the client is lacking reliable PMTU data to inform
   fragmentation (e.g., using [RFC1981] or [RFC1191]) can put a cost on
   the constrained implementation in terms of memory (due to re-
   buffering) and latency (due to re-transmission) much higher than the
   benefit that it would get from a shorter handshake.

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

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

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

Authors' Addresses









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

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


   Thomas Fossati
   Alcatel-Lucent
   3 Ely Road
   Milton, Cambridge  CB24 6DD
   UK

   Email: thomas.fossati@alcatel-lucent.com


































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