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Versions: (draft-tschofenig-layered-tls) 00 01 02 03

Network Working Group                                           O. Friel
Internet-Draft                                                 R. Barnes
Intended status: Standards Track                             M. Pritikin
Expires: September 12, 2019                                        Cisco
                                                           H. Tschofenig
                                                             ARM Limited
                                                              M. Baugher
                                                              Consultant
                                                          March 11, 2019


                         Application-Layer TLS
                        draft-friel-tls-atls-02

Abstract

   This document specifies how TLS sessions can be established at the
   application layer over untrusted transport between clients and
   services for the purposes of establishing secure end-to-end encrypted
   communications channels.  Transport layer encodings for application
   layer TLS records are specified for HTTP and CoAP transport.
   Explicit identification of application layer TLS packets enables
   middleboxes to provide transport services and enforce suitable
   transport policies for these payloads, without requiring access to
   the unencrypted payload content.  Multiple scenarios are presented
   identifying the need for end-to-end application layer encryption
   between clients and services, and the benefits of reusing the well-
   defined TLS protocol, and a standard TLS stack, to accomplish this
   are described.  Application software architectures for building, and
   network architectures for deploying application layer TLS are
   outlined.

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 https://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 September 12, 2019.



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Copyright Notice

   Copyright (c) 2019 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
   (https://trustee.ietf.org/license-info) in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  Application Layer End-to-End Security Use Cases . . . . . . .   4
     3.1.  Bootstrapping Devices . . . . . . . . . . . . . . . . . .   4
     3.2.  Constrained Devices . . . . . . . . . . . . . . . . . . .   5
       3.2.1.  Constrained Device Connecting over a Closed Network .   5
       3.2.2.  Constrained Device Connecting over the Internet . . .   6
   4.  Current Approaches to Application Layer End-to-End Security .   7
     4.1.  Noise . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.2.  Signal  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     4.3.  Google ALTS . . . . . . . . . . . . . . . . . . . . . . .   7
     4.4.  Ephemeral Diffie-Hellman Over COSE  . . . . . . . . . . .   8
   5.  ATLS Goals  . . . . . . . . . . . . . . . . . . . . . . . . .   8
   6.  Architecture Overview . . . . . . . . . . . . . . . . . . . .   8
     6.1.  Application Architecture  . . . . . . . . . . . . . . . .   8
       6.1.1.  Application Architecture Benefits . . . . . . . . . .  11
       6.1.2.  ATLS Packet Identification  . . . . . . . . . . . . .  12
       6.1.3.  ATLS Session Tracking . . . . . . . . . . . . . . . .  12
       6.1.4.  ATLS Record Inspection  . . . . . . . . . . . . . . .  12
       6.1.5.  Implementation  . . . . . . . . . . . . . . . . . . .  12
     6.2.  Functional Design . . . . . . . . . . . . . . . . . . . .  13
     6.3.  Network Architecture  . . . . . . . . . . . . . . . . . .  14
   7.  Key Exporting and Application Data Encryption . . . . . . . .  16
   8.  ATLS Session Establishment  . . . . . . . . . . . . . . . . .  17
   9.  ATLS over HTTP Transport  . . . . . . . . . . . . . . . . . .  19
     9.1.  Protocol Summary  . . . . . . . . . . . . . . . . . . . .  20
     9.2.  Content-Type Header . . . . . . . . . . . . . . . . . . .  20
     9.3.  HTTP Status Codes . . . . . . . . . . . . . . . . . . . .  20
     9.4.  ATLS Session Tracking . . . . . . . . . . . . . . . . . .  20
     9.5.  Session Establishment and Key Exporting . . . . . . . . .  20
     9.6.  Illustrative ATLS over HTTP Session Establishment . . . .  21



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     9.7.  ATLS and HTTP CONNECT . . . . . . . . . . . . . . . . . .  21
   10. ATLS over CoAP Transport  . . . . . . . . . . . . . . . . . .  24
   11. The "oscore_connection_id" Extension  . . . . . . . . . . . .  25
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  26
     12.1.  "oscore_connection_id" TLS extension . . . . . . . . . .  26
     12.2.  .well-known URI Registry . . . . . . . . . . . . . . . .  26
     12.3.  Media Types Registry . . . . . . . . . . . . . . . . . .  27
     12.4.  HTTP Content-Formats Registry  . . . . . . . . . . . . .  28
     12.5.  CoAP Content-Formats Registry  . . . . . . . . . . . . .  28
     12.6.  TLS Key Extractor Label  . . . . . . . . . . . . . . . .  28
   13. Security Considerations . . . . . . . . . . . . . . . . . . .  28
   14. Informative References  . . . . . . . . . . . . . . . . . . .  29
   Appendix A.  Pseudo Code  . . . . . . . . . . . . . . . . . . . .  31
     A.1.  OpenSSL . . . . . . . . . . . . . . . . . . . . . . . . .  31
     A.2.  Java JSSE . . . . . . . . . . . . . . . . . . . . . . . .  33
   Appendix B.  Example ATLS Handshake . . . . . . . . . . . . . . .  35
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  35

1.  Introduction

   There are multiple scenarios where there is a need for application
   layer end-to-end security between clients and application services.
   Two examples include:

   o  Bootstrapping devices that must connect to HTTP application
      services across untrusted TLS interception middleboxes

   o  Constrained devices connecting via gateways to application
      services, where different transport layer protocols may be in use
      on either side of the gateway, with the gateway transcoding
      between the different transport layer protocols.

   These two scenarios are described in more detail in Section 3.

   This document describes how clients and applications can leverage
   standard TLS software stacks to establish secure end-to-end encrypted
   connections at the application layer.  The connections may establish
   TLS [RFC5246] [I-D.ietf-tls-tls13] or DTLS [RFC6347]
   [I-D.ietf-tls-dtls13] sessions.  There are multiple advantages to
   reuse of existing TLS software stacks for establishment of
   application layer secure connections.  These include:

   o  many clients and application services already include a TLS
      software stack, so there is no need to include yet another
      software stack in the software build

   o  no need to define a new cryptographic negotiation, authentication,
      and key exchange protocol between clients and services



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   o  provides standards based PKI mutual authentication between clients
      and services

   o  no need to train software developers on how to use a new
      cryptographic protocols or libraries

   o  automatically benefit from new cipher suites by simply upgrading
      the TLS software stack

   o  automatically benefit from new features, bugfixes, etc. in TLS
      software stack upgrades

   This document also explicitly defines how application layer TLS
   connections can be established using HTTP [RFC7230] [RFC7540] or CoAP
   as transport layers.  This document does not preclude the use of
   other transport layers.  However, defining how application layer TLS
   connections can be established over other transport layers, such as
   [ZigBee] or [Bluetooth], is beyond the scope of this document.

   Explicitly identifying application layer TLS packets enables
   transport layer middleboxes to provide transport capabilities and
   enforce suitable transport policies for these payloads, without
   requiring access to unencrypted application data.

2.  Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

   Application layer TLS is referred to as ATLS throughout this
   document.

3.  Application Layer End-to-End Security Use Cases

   This section describes in more detail the bootstrapping and
   constrained device use cases mentioned in the introduction.

3.1.  Bootstrapping Devices

   There are far more classes of clients being deployed on today's
   networks than at any time previously.  This poses challenges for
   network administrators who need to manage their network and the
   clients connecting to their network, and poses challenges for client
   vendors and client software developers who must ensure that their
   clients can connect to all required services.



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   One common example is where a client is deployed on a local domain
   TCP/IP network that protects its perimeter using a TLS terminating
   middlebox, and the client needs to establish a secure connection to a
   service in a different network via the middlebox.  This is
   illustrated in Figure 1.

   Traditionally, this has been enabled by the network administrator
   deploying the necessary certificate authority trusted roots on the
   client.  This can be achieved at scale using standard tools that
   enable the administrator to automatically push trusted roots out to
   all client machines in the network from a centralized domain
   controller.  This works for personal computers, laptops and servers
   running standard Operating Systems that can be centrally managed.
   This client management process breaks for multiple classes of clients
   that are being deployed today, there is no standard mechanism for
   configuring trusted roots on these clients, and there is no standard
   mechanism for these clients to securely traverse middleboxes.

   +--------+    C->M TLS    +-----------+   M->S TLS   +---------+
   | Client |--------------->| Middlebox |------------->| Service |
   +--------+                +-----------+              +---------+
       ^                                                     ^
       |                                                     |
       +-----------Client to Service ATLS Connection---------+

                      Figure 1: Bootstrapping Devices

   The ATLS mechanism defined in this document enables clients to
   traverse middleboxes and establish secure connections to services
   across network domain boundaries.  The purpose of this connection may
   simply be to facilitate a bootstrapping process, for example
   [I-D.ietf-anima-bootstrapping-keyinfra], whereby the client securely
   discovers the local domain certificate authorities required to
   establish a trusted network layer TLS connection to the middlebox.

3.2.  Constrained Devices

   Two constrained device use cases are outlined here.

3.2.1.  Constrained Device Connecting over a Closed Network

   There are industry examples of home smart lighting systems where the
   smart light bulbs connect using ZigBee to a gateway device.  A
   controller application running on a mobile device connects to the
   gateway using CoAP over DTLS.  The controller can then control the
   light bulbs by sending messages and commands via the gateway.  The
   gateway device has full access to all messages sent between the light
   bulbs and the controller application.



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   A generic use case similar to the smart lighting system outlined
   above has an IoT device talking ZigBee to a gateway, with the gateway
   in turn talking CoAP over DTLS to a controller application running on
   a mobile device.  This is illustrated in Figure 2.

   There are scenarios where the messages sent between the IoT device
   and the controller application must not be exposed to the gateway
   function.  Additionally, the end devices (the IoT device and the
   controller application service) have no visibility to and no
   guarantees about what transport layer security and encryption is
   enforced across all hops end-to-end as they only have visibility to
   their immediate next hop.  ATLS addresses these concerns.

   +--------+    ZigBee     +---------+  CoAP/DTLS   +------------+
   | Device |-------------->| Gateway |------------->| Mobile App |
   +--------+               +---------+              +------------+
       ^                                                   ^
       |                                                   |
       +--------Device to Mobile App ATLS Connection-------+

                   Figure 2: IoT Closed Network Gateway

3.2.2.  Constrained Device Connecting over the Internet

   In this example an IoT device connecting to a gateway using a
   suitable transport mechanism, such as ZigBee, CoAP, MQTT, etc.  The
   gateway function in turn talks HTTP over TLS (or, for example, HTTP
   over QUIC) to an application service over the Internet.  This is
   illustrated in Figure 3.

   The gateway may not be trusted and all messages between the IoT
   device and the application service must be end-to-end encrypted.
   Similar to the previous use case, the endpoints have no guarantees
   about what level of transport layer security is enforced across all
   hops.  Again, ATLS addresses these concerns.

   +--------+  CoAP/DTLS    +------------------+  HTTP/TLS   +---------+
   | Device |-------------->| Internet Gateway |------------>| Service |
   +--------+               +------------------+             +---------+
       ^                                                          ^
       |                                                          |
       +---------Device to Cloud Service ATLS Connection----------+

                      Figure 3: IoT Internet Gateway







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4.  Current Approaches to Application Layer End-to-End Security

   End-to-end security at the application layer is increasing seen as a
   key requirement across multiple applications and services.  Some
   examples of end-to-end security mechanisms are outlined here.  All
   the solutions outlined here have some common characteristics.  The
   solutions:

   o  do not rely on transport layer security

   o  define a new handshake protocol for establishment of a secure end-
      to-end session

4.1.  Noise

   [Noise] is a framework for cryptographic protocols based on Elliptic
   Curve Diffie-Hellman (ECDH) key agreement, AEAD encryption, and
   BLAKE2 and SHA2 hash functions.  Noise is currently used by WhatsApp,
   WireGuard, and Lightning.

   The current Noise protocol framework defines mechanisms for proving
   possession of a private key, but does not define authentication
   mechanisms.  Section 14 "Security Considerations" of Noise states:
   ~~~ it's up to the application to determine whether the remote
   party's static public key is acceptable ~~~

4.2.  Signal

   The [Signal] protocol provides end-to-end encryption and uses EdDSA
   signatures, Triple Diffie-Hellman handshake for shared secret
   establishment, and the Double Ratchet Algorithm for key management.
   It is used by Open Whisper Systems, WhatsApp and Google.

   Similar to Noise, Signal does not define an authentication mechanism.
   The current [X3DH] specification states in Section 4.1
   "Authentication":

   Methods for doing this are outside the scope of this document

4.3.  Google ALTS

   Google's Application Layer Transport Security [ALTS] is a mutual
   authentication and transport encryption system used for securing
   Remote Procedure Call (RPC) communications within Google's
   infrastructure.  ALTS uses an ECDH handshake protocol and a record
   protocol containing AES encrypted payloads.





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4.4.  Ephemeral Diffie-Hellman Over COSE

   There is ongoing work to standardise [I-D.selander-ace-cose-ecdhe],
   whiich defines a SIGMA-I based authenticated key exchange protocol
   using COSE and CBOR.

5.  ATLS Goals

   The high level goals driving the design of this mechanism are:

   o  enable authenticated key exchange at the application layer by
      reusing existing technologies

   o  ensure that ATLS packets are explicitly identified thus ensuring
      that any middleboxes or gateways at the transport layer are
      content aware

   o  leverage existing TLS stacks and handshake protocols thus avoiding
      introducing new software or protocol dependencies in clients and
      applications

   o  reuse existing TLS [RFC5246] [I-D.ietf-tls-tls13] and DTLS
      [RFC6347] [I-D.ietf-tls-dtls13] specifications as is without
      requiring any protocol changes or software stack changes

   o  do not mandate constraints on how the TLS stack is configured or
      used

   o  be forward compatible with future TLS versions

   o  avoid introducing TLS protocol handling logic or semantics into
      the application layer, i.e. TLS protocol knowledge and logic is
      handled by the TLS stack, not the application

   o  ensure the client and server software implementations are as
      simple as possible

6.  Architecture Overview

6.1.  Application Architecture

   TLS software stacks allow application developers to 'unplug' the
   default network socket transport layer and read and write TLS records
   directly from byte buffers.  This enables application developers to
   create application layer TLS sessions, extract the raw TLS record
   bytes from the bottom of the TLS stack, and transport these bytes
   over any suitable transport.  The TLS software stacks can generate
   byte streams of full TLS flights which may include multiple TLS



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   records.  Additionally, TLS software stacks support Keying Material
   Exporters [RFC5705] and allow applications to export keying material
   from established TLS sessions.  This keying material can then be used
   by the application for encryption of data outside the context of the
   TLS session.  This is illustrated in Figure 4 below.

                       +------------+                    +---------+
    Handshake Records  |            | Handshake Records  |         |
   ------------------->|            |------------------->|         |
                       |            |                    |  Byte   |
    Unencrypted Data   |    TLS     | Encrypted Data     |         |
   ------------------->|            |------------------->| Buffers |
                       |  Software  |                    |         |
    Encrypted Data     |            | Unencrypted Data   |         |
   ------------------->|   Stack    |------------------->|         |
                       |            |                    +---------+
    Keying Material    |            |
   <-------------------|            |
                       + -----------+

                      Figure 4: TLS Stack Interfaces

   These TLS software stack APIs enable application developers to build
   the software architectures illustrated in Figure 5 and Figure 6.

   In both architectures, the application creates and interacts with an
   application layer TLS session in order to generate and consume raw
   TLS records.  The application transports these raw TLS records inside
   transport layer message bodies using whatever standard transport
   layer stack is suitable for the application or architecture.  This
   document does not place any restrictions on the choice of transport
   layer and any suitable protocol such as HTTP, TCP, CoAP, ZigBee,
   Bluetooth, etc. could be used.

   The transport layer will typically encrypt data, and this encryption
   is completely independent from any application layer encryption.  The
   transport stack may create a transport layer TLS session.  The
   application layer TLS session and transport layer TLS session can
   both leverage a shared, common TLS software stack.  This high level
   architecture is applicable to both clients and application services.
   The key differences between the architectures are as follows.

   In the model illustrated in Figure 5, the application sends all
   sensitive data that needs to be securely exchanged with the peer
   application through the Application TLS session in order to be
   encrypted and decrypted.  All sensitive application data is thus
   encoded within TLS records by the TLS stack, and these TLS records
   are transmitted over the transport layer.



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   +-------------+
   |             |    App
   |             |    Data    +---------+
   | Application |<---------->|   App   |      +---------+
   |             |    TLS     |   TLS   |----->|   TLS   |
   |             |  Records   | Session |      |  Stack  |
   |        +--->|<---------->|         |      +---------+
   |        |    |            +---------+           ^
   |        |    |                                  |?
   |        |    | Transport +-----------+    +------------+
   |        |    |  Payload  | Transport |    | Transport  |
   |        +--->|<--------->|   Stack   |--->| Encryption |-->Packets
   +-------------+           +-----------+    +------------+

             Figure 5: TLS Stack used for all data encryption

   In the model illustrated in Figure 6, the application establishes an
   application layer TLS session purely for the purposes of key
   exchange.  Therefore, the only TLS records that are sent or received
   by the application layer are TLS handshake records.  Once the
   application layer TLS session is established, the application uses
   Keying Material Exporter [RFC5705] APIs to export keying material
   from the TLS stack from this application layer TLS session.  The
   application can then use these exported keys to derive suitable
   shared encryption keys with its peer for exchange of encrypted data.
   The application encrypts and decrypts sensitive data using these
   shared encryption keys using any suitable cryptographic library
   (which may be part of the same library that provides the TLS stack),
   and transports the encrypted data directly over the transport layer.






















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   +--------------+
   |              |
   | Application  |
   |              |
   | +-------+    |            +---------+
   | | App   |    | Key Export |         |
   | | Data  |<---|<-----------|         |
   | | Crypto|    |            |   App   |
   | +-------+    |    TLS     |   TLS   |      +---------+
   |    ^         | Handshake  | Session |----->|   TLS   |
   |    |         |  Records   |         |      |  Stack  |
   |    |    +--->|<---------->|         |      +---------+
   |    |    |    |            +---------+           ^
   |    |    |    |                                  |?
   |    |    |    | Transport +-----------+    +------------+
   |    |    |    |  Payload  | Transport |    | Transport  |
   |    +----+--->|<--------->|   Stack   |--->| Encryption |-->Packets
   +--------------+           +-----------+    +------------+

         Figure 6: TLS stack used for key agreement and exporting

   The choice of which application architecture to use will depend on
   the overall solution architecture, and the underlying transport layer
   or layers in use.  While the choice of application architecture is
   outside the scope of this document, some considerations are outlined
   here.

   o  in some IoT use cases reducing the number of bytes transmitted is
      important.  [I-D.mattsson-core-security-overhead] analyses the
      overhead of TLS headers compared with OSCORE
      [I-D.ietf-core-object-security] illustrating the additional
      overhead associated with TLS headers.  The overhead varies between
      the different TLS versions and also between TLS and DTLS.  It may
      be more appropriate to use the architecture defined in Figure 6 in
      order to establish shared encryption keys, and then transport
      encrypted data directly without the overhead of unwanted TLS
      record headers.

   o  when using HTTP as a transport layer, it may be more appropriate
      to use the architecture defined in Figure 6 in order to avoid any
      TLS session vs. HTTP session affinity issues.

6.1.1.  Application Architecture Benefits

   There are several benefits to using a standard TLS software stack to
   establish an application layer secure communications channel between
   a client and a service.  These include:




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   o  no need to define a new cryptographic negotiation and exchange
      protocol between client and service

   o  automatically benefit from new cipher suites by simply upgrading
      the TLS software stack

   o  automatically benefit from new features, bugfixes, etc. in TLS
      software stack upgrades

6.1.2.  ATLS Packet Identification

   It is recommended that ATLS packets are explicitly identified by a
   standardized, transport-specific identifier enabling any gateways and
   middleboxes to identify ATLS packets.  Middleboxes have to contend
   with a vast number of applications and network operators have
   difficulty configuring middleboxes to distinguish unencrypted but not
   explicitly identified application data from end-to-end encrypted
   data.  This specification aims to assist network operators by
   explicitly identifying ATLS packets.  The HTTP and CoAP encodings
   documented in Section 9 and Section 10 explicitly identify ATLS
   packets.

6.1.3.  ATLS Session Tracking

   The ATLS application service establishes multiple ATLS sessions with
   multiple clients.  As TLS sessions are stateful, the application
   service must be able to correlate ATLS records from different clients
   across the relevant ATLS sessions.  The details of how session
   tracking is implemented are outside the scope of this document.
   Recommendations are given in Section 9 and Section 10, but session
   tracking is application and implementation specific.

6.1.4.  ATLS Record Inspection

   It should not be necessary for the application layer to have to
   inspect, parse or understand the contents of ATLS records.  No
   constraints are placed on the ContentType contained within the
   transported TLS records.  The TLS records may contain handshake,
   application_data, alert or change_cipher_spec messages.  If new
   ContentType messages are defined in future TLS versions, these may
   also be transported using this protocol.

6.1.5.  Implementation

   Pseudo code illustrating how to read and write TLS records directly
   from byte buffers using both OpenSSL BIO functions and Java JSSE
   SSLEngine is given in the appendices.  A blog post by [Norrell]




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   outlines a similar approach to leveraging OpenSSL BIO functions, and
   Oracle publish example code for leveraging [SSLEngine].

6.2.  Functional Design

   The functional design assumes that an authorization system has
   established operational keys for authenticating endpoints.  In a
   layered design, this needs to be done for each layer, which may
   operate in two separate authorization domains.  Note that Figure 7
   shows a generic setup where TLS/DTLS is used at two layers.  In some
   cases, use of TLS/DTLS at the application layer may be sufficient
   where lower layer security mechanisms provide protection of the
   transport-specific headers.

             +-------------------------------------------------------+
             |               +---+               +---+               |
             |  +--------+   |APP|               |APP|   +--------+  |
             |  |security|   +---+               +---+   |security|  |
             |  |--------+     ^                   ^     |--------+  |
             |  |policies|     |                   |     |policies|  |
             |  |LAYER 0 |     |                   |     |LAYER 0 |  |
             |  +--------+     v                   v     +--------+  |
             |       +      +------+    APP    +------+      +       |
             |       |      | TLS- |<--------->| TLS- |      |       |
             |       +----->|SERVER|   LAYER   |CLIENT|<-----+       |
             |              +------+           +------+              |
             | TOP LAYER       ^                   ^                 |
             +-----------------|-------------------|-----------------+
             | BOTTTOM LAYER   |                   |                 |
             |                 v                   v                 |
             |              +------+ TRANSPORT +------+              |
             |              | TLS- |<--------->| TLS- |              |
             |  +--------+  |SERVER|   LAYER   |CLIENT|  +--------+  |
             |  |security|  +------+           +------+  |security|  |
             |  |--------+     ^                   ^     |--------+  |
             |  |policies|     |                   |     |policies|  |
             |  |LAYER 1 +-----+                   +-----+LAYER 1 |  |
             |  +--------+                               +--------+  |
             |                                                       |
             +-------------------------------------------------------+

                        Figure 7: Functional Design

   The security policies of one layer are distinct from those of another
   in Figure 7.  They may overlap, but that is not necessary or perhaps
   even likely since the key exchanges at the different layers terminate
   at different endpoints and the two often have different authorization
   domains.



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   TLS can protect IoT device-to-gateway communications "on the wire"
   using the "bottom layer" of Figure 7, and it can protect application
   data from the device to the application server using the "top layer."
   Application and transport security each have a role to play.
   Transport security restricts access to messages on the networks,
   notably application headers and application-layer TLS restricts
   access to the application payloads.

   As shown in Figure 7, an application-layer message, which gets
   encrypted and integrity protected and, in the generic case, the the
   resulting TLS message and headers are passed to a TLS socket at the
   bottom layer, which may have a different security policy than the
   application layer.

6.3.  Network Architecture

   An example network deployment is illustrated in Figure 8.  It shows a
   constrained client connecting to an application service via an
   internet gateway.  The client uses CoAP over DTLS to communicate with
   the gateway.  The gateway extracts the messages the client sent over
   CoAP and sends these messages inside HTTP message bodies to the
   application service.  It also shows a TLS terminator deployed in
   front of the application service.  The client establishes a transport
   layer CoAP/DTLS connection with the gateway (C->G DTLS), the gateway
   in turn opens a transport layer TLS connection with the TLS
   terminator deployed in front of the service (G->T TLS).  The client
   can ignore any certificate validation errors when it connects to the
   gateway.  CoAP messages are transported between the client and the
   gateway, and HTTP messages are transported between the client and the
   service.  Finally, application layer TLS messages are exchanged
   inside the CoAP and HTTP message bodies in order to establish an end-
   to-end TLS session between the client and the service (C->S TLS).



















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          +----------+        +----------+
          | App Data |        | App Data |
          +----------+        +----------+         +----------+
          | C->S TLS |        | C->S TLS |         | App Data |
          +----------+        +----------+         +----------+
          |   CoAP   |        |   HTTP   |         | C->S TLS |
          +----------+        +----------+         +----------+
          | C->G DTLS|        | M->T TLS |         |   HTTP   |
          +----------+        +----------+         +----------+
          |   UDP    |        |   TCP    |         |   TCP    |
          +----------+        +----------+         +----------+

   +--------+      +-----------+      +----------------+     +---------+
   | Client |----->|  Gateway  |----->| TLS Terminator |---->| Service |
   +--------+      +-----------+      +----------------+     +---------+
      ^                                                           ^
      |                                                           |
      +-------------Client to Service ATLS Connection-------------+

         Figure 8: Constrained Device Gateway Network Architecture

   Another typical network deployment is illustrated in Figure 9.  It
   shows a client connecting to a service via a middlebox.  It also
   shows a TLS terminator deployed in front of the service.  The client
   establishes a transport layer TLS connection with the middlebox (C->M
   TLS), the middlebox in turn opens a transport layer TLS connection
   with the TLS terminator deployed in front of the service (M->T TLS).
   The client can ignore any certificate validation errors when it
   connects to the middlebox.  HTTP messages are transported over this
   layer between the client and the service.  Finally, application layer
   TLS messages are exchanged inside the HTTP message bodies in order to
   establish an end-to-end TLS session between the client and the
   service (C->S TLS).


















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          +----------+        +----------+
          | App Data |        | App Data |
          +----------+        +----------+         +----------+
          | C->S TLS |        | C->S TLS |         | App Data |
          +----------+        +----------+         +----------+
          |   HTTP   |        |   HTTP   |         | C->S TLS |
          +----------+        +----------+         +----------+
          | C->M TLS |        | M->T TLS |         |   HTTP   |
          +----------+        +----------+         +----------+
          |   TCP    |        |   TCP    |         |   TCP    |
          +----------+        +----------+         +----------+

   +--------+      +-----------+      +----------------+     +---------+
   | Client |----->| Middlebox |----->| TLS Terminator |---->| Service |
   +--------+      +-----------+      +----------------+     +---------+
      ^                                                           ^
      |                                                           |
      +-------------Client to Service ATLS Connection-------------+

               Figure 9: HTTP Middlebox Network Architecture

7.  Key Exporting and Application Data Encryption

   When solutions implement the architecture described in Figure 6, they
   leverage [RFC5705] for exporting keys.  When the OSCORE mode has been
   agreed using the "oscore_connection_id" extension defined in this
   document, different keys are used for ordinary DTLS/TLS record
   protection and OSCORE packet protection.  These keys are produced
   using a TLS exporter [RFC5705] and the exporter takes three input
   values:

   o  a disambiguating label string,

   o  a per-association context value provided by the application using
      the exporter, and

   o  a length value.

   The label string for use with this specification is defined as
   'application-layer-tls'.  The per-association context value is empty.

   The length value is twice the size of the key size utilized by the
   negotiated algorithm since the lower-half is used for the Master
   Secret and the upper-half is used for the Master Salt.

   For example, if a TLS/DTLS 1.2 handshake negotiated the
   TLS_PSK_WITH_AES_128_CCM_8 ciphersuite then the key size utilized by




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   the negotiated algorithm, i.e. AES 128, is 128 bit.  Hence, the key
   extractor is requested to produce 2 x 128 bit keying material.

   The following parameters are needed for use with OSCORE:

   o  Master Secret: The master secret is described as described above.

   o  Sender ID: This values is negotiated using the
      "oscore_connection_id" extension, as described in Section 11.

   o  Recipient ID: This values is negotiated using the
      "oscore_connection_id" extension, as described in Section 11.

   o  AEAD Algorithm: This value is negotiated using the ciphersuite
      exchange provided by the TLS/DTLS handshake.  For example, if a
      TLS/DTLS 1.2 handshake negotiated the TLS_PSK_WITH_AES_128_CCM_8
      ciphersuite then AEAD algorithm identifier is AES_128_CCM_8, which
      corresponds to COSE algorithms AES-CCM-64-64-128 or AES-CCM-
      16-64-128, whereby the former uses a 7-byte nonce and the later
      13-byte nonce.  Since in TLS/DTLS the nonce value is not
      negotiated but rather fixed, a 7-byte nonce value is assumed as a
      default in this document.

   o  Master Salt: The master salt is described as described above.

   o  HKDF Algorithm: This value is negotiated using the ciphersuite
      exchange provided by the TLS/DTLS handshake.  As a default,
      SHA-256 is assumed as a HKDF algorithm.

   o  Replay Window: A default window size of 32 packets is assumed.

   A future version of this specification will describe how to establish
   keying material and parameters for security contexts other than
   OSCORE.

8.  ATLS Session Establishment

   Figure 10 illustrates how an ATLS session is established using the
   key exporting architectural model shown in Figure 6.  The number of
   RTTs that take place when establishing a TLS session depends on the
   version of TLS and what capabilities are enabled on the TLS software
   stack.  For example, a 0-RTT exchange is possible with TLS 1.3.  If
   applications wish to ensure a predictable number of RTTs when
   establishing an application layer TLS connection, this may be
   achieved by configuring the TLS software stack appropriately.

   The outline is as follows:




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   o  the client creates an ATLS session object

   o  the client initiates a TLS handshake on the session

   o  the client extracts the TLS records for the first TLS flight (the
      first RTT)

   o  the client sends the TLS records over the transport layer to the
      server

   o  on receipt of the TLS flight, the server creates an ATLS session
      object

   o  the server injects the received TLS flight into the session

   o  the server extracts the TLS records for the first TLS flight
      response

   o  the server sends the TLS response records over the transport layer
      to the client

   o  the client injects the received TLS records into its TLS session
      completing the first full RTT

   o  the client and server repeat the above process and complete the
      second RTT

   o  once the ATLS session is up, both sides export keying material

   o  both sides now can exchange data encrypted using shared keys
      derived from the keying material

   +-------------------------------+  +-------------------------------+
   |             Client            |  |           ATLS Server         |
   +---------+---+-----+-+---------+  +---------+--+-----+--+---------+
   |  ATLS   |   | App | |Transport|  |Transport|  | App |  |  ATLS   |
   | Session |   +-----+ |  Stack  |  |  Stack  |  +-----+  | Session |
   +---------+     |     +---------+  +---------+     |     +---------+
        |          |         |             |          |          |
        |          |         |             |          |          |
        |          |         |             |          |          |
        |  Create  |         |             |          |          |
        |  Session |         |             |          |          |
    +   |<---------|         |             |          |          |
    |   |  Start   |         |             |          |          |
    |   | Handshake|         |             |          |          |
    |   |<---------|         |             |          |          |
    |   |   TLS    |         |             |          |          |



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    |   | Records  |  Pack   |             |          |          |
    |   |--------->| Records |             |          |          |
        |          |-------->| send packet | Unpack   |          |
    R   |          |         |------------>| Records  | Create   |
    T   |          |         |             |--------->| Session  |
    T   |          |         |             |          |--------->|
        |          |         |             |          |   TLS    |
    1   |          |         |             |          | Records  |
        |          |         |             |          |--------->|
    |   |          |         |             |          |   TLS    |
    |   |          |         |             |  Pack    | Records  |
    |   |          |         |             | Records  |<---------|
    |   |          | Unpack  |send response|<---------|          |
    |   |   TLS    | Records |<------------|          |          |
    |   | Records  |<--------|             |          |          |
    +   |<---------|         |             |          |          |
        |   TLS    |         |             |          |          |
        | Records  |         |             |          |          |
    +   |--------->|-------->|------------>|--------->|--------->|
    |   |          |         |             |          |          |
        |          |         |             |          | Session  |
    R   |          |         |             |          |    Up    |
    T   |          |         |             |          |<---------|
    T   |          |         |             |          |   TLS    |
        |          |         |             |          | Records  |
    2   |<---------|<--------|<------------|<---------|<---------|
        | Session  |         |             |          |          |
    |   |    Up    |         |             |          |          |
    +   |--------->|         |             |          |          |
        |  Export  |         |             |          |  Export  |
        |   Keys   |         |             |          |   Keys   |
        |--------->|         | E2E Session |          |<---------|
        |          |<--------|-------------|--------->|          |

                   Figure 10: ATLS Session Establishment

9.  ATLS over HTTP Transport

   The assumption is that the client will establish a transport layer
   connection to the server for exchange of HTTP messages.  The
   underlying transport layer connection could be over TCP or TLS.  The
   client will then establish an application layer TLS connection with
   the server by exchanging TLS records with the server inside HTTP
   message request and response bodies.







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9.1.  Protocol Summary

   All ATLS records are transported unmodified as binary data within
   HTTP message bodies.  The application simply extracts the TLS records
   from the TLS stack and inserts them directly into HTTP message
   bodies.  Each message body contains a full TLS flight, which may
   contain multiple TLS records.

   The client sends all ATLS records to the server in the bodies of POST
   requests.

   The server sends all ATLS records to the client in the bodies of 200
   OK responses to the POST requests.

   The URI path used by ATLS is "/.well-known/atls".

9.2.  Content-Type Header

   A new Content-Type header value is defined:

   Content-type: application/atls

   All message bodies containing ATLS records must set this Content-
   Type.  This enables middleboxes to readily identify ATLS payloads.

9.3.  HTTP Status Codes

   This document does not define any new HTTP status codes, and does not
   specify additional semantics or refine existing semantics for status
   codes.  This is the best current practice as outlined in
   [I-D.ietf-httpbis-bcp56bis].

9.4.  ATLS Session Tracking

   The application service needs to track multiple client application
   layer TLS sessions so that it can correlate TLS records received in
   HTTP message bodies with the appropriate TLS session.  The
   application service should use stateful cookies [RFC6265] in order to
   achieve this as recommended in [I-D.ietf-httpbis-bcp56bis].

9.5.  Session Establishment and Key Exporting

   It is recommended that applications using ATLS over HTTP transport
   only use ATLS for session establishment and key exchange, resulting
   in only 2 ATLS RTTs between the client and the application service.

   Key exporting must be carried out as described in Section 7.




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9.6.  Illustrative ATLS over HTTP Session Establishment

   A client initiates an ATLS session by sending the first TLS flight in
   a POST request message body to the ATLS server.

   POST /.well-known/atls
   Content-Type: application/atls

   <binary TLS client flight 1 records>

   The server handles the request, creates an ATLS session object, and
   replies by including its first TLS flight in a 200 OK message body.
   The server also sets a suitable cookie for session tracking purposes.

   200 OK
   Content-Type: application/atls
   Set-Cookie: my-atls-cookie=my-cookie-value

   <binary TLS server flight 1 records>

   The client handles the server first flight TLS records and replies
   with its second flight.

   POST /.well-known/atls
   Content-Type: application/atls
   Cookie: my-atls-cookie=my-cookie-value

   <binary TLS client flight 2 records>

   The server handles the second flight, establishes the ATLS session,
   and replies with its second flight.

   200 OK
   Content-Type: application/atls

   <binary TLS server flight 2 records>

9.7.  ATLS and HTTP CONNECT

   It is worthwhile comparing and contrasting ATLS with HTTP CONNECT
   tunneling.

   First, let us introduce some terminology:

   o  HTTP Proxy: A HTTP Proxy operates at the application layer,
      handles HTTP CONNECT messages from clients, and opens tunnels to
      remote origin servers on behalf of clients.  If a client
      establishes a tunneled TLS connection to the origin server, the



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      HTTP Proxy does not attempt to intercept or inspect the HTTP
      messages exchanged between the client and the server

   o  middlebox: A middlebox operates at the transport layer, terminates
      TLS connections from clients, and originates new TLS connections
      to services.  A middlebox inspects all messages sent between
      clients and services.  Middleboxes are generally completely
      transparent to applications, provided that the necessary PKI root
      Certificate Authority is installed in the client's trust store.

   HTTP Proxies and middleboxes are logically separate entities and one
   or both of these may be deployed in a network.

   HTTP CONNECT is used by clients to instruct a HTTP Forward Proxy
   deployed in the local domain to open up a tunnel to a remote origin
   server that is typically deployed in a different domain.  Assuming
   that TLS transport is used between both client and proxy, and proxy
   and origin server, the network architecture is as illustrated in
   Figure 11.  Once the proxy opens the transport tunnel to the service,
   the client establishes an end-to-end TLS session with the service,
   and the proxy is blindly transporting TLS records (the C->S TLS
   session records) between the client and the service.  From the client
   perspective, it is tunneling a TLS session to the service inside the
   TLS session it has established to the proxy (the C->P TLS session).
   No middlebox is attempting to intercept or inspect the HTTP messages
   between the client and the service.

          +----------+        +----------+
          | C->S HTTP|        | C->S HTTP|
          +----------+        +----------+
          | C->S TLS |        | C->S TLS |
          +----------+        +----------+
          | C->P TLS |        | P->S TCP |
          +----------+        +----------+
          | C->P TCP |
          +----------+

   +--------+      +------------+      +---------+
   | Client |----->| HTTP Proxy |----->| Service |
   +--------+      +------------+      +---------+

                  Figure 11: HTTP Proxy transport layers

   A more complex network topology where the network operator has both a
   HTTP Proxy and a middlebox deployed is illustrated in Figure 12.  In
   this scenario, the proxy has tunneled the TLS session from the client
   towards the origin server, however the middlebox is intercepting and
   terminating this TLS session.  A TLS session is established between



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   the client and the middlebox (C->M TLS), and not end-to-end between
   the client and the server.  It can clearly be seen that HTTP CONNECT
   and HTTP Proxies serve completely different functions than
   middleboxes.

   Additionally, the fact that the TLS session is established between
   the client and the middlebox can be problematic for two reasons:

   o  the middle box is inspecting traffic that is sent between the
      client and the service

   o  the client may not have the necessary PKI root Certificate
      Authority installed that would enable it to validate the TLS
      connection to the middlebox.  This is the scenario outlined in
      Section 3.1.

          +----------+        +----------+       +----------+
          | C->S HTTP|        | C->S HTTP|       | C->S HTTP|
          +----------+        +----------+       +----------+
          | C->M TLS |        | C->M TLS |       | M->S TLS |
          +----------+        +----------+       +----------+
          | C->P TLS |        | P->M TCP |       | M->S TCP |
          +----------+        +----------+       +----------+
          | C->P TCP |
          +----------+

   +--------+      +------------+      +-----------+      +---------+
   | Client |----->| HTTP Proxy |----->| Middlebox |----->| Service |
   +--------+      +------------+      +-----------+      +---------+

           Figure 12: HTTP Proxy and middlebox transport layers

   As HTTP CONNECT can be used to establish a tunneled TLS connection,
   one hypothetical solution to this middlebox issue is for the client
   to issue a HTTP CONNECT command to a HTTP Reverse Proxy deployed in
   front of the origin server.  This solution is not practical for
   several reasons:

   o  if there is a local domain HTTP Forward Proxy deployed, this would
      result in the client doing a first HTTP CONNECT to get past the
      Forward Proxy, and then a second HTTP CONNECT to get past the
      Reverse Proxy.  No client or client library supports the concept
      of HTTP CONNECT inside HTTP CONNECT.

   o  if there is no local domain HTTP Proxy deployed, the client still
      has to do a HTTP CONNECT to the HTTP Reverse Proxy.  This breaks
      with standard and expected HTTP CONNECT operation, as HTTP CONNECT
      is only ever called if there is a local domain proxy.



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   o  clients cannot generate CONNECT from XHR in web applications.

   o  this would require the deployment of a Reverse Proxy in front of
      the origin server, or else support of the HTTP CONNECT method in
      standard web frameworks.  This is not an elegant design.

   o  using HTTP CONNECT with HTTP 1.1 to a Reverse Proxy will break
      middleboxes inspecting HTTP traffic, as the middlebox would see
      TLS records when it expects to see HTTP payloads.

   In contrast to trying to force HTTP CONNECT to address a problem for
   which it was not designed to address, and having to address all the
   issues just outlined; ATLS is specifically designed to address the
   middlebox issue in a simple, easy to develop, and easy to deploy
   fashion.

   o  ATLS works seamlessly with HTTP Proxy deployments

   o  no changes are required to HTTP CONNECT semantics

   o  no changes are required to HTTP libraries or stacks

   o  no additional Reverse Proxy is required to be deployed in front of
      origin servers

   It is also worth noting that if HTTP CONNECT to a Reverse Proxy were
   a conceptually sound solution, the solution still ultimately results
   in encrypted traffic traversing the middlebox that the middlebox
   cannot intercept and inspect.  That is ultimately what ATLS results
   in - traffic traversing the middle box that the middlebox cannot
   intercept and inspect.  Therefore, from a middlebox perspective, the
   differences between the two solutions are in the areas of solution
   complexity and protocol semantics.  It is clear that ATLS is a
   simpler, more elegant solution that HTTP CONNECT.

10.  ATLS over CoAP Transport

   To carry TLS messages over CoAP it is recommended to use Confirmable
   messages while DTLS payloads may as well use non-confirmable
   messages.  The exchange pattern in CoAP uses the following style: A
   request from the CoAP client to the CoAP server uses a POST with the
   ATLS message contained in the payload of the request.  An ATLS
   response is returned by the CoAP server to the CoAP client in a 2.04
   (Changed) message.

   When DTLS messages are conveyed in CoAP over UDP then the DDoS
   protection offered by DTLS MAY be used instead of replicating the
   functionality at the CoAP layer.  If TLS is conveyed in CoAP over UDP



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   then DDoS protection by CoAP has to be utilized.  Carrying ATLS
   messages in CoAP over TCP does not require any additional DDoS
   protection.

   The URI path used by ATLS is "/.well-known/atls".

   {{coap-example} shows a TLS 1.3 handshake inside CoAP graphically.

       Client    Server
         |          |
         +--------->| Header: POST (Code=0.02)
         |   POST   | Uri-Path: "/.well-known/atls"
         |          | Content-Format: application/atls
         |          | Payload: ATLS (ClientHello)
         |          |
         |<---------+ Header: 2.04 Changed
         |   2.04   | Content-Format: application/atls
         |          | Payload: ATLS (ServerHello,
         |          | {EncryptedExtensions}, {CertificateRequest*}
         |          | {Certificate*}, {CertificateVerify*} {Finished})
         |          |
         +--------->| Header: POST (Code=0.02)
         |   POST   | Uri-Path: "/.well-known/atls"
         |          | Content-Format: application/atls
         |          | Payload: ATLS ({Certificate*},
         |          | {CertificateVerify*}, {Finished})
         |          |
         |<---------+ Header: 2.04 Changed
         |   2.04   |
         |          |

                   Figure 13: Transferring ATLS in CoAP

   Note that application data can already be sent by the server in the
   second message and by the client in the third message, in case of the
   full TLS 1.3 handshake.  In case of the 0-RTT handshake application
   data can be sent earlier.  To mix different media types in the same
   CoAP payload the application/multipart-core content type is used.

   Note also that CoAP blockwise transfer MAY be used if the payload
   size, for example due to the size of the certificate chain, exceeds
   the MTU size.

11.  The "oscore_connection_id" Extension

   This document defines the "oscore_connection_id" extension, which is
   used in ClientHello and ServerHello messages.  It is used only for
   establishing the client's OSCORE Sender ID and the server's OSCORE



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   Sender ID.  The client's OSCORE Sender ID maps to the CID provided by
   the server in the ServerHello and the server's OSCORE Sender ID maps
   to the CID provided by the client in the ClientHello.

   The negotiation mechanism follows the procedure used in
   [I-D.ietf-tls-dtls-connection-id] with the exception that the
   negotiated CIDs agreed with the "oscore_connection_id" extension is
   only used with OSCORE and does not impact the record layer format of
   the DTLS/TLS payloads nor the MAC calculation used by DTLS/TLS.  As
   such, this extension can be used with DTLS as well as with TLS when
   those protocols are used at the application layer.

   The extension type is specified as follows.

   enum { oscore_connection_id(TBD), (65535) } ExtensionType;

   struct { opaque cid<0..2^8-1>; } ConnectionId;

   Note: This extension allows a client and a server to determine
   whether an OSCORE security context should be established.

   A future version of this specification may extend the negotiation
   capabilities.

12.  IANA Considerations

12.1.  "oscore_connection_id" TLS extension

   IANA is requested to allocate an entry to the existing TLS
   "ExtensionType Values" registry, defined in [RFC5246], for
   oscore_connection_id(TBD) defined in this document.

12.2.  .well-known URI Registry

   IANA is requested to add the well-known URI 'atls' to the Well-Known
   URIs registry.

   o  URI suffix: atls

   o  Change controller: IETF

   o  Specification document(s): [[this document]]

   o  Related information: None







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12.3.  Media Types Registry

   IANA is requested to add the media type 'application/atls' to the
   Media Types registry.

   o  Type name: application

   o  Subtype name: atls

   o  Required parameters: N/A

   o  Optional parameters: N/A

   o  Encoding considerations: binary

   o  Security considerations: See Security Considerations section of
      this document.

   o  Interoperability considerations: N/A

   o  Published specification: [[this document]] (this document)

   o  Applications that use this media type: Potentially any

   o  Fragment identifier considerations: N/A

   o  Additional information:

      *  Magic number(s): N/A

      *  File extension(s): N/A

      *  Macintosh file type code(s): N/A

   o  Person & email address to contact for further information: See
      "Authors' Addresses" section.

   o  Intended usage: COMMON

   o  Restrictions on usage: N/A

   o  Author: See "Authors' Addresses" section.

   o  Change Controller: IESG







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12.4.  HTTP Content-Formats Registry

   IANA is requested to add the media type 'application/atls' to the
   HTTP Content-Formats registry.

   o  Media Type: application/atls

   o  Encoding: binary

   o  ID: TBD

   o  Reference: [[this document]]

12.5.  CoAP Content-Formats Registry

   IANA is requested to add the media type 'application/atls' to the
   CoAP Content-Formats registry.

   o  Media Type: application/atls

   o  Encoding: binary

   o  ID: TBD

   o  Reference: [[this document]]

12.6.  TLS Key Extractor Label

   IANA is requested to register the "application-layer-tls" label in
   the TLS Extractor Label Registry to correspond to this specification.

13.  Security Considerations

   This specification re-uses the TLS and DTLS and hence the security
   considerations of the respective TLS/DTLS version applies.  As
   described in Section 6.2, implementers need to take the policy
   configuration into account when applying security protection at
   various layers of the stack even if the same protocol is used since
   the communiation endpoints and the security requirements are likely
   going to vary.

   For use in the IoT environment the considerations described in
   [RFC7925] apply and other environments the guidelines in [RFC7525]
   are applicable.







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14.  Informative References

   [ALTS]     Google, "Application Layer Transport Security", December
              2017, <https://cloud.google.com/security/encryption-in-
              transit/application-layer-transport-security/>.

   [Bluetooth]
              Bluetooth, "Bluetooth Core Specification v5.0", 2016,
              <https://www.bluetooth.com/>.

   [I-D.ietf-anima-bootstrapping-keyinfra]
              Pritikin, M., Richardson, M., Behringer, M., Bjarnason,
              S., and K. Watsen, "Bootstrapping Remote Secure Key
              Infrastructures (BRSKI)", draft-ietf-anima-bootstrapping-
              keyinfra-19 (work in progress), March 2019.

   [I-D.ietf-core-object-security]
              Selander, G., Mattsson, J., Palombini, F., and L. Seitz,
              "Object Security for Constrained RESTful Environments
              (OSCORE)", draft-ietf-core-object-security-16 (work in
              progress), March 2019.

   [I-D.ietf-httpbis-bcp56bis]
              Nottingham, M., "Building Protocols with HTTP", draft-
              ietf-httpbis-bcp56bis-08 (work in progress), November
              2018.

   [I-D.ietf-tls-dtls-connection-id]
              Rescorla, E., Tschofenig, H., and T. Fossati, "Connection
              Identifiers for DTLS 1.2", draft-ietf-tls-dtls-connection-
              id-04 (work in progress), March 2019.

   [I-D.ietf-tls-dtls13]
              Rescorla, E., Tschofenig, H., and N. Modadugu, "The
              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", draft-ietf-tls-dtls13-30 (work in progress),
              November 2018.

   [I-D.ietf-tls-tls13]
              Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", draft-ietf-tls-tls13-28 (work in progress),
              March 2018.

   [I-D.mattsson-core-security-overhead]
              Mattsson, J., "Message Size Overhead of CoAP Security
              Protocols", draft-mattsson-core-security-overhead-02 (work
              in progress), November 2017.




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   [I-D.selander-ace-cose-ecdhe]
              Selander, G., Mattsson, J., and F. Palombini, "Ephemeral
              Diffie-Hellman Over COSE (EDHOC)", draft-selander-ace-
              cose-ecdhe-12 (work in progress), February 2019.

   [LwM2M]    Open Mobile Alliance, "Lightweight Machine to Machine
              Requirements", December 2017,
              <http://www.openmobilealliance.org/>.

   [Noise]    Perrin, T., "Noise Protocol Framework", October 2017,
              <http://noiseprotocol.org/>.

   [Norrell]  Norrell, ., "Use SSL/TLS within a different protocol with
              BIO pairs", 2016,
              <https://thekerneldiaries.com/2016/06/13/
              openssl-ssltls-within-a-different-protocol/>.

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

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

   [RFC5705]  Rescorla, E., "Keying Material Exporters for Transport
              Layer Security (TLS)", RFC 5705, DOI 10.17487/RFC5705,
              March 2010, <https://www.rfc-editor.org/info/rfc5705>.

   [RFC6265]  Barth, A., "HTTP State Management Mechanism", RFC 6265,
              DOI 10.17487/RFC6265, April 2011,
              <https://www.rfc-editor.org/info/rfc6265>.

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

   [RFC7230]  Fielding, R., Ed. and J. Reschke, Ed., "Hypertext Transfer
              Protocol (HTTP/1.1): Message Syntax and Routing",
              RFC 7230, DOI 10.17487/RFC7230, June 2014,
              <https://www.rfc-editor.org/info/rfc7230>.








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

   [RFC7540]  Belshe, M., Peon, R., and M. Thomson, Ed., "Hypertext
              Transfer Protocol Version 2 (HTTP/2)", RFC 7540,
              DOI 10.17487/RFC7540, May 2015,
              <https://www.rfc-editor.org/info/rfc7540>.

   [RFC7925]  Tschofenig, H., Ed. and T. Fossati, "Transport Layer
              Security (TLS) / Datagram Transport Layer Security (DTLS)
              Profiles for the Internet of Things", RFC 7925,
              DOI 10.17487/RFC7925, July 2016,
              <https://www.rfc-editor.org/info/rfc7925>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

   [Signal]   Open Whisper Systems, "Signal Protocol", 2016,
              <https://signal.org/>.

   [SSLEngine]
              Oracle, "SSLEngineSimpleDemo.java", 2004, <https://docs.or
              acle.com/javase/7/docs/technotes/guides/security/jsse/samp
              les/sslengine/SSLEngineSimpleDemo.java>.

   [ZigBee]   ZigBee Alliance, "ZigBee Specification", 2012,
              <http://www.zigbee.org>.

Appendix A.  Pseudo Code

   This appendix gives both C and Java pseudo code illustrating how to
   inject and extract raw TLS records from a TLS software stack.  Please
   not that this is illustrative, non-functional pseudo code that does
   not compile.  Functioning proof-of-concept code is available on the
   following public repository [[ EDITOR'S NOTE: Add the URL here ]].

A.1.  OpenSSL

   OpenSSL provides a set of Basic Input/Output (BIO) APIs that can be
   used to build a custom transport layer for TLS connections.  This
   appendix gives pseudo code on how BIO APIs could be used to build a
   client application that completes a TLS handshake and exchanges
   application data with a service.




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   char inbound[MAX];
   char outbound[MAX];
   int rx_bytes;
   SSL_CTX *ctx = SSL_CTX_new();
   SSL *ssl = SSL_new(ctx);

   // Create in-memory BIOs and plug in to the SSL session
   BOI* bio_in = BIO_new(BIO_s_mem());
   BOI* bio_out = BIO_new(BIO_s_mem());
   SSL_set_bio(ssl, bio_in, bio_out);

   // We are a client
   SSL_set_connect_state(ssl);

   // Loop through TLS flights until we are done
   do {
     // Calling SSL_do_handshake() will result in a full
     // TLS flight being written to the BIO buffer
     SSL_do_handshake(ssl);

     // Read the client flight that the TLS session
     // has written to memory
     BIO_read(bio_out, outbound, MAX);

     // POST the outbound bytes to the server using a suitable
     // function. Lets assume that the server response will be
     // written to the 'inbound' buffer
     num_bytes = postTlsRecords(outbound, inbound);

     // Write the server flight to the memory BIO so the TLS session
     // can read it. The next call to SSL_do_handshake() will handle
     // this received server flight
     BIO_write(bio_in, inbound, num_bytes);

   } while (!SSL_is_init_finished(ssl));

   // Send a message to the server. Calling SSL_write() will run the
   // plaintext through the TLS session and write the encrypted TLS
   // records to the BIO buffer
   SSL_write(ssl, "Hello World", strlen("Hello World"));

   // Read the TLS records from the BIO buffer and
   // POST them to the server
   BIO_read(bio_out, outbound, MAX);
   num_bytes = postTlsRecords(outbound, inbound);






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A.2.  Java JSSE

   The Java SSLEngine class "enables secure communications using
   protocols such as the Secure Sockets Layer (SSL) or IETF RFC 2246
   "Transport Layer Security" (TLS) protocols, but is transport
   independent".  This pseudo code illustrates how a server could use
   the SSLEngine class to handle an inbound client TLS flight and
   generate an outbound server TLS flight response.











































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   SSLEngine sslEngine = SSLContext.getDefault().createSSLEngine();
   sslEngine.setUseClientMode(false);
   sslEngine.beginHandshake();

   // Lets assume 'inbound' has been populated with
   // the Client 1st Flight
   ByteBuffer inbound;

   // 'outbound' will be populated with the
   // Server 1st Flight response
   ByteBuffer outbound;

   // SSLEngine handles one TLS Record per call to unwrap().
   // Loop until the engine is finished unwrapping.
   while (sslEngine.getHandshakeStatus() ==
          HandshakeStatus.NEED_UNWRAP) {
     SSLEngineResult res = sslEngine.unwrap(inbound, outbound);

     // SSLEngine may need additional tasks run
     if (res.getHandshakeStatus() == NEED_TASK) {
       Runnable run = sslEngine.getDelegatedTask();
       run.run();
     }
   }

   // The SSLEngine has now finished handling all inbound TLS Records.
   // Check if it wants to generate outbound TLS Records. SSLEngine
   // generates one TLS Record per call to wrap().
   // Loop until the engine is finished wrapping.
   while (sslEngine.getHandshakeStatus() ==
          HandshakeStatus.NEED_WRAP) {
     SSLEngineResult res = sslEngine.wrap(inbound, outbound);

     // SSLEngine may need additional tasks run
     if (res.getHandshakeStatus() == NEED_TASK) {
       Runnable run = sslEngine.getDelegatedTask();
       run.run();
     }
   }

   // outbound ByteBuffer now contains a complete server flight
   // containing multiple TLS Records
   // Rinse and repeat!








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Appendix B.  Example ATLS Handshake

   [[ EDITOR'S NOTE: For completeness, include a simple full TLS
   handshake showing the raw binary flights, along with the HTTP
   request/response/headers.  And also the raw hex TLS records showing
   protocol bits ]]

Authors' Addresses

   Owen Friel
   Cisco

   Email: ofriel@cisco.com


   Richard Barnes
   Cisco

   Email: rlb@ipv.sx


   Max Pritikin
   Cisco

   Email: pritikin@cisco.com


   Hannes Tschofenig
   ARM Limited

   Email: hannes.tschofenig@gmx.net


   Mark Baugher
   Consultant

   Email: mark@mbaugher.com














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