HTTP Transport for Trusted Execution Environment Provisioning: Agent-to- TAM Communication
draft-ietf-teep-otrp-over-http-07

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TEEP WG                                                        D. Thaler
Internet-Draft                                                 Microsoft
Intended status: Informational                             July 23, 2020
Expires: January 24, 2021

HTTP Transport for Trusted Execution Environment Provisioning: Agent-to-
                           TAM Communication
                   draft-ietf-teep-otrp-over-http-07

Abstract

   The Trusted Execution Environment Provisioning (TEEP) Protocol is
   used to manage code and configuration data in a Trusted Execution
   Environment (TEE).  This document specifies the HTTP transport for
   TEEP communication where a Trusted Application Manager (TAM) service
   is used to manage code and data in TEEs on devices that can initiate
   communication to the TAM.  An implementation of this document can (if
   desired) run outside of any TEE, but interacts with a TEEP
   implementation that runs inside a TEE.

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 January 24, 2021.

Copyright Notice

   Copyright (c) 2020 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

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   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  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   3.  TEEP Broker Models  . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Use of Abstract APIs  . . . . . . . . . . . . . . . . . .   5
   4.  Use of HTTP as a Transport  . . . . . . . . . . . . . . . . .   6
   5.  TEEP/HTTP Client Behavior . . . . . . . . . . . . . . . . . .   7
     5.1.  Receiving a request to install a new Trusted Application    7
       5.1.1.  Session Creation  . . . . . . . . . . . . . . . . . .   8
     5.2.  Getting a message buffer back from a TEEP Agent . . . . .   8
     5.3.  Receiving an HTTP response  . . . . . . . . . . . . . . .   9
     5.4.  Handling checks for policy changes  . . . . . . . . . . .   9
     5.5.  Error handling  . . . . . . . . . . . . . . . . . . . . .  10
   6.  TEEP/HTTP Server Behavior . . . . . . . . . . . . . . . . . .  10
     6.1.  Receiving an HTTP POST request  . . . . . . . . . . . . .  10
     6.2.  Getting an empty buffer back from the TAM . . . . . . . .  10
     6.3.  Getting a message buffer from the TAM . . . . . . . . . .  10
     6.4.  Error handling  . . . . . . . . . . . . . . . . . . . . .  11
   7.  Sample message flow . . . . . . . . . . . . . . . . . . . . .  11
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  13
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  13
     10.2.  Informative References . . . . . . . . . . . . . . . . .  14
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  15

1.  Introduction

   A Trusted Execution Environment (TEE) is an environment that enforces
   that any code within that environment cannot be tampered with, and
   that any data used by such code cannot be read or tampered with by
   any code outside that environment.  The Trusted Execution Environment
   Provisioning (TEEP) protocol is designed to provision authorized code
   and configuration into TEEs.

   To be secure against malware, a TEEP implementation (referred to as a
   TEEP "Agent" on the client side, and a "Trusted Application Manager
   (TAM)" on the server side) SHOULD themselves run inside a TEE,
   although a TAM running outside a TEE is also supported.  However, the
   transport for TEEP, along with the underlying TCP/IP stack, does not
   necessarily run inside a TEE.  This split allows the set of highly
   trusted code to be kept as small as possible, including allowing code

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   (e.g., TCP/IP or QUIC [I-D.ietf-quic-transport]) that only sees
   encrypted messages, to be kept out of the TEE.

   The TEEP specification [I-D.ietf-teep-protocol] (like its
   predecessors [I-D.ietf-teep-opentrustprotocol] and [GP-OTrP])
   describes the behavior of TEEP Agents and TAMs, but does not specify
   the details of the transport.  The purpose of this document is to
   provide such details.  That is, a TEEP-over-HTTP (TEEP/HTTP)
   implementation delivers messages up to a TEEP implementation, and
   accepts messages from the TEEP implementation to be sent over a
   network.  The TEEP-over-HTTP implementation can be implemented either
   outside a TEE (i.e., in a TEEP "Broker") or inside a TEE.

   There are two topological scenarios in which TEEP could be deployed:

   1.  TAMs are reachable on the Internet, and Agents are on networks
       that might be behind a firewall or stateful NAT, so that
       communication must be initiated by an Agent.  Thus, the Agent has
       an HTTP Client and the TAM has an HTTP Server.

   2.  Agents are reachable on the Internet, and TAMs are on networks
       that might be behind a firewall or stateful NAT, so that
       communication must be initiated by a TAM.  Thus, the Agent has an
       HTTP Server and the TAM has an HTTP Client.

   The remainder of this document focuses primarily on the first
   scenario as depicted in Figure 1, but some sections (Section 4 and
   Section 8) may apply to the second scenario as well.  A fuller
   discussion of the second scenario may be handled by a separate
   document.

      +------------------+           TEEP           +------------------+
      |    TEEP Agent    | <----------------------> |        TAM       |
      +------------------+                          +------------------+
               |                                              |
      +------------------+      TEEP-over-HTTP      +------------------+
      | TEEP/HTTP Client | <----------------------> | TEEP/HTTP Server |
      +------------------+                          +------------------+
               |                                              |
      +------------------+           HTTP           +------------------+
      |    HTTP Client   | <----------------------> |    HTTP Server   |
      +------------------+                          +------------------+

                   Figure 1: Agent-to-TAM Communication

   This document specifies the middle layer (TEEP-over-HTTP), whereas
   the top layer (TEEP) is specified in [I-D.ietf-teep-protocol].

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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.

   This document also uses various terms defined in
   [I-D.ietf-teep-architecture], including Trusted Execution Environment
   (TEE), Trusted Application (TA), Trusted Application Manager (TAM),
   TEEP Agent, TEEP Broker, and Rich Execution Environment (REE).

3.  TEEP Broker Models

   Section 6 of the TEEP architecture [I-D.ietf-teep-architecture]
   defines a TEEP "Broker" as being a component on the device, but
   outside the TEE, that facilitates communication with a TAM.  As
   depicted in Figure 2, there are multiple ways in which this can be
   implemented, with more or fewer layers being inside the TEE.  For
   example, in model A, the model with the smallest TEE footprint, only
   the TEEP implementation is inside the TEE, whereas the TEEP/HTTP
   implementation is in the TEEP Broker outside the TEE.

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                           Model:    A      B      C     ...

                                    TEE    TEE    TEE
        +----------------+           |      |      |
        |      TEEP      |     Agent |      |      | Agent
        | implementation |           |      |      |
        +----------------+           v      |      |
                 |                          |      |
        +----------------+           ^      |      |
        |    TEEP/HTTP   |    Broker |      |      |
        | implementation |           |      |      |
        +----------------+           |      v      |
                 |                   |             |
        +----------------+           |      ^      |
        |      HTTP      |           |      |      |
        | implementation |           |      |      |
        +----------------+           |      |      v
                 |                   |      |
        +----------------+           |      |      ^
        |   TCP or QUIC  |           |      |      | Broker
        | implementation |           |      |      |
        +----------------+           |      |      |
                                    REE    REE    REE

                       Figure 2: TEEP Broker Models

   In other models, additional layers are moved into the TEE, increasing
   the TEE footprint, with the Broker either containing or calling the
   topmost protocol layer outside of the TEE.  An implementation is free
   to choose any of these models, although model A is the one we will
   use in our examples.

   Passing information from an REE component to a TEE component is
   typically spoken of as being passed "in" to the TEE, and informaton
   passed in the opposite direction is spoken of as being passed "out".
   In the protocol layering sense, information is typically spoken of as
   being passed "up" or "down" the stack.  Since the layer at which
   information is passed in/out may vary by implementation, we will
   generally use "up" and "down" in this document.

3.1.  Use of Abstract APIs

   This document refers to various APIs between a TEEP implementation
   and a TEEP/HTTP implementation in the abstract, meaning the literal
   syntax and programming language are not specified, so that various
   concrete APIs can be designed (outside of the IETF) that are
   compliant.

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   Some TEE architectures (e.g., SGX) may support API calls both into
   and out of a TEE.  In other TEE architectures, there may be no calls
   out from a TEE, but merely data returned from calls into a TEE.  This
   document attempts to be agnostic as to the concrete API architecture
   for Broker/Agent communication.  Since in model A, the Broker/Agent
   communication is done at the layer between the TEEP and TEEP/HTTP
   implementations, and there may be some architectures that do not
   support calls out of the TEE (which would be downcalls from TEEP in
   model A), we will refer to passing information up to the TEEP
   implementation as API calls, but will simply refer to "passing data"
   back down from a TEEP implementation.  A concrete API might pass data
   back via an API downcall or via data returned from an API upcall.

   This document will also refer to passing "no" data back out of a TEEP
   implementation.  In a concrete API, this might be implemented by not
   making any downcall, or by returning 0 bytes from an upcall, for
   example.

4.  Use of HTTP as a Transport

   This document uses HTTP [I-D.ietf-httpbis-semantics] as a transport.
   For the motivation behind the HTTP recommendations in this document,
   see the discussion of HTTP as a transport in
   [I-D.ietf-httpbis-bcp56bis].

   Redirects MAY be automatically followed, and no additional request
   headers beyond those specified by HTTP need be modified or removed
   upon following such a redirect.  Cookies are not used.

   Content is not intended to be treated as active by browsers and so
   HTTP responses with content SHOULD have the following headers as
   explained in Section 4.12 of [I-D.ietf-httpbis-bcp56bis] (using the
   relevant TEEP content type defined in [I-D.ietf-teep-protocol]):

       Content-Type: application/teep+cbor
       Cache-Control: no-store
       X-Content-Type-Options: nosniff
       Content-Security-Policy: default-src 'none'
       Referrer-Policy: no-referrer

   The "Cache-control" header SHOULD be set to no-store to disable
   caching of any TEEP protocol messages by HTTP intermediaries.
   Otherwise, there is the risk of stale TEEP messages.

   Only the POST method is specified for TAM resources exposed over
   HTTP.  A URI of such a resource is referred to as a "TAM URI".  A TAM
   URI can be any HTTP(S) URI.  The URI to use is configured in a TEEP
   Agent via an out-of-band mechanism, as discussed in the next section.

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   It is strongly RECOMMENDED that implementations use HTTPS.  Although
   TEEP is protected end-to-end inside of HTTP, there is still value in
   using HTTPS for transport, since HTTPS can provide additional
   protections as discussed in Sections 4.4.2 and 6 of
   [I-D.ietf-httpbis-bcp56bis].

   However, there may be constrained nodes where code space is an issue.
   [RFC7925] provides TLS profiles that can be used in many constrained
   nodes, but in rare cases the most constrained nodes might need to use
   HTTP without a TLS stack, relying on the end-to-end security provided
   by the TEEP protocol.

   When HTTPS is used, TLS certificates MUST be checked according to
   [RFC2818], as well as [RFC6125] if PKIX certificates are used.  See
   [BCP195] for additional TLS recommendations and [RFC7925] for TLS
   recommandations related to IoT devices.

5.  TEEP/HTTP Client Behavior

5.1.  Receiving a request to install a new Trusted Application

   In some environments, an application installer can determine (e.g.,
   from an app manifest) that the application being installed or updated
   has a dependency on a given Trusted Application (TA) being available
   in a given type of TEE.  In such a case, it will notify a TEEP
   Broker, where the notification will contain the following:

   -  A unique identifier of the TA

   -  Optionally, any metadata to provide to the TEEP Agent.  This might
      include a TAM URI provided in the application manifest, for
      example.

   -  Optionally, any requirements that may affect the choice of TEE, if
      multiple are available to the TEEP Broker.

   When a TEEP Broker receives such a notification, it first identifies
   in an implementation-dependent way which TEE (if any) is most
   appropriate based on the constraints expressed.  If there is only one
   TEE, the choice is obvious.  Otherwise, the choice might be based on
   factors such as capabilities of available TEE(s) compared with TEE
   requirements in the notification.  Once the TEEP Broker picks a TEE,
   it passes the notification to the TEEP/HTTP Client for that TEE.

   The TEEP/HTTP Client then informs the TEEP Agent in that TEE by
   invoking an appropriate "RequestTA" API that identifies the TA needed
   and any other associated metadata.  The TEEP/HTTP Client need not

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   know whether the TEE already has such a TA installed or whether it is
   up to date.

   The TEEP Agent will either (a) pass no data back, (b) pass back a TAM
   URI to connect to, or (c) pass back a message buffer and TAM URI to
   send it to.  The TAM URI passed back may or may not be the same as
   the TAM URI, if any, provided by the TEEP/HTTP Client, depending on
   the TEEP Agent's configuration.  If they differ, the TEEP/HTTP Client
   MUST use the TAM URI passed back.

5.1.1.  Session Creation

   If no data is passed back, the TEEP/HTTP Client simply informs its
   caller (e.g., the application installer) of success.

   If the TEEP Agent passes back a TAM URI with no message buffer, the
   TEEP/HTTP Client attempts to create session state, then sends an
   HTTP(S) POST to the TAM URI with an Accept header with the TEEP media
   type requested, and an empty body.  The HTTP request is then
   associated with the TEEP/HTTP Client's session state.

   If the TEEP Agent instead passes back a TAM URI with a message
   buffer, the TEEP/HTTP Client attempts to create session state and
   handles the message buffer as specified in Section 5.2.

   Session state consists of:

   -  Any context (e.g., a handle) that identifies the API session with
      the TEEP Agent.

   -  Any context that identifies an HTTP request, if one is
      outstanding.  Initially, none exists.

5.2.  Getting a message buffer back from a TEEP Agent

   When a TEEP Agent passes a message buffer (and TAM URI) to a TEEP/
   HTTP Client, the TEEP/HTTP Client MUST do the following, using the
   TEEP/HTTP Client's session state associated with its API call to the
   TEEP Agent.

   The TEEP/HTTP Client sends an HTTP POST request to the TAM URI with
   Accept and Content-Type headers with the TEEP media type in use, and
   a body containing the TEEP message buffer provided by the TEEP Agent.
   The HTTP request is then associated with the TEEP/HTTP Client's
   session state.

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5.3.  Receiving an HTTP response

   When an HTTP response is received in response to a request associated
   with a given session state, the TEEP/HTTP Client MUST do the
   following.

   If the HTTP response body is empty, the TEEP/HTTP Client's task is
   complete, and it can delete its session state, and its task is done.

   If instead the HTTP response body is not empty, the TEEP/HTTP Client
   passes (e.g., using "ProcessTeepMessage" API as mentioned in
   Section 6.2.1 of [I-D.ietf-teep-architecture]) the response body up
   to the TEEP Agent associated with the session.  The TEEP Agent will
   then either pass no data back, or pass back a message buffer.

   If no data is passed back, the TEEP/HTTP Client's task is complete,
   and it can delete its session state, and inform its caller (e.g., the
   application installer) of success.

   If instead the TEEP Agent passes back a message buffer, the TEEP/HTTP
   Client handles the message buffer as specified in Section 5.2.

5.4.  Handling checks for policy changes

   An implementation MUST provide a way to periodically check for TAM
   policy changes, such as a Trusted Application needing to be deleted
   from a TEE because it is no longer permitted, or needing to be
   updated to a later version.  This can be done in any implementation-
   specific manner, such as:

   A) The TEEP/HTTP Client might call up to the TEEP Agent at an
   interval previously specified by the TEEP Agent.  This approach
   requires that the TEEP/HTTP Client be capable of running a periodic
   timer.

   B) The TEEP/HTTP Client might be informed when an existing TA is
   invoked, and call up to the TEEP Agent if more time has passed than
   was previously specified by the TEEP Agent.  This approach allows the
   device to go to sleep for a potentially long period of time.

   C) The TEEP/HTTP Client might be informed when any attestation
   attempt determines that the device is out of compliance, and call up
   to the TEEP Agent to remediate.

   The TEEP/HTTP Client informs the TEEP Agent by invoking an
   appropriate "RequestPolicyCheck" API.  The TEEP Agent will either (a)
   pass no data back, (b) pass back a TAM URI to connect to, or (c) pass

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   back a message buffer and TAM URI to send it to.  Processing then
   continues as specified in Section 5.1.1.

5.5.  Error handling

   If any local error occurs where the TEEP/HTTP Client cannot get a
   message buffer (empty or not) back from the TEEP Agent, the TEEP/HTTP
   Client deletes its session state, and informs its caller (e.g., the
   application installer) of a failure.

   If any HTTP request results in an HTTP error response or a lower
   layer error (e.g., network unreachable), the TEEP/HTTP Client calls
   the TEEP Agent's "ProcessError" API, and then deletes its session
   state and informs its caller of a failure.

6.  TEEP/HTTP Server Behavior

6.1.  Receiving an HTTP POST request

   If the TAM does not receive the appropriate Content-Type and Accept
   header fields, the TAM SHOULD fail the request, returning a 406 (not
   acceptable) response.  Otherwise, processing continues as follows.

   When an HTTP POST request is received with an empty body, the TEEP/
   HTTP Server invokes the TAM's "ProcessConnect" API.  The TAM will
   then pass back a (possibly empty) message buffer.

   When an HTTP POST request is received with a non-empty body, the
   TEEP/HTTP Server passes the request body to the TAM (e.g., using the
   "ProcessTeepMessage" API mentioned in [I-D.ietf-teep-architecture]).
   The TAM will then pass back a (possibly empty) message buffer.

6.2.  Getting an empty buffer back from the TAM

   If the TAM passes back an empty buffer, the TEEP/HTTP Server sends a
   successful (2xx) response with no body.  It SHOULD be status 204 (No
   Content).

6.3.  Getting a message buffer from the TAM

   If the TAM passes back a non-empty buffer, the TEEP/HTTP Server
   generates a successful (2xx) response with a Content-Type header with
   the appropriate media type in use, and with the message buffer as the
   body.

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6.4.  Error handling

   If any error occurs where the TEEP/HTTP Server cannot get a message
   buffer (empty or not) back from the TAM, the TEEP/HTTP Server
   generates an appropriate HTTP 5xx error response.

7.  Sample message flow

   The following shows a sample TEEP message flow that uses application/
   teep+cbor as the Content-Type.

   1.   An application installer determines (e.g., from an app manifest)
        that the application has a dependency on TA "X", and passes this
        notification to the TEEP Broker.  The TEEP Broker picks a TEE
        (e.g., the only one available) based on this notification, and
        passes the information to the TEEP/HTTP Cient for that TEE.

   2.   The TEEP/HTTP Client calls the TEEP Agent's "RequestTA" API,
        passing TA Needed = X.

   3.   The TEEP Agent finds that no such TA is already installed, but
        that it can be obtained from a given TAM.  The TEEP Agent passes
        the TAM URI (e.g., "https://example.com/tam") to the TEEP/HTTP
        Client.

   4.   The TEEP/HTTP Client sends an HTTP POST request to the TAM URI:

           POST /tam HTTP/1.1
           Host: example.com
           Accept: application/teep+cbor
           Content-Length: 0
           User-Agent: Foo/1.0

        where the TEEP/HTTP Client fills in an implementation-specific
        value in the User-Agent header.

   5.   On the TAM side, the TEEP/HTTP Server receives the HTTP POST
        request, and calls the TAM's "ProcessConnect" API.

   6.   The TAM generates a TEEP message (where typically QueryRequest
        is the first message) and passes it to the TEEP/HTTP Server.

   7.   The TEEP/HTTP Server sends an HTTP successful response with the
        TEEP message in the body:

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           HTTP/1.1 200 OK
           Content-Type: application/teep+cbor
           Content-Length: [length of TEEP message here]
           Server: Bar/2.2
           Cache-Control: no-store
           X-Content-Type-Options: nosniff
           Content-Security-Policy: default-src 'none'
           Referrer-Policy: no-referrer

           [TEEP message here]

        where the TEEP/HTTP Server fills in an implementation-specific
        value in the Server header.

   8.   Back on the TEEP Agent side, the TEEP/HTTP Client gets the HTTP
        response, extracts the TEEP message and pass it up to the TEEP
        Agent.

   9.   The TEEP Agent processes the TEEP message, and generates a TEEP
        response (e.g., QueryResponse) which it passes back to the TEEP/
        HTTP Client.

   10.  The TEEP/HTTP Client gets the TEEP message buffer and sends an
        HTTP POST request to the TAM URI, with the TEEP message in the
        body:

          POST /tam HTTP/1.1
          Host: example.com
          Accept: application/teep+cbor
          Content-Type: application/teep+cbor
          Content-Length: [length of TEEP message here]
          User-Agent: Foo/1.0

          [TEEP message here]

   11.  The TEEP/HTTP Server receives the HTTP POST request, and passes
        the payload up to the TAM.

   12.  Steps 6-11 are then repeated until the TAM passes no data back
        to the TEEP/HTTP Server in step 6.

   13.  The TEEP/HTTP Server sends an HTTP successful response with no
        body:

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          HTTP/1.1 204 No Content
          Server: Bar/2.2

   14.  The TEEP/HTTP Client deletes its session state.

8.  Security Considerations

   Section 4 discussed security recommendations for HTTPS transport of
   TEEP messages.  See Section 6 of [I-D.ietf-httpbis-bcp56bis] for
   additional discussion of HTTP(S) security considerations.

9.  IANA Considerations

   This document has no actions for IANA.

10.  References

10.1.  Normative References

   [BCP195]   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>.

   [I-D.ietf-httpbis-semantics]
              Fielding, R., Nottingham, M., and J. Reschke, "HTTP
              Semantics", draft-ietf-httpbis-semantics-10 (work in
              progress), July 2020.

   [I-D.ietf-teep-protocol]
              Tschofenig, H., Pei, M., Wheeler, D., Thaler, D., and A.
              Tsukamoto, "Trusted Execution Environment Provisioning
              (TEEP) Protocol", draft-ietf-teep-protocol-03 (work in
              progress), July 2020.

   [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>.

   [RFC2818]  Rescorla, E., "HTTP Over TLS", RFC 2818,
              DOI 10.17487/RFC2818, May 2000, <https://www.rfc-
              editor.org/info/rfc2818>.

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   [RFC6125]  Saint-Andre, P. and J. Hodges, "Representation and
              Verification of Domain-Based Application Service Identity
              within Internet Public Key Infrastructure Using X.509
              (PKIX) Certificates in the Context of Transport Layer
              Security (TLS)", RFC 6125, DOI 10.17487/RFC6125, March
              2011, <https://www.rfc-editor.org/info/rfc6125>.

   [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>.

10.2.  Informative References

   [GP-OTrP]  Global Platform, "TEE Management Framework: Open Trust
              Protocol (OTrP) Profile Version 1.0", Global
              Platform GPD_SPE_123, May 2019,
              <https://globalplatform.org/specs-library/tee-management-
              framework-open-trust-protocol/>.

   [I-D.ietf-httpbis-bcp56bis]
              Nottingham, M., "Building Protocols with HTTP", draft-
              ietf-httpbis-bcp56bis-09 (work in progress), November
              2019.

   [I-D.ietf-quic-transport]
              Iyengar, J. and M. Thomson, "QUIC: A UDP-Based Multiplexed
              and Secure Transport", draft-ietf-quic-transport-29 (work
              in progress), June 2020.

   [I-D.ietf-teep-architecture]
              Pei, M., Tschofenig, H., Thaler, D., and D. Wheeler,
              "Trusted Execution Environment Provisioning (TEEP)
              Architecture", draft-ietf-teep-architecture-12 (work in
              progress), July 2020.

   [I-D.ietf-teep-opentrustprotocol]
              Pei, M., Atyeo, A., Cook, N., Yoo, M., and H. Tschofenig,
              "The Open Trust Protocol (OTrP)", draft-ietf-teep-
              opentrustprotocol-03 (work in progress), May 2019.

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Author's Address

   Dave Thaler
   Microsoft

   EMail: dthaler@microsoft.com

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