TEEP WG                                                        D. Thaler
Internet-Draft                                                 Microsoft
Intended status: Informational                             July 08,                          October 22, 2019
Expires: January 9, April 24, 2020

HTTP Transport for the Open Trust Protocol (OTrP)
                   draft-ietf-teep-otrp-over-http-01 Trusted Execution Environment Provisioning: Agent-to-
                           TAM Communication
                   draft-ietf-teep-otrp-over-http-02

Abstract

   This document specifies the HTTP transport for the

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

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   This Internet-Draft will expire on January 9, April 24, 2020.

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   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3   4
   3.  TEEP Broker Models  . . . . . . . . . . . . . . . . . . . . .   4
     3.1.  Use of Abstract APIs  . . . . . . . . . . . . . . . . . . . .   3   5
   4.  Use of HTTP as a Transport  . . . . . . . . . . . . . . . . .   3   6
   5.  TEEP Broker  OTrP/HTTP Client Behavior . . . . . . . . . . . . . . . . . . . .   4   7
     5.1.  Receiving a request to install a new Trusted Application    4    7
       5.1.1.  Session Creation  . . . . . . . . . . . . . . . . . .   5   7
     5.2.  Getting a message buffer back from an TEEP Agent  . . . .   5 OTrP implementation   8
     5.3.  Receiving an HTTP response  . . . . . . . . . . . . . . .   6   8
     5.4.  Handling checks for policy changes  . . . . . . . . . . .   6   9
     5.5.  Error handling  . . . . . . . . . . . . . . . . . . . . .   7   9
   6.  TAM Broker  OTrP/HTTP Server Behavior . . . . . . . . . . . . . . . . . . . . .   7  10
     6.1.  Receiving an HTTP POST request  . . . . . . . . . . . . .   7  10
     6.2.  Getting an empty buffer back from the TAM . . . . . . . .   7 OTrP implementation  10
     6.3.  Getting a message buffer from the TAM . . . . . . . . OTrP implementation . .   7  10
     6.4.  Error handling  . . . . . . . . . . . . . . . . . . . . .   7  10
   7.  Sample message flow . . . . . . . . . . . . . . . . . . . . .   7  10
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .   9  12
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  10  12
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  10  13
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  10  13
     10.2.  Informative References . . . . . . . . . . . . . . . . .  10  13
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . .  11  14

1.  Introduction

   Trusted Execution Environments (TEEs), including environments based
   on Intel SGX, ARM TrustZone, Secure Elements, and others, enforce
   that only authorized code can execute within the TEE, and any memory
   used by such code is protected against tampering or disclosure
   outside the TEE.  The Open Trust Protocol (OTrP) is designed to
   provision authorized code and configuration into TEEs.

   To be secure against malware, an OTrP implementation (referred to as
   an OTrP
   a TEEP "Agent" on the client side, and a "Trusted Application Manager
   (TAM)" on the server side) must themselves run inside a TEE.
   However, the transport for OTrP, along with typical networking
   stacks, need 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 (e.g., TCP/IP) that only sees encrypted messages to be
   kept out of the TEE.

   The OTrP specification [I-D.ietf-teep-opentrustprotocol] ([I-D.ietf-teep-opentrustprotocol] or
   [I-D.tschofenig-teep-otrp-v2]) describes the behavior of TEEP Agents
   and TAMs, but does not specify the details of the transport, an implementation of which is referred to
   as a "Broker". transport.  The
   purpose of this document is to provide such details.  That is, the HTTP transport for an
   OTrP is implemented in a
   Broker (typically outside a TEE) that over HTTP (OTrP/HTTP) implementation delivers messages up to an
   OTrP implementation, and accepts messages from the OTrP
   implementation to be sent over a network.

2.  Terminology  The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" OTrP over HTTP
   implementation can be implemented either outside a TEE (i.e., in this document a
   TEEP "Broker") or inside a TEE.

   There are to be interpreted as described two topological scenarios in BCP
   14 [RFC2119] [RFC8174] when, which OTrP could be deployed:

   1.  TAMs are reachable on the Internet, and only when, they appear in all
   capitals, as shown here.

   This document also uses various terms defined in Agents are on networks
       that might be behind a firewall, 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, 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.

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

                   Figure 1: Agent-to-TAM Communication

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, and TEEP Broker. 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 OTrP implementation is inside the TEE, whereas the OTrP/HTTP
   implementation is in the TEEP Broker outside the TEE.

                           Model:    A      B      C     ...

                                    TEE    TEE    TEE
        +----------------+           |      |      |
        |      OTrP      |     Agent |      |      | Agent
        | implementation |           |      |      |
        +----------------+           v      |      |
                 |                          |      |
        +----------------+           ^      |      |
        |    OTrP/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 Broker and an OTrP implementation
   and an OTrP/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.

   It is common in some

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

   This document will also refer to passing "no" data back out of a TA.
   In an OCALL-based architecture,
   OTrP implementation.  In a concrete API, this might be implemented by
   not making any such call.  In a return-based architecture, this might be
   implemented downcall, or by returning 0 bytes. 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.
   When not called out explicitly in this document, all implementation
   recommendations in [I-D.ietf-httpbis-bcp56bis] apply to use of HTTP
   by OTrP.

   Redirects MAY be automatically followed, and no additional request
   headers beyond those specified by HTTP need be modified or removed
   upon a following such a redirect.

   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] (replacing
   the content type with the relevant OTrP content type per the OTrP
   specification):

       Content-Type: <content type>
       Cache-Control: no-store
       X-Content-Type-Options: nosniff
       Content-Security-Policy: default-src 'none'
       Referrer-Policy: no-referrer

   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 an a TEEP
   Agent via an out-of-band mechanism, as discussed in the next section.

   When HTTPS is used, TLS certificates MUST be checked according to
   [RFC2818].

5.  TEEP Broker  OTrP/HTTP Client Behavior

5.1.  Receiving a request to install a new Trusted Application

   When the TEEP Broker receives a notification

   In some environments, an application installer can determine (e.g.,
   from an
   application installer) app manifest) that an the application being installed or updated
   has a dependency on a given Trusted Application (TA) being available
   in a given type of
   TEE, 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 pass provide to the TEEP Agent. OTrP implementation.
      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 such a notification is received, the 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.

   The  Once the TEEP Broker picks a TEE,
   it passes the notification to the OTrP/HTTP Cient for that TEE.

   The OTrP/HTTP Client then informs the TEEP Agent OTrP implementation in that TEE
   by invoking an appropriate "RequestTA" API that identifies the TA
   needed and any other associated metadata.  The TEEP Broker OTrP/HTTP Client need
   not know whether the TEE already has such a TA installed or whether
   it is up to date.

   The TEEP Agent OTrP implementation 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 broker, OTrP/HTTP Client,
   depending on the TEEP
   Agent's OTrP implementation's configuration.  If they
   differ, the TEEP Broker OTrP/HTTP Client MUST use the TAM URI passed back.

5.1.1.  Session Creation

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

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

   If the TEEP Agent OTrP implementation instead passes back a TAM URI with a
   message buffer, the TEEP Broker OTrP/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. OTrP implementation.

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

5.2.  Getting a message buffer back from an TEEP Agent OTrP implementation

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

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

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 Broker OTrP/HTTP Client MUST do the
   following.

   If the HTTP response body is empty, the TEEP Broker's OTrP/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 Broker OTrP/HTTP Client
   calls a "ProcessOTrPMessage" API (Section 6.2 of
   [I-D.ietf-teep-opentrustprotocol]) to pass the response body up to
   the
   TEEP Agent OTrP implementation associated with the session.  The TEEP Agent OTrP
   implementation will then either pass no data back, or pass pack back a
   message buffer.

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

   If instead the TEEP Agent OTrP implementation passes back a message buffer, the TEEP
   Broker
   OTrP/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 OTrP
   policy changes.  This can be done in any implementation-specific
   manner, such as:

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

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

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

   The TEEP Broker OTrP/HTTP Client informs the TEEP Agent OTrP implementation by invoking an
   appropriate "RequestPolicyCheck" API.  The TEEP Agent OTrP implementation 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.
   Processing then continues as specified in Section 5.1.1.

5.5.  Error handling

   If any local error occurs where the TEEP Broker OTrP/HTTP Client cannot get a
   message buffer (empty or not) back from the TEEP Agent, OTrP implementation, the TEEP Broker
   OTrP/HTTP Client deletes its session state, and informs its client 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 Broker OTrP/HTTP Client calls
   the
   TEEP Agent's OTrP implementation's "ProcessError" API, and then deletes its
   session state and informs its client caller of a failure.

6.  TAM Broker  OTrP/HTTP Server Behavior

6.1.  Receiving an HTTP POST request

   When an HTTP POST request is received with an empty body, the TAM
   Broker OTrP/
   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 TAM
   Broker
   OTrP/HTTP Server calls the TAM's "ProcessOTrPMessage" API to pass it
   the request body.  The TAM will then pass back a (possibly empty)
   message buffer.

6.2.  Getting an empty buffer back from the TAM OTrP implementation

   If the TAM OTrP implementation passes back an empty buffer, the TAM Broker OTrP/HTTP
   Server sends a successful (2xx) response with no body.

6.3.  Getting a message buffer from the TAM OTrP implementation

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

6.4.  Error handling

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

7.  Sample message flow

   The following shows a sample OTrP message flow that uses application/
   otrp+json 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 an TEEP
        Agent a TEE
        (e.g., the only one available) based on this notification. notification, and
        passes the information to the OTrP/HTTP Cient for that TEE.

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

   3.   The TEEP Agent OTrP implementation 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
        Broker. OTrP/HTTP Client.  (If the TEEP Agent OTrP implementation already
        had a cached TAM certificate that it trusts, it could skip to
        step 9 instead and generate a GetDeviceStateResponse.)

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

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

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

   6.   The TAM OTrP implementation generates an OTrP message (typically (where
        typically GetDeviceStateRequest is the first message) and passes
        it to the
        TAM Broker. OTrP/HTTP Server.

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

           HTTP/1.1 200 OK
           Content-Type: application/otrp+json
           Content-Length: [length of OTrP 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

           [OTrP message here]

   8.   The   Back on the TEEP Broker Agent side, the OTrP/HTTP Client gets the HTTP
        response, extracts the OTrP message and calls the TEEP Agent's OTrP
        implementation's "ProcessOTrPMessage" API to pass it the
        message.

   9.   The TEEP Agent OTrP implementation processes the OTrP message, and
        generates an OTrP response (e.g., GetDeviceStateResponse) which
        it passes back to the TEEP Broker. OTrP/HTTP Client.

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

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

          [OTrP message here]

   11.  The TAM Broker OTrP/HTTP Server receives the HTTP POST request, and calls
        the
        TAM's OTrP implementation's "ProcessOTrPMessage" API.

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

   13.  The TAM Broker OTrP/HTTP Server sends an HTTP successful response with no
        body:

          HTTP/1.1 204 No Content
          Server: Bar/2.2

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

8.  Security Considerations

   Although OTrP 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 Section 6 of
   [I-D.ietf-httpbis-bcp56bis].  As such, Broker OTrP/HTTP implementations MUST
   support HTTPS.  The choice of HTTP vs HTTPS at runtime is up to
   policy, where an administrator configures the TAM URI to be used, but
   it is expected that real deployments will always use HTTPS TAM URIs.

9.  IANA Considerations

   This document has no actions for IANA.

10.  References

10.1.  Normative References

   [I-D.ietf-httpbis-semantics]
              Fielding, R., Nottingham, M., and J. Reschke, "HTTP
              Semantics", draft-ietf-httpbis-semantics-05 (work in
              progress), July 2019.

   [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.

   [I-D.tschofenig-teep-otrp-v2]
              Pei, M., Tschofenig, H., and D. Wheeler, "The Open Trust
              Protocol (OTrP) v2", draft-tschofenig-teep-otrp-v2-00
              (work in progress), July 2019.

   [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>.

   [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

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

   [I-D.ietf-teep-architecture]
              Pei, M., Tschofenig, H., Wheeler, D., Atyeo, A., and D.
              Liu, "Trusted Execution Environment Provisioning (TEEP)
              Architecture", draft-ietf-teep-architecture-02 draft-ietf-teep-architecture-03 (work in
              progress), March July 2019.

Author's Address

   Dave Thaler
   Microsoft

   EMail: dthaler@microsoft.com