drip                                                             S. Card
Internet-Draft                                           A. Wiethuechter
Intended status: Informational                             AX Enterprize
Expires: 28 April 14 May 2022                                        R. Moskowitz
                                                          HTT Consulting
                                                        S. Zhao (Editor)
                                                               A. Gurtov
                                                    Linköping University
                                                         25 October
                                                        10 November 2021

        Drone Remote Identification Protocol (DRIP) Architecture


   This document describes an architecture for protocols and services to
   support Unmanned Aircraft System Remote Identification and tracking
   (UAS RID), plus RID-related communications.  This architecture
   adheres to the requirements listed in the DRIP Requirements document.

Status of This Memo

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   This Internet-Draft will expire on 28 April 14 May 2022.

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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Overview of Unmanned Aircraft System (UAS) Remote ID (RID)
           and Standardization . . . . . . . . . . . . . . . . . . .   3
     1.2.  Overview of Types of UAS Remote ID  . . . . . . . . . . .   4
       1.2.1.  Broadcast RID . . . . . . . . . . . . . . . . . . . .   4
       1.2.2.  Network RID . . . . . . . . . . . . . . . . . . . . .   5
     1.3.  Overview of USS Interoperability  . . . . . . . . . . . .   7
     1.4.  Overview of DRIP Architecture . . . . . . . . . . . . . .   8
   2.  Terms and Definitions . . . . . . . . . . . . . . . . . . . .   9  10
     2.1.  Architecture Terminology  . . . . . . . . . . . . . . . .   9  10
     2.2.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   9  10
     2.3.  Additional Definitions  . . . . . . . . . . . . . . . . .  10
   3.  Claims, Assertions, Attestations, and Certificates  . . . . .  10
   4.  HHIT as the DRIP Entity Identifier  . . . . . . . . . . . . .  11
     4.1.  UAS Remote Identifiers Problem Space  . . . . . . . . . .  11  12
     4.2.  HHIT as A Trustworthy DRIP Entity Identifier  . . . . . .  12
     4.3.  HHIT for DRIP Identifier Registration and Lookup  . . . .  13  14
     4.4.  HHIT as a Cryptographic Identifier  . . . . . . . . . . .  13  14
   5.  DRIP Identifier Registration and Registries . . . . . . . . .  14
     5.1.  Public Information Registry . . . . . . . . . . . . . . .  14  15
       5.1.1.  Background  . . . . . . . . . . . . . . . . . . . . .  14  15
       5.1.2.  DNS as the Public DRIP Identifier Registry  . . . . .  14  15
     5.2.  Private Information Registry  . . . . . . . . . . . . . .  14  15
       5.2.1.  Background  . . . . . . . . . . . . . . . . . . . . .  14  15
       5.2.2.  EPP and RDAP as the Private DRIP Identifier
               Registry  . . . . . . . . . . . . . . . . . . . . . .  15  16
       5.2.3.  Alternative Private DRIP Registry methods . . . . . .  15  16
   6.  DRIP Identifier Trust . . . . . . . . . . . . . . . . . . . .  15  16
   7.  Harvesting Broadcast Remote ID messages for UTM Inclusion . .  16  17
     7.1.  The CS-RID Finder . . . . . . . . . . . . . . . . . . . .  16  18
     7.2.  The CS-RID SDSP . . . . . . . . . . . . . . . . . . . . .  17  18
   8.  DRIP Contact  . . . . . . . . . . . . . . . . . . . . . . . .  17  18
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  18  19
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  18  19
   11. Privacy & Transparency Considerations . . . . . . . . . . . .  19
   12. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19  20
     12.1.  Normative References . . . . . . . . . . . . . . . . . .  19  20
     12.2.  Informative References . . . . . . . . . . . . . . . . .  19  20
   Appendix A.  Overview of Unmanned Aircraft Systems (UAS) Traffic
           Management (UTM)  . . . . . . . . . . . . . . . . . . . .  22  23
     A.1.  Operation Concept . . . . . . . . . . . . . . . . . . . .  23
     A.2.  UAS Service Supplier (USS)  . . . . . . . . . . . . . . .  23  24
     A.3.  UTM Use Cases for UAS Operations  . . . . . . . . . . . .  24
   Appendix B.  Automatic Dependent Surveillance Broadcast
           (ADS-B) . . . . . . . . . . . . . . . . . . . . . . . . .  24  25
   Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . .  25
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  25

1.  Introduction

   This document describes an architecture for protocols and services to
   support Unmanned Aircraft System Remote Identification and tracking
   (UAS RID), plus RID-related communications.  The architecture takes
   into account both current (including proposed) regulations and non-
   IETF technical standards.

   The architecture adheres to the requirements listed in the DRIP
   Requirements document [I-D.ietf-drip-reqs].  The requirements
   document provides an extended introduction to the problem space and
   use cases.

1.1.  Overview of Unmanned Aircraft System (UAS) Remote ID (RID) and

   UAS Remote Identification (RID) is an application enabler for a UAS
   to be identified by Unmanned Aircraft Systems Traffic Management
   (UTM) and UAS Service Supplier (USS) (Appendix A) or third parties
   entities such as law enforcement.  Many considerations (e.g., safety)
   dictate that UAS be remotely identifiable.

   Civil Aviation Authorities (CAAs) worldwide are mandating UAS RID.
   CAAs currently promulgate performance-based regulations that do not
   specify techniques, but rather cite industry consensus technical
   standards as acceptable means of compliance.

   Federal Aviation Administration (FAA)

      The FAA published a Notice of Proposed Rule Making [NPRM] in 2019
      and thereafter published the a "Final Rule" in 2021 [FAA_RID].
      Under the Final Rule, [FAA_RID],
      imposing requirements on UAS manufacturers, producers, manufacturers and operators, both
      commercial and recreational UAS drone pilots must comply with the final
      rule's requirements.

         In FAA's final rule, it is recreational.  The rule clearly stated states that
      Automatic Dependent Surveillance Broadcast (ADS-B) Out and
         can not cannot be used to serve satisfy the purpose of an remote
         identification.  More details about ADS-B can be found in RID requirements on UAS
      to which the rule applies (see Appendix B. B).

   European Union Aviation Safety Agency (EASA)
      The EASA has published the a [Delegated] regulation in 2019 to provide
      regulations for unmanned aircraft systems (UAS) and imposing
      requirements on third-
      country operators of UAS manufacturers and followed with implementation third-country operators,
      including but not limited to RID requirements.  The EASA also
      published in 2019 an [Implementing] regulation on the laying down
      detailed rules and procedures for the operation of
      unmanned aircraft Regulations [Implementing]. UAS operations and operating

   American Society for Testing and Materials (ASTM)

      ASTM International, Technical Committee F38 (UAS), Subcommittee
      F38.02 (Aircraft Operations), Work Item WK65041, developed the
      ASTM [F3411-19] [F3411] Standard Specification for Remote ID and Tracking.

      ASTM defines one set of RID information and two means, MAC-layer
      broadcast and IP-layer network, of communicating it.  If an UAS
      uses both communication methods, the same information must be
      provided via both means.  [F3411-19]  [F3411] is cited by FAA in its RID final
      rule [FAA_RID] as "a potential means of compliance" to a Remote ID

   The 3rd Generation Partnership Project (3GPP)

      With release 16, the 3GPP completed the UAS RID requirement study
      [TS-22.825] and proposed a set of use cases in the mobile network
      and the services that can be offered based on RID.  Release 17
      specification focuses on enhanced UAS service requirements and
      provides the protocol and application architecture support that
      will be applicable for both 4G and 5G networks.

1.2.  Overview of Types of UAS Remote ID

1.2.1.  Broadcast RID


   [F3411] defines a set of RID messages for direct, one-way, broadcast
   transmissions from the UA over Bluetooth or Wi-Fi.  These are
   currently defined as MAC-Layer messages.  Internet (or other Wide
   Area Network) connectivity is only needed for UAS registry
   information lookup by Observers using the directly received UAS ID.
   Broadcast RID should be functionally usable in situations with no
   Internet connectivity.

   The minimum Broadcast RID data flow is illustrated in Figure 1.

                     | Unmanned Aircraft (UA) |
                                 | app messages directly over
                                 | one-way RF data link (no IP)
              | Observer's device (e.g., smartphone) |

                                  Figure 1

   Broadcast RID provides information only about UA (unmanned aircraft) unmanned aircraft (UA)
   within direct RF LOS, typically similar to visual Light-Of-Sight
   (LOS), with a range on the order of up to approximately 1 km.  This information may
   be 'harvested' from received broadcasts and made available via the
   Internet (see Section 7),
   Internet, enabling surveillance of areas too large for local direct
   visual observation or direct RF link based ID (e.g.
   around airports, public gatherings, and other sensitive areas). (see Section 7).

1.2.2.  Network RID


   [F3411], using the same data dictionary that is the basis of
   Broadcast RID messages, defines a Network Remote ID (Net-RID) data dictionary and Identification (Net-
   RID) data flow for Internet based as follows.

   *  The information delivery.  This data flow to be reported via RID is
   emitted from an generated by the UAS
      (typically some by the UA and some by the GCS, e.g. their
      respective GNSS derived locations).

   *  The information is sent by the UAS (UA or GCS) via unspecified
      means (but at least in part over
   the Internet) to a Net-RID the cognizant Network Remote Identification Service
      Provider (Net-RID SP).  A SP), typically the USS under which the UAS is
      operating if participating in UTM.

   *  The Net-RID SP provides publishes via the RID data to Net-RID Display Providers (Net-RID DP).
   It is Discovery and Synchronization
      Service (DSS) over the Net-RID DP Internet that responds to queries from it has operations in various
      4-D airspace volumes, describing the volumes but not the

   *  An Observer's Net-RID
   device (expected typically, device, expected typically but not specified exclusively, to be web-
   based) specifying airspace volumes of interest.  Net-RID depends upon
   internet connectivity to fulfill Observer's
      web based, queries to the Net-RID
   DP.  The summary of Net-RID data flows work as follows:

   *  The UA's RID data is generated from a UAS which consists of UAs
      and GCSs.

   *  The GCS or UA (e.g.  BVLOS and autonomous operation) provides the
      UA's RID data to Network Remote Identification Display
      Provider (Net-RID DP), typically also a Net-RID SP via USS, about any operations
      in a secure internet connection. specific 4-D airspace volume.

   *  Using fully specified web based methods over the Internet, the
      Net-RID DP as a queries all Net-RID SP subscriber satisfies that have operations in volumes
      intersecting that of the Observer's query request via a secure internet connection. for details on all such

   *  The Net-RID DP aggregates information received from all such Net-
      RID SP and responds to the Observer's query.

   The minimum Net-RID data flow is illustrated in Figure 2:

          +-------------+     ******************
          |     UA      |     *    Internet    *
          +--o-------o--+     *                *
             |       |        *                *
             |       |        *                *     +------------+
             |       '--------*--(+)-----------*-----o            |
             |                *   |            *     |            |
             |       .--------*--(+)-----------*-----o NET-RID SP |
             |       |        *                *     |            |
             |       |        *         .------*-----o            |
             |       |        *         |      *     +------------+
             |       |        *         |      *
             |       |        *         |      *     +------------+
             |       |        *         '------*-----o            |
             |       |        *                *     | NET-RID DP |
             |       |        *         .------*-----o            |
             |       |        *         |      *     +------------+
             |       |        *         |      *
             |       |        *         |      *     +------------+
          +--o-------o--+     *         '------*-----o Observer's |
          |     GCS     |     *                *     | Device     |
          +-------------+     ******************     +------------+

                                  Figure 2

   Command and Control (C2) must flow from the GCS to the UA via some
   path, currently (in the year of 2021) typically a direct RF link, but
   with increasing beyond Visual Line of Sight (BVLOS) operations
   expected often to be wireless links at either end with the Internet

   Telemetry (at least UA's position and heading) flows from the UA to
   the GCS via some path, typically the reverse of the C2 path.  Thus,
   RID information pertaining to both the GCS and the UA can be sent, by
   whichever has Internet connectivity, to the Net-RID SP, typically the
   USS managing the UAS operation.

   The Net-RID SP forwards RID information via the Internet to
   subscribed Net-RID DP, typically USS.  Subscribed Net-RID DP forward
   RID information via the Internet to subscribed Observer devices.
   Regulations require and [F3411-19] [F3411] describes RID data elements that must
   be transported end-to-end from the UAS to the subscribed Observer


   [F3411] prescribes the protocols between the Net-RID SP, Net-RID DP,
   and the Discovery and Synchronization Service (DSS).  It also
   prescribes data elements (in JSON) between Observer and USS. Net-RID DP.
   DRIP could address standardization of secure protocols between the UA
   and GCS (over direct wireless and Internet connection), between the
   UAS and the Net-RID SP, and/or between the Net-RID DP and Observer

         Informative note: Neither link layer protocols nor the use of
         links (e.g., the link often existing between the GCS and the
         UA) for any purpose other than carriage of RID information is
         in the scope of [F3411-19] [F3411] Network RID.

1.3.  Overview of USS Interoperability

   With Net-RID, there is direct communication between the UAS and its
   USS.  With Broadcast-RID and UTM, the UAS Operator has either pre-
   filed a 4D space volume for USS operational knowledge and/or
   Observers can be providing information about observed UA to a
   Surveillance Supplemental Data Service Provider (SDSP).  USS exchange
   information via a Discovery and Synchronization Service (DSS) so all
   USS collectively have knowledge about all activities in a 4D

   The interactions among Observer, UA, and USS are shown in Figure 3.

                               | Observer |
                              /            \
                             /              \
                      +------+               +------+
                      | UAS1 |               | UAS2 |
                      +------+               +------+
                             \              /
                              \            /
                               | Internet |
                              /            \
                             /              \
                       +------+           +------+
                       | USS1 | <-------> | USS2 |
                       +------+           +------+
                                \         /
                                 \       /
                                 |  DSS |

                                  Figure 3

   Editor-note-1: (Stu) re-draw this figure and propose text.  Then
   double check the langauge in Editor-note-8

1.4.  Overview of DRIP Architecture

   The requirements document [I-D.ietf-drip-reqs] provides an extended
   introduction to the problem space and use cases.  Only

   Figure 4 illustrates a brief summary of that introduction is restated here as context, with
   reference to the general UAS RID usage
   scenarios shown in Figure 4. DRIP.

         General      x                           x     Public
         Public     xxxxx                       xxxxx   Safety
         Observer     x                           x     Observer
                      x                           x
                     x x ---------+  +---------- x x
                    x   x         |  |          x   x
                                  |  |
            UA1 x x               |  |  +------------ x x UA2
               xxxxx              |  |  |            xxxxx
                  |               +  +  +              |
                  |            xxxxxxxxxx              |
                  |           x          x             |
                  +----------+x Internet x+------------+
       UA1        |           x          x             |       UA1
      Pilot     x |            xxxxxxxxxx              | x    Pilot
     Operator  xxxxx              + + +                xxxxx Operator
      GCS1      x                 | | |                  x    GCS2
                x                 | | |                  x
               x x                | | |                 x x
              x   x               | | |                x   x
                                  | | |
                +----------+      | | |       +----------+
                |          |------+ | +-------|          |
                | Public   |        |         | Private  |
                | Registry |     +-----+      | Registry |
                |          |     | DNS |      |          |
                +----------+     +-----+      +----------+

                                  Figure 4

   Editor-note-2: Stu: replace figure 4

   DRIP is meant to leverage existing Internet resources (standard
   protocols, services, infrastructures, and business models) to meet
   UAS RID and closely related needs.  DRIP will specify how to apply
   IETF standards, complementing [F3411-19] [F3411] and other external standards,
   to satisfy UAS RID requirements.

   This document outlines the DRIP architecture in the context of the
   UAS RID architecture.  This includes presenting the gaps between the
   CAAs' Concepts of Operations and [F3411-19] [F3411] as it relates to the use of
   Internet technologies and UA direct RF communications.  Issues
   include, but are not limited to:


         o  Design of trustworthy remote ID and trust in RID messages identifiers (Section 4) 4).

      -  Mechanisms to leverage Domain Name System (DNS: (DNS [RFC1034]),
         Extensible Provisioning Protocol (EPP [RFC5731]) and
         Registration Data Access Protocol (RDAP) ([RFC7482]) for
         publishing public and private information (see Section 5.1 and
         Section 5.2).

      -  Specific authentication methods and message payload formats to
         enable verification that Broadcast RID messages were sent by
         the claimed sender (Section 6) and that sender is in the
         claimed registry (Section 5 and Section 6).

      -  Harvesting broadcast RID messages for UTM inclusion
         (Section 7). 7).

      -  Methods for instantly establishing secure communications
         between an Observer and the pilot of an observed UAS
         (Section 8).

      -  Privacy in RID messages (PII protection) (Section 11).

2.  Terms and Definitions

2.1.  Architecture Terminology

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "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 above.

2.2.  Abbreviations

   DSS:        Discovery & Synchronization Service

   EdDSA:      Edwards-Curve Digital Signature Algorithm

   GCS:        Ground Control Station

   HHIT:       Hierarchical HIT

   HIP:        Host Identity Protocol

   HIT:        Host Identity Tag

   RID:        Remote ID

   Net-RID SP: Network RID Service Provider

   Net-RID DP: Network RID Display Provider.

   PII:        Personally Identifiable Information

   RF:         Radio Frequency
   SDSP:       Supplemental Data Service Provider

   UA:         Unmanned Aircraft

   UAS:        Unmanned Aircraft System

   USS:        UAS Service Supplier

   UTM:        UAS Traffic Management

2.3.  Additional Definitions

   This document uses terms defined in [I-D.ietf-drip-reqs].

3.  Claims, Assertions, Attestations, and Certificates

   Editor-note-7: (Bob) move section 3 to Section 2.4?
   This section introduces the terms "Claims", "Assertions",
   "Attestations", and "Certificates" as used in DRIP.  DRIP certificate
   has a different context compared with security certificates and
   Public Key Infrastructure used in X.509.


      A claim in DRIP is a predicate (e.g., "X is Y", "X has property
      Y", and most importantly "X owns Y" or "X is owned by Y").


      An assertion in DRIP is a set of claims.  This definition is
      borrowed from JWT [RFC7519] and CWT [RFC8392].


      An attestation in DRIP is a signed assertion.  The signer may be
      the claimant or a related party with stake in the assertion(s).
      Under DRIP this is normally used when an entity asserts a
      relationship with another entity, along with other information,
      and the asserting entity signs the assertion, thereby making it an


      A certificate in DRIP is an attestation, strictly over identity
      information, signed by a third party.  This third party should be
      one with no stake in the attestation(s) its signing over.

4.  HHIT as the DRIP Entity Identifier

   This section describes the DRIP architectural approach to meeting the
   basic requirements of a DRIP entity identifier within external
   technical standard ASTM [F3411-19] [F3411] and regulatory constraints.  It
   justifies and explains the use of Hierarchical Host Identity Tags
   (HHITs) as self-asserting IPv6 addresses suitable as a UAS ID type
   and more generally as trustworthy multipurpose remote identifiers.

   Self-asserting in this usage is given the Host Identity (HI), the
   HHIT ORCHID construction and a signature of the HHIT by the HI can
   both be validated.  The explicit registration hierarchy within the
   HHIT provides registry discovery (managed by a Registrar) to either
   yield the HI for 3rd-party (who is looking for ID attestation)
   validation or prove the HHIT and HI have uniquely been registered.

4.1.  UAS Remote Identifiers Problem Space

   A DRIP entity identifier needs to be "Trustworthy" (See DRIP
   Requirement GEN-1, ID-4 and ID-5 in [I-D.ietf-drip-reqs]).  This
   means that given a sufficient collection of RID messages, an Observer
   can establish that the identifier claimed therein uniquely belongs to
   the claimant: that the only way for any other entity to prove
   ownership of that identifier would be to obtain information that
   ought to be available only to the legitimate owner of the identifier
   (e.g., a cryptographic private key).

   To satisfy DRIP requirements and maintain important security
   properties, the DRIP identifier should be self-generated by the
   entity it names (e.g., a UAS) and registered (e.g., with a USS, see
   Requirements GEN-3 and ID-2).

   Broadcast RID, especially its support for Bluetooth 4, imposes severe
   constraints.  ASTM [F3411-19] RID [F3411] allows a UAS ID of types 1, 2 and 3 of
   20 bytes; the new a revision to [F3411], currently in balloting (as of Oct
   2021), adds type 4, created to enable Session IDs IDs, to be standardized by IETF and other
   standard development organizations (SDOs) as extensions to ASTM [F3411-19], RID,
   consumes one of those bytes to index the sub-type, leaving only 19
   for the identifier (see DRIP Requirement ID-1).  Likewise, the
   maximum ASTM [F3411-19] RID [F3411] Authentication Message payload is 201 bytes
   for most authentication types, but for type 5, created also added in this
   revision, for IETF and other SDOs to develop Specific Authentication
   Methods as extensions to ASTM [F3411-19], RID, one byte is consumed to index the
   sub-type, leaving only 200 for DRIP authentication payloads,
   including one or more DRIP entity identifiers and associated
   authentication data.

4.2.  HHIT as A Trustworthy DRIP Entity Identifier

   A Remote ID that can be trustworthily used in the RID Broadcast mode
   can be built from an asymmetric keypair.  Rather than using a key
   signing operation to claim ownership of an ID that does not guarantee
   name uniqueness, in this method the ID is cryptographically derived
   directly from the public key.  The proof of ID ownership (verifiable
   attestation, versus mere claim) is guaranteed by signing this
   cryptographic ID with the associated private key.  The association
   between the ID and the private key is ensured by cryptographically
   binding the public key with the ID, more specifically the ID results
   from the hash of the public key.  It is statistically hard for
   another entity to create a public key that would generate (spoof) the

   The basic HIT is designed statistically unique through the
   cryptographic hash feature of second-preimage resistance.  The
   cryptographically-bound addition of the Hierarchy and an HHIT
   registration process (e.g. based on Extensible Provisioning Protocol,
   [RFC5730]) provide complete, global HHIT uniqueness.  This
   registration forces the attacker to generate the same public key
   rather than a public key that generates the same HHIT.  This is in
   contrast to general IDs (e.g. a UUID or device serial number) as the
   subject in an X.509 certificate.

   A DRIP identifier can be assigned to a UAS as a static HHIT by its
   manufacturer, such as a single HI and derived HHIT encoded as a
   hardware serial number per [CTA2063A].  Such a static HHIT SHOULD
   only be used to bind one-time use DRIP identifiers to the unique UA.
   Depending upon implementation, this may leave a HI private key in the
   possession of the manufacturer (more details in Section 10).

   A UAS UA equipped for Broadcast RID SHOULD be provisioned not only with
   its HHIT but also with the HI public key from which the HHIT was
   derived and the corresponding private key, to enable message
   signature.  A UAS equipped for Network RID SHOULD be provisioned
   likewise; the private key resides only in the ultimate source of
   Network RID messages (i.e. on the UA itself if the GCS is merely
   relaying rather than sourcing Network RID messages).  Each Observer
   device SHOULD be provisioned either with public keys of the DRIP
   identifier root registries or certificates for subordinate

   HHITs can also be used throughout the USS/UTM system.  The Operators,
   Private Information Registries, as well as other UTM entities, can
   use HHITs for their IDs.  Such HHITs can facilitate DRIP security
   functions such as used with HIP to strongly mutually authenticate and
   encrypt communications.

   A self-attestation of a HHIT used as a UAS ID can be done in as
   little as 84 bytes, by avoiding an explicit encoding technology like
   ASN.1 or Concise Binary Object Representation (CBOR [RFC8949]).  This
   attestation consists of only the HHIT, a timestamp, and the EdDSA
   signature on them.

   An Observer would need Internet access to validate a self-
   attestations claim.  A third-party Certificate can be validated via a
   small credential cache in a disconnected environment.  This third-
   party Certificate is possible when the third-party also uses HHITs
   for its identity and the UA has the public key and the Certificate
   for that HHIT.

   Editor-note-3: review the last/above pragraph.

4.3.  HHIT for DRIP Identifier Registration and Lookup

   Remote ID needs a deterministic lookup mechanism that rapidly
   provides actionable information about the identified UA.  Given the
   size constraints imposed by the Bluetooth 4 broadcast media, the UAS
   ID itself needs to be a non-spoofable inquiry input into the lookup.

   A DRIP registration process based on the explicit hierarchy within a
   HHIT provides manageable uniqueness of the HI for the HHIT (defense HHIT.  This is
   the defense against a cryptographic hash second pre-image attach attack on
   the HHIT (e.g. multiple HIs yielding the same HHIT, see Requirement
   ID-3).  A lookup of the HHIT into this registration data provides the
   registered HI for HHIT proof.  A first-come-first-serve registration
   for a HHIT provides deterministic access to any other needed
   actionable information based on inquiry access authority (more
   details in Section 5.2).

4.4.  HHIT as a Cryptographic Identifier

   The only (known to the authors at the time of this writing) extant
   types of IP address compatible identifiers cryptographically derived
   from the public keys of the identified entities are Cryptographically
   Generated Addresses (CGAs) [RFC3972] and Host Identity Tags (HITs)
   [RFC7401].  CGAs and HITs lack registration/retrieval capability.  To
   provide this, each HHIT embeds plaintext information designating the
   hierarchy within which it is registered and a cryptographic hash of that
   information concatenated with the entity's public key, etc.  Although
   hash collisions may occur, the registrar can detect them and reject
   registration requests rather than issue credentials, e.g., by
   enforcing a first-claimed, first-attested policy.  Pre-image hash
   attacks are also mitigated through this registration process, locking
   the HHIT to a specific HI

5.  DRIP Identifier Registration and Registries

   Editor-note-4: Section 5 needs to cite the corresponding numbered
   requirement that it supports.

   DRIP registries hold both public and private UAS information
   resulting from the DRIP identifier registration process.  Given these
   different uses, and to improve scalability, security, and simplicity
   of administration, the public and private information can be stored
   in different registries.  This section introduces the public and
   private information registries for DRIP identifiers.

5.1.  Public Information Registry

5.1.1.  Background

   The public registry provides trustable information such as
   attestations of RID ownership and registration with the HDA
   (Hierarchical HIT Domain Authority).  Optionally, pointers to the
   registries for the HDA and RAA (Registered Assigning
   Authority)implicit in the RID can be included (e.g., for HDA and RAA
   HHIT|HI used in attestation signing operations).  This public
   information will be principally used by Observers of Broadcast RID
   messages.  Data on UAS that only use Network RID, is available via an
   Observer's Net-RID DP that would tend to directly provide all public
   registry information.  The Observer may visually "see" these Net-RID
   UAS, but they may be silent to the Observer.  The Net-RID DP is the
   only source of information based on a query for an airspace volume.

5.1.2.  DNS as the Public DRIP Identifier Registry

   A DRIP identifier SHOULD be registered as an Internet domain name (at
   an arbitrary level in the hierarchy, e.g. in .ip6.arpa).  Thus DNS
   can provide all the needed public DRIP information.  A standardized
   HHIT FQDN (Fully Qualified Domain Name) can deliver the HI via a HIP
   RR (Resource Record) [RFC8005] and other public information (e.g.,
   RRA and HDA PTRs, and HIP RVS (Rendezvous Servers) [RFC8004]).  These
   public information registries can use secure DNS transport (e.g.  DNS
   over TLS) to deliver public information that is not inherently
   trustable (e.g. everything other than attestations).

5.2.  Private Information Registry

5.2.1.  Background

   The private information required for DRIP identifiers is similar to
   that required for Internet domain name registration.  A DRIP
   identifier solution can leverage existing Internet resources:
   registration protocols, infrastructure, and business models, by
   fitting into an ID structure compatible with DNS names.  The HHIT
   hierarchy can provide the needed scalability and management
   structure.  It is expected that the private registry function will be
   provided by the same organizations that run a USS, and likely
   integrated with a USS.  The lookup function may be implemented by the
   Net-RID DPs.

5.2.2.  EPP and RDAP as the Private DRIP Identifier Registry

   A DRIP private information registry supports essential registry
   operations (e.g. add, delete, update, query) using interoperable open
   standard protocols.  It can accomplish this by using the Extensible
   Provisioning Protocol (EPP [RFC5730]) and the Registry Data Access
   Protocol (RDAP RFC7480] [RFC7482] [RFC7483]).  The DRIP private
   information registry in which a given UAS is registered needs to be
   findable, starting from the UAS ID, using the methods specified in

5.2.3.  Alternative Private DRIP Registry methods

   A DRIP private information registry might be an access controlled DNS
   (e.g. via DNS over TLS).  Additionally, WebFinger [RFC7033] can be
   deployed.  These alternative methods may be used by Net-RID DP with
   specific customers.

6.  DRIP Identifier Trust

   Editor-note-5: Section 6 doesn't use the word "authentication" in the
   section title, is there a reason to avoid it?

   While the DRIP entity identifier is self-asserting, it alone does not
   provide the "trustworthiness" specified in [I-D.ietf-drip-reqs].  For
   that it MUST be registered (under DRIP Registries) and be actively
   used by the party (in most cases the UA).  For Broadcast RID this is
   a challenge to balance the original requirements of Broadcast RID and
   the efforts needed to satisfy the DRIP requirements all under severe

   An optimization of different DRIP Authentication Messages allows an
   Observer, offline without Internet connection (offline) or online, with (online), to
   be able to validate a UAS DRIP ID in real-
   time. real-time.  First is the sending
   of various BroadcastAttestastion's Broadcast Attestations (over DRIP Link Authentication Messages)
   containing the relevant registry
   hierarchy from the Root all registration of the way to UA's DRIP ID in the
   claimed Registry.  Next is sending DRIP Wrapper Authentication
   Messages that sign over both static (e.g. above registration) and
   dynamically changing data (such as UA location data).  Combining
   these two sets of information an Observer can piece together a chain
   of trust and real-time evidence to make their determination of the
   UAs claims.

   This process (combining the DRIP entity identifier, Registries and
   Authentication Formats for Broadcast RID) can satisfy the following
   DRIP requirement defined in [I-D.ietf-drip-reqs]: GEN-1, GEN-2, GEN-
   3, ID-2, ID-3, ID-4 and ID-5.

7.  Harvesting Broadcast Remote ID messages for UTM Inclusion

   Editor-note-6: Section 7 needs to cite the corresponding numbered
   requirement that it supports.

   ASTM anticipated that regulators would require both Broadcast RID and
   Network RID for large UAS, but allow RID requirements for small UAS
   to be satisfied with the operator's choice of either Broadcast RID or
   Network RID.  The EASA initially specified Broadcast RID for UAS of
   essentially all UAS and is now also considering Network RID.  The FAA
   RID Final Rules [FAA_RID] permit only Broadcast RID for rule
   compliance, but still encourage Network RID for complementary
   functionality, especially in support of UTM.

   One obvious opportunity is to enhance the architecture with gateways
   from Broadcast RID to Network RID.  This provides the best of both
   and gives regulators and operators flexibility.  It offers advantages
   over either form of RID alone: greater fidelity than Network RID
   reporting of planned area operations; surveillance of areas too large
   for local direct visual observation and direct RF-LOS link based
   Broadcast RID (e.g., a city or a national forest).

   These gateways could be pre-positioned (e.g. around airports, public
   gatherings, and other sensitive areas) and/or crowd-sourced (as
   nothing more than a smartphone with a suitable app is needed).  As
   Broadcast RID media have limited range, gateways receiving messages
   claiming locations far from the gateway can alert authorities or a
   SDSP to the failed sanity check possibly indicating intent to
   deceive.  Surveillance SDSPs can use messages with precise date/time/
   position stamps from the gateways to multilaterate UA location,
   independent of the locations claimed in the messages, which are
   entirely operator self-reported in UAS RID and UTM, and thus are
   subject not only to natural time lag and error but also operator
   misconfiguration or intentional deception.

   Further, gateways with additional sensors (e.g. smartphones with
   cameras) can provide independent information on the UA type and size,
   confirming or refuting those claims made in the RID messages.  This
   Crowd Sourced Remote ID (CS-RID) would be a significant enhancement,
   beyond baseline DRIP functionality; if implemented, it adds two more
   entity types.

7.1.  The CS-RID Finder

   A CS-RID Finder is the gateway for Broadcast Remote ID Messages into
   the UTM.  It performs this gateway function via a CS-RID SDSP.  A CS-
   RID Finder could implement, integrate, or accept outputs from, a
   Broadcast RID receiver.  However, it should not depend upon a direct
   interface with a GCS, Net-RID SP, Net-RID DP or Network RID client.
   It would present a TBD interface to a CS-RID SDSP, similar to but
   readily distinguishable from that between a GCS and a Net-RID SP.

7.2.  The CS-RID SDSP

   A CS-RID SDSP aggregates and processes (e.g., estimates UA location
   using including using multilateration when possible) information
   collected by CS-RID Finders.  A CS-RID SDSP should appear (i.e.
   present the same interface) to a Net-RID SP as a Net-RID DP.  A CS-RID SDSP aggregates and processes
   (e.g., estimates UA location using) information collected by CS-RID

   Editor-note-8: double check above paragraph after Editor-note-1 is

8.  DRIP Contact

   One of the ways in which DRIP is to can enhance [F3411-19] [F3411] with immediately
   actionable information is by enabling an Observer to instantly
   initiate secure communications with the UAS remote pilot, Pilot In
   Command, operator, USS under which the operation is being flown, or
   other entity potentially able to furnish further information
   regarding the operation and its intent and/or to immediately
   influence further conduct or termination of the operation (e.g., land
   or otherwise exit an airspace volume).  Such potentially distracting
   communications demand strong "AAA" (Authentication, Attestation,
   Authorization, Access Control, Accounting, Attribution, Audit) per
   applicable policies (e.g., of the cognizant CAA).

   A DRIP entity identifier based on a HHIT as outlined in Section 4
   embeds an identifier of the registry in which it can be found
   (expected typically to be the USS under which the UAS is flying) and
   the procedures outlined in Section 6 enable Observer verification of
   that relationship.  A DRIP entity identifier with suitable records in
   public and private registries as outlined in Section 5 can enable
   lookup not only of information regarding the UAS but also identities
   of and pointers to information regarding the various associated
   entities (e.g., the USS under which the UAS is flying an operation),
   including means of contacting those associated entities (i.e.,
   locators, typically IP addresses).  An Observer equipped with HIP can
   initiate a Base Exchange (BEX) and establish a Bound End to End
   Tunnel (BEET) protected by IPsec Encapsulating Security Payload (ESP)
   encryption to a likewise equipped and identified entity: the UA
   itself, if operating autonomously; the GCS, if the UA is remotely
   piloted and the necessary records have been populated in DNS;
   likewise the USS, etc.  Certain preconditions are necessary: each
   party to the communication needs a currently usable means (typically
   DNS) of resolving the other party's DRIP entity identifier to a
   currently usable locator (IP address); and there must be currently
   usable bidirectional IP (not necessarily Internet) connectivity
   between the parties.  Given a BEET, arbitrary standard higher layer
   protocols can then be used for Observer to Pilot (O2P) communications
   (e.g., SIP [RFC3261] et seq), V2X communications (e.g., [MAVLink]),
   etc.  This approach satisfies DRIP requirement GEN-6 Contact,
   supports satisfaction of requirements [I-D.ietf-drip-reqs] GEN-8,
   GEN-9, PRIV-2, PRIV-5 and REG-3, and is compatible with all other
   DRIP requirements.

9.  IANA Considerations

   This document does not make any IANA request.

10.  Security Considerations

   The security provided by asymmetric cryptographic techniques depends
   upon protection of the private keys.  A manufacturer that embeds a
   private key in an UA may have retained a copy.  A manufacturer whose
   UA are configured by a closed source application on the GCS which
   communicates over the Internet with the factory may be sending a copy
   of a UA or GCS self-generated key back to the factory.  Keys may be
   extracted from a GCS or UA.  The RID sender of a small harmless UA
   (or the entire UA) could be carried by a larger dangerous UA as a
   "false flag."  Compromise of a registry private key could do
   widespread harm.  Key revocation procedures are as yet to be
   determined.  These risks are in addition to those involving Operator
   key management practices.

11.  Privacy & Transparency Considerations

   Broadcast RID messages can contain Personally Identifiable
   Information (PII).  A viable architecture for PII protection would be
   symmetric encryption of the PII using a session key known to the UAS
   and its USS.  Authorized Observers could obtain plaintext in either
   of two ways.  An Observer can send the UAS ID and the cyphertext to a
   server that offers decryption as a service.  An Observer can send the
   UAS ID only to a server that returns the session key, so that
   Observer can directly locally decrypt all cyphertext sent by that UA
   during that session (UAS operation).  In either case, the server can
   be: a Public Safety USS; the Observer's own USS; or the UA's USS if
   the latter can be determined (which under DRIP it can be, from the
   UAS ID itself).  PII can be protected unless the UAS is informed
   otherwise.  This could come as part of UTM operation authorization.
   It can be special instructions at the start or during an operation.
   PII protection MUST not be used if the UAS loses connectivity to the
   USS.  The UAS always has the option to abort the operation if PII
   protection is disallowed.

12.  References

12.1.  Normative References

              Card, S. W., Wiethuechter, A., Moskowitz, R., and A.
              Gurtov, "Drone Remote Identification Protocol (DRIP)
              Requirements", Work in Progress, Internet-Draft, draft-
              ietf-drip-reqs-18, 8 September 2021,

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [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>.

12.2.  Informative References

   [CTA2063A] ANSI, "Small Unmanned Aerial Systems Serial Numbers",

              European Union Aviation Safety Agency (EASA), "EU
              Commission Delegated Regulation 2019/945 of 12 March 2019
              on unmanned aircraft systems and on third-country
              operators of unmanned aircraft systems", 2019.

   [F3411-19] ASTM,

   [F3411]    ASTM International, "Standard Specification for Remote ID
              and Tracking",
              2019. February 2020,

   [FAA_RID]  United States Federal Aviation Administration (FAA),
              "Remote Identification of Unmanned Aircraft", 2021,

              United States Federal Aviation Administration (FAA),
              "Unmanned Aircraft System (UAS) Traffic Management (UTM)
              Concept of Operations (V2.0)", 2020,

              Moskowitz, R., Card, S. W., Wiethuechter, A., and A.
              Gurtov, "DRIP Entity Tag (DET) for Unmanned Aircraft
              System Remote Identification (UAS RID)", Work in Progress,
              Internet-Draft, draft-ietf-drip-rid-11, 20 October 2021,

              European Union Aviation Safety Agency (EASA), "EU
              Commission Implementing Regulation 2019/947 of 24 May 2019
              on the rules and procedures for the operation of unmanned
              aircraft", 2019.

   [LAANC]    United States Federal Aviation Administration (FAA), "Low
              Altitude Authorization and Notification Capability", n.d.,

   [MAVLink]  "Micro Air Vehicle Communication Protocol", n.d.. 2021,

   [NPRM]     United States Federal Aviation Administration (FAA),
              "Notice of Proposed Rule Making on Remote Identification
              of Unmanned Aircraft Systems", 2019.

   [RFC1034]  Mockapetris, P., "Domain names - concepts and facilities",
              STD 13, RFC 1034, DOI 10.17487/RFC1034, November 1987,

   [RFC3261]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
              A., Peterson, J., Sparks, R., Handley, M., and E.
              Schooler, "SIP: Session Initiation Protocol", RFC 3261,
              DOI 10.17487/RFC3261, June 2002,

   [RFC3972]  Aura, T., "Cryptographically Generated Addresses (CGA)",
              RFC 3972, DOI 10.17487/RFC3972, March 2005,

   [RFC5730]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)",
              STD 69, RFC 5730, DOI 10.17487/RFC5730, August 2009,

   [RFC5731]  Hollenbeck, S., "Extensible Provisioning Protocol (EPP)
              Domain Name Mapping", STD 69, RFC 5731,
              DOI 10.17487/RFC5731, August 2009,

   [RFC7033]  Jones, P., Salgueiro, G., Jones, M., and J. Smarr,
              "WebFinger", RFC 7033, DOI 10.17487/RFC7033, September
              2013, <https://www.rfc-editor.org/info/rfc7033>.

   [RFC7401]  Moskowitz, R., Ed., Heer, T., Jokela, P., and T.
              Henderson, "Host Identity Protocol Version 2 (HIPv2)",
              RFC 7401, DOI 10.17487/RFC7401, April 2015,

   [RFC7482]  Newton, A. and S. Hollenbeck, "Registration Data Access
              Protocol (RDAP) Query Format", RFC 7482,
              DOI 10.17487/RFC7482, March 2015,

   [RFC7483]  Newton, A. and S. Hollenbeck, "JSON Responses for the
              Registration Data Access Protocol (RDAP)", RFC 7483,
              DOI 10.17487/RFC7483, March 2015,

   [RFC7484]  Blanchet, M., "Finding the Authoritative Registration Data
              (RDAP) Service", RFC 7484, DOI 10.17487/RFC7484, March
              2015, <https://www.rfc-editor.org/info/rfc7484>.

   [RFC7519]  Jones, M., Bradley, J., and N. Sakimura, "JSON Web Token
              (JWT)", RFC 7519, DOI 10.17487/RFC7519, May 2015,

   [RFC8002]  Heer, T. and S. Varjonen, "Host Identity Protocol
              Certificates", RFC 8002, DOI 10.17487/RFC8002, October
              2016, <https://www.rfc-editor.org/info/rfc8002>.

   [RFC8004]  Laganier, J. and L. Eggert, "Host Identity Protocol (HIP)
              Rendezvous Extension", RFC 8004, DOI 10.17487/RFC8004,
              October 2016, <https://www.rfc-editor.org/info/rfc8004>.

   [RFC8005]  Laganier, J., "Host Identity Protocol (HIP) Domain Name
              System (DNS) Extension", RFC 8005, DOI 10.17487/RFC8005,
              October 2016, <https://www.rfc-editor.org/info/rfc8005>.

   [RFC8032]  Josefsson, S. and I. Liusvaara, "Edwards-Curve Digital
              Signature Algorithm (EdDSA)", RFC 8032,
              DOI 10.17487/RFC8032, January 2017,

   [RFC8392]  Jones, M., Wahlstroem, E., Erdtman, S., and H. Tschofenig,
              "CBOR Web Token (CWT)", RFC 8392, DOI 10.17487/RFC8392,
              May 2018, <https://www.rfc-editor.org/info/rfc8392>.

   [RFC8949]  Bormann, C. and P. Hoffman, "Concise Binary Object
              Representation (CBOR)", STD 94, RFC 8949,
              DOI 10.17487/RFC8949, December 2020,

              3GPP, "Study on Remote Identification of Unmanned Aerial
              Systems (UAS)", n.d.,

   [U-Space]  European Organization for the Safety of Air Navigation
              (EUROCONTROL), "U-space Concept of Operations", 2019,

Appendix A.  Overview of Unmanned Aircraft Systems (UAS) Traffic
             Management (UTM)

A.1.  Operation Concept

   The National Aeronautics and Space Administration (NASA) and FAA's
   effort of integrating UAS's operation into the national airspace
   system (NAS) led to the development of the concept of UTM and the
   ecosystem around it.  The UTM concept was initially presented in 2013
   and version 2.0 was published in 2020 [FAA_UAS_Concept_Of_Ops].

   The eventual concept refinement, initial prototype implementation implementation,
   and testing were conducted by the UTM research transition team which
   is the joint workforce by FAA and NASA.  World efforts took place
   afterward.  The Single European Sky ATM Research (SESAR) started the
   CORUS project to research its UTM counterpart concept, namely
   [U-Space].  This effort is led by the European Organization for the
   Safety of Air Navigation (Eurocontrol).

   Both NASA and SESAR have published the UTM concept of operations to
   guide the development of their future air traffic management (ATM)
   system and ensure safe and efficient integration of manned and
   unmanned aircraft into the national airspace.

   The UTM comprises UAS operation infrastructure, procedures and local
   regulation compliance policies to guarantee safe UAS integration and
   operation.  The main functionality of a UTM includes, but is not
   limited to, providing means of communication between UAS operators
   and service providers and a platform to facilitate communication
   among UAS service providers.

A.2.  UAS Service Supplier (USS)

   A USS plays an important role to fulfill the key performance
   indicators (KPIs) that a UTM has to offer.  Such an Entity acts as a
   proxy between UAS operators and UTM service providers.  It provides
   services like real-time UAS traffic monitoring and planning,
   aeronautical data archiving, airspace and violation control,
   interacting with other third-party control entities, etc.  A USS can
   coexist with other USS to build a large service coverage map which that can
   load-balance, relay relay, and share UAS traffic information.

   The FAA works with UAS industry shareholders and promotes the Low
   Altitude Authorization and Notification Capability [LAANC] program
   which is the first system to realize some of the UTM envisioned
   functionality.  The LAANC program can automate the UAS operational
   intent (flight plan) submission and application for airspace
   authorization in real-time by checking against multiple aeronautical
   databases such as airspace classification and fly operating rules
   associated with it, FAA UAS facility map, special use airspace,
   Notice to Airmen (NOTAM), and Temporary Flight Restriction (TFR).

A.3.  UTM Use Cases for UAS Operations

   This section illustrates a couple of use case scenarios where UAS
   participation in UTM has significant safety improvement.

   1.  For a UAS participating in UTM and taking off or landing in a
       controlled airspace (e.g., Class Bravo, Charlie, Delta Delta, and Echo
       in the United States), the USS under which the UAS is operating
       is responsible for verifying UA registration, authenticating the
       UAS operational intent (flight plan) by checking against
       designated UAS facility map database, obtaining the air traffic
       control (ATC) authorization authorization, and monitor monitoring the UAS flight path
       in order to maintain safe margins and follow the pre-authorized
       sequence of authorized 4-D volumes (route).

   2.  For a UAS participating in UTM and taking off or landing in an
       uncontrolled airspace (ex.  Class Golf in the United States),
       pre-flight authorization must be obtained from a USS when
       operating beyond-visual-of-sight (BVLOS).  The USS either accepts
       or rejects the received operational intent (flight plan) from the
       UAS.  Accepted UAS operation may share its current flight data
       such as GPS position and altitude to USS.  The USS may keep the
       UAS operation status near real-time and may keep it as a record
       for overall airspace air traffic monitoring.

Appendix B.  Automatic Dependent Surveillance Broadcast (ADS-B)

   The ADS-B is the de jure technology used in manned aviation for
   sharing location information, from the aircraft to ground and
   satellite-based systems, designed in the early 2000s.  Broadcast RID
   is conceptually similar to ADS-B, but with the receiver target being
   the general public on generally available devices (e.g. smartphones).

   For numerous technical reasons, ADS-B itself is not suitable for low-
   flying small UA.  Technical reasons include but not limited to the

   1.  Lack of support for the 1090 MHz ADS-B channel on any consumer
       handheld devices

   2.  Weight and cost of ADS-B transponders on CSWaP constrained UA

   3.  Limited bandwidth of both uplink and downlink, which would likely
       be saturated by large numbers of UAS, endangering manned aviation

   Understanding these technical shortcomings, regulators worldwide have
   ruled out the use of ADS-B for the small UAS for which UAS RID and
   DRIP are intended.


   The work of the FAA's UAS Identification and Tracking (UAS ID)
   Aviation Rulemaking Committee (ARC) is the foundation of later ASTM
   and proposed IETF DRIP WG efforts.  The work of ASTM F38.02 in
   balancing the interests of diverse stakeholders is essential to the
   necessary rapid and widespread deployment of UAS RID.  IETF
   volunteers who have contributed  Thanks to this draft include Amelia
   Alexandre Petrescu and Stephan Wenger for the helpful and positive
   comments.  Thanks to chairs Daniel Migault and Mohamed Boucadair. Boucadair for
   direction of our team of authors and editor, some of whom are
   newcomers to writing IETF documents.  Thanks especially to Internet
   Area Director Eric Vyncke for guidance and support.

Authors' Addresses

   Stuart W. Card
   AX Enterprize
   4947 Commercial Drive
   Yorkville, NY,  13495
   United States of America

   Email: stu.card@axenterprize.com
   Adam Wiethuechter
   AX Enterprize
   4947 Commercial Drive
   Yorkville, NY,  13495
   United States of America

   Email: adam.wiethuechter@axenterprize.com

   Robert Moskowitz
   HTT Consulting
   Oak Park, MI,  48237
   United States of America

   Email: rgm@labs.htt-consult.com

   Shuai Zhao
   2747 Park Blvd
   Palo Alto,  94588
   United States of America

   Email: shuai.zhao@ieee.org

   Andrei Gurtov
   Linköping University
   SE-58183 Linköping Linköping

   Email: gurtov@acm.org