drip                                                             S. Card
Internet-Draft                                           A. Wiethuechter
Intended status: Informational                             AX Enterprize
Expires: 26 August 2021                                     R. Moskowitz
                                                          HTT Consulting
                                                        S. Zhao (Editor)
                                                               A. Gurtov
                                                   Linkoeping University
                                                        22 February 2021

        Drone Remote Identification Protocol (DRIP) Architecture


   This document defines an architecture for protocols and services to
   support Unmanned Aircraft System Remote Identification and tracking
   (UAS RID), plus RID-related communications, including required
   architectural building blocks and their interfaces.

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
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 26 August 2021.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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

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

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Overview UAS Remote ID (RID) and RID 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  . . . . . . . . . . . .   6
     1.4.  Overview of DRIP Architecture . . . . . . . . . . . . . .   7
   2.  Conventions . . . . . . . . . . . . . . . . . . . . . . . . .   9
   3.  Definitions and Abbreviations . . . . . . . . . . . . . . . .   9
     3.1.  Additional Definitions  . . . . . . . . . . . . . . . . .   9
     3.2.  Abbreviations . . . . . . . . . . . . . . . . . . . . . .   9
     3.3.  Claims, Assertions, Attestations, and Certificates  . . .  10
   4.  HHIT for DRIP Entity Identifier . . . . . . . . . . . . . . .  11
     4.1.  UAS Remote Identifiers Problem Space  . . . . . . . . . .  11
     4.2.  HIT as A Trustworthy DRIP Entity Identifier . . . . . . .  12
     4.3.  HHIT for DRIP Identifier Registration and Lookup  . . . .  12
     4.4.  HHIT for DRIP Identifier Cryptographic  . . . . . . . . .  13
   5.  DRIP Identifier Registration and Registries . . . . . . . . .  14
     5.1.  Public Information Registry . . . . . . . . . . . . . . .  14
       5.1.1.  Background  . . . . . . . . . . . . . . . . . . . . .  14
       5.1.2.  Proposed Approach . . . . . . . . . . . . . . . . . .  14
     5.2.  Private Information Registry  . . . . . . . . . . . . . .  14
       5.2.1.  Background  . . . . . . . . . . . . . . . . . . . . .  15
       5.2.2.  Proposed Approach . . . . . . . . . . . . . . . . . .  15
   6.  Harvesting Broadcast Remote ID messages for UTM Inclusion . .  15
     6.1.  The CS-RID Finder . . . . . . . . . . . . . . . . . . . .  16
     6.2.  The CS-RID SDSP . . . . . . . . . . . . . . . . . . . . .  16
   7.  Privacy for Broadcast PII . . . . . . . . . . . . . . . . . .  16
   8.  Security Considerations . . . . . . . . . . . . . . . . . . .  17
   9.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  17
   10. References  . . . . . . . . . . . . . . . . . . . . . . . . .  17
     10.1.  Normative References . . . . . . . . . . . . . . . . . .  17
     10.2.  Informative References . . . . . . . . . . . . . . . . .  18
   Appendix A.  Overview of Unmanned Aircraft Systems (UAS) Traffic
           Management (UTM)  . . . . . . . . . . . . . . . . . . . .  20
     A.1.  Operation Concept . . . . . . . . . . . . . . . . . . . .  20
     A.2.  UAS Service Supplier (USS)  . . . . . . . . . . . . . . .  21
     A.3.  UTM Use Cases for UAS Operations  . . . . . . . . . . . .  21
     A.4.  Automatic Dependent Surveillance Broadcast (ADS-B)  . . .  22
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  22

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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, conforming to proposed
   and final regulations plus external technical standards, satisfying
   the requirements listed in the companion requirements document

1.1.  Overview UAS Remote ID (RID) and RID Standardization

   UAS Remote Identification (RID) is an application enabler for a UAS
   to be identified by UTM/USS or third parties entities such as law
   enforcement.  Many safety and other considerations dictate that UAS
   be remotely identifiable.  CAAs worldwide are mandating UAS RID.  The
   European Union Aviation Safety Agency (EASA) has published
   [Delegated] and [Implementing] Regulations.

   CAAs currently promulgate performance-based regulations that do not
   specify techniques, but rather cite industry consensus technical
   standards as acceptable means of compliance.


      The FAA published a Notice of Proposed Rule Making [NPRM] in 2019
      and whereafter published the Final Rule [FAA_RID] in 2021.


      ASTM International, Technical Committee F38 (UAS), Subcommittee
      F38.02 (Aircraft Operations), Work Item WK65041, developed the
      ASTM [F3411-19] 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 a UAS
      uses both communication methods, the same information must be
      provided via both means.  The [F3411-19] is cited by FAA in its
      RID final rule [FAA_RID] as "a potential means of compliance" to a
      Remote ID rule.


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

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1.2.  Overview of Types of UAS Remote ID

1.2.1.  Broadcast RID

   A set of RID messages are defined 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 locally directly received
   UAS RID as a key.  Broadcast RID should be functionally usable in
   situations with no Internet connectivity.

   The Broadcast RID is illustrated in Figure 1 below.

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

                                  Figure 1

   With Broadcast RID, an Observer is limited to their radio "visible"
   airspace for UAS awareness and information.  With Internet queries
   using harvested RID (see Section 6), the Observer may gain more
   information about those visible UAS.

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1.2.2.  Network RID

   A RID data dictionary and data flow for Network RID are defined in
   [F3411-19].  This data flow is from a UAS via unspecified means (but
   at least in part over the Internet) to a Network Remote ID Service
   Provider (Net-RID SP).  These Net-RID SPs provide the RID data to
   Network Remote ID Display Providers (Net-RID DP).  It is the Net-RID
   DP that responds to queries from Network Remote ID Observers
   (expected typically, but not specified exclusively, to be web-based)
   specifying airspace volumes of interest.  Network RID depends upon
   connectivity, in several segments, via the Internet, from the UAS to
   the Observer.

   The Network RID is illustrated in Figure 2 below:

               x x  UA
               xxxxx       ********************
                |   \    *                ------*---+------------+
                |    \   *              /       *  | NET_RID_SP |
                |     \  * ------------/    +---*--+------------+
                | RF   \ */                 |   *
                |        *      INTERNET    |   *  +------------+
                |       /*                  +---*--| NET_RID_DP |
                |      / *                  +---*--+------------+
                +     /   *                 |   *
                 x   /     *****************|***      x
               xxxxx                        |       xxxxx
                 x                          +-------  x
                 x                                    x
                x x   Operator (GCS)      Observer   x x
               x   x                                x   x

                                  Figure 2

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   Command and Control (C2) must flow from the GCS to the UA via some
   path (ex. a direct RF link, but with increasing BVLOS operations
   expected often to be wireless links at either end with the Internet
   between).  For all but the simplest hobby aircraft, telemetry (at
   least 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 other USS).  Subscribed
   Net-RID DP forward RID information via the Internet to subscribed
   Observer devices.  Regulations require and [F3411-19] describes RID
   data elements end-to-end.  [F3411-19] prescribes the protocol only
   among Net-RID SP, Net-RID DP, and the Discovery and Synchronization
   Service (DSS).

         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] Network RID..

1.3.  Overview of USS Interoperability

   Each UAS is registered to at least one USS.  With Net-RID, there is
   direct communication between the UAS and its USS.  With Broadcast-
   RID, 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 USS.  USS exchange information via a Discovery
   and Synchronization Service (DSS) so all USS collectively have
   knowledge about all activities in a 4D airspace.

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

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                               | Observer |
                              /            \
                             /              \
                      +-----+                +-----+
                      | UA1 |                | UA2 |
                      +-----+                +-----+
                             \              /
                              \            /
                               | Internet |
                              /            \
                             /              \
                       +-------+           +-------+
                       | USS-1 | <-------> | USS-2 |
                       +-------+           +-------+
                                \         /
                                 \       /
                                 |  DSS |

                                  Figure 3

1.4.  Overview of DRIP Architecture

   The requirements document [I-D.ietf-drip-reqs] also provides an
   extended introduction to the problem space, use cases, etc.  Only a
   brief summary of that introduction will be restated here as context,
   with reference to the general UAS RID usage scenarios shown in
   Figure 4 below.

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         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

   DRIP will enable leveraging existing Internet resources (standard
   protocols, services, infrastructure, and business models) to meet UAS
   RID and closely related needs.  DRIP will specify how to apply IETF
   standards, complementing [F3411-19] and other external standards, to
   satisfy UAS RID requirements.  DRIP will update existing and develop
   new protocol standards as needed to accomplish the foregoing.

   This document will outline the UAS RID architecture into which DRIP
   must fit and the architecture for DRIP itself.  This includes
   presenting the gaps between the CAAs' Concepts of Operations and
   [F3411-19] as it relates to the use of Internet technologies and UA
   direct RF communications.  Issues include, but are not limited to:

      -  Design of trustworthy remote ID and trust in RID messages
         (Section 4)

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

      -  Harvesting broadcast remote ID messages for UTM inclusion
         (Section 6)

      -  Privacy in RID messages (PII protection) (Section 7)

2.  Conventions

   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.

3.  Definitions and Abbreviations

3.1.  Additional Definitions

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

3.2.  Abbreviations

   ADS-B:      Automatic Dependent Surveillance Broadcast

   DSS:        Discovery & Synchronization Service

   EdDSA:      Edwards-Curve Digital Signature Algorithm

   GCS:        Ground Control Station

   HHIT:       Hierarchical HIT Registries

   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

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   RF:         Radio Frequency

   SDSP:       Supplemental Data Service Provider

   UA:         Unmanned Aircraft

   UAS:        Unmanned Aircraft System

   USS:        UAS Service Supplier

   UTM:        UAS Traffic Management

3.3.  Claims, Assertions, Attestations, and Certificates

   This section introduces the terms "Claims", "Assertions",
   "Attestations", and "Certificates" as used in DRIP.

   This is due to the term "certificate" having significant
   technological and legal baggage associated with it, specifically
   around X.509 certificates.  These types of certificates and Public
   Key Infrastructure invoke more legal and public policy considerations
   than probably any other electronic communication sector.  It emerged
   as a governmental platform for trusted identity management and was
   pursued in intergovernmental bodies with links into treaty


      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").  One
      basic use case of a claim is an entity using an HHIT as an
      identifier, e.g., a UAS using an HHIT as a UAS ID.


      An assertion in DRIP is a set of claims.  This definition is
      borrowed from JWT/CWT.  An HHIT of itself can be seen as an
      assertion: a claim that the identifier is a handle to an
      asymmetric keypair owned by the entity, and a claim that the
      identifier is in the registry specified by the HID embedded in the


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      An attestation in DRIP is a signed assertion.  The signer may be a
      claimant or a third party.  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 attestation.


      A certificate in DRIP is an attestation, strictly over identity
      information, signed by a third party.

4.  HHIT for DRIP Entity Identifier

   This section describes the basic requirements of a DRIP entity
   identifier per regulation constrains from ASTM [F3411-19] and
   explains the use of Hierarchical Host Identity Tags (HHITs) as self-
   asserting IPv6 addresses and thereby a trustable DRIP identifier for
   use as the UAS Remote ID.  HHITs self-attest to the included explicit
   hierarchy that provides Registrar discovery for 3rd-party ID

4.1.  UAS Remote Identifiers Problem Space

   A DRIP entity identifier needs to be "Trustworthy".  This means that
   within the framework of the RID messages, an Observer can establish
   that the DRIP identifier used does uniquely belong to the UAS.  That
   the only way for any other UAS to assert this DRIP identifier would
   be to steal something from within the UAS.  The DRIP identifier is
   self-generated by the UAS (either UA or GCS) and registered with the

   The data communication of using Broadcast RID faces extreme
   challenges due to the limitation of the demanding support for
   Bluetooth.  The ASTM [F3411-19] defines the basic RID message which
   is expected to contain certain RID data and the Authentication
   message.  The Basic RID message has a maximum payload of 25 bytes and
   the maximum size allocated by ASTM for the RID is 20 bytes and only 3
   bytes are left unused. currently, the authentication maximum payload
   is defined to be 201 bytes.

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   Standard approaches like X.509 and PKI will not fit these
   constraints, even using the new EdDSA [RFC8032] algorithm.  An
   example of a technology that will fit within these limitations is an
   enhancement of the Host Identity Tag (HIT) of HIPv2 [RFC7401] using
   Hierarchical HITs (HHITs) for UAS RID is outlined in HHIT based UAS
   RID [I-D.ietf-drip-rid].  As PKI with X.509 is being used in other
   systems with which UAS RID must interoperate (e.g.  Discovery and
   Synchronization Service and any other communications involving USS)
   mappings between the more flexible but larger X.509 certificates and
   the HHIT-based structures must be devised.

   By using the EdDSA HHIT suite, the self-attestations of the RID can
   be done in as little as 84 bytes.  Third-party Certificates can be
   done in 200 bytes.  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.

4.2.  HIT as A Trustworthy DRIP Entity Identifier

   For a Remote ID to be trustworthy in the Broadcast mode, it is better
   to have an asymmetric keypair for proof of ID ownership.  The common
   method of using a key signing operation to assert ownership of an ID,
   does not guarantee name uniqueness.  Any entity can sign an ID,
   claiming ownership.  To mitigate spoofing risks, the ID needs to be
   cryptographically generated from the public key, in such a manner
   that it is statistically hard for an entity to create a public key
   that would generate (spoof) the ID.  Thus the signing of such an ID
   becomes an a proof (verifiable attestation, versus mere claim) of

   HITs are 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 is in contrast to general IDs (e.g. a UUID or
   device serial number) as the subject in an X.509 certificate.

4.3.  HHIT for DRIP Identifier Registration and Lookup

   DRIP identifiers need a deterministic lookup mechanism that rapidly
   provides actionable information about the identified UA.  The
   identifier itself needs to be the inquiry input into the lookup given
   the constraints imposed by some of the broadcast media.  This can
   best be achieved by an Identifier registration hierarchy
   cryptographically embedded within the Identifier.

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   A HHIT itself consists of a registration hierarchy, the hashing
   crypto suite information, and the hash of these items along with the
   underlying public key.  Additional information, e.g. an IPv6 prefix,
   can enhance the HHITs use beyond the basic Remote ID function (e.g
   use in HIP, [RFC7401]).

   Therefore, a DRIP identifier can be represented as a HHIT.  It can be
   self-generated by a UAS (either UA or GCS) and registered with the
   Private Information Registry (More details in Section 5.2) identified
   in its hierarchy fields.  Each DRIP identifier represented as an HHIT
   can not be used more than once.

   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 can 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 8).

   In another case, a UAS equipped for Broadcast RID can 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 can 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 can be provisioned either with public keys of the DRIP
   identifier root registries or certificates for subordinate

   The Operators, Private Information Registries as well as other UTM
   entities can possess UAS ID style HHITs.  When present, such HHITs
   can be used with HIP to strongly mutually authenticate and optionally
   encrypt communications.

4.4.  HHIT for DRIP Identifier Cryptographic

   The only (known to the authors of this document at the time of its
   writing) extant fixed-length ID cryptographically derived from a
   public key are the Host Identity Tag [RFC7401], HITs, and
   Cryptographically Generated Addresses [RFC3972], CGAs.  However, both
   HITs and CGAs lack registration/retrieval capability.  HHIT, on the
   other hand, is capable of providing a cryptographic hashing function,
   along with a registration process to mitigate the probability of a
   hash collision (first registered, first allowed).

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5.  DRIP Identifier Registration and Registries

   UAS registries can 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.  A DRIP identifier is amenable to handling
   as an Internet domain name (at an arbitrary level in the hierarchy).
   It also can be registered in at least a pseudo-domain (e.g. .ip6.arpa
   for reverse lookup), or as a sub-domain (for forward lookup).  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 HDA registration.  Optionally,
   pointers to the repositories for the HDA and RAA 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 only available via an Observer's Net-RID DP that
   would tend to provide all public registry information directly.  The
   Observer can visually "see" these UAS, but they are 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.  Proposed Approach

   A DRIP public information registry can respond to standard DNS
   queries, in the definitive public Internet DNS hierarchy.  If a DRIP
   public information registry lists, in a HIP RR, any HIP RVS servers
   for a given DRIP identifier, those RVS servers can restrict relay
   services per AAA policy; this requires extensions to [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

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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.  This implies
   some sort of hierarchy, for scalability, and management of this
   hierarchy.  It is expected that the private registry function will be
   provided by the same organizations that run USS, and likely
   integrated with USS.

5.2.2.  Proposed Approach

   A DRIP private information registry can support essential Internet
   domain name registry operations (e.g. add, delete, update, query)
   using interoperable open standard protocols.  It can also support the
   Extensible Provisioning Protocol (EPP) and the Registry Data Access
   Protocol (RDAP) with access controls.  It might be listed in a DNS:
   that DNS could be private; but absent any compelling reasons for use
   of private DNS, a public DNS hierarchy needs to be in place.  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 [RFC7484].  A DRIP private information registry can also
   support WebFinger as specified in [RFC7033].

6.  Harvesting Broadcast Remote ID messages for UTM Inclusion

   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 only specifies Broadcast RID for UAS, however, still
   encourages 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
   considerable enhancement over some Network RID options such as only
   reporting planned 4D operation space by the operator.

   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

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   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 may have
   a natural time lag as it is), which are entirely operator self-
   reported in UAS RID and UTM.

   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.

6.1.  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 can not interface directly with
   a GCS, Net-RID SP, Net-RID DP or Network RID client.  It would
   present a TBD interface to a CS-RID SDSP; this interface needs to be
   based upon but readily distinguishable from that between a GCS and a
   Net-RID SP.

6.2.  The CS-RID SDSP

   A CS-RID SDSP would appear (i.e. present the same interface) to a
   Net-RID SP as a Net-RID DP.  A CS-RID SDSP can not present a standard
   GCS-facing interface as if it were a Net-RID SP.  A CS-RID SDSP would
   present a TBD interface to a CS-RID Finder; this interface can be
   based upon but readily distinguishable between a GCS and a Net-RID

7.  Privacy for Broadcast PII

   Broadcast RID messages can contain PII.  A viable architecture for
   PII protection would be symmetric encryption of the PII using a key
   known to the UAS and its USS.  An authorized Observer could send the
   encrypted PII along with the UAS ID (to entities such as USS of the
   Observer, or to the UAS in which the UAS ID is registered if that can
   be determined from the UAS ID itself or to a Public Safety USS) to
   get the plaintext.  Alternatively, the authorized Observer can
   receive the key to directly decrypt all future PII content from the

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   PII can be protected unless the UAS is informed otherwise.  This
   could come from operational instructions to even permit flying in a
   space/time.  It can be special instructions at the start or during an
   operation.  PII protection can 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.

   An authorized Observer can instruct a UAS via the USS that conditions
   have changed mandating no PII protection or land the UA (abort the

8.  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.

9.  Acknowledgements

   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 to this draft include Amelia
   Andersdotter and Mohamed Boucadair.

10.  References

10.1.  Normative References

              Card, S., Wiethuechter, A., Moskowitz, R., and A. Gurtov,
              "Drone Remote Identification Protocol (DRIP)
              Requirements", Work in Progress, Internet-Draft, draft-
              ietf-drip-reqs-06, 1 November 2020, <http://www.ietf.org/

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   [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>.

10.2.  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, "Standard Specification for Remote ID and Tracking",

   [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., Wiethuechter, A., and A. Gurtov,
              "UAS Remote ID", Work in Progress, Internet-Draft, draft-
              ietf-drip-rid-06, 31 December 2020, <http://www.ietf.org/

              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.

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

   [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,

   [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,

   [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>.

   [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>.

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

              3GPP, "UAS RID requirement study", 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 FAAs'
   effort of integrating UAS's operation into the national airspace
   system (NAS) leads 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 is published in 2020 [FAA_UAS_Concept_Of_Ops].

   The eventual development and implementation are 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 make sure safe and efficient integrations of manned and
   unmanned aircraft into the national airspace.

   The UTM composes of UAS operation infrastructure, procedures and
   local regulation compliance policies to guarantee UAS's safe
   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.

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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 Entity acts as a
   proxy between UAS operators and UTM service providers.  It provides
   services like real-time UAS traffic monitor and planning,
   aeronautical data archiving, airspace and violation control,
   interacting with other third-party control entities, etc.  A USS can
   coexist with other USS(s) to build a large service coverage map which
   can load-balance, 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's flight plan
   application and approval process for airspace authorization in real-
   time by checking against multiple aeronautical databases such as
   airspace classification and fly rules associated with it, FAA UAS
   facility map, special use airspace, Notice to Airman (NOTAM), and
   Temporary Flight Rule (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 takeoff or land in a
       controlled airspace (e.g., Class Bravo, Charlie, Delta and Echo
       in United States), the USS where UAS is currently communicating
       with is responsible for UAS's registration, authenticating the
       UAS's fly plan by checking against designated UAS fly map
       database, obtaining the air traffic control (ATC) authorization
       and monitor the UAS fly path in order to maintain safe boundary
       and follow the pre-authorized route.

   2.  For a UAS participating in UTM and take off or land in an
       uncontrolled airspace (ex.  Class Golf in the United States),
       pre-fly authorization must be obtained from a USS when operating
       beyond-visual-of-sight (BVLOS) operation.  The USS either accepts
       or rejects received intended fly plan from the UAS.  Accepted UAS
       operation may share its current fly 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 monitor.

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A.4.  Automatic Dependent Surveillance Broadcast (ADS-B)

   The ADS-B is the de facto technology used in manned aviation for
   sharing location information, which is a ground and satellite based
   system designed in the early 2000s.  Broadcast RID is conceptually
   similar to ADS-B.  However, for numerous technical and regulatory
   reasons, ADS-B itself is not suitable for low-flying small UA.
   Technical reasons include: needing RF-LOS to large, expensive (hence
   scarce) ground stations; needing both a satellite receiver and 1090
   MHz transceiver onboard CSWaP constrained UA; the limited bandwidth
   of both uplink and downlink, which are adequate for the current
   manned aviation traffic volume, but would likely be saturated by
   large numbers of UAS, endangering manned aviation; etc.
   Understanding these technical shortcomings, regulators world-wide
   have ruled out use of ADS-B for the small UAS for which UAS RID and
   DRIP are intended.

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

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   Palo Alto,  94588
   United States of America

   Email: shuai.zhao@ieee.org

   Andrei Gurtov
   Linkoeping University
   SE-58183 Linkoeping Linkoeping

   Email: gurtov@acm.org

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