Drone Remote Identification Protocol (DRIP) Architecture

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

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 a "Final Rule" in 2021 [FAA_RID], imposing requirements on UAS manufacturers and operators, both commercial and recreational. The rule clearly states that Automatic Dependent Surveillance Broadcast (ADS-B) Out and transponders cannot be used to satisfy the RID requirements on UAS to which the rule applies (see Appendix B).¶

European Union Aviation Safety Agency (EASA)¶

• The EASA published a [Delegated] regulation in 2019 imposing requirements on UAS manufacturers and third-country operators, including but not limited to RID requirements. The EASA also published in 2019 an [Implementing] regulation laying down detailed rules and procedures for UAS operations and operating personnel.¶

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] 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] is cited by FAA in its RID final rule [FAA_RID] as "a potential means of compliance" to a Remote ID rule.¶

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

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

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

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 airspace.¶

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

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

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

Figure 4 illustrates a brief summary of the general UAS RID usage scenarios in 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 |      |          |
             +----------+     +-----+      +----------+

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] 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] 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 identifiers (Section 4).¶

• • Mechanisms to leverage Domain Name System (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).¶

• • 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).¶

2.1. Architecture Terminology

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

2.2. Abbreviations

EdDSA:      Edwards-Curve Digital Signature Algorithm¶

HHIT:       Hierarchical HIT¶

HIP:        Host Identity Protocol¶

HIT:        Host Identity Tag¶

2.3. Additional Definitions

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

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 RID [F3411] allows a UAS ID of types 1, 2 and 3 of 20 bytes; a revision to [F3411], currently in balloting (as of Oct 2021), adds type 4, Session IDs, to be standardized by IETF and other standard development organizations (SDOs) as extensions to ASTM 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 RID [F3411] Authentication Message payload is 201 bytes for most authentication types, but for type 5, also added in this revision, for IETF and other SDOs to develop Specific Authentication Methods as extensions to ASTM 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 ID.¶

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 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 registries.¶

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. This is the defense against a cryptographic hash second pre-image 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 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¶

5.1. Public Information Registry

5.2. Private Information Registry

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.¶

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

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, 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 that 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 operational intent (flight plan) submission and application for airspace authorization in real-time by checking against multiple aeronautical databases such as airspace classification and 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, 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, and 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 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.¶