| Internet-Draft | DRIP Reqs | May 2020 |
| Card, et al. | Expires 26 November 2020 | [Page] |
This document defines the requirements for Drone Remote Identification Protocol (DRIP) Working Group protocols to support Unmanned Aircraft System Remote Identification and tracking (UAS RID) for safety, regulatory compliance and other purposes.¶
Complementing external technical standards as regulator-accepted means of compliance with UAS RID regulations, DRIP will:¶
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Many considerations (especially safety) dictate that UAS be remotely identifiable. Any observer with responsibilities involving aircraft inherently must classify them situationally according to basic considerations, as illustrated notionally in Figure 1 below.¶
xxxxxxx +--------------+
x x No | |
x ID? x+---->| UNIDENTIFIED |
x x | |
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+
| Yes
v
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x x
+---------+x TYPE? x+----------+
| x x |
| xxxxxxx |
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v v v
+--------------+ +--------------+ +--------------+
| | | | | |
| TASKABLE | | LOW CONCERN | | HIGH CONCERN |
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+--------------+ +--------------+ +--------------+
Civil Aviation Authorities (CAAs) worldwide are mandating Unmanned Aircraft System Remote Identification and tracking (UAS RID). The European Union Aviation Safety Agency (EASA) has published [Delegated] and [Implementing] Regulations. The United States (US) Federal Aviation Administration (FAA) has published a Notice of Proposed Rule Making ([NPRM]) and has described the key role that UAS RID plays in UAS Traffic Management (UTM [CONOPS] especially Section 2.6). CAAs currently (2020) promulgate performance-based regulations that do not specify techniques, but rather cite industry consensus technical standards as acceptable means of compliance.¶
ASTM International, Technical Committee F38 (UAS), Subcommittee F38.02 (Aircraft Operations), Work Item WK65041, developed ASTM F3411-19 [F3411-19] Standard Specification for Remote ID and Tracking. It defines two means of UAS RID:¶
Generally the same information must provided via both means. Network RID depends upon Internet connectivity in several segments from the UAS to the observer. Broadcast RID should need Internet (or other Wide Area Network) connectivity only for UAS registry information lookup using the directly locally received UAS ID as a key.¶
[F3411-19] specifies 3 UAS ID types:¶
The EU allows only Type 1; the US allows Types 1 and 3, but requires Type 3 IDs (if used) each to be used only once (for a single UAS flight, which in the context of UTM is called an "operation"). [F3411-19] Broadcast RID transmits all information in the clear as plaintext (ASCII or binary), so static IDs enable trivial correlation of patterns of use, unacceptable in many applications, e.g., package delivery routes of competitors.¶
An ID is not an end in itself; it exists to enable lookups and provision of services complementing mere identification.¶
Minimal specified information must be made available to the public; access to other data, e.g., UAS operator Personally Identifiable Information (PII), must be limited to strongly authenticated personnel, properly authorized per policy. The balance between privacy and transparency remains a subject for public debate and regulatory action; DRIP can only offer tools to expand the achievable trade space and enable trade-offs within that space. [F3411-19] specifies only how to get the UAS ID to the observer; how the observer can perform these lookups, and how the registries first can be populated with information, is unspecified.¶
Using UAS RID to facilitate vehicular (V2X) communications and applications such as Detect And Avoid (DAA, which would impose tighter latency bounds than RID itself) is an obvious possibility, explicitly contemplated in the FAA NPRM. However, applications of RID beyond RID itself have been omitted from [F3411-19]; DAA has been explicitly declared out of scope in ASTM working group discussions, based on a distinction between RID as a security standard vs DAA as a safety application. Although dynamic establishment of secure communications between the observer and the UAS pilot seems to have been contemplated by the FAA UAS ID and Tracking Aviation Rulemaking Committee (ARC) in their [Recommendations], it is not addressed in any of the subsequent proposed regulations or technical specifications.¶
The need for near-universal deployment of UAS RID is pressing. This implies the need to support use by observers of already ubiquitous mobile devices (typically smartphones and tablets). Anticipating likely CAA requirements to support legacy devices, especially in light of [Recommendations], [F3411-19] specifies that any UAS sending Broadcast RID over Bluetooth must do so over Bluetooth 4, regardless of whether it also does so over newer versions; as UAS sender devices and observer receiver devices are unpaired, this implies extremely short "advertisement" (beacon) frames.¶
UA onboard RID devices are severely constrained in Cost, Size, Weight and Power ($SWaP). Cost is a significant impediment to the necessary near-universal adoption of UAS send and observer receive RID capabilities. $SWaP is a burden not only on the designers of new UA for production and sale, but also on owners of existing UA that must be retrofit. Radio Controlled (RC) aircraft modelers, "hams" who use licensed amateur radio frequencies to control UAS, drone hobbyists and others who custom build UAS all need means of participating in UAS RID sensitive to both generic $SWaP and application-specific considerations.¶
To accommodate the most severely constrained cases, all these conspire to motivate system design decisions, especially for the Broadcast RID data link, which complicate the protocol design problem: one-way links; extremely short packets; and Internet-disconnected operation of UA onboard devices. Internet-disconnected operation of observer devices has been deemed by ASTM F38.02 too infrequent to address, but for some users is important and presents further challenges.¶
Despite work by regulators and Standards Development Organizations (SDOs), there are substantial gaps in UAS standards generally and UAS RID specifically. [Roadmap] especially Section 7.8 catalogs UAS RID standards, ongoing standardization activities and gaps.¶
Given not only packet payload length and bandwidth, but also processing and storage within the $SWaP constraints of very small (e.g. consumer toy) UA, heavyweight cryptographic security protocols are infeasible, yet trustworthiness of UAS RID information is essential. Under [F3411-19], even the most basic datum, the UAS ID string (typically number) itself can be merely an unsubstantiated claim. Observer devices being ubiquitous, thus popular targets for malware or other compromise, cannot be generally trusted (although the user of each device is compelled to trust that device, to some extent); a "fair witness" functionality (inspired by [Stranger]) may be desirable.¶
DRIP's initial goal is to make RID immediately actionable, in both Internet and local-only connected scenarios (especially emergencies), in severely constrained UAS environments, balancing legitimate (e.g., public safety) authorities' Need To Know trustworthy information with UAS operators' privacy. By "immediately actionable" is meant information of sufficient precision, accuracy, timeliness, etc. for an observer to use it as the basis for immediate decisive action, whether that be to trigger a defensive counter-UAS system, to attempt to initiate communications with the UAS operator, to accept the presence of the UAS in the airspace where/when observed as not requiring further action, or whatever, with potentially severe consequences of any action or inaction chosen based on that information. Potential follow-on goals may extend beyond providing timely and trustworthy identification data, to using it to enable identity-oriented networking of UAS.¶
DRIP (originally Trustworthy Multipurpose Remote Identification, TM-RID) potentially could be applied to verifiably identify other types of registered things reported to be in specified physical locations, but the urgent motivation and clear initial focus is UAS. Existing Internet resources (protocol standards, services, infrastructure, and business models) should be leveraged. A natural Internet based architecture for UAS RID conforming to proposed regulations and external technical standards is described in a companion architecture document [I-D.ietf-drip-arch]; this document describes only relevant requirements.¶
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all capitals, as shown here.¶
This section defines a set of terms that are used in DRIP documents. This list is meant to be the DRIP terminology reference. Some of the terms listed below are not used in this document.¶
UA may be fixed wing Short Take-Off and Landing (STOL), rotary wing (e.g., helicopter) Vertical Take-Off and Landing (VTOL), or hybrid. They may be single engine or multi engine. The most common today are multicopters: rotary wing, multi engine. The explosion in UAS was enabled by hobbyist development, for multicopters, of advanced flight stability algorithms, enabling even inexperienced pilots to take off, fly to a location of interest, hover, and return to the take-off location or land at a distance. UAS can be remotely piloted by a human (e.g., with a joystick) or programmed to proceed from Global Positioning System (GPS) waypoint to waypoint in a weak form of autonomy; stronger autonomy is coming. UA are "low observable": they typically have a small radar cross section; they make noise quite noticeable at short range but difficult to detect at distances they can quickly close (500 meters in under 17 seconds at 60 knots); they typically fly at low altitudes (for the small UAS to which RID applies in the US, under 400 feet AGL); they are highly maneuverable so can fly under trees and between buildings.¶
UA can carry payloads including sensors, cyber and kinetic weapons, or can be used themselves as weapons by flying them into targets. They can be flown by clueless, careless or criminal operators. Thus the most basic function of UAS RID is "Identification Friend or Foe" (IFF) to mitigate the significant threat they present. Numerous other applications can be enabled or facilitated by RID: consider the importance of identifiers in many Internet protocols and services.¶
Network RID from the UA itself (rather than from its GCS) and Broadcast RID require one or more wireless data links from the UA, but such communications are challenging due to $SWaP constraints and low altitude flight amidst structures and foliage over terrain.¶
Disambiguation of multiple UA flying in close proximity may be very challenging, even if each is reporting its identity, position and velocity as accurately as it can.¶
Network RID has several variants. The UA may have persistent onboard Internet connectivity, in which case it can consistently source RID information directly over the Internet. The UA may have intermittent onboard Internet connectivity, in which case the GCS must source RID information whenever the UA itself is offline. The UA may not have Internet connectivity of its own, but have instead some other form of communications to another node that can relay RID information to the Internet; this would typically be the GCS (which to perform its function must know where the UA is).¶
The UA may have no means of sourcing RID information, in which case the GCS must source it; this is typical under FAA NPRM Limited RID proposed rules, which require providing the location of the GCS (not that of the UA). In the extreme case, this could be the pilot using a web browser to designate, to an UAS Service Supplier (USS) or other UTM entity, a time-bounded airspace volume in which an operation will be conducted; this may impede disambiguation of ID if multiple UAS operate in the same or overlapping spatio-temporal volumes.¶
In most cases in the near term, if the RID information is fed to the Internet directly by the UA or GCS, the first hop data links will be cellular Long Term Evolution (LTE) or Wi-Fi, but provided the data link can support at least UDP/IP and ideally also TCP/IP, its type is generally immaterial to the higher layer protocols. An UAS as the ultimate source of Network RID information feeds an USS acting as a Network RID Service Provider (Net-RID SP), which essentially proxies for that and other sources; an observer or other ultimate consumer of Network RID information obtains it from a Network RID Display Provider (Net-RID DP), which aggregates information from multiple Net-RID SPs to offer coverage of an airspace volume of interest. Network RID Service and Display providers are expected to be implemented as servers in well-connected infrastructure, accessible via typical means such as web APIs/browsers.¶
Network RID is the more flexible and less constrained of the defined UAS RID means, but is only partially specified in [F3411-19]. It is presumed that IETF efforts supporting Broadcast RID (see next section) can be easily generalized for Network RID.¶
[F3411-19] specifies three Broadcast RID data links: Bluetooth 4.X; Bluetooth 5.X Long Range; and Wi-Fi with Neighbor Awareness Networking (NAN). For compliance with this standard, an UA must broadcast (using advertisement mechanisms where no other option supports broadcast) on at least one of these; if broadcasting on Bluetooth 5.x, it is also required concurrently to do so on 4.x (referred to in [F3411-19] as Bluetooth Legacy).¶
The selection of the Broadcast media was driven by research into what is commonly available on 'ground' units (smartphones and tablets) and what was found as prevalent or 'affordable' in UA. Further, there must be an Application Programming Interface (API) for the observer's receiving application to have access to these messages. As yet only Bluetooth 4.X support is readily available, thus the current focus is on working within the 26 byte limit of the Bluetooth 4.X "Broadcast Frame" transmitted on beacon channels. After nominal overheads, this limits the UAS ID string to a maximum length of 20 bytes, and precludes the same frame carrying position, velocity and other information that should be bound to the UAS ID, much less strong authentication data. This requires segmentation ("paging") of longer messages or message bundles ("Message Pack"), and/or correlation of short messages (anticipated by ASTM to be done on the basis of Bluetooth 4 MAC address, which is weak and unverifiable).¶
DRIP will focus on making information obtained via UAS RID immediately usable:¶
Any UA can assert any ID using the [F3411-19] required Basic ID message, which lacks any provisions for verification. The Position/Vector message likewise lacks provisions for verification, and does not contain the ID, so must be correlated somehow with a Basic ID message: the developers of [F3411-19] have suggested using the MAC addresses, but these may be randomized by the operating system stack to avoid the adversarial correlation problems of static identifiers.¶
The [F3411-19] optional Authentication Message specifies framing for authentication data, but does not specify any authentication method, and the maximum length of the specified framing is too short for conventional digital signatures and far too short for conventional certificates. The one-way nature of Broadcast RID precludes challenge-response security protocols (e.g., observers sending nonces to UA, to be returned in signed messages). An observer would be seriously challenged to validate the asserted UAS ID or any other information about the UAS or its operator looked up therefrom.¶
Further, [F3411-19] provides very limited choices for an observer to communicate with the pilot, e.g., to request further information on the UAS operation or exit from an airspace volume in an emergency. The System Message provides the location of the pilot/GCS, so an observer could physically go to the asserted GCS location to look for the remote pilot. An observer with Internet connectivity could look up operator PII in a registry, then call a phone number in hopes someone who can immediately influence the UAS operation will answer promptly during that operation.¶
Thus complementing [F3411-19] with protocols enabling strong authentication, preserving operator privacy while enabling immediate use of information by authorized parties, is critical to achieve widespread adoption of a RID system supporting safe and secure operation of UAS.¶
Whether a UAS ID is generated by the operator, GCS, UA, USS or registry, or some collaboration thereamong, is unspecified; however, there must be agreement on the UAS ID among these entities.¶
As satisfying these requirements may require that authorized actors have connectivity to third parties, e.g., Internet to a Remote ID USS, to enable decryption, and such connectivity cannot be assured, DRIP SHOULD provide automatic fallback to plaintext transmission of safety-critical information when necessary.¶
This document is largely based on the process of one SDO, ASTM. Therefore, it is tailored to specific needs and data formats of this standard. Other organizations, for example in EU, do not necessary follow the same architecture. IETF traditionally operates assuming the source material for the standardization process is publicly available. However, ASTM standards require a fee for download. Therefore a double-liaison program at IETF might need to be activated, providing free access to ASTM specifications for contributors to IETF documents.¶
The need for drone ID and operator privacy is an open discussion topic. For instance, in the ground vehicular domain each car carries a publicly visible plate number. In some countries, for nominal cost or even for free, anyone can resolve the identity and contact information of the owner. Civil commercial aviation and maritime industries also have a tradition of broadcasting plane or ship ID, coordinates and even flight plans in plain text. Community networks such as OpenSky and Flightradar use this open information through ADS-B to deploy public services of flight tracking. Many researchers also use these data to perform optimization of routes and airport operations. Such ID information should be integrity protected, but not necessarily confidential.¶
In civil aviation, aircraft identity is broadcast by a device known as transponder. It transmits a four-digit squawk code, which is assigned by a traffic controller to an airplane after approving a flight plan. There are several reserved codes such as 7600 which indicate radio communication failure. The codes are unique in each traffic area and can be re-assigned when entering another control area. The code is transmitted in plain text by the transponder and also used for collision avoidance by a system known as Traffic alert and Collision Avoidance System (TCAS). The system could be used for UAS as well initially, but the code space is quite limited and likely to be exhausted soon. The number of UAS far exceeds the number of civil airplanes in operation.¶
The ADS-B system is utilized in civil aviation for each "ADS-B Out" equipped airplane to broadcast its ID, coordinates and altitude for other airplanes and ground control stations. If this system is adopted for drone IDs, it has additional benefit with backward compatibility with civil aviation infrastructure; then, pilots and dispatchers will be able to see UA on their control screens and take those into account. If not, a gateway translation system between the proposed drone ID and civil aviation system should be implemented. Again, system saturation due to large numbers of UAS is a concern.¶
Wi-Fi and Bluetooth are two wireless technologies currently recommended by ASTM specifications due to their widespread use and broadcast nature. However, those have limited range (max 100s of meters) and may not reliably deliver UAS ID at high altitude or distance. Therefore, a study should be made of alternative technologies from the telecom domain (WiMax, 5G) or sensor networks (Sigfox, LORA). Such transmission technologies can impose additional restrictions on packet sizes and frequency of transmissions, but could provide better energy efficiency and range. In civil aviation, Controller-Pilot Data Link Communications (CPDLC) is used to transmit command and control between the pilots and ATC. It could be considered for UAS as well due to long range and proven use despite its lack of security [cpdlc].¶
L-band Digital Aeronautical Communications System (LDACS) is being standardized by ICAO and IETF for use in future civil aviation [I-D.maeurer-raw-ldacs]. It provides secure communication, positioning and control for aircraft using a dedicated radio band. It should be analyzed as a potential provider for UAS RID as well. This will bring the benefit of a global integrated system creating a global airspace use awareness.¶
This document does not make any IANA request.¶
DRIP is all about safety and security, so content pertaining to such is not limited to this section. DRIP information falls into two classes: that which, to achieve the purpose, must be published openly in clear plaintext, for the benefit of any observer; and that which must be protected (e.g., PII of pilots) but made available to properly authorized parties (e.g., public safety personnel who urgently need to contact pilots in emergencies). This classification must be made explicit and reflected with markings, design, etc. Classifying the information will be addressed primarily in external standards; herein it will be regarded as a matter for CAA, registry and operator policies, for which enforcement mechanisms will be defined within the scope of DRIP WG and offered. Details of the protection mechanisms will be provided in other DRIP documents. Mitigation of adversarial correlation will also be addressed.¶
The work of the FAA's UAS Identification and Tracking (UAS ID) Aviation Rulemaking Committee (ARC) is the foundation of later ASTM [F3411-19] and 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.¶
The work of the FAA's UAS Identification and Tracking (UAS ID) Aviation Rulemaking Committee (ARC) is the foundation of later ASTM [F3411-19] and IETF DRIP 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, Mohamed Boucadair, Toerless Eckert, Susan Hares, Mika Järvenpää, Daniel Migault, Saulo Da Silva and Shuai Zhao.¶