| Internet-Draft | DRIP Architecture | May 2021 |
| Card, et al. | Expires 11 November 2021 | [Page] |
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 [I-D.ietf-drip-reqs].¶
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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 [I-D.ietf-drip-reqs].¶
This document assumes the reader is familiar with [I-D.ietf-drip-reqs].¶
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 safety and other considerations dictate that UAS be remotely identifiable. Civil Aviation Authorities (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.¶
Federal Aviation Administration (FAA)¶
American Society for Testing and Materials (ASTM)¶
The 3rd Generation Partnership Project (3GPP)¶
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
xxxxx
|
|
| app messages directly over
| one-way RF data link (no IP)
|
|
+
x
xxxxx
x
x
x x Observer's device (e.g. smartphone)
x x
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.¶
A RID data dictionary and data flow for Network RID are defined in [F3411-19]. This data flow is emitted 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
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 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 a USS. Subscribed Net-RID DP forward RID information via the Internet to subscribed Observer devices. Regulations require and [F3411-19] describes RID data elements that must be transported end-to-end from the UAS to the subscribed Observer devices.¶
[F3411-19] prescribes the protocols only between the Net-RID SP, Net-RID DP, and the Discovery and Synchronization Service (DSS). DRIP may also address standardization of protocols between the UA and GCS, between the UAS and the Net-RID SP, and/or between the Net-RID DP and Observer devices.¶
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.¶
+----------+
| Observer |
+----------+
/ \
/ \
+-----+ +-----+
| UA1 | | UA2 |
+-----+ +-----+
\ /
\ /
+----------+
| Internet |
+----------+
/ \
/ \
+-------+ +-------+
| USS-1 | <-------> | USS-2 |
+-------+ +-------+
\ /
\ /
+------+
| DSS |
+------+
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.¶
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 | | |
+----------+ +-----+ +----------+
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:¶
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.¶
This document uses terms defined in [I-D.ietf-drip-reqs].¶
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¶
RF: Radio Frequency¶
SDSP: Supplemental Data Service Provider¶
UA: Unmanned Aircraft¶
UAS: Unmanned Aircraft System¶
USS: UAS Service Supplier¶
UTM: UAS Traffic Management¶
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 instruments.¶
Claims:¶
Assertions:¶
Attestations:¶
Certificates:¶
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 attestation.¶
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 USS.¶
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.¶
Standard approaches like X.509 and PKI will not fit these constraints, even using the new EdDSA [RFC8032] algorithm cannot fit within the maximum 201 byte limit, due in large measure to ASN.1 encoding format overhead.¶
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. This could be as in [RFC8002] or simply the HHIT as Subject Alternative Name (SAN) and no Distinguished Name (DN).¶
A self-attestation of the HHIT RID 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 compressed attestation consists of only the HHIT, a timestamp, and the EdDSA signature on them. The HHIT prefix and suiteID provide crypto agility and implicit encoding rules. Similarly, a self-attestation of the Hierarchical registration of the RID (an attestation of a RID third-party registration "certificate") can be done in 200 bytes. Both these are detailed in UAS RID [I-D.ietf-drip-rid].¶
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.¶
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) comes from signing this cryptographic ID with the associated private key. It is statistically hard for another entity to create a public key that would generate (spoof) the ID.¶
HITs are so designed; they 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 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.¶
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 Remote ID itself needs to be the inquiry input into the lookup. An HHIT DRIP identifier contains cryptographically embedded registration information. This HHIT registration hierarchy, along with the IPv6 prefix, is trustable and sufficient information that can be used to perform such a lookup. Additionally, the 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 registries.¶
HHITs can be used throughout the UAS/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.¶
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).¶
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.¶
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.¶
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).¶
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.¶
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].¶
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 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.¶
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.¶
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 SP.¶
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 UA.¶
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 operation).¶
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.¶
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.¶
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.¶
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).¶
This section illustrates a couple of use case scenarios where UAS participation in UTM has significant safety improvement.¶
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 following:¶
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.¶