| Internet-Draft | drip-arch | December 2020 |
| Card, et al. | Expires 3 July 2021 | [Page] |
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.¶
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This document describes a natural Internet and MAC-layer broadcast-based architecture for Unmanned Aircraft System Remote Identification and tracking (UAS RID), conforming to proposed regulations and external technical standards, satisfying the requirements listed in the companion requirements document [I-D.ietf-drip-reqs].¶
Many considerations (especially safety) dictate that UAS be remotely identifiable. Civil Aviation Authorities (CAAs) worldwide are mandating Unmanned Aircraft Systems (UAS) Remote Identification (RID). CAAs currently (2020) promulgate performance-based regulations that do not specify techniques, but rather cite industry consensus technical standards as acceptable means of compliance.¶
A RID is an application enabler for a UAS to be identified by a 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. The FAA has published a Notice of Proposed Rule Making [NPRM]. CAAs currently promulgate performance-based regulations that do not specify techniques, but rather cite industry consensus technical standards as acceptable means of compliance.¶
ASTM¶
3GPP¶
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 this information to Network Remote ID Display Providers (Net-RID DP). It is the Net-RID DP that respond to queries from Network Remote ID clients (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 1 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
Via the direct Radio Frequency (RF) link between the UA and GCS, Command and Control (C2) flows between the GCS to the UA such that either can communicate with the Net-RID SP. For all but the simplest hobby aircraft, position and status flow from the UA to the GCS and on to the Net-RID SP. Thus via the Internet, through three distinct segments, Network RID information flows from the UAS to the Observer.¶
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 2 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, the Observer may gain more information about those visible UAS.¶
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 have knowledge about all activities in a 4D airspace. The interactions among observer, UA and USS is 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 architecture 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 | | |
+----------+ +-----+ +----------+
Editor's note 1: the architecture may need more clarification, and address the following:¶
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 an architecture for DRIP itself. This includes presenting the gaps between the CAAs' Concepts of Operations and [F3411-19] as it relates to 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 meaning of "Claims", "Assertions", "Attestations", and "Certificates" in the context of DRIP.¶
This is due, in part, to the term "certificate" having significant technologic and legal baggage associated with it, specifically around X.509 certificates. These type of certificates and Public Key Infrastructure invokes 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. As such the following terms are being used in DRIP.¶
Claims:¶
Assertions:¶
Attestations:¶
Certificates:¶
This section describes the basic requirements of a DRIP UAS remote ID 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 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 UAS ID needs to be "Trustworthy". This means that within the framework of the RID messages, an observer can establish that the RID used does uniquely belong to the UA. That the only way for any other UA to assert this RID would be to steal something from within the UA. The RID is self-generated by the UAS (either UA or GCS) and registered with the USS.¶
Within the limitations of Broadcast RID, this is extremely challenging as:¶
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] introducing hierarchy as defined in HHIT [I-D.ietf-drip-rid]; using Hierarchical HITs 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. the UTM Discovery and Synchronization Service and the UTM InterUSS protocol) mappings between the more flexible but larger X.509 certificates and the HHIT based structures must be devised.¶
By using the EdDSA HHIT suite, self-assertions of the RID can be done in as little as 84 bytes. Third-party assertions can be done in 200 bytes. An observer would need Internet access to validate a self-assertion claim. A third-party assertion can be validated via a small credential cache in a disconnected environment. This third-party assertion is possible when the third-party also uses HHITs for its identity and the UA has the public key for that HHIT¶
For a Remote ID to be trustworthy in the Broadcast mode, there MUST be 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 attestation (compared to claim) of ownership.¶
HITs are statistically unique through the cryptographic hash feature of second-preimage resistance. The cryptographically-bound addition of the Hierarchy and a 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.¶
Remote IDs need a deterministic lookup mechanism that rapidly provides actionable information about the identified UA. The ID itself needs to be the key into the lookup given the constraints imposed by some of the broadcast media. This can best be achieved by an ID registration hierarchy cryptographically embedded within the ID.¶
The original proposal for HITs included a registration hierarchy scheme. This was dropped during HIP development for lack of a use case. No similar mechanism is possible within CGAs. It is a rather straightforward design update to HITs to Hierarchical HITs (HHITs) to meet the UAS Remote ID use case.¶
The HHIT needs to consist 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, may enhance the HHITs use beyond the basic Remote ID function (e.g. use in HIP, [RFC7401]).¶
A DRIP UAS ID SHOULD be a HHIT. It SHOULD be self-generated by the UAS (either UA or GCS) and MUST be registered with the Private Information Registry identified in its hierarchy fields. Each UAS ID HHIT MUST NOT be used more than once, with one exception as follows.¶
Each UA MAY be assigned, by its manufacturer, a single HI and derived HHIT encoded as a hardware serial number per [CTA2063A]. Such a static HHIT SHOULD be used only to bind one-time use UAS IDs (other HHITs) to the unique UA. Depending upon implementation, this may leave a HI private key in the possession of the manufacturer (see Security Considerations).¶
Each UA equipped for Broadcast RID MUST 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. Each UAS equipped for Network RID MUST be provisioned likewise; the private key SHOULD reside 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 MUST be provisioned with public keys of the UAS RID root registries and MAY be provisioned with public keys or certificates for subordinate registries.¶
Operators and Private Information Registries MUST possess and other UTM entities MAY possess UAS ID style HHITs. When present, such HHITs SHOULD be used with HIP to strongly mutually authenticate and optionally encrypt communications.¶
The only (at time of Trustworthy Remote ID design) extant fixed length ID cryptographically derived from a public key are the Host Identity Tag [RFC7401], HITs, and Cryptographically Generated Addresses [RFC3972], CGAs. Both lack a registration/retrieval capability and CGAs have only a limited crypto agility [RFC4982]. Distributed Hash Tables have been tried for HITs [RFC6537]; this is really not workable for a globally deployed UAS Remote ID scheme.¶
The security of HHITs is achieved first through the cryptographic hashing function of the above information, along with a registration process to mitigate the probability of a hash collision (first registered, first allowed).¶
The DRIP HHIT RID registration goes beyond what is currently envisioned in UTM for the UAS to USS registration/subscription process.¶
UAS registries hold both public and private UAS information resulting from the UAS RID registration. Given these different uses, and to improve scalability, security and simplicity of administration, the public and private information can be stored in different registries, indeed different types of registry.¶
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 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 directly provide all public registry information directly. The observer may 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. Thus there is no need for information on them in a Public Registry.¶
A DRIP public information registry MUST respond to standard DNS queries, in the definitive public Internet DNS hierarchy. It MUST support NS, MX, SRV, TXT, AAAA, PTR, CNAME and HIP RR (the last per [RFC8005]) types. If a DRIP public information registry lists, in a HIP RR, any HIP RVS servers for a given DRIP UAS ID, those RVS servers MUST restrict relay services per AAA policy; this may require extensions to [RFC8004]. These public information registries SHOULD 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 RID is similar to that required for Internet domain name registration. This information SHOULD be available for ALL UAS, including those that only use Network RID. A DRIP RID 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 RID MUST be amenable to handling as an Internet domain name (at an arbitrary level in the hierarchy), MUST be registered in at least a pseudo-domain (e.g. .ip6.arpa for reverse lookup), and MAY be registered as a sub-domain (for forward lookup). This DNS information MAY be protected with DNSSEC. Its access SHOULD be protected with a secure DNS transport (e.g. DNS over TLS).¶
A DRIP private information registry MUST support essential Internet domain name registry operations (e.g. add, delete, update, query) using interoperable open standard protocols. It SHOULD support the Extensible Provisioning Protocol (EPP) and the Registry Data Access Protocol (RDAP) with access controls. It MAY use XACML to specify those access controls. It MUST be listed in a DNS: that DNS MAY be private; but absent any compelling reasons for use of private DNS, SHOULD be the definitive public Internet DNS hierarchy. The DRIP private information registry in which a given UAS is registered MUST be findable, starting from the UAS ID, using the methods specified in [RFC7484]. A DRIP private information registry MAY 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 NPRM requires both for Standard RID and specifies Network RID only for Limited RID.¶
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 must implement, integrate, or accept outputs from, a Broadcast RID receiver. It MUST NOT interface directly with a GCS, Net-RID SP, Net- RID DP or Network RID client. It MUST present a TBD interface to a CS-RID SDSP; this interface SHOULD be based upon but readily distinguishable from that between a GCS and a Net-RID SP.¶
A CS-RID SDSP MUST appear (i.e. present the same interface) to a Net- RID SP as a Net-RID DP. A CS-RID SDSP MUST appear to a Net-RID DP as a Net-RID SP. A CS-RID SDSP MUST NOT present a standard GCS-facing interface as if it were a Net-RID SP. A CS-RID SDSP MUST NOT present a standard client-facing interface as if it were a Net-RID DP. A CS- RID SDSP MUST present a TBD interface to a CS-RID Finder; this interface SHOULD be based upon but readily distinguishable from that between a GCS and a Net-RID SP.¶
The UTM (U-SPACE) architecture leaves much about all the operators/UAS to the various USS. Each CAA will have some registration requirements on operators (FAA part 105 is considered very minimal by some CAA), along with some UAS and operation registration. DRIP leverages this model with Identities for each component that augment the DRIP RID and transactions to support these Identities.¶
To this end, in DRIP, each Operator MUST generate a Host Identity of the Operator (HIo) and derived Hierarchical HIT of the Operator (HHITo). These are registered with a Private Information Registry along with whatever Operator data (inc. PII) is required by the cognizant CAA and the registry. In response, the Operator will obtain a Certificate from the Registry, an Operator (Cro), signed with the Host Identity of the Registry private key (HIr(priv)) proving such registration.¶
An Operator may now add a UA.¶
The operator then MUST provision the UA with HIa, HIa(priv), HHITa and Cra.¶
Broadcast RID messages may contain PII. This may be information about the UA such as its destination or Operator information such as GCS location. There is no absolute "right" in hiding PII, as there will be times (e.g., disasters) and places (buffer zones around airports and sensitive facilities) where policy may mandate all information be sent as cleartext. Otherwise, the modern general position (consistent with, e.g., the EU General Data Protection Regulation) is to hide PII unless otherwise instructed. While some have argued that a system like that of automobile registration plates should suffice for UAS, others have argued persuasively that each generation of new identifiers should take advantage of advancing technology to protect privacy, to the extent compatible with the transparency needed to protect safety.¶
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 may send the encrypted PII along with the Remote ID (to their UTM Service Provider) to get the plaintext. Alternatively, the authorized Observer may receive the key to directly decrypt all future PII content from the UA.¶
PII SHOULD protected unless the UAS is informed otherwise. This may come from operational instructions to even permit flying in a space/time. It may be special instructions at the start or during an operation. PII protection should 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 may 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. 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 implementation to realize UTM's functionality. The LAANC program can automate the UAS's fly 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 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.¶