Internet Engineering Task Force (IETF) N. Mäurer, Ed.
Request for Comments: 9372 T. Gräupl, Ed.
Category: Informational German Aerospace Center (DLR)
ISSN: 2070-1721 C. Schmitt, Ed.
Research Institute CODE, UniBwM
February
March 2023
L-Band Digital Aeronautical Communications System (LDACS)
Abstract
This document gives an overview of the L-band Digital Aeronautical
Communications System (LDACS) architecture, which provides a secure,
scalable, and spectrum-efficient terrestrial data link for civil
aviation. LDACS is a scheduled and reliable multi-application
cellular broadband system with support for IPv6. It is part of a
larger shift of flight guidance communication moving to IP-based
communication. High reliability and availability of IP connectivity
over LDACS, as well as security, are therefore essential. The intent
of this document is to introduce LDACS to the IETF community, raise
awareness on related activities inside and outside of the IETF, and
to seek expertise in shaping the shift of aeronautics to IP.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are candidates for any level of Internet
Standard; see Section 2 of RFC 7841.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
https://www.rfc-editor.org/info/rfc9372.
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Table of Contents
1. Introduction
2. Acronyms
3. Motivation and Use Cases
3.1. Voice Communications Today
3.2. Data Communications Today
4. Provenance and Documents
5. Applicability
5.1. Advances beyond the State of the Art
5.1.1. Priorities
5.1.2. Security
5.1.3. High Data Rates
5.2. Application
5.2.1. Air/Ground Multilink
5.2.2. Air/Air Extension for LDACS
5.2.3. Flight Guidance
5.2.4. Business Communications of Airlines
5.2.5. LDACS-Based Navigation
6. Requirements
7. Characteristics
7.1. LDACS Access Network
7.2. Topology
7.3. LDACS Protocol Stack
7.3.1. LDACS Physical Layer
7.3.2. LDACS Data Link Layer
7.3.3. LDACS Subnetwork Layer and Protocol Services
7.4. LDACS Mobility
7.5. LDACS Management Interfaces and Protocols
8. Reliability and Availability
8.1. Below Layer 1
8.2. Layers 1 and 2
8.3. Beyond Layer 2
9. Security Considerations
9.1. Security in Wireless Digital Aeronautical Communications
9.2. Security in Depth
9.3. LDACS Security Requirements
9.4. LDACS Security Objectives
9.5. LDACS Security Functions
9.6. LDACS Security Architecture
9.6.1. Entities
9.6.2. Entity Identification
9.6.3. Entity Authentication and Key Establishment
9.6.4. Message-In-Transit Confidentiality, Integrity, and
Authenticity
9.7. Considerations on LDACS Security Impact on IPv6 Operational
Security
10. IANA Considerations
11. Informative References
Appendix A. Selected Information from DO-350A
Acknowledgements
Authors' Addresses
1. Introduction
One of the main pillars of the modern Air Traffic Management (ATM)
system is the existence of a communications infrastructure that
enables efficient aircraft control and safe aircraft separation in
all phases of flight. Current systems are technically mature, but
they are suffering from the Very High Frequency (VHF) band's
increasing saturation in high-density areas and the limitations posed
by analog radio communications. Therefore, aviation strives for a
sustainable modernization of the aeronautical communications
infrastructure on the basis of IP.
This modernization is realized in two steps: (1) the transition of
communications data links from analog to digital technologies and (2)
the introduction of IPv6-based networking protocols [RFC8200] in
aeronautical networks [ICAO2015].
Step (1) is realized via ATM communications transitioning from analog
VHF voice [KAMA2010] to more spectrum-efficient digital data
communication. For terrestrial communications, the Global Air
Navigation Plan (GANP) created by the International Civil Aviation
Organization (ICAO) foresees this transition to be realized by the
development of the L-band Digital Aeronautical Communications System
(LDACS). Since Central Europe has been identified as the area of the
world that suffers the most from increased saturation of the VHF
band, the initial rollout of LDACS will likely start there and
continue to other increasingly saturated zones such as the East and
West Coast of the US and parts of Asia [ICAO2018].
Technically, LDACS enables IPv6-based Air/Ground (A/G) communication
related to aviation safety and regularity of flight [ICAO2015].
Passenger communication and similar services are not supported since
only communications related to "safety and regularity of flight" are
permitted in protected aviation frequency bands. The particular
challenge is that no additional frequencies can be made available for
terrestrial aeronautical communication; thus, it was necessary to
develop coexistence mechanisms and procedures to enable the
interference-free operation of LDACS in parallel with other
aeronautical services and systems in the protected frequency band.
Since LDACS will be used for aircraft guidance, high reliability and
availability for IP connectivity over LDACS are essential.
LDACS is standardized in ICAO and the European Organization for Civil
Aviation Equipment (EUROCAE).
This document provides information to the IETF community about the
aviation industry transition of flight guidance systems from analog
to digital, provides context for LDACS relative to related IETF
activities [LISP-GB-ATN], and seeks expertise on realizing reliable
IPv6 over LDACS for step (1). This document does not intend to
advance LDACS as an IETF Standards Track document.
Step (2) is a strategy for the worldwide rollout of IPv6-capable
digital aeronautical internetworking. This is called the
Aeronautical Telecommunications Network (ATN) / Internet Protocol
Suite (IPS) (hence, ATN/IPS). It is specified in the ICAO document
Doc 9896 [ICAO2015], the Radio Technical Commission for Aeronautics
(RTCA) document DO-379 [RTCA2019], the EUROCAE document ED-262
[EURO2019], and the Aeronautical Radio Incorporated (ARINC) document
858 [ARI2021]. LDACS is subject to these regulations since it
provides an "access network" (link-layer data link) to the ATN/IPS.
ICAO has chosen IPv6 as a basis for the ATN/IPS mostly for historical
reasons since a previous architecture based on ISO/OSI protocols (the
ATN/OSI) failed in the marketplace.
In the context of safety-related communications, LDACS will play a
major role in future ATM. ATN/IPS data links will provide
diversified terrestrial and space-based connectivity in a multilink
concept called the Future Communications Infrastructure (FCI)
[VIR2021]. From a technical point of view, the FCI will realize
airborne and multihomed IPv6 networks connected to a global ground
network via at least two independent communication technologies.
This is considered in more detail in related IETF Internet-Drafts documents
[LISP-GB-ATN] [RTGWG-ATN-BGP]. As such, ICAO has actively sought out
the support of IETF to define a mobility solution for step (2), which
is currently the Locator/ID Separation Protocol (LISP).
In the context of the Reliable and Available Wireless (RAW) Working
Group, developing options, such as intelligent switching between data
links, for reliably delivering content from and to endpoints is
foreseen. As LDACS is part of such a concept, the work of RAW is
immediately applicable. In general, with the aeronautical
communications system transitioning to ATN/IPS and data being
transported via IPv6, closer cooperation and collaboration between
the aeronautical and IETF community is desirable.
LDACS standardization within the framework of ICAO started in
December 2016. As of 2022, the ICAO standardization group has
produced the final Standards and Recommended Practices (SARPS)
document [ICAO2022] that defines the general characteristics of
LDACS. By the end of 2023, the ICAO standardization group plans to
have developed an ICAO technical manual, which is the ICAO equivalent
to a technical standard. The LDACS standardization is not finished
yet; therefore, this document is a snapshot of the current status.
The physical characteristics of an LDACS installation (form, fit, and
function) will be standardized by EUROCAE. Generally, the group is
open to input from all sources and encourages cooperation between the
aeronautical and IETF communities.
2. Acronyms
The following terms are used in the context of RAW in this document:
A/A: Air/Air
A/G: Air/Ground
A2G: Air-to-Ground
ACARS: Aircraft Communications Addressing and Reporting System
AC-R: Access Router
ADS-B: Automatic Dependent Surveillance - Broadcast
ADS-C: Automatic Dependent Surveillance - Contract
AeroMACS: Aeronautical Mobile Airport Communications System
ANSP: Air Traffic Network Service Provider
AOC: Aeronautical Operational Control
ARINC: Aeronautical Radio Incorporated
ARQ: Automatic Repeat reQuest
AS: Aircraft Station
ATC: Air Traffic Control
ATM: Air Traffic Management
ATN: Aeronautical Telecommunications Network
ATS: Air Traffic Service
BCCH: Broadcast Channel
CCCH: Common Control Channel
CM: Context Management
CNS: Communication Navigation Surveillance
COTS: Commercial Off-The-Shelf
CPDLC: Controller-Pilot Data Link Communications
CSP: Communications Service Provider
DCCH: Dedicated Control Channel
DCH: Data Channel
Diffserv: Differentiated Services
DLL: Data Link Layer
DLS: Data Link Service
DME: Distance Measuring Equipment
DSB-AM: Double Side-Band Amplitude Modulation
DTLS: Datagram Transport Layer Security
EUROCAE: European Organization for Civil Aviation Equipment
FAA: Federal Aviation Administration
FCI: Future Communications Infrastructure
FDD: Frequency Division Duplex
FL: Forward Link
GANP: Global Air Navigation Plan
GBAS: Ground-Based Augmentation System
GNSS: Global Navigation Satellite System
GS: Ground-Station
G2A: Ground-to-Air
HF: High Frequency
ICAO: International Civil Aviation Organization
IP: Internet Protocol
IPS: Internet Protocol Suite
kbit/s: kilobit per second
LDACS: L-band Digital Aeronautical Communications System
LISP: Locator/ID Separation Protocol
LLC: Logical Link Control
LME: LDACS Management Entity
MAC: Medium Access Control
MF: Multiframe
NETCONF: Network Configuration Protocol
OFDM: Orthogonal Frequency Division Multiplexing
OFDMA: Orthogonal Frequency Division Multiplexing Access
OSI: Open Systems Interconnection
PHY: Physical Layer
QPSK: Quadrature Phase-Shift Keying
RACH: Random-Access Channel
RL: Reverse Link
RTCA: Radio Technical Commission for Aeronautics
SARPS: Standards and Recommended Practices
SDR: Software-Defined Radio
SESAR: Single European Sky ATM Research
SF: Super-Frame
SNMP: Simple Network Management Protocol
SNP: Subnetwork Protocol
VDLm2: VHF Data Link mode 2
VHF: Very High Frequency
VI: Voice Interface
3. Motivation and Use Cases
Aircraft are currently connected to Air Traffic Control (ATC) and
Aeronautical Operational Control (AOC) services via voice and data
communications systems through all phases of flight. ATC refers to
communication for flight guidance. AOC is a generic term referring
to the business communication of airlines and refers to the mostly
proprietary exchange of data between the aircraft of the airline and
the airline's operation centers and service partners. The ARINC
document 633 was developed and first released in 2007 [ARI2019] with
the goal to standardize these messages for interoperability, e.g.,
messages between the airline and fueling or de-icing companies.
Within the airport and terminal area, connectivity is focused on high
bandwidth communications. However, in the en route domain, high
reliability, robustness, and range are the main foci. Voice
communications may use the same or different equipment as data
communications systems. In the following, the main differences
between voice and data communications capabilities are summarized.
The assumed list of use cases for LDACS complements the list of use
cases stated in [RAW-USE-CASES] and the list of reliable and
available wireless technologies presented in [RAW-TECHNOS].
3.1. Voice Communications Today
Voice links are used for Air/Ground (A/G) and Air/Air (A/A)
communications. The communications equipment can be installed on
ground or in the aircraft, in which cases the High Frequency (HF) or
VHF frequency band is used. For remote domains, voice communications
can also be satellite-based. All VHF and HF voice communications are
operated via open Broadcast Channels (BCCHs) without authentication,
encryption, or other protective measures. The use of well-proven
communications procedures via BCCHs, such as phraseology or read-
backs, requiring well-trained personnel help to enhance the safety of
communications but does not replace necessary cryptographical
security mechanisms. The main voice communications media is still
the analog VHF Double Side-Band Amplitude Modulation (DSB-AM)
communications technique supplemented by HF single side-band
amplitude modulation and satellite communications for remote and
oceanic regions. DSB-AM has been in use since 1948, works reliably
and safely, and uses low-cost communication equipment. These are the
main reasons why VHF DSB-AM communications are still in use, and it
is likely that this technology will remain in service for many more
years. However, this results in current operational limitations and
impediments in deploying new ATM applications, such as flight-centric
operation with point-to-point communications between pilots and ATC
officers [BOE2019].
3.2. Data Communications Today
Like for voice communications, data communications into the cockpit
are currently provided by ground-based equipment operating either on
HF or VHF radio bands or by legacy satellite systems. All these
communication systems use narrowband radio channels with a data
throughput capacity in the order of kbit/s. Additional
communications systems are available while the aircraft is on the
ground, such as the Aeronautical Mobile Airport Communications System
(AeroMACS) or public cellular networks, that operate in the Airport
(APT) domain and are able to deliver broadband communications
capability [BOE2019].
For regulatory reasons, the data communications networks used for the
transmission of data relating to the safety and regularity of flight
must be strictly isolated from those providing entertainment services
to passengers. This leads to a situation where the flight crews are
supported by narrowband services during flight while passengers have
access to in-flight broadband services. The current HF and VHF data
links cannot provide broadband services now or in the future due to
the lack of available spectrum. This technical shortcoming is
becoming a limitation to enhanced ATM operations, such as trajectory-
based operations and 4D trajectory negotiations [BOE2019].
Satellite-based communications are currently under investigation, and
enhanced capabilities that will be able to provide in-flight
broadband services and communications supporting the safety and
regularity of flight are under development. In parallel, the ground-
based broadband data link technology LDACS is being standardized by
ICAO and has recently shown its maturity during flight tests
[MAE20211] [BEL2021]. The LDACS technology is scalable, secure, and
spectrum-efficient, and it provides significant advantages to the
users and service providers. It is expected that both satellite
systems and LDACS will be deployed to support the future aeronautical
communication needs as envisaged by the ICAO GANP [BOE2019].
4. Provenance and Documents
The development of LDACS has already made substantial progress in the
Single European Sky ATM Research (SESAR) framework and is currently
being continued in the follow-up program SESAR2020 [RIH2018]. A key
objective of these activities is to develop, implement, and validate
a modern aeronautical data link that is able to evolve with aviation
needs over the long term. To this end, an LDACS specification has
been produced [GRA2020] and is continuously updated. Transmitter
demonstrators were developed to test the spectrum compatibility of
LDACS with legacy systems operating in the L-band [SAJ2014], and the
overall system performance was analyzed by computer simulations,
indicating that LDACS can fulfill the identified requirements
[GRA2011].
Up to now, LDACS standardization has been focused on the development
of the Physical Layer (PHY) and the Data Link Layer (DLL). Only
recently have higher layers come into the focus of the LDACS
development activities. Currently no "IPv6 over LDACS" specification
is defined; however, SESAR2020 has started experimenting with
IPv6-based LDACS and ICAO plans to seek guidance from IETF to develop
IPv6 over LDACS. As of May 2022, LDACS defines 1536-byte user data
packets [GRA2020] in which IPv6 traffic shall be encapsulated.
Additionally, Robust Header Compression (ROHC) [RFC5795] is
considered on the LDACS Subnetwork Protocol (SNP) layer
(cf. Section 7.3.3).
The IPv6 architecture for the aeronautical telecommunication network
is called the ATN/IPS. Link-layer technologies within the ATN/IPS
encompass LDACS [GRA2020], AeroMACS [KAMA2018], and several SatCOM
candidates; combined with the ATN/IPS, these are called the "FCI".
The FCI will support quality of service, link diversity, and mobility
under the umbrella of the "multilink concept". The "multilink
concept" describes the idea that depending on link quality,
communication can be switched seamlessly from one data link
technology to another. This work is led by the ICAO Communication
Panel Working Group (WG-I).
In addition to standardization activities, several industrial LDACS
prototypes have been built. One set of LDACS prototypes has been
evaluated in flight trials confirming the theoretical results
predicting the system performance [GRA2018] [MAE20211] [BEL2021].
5. Applicability
LDACS is a multi-application cellular broadband system capable of
simultaneously providing various kinds of Air Traffic Services (ATSs)
including ATS-B3 and AOC communications services from deployed
Ground-Stations (GSs). The physical layer and data link layer of
LDACS are optimized for Controller-Pilot Data Link Communications
(CPDLC), but the system also supports digital A/G voice
communications.
LDACS supports communications in all airspaces (airport, terminal
maneuvering area, and en route) and on the airport surface. The
physical LDACS cell coverage is effectively decoupled from the
operational coverage required for a particular service. This is new
in aeronautical communications. Services requiring wide-area
coverage can be installed at several adjacent LDACS cells. The
handover between the involved LDACS cells is seamless, automatic, and
transparent to the user. Therefore, the LDACS communications concept
enables the aeronautical communication infrastructure to support
future dynamic airspace management concepts.
5.1. Advances beyond the State of the Art
LDACS will offer several capabilities that are not yet provided in
contemporarily deployed aeronautical communications systems. These
capabilities were already tested and confirmed in lab or flight
trials with available LDACS prototype hardware [BEL2021] [MAE20211].
5.1.1. Priorities
LDACS is able to manage service priorities, which is an important
feature that is not available in some of the current data link
deployments. Thus, LDACS guarantees bandwidth availability, low
latency, and high continuity of service for safety-critical ATS
applications while simultaneously accommodating less safety-critical
AOC services.
5.1.2. Security
LDACS is a secure data link with built-in security mechanisms. It
enables secure data communications for ATS and AOC services,
including secured private communications for aircraft operators and
Air Traffic Network Service Providers (ANSPs). This includes
concepts for key and trust management, Mutual Authentication and Key
Establishment (MAKE) protocols, key derivation measures, user and
control message-in-transit protection, secure logging, and
availability and robustness measures [MAE20182] [MAE2021].
5.1.3. High Data Rates
The user data rate of LDACS is 315 kbit/s to 1428 kbit/s on the
Forward Link (FL) for the Ground-to-Air (G2A) connection, and 294
kbit/s to 1390 kbit/s on the Reverse Link (RL) for the Air-to-Ground
(A2G) connection, depending on coding and modulation. This is up to
two orders of magnitude greater than what current terrestrial digital
aeronautical communications systems, such as the VHF Data Link mode 2
(VDLm2), provide; see [ICAO2019] [GRA2020].
5.2. Application
LDACS will be used by several aeronautical applications ranging from
enhanced communications protocol stacks (multihomed mobile IPv6
networks in the aircraft and potentially ad-hoc networks between
aircraft) to broadcast communication applications (Global Navigation
Satellite System (GNSS) correction data) and integration with other
service domains (using the communications signal for navigation)
[MAE20211]. Also, a digital voice service offering better quality
and service than current HF and VHF systems is foreseen.
5.2.1. Air/Ground Multilink
It is expected that LDACS, together with upgraded satellite-based
communications systems, will be deployed within the FCI and
constitute one of the main components of the multilink concept within
the FCI.
Both technologies, LDACS and satellite systems, have their specific
benefits and technical capabilities that complement each other.
Satellite systems are especially well-suited for large coverage areas
with less dense air traffic, e.g., oceanic regions. LDACS is well-
suited for dense air traffic areas, e.g., continental areas or
hotspots around airports and terminal airspace. In addition, both
technologies offer comparable data link capacity; thus, both are
well-suited for redundancy, mutual back-up, or load balancing.
Technically, the FCI multilink concept will be realized by multihomed
mobile IPv6 networks in the aircraft. The related protocol stack is
currently under development by ICAO, within SESAR, and the IETF.
Currently, two layers of mobility are foreseen. Local mobility
within the LDACS access network is realized through Proxy Mobile IPv6
(PMIPv6), and global mobility between "multilink" access networks
(which need not be LDACS) is implemented on top of LISP [LISP-GB-ATN]
[RFC9300] [RFC9301].
5.2.2. Air/Air Extension for LDACS
A potential extension of the multilink concept is its extension to
the integration of ad-hoc networks between aircraft.
Direct A/A communication between aircraft in terms of ad-hoc data
networks is currently considered a research topic since there is no
immediate operational need for it, although several possible use
cases are discussed (Automatic Dependent Surveillance - Broadcast
(ADS-B), digital voice, wake vortex warnings, and trajectory
negotiation) [BEL2019]. It should also be noted that currently
deployed analog VHF voice radios support direct voice communication
between aircraft, making a similar use case for digital voice
plausible.
LDACS A/A is currently not a part of the standardization process and
will not be covered within this document. However, it is planned
that LDACS A/A will be rolled out after the initial deployment of
LDACS A/G and seamlessly integrated in the existing LDACS ground-
based system.
5.2.3. Flight Guidance
The FCI (and therefore LDACS) is used to provide flight guidance.
This is realized using three applications:
1. Context Management (CM): The CM application manages the automatic
logical connection to the ATC center currently responsible to
guide the aircraft. Currently, this is done by the air crew
manually changing VHF voice frequencies according to the progress
of the flight. The CM application automatically sets up
equivalent sessions.
2. Controller-Pilot Data Link Communications (CPDLC): The CPDLC
application provides the air crew with the ability to exchange
data messages similar to text messages with the currently
responsible ATC center. The CPDLC application takes over most of
the communication currently performed over VHF voice and enables
new services that do not lend themselves to voice communication
(i.e., trajectory negotiation).
3. Automatic Dependent Surveillance - Contract (ADS-C): ADS-C
reports the position of the aircraft to the currently active ATC
center. Reporting is bound to "contracts", i.e., pre-defined
events related to the progress of the flight (i.e., the
trajectory). ADS-C and CPDLC are the primary applications used
for implementing in-flight trajectory management.
CM, CPDLC, and ADS-C are available on legacy data links but are not
widely deployed and with limited functionality.
Further ATC applications may be ported to use the FCI or LDACS as
well. A notable application is the Ground-Based Augmentation System
(GBAS) for secure, automated landings. The GNSS-based GBAS is used
to improve the accuracy of GNSS to allow GNSS-based instrument
landings. This is realized by sending GNSS correction data (e.g.,
compensating ionospheric errors in the GNSS signal) to the aircraft's
GNSS receiver via a separate data link. Currently, the VHF Data
Broadcast (VDB) data link is used. VDB is a narrowband one-way,
single-purpose data link without advanced security and is only used
to transmit GBAS correction data. These shortcomings show a clear
need to replace VDB. A natural candidate to replace it is LDACS,
because it is a bidirectional data link, also operates in non-line-of
sight scenarios, offers strong integrated link-layer security, and
has a considerably larger operational range than VDB [MAE20211].
5.2.4. Business Communications of Airlines
In addition to ATSs, AOC services are transmitted over LDACS. AOC is
a generic term referring to the business communication of airlines
between the airlines and service partners on the ground and their own
aircraft in the air. Regulatory-wise, this is considered related to
safety and regularity of flight; therefore, it may be transmitted
over LDACS. AOC communication is considered the main business case
for LDACS communications service providers since modern aircraft
generate significant amounts of data (e.g., engine maintenance data).
5.2.5. LDACS-Based Navigation
Beyond communications, radio signals can always be used for
navigation as well. This fact is used for the LDACS navigation
concept.
For future aeronautical navigation, ICAO recommends the further
development of GNSS-based technologies as primary means for
navigation. However, due to the large separation between
navigational satellites and aircraft, the power of the GNSS signals
received by the aircraft is very low. As a result, GNSS disruptions
might occasionally occur due to unintentional interference or
intentional jamming. Yet, the navigation services must be available
with sufficient performance for all phases of flight. Therefore,
during GNSS outages or blockages, an alternative solution is needed.
This is commonly referred to as Alternative Positioning, Navigation,
and Timing (APNT).
One such APNT solution is based on exploiting the built-in navigation
capabilities of LDACS operation. That is, the normal operation of
LDACS for ATC and AOC communications would also directly enable the
aircraft to navigate and obtain a reliable timing reference from the
LDACS GSs. Current cell planning for Europe shows 84 LDACS cells to
be sufficient [MOST2018] to cover the continent at a sufficient
service level. If more than three GSs are visible by the aircraft,
via knowing the exact positions of these and having a good channel
estimation (which LDACS does due to numerous works mapping the L-band
channel characteristics [SCHN2018]), it is possible to calculate the
position of the aircraft via measuring signal propagation times to
each GS. In flight trials in 2019 with one aircraft (and airborne
radio inside it) and just four GSs, navigation feasibility was
demonstrated within the footprint of all four GSs with a 95th
percentile position-domain error of 171.1m [OSE2019] [BEL2021]
[MAE20211]. As such, LDACS can be used independently of GNSS as a
navigation alternative. Positioning errors will decrease markedly as
more GSs are deployed [OSE2019] [BEL2021] [MAE20211].
LDACS navigation has already been demonstrated in practice in two
flight measurement campaigns [SHU2013] [BEL2021] [MAE20211].
6. Requirements
The requirements for LDACS are mostly defined by its application
area: communications related to safety and regularity of flight.
A particularity of the current aeronautical communication landscape
is that it is heavily regulated. Aeronautical data links (for
applications related to safety and regularity of flight) may only use
spectrum licensed to aviation and data links endorsed by ICAO.
Nation states can change this locally; however, due to the global
scale of the air transportation system, adherence to these practices
is to be expected.
Aeronautical data links for the ATN are therefore expected to remain
in service for decades. The VDLm2 data link currently used for
digital terrestrial internetworking was developed in the 1990s (the
use of the Open Systems Interconnection (OSI) stack indicates that as
well). VDLm2 is expected to be used at least for several decades to
come. In this respect, aeronautical communications for applications
related to safety and regularity of flight is more comparable to
industrial applications than to the open Internet.
Internetwork technology is already installed in current aircraft.
Current ATS applications use either the Aircraft Communications
Addressing and Reporting System (ACARS) or the OSI stack. The
objective of the development effort of LDACS, as part of the FCI, is
to replace legacy OSI stack and proprietary ACARS internetwork
technologies with industry standard IP technology. It is anticipated
that the use of Commercial Off-The-Shelf (COTS) IP technology mostly
applies to the ground network. The avionics networks on the aircraft
will likely be heavily modified versions of Ethernet or proprietary.
Currently, AOC applications mostly use the same stack (although some
applications, like the graphical weather service, may use the
commercial passenger network). This creates capacity problems
(resulting in excessive amounts of timeouts) since the underlying
terrestrial data links do not provide sufficient bandwidth (i.e.,
with VDLm2 currently in the order of 10 kbit/s). The use of non-
aviation-specific data links is considered a security problem.
Ideally, the aeronautical IP internetwork (hence the ATN over which
only communications related to safety and regularity of flight is
handled) and the Internet should be completely separated at Layer 3.
The objective of LDACS is to provide a next-generation terrestrial
data link designed to support IP addressing and provide much higher
bandwidth to avoid the operational problems that are currently
experienced.
The requirement for LDACS is therefore to provide a terrestrial high-
throughput data link for IP internetworking in the aircraft.
In order to fulfill the above requirement, LDACS needs to be
interoperable with IP (and IP-based services like Voice-over-IP) at
the gateway connecting the LDACS network to other aeronautical ground
networks (i.e., the ATN). On the avionics side, in the aircraft,
aviation-specific solutions are to be expected.
In addition to these functional requirements, LDACS and its IP stack
need to fulfill the requirements defined in RTCA DO-350A/EUROCAE ED-
228A [DO350A]. This document defines continuity, availability, and
integrity requirements at different scopes for each ATM application
(CPDLC, CM, and ADS-C). The scope most relevant to IP over LDACS is
the Communications Service Provider (CSP) scope.
Continuity, availability, and integrity requirements are defined in
Volume 1 of [DO350A] in Tables 5-14 and 6-13. Appendix A presents
the required information.
In a similar vein, requirements to fault management are defined in
the same tables.
7. Characteristics
LDACS will become one of several wireless access networks connecting
aircraft to the ATN implemented by the FCI.
The current LDACS design is focused on the specification of Layers 1
and 2. However, for the purpose of this work, only Layer 2 details
are discussed here.
Achieving the stringent continuity, availability, and integrity
requirements defined in [DO350A] will require the specification of
Layer 3 and above mechanisms (e.g., reliable crossover at the IP
layer). Fault management mechanisms are similarly unspecified as of
November 2022. Current regulatory documents do not fully specify the
above mechanism yet. However, a short overview of the current state
shall be given throughout each section here.
7.1. LDACS Access Network
An LDACS access network contains an Access Router (AC-R) and several
GSs, each of them providing one LDACS radio cell.
User-plane interconnection to the ATN is facilitated by the AC-R
peering with an A/G Router connected to the ATN.
The internal control plane of an LDACS access network interconnects
the GSs. An LDACS access network is illustrated in Figure 1. Dashes
denote the user plane and points denote the control plane.
wireless user
link plane
AS-------------GS---------------AC-R---A/G-----ATN
.............. | Router
control . |
plane . |
. |
GS----------------|
. |
. |
GS----------------+
Figure 1: LDACS Access Network with Three GSs and One AS
7.2. Topology
LDACS is a cellular point-to-multipoint system. It assumes a star
topology in each cell where Aircraft Stations (ASs) belonging to
aircraft within a certain volume of space (the LDACS cell) are
connected to the controlling GS. The LDACS GS is a centralized
instance that controls LDACS A/G communications within its cell. The
LDACS GS can simultaneously support multiple bidirectional
communications to the ASs under its control. LDACS's GSs themselves
are connected to each other and the AC-R.
Prior to utilizing the system, an aircraft has to register with the
controlling GS to establish dedicated logical channels for user and
control data. Control channels have statically allocated resources
while user channels have dynamically assigned resources according to
the current demand. Logical channels exist only between the GS and
the AS.
7.3. LDACS Protocol Stack
The protocol stack of LDACS is implemented in the AS and GS. It
consists of the PHY with five major functional blocks above it. Four
are placed in the DLL of the AS and GS: Medium Access Control (MAC)
layer, Voice Interface (VI), Data Link Service (DLS), and LDACS
Management Entity (LME). The fifth entity, the SNP, resides within
the subnetwork layer. The LDACS radio is externally connected to a
voice unit and radio control unit via the AC-R to the ATN network.
LDACS is considered an ATN/IPS radio access technology from the view
of ICAO's regulatory framework. Hence, the interface between ATN and
LDACS must be IPv6-based, as regulatory documents such as ICAO Doc
9896 [ICAO2015] and DO-379 [RTCA2019] clearly foresee that. The
translation between the IPv6 layer and SNP layer is currently the
subject of ongoing standardization efforts and not finished yet at
the time of writing.
Figure 2 shows the protocol stack of LDACS as implemented in the AS
and GS. Acronyms used here are introduced throughout the upcoming
sections.
IPv6 Network Layer
|
Airborne Voice |
Interface (AVI) / | Radio Control Unit (RCU)
Voice Unit (VU) |
| |
| +------------------+ +----+
| | SNP |--| | Subnetwork
| | | | | Layer
| +------------------+ | |
| | | LME|
+-----+ +------------------+ | |
| VI | | DLS | | | LLC Layer
+-----+ +------------------+ +----+
| | |
DCH DCH DCCH/CCCH
| RACH/BCCH
| |
+-------------------------------------+
| MAC | Medium Access
| | Layer
+-------------------------------------+
|
+-------------------------------------+
| PHY | Physical Layer
+-------------------------------------+
|
|
((*))
FL/RL radio channels
separated by FDD
Figure 2: LDACS Protocol Stack in the AS and GS
7.3.1. LDACS Physical Layer
The physical layer provides the means to transfer data over the radio
channel. The LDACS GS supports bidirectional links to multiple
aircraft under its control. The FL direction at the G2A connection
and the RL direction at the A2G connection are separated by Frequency
Division Duplex (FDD). FL and RL use a 500 kHz channel each. The GS
transmits a continuous stream of Orthogonal Frequency Division
Multiplexing Access (OFDM) symbols on the FL. In the RL, different
aircraft are separated in time and frequency using Orthogonal
Frequency Division Multiple Access (OFDMA). Thus, aircraft transmit
discontinuously on the RL via short radio bursts sent in precisely
defined transmission opportunities allocated by the GS.
7.3.2. LDACS Data Link Layer
The data link layer provides the necessary protocols to facilitate
concurrent and reliable data transfer for multiple users. The LDACS
data link layer is organized in two sub-layers: the medium access
sub-layer and the Logical Link Control (LLC) sub-layer. The medium
access sub-layer manages the organization of transmission
opportunities in slots of time and frequency. The LLC sub-layer
provides acknowledged point-to-point logical channels between the
aircraft and the GS using an Automatic Repeat reQuest (ARQ) protocol.
LDACS also supports unacknowledged point-to-point channels and G2A
broadcast transmission.
7.3.2.1. Medium Access Control (MAC) Services
The MAC time framing service provides the frame structure necessary
to realize slot-based time-division multiplex-access on the physical
link. It provides the functions for the synchronization of the MAC
framing structure and the PHY layer framing. The MAC time framing
provides a dedicated time slot for each logical channel.
The MAC sub-layer offers access to the physical channel to its
service users. Channel access is provided through transparent
logical channels. The MAC sub-layer maps logical channels onto the
appropriate slots and manages the access to these channels. Logical
channels are used as interface between the MAC and LLC sub-layers.
7.3.2.2. Data Link Services (DLSs)
The DLS provides acknowledged and unacknowledged (including broadcast
and packet mode voice) bidirectional exchange of user data. If user
data is transmitted using the acknowledged DLS, the sending DLS
entity will wait for an acknowledgement from the receiver. If no
acknowledgement is received within a specified time frame, the sender
may automatically try to retransmit its data. However, after a
certain number of failed retries, the sender will suspend further
retransmission attempts and inform its client of the failure.
The DLS uses the logical channels provided by the MAC:
1. A GS announces its existence and access parameters in the
Broadcast Channel (BCCH).
2. The Random-Access Channel (RACH) enables the AS to request access
to an LDACS cell.
3. In the FL, the Common Control Channel (CCCH) is used by the GS to
grant access to Data Channel (DCH) resources.
4. The reverse direction is covered by the RL, where ASs need to
request resources before sending. This happens via the Dedicated
Control Channel (DCCH).
5. User data itself is communicated in the DCH on the FL and RL.
Access to the FL and RL DCH is granted by the scheduling mechanism
implemented in the LME discussed below.
7.3.2.3. Voice Interface (VI) Services
The VI provides support for virtual voice circuits. Voice circuits
may be either set up permanently by the GS (e.g., to emulate voice
party line) or created on demand.
7.3.2.4. LDACS Management Entity (LME) Services
The mobility management service in the LME provides support for
registration and de-registration (cell entry and cell exit), scanning
RF channels of neighboring cells, and handover between cells. In
addition, it manages the addressing of aircraft within cells.
The resource management service provides link maintenance (power,
frequency, and time adjustments), support for adaptive coding and
modulation, and resource allocation.
The resource management service accepts resource requests from/for
different ASs and issues resource allocations accordingly. While the
scheduling algorithm is not specified and a point of possible vendor
differentiation, it is subject to the following requirements:
1. Resource scheduling must provide channel access according to the
priority of the request.
2. Resource scheduling must support "one-time" requests.
3. Resource scheduling must support "permanent" requests that
reserve a resource until the request is canceled (e.g., for
digital voice circuits).
7.3.3. LDACS Subnetwork Layer and Protocol Services
Lastly, the SNP layer of LDACS directly interacts with IPv6 traffic.
Incoming ATN/IPS IPv6 packets are forwarded over LDACS from and to
the aircraft. The final IP addressing structure in an LDACS subnet
still needs to be defined; however, the current layout consists of
the five network segments: Air Core Net, Air Management Net, Ground
Core Net, Ground Management Net, and Ground Net. Any protocols that
the ATN/IPS [ICAO2015] defines as mandatory will reach the aircraft;
however, listing these here is out of scope. For more information on
the technicalities of the above ATN/IPS layer, please refer to
[ICAO2015], [RTCA2019], and [ARI2021].
The DLS provides functions that are required for the transfer of
user-plane data and control plane data over the LDACS access network.
The security service provides functions for secure user data
communication over the LDACS access network. Note that the SNP
security service applies cryptographic measures as configured by the
GS.
7.4. LDACS Mobility
LDACS supports Layer 2 handovers to different LDACS cells. Handovers
may be initiated by the aircraft (break-before-make) or by the GS
(make-before-break). Make-before-break handovers are only supported
between GSs connected to each other and usually GSs operated by the
same service provider.
When a handover between the AS and two interconnected GSs takes
place, it can be triggered by the AS or GS. Once that is done, new
security information is exchanged between the AS, GS1, and GS2 before
the "old" connection is terminated between the AS and GS1 and a "new"
connection is set up between the AS and GS2. As a last step,
accumulated user data at GS1 is forwarded to GS2 via a ground
connection before it is sent via GS2 to the AS. While some
information for handover is transmitted in the LDACS DCH, the
information remains in the "control plane" part of LDACS and is
exchanged between LMEs in the AS, GS1, and GS2. As such, local
mobility takes place entirely within the LDACS network and utilizes
the PMIPv6 protocol [RFC5213]. The use of PMIPv6 is currently not
mandated by standardization and may be vendor-specific. External
handovers between non-connected LDACS access networks or different
aeronautical data links are handled by the FCI multilink concept.
7.5. LDACS Management Interfaces and Protocols
LDACS management interfaces and protocols are currently not be
mandated by standardization. The implementations currently available
use SNMP for management and Radius for Authentication, Authorization,
and Accounting (AAA). Link state (link up, link down) is reported
using the ATN/IPS Aircraft Protocol (AIAP) mandated by ICAO WG-I for
multilink.
8. Reliability and Availability
8.1. Below Layer 1
Below Layer 1, aeronautics usually rely on hardware redundancy. To
protect availability of the LDACS link, an aircraft equipped with
LDACS will have access to two L-band antennae with triple redundant
radio systems as required for any safety relevant aeronautical
systems by ICAO.
8.2. Layers 1 and 2
LDACS has been designed with applications related to the safety and
regularity of flight in mind; therefore, it has been designed as a
deterministic wireless data link (as far as this is possible).
Based on channel measurements of the L-band channel, LDACS was
designed from the PHY layer up with robustness in mind. Channel
measurements of the L-band channel [SCH2016] confirmed LDACS to be
well adapted to its channel.
In order to maximize the capacity per channel and to optimally use
the available spectrum, LDACS was designed as an OFDM-based FDD
system that supports simultaneous transmissions in FL in the G2A
connection and RL in the A2G connection. The legacy systems already
deployed in the L-band limit the bandwidth of both channels to
approximately 500 kHz.
The LDACS physical layer design includes propagation guard times
sufficient for operation at a maximum distance of 200 nautical miles
(nm) from the GS. In actual deployment, LDACS can be configured for
any range up to this maximum range.
The LDACS physical layer supports adaptive coding and modulation for
user data. Control data is always encoded with the most robust
coding and modulation (FL: Quadrature Phase-Shift Keying (QPSK),
coding rate 1/2; RL: QPSK, coding rate 1/3).
LDACS medium access layer on top of the physical layer uses a static
frame structure to support deterministic timer management. As shown
in Figures 3 and 4, LDACS framing structure is based on Super-Frames
(SFs) of 240 ms (milliseconds) duration corresponding to 2000 OFDM
symbols. OFDM symbol time is 120 microseconds, sampling time is 1.6
microseconds, and guard time is 4.8 microseconds. The structure of
an SF is depicted in Figure 3 along with its structure and timings of
each part. FL and RL boundaries are aligned in time (from the GS
perspective) allowing for deterministic slots for control and DCHs.
This initial AS time synchronization and time synchronization
maintenance is based on observing the synchronization symbol pairs
that repetitively occur within the FL stream being sent by the
controlling GS [GRA2020]. As already mentioned, LDACS data
transmission is split into user data (DCH) and control (BCCH and CCCH
in FL; RACH and DCCH in RL) as depicted with corresponding timings in
Figure 4.
^
| +---------+------------+------------+------------+------------+
| FL | BCCH | MF | MF | MF | MF |
| | 6.72 ms | 58.32 ms | 58.32 ms | 58.32 ms | 58.32 ms |
F +---------+------------+------------+------------+------------+
r <----------------- Super-Frame (SF) - 240 ms ----------------->
e
q +---------+------------+------------+------------+------------+
u RL | RACH | MF | MF | MF | MF |
e | 6.72 ms | 58.32 ms | 58.32 ms | 58.32 ms | 58.32 ms |
n +---------+------------+------------+------------+------------+
c <----------------- Super-Frame (SF) - 240 ms ----------------->
y
------------------------------ Time -------------------------------->
|
Figure 3: SF Structure for LDACS
^
| +--------------+-----------------+------------------+
| FL | DCH | CCCH | DCH |
| | 25.92 ms | 2.16 - 17.28 ms | 15.12 - 30.24 ms |
F +--------------+-----------------+------------------+
r <----------- Multiframe (MF) - 58.32 ms ----------->
e
q +---------------+----------------------------------+
u RL | DCCH | DCH |
e | 2.8 - 24.4 ms | 33.84 - 55.44 ms |
n +---------------+----------------------------------+
c <----------- Multiframe (MF) - 58.32 ms ---------->
y
----------------------------- Time ---------------------->
|
Figure 4: MF Structure for LDACS
LDACS cell entry is conducted with an initial control message
exchange via the RACH and the BCCH.
After cell entry, LDACS medium access is always under the control of
the GS of a radio cell. Any medium access for the transmission of
user data on a DCH has to be requested with a resource request
message stating the requested amount of resources and class of
service. The GS performs resource scheduling on the basis of these
requests and grants resources with resource allocation messages.
Resource request and allocation messages are exchanged over dedicated
contention-free control channels (DCCH and CCCH).
The purpose of QoS in LDACS medium access is to provide prioritized
medium access at the bottleneck (the wireless link). Signaling of
higher-layer QoS requests to LDACS is implemented on the basis of
Differentiated Services (Diffserv) classes CS01 (lowest priority) to
CS07 (highest priority).
In addition to having full control over resource scheduling, the GS
can send forced handover commands for off-loading or channel
management, e.g., when the signal quality declines and a more
suitable GS is in the AS's reach. With robust resource management of
the capacities of the radio channel, reliability and robustness
measures are also anchored in the LME.
In addition to radio resource management, the LDACS control channels
are also used to send keepalive messages when they are not otherwise
used. Since the framing of the control channels is deterministic,
missing keepalive messages can be immediately detected. This
information is made available to the multilink protocols for fault
management.
The protocol used to communicate faults is not defined in the LDACS
specification. It is assumed that vendors would use industry
standard protocols like the Simple Network Management Protocol or the
Network Configuration Protocol (NETCONF) where security permits.
The LDACS data link layer protocol, running on top of the medium
access sub-layer, uses ARQ to provide reliable data transmission on
the DCH. It employs selective repeat ARQ with transparent
fragmentation and reassembly to the resource allocation size to
minimize latency and overhead without losing reliability. It ensures
correct order of packet delivery without duplicates. In case of
transmission errors, it identifies lost fragments with deterministic
timers synced to the medium access frame structure and initiates
retransmission.
8.3. Beyond Layer 2
LDACS availability can be increased by appropriately deploying LDACS
infrastructure. This means proliferating the number of terrestrial
GSs. However, there are four aspects that need to be taken into
consideration: (1) scarcity of aeronautical spectrum for data link
communication (tens of MHz in the L-band in the case of LDACS), (2)
an increase in the number of GSs also increases the individual
bandwidth for aircraft in the cell, as fewer aircraft have to share
the spectrum, (3) covering worldwide terrestrial ATM via LDACS is
also a question of cost and the possible reuse of spectrum, which
makes it not always possible to decrease cell sizes, and (4) the
Distance Measuring Equipment (DME) is the primary user of the
aeronautical L-band, which means any LDACS deployment has to take DME
frequency planning into account.
While aspect (2) provides a good reason alongside increasing
redundancy for smaller cells than the maximum range LDACS was
developed for (200 nm), the other three need to be respected when
doing so. There are preliminary works on LDACS cell planning, such
as [MOST2018], where the authors concluded that 84 LDACS cells in
Europe would be sufficient to serve European air traffic for the next
20 years.
For redundancy reasons, the aeronautical community has decided not to
rely on a single communication system or frequency band. It is
envisioned to have multiple independent data link technologies in the
aircraft (e.g., terrestrial and satellite communications) in addition
to legacy VHF voice.
However, as of now, no reliability and availability mechanisms that
could utilize the multilink architecture have been specified on Layer
3 and above. Even if LDACS has been designed for reliability, the
wireless medium presents significant challenges to achieve
deterministic properties such as low packet error rate, bounded
consecutive losses, and bounded latency. Support for high
reliability and availability for IP connectivity over LDACS is highly
desirable, but support needs to be adapted to the specific use case.
9. Security Considerations
The goal of this section is to inform the reader about the state of
security in aeronautical communications and the state security
considerations applicable for all ATN/IPS traffic and to provide an
overview of the LDACS link-layer security capabilities.
9.1. Security in Wireless Digital Aeronautical Communications
Aviation will require secure exchanges of data and voice messages for
managing the air traffic flow safely through the airspaces all over
the world. Historically, Communication Navigation Surveillance (CNS)
wireless communications technology emerged from the military and a
threat landscape where inferior technological and financial
capabilities of adversaries were assumed [STR2016]. The main
communications method for ATC today is still an open analog voice
broadcast within the aeronautical VHF band. Currently, information
security is mainly procedural and based by using well-trained
personnel and proven communications procedures. This communication
method has been in service since 1948. However, the world has
changed since the emergence of civil aeronautical CNS applications in
the 70s.
Civil applications have significant lower spectrum available than
military applications. This means that several military defense
mechanisms such as frequency hopping or pilot symbol scrambling (and
thus a defense-in-depth approach starting at the physical layer) are
infeasible for civil systems. With the rise of cheap Software-
Defined Radios (SDRs), the previously existing financial barrier is
almost gone, and open source projects such as GNU radio [GNU2021]
allow for a new type of unsophisticated listener and possible
attacker.
Most CNS technology developed in ICAO relies on open standards; thus,
syntax and semantics of wireless digital aeronautical communications
should be expected to be common knowledge for attackers. With
increased digitization and automation of civil aviation, the human as
control instance is being taken gradually out of the loop.
Autonomous transport drones or single-piloted aircraft demonstrate
this trend. However, without profound cybersecurity measures, such
as authenticity and integrity checks of messages in-transit on the
wireless link or mutual entity authentication, this lack of a control
instance can prove disastrous. Thus, future digital communications
will need additional embedded security features to fulfill modern
information security requirements like authentication and integrity.
These security features require sufficient bandwidth, which is beyond
the capabilities of currently deployed VHF narrowband communications
systems. For voice and data communications, sufficient data
throughput capability is needed to support the security functions
while not degrading performance. LDACS is a data link technology
with sufficient bandwidth to incorporate security without losing too
much user data throughput.
9.2. Security in Depth
ICAO Doc 9896 [ICAO2015] foresees transport layer security for all
aeronautical data transmitted via the ATN/IPS, as described in ARINC
858 [ARI2021]. This is realized via Datagram Transport Layer
Security (DTLS) 1.3 [RFC9147].
LDACS also needs to comply with in-depth security requirements as
stated in ARINC 858 for the radio access technologies transporting
ATN/IPS data. These requirements imply that LDACS must provide Layer
2 security in addition to any higher-layer mechanisms. Specifically,
ARINC 858 [ARI2021] states that data links within the FCI need to
provide
| a secure channel between the airborne radio systems and the peer
| radio access endpoints on the ground [...] to ensure
| authentication and integrity of air-ground message exchanges in
| support of an overall defense-in-depth security strategy.
9.3. LDACS Security Requirements
Overall, cybersecurity for CNS technology shall protect the following
business goals [MAE20181]:
1. Safety: The system must sufficiently mitigate attacks that
contribute to safety hazards.
2. Flight regularity: The system must sufficiently mitigate attacks
that contribute to delays, diversions, or cancelations of
flights.
3. Protection of business interests: The system must sufficiently
mitigate attacks that result in financial loss, reputation
damage, disclosure of sensitive proprietary information, or
disclosure of personal information.
To further analyze assets, derive threats, and create protection
scenarios, several threat and risk analyses were performed for LDACS
[MAE20181] [MAE20191]. These results allowed the derivation of
security scope and objectives from the requirements and the conducted
threat and risk analysis. Note, IPv6 security considerations are
briefly discussed in Section 9.7 while a summary of security
requirements for link-layer candidates in the ATN/IPS is given in
[ARI2021], which states:
| Since the communication radios connect to local airborne networks
| in the aircraft control domain, [...] the airborne radio systems
| represent the first point of entry for an external threat to the
| aircraft. Consequently, a secure channel between the airborne
| radio systems and the peer radio access endpoints on the ground is
| necessary to ensure authentication and integrity of air-ground
| message exchanges in support of an overall defense-in-depth
| security strategy.
9.4. LDACS Security Objectives
Security considerations for LDACS are defined by the official SARPS
document by ICAO [ICAO2022]:
* LDACS shall provide a capability to protect the availability and
continuity of the system.
* LDACS shall provide a capability including cryptographic
mechanisms to protect the integrity of messages in transit.
* LDACS shall provide a capability to ensure the authenticity of
messages in transit.
* LDACS should provide a capability for non-repudiation of origin
for messages in transit.
* LDACS should provide a capability to protect the confidentiality
of messages in transit.
* LDACS shall provide an authentication capability.
* LDACS shall provide a capability to authorize the permitted
actions of users of the system and to deny actions that are not
explicitly authorized.
* If LDACS provides interfaces to multiple domains, LDACS shall
provide capability to prevent the propagation of intrusions within
LDACS domains and towards external domains.
Work in 2022 includes a change request for these SARPS aims to limit
the "non-repudiation of origin of messages in transit" requirement
only to the authentication and key establishment messages at the
beginning of every session.
9.5. LDACS Security Functions
These objectives were used to derive several security functions for
LDACS required to be integrated in the LDACS cybersecurity
architecture: Identification, Authentication, Authorization,
Confidentiality, System Integrity, Data Integrity, Robustness,
Reliability, Availability, and Key and Trust Management. Several
works investigated possible measures to implement these security
functions [BIL2017] [MAE20181] [MAE20191].
9.6. LDACS Security Architecture
The requirements lead to an LDACS security model, including different
entities for identification, authentication, and authorization
purposes ensuring integrity, authenticity, and confidentiality of
data. A draft of the cybersecurity architecture of LDACS can be
found in [ICAO2022] and [MAE20182], and respective updates can be
found in [MAE20191], [MAE20192], [MAE2020], and [MAE2021].
9.6.1. Entities
A simplified LDACS architectural model requires the following
entities: network operators such as the Societe Internationale de
Telecommunications Aeronautiques (SITA) [SIT2020] and ARINC
[ARI2020]; both entities provide access to the ground IPS network via
an A/G LDACS router. This router is attached to an internal LDACS
access network that connects via further AC-Rs to the different LDACS
cell ranges, each controlled by a GS (serving one LDACS cell), with
several interconnected GSs spanning a local LDACS access network.
Via the A/G wireless LDACS data link AS, the aircraft is connected to
the ground network. Via the aircraft's VI and network interface, the
aircraft's data can be sent via the AS back to the GS, then to the
LDACS local access network, AC-Rs, LDACS access network, A/G LDACS
router, and finally to the ground IPS network [ICAO2015].
9.6.2. Entity Identification
LDACS needs specific identities for the AS, the GS, and the network
operator. The aircraft itself can be identified using the 24-bit
ICAO identifier of an aircraft [ICAO2022], the call sign of that
aircraft, or the recently founded privacy ICAO address of the Federal
Aviation Administration (FAA) program with the same name [FAA2020].
It is conceivable that the LDACS AS will use a combination of
aircraft identification, radio component identification, and even
operator feature identification to create a unique LDACS AS
identification tag. Similar to a 4G's eNodeB-serving network
identification tag, a GS could be identified using a similar field.
The identification of the network operator is similar to 4G (e.g.,
E-Plus, AT&T, and TELUS), in the way that the aeronautical network
operators are listed (e.g., ARINC [ARI2020] and SITA [SIT2020]).
9.6.3. Entity Authentication and Key Establishment
In order to anchor trust within the system, all LDACS entities
connected to the ground IPS network will be rooted in an LDACS-
specific chain-of-trust and PKI solution, quite similar to AeroMACS's
approach [CRO2016]. These certificates, residing at the entities and
incorporated in the LDACS PKI, provide proof of the ownership of
their respective public key and include information about the
identity of the owner and the digital signature of the entity that
has verified the certificate's content. First, all ground
infrastructures must mutually authenticate to each other, negotiate
and derive keys, and then secure all ground connections. How this
process is handled in detail is still an ongoing discussion.
However, established methods to secure the user plane by IPsec
[RFC4301] and IKEv2 [RFC7296] or the application layer via TLS 1.3
[RFC8446] are conceivable. The LDACS PKI with its chain-of-trust
approach, digital certificates, and public entity keys lay the
groundwork for this step. In a second step, the AS with the LDACS
radio aboard approaches an LDACS cell and performs a cell-attachment
procedure with the corresponding GS. This procedure consists of (1)
the basic cell entry [GRA2020] and (2) a MAKE procedure [MAE2021].
Note that LDACS will foresee multiple security levels. To address
the issue of the long service life of LDACS (i.e., possibly greater
than 30 years) and the security of current pre-quantum cryptography,
these security levels include pre-quantum and post-quantum
cryptographic solutions. Limiting security data on the LDACS data
link as much as possible to reserve as much space for actual user
data transmission is key in the LDACS security architecture. This is
also reflected in the underlying cryptography. Pre-quantum solutions
will rely on elliptic curves [NIST2013], while post-quantum solutions
consider Falcon [SON2021] [MAE2021] or similar lightweight PQC
signature schemes and CRYSTALS-KYBER or SABER as key establishment
options [AVA2021] [ROY2020].
9.6.4. Message-In-Transit Confidentiality, Integrity, and Authenticity
The key material from the previous step can then be used to protect
LDACS Layer 2 communications via applying encryption and integrity
protection measures on the SNP layer of the LDACS protocol stack. As
LDACS transports AOC and ATS data, the integrity of that data is most
important while confidentiality only needs to be applied to AOC data
to protect business interests [ICAO2022]. This possibility of
providing low-layered confidentiality and integrity protection
ensures a secure delivery of user data over the wireless link.
Furthermore, it ensures integrity protection of LDACS control data.
9.7. Considerations on LDACS Security Impact on IPv6 Operational
Security
In this part, considerations on IPv6 operational security in
[RFC9099] and interrelations with the LDACS security additions are
compared and evaluated to identify further protection demands. As
IPv6 heavily relies on the Neighbor Discovery Protocol (NDP)
[RFC4861], integrity and authenticity protection on the link layer,
as provided by LDACS, already help mitigate spoofing and redirection
attacks. However, to also mitigate the threat of remote DDoS
attacks, neighbor solicitation rate-limiting is recommended by
[RFC9099]. To prevent the threat of DDoS and DoS attacks in general
on the LDACS access network, rate-limiting needs to be performed on
each network node in the LDACS access network. One approach is to
filter for the total amount of possible LDACS AS-GS traffic per cell
(i.e., of up to 1.4 Mbit/s user data per cell and up to the amount of
GS per service provider network times 1.4 Mbit/s).
10. IANA Considerations
This document has no IANA actions.
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Appendix A. Selected Information from DO-350A
This appendix includes the continuity, availability, and integrity
requirements applicable for LDACS defined in [DO350A].
The following terms are used here:
CPDLC: Controller-Pilot Data Link Communications
DT: Delivery Time (nominal) value for RSP
ET: Expiration Time value for RCP
FH: Flight Hour
MA: Monitoring and Alerting criteria
OT: Overdue Delivery Time value for RSP
RCP: Required Communication Performance
RSP: Required Surveillance Performance
TT: Transaction Time (nominal) value for RCP
+========================+=============+=============+
| | RCP 130 | RCP 130 |
+========================+=============+=============+
| Parameter | ET | TT95% |
+------------------------+-------------+-------------+
| Transaction Time (sec) | 130 | 67 |
+------------------------+-------------+-------------+
| Continuity | 0.999 | 0.95 |
+------------------------+-------------+-------------+
| Availability | 0.989 | 0.989 |
+------------------------+-------------+-------------+
| Integrity | 1E-5 per FH | 1E-5 per FH |
+------------------------+-------------+-------------+
Table 1: CPDLC Requirements for RCP 130
+========================+=========+=========+=========+=========+
| | RCP 240 | RCP 240 | RCP 400 | RCP 400 |
+========================+=========+=========+=========+=========+
| Parameter | ET | TT95% | ET | TT95% |
+------------------------+---------+---------+---------+---------+
| Transaction Time (sec) | 240 | 210 | 400 | 350 |
+------------------------+---------+---------+---------+---------+
| Continuity | 0.999 | 0.95 | 0.999 | 0.95 |
+------------------------+---------+---------+---------+---------+
| Availability | 0.989 | 0.989 | 0.989 | 0.989 |
+------------------------+---------+---------+---------+---------+
| Integrity | 1E-5 | 1E-5 | 1E-5 | 1E-5 |
| | per FH | per FH | per FH | per FH |
+------------------------+---------+---------+---------+---------+
Table 2: CPDLC Requirements for RCP 240/400
RCP Monitoring and Alerting Criteria in case of CPDLC:
MA-1: The system shall be capable of detecting failures and
configuration changes that would cause the communication service
to no longer meet the RCP specification for the intended use.
MA-2: When the communication service can no longer meet the RCP
specification for the intended function, the flight crew and/or
the controller shall take appropriate action.
+==============+========+========+========+========+========+=======+
| | RSP | RSP | RSP | RSP | RSP | RSP |
| | 160 | 160 | 180 | 180 | 400 | 400 |
+==============+========+========+========+========+========+=======+
| Parameter | OT | DT95% | OT | DT95% | OT | DT95% |
+--------------+--------+--------+--------+--------+--------+-------+
| Transaction | 160 | 90 | 180 | 90 | 400 | 300 |
| Time (sec) | | | | | | |
+--------------+--------+--------+--------+--------+--------+-------+
| Continuity | 0.999 | 0.95 | 0.999 | 0.95 | 0.999 | 0.95 |
+--------------+--------+--------+--------+--------+--------+-------+
| Availability | 0.989 | 0.989 | 0.989 | 0.989 | 0.989 | 0.989 |
+--------------+--------+--------+--------+--------+--------+-------+
| Integrity | 1E-5 | 1E-5 | 1E-5 | 1E-5 | 1E-5 | 1E-5 |
| | per FH | per FH | per FH | per FH | per | per |
| | | | | | FH | FH |
+--------------+--------+--------+--------+--------+--------+-------+
Table 3: ADS-C Requirements
RCP Monitoring and Alerting Criteria:
MA-1: The system shall be capable of detecting failures and
configuration changes that would cause the ADS-C service to no
longer meet the RSP specification for the intended function.
MA-2: When the ADS-C service can no longer meet the RSP
specification for the intended function, the flight crew and/or
the controller shall take appropriate action.
Acknowledgements
Thanks to all contributors to the development of LDACS and ICAO
Project Team Terrestrial (PT-T), as well as to all in the RAW Working
Group for deep discussions and feedback.
Thanks to Klaus-Peter Hauf, Bart Van Den Einden, and Pierluigi
Fantappie for their comments on this document.
Thanks to the Chair of Network Security for input and to the Research
Institute CODE for their comments and improvements.
Thanks to the colleagues of the Research Institute CODE at the
UniBwM, who are working on the AMIUS project funded under the
Bavarian Aerospace Program by the Bavarian State Ministry of
Economics, Regional Development and Energy with the GA ROB-
2-3410.20-04-11-15/HAMI-2109-0015, for fruitful discussions on
aeronautical communications and relevant security incentives for the
target market.
Thanks to SBA Research Vienna for continuous discussions on security
infrastructure issues in quickly developing markets such as the air
space and potential economic spillovers to used technologies and
protocols.
Thanks to the Aeronautical Communications group at the Institute of
Communications and Navigation of the German Aerospace Center (DLR).
With that, the authors would like to explicitly thank Miguel Angel
Bellido-Manganell and Lukas Marcel Schalk for their thorough
feedback.
Authors' Addresses
Nils Mäurer (editor)
German Aerospace Center (DLR)
Münchner Strasse 20
82234 Wessling
Germany
Email: Nils.Maeurer@dlr.de
Thomas Gräupl (editor)
German Aerospace Center (DLR)
Münchner Strasse 20
82234 Wessling
Germany
Email: Thomas.Graeupl@dlr.de
Corinna Schmitt (editor)
Research Institute CODE, UniBwM
Werner-Heisenberg-Weg 39
85577 Neubiberg
Germany
Email: corinna.schmitt@unibw.de