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L-Band Digital Aeronautical Communications System (LDACS)
RFC 9372

Document Type RFC - Informational (March 2023)
Authors Nils Mäurer , Thomas Gräupl , Corinna Schmitt
Last updated 2023-12-12
RFC stream Internet Engineering Task Force (IETF)
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IESG Responsible AD John Scudder
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RFC 9372

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
                                                              March 2023

       L-Band Digital Aeronautical Communications System (LDACS)


   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

Copyright Notice

   Copyright (c) 2023 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Revised BSD License text as described in Section 4.e of the
   Trust Legal Provisions and are provided without warranty as described
   in the Revised BSD License.

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
     9.7.  Considerations on LDACS Security Impact on IPv6 Operational
   10. IANA Considerations
   11. Informative References
   Appendix A.  Selected Information from DO-350A
   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 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

   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

   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

   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

   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

   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
       ..............                 |   Router
          control   .                 |
          plane     .                 |
                    .                 |
                    .                 |
                    .                 |

          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

                      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.  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.  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.  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.  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

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

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 ----------------->
   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 ----------------->
   ------------------------------ 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 ----------->
   q     +---------------+----------------------------------+
   u  RL |    DCCH       |                DCH               |
   e     | 2.8 - 24.4 ms |           33.84 - 55.44 ms       |
   n     +---------------+----------------------------------+
   c     <-----------  Multiframe (MF) - 58.32 ms ---------->
   ----------------------------- 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

   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

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

   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

   |  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
   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

   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.

11.  Informative References

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              Technology for the Next Generation of Air Transportation
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              Maino, F., and B. Venkatachalapathy, "Ground-Based LISP
              for the Aeronautical Telecommunications Network", Work in
              Progress, Internet-Draft, draft-haindl-lisp-gb-atn-08, 23
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              security architecture for LDACS: A datalink security
              threat and risk analysis", IEEE Integrated Communications,
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              DOI 10.1109/ICNSURV.2018.8384828, April 2018,

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              for the L-band Digital Aeronautical Communications System
              (LDACS)", IEEE/AIAA 37th Digital Avionics Systems
              Conference (DASC), pp. 1-10,
              DOI 10.1109/DASC.2018.8569878, September 2018,

   [MAE20191] Mäurer, N., Gräupl, T., and C. Schmitt, "Evaluation of the
              LDACS Cybersecurity Implementation", IEEE 38th Digital
              Avionics Systems Conference (DASC), pp. 1-10,
              DOI 10.1109/DASC43569.2019.9081786, September 2019,

   [MAE20192] Mäurer, N. and C. Schmitt, "Towards Successful Realization
              of the LDACS Cybersecurity Architecture: An Updated
              Datalink Security Threat- and Risk Analysis", IEEE
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              Conference (ICNS), pp. 1-13,
              DOI 10.1109/ICNSURV.2019.8735139, April 2019,

   [MAE2020]  Mäurer, N., Gräupl, T., Gentsch, C., and C. Schmitt,
              "Comparing Different Diffie-Hellman Key Exchange Flavors
              for LDACS", IEEE/AIAA 39th Digital Avionics Systems
              Conference (DASC), pp. 1-10,
              DOI 10.1109/DASC50938.2020.9256746, October 2020,

   [MAE2021]  Mäurer, N., Gräupl, T., Gentsch, C., Guggemos, T.,
              Tiepelt, M., Schmitt, C., and G. Dreo Rodosek, "A Secure
              Cell-Attachment Procedure for LDACS", IEEE European
              Symposium on Security and Privacy Workshops (EuroS&PW),
              pp. 1-10, DOI 10.1109/EuroSPW54576.2021.00019, September
              2021, <>.

   [MAE20211] Mäurer, N., Gräupl, T., Bellido-Manganell, M.A., Mielke,
              D.M., Filip-Dhaubhadel, A., Heirich, O., Gerberth, D.,
              Felux, M., Schalk, L.M., Becker, D., Schneckenburger, N.,
              and M. Schnell, "Flight Trial Demonstration of Secure GBAS
              via the L-band Digital Aeronautical Communications System
              (LDACS)", IEEE Aerospace and Electronic Systems Magazine,
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   [MOST2018] Mostafa, M., Bellido-Manganell, M.A.., and T. Gräupl,
              "Feasibility of Cell Planning for the L-Band Digital
              Aeronautical Communications System Under the Constraint of
              Secondary Spectrum Usage", IEEE Transactions on Vehicular
              Technology, Vol. 67, Issue 10, pp. 9721-9733,
              DOI 10.1109/TVT.2018.2862829, October 2018,

   [NIST2013] National Institute of Standards and Technology (NIST),
              "Digital Signature Standard (DSS)", FIPS PUB 186-4,
              DOI 10.6028/NIST.FIPS.186-4, July 2013,

   [OSE2019]  Osechas, O., Narayanan, S., Crespillo, O.G., Zampieri, G.,
              Battista, G., Kumar, R., Schneckenburger, N., Lay, E.,
              Belabbas, B., and M. Meurer, "Feasibility Demonstration of
              Terrestrial RNP with LDACS", 32nd International Technical
              Meeting of the Satellite Division of The Institute of
              Navigation, pp. 3254-3265, DOI 10.33012/2019.17119,
              September 2019, <>.

              Thubert, P., Ed., Cavalcanti, D., Vilajosana, X., Schmitt,
              C., and J. Farkas, "Reliable and Available Wireless
              Technologies", Work in Progress, Internet-Draft, draft-
              ietf-raw-technologies-06, 30 November 2022,

              Bernardos, C. J., Ed., Papadopoulos, G. Z., Thubert, P.,
              and F. Theoleyre, "RAW Use-Cases", Work in Progress,
              Internet-Draft, draft-ietf-raw-use-cases-09, 13 March
              2023, <

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <>.

   [RFC4861]  Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
              "Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
              DOI 10.17487/RFC4861, September 2007,

   [RFC5213]  Gundavelli, S., Ed., Leung, K., Devarapalli, V.,
              Chowdhury, K., and B. Patil, "Proxy Mobile IPv6",
              RFC 5213, DOI 10.17487/RFC5213, August 2008,

   [RFC5795]  Sandlund, K., Pelletier, G., and L. Jonsson, "The RObust
              Header Compression (ROHC) Framework", RFC 5795,
              DOI 10.17487/RFC5795, March 2010,

   [RFC7296]  Kaufman, C., Hoffman, P., Nir, Y., Eronen, P., and T.
              Kivinen, "Internet Key Exchange Protocol Version 2
              (IKEv2)", STD 79, RFC 7296, DOI 10.17487/RFC7296, October
              2014, <>.

   [RFC8200]  Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", STD 86, RFC 8200,
              DOI 10.17487/RFC8200, July 2017,

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,

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              Datagram Transport Layer Security (DTLS) Protocol Version
              1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,

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              Cabellos, Ed., "The Locator/ID Separation Protocol
              (LISP)", RFC 9300, DOI 10.17487/RFC9300, October 2022,

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              Ed., "Locator/ID Separation Protocol (LISP) Control
              Plane", RFC 9301, DOI 10.17487/RFC9301, October 2022,

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              Coprocessor for Lattice-based Key Encapsulation Mechanism:
              Saber in Hardware", IACR Transactions on Cryptographic
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              conformance and compatibility assessment", IEEE/AIAA 33rd
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              DOI 10.1109/DASC.2014.6979447, October 2014,

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              Real-Time Monitoring and Control in the Future
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              DOI 10.1109/TITS.2020.2983614, August 2021,

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.


   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

   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

Authors' Addresses

   Nils Mäurer (editor)
   German Aerospace Center (DLR)
   Münchner Strasse 20
   82234 Wessling

   Thomas Gräupl (editor)
   German Aerospace Center (DLR)
   Münchner Strasse 20
   82234 Wessling

   Corinna Schmitt (editor)
   Research Institute CODE, UniBwM
   Werner-Heisenberg-Weg 39
   85577 Neubiberg