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L-band Digital Aeronautical Communications System (LDACS)

The information below is for an old version of the document.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 9372.
Authors Nils Mäurer , Thomas Gräupl , Corinna Schmitt
Last updated 2022-04-21 (Latest revision 2022-03-21)
Replaces draft-maeurer-raw-ldacs
RFC stream Internet Engineering Task Force (IETF)
Additional resources Mailing list discussion
Stream WG state Submitted to IESG for Publication
Associated WG milestones
Oct 2020
Working Group Adoption of LDACS Document
Jun 2021
LDACS Document submitted to IESG
Document shepherd Pascal Thubert
Shepherd write-up Show Last changed 2022-02-27
IESG IESG state Became RFC 9372 (Informational)
Consensus boilerplate Yes
Telechat date (None)
Has enough positions to pass.
Responsible AD John Scudder
Send notices to
IANA IANA review state IANA OK - No Actions Needed
RAW                                                      N. Maeurer, Ed.
Internet-Draft                                           T. Graeupl, Ed.
Intended status: Informational             German Aerospace Center (DLR)
Expires: 22 September 2022                               C. Schmitt, Ed.
                                         Research Institute CODE, UniBwM
                                                           21 March 2022

       L-band Digital Aeronautical Communications System (LDACS)


   This document gives an overview of the architecture of the L-band
   Digital Aeronautical Communications System (LDACS), which provides a
   secure, scalable and spectrum efficient terrestrial data link for
   civil aviation.  LDACS is a scheduled, reliable multi-application
   cellular broadband system with support for IPv6.  LDACS provides a
   data link for IPv6 network-based aircraft guidance.  High reliability
   and availability for IP connectivity over LDACS, as well as security,
   are therefore essential.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   This Internet-Draft will expire on 22 September 2022.

Copyright Notice

   Copyright (c) 2022 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 (
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights

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   and restrictions with respect to this document.  Code Components
   extracted from this document must include Revised BSD License text as
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   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   3.  Motivation and Use Cases  . . . . . . . . . . . . . . . . . .   6
     3.1.  Voice Communications Today  . . . . . . . . . . . . . . .   7
     3.2.  Data Communications Today . . . . . . . . . . . . . . . .   7
   4.  Provenance and Documents  . . . . . . . . . . . . . . . . . .   8
   5.  Applicability . . . . . . . . . . . . . . . . . . . . . . . .   9
     5.1.  Advances Beyond the State-of-the-Art  . . . . . . . . . .   9
       5.1.1.  Priorities  . . . . . . . . . . . . . . . . . . . . .   9
       5.1.2.  Security  . . . . . . . . . . . . . . . . . . . . . .  10
       5.1.3.  High Data Rates . . . . . . . . . . . . . . . . . . .  10
     5.2.  Application . . . . . . . . . . . . . . . . . . . . . . .  10
       5.2.1.  Air/Ground Multilink  . . . . . . . . . . . . . . . .  10
       5.2.2.  Air/Air Extension for LDACS . . . . . . . . . . . . .  11
       5.2.3.  Flight Guidance . . . . . . . . . . . . . . . . . . .  11
       5.2.4.  Business Communications of Airlines . . . . . . . . .  12
       5.2.5.  LDACS-based Navigation  . . . . . . . . . . . . . . .  12
   6.  Requirements  . . . . . . . . . . . . . . . . . . . . . . . .  13
   7.  Characteristics . . . . . . . . . . . . . . . . . . . . . . .  14
     7.1.  LDACS Sub-Network . . . . . . . . . . . . . . . . . . . .  14
     7.2.  Topology  . . . . . . . . . . . . . . . . . . . . . . . .  15
     7.3.  LDACS Protocol Stack  . . . . . . . . . . . . . . . . . .  16
       7.3.1.  LDACS Physical Layer  . . . . . . . . . . . . . . . .  17
       7.3.2.  LDACS Data Link Layer . . . . . . . . . . . . . . . .  18
       7.3.3.  LDACS Sub-Network Layer and Protocol Services . . . .  19
     7.4.  LDACS Mobility  . . . . . . . . . . . . . . . . . . . . .  20
   8.  Reliability and Availability  . . . . . . . . . . . . . . . .  20
     8.1.  Below Layer 1 . . . . . . . . . . . . . . . . . . . . . .  20
     8.2.  Layer 1 and 2 . . . . . . . . . . . . . . . . . . . . . .  20
     8.3.  Beyond Layer 2  . . . . . . . . . . . . . . . . . . . . .  23
   9.  Security  . . . . . . . . . . . . . . . . . . . . . . . . . .  23
     9.1.  Security in Wireless Digital Aeronautical
           Communications  . . . . . . . . . . . . . . . . . . . . .  24
     9.2.  LDACS Requirements  . . . . . . . . . . . . . . . . . . .  25
     9.3.  LDACS Security Objectives . . . . . . . . . . . . . . . .  25
     9.4.  LDACS Security Functions  . . . . . . . . . . . . . . . .  26
     9.5.  LDACS Security Architecture . . . . . . . . . . . . . . .  26
       9.5.1.  Entities  . . . . . . . . . . . . . . . . . . . . . .  26
       9.5.2.  Entity Identification . . . . . . . . . . . . . . . .  27
       9.5.3.  Entity Authentication and Key Establishment . . . . .  27

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       9.5.4.  Message-in-transit Confidentiality, Integrity and
               Authenticity  . . . . . . . . . . . . . . . . . . . .  28
   10. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  28
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  28
   12. Normative References  . . . . . . . . . . . . . . . . . . . .  28
   13. Informative References  . . . . . . . . . . . . . . . . . . .  28
   Appendix A.  Selected Information from DO-350A  . . . . . . . . .  34
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  36

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
   suffering from the Very High Frequency (VHF) band's increasing
   saturation in high- density areas and the limitations posed by
   analogue radio communications.  Therefore, aviation globally, and the
   European Union (EU) in particular, strives for a sustainable
   modernization of the aeronautical communications infrastructure.

   This modernization is realized in two steps: (1) the transition of
   communications datalinks from analogue to digital technologies and,
   (2) the introduction of IPv6 based networking protocols in
   aeronautical networks [RFC4291], [RFC7136], [ICAO2015].

   Step (1) is realized via ATM communications transitioning from
   analogue VHF voice [KAMA2010] to more spectrum efficient digital data
   communication.  For terrestrial communications the European ATM
   Master Plan 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 roll-out of LDACS will likely start there, and
   continue to other increasingly saturated zones as the east- and west-
   cost of the US and parts of Asia [ICAO2018].

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   Technically LDACS enables IPv6 based air- ground 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.  It was thus necessary to
   develop co-existence mechanism/procedures to enable the interference
   free operation of LDACS in parallel with other aeronautical services/
   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.

   Step (2) is a strategy for the worldwide roll-out of IPv6 capable
   digital aeronautical inter-networking.  This is called the
   Aeronautical Telecommunications Network (ATN)/Internet Protocol Suite
   (IPS) (hence, ATN/IPS).  It is specified in the International Civil
   Aviation Organization (ICAO) document Doc 9896 [ICAO2015], the Radio
   Technical Commission for Aeronautics (RTCA) document DO-379
   [RTCA2019], the European Organization for Civil Aviation Equipment
   (EUROCAE) document ED-262 [EURO2019], and the Aeronautical Radio
   Incorporated (ARINC) document P858 [ARI2021].  LDACS is subject to
   these regulations since it provides access subnets to the ATN/IPS.

   ICAO has chosen IPv6 as basis for the ATN/IPS mostly for historical
   reasons, since a previous architecture based on ISO/OSI protocols,
   the ATN/OSI, failed in the market place.

   In the context of safety-related communications, LDACS will play a
   major role in future ATM.  ATN/IPS datalinks will provide diversified
   terrestrial and space-based connectivity in a multi-link concept,
   called the Future Communications Infrastructure (FCI) [VIR2021].
   From a technical point of view the FCI will realize airborne multi-
   homed IPv6 networks connected to a global ground network via at least
   two independent communication technologies.  This is considered in
   more detail in related IETF work in progress [I-D.haindl-lisp-gb-atn]

   In the context of the Reliable and Available Wireless (RAW) working
   group, developing options, such as intelligent switching between
   datalinks, 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.

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   LDACS standardization within the framework of ICAO started in
   December 2016.  The ICAO standardization group has produced an
   initial Standards and Recommended Practices (SARPS) document
   [ICA2018].  It defines the general characteristics of LDACS.  The
   ICAO standardization group plans to produce an ICAO technical manual
   - the ICAO equivalent to a technical standard - within the next
   years.  Generally, the group is open to input from all sources and
   encourages cooperation between the aeronautical and the IETF

2.  Terminology

   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
   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
   AR  Access Router
   ARINC  Aeronautical Radio, Incorporated
   ARQ  Automatic Repeat reQuest
   AS  Aircraft Station
   ATC  Air Traffic Control
   ATM  Air Traffic Management
   ATN  Aeronautical Telecommunication 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
   CRL  Certificate Revocation List
   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

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   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
   LLC  Logical Link Control
   LME  LDACS Management Entity
   MAC  Medium Access Control
   MF  Multi Frame
   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
   SNP  Sub-Network 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.  It refers to the mostly
   proprietary exchange of data between the aircraft of the airline, its
   operation centers, and its service partners.  ARINC document 633 was
   developed and first released in 2007 [ARI2019] with the goal to
   standardize these messages for interoperability, e.g., messages

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   between the airline and fueling or de-icing companies.  Within the
   airport terminal, connectivity is focused on high bandwidth
   communications, while during en-route, high reliability, robustness,
   and range is the main focus.  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 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

3.1.  Voice Communications Today

   Voice links are used for Air/Ground (A/G) and Air/Air (A/A)
   communications.  The communications equipment is either ground-based
   working in the High Frequency (HF) or VHF frequency band or
   satellite-based.  All VHF and HF voice communications are operated
   via open broadcast channels without authentication, encryption or
   other protective measures.  The use of well-proven communications
   procedures via broadcast channels, 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 analogue 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.  This however, 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 air
   traffic control officers.  [BOE2019]

3.2.  Data Communications Today

   Like for voice, 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 are using narrowband radio channels with a data throughput
   capacity in the order of kilobits per second.  While the aircraft is
   on ground, some additional communications systems are available, like
   the Aeronautical Mobile Airport Communications System (AeroMACS) or
   public cellular networks, operating in the Airport (APT) domain and
   able to deliver broadband communications capability.  [BOE2019]

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   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 that the flight
   crews are supported by narrowband services during flight while
   passengers have access to inflight 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.

   Satellite-based communications are currently under investigation and
   enhanced capabilities are under development which will be able to
   provide inflight broadband services and communications supporting the
   safety and regularity of flight.  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 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 Global Air Navigation
   Plan (GNAP).  [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 able to evolve with aviation needs over
   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 fulfil the identified requirements [GRA2011].

   Up to now LDACS standardization has been focused on the development
   of the physical layer and the data link layer.  Only recently have
   higher layers have come into the focus of the LDACS development
   activities.  There is currently no "IPv6 over LDACS" specification
   publicly available; however, SESAR2020 has started the testing of
   IPv6-based LDACS testbeds.

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   The IPv6 architecture for the aeronautical telecommunication network
   is called the FCI.  The FCI will support quality of service,
   diversity, and mobility under the umbrella of the "multi-link
   concept".  This work is led by ICAO Communication Panel working group

   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 (ATS)
   including ATS-B3, and AOC communications services from deployed
   Ground-Stations (GS).  The physical layer and data link layer of
   LDACS are optimized for controller-pilot data link communications,
   but the system also supports digital air-ground 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 de-coupled 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 offers several capabilities, not yet provided in contemporarily
   deployed aeronautical communications systems.

5.1.1.  Priorities

   LDACS is able to manage service priorities, an important feature 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.

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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 (ANSP).  This includes concepts
   for key and trust management, mutual authentication and key
   establishment 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 current terrestrial digital
   aeronautical communications systems, such as the VHF Data Link mode 2
   (VDLm2), provide [ICAO2019] [GRA2020].

5.2.  Application

   LDACS will be used by several aeronautical applications ranging from
   enhanced communications protocol stacks (multi-homed mobile IPv6
   networks in the aircraft and potentially ad-hoc networks between
   aircraft) to broadcast communication applications (sending Ground
   Based Augmentation System (GBAS) correction data) and integration
   with other service domains (using the communications signal for
   navigation) [MAE20211].

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 which complement each other.
   Especially, satellite systems are 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
   hot-spots around airports and terminal airspace.  In addition, both
   technologies offer comparable data link capacity and, thus, are well-
   suited for redundancy, mutual back-up, or load balancing.

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   Technically the FCI multilink concept will be realized by multi-
   homed mobile IPv6 networks in the aircraft.  The related protocol
   stack is currently under development by ICAO, within SESAR, and the
   IETF [I-D.haindl-lisp-gb-atn] [I-D.ietf-rtgwg-atn-bgp].

5.2.2.  Air/Air Extension for LDACS

   A potential extension of the multi-link 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 are 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 part of the standardization process and
   will not be covered within this document.

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.

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   CM, CPDLC, and ADS-C are available on legacy datalinks, 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 GBAS for secure, automated landings:
   The Global Navigation Satellite System (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 VDB data link is
   used.  VDB is a narrow-band single-purpose datalink without advanced
   security only used to transmit GBAS correction data.  This makes VDB
   a natural candidate for replacement by LDACS [MAE20211].

5.2.4.  Business Communications of Airlines

   In addition to air traffic services, 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 and may
   therefore 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 also be used for
   navigation.  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.  Due to the large separation between navigational
   satellites and aircraft, the power of the GNSS signals received by
   the aircraft is, however, 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 of such APNT solutions consists of 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.

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   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 1990ies (the
   use of the Open Systems Interconnection (OSI) stack indicates that as
   well).  VDLm2 is expected to be used at least for several decades.
   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 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.

   AOC applications currently 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.

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   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 currently experienced operational problems.

   The requirement for LDACS is therefore to provide a terrestrial high-
   throughput data link for IP internetworking in the aircraft.

   In order to fulfil 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 fulfil the requirements defined in RTCA DO-350A/EUROCAE ED-
   228A [DO350A].  This document defines continuity, availability, and
   integrity requirements at different scopes for each air traffic
   management 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
   [DO350A] volume 1 Table 5-14, and Table 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 layer one
   and two.  However, for the purpose of this work, only layer two
   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 undefined.

7.1.  LDACS Sub-Network

   An LDACS sub-network contains an Access Router (AR) and several GS,
   each of them providing one LDACS radio cell.

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   User plane interconnection to the ATN is facilitated by the AR
   peering with an A/G Router connected to the ATN.

   The internal control plane of an LDACS sub-network interconnects the
   GSs.  An LDACS sub-network is illustrated in Figure 1.

   wireless      user
   link          plane
                    .                |   Router
                    . control        |
                    . plane          |
                    .                |
                    .                |
                    .                |

           Figure 1: LDACS sub-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 (AS) belonging to
   aircraft within a certain volume of space (the LDACS cell) is
   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 bi-directional
   communications to the ASs under its control.  LDACS's GSs themselves
   are connected to each other and the AR.

   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.

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7.3.  LDACS Protocol Stack

   The protocol stack of LDACS is implemented in the AS and GS: It
   consists of the Physical Layer (PHY) with five major, functional
   blocks above it.  Four are placed in the Data Link Layer (DLL) of the
   AS and GS: (1) Medium Access Control (MAC) Layer, (2) Voice Interface
   (VI), (3) Data Link Service (DLS), and (4) LDACS Management Entity
   (LME).  The last entity resides within the sub-network layer: the
   Sub-Network Protocol (SNP).  The LDACS network is externally
   connected to voice units, radio control units, and 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 IPv6 layer and SNP layer is currently subject of
   ongoing standardization efforts and at the time of writing not
   finished yet.

   Figure 2 shows the protocol stack of LDACS as implemented in the AS
   and GS.  Acronyms used here are introduced throughout the upcoming

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            IPv6                   Network Layer
   +------------------+  +----+
   |        SNP       |--|    |   Sub-Network
   |                  |  |    |   Layer
   +------------------+  |    |
             |           | LME|
   +------------------+  |    |
   |        DLS       |  |    |   LLC Layer
   +------------------+  +----+
             |             |
            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 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 bi-directional 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).  Aircraft thus transmit
   discontinuously on the RL via short radio bursts sent in precisely
   defined transmission opportunities allocated by the GS.

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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 supports also 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 Service (DLS) Services

   The DLS provides acknowledged and unacknowledged (including broadcast
   and packet mode voice) bi-directional 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 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 resources.
   4.  The reverse direction is covered by the RL, where ASs need to

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      request resources before sending.  This happens via the Dedicated
      Control Channel (DCCH).
   5.  User data itself is communicated in the Data Channel (DCH) on the
      FL and RL.

   Access to the FL and RL data channel 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 either be set-up permanently by the GS (e.g., to emulate voice
   party line) or may be 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 AS 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 Sub-Network Layer and Protocol Services

   Lastly, the SNP handles the transition from IPv6 packts to LDACS
   internal packet structures.  This work is ongoing and not part of
   this document.  The DLS provides functions required for the transfer
   of user plane data and control plane data over the LDACS sub-network.
   The security service provides functions for secure user data
   communication over the LDACS sub-network.  Note that the SNP security
   service applies cryptographic measures as configured by the GS.

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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.

   External handovers between non-connected LDACS sub-networks or
   different aeronautical data links are handled by the FCI multi- link

8.  Reliability and Availability

8.1.  Below Layer 1

   Below Layer 2, aeronautics usually relies 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.  Layer 1 and 2

   LDACS has been designed with applications related to the safety and
   regularity of flight in mind.  It has therefore 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, supporting 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 the operation at a maximum distance of 200 nautical
   miles 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).

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   LDACS medium access layer on top of the physical layer uses a static
   frame structure to support deterministic timer management.  As shown
   in Figure 3 and Figure 4, LDACS framing structure is based on Super-
   Frames (SF) of 240ms duration corresponding to 2000 OFDM symbols.  FL
   and RL boundaries are aligned in time (from the GS perspective)
   allowing for deterministic slots for control and data channels.  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].

   |     +------+------------+------------+------------+------------+
   |  FL | BCCH |     MF     |     MF     |     MF     |     MF     |
   F     +------+------------+------------+------------+------------+
   r     <---------------- Super-Frame (SF) - 240ms ---------------->
   q     +------+------------+------------+------------+------------+
   u  RL | RACH |     MF     |     MF     |     MF     |     MF     |
   e     +------+------------+------------+------------+------------+
   n     <---------------- Super-Frame (SF) - 240ms ---------------->
   ----------------------------- Time ------------------------------>

                      Figure 3: SF structure for LDACS

   |     +-------------+------+-------------+
   |  FL |     DCH     | CCCH |     DCH     |
   F     +-------------+------+-------------+
   r     <---- Multi-Frame (MF) - 58.32ms -->
   q     +------+---------------------------+
   u  RL | DCCH |             DCH           |
   e     +------+---------------------------+
   n     <---- Multi-Frame (MF) - 58.32ms -->
   -------------------- Time ------------------>

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                      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 quality-of-service in LDACS medium access is to
   provide prioritized medium access at the bottleneck (the wireless
   link).  The signaling of higher layer quality-of-service requirements
   to LDACS is yet to be defined.  A Differentiated Services- (DiffServ)
   based solution with a small number of priorities is to be expected.

   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 therefore also anchored in the LME.

   In addition to radio resource management, the LDACS control channels
   are also used to send keep-alive messages, when they are not
   otherwise used.  Since the framing of the control channels is
   deterministic, missing keep-alive messages can thus be immediately
   detected.  This information is made available to the multi-link
   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, 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 data channel.

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   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
   ground stations.  However, the scarcity of aeronautical spectrum for
   data link communication (in the case of LDACS: tens of MHz in the
   L-band) and the long range (in the case of LDACS: up to 200 nautical
   miles) make this quite hard.  While the deployment of a larger number
   of small cells is one possible solution, this also suffers from the
   spectrum scarcity.  An additional constraint to consider, is that
   Distance Measuring Equipment (DME) is the primary user of the
   aeronautical L-band.  That is, any LDACS deployment has to take DME
   frequency planning into account.

   The aeronautical community has therefore 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 multi-link 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
   certainly highly desirable but needs to be adapted to the specific
   use case.

9.  Security

   ICAO Doc 9896 foresees transport layer security [ICAO2015] for all
   aeronautical data as described in ARINC P858 [ARI2021], most likely
   realized via Datagram Transport Layer Security (DTLS) [RFC6012]

   LDACS also needs to comply with in-depth security requirements,
   stated in P858, for the radio access technologies transporting ATN/
   IPS data [ARI2021].  These requirements imply that LDACS must provide
   layer 2 security in addition to any higher layer mechanisms.

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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 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 analogue voice broadcast within the
   aeronautical VHF band.  Currently, information security is mainly
   procedural, based by using well-trained personnel and proven
   communications procedures.  This communication method has been in
   service since 1948.  However, since the emergence of civil
   aeronautical CNS applications in the 70s, and today, the world has

   Civil applications have significant lower spectrum available than
   military applications.  This means several military defense
   mechanisms, such as frequency hopping or pilot symbol scrambling and,
   thus, a defense-in- depth approach starting at the physical layer, is
   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 a new type of unsophisticated listeners and possible attackers.

   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.

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9.2.  LDACS Requirements

   Overall, there are several business goals for cybersecurity to
   protect, within the FCI in civil aviation:

   1.  Safety: The system must sufficiently mitigate attacks, which
      contribute to safety hazards.
   2.  Flight regularity: The system must sufficiently mitigate attacks,
      which contribute to delays, diversions, or cancellations of
   3.  Protection of business interests: The system must sufficiently
      mitigate attacks which result in financial loss, reputation
      damage, disclosure of sensitive proprietary information, or
      disclosure of personal information.

   To further analyze assets and derive threats and thus protection
   scenarios several threat-and risk analyses were performed for LDACS
   [MAE20181] , [MAE20191].  These results allowed deriving security
   scope and objectives from the requirements and the conducted threat-
   and risk analysis.

9.3.  LDACS Security Objectives

   Security considerations for LDACS are defined by the official SARPS
   document by ICAO [ICA2018]:

   1.  LDACS shall provide a capability to protect the availability and
      continuity of the system.
   2.  LDACS shall provide a capability including cryptographic
      mechanisms to protect the integrity of messages in transit.
   3.  LDACS shall provide a capability to ensure the authenticity of
      messages in transit.
   4.  LDACS should provide a capability for nonrepudiation of origin
      for messages in transit.
   5.  LDACS should provide a capability to protect the confidentiality
      of messages in transit.
   6.  LDACS shall provide an authentication capability.
   7.  LDACS shall provide a capability to authorize the permitted
      actions of users of the system and to deny actions that are not
      explicitly authorized.
   8.  If LDACS provides interfaces to multiple domains, LDACS shall
      provide capability to prevent the propagation of intrusions within
      LDACS domains and towards external domains.

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   Currently, 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.

9.4.  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].

9.5.  LDACS Security Architecture

   The requirements lead to a 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 [ICA2018] and [MAE20182] and respective updates in
   [MAE20191], [MAE20192], [MAE2020], and most recently [MAE2021].

9.5.1.  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]
   are providing access to the ground IPS network via an A/G LDACS
   router.  This router is attached to a closed off LDACS access
   network, which connects via further access routers to the different
   LDACS cell ranges, each controlled by a GS (serving one LDACS cell),
   with several interconnected GS spanning a local LDACS access network.
   Via the A/G wireless LDACS data link AS the aircraft is connected to
   the ground network and via the aircraft's VI and aircraft's network
   interface, aircraft's data can be sent via the AS back to the GS,
   then to the LDACS local access network, access routers, LDACS access
   network, A/G LDACS router and finally to the ground IPS network

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9.5.2.  Entity Identification

   LDACS needs specific identities for the AS, the GS, and the network
   operator.  The aircraft itself can be identified using the ICAO
   unique address of an aircraft, 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 AS LDACS 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 again 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]).

9.5.3.  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, providing proof the ownership of their
   respective public key, 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,
   thus, secure all ground connections.  How this process is handled in
   detail is still an ongoing discussion.  However, established methods
   to secure user plane by IPSec [RFC4301] and IKEv2 [RFC7296] or the
   application layer via TLS 1.3 [RFC8446] are conceivable.  The LDACS
   PKI with their 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 Mutual Authentication and Key Establishment (MAKE) procedure

   Note, that LDACS will foresee multiple security levels.  To address
   the issue of the long service life of LDACS (i.e., possibly >30
   years) and the security of current pre-quantum cryptography, these
   security levels include pre- and post-quantum cryptographic
   solutions.  Limiting security data on the LDACS datalink 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 [KOB1987], while post-quantum solutions consider

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   Falcon [SON2021] [MAE2021] or similar lightweight PQC signature
   schemes, and SIKE or SABER as key establishment options [SIK2021]

9.5.4.  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 [ICA2018].  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.

10.  IANA Considerations

   This memo includes no request to IANA.

11.  Acknowledgements

   Thanks to all contributors to the development of LDACS and ICAO PT-T.

   Thanks to Klaus-Peter Hauf, Bart Van Den Einden, and Pierluigi
   Fantappie for further input to this draft.

   Thanks to the Chair for Network Security and the research institute
   CODE for their comments and improvements.

   Thanks to SBA Research Vienna for fruitful discussions on
   aeronautical communications concerning security incentives for
   industry and potential economic spillovers.

   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

12.  Normative References

13.  Informative References

   [RFC4291]  Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 4291, DOI 10.17487/RFC4291, February
              2006, <>.

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

   [RFC6012]  Salowey, J., Petch, T., Gerhards, R., and H. Feng,
              "Datagram Transport Layer Security (DTLS) Transport
              Mapping for Syslog", RFC 6012, DOI 10.17487/RFC6012,
              October 2010, <>.

   [RFC6347]  Rescorla, E. and N. Modadugu, "Datagram Transport Layer
              Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
              January 2012, <>.

   [RFC7136]  Carpenter, B. and S. Jiang, "Significance of IPv6
              Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136,
              February 2014, <>.

   [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, <>.

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

   [GRA2020]  Graeupl, T., Rihacek, C., and B. Haindl, "LDACS A/G
              Specification", SESAR2020 PJ14-02-01 D3.3.030 , 2020,

   [ARI2021]  ARINC, "Internet Protocol Suite (IPS) For Aeronautical
              Safety Services Part 1- Airborne IP System Technical
              Requirements, ARINC SPECIFICATION 858 P1", June 2021,

   [EURO2019] European Organization for Civil Aviation Equipment
              (EUROCAE), "Technical Standard of Aviation Profiles for
              ATN/IPS, ED-262", September 2019,

   [ICAO2015] International Civil Aviation Organization (ICAO), "Manual
              on the Aeronautical Telecommunication Network (ATN) using
              Internet Protocol Suite (IPS) Standards and Protocols, Doc
              9896", January 2015,

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   [RTCA2019] Radio Technical Commission for Aeronautics (RTCA),
              "Internet Protocol Suite Profiles, DO-379", September
              2019, <>.

   [SCH2016]  Schneckenburger, N., Jost, T., Shutin, D., Walter, M.,
              Thiasiriphet, T., Schnell, M., and U.C. Fiebig,
              "Measurement of the L-band Air-to-Ground Channel for
              Positioning Applications", IEEE Transactions on Aerospace
              and Electronic Systems, 52(5), pp.2281-229 , 2016.

   [MAE20191] Maeurer, N., Graeupl, T., and C. Schmitt, "Evaluation of
              the LDACS Cybersecurity Implementation", IEEE 38th Digital
              Avionics Systems Conference (DACS), pp. 1-10, San Diego,
              CA, USA , 2019.

   [MAE20192] Maeurer, N. and C. Schmitt, "Towards Successful
              Realization of the LDACS Cybersecurity Architecture: An
              Updated Datalink Security Threat- and Risk Analysis", IEEE
              Integrated Communications, Navigation and Surveillance
              Conference (ICNS), pp. 1-13, Herndon, VA, USA , 2019.

   [MAE20182] Maeurer, N. and A. Bilzhause, "A Cybersecurity
              Architecture for the L-band Digital Aeronautical
              Communications System (LDACS)", IEEE 37th Digital Avionics
              Systems Conference (DASC), pp. 1-10, London, UK , 2017.

   [GRA2011]  Graeupl, T. and M. Ehammer, "L-DACS1 Data Link Layer
              Evolution of ATN/IPS", 30th IEEE/AIAA Digital Avionics
              Systems Conference (DASC), pp. 1-28, Seattle, WA, USA ,

   [GRA2018]  Graeupl, T., Schneckenburger, N., Jost, T., Schnell, M.,
              Filip, A., Bellido-Manganell, M.A., Mielke, D.M., Maeurer,
              N., Kumar, R., Osechas, O., and G. Battista, "L-band
              Digital Aeronautical Communications System (LDACS) flight
              trials in the national German project MICONAV", Integrated
              Communications, Navigation, Surveillance Conference
              (ICNS), pp. 1-7, Herndon, VA, USA , 2018.

   [ICA2018]  International Civil Aviation Organization (ICAO), "L-Band
              Digital Aeronautical Communication System (LDACS)",
              International Standards and Recommended Practices Annex 10
              - Aeronautical Telecommunications, Vol. III -
              Communication Systems , 2018.

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   [SAJ2014]  Haindl, B., Meser, J., Sajatovic, M., Mueller, S.,
              Arthaber, H., Faseth, T., and M. Zaisberger, "LDACS1
              Conformance and Compatibility Assessment", IEEE/AIAA 33rd
              Digital Avionics Systems Conference (DASC), pp. 1-11,
              Colorado Springs, CO, USA , 2014.

   [RIH2018]  Rihacek, C., Haindl, B., Fantappie, P., Pierattelli, S.,
              Graeupl, T., Schnell, M., and N. Fistas, "L-band Digital
              Aeronautical Communications System (LDACS) Activities in
              SESAR2020", Integrated Communications Navigation and
              Surveillance Conference (ICNS), pp. 1-8, Herndon, VA,
              USA , 2018.

   [BEL2019]  Bellido-Manganell, M. A. and M. Schnell, "Towards Modern
              Air-to-Air Communications: the LDACS A2A Mode", IEEE/AIAA
              38th Digital Avionics Systems Conference (DASC), pp. 1-10,
              San Diego, CA, USA , 2019.

   [CRO2016]  Crowe, B., "Proposed AeroMACS PKI Specification is a Model
              for Global and National Aeronautical PKI Deployments",
              WiMAX Forum at 16th Integrated Communications, Navigation
              and Surveillance Conference (ICNS), pp. 1-19, New York,
              NY, USA , 2016.

   [MAE2020]  Maeurer, N., Graeupl, T., and C. Schmitt, "Comparing
              Different Diffie-Hellman Key Exchange Flavors for LDACS",
              IEEE/AIAA 39th Digital Avionics Systems Conference (DASC),
              pp. 1-10, San Antonio, TX, USA , 2020.

   [STR2016]  Strohmeier, M., Schaefer, M., Pinheiro, R., Lenders, V.,
              and I. Martinovic, "On Perception and Reality in Wireless
              Air Traffic Communication Security", IEEE Transactions on
              Intelligent Transportation Systems, 18(6), pp. 1338-1357,
              New York, NY, USA , 2016.

   [BIL2017]  Bilzhause, A., Belgacem, B., Mostafa, M., and T. Graeupl,
              "Datalink Security in the L-band Digital Aeronautical
              Communications System (LDACS) for Air Traffic Management",
              IEEE Aerospace and Electronic Systems Magazine, 32(11),
              pp. 22-33, New York, NY, USA , 2017.

   [MAE20181] Maeurer, N. and A. Bilzhause, "Paving the Way for an IT
              Security Architecture for LDACS: A Datalink Security
              Threat and Risk Analysis", IEEE Integrated Communications,
              Navigation, Surveillance Conference (ICNS), pp. 1-11, New
              York, NY, USA , 2018.

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   [FAA2020]  FAA, "Federal Aviation Administration. ADS-B Privacy.",
              August 2020,

   [GNU2021]  GNU Radio project, "GNU radio", October 2021,

   [SIT2020]  SITA, "Societe Internationale de Telecommunications
              Aeronautiques", August 2020, <>.

   [ARI2020]  ARINC, "Aeronautical Radio Incorporated", August 2020,

   [DO350A]   RTCA SC-214, "Safety and Performance Standard for Baseline
              2 ATS Data Communications (Baseline 2 SPR Standard)", May
              2016, <

   [ICAO2019] International Civil Aviation Organization (ICAO), "Manual
              on VHF Digital Link (VDL) Mode 2, Doc 9776", January 2019,

   [KAMA2010] Kamali, B., "An Overview of VHF Civil Radio Network and
              the Resolution of Spectrum Depletion", Integrated
              Communications, Navigation, and Surveillance Conference,
              pp. F4-1-F4-8 , May 2010.

   [SON2021]  Soni, D., Basu, K., Nabeel, M., Aaraj, N., Manzano, M.,
              and R. Karri, "FALCON", Hardware Architectures for Post-
              Quantum Digital Signature Schemes, pp. 31-41 , November

   [SIK2021]  SIKE, "SIKE – Supersingular Isogeny Key Encapsulation",
              October 2021, <>.

   [ROY2020]  Roy, S.S.. and A. Basso, "High-Speed Instruction-Set
              Coprocessor For Lattice-Based Key Encapsulation Mechanism:
              Saber In Hardware", IACR Transactions on Cryptographic
              Hardware and Embedded Systems, 443-466. , August 2020.

              Thubert, P., Cavalcanti, D., Vilajosana, X., Schmitt, C.,
              and J. Farkas, "Reliable and Available Wireless
              Technologies", Work in Progress, Internet-Draft, draft-
              ietf-raw-technologies-05, 2 February 2022,

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              Bernardos, C. J., Papadopoulos, G. Z., Thubert, P., and F.
              Theoleyre, "RAW use-cases", Work in Progress, Internet-
              Draft, draft-ietf-raw-use-cases-05, 23 February 2022,

              Haindl, B., Lindner, M., Rahman, R., Comeras, M. P.,
              Moreno, V., Maino, F., and B. Venkatachalapathy, "Ground-
              Based LISP for the Aeronautical Telecommunications
              Network", Work in Progress, Internet-Draft, draft-haindl-
              lisp-gb-atn-06, 6 March 2021,

              Templin, F. L., Saccone, G., Dawra, G., Lindem, A., and V.
              Moreno, "A Simple BGP-based Mobile Routing System for the
              Aeronautical Telecommunications Network", Work in
              Progress, Internet-Draft, draft-ietf-rtgwg-atn-bgp-14, 14
              February 2022, <

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              "Handbook on Radio Frequency Spectrum Requirements for
              Civil Aviation, Doc 9718, Volume 1, ICAO Spectrum
              Strategy, Policy Statements and Related Information", July
              2018, <

   [ARI2019]  ARINC, "AOC Air-Ground Data And Message Exchange Format,
              ARINC 633", January 2019,

   [VIR2021]  Virdia, A., Stea, G., and G. Dini, "SAPIENT: Enabling
              Real-Time Monitoring and Control in the Future
              Communication Infrastructure of Air Traffic Management",
              IEEE Transactions on Intelligent Transportation Systems,
              22(8):4864-4875 , August 2021.

   [SHU2013]  Shutin, D., Schneckenburger, N., Walter, M., and M.
              Schnell, "LDACS1 Ranging Performance - An Analysis Of
              Flight Measurement Results", IEEE 32th Digital Avionics
              Systems Conference (DASC), pp. 1-10, East Syracuse, NY,
              USA , October 2013.

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   [BEL2021]  Bellido-Manganell, M.A., Graeupl, T., Heirich, O.,
              Maeurer, N., Filip-Dhaubhadel, A., Mielke, D.M., Schalk,
              L.M., Becker, D., Schneckenburger, N., and M. Schnell,
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              Analysis of the Future Aeronautical Communications
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              Cell-Attachment Procedure for LDACS", 1st Workshop on
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              October 2019.

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 Communication
   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

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          |                        |   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
      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.

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   |              |  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 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.

Authors' Addresses

   Nils Maeurer (editor)
   German Aerospace Center (DLR)
   Muenchner Strasse 20
   82234 Wessling

   Thomas Graeupl (editor)
   German Aerospace Center (DLR)
   Muenchner Strasse 20
   82234 Wessling

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   Corinna Schmitt (editor)
   Research Institute CODE, UniBwM
   Werner-Heisenberg-Weg 28
   85577 Neubiberg

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