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RAW                                                      N. Maeurer, Ed.
Internet-Draft                                           T. Graeupl, Ed.
Intended status: Informational             German Aerospace Center (DLR)
Expires: 5 April 2021                                    C. Schmitt, Ed.
                                         Research Institute CODE, UniBwM
                                                          2 October 2020


       L-band Digital Aeronautical Communications System (LDACS)
                       draft-maeurer-raw-ldacs-06

Abstract

   This document provides 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 shall provide
   a data link for IP network-based aircraft guidance.  High reliability
   and availability for IP connectivity over LDACS 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 5 April 2021.

Copyright Notice

   Copyright (c) 2020 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 (https://trustee.ietf.org/
   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 Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Motivation and Use Cases  . . . . . . . . . . . . . . . . . .   5
     3.1.  Voice Communications Today  . . . . . . . . . . . . . . .   5
     3.2.  Data Communications Today . . . . . . . . . . . . . . . .   6
   4.  Provenance and Documents  . . . . . . . . . . . . . . . . . .   7
   5.  Applicability . . . . . . . . . . . . . . . . . . . . . . . .   8
     5.1.  Advances Beyond the State-of-the-Art  . . . . . . . . . .   8
       5.1.1.  Priorities  . . . . . . . . . . . . . . . . . . . . .   8
       5.1.2.  Security  . . . . . . . . . . . . . . . . . . . . . .   8
       5.1.3.  High Data Rates . . . . . . . . . . . . . . . . . . .   9
     5.2.  Application . . . . . . . . . . . . . . . . . . . . . . .   9
       5.2.1.  Air-to-Ground Multilink . . . . . . . . . . . . . . .   9
       5.2.2.  Air-to-Air Extension for LDACS  . . . . . . . . . . .   9
       5.2.3.  Flight Guidance . . . . . . . . . . . . . . . . . . .  10
       5.2.4.  Business Communication of Airlines  . . . . . . . . .  11
       5.2.5.  LDACS Navigation  . . . . . . . . . . . . . . . . . .  11
   6.  Requirements to LDACS . . . . . . . . . . . . . . . . . . . .  12
   7.  Characteristics of LDACS  . . . . . . . . . . . . . . . . . .  15
     7.1.  LDACS Sub-Network . . . . . . . . . . . . . . . . . . . .  16
     7.2.  Topology  . . . . . . . . . . . . . . . . . . . . . . . .  16
     7.3.  LDACS Physical Layer  . . . . . . . . . . . . . . . . . .  17
     7.4.  LDACS Data Link Layer . . . . . . . . . . . . . . . . . .  17
     7.5.  LDACS Mobility  . . . . . . . . . . . . . . . . . . . . .  17
   8.  Reliability and Availability  . . . . . . . . . . . . . . . .  18
     8.1.  Layer 2 . . . . . . . . . . . . . . . . . . . . . . . . .  18
     8.2.  Beyond Layer 2  . . . . . . . . . . . . . . . . . . . . .  20
   9.  Protocol Stack  . . . . . . . . . . . . . . . . . . . . . . .  21
     9.1.  Medium Access Control (MAC) Entity Services . . . . . . .  22
     9.2.  Data Link Service (DLS) Entity Services . . . . . . . . .  24
     9.3.  Voice Interface (VI) Services . . . . . . . . . . . . . .  25
     9.4.  LDACS Management Entity (LME) Services  . . . . . . . . .  25
     9.5.  Sub-Network Protocol (SNP) Services . . . . . . . . . . .  25
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  25
     10.1.  Reasons for Wireless Digital Aeronautical
            Communications . . . . . . . . . . . . . . . . . . . . .  25
     10.2.  Requirements for LDACS . . . . . . . . . . . . . . . . .  26
     10.3.  Security Objectives for LDACS  . . . . . . . . . . . . .  27
     10.4.  Security Functions for LDACS . . . . . . . . . . . . . .  27
     10.5.  Security Architectural Details for LDACS . . . . . . . .  28
       10.5.1.  Entities in LDACS Security Model . . . . . . . . . .  28



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       10.5.2.  Matter of LDACS Entity Identification  . . . . . . .  28
       10.5.3.  Matter of LDACS Entity Authentication and Key
               Negotiation . . . . . . . . . . . . . . . . . . . . .  29
       10.5.4.  Matter of LDACS Message-in-transit Confidentiality,
               Integrity and Authenticity  . . . . . . . . . . . . .  29
     10.6.  Security Architecture for LDACS  . . . . . . . . . . . .  30
   11. Privacy Considerations  . . . . . . . . . . . . . . . . . . .  30
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  30
   13. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  30
   14. Normative References  . . . . . . . . . . . . . . . . . . . .  30
   15. Informative References  . . . . . . . . . . . . . . . . . . .  31
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  34

1.  Introduction

   One of the main pillars of the modern Air Traffic Management (ATM)
   system is the existence of a communication infrastructure that
   enables efficient aircraft control and safe separation in all phases
   of flight.  Current systems are technically mature but suffering from
   the 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 communication
   infrastructure.

   In the long-term, ATM communication shall transition from analogue
   VHF voice and VDLM2 communication to more spectrum efficient digital
   data communication.  The European ATM Master Plan foresees this
   transition to be realized for terrestrial communications by the
   development (and potential implementation) of the L-band Digital
   Aeronautical Communications System (LDACS).  LDACS shall enable IPv6
   based air- ground communication related to the aviation safety and
   regularity of flight.  The particular challenge is that no additional
   spectrum 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
   same frequency band.

   Since LDACS shall be used for aircraft guidance, high reliability and
   availability for IP connectivity over LDACS are essential.

2.  Terminology

   The following terms are used in the context of RAW in this document:

   A2A  Air-to-Air
   LDACS A2A  LDACS Air-to-Air



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   AeroMACS  Aeronautical Mobile Airport Communication System
   A2G  Air-to-Ground
   ACARS  Aircraft Communications Addressing and Reporting System
   ADS-C  Automatic Dependent Surveillance - Contract
   AM(R)S  Aeronautical Mobile (Route) Service
   ANSP  Air traffic Network Service Provider
   AOC  Aeronautical Operational Control
   AS  Aircraft Station
   ATC  Air-Traffic Control
   ATM  Air-Traffic Management
   ATN  Aeronautical Telecommunication Network
   ATS  Air Traffic Service
   CCCH  Common Control Channel
   COTS IP  Commercial Off-The-Shelf
   CM  Context Management
   CNS  Communication Navigation Surveillance
   CPDLC  Controller Pilot Data Link Communication
   DCCH  Dedicated Control Channel
   DCH  Data Channel
   DLL  Data Link Layer
   DLS  Data Link Service
   DME  Distance Measuring Equipment
   DSB-AM  Double Side-Band Amplitude Modulation
   FAA  Federal Aviation Administration
   FCI  Future Communication Infrastructure
   FDD  Frequency Division Duplex
   FL  Forward Link
   GANP  Global Air Navigation Plan
   GNSS  Global Navigation Satellite System
   GS  Ground Station
   GSC  Ground-Station Controller
   G2A  Ground-to-Air
   HF  High Frequency
   ICAO  International Civil Aviation Organization
   IP  Internet Protocol
   kbit/s  kilobit per second
   LDACS  L-band Digital Aeronautical Communications System
   LLC  Logical Link Layer
   LME  LDACS Management Entity
   MAC  Medium Access Layer
   MF  Multi Frame
   OFDM  Orthogonal Frequency-Division Multiplexing
   OFDMA  Orthogonal Frequency-Division Multiplexing Access
   OSI  Open Systems Interconnection
   PDU  Protocol Data Units
   PHY  Physical Layer
   QoS  Quality of Service
   RL  Reverse Link



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   SARPs  Standards And Recommended Practices
   SDR  Software Defined Radio
   SESAR  Single European Sky ATM Research
   SF  Super-Frame
   SNP  Sub-Network Protocol
   SSB-AM  Single Side-Band Amplitude Modulation
   TBO  Trajectory-Based Operations
   TDM  Time Division Multiplexing
   TDMA  Time-Division Multiplexing-Access
   VDLM1  VHF Data Link mode 1
   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) via voice and data
   communications systems through all phases of a flight.  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
   completes 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-to-Ground (A2G) and Air-to-Air (A2A)
   communications.  The communication equipment is either ground-based
   working in the High Frequency (HF) or Very High Frequency (VHF)
   frequency band or satellite-based.  All VHF and HF voice
   communications is operated via open broadcast channels without
   authentication, encryption or other protective measures.  The use of
   well-proven communication procedures via broadcast channels helps to
   enhance the safety of communications by taking into account that
   other users may encounter communication problems and may be
   supported, if required.  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 (SSB-AM) and satellite communications for remote
   and oceanic areas.  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 is still in use, and it is



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   likely that this technology will remain in service for many more
   years.  This however results in current operational limitations and
   impediments in deploying new Air-Traffic Management (ATM)
   applications, such as flight-centric operation with Point-to-Point
   communications.

3.2.  Data Communications Today

   Like for voice, data communications into the cockpit is 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 order of kilobits per second.  While the aircraft is on
   ground some additional communications systems are available, like
   Aeronautical Mobile Airport Communication System (AeroMACS; as of now
   not widely used) or public cellular networks, operating in the
   Airport (APT) domain and able to deliver broadband communication
   capability.

   The data communication networks used for the transmission of data
   relating to the safety and regularity of the 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 (TBO) 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 [SCH20191].
   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 (GANP).










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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 the SESAR 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 [GRA2019] 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].

   LDACS standardization within the framework of the ICAO started in
   December 2016.  The ICAO standardization group has produced an
   initial Standards and Recommended Practices (SARPs) document
   [ICA2018].  The SARPs document 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 develops LDACS in the open.

   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 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.

   The IPv6 architecture for the aeronautical telecommunication network
   is called the Future Communications Infrastructure (FCI).  FCI shall
   support quality of service, diversity, and mobility under the
   umbrella of the "multi-link concept".  This work is conducted by 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] [SCH20191].










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5.  Applicability

   LDACS is a multi-application cellular broadband system capable of
   simultaneously providing various kinds of Air Traffic Services
   (including ATS-B3) and Aeronautical Operational Control (AOC)
   communications services from deployed Ground Stations (GS).  The
   LDACS A2G sub-system physical layer and data link layer are optimized
   for data link communications, but the system also supports digital
   air-ground voice communications.

   LDACS supports communication 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 A2G 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 that are not provided in
   contemporarily deployed aeronautical communication systems.

5.1.1.  Priorities

   LDACS is able to manage services priorities, an important feature not
   available in some of the current data link deployments.  Thus, LDACS
   guarantees bandwidth, 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
   ANSPs (Air Navigation Service Providers).  This includes concepts for
   key and trust management, mutual authenticated key exchange
   protocols, key derivation measures, user and control message-in-
   transit confidentiality and authenticity protection, secure logging
   and availability and robustness measures [MAE20181], [MAE20191],
   [MAE20192].






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5.1.3.  High Data Rates

   The user data rate of LDACS is 315 kbit/s to 1428 kbit/s on the
   forward link (Ground-to-Air), and 294 kbit/s to 1390 kbit/s on the
   reverse link (Air-to-Ground), depending on coding and modulation.
   This is 50 times the amount terrestrial digital aeronautical
   communications systems such as VDLM2 provide [SCH20191].

5.2.  Application

   LDACS shall be used by several aeronautical applications ranging from
   enhanced communication protocol stacks (multi-homed mobile IPv6
   networks in the aircraft and potentially ad-hoc networks between
   aircraft) to classical communication applications (sending GBAS
   correction data) and integration with other service domains (using
   the communication signal for navigation).

5.2.1.  Air-to-Ground Multilink

   It is expected that LDACS together with upgraded satellite-based
   communications systems will be deployed within the Future
   Communication Infrastructure (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.

   Technically the FCI multilink concept shall be realized by multi-
   homed mobile IPv6 networks in the aircraft.  The related protocol
   stack is currently under development by ICAO and SESAR.

5.2.2.  Air-to-Air Extension for LDACS

   A potential extension of the multi-link concept is its extension to
   ad-hoc networks between aircraft.










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   Direct Air-to-Air (A2A) communication between aircrafts 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 (digital voice, wake vortex
   warnings, and trajectory negotiation) [BEL2019].  It should also be
   noted that currently deployed analog VHF voice radios support direct
   voice communication between aircraft, making a similar use case for
   digital voice plausible.

   LDACS direct A2A is currently not part of standardization.

5.2.3.  Flight Guidance

   The FCI (and therefore LDACS) shall be used to host flight guidance.
   This is realized using three applications:

   1.  Context Management (CM): The CM application shall manage 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 Communication (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 shall take over
      most of the communication currently performed over VHF voice and
      enable new services that do not lend themselves to voice
      communication (e.g., 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 to
      implement in-flight trajectory management.

   CM, CPDLC, and ADS-C are available on legacy datalinks, but not
   widely deployed and with limited functionality.













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   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 Ground Based
   Augmentation System (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 airborne 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.

5.2.4.  Business Communication of Airlines

   In addition to air traffic services AOC services shall be transmitted
   over LDACS.  AOC is a generic term referring to the business
   communication of airlines.  Regulatory this is considered related to
   the safety and regularity of flight and may therefore be transmitted
   over LDACS.

   AOC communication is considered the main business case for LDACS
   communication service providers since modern aircraft generate
   significant amounts of data (e.g., engine maintenance data).

5.2.5.  LDACS Navigation

   Beyond communication radio signals can always also be used for
   navigation.  LDACS takes this into account.

   For future aeronautical navigation, ICAO recommends the further
   development of Global Navigation Satellite System (GNSS) based
   technologies as primary means for navigation.  However, the drawback
   of GNSS is its inherent single point of failure - the satellite.  Due
   to the large separation between navigational satellites and aircraft,
   the received power of GNSS signals on the ground 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 of such APNT solution consists of integrating the navigation
   functionality into LDACS.  The ground infrastructure for APNT is
   deployed through the implementation of LDACS ground stations and the
   navigation capability comes "for free".





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   LDACS navigation has already been demonstrated in practice in a
   flight measurement campaign [SCH20191].

6.  Requirements to LDACS

   The requirements to LDACS are mostly defined by its application area:
   Communication 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 Aeronautical Telecommunication
   Network (ATN) are therefore expected to remain in service for
   decades.  The VDLM2 data link currently used for digital terrestrial
   internetworking was developed in the 1990es (the use of the OSI
   internetwork stack indicates that as well).  VDLM2 is expected to be
   used at least for several decades.  In this respect aeronautical
   communication (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 Open Systems
   Interconnection (OSI) stack.  The objective of the development effort
   LDACS is part of (FCI) is to replace legacy (OSI) 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 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 (VDLM1/2) do not provide sufficient bandwidth.
   The use of non-aviation specific data links is considered a security
   problem.  Ideally the aeronautical IP internetwork and the Internet
   should be completely separated.






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   The objective of LDACS is to provide a next generation terrestrial
   data link designed to support IP 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 e.g.  VoIP) at the
   gateway connecting the LDACS network to other aeronautical ground
   networks (the totality of them being the ATN).  On the avionics side
   in the aircraft aviation specific solutions are to be expected.

   In addition to the 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 CSP (Communication Service Provider)
   scope.

   The upcoming Figures Figure 1 and Figure 2 summarize the main
   seetings based on volume 1 Table 5-14, and Table 6-13 defined in
   [DO350A].  In a similar vein, requirements to fault management are
   defined in the same tables.


























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+--------------+----------------+---------------------+----------------+
|              | ECP 130        | RCP 240             | RCP 400        |
+--------------+-------+--------+----------+----------+-------+--------+
| Parameter    | ET    | TT_95% | ET       | TT_95%   | ET    | TT_95% |
+--------------+-------+--------+----------+----------+-------+--------+
| Transaction  | 130   | 67     | 240      | 210      | 400   | 350    |
| Time (Sec)   |       |        |          |          |       |        |
+--------------+-------+--------+----------+----------+-------+--------+
| Continuity   | 0.999 | 0.95   | 0.999    | 0.95     | 0.999 | 0.95   |
+--------------+-------+--------+----------+----------+-------+--------+
| Availability | 0.989          | 0.989  (safety)     | 0.989          |
|              |                | 0.9899 (efficiency) |                |
+--------------+----------------+---------------------+----------------+
| Integrity    | 1E-5 per FH    | 1E-5 per FH         | 1E-5 per FH    |
+--------------+----------------+---------------------+----------------+
| RCP Monitoring and Alerting Criteria                                 |
+--------------+-------------------------------------------------------+
| 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.                     |
+--------------+-------------------------------------------------------+


                   Figure 1: Requirements for CPDLC





















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+--------------+----------------+---------------------+----------------+
|              | RSP 160        | RSP 180             | RSP 400        |
+--------------+-------+--------+----------+----------+-------+--------+
| Parameter    | OT    | DT 95% | OT       | DT 95%   | OT    | DT 95% |
+--------------+-------+--------+----------+----------+-------+--------+
| 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  (safety)     | 0.989          |
|              |                | 0.9899 (efficiency) |                |
+--------------+----------------+---------------------+----------------+
| Integrity    | 1E-5 per FH    | 1E-5 per FH         | 1E-5 per FH    |
+--------------+----------------+---------------------+----------------+
| 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.                     |
+--------------+-------------------------------------------------------+


                   Figure 2: Requirements for ADS-C

7.  Characteristics of LDACS

   LDACS will become one of several wireless access networks connecting
   aircraft to the ATN implemented by the FCI and possibly ACARS/FANS
   networks [FAN2019].

   The current LDACS design is focused on the specification of layer 2.

   Achieving stringent the 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.  Input
   from the working group will be appreciated here.







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7.1.  LDACS Sub-Network

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

   User plane interconnection to the ATN is facilitated by the Access
   Router (AR) peering with an Air-to-Ground Router (A2G Router)
   connected to the ATN.  It is up to implementer's choice to keep
   Access Router and Air-Ground Router functions separated, or to merge
   them.

   The internal control plane of an LDACS sub-network is managed by the
   GSC.  An LDACS sub-network is illustrated in Figure 3.



   wireless      user
   link          plane
     A--------------G-------------Access---A2G-----ATN
     S..............S             Router   Router
                    . control      . |
                    . plane        . |
                    .              . |
                    GSC..............|
                    .                |
                    .                |
                    GS---------------+




            Figure 3: LDACS sub-network with two GSs and one AS

7.2.  Topology

   LDACS operating in A2G mode is a cellular point-to-multipoint system.
   The A2G mode 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 A2G communications within
   its cell.  The LDACS GS can simultaneously support multiple bi-
   directional communications to the ASs under its control.  LDACS
   ground stations themselves are connected to a GSC controlling the
   LDACS sub-network.






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   Prior to utilizing the system an AS 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.

   The LDACS wireless link protocol stack defines two layers, the
   physical layer and the data link layer.

7.3.  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 forward link direction (FL; G2A) and
   the reverse link direction (RL; A2G) are separated by frequency
   division duplex.  Forward link and reverse link use a 500 kHz channel
   each.  The ground-station transmits a continuous stream of Orthogonal
   Frequency-Division Multiplexing (OFDM) symbols on the forward link.
   In the reverse link different aircraft are separated in time and
   frequency using a combination of Orthogonal Frequency-Division
   Multiple-Access (OFDMA) and Time-Division Multiple-Access (TDMA).
   Aircraft thus transmit discontinuously on the reverse link with radio
   bursts sent in precisely defined transmission opportunities allocated
   by the ground-station.

7.4.  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 sub-layer.  The medium access
   sub-layer manages the organization of transmission opportunities in
   slots of time and frequency.  The logical link control sub-layer
   provides acknowledged point-to-point logical channels between the
   aircraft and the ground-station using an automatic repeat request
   protocol.  LDACS supports also unacknowledged point-to-point channels
   and G2A broadcast.

7.5.  LDACS Mobility

   LDACS supports layer 2 handovers to different LDACS channels.
   Handovers may be initiated by the aircraft (break-before-make) or by
   the GS (make-before-break).  Make-before-break handovers are only
   supported for ground-stations connected to the same GSC.






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

8.  Reliability and Availability

8.1.  Layer 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 [SCHN2016] and
   respecting the specific nature of the area of application, LDACS was
   designed from the PHY layer up with robustness in mind.

   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 Forward Link (FL;
   G2A) and Reverse Link (RL; A2G).  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 FL physical layer is a continuous OFDM transmission.  LDACS
   RL transmission is based on OFDMA-TDMA bursts, with silence between
   such bursts.  The RL resources (i.e. bursts) are assigned to
   different users (ASs) on demand by the ground station (GS).

   The LDACS physical layer supports adaptive coding and modulation for
   user data.  Control data is always encoded with the most robust
   coding and modulation (QPSK coding rate 1/2).

   LDACS medium access on top of the physical layer uses a static frame
   structure to support deterministic timer management.  As shown in
   figure 3 and 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 sending windows for KEEP ALIVE messages and control and
   data channels in general.

   LDACS medium access is always under the control of the GS of a radio
   cell.  Any medium access for the transmission of user data has to be
   requested with a resource request message stating the requested



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   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.

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

   LDACS has two mechanisms to request resources from the scheduler in
   the GS.

   Resources can either be requested "on demand" with a given priority.
   On the forward link, this is done locally in the GS, on the reverse
   link a dedicated contention-free control channel is used called
   Dedicated Control Channel (DCCH; roughly 83 bit every 60 ms).  A
   resource allocation is always announced in the control channel of the
   forward link (Common Control Channel (CCCH); variably sized).  Due to
   the spacing of the reverse link control channels every 60 ms, a
   medium access delay in the same order of magnitude is to be expected.

   Resources can also be requested "permanently".  The permanent
   resource request mechanism supports requesting recurring resources in
   given time intervals.  A permanent resource request has to be
   canceled by the user (or by the ground-station, which is always in
   control).

   User data transmissions over LDACS are therefore always scheduled by
   the GS, while control data uses statically (i.e. at cell entry)
   allocated recurring resources (DCCH and CCCH).  The current
   specification specifies no scheduling algorithm.  Scheduling of
   reverse link resources is done in physical Protocol Data Units (PDU)
   of 112 bit (or larger if more aggressive coding and modulation is
   used).  Scheduling on the forward link is done Byte- wise since the
   forward link is transmitted continuously by the GS.

   In addition to having full control over resource scheduling, the GS
   can send forced Handover (HO) commands for off-loading or RF channel
   management, e.g. when the signal quality declines and a more suitable
   GS is in the AS reach.  With robust resource management of the
   capacities of the radio channel, reliability and robustness measures
   are therefore also anchored in the LDACS management entity.






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   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
   layer 2.

   It employs selective repeat ARQ with transparent fragmentation and
   reassembly to the resource allocation size to achieve low latency and
   a low 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.
   Additionally, the priority mechanism of LDACS ensures the timely
   delivery of messages with high importance.

8.2.  Beyond Layer 2

   LDACS availability can be increased by appropriately deploying LDACS
   infrastructure: This means proliferating the number of terrestrial
   base 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 400 km) make
   this quite hard.  The deployment of a larger number of small cells is
   certainly possible, suffers, however, also from the scarcity of
   spectrum.  An additional constraint to take into account, 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, too.

   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 SatCom) in addition to legacy VHF voice.

   However, as of now no reliability and availability mechanisms that
   could utilize the multi-link have been specified on Layer 3 and
   above.




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   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 system by ICAO.

9.  Protocol Stack

   The protocol stack of LDACS is implemented in the AS, GS, and GSC: 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 Layer (MAC), (2) Voice Interface (VI),
   (3) Data Link Service (DLS), (4) LDACS Management Entity (LME).  The
   last entity resides within the Sub-Network Layer: Sub-Network
   Protocol (SNP).  The LDACS network is externally connected to voice
   units, radio control units, and the ATN Network Layer.

   Figure 4 shows the protocol stack of LDACS as implemented in the AS
   and GS.

































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            IPv6                   Network Layer
             |
             |
   +------------------+  +----+
   |        SNP       |--|    |   Sub-Network
   |                  |  |    |   Layer
   +------------------+  |    |
             |           | LME|
   +------------------+  |    |
   |        DLS       |  |    |   Logical Link
   |                  |  |    |   Control Layer
   +------------------+  +----+
             |             |
            DCH         DCCH/CCCH
             |          RACH/BCCH
             |             |
   +--------------------------+
   |           MAC            |   Medium Access
   |                          |   Layer
   +--------------------------+
                |
   +--------------------------+
   |           PHY            |   Physical Layer
   +--------------------------+
                |
                |
              ((*))
              FL/RL              radio channels
                                 separated by FDD



                Figure 4: LDACS protocol stack in AS and GS

9.1.  Medium Access Control (MAC) Entity Services

   The MAC time framing service provides the frame structure necessary
   to realize slot-based Time Division Multiplex (TDM) 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.




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   The LDACS framing structure for FL and RL is based on Super-Frames
   (SF) of 240 ms duration.  Each SF corresponds to 2000 OFDM symbols.
   The FL and RL SF boundaries are aligned in time (from the view of the
   GS).

   In the FL, an SF contains a Broadcast Frame of duration 6.72 ms (56
   OFDM symbols) for the Broadcast Control Channel (BCCH), and four
   Multi-Frames (MF), each of duration 58.32 ms (486 OFDM symbols).

   In the RL, each SF starts with a Random Access (RA) slot of length
   6.72 ms with two opportunities for sending reverse link random access
   frames for the Random Access Channel (RACH), followed by four MFs.
   These MFs have the same fixed duration of 58.32 ms as in the FL, but
   a different internal structure

   Figure 5 and Figure 6 illustrates the LDACS frame structure.


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


                   Figure 5: LDACS super-frame structure















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


                 Figure 6: LDACS multi-frame (MF) structure

9.2.  Data Link Service (DLS) Entity 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 data link service, 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 data link service uses the logical channels provided by the MAC:

   1.  A ground-stations announces its existence and access parameters
      in the Broadcast Channel (BC).
   2.  The Random Access Channel (RA) enables AS to request access to an
      LDACS cell.
   3.  In the Forward Link (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 Reverse Link (RL), where
      aircraft-stations need to request resources before sending.  This
      happens via the Dedicated Common Control Channel (DCCH).
   5.  User data itself is communicated in the Data Channel (DCH) on the
      FL and RL.







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9.3.  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.  The creation and selection
   of voice circuits is performed in the LME.  The VI provides only the
   transmission services.

9.4.  LDACS Management Entity (LME) Services

   The mobility management service in the LME provides support for
   registration and de-registration (cell entry and cell exit), scanning
   RF channels of neighboring cells and handover between cells.  In
   addition, it manages the addressing of aircraft/ ASs within cells.
   It is controlled by the network management service in the GSC.

   The resource management service provides link maintenance (power,
   frequency and time adjustments), support for adaptive coding and
   modulation (ACM), and resource allocation.

9.5.  Sub-Network Protocol (SNP) Services

   The data link service 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 communication over
   the LDACS sub-network.  Note that the SNP security service applies
   cryptographic measures as configured by the ground station
   controller.

10.  Security Considerations

10.1.  Reasons for 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 communication method
   for ATC today is still an open analogue voice broadcast within the
   aeronautical VHF band.  Currently, the information security is purely
   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 application and today, the world has changed.  First
   of all civil applications have significant lower spectrum available
   than military applications.  This means several military defense



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   mechanisms such as frequency hopping or pilot symbol scrambling and
   thus a defense-in-depth approach starting at the physical layer is
   impossible for civil systems.  With the rise of cheap Software
   Defined Radios (SDR), the previously existing financial barrier is
   almost gone and open source projects such as GNU radio [GNU2012]
   allow the new type of unsophisticated listeners and possible
   attackers.  Furthermore most CNS technology developed in ICAO relies
   on open standards, thus syntax and semantics of wireless digital
   aeronautical communications can be common knowledge for attackers.
   Finally 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 waveforms will need additional embedded security
   features to fulfill modern information security requirements like
   authentication and integrity.  However, these security features
   require sufficient bandwidth which is beyond the capabilities of a
   VHF narrowband communications system.  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 throughput.

   As digitalization progresses even further with LDACS and automated
   procedures such as 4D-Trajectories allowing semi-automated en-route
   flying of aircraft, LDACS requires stronger cybersecurity measures.

10.2.  Requirements for LDACS

   Overall there are several business goals for cybersecurity to protect
   in future communication infrastructure 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
      flights.
   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.







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

10.3.  Security Objectives for LDACS

   Security considerations for LDACS are defined by the official ICAO
   SARPS [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.


10.4.  Security Functions for LDACS

   These objectives were used to derive several security functions for
   LDACS required to be integrated in the LDACS cybersecurity
   architecture: (1) Identification, (2) Authentication, (3)
   Authorization, (4) Confidentiality, (5) System Integrity, (6) Data
   Integrity, (7) Robustness, (8) Reliability, (9) Availability, and
   (10) Key and Trust Management.  Several works investigated possible
   measures to implement these security functions [BIL2017], [MAE20181],
   [MAE20191].  Having identified security requirements, objectives and
   functions now we must look at the scope of the applicability of these
   functions.









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10.5.  Security Architectural Details for LDACS

   With requirements out of the way, we want to have a look at the scope
   of the LDACS security model.  This includes looking at the entities,
   identification, authentication and authorization of entities,
   integrity, authenticity and confidentiality of data in-transit and
   more.

10.5.1.  Entities in LDACS Security Model

   First of all the question is what entities do we have in a simplified
   LDACS architectural model: Network operators such as the Societe
   Internationale de Telecommunications Aeronautiques (SITA) [SIT2020]
   and ARINC [ARI2020] are providing access to the (1) Ground IPS
   network via an (2) A2G LDACS Router.  This router is attached to a
   closed off LDACS Access Network (3) which connects via further (4)
   Access Routers to the different (5) LDACS Cell Ranges, each
   controlled by a (6) Ground Station Controller (GSC) and spanning a
   local LDACS Access Network connecting to the (7) Ground Stations (GS)
   that serve one LDACS cell.  Via the (8) A2G wireless LDACS data link
   (9) Airborne Stations (AS) the aircraft is connected to the ground
   network and via the (10) airborne voice interface and (11) airborne
   network interface, airborne data can be sent via the AS back to the
   GS and the forwarded back via GSC, LDACS local access network, access
   routers, LDACS access network, A2G LDACS router to the ground IPS
   network.

10.5.2.  Matter of LDACS Entity Identification

   Each entity described in the sections above must be uniquely
   identified within the LDACS network thus we need LDACS specific
   identities for (1) the Aircraft Station (AS), (2) Ground Station
   (GS), (3) Ground Station Controller (GSC) and (4) Network Operator
   (NO).  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 (PIA) program [FAA2020].  It is
   conceivable that the LDACS AS will use a combination of aircraft
   identification, radio component identification such as MAC addresses
   and even operator features identification to create a unique AS LDACS
   identification tag.  Similar to a 4G's eNodeB Serving Network (SN)
   Identification tag, a GS could be identified using a similar field.
   And again similar to 4G's Mobility Management Entities (MME), a GSC
   could be identified using similar identification fields within the
   LDACS network.  The identification of the network operator is again
   similar to 4G (e.g., E-Plus, AT&T, TELUS, ...), in the way that the
   aeronautical network operators are listed (e.g., ARINC [ARI2020] and
   SITA [SIT2020]).




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10.5.3.  Matter of LDACS Entity Authentication and Key Negotiation

   In order to anchor Trust within the system all LDACS entities
   connected to the ground IPS network shall be rooted in an LDACS
   specific chain-of-trust and PKI solution, quite similar to AeroMACS
   approach [CRO2016].  These X.509 certificates [RFC5280] residing at
   the entities and incorporated in the LDACS PKI 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 aircraft with the LDACS radio (AS) approaches an LDACS cell
   and performs a cell entry with the corresponding groundstation (GS).
   Similar to the LTE cell attachment process [TS33.401], where
   authentication happens after basic communication has been enabled
   between AS and GS (step 5a in the UE attachment process [TS33.401]),
   the next step is mutual authentication and key exchange.  Thus in
   step three using the identity based Station-to-Station (STS) protocol
   with Diffie-Hellman Key Exchange [MAE2020], AS and GS establish
   mutual trust by authenticating each other, exchanging key material
   and finally both ending up with derived key material.  A key
   confirmation is mandatory before the communication channel AS-GS can
   be opened for user-data communications.

10.5.4.  Matter of LDACS Message-in-transit Confidentiality, Integrity
         and Authenticity

   The subsequent 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 air gap.
   Furthermore it ensures integrity protection of LDACS control data.









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10.6.  Security Architecture for LDACS

   Summing up all previous paragraphs, a draft of the cybersecurity
   architecture of LDACS can be found in [ICA2018], [MAE20182] and
   updates in [MAE20191], [MAE20192], [MAE2020].  It proposes the use of
   an own LDACS PKI, identity management based on aircraft identities
   and network operator identities (e.g., SITA and ARINC), public key
   certificates incorporated in the PKI based chain-of-trust and stored
   in the entities allowing for mutual authentication and key exchange
   procedures, key derivation mechanisms for perfect forward secrecy and
   user/control plane message-in-transit integrity and confidentiality
   protection.  This secures data traveling over the airgap between
   aircraft and groundstation and also between groundstation and Air
   Navigation Service Provider regardless of the secure or unsecure
   nature of application data.  Of course application data itself must
   be additionally secured to achieve end-to-end security (secure
   dialogue service), however the LDACS datalinks aims to provide an
   additional layer of protection just for this network segment.

11.  Privacy Considerations

   LDACS provides a Quality of Service (QoS), and the generic
   considerations for such mechanisms apply.

12.  IANA Considerations

   This memo includes no request to IANA.

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

14.  Normative References

   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, DOI 10.17487/RFC4301,
              December 2005, <https://www.rfc-editor.org/info/rfc4301>.







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   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, DOI 10.17487/RFC5280, May 2008,
              <https://www.rfc-editor.org/info/rfc5280>.

   [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, <https://www.rfc-editor.org/info/rfc7296>.

   [RFC8446]  Rescorla, E., "The Transport Layer Security (TLS) Protocol
              Version 1.3", RFC 8446, DOI 10.17487/RFC8446, August 2018,
              <https://www.rfc-editor.org/info/rfc8446>.

15.  Informative References

   [SCHN2016] 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.

   [GRA2019]  Graeupl, T., Rihacek, C., and B. Haindl, "LDACS A/G
              Specification", SESAR2020 PJ14-02-01 D3.3.030 , 2019.

   [FAN2019]  Pierattelli, S., Fantappie, P., Tamalet, S., van den
              Einden, B., Rihacek, C., and T. Graeupl, "LDACS Deployment
              Options and Recommendations", SESAR2020 PJ14-02-01
              D3.4.020 , 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.





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   [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 ,
              2011.

   [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.

   [SCH20191] Schnell, M., "DLR Tests Digital Communications
              Technologies Combined with Additional Navigation Functions
              for the First Time", 2019.

   [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.

   [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.

   [TS33.401] Zhang, D., "3GPP System Architecture Evolution (SAE);
              Security architecture", T33.401, 3GPP , 2012.








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   [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.

   [FAA2020]  FAA, "Federal Aviation Administration. ADS-B Privacy.",
              August 2020,
              <https://www.faa.gov/nextgen/equipadsb/privacy/>.

   [GNU2012]  GNU Radio project, "GNU radio", August 2012,
              <http://gnuradio.org>.

   [SIT2020]  SITA, "Societe Internationale de Telecommunications
              Aeronautiques", August 2020, <https://www.sita.aero/>.

   [ARI2020]  ARINC, "Aeronautical Radio Incorporated", August 2020,
              <https://www.aviation-ia.com/>.

   [DO350A]   RTCA SC-214, "Safety and Performance Standard for Baseline
              2 ATS Data Communications (Baseline 2 SPR Standard)", May
              2016, <https://standards.globalspec.com/std/10003192/rtca-
              do-350-volume-1-2>.





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   [RAW-TECHNOS]
              Thubert, P., Cavalcanti, D., Vilajosana, X., Schmitt, C.,
              and J. Farkas, "Reliable and Available Wireless
              Technologies", Work in Progress, Internet-Draft, draft-
              thubert-raw-technologies-05, 18 May 2020,
              <https://tools.ietf.org/html/draft-thubert-raw-
              technologies-05>.

   [RAW-USE-CASES]
              Papadopoulos, G., Thubert, P., Theoleyre, F., and C.
              Bernardos, "RAW use cases", Work in Progress, Internet-
              Draft, draft-bernardos-raw-use-cases-04, 13 July 2020,
              <https://tools.ietf.org/html/draft-bernardos-raw-use-
              cases-04>.

Authors' Addresses

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

   Email: Nils.Maeurer@dlr.de


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

   Email: Thomas.Graeupl@dlr.de


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

   Email: corinna.schmitt@unibw.de









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