L-band Digital Aeronautical Communications System (LDACS)
draft-ietf-raw-ldacs-11
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 | 2022-07-08 | ||
| Replaces | draft-maeurer-raw-ldacs | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
| Formats | |||
| Reviews |
GENART Last Call review
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by Dale Worley
Ready w/issues
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| Additional resources | Mailing list discussion | ||
| Stream | WG state | Submitted to IESG for Publication | |
| Associated WG milestones |
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||
| 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 |
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Has enough positions to pass. |
||
| Responsible AD | John Scudder | ||
| Send notices to | pthubert@cisco.com | ||
| IANA | IANA review state | Version Changed - Review Needed |
draft-ietf-raw-ldacs-11
RAW N. Mäurer, Ed.
Internet-Draft T. Gräupl, Ed.
Intended status: Informational German Aerospace Center (DLR)
Expires: 9 January 2023 C. Schmitt, Ed.
Research Institute CODE, UniBwM
8 July 2022
L-band Digital Aeronautical Communications System (LDACS)
draft-ietf-raw-ldacs-11
Abstract
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. It is part of a
larger shift of flight guidance communication moving to IP-based
communication. High reliability and availability of IP connectivity
over LDACS, as well as security, are therefore essential. The intent
of this document is to introduce LDACS to the IETF community, raise
awareness on related activities inside and outside of the IETF, and
to seek expertise in shaping the shift of aeronautics to IP.
Status of This Memo
This 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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 9 January 2023.
Copyright Notice
Copyright (c) 2022 IETF Trust and the persons identified as the
document authors. All rights reserved.
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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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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Acronyms . . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Motivation and Use Cases . . . . . . . . . . . . . . . . . . 7
3.1. Voice Communications Today . . . . . . . . . . . . . . . 7
3.2. Data Communications Today . . . . . . . . . . . . . . . . 8
4. Provenance and Documents . . . . . . . . . . . . . . . . . . 8
5. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Advances Beyond the State-of-the-Art . . . . . . . . . . 10
5.1.1. Priorities . . . . . . . . . . . . . . . . . . . . . 10
5.1.2. Security . . . . . . . . . . . . . . . . . . . . . . 10
5.1.3. High Data Rates . . . . . . . . . . . . . . . . . . . 10
5.2. Application . . . . . . . . . . . . . . . . . . . . . . . 10
5.2.1. Air/Ground Multilink . . . . . . . . . . . . . . . . 11
5.2.2. Air/Air Extension for LDACS . . . . . . . . . . . . . 11
5.2.3. Flight Guidance . . . . . . . . . . . . . . . . . . . 12
5.2.4. Business Communications of Airlines . . . . . . . . . 12
5.2.5. LDACS-based Navigation . . . . . . . . . . . . . . . 13
6. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 13
7. Characteristics . . . . . . . . . . . . . . . . . . . . . . . 15
7.1. LDACS Access Network . . . . . . . . . . . . . . . . . . 15
7.2. Topology . . . . . . . . . . . . . . . . . . . . . . . . 16
7.3. LDACS Protocol Stack . . . . . . . . . . . . . . . . . . 16
7.3.1. LDACS Physical Layer . . . . . . . . . . . . . . . . 18
7.3.2. LDACS Data Link Layer . . . . . . . . . . . . . . . . 18
7.3.3. LDACS Sub-Network Layer and Protocol Services . . . . 20
7.4. LDACS Mobility . . . . . . . . . . . . . . . . . . . . . 20
7.5. LDACS Management - Interfaces and Protocols . . . . . . . 21
8. Reliability and Availability . . . . . . . . . . . . . . . . 21
8.1. Below Layer 1 . . . . . . . . . . . . . . . . . . . . . . 21
8.2. Layer 1 and 2 . . . . . . . . . . . . . . . . . . . . . . 21
8.3. Beyond Layer 2 . . . . . . . . . . . . . . . . . . . . . 24
9. Security . . . . . . . . . . . . . . . . . . . . . . . . . . 25
9.1. Security in Wireless Digital Aeronautical
Communications . . . . . . . . . . . . . . . . . . . . . 25
9.2. Security in Depth . . . . . . . . . . . . . . . . . . . . 26
9.3. LDACS Security Requirements . . . . . . . . . . . . . . . 26
9.4. LDACS Security Objectives . . . . . . . . . . . . . . . . 27
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9.5. LDACS Security Functions . . . . . . . . . . . . . . . . 28
9.6. LDACS Security Architecture . . . . . . . . . . . . . . . 28
9.6.1. Entities . . . . . . . . . . . . . . . . . . . . . . 28
9.6.2. Entity Identification . . . . . . . . . . . . . . . . 29
9.6.3. Entity Authentication and Key Establishment . . . . . 29
9.6.4. Message-in-transit Confidentiality, Integrity and
Authenticity . . . . . . . . . . . . . . . . . . . . 30
9.7. Considerations on LDACS Security Impact on IPv6 Operational
Security . . . . . . . . . . . . . . . . . . . . . . . . 30
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 30
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 30
12. Normative References . . . . . . . . . . . . . . . . . . . . 31
13. Informative References . . . . . . . . . . . . . . . . . . . 31
Appendix A. Selected Information from DO-350A . . . . . . . . . 38
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 40
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 strives for a
sustainable modernization of the aeronautical communications
infrastructure on the basis of IP.
This modernization is realized in two steps: (1) the transition of
communications datalinks from analogue to digital technologies
and,(2) the introduction of IPv6 based networking protocols [RFC8200]
in aeronautical networks [ICAO2015].
Step (1) is realized via ATM communications transitioning from
analogue VHF voice [KAMA2010] to more spectrum efficient digital data
communication. For terrestrial communications the International
Civil Aviation Organization (ICAO)'s Global Air Navigation Plan
(GANP) 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 Coast of the
US and parts of Asia [ICAO2018].
Technically LDACS enables IPv6 based air- groundTechnically LDACS
enables IPv6-based air-ground communication related to aviation
safety and regularity of flight [ICAO2015]. Passenger communication
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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 andavailability for IP
connectivity over LDACS are essential.
LDACS is standardized in ICAO and European Organization for Civil
Aviation Equipment (EUROCAE).
This document provides a description by matter specialists on the
subject to inform the IETF community about the current transition of
the aviation industry from analogue to digital flight guidance, to
put LDACS in context of related activities at IETF
[I-D.haindl-lisp-gb-atn], and to seek expertise on reliably using
IPv6 over LDACS for step (1). This document does not intend to
standardize LDACS in the scope of IETF.
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 EUROCAE document ED-262 [EURO2019], and the
Aeronautical Radio Incorporated (ARINC) document P858 [ARI2021].
LDACS is subject to these regulations since it provides an "access
network" - link-layer datalink - 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 marketplace.
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 multilink 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]
[I-D.ietf-rtgwg-atn-bgp]. As such, ICAO has actively sought out the
support of IETF to define a mobility solution for step (2), which is
currently the Locator/ID Separation Protocol (LISP).
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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.
LDACS standardization within the framework of ICAO started in
December 2016. The ICAO standardization group has produced the final
Standards and Recommended Practices (SARPS) document as of 2022
[ICAO2022]. 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 2022. As such
LDACS standardization is not actually finished, and therefore this
document is a snapshot of current status. The physical
characteristics of an LDACS installation (form, fit, and function)
will be standardized by EUROCAE. Generally, the group is open to
input from all sources and encourages cooperation between the
aeronautical and the IETF community.
2. Acronyms
The following terms are used in the context of RAW in this document:
A/A Air/Air
A/G Air/Ground
A2G Air-to-Ground
ACARS Aircraft Communications Addressing and Reporting System
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
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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
FAA Federal Aviation Administration
FCI Future Communications Infrastructure
FDD Frequency Division Duplex
FL Forward Link
GANP Global Air Navigation Plan
GBAS Ground Based Augmentation System
GNSS Global Navigation Satellite System
GS Ground-Station
G2A Ground-to-Air
HF High Frequency
ICAO International Civil Aviation Organization
IP Internet Protocol
IPS Internet Protocol Suite
kbit/s kilobit per second
LDACS L-band Digital Aeronautical Communications System
LISP Locator/ID Separation Protocol
LLC Logical Link Control
LME LDACS Management Entity
MAC Medium Access Control
MF Multi Frame
NETCONF NETCONF Network Configuration Protocol
OFDM Orthogonal Frequency-Division Multiplexing
OFDMA Orthogonal Frequency-Division Multiplexing Access
OSI Open Systems Interconnection
PHY Physical Layer
QPSK Quadrature Phase-Shift Keying
RACH Random Access Channel
RL Reverse Link
RTCA Radio Technical Commission for Aeronautics
SARPS Standards and Recommended Practices
SDR Software Defined Radio
SESAR Single European Sky ATM Research
SF Super-Frame
SNMP Simple Network Management Protocol
SNP Sub-Network Protocol
VDLm2 VHF Data Link mode 2
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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 and
the airline's operation centers and service partners. The ARINC
document 633 was developed and first released in 2007 [ARI2019] with
the goal to standardize these messages for interoperability, e.g.,
messages between the airline and fueling or de-icing companies.
Within the airport 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 [RAW-TECHNOS].
3.1. Voice Communications Today
Voice links are used for Air/Ground (A/G) and Air/Air (A/A)
communications. The communications equipment can be installed on
ground or in the aircraft, in which cases the High Frequency (HF) or
VHF frequency band is used. For remote domains voice communications
can also be satellite-based. All VHF and HF voice communications are
operated via open broadcast channels 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
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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 kbit/s. While the aircraft is on the
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]
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.
[BOE2019]
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
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[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. Currently no "IPv6 over LDACS" specification is defined;
however, SESAR2020 has started experimenting with IPv6-based LDACS
testbeds and ICAO plans to seek guidance from IETF to develop IPv6
over LDACS. As of May 2022, LDACS defines 1536 Byte user-data
packets [GRA2020] in which IPv6 traffic shall be encapsulated.
Additionally, Robust Header Compression (ROHC) is considered on LDACS
Sub-Network Protocol (SNP) layer (cf. Section 7.3.3.) [RFC5795].
The IPv6 architecture for the aeronautical telecommunication network
is called the ATN/IPS. Link-layer technologies within the ATN/IPS
encompass LDACS [GRA2020], AeroMACS [KAMA2018] and several SatCOM
candidates and combined with the ATN/IPS, are called the FCI. The
FCI will support quality of service, link diversity, and mobility
under the umbrella of the "multilink concept". The "multilink
concept" describing the idea that depending on link quality,
communication can be switched seamlessly from one datalink technology
to another. This work is led 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] [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
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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.
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 (GNSS correction
data) and integration with other service domains (using the
communications signal for navigation) [MAE20211]. Also, a digital
voice service offering better quality and service than current HF and
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VHF systems is foreseen.
5.2.1. Air/Ground Multilink
It is expected that LDACS, together with upgraded satellite-based
communications systems, will be deployed within the FCI and
constitute one of the main components of the multilink concept within
the FCI.
Both technologies, LDACS and satellite systems, have their specific
benefits and technical capabilities 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 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.
Currently two layers of mobility are foreseen. Local mobility within
the LDACS access network is realized through PMIPv6, global mobility
between "multi-link" access networks (which need not be LDACS) is
implemented on top of LISP [I-D.haindl-lisp-gb-atn]
[I-D.ietf-lisp-rfc6830bis] [I-D.ietf-lisp-rfc6833bis].
5.2.2. Air/Air Extension for LDACS
A potential extension of the multilink concept is its extension to
the integration of ad-hoc networks between aircraft.
Direct A/A communication between aircraft in terms of ad-hoc data
networks 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
plausible.
LDACS A/A is currently not part of the standardization process and
will not be covered within this document. However, it is planned
that LDACS A/A will be rolled out after the initial deployment of
LDACS A/G, then being seamlessly integrated in the existing LDACS
ground-based system.
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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 manually according to the
progress of the flight. The CM application automatically sets up
equivalent sessions.
2. Controller Pilot Data Link Communications (CPDLC): The CPDLC
application provides the air crew with the ability to exchange
data messages similar to text messages with the currently
responsible ATC center. The CPDLC application takes over most of
the communication currently performed over VHF voice and enables
new services that do not lend themselves to voice communication
(i.e., trajectory negotiation).
3. Automatic Dependent Surveillance - Contract (ADS-C): ADS-C
reports the position of the aircraft to the currently active ATC
center. Reporting is bound to "contracts", i.e., pre-defined
events related to the progress of the flight (i.e., the
trajectory). ADS-C and CPDLC are the primary applications used
for implementing in-flight trajectory management.
CM, CPDLC, and ADS-C are available on legacy 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 narrowband 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
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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 such APNT solution is based on exploiting the built-in navigation
capabilities of LDACS operation. That is, the normal operation of
LDACS for ATC and AOC communications would also directly enable the
aircraft to navigate and obtain a reliable timing reference from the
LDACS GSs. Current cell planning for Europe shows 84 LDACS cells to
be sufficient [MOST2018] to cover the continent at sufficient service
level. If more than three Ground Stations (GS) are visible by the
aircraft, via knowing the exact positions of these and having a good
channel estimation (which LDACS does due to numerous works mapping
the L-band channel characteristics [SCHN2018] ) it is possible to
calculate the position of the aircraft via measuring signal
propagation times to each GS. In flight trials in 2019 with one
aircraft (and airborne radio inside it) and just four GS, navigation
feasibility was demonstrated within the footprint of all four GS with
a 95th percentile position-domain error of 171.1m [OSE2019] [BEL2021]
[MAE20211]. As such LDACS can be used independent of GNSS as
navigation alternative with way smaller position-domain errors with
more deployed GS [OSE2019] [BEL2021] [MAE20211].
LDACS navigation has already been demonstrated in practice in two
flight measurement campaigns [SHU2013] [BEL2021] [MAE20211].
6. Requirements
The requirements for LDACS are mostly defined by its application
area: Communications related to safety and regularity of flight.
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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.
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.
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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 unspecified as of
May 2022. While current regulatory documents at ICAO, as well as
this document do not specify the above mechanism, a short overview of
the current state shall throughout each section.
7.1. LDACS Access Network
An LDACS access network contains an Access Router (AR) and several
GS, each of them providing one LDACS radio cell.
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 access network interconnects
the GSs. An LDACS access network is illustrated in Figure 1.
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wireless user
link plane
AS-------------GS---------------AR---A/G-----ATN
.............. | Router
control . |
plane . |
. |
GS---------------|
. |
. |
GS---------------+
Figure 1: LDACS access network with three GSs and one AS. dashes
denotes the user plane and points the control plane
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) are
connected to the controlling GS. The LDACS GS is a centralized
instance that controls LDACS A/G communications within its cell. The
LDACS GS can simultaneously support multiple bidirectional
communications to the ASs under its control. LDACS's GSs themselves
are connected to each other and the 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.
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 fifth entity resides within the sub-network layer: (5)
the Sub-Network Protocol (SNP). The LDACS radio is externally
connected to a voice unit, radio control unit, and via the AC-R to
the ATN network.
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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
sections.
IPv6 Network Layer
|
Airborne Voice |
Interface (AVI)/ | Radio Control Unit (RCU)
Voice Unit (VU) |
| |
| +------------------+ +----+
| | SNP |--| | Sub-Network
| | | | | Layer
| +------------------+ | |
| | | LME|
+-----+ +------------------+ | |
| VI | | DLS | | | LLC Layer
+-----+ +------------------+ +----+
| | |
DCH DCH DCCH/CCCH
| RACH/BCCH
| |
+-------------------------------------+
| MAC | Medium Access
| | Layer
+-------------------------------------+
|
+-------------------------------------+
| PHY | Physical Layer
+-------------------------------------+
|
|
((*))
FL/RL radio channels
separated by FDD
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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 bidirectional links to multiple
aircraft under its control. The FL direction at the G2A connection
and the RL direction at the A2G connection are separated by Frequency
Division Duplex (FDD). FL and RL use a 500 kHz channel each. The GS
transmits a continuous stream of Orthogonal Frequency-Division
Multiplexing Access (OFDM) symbols on the FL. In the RL different
aircraft are separated in time and frequency using Orthogonal
Frequency-Division Multiple Access (OFDMA). Aircraft thus transmit
discontinuously on the RL via short radio bursts sent in precisely
defined transmission opportunities allocated by the GS.
7.3.2. LDACS Data Link Layer
The data-link layer provides the necessary protocols to facilitate
concurrent and reliable data transfer for multiple users. The LDACS
data link layer is organized in two sub-layers: The medium access
sub-layer and the Logical Link Control (LLC) sub-layer. The medium
access sub-layer manages the organization of transmission
opportunities in slots of time and frequency. The LLC sub-layer
provides acknowledged point-to-point logical channels between the
aircraft and the GS using an Automatic Repeat reQuest (ARQ) protocol.
LDACS supports also unacknowledged point-to-point channels and G2A
Broadcast transmission.
7.3.2.1. Medium Access Control (MAC) Services
The MAC time framing service provides the frame structure necessary
to realize slot-based time-division multiplex-access on the physical
link. It provides the functions for the synchronization of the MAC
framing structure and the PHY Layer framing. The MAC time framing
provides a dedicated time slot for each logical channel.
The MAC sub-layer offers access to the physical channel to its
service users. Channel access is provided through transparent
logical channels. The MAC sub-layer maps logical channels onto the
appropriate slots and manages the access to these channels. Logical
channels are used as interface between the MAC and LLC sub-layers.
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7.3.2.2. Data Link Service (DLS) Services
The DLS provides acknowledged and unacknowledged (including broadcast
and packet mode voice) bidirectional exchange of user data. If user
data is transmitted using the acknowledged DLS, the sending DLS
entity will wait for an acknowledgement from the receiver. If no
acknowledgement is received within a specified time frame, the sender
may automatically try to retransmit its data. However, after a
certain number of failed retries, the sender will suspend further
retransmission attempts and inform its client of the failure.
The DLS uses the logical channels provided by the MAC:
1. A GS announces its existence and access parameters in the
Broadcast Channel (BCCH).
2. The Random Access Channel (RACH) enables 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
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.
7.3.2.3. 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.
7.3.2.4. LDACS Management Entity (LME) Services
The mobility management service in the LME provides support for
registration and de-registration (cell entry and cell exit), scanning
RF channels of neighboring cells and handover between cells. In
addition, it manages the addressing of aircraft within cells.
The resource management service provides link maintenance (power,
frequency and time adjustments), support for adaptive coding and
modulation, and resource allocation.
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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 layer of LDACS directly interacts with IPv6 traffic.
Incoming ATN/IPS IPv6 packets are forwarded over LDACS from and to
the aircraft. The final IP addressing structure in an LDACS subnet
still needs to be defined, however the current layout is considered
to consist of the five network segments: Air Core Net, Air Management
Net, Ground Core Net, Ground Management Net, Ground Net. Any
protocols that the ATN/IPS [ICAO2015] defines as mandatory will reach
the aircraft, however listing these here is out of its actual scope.
For more information on the technicalities of the above ATN/IPS
layer, please refer to [ICAO2015] [RTCA2019] [ARI2021].
The DLS provides functions required for the transfer of user plane
data and control plane data over the LDACS access network. The
security service provides functions for secure user data
communication over the LDACS access network. Note that the SNP
security service applies cryptographic measures as configured by the
GS.
7.4. LDACS Mobility
LDACS supports layer 2 handovers to different LDACS cells. Handovers
may be initiated by the aircraft (break-before-make) or by the GS
(make-before-break). Make-before-break handovers are only supported
between GSs connected to each other, usually GS operated by the same
service provider.
When a handover between AS and two interconnected GS takes place, it
can be triggered by AS or GS. Once that is done, new security
information is exchanged between AS, GS1 and GS2, before the "old"
connection is terminated between AS and GS1 and a "new" connection is
set up between AS and GS2. As a last step, accumulated user-data at
GS1 is forwarded to GS2 via a ground connection, before that is sent
via GS2 to the AS. While some information for handover is
transmitted in the LDACS DCH, the information remains in the
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"control-plane" part of LDACS and is exchanged between LMEs in AS,
GS1 and GS2. As such, local mobility takes place entirely within the
LDACS network, utilizing the PMIPv6 protocol [RFC5213]. The use of
PMIPv6 is currently not be mandated by standardization, and become
vendor-specific.
External handovers between non-connected LDACS access networks or
different aeronautical data links are handled by the FCI multi- link
concept.
7.5. LDACS Management - Interfaces and Protocols
LDACS management interfaces and protocols are currently not be
mandated by standardization. The implementations currently available
use SNMP for management and Radius for AAA. Link state (link up,
link down) is reported using the ATN/IPS Aircraft Protocol (AIAP)
mandated by ICAO WG-I for 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.
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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).
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.
OFDM symbol time is 120 microseconds, sampling time 1.6 microseconds
and a guard time of 4.8 microseconds. The structure of a SF is
depicted in Figure 3 along with its structure and timings of each
part. FL and RL boundaries are aligned in time (from the GS
perspective) allowing for deterministic slots for control and 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]. As already mentioned, LDACS data
transmission is split into user-data (DCH) and control (BCCH, CCCH in
FL; RACH, DCCH in RL) as depicted with corresponding timings in
Figure 4.
^
| +--------+------------+------------+------------+------------+
| FL | BCCH | MF | MF | MF | MF |
| | 6.72ms | 58.32ms | 58.32ms | 58.32ms | 58.32ms |
F +--------+------------+------------+------------+------------+
r <----------------- Super-Frame (SF) - 240ms ----------------->
e
q +--------+------------+------------+------------+------------+
u RL | RACH | MF | MF | MF | MF |
e | 6.72ms | 58.32ms | 58.32ms | 58.32ms | 58.32ms |
n +--------+------------+------------+------------+------------+
c <----------------- Super-Frame (SF) - 240ms ----------------->
y
------------------------------ Time ------------------------------->
|
Figure 3: SF structure for LDACS
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^
| +-------------+----------------+-----------------+
| FL | DCH | CCCH | DCH |
| | 25.92ms | 2.16 - 17.28ms | 15.12 - 30.24ms |
F +-------------+----------------+-----------------+
r <----------- Multi-Frame (MF) - 58.32ms --------->
e
q +--------------+---------------------------------+
u RL | DCCH | DCH |
e | 2.8 - 24.4ms | 33.84 - 55.44ms |
n +--------------+---------------------------------+
c <----------- Multi-Frame (MF) - 58.32ms --------->
y
----------------------------- Time -------------------->
|
Figure 4: MF structure for LDACS
LDACS cell entry is conducted with an initial control message
exchange via the RACH and the BCCH.
After cell entry, LDACS medium access is always under the control of
the GS of a radio cell. Any medium access for the transmission of
user data on a DCH has to be requested with a resource request
message stating the requested amount of resources and class of
service. The GS performs resource scheduling on the basis of these
requests and grants resources with resource allocation messages.
Resource request and allocation messages are exchanged over dedicated
contention-free control channels (DCCH and CCCH).
The purpose of quality-of-service in LDACS medium access is to
provide prioritized medium access at the bottleneck (the wireless
link). The requested calls of service is signaled to LDACS through
the Signaling of higher layer quality-of-service requests to LDACS is
implemented on the basis of Differentiated Services-
(DiffServ)classes CS01 (lowest priority) to CS07 (highest priority).
In addition to having full control over resource scheduling, the GS
can send forced handover commands for off-loading or channel
management, e.g., when the signal quality declines and a more
suitable GS is in the AS's reach. With robust resource management of
the capacities of the radio channel, reliability and robustness
measures are 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
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deterministic, missing keep-alive messages can thus be immediately
detected. This information is made available to the multilink
protocols for fault management.
The protocol used to communicate faults is not defined in the LDACS
specification. It is assumed that vendors would use industry
standard protocols like the Simple Network Management Protocol or the
Network Configuration Protocol, 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.
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, there are four aspect that need to be
taken into consideration: (1) scarcity of aeronautical spectrum for
data link communication (in the case of LDACS: tens of MHz in the
L-band), (2) an increase in the amount of ground stations also
increases the individual bandwidth for aircraft in the cell, as fewer
aircraft have to share the spectrum, (3) to cover worldwide
terrestrial ATM via LDACS is also a question of cost and the possible
reuse of spectrum which makes it not always possible to decrease cell
sizes and (4) the Distance Measuring Equipment (DME) is the primary
user of the aeronautical L-band, which means any LDACS deployment has
to take DME frequency planning into account.
While aspect (2) provides a good reason, alongside increasing
redundancy, for smaller cells than the maximum range LDACS was
developed for (200 Nautical Miles (NM)), the other three need to be
respected when doing so. There are preliminary works on LDACS cell
planning, such as [MOST2018], where the authors reach the conclusion
that 84 LDACS cells in Europe would be sufficient to serve European
air traffic for the next 20 years.
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For redundancy reasons, the aeronautical community has decided not to
rely on a single communication system or frequency band. It is
envisioned to have multiple independent data link technologies in the
aircraft (e.g., terrestrial and satellite communications) in addition
to legacy VHF voice.
However, as of now, no reliability and availability mechanisms that
could utilize the multilink architecture, have been specified on
Layer 3 and above. Even if LDACS has been designed for reliability,
the wireless medium presents significant challenges to achieve
deterministic properties such as low packet error rate, bounded
consecutive losses, and bounded latency. Support for high
reliability and availability for IP connectivity over LDACS is
certainly highly desirable but needs to be adapted to the specific
use case.
9. Security
The goal of this Section is to inform the reader about the state of
security in aeronautical communications, state security
considerations applicable for all ATN/IPS traffic and to provide an
overview of the LDACS link-layer security capabilities
9.1. Security in Wireless Digital Aeronautical Communications
Aviation will require secure exchanges of data and voice messages for
managing the air traffic flow safely through the airspaces all over
the world. Historically Communication Navigation Surveillance (CNS)
wireless communications technology emerged from 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
changed.
Civil applications have significant lower spectrum available than
military applications. This means several military defenses
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 (SDR), 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.
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Most CNS technology developed in ICAO relies on open standards, thus
syntax and semantics of wireless digital aeronautical communications
should be expected to be common knowledge for attackers. With
increased digitization and automation of civil aviation, the human as
control instance, is being taken gradually out of the loop.
Autonomous transport drones or single piloted aircraft demonstrate
this trend. However, without profound cybersecurity measures such as
authenticity and integrity checks of messages in-transit on the
wireless link or mutual entity authentication, this lack of a control
instance can prove disastrous. Thus, future digital communications
will need additional embedded security features to fulfill modern
information security requirements like authentication and integrity.
These security features require sufficient bandwidth which is beyond
the capabilities of currently deployed VHF narrowband communications
systems. For voice and data communications, sufficient data
throughput capability is needed to support the security functions
while not degrading performance. LDACS is a data link technology
with sufficient bandwidth to incorporate security without losing too
much user data throughput.
9.2. Security in Depth
ICAO Doc 9896 foresees transport layer security [ICAO2015] for all
aeronautical data transmitted via the ATN/IPS, as described in ARINC
P858 [ARI2021]. This is realized via Datagram Transport Layer
Security (DTLS) 1.3 [RFC9147].
LDACS also needs to comply with in-depth security requirements,
stated in ARINC 858, for the radio access technologies transporting
ATN/IPS data. These requirements imply that LDACS must provide layer
2 security in addition to any higher layer mechanisms. Specifically,
ARINC 858 states that [datalinks within the FCI need to provide] "a
secure channel between the airborne radio systems and the peer radio
access endpoints on the ground [...] to ensure authentication and
integrity of air-ground message exchanges in support of an overall
defense-in-depth security strategy." [ARI2021]
9.3. LDACS Security Requirements
Overall, cybersecurity for CNS technology shall protect the following
business goals [MAE20181]:
1. Safety: The system must sufficiently mitigate attacks, 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
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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]. While all LDACS These results allowed
deriving security scope and objectives from the requirements and the
conducted threat-and risk analysis. Please note, IPv6 security
considerations are briefly discussed in Section 9.7 while a summary
of security requirements for link-layer candidates in the ATN/IPS is
given in [ARI2021], which states: "Since the communication radios
connect to local airborne networks in the aircraft control domain,
[...] the airborne radio systems represent the first point of entry
for an external threat to the aircraft. Consequently, a secure
channel between the airborne radio systems and the peer radio access
endpoints on the ground is necessary to ensure authentication and
integrity of air-ground message exchanges in support of an overall
defense-in-depth security strategy".
9.4. LDACS Security Objectives
Security considerations for LDACS are defined by the official SARPS
document by ICAO [ICAO2022]:
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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Work in 2022 includes a change request for these SARPS aims to limit
the "non-repudiation of origin of messages in transit" requirement
only to the authentication and key establishment messages at the
beginning of every session.
9.5. LDACS Security Functions
These objectives were used to derive several security functions for
LDACS required to be integrated in the LDACS cybersecurity
architecture: Identification, Authentication, Authorization,
Confidentiality, System Integrity, Data Integrity, Robustness,
Reliability, Availability, and Key and Trust Management. Several
works investigated possiblemeasures to implement these security
functions [BIL2017], [MAE20181], [MAE20191].
9.6. 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 [ICAO2022] and [MAE20182] and respective updates in
[MAE20191], [MAE20192], [MAE2020], [MAE2021].
9.6.1. Entities
A simplified LDACS architectural model requires the following
entities: Network operators such as the Societe Internationale de
Telecommunications Aeronautiques (SITA) [SIT2020] and ARINC [ARI2020]
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
[ICAO2015].
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9.6.2. Entity Identification
LDACS needs specific identities for the AS, the GS, and the network
operator. The aircraft itself can be identified using the 24-bit
ICAO identifier of an aircraft [ICAO2022], the call sign of that
aircraft or the recently founded privacy ICAO address of the Federal
Aviation Administration (FAA) program with the same name [FAA2020].
It is conceivable that the LDACS AS will use a combination of
aircraft identification, radio component identification and even
operator feature identification to create a unique 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]).
9.6.3. Entity Authentication and Key Establishment
In order to anchor trust within the system, all LDACS entities
connected to the ground IPS network will be rooted in an LDACS
specific chain-of-trust and PKI solution, quite similar to AeroMACS's
approach [CRO2016]. These certificates, residing at the entities and
incorporated in the LDACS PKI, 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
[MAE2021].
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
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elliptic curves [NIST2013], while post-quantum solutions consider
Falcon [SON2021] [MAE2021] or similar lightweight PQC signature
schemes, and SIKE or SABER as key establishment options [SIK2021]
[ROY2020].
9.6.4. Message-in-transit Confidentiality, Integrity and Authenticity
The key material from the previous step can then be used to protect
LDACS Layer 2 communications via applying encryption and integrity
protection measures on the SNP layer of the LDACS protocol stack. As
LDACS transports AOC and ATS data, the integrity of that data is most
important, while confidentiality only needs to be applied to AOC data
to protect business interests [ICAO2022]. This possibility of
providing low layered confidentiality and integrity protection
ensures a secure delivery of user data over the wireless link.
Furthermore, it ensures integrity protection of LDACS control data.
9.7. Considerations on LDACS Security Impact on IPv6 Operational
Security
In this part, considerations on IPv6 operational security in
[RFC9099] and interrelations with the LDACS security additions are
compared and evaluated to identify further protection demands. As
IPv6 heavily relies on the Neighbor Discovery Protocol (NDP)
[RFC4861], integrity and authenticity protection on the link-layer,
as provided by LDACS, already help mitigate spoofing and redirection
attacks. However, to also mitigate the threat of remote DDoS
attacks, neighbor solicitation rate-limiting is recommended by RFC
9099. To prevent the threat of (D)DoS attacks in general on the
LDACS access network, rate-limiting need to be performed on each
network node in the network access. One approach is to filter for
the total amount of possible LDACS AS-GS traffic per cell - i.e., of
up to 1.4 Mbps user-data per cell and up to the amount of GS per
service provider network times 1.4 Mbps.
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 their comments to this draft.
Thanks to the Chair for Network Security and the research institute
CODE for their comments and improvements.
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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
feedback.
12. Normative References
13. Informative 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>.
[RFC4861] Narten, T., Nordmark, E., Simpson, W., and H. Soliman,
"Neighbor Discovery for IP version 6 (IPv6)", RFC 4861,
DOI 10.17487/RFC4861, September 2007,
<https://www.rfc-editor.org/info/rfc4861>.
[RFC5213] Gundavelli, S., Ed., Leung, K., Devarapalli, V.,
Chowdhury, K., and B. Patil, "Proxy Mobile IPv6",
RFC 5213, DOI 10.17487/RFC5213, August 2008,
<https://www.rfc-editor.org/info/rfc5213>.
[RFC5795] Sandlund, K., Pelletier, G., and L-E. Jonsson, "The RObust
Header Compression (ROHC) Framework", RFC 5795,
DOI 10.17487/RFC5795, March 2010,
<https://www.rfc-editor.org/info/rfc5795>.
[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>.
[RFC8200] Deering, S. and R. Hinden, "Internet Protocol, Version 6
(IPv6) Specification", STD 86, RFC 8200,
DOI 10.17487/RFC8200, July 2017,
<https://www.rfc-editor.org/info/rfc8200>.
[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>.
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[RFC9099] Vyncke, É., Chittimaneni, K., Kaeo, M., and E. Rey,
"Operational Security Considerations for IPv6 Networks",
RFC 9099, DOI 10.17487/RFC9099, August 2021,
<https://www.rfc-editor.org/info/rfc9099>.
[RFC9147] Rescorla, E., Tschofenig, H., and N. Modadugu, "The
Datagram Transport Layer Security (DTLS) Protocol Version
1.3", RFC 9147, DOI 10.17487/RFC9147, April 2022,
<https://www.rfc-editor.org/info/rfc9147>.
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<https://standards.globalspec.com/std/14391274/858p1>.
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<https://eshop.eurocae.net/eurocae-documents-and-reports/
ed-262/>.
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<https://standards.globalspec.com/std/10026940/icao-9896>.
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Battista, G., Kumar, R., Schneckenburger, N., Lay, E.,
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Terrestrial RNP with LDACS", 32nd International Technical
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[SAJ2014] Haindl, B., Meser, J., Sajatovic, M., Müller, S.,
Arthaber, H., Faseth, T., and M. Zaisberger, "LDACS1
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[RIH2018] Rihacek, C., Haindl, B., Fantappie, P., Pierattelli, S.,
Gräupl, T., Schnell, M., and N. Fistas, "L-band Digital
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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,
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[CRO2016] Crowe, B., "Proposed AeroMACS PKI Specification is a Model
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[MAE2020] Mäurer, N., Gräupl, 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., Schäfer, 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. Gräupl,
"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.
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[MAE20181] Mäurer, N. and A. Bilzhause, "Paving the Way for an IT
Security Architecture for LDACS: A Datalink Security
Threat and Risk Analysis", IEEE Integrated Communications,
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York, NY, USA , 2018.
[FAA2020] FAA, "Federal Aviation Administration. ADS-B Privacy.",
August 2020,
<https://www.faa.gov/nextgen/equipadsb/privacy/>.
[GNU2021] GNU Radio project, "GNU radio", October 2021,
<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>.
[ICAO2019] International Civil Aviation Organization (ICAO), "Manual
on VHF Digital Link (VDL) Mode 2, Doc 9776", January 2019,
<https://store.icao.int/en/manual-on-vhf-digital-link-vdl-
mode-2-doc-9776>.
[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.
[KAMA2018] Kamali, B., "AeroMACS: An IEEE 802.16 Standard-based
Technology for the Next Generation of Air Transportation
Systems", John Wiley and Sons, DOI:
10.1002/9781119281139 , September 2018.
[NIST2013] Barker, E., "Digital Signature Standard (DSS)", National
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[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
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[SIK2021] SIKE, "SIKE – Supersingular Isogeny Key Encapsulation",
October 2021, <https://sike.org/>.
[ROY2020] Roy, S.S.. and A. Basso, "High-Speed Instruction-Set
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technologies-05>.
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Theoleyre, "RAW use-cases", Work in Progress, Internet-
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cases-05>.
[I-D.haindl-lisp-gb-atn]
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haindl-lisp-gb-atn-07>.
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[I-D.ietf-lisp-rfc6830bis]
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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 Mäurer (editor)
German Aerospace Center (DLR)
Münchner Strasse 20
82234 Wessling
Germany
Email: Nils.Maeurer@dlr.de
Thomas Gräupl (editor)
German Aerospace Center (DLR)
Münchner Strasse 20
82234 Wessling
Germany
Email: Thomas.Graeupl@dlr.de
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Corinna Schmitt (editor)
Research Institute CODE, UniBwM
Werner-Heisenberg-Weg 39
85577 Neubiberg
Germany
Email: corinna.schmitt@unibw.de
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