L-band Digital Aeronautical Communications System (LDACS)
draft-ietf-raw-ldacs-09
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.
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|
|---|---|---|---|
| Authors | Nils Mäurer , Thomas Gräupl , Corinna Schmitt | ||
| Last updated | 2022-01-20 (Latest revision 2021-10-22) | ||
| Replaces | draft-maeurer-raw-ldacs | ||
| RFC stream | Internet Engineering Task Force (IETF) | ||
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| Reviews |
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| Stream | WG state | Submitted to IESG for Publication | |
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| Document shepherd | Pascal Thubert | ||
| Shepherd write-up | Show Last changed 2021-06-07 | ||
| IESG | IESG state | Became RFC 9372 (Informational) | |
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| Responsible AD | John Scudder | ||
| Send notices to | pthubert@cisco.com |
draft-ietf-raw-ldacs-09
RAW N. Maeurer, Ed.
Internet-Draft T. Graeupl, Ed.
Intended status: Informational German Aerospace Center (DLR)
Expires: 25 April 2022 C. Schmitt, Ed.
Research Institute CODE, UniBwM
22 October 2021
L-band Digital Aeronautical Communications System (LDACS)
draft-ietf-raw-ldacs-09
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. LDACS provides a
data link for IPv6 network-based aircraft guidance. High reliability
and availability for IP connectivity over LDACS, as well as security,
are therefore essential.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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 25 April 2022.
Copyright Notice
Copyright (c) 2021 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
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provided without warranty as described in the Simplified BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
3. Motivation and Use Cases . . . . . . . . . . . . . . . . . . 6
3.1. Voice Communications Today . . . . . . . . . . . . . . . 7
3.2. Data Communications Today . . . . . . . . . . . . . . . . 7
4. Provenance and Documents . . . . . . . . . . . . . . . . . . 8
5. Applicability . . . . . . . . . . . . . . . . . . . . . . . . 9
5.1. Advances Beyond the State-of-the-Art . . . . . . . . . . 9
5.1.1. Priorities . . . . . . . . . . . . . . . . . . . . . 9
5.1.2. Security . . . . . . . . . . . . . . . . . . . . . . 9
5.1.3. High Data Rates . . . . . . . . . . . . . . . . . . . 10
5.2. Application . . . . . . . . . . . . . . . . . . . . . . . 10
5.2.1. Air/Ground Multilink . . . . . . . . . . . . . . . . 10
5.2.2. Air/Air Extension for LDACS . . . . . . . . . . . . . 10
5.2.3. Flight Guidance . . . . . . . . . . . . . . . . . . . 11
5.2.4. Business Communications of Airlines . . . . . . . . . 12
5.2.5. LDACS-based Navigation . . . . . . . . . . . . . . . 12
6. Requirements . . . . . . . . . . . . . . . . . . . . . . . . 12
7. Characteristics . . . . . . . . . . . . . . . . . . . . . . . 14
7.1. LDACS Sub-Network . . . . . . . . . . . . . . . . . . . . 14
7.2. Topology . . . . . . . . . . . . . . . . . . . . . . . . 15
7.3. LDACS Protocol Stack . . . . . . . . . . . . . . . . . . 15
7.3.1. LDACS Physical Layer . . . . . . . . . . . . . . . . 17
7.3.2. LDACS Data Link Layer . . . . . . . . . . . . . . . . 17
7.3.3. LDACS Sub-Network Layer and Protocol Services . . . . 19
7.4. LDACS Mobility . . . . . . . . . . . . . . . . . . . . . 19
8. Reliability and Availability . . . . . . . . . . . . . . . . 19
8.1. Below Layer 1 . . . . . . . . . . . . . . . . . . . . . . 19
8.2. Layer 1 and 2 . . . . . . . . . . . . . . . . . . . . . . 19
8.3. Beyond Layer 2 . . . . . . . . . . . . . . . . . . . . . 23
9. Security . . . . . . . . . . . . . . . . . . . . . . . . . . 23
9.1. Security in Wireless Digital Aeronautical
Communications . . . . . . . . . . . . . . . . . . . . . 24
9.2. LDACS Requirements . . . . . . . . . . . . . . . . . . . 25
9.3. LDACS Security Objectives . . . . . . . . . . . . . . . . 25
9.4. LDACS Security Functions . . . . . . . . . . . . . . . . 26
9.5. LDACS Security Architecture . . . . . . . . . . . . . . . 26
9.5.1. Entities . . . . . . . . . . . . . . . . . . . . . . 26
9.5.2. Entity Identification . . . . . . . . . . . . . . . . 27
9.5.3. Entity Authentication and Key Establishment . . . . . 27
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9.5.4. Message-in-transit Confidentiality, Integrity and
Authenticity . . . . . . . . . . . . . . . . . . . . 28
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 28
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 28
12. Normative References . . . . . . . . . . . . . . . . . . . . 28
13. Informative References . . . . . . . . . . . . . . . . . . . 29
Appendix A. Selected Information from DO-350A . . . . . . . . . 35
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 37
1. Introduction
One of the main pillars of the modern Air Traffic Management (ATM)
system is the existence of a communications infrastructure that
enables efficient aircraft control and safe aircraft separation in
all phases of flight. Current systems are technically mature but
suffering from the Very High Frequency (VHF) band's increasing
saturation in high- density areas and the limitations posed by
analogue radio communications. Therefore, aviation globally, and the
European Union (EU) in particular, strives for a sustainable
modernization of the aeronautical communications infrastructure.
This modernization is realized in two steps: (1) the transition of
communications datalinks from analogue to digital technologies and,
(2) the introduction of IPv6 based networking protocols in
aeronautical networks [RFC4291], [RFC7136], [ICAO2015].
Step (1) is realized via ATM communications transitioning from
analogue VHF voice [KAMA2010] to more spectrum efficient digital data
communication. For terrestrial communications the European ATM
Master Plan foresees this transition to be realized by the
development of the L-band Digital Aeronautical Communications System
(LDACS). Since central Europe has been identified as the area of the
world, that suffers the most from increased saturation of the VHF
band, the initial roll-out of LDACS will likely start there, and
continue to other increasingly saturated zones as the east- and west-
cost of the US and parts of Asia [ICAO2018].
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Technically LDACS enables IPv6 based air- ground communication
related to aviation safety and regularity of flight [ICAO2015].
Passenger communication and similar services are not supported, since
only communications related to "safety and regularity of flight" are
permitted in protected aviation frequency bands. The particular
challenge is that no additional frequencies can be made available for
terrestrial aeronautical communication. It was thus necessary to
develop co-existence mechanism/procedures to enable the interference
free operation of LDACS in parallel with other aeronautical services/
systems in the protected frequency band. Since LDACS will be used
for aircraft guidance, high reliability and availability for IP
connectivity over LDACS are essential.
Step (2) is a strategy for the worldwide roll-out of IPv6 capable
digital aeronautical inter-networking. This is called the
Aeronautical Telecommunications Network (ATN)/Internet Protocol Suite
(IPS) (hence, ATN/IPS). It is specified in the International Civil
Aviation Organization (ICAO) document Doc 9896 [ICAO2015], the Radio
Technical Commission for Aeronautics (RTCA) document DO-379
[RTCA2019], the European Organization for Civil Aviation Equipment
(EUROCAE) document ED-262 [EURO2019], and the Aeronautical Radio
Incorporated (ARINC) document P858 [ARI2021]. LDACS is subject to
these regulations since it provides access subnets to the ATN/IPS.
ICAO has chosen IPv6 as basis for the ATN/IPS mostly for historical
reasons, since a previous architecture based on ISO/OSI protocols,
the ATN/OSI, failed in the market place.
In the context of safety-related communications, LDACS will play a
major role in future ATM. ATN/IPS datalinks will provide diversified
terrestrial and space-based connectivity in a multi-link concept,
called the Future Communications Infrastructure (FCI) [VIR2021].
From a technical point of view the FCI will realize airborne multi-
homed IPv6 networks connected to a global ground network via at least
two independent communication technologies. This is considered in
more detail in related IETF work in progress [I-D.haindl-lisp-gb-atn]
[I-D.ietf-rtgwg-atn-bgp].
In the context of WG-RAW, developing options, such as intelligent
switching between datalinks, for reliably delivering content from and
to endpoints, is foreseen. As LDACS is part of such a concept, the
work of RAW is immediately applicable. In general, with the
aeronautical communications system transitioning to ATN/IPS, and data
being transported via IPv6, closer cooperation and collaboration
between the aeronautical and IETF community is desirable.
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LDACS standardization within the framework of ICAO started in
December 2016. The ICAO standardization group has produced an
initial Standards and Recommended Practices (SARPS) document
[ICA2018]. It defines the general characteristics of LDACS. The
ICAO standardization group plans to produce an ICAO technical manual
- the ICAO equivalent to a technical standard - within the next
years. Generally, the group is open to input from all sources and
encourages cooperation between the aeronautical and the IETF
community.
2. Terminology
The following terms are used in the context of RAW in this document:
A/A Air/Air
A/G Air/Ground
A2G Air-to-Ground
ACARS Aircraft Communications Addressing and Reporting System
ADS-B Automatic Dependent Surveillance - Broadcast
ADS-C Automatic Dependent Surveillance - Contract
AeroMACS Aeronautical Mobile Airport Communications System
ANSP Air Traffic Network Service Provider
AOC Aeronautical Operational Control
AR Access Router
ARINC Aeronautical Radio, Incorporated
ARQ Automatic Repeat reQuest
AS Aircraft Station
ATC Air Traffic Control
ATM Air Traffic Management
ATN Aeronautical Telecommunication Network
ATS Air Traffic Service
BCCH Broadcast Channel
CCCH Common Control Channel
CM Context Management
CNS Communication Navigation Surveillance
COTS Commercial Off-The-Shelf
CPDLC Controller Pilot Data Link Communications
CRL Certificate Revocation List
CSP Communications Service Provider
DCCH Dedicated Control Channel
DCH Data Channel
DiffServ Differentiated Services
DLL Data Link Layer
DLS Data Link Service
DME Distance Measuring Equipment
DSB-AM Double Side-Band Amplitude Modulation
DTLS Datagram Transport Layer Security
EUROCAE European Organization for Civil Aviation Equipment
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FAA Federal Aviation Administration
FCI Future Communications Infrastructure
FDD Frequency Division Duplex
FL Forward Link
GANP Global Air Navigation Plan
GBAS Ground Based Augmentation System
GNSS Global Navigation Satellite System
GS Ground-Station
G2A Ground-to-Air
HF High Frequency
ICAO International Civil Aviation Organization
IP Internet Protocol
IPS Internet Protocol Suite
kbit/s kilobit per second
LDACS L-band Digital Aeronautical Communications System
LLC Logical Link Control
LME LDACS Management Entity
MAC Medium Access Control
MF Multi Frame
OFDM Orthogonal Frequency-Division Multiplexing
OFDMA Orthogonal Frequency-Division Multiplexing Access
OSI Open Systems Interconnection
PHY Physical Layer
QPSK Quadrature Phase-Shift Keying
RACH Random Access Channel
RL Reverse Link
RTCA Radio Technical Commission for Aeronautics
SARPS Standards and Recommended Practices
SDR Software Defined Radio
SESAR Single European Sky ATM Research
SF Super-Frame
SNP Sub-Network Protocol
VDLm2 VHF Data Link mode 2
VHF Very High Frequency
VI Voice Interface
3. Motivation and Use Cases
Aircraft are currently connected to Air Traffic Control (ATC) and
Aeronautical Operational Control (AOC) services via voice and data
communications systems through all phases of flight. ATC refers to
communication for flight guidance. AOC is a generic term referring
to the business communication of airlines. It refers to the mostly
proprietary exchange of data between the aircraft of the airline, its
operation centers, and its service partners. ARINC document 633 was
developed and first released in 2007 [ARI2019] with the goal to
standardize these messages for interoperability, e.g., messages
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between the airline and fueling or de-icing companies. Within the
airport terminal, connectivity is focused on high bandwidth
communications, while during en-route, high reliability, robustness,
and range is the main focus. Voice communications may use the same
or different equipment as data communications systems. In the
following, the main differences between voice and data communications
capabilities are summarized. The assumed use cases for LDACS
complements the list of use cases stated in [RAW-USE-CASES] and the
list of reliable and available wireless technologies presented in
[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 is either ground-based
working in the High Frequency (HF) or VHF frequency band or
satellite-based. All VHF and HF voice communications are operated
via open broadcast channels without authentication, encryption or
other protective measures. The use of well-proven communications
procedures via broadcast channels can help to enhance the safety of
communications. The main voice communications media is still the
analogue VHF Double Side-Band Amplitude Modulation (DSB-AM)
communications technique, supplemented by HF single side-band
amplitude modulation and satellite communications for remote and
oceanic regions. DSB-AM has been in use since 1948, works reliably
and safely, and uses low-cost communication equipment. These are the
main reasons why VHF DSB-AM communications are still in use, and it
is likely that this technology will remain in service for many more
years. This however, results in current operational limitations and
impediments in deploying new ATM applications, such as flight-centric
operation with point-to-point communications between pilots and air
traffic control officers. [BOE2019]
3.2. Data Communications Today
Like for voice, data communications into the cockpit, are currently
provided by ground-based equipment operating either on HF or VHF
radio bands or by legacy satellite systems. All these communication
systems are using narrowband radio channels with a data throughput
capacity in the order of kilobits per second. While the aircraft is
on ground, some additional communications systems are available, like
the Aeronautical Mobile Airport Communications System (AeroMACS) or
public cellular networks, operating in the Airport (APT) domain and
able to deliver broadband communications capability. [BOE2019]
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.
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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
[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].
Up to now LDACS standardization has been focused on the development
of the physical layer and the data link layer. Only recently have
higher layers have come into the focus of the LDACS development
activities. There is currently no "IPv6 over LDACS" specification
publicly available; however, SESAR2020 has started the testing of
IPv6-based LDACS testbeds.
The IPv6 architecture for the aeronautical telecommunication network
is called the FCI. The FCI will support quality of service,
diversity, and mobility under the umbrella of the "multi-link
concept". This work is led by ICAO Communication Panel working group
WG-I.
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In addition to standardization activities several industrial LDACS
prototypes have been built. One set of LDACS prototypes has been
evaluated in flight trials confirming the theoretical results
predicting the system performance [GRA2018] [MAE20211] [BEL2021].
5. Applicability
LDACS is a multi-application cellular broadband system capable of
simultaneously providing various kinds of Air Traffic Services (ATS)
including ATS-B3, and AOC communications services from deployed
Ground-Stations (GS). The physical layer and data link layer of
LDACS are optimized for controller-pilot data link communications,
but the system also supports digital air-ground voice communications.
LDACS supports communications in all airspaces (airport, terminal
maneuvering area, and en-route), and on the airport surface. The
physical LDACS cell coverage is effectively de-coupled from the
operational coverage required for a particular service. This is new
in aeronautical communications. Services requiring wide-area
coverage can be installed at several adjacent LDACS cells. The
handover between the involved LDACS cells is seamless, automatic, and
transparent to the user. Therefore, the LDACS communications concept
enables the aeronautical communication infrastructure to support
future dynamic airspace management concepts.
5.1. Advances Beyond the State-of-the-Art
LDACS offers several capabilities, not yet provided in contemporarily
deployed aeronautical communications systems.
5.1.1. Priorities
LDACS is able to manage service priorities, an important feature not
available in some of the current data link deployments. Thus, LDACS
guarantees bandwidth availability, low latency, and high continuity
of service for safety critical ATS applications while simultaneously
accommodating less safety-critical AOC services.
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].
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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 (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] [GRA2019].
5.2. Application
LDACS will be used by several aeronautical applications ranging from
enhanced communications protocol stacks (multi-homed mobile IPv6
networks in the aircraft and potentially ad-hoc networks between
aircraft) to broadcast communication applications (sending Ground
Based Augmentation System (GBAS) correction data) and integration
with other service domains (using the communications signal for
navigation) [MAE20211].
5.2.1. Air/Ground Multilink
It is expected that LDACS, together with upgraded satellite-based
communications systems, will be deployed within the FCI and
constitute one of the main components of the multilink concept within
the FCI.
Both technologies, LDACS and satellite systems, have their specific
benefits and technical capabilities which complement each other.
Especially, satellite systems are well-suited for large coverage
areas with less dense air traffic, e.g. oceanic regions. LDACS is
well-suited for dense air traffic areas, e.g., continental areas or
hot-spots around airports and terminal airspace. In addition, both
technologies offer comparable data link capacity and, thus, are well-
suited for redundancy, mutual back-up, or load balancing.
Technically the FCI multilink concept will be realized by multi-
homed mobile IPv6 networks in the aircraft. The related protocol
stack is currently under development by ICAO, within SESAR, and the
IETF [I-D.haindl-lisp-gb-atn] [I-D.ietf-rtgwg-atn-bgp].
5.2.2. Air/Air Extension for LDACS
A potential extension of the multi-link concept is its extension to
the integration of ad-hoc networks between aircraft.
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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.
5.2.3. Flight Guidance
The FCI (and therefore LDACS) is used to provide flight guidance.
This is realized using three applications:
1. Context Management (CM): The CM application manages the automatic
logical connection to the ATC center currently responsible to
guide the aircraft. Currently this is done by the air crew
manually changing VHF voice frequencies according to the progress
of the flight. The CM application automatically sets up
equivalent sessions.
2. Controller Pilot Data Link Communications (CPDLC): The CPDLC
application provides the air crew with the ability to exchange
data messages similar to text messages with the currently
responsible ATC center. The CPDLC application takes over most of
the communication currently performed over VHF voice and enables
new services that do not lend themselves to voice communication
(i.e., trajectory negotiation).
3. Automatic Dependent Surveillance - Contract (ADS-C): ADS-C
reports the position of the aircraft to the currently active ATC
center. Reporting is bound to "contracts", i.e., pre-defined
events related to the progress of the flight (i.e., the
trajectory). ADS-C and CPDLC are the primary applications used
for implementing in-flight trajectory management.
CM, CPDLC, and ADS-C are available on legacy 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
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used. VDB is a narrow-band single-purpose datalink without advanced
security only used to transmit GBAS correction data. This makes VDB
a natural candidate for replacement by LDACS [MAE20211].
5.2.4. Business Communications of Airlines
In addition to air traffic services, AOC services are transmitted
over LDACS. AOC is a generic term referring to the business
communication of airlines, between the airlines and service partners
on the ground and their own aircraft in the air. Regulatory-wise,
this is considered related to safety and regularity of flight and may
therefore be transmitted over LDACS. AOC communication is considered
the main business case for LDACS communications service providers
since modern aircraft generate significant amounts of data (i.e.,
engine maintenance data).
5.2.5. LDACS-based Navigation
Beyond communications, radio signals can always also be used for
navigation. This fact is used for the LDACS navigation concept.
For future aeronautical navigation, ICAO recommends the further
development of GNSS based technologies as primary means for
navigation. Due to the large separation between navigational
satellites and aircraft, the power of the GNSS signals received by
the aircraft is, however, very low. As a result, GNSS disruptions
might occasionally occur due to unintentional interference, or
intentional jamming. Yet the navigation services must be available
with sufficient performance for all phases of flight. Therefore,
during GNSS outages, or blockages, an alternative solution is needed.
This is commonly referred to as Alternative Positioning, Navigation,
and Timing (APNT).
One of such APNT solutions consists of exploiting the built-in
navigation capabilities of LDACS operation. That is, the normal
operation of LDACS for ATC and AOC communications would also directly
enable the aircraft to navigate and obtain a reliable timing
reference from the LDACS GSs.
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 and the Internet should be
completely separated.
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 undefined. Input
from the working group will be appreciated here.
7.1. LDACS Sub-Network
An LDACS sub-network contains an Access Router (AR) and several GS,
each of them providing one LDACS radio cell.
User plane interconnection to the ATN is facilitated by the AR
peering with an A/G Router connected to the ATN.
The internal control plane of an LDACS sub-network interconnects the
GSs. An LDACS sub-network is illustrated in Figure 1.
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wireless user
link plane
AS-------------GS---------------AR---A/G-----ATN
. | Router
. control |
. plane |
. |
GS---------------|
. |
. |
GS---------------+
Figure 1: LDACS sub-network with three GSs and one AS
7.2. Topology
LDACS is a cellular point-to-multipoint system. It assumes a star-
topology in each cell where Aircraft Stations (AS) belonging to
aircraft within a certain volume of space (the LDACS cell) is
connected to the controlling GS. The LDACS GS is a centralized
instance that controls LDACS A/G communications within its cell. The
LDACS GS can simultaneously support multiple bi-directional
communications to the ASs under its control. LDACS's GSs themselves
are connected to each other and the AR.
Prior to utilizing the system an aircraft has to register with the
controlling GS to establish dedicated logical channels for user and
control data. Control channels have statically allocated resources,
while user channels have dynamically assigned resources according to
the current demand. Logical channels exist only between the GS and
the AS.
7.3. LDACS Protocol Stack
The protocol stack of LDACS is implemented in the AS and GS: It
consists of the Physical Layer (PHY) with five major, functional
blocks above it. Four are placed in the Data Link Layer (DLL) of the
AS and GS: (1) Medium Access Control (MAC) Layer, (2) Voice Interface
(VI), (3) Data Link Service (DLS), and (4) LDACS Management Entity
(LME). The last entity resides within the sub-network layer: the
Sub-Network Protocol (SNP). The LDACS network is externally
connected to voice units, radio control units, and the ATN network
layer.
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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
|
|
+------------------+ +----+
| SNP |--| | Sub-Network
| | | | Layer
+------------------+ | |
| | LME|
+------------------+ | |
| DLS | | | LLC Layer
+------------------+ +----+
| |
DCH DCCH/CCCH
| RACH/BCCH
| |
+--------------------------+
| MAC | Medium Access
| | Layer
+--------------------------+
|
+--------------------------+
| PHY | Physical Layer
+--------------------------+
|
|
((*))
FL/RL radio channels
separated by FDD
Figure 2: LDACS protocol stack in AS and GS
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7.3.1. LDACS Physical Layer
The physical layer provides the means to transfer data over the radio
channel. The LDACS GS supports bi-directional links to multiple
aircraft under its control. The FL direction at the G2A connection
and the RL direction at the A2G connection are separated by Frequency
Division Duplex (FDD). FL and RL use a 500 kHz channel each. The GS
transmits a continuous stream of Orthogonal Frequency-Division
Multiplexing Access (OFDM) symbols on the FL. In the RL different
aircraft are separated in time and frequency using Orthogonal
Frequency-Division Multiple Access (OFDMA). Aircraft thus transmit
discontinuously on the RL via short radio bursts sent in precisely
defined transmission opportunities allocated by the GS.
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) bi-directional exchange of user data. If user
data is transmitted using the acknowledged DLS, the sending DLS
entity will wait for an acknowledgement from the receiver. If no
acknowledgement is received within a specified time frame, the sender
may automatically try to retransmit its data. However, after a
certain number of failed retries, the sender will suspend further
retransmission attempts and inform its client of the failure.
The DLS uses the logical channels provided by the MAC:
1. A GS announces its existence and access parameters in the
Broadcast Channel (BCCH).
2. The Random Access Channel (RACH) enables AS to request access to
an LDACS cell.
3. In the FL the Common Control Channel (CCCH) is used by the GS to
grant access to data channel resources.
4. The reverse direction is covered by the RL, where ASs need to
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. The creation and selection
of voice circuits is performed.
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 handles the transition from IPv6 packts to LDACS
internal packet structures. This work is ongoing and not part of
this document. The DLS provides functions required for the transfer
of user plane data and control plane data over the LDACS sub-network.
The security service provides functions for secure user data
communication over the LDACS sub-network. Note that the SNP security
service applies cryptographic measures as configured by the GS.
7.4. LDACS Mobility
LDACS supports layer 2 handovers to different LDACS cells. Handovers
may be initiated by the aircraft (break-before-make) or by the GS
(make-before-break). Make-before-break handovers are only supported
between GSs connected to each other.
External handovers between non-connected LDACS sub-networks or
different aeronautical data links are handled by the FCI multi- link
concept.
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).
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Based on channel measurements of the L-band channel LDACS was
designed from the PHY layer up with robustness in mind. Channel
measurements of the L-band channel [SCH2016] confirmed LDACS to be
well adapted to its channel.
In order to maximize the capacity per channel and to optimally use
the available spectrum, LDACS was designed as an OFDM-based FDD
system, supporting simultaneous transmissions in FL in the G2A
connection and RL in the A2G connection. The legacy systems already
deployed in the L-band limit the bandwidth of both channels to
approximately 500 kHz.
The LDACS physical layer design includes propagation guard times
sufficient for the operation at a maximum distance of 200 nautical
miles from the GS. In actual deployment, LDACS can be configured for
any range up to this maximum range.
The LDACS physical layer supports adaptive coding and modulation for
user data. Control data is always encoded with the most robust
coding and modulation (FL: Quadrature Phase-Shift Keying (QPSK),
coding rate 1/2, RL: QPSK, coding rate 1/3).
LDACS medium access layer on top of the physical layer uses a static
frame structure to support deterministic timer management. As shown
in Figure 3 and Figure 4, LDACS framing structure is based on Super-
Frames (SF) of 240ms duration corresponding to 2000 OFDM symbols. FL
and RL boundaries are aligned in time (from the GS perspective)
allowing for deterministic slots for control and data channels. This
initial AS time synchronization and time synchronization maintenance
is based on observing the synchronization symbol pairs that
repetitively occur within the FL stream, being sent by the
controlling GS [GRA2019].
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^
| +------+------------+------------+------------+------------+
| 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 3: SF structure for LDACS
^
| +-------------+------+-------------+
| 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 4: MF structure for LDACS
LDACS cell entry is conducted with an initial control message
exchange via the RACH and the BCCH.
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After cell entry, LDACS medium access is always under the control of
the GS of a radio cell. Any medium access for the transmission of
user data on a DCH has to be requested with a resource request
message stating the requested amount of resources and class of
service. The GS performs resource scheduling on the basis of these
requests and grants resources with resource allocation messages.
Resource request and allocation messages are exchanged over dedicated
contention-free control channels (DCCH and CCCH).
The purpose of quality-of-service in LDACS medium access is to
provide prioritized medium access at the bottleneck (the wireless
link). The signaling of higher layer quality-of-service requirements
to LDACS is yet to be defined. A Differentiated Services- (DiffServ)
based solution with a small number of priorities is to be expected.
In addition to having full control over resource scheduling, the GS
can send forced handover commands for off-loading or channel
management, e.g., when the signal quality declines and a more
suitable GS is in the AS's reach. With robust resource management of
the capacities of the radio channel, reliability and robustness
measures are therefore also anchored in the LME.
In addition to radio resource management, the LDACS control channels
are also used to send keep-alive messages, when they are not
otherwise used. Since the framing of the control channels is
deterministic, missing keep-alive messages can thus be immediately
detected. This information is made available to the multi-link
protocols for fault management.
The protocol used to communicate faults is not defined in the LDACS
specification. It is assumed that vendors would use industry
standard protocols like the Simple Network Management Protocol or the
Network Configuration Protocol, where security permits.
The LDACS data link layer protocol, running on top of the medium
access sub-layer, uses ARQ to provide reliable data transmission on
the data channel.
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.
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8.3. Beyond Layer 2
LDACS availability can be increased by appropriately deploying LDACS
infrastructure: This means proliferating the number of terrestrial
ground stations. However, the scarcity of aeronautical spectrum for
data link communication (in the case of LDACS: tens of MHz in the
L-band) and the long range (in the case of LDACS: up to 200 nautical
miles) make this quite hard. The deployment of a larger number of
small cells is certainly possible, suffers, however, also from the
scarcity of spectrum. An additional constraint to consider, is that
Distance Measuring Equipment (DME) is the primary user of the
aeronautical L-band. That is, any LDACS deployment has to take DME
frequency planning into account.
The aeronautical community has therefore decided not to rely on a
single communication system or frequency band. It is envisioned to
have multiple independent data link technologies in the aircraft
(e.g., terrestrial and satellite communications) in addition to
legacy VHF voice.
However, as of now, no reliability and availability mechanisms that
could utilize the multi-link architecture, have been specified on
Layer 3 and above. Even if LDACS has been designed for reliability,
the wireless medium presents significant challenges to achieve
deterministic properties such as low packet error rate, bounded
consecutive losses, and bounded latency. Support for high
reliability and availability for IP connectivity over LDACS is
therefore, highly desirable, needs, however, to be adapted to the
specific use case.
9. Security
ICAO Doc 9896 foresees transport layer security [ICAO2015] for all
aeronautical data as described in ARINC P858 [ARI2021], most likely
realized via Datagram Transport Layer Security (DTLS) [RFC6012]
[RFC6347].
LDACS also needs to comply with in-depth security requirements,
stated in P858, for the radio access technologies transporting ATN/
IPS data [ARI2021]. These requirements imply that LDACS must provide
layer 2 security in addition to any higher layer mechanisms.
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9.1. Security in Wireless Digital Aeronautical Communications
Aviation will require secure exchanges of data and voice messages for
managing the air traffic flow safely through the airspaces all over
the world. Historically Communication Navigation Surveillance (CNS)
wireless communications technology emerged from military and a threat
landscape where inferior technological and financial capabilities of
adversaries were assumed [STR2016]. The main communications method
for ATC today is still an open analogue voice broadcast within the
aeronautical VHF band. Currently, information security is mainly
procedural, based by using well-trained personnel and proven
communications procedures. This communication method has been in
service since 1948. However, since the emergence of civil
aeronautical CNS applications in the 70s, and today, the world has
changed.
Civil applications have significant lower spectrum available than
military applications. This means several military defense
mechanisms, such as frequency hopping or pilot symbol scrambling and,
thus, a defense-in- depth approach starting at the physical layer, is
infeasible for civil systems. With the rise of cheap Software
Defined Radios (SDRs), the previously existing financial barrier is
almost gone and open source projects such as GNU radio [GNU2021]
allow a new type of unsophisticated listeners and possible attackers.
Most CNS technology developed in ICAO relies on open standards, thus
syntax and semantics of wireless digital aeronautical communications
should be expected to be common knowledge for attackers. With
increased digitization and automation of civil aviation, the human as
control instance, is being taken gradually out of the loop.
Autonomous transport drones or single piloted aircraft demonstrate
this trend. However, without profound cybersecurity measures such as
authenticity and integrity checks of messages in-transit on the
wireless link or mutual entity authentication, this lack of a control
instance can prove disastrous. Thus, future digital communications
waveforms will need additional embedded security features to fulfill
modern information security requirements like authentication and
integrity. These security features require sufficient bandwidth
which is beyond the capabilities of currently deployed VHF narrowband
communications systems. For voice and data communications,
sufficient data throughput capability is needed to support the
security functions while not degrading performance. LDACS is a data
link technology with sufficient bandwidth to incorporate security
without losing too much user data throughput.
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9.2. LDACS Requirements
Overall, there are several business goals for cybersecurity to
protect, within the FCI in civil aviation:
1. Safety: The system must sufficiently mitigate attacks, which
contribute to safety hazards.
2. Flight regularity: The system must sufficiently mitigate attacks,
which contribute to delays, diversions, or cancellations of
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.
To further analyze assets and derive threats and thus protection
scenarios several threat-and risk analyses were performed for LDACS
[MAE20181] , [MAE20191]. These results allowed deriving security
scope and objectives from the requirements and the conducted threat-
and risk analysis.
9.3. LDACS Security Objectives
Security considerations for LDACS are defined by the official SARPS
document by ICAO [ICA2018]:
1. LDACS shall provide a capability to protect the availability and
continuity of the system.
2. LDACS shall provide a capability including cryptographic
mechanisms to protect the integrity of messages in transit.
3. LDACS shall provide a capability to ensure the authenticity of
messages in transit.
4. LDACS should provide a capability for nonrepudiation of origin
for messages in transit.
5. LDACS should provide a capability to protect the confidentiality
of messages in transit.
6. LDACS shall provide an authentication capability.
7. LDACS shall provide a capability to authorize the permitted
actions of users of the system and to deny actions that are not
explicitly authorized.
8. If LDACS provides interfaces to multiple domains, LDACS shall
provide capability to prevent the propagation of intrusions within
LDACS domains and towards external domains.
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Currently, a change request for these SARPS aims to limit the "non-
repudiation of origin of messages in transit" requirement only to the
authentication and key establishment messages at the beginning of
every session.
9.4. LDACS Security Functions
These objectives were used to derive several security functions for
LDACS required to be integrated in the LDACS cybersecurity
architecture: Identification, Authentication, Authorization,
Confidentiality, System Integrity, Data Integrity, Robustness,
Reliability, Availability, and Key and Trust Management. Several
works investigated possible measures to implement these security
functions [BIL2017], [MAE20181], [MAE20191].
9.5. LDACS Security Architecture
The requirements lead to a LDACS security model, including different
entities for identification, authentication and authorization
purposes, ensuring integrity, authenticity and confidentiality of
data. A draft of the cybersecurity architecture of LDACS can be
found in [ICA2018] and [MAE20182] and respective updates in
[MAE20191], [MAE20192], [MAE2020], and most recently [MAE2021].
9.5.1. Entities
A simplified LDACS architectural model requires the following
entities: Network operators such as the Societe Internationale de
Telecommunications Aeronautiques (SITA) [SIT2020] and ARINC [ARI2020]
are providing access to the ground IPS network via an A/G LDACS
router. This router is attached to a closed off LDACS access
network, which connects via further (access routers to the different
LDACS cell ranges, each controlled by a GS (serving one LDACS cell),
with several interconnected GS spanning a local LDACS access network.
Via the A/G wireless LDACS data link AS the aircraft is connected to
the ground network and via the aircraft's VI and aircraft's network
interface, aircraft's data can be sent via the AS back to the GS,
then to the LDACS local access network, access routers, LDACS access
network, A/G LDACS router and finally to the ground IPS network
[ICAO2015].
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9.5.2. Entity Identification
LDACS needs specific identities for the AS, the GS, and the network
operator. The aircraft itself can be identified using the ICAO
unique address of an aircraft, the call sign of that aircraft or the
recently founded privacy ICAO address of the Federal Aviation
Administration (FAA) program with the same name [FAA2020]. It is
conceivable that the LDACS AS will use a combination of aircraft
identification, radio component identification and even operator
feature identification to create a unique AS LDACS identification
tag. Similar to a 4G's eNodeB serving network identification tag, a
GS could be identified using a similar field. The identification of
the network operator is again similar to 4G (e.g., E-Plus, AT&T, and
TELUS), in the way that the aeronautical network operators are listed
(e.g., ARINC [ARI2020] and SITA [SIT2020]).
9.5.3. Entity Authentication and Key Establishment
In order to anchor trust within the system, all LDACS entities
connected to the ground IPS network will be rooted in an LDACS
specific chain-of-trust and PKI solution, quite similar to AeroMACS's
approach [CRO2016]. These certificates, residing at the entities and
incorporated in the LDACS PKI, providing proof the ownership of their
respective public key, include information about the identity of the
owner and the digital signature of the entity that has verified the
certificate's content. First, all ground infrastructures must
mutually authenticate to each other, negotiate and derive keys and,
thus, secure all ground connections. How this process is handled in
detail is still an ongoing discussion. However, established methods
to secure user plane by IPSec [RFC4301] and IKEv2 [RFC7296] or the
application layer via TLS 1.3 [RFC8446] are conceivable. The LDACS
PKI with their chain-of-trust approach, digital certificates and
public entity keys lay the groundwork for this step. In a second
step, the AS with the LDACS radio aboard, approaches an LDACS cell
and performs a cell-attachment procedure with the corresponding GS.
This procedure consists of (1) the basic cell entry [GRA2019] 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
elliptic curves [KOB1987], while post-quantum solutions consider
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Falcon [SON2021] [MAE2021] or similar lightweight PQC signature
schemes, and SIKE or SABER as key establishment options [SIK2021]
[ROY2020].
9.5.4. Message-in-transit Confidentiality, Integrity and Authenticity
The key material from the previous step can then be used to protect
LDACS Layer 2 communications via applying encryption and integrity
protection measures on the SNP layer of the LDACS protocol stack. As
LDACS transports AOC and ATS data, the integrity of that data is most
important, while confidentiality only needs to be applied to AOC data
to protect business interests [ICA2018]. This possibility of
providing low layered confidentiality and integrity protection
ensures a secure delivery of user data over the air gap.
Furthermore, it ensures integrity protection of LDACS control data.
10. IANA Considerations
This memo includes no request to IANA.
11. Acknowledgements
Thanks to all contributors to the development of LDACS and ICAO PT-T.
Thanks to Klaus-Peter Hauf, Bart Van Den Einden, and Pierluigi
Fantappie for further input to this draft.
Thanks to the Chair for Network Security and the research institute
CODE for their comments and improvements.
Thanks to SBA Research Vienna for fruitful discussions on
aeronautical communications concerning security incentives for
industry and potential economic spillovers.
Thanks to the Aeronautical Communications group at the Institute of
Communications and Navigation of the German Aerospace Center (DLR).
With that, the authors would like to explicitly thank Miguel Angel
Bellido-Manganell and Lukas Marcel Schalk for their thorough
feedback.
12. Normative References
[GRA2019] Graeupl, T., Rihacek, C., and B. Haindl, "LDACS A/G
Specification", SESAR2020 PJ14-02-01 D3.3.030 , 2019.
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[ICAO2015] International Civil Aviation Organization (ICAO), "Manual
on the Aeronautical Telecommunication Network (ATN) using
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[RTCA2019] Radio Technical Commission for Aeronautics (RTCA),
"Internet Protocol Suite Profiles, DO-379", September
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[EURO2019] European Organization for Civil Aviation Equipment
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ATN/IPS, ED-262", September 2019,
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[ARI2021] ARINC, "Internet Protocol Suite (IPS) For Aeronautical
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Requirements, ARINC SPECIFICATION 858 P1", June 2021,
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13. Informative References
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[RFC4291] Hinden, R. and S. Deering, "IP Version 6 Addressing
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[RFC4301] Kent, S. and K. Seo, "Security Architecture for the
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[RFC4493] Song, JH., Poovendran, R., Lee, J., and T. Iwata, "The
AES-CMAC Algorithm", RFC 4493, DOI 10.17487/RFC4493, June
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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
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[RFC5869] Krawczyk, H. and P. Eronen, "HMAC-based Extract-and-Expand
Key Derivation Function (HKDF)", RFC 5869,
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[RFC6012] Salowey, J., Petch, T., Gerhards, R., and H. Feng,
"Datagram Transport Layer Security (DTLS) Transport
Mapping for Syslog", RFC 6012, DOI 10.17487/RFC6012,
October 2010, <https://www.rfc-editor.org/info/rfc6012>.
[RFC6347] Rescorla, E. and N. Modadugu, "Datagram Transport Layer
Security Version 1.2", RFC 6347, DOI 10.17487/RFC6347,
January 2012, <https://www.rfc-editor.org/info/rfc6347>.
[RFC7136] Carpenter, B. and S. Jiang, "Significance of IPv6
Interface Identifiers", RFC 7136, DOI 10.17487/RFC7136,
February 2014, <https://www.rfc-editor.org/info/rfc7136>.
[RFC7236] Reschke, J., "Initial Hypertext Transfer Protocol (HTTP)
Authentication Scheme Registrations", RFC 7236,
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<https://www.rfc-editor.org/info/rfc7236>.
[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
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[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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[GRA2018] Graeupl, T., Schneckenburger, N., Jost, T., Schnell, M.,
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[RIH2018] Rihacek, C., Haindl, B., Fantappie, P., Pierattelli, S.,
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[BEL2019] Bellido-Manganell, M. A. and M. Schnell, "Towards Modern
Air-to-Air Communications: the LDACS A2A Mode", IEEE/AIAA
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[TS33.401] Zhang, D., "3GPP System Architecture Evolution (SAE);
Security architecture", T33.401, 3GPP , 2012.
[CRO2016] Crowe, B., "Proposed AeroMACS PKI Specification is a Model
for Global and National Aeronautical PKI Deployments",
WiMAX Forum at 16th Integrated Communications, Navigation
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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
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[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.
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Security Architecture for LDACS: A Datalink Security
Threat and Risk Analysis", IEEE Integrated Communications,
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[FAA2020] FAA, "Federal Aviation Administration. ADS-B Privacy.",
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[GNU2021] GNU Radio project, "GNU radio", October 2021,
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[SIT2020] SITA, "Societe Internationale de Telecommunications
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[ARI2020] ARINC, "Aeronautical Radio Incorporated", August 2020,
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2 ATS Data Communications (Baseline 2 SPR Standard)", May
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do-350-volume-1-2>.
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on VHF Digital Link (VDL) Mode 2, Doc 9776", January 2019,
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mode-2-doc-9776>.
[KAMA2010] Kamali, B., "An Overview of VHF Civil Radio Network and
the Resolution of Spectrum Depletion", Integrated
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[SON2021] Soni, D., Basu, K., Nabeel, M., Aaraj, N., Manzano, M.,
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Coprocessor For Lattice-Based Key Encapsulation Mechanism:
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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-
ietf-raw-technologies-04, 3 August 2021,
<https://datatracker.ietf.org/doc/html/draft-ietf-raw-
technologies-04>.
[RAW-USE-CASES]
Papadopoulos, G. Z., Thubert, P., Theoleyre, F., and C. J.
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[I-D.haindl-lisp-gb-atn]
Haindl, B., Lindner, M., Rahman, R., Comeras, M. P.,
Moreno, V., Maino, F., and B. Venkatachalapathy, "Ground-
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Network", Work in Progress, Internet-Draft, draft-haindl-
lisp-gb-atn-06, 6 March 2021,
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Appendix A. Selected Information from DO-350A
This appendix includes the continuity, availability, and integrity
requirements applicable for LDACS defined in [DO350A].
The following terms are used here:
CPDLC Controller Pilot Data Link 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 | 240 | 210 | 400 | 350 |
| Time (sec) | | | | |
+--------------+----------+--------------+---------+---------+
| Continuity | 0.999 | 0.95 | 0.999 | 0.95 |
+--------------+----------+--------------+---------+---------+
| Availability | 0.989 | 0.989 | 0.989 | 0.989 |
| | (safety) | (efficiency) | | |
+--------------+----------+--------------+---------+---------+
| Integrity | 1E-5 per | 1E-5 per FH | 1E-5 | 1E-5 |
| | 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 180 | RSP 180 | RSP |RSP 400|
| | 160 | 160 | | | 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 |
| | | | (safety) | (efficiency) | | |
+--------------+-----+-----+----------+--------------+------+-------+
| Integrity | 1E-5| 1E-5| 1E-5 per | 1E-5 per FH | 1E-5 | 1E-5 |
| | per | per | FH | |per FH| per FH|
| | FH | FH | | | | |
+--------------+-----+-----+----------+--------------+------+-------+
Table 3: ADS-C Requirements
RCP Monitoring and Alerting Criteria:
- MA-1: The system shall be capable of detecting failures and
configuration changes that would cause the ADS-C service no longer
meet the RSP specification for the intended function.
- MA-2: When the ADS-C service can no longer meet the RSP
specification for the intended function, the flight crew and/or
the controller shall take appropriate action.
Authors' Addresses
Nils Maeurer (editor)
German Aerospace Center (DLR)
Muenchner Strasse 20
82234 Wessling
Germany
Email: Nils.Maeurer@dlr.de
Thomas Graeupl (editor)
German Aerospace Center (DLR)
Muenchner Strasse 20
82234 Wessling
Germany
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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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