Efficient Air-Ground Communications
draft-moskowitz-drip-efficient-a2g-comm-00
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| Authors | Robert Moskowitz , Stuart W. Card , Andrei Gurtov | ||
| Last updated | 2023-04-04 | ||
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draft-moskowitz-drip-efficient-a2g-comm-00
DRIP R. Moskowitz
Internet-Draft HTT Consulting
Intended status: Standards Track S. Card
Expires: 6 October 2023 AX Enterprize
A. Gurtov
Linköping University
4 April 2023
Efficient Air-Ground Communications
draft-moskowitz-drip-efficient-a2g-comm-00
Abstract
This document defines protocols to provide efficient air-ground
communications without associated need for aircraft to maintain
stateful connection to ground-tower infrastructure. Instead, a
secure source-routed ground infrastructure will not only provide the
needed routing intelligence, but also reliable packet delivery
through inclusion of Automatic Repeat reQuest (ARQ) and Forward Error
Correction (FEC) to address both reliable wireless packet delivery,
and assured terrestrial packet delivery.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on 6 October 2023.
Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Terms and Definitions . . . . . . . . . . . . . . . . . . . . 3
2.1. Requirements Terminology . . . . . . . . . . . . . . . . 3
2.2. Definitions . . . . . . . . . . . . . . . . . . . . . . . 3
3. Enabling and Enhancing Functions . . . . . . . . . . . . . . 3
3.1. Enabling Requirements . . . . . . . . . . . . . . . . . . 3
3.2. Enhancing Security Requirement . . . . . . . . . . . . . 4
3.3. Enhancing Performance Requirements . . . . . . . . . . . 4
4. Background Discussion . . . . . . . . . . . . . . . . . . . . 5
4.1. The problem and simple solution using IPnIP . . . . . . . 5
4.2. Improved tower trust through digital signing . . . . . . 6
4.3. Inclusion of mobile ground systems . . . . . . . . . . . 7
4.4. Improved uplink reliability . . . . . . . . . . . . . . . 7
4.5. Alternative dedicated Tower-GS tunneling . . . . . . . . 8
5. Aircraft to GS Messaging . . . . . . . . . . . . . . . . . . 8
5.1. The Tower to GS tunnel . . . . . . . . . . . . . . . . . 10
6. GS to Aircraft Messaging . . . . . . . . . . . . . . . . . . 11
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 12
8. Security Considerations . . . . . . . . . . . . . . . . . . . 12
9. References . . . . . . . . . . . . . . . . . . . . . . . . . 12
9.1. Normative References . . . . . . . . . . . . . . . . . . 12
9.2. Informative References . . . . . . . . . . . . . . . . . 12
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 13
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 14
1. Introduction
The goal of this design approach is to place minimal network
intelligence in the aircraft and even the wireless towers.
Practically all the networking intelligence is placed within the
Ground Station (GS). The justification for this approach is
intelligence in the aircraft has disproportional costs to that in the
GS; there are many factors in this claim. Lower intelligence
requirements in the towers will make the technology more attractive
to tower owners, provided there is an associated functional payment
mechanism for them for the service.
The wireless downlink from the aircraft is treated as a broadcast
message, with every receiving tower forwarding messages to the GS.
The GS, in turns, notes which towers are in contact with the aircraft
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and sends uplink messages through them to the aircraft. There is no
need for complex aircraft/tower connection technologies. At most,
for billing purposes, the towers are aware of aircraft and GS that
will use their connectivity services via their source IP addresses.
2. Terms and Definitions
2.1. Requirements Terminology
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
2.2. Definitions
A2G
Communications from an aircraft to some ground equipment. Also
somethings GS.
3. Enabling and Enhancing Functions
The following is a list of enabling and enhancing functions.
3.1. Enabling Requirements
The aircraft:
* Support end-to-end secure communications with the GS and start the
operation with the pre-configured GS IP address. The aircraft
sends the first message, to the GS, to establish the routing
knowledge in the GS,
* Use a fixed IP address for itself for the duration of the
operation, and
* be able to process multiple copies of messages from the GS,
received potentially from multiple towers.
The tower:
* Support digital signing of messages from the aircraft, and the
tower's IP address, and forward these objects to the GS.
The GS:
* Support end-to-end secure communications with the aircraft,
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* support processing multiple copies of messages from the aircraft,
* support digital signing by the tower, and
* maintain a list/map of towers forwarding aircraft messages from
the aircraft for messaging to the aircraft. The list of trusted
tower IP addresses is constructed from within the tower signed
objects.
3.2. Enhancing Security Requirement
The GS should:
* Support digital signing for the tower to trust messages from the
GS.
3.3. Enhancing Performance Requirements
The aircraft may:
* Support uplink usage optimizations like FEC and ARQ, and
* support GS IP address mobility (e.g. via HIP, [RFC7401]).
The tower may:
* Include information like timestamp and its GPS-derived location
(and accuracy of same) in the signed object delivered to the GS,
* may be IP address mobile, if so, then MUST provide its IP address
within the signed object,
* support multicast and DETNET (rfc8938) for efficient and reliable
communications with the GS, and
* use a subscription model to filter messages supported for
forwarding. If done with a list of registered IP addresses it
MUST support GS IP address mobility.
The GS may:
* Support intelligent operation routing and tower contact
information to select towers to use to send messages to the
aircraft,
* support tower subscription for tower communication filtering,
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* support multicast and DETNET for efficient communications with the
towers,and
* support FEC and ARQ for efficient use of uplinks to the aircraft.
4. Background Discussion
The following considers the possible technologies, some challenges,
and final proposed solution.
4.1. The problem and simple solution using IPnIP
Internet Protocol (IP) transmissions from the aircraft to ground,
though unicast in construction (i.e. IP destination/source paired),
are really broadcasts, as a practical matter, to all available ground
towers. Such towers can simply send the packets on their way and
they will naturally get routed, i.e. relayed, to the GS which
correspondingly simply recognizes and processes potential multiple
receipts via the many relaying towers. The problem is only the
uplink: how to get IP transmissions from the GS to the aircraft.
The GS needs to ‘know’ which towers can likely transmit up to the
aircraft and how to route packets through them. A simple solution is
to use IP-in-IP (IPnIP) tunneling protocol [RFC1853]. Here, each
receiving tower wraps the downlink message in IPnIP with the outer
source address being the the tower’s address. The aircraft always
uses a fixed source address (e.g. their respective DET [RFC9374]).
The GS maintains an IPnIP tunneling table for each aircraft DET of
the tower addresses. Packets inbound to the GS update this table
(stale entries are purged) and the IPnIP service unwraps and forwards
the inner content back through the IP kernel for sending up to the
application. Packets outbound to the aircraft address get routed
internally to the IPnIP process which ends up sending out multiple
copies to each of the tower addresses in the table. Each receiving
tower then simply unwarps and uplinks the content to the aircraft.
Though this approach works, it has security and traffic management
challenges. First and foremost, the aircraft must know the GS IP
address. It either needs to be fixed or the aircraft needs a
separate process to update its knowledge of the GS address. The GS
should have the aircraft address prior to operation start or can
simply learn them through received messages.
There are two security issues associated with the GS processing
messages from any random aircraft address:
* either these addresses are preset (e.g. registered DET), or there
is some process for the GS to dynamically learn which to trust.
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* a larger security challenge is why should the GS trust the address
for the towers as a route to the aircraft. A malicious source
could provide bad tower addresses resulting in loss of aircraft
contact at worst, or consumption, through DOS attacks, of both GS
processing resources and tower uplink bandwidth.
An additional challenge for the GS is determining which set of towers
to use to send messages uplinked to the aircraft. Which of the
towers last sending messages from the aircraft are still in RF reach
of the aircraft and are there now towers better able to message the
aircraft? If the GS can trust the towers and know their GPS location
and the signal strengths of messages from the aircraft, the GS can
use this map along with the map of the planned operation to better
select towers for uplinking messages.
With all these stated concerns, the IPnIP approach should only be
used for PoC and general testing. It presents too good of a DOS
attack scenario for production deployments.
4.2. Improved tower trust through digital signing
Trust in tower messaging can be achieved by each tower
cryptographically signing the received aircraft messages before
forwarding them to the GS. This must be a specific signed object,
perhaps in COSE format [RFC8152]. Not only would it contain the
aircraft message with the tower’s digital ID and signature, it should
also minimally include a timestamp, the tower’s GPS location plus GPS
accuracy, and signal strength. With this information, a process on
the GS can put the tower on a map with the planned aircraft flight
plan. If three or more towers forwarded the message, the GS can also
multilaterate the aircraft location for accurate location on the map.
This information would allow the GS to predict which towers are still
in range, or soon in range (i.e. predicting new towers for
communications) of the aircraft for uplink messaging.
This signing method is preferable to secure tunnels from the tower to
the GS as there will be thousands of GS using a small number of
towers. How should tunnels be set up and torn down recognizing the
cost to the tower system? It is preferable for the towers to be
stateless in their forwarding to the GS. Also, it is questionable
whether the GS should sign messages for the uplink. Doing so would
potentially place the burden of processing cost on the tower, and
analysis would be needed to avoid denial of service (DOS) attacks
against towers and their uplink capacity.
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The inclusion of GPS accuracy supports improved mobile tower
multilateration. The timestamp also enables multilateration, in
aging towers out of the reachable list of towers against the flight
plan.
Thus, inbound processing on the GS would first place tower
information into the aircraft reachable table mentioned above, then
forward the aircraft message up to the application. For outbound,
the aircraft address could result in passing the message to an IPnIP
service for simple forwarding to the tower as above, but as mentioned
above IPnIP has DOS risks.
4.3. Inclusion of mobile ground systems
If the air-ground communications are secured with the Host Identity
Protocol (HIP, [RFC7401]), the HIP mobility function can update the
aircraft with any changes in the GS IP address. DTLS 1.3 [RFC9147]
can be used only if the aircraft is the server as these support
client, not server, mobility and the aircraft can learn of new GS
addressing as it processes uplinked messages from the new addresses.
In any case, the aircraft address should be its DET and be unchanging
for the flight duration.
4.4. Improved uplink reliability
If three or more towers provide the uplink, the GS can use Forward
Error Correction (FEC) and send the fragments to different towers.
The aircraft need only receive the proper set of fragments to
reconstruct the full message. This both reduces the packet size on
the uplink, conserving uplink capacity and increases both ground and
wireless delivery reliability. Static Context Header Compression
(SCHC, [RFC8724]) should also be used to reduce the size of the
aircraft-ground messages. SCHC Automatic Repeat reQuest (ARQ) may
also be used and will soon directly support FEC.
The ground communications path reliability can be further improved
through use of a subset of Deterministic Networking (DETNET) (tbd)
and Bit Indexing Explicit Replication (BIER) multicasting from the GS
to the towers.
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4.5. Alternative dedicated Tower-GS tunneling
There will be areas where significant traffic exists between a tower
(or group of towers) and a GS. An example of such an area is around
an aerodrome and its supporting systems. Here it makes performance
sense that a secure tunneling technology (e.g ESP, [RFC4303]) be used
between the tower(s) and GS(s) rather than digitally signing
individual messages. Often, in such cases the ground network can be
deployed to ensure reliable delivery.
5. Aircraft to GS Messaging
The aircraft and GS MAY have a pre-configured secure connection using
technologies like DTLS, IPsec, or HIP. The aircraft SHOULD use its
DET as its IPv6 address, and underlying HI for the rawPublicKey to
establish the connection. Examples of this type of secure aircraft
to GS is discussed in [drip-secure-nrid-c2].
There is a bit of chicken-and-egg here if the initial connection
setup is not over a single link, as DETs are not easy to route over
an IPv6 network. In such a case a tunnel, as discussed later, needs
to be in place between the first hop from the aircraft (e.g. WiFi
Access Point) and the GS.
In some instances, a pre-established aircraft-GS session is not
practical (e.g. aircraft to airport traffic control). A variant of
Section 3.2 of [drip-a2x-adhoc-session] (Compressed UA Signed
Evidence of the A2X message) can be sent to the pre-configured GS
IPv6 address:
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+=======+=================+=======================================+
| Bytes | Name | Explanation |
+=======+=================+=======================================+
| 16 | DET of Aircraft | DRIP Entity Tag of Aircraft |
+-------+-----------------+---------------------------------------+
| 16 | Destination | IPv6 address of GS |
| | Address | |
+-------+-----------------+---------------------------------------+
| 4 | VNA Timestamp | Timestamp denoting recommended time |
| | | to trust Evidence |
+-------+-----------------+---------------------------------------+
| 1 | Message ID | A2G Message ID Number |
+-------+-----------------+---------------------------------------+
| n | A2G Message | Actual A2G Message |
+-------+-----------------+---------------------------------------+
| 64 | Signature by | Signature over preceding fields using |
| | Aircraft | the keypair of the Aircraft DET |
+-------+-----------------+---------------------------------------+
Table 1: 101+n Byte Aircraft Signed A2G message
This message is a SCHC compressed IPv6/UDP datagram. The signature
is on the whole datagram. The wireless transport will have some
mechanism (e.g. SCHC as Ethertype) to trigger the SCHC rule
processing to compress the datagram for transmission. Depending on
the wireless technology there will be a 1-byte SCHC RuleID after the
SCHC Ethertype (or equivalent). If the IP Header is sent without
SCHC compression, then SCHC will need to be the Next Header in the
IPv6 Header and the SCHC RuleID will immediately follow the IPv6
Header.
The full uncompressed message is:
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+=======+===============+=======================================+
| Bytes | Name | Explanation |
+=======+===============+=======================================+
| 40 | IPv6 Header | IPv6 Header from Aircraft to GS |
+-------+---------------+---------------------------------------+
| 8 | UDP Header | Full UDP Header |
+-------+---------------+---------------------------------------+
| 4 | VNA Timestamp | Timestamp denoting recommended time |
| | | to trust Evidence |
+-------+---------------+---------------------------------------+
| 1 | Message ID | A2G Message ID Number |
+-------+---------------+---------------------------------------+
| n | A2G Message | Actual A2G Message |
+-------+---------------+---------------------------------------+
| 64 | Signature by | Signature over preceding fields using |
| | Aircraft | the keypair of the Aircraft DET |
+-------+---------------+---------------------------------------+
Table 2: IPv6 117+m+n Byte Aircraft Signed A2G message
Any tower that receives these messages and has a tunnel to the
destination IPv6 address uses it to forward the message to the GS.
The GS will use the aircraft DET to retrieve, via DNS, the HDA
Endorsement of the DET. This will provide the aircraft HI to
validate the signature.
A tower MAY validate the signature by using the aircraft DET to
retrieve via DNS the HDA Endorsement of the aircraft DET. The tower
may choose to leave this validation to the GS as it is terrestrial
network that may be DOSed from wireless transmissions.
5.1. The Tower to GS tunnel
It is impractical for most towers to maintain long-lived static
tunnels as described in Section 4.5. Too many towers will need to
forward messages to too many GS for static tunneling. Rather, per-
packet tunneling will be frequently used. These tunnels are the
Aircraft-GS packets wrapped in a signed IPv6 datagram from the
tower's IPv6 address to the GS's address that is in the A-GS packet:
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+=======+===============+====================================+
| Bytes | Name | Explanation |
+=======+===============+====================================+
| 40 | IPv6 Header | IPv6 Header from Tower to GS |
+-------+---------------+------------------------------------+
| 8 | UDP Header | Full UDP Header |
+-------+---------------+------------------------------------+
| 16 | DET of Tower | DRIP Entity Tag of Tower |
+-------+---------------+------------------------------------+
| 4 | VNA Timestamp | Timestamp from tower denoting |
| | | recommended time to trust Evidence |
+-------+---------------+------------------------------------+
| m | Tower | Optional tower location |
| | Location | |
+-------+---------------+------------------------------------+
| m | A2G Message | Full A2G Message |
+-------+---------------+------------------------------------+
| 64 | Signature by | Signature over preceding fields |
| | Tower | using the keypair of the Tower DET |
+-------+---------------+------------------------------------+
Table 3: IPv6 117+n Byte Aircraft Signed tunnel message
The GS will use the tower DET to retrieve, via DNS, the HDA
Endorsement of the tower. This will provide the tower HI to validate
the signature.
The UDP Destination Port can be the indicator of the presence of the
Tower Location information. If absent, this information needs to be
accessible via DNS using the Tower's DET (or pre-configured in the
GS). If the tower is physically mobile, this information SHOULD be
included.
The GS MUST be able to handle multiple copies of the A2G message. It
MUST use the Tower location information to maintain a mapping for
routing messages to the aircraft. It MAY use knowledge of the
aircraft's planned flight to adjust this routing information as to
which tower's are likely to be within reach of the aircraft.
6. GS to Aircraft Messaging
In most cases, the GS to aircraft messaging is the mirror of aircraft
to GS. The important difference is how the GS selects towers for
forwarding G2A messages and how the towers pre-process these messages
before using precious wireless bandwidth in sending messages.
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The GS uses some process to select towers from the list of towers
last forwarding aircraft messages to the GS plus knowledge of the
aircraft flight and other towers in the area.
The GS to tower tunnel is the mirror of Section 5.1 without the
location information. The tower SHOULD validate the authenticity of
the GS via DNS retrieved HDA Endorsement of the GS DET. It MAY also
filter messages based on having recently received aircraft to GS
messages.
The tower takes the G2A message from within the tunnel, adding any
needed wireless heading and transmits the datagram.
The aircraft MUST be able to process multiple copies of an G2A
message coming from multiple towers.
7. IANA Considerations
TBD
8. Security Considerations
TBD
9. References
9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
[RFC9374] Moskowitz, R., Card, S., Wiethuechter, A., and A. Gurtov,
"DRIP Entity Tag (DET) for Unmanned Aircraft System Remote
ID (UAS RID)", RFC 9374, DOI 10.17487/RFC9374, March 2023,
<https://www.rfc-editor.org/info/rfc9374>.
9.2. Informative References
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[drip-a2x-adhoc-session]
Moskowitz, R., Card, S. W., and A. Gurtov, "Aircraft to
Anything AdHoc Broadcasts and Session", Work in Progress,
Internet-Draft, draft-moskowitz-drip-a2x-adhoc-session-01,
4 April 2023, <https://datatracker.ietf.org/doc/html/
draft-moskowitz-drip-a2x-adhoc-session-01>.
[drip-secure-nrid-c2]
Moskowitz, R., Card, S. W., Wiethuechter, A., and A.
Gurtov, "Secure UAS Network RID and C2 Transport", Work in
Progress, Internet-Draft, draft-moskowitz-drip-secure-
nrid-c2-12, 26 March 2023,
<https://datatracker.ietf.org/doc/html/draft-moskowitz-
drip-secure-nrid-c2-12>.
[RFC1853] Simpson, W., "IP in IP Tunneling", RFC 1853,
DOI 10.17487/RFC1853, October 1995,
<https://www.rfc-editor.org/info/rfc1853>.
[RFC4303] Kent, S., "IP Encapsulating Security Payload (ESP)",
RFC 4303, DOI 10.17487/RFC4303, December 2005,
<https://www.rfc-editor.org/info/rfc4303>.
[RFC7401] Moskowitz, R., Ed., Heer, T., Jokela, P., and T.
Henderson, "Host Identity Protocol Version 2 (HIPv2)",
RFC 7401, DOI 10.17487/RFC7401, April 2015,
<https://www.rfc-editor.org/info/rfc7401>.
[RFC8152] Schaad, J., "CBOR Object Signing and Encryption (COSE)",
RFC 8152, DOI 10.17487/RFC8152, July 2017,
<https://www.rfc-editor.org/info/rfc8152>.
[RFC8724] Minaburo, A., Toutain, L., Gomez, C., Barthel, D., and JC.
Zuniga, "SCHC: Generic Framework for Static Context Header
Compression and Fragmentation", RFC 8724,
DOI 10.17487/RFC8724, April 2020,
<https://www.rfc-editor.org/info/rfc8724>.
[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>.
Acknowledgments
Adam Wiethuechter of AX Enterprize provided review and implementation
insights. Michael Baum provided extensive review of the contents in
chapters 3 and 4 in a prior white paper.
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Authors' Addresses
Robert Moskowitz
HTT Consulting
Oak Park, MI 48237
United States of America
Email: rgm@labs.htt-consult.com
Stuart W. Card
AX Enterprize
4947 Commercial Drive
Yorkville, NY 13495
United States of America
Email: stu.card@axenterprize.com
Andrei Gurtov
Linköping University
IDA
SE-58183 Linköping
Sweden
Email: gurtov@acm.org
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