The International Civil Aviation Organization (ICAO) is investigating
mobile routing solutions for a worldwide Aeronautical Telecommunications
Network with Internet Protocol Services (ATN/IPS). The ATN/IPS will
eventually replace existing communication services with an IP-based
service supporting pervasive Air Traffic Management (ATM) for Air
Traffic Controllers (ATC), Airline Operations Controllers (AOC), and all
commercial aircraft worldwide. This informational document describes a
simple and extensible mobile routing service based on industry-standard
BGP to address the ATN/IPS requirements.¶
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.¶
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and may be updated, replaced, or obsoleted by other documents at any
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This Internet-Draft will expire on 25 April 2024.¶
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The worldwide Air Traffic Management (ATM) system today uses a
service known as Aeronautical Telecommunications Network based on Open
Systems Interconnection (ATN/OSI). The service is used to augment
controller to pilot voice communications with rudimentary short text
command and control messages. The service has seen successful deployment
in some worldwide ATM domains.¶
The International Civil Aviation Organization (ICAO) is now
engaged in the development of a next-generation replacement for ATN/OSI
known as Aeronautical Telecommunications Network with Internet Protocol
Services (ATN/IPS) [ATN][ATN-IPS]. ATN/IPS
will eventually provide an Internetworking service supporting pervasive
ATM for Air Traffic Controllers (ATC), Airline Operations Controllers
(AOC), and all commercial aircraft worldwide. The ATN/IPS will require
a new mobile routing service as a core element of the architecture.
This document presents an approach based on the Border Gateway
Protocol (BGP) [RFC4271].¶
Aircraft communicate via wireless aviation data links that typically
support much lower data rates than terrestrial wireless and wired-line
communications. For example, some Very High Frequency (VHF)-based data
links only support data rates on the order of 32Kbps and an emerging
L-Band data link that is expected to play a key role in future
aeronautical communications only supports rates on the order of 1Mbps.
Although satellite data links can provide much higher data rates during
optimal conditions, like any other aviation data link they are subject
to errors, delay, disruption, signal intermittence, degradation due to
atmospheric conditions, etc. The well-connected ground domain ATN/IPS
network should therefore treat each safety-of-flight critical packet
produced by (or destined to) an aircraft as a precious commodity and
strive for an optimized service that provides the highest possible
degree of reliability. Furthermore, continuous performance-intensive
control messaging services such as BGP peering sessions must be carried
only over the well-connected ground domain ATN/IPS network and never
over low-end aviation data links.¶
The ATN/IPS is an IP-based overlay network configured over one or
more Internetworking underlays ("INETs") maintained by aeronautical
network service providers such as ARINC, SITA and Inmarsat. The Overlay
Multilink Network Interface (OMNI) [I-D.templin-intarea-omni]
uses an adaptation layer encapsulation to create a Non-Broadcast, Multiple
Access (NBMA) virtual link spanning the entire ATN/IPS. Each aircraft
connects to the OMNI link via an OMNI interface configured over the
aircraft's underlying physical and/or virtual access network interfaces.¶
Each underlying INET comprises one or more "partitions" where all
nodes within a partition can exchange packets with all other nodes,
i.e., the partition is connected internally. There is no requirement
that each INET partition uses the same IP protocol version nor has
consistent IP addressing plans in relation to other partitions.
Instead, the OMNI link sees each partition as a "segment" of a
lower layer topology concatenated by a service known as the
OMNI Adaptation Layer (OAL) [I-D.templin-intarea-omni]
based on IPv6 encapsulation [RFC2473].¶
The IPv6 addressing architecture provides different classes of
addresses, including Global Unicast Addresses (GUAs), Unique Local
Addresses (ULAs) and Link-Local Addresses (LLAs) [RFC4291][RFC4193]. The ATN/IPS receives an IPv6
GUA Mobility Service Prefix (MSP) from an Internet assigned numbers
authority, and each aircraft will receive a Mobile Network Prefix (MNP)
delegation from the MSP that accompanies the aircraft wherever it
travels. ATCs and AOCs will likewise receive MNPs, but they would
typically appear in static (not mobile) deployments such as air traffic
control towers, airline headquarters, etc. (Note that while IPv6 GUAs
are assumed for ATN/IPS, IPv4 with public/private addresses could
also be used.)¶
The OAL uses ULAs for adaptation layer addressing. Each ULA includes
a prefix beginning with "fd00::/8" followed by a 40-bit Global ID and a
16-bit Subnet ID as "fd{Global ID}:{Subnet ID}::/64". Each aircraft ULA
includes an MNP in the interface identifier ("ULA-MNP"), as discussed in
[I-D.templin-intarea-omni]. Due to MNP delegation policies
and random node mobility properties, ULA-MNPs are generally not aggregable
in the BGP routing service and are represented as many more-specific
prefixes instead of a smaller number of aggregated prefixes.¶
BGP routing service infrastructure nodes additionally configure
ULAs with randomized interface identifiers ("ULA-RND") that are
statically-assigned and derived from a shorter ULA prefix assigned to
their BGP network partitions. Unlike ULA-MNPs, the ULA-RNDs are
persistently present and unchanging in the routing system. The BGP
routing services therefore establish forwarding table entries based on
these adaptation layer ULA-MNPs and ULA-RNDs instead of based on the
network layer GUA MNPs. However, nodes set the 40-bit Global ID and
16-bit Subnet ID to 0 ("wildcard") when they advertise ULA-MNPs in
BGP routing exchanges and/or install ULA-MNPs in forwarding tables
since the MNP uniquely addresses the aircraft regardless of its
current BGP network partition affiliation(s).¶
When an OMNI interface forwards an original IP packet, the OAL performs
IPv6 encapsulation using ULA-RNDs and/or ULA-MNPs as source and destination
addresses. The OAL next subjects each resulting "OAL packet" to IPv6
fragmentation and header compression, then encapsulates each fragment
in INET headers specific to the underlying Internetwork. These resulting
"carrier packets" then traverse a series of Internetworks connected by
OAL intermediate nodes which re-encapsulate them in new INET headers for
traversal of the next Internetwork in succession. (The carrier packets
may themselves be subject to IP layer fragmentation/reassembly during
each successive Internetwork traversal at a layer below the OAL.)
The carrier packets finally arrive at the OAL destination which performs
adaptation layer decapsulation and reassembly to restore the original
IP packet. A high-level ATN/IPS network diagram is shown in Figure 1:¶
Connexion By Boeing [CBB] was an early
aviation mobile routing service based on dynamic updates in the global
public Internet BGP routing system. Practical experience with the
approach has shown that frequent injections and withdrawals of prefixes
in the Internet routing system can result in excessive BGP update
messaging, slow routing table convergence times, and extended outages
when no route is available. This is due to both conservative default BGP
protocol timing parameters (see Section 6) and the complex
peering interconnections of BGP routers within the global Internet
infrastructure. The situation is further exacerbated by frequent
aircraft mobility events that each result in BGP updates which must be
propagated to all BGP routers in the Internet with full routing tables.¶
We therefore consider a routing system using an overlay network
that maintains a private BGP routing protocol instance between ATN/IPS
Autonomous System (AS) Border Routers (ASBRs). The private BGP instance
does not interact with the native BGP routing systems in underlying
INETs, and BGP updates are unidirectional from "stub" ASBRs (s-ASBRs)
to a small set of "core" ASBRs (c-ASBRs) in a hub-and-spokes topology.
No extensions to the BGP protocol are necessary, and BGP routing is
based on (intermediate-layer) ULAs instead of upper- or lower-layer
public/private IP prefixes. This allows ASBRs to perform adaptation
layer forwarding based on intermediate layer IPv6 header information
instead of network layer forwarding based on upper layer IP header
information or link layer forwarding based on lower layer IP header
information.¶
The s-ASBRs for each stub AS connect to a small number of c-ASBRs
via secured direct point-to-point links and/or secured tunnels (e.g.,
IPsec [RFC4301], etc.) over the underlying INET.
Neighboring c-ASBRs should also use IP layer or lower-layer security
services over their connecting links to ensure INET layer security.¶
The s-ASBRs engage in external BGP (eBGP) peerings with their
respective c-ASBRs, and only maintain routing table entries for the
ULA-MNPs currently active within the stub AS. The s-ASBRs send BGP
updates for ULA-MNP injections or withdrawals to c-ASBRs but do not
receive any BGP updates from c-ASBRs. Instead, the s-ASBRs maintain
default routes with their c-ASBRs as the next hop, and therefore hold
only partial topology information.¶
The c-ASBRs connect to other c-ASBRs within the same partition using
internal BGP (iBGP) peerings over which they collaboratively maintain a
full routing table for all active ULA-MNPs currently in service within
the partition. Therefore, only the c-ASBRs maintain a full BGP routing
table and never send any BGP updates to s-ASBRs. This simple routing
model therefore greatly reduces the number of BGP updates that need to
be synchronized among peers, and the number is reduced further still
when intradomain routing changes within stub ASes are processed within
the AS instead of being propagated to the core. BGP Route Reflectors
(RRs) [RFC4456] can also be used to support increased
scaling properties.¶
When there are multiple INET partitions, the c-ASBRs of each
partition use eBGP to peer with the c-ASBRs of other partitions so that
the full set of ULAs for all partitions are known globally among all of
the c-ASBRs. Each c/s-ASBR further configures a ULA-RND taken from a
ULA prefix assigned to each partition, as well as static forwarding
table entries for all other OMNI link partition prefixes.¶
With these intra- and inter-INET BGP peerings in place, a forwarding
plane spanning tree is established that properly covers the entire
operating domain. All nodes in the network can be visited using strict
spanning tree hops, but in many instances this may result in longer
paths than are necessary. AERO [I-D.templin-intarea-aero]
provides an example service for discovering and utilizing (route-optimized)
shortcuts that do not always follow strict spanning tree paths.¶
The remainder of this document discusses the proposed BGP-based
ATN/IPS mobile routing service.¶
The terms Autonomous System (AS) and Autonomous System Border Router
(ASBR) are the same as defined in [RFC4271]. The term
Internet Protocol (IP) refers generically to either protocol version
unless specifically qualified as IPv4 [RFC0791] or
IPv6 [RFC8200].¶
The following terms are defined for the purposes of this
document:¶
Air Traffic Management (ATM)
The worldwide
service for coordinating safe aviation operations.¶
Air Traffic Controller (ATC)
A government
agent responsible for coordinating with aircraft within a defined
operational region via voice and/or data Command and Control
messaging.¶
Airline Operations Controller (AOC)
An
airline agent responsible for tracking and coordinating with
aircraft within their fleet.¶
Aeronautical Telecommunications Network with Internet Protocol Services (ATN/IPS)
A future aviation network for
ATCs and AOCs to coordinate with all aircraft operating worldwide.
The ATN/IPS will provide an IP-based overlay network service that
connects access networks via encapsulation and forwarding over one
or more Internetworking underlays.¶
Internetworking underlay ("INET")
A
wide-area network that supports overlay network encapsulation/forwarding
and connects Radio Access Networks to the rest of the ATN/IPS. Example
INET service providers for civil aviation include ARINC, SITA and
Inmarsat.¶
(Radio) Access Network ("ANET")
An
aviation radio data link service provider's network, including radio
transmitters and receivers as well as supporting ground-domain
infrastructure needed to convey a customer's data packets to outside
INETs. The term ANET is intended in the same spirit as for
radio-based Internet service provider networks (e.g., cellular
operators), but can also refer to ground-domain networks that
connect AOCs and ATCs.¶
partition (or "segment")
A
fully-connected internal subnetwork of an INET in which all nodes
can communicate with all other nodes within the same partition using
the same IP protocol version and addressing plan. Each INET consists
of one or more partitions.¶
Overlay Multilink Network Interface (OMNI)
A
virtual layer 2 bridging service that presents an ATN/IPS overlay
unified link view even though the underlay may consist of multiple
INET partitions. The OMNI virtual link is manifested through nested
encapsulation in which original IP packets from the ATN/IPS are
first encapsulated in ULA-addressed IPv6 headers which are then
forwarded to the next hop using INET encapsulation if necessary.
Forwarding over the OMNI virtual link is therefore based on ULAs
instead of the original IP addresses. In this way, packets sent
from a source can be conveyed over the OMNI virtual link even
though there may be many underlying INET partitions in the
path to the destination.¶
OMNI Adaptation Layer (OAL)
A middle layer
below the IP layer but above the INET layer that forwards original
IP packets by applying IPv6 encapsulation, fragmentation and header
compression followed by INET encapsulation to form "carrier packets".
End systems that configure OMNI interfaces act as the OAL source and
destination, while intermediate systems with OMNI interfaces act as
OAL forwarding nodes. Regardless of the number of intermediate
systems in the path, the network layer IP TTL/Hop Limit is not
decremented during (OAL layer) forwarding. Further details on
OMNI and the OAL are found in [I-D.templin-intarea-omni].¶
OAL Autonomous System (OAL AS)
A "hub-of-hubs"
autonomous system maintained through peerings between the core
autonomous systems of different OMNI virtual link partitions.¶
Core Autonomous System Border Router (c-ASBR)
A
BGP router located in the hub of the INET partition hub-and-spokes
overlay network topology.¶
Core Autonomous System (Core AS)
The "hub"
autonomous system maintained by all c-ASBRs within the same
partition.¶
Stub Autonomous System Border Router (s-ASBR)
A
BGP router configured as a spoke in the INET partition
hub-and-spokes overlay network topology.¶
Stub Autonomous System (Stub AS)
A logical
grouping that includes all Clients currently associated with a given
s-ASBR.¶
Client
An ATC, AOC or aircraft that connects
to the ATN/IPS as a leaf node. The Client could be a singleton host,
or a router that connects a mobile or fixed network.¶
Proxy/Server
An ANET/INET border node that
acts as a transparent intermediary between Clients and s-ASBRs. From
the Client's perspective, the Proxy/Server presents the appearance
that the Client is communicating directly with the s-ASBR. From the
s-ASBR's perspective, the Proxy/Server presents the appearance that
the s-ASBR is communicating directly with the Client.¶
Mobile Network Prefix (MNP)
An IP prefix
that is delegated to any ATN/IPS end system, including ATCs, AOCs,
and aircraft.¶
Mobility Service Prefix (MSP)
An aggregated
IP prefix assigned to the ATN/IPS by an Internet assigned numbers
authority, and from which all MNPs are delegated (e.g., up to 2**32
IPv6 /56 MNPs could be delegated from a /24 MSP).¶
The ATN/IPS routing system comprises a private BGP instance
coordinated in an overlay network via tunnels between neighboring ASBRs
over one or more underlying INETs. The ATN/IPS routing system interacts
with underlying INET BGP routing systems only through the static
advertisement of a small and unchanging set of MSPs instead of the full
dynamically changing set of MNPs.¶
Within each INET partition, each s-ASBR connects a stub AS to the
INET partition core using a distinct stub AS Number (ASN). Each s-ASBR
further uses eBGP to peer with one or more c-ASBRs. All c-ASBRs are
members of the INET partition core AS, and use a shared core ASN. Unique
ASNs are assigned according to the standard 32-bit ASN format [RFC4271][RFC6793]. Since the BGP instance does
not connect with any INET BGP routing systems, the ASNs can be assigned
from the [RFC6996] 32-bit ASN space which reserves
94,967,295 numbers for private use. The ASNs must be allocated and
managed by an ATN/IPS assigned numbers authority established by ICAO,
which must ensure that ASNs are responsibly distributed without
duplication and/or overlap.¶
The c-ASBRs use iBGP to maintain a synchronized consistent view of
all active ULA-MNPs currently in service within the INET partition.
Figure 2 below represents the reference INET partition
deployment. (Note that the figure shows details for only two s-ASBRs
(s-ASBR1 and s-ASBR2) due to space constraints, but the other s-ASBRs
should be understood to have similar Stub AS, MNP and eBGP peering
arrangements.) The solution described in this document is flexible
enough to extend to these topologies.¶
In the reference deployment, each s-ASBR maintains routes for
active ULA-MNPs that currently belong to its stub AS. In response to
"Inter-domain" mobility events, each s-ASBR dynamically announces new
ULA-MNPs and withdraws departed ULA-MNPs in its eBGP updates to c-ASBRs.
Since ATN/IPS end systems are expected to remain within the same stub AS
for extended timeframes, however, intra-domain mobility events (such as
an aircraft handing off between cell towers) are handled within the stub
AS instead of being propagated as inter-domain eBGP updates.¶
Each c-ASBR configures a black-hole route for each of its MSPs. By
black-holing the MSPs, the c-ASBR maintains forwarding table entries
only for the ULA-MNPs that are currently active. If an arriving packet
matches a black-hole route without matching an ULA-MNP, the c-ASBR
should drop the packet and may also generate an ICMPv6 Destination
Unreachable message [RFC4443], i.e., without forwarding
the packet outside of the ATN/IPS overlay based on a less-specific
route.¶
The c-ASBRs do not send BGP updates for ULA-MNPs to s-ASBRs, but
instead originate a default route. In this way, s-ASBRs have only
partial topology knowledge (i.e., they know only about the active
ULA-MNPs currently within their stub ASes) and they forward all other
packets to c-ASBRs which have full topology knowledge.¶
Each s-ASBR and c-ASBR configures an ULA-RND that is aggregable
within an INET partition, and each partition configures a unique ULA
prefix that is permanently announced into the routing system. The core
ASes of each INET partition are joined together through external BGP
peerings. The c-ASBRs of each partition establish external peerings with
the c-ASBRs of other partitions to form a "core-of-cores" OMNI link AS.
The OMNI link AS contains the global knowledge of all ULA-MNPs deployed
worldwide, and supports ATN/IPS overlay communications between nodes
located in different INET partitions by virtue of OAL encapsulation.
OMNI link nodes can then navigate to ASBRs by including an ULA-RND or
directly to an end system by including an ULA-MNP in the destination
address of an OAL-encapsulated packet (see: [I-D.templin-intarea-aero]). Figure 3
shows a reference OAL topology.¶
Scaling properties of this ATN/IPS routing system are limited by the
number of BGP routes that can be carried by the c-ASBRs. A 2015 study
showed that BGP routers in the global public Internet at that time
carried more than 500K routes with linear growth and no signs of router
resource exhaustion [BGP]. A more recent network
emulation study also showed that a single c-ASBR can accommodate at
least 1M dynamically changing BGP routes even on a lightweight virtual
machine. Commercially-available high-performance dedicated router
hardware can support many millions of routes.¶
Therefore, assuming each c-ASBR can carry 1M or more routes, this
means that at least 1M ATN/IPS end system ULA-MNPs can be serviced by a
single set of c-ASBRs and that number could be further increased by
using RRs and/or more powerful routers. Another means of increasing
scale would be to assign a different set of c-ASBRs for each set of
MSPs. In that case, each s-ASBR still peers with one or more c-ASBRs
from each set of c-ASBRs, but the s-ASBR institutes route filters so
that it only sends BGP updates to the specific set of c-ASBRs that
aggregate the MSP. In this way, each set of c-ASBRs maintains separate
routing and forwarding tables so that scaling is distributed across
multiple c-ASBR sets instead of concentrated in a single c-ASBR set. For
example, a first c-ASBR set could aggregate an MSP segment A::/32, a
second set could aggregate B::/32, a third could aggregate C::/32, etc.
The union of all MSP segments would then constitute the collective
MSP(s) for the entire ATN/IPS, with potential for supporting many
millions of mobile networks or more.¶
In this design, each set of c-ASBRs services a specific set of MSPs,
and each s-ASBR configures MSP-specific routes that list the correct
set of c-ASBRs as next hops. This also allows for natural incremental
deployment, and can support initial medium-scale deployments followed by
dynamic deployment of additional ATN/IPS infrastructure elements without
disturbing the already-deployed base. For example, additional c-ASBRs
can be added later if the MNP service demand ever outgrows the initial
deployment. For larger-scale applications (such as unmanned air vehicles
and terrestrial vehicles) even larger scales can be accommodated by
adding more c-ASBRs.¶
Consider now that the c-ASBRs provide adaptation layer gateways
between independent Internetworks to form a true network-of-networks
supporting the ATN/IPS overlay. This same arrangement was first
envisioned by the "Catenet Model for Internetworking" [IEN48][IEN48-2] circa 1978.¶
(Radio) Access Networks (ANETs) connect end system Clients such as
aircraft, ATCs, AOCs etc. to the ATN/IPS routing system. Clients may
connect to multiple ANETs at once, for example, when they have
satellite, cellular, WiFi and/or other data links activated
simultaneously. Each Client configures an OMNI interface
[I-D.templin-intarea-omni] over its underlying ANET
interfaces as a connection to an NBMA virtual link (manifested by
the OAL) that spans the entire ATN/IPS. Clients may further move
between ANETs in a manner that is perceived as a network layer
mobility event. Clients should therefore employ a multilink/mobility
routing service such as those discussed in Section 7.¶
Clients register their active data link connections with their
serving s-ASBRs as discussed in Section 3. Clients may
connect to s-ASBRs either directly, or via a Proxy/Server at the
ANET/INET boundary.¶
Figure 4 shows the ATN/IPS ANET model where Clients
connect to ANETs via aviation data links. Clients register their ANET
addresses with a nearby s-ASBR, where the registration process may be
brokered by a Proxy/Server at the edge of the ANET.¶
When a Client connects to an ANET it specifies a nearby s-ASBR that
it has selected to connect to the ATN/IPS. The login process is
transparently brokered by a Proxy/Server at the border of the ANET which
then conveys the connection request to the s-ASBR via adaptation layer
encapsulation and forwarding across the OMNI virtual link. Each ANET
border Proxy/Server is also equally capable of serving in the s-ASBR
role so that a first on-link Proxy/Server can be selected as the s-ASBR
while all others perform the Proxy/Server role in a hub-and-spokes
arrangement. An on-link Proxy/Server is selected to serve the s-ASBR
role when it receives a control message from a Client specifically
requesting that service.¶
The Client can coordinate with a network-based s-ASBR over additional
ANETs after it has already coordinated with a first-hop Proxy/Server
over a first ANET. If the Client connects to multiple ANETs, the s-ASBR
will register the individual ANET Proxy/Servers as conduits through
which the Client can be reached. The Client then sees the s-ASBR as the
"hub" in a "hub-and-spokes" arrangement with the first-hop Proxy/Servers
as spokes. Selection of a network-based s-ASBR is through the discovery
methods specified in relevant mobility and virtual link coordination
specifications (e.g., see AERO [I-D.templin-intarea-aero]
and OMNI [I-D.templin-intarea-omni]).¶
The s-ASBR represents all of its active Clients as ULA-MNP routes in
the ATN/IPS BGP routing system. The s-ASBR's stub AS is therefore used
only to advertise the set of MNPs of all its active Clients to its BGP
peer c-ASBRs and not to peer with other s-ASBRs (i.e., the stub AS is a
logical construct and not a physical one). The s-ASBR injects the
ULA-MNPs of its active Clients and withdraws the ULA-MNPs of its
departed Clients via BGP updates to c-ASBRs, which further propagate the
ULA-MNPs to other c-ASBRs within the OAL AS. Since Clients are expected
to remain associated with their current s-ASBR for extended periods, the
level of ULA-MNP injections and withdrawals in the BGP routing system
will be on the order of the numbers of network joins, leaves and s-ASBR
handovers for aircraft operations (see: Section 6). It is
important to observe that fine-grained events such as Client mobility
and Quality of Service (QoS) signaling are coordinated only by the
Client's current s-ASBRs, and do not involve other ASBRs in the routing
system. In this way, intradomain routing changes within the stub AS are
not propagated into the rest of the ATN/IPS BGP routing system.¶
ATN/IPS end systems will frequently need to communicate with
correspondents associated with other s-ASBRs. In the BGP peering
topology discussed in Section 3, this can initially only
be accommodated by including multiple extraneous hops and/or spanning
tree segments in the forwarding path. In many cases, it would be
desirable to establish a "short cut" around this "dogleg" route so that
packets can traverse a minimum number of forwarding hops across the OMNI
virtual link. ATN/IPS end systems could therefore employ a route
optimization service according to the mobility service employed (see:
Section 7).¶
Each s-ASBR provides designated routing services for only a subset of
all active Clients, and instead acts as a simple Proxy/Server for other
Clients. As a designated router, the s-ASBR advertises the MNPs of each
of its active Clients into the ATN/IPS routing system and provides basic
(unoptimized) forwarding services when necessary. An s-ASBR could be the
first-hop ATN/IPS service access point for some, all or none of a
Client's underlying interfaces, while the Client's other underlying
interfaces employ the Proxy/Server function of other s-ASBRs. Route
optimization allows Client-to-Client communications while bypassing
s-ASBR designated routing services whenever possible.¶
A route optimization example is shown in Figure 5 and
Figure 6 below. In the first figure, multiple spanning
tree segments between Proxy/Servers and ASBRs are necessary to convey
packets between Clients associated with different s-ASBRs. In the second
figure, the optimized route forwards encapsulated packets directly between
Proxy/Servers without involving the ASBRs.¶
These route optimized paths are established through secured control
plane messaging (i.e., over secured tunnels and/or using higher-layer
control message authentications) but do not provide lower-layer security
for the data plane. Data communications over these route optimized paths
should therefore employ higher-layer security.¶
The number of eBGP peering sessions that each c-ASBR must service is
proportional to the number of s-ASBRs in its local partition. Network
emulations with lightweight virtual machines have shown that a single
c-ASBR can service at least 100 eBGP peerings from s-ASBRs that each
advertise 10K ULA-MNP routes (i.e., 1M total). It is expected that
robust c-ASBRs can service many more peerings than this - possibly by
multiple orders of magnitude. But even assuming a conservative limit,
the number of s-ASBRs could be increased by also increasing the number
of c-ASBRs. Since c-ASBRs also peer with each other using iBGP, however,
larger-scale c-ASBR deployments may need to employ an adjunct facility
such as BGP Route Reflectors (RRs)[RFC4456].¶
The number of aircraft in operation at a given time worldwide is
likely to be significantly less than 1M, but we will assume this number
for a worst-case analysis. Assuming a worst-case average 1 hour flight
profile from gate-to-gate with 10 service region transitions per flight,
the entire system will need to service at most 10M BGP updates per hour
(2778 updates per second). This number is within the realm of the peak
BGP update messaging seen in the global public Internet today [BGP2]. Assuming a BGP update message size of 100 octets
(800bits), the total amount of BGP control message traffic to a single
c-ASBR will be less than 2.5Mbps which is a nominal rate for modern data
links.¶
Industry standard BGP routers provide configurable parameters with
conservative default values. For example, the default hold time is 90
seconds, the default keepalive time is 1/3 of the hold time, and the
default MinRouteAdvertisementinterval is 30 seconds for eBGP peers and 5
seconds for iBGP peers (see Section 10 of [RFC4271]). For
the simple mobile routing system described herein, these parameters can
be set to more aggressive values to support faster neighbor/link failure
detection and faster routing protocol convergence times. For example, a
hold time of 3 seconds and a MinRouteAdvertisementinterval of 0 seconds
for both iBGP and eBGP.¶
Instead of adjusting BGP default time values, BGP routers can use the
Bidirectional Forwarding Detection (BFD) protocol [RFC5880]
to quickly detect link failures that do not result in interface state
changes, BGP peer failures, and administrative state changes. BFD is
important in environments where rapid response to failures is required
for routing reconvergence and, hence, communications continuity.¶
Each c-ASBR will be using eBGP both in the ATN/IPS and the INET
with the ATN/IPS unicast IP routes resolving over INET routes.
Consequently, c-ASBRs and potentially s-ASBRs will need to support
separate local ASes for the two BGP routing domains and routing policy
or assure routes are not propagated between the two BGP routing domains.
From a conceptual, operational and correctness standpoint, the
implementation should provide isolation between the two BGP routing
domains (e.g., separate BGP instances).¶
This gives rise to a BGP routing system that must accommodate large
numbers of long and non-aggregable ULA-MNP prefixes as well as moderate
numbers of long and semi-aggregable ULA-RND prefixes. The system is kept
stable and scalable through the s-ASBR / c-ASBR hub-and-spokes topology
which ensures that mobility-related churn is not exposed to the
core.¶
Stub ASes maintain intradomain routing information for mobile node
clients, and are responsible for all localized mobility signaling
without disturbing the BGP routing system. Clients can enlist the
services of a candidate mobility service such as Mobile IPv6 (MIPv6)
[RFC6275], LISP [I-D.ietf-lisp-rfc6830bis]
or AERO [I-D.templin-intarea-aero] according to the service
offered by the stub AS. Further details of mobile routing services are
out of scope for this document.¶
The BGP routing topology described in this document has been modeled
in realistic network emulations showing that at least 1 million ULA-MNPs
can be propagated to each c-ASBR even on lightweight virtual machines.
No BGP routing protocol extensions need to be adopted.¶
ATN/IPS ASBRs on the open Internet are susceptible to the same attack
profiles as for any Internet nodes. For this reason, ASBRs should employ
physical security and/or IP securing mechanisms such as IPsec [RFC4301], etc.¶
ATN/IPS ASBRs present targets for Distributed Denial of Service
(DDoS) attacks. This concern is no different than for any node on the
open Internet, where attackers could send spoofed packets to the node at
high data rates. This can be mitigated by connecting ATN/IPS ASBRs over
dedicated links with no connections to the Internet and/or when ASBR
connections to the Internet are only permitted through well-managed
firewalls.¶
ATN/IPS s-ASBRs should institute rate limits to protect low data rate
aviation data links from receiving DDoS packet floods.¶
BGP protocol message exchanges and control message exchanges used
for route optimization must be secured to ensure the integrity of the
system-wide routing information base. Security is based on IP layer
security associations between peers which ensure confidentiality,
integrity and authentication over secured tunnels (see above). Higher
layer security protection such as TCP-AO [RFC5926] is
therefore optional, since it would be redundant with the security
provided at lower layers.¶
Data communications over route optimized paths should employ
end-to-end higher-layer security since only the control plane and
unoptimized paths are protected by lower-layer security. End-to-end
higher-layer security mechanisms include QUIC-TLS [RFC9001], TLS [RFC8446], DTLS [RFC6347], SSH [RFC4251], etc. applied in a
manner outside the scope of this document.¶
This document does not include any new specific requirements for
mitigation of DDoS.¶
In development of the overall ATN/IPS operational concept, ICAO
addressed the security concerns in multiple ways to ensure
coordination and consistency across the various groups. This also
avoided potential duplicative work. Technical provisions related
specifically to the operation of ATN/IPS are specified in supporting
ATN/IPS standards. However, other considerations such as the
establishment of a PKI, were determined to have an impact beyond
ATN/IPS. ICAO created a Trust Framework Study Group (TFSG) to define
various governance, policy, procedures and overall technical
performance requirements for system connectivity and
interoperability.¶
As part of their charter, the TSFG is specifically developing a
concept of operations for a common aviation digital trust framework
and principles to facilitate an interoperable secure, cyber resilient
and seamless exchange of information in a digitally connected
environment. They are also developing governance principles, policy,
procedures and requirements for establishing digital identity for a
global trust framework that will consider any exchange of information
among users of the aviation ecosystem, and to promote these concepts
with all relevant stakeholders.¶
ATN/IPS will take advantage of the developments of TFSG within the
overall ATN/IPS operational concept. As such, this will include the
usage of the PKI specification resulting from the TFSG.¶
This work is aligned with the FAA as per the SE2025 contract number
DTFAWA-15-D-00030.¶
This work is aligned with the NASA Safe Autonomous Systems Operation
(SASO) program under NASA contract number NNA16BD84C.¶
This work is aligned with the Boeing Commercial Airplanes (BCA)
Internet of Things (IoT) and autonomy programs.¶
This work is aligned with the Boeing Information Technology (BIT)
MobileNet program.¶
The following individuals contributed insights that have improved the
document: Ahmad Amin, Mach Chen, Russ Housley, Erik Kline, Hubert
Kuenig, Tony Li, Gyan Mishra, Alexandre Petrescu, Dave Thaler, Pascal
Thubert, Michael Tuxen, Eric Vyncke, Tony Whyman. Vaughn Maiolla is
further remembered for his support and guidance.¶
Honoring life, liberty and the pursuit of happiness.¶
Conta, A., Deering, S., and M. Gupta, Ed., "Internet Control Message Protocol (ICMPv6) for the Internet Protocol Version 6 (IPv6) Specification", STD 89, RFC 4443, DOI 10.17487/RFC4443, , <https://www.rfc-editor.org/info/rfc4443>.
[RFC4456]
Bates, T., Chen, E., and R. Chandra, "BGP Route Reflection: An Alternative to Full Mesh Internal BGP (IBGP)", RFC 4456, DOI 10.17487/RFC4456, , <https://www.rfc-editor.org/info/rfc4456>.
Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6) Specification", STD 86, RFC 8200, DOI 10.17487/RFC8200, , <https://www.rfc-editor.org/info/rfc8200>.
Maiolla, V., "The OMNI Interface - An IPv6 Air/Ground Interface for Civil Aviation, IETF Liaison Statement #1676, https://datatracker.ietf.org/liaison/1676/", .
[ATN-IPS]
WG-I, ICAO., "ICAO Document 9896 (Manual on the Aeronautical Telecommunication Network (ATN) using Internet Protocol Suite (IPS) Standards and Protocol), Draft Edition 3 (work-in-progress)", .
Cerf, V., "The Catenet Model For Internetworking, https://www.rfc-editor.org/ien/ien48.txt", .
[IEN48-2]
Cerf, V., "The Catenet Model For Internetworking (with figures), http://www.postel.org/ien/pdf/ien048.pdf", .
[RFC4251]
Ylonen, T. and C. Lonvick, Ed., "The Secure Shell (SSH) Protocol Architecture", RFC 4251, DOI 10.17487/RFC4251, , <https://www.rfc-editor.org/info/rfc4251>.
Lebovitz, G. and E. Rescorla, "Cryptographic Algorithms for the TCP Authentication Option (TCP-AO)", RFC 5926, DOI 10.17487/RFC5926, , <https://www.rfc-editor.org/info/rfc5926>.
Vohra, Q. and E. Chen, "BGP Support for Four-Octet Autonomous System (AS) Number Space", RFC 6793, DOI 10.17487/RFC6793, , <https://www.rfc-editor.org/info/rfc6793>.
Experimental evidence has shown that BGP convergence time required
after an ULA-MNP is asserted at a new location or withdrawn from an old
location can be several hundred milliseconds even under optimal AS
peering arrangements. This means that packets in flight destined to an
ULA-MNP route that has recently been changed can be (mis)delivered to an
old s-ASBR after a Client has moved to a new s-ASBR.¶
To address this issue, the old s-ASBR can maintain temporary state
for a "departed" Client that includes an OAL address for the new s-ASBR.
The OAL address never changes since ASBRs are fixed infrastructure
elements that never move. Hence, packets arriving at the old s-ASBR can
be forwarded to the new s-ASBR while the BGP routing system is still
undergoing reconvergence. Therefore, as long as the Client associates
with the new s-ASBR before it departs from the old s-ASBR (while
informing the old s-ASBR of its new location) packets in flight during
the BGP reconvergence window are accommodated without loss.¶
The AERO/OMNI services establish an adaptation layer for the OSI reference
model appearing as "the layer below the network layer but above the data link
layer". The adaptation layer presents a virtual bridging service from the
network layer's perspective and a BGP-based IPv6 routing service from the
data link layer's perspective.¶
AERO/OMNI overlay networks include s-ASBRs (aka Proxy/Servers) and c-ASBRs
(aka Gateways) as the vertices in a graph with a typically sparse collection
of edges between pairwise vertices. The graph minimally comprises a true
spanning tree connecting all vertices but may also include additional edges
in the spirit of IEEE 802.1aq Shortest Path Bridging. BGP routing then
ensures loop freedom even if this "augmented" spanning tree includes cycles.¶
Each edge in the spanning tree corresponds to one or more connecting links
which may be physical (e.g., fiber/free-space optics, etc.) or virtual (an IP
tunnel over an underlying Internetwork). The OMNI interface provides a nexus
for link selection, where control messages between vertices must be forwarded
over edge links that are secured at the network layer or below while data
messages may be forwarded over unsecured links. AERO/OMNI refer to these
distinct forwarding contexts as the "secured spanning tree" and "unsecured
spanning tree", respectively.¶
AERO route optimization provides an essential service to establish
shortcut paths in the data plane that do not necessarily follow strict
spanning tree paths. The shortcuts are formed through secured spanning
tree control message exchanges which establish shortest path forwarding
state in Clients, Proxy/Servers and intermediate Gateways. Route
optimization therefore reduces spanning tree traffic concentration
and instead distributes traffic over a diversity of underlying
Internetwork paths.¶