Problems and Requirements of Addressing in Integrated Space-Terrestrial NetworkTsinghua UniversityBeijing 100084Chinayuanjiel@tsinghua.edu.cnTsinghua UniversityBeijing 100084Chinalihewu@cernet.edu.cnTsinghua UniversityBeijing 100084Chinaliu-jy21@mails.tsinghua.edu.cn
General
Internet Engineering Task ForcetemplateThis document presents a detailed analysis of the problems and requirements of network addressing in "Internet in space" for terrestrial users.
It introduces the basics of satellite mega-constellations, terrestrial terminals/ground stations, and their inter-networking.
Then it explicitly analyzes how space-terrestrial mobility yeilds challenges for the logical topology, addressing, and their impact on routing.
The requirements of addressing in the space-terrestrial network are discussed in detail, including uniqueness, stability, locality, scalability, efficiency and backward compatibility with terrestrial Internet.
The problems and requirements of network addressing in space-terrestrial networks are finally outlined.Introduction
The future Internet is up in the sky.
We have seen a rocket-fast deployment of mega-constellations with 100s-10,000s of low-earth-orbit (LEO) satellites, such as Starlink,
Kuiper and OneWeb.
These constellations promise competitive low latency and high capacity to terrestrial networks.
They expand global high-speed Internet to remote areas that were not reachable by terrestrial networks,
resulting in a tens-of-billions-of-dollar market with 3.7 billion users in rural areas, developing countries, aircraft, or oceans.
A salient feature for LEO mega-constellations is their high relative motions to the rotating earth.
Unlike geosynchronous satellite or terrestrial networks, each LEO satellite moves fast (e.g., 28,080 km/h for Starlink), causing short-lived coverage for terrestrial users (less than 3 minutes).
This yields diverse challenges for the traditional network designs.
This memo outlines the problems and requirements of addressing in integrated space-terrestrial network.
It starts with the basics of satellite mega-constellations, terrestrial ground stations/terminals, and their inter-networking.
It analyzes how high space-terrestrial relative motions yields challenges for logical topology, addressing and their impacts on routing.
Then it discusses the requirements of network addressing in space-terrestrial network for uniqueness, stability, locality, scalability, efficiency and backward compatibility with terrestrial Internet.
TerminologyGSO: Geosynchronous orbit (at the altitude of 35,786 km).NGSO: Non-geosynchronous orbit.LEO: Low Earth Orbit (at the altitude of 180-2,000 km).MEO: Medium Earth Orbit (at the altitude of 180-35,786 km).ISL: Inter Satellite Link.NAT: Network Address Translation. GS: Ground Station, a device on ground connecting the satellite.FIB: Forwarding Information Base.Basics of Space-Terrestrial NetworkAs shown in Figure 1, a space-terrestrial network for terrestrial users consists of the space segment and ground segment. The space segment
includes the satellite or constellation. The ground segment comprises satellite terminals and ground stations.Space SegmentSatellites can be classified based on their relative motions to the earth. A satellite can operate at the
geosynchronous orbit (GSO, at about 35,786 km altitude) or non-geosynchronous orbits, such as low earth orbits (LEO, <=2,000km) and medium earth
orbits (MEO, between 2,000 km and 35,786 km). Satellites at higher altitudes offer broader coverage, while satellites at lower altitudes move
faster.Historically, communications in space were dominated by GSO satellites. As shown in , GSO offers excellent coverage at high
altitudes, but at the cost of long space-terrestrial RTT (>=200ms) and low bandwidth (<=10Mbps, due to bit errors in
long distance transmission). Instead, recent efforts seek to adopt satellites at lower non-geosynchronous orbits,
with a special interest in low-earth orbits. Together with Ku (12-18 GHz) and Ka (26.5-40GHz) bands, these satellites
promise competitive bandwidth and latency to terrestrial networks( ,
, ).
Due to low coverage for each LEO satellite, a mega-constellation is necessary to retain global coverage.
exemplifies popular LEO mega-costellations in operation.
They are enabled by recent advances in satellite miniaturization and rocket reusability.
Differences between GSO and NGSO.
Orbit
Altitude (km)
RTT (ms)
GSO
GEO
35,786
240
NGSO
MEO
2,000-35,786
12-240
LEO
500-2,000
2-12
Low-earth-orbit (LEO) satellite mega-constellations in operation.
Constellation
Num. satellites
Num. orbits
Altitude (km)
Starlink
1584
72
540/550
720
36
570
348
6
560
172
4
560
Kuiper
1156
34
630
1296
36
610
784
28
590
Telesat
351
27
1015
1320
40
1325
Iridium
66
6
780
Ground SegmentTerrestrial users access satellite networks via terminals (e.g., satellite phones, onboard dishes, IoT endpoints)
or ground stations. Ground stations can serve as network gateways (e.g., carrier-grade NAT in Starlink and Kuiper) and
remote satellite controllers (e.g., telemetry, tracking, orbital update commands, or centralized routing control).Space-Ground InternetworkingEarly satellite communications favor the simple "bent-pipe-only" model (Figure 1), i.e., satellites only relay terrestrial
users' radio signals to the fixed ground stations without ISLs or routing. This model has been popular in GSO
satellites with broad coverage (2G GMR , 3G BGAN , and DVB-S ),
and recently adopted by LEO satellites in OneWeb (4G) and 5G NTN . However, this model suffers from low LEO satellite coverage.
To access the network, both terrestrial users and ground stations must reside inside the satellite's coverage. Due to each LEO satellite's low coverage,
most users in remote areas with sparse or no ground stations cannot be served. As shown in , under current Starlink (no ISLs so far) and ground station deployments
(, , , , , ),
27%-52% global populations cannot be served by the
"bent-pipe-only" model (depending on how many satellites each ground station can simultaneously associate to).
Most under-served users are from remote areas (e.g., Africa), thus causing revenue loss for operators. Deploying dense
ground stations in these remote areas could mitigate this. However, it is expensive and lowers commercial competitive
advantages to terrestrial networks.Instead, modern LEO mega-constellations favor satellite routing to expand global coverage or enable Internet backbones . To date, inter-satellite links are still under early adoption(, ).
The recent "burn on re-entry" regulations from FCC also slows down the adoption of ISLs.
As a near-term remedy, ground station-assisted routing is currently adopted. There are two variants. The GS-as-gateway is adopted by Starlink and Kuiper. Each ground station is a
carrier-grade NAT that offers private IP for terrestrial users. The GS-as-relay mitigates ISLs with ground
station-assisted routing, but is vulnerable to intermittent space-terrestrial links in Ku/Ka-bands. Like the
"bent-pipe only" model (Figure 1), both heavily rely on ubiquitous ground station deployments in remote areas and even oceans,
thus lowering competitive edges to terrestrial networks. Instead, we take a forward-looking view to exploring
inter-satellite routing for its long-term success.
Global population that could access Starlink in its current "bent-pipe-only" model.
Global
Africa
Oceania
South America
Asia
European
North America
1-SAT assicoation
48.71%
19.52%
42.85%
49.63%
43.49%
91.00%
87.50%
2-SAT assicoation
57.30%
24.37%
56.58%
53.90%
55.91%
94.33%
91.23%
4-SAT assicoation
67.04%
26.13%
60.31%
63.16%
71.34%
95.46%
95.04%
8-SAT assicoation
73.04%
29.17%
60.68%
65.65%
80.28%
96.91%
98.86%
Problems in Space-Terrestrial Network AddressingIn terrestrial and GEO satellite networks, the logical network topology, addresses, and routes are mostly stationary due to fixed infrastructure.
Instead, LEO mega-constellations hardly enjoy this luxury, whose satellites move at high speeds (about 28,080 km/h).
The earth's rotation further complicates the relative motions between space and ground.
In this section, we will analyze how high relative motion between space and ground challenges addressing due to topology instability, and its impact on routing.
Unstable Space-Terrestrial TopologyHigh physical mobility incurs frequent link churns between space and terrestrial nodes, thus causing frequent logical network topology changes. For all mega-constellations in Table 2, the topology changes every 10s of seconds. The link churn populates with more satellites and ground stations. In terrestrial mobile networks (e.g., 4G/5G), such physical link churn can be masked by handoffs without incurring logical topology changes. This method works based on two premises. First, all link churns occur at the last-hop radio due to user mobility, without affecting the infrastructure topology. Second, all cellular infrastructure nodes are fixed, resulting in a stable logical topology as "anchors". However, neither premise holds in non-geosynchronous constellations. Instead, infrastructure mobility between satellites
and ground stations becomes a norm rather than an exception. This voids cellular handoffs' merits to avoid propagation of
physical link churns to logical network topology: They are designed for user mobility only, and heavily rely on the fixed
infrastructure as "anchors." Therefore, 5G NTN lists satellite handoffs as an unsolved problem (, ),
and the latest 3GPP 5G release 17 defers its mobility support for satellites due to significant architectural changes. While Starlink uses handoffs to migrate
physical links between satellites and ground stations (every 15s ), its logical topology and routing are still be repeatedly
updated at high costs.Inconsistent "Locations" for Space/Terrestrial NodesEach space/terrestrial node has two notions of "locations": The logical location in its topological address, and the physical location in reality. With repetitive topology changes, a static network address can hardly ensure its logical location in the topology is consistent with the fast-moving node's physical location in reality. Then to correctly forward data, a network should choose one of the following designs:
Dynamic address updatesA node can repetitively re-bind its physical location to its logical network address, thus incurring frequent address updates or re-binding.
Under high mobility, this could severely disrupt user experiences or incur heavy signaling overhead.
and project the address update frequency when using legacy IP addresses for logical interfaces.
In this scheme, the terrestrial users' logical IP address changes if it re-associates to a new satellite (thus new interfaces and subnets) to retain its Internet access.
Due to high LEO satellite mobility, each user is forced to change its logical IP address every 133-510s.
Every second, we observe 2,082-7,961 global users per second should change their IP addresses.
Frequency of each user's logical IP address update.
Starlink
Telesat
Kuiper
Iridium
Every 133s
Every 510s
Every 510s
Every 458s
Number of terrestrial users that change logical IP address per second.
Starlink
Telesat
Kuiper
Iridium
7961
2082
5673
2379
Static address binding to a fixed gatewayThis is adopted by the cellular networks and Starlink and Kuiper's initial rollouts. Each user gets a static address from the remote ground station (via carrier-grade NAT), which masks the external address changes and redirects users' traffic. This mitigates user address updates, but cannot avoid gateway's external address updates when changing satellite interfaces (detailed below). It also incurs detours and long routing latencies for remote users from ground stations (e.g., 18,000 km detours and 370 ms extra delays in ).
Impact on Routing: Frequent Routing Updates
The inconsistent locations in addressing further impact the network routing.
As space and terrestrial infrastructure nodes physically move fast, the logical routing in cyberspace expires frequently. It must be updated frequently, thus threatening various routing schemes:
Distributed routing: Repetitive re-convergence.In distributed routing, network nodes distribute topology information to others, locally compute forwarding tables,
and eventually reach a global consensus on routing paths (i.e., convergence). Before global routing convergence, there
is no guaranteed network reachability. With high mobility, each LEO satellite can only offer very short-lived access
for a ground station(<=3 minutes in Starlink). Frequent topology updates cause repetitive routing re-convergence and thus
low network usability.
For intra-domain routing (e.g., OSPF, IS-IS),
most mega-constellations suffer from low network usability.
Even the the size of constellation is samll, the network needs more than fifty seconds to converge after each handoff while using OSPF.
For inter-domain routing (e.g., BGP), and show frequent logical topology changes cause BGP re-peering, thus sharpening the instability of
global Internet routing.
Centralized routing: Repetitive global updates.
In the centralized routing, a ground station predicts the temporal evolution of topology based on satellites' orbital patterns, divides it into a series of semi-static topology snapshots, schedules the forthcoming global routing tables for each snapshot, and remotely updates the routing tables to all satellites (e.g., via SDN, MPLS, or SRv6).
LEO mega-constellations pose stresses on centralized routing's scalability, stability, and complexity.
Due to frequent link churns, the topology snapshots and FIBs explode with more satellites, ground stations, and faster mobility.
Moreover, every satellite should locally load these new FIBs upon snapshot changes, which is vulnerable to transient global routing inconsistencies and thus black holes or loops.
Requirements of Addressing in Space-Terrestrial NetworkExcept from the basic properties like clusterability, network addressing in space-terrestrial network should
also meet the following requirements: Uniqueness
In integrated space-terrestrial networks, each user's address should be globally unique.
This property calls for address allocation and duplicate address detection mechanisms.
StabilityThis property ensures that the addressing of each node does not change with the movement of users or satellites. With this property, location-based
routing will be more stable, avoiding routing convergence caused by the high dynamics of integrated space-terrestrial network.
It also reduces the frequency of network addresses updates and reduces the impact on users' network services.LocalityFor any two users or satellites, this property ensures that if their addresses are closer, their actual physical distances should be closer.
It not only guarantees the unified logical and physical locations, but also simplifies the design and implementation of location-based routing.ScalabilityThe addressing of integrated space-terrestrial network should be designed to accommodate as many satellites and terrestrial
nodes as possible. What's more, it should scale to more satellites and terrestrial users if needed.EfficiencyIn order to avoid frequent addresses updates, the design of addressing in space-terrestrial network should not
require static address binding to remote gateways (e.g., carrier-grade NAT). The addressing method should ensure consistent
cyber-physical locations, thus easing physically shortest paths without detours.Backward Compatibility with Terrestrial Internet
To ensure seamless expansion of the global Internet ecosystem, the addressing in space-terrestrial network should be backward compatible with terrestrial Internet. It should be compatible the standard IPv6 addressing formats, and facilitates inter-networking to external networks without modifying terrestrial infrastructure. For backward compatibility with IPv4, we recommend adopting 4over6 transition for integrated space-terrestrial networks.
IANA ConsiderationsThis memo includes no request to IANA.Security ConsiderationsThe present memo does not introduce any new technology and/or mechanism and as such does not introduce any security
threat to the TCP/IP protocol suite.ReferencesNormative ReferencesA Border Gateway Protocol 4 (BGP-4)This document discusses the Border Gateway Protocol (BGP), which is an inter-Autonomous System routing protocol.OSPF Version 2This memo documents version 2 of the OSPF protocol. OSPF is a link- state routing protocol. [STANDARDS-TRACK]Internet ProtocolOSI IS-IS Intra-domain Routing ProtocolThis RFC is a republication of ISO DP 10589 as a service to the Internet community. This is not an Internet standard.IPv6 Segment Routing Header (SRH)Segment Routing can be applied to the IPv6 data plane using a new type of Routing Extension Header called the Segment Routing Header (SRH). This document describes the SRH and how it is used by nodes that are Segment Routing (SR) capable.Software-Defined Networking (SDN): Layers and Architecture TerminologySoftware-Defined Networking (SDN) refers to a new approach for network programmability, that is, the capacity to initialize, control, change, and manage network behavior dynamically via open interfaces. SDN emphasizes the role of software in running networks through the introduction of an abstraction for the data forwarding plane and, by doing so, separates it from the control plane. This separation allows faster innovation cycles at both planes as experience has already shown. However, there is increasing confusion as to what exactly SDN is, what the layer structure is in an SDN architecture, and how layers interface with each other. This document, a product of the IRTF Software-Defined Networking Research Group (SDNRG), addresses these questions and provides a concise reference for the SDN research community based on relevant peer-reviewed literature, the RFC series, and relevant documents by other standards organizations.Multiprotocol Label Switching ArchitectureThis document specifies the architecture for Multiprotocol Label Switching (MPLS). [STANDARDS-TRACK]Informative ReferencesSpaceX StarlinkAmazon receives FCC approval for project Kuiper satellite constellation.OneWeb constellationDelay is Not an Option: Low Latency Routing in SpaceGearing up for the 21st century space raceNetwork Topology Design at 27,000 km/hourPetition of Starlink Services, LLC for Designation as an Eligible Telecommunication CarrierAmazon KuiperGEO-Mobile Radio InterfaceETSI. TS 101 376-1-3: GEO-Mobile Radio Interface Specifications; Part 1: General specifications; Sub-part 3: General System DescriptionBroadband Global Area Network (BGAN)ETSI. TS 102 744-3-6: Satellite Earth Stations and Systems (SES); Part 3: Control Plane and User Plane Specifications; Sub-part 6: Adaptation Layer OperationSatellite communications. McGraw-Hill EducationStudy on New Radio (NR) to support nonterrestrial networksSolutions for NR to support non-terrestrial networks (NTN)Internet backbones in spaceChina tests inter-satellite links of BeiDou navigation systemWith latest Starlink launch, SpaceX touts 100 Mbps download speeds and "space lasers" (though the system still has a ways to go)Spacex claims to have redesigned its starlink satellites to eliminate casualty risksUsing ground relays for low-latency wide-area routing in megaconstellationsTechnical Specification Group Meeting #91ENetworking in heaven as on earthA Timeslot Division Strategy for Availability in Integrated Satellite and Terrestrial NetworkCooperatively constructing cost-effective content distribution networks upon emerging low earth orbit satellites and cloudsITU. Measuring digital development: Facts and figures 2020, 2020Tesmanian. SpaceX Starlink Gateway Stations Found In The United States and Abroad, 2021Microsoft Azure. New Azure Orbital, ground station as a service, now in preview, 2020Amazon AWS Ground station: Easily control satellites and ingest data with fully managed Ground Station as a Service, 2021ZDNet. SpaceX to put Starlink ground stations in Google data centres, 2021CNBC. Microsoft partners with SpaceX to connect Azure cloud to Musk's Starlink satellite Internet, 2020SpaceX Starlink Ground Station Map, 2021"Internet in Space" for Terrestrial Users via Cyber-Physical Convergence