Internet DRAFT - draft-draft-li-istn-addressing-requirement

draft-draft-li-istn-addressing-requirement







Internet Engineering Task Force                                    Y. Li
Internet-Draft                                                     H. Li
Intended status: Informational                                    J. Liu
Expires: 28 April 2022                               Tsinghua University
                                                         25 October 2021


Problems and Requirements of Addressing in Integrated Space-Terrestrial
                                Network
                draft-li-istn-addressing-requirement-00

Abstract

   This 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.

Status of This Memo

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   provisions of BCP 78 and BCP 79.

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   This Internet-Draft will expire on 28 April 2022.

Copyright Notice

   Copyright (c) 2021 IETF Trust and the persons identified as the
   document authors.  All rights reserved.




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   This document is subject to BCP 78 and the IETF Trust's Legal
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   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Basics of Space-Terrestrial Network . . . . . . . . . . . . .   3
     3.1.  Space Segment . . . . . . . . . . . . . . . . . . . . . .   4
     3.2.  Ground Segment  . . . . . . . . . . . . . . . . . . . . .   5
     3.3.  Space-Ground Internetworking  . . . . . . . . . . . . . .   6
   4.  Problems in Space-Terrestrial Network Addressing  . . . . . .   7
     4.1.  Unstable Space-Terrestrial Topology . . . . . . . . . . .   7
     4.2.  Inconsistent "Locations" for Space/Terrestrial Nodes  . .   8
     4.3.  Impact on Routing: Frequent Routing Updates . . . . . . .   9
   5.  Requirements of Addressing in Space-Terrestrial Network . . .  10
     5.1.  Uniqueness  . . . . . . . . . . . . . . . . . . . . . . .  10
     5.2.  Stability . . . . . . . . . . . . . . . . . . . . . . . .  10
     5.3.  Locality  . . . . . . . . . . . . . . . . . . . . . . . .  11
     5.4.  Scalability . . . . . . . . . . . . . . . . . . . . . . .  11
     5.5.  Efficiency  . . . . . . . . . . . . . . . . . . . . . . .  11
     5.6.  Backward Compatibility with Terrestrial Internet  . . . .  11
   6.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  11
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  11
   8.  References  . . . . . . . . . . . . . . . . . . . . . . . . .  11
     8.1.  Normative References  . . . . . . . . . . . . . . . . . .  11
     8.2.  Informative References  . . . . . . . . . . . . . . . . .  12
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  16

1.  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 [STARLINK], Kuiper [KUIPER]
   and OneWeb [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[ITU-Measure], developing
   countries, aircraft, or oceans.





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   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.

2.  Terminology

   GSO: 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.

3.  Basics of Space-Terrestrial Network

   As 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.










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3.1.  Space Segment

   Satellites 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 Table 1, 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( [LOWLATENCY-ROUTING-SPACE],
   [SPACE-RACE], [NETWORK-TOPO-DESIGN]).  Due to low coverage for each
   LEO satellite, a mega-constellation is necessary to retain global
   coverage.  Table 2 exemplifies popular LEO mega-costellations in
   operation.  They are enabled by recent advances in satellite
   miniaturization and rocket reusability.


            +-----------+                 +-----------+
            | Satellite |_______ISL_______| Satellite | (Space Segment)
         _ _|___________|              _ _|___________|
            /|           \                /|
           / |            \              / |
          /   (Bent pipe)  \ |          /
         /               _ _\|         /
   +---------+               +------------+
   |  Mobile |               |   Ground   |
   | Station |               |  Station   |            (Ground Segment)
   |_________|               |____________|




       Figure 1: A simplified space-terrestrial network architecture










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                 +============+===============+==========+
                 |   Orbit    | Altitude (km) | RTT (ms) |
                 +======+=====+===============+==========+
                 | GSO  | GEO |     35,786    |   240    |
                 +------+-----+---------------+----------+
                 | NGSO | MEO |  2,000-35,786 |  12-240  |
                 |      +-----+---------------+----------+
                 |      | LEO |   500-2,000   |   2-12   |
                 +------+-----+---------------+----------+

                    Table 1: Differences between GSO and
                                   NGSO.

     +===============+=================+=============+===============+
     | 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      |
     +---------------+-----------------+-------------+---------------+

        Table 2: Low-earth-orbit (LEO) satellite mega-constellations
                               in operation.

3.2.  Ground Segment

   Terrestrial 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 [STARLINK-CGNAT] and Kuiper [KUIPER-CGNAT]) and
   remote satellite controllers (e.g., telemetry, tracking, orbital
   update commands, or centralized routing control).



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3.3.  Space-Ground Internetworking

   Early 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 [GEO-MOBILE-RADIO-INTERFACE] [ETSI-TS-101], 3G BGAN [BGAN]
   [ETSI-TS-102], and DVB-S [SATELLITE-COMMUNICATIONS]), and recently
   adopted by LEO satellites in OneWeb (4G) [ONEWEB] and 5G NTN
   [STUDY-NR-SUPPORT] [SOLUTION-NR-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 Table 3, under current Starlink (no ISLs so far)
   and ground station deployments ([STARLINK-GS-FOUND], [AZURE-GS],
   [AWS-GS], [GOOGLE-DATA-CENTER], [AZURE-CLOUD-STARLINK],
   [STARLINK-GS-MAP]), 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
   [Giuliari20Internet].  To date, inter-satellite links are still under
   early adoption([BEIDOU-TEST], [TheVerge-STARLINK-SPEED]).  The recent
   "burn on re-entry" regulations from FCC also slows down the adoption
   of ISLs[SPACEX-CLAIM].  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[RFC0791] for
   terrestrial users.  The GS-as-relay [USE-GROUND-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.









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   +=============+======+======+=======+=======+======+========+=======+
   |             |Global|Africa|Oceania| South | Asia |European| North |
   |             |      |      |       |America|      |        |America|
   +=============+======+======+=======+=======+======+========+=======+
   |    1-SAT    |48.71%|19.52%| 42.85%| 49.63%|43.49%| 91.00% | 87.50%|
   | assicoation |      |      |       |       |      |        |       |
   +-------------+------+------+-------+-------+------+--------+-------+
   |    2-SAT    |57.30%|24.37%| 56.58%| 53.90%|55.91%| 94.33% | 91.23%|
   | assicoation |      |      |       |       |      |        |       |
   +-------------+------+------+-------+-------+------+--------+-------+
   |    4-SAT    |67.04%|26.13%| 60.31%| 63.16%|71.34%| 95.46% | 95.04%|
   | assicoation |      |      |       |       |      |        |       |
   +-------------+------+------+-------+-------+------+--------+-------+
   |    8-SAT    |73.04%|29.17%| 60.68%| 65.65%|80.28%| 96.91% | 98.86%|
   | assicoation |      |      |       |       |      |        |       |
   +-------------+------+------+-------+-------+------+--------+-------+

        Table 3: Global population that could access Starlink in its
                      current "bent- pipe-only" model.

4.  Problems in Space-Terrestrial Network Addressing

   In 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[INTERNET-IN-SPACE].

4.1.  Unstable Space-Terrestrial Topology

   High 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[INTERNET-IN-SPACE].  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".





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   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 ([STUDY-NR-SUPPORT], [SOLUTION-NR-NTN]), and the latest 3GPP
   5G release 17 defers its mobility support for satellites
   [TEC-SPECI-GROUP-MEETING] due to significant architectural changes.
   While Starlink uses handoffs to migrate physical links between
   satellites and ground stations (every 15s [STARLINK-CGNAT]), its
   logical topology and routing are still be repeatedly updated at high
   costs.

4.2.  Inconsistent "Locations" for Space/Terrestrial Nodes

   Each 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:

   a.  Dynamic address updates

       A 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.  Table 4 and
       Table 5 project the address update frequency when using legacy IP
       addresses[RFC0791] for logical interfaces.  In this scheme, the
       terrestrial users' logical IP[RFC0791] 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[RFC0791] every 133-510s.  Every second, we observe
       2,082-7,961 global users per second should change their IP
       addresses.











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             +============+============+============+============+
             |  Starlink  |  Telesat   |   Kuiper   |  Iridium   |
             +============+============+============+============+
             | Every 133s | Every 510s | Every 510s | Every 458s |
             +------------+------------+------------+------------+

                  Table 4: Frequency of each user's logical IP
                                address update.

                   +==========+=========+========+=========+
                   | Starlink | Telesat | Kuiper | Iridium |
                   +==========+=========+========+=========+
                   |   7961   |   2082  |  5673  |   2379  |
                   +----------+---------+--------+---------+

                      Table 5: Number of terrestrial users
                       that change logical IP address per
                                    second.

   b.  Static address binding to a fixed gateway

       This is adopted by the cellular networks and Starlink
       [STARLINK-CGNAT] and Kuiper's[KUIPER-CGNAT] 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
       [lai2021icnp]).

4.3.  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:

   a.  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



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       cause repetitive routing re-convergence and thus low network
       usability.  For intra-domain routing (e.g., OSPF[RFC2328], IS-
       IS[RFC1142]), 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[TIMESLOT-DIVISION].  For inter-domain
       routing (e.g., BGP[RFC4271]), [Giuliari20Internet] and
       [NETWORK-IN-HEAVEN] show frequent logical topology changes cause
       BGP[RFC4271] re-peering, thus sharpening the instability of
       global Internet routing.

   b.  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[RFC7426], MPLS[RFC3031], or
       SRv6[RFC8754]).  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.

5.  Requirements of Addressing in Space-Terrestrial Network

   Except from the basic properties like clusterability, network
   addressing in space-terrestrial network should also meet the
   following requirements:

5.1.  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.

5.2.  Stability

   This 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.




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5.3.  Locality

   For 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.

5.4.  Scalability

   The 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.

5.5.  Efficiency

   In 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.

5.6.  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.

6.  IANA Considerations

   This memo includes no request to IANA.

7.  Security Considerations

   The 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.

8.  References

8.1.  Normative References





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   [RFC0791]  Postel, J., "Internet Protocol", STD 5, RFC 791,
              DOI 10.17487/RFC0791, September 1981,
              <https://www.rfc-editor.org/info/rfc791>.

   [RFC1142]  Oran, D., "OSI IS-IS Intra-domain Routing Protocol",
              RFC 1142, DOI 10.17487/RFC1142, February 1990,
              <https://www.rfc-editor.org/info/rfc1142>.

   [RFC2328]  Moy, J., "OSPF Version 2", STD 54, RFC 2328,
              DOI 10.17487/RFC2328, April 1998,
              <https://www.rfc-editor.org/info/rfc2328>.

   [RFC3031]  Rosen, E., Viswanathan, A., and R. Callon, "Multiprotocol
              Label Switching Architecture", RFC 3031,
              DOI 10.17487/RFC3031, January 2001,
              <https://www.rfc-editor.org/info/rfc3031>.

   [RFC4271]  Rekhter, Y., Li, T., and S. Hares, "A Border Gateway
              Protocol 4 (BGP-4)", RFC 4271, DOI 10.17487/RFC4271,
              January 2006, <https://www.rfc-editor.org/info/rfc4271>.

   [RFC7426]  Haleplidis, E., Ed., Pentikousis, K., Ed., Denazis, S.,
              Hadi Salim, J., Meyer, D., and O. Koufopavlou, "Software-
              Defined Networking (SDN): Layers and Architecture
              Terminology", RFC 7426, DOI 10.17487/RFC7426, January
              2015, <https://www.rfc-editor.org/info/rfc7426>.

   [RFC8754]  Filsfils, C., Ed., Dukes, D., Ed., Previdi, S., Leddy, J.,
              Matsushima, S., and D. Voyer, "IPv6 Segment Routing Header
              (SRH)", RFC 8754, DOI 10.17487/RFC8754, March 2020,
              <https://www.rfc-editor.org/info/rfc8754>.

8.2.  Informative References

   [AWS-GS]   "Amazon AWS Ground station: Easily control satellites and
              ingest data with fully managed Ground Station as a
              Service, 2021", <https://aws.amazon.com/ground-station/>.

   [AZURE-CLOUD-STARLINK]
              "CNBC. Microsoft partners with SpaceX to connect Azure
              cloud to Musk's Starlink satellite Internet, 2020",
              <https://tinyurl.com/ybcwn7ft>.

   [AZURE-GS] "Microsoft Azure. New Azure Orbital, ground station as a
              service, now in preview, 2020",
              <https://tinyurl.com/ejfpre>.





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Internet-Draft   Problems and Requirements of Addressing    October 2021


   [BEIDOU-TEST]
              "China tests inter-satellite links of BeiDou navigation
              system", <https://tinyurl.com/3cufz6dt>.

   [BGAN]     "Broadband Global Area Network (BGAN)",
              <https://en.wikipedia.org/wiki/
              Broadband_Global_Area_Network>.

   [ETSI-TS-101]
              "ETSI. TS 101 376-1-3: GEO-Mobile Radio Interface
              Specifications; Part 1: General specifications; Sub-part
              3: General System Description",
              <https://en.wikipedia.org/wiki/ETSI>.

   [ETSI-TS-102]
              "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 Operation",
              <https://en.wikipedia.org/wiki/ETSI>.

   [GEO-MOBILE-RADIO-INTERFACE]
              "GEO-Mobile Radio Interface",
              <https://en.wikipedia.org/wiki/GEO-
              Mobile_Radio_Interface>.

   [Giuliari20Internet]
              Giuliari, G., Klenze, T., Legner, M., Basin, D., Perrig,
              A., and A. Singla, "Internet backbones in space",  ACM
              SIGCOMM Computer Communication Review, 2020, 50(1): 25-37,
              <https://dl.acm.org/doi/abs/10.1145/3390251.3390256>.

   [GOOGLE-DATA-CENTER]
              "ZDNet. SpaceX to put Starlink ground stations in Google
              data centres, 2021", <https://tinyurl.com/pzy8vstx>.

   [INTERNET-IN-SPACE]
              Li, Y., Li, H., Liu, L., Liu, W., Liu, J., Wu, J., Wu, Q.,
              Liu, J., and Z. Lai, ""Internet in Space" for Terrestrial
              Users via Cyber-Physical Convergence",  Proceedings of the
              20th ACM Workshop on Hot Topics in Networks. 2021,
              <https://conferences.sigcomm.org/hotnets/2021/>.

   [ITU-Measure]
              "ITU. Measuring digital development: Facts and figures
              2020, 2020".

   [KUIPER]   "Amazon receives FCC approval for project Kuiper satellite
              constellation.", <https://tinyurl.com/bs7syjnk>.



Li, et al.                Expires 28 April 2022                [Page 13]

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   [KUIPER-CGNAT]
              "Amazon Kuiper", <https://eurospace.org/wp-
              content/uploads/2020/11/information-note-amazon-
              kuiper_18112020.pdf>.

   [lai2021icnp]
              Lai, Z., Li, H., Zhang, Q., Wu, Q., and J. Wu,
              "Cooperatively constructing cost-effective content
              distribution networks upon emerging low earth orbit
              satellites and clouds",  2021 IEEE 29th International
              Conference on Network Protocols (ICNP). IEEE, 2021.

   [LOWLATENCY-ROUTING-SPACE]
              Handley, M., "Delay is Not an Option: Low Latency Routing
              in Space",  Proceedings of the 17th ACM Workshop on Hot
              Topics in Networks. 2018,
              <https://dl.acm.org/doi/abs/10.1145/3286062.3286075>.

   [NETWORK-IN-HEAVEN]
              Klenze, T., Giuliari, G., Pappas, C., Perrig, A., and D.
              Basin, "Networking in heaven as on earth",  Proceedings of
              the 17th ACM Workshop on Hot Topics in Networks. 2018:
              22-28,
              <https://dl.acm.org/doi/abs/10.1145/3286062.3286066>.

   [NETWORK-TOPO-DESIGN]
              Bhattacherjee, D. and A. Singla, "Network Topology Design
              at 27,000 km/hour",  Proceedings of the 15th International
              Conference on Emerging Networking Experiments And
              Technologies. 2019,
              <https://dl.acm.org/doi/abs/10.1145/3359989.3365407>.

   [ONEWEB]   "OneWeb constellation", <https://www.oneweb.world/>.

   [SATELLITE-COMMUNICATIONS]
              Roddy, D., "Satellite communications. McGraw-Hill
              Education",
              <https://ui.adsabs.harvard.edu/abs/1977ph...book.....S/
              abstract>.

   [SOLUTION-NR-NTN]
              "Solutions for NR to support non-terrestrial networks
              (NTN)",
              <https://portal.3gpp.org/desktopmodules/Specifications/
              SpecificationDetails.aspx?specificationId=3525>.






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   [SPACE-RACE]
              Bhattacherjee, D., Aqeel, W., Bozkurt, I.N., Aguirre, A.,
              Chandrasekaran, B., Godfrey, P.B., Laughlin, G., Maggs,
              B., and A. Singla, "Gearing up for the 21st century space
              race",  Proceedings of the 17th ACM Workshop on Hot Topics
              in Networks. 2018,
              <https://dl.acm.org/doi/abs/10.1145/3286062.3286079>.

   [SPACEX-CLAIM]
              "Spacex claims to have redesigned its starlink satellites
              to eliminate casualty risks",
              <https://tinyurl.com/yryp2upy>.

   [STARLINK] "SpaceX Starlink", <http://www.starlink.com/>.

   [STARLINK-CGNAT]
              "Petition of Starlink Services, LLC for Designation as an
              Eligible Telecommunication Carrier",
              <https://tinyurl.com/ury6rzw5>.

   [STARLINK-GS-FOUND]
              "Tesmanian. SpaceX Starlink Gateway Stations Found In The
              United States and Abroad, 2021",
              <https://tinyurl.com/4m5uah43>.

   [STARLINK-GS-MAP]
              "SpaceX Starlink Ground Station Map, 2021",
              <https://tinyurl.com/pu59m7j3>.

   [STUDY-NR-SUPPORT]
              "Study on New Radio (NR) to support nonterrestrial
              networks",
              <https://portal.3gpp.org/desktopmodules/Specifications/
              SpecificationDetails.aspx?specificationId=3234>.

   [TEC-SPECI-GROUP-MEETING]
              "Technical Specification Group Meeting #91E",
              <https://www.3gpp.org/ftp/tsg_ran/TSG_RAN/TSGR_91e/Inbox/
              RP-210915.zip>.

   [TheVerge-STARLINK-SPEED]
              "With latest Starlink launch, SpaceX touts 100 Mbps
              download speeds and "space lasers" (though the system
              still has a ways to go)", <https://tinyurl.com/hj8juyun>.

   [TIMESLOT-DIVISION]
              Li, J., Li, H., Liu, J., Lai, Z., Wu, Q., and X. Wang, "A
              Timeslot Division Strategy for Availability in Integrated



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              Satellite and Terrestrial Network",  2021 IEEE Wireless
              Communications and Networking Conference (WCNC). IEEE,
              2021: 1-7, <https://ieeexplore.ieee.org/document/9417256>.

   [USE-GROUND-RELAY]
              Handley, M., "Using ground relays for low-latency wide-
              area routing in megaconstellations",  Proceedings of the
              18th ACM Workshop on Hot Topics in Networks. 2019:
              125-132,
              <https://dl.acm.org/doi/abs/10.1145/3365609.3365859>.

Authors' Addresses

   Yuanjie Li
   Tsinghua University
   Beijing 100084
   China

   Email: yuanjiel@tsinghua.edu.cn


   Hewu Li
   Tsinghua University
   Beijing 100084
   China

   Email: lihewu@cernet.edu.cn


   Jiayi Liu
   Tsinghua University
   Beijing 100084
   China

   Email: liu-jy21@mails.tsinghua.edu.cn
















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