Problems and Requirements of Satellite Constellation for Internet
draft-lhan-problems-requirements-satellite-net-01

Document Type Active Internet-Draft (individual)
Authors Lin Han  , Richard Li 
Last updated 2021-10-19
Stream (None)
Intended RFC status (None)
Formats pdf htmlized bibtex
Stream Stream state (No stream defined)
Consensus Boilerplate Unknown
RFC Editor Note (None)
IESG IESG state I-D Exists
Telechat date
Responsible AD (None)
Send notices to (None)
Network Working Group                                        L. Han, Ed.
Internet-Draft                                                     R. Li
Intended status: Informational              Futurewei Technologies, Inc.
Expires: 22 April 2022                                   19 October 2021

   Problems and Requirements of Satellite Constellation for Internet
           draft-lhan-problems-requirements-satellite-net-01

Abstract

   This document presents the detailed analysis about the problems and
   requirements of satellite constellation used for Internet.  It starts
   from the satellite orbit basics, coverage calculation, then it
   estimates the time constraints for the communications between
   satellite and ground-station, also between satellites.  How to use
   satellite constellation for Internet is discussed in detail including
   the satellite relay and satellite networking.  The problems and
   requirements of using traditional network technology for satellite
   network integrating with Internet are finally outlined.

Status of This Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at https://datatracker.ietf.org/drafts/current/.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on 22 April 2022.

Copyright Notice

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

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents (https://trustee.ietf.org/
   license-info) in effect on the date of publication of this document.
   Please review these documents carefully, as they describe your rights
   and restrictions with respect to this document.  Code Components

Han & Li                  Expires 22 April 2022                 [Page 1]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

   extracted from this document must include Simplified BSD License text
   as described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
   2.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   3
   3.  Overview  . . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Basics of Satellite Constellation . . . . . . . . . . . . . .   6
     4.1.  Satellite Orbit . . . . . . . . . . . . . . . . . . . . .   6
     4.2.  Coverage of LEO and VLEO Satellites and Minimum Number
           Required  . . . . . . . . . . . . . . . . . . . . . . . .   6
     4.3.  Real Deployment of LEO and VLEO for Satellite Network . .   9
   5.  Communications for Satellite Constellation  . . . . . . . . .  10
     5.1.  Dynamic Ground-station-Satellite Communication  . . . . .  11
     5.2.  Dynamic Inter-satellite Communication . . . . . . . . . .  12
       5.2.1.  Inter-satellite Communication Overview  . . . . . . .  12
       5.2.2.  Satellites on Adjacent Orbit Planes with Same
               Altitude  . . . . . . . . . . . . . . . . . . . . . .  15
       5.2.3.  Satellites on Adjacent Orbit Planes with Different
               Altitude  . . . . . . . . . . . . . . . . . . . . . .  17
   6.  Use Satellite Network for Internet  . . . . . . . . . . . . .  19
   7.  Problems and Requirements for Satellite Constellation for
           Internet  . . . . . . . . . . . . . . . . . . . . . . . .  22
     7.1.  Common Problems and Requirements  . . . . . . . . . . . .  22
     7.2.  Satellite Relay . . . . . . . . . . . . . . . . . . . . .  24
       7.2.1.  One Satellite Relay . . . . . . . . . . . . . . . . .  24
       7.2.2.  Multiple Satellite Relay  . . . . . . . . . . . . . .  26
     7.3.  Satellite Networking  . . . . . . . . . . . . . . . . . .  27
       7.3.1.  L2 or L3 network  . . . . . . . . . . . . . . . . . .  27
       7.3.2.  Inter-satellite-Link Lifetime . . . . . . . . . . . .  28
       7.3.3.  Problems for Traditional Routing Technologies . . . .  28
   8.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  32
   9.  Contributors  . . . . . . . . . . . . . . . . . . . . . . . .  32
   10. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  32
   11. References  . . . . . . . . . . . . . . . . . . . . . . . . .  32
     11.1.  Normative References . . . . . . . . . . . . . . . . . .  32
     11.2.  Informative References . . . . . . . . . . . . . . . . .  33
   Appendix A.  Change Log . . . . . . . . . . . . . . . . . . . . .  35
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  35

Han & Li                  Expires 22 April 2022                 [Page 2]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

1.  Introduction

   Satellite constellation for Internet is emerging.  Even there is no
   constellation network established completely yet at the time of the
   publishing of the draft (June 2021), some basic internet service has
   been provided and has demonstrated competitive quality to traditional
   broadband service.

   This memo will analyze the challenges for satellite network used in
   Internet by traditional routing and switching technologies.  It is
   based on the analysis of the dynamic characters of both ground-
   station-to-satellite and inter-satellite communications and its
   impact to satellite constellation networking.

   The memo also provides visions for the future solution, such as in
   routing and forwarding.

   The memo focuses on the topics about how the satellite network can
   work with Internet.  It does not focus on physical layer technologies
   (wireless, spectrum, laser, mobility, etc.) for satellite
   communication.

2.  Terminology

   LEO               Low Earth Orbit with the altitude from 180 km to
                     2000 km.

   VLEO              Very Low Earth Orbit with the altitude below 450 km

   MEO               Medium Earth Orbit with the altitude from 2000 km
                     to 35786 km

   GEO               Geosynchronous orbit with the altitude 35786 km

   GSO               Geosynchronous satellite on GEO

   ISL               Inter Satellite Link

   ISLL              Inter Satellite Laser Link

   EIRP              Effective isotropic radiated power

   P2MP              Point to Multiple Points

   GS                Ground Station, a device on ground connecting the

Han & Li                  Expires 22 April 2022                 [Page 3]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

                     satellite.  In the document, GS will hypothetically
                     provide L2 and/or L3 functionality in addition to
                     process/send/receive radio wave.  It might be
                     different as the reality that the device to
                     process/send/receive radio wave and the device to
                     provide L2 and/or L3 functionality could be
                     separated.

   SGS               Source ground station.  For a specified flow, a
                     ground station that will send data to a satellite
                     through its uplink.

   DGS               Destination ground station.  For a specified flow,
                     a ground station that is connected to a local
                     network or Internet, it will receive data from a
                     satellite through its downlink and then forward to
                     a local network or Internet.

   PGW               Packet Gateway

   UPF               User Packet Function

   PE router         Provider Edge router

   CE router         Customer Edge router

   P router          Provider router

   LSA               Link-state advertisement

   LSP               Link-State PDUs

   L1                Layer 1, or Physical Layer in OSI model [OSI-Model]

   L2                Layer 2, or Data Link Layer in OSI model
                     [OSI-Model]

   L3                Layer 3, or Network Layer in OSI model [OSI-Model],
                     it is also called IP layer in TCP/IP model

   BGP               Border Gateway Protocol [RFC4271]

   eBGP              External Border Gateway Protocol, two BGP peers
                     have different Autonomous Number

   iBGP              Internal Border Gateway Protocol, two BGP peers
                     have same Autonomous Number

Han & Li                  Expires 22 April 2022                 [Page 4]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

   IGP               Interior gateway protocol, examples of IGPs include
                     Open Shortest Path First (OSPF [RFC2328]), Routing
                     Information Protocol (RIP [RFC2453]), Intermediate
                     System to Intermediate System (IS-IS [RFC7142]) and
                     Enhanced Interior Gateway Routing Protocol (EIGRP
                     [RFC7868]).

3.  Overview

   The traditional satellite communication system is composed of few GSO
   and ground stations.  For this system, each GSO can cover 42% Earth's
   surface [GEO-Coverage], so as few as three GSO can provide the global
   coverage theoretically.  With so huge coverage, GSO only needs to
   amplify signals received from uplink of one ground station and relay
   to the downlink of another ground station.  There is no inter-
   satellite communications needed.  Also, since the GSO is stationary
   to the ground station, there is no mobility issue involved.

   Recently, more and more LEO and VLEO satellites have been launched,
   they attract attentions due to their advantages over GSO and MEO in
   terms of higher bandwidth, lower cost in satellite, launching, ground
   station, etc.  Some organizations [ITU-6G][Surrey-6G][Nttdocomo-6G]
   have proposed the non-terrestrial network using LEO, VLEO as
   important parts for 6G to extend the coverage of Internet.  SpaceX
   has started to build the satellite constellation called StarLink that
   will deploy over 10 thousand LEO and VLEO satellites finally
   [StarLink].  China also started to request the spectrum from ITU to
   establish a constellation that has 12992 satellites
   [China-constellation].  European Space Agency (ESA) has proposed
   "Fiber in the sky" initiative to connect satellites with fiber
   network on Earth [ESA-HydRON].

   When satellites on MEO, LEO and VLEO are deployed, the communication
   problem becomes more complicated than for GSO.  This is because the
   altitude of MEO/LEO/VLEO satellites are much lower.  As a result, the
   coverage of each satellite is much smaller than for GSO, and the
   satellite is not relatively stationary to the ground.  This will lead
   to:

   1.  More satellites than GSO are needed to provide the global
       coverage.  Section 4.2 will analyze the coverage area, and the
       minimum number of satellites required to cover the earth surface.

   2.  The point-to-point communication between satellite and ground
       station will not be static.  Mobility issue has to be considered.
       Detailed analysis will be done in Section 5.1.

Han & Li                  Expires 22 April 2022                 [Page 5]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

   3.  The inter-satellite communication is needed, and all satellites
       need to form a network. details are described in Section 5.2.

   In addition to above context, Section 7 will address the problem and
   requirements when satellite constellation is joining Internet.

   As the 1st satellite constellation company in history, the SpaceX/
   StarLink will be inevitably mentioned in the draft.  But it must be
   noted that all information about SpaceX/StarLink in the draft are
   from public.  Authors of the draft have no relationship or relevant
   inside knowledge of SpaceX/Starlink.

4.  Basics of Satellite Constellation

   This section will introduce some basics for satellite such as orbit
   parameters, coverage estimation, minimum number of satellite and
   orbit plane required, real deployments.

4.1.  Satellite Orbit

   The orbit of a satellite can be either circular or ecliptic, it can
   be described by following Keplerian elements [KeplerianElement]:

   1.  Inclination (i)

   2.  Longitude of the ascending node (Omega)

   3.  Eccentricity (e)

   4.  Semimajor axis (a)

   5.  Argument of periapsis (omega)

   6.  True anomaly (nu)

   For a circular orbit, two parameters, Inclination and Longitude of
   the ascending node, will be enough to describe the orbit.

4.2.  Coverage of LEO and VLEO Satellites and Minimum Number Required

   The coverage of a satellite is determined by many physical factors,
   such as spectrum, transmitter power, the antenna size, the altitude
   of satellite, the air condition, the sensitivity of receiver, etc.
   EIRP could be used to measure the real power distribution for
   coverage.  It is not deterministic due to too many variants in a real
   environment.  The alternative method is to use the minimum elevation
   angle from user terminals or gateways to a satellite.  This is easier
   and more deterministic.  [SpaceX-Non-GEO] has suggested originally

Han & Li                  Expires 22 April 2022                 [Page 6]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

   the minimum elevation angle of 35 degrees and deduced the radius of
   the coverage area is about 435km and 1230km for VLEO (altitude
   335.9km) and LEO (altitude 1150km) respectively.  The details about
   how the coverage is calculated from the satellite elevation angle can
   be found in [Satellite-coverage].

   Using this method to estimate the coverage, we can also estimate the
   minimum number of satellites required to cover the earth surface.

   It must be noted, SpaceX has recently reduced the required minimum
   elevation angle from 35 degrees to 25 degrees.  The following
   analysis still use 35 degrees.

   Assume there is multiple orbit planes with the equal angular interval
   across the earth surface (The Longitude of the ascending node for
   sequential orbit plane is increasing with a same angular interval).
   Each orbit plane will have:

   1.  The same altitude.

   2.  The same inclination of 90 degree.

   3.  The same number of satellites.

   With such deployment, all orbit planes will meet at north and south
   pole.  The density of satellite is not equal.  Satellite is more
   dense in the space above the polar area than in the space above the
   equator area.  Below estimations are made in the worst covered area,
   or the area of equator where the satellite density is the minimum.

   Figure 1 illustrates the coverage area on equator area, and each
   satellite will cover one hexagon area.  The figure is based on plane
   geometry instead of spherical geometry for simplification, so, the
   orbit is parallel approximately.

   Figure 2 shows how to calculate the radius (Rc) of coverage area from
   the satellite altitude (As) and the elevation angle (b).

Han & Li                  Expires 22 April 2022                 [Page 7]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

                                   x
                        |          |
                        |          |
                        x      *********
                        |     *    |    *
                        |    *     |     *
                    *********      x      *
                   *    |    *     |     *
                  *     |     *    |    *
                 *      x      *********
                  *     |     *    |
                   *    |    *     |
                    *********      x
                        |          |
                        |       orbit 2          ^ north
                        x                        |
                        |                        |
                        |                        |
                      orbit 1                    +-------> east

                   Figure 1: Satellite coverage on ground

                           |<---  2*Rc --->|

                                   + Satellite
                                  /|
                                 / |
                                /  |
                               / b |
                              /-\  +
                             /   * |     __Earth surface
                            /  *   |    /
                           / *_----+----__
                           +               +
                            *             *
                             *           *
                              *   2*a   *
                               *  ___  *
                                *-   -*
                                 *   *
                                  * *
                                   * Earth center

                  Figure 2: Satellite coverage estimation

   x   The vertical projection of satellite to Earth

   Re  The radius of the Earth, Re=6378(km)

Han & Li                  Expires 22 April 2022                 [Page 8]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

   As  The altitude of a satellite

   Rc  The radius (arc length) of the coverage, or, the arc length of
       hexagon center to its 6 vertices.  Rc=Re*(a*pi)/180

   a   The cap angle for the coverage area (the RC arc).  a =
       arccos((Re/(Re+As))*cos(b))-b.

   b   The least elevation angle that a ground station or a terminal can
       communicate with a satellite, b = 35 degree.

   Ns  The minimum number of satellites on one orbit plane, it is equal
       to the number of the satellite's vertical projection on Earth,
       so, Ns = 180/(a*cos(30))

   No  The minimum number of orbit (with same inclination), it is equal
       to the number of the satellite orbit's vertical projection, so,
       No = 360/(a*(1+sin(30)))

   For a example of two type of satellite LEO and VEO, the coverages are
   calculated as in Table 1:

             +============+=======+=======+========+========+
             | Parameters | VLEO1 | VLEO2 |  LEO1  |  LEO2  |
             +============+=======+=======+========+========+
             |   As(km)   | 335.9 |  450  |  1100  |  1150  |
             +------------+-------+-------+--------+--------+
             | a(degree)  | 3.907 | 5.078 | 10.681 | 11.051 |
             +------------+-------+-------+--------+--------+
             |   Rc(km)   |  435  |  565  |  1189  |  1230  |
             +------------+-------+-------+--------+--------+
             |     Ns     |   54  |   41  |   20   |   19   |
             +------------+-------+-------+--------+--------+
             |     No     |   62  |   48  |   23   |   22   |
             +------------+-------+-------+--------+--------+

                Table 1: Satellite coverage estimation for
                          LEO and VLEO examples

4.3.  Real Deployment of LEO and VLEO for Satellite Network

   Obviously, the above orbit parameter setup is not optimal since the
   sky in the polar areas will have the highest density of satellite.

Han & Li                  Expires 22 April 2022                 [Page 9]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

   In the real deployment, to provide better coverage for the areas with
   denser population, to get redundance and better signal quality, and
   to make the satellite distance within the range of inter-satellite
   communication (2000km [Laser-communication-range]), more than the
   minimum number of satellites are launched.  For example, different
   orbit planes with different inclination/altitude are used.

   Normally, all satellites are grouped by orbit planes, each group has
   a number of orbit planes and each orbit plane has the same orbit
   parameters, so, each orbit in the same group will have:

   1.  The same altitude

   2.  The same inclination, but the inclination is less than 90
       degrees.  This will result in the empty coverage for polar areas
       and better coverage in other areas.  See the orbit picture for
       phrase 1 for [StarLink].

   3.  The same number of satellites

   4.  The same moving direction for all satellites

   The proposed deployment of SpaceX can be seen in [SpaceX-Non-GEO] for
   StarLink.

   The China constellation deployment and orbit parameters can be seen
   in [China-constellation].

5.  Communications for Satellite Constellation

   Unlike the communication on ground, the communication for satellite
   constellation is much more complicated.  There are two mobility
   aspects, one is between ground-station and satellite, another is
   between satellites.

   In the traditional mobility communication system, only terminal is
   moving, the mobile core network including base station, front haul
   and back haul are static, thus an anchor point, i.e., PGW in 4G or
   UPF in 5G, can be selected for the control of mobility session.
   Unfortunately, when satellite constellation joins the static network
   system of Internet on ground, there is no such anchor point can be
   selected since the whole satellite constellation network is moving.

   Another special aspect that can impact the communication is that the
   fast moving speed of satellite will cause frequent changes of
   communication peers and link states, this will make big challenges to
   the network side for the packet routing and delivery, session control
   and management, etc.

Han & Li                  Expires 22 April 2022                [Page 10]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

5.1.  Dynamic Ground-station-Satellite Communication

   All satellites are moving and will lead to the communication between
   ground station and satellite can only last a certain period of time.
   This will greatly impact the technologies for the satellite
   networking.  Below illustrates the approximate speed and the time for
   a satellite to pass through its covered area.

   In Table 2, VLEO1 and LEO3 have the lowest and highest altitude
   respectively, VLEO2 is for the highest altitude for VLEO.  We can see
   that longest communication time of ground-station-satellite is less
   than 400 seconds, the longest communication time for VLEO ground-
   station-satellite is less than 140 seconds.

   The "longest communication time" is for the scenario that the
   satellite will fly over the receiver ground station exactly above the
   head, or the ground station will be on the diameter line of satellite
   coverage circular area, see Figure 1.

   Re  The radius of the Earth, Re=6378(km)

   As  The altitude of a satellite

   AL  The arc length(in km) of two neighbor satellite on the same orbit
       plane, AL=2*cos(30)*(Re+As)*(a*pi)/180

   SD  The space distance(in km) of two neighbor satellite on the same
       orbir plane, SD=2*(Re+As)*sin(AL/(2*(Re+As))).

   V   the velocity (in m/s) of satellite, V=sqrt(G*M/(Re+As))

   G   Gravitational constant, G=6.674*10^(-11)(m^3/(kg*s^2))

   M   Mass of Earth, M=5.965*10^24 (kg)

   T   The time (in second) for a satellite to pass through its cover
       area, or, the time for the station-satellite communication.  T=
       ALs/V

Han & Li                  Expires 22 April 2022                [Page 11]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

        +============+=======+========+========+========+========+
        | Parameters | VLEO1 | VLEO2  |  LEO1  |  LEO2  |  LEO3  |
        +============+=======+========+========+========+========+
        |   As(km)   | 335.9 |  450   |  1100  |  1150  |  1325  |
        +------------+-------+--------+--------+--------+--------+
        | a(degree)  | 3.907 | 5.078  | 10.681 | 11.051 | 12.293 |
        +------------+-------+--------+--------+--------+--------+
        |   AL(km)   |  793  |  1048  |  2415  |  2515  |  2863  |
        +------------+-------+--------+--------+--------+--------+
        |   SD(km)   | 792.5 | 1047.2 |  2404  | 2503.2 | 2846.1 |
        +------------+-------+--------+--------+--------+--------+
        |  V(km/s)   |  7.7  | 7.636  | 7.296  | 7.272  | 7.189  |
        +------------+-------+--------+--------+--------+--------+
        |    T(s)    |  103  |  137   |  331   |  346   |  398   |
        +------------+-------+--------+--------+--------+--------+

            Table 2: The time for the ground-station-satellite
                              communication

5.2.  Dynamic Inter-satellite Communication

5.2.1.  Inter-satellite Communication Overview

   In order to form a network by satellites, there must be an inter-
   satellite communication.  Traditionally, inter-satellite
   communication uses the microwave technology, but it has following
   disadvantages:

   1.  Bandwidth is limited and only up to 600M bps
       [Microwave-vs-Laser-communication].

   2.  Security is a concern since the microwave beam is relatively wide
       and it is easy for 3rd party to sniff or attack.

   3.  Big antenna size.

   4.  Power consumption is high.

   5.  High cost per bps.

   Recently, laser is used for the inter-satellite communication, it has
   following advantages, and will be the future for inter-satellite
   communication.

   1.  Higher bandwidth and can be up to 10G bps
       [Microwave-vs-Laser-communication].

Han & Li                  Expires 22 April 2022                [Page 12]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

   2.  Better security since the laser beam size is much narrower than
       microwave, it is harder for sniffing.

   3.  The size of optical lens for laser is much smaller than
       microwave's antenna size.

   4.  Power saving compared with microwave.

   5.  Lower cost per bps.

   The range for satellite-to-satellite communications has been
   estimated to be approximately 2,000 km currently
   [Laser-communication-range].

   From Table 2, we can see the Space Distance (SD) for some LEO
   (altitude over 1100km) are exceeding the celling of the range of
   laser communication, so, the satellite and orbit density for LEO need
   to be higher than the estimation values in the Table 1.

   Assume the laser communication is used for inter-satellite
   communication, then we can analyze the lifetime of inter-satellite
   communication when satellites are moving.  The Figure 3 illustrates
   the movement and relative position of satellites on three orbits.
   The inclination of orbit planes is 90 degrees.

                                     + North pole
                                    /|\
                                   | s |
                                  s  |  s
                                 /   s   \
                                 s   |   s
                                 |   s1  |
                                 s4  |   s6
                                 |   s2  |   -------- Equator
                                 s5  |   s7
                                 |   s3  |
                                 s   |   s
                                 \   s   /
                                  s  |  s
                                   | s |
                                    \|/
                                     +  South pole

                        Figure 3: Satellite movement

   There are four scenarios:

Han & Li                  Expires 22 April 2022                [Page 13]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

   1.  For satellites within the same orbit
       The satellites in the same orbit will move to the same direction
       with the same speed, thus the interval between satellites is
       relatively steady.  Each satellite can communicate with its front
       and back neighbor satellite as long as satellite's orbit is
       maintained in its life cycle.  For example, in Figure 3, s2 can
       communication with s1 and s3.

   2.  For satellites between neighbor orbits in the same group at
   non-polar areas
       The orbits for the same group will share the same orbit altitude
       and inclination.  So, the satellite speed in different orbit are
       also same, but the moving direction may be same or different.
       Figure 4 illustrates this scenario.  When the moving direction is
       the same, it is similar to the scenario 1, the relative position
       of satellites in different orbit are relatively steady as long as
       satellite's orbit is maintained in its life cycle.  When the
       moving direction is different, the relative position of
       satellites in different orbit are un-steady, this scenario will
       be analyzed in more details in Section 5.2.2.

   3.  For satellites between neighbor orbits in the same group at
   polar areas
       For satellites between neighbor orbits with the same speed and
       moving direction, the relative position is steady as described in
       #2 above, but the steady position is only valid at areas other
       than polar area.  When satellites meet in the polar area, the
       relative position will change dramatically.  Figure 5 shows two
       satellites meet in polar area and their ISL facing will be
       swapped.  So, if the range of laser pointing angle is 360 degrees
       and tracking technology supports, the ISL will not be flipping
       after passing polar area; Otherwise, the link will be flipping
       and inter-satellite communication will be interrupted.

   4.  For satellites between different orbits in the different group
       The orbits for the different group will have different orbit
       altitude, inclination and speed.  So, the relative position of
       satellite is not static.  The inter-satellite communication can
       only last for a while when the distance between two satellite is
       within the limit of inter-satellite communication, that is 2000km
       for laser [Laser-communication-range], this scenario will be
       analyzed in more details in Section 5.2.3

Han & Li                  Expires 22 April 2022                [Page 14]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

                   i+N/2         i+1+N/2       i+2+N/2
                 / \           / \           / \
                /   \         /   \         /   \
               S1    \       S2    \       S3    \
              /       S4    /      S5     /      S6
             /         \   /         \   /         \
            /           \ /           \ /           \
            i-1           i             i+1

       * The total number of orbit planes are N
       * The number (i-1, i, i+1,...) represents the Orbit index
       * The bottom numbers (i-1, i, i+1) are for orbit planes on
         which satellites (S1, S2, S3) are moving from bottom to up.
       * The top numbers (i+N/2, i+1+N/2, i+2+N/2) are for orbit
         planes on which satellites (S4, S5, S6) are moving from up
         to bottom.

      Figure 4: Two satellites with same altitude and inclination (i)
                   move in the same or opposite direction

                   \      /
                    P3   P4
                     \  /
                      \/
                      /\
                     /  \
                    P1   P2
                   /      \

      * Two satellites S1 and S2 are at position P1 and P2 at time T1
      * S1's right facing ISL connected to S2's left facing ISL
      * S1 and S2 move to the position P4 and P3 at time T2
      * S1's left facing ISL connected to S2's right facing ISL

       Figure 5: Two satellites meeting in the polar area will change
                             its facing of ISL

5.2.2.  Satellites on Adjacent Orbit Planes with Same Altitude

   For satellites on different orbit planes with same altitude, the
   estimation of the lifetime when two satellite can communicate are as
   follows.

   Figure 6 illustrates a general case that two satellites move and
   intersect with an angle A.

Han & Li                  Expires 22 April 2022                [Page 15]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

                                          ^ V2
                                         /
                                        /
                                       +-
                                      /  \ A
                        -------------+----+----> V1
                                    /
                                   /

      Figure 6: Two satellites (speed vector V1 and V2) intersect with
                                  angle A

   More specifically, for orbit planes with the inclination angle i,
   Figure 7 illustrates two satellites move in the opposite direction
   and intersect with an angle 2*i.

                                   ^ move from south to north
                           \      /
                            \    /
                             \  /\
                              \/  | A = 2*i
                              /\  |
                             /  \/
                            /    \
                           /      V move from north to south

      Figure 7: Two satellites with same altitude and inclination (i)
                         intersect with angle A=2*i

   Follows are the math to calculate the lifetime of communication.
   Table 3 are the results using the math for two satellites with
   different altitudes and different inclination angles.

   Dl  The laser communication limit, Dl=2000km
       [Laser-communication-range]

   A   The angle between two orbit's vertical projection on Earth.
       A=2*i

   V1  The speed vector of satellite on orbit1

   V2  The speed vector of satellite on orbit2

   |V|  the magnitude of the difference of two speed vector V1 and
       V2, |V|=|V1-V2|=sqrt((V1-V2*cos(A))^2+(V2*sin(A))^2).  For
       satellites with the same altitude and inclination angle i, V1=V2,
       so, |V|=V1*sqrt(2-2*cos(2*i))=2V1*sin(i)

Han & Li                  Expires 22 April 2022                [Page 16]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

   T   The lifetime two satellites can communicate, or the time of two
       satellites' distance is within the range of communication, T =
       2*Dl/|V|.

       +============+=======+=======+=======+======+=======+=======+
       | i (degree) |   80  |   80  |   65  |  65  |   50  |   50  |
       +============+=======+=======+=======+======+=======+=======+
       |  Alt (km)  |  500  |  800  |  500  | 800  |  500  |  800  |
       +============+=======+=======+=======+======+=======+=======+
       | |V| (km/s) | 14.98 | 14.67 | 13.79 | 13.5 | 11.66 | 11.41 |
       +------------+-------+-------+-------+------+-------+-------+
       |    T(s)    |  267  |  273  |  290  | 296  |  343  |  350  |
       +------------+-------+-------+-------+------+-------+-------+

         Table 3: The lifetime of communication for two LEOs (with
                two altitudes and three inclination angles)

5.2.3.  Satellites on Adjacent Orbit Planes with Different Altitude

   For satellites on different orbit planes with different altitude, the
   estimation of the lifetime when two satellite can communicate are as
   follows.

   Figure 8 illustrates two satellites (with the altitude difference Da)
   move and intersect with an angle A.

                                        ^ V2
                                       /
                                      /
                              -------+  /
                              Da    /| +-
                                   / |/  \ A
                        ----------/--+----+----> V1
                                 /  /
                                   /
                                  /
                                 /

      Figure 8: Satellite (speed vector V1 and V2, Altitude difference
                        Da) intersects with Angle A

   Follows are the math to calculate the lifetime of communication

   Dl  The laser communication limit, Dl=2000km
       [Laser-communication-range]

   Da  Altitude difference (in km) for two orbit planes

Han & Li                  Expires 22 April 2022                [Page 17]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

   A   The angle between two orbit's vertical projection on Earth

   Vl  The speed vector of satellite on orbit 1

   V2  The speed vector of satellite on orbit 2

   |V|  the magnitude of the difference of two speed vector V1 and
       V2, |v|=|V1-V2|=sqrt((V1-V2*cos(A))^2+(V2*sin(A))^2)

   T   The lifetime two satellites can communicate, or the time of two
       satellites' distance is within the range of communication, T =
       2*sqrt(Dl^2-Da^2)/|V|

   Using formulas above, below is the estimation for the life of
   communication of two satellites when they intersect.  Table 4 and
   Table 5 are for two VLEOs with the difference of 114.1km for
   altitude.  (VLEO1 and VLEO2 on Table 2).  Table 6 and Table 7 are for
   two LEOs with the difference of 175km for altitude (LEO2 and LEO3 on
   Table 2).

                      +============+=======+=======+
                      | Parameters | VLEO1 | VLEO2 |
                      +============+=======+=======+
                      |   As(km)   | 335.9 |  450  |
                      +------------+-------+-------+
                      |  V (km/s)  |  7.7  | 7.636 |
                      +------------+-------+-------+

                          Table 4: Two VLEO with
                          different altitude and
                                  speed

     +============+=======+=======+=======+========+========+========+
     | A (degree) |   0   |   10  |   45  |   90   |  135   |  180   |
     +============+=======+=======+=======+========+========+========+
     | |V| (km/s) | 0.065 | 1.338 | 5.869 | 10.844 | 14.169 | 15.336 |
     +------------+-------+-------+-------+--------+--------+--------+
     |    T(s)    | 61810 |  2984 |  680  |  368   |  282   |  260   |
     +------------+-------+-------+-------+--------+--------+--------+

     Table 5: Two VLEO intersects with different angle and the life of
                               communication

Han & Li                  Expires 22 April 2022                [Page 18]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

                      +============+=======+=======+
                      | Parameters |  LEO1 |  LEO2 |
                      +============+=======+=======+
                      |   As(km)   |  1150 |  1325 |
                      +------------+-------+-------+
                      |  V (km/s)  | 7.272 | 7.189 |
                      +------------+-------+-------+

                          Table 6: Two LEO with
                          different altitude and
                                  speed

     +============+=======+=======+=======+========+========+========+
     | A (degree) |   0   |   10  |   45  |   90   |  135   |  180   |
     +============+=======+=======+=======+========+========+========+
     | |V| (km/s) | 0.083 | 1.263 | 5.535 | 10.226 | 13.360 | 14.461 |
     +------------+-------+-------+-------+--------+--------+--------+
     |    T(s)    | 47961 |  3155 |  720  |  390   |  298   |  276   |
     +------------+-------+-------+-------+--------+--------+--------+

       Table 7: Two LEO intersects with different angle and the life
                              of communication

6.  Use Satellite Network for Internet

   Since there is no complete satellite network established yet, all
   following analysis is based on the predictions from the traditional
   GEO communication.  The analysis also learnt how other type of
   network has been used in Internet, such as Broadband access network,
   Mobile access network, Enterprise network and Service Provider
   network.

   As a criteria to be part of Internet, any device connected to any
   satellite should be able to communicate with any public IP4 or IPv6
   address in Internet.  There could be three types of methods to
   deliver IP packet from source to destination by satellite:

   1.  Data packet is relayed between ground station and satellite.
       For this method, there is no inter-satellite communication and
       networking.  Data packet is bounced once or couple times between
       ground stations and satellites until the packet arrives at the
       destination in Internet.

   2.  Data packet is delivered by inter-satellite networking.
       For this method, the data packet traverses with multiple
       satellites and inter-satellite networking is used to deliver the
       packet to the destination in Internet.

Han & Li                  Expires 22 April 2022                [Page 19]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

   3.  Both satellite relay and inter-satellite networking are used.
       For this method, the data packet is relayed in some segments and
       traverse with multiple satellites in other segments.  It is a
       combination of the method 1 and method 2.

   Using the above methods, follows are typical deployment scenarios
   that a Satellite network is integrated with Internet:

   1.  The end user terminal access Internet through satellite relay
       (Figure 9 for one satellite relay, Figure 10 for multiple
       satellite relay).

   2.  The end user terminal access Internet through inter-satellite-
   networking
       (Figure 11).

   3.  The local network access Internet through satellite relay
       (Figure 12 for one satellite relay, Figure 13 for multiple
       satellite relay).

   4.  The local network access Internet through inter-satellite-
   networking
       (Figure 14).

                          S1----\            /-----------\
                         /       \          /             \
                T---GW--GS1--S2--GS2-------PE   Internet   +
                         \       /          \             /
                          \---S3/            \-----------/

     Figure 9: End user terminal access Internet through one satellite
                                   relay

                       S1----\    S4----\       /-----------\
                      /       \  /       \     /             \
             T---GW--GS1--S2--GS2---S5--GS3---PE   Internet   +
                      \       /  \       /     \             /
                       \---S3/    \---S6/       \-----------/

       Figure 10: End user terminal access Internet through multiple
                              satellite relay

                    S1-----S2-----S3--\            /----------\
                   /                   \          /            \
          T---GW--GS1--S4----S5---S6---GS2-------PE  Internet   +
                   \                   /          \            /
                    \---S7----S8----S9/            \----------/

Han & Li                  Expires 22 April 2022                [Page 20]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

        Figure 11: End user terminal access Internet through inter-
                            satellite-networking

            /-----------\           S1----\           /-------\
           /             \         /       \         /         \
          + Local network CE------GS1--S4--GS2-------PE Internet +
           \             /         \       /         \         /
            \-----------/           \---S7/           \-------/

    Figure 12: Local network access Internet through one satellite relay

          /-----------\         S1----\   S4----\       /-------\
         /             \       /       \  /      \     /         \
        + Local network CE----GS1--S2--GS2--S5--GS3---PE Internet +
         \             /       \       / \       /     \         /
          \-----------/         \---S3/   \---S6/       \-------/

         Figure 13: Local network access Internet through multiple
                              satellite relay

      /-----------\          S1-----S2-----S3---\            /------\
     /             \         /                   \          /        \
    + Local network CE------GS1--S4----S5---S6---GS2-------PE Internet+
     \             /         \                   /          \        /
      \-----------/           \---S7----S8----S9/            \------/

     Figure 14: Local network access Internet through inter-satellite-
                                 networking

   In above Figure 9 to Figure 14, the meaning of symbols are as
   follows:

   T               The end user terminal

   GW              Gateway router

   GS1, GS2, GS3   Ground station with L2/L3 routing/switch
                   functionality.

   S1 to S9        Satellites

   PE              Provider Edge Router

   CE              Customer Edge Router

Han & Li                  Expires 22 April 2022                [Page 21]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

7.  Problems and Requirements for Satellite Constellation for Internet

   As described in Section 6, satellites in a satellite constellation
   can either relay internet traffic or multiple satellites can form a
   network to deliver internet traffic.  More detailed analysis are in
   following sub sections.  There might have multiple solutions for each
   method described in Section 6, following contexts only discuss the
   most plausible solution from networking perspectives.

   Section 7.1 will list the common problems and requirements for both
   satellite relay and satellite networking.

   Section 7.2 and Section 7.3 will describe key problems, requirement
   and potential solution from the networking perspective for these two
   cases respectively.

7.1.  Common Problems and Requirements

   For both satellite relay and satellite networking, satellite-ground-
   station must be used, so, the problems and requirements for the
   satellite-ground-station communication is common and will apply for
   both methods.

   When one satellite is communicating with ground station, the
   satellite only needs to receive data from uplink of one ground
   station, process it and then send to the downlink of another ground
   station.  Figure 9 illustrates this case.  Normally microwave is used
   for both links.

   Additionally, from the coverage analysis in Section 4.2 and real
   deployment in Section 4.3, we can see one ground station may
   communicate with multiple satellites.  Similarly, one satellite may
   communicate with multiple ground stations.  The characters for
   satellite-ground-station communication are:

   1.  Satellite-ground-station communication is P2MP.
       Since microwave physically is the carrier of broadcast
       communication, one satellite can send data while multiple ground
       stations can receive it.  Similarly, one ground station can send
       data and multiple satellites can receive it.

   2.  Satellite-ground-station communication is in open space and
   not secure.
       Since electromagnetic fields for microwave physically are
       propagating in open space.  The satellite-ground-station
       communication is also in open space.  It is not secure naturally.

Han & Li                  Expires 22 April 2022                [Page 22]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

   3.  Satellite-ground-station communication is not steady.
       Since the satellite is moving with high speed, from Section 5.1,
       the satellite-ground-station communication can only last a
       certain period of time.  The communication peers will keep
       changing.

   4.  Satellite-to-Satellite communication is not steady.
       For some satellites, even they are in the same altitude and move
       in the same speed, but they move in the opposite direction, from
       Section 5.2.2, the satellite-to-satellite communication can only
       last a certain period of time.  The communication peers will keep
       changing.

   5.  Satellite-to-Satellite distance is not steady.
       For satellites with the same altitude and same moving direction,
       even their relative position is steady, but the distance between
       satellites are not steady.  This will lead to the inter-
       satellite-communication's bandwidth and latency keep changing.

   6.  Satellite physical resource is limited.
       Due to the weight, complexity and cost constraint, the physical
       resource on a satellite, such as power supply, memory, link
       speed, are limited.  It cannot be compared with the similar
       device on ground.  The design and technology used should consider
       these factors and take the appropriate approach if possible.

   The requirements of satellite-ground-station communication are:

   R1.  The bi-directional communication capability
       Both satellites and ground stations have the bi-directional
       communication capability

   R2.  The identifier for satellites and ground stations
       Satellites and ground stations should have Ethernet and/or IP
       address configured for the device and each link.  More detailed
       address configuration can be seen in each solution.

   R3.  The capability to decide where the IP packet is forwarded to.
       In order to send Internet traffic or IP date to destination
       correctly, satellites and ground station must have Ethernet hub
       or switching or IP routing capability.  More detailed capability
       can be seen in each solution.

   R4.  The protocol to establish the satellite-ground-station
   communication.
       For security and management purpose, the satellite-ground-station
       communication is only allowed after both sides agree through a
       protocol.  The protocol should be able to establish a secured

Han & Li                  Expires 22 April 2022                [Page 23]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

       channel for the communication when a new communication peer comes
       up.  Each ground station should be able to establish multiple
       channels to communicate with multiple satellites.  Similarly,
       each satellite should be able to establish multiple channels to
       communicate to multiple ground stations.

   R5.  The protocol to discover the state of communication peer.
       The discover protocol is needed to detect the state of
       communication peer such as peer's identity, the state of the peer
       and other info of the peer.  The protocol must be running
       securely without leaking the discovered info.

   R6.  The internet data packet is forwarded securely.
       When satellite or ground station is sending the IP packet to its
       peer, the packet must be relayed securely without leaking the
       user data.

   R7.  The internet data packet is processed efficiently on
   satellite
       Due to the resource constraint on a satellite, the packet may
       need more efficient mechanism to be processed on satellite.  The
       process on satellite should be very minimal and offloaded to
       ground as much as possible.

7.2.  Satellite Relay

   One of the reasons to use satellite constellation for internet access
   is it can provide shorter latency than using the fiber underground.
   But using ISL for inter-satellite communication is the premise for
   such benefit in latency.  Since the ISL is still not mature and
   adopted commercially, satellite relay is a only choice currently for
   satellite constellation used for internet access.  In
   [UCL-Mark-Handley], detailed simulations have demonstrated better
   latency than fiber network by satellite relay even the ISL is not
   present.

7.2.1.  One Satellite Relay

   One satellite relay is the simplest method for satellite
   constellation to provide Internet service.  By this method, IP
   traffic will be relayed by one satellite to reach the DGS and go to
   Internet.

   The solution option and associated requirements are:

   S1.  The satellite only does L1 relay or the physical signal process.

Han & Li                  Expires 22 April 2022                [Page 24]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

   For this solution, a satellite only receives physical signal, amplify
   it and broadcast to ground stations.  It has no further process for
   packet, such as L2 packet compositing and processing, etc.  All
   packet level work is done only at ground station.  The requirements
   for the solution are:

   R1-1.  SGS and BGS are configured as IP routing node.  Routing
   protocol is running in SGS and BGS
      SGS and BGS is a IP peer for a routing protocol (IGP or BGP).  SGS
      will send internet traffic to DGS as next hop through satellite
      uplink and downlink.

   R1-2.  DGS must be connected with Internet.
      DGS can process received packet from satellite and forward the
      packet to the destination in Internet.

   In addition to the above requirements, following problem should be
   solved:

   P1-1.  IP continuity between two ground stations
      This problem is that two ground stations are connected by one
      satellite relay.  Since the satellite is moving, the IP continuity
      between ground stations is interrupted by satellite changing
      periodically.  Even though this is not killing problem from the
      view point that IP service traditionally is only a best effort
      service, it will benefit the service if the problem can be solved.
      Different approaches may exist, such as using hands off protocols,
      multipath solutions, etc.

   S2.  The satellite does the L2 relay or L2 packet process.

   For this solution, IP packet is passing through individual satellite
   as an L2 capable device.  Unlike in the solution S1, satellite knows
   which ground station it should send based on packet's destination MAC
   address after L2 processing.  The advantage of this solution over S1
   is it can use narrower beam to communicate with DGS and get higher
   bandwidth and better security.  The requirements for the solution
   are:

   R2-1.  Satellite must have L2 bridge or switch capability
      In order to forward packet to properly, satellite should run some
      L2 process such as MAC learning, MAC switching.  The protocol
      running on satellite must consider the fast movement of satellite
      and its impact to protocol convergence, timer configuration, table
      refreshment, etc.

   R2-2. same as R1-1 in S1

Han & Li                  Expires 22 April 2022                [Page 25]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

   R2-3. same as R1-2 in S1

   In addition to the above requirements, the problem P1-1 for S1 should
   also apply.

7.2.2.  Multiple Satellite Relay

   For this method, packet from SGS will be relayed through multiple
   intermediate satellites and ground station until reaching a DGS.

   This is more complicated than one satellite relay described in
   Section 7.2.1.

   One general solution is to configure both satellites and ground-
   stations as IP routing nodes, proper routing protocols are running in
   this network.  The routing protocol will dynamically determine
   forwarding path.  The obvious challenge for this solution is that all
   links between satellite and ground station are not static, according
   to the analysis in Section 5.1, the lifetime of each link may last
   only couple of minutes.  This will result in very quick and constant
   topology changes in both link state and IP adjacency, it will cause
   the distributed routing algorithms may never converge.  So this
   solution is not feasible.

   Another plausible solution is to specify path statically.  The path
   is composed of a serials of intermediate ground stations plus SGS and
   DGS.  This idea will make ground stations static and leave the
   satellites dynamic.  It will reduce the fluctuation of network path,
   thus provide more steady service.  One variant for the solution is
   whether the intermediate ground stations are connected to Internet.
   Separated discussion is as below:

   S1.  Manual configuring routing path and table

   For this solution, the intermediate ground stations and DGS are
   specified and configured manually during the stage of network
   planning and provisioning.  Following requirements apply:

   R1-1.  Specify a path from SGS to DGS via a list of intermediate
   ground stations.
      The specified DGS must be connected with internet.  Other
      specified intermediate ground stations does not have to

   R1-2.  All Ground stations are configured as IP routing node.
      Static routing table on all ground stations must be pre-
      configured, the next hop of routes to Internet destination in any
      ground station is configured to going through uplink of satellite
      to the next ground station until reaching the DGS.

Han & Li                  Expires 22 April 2022                [Page 26]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

   R1-3.  All Satellites are configured as either L1 relay or L2
   relay.
      The Satellite can be configured as L1 relay or L2 relay described
      in S1 and S2 respectively in Section 7.2.1

   In addition to the above requirements, the problem P1-1 in
   Section 7.2.1 should also apply.

   S2.  Automatic decision by routing protocol.

   This solution is only feasible after the IP continuity problem (P1-1
   in Section 7.2.1) is solved.  Following requirements apply:

   R2-1.  All Ground stations are configured as IP routing node.
   Proper routing protocols are configured as well.
      The satellite link cost is configured to be lower than the ground
      link.  In such a way, the next hop of routes for the IP forwarding
      to Internet destination in any ground station will be always going
      through the uplink of satellite to the next ground station until
      reaching the DGS.

   R2-2.  All Satellites are configured as either L1 relay or L2
   relay.
      The Satellite can be configured as L1 relay or L2 relay described
      in S1 and S2 respectively in Section 7.2.1

   In addition to the above requirements, the problem P1-1 in
   Section 7.2.1 should also apply.

7.3.  Satellite Networking

   In the draft, satellite Network is defined as a network that
   satellites are inter-connected by inter-satellite links (ISL).  One
   of the major difference of satellite network with the other type of
   network on ground (telephone, fiber, etc.) is its topology and links
   are not stationary, some new issues have to be considered and solved.
   Follows are the factors that impact the satellite networking.

7.3.1.  L2 or L3 network

   The 1st question to answer is should the satellite network be
   configured as L2 or L3 network?  As analyzed in Section 4.2 and
   Section 4.3, since there are couple of hundred or over ten thousand
   satellites in a network, L2 network is not a good choice, instead, L3
   or IP network is more appropriate for such scale of network.

Han & Li                  Expires 22 April 2022                [Page 27]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

7.3.2.  Inter-satellite-Link Lifetime

   If we assume the orbit is circular and ignore other trivial factors,
   the satellite speed is approximately determined by the orbit altitude
   as described in the Section 5.1.  The satellite orbit can determine
   if the dynamic position of two satellites is within the range of the
   inter-satellite communication.  That is 2000km for laser
   communication [Laser-communication-range] by Inter Satellite Laser
   Link (ISLL).

   When two satellites' orbit planes belong to the same group, or two
   orbit planes share the same altitude and inclination, and when the
   satellites move in the same direction, the relative positions of two
   satellites are relatively stationary, and the inter-satellite
   communication is steady.  But when the satellites move in the
   opposite direction, the relative positions of two satellites are not
   stationary, the communication lifetime is couple of minutes.  The
   Section 5.2.2 has analyzed the scenario.

   When two satellites' orbit planes belong to the different group, or
   two orbit planes have different altitude, the relative position of
   two satellite are unstable, and the inter-satellite communication is
   not steady.  As described in Section 5.2, The life of communication
   for two satellites depends on the following parameters of two
   satellites:

   1.  The speed vectors.

   2.  The altitude difference

   3.  The intersection angle

   From the examples shown in Table 4 to Table 7, we can see that the
   lifetime of inter-satellite communication for the different group of
   orbit planes are from couple of hundred seconds to about 18 hours.
   This fact will impact the routing technologies used for satellite
   network and will be discussed in Section 7.3.3.

7.3.3.  Problems for Traditional Routing Technologies

   When the satellite network is integrated with Internet by traditional
   routing technologies, following provisioning and configuration (see
   Figure 15) will apply:

   1.  The ground stations connected to local network and internet are
       treated as PE router for satellite network (called PE_GS1 and
       PE_GS2 in the following context), and all satellites are treated
       as P router.

Han & Li                  Expires 22 April 2022                [Page 28]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

   2.  All satellites in the network and ground stations are configured
       to run IGP.

   3.  The eBGP is configured between PE_GS and its peered network's PE
       or CE.

   The work on PE_GS1 are:

   *  The local network routes are received at PE_GS1 from CE by eBGP.
      The routes are redistributed to IGP and then IGP flood them to all
      satellites.  (Other more efficient methods, such as iBGP or BGP
      reflectors are hard to be used, since the satellite is moving and
      there is no easy way to configure a full meshed iBGP session for
      all satellites, or configure one satellite as BGP reflector in
      satellite network.)

   *  The internet routes are redistributed from IGP to eBGP running on
      PE_GS1, and eBGP will advertise them to CE.

   The work on PE_GS2 are:

   *  The Internet routes are received at PE_GS2 from PE by eBGP.  The
      routes are redistributed to IGP and then IGP flood them to all
      satellites.  (Similar as in PE_GS1, Other more efficient methods,
      such as iBGP or BGP reflector cannot be used.)

   *  The local network routes are redistributed from IGP to eBGP
      running on PE_GS2, and eBGP will advertise them to Internet.

     /--------\             S1---S2----S3----\               /------\
    /          \            /    IGP domain   \             /        \
   + Local net CE--eBGP--PE_GS1---S4---S5---PE_GS2--eBGP--PE Internet +
    \          /            \                 /             \        /
     \--------/              \---S6---S7---S8/               \------/

    Figure 15: Local access Internet through inter-satellite-networking

   Local access Internet through inter-satellite-networking

   On PE-GS1, due to the fact that IGP link between PE_GS1 and satellite
   is not steady; this will lead to following routing activity:

   1.  When one satellite is connecting with PE_GS1, the satellite and
       PE_GS1 form a IGP adjacency.  IGP starts to exchange the link
       state update.

Han & Li                  Expires 22 April 2022                [Page 29]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

   2.  The local network routes received by eBGP in PE_GS1 from CE are
       redistributed to IGP, and IGP starts to flood link state update
       to all satellites.

   3.  Meanwhile, the Internet routes learnt from IGP in PE_GS1 will be
       redistributed to eBGP. eBGP starts to advertise to CE.

   4.  Every satellite will update its routing table (RIB) and
       forwarding table (FIB) after IGP finishes the SPF algorithm.

   5.  When the satellite is disconnecting with PE-GS1, the IGP
       adjacency between satellite and PE_GS1 is gone.  IGP starts to
       exchange the link state update.

   6.  The routes of local network and satellite network that were
       redistributed to IGP in step 2 will be withdrawn, and IGP starts
       to flood link state update to all satellites.

   7.  Meanwhile, the Internet routes previously redistributed to eBGP
       in step 3 will also be withdrawn. eBGP starts to advertise route
       withdraw to CE.

   8.  Every satellite will update its routing table (RIB) and
       forwarding table (FIB) after the SPF algorithm.

   Similarly on PE_GS2, due to the fact that IGP link between PE_GS2 and
   satellite is not steady; this will lead to following routing
   activity:

   1.  When one satellite is connecting with PE_GS2, the satellite and
       PE_GS2 form a IGP adjacency.  IGP starts to exchange the link
       state update.

   2.  The Internet routes previously received by eBGP in PE_GS2 from PE
       are redistributed to IGP, IGP starts to flood the new link state
       update to all satellites.

   3.  Meanwhile, the routes of local network and satellite network
       learnt from IGP in PE_GS2 will be redistributed to eBGP. eBGP
       starts to advertise to Internet peer PE.

   4.  Every satellite will update its routing table (RIB) and
       forwarding table (FIB) after IGP finishes the SPF algorithm.

   5.  When the satellite is disconnecting with PE-GS2, the IGP
       adjacency between satellite and PE_GS2 is gone.  IGP starts to
       exchange the link state update.

Han & Li                  Expires 22 April 2022                [Page 30]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

   6.  The internet routes previously redistributed to IGP in step 2
       will be withdrawn, and IGP starts to flood link state update to
       all satellites

   7.  Meanwhile, the routes of local network and satellite network
       previously redistributed to eBGP in step 3 will also be
       withdrawn. eBGP starts to advertise route withdraw to PE.

   8.  Every satellite will update its routing table (RIB) and
       forwarding table (FIB) after the SPF algorithm.

   For the analysis of detailed events above, the estimated time
   interval between event 1 and 5 for PE_GS1 and PE_GS2 can use the
   analysis in Section 5.1.  For example, it is about 398s for LEO and
   103s for VLEO.  Within this time interval, the satellite network
   including all satellites and two ground stations must finish the
   works from 1 to 4 for PE_GS1 and PE_GS2.  The normal internet IPv6
   and IPv4 BGP routes size are about 850k v4 routes + 100K v6 routes
   [BGP-Table-Size].  There are couple critical problems associated with
   the events:

   P1.  Frequent IGP update for its link cost
      Even for satellites in different orbit with the steady relative
      positions, the distance between satellites is keep changing.  If
      the distance is used as the link cost, it means the IGP has to
      update the link cost frequently.  This will make IGP keep running
      and update its routing table.

   P2.  Frequent IGP flooding for the internet routes
      Whenever the IGP adjacency changes (step 1 and 5 for PE_GS2), it
      will trigger the massive IGP flooding for the link state update
      for massive internet routes learnt from eBGP.  This will result in
      the IGP re-convergency, RIB and FIP update.

   P3.  Frequent BGP advertisement for the internet routes
      Whenever the IGP adjacency changes (step 3 and 7 for PE_GS1), it
      will trigger the massive BGP advertisement for the internet routes
      learnt from IGP.  This will result in the BGP re-convergency, RIB
      and FIB update.  BGP convergency time is longer than IGP.  The
      document [BGP-Converge-Time1] has shown that the BGP convergence
      time varies from 50sec to couple of hundred seconds.  The analysis
      [BGP-Converge-Time2] indicated that per entry update takes about
      150us, and it takes o(75s) for 500k routes, or o(150s) for 1M
      routes.

   P4.  More frequent IGP flooding and BGP update in whole satellite
   network
      To provide the global coverage, a satellite constellation will

Han & Li                  Expires 22 April 2022                [Page 31]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

      have many ground stations deployed.  For example, StarLink has
      applied for the license for up to one million ground stations
      [StarLink-Ground-Station-Fcc], in which, more than 50 gateway
      ground stations (equivalent to the PE_GS2) have been registered
      [SpaceX-Ground-Station-Fcc] and deployed in U.S.
      [StarLink-GW-GS-map].  It is expected that the gateway ground
      station will grow quickly to couple of thousands
      [Tech-Comparison-LEOs].  This means almost each satellite in the
      satellite network would have a ground station connected. , Due to
      the fact that all satellites are moving, many IGP adjacency
      changes may occur in a shorter period of time described in
      Section 5.1 and result in the problem P1 and P2 constantly occur.

   P5.  Service is not steady
      Due to the problems P1 to P3, the service provider of satellite
      constellation is hard to provide a steady service for broadband
      service by using inter-satellite network and traditional routing
      technologies.

   As a summary, the traditional routing technology is problematic for
   large scale inter-satellite networking for Internet.  Enhancements on
   traditional technologies, or new technologies are expected to solve
   the specific issues associated with satellite networking.

8.  IANA Considerations

   This memo includes no request to IANA.

9.  Contributors

10.  Acknowledgements

11.  References

11.1.  Normative References

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

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

Han & Li                  Expires 22 April 2022                [Page 32]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

   [RFC7142]  Shand, M. and L. Ginsberg, "Reclassification of RFC 1142
              to Historic", RFC 7142, DOI 10.17487/RFC7142, February
              2014, <https://www.rfc-editor.org/info/rfc7142>.

   [RFC2453]  Malkin, G., "RIP Version 2", STD 56, RFC 2453,
              DOI 10.17487/RFC2453, November 1998,
              <https://www.rfc-editor.org/info/rfc2453>.

   [RFC7868]  Savage, D., Ng, J., Moore, S., Slice, D., Paluch, P., and
              R. White, "Cisco's Enhanced Interior Gateway Routing
              Protocol (EIGRP)", RFC 7868, DOI 10.17487/RFC7868, May
              2016, <https://www.rfc-editor.org/info/rfc7868>.

11.2.  Informative References

   [KeplerianElement]
              "Keplerian elements",
              <https://en.wikipedia.org/wiki/Orbital_elements>.

   [GEO-Coverage]
              "Coverage of a geostationary satellite at Earth",
              <https://www.planetary.org/space-images/coverage-of-
              a-geostationary>.

   [Nttdocomo-6G]
              "NTTDPCOM 6G White Paper",
              <https://www.nttdocomo.co.jp/english/binary/pdf/corporate/
              technology/whitepaper_6g/
              DOCOMO_6G_White_PaperEN_20200124.pdf>.

   [ITU-6G]   "ITU 6G vision", <https://www.itu.int/dms_pub/itu-
              s/opb/itujnl/S-ITUJNL-JFETF.V1I1-2020-P09-PDF-E.pdf>.

   [Surrey-6G]
              "Surrey 6G vision",
              <https://www.surrey.ac.uk/sites/default/files/2020-11/6g-
              wireless-a-new-strategic-vision-paper.pdf>.

   [OSI-Model]
              "OSI Model", <https://en.wikipedia.org/wiki/OSI_model>.

   [StarLink] "Star Link", <https://en.wikipedia.org/wiki/Starlink>.

   [China-constellation]
              "China Constellation", <https://www.itu.int/ITU-
              R/space/asreceived/Publication/DisplayPublication/23706>.

Han & Li                  Expires 22 April 2022                [Page 33]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

   [ESA-HydRON]
              "HydRON: Fiber in the sky",
              <https://www.esa.int/ESA_Multimedia/Videos/2021/04/
              HydRON_Fibre_in_the_sky>.

   [SpaceX-Non-GEO]
              "FCC report: SPACEX V-BAND NON-GEOSTATIONARY SATELLITE
              SYSTEM", <https://fcc.report/IBFS/SAT-LOA-
              20170301-00027/1190019.pdf>.

   [Satellite-coverage]
              Alan R.Washburn, Department of Operations Research, Naval
              Postgraduate School, "Earth Coverage by Satellites in
              Circular Orbit",
              <https://faculty.nps.edu/awashburn/Files/Notes/
              EARTHCOV.pdf>.

   [Microwave-vs-Laser-communication]
              International Journal for Research in Applied Science and
              Engineering Technology (IJRASET), "Comparison of Microwave
              and Optical Wireless Inter-Satellite Links",
              <https://www.ijraset.com/fileserve.php?FID=7815>.

   [Laser-communication-range]
              "Interferometric optical communications can potentially
              lead to robust, secure, and naturally encrypted long-
              distance laser communications in space by taking advantage
              of the underlying physics of quantum entanglement.",
              <https://www.laserfocusworld.com/optics/article/16551652/
              interferometry-quantum-entanglement-physics-secures-
              spacetospace-interferometric-communications>.

   [BGP-Table-Size]
              "BGP in 2020 - BGP table",
              <https://blog.apnic.net/2021/01/05/bgp-in-2020-the-bgp-
              table/>.

   [BGP-Converge-Time1]
              "BGP in 2020 - BGP Update Churn",
              <https://labs.apnic.net/?p=1397>.

   [BGP-Converge-Time2]
              "Bringing SDN to the Internet, one exchange point at the
              time",
              <https://www.cs.princeton.edu/courses/archive/fall14/
              cos561/docs/SDX.pdf>.

Han & Li                  Expires 22 April 2022                [Page 34]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

   [StarLink-Ground-Station-Fcc]
              "APPLICATION FOR BLANKET LICENSED EARTH STATIONS",
              <https://fcc.report/IBFS/SES-LIC-INTR2019-00217/1616678>.

   [SpaceX-Ground-Station-Fcc]
              "List of SpaceX applications for ground stations",
              <https://fcc.report/IBFS/Company/Space-Exploration-
              Technologies-Corp-SpaceX>.

   [Tech-Comparison-LEOs]
              "A Technical Comparison of Three Low Earth Orbit Satellite
              Constellation Systems to Provide Global Broadband",
              <http://www.mit.edu/~portillo/files/Comparison-LEO-IAC-
              2018-slides.pdf>.

   [StarLink-GW-GS-map]
              "StarLink gateway ground station map",
              <https://www.google.com/maps/d/u/0/
              viewer?mid=1H1x8jZs8vfjy60TvKgpbYs_grargieVw>.

   [UCL-Mark-Handley]
              "Using ground relays for low-latency wide-area routing in
              megaconstellations",
              <https://discovery.ucl.ac.uk/id/eprint/10090242/1/hotnets-
              ucl.pdf>.

Appendix A.  Change Log

   *  Initial version, 07/03/2021

   *  01 version, 10/20/2021

Authors' Addresses

   Lin Han (editor)
   Futurewei Technologies, Inc.
   2330 Central Expy
   Santa Clara, CA 95050,
   United States of America

   Email: lhan@futurewei.com

Han & Li                  Expires 22 April 2022                [Page 35]
Internet-Draft  Problems, Requirements for Satellite Net    October 2021

   Richard Li
   Futurewei Technologies, Inc.
   2330 Central Expy
   Santa Clara, CA 95050,
   United States of America

   Email: rli@futurewei.com

Han & Li                  Expires 22 April 2022                [Page 36]