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Satellite Network Routing Use Cases
draft-hou-tvr-satellite-network-usecases-00

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This is an older version of an Internet-Draft whose latest revision state is "Expired".
Authors Hou Dongxu , Fenlin Zhou , Dongyu Yuan , Xiao Min
Last updated 2023-03-12
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draft-hou-tvr-satellite-network-usecases-00
TVR                                                          D. Hou, Ed.
Internet-Draft                                                   F. Zhou
Intended status: Standards Track                                 D. Yuan
Expires: 10 September 2023                                       M. Xiao
                                                         ZTE Corporation
                                                            9 March 2023

                  Satellite Network Routing Use Cases
              draft-hou-tvr-satellite-network-usecases-00

Abstract

   Time-Variant Routing (TVR) is chartered and proposed to solve the
   problem of time-based, scheduled changes, including the variations of
   links, adjacencies, cost, and traffic volumes in some cases.  In a
   satellite network, the network is in continual motion which will
   cause detrimental consequences on the routing issue.  However, each
   network node in a satellite network follows a predefined orbit around
   the Earth and represents an appropriate example of time-based
   scheduled mobility.  Therefore, TVR can be implemented to improve the
   routing and forwarding process in satellite networks.  This document
   mainly focuses on the use cases in this scenario.

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
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   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 10 September 2023.

Copyright Notice

   Copyright (c) 2023 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
   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
   extracted from this document must include Revised BSD License text as
   described in Section 4.e of the Trust Legal Provisions and are
   provided without warranty as described in the Revised BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   5
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   5
   4.  Problem Analysis  . . . . . . . . . . . . . . . . . . . . . .   5
   5.  Solution  . . . . . . . . . . . . . . . . . . . . . . . . . .   6
   6.  Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . .   7
     6.1.  Scenario 1: Predictable changes in connectivities between
           satellites  . . . . . . . . . . . . . . . . . . . . . . .   8
     6.2.  Scenario 2: Predictable property changes of satellite
           links . . . . . . . . . . . . . . . . . . . . . . . . . .  10
   7.  Security Considerations . . . . . . . . . . . . . . . . . . .  13
   8.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  13
   9.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .  13
   10. Normative References  . . . . . . . . . . . . . . . . . . . .  13
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  14

1.  Introduction

   Since the beginning of the 21st century, the satellite network has
   become a significant part of information and communication
   infrastructure.  The large-scale sallite network composed of
   thousands or even tens of thousands of LEO satellites, MEO satellites
   and GEO satellites can overcome the limitations of the conventional
   terrestrial network, achieving global signal coverage, and providing
   large broadband as well as low-latency network services for global
   users.  The global communications ecosystem believes that satellite-
   based communication will become an important part of 5G-advanced and
   6G.

   In a satellite network, satellites move along the orbit, which can be
   divided into circular orbit satellites and elliptical orbit
   satellites.  Different orbits can be described by Keplerian
   parameters, including inclination, longitude of the ascending node,
   eccentricity, semimajor axis, argument of periapsis, true anomaly.
   At present, the mainstream of satellite networks basically adopt
   circular orbit.

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   When links between satellites are established for end-to-end
   communication, each satellite usually has a fixed number of links
   which communicate with neighboring nodes, and considering the cost of
   satellite links and power restrictions of satellites, satellite links
   are generally limited to direct connections between adjacent nodes.
   In a single-layer satellite constellation, each satellite may have
   four types of contiguous neighbour satellites and each type refers to
   a direction.  The number of neighbor satellites distributed in one
   direction is determined by the number of antennas deployed on the
   satellite for communication.  If the satellite contains a single
   antenna in each direction, the connection relationship between the
   satellite N5 and its two satellites in the same orbit and two
   satellites in different adjacent orbits is shown in Figure 1.  N2 and
   N8 are front and rear adjacent satellites in the same orbit plane
   which includes N5.  N4 and N6 are left and right adjacent satellites
   which are adjacent to N5 locate in different orbit planes.  In a
   multi-tier satellite constellation, each satellite may have two
   additional types of adjacent satellites, upper level satellites and
   lower level satellites in different tiers.

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              ^                      ^                      ^
              |                      |                      |
              |                      |                      |
              v                      v                      v
             .--.                   .--.                   .--.
 <---> ####-| N1 |-#### <---> ####-| N2 |-#### <---> ####-| N3 |-#### <--->
             \__/                   \__/                   \__/
              ^                      ^                      ^
              |                      |                      |
              |                      |                      |
              v                      v                      v
             .--.                   .--.                   .--.
 <---> ####-| N4 |-#### <---> ####-| N5 |-#### <---> ####-| N6 |-#### <--->
             \__/                   \__/                   \__/
              ^                      ^                      ^
              |                      |                      |
              |                      |                      |
              v                      v                      v
             .--.                   .--.                   .--.
 <---> ####-| N7 |-#### <---> ####-| N8 |-#### <---> ####-| N9 |-#### <--->
             \__/                   \__/                   \__/
              ^                      ^                      ^
              |                      |                      |             N
              |                      |                      |             ^
              v                      v                      v             |
         Orbit plane 1          Orbit plane 2          Orbit plane 3      |
                                                                          S

                                                                     Moving
                                                                  Direction

               Figure 1: N5 and its adjacent satellites

   The satellite orbit velocity is related to the satellite orbit
   altitude (in a circular orbit), and satellites at the same altitude
   move at the same speed.  Therefore, the relative position between
   satellites in the same orbit plane is stable and the intra-satellite
   links can always be connected, and the link distance is basically
   unchanged, such as N2 and N5.  The relative position between
   satellites in different orbit planes changes dynamically, inter-
   satellite links may be interrrupted due to antenna tracking
   difficulties or limited communication range, and the physical
   distance of the link is constantly altering, such as N4 and N5.

   As one of the indispensable issues for communication, routing
   strategies directly affects the transmission efficiency and the
   quality of network services.  However, due to the particularity of

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   the satellite network, such as the high frequency and intensity
   changes in network topology, the relatively mature terrestrial
   network routing technologies can not be directly applied to the
   satellite network.  In view of the mentioned characteristics, and
   considering the combination of satellite networks and TVR, this
   document includes the following information:

   1.  The core problems of routing issues in satellite networks are
       stated and analyzed.

   2.  This paper discusses the unique time-based predictable network
       information in satellite networks, and proposes a routing
       optimization method based on this information.

   3.  The relevant application scenarios are given and illustrated.

2.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
   "OPTIONAL" in this document are to be interpreted as described in BCP
   14 [RFC2119] [RFC8174] when, and only when, they appear in all
   capitals, as shown here.

3.  Terminology

   *  LEO: Low Earth Orbit.

   *  MEO: Middle Earth Orbit.

   *  GEO: GEostationary Orbit.

   *  Intra-satellite links: Links between adjacent satellites in the
      same orbit.

   *  Intra-satellite links: Links between adjacent satellites in the
      different orbits.

   *  SGP4: Simplified Perturbations Models

4.  Problem Analysis

   The dynamic nature of nodes is the most significant feature of
   satellite networks compared to conventional terrestrial networks.  In
   LEO mega-constellations, this feature becomes more obvious and
   prominent.  Typical phenomena in satellite networks are listed here:

   1.  LEOs move at a relatively high speed for over 7km/s.

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   2.  Half of LEOs in the network move in the same direction which is
       the opposite to the other half.

   3.  A great number of links between satellites or between satellites
       and ground-stations.

   4.  A large part of above links may be interrupted at specific areas.

   5.  All metrics of inter-satellite links are constantly changing.

   6.  All metrics of links between satellites and ground-stations are
       constantly changing.

   Existing routing protocols are designated to maintain contemporaneous
   end-to-end connections across a network.  Once the network topology
   or connection of links changes, corresponding operations and
   procedures are adopted to recover and maintain the reachability
   between various pairs.  Representative procedures in traditional
   protocols may consist of attempting to re-establish lost adjacencies
   ,recalculating or rediscovering a valid path.  The dynamic changes of
   network topologies and links in satellite networks will constantly
   trigger the process of routing re-convergence process with existing
   routing protocols, resulting in routing shocks, which makes it
   inappropriate for existing routing protocols to be directly applied
   in satellite networks.

5.  Solution

   The process of satellite motion along the orbit is periodic and
   predictable.  Predictable information in a satellite network includes
   satellite real-time positions in the space, satellite link
   connectivity, and satellite link real-time metrics.  The satellite-
   ground link also has similar characteristics, which have been
   described in [I-D.birrane-tvr-use-cases] and will not be repeated
   here.

   (1) The real-time position of a satellite is predictable.

   Satellites move around the earth in a predetermined orbit and are
   endowed with a unified and accurate count of time by a ground network
   control center or some specific designated nodes.  Thus, the real-
   time position in the space of a satellite can be predicted in advance
   according to the satellite orbit parameters, the orbit injection
   moment and the satellite operation time.

   (2) The connectivity of satellite links are predictable.

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   Due to the influence of the relative position changes between
   adjacent satellites in different orbits and the restrictions of
   current communication technologies, the inter-satellite link will be
   interrupted when entering a specific area and restored after leaving
   this regions.  According to the satellite orbit parameters, satellite
   operation time, satellite attitude, and the communication range of
   the satellite antenna, combined with some specific algorithms, SGP4
   (Simplified Perturbations Models) for instance, the connectivity of
   satellite links can be predicted in advance.

   (3) The characteristics of satellite links are predictable.

   Affected by the change of relative position between adjacent
   satellites in different orbits, the communication distance between
   satellites is constantly changing.  This distance reaches the largest
   near the equator and declines to the smallest while moving to the
   pole.  The changes in inter-satellite communication distance will
   further lead to the time-varying characteristic of inter-satellite
   links, such as propagation delay and bit error rate.  According to
   satellite orbit parameters, satellite operation time, antenna
   transmission power, space propagation loss and so on, combined with
   proper algorithms, the characteristics of inter-satellite links can
   be predicted in advance.

   As analyzed aboved, the management plane, the control plane and the
   forwarding plane of the network can be adaptively improved by
   utilizing time-based predictable information and combining the
   characteristics of inter satellite and satellite to ground
   transmission conditions, so as to ensure a stable and optimal end-to-
   end reachable path between a pair of satellites, such as:

   (1) By improving the control plane protocol based on the
   predictability of the interruption/recovery of the satellite links,
   on one hand, the flooding of routing convergence information caused
   by network topology changes can be avoided, and on the other hand,
   the routing re-calculation is able to be fulfilled in advance before
   the satellite network topology changes, and thus the calculated
   results can be updated immediately and timely.The same methods can
   also apply to predictable changes in the characteristics of satellite
   links.

   (2) By using the predictability of satellite spatial location, the
   routing algorithm can be improved, such as Dijkstra algorithm, which
   could screen relay nodes in the path without traversing all possible
   choices, and further reduce the complexity of the routing algorithm.

6.  Use Cases

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6.1.  Scenario 1: Predictable changes in connectivities between
      satellites

   As shown in Figure 2, N1, N2 and N3 are adjacent satellites in
   different orbit planes at the same altitude, moving from south to
   north.  At T2 and T3, N3 and N2 enter a specific area (such as the
   polar region) in turn, and inter-satellite links are interrupted due
   to the difficulty in alignment of the on-board antenna.  When the
   node leaves the specific area, the on-board antenna is re-aligned and
   the inter-satellite link is restored.

            .--.                     .--.                     .--.
 t1   ####-| N1 |-####  <--->  ####-| N2 |-####  <--->  ####-| N3 |-####
            \__/                     \__/                     \__/

            .--.                     .--.                     .--.
 t2   ####-| N1 |-####  <--->  ####-| N2 |-####         ####-| N3 |-####
            \__/                     \__/                     \__/

            .--.                     .--.                     .--.
 t3   ####-| N1 |-####         ####-| N2 |-####         ####-| N3 |-####
            \__/                     \__/                     \__/

    Figure 2: Changes in connectivity between adjacent satellites in
                           different orbits.

   For any satellite in the network, the change of the connectivity
   failure/recovery state of the satellite links can be predicted in
   advance through pre-calculation.  Therefore, N2 and N3 do not need to
   perform the flooding notification of the link state changes, and the
   nodes in the network can calculate the route in advance according to
   the predicted network topology, and timely complete the route update
   procedures when the topology changes.

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                 N2-N3                N1-N2
                   |                    |
                   |--------+           |-------------+
                   |        |           |             |
                   |        |           |             |
                   |        +--------   |             +---
                   |                    |
                   +---++---++---++--   +---++---++---++--
                       t1   t2   t3         t1   t2   t3
                          Time                 Time

                Figure 3: Inter-satellite link connectivity.

   At time T1, both links between N1 and N2 and between N2 and N3 are
   connected, and the end-to-end path from N1 to N3 will be forwarded
   through N2, as shown in Figure 4.  As the nodes move, the links
   between N1 and N2 and between N2 and N3 will predictably fail at time
   T3, as shown in Figure 3.  In response to this predictable change in
   network topology, the relevant satellite nodes may perform routing
   calculations in advance, and the end-to-end path from N1 to N3 will
   be forwarded through N4, N5, N6 as shown in Figure 5.

             ^                      ^                      ^
             |                      |                      |
             |                      |                      |
             v  Src                 v                      v  Dst
            .--.                   .--.                   .--.
 <->  ####-| N1 |-####  <->  ####-| N2 |-####  <->  ####-| N3 |-####  <->
            \__/      ------>      \__/      ------>      \__/
             ^                      ^                      ^
             |                      |                      |
             |                      |                      |
             v                      v                      v
            .--.                   .--.                   .--.
<---> ####-| N4 |-#### <---> ####-| N5 |-#### <---> ####-| N6 |-#### <--->
            \__/                   \__/                   \__/
             ^                      ^                      ^
             |                      |                      |             N
             |                      |                      |             ^
             v                      v                      v             |
        Orbit plane 1          Orbit plane 2          Orbit plane 3      |
                                                                         S

                                                                    Moving
                                                                 Direction

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                 Figure 4: Path from N1 to N3 at T1.

             ^                      ^                      ^
             |                      |                      |
             |                      |                      |
             v  Src                 v                      v  Dst
            .--.                   .--.                   .--.
      ####-| N1 |-####       ####-| N2 |-####       ####-| N3 |-####
            \__/                   \__/                   \__/
             ^|                     ^                      ^^
             ||                     |                      ||
             ||                     |                      ||
             vv                     v                      |v
            .--.      ------>      .--.      ------>      .--.
<---> ####-| N4 |-#### <---> ####-| N5 |-#### <---> ####-| N6 |-#### <--->
            \__/                   \__/                   \__/
             ^                      ^                      ^
             |                      |                      |             N
             |                      |                      |             ^
             v                      v                      v             |
        Orbit plane 1          Orbit plane 2          Orbit plane 3      |
                                                                         S
                                                                    Moving
                                                                 Direction

                 Figure 5: Path from N1 to N3 at T3.

6.2.  Scenario 2: Predictable property changes of satellite links

   As shown in Figure 6, N1 and N2 are adjacent satellites at the same
   altitude and in different orbit planes, moving from the equator to
   the polar region from south to north.  At time T1, the distance
   between N1 and N2 is the largest, and at time T3, the distance
   between N1 and N2 is the smallest.  For any satellite in the network,
   the changes in satellite communication distances will influence the
   characteristics of satellite links, including delay and error rate.
   Each satellite in the network can predict these changes in advance
   through pre-calculation, and update the link cost correspondingly.
   Therefore, N1 and N2 do not need to perform the flooding notification
   of the link state changes, and the nodes in the network can calculate
   the route in advance according to the predicted link cost change and
   switch the routing path at an appropriate time.

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                     .--.                              .--.
          t1   ####-| N1 |-####  <------------>  ####-| N2 |-####
                     \__/                              \__/

                     .--.                           .--.
          t2   ####-| N1 |-####  <--------->  ####-| N2 |-####
                     \__/                           \__/

                     .--.                        .--.
          t3   ####-| N1 |-####  <------>  ####-| N2 |-####
                     \__/                        \__/

        Figure 6: Changes of communication distance between adjacent
                      satellites in different orbits.

   At time T1, N7 and N3 are symmetrically located on both sides of the
   equator, and N4, N5 and N6 are located in the equatorial region.
   Therefore, the communication distance between N4 and N5 and between
   N5 and N6 is the largest, and the corresponding link cost is also
   higher.  Therefore, the end-to-end path from N7 to N3 does not
   include the N4, N5, and N6, but forwards through N8, N5, and N2 which
   is shown in Figure 7.

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             ^                     ^                     ^
             |                     |                     |
             |                     |                     |
             v                     v                     v  Dst
            .--.                  .--.                  .--.
 <--> ####-| N1 |-#### <--> ####-| N2 |-#### <--> ####-| N3 |-#### <-->
            \__/                  \__/                  \__/
             ^                     ^^      ------->      ^
             |                     ||                    |
             |                     ||                    |
             v                     v|                    v
            .--.                  .--.                  .--.
<---->####-| N4 |-####<---->####-| N5 |-####<---->####-| N6 |-####<---->
            \__/                  \__/                  \__/
             ^                     ^^                    ^
             |                     ||                    |
             |                     ||                    |
             v       ------->      v|                    v
            .--.                  .--.                  .--.
 <--> ####-| N7 |-#### <--> ####-| N8 |-#### <--> ####-| N9 |-#### <-->
            \__/                  \__/                  \__/
             ^  Src                ^                     ^
             |                     |                     |             N
             |                     |                     |             ^
             v                     v                     v             |
        Orbit plane 1         Orbit plane 2         Orbit plane 3      |
                                                                       S

                                                                  Moving
                                                               Direction

                 Figure 7: Path from N7 to N3 at T3.

   With the continuous movement of the node, at time T3, the source
   satellite N7 and the destination satellite N3 both move across the
   equator and enter the northern hemisphere, while N1, N2 and N3 are in
   a relatively near-polar region.  Therefore, the communication
   distance between N1, N2, and N3 is the smallest compared to other
   inter-satellite links, and the corresponding link cost is also lower.
   Thus, the end-to-end path from N7 to N3 includes N4, N1, N2 which is
   shown in Figure 8.

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              ^                      ^                      ^
              |                      |                      |
              |                      |                      |
              v                      v                      v  Dst
             .--.                   .--.                   .--.
  <->  ####-| N1 |-####  <->  ####-| N2 |-####  <->  ####-| N3 |-####  <->
             \__/                   \__/                   \__/
              ^^        ---->        ^         ----->       ^
              ||                     |                      |
              ||                     |                      |
              v|                     v                      v
             .--.                   .--.                   .--.
 <---> ####-| N4 |-#### <---> ####-| N5 |-#### <---> ####-| N6 |-#### <--->
             \__/                   \__/                   \__/
              ^^                     ^                      ^
              ||                     |                      |
              ||                     |                      |
              v|                     v                      v
             .--.                   .--.                   .--.
<----->####-| N7 |-####<----->####-| N8 |-####<----->####-| N9 |-####<----->
             \__/                   \__/                   \__/
              ^  Src                 ^                      ^
              |                      |                      |              N
              |                      |                      |              ^
              v                      v                      v              |
         Orbit plane 1          Orbit plane 2          Orbit plane 3       |
                                                                           S

                                                                      Moving
                                                                   Direction

                 Figure 8: Path from N7 to N3 at T1.

7.  Security Considerations

   TBA

8.  Acknowledgements

   TBA

9.  IANA Considerations

   TBA

10.  Normative References

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   [I-D.birrane-tvr-use-cases]
              Birrane, E. J., "TVR (Time-Variant Routing) Use Cases",
              Work in Progress, Internet-Draft, draft-birrane-tvr-use-
              cases-00, 24 October 2022,
              <https://datatracker.ietf.org/doc/html/draft-birrane-tvr-
              use-cases-00>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,
              <https://www.rfc-editor.org/info/rfc2119>.

   [RFC8174]  Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
              2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
              May 2017, <https://www.rfc-editor.org/info/rfc8174>.

Authors' Addresses

   Dongxu Hou (editor)
   ZTE Corporation
   No.50 Software Avenue
   Nanjing
   Jiangsu, 210012
   China
   Email: hou.dongxu@zte.com.cn

   Fenlin Zhou
   ZTE Corporation
   No.50 Software Avenue
   Nanjing
   Jiangsu, 210012
   China
   Email: zhou.fenlin@zte.com.cn

   Dongyu Yuan
   ZTE Corporation
   No.50 Software Avenue
   Nanjing
   Jiangsu, 210012
   China
   Email: yuan.dongyu@zte.com.cn

   Min Xiao
   ZTE Corporation
   No.50 Software Avenue

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   Nanjing
   Jiangsu, 210012
   China
   Email: xiao.min2@zte.com.cn

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