Internet DRAFT - draft-hou-satellite-routing-framework

draft-hou-satellite-routing-framework







Network Working Group                                     H. Dongxu, Ed.
Internet-Draft                                                    X. Min
Intended status: Informational                                   F. Zhou
Expires: 20 August 2024                                  ZTE Corporation
                                                           February 2024


 Routing Framework for LEO Mega-constellation Based on Region Division
                draft-hou-satellite-routing-framework-02

Abstract

   The inter-satellite routing is the premis to ensure that the
   satellite network provides end-to-end stable service covering the
   whole globe.  However, the mature terrestrial network technologies
   are difficult to directly apply to the satellite network because of
   the highly dynamic network topology and the limited on-board
   resources.  This issue is further exacerbated in LEO mega-
   constellations.  In view of this challenge, this document presents a
   routing framework for LEO mega-constellation.  Based on the orbit
   position characteristic and the predictable topology, this framwork
   realizes flexible region division, establishes intra-region and
   inter-region path, as well as completes end-to-end data forwarding.

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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   This Internet-Draft will expire on 4 August 2024.

Copyright Notice

   Copyright (c) 2024 IETF Trust and the persons identified as the
   document authors.  All rights reserved.






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   This document is subject to BCP 78 and the IETF Trust's Legal
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   Please review these documents carefully, as they describe your rights
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Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   2
   2.  Requirements Language . . . . . . . . . . . . . . . . . . . .   4
   3.  Terminology . . . . . . . . . . . . . . . . . . . . . . . . .   4
   4.  Region Planning . . . . . . . . . . . . . . . . . . . . . . .   4
     4.1.  Region Model  . . . . . . . . . . . . . . . . . . . . . .   5
     4.2.  Mapping Relationship  . . . . . . . . . . . . . . . . . .   6
   5.  Node Capability . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Node Type . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.2.  Border Node Capability  . . . . . . . . . . . . . . . . .   8
     5.3.  Shared Node Capability  . . . . . . . . . . . . . . . . .   9
   6.  Routing Computation . . . . . . . . . . . . . . . . . . . . .  11
     6.1.  Information for Routing Calculation . . . . . . . . . . .  11
     6.2.  Routing Computation . . . . . . . . . . . . . . . . . . .  12
   7.  Data Forwarding . . . . . . . . . . . . . . . . . . . . . . .  13
     7.1.  Scenario 1: Intra-region data forwarding  . . . . . . . .  13
     7.2.  Scenario 2: Inter-region data forwarding  . . . . . . . .  14
   8.  Sudden Failure Handling . . . . . . . . . . . . . . . . . . .  16
     8.1.  Unreachable node in a region  . . . . . . . . . . . . . .  17
     8.2.  Complete link break in a region . . . . . . . . . . . . .  17
     8.3.  Complete link break between neighboring regions . . . . .  18
   9.  Extensions and Future Works . . . . . . . . . . . . . . . . .  19
   10. Security Considerations . . . . . . . . . . . . . . . . . . .  19
   11. Acknowledgements  . . . . . . . . . . . . . . . . . . . . . .  19
   12. IANA Considerations . . . . . . . . . . . . . . . . . . . . .  19
   13. References  . . . . . . . . . . . . . . . . . . . . . . . . .  19
     13.1.  Normative References . . . . . . . . . . . . . . . . . .  19
     13.2.  Informative References . . . . . . . . . . . . . . . . .  19
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .  20

1.  Introduction

   The LEO mega-constellation networks can overcome the limitations of
   the conventional terrestial network, achieving global signal
   coverage, and providing large broadband and low-latency network
   services for global users.  The global communications ecosystem
   suggests that satellite-based communication will become an important
   part of 5G-advanced and 6G.



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   Routing issues are the premise to ensure the stable worldwide end-to-
   end service based on the satellite network.  Compared with the
   terrestial network, the satellite network has the characteristics of
   highly dynamic nodes, time-varying network topology, and limited on-
   board resources.  So the mature terrestrial nework routing technology
   is difficult to directly apply.

   In view of this situation, some improved routing methods have been
   proposed based on the predictable topology changes in the satellite
   network.  However, the development trend of the satellite network is
   becoming more and more low-orbit and large-scale, which further
   intensifies the dynamic property of the network.  More topology
   changes as well as information announcements are happend, which would
   result in frequent routing calculations, much more protocol overhead,
   and even route flapping.

   To tackle the above issues, the terrestial network generally adopts
   the manner of area division to imporve the efficiency of routing
   convergence and network management.  The existing area division
   method typically divides a network into a backbone area and multiple
   non-backbone areas.  The backbone area is unique and has routing
   informations of all areas.  The non-backbone areas establish fixed
   connections around the backbone area.  Thus, nodes in the backbone
   area are responsible for traffic forwarding between all areas, and
   require higher performance.  However, the performance of satellites
   is peer-to-peer and limited, especially in the single-layer satellite
   constellation.  The current area division method could easily make
   the backbone area congested in the satellite network.  Besides, due
   to the permanent changes of the satellite network topology, the
   connection relationship between different areas is unable to maintain
   stability, and each area need to exchange routing informations
   continuously.  So the existing area division method is not only
   inefficient but also lacks sufficient flexibility, and can't meet the
   requirement of routing in the satellite network.

   Considering these problems, this document proposes a routing
   framework for LEO mega-constellation networks based on region
   division.  This framework can be implemented on the basis of existing
   IGP.  The details of this framework are as follows:

   (1) Firstly, based on the relative position between satellites, the
   new node type and capability are defined to realize flexible region
   divesion.

   (2) Then, the mapping relationship between nodes and regions is
   established based on the satellite orbit position to reduce the
   inter-region network information announcement.




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   (3) Finally, based on the predictable network topology in the
   satellite network, the inter-region and intra-region paths are
   constructed to complete the end-to-end data forwarding.

   Further details are discussed in subsequent sections.

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

   *  VLEO: Very Low Earth Orbit with the altitude below 450 km.

   *  LEO: Low Earth Orbit with the altitude between 180 km and 2000 km.

   *  MEO: Medium Earth Orbit with the altitude between 2000 km and
      35786 km.

   *  GEO: Geosynchronous Orbit with the altitude 35786 km.

   *  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

   *  BGP: Border Gateway Protocol [RFC4271]

   *  IGP: Interior Gateway Protocol, examples of IGPs inculde 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]).

4.  Region Planning









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4.1.  Region Model

   In the satellite network, each satellite moves along the orbit, which
   can be divided into circular orbit satellites and elliptical orbit
   satellites.  Different orbits can be described by Keplerian
   parameters.  At present, the mainstream of satellite networks
   basically adopt circular orbit.


                          Orbit     Orbit     Orbit
                         Plane 1   Plane 2   Plane 3

                           |         |         |
                         +---+     +---+     +---+
                      ---| 1 |-----| 4 |-----| 7 |---
                         +---+     +---+     +---+
                           |         |         |
                           |         |         |
                         +---+     +---+     +---+
                      ---| 2 |-----| 5 |-----| 8 |---
                         +---+     +---+     +---+
                           |         |         |
                           |         |         |
                         +---+     +---+     +---+
                      ---| 3 |-----| 6 |-----| 9 |---
                         +---+     +---+     +---+
                           |         |         |

                  Figure 1: N5 and its adjacent satellites


   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.  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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   Therefore, the quadrangle is adopted as the basic model of region
   division, that is, a region also have four types of contiguous
   neighbour regions and each type refers to a direction, as shown in
   Figure 2.  In this way, satellites in each region and the adjacency
   relationship between regions could remain unchanged with the dynamic
   change of satellite topology.


                         Orbit     Orbit     Orbit
                        Plane 1   Plane 2   Plane 3

                         +---+     +---+     +---+
                         | 3 |-----| 6 |-----| 9 |
                      |  +---+     +---+     +---+  |
                      |    |   A18   |         |    |
                  ----+----+---------+---------+----+----
               +---+  |  +---+     +---+     +---+  |  +---+
               | 7 |--+--| 1 |-----| 4 |-----| 7 |--+--| 1 |
               +---+  |  +---+     +---+     +---+  |  +---+
                 | A14|    |   A19   |         |    |A24 |
                 |    |    |         |         |    |    |
               +---+  |  +---+     +---+     +---+  |  +---+
               | 8 |--+--| 2 |-----| 5 |-----| 8 |--+--| 2 |
               +---+  |  +---+     +---+     +---+  |  +---+
                 |    |    |         |         |    |    |
                 |    |    |         |         |    |    |
               +---+  |  +---+     +---+     +---+  |  +---+
               | 9 |--+--| 3 |-----| 6 |-----| 9 |--+--| 3 |
               +---+  |  +---+     +---+     +---+  |  +---+
                  ----+----+---------+---------+----+----
                      |    |   A20   |         |    |   l=3
                      |  +---+     +---+     +---+  |
                     h=3 | 1 |-----| 4 |-----| 7 |
                         +---+     +---+     +---+

                  Figure 2: Basic model of region division


4.2.  Mapping Relationship

   In a satellite constellation which has M orbit planes and N nodes per
   orbit plane, the orbit position information of a satellite can be
   marked with (X, Y), namely the node identifier.  The corresponding
   region number k of this satellite can be calculated by Formula 1.

   k=ceil(X/l)*(M/l)+ceil(Y/h)+delta





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   The X is the orbit plane of the satellite.  The Y is the sequence in
   the orbit plane of the satellite.  The h is the number of satellites
   included in the region in a single orbit plane.  The l is the number
   of orbits spanned by the region.  The h and the l are region range
   parameters which can be pre-planned by the control layer of the
   network.  The delta is an offset value when calculating the region
   number.

   Based on the orbit position information, the communication port of
   the satellite is addressed.  During the data forwarding process, each
   relay satellite resolves the destination node identifier from the
   destination address, and then calculates the region number of the
   destination node via Formula 1.  Through this method, each node can
   determine the mapping relationship between regions and nodes.

5.  Node Capability

   In order to realize the region division described in the previous
   section, the corresponding capability of the satellite node should be
   defined.

   It should be noted that the process of satellite motion along the
   orbit is periodic and predictable.  Predictable information in a
   satellite network includes the real-time position of a satellite, the
   connectivity of satellite links, the characteristics of satellite
   links.

   Based on these predictable information, the management plane, the
   control plane and the forwarding plane of the network can be
   adaptively improved to ensure a stable and optimal end-to-end
   reachable path between a pair of satellites.  On one hand, the
   flooding of routing convergence information caused by network
   topology changes can be avoided.  On the other hand, the routing re-
   calculation is able to be fulfilled in advance before the satellite
   network topology changes.  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.  The
   node capabilities described in this section incorporate this manner.

5.1.  Node Type

   According to the relative position of a satellite in the rigion, the
   type of the satellite can be classified as the internal node and the
   border node.  The internal node doesn't connect nodes outside of the
   rigion.  The border node is part of the rigion and has connections to
   nodes outside of the rigion.  The internal node and the border node
   are collectively referred as the intra-region nodes.  The node which
   have connections to the border node in the neighbor rigion is called



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   the cross-rigion neighbor node.

5.2.  Border Node Capability

   Each region is abstracted as a single virtual node to hide the
   network information and the topology change from the external
   network.  The virtual node represents the time-varying geographical
   location of a region in the satellite network.  The geographical
   location of the virtual node corresponds to the specified node in the
   region.  The specified node could be configured by the network
   administrator or elected via protocol mechanism.  It should be noted
   that there is only one edge between any two adjacent virtual nodes.

   In order to shield the network information in a region from the
   external network, the border node should have the following features:

   (1) The capability of establishing connections with cross-rigion
   neighbor nodes.

   The border node is the agent of the external connection for the local
   region, and is responsible for establishing the connection with the
   cross-rigion neighbor node.  During the connection establishment
   process, these nodes build the adjacency relationship and announce
   the region number with each other.  After the border node receives
   the region number of the neighbor region, the border node informs
   this information to other nodes on the local region.

   (2) The capability of maintaining connections with cross-rigion
   neighbor nodes.

   The change of the connection relationship between the border node and
   the cross-rigion neighbor node can be divided into predictable and
   unpredictable.  The predictable connection change can be self-
   calculated and obtained by each node according to the orbit
   parameters.  Therefore, the border node only notifiy the
   unpredictable connection change to nodes in the same region.
   Figure 3 presents a unpredictable link failure between A19 adn A24,
   and N8 is responsible for informing other nodes in the same region.













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                         Orbit     Orbit     Orbit
                        Plane 1   Plane 2   Plane 3

                         +---+     +---+     +---+
                         | 3 |-----| 6 |-----| 9 |
                      |  +---+     +---+     +---+  |
                      |    |   A18   |         |    |
                  ----+----+---------+---------+----+----
               +---+  |  +---+     +---+     +---+  |  +---+
               | 7 |--+--| 1 |-----| 4 |-----| 7 |--+--| 1 |
               +---+  |  +---+     +---+     +---+  |  +---+
                 | A14|    |   A19   |         |    |A24 |
                 |    |    |         |         |    |    |
               +---+  |  +---+     +---+     +---+ \|/ +---+
               | 8 |--+--| 2 |-----| 5 |-----| 8 |--X--| 2 |
               +---+  |  +---+     +---+     +---+ /|\ +---+
                 |    |    |         |         |    |    |
                 |    |    |         |         |    |    |
               +---+  |  +---+     +---+     +---+  |  +---+
               | 9 |--+--| 3 |-----| 6 |-----| 9 |--+--| 3 |
               +---+  |  +---+     +---+     +---+  |  +---+
                  ----+----+---------+---------+----+----
                      |    |   A20   |         |    |
                      |  +---+     +---+     +---+  |
                         | 1 |-----| 4 |-----| 7 |
                         +---+     +---+     +---+

                    Figure 3: Inter-region link failure


   (3) The capability of isolating network information advertisements
   between different regions.

   If the network variation is happened in a region (such as link
   interruption, node failure, and etc.), the affected nodes will
   typically flood routing information to other nodes.  When the
   flooding information reaches the border node, this information will
   be blocked, that is, the routing information in a region can't pass
   through the border node to the adjacent region.

5.3.  Shared Node Capability

   In addition to the above unique features of the border node, both the
   border node and the internal node share the following functions:

   (1) The capability of maintaining intra-region network information.





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   When a region is initially created, the node establishes a connection
   with a neighbor node, determines the node of the region based on the
   planned region parameters, generates a network topology in the region
   according to the orbit parameters, notifies its own network
   information to other nodes in the region, and continuously monitors
   its own direct links.  Then, the node calculates and updates the
   intra-region network topology according to the planned time interval
   by the predictability of satellite orbital movement.  As a sudden
   failure occurs, the node notifies it and updates the local network
   information and the intra-region topology.  Figure 4 presents a
   unpredictable link failure between N5 adn N8.  N5 and N8 are
   responsible for informing other nodes in the same region.


                         Orbit     Orbit     Orbit
                        Plane 1   Plane 2   Plane 3

                         +---+     +---+     +---+
                         | 3 |-----| 6 |-----| 9 |
                      |  +---+     +---+     +---+  |
                      |    |   A18   |         |    |
                  ----+----+---------+---------+----+----
               +---+  |  +---+     +---+     +---+  |  +---+
               | 7 |--+--| 1 |-----| 4 |-----| 7 |--+--| 1 |
               +---+  |  +---+     +---+     +---+  |  +---+
                 | A14|    |   A19   |         |    |A24 |
                 |    |    |         |         |    |    |
               +---+  |  +---+     +---+ \ / +---+  |  +---+
               | 8 |--+--| 2 |-----| 5 |--X--| 8 |--+--| 2 |
               +---+  |  +---+     +---+ / \ +---+  |  +---+
                 |    |    |         |         |    |    |
                 |    |    |         |         |    |    |
               +---+  |  +---+     +---+     +---+  |  +---+
               | 9 |--+--| 3 |-----| 6 |-----| 9 |--+--| 3 |
               +---+  |  +---+     +---+     +---+  |  +---+
                  ----+----+---------+---------+----+----
                      |    |   A20   |         |    |
                      |  +---+     +---+     +---+  |
                         | 1 |-----| 4 |-----| 7 |
                         +---+     +---+     +---+

                    Figure 4: Intra-region link failure


   (2) The capability of maintaining cross-region connection
   relationships.





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   When a region is initially created, the node generates a mapping
   relationship between the border node and the adjacent region
   according to the notification information of the border node.  Then,
   the node periodically calculates the connectivity between border
   nodes and connected cross-region neighbor nodes based on the
   predictability of satellite orbital movement, and updates the mapping
   relationship.  As a sudden failure occurs, each node in the local
   region receives the routing information announced by the border node,
   and updates the mapping relationship.

   (3) The capability of maintaining inter-region network topology.

   Based on the orbit parameters of the designated nodes in the region
   corresponding to the virtual nodes, and the planned region range, the
   node calculate the geographical positions of other virtual nodes in
   the entire network to obtain the inter-region network topology.

6.  Routing Computation

6.1.  Information for Routing Calculation

   Through above node capabilities, each node periodicly predict time-
   varying network states based on the satellite parameters to shield
   the dynamic of network.  These network states include intra-region
   topology relationship, the inter-region topology relationship, as
   well as the mapping relationship between the border node and the
   adjacent region.

   (1) Inter-region topology relationship

   Based on the satellite parameters (such as orbit parameters,
   satellite running time, and etc.), each satellite calculates the
   geographical location of virtual node corresponding to each region
   and constructs the inter-region topology relationship.  Then,
   according to the inter-region topology relationship and the route
   computation algorithm, inter-region paths between the local region
   and others are obtained.  Finally, the next hop regions to reach
   destination regions is also achieved.  Figure 5 shows the
   representation of the inter-region topology relationship.


                          +------+------+------+
                          | Area1| Area2| Cost |
                          +------+------+------+
                          |  A14 |  A19 |  10  |
                          +------+------+------+

                Figure 5: Inter-region topology relationship



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   (2) Intra-region topology relationship

   Each satellite calculates the connection relationship between any two
   nodes in the local region based on the satellite parameters.
   According to the connection relationship, intra-region topology
   relationship is constructed, which shows in Figure 6.


                          +------+------+------+
                          | Node1| Node2| Cost |
                          +------+------+------+
                          |  N1  |  N2  |  6   |
                          +------+------+------+

                Figure 6: Intra-region topology relationship


   (3) Mapping relationship

   In a region, each satellite receives the neighbor region number
   advertised by the border node.  Then, a mapping relationship between
   the neighbor region and the border node is constructed.  Figure 7
   shows the representation of the mapping relationship.


                         +-----------+----------+
                         | Edge Node | Nig Area |
                         +-----------+----------+
                         |    N1     |   INF    |
                         +-----------+----------+
                         |    N2     |   A14    |
                         +-----------+----------+

                       Figure 7: Mapping relationship


6.2.  Routing Computation

   Based on these network information, the intra-region routing table
   and inter-region routing table could be constructed.

   (1) Intra-region routing









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   When the destination address of the data packet is identified in the
   local region, the intra-region routing table is used for the data
   forwarding.  The data forwarding process is consistent with the
   terrestrial network.  The intra-region routing table entries include
   destination address, network mask, routing priority, routing
   overhead, forwarding interface, and next hop address.

   (2) Inter-region routing

   When the destination address of the data packet is identified in the
   remote region, the inter-region routing table is used for the data
   forwarding.  The path calculation process of each node is as follows:

   a.  Firstly, based on the inter-region topology relationship, the
   inter-region path from the region where the node is located to other
   regions is calculated.

   b.  Then, according to the mapping relationship and the intra-region
   topology relationship, the intra-region path to the next hop region
   is calculated.

   The inter-region routing table entries replaces the destination
   address and network mask with the destination area, and adds the next
   hop forwarding area.  Figure 8 presents an example of the inter-
   region routing table.


           +--------+---------+-----+------+-----+------------+
           |Dst Area| Nxt Area| Pri | Cost | Inf | Nxt Hop Adr|
           +--------+---------+-----+------+-----+------------+
           |   22   |   18    |  1  |  6   | eth |  x.x.x.x   |
           +--------+---------+-----+------+-----+------------+

                    Figure 8: Inter-region routing table


7.  Data Forwarding

7.1.  Scenario 1: Intra-region data forwarding

   As shown in Figure 9, N2 and N4 are located in the same region.  N2
   serves as the source end to send data to the destination end N4.  The
   data forwarding path between N2 and N4 pass through N2, N5, and N4 in
   turn.  The routing process is consistent with that of the terrestial
   network.  So, this document doesn't describe the process here.






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                     Orbit plane 1       Orbit plane 2

                 |       .--.                .--.       |
                 |  ###-| N3 |-### <--> ###-| N6 |-###  |
                 |       \__/                \__/       |
                 |        ^        A18        ^         |
   --------------+--------+-------------------+---------+--------------
        A14      |        v        A19        v         |     A24
     .--.        |       .--.                .--.       |        .--.
###-| N4 |-### <-+> ###-| N1 |-### <--> ###-| N4 |-### <+-> ###-| N1 |-###
     \__/        |       \__/                \__/       |        \__/
      ^          |        ^                   ^  ^      |         ^
      |          |        |                   |  |      |         |
      v          |        v ----------------> v  |      |         v
     .--.        |       .--.                .--.       |        .--.
###-| N5 |-### <-+> ###-| N2 |-### <--> ###-| N5 |-### <+-> ###-| N2 |-###
     \__/        |       \__/                \__/       |        \__/
      ^          |        ^                   ^         |         ^
      |          |        |                   |         |         |
      v          |        v                   v         |         v
     .--.        |       .--.                .--.       |        .--.
###-| N6 |-### <-+> ###-| N3 |-### <--> ###-| N6 |-### <+-> ###-| N3 |-###
     \__/        |       \__/                \__/       |        \__/
                 |        ^                   ^         |
   --------------+--------+-------------------+---------+--------------  N
                 |        v        A20        v         |                ^
                 |       .--.                .--.       |                |
                 |  ###-| N1 |-### <--> ###-| N4 |-###  |         Moving |
                 |       \__/                \__/       |      Direction S

                Figure 9: Intra-region data forwarding


7.2.  Scenario 2: Inter-region data forwarding

















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                   +-----+-----+-----+-----+-----+-----+
                   | A1  | A6  | A11 | A16 | A21 | A26 |
                   |     |     |     |     |     |     |
                   +-----+-----+-----+-----+-----+-----+
                   | A2  | A7  | A12 | A17 | A22 | A27 |
                   |     |     |     |    ^>>    |     |
                   +-----+-----+-----+----^+-----+-----+
                   | A3  | A8  | A13 | A18^| A23 | A28 |
                   |     |     |     |    ^|     |     |
                   +-----+-----+-----+----^+-----+-----+
                   | A4  | A9  | A14 | A19^| A24 | A29 |
                   |   >>>>>>>>>>>>>>>>>>>>|     |     |
                   +-----+-----+-----+-----+-----+-----+
                   | A5  | A10 | A15 | A20 | A25 | A30 |
                   |     |     |     |     |     |     |
                   +-----+-----+-----+-----+-----+-----+

                  Figure 10: End-to-end inter-region path


   As shown in Figure 10, the satellite network is divided into 30
   regions.  At a certaion time, Sat1 is located in region 4 and Sat2 is
   located in region 22.  Sat1 serves as the source end to send data to
   the destination end Sat2.  The data forwarding path between Sat1 and
   Sat2 pass through region 4, 9, 14, 19, 18, 17, and 22 in turn.

   As shown in Figure 11, the data is forwarded from Node 6 in region 14
   to Node 3 in region 19 20.  Node 3 resolves the destination node
   identifier (x_dst, y_dst) from the destination address, and then
   calculates the region number of the destination node as 22 via
   Formula 1.

   Node 3 queries the inter-region routing table to forward data.
   According to the inter-region routing table record, if the
   destination node is located in region 22, the data packet is
   forwarded via region 18, and the next hop node is Node 6.  Each node
   in region 19 repeats the above operations, and finally forwards the
   packet to region 18.  For the border node, the next hop address in
   the inter-region routing table is the port address of the cross-
   rigion neighbor satellite.











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                     Orbit plane 1       Orbit plane 2

                 |       .--.                .--.       |
                 |  ###-| N3 |-### <--> ###-| N6 |-###  |
                 |       \__/                \__/       |
                 |        ^        A18        ^  ^      |
   --------------+--------+-------------------+--|------+--------------
        A14      |        v        A19        v  |      |     A24
     .--.        |       .--.                .--.       |        .--.
###-| N4 |-### <-+> ###-| N1 |-### <--> ###-| N4 |-### <+-> ###-| N1 |-###
     \__/        |       \__/                \__/       |        \__/
      ^          |        ^                   ^  ^      |         ^
      |          |        |                   |  |      |         |
      v          |        v                   v  |      |         v
     .--.        |       .--.                .--.       |        .--.
###-| N5 |-### <-+> ###-| N2 |-### <--> ###-| N5 |-### <+-> ###-| N2 |-###
     \__/        |       \__/                \__/       |        \__/
      ^          |        ^                   ^  ^      |         ^
      |          |        |                   |  |      |         |
      v ----------------> v ----------------> v  |      |         v
     .--.        |       .--.                .--.       |        .--.
###-| N6 |-### <-+> ###-| N3 |-### <--> ###-| N6 |-### <+-> ###-| N3 |-###
     \__/        |       \__/                \__/       |        \__/
                 |        ^                   ^         |
   --------------+--------+-------------------+---------+--------------  N
                 |        v        A20        v         |                ^
                 |       .--.                .--.       |                |
                 |  ###-| N1 |-### <--> ###-| N4 |-###  |         Moving |
                 |       \__/                \__/       |      Direction S

               Figure 11: Inter-region data forwarding


8.  Sudden Failure Handling

   The above routing calculation process presents the inter-satellite
   routing in the large-scale constellation scenario.  However, sudden
   failures in this scenario have not been explained.

   For sudden failures within or between regions which don't trigger
   changes in node reachability or network connectivity, the mentioned
   node capabilities and link state flooding methods can perform.

   For sudden failures which trigger node reachability or network
   connectivity changes, the network-wide routing information needs to
   be advertised.  This scenario can be divided into three categories:





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8.1.  Unreachable node in a region

   As shown in Figure 11, Sat2 is disconnected from Sat1, Sat3, Sat4,
   and Sat5 for a sudden failure.  After completing the intra-region
   advertisement, each node in the region finds that the Sat2 is
   unreachable based on the intra-region topology relationship table.
   The region edge node will notify other regions the unreachability
   information.


               .--.                .--.                .--.
          ###-| N1 |-### <--> ###-| N4 |-### <--> ###-| N7 |-###
               \__/                \__/                \__/
                ^                   ^                   ^
                |                   X                   |
                v                   v                   v
               .--.                .--.                .--.
          ###-| N2 |-### <-X> ###-| N5 |-### <X-> ###-| N8 |-###
               \__/                \__/                \__/
                ^                   ^                   ^
                |                   X                   |
                v                   v                   v
               .--.                .--.                .--.
          ###-| N3 |-### <--> ###-| N6 |-### <--> ###-| N9 |-###
               \__/                \__/                \__/

                  Figure 12: Unreachable node in a region

8.2.  Complete link break in a region

   As shown in Figure 12, a region is divided for sudden failures.
   After the intra-region advertisement, each node in the region finds
   that the region is divided into two disconnected parts based on the
   intra-region topology relationship table.  The region edge node will
   notify other regions that the region is unreachability.
















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        +--------------------------------------------------------+
        |      .--.                .--.                .--.      |
        | ###-| N1 |-### <--> ###-| N4 |-### <X-> ###-| N7 |-### |
        |      \__/                \__/                \__/      |
        |       ^                   ^                   ^        |
        |       |                   |                   |        |
        |       v                   v                   v        |
        |      .--.                .--.                .--.      |
        | ###-| N2 |-### <--> ###-| N5 |-### <X-> ###-| N8 |-### |
        |      \__/                \__/                \__/      |
        |       ^                   ^                   ^        |
        |       |                   |                   |        |
        |       v                   v                   v        |
        |      .--.                .--.                .--.      |
        | ###-| N3 |-### <--> ###-| N6 |-### <X-> ###-| N9 |-### |
        |      \__/                \__/                \__/      |
        +--------------------------------------------------------+

                  Figure 13: Inter-region data forwarding

8.3.  Complete link break between neighboring regions

   As shown in Figure 13, connections between two regions are
   disconnected for sudden failures.  According to the mapping
   relationship between region edge nodes and adjacent regions, each
   directly related node gets this network change.  Then, these nodes
   will notify other nodes the connectivity change.  Nodes in other
   regions will update their inter-region topology relationship table
   based on the advertisement information.


                  ----------------+    +----------------
                         .--.     |    |     .--.
                    ###-| N7 |-###|<-X>|###-| N1 |-###
                         \__/     |    |     \__/
                          ^       |    |      ^
                          |       |    |      X
                          v       |    |      v
                         .--.     |    |     .--.
                    ###-| N8 |-###|<-X>|###-| N2 |-###
                         \__/     |    |     \__/
                          ^       |    |      ^
                          |       |    |      X
                          v       |    |      v
                         .--.     |    |     .--.
                    ###-| N9 |-###|<-X>|###-| N3 |-###
                         \__/     |    |     \__/
                  ----------------+    +----------------



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         Figure 14: Complete link break between neighboring regions

9.  Extensions and Future Works

   In the future work, the extension of the current routing protocol to
   support the framework described in this document would be taken in
   mind.

10.  Security Considerations

   TBA

11.  Acknowledgements

   TBA

12.  IANA Considerations

   This document has no IANA actions.

13.  References

13.1.  Normative References

   [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>.

13.2.  Informative References

   [I-D.hou-tvr-satellite-network-usecases]
              Dongxu, H., Min, X., Zhou, F., and D. Yuan, "Satellite
              Network Routing Use Cases", Work in Progress, Internet-
              Draft, draft-hou-tvr-satellite-network-usecases-01, 13
              March 2023, <https://datatracker.ietf.org/doc/html/draft-
              hou-tvr-satellite-network-usecases-01>.

   [I-D.ietf-tvr-use-cases]
              Birrane, E. J., Kuhn, N., and Y. Qu, "TVR (Time-Variant
              Routing) Use Cases", Work in Progress, Internet-Draft,
              draft-ietf-tvr-use-cases-00, 15 April 2023,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tvr-use-
              cases-00>.



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   [I-D.lhan-satellite-semantic-addressing]
              Han, L., Li, R., Retana, A., chenmeiling, and N. Wang,
              "Satellite Semantic Addressing for Satellite
              Constellation", Work in Progress, Internet-Draft, draft-
              lhan-satellite-semantic-addressing-03, 3 March 2023,
              <https://datatracker.ietf.org/doc/html/draft-lhan-
              satellite-semantic-addressing-03>.

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

   [Large-Scale-LEO-Network-Routing]
              "Large Scale LEO Satellite Networks for the Future
              Internet: Challenges and Solutions to Addressing and
              Routing," Computer Networks and Communications, 1(1),
              31-58", <https://ojs.wiserpub.com/index.php/CNC/article/
              view/2105>.

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

   [ThreeGPP] "3GPP", <https://www.3gpp.org/>.

Authors' Addresses

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


   Xiao Min
   ZTE Corporation
   No.50 Software Avenue
   Nanjing
   Jiangsu, 210012
   China
   Email: xiao.min2@zte.com.cn


   Fenlin Zhou
   ZTE Corporation
   No.50 Software Avenue
   Nanjing
   Jiangsu, 210012
   China



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   Email: zhou.fenlin@zte.com.cn


















































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