



Independent Submission                                             T. Li
Request for Comments: 9717                              Juniper Networks
Category: Informational                                     January 2025
ISSN: 2070-1721


             A Routing Architecture for Satellite Networks

Abstract

   Satellite networks present some interesting challenges for packet
   networking.  The entire topology is continually in motion, with links
   far less reliable than what is common in terrestrial networks.  Some
   changes to link connectivity can be anticipated due to orbital
   dynamics.

   This document proposes a scalable routing architecture for satellite
   networks based on existing routing protocols and mechanisms that is
   enhanced with scheduled link connectivity change information.  This
   document proposes no protocol changes.

   This document presents the author's view and is neither the product
   of the IETF nor a consensus view of the community.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for informational purposes.

   This is a contribution to the RFC Series, independently of any other
   RFC stream.  The RFC Editor has chosen to publish this document at
   its discretion and makes no statement about its value for
   implementation or deployment.  Documents approved for publication by
   the RFC Editor are not candidates for any level of Internet Standard;
   see Section 2 of RFC 7841.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   https://www.rfc-editor.org/info/rfc9717.

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Table of Contents

   1.  Introduction
     1.1.  Related Work
     1.2.  Terms and Abbreviations
   2.  Overview
     2.1.  Topological Considerations
     2.2.  Link Changes
     2.3.  Scalability
     2.4.  Assumptions
       2.4.1.  Traffic Patterns
       2.4.2.  User Station Constraints
       2.4.3.  Stochastic Connectivity
     2.5.  Problem Statement
   3.  Forwarding Plane
   4.  IGP Suitability and Scalability
   5.  Stripes and Areas
   6.  Traffic Forwarding and Traffic Engineering
   7.  Scheduling
   8.  Future Work
   9.  Deployment Considerations
   10. Security Considerations
   11. IANA Considerations
   12. References
     12.1.  Normative References
     12.2.  Informative References
   Acknowledgements
   Author's Address

1.  Introduction

   Satellite networks present some interesting challenges for packet
   networking.  The entire topology is continually in motion, with links
   far less reliable than what is common in terrestrial networks.  Some
   changes to link connectivity can be anticipated due to orbital
   dynamics.

   This document proposes a scalable routing architecture for satellite
   networks based on existing routing protocols and mechanisms that is
   enhanced with scheduled link connectivity change information.  This
   document proposes no protocol changes.

   Large-scale satellite networks are being deployed, presenting an
   unforeseen application for conventional routing protocols.  The high
   rate of intentional topological change and the extreme scale are
   unprecedented in terrestrial networking.  Links between satellites
   can utilize free-space optics technology that allows liberal
   connectivity.  Still, there are limitations due to the range of the
   links and conjunction with the sun, resulting in links that are far
   less reliable than network designers are used to.  In addition, links
   can change their endpoints dynamically, resulting in structural
   changes to the topology.

   Current satellite networks are proprietary, and little information is
   generally available for analysis and discussion.  This document is
   based on what is currently accessible.

   This document proposes one approach to provide a routing architecture
   for such networks utilizing current standards-based routing
   technology and to provide a solution for the scalability of the
   network while incorporating the rapid rate of topological change.
   This document intends to provide some initial guidance for satellite
   network operators, but without specific details, this document can
   only provide the basis for a more complete analysis and design.

   This document presents the author's view and is neither the product
   of the IETF nor a consensus view of the community.

1.1.  Related Work

   A survey of related work can be found in [Westphal].  Link-state
   routing for satellite networks has been considered in [Cao] and
   [Zhang].

1.2.  Terms and Abbreviations

   Constellation:  A set of satellites.

   Downlink:  The half of a ground link leading from a satellite to an
      Earth station.

   Earth station:  A node in the network that is on or close to the
      planetary surface and has a link to a satellite.  This includes
      ships, aircraft, and other vehicles below LEO [ITU].

   Gateway:  An Earth station that participates in the network and acts
      as the interconnect between satellite constellations and the
      planetary network.  Gateways have a much higher bandwidth than
      user stations, have ample computing capabilities, and perform
      traffic engineering duties, subsuming the functionality of a
      network controller or Path Computation Element (PCE) [RFC4655].
      Multiple gateways are assumed to exist, and each serves a portion
      of the network.

   GEO:  Geostationary Earth Orbit.  A satellite in GEO has an orbit
      that is synchronized to planetary rotation, so it effectively sits
      over one spot on the planet.

   Ground link:  A link between a satellite and an Earth station,
      composed of a downlink and an uplink.

   IGP:  Interior Gateway Protocol.  A routing protocol that is used
      within a single administrative domain.  Note that 'gateway' in
      this context is semantically equivalent to 'router' and has no
      relationship to the 'gateway' used in the rest of this document.

   IS-IS:  Intermediate System to Intermediate System.  An IGP that is
      commonly used by service providers [ISO10589] [RFC1195].

   ISL:  Inter-Satellite Link.  Frequently implemented with free-space
      optics that allow signaling using photons without any intervening
      medium [Bell].

   L1:  IS-IS Level 1

   L1L2:  IS-IS Level 1 and Level 2

   L2:  IS-IS Level 2

   LEO:  Low Earth Orbit.  A satellite in LEO has an altitude of 2,000
      km or less.

   Local gateway:  Each user station is associated with a single gateway
      in its region.

   LSP:  Link State Protocol Data Unit.  An IS-IS LSP is a set of
      packets that describe a node's connectivity to other nodes.

   MEO:  Medium Earth Orbit.  A satellite in MEO is between LEO and GEO
      and has an altitude between 2,000 km and 35,786 km.

   SID:  Segment Identifier [RFC8402]

   Stripe:  A set of satellites in a few adjacent orbits.  These form an
      IS-IS L1 area.

   SR:  Segment Routing [RFC8402]

   Uplink:  The half of a link leading from an Earth station to a
      satellite.

   User station:  An Earth station interconnected with a small end-user
      network.

2.  Overview

2.1.  Topological Considerations

   Satellites travel in specific orbits around their parent planets.
   Some of them have their orbital periods synchronized to planetary
   rotation, so they are effectively stationary over a single point.
   Other satellites have orbits that cause them to travel across regions
   of the planet either gradually or quite rapidly.  Respectively, these
   are typically known as the Geostationary Earth Orbit (GEO), Medium
   Earth Orbit (MEO), or Low Earth Orbit (LEO), depending on the
   altitude.  This discussion is not Earth-specific; as we get to other
   planets, we can test this approach's generality.

   Satellites may have data interconnections with one another through
   Inter-Satellite Links (ISLs).  Due to differences in orbits, ISLs may
   be connected temporarily with periods of potential connectivity
   computed through orbital dynamics.  Multiple satellites may be in the
   same orbit but separated in space with a roughly constant separation.
   Satellites in the same orbit may have ISLs that have a higher duty
   cycle than ISLs between different orbits, but they are still not
   guaranteed to be always connected.

     User           +-----------------+    Local
     Stations   --- |   Satellites    |--- Gateway --- Internet
                    +-----------------+

                   Figure 1: Overall Network Architecture

   Earth stations can communicate with one or more satellites in their
   region.  User stations are Earth stations with a limited capacity
   that communicate with only a single satellite at a time.  Other Earth
   stations that may have richer connectivity and higher bandwidth are
   commonly called "gateways" and provide connectivity between the
   satellite network and conventional wired networks.  Gateways serve
   user stations in their geographic proximity and are replicated
   globally as necessary to provide coverage and to meet service density
   goals.  User stations are associated with a single local gateway.
   Traffic from one Earth station to another may need to traverse a path
   across multiple satellites via ISLs.

2.2.  Link Changes

   Like conventional network links, ISLs and ground links can fail
   without warning.  However, unlike terrestrial links, there are
   predictable times when ISLs and ground links can potentially connect
   and disconnect.  These predictions can be computed and cataloged in a
   schedule that can be distributed to relevant network elements.
   Predictions of a link connecting are not guaranteed: A link may not
   connect for many reasons.  Link disconnection predictions due to
   orbital dynamics are effectively guaranteed, as the underlying
   physics will not improve unexpectedly.

2.3.  Scalability

   Some proposed satellite networks are fairly large, with tens of
   thousands of proposed satellites [CNN].  A key concern is the ability
   to reach this scale and larger, as useful networks tend to grow.

   As we know, the key to scalability is the ability to create
   hierarchical abstractions, so a key question of any routing
   architecture will be about the abstractions that can be created to
   contain topological information.

   Normal routing protocols are architected to operate with a static but
   somewhat unreliable topology.  Satellite networks lack the static
   organization of terrestrial networks, so normal architectural
   practices for scalability may not apply, and alternative approaches
   may need consideration.

   In a typical deployment of a link-state routing protocol, current
   implementations can be deployed with a single area that spans a few
   thousand routers.  A single area would also provide no isolation for
   topological changes, causing every link change to be propagated
   throughout the entire network.  This would be insufficient for the
   needs of large satellite networks.

   Multiple areas or multiple instances of an Interior Gateway Protocol
   (IGP) can be used to improve scalability, but there are limitations
   to typical approaches.

   Currently, the IETF actively supports two link-state IGPs: OSPF
   [RFC2328] [RFC5340] and IS-IS.

   OSPF requires that the network operate around a backbone area, with
   subsidiary areas hanging off of the backbone.  While this works well
   for typical terrestrial networks, this does not seem appropriate for
   satellite networks, where there is no centralized portion of the
   topology.

   IS-IS has a different hierarchical structure, where Level 1 (L1)
   areas are connected sets of nodes, and then Level 2 (L2) is a
   connected subset of the topology that intersects all of the L1 areas.
   Individual nodes can be L1, L2, or both (L1L2).  Typical IS-IS
   designs require that any node or link that is to be used as transit
   between L2 areas must appear as part of the L2 topology.  In a
   satellite network, any satellite could end up being used for L2
   transit, and so every satellite and link would be part of L2,
   negating any scalability benefits from IS-IS's hierarchical
   structure.

   We elaborate on considerations specific to IS-IS in Section 4.

2.4.  Assumptions

   In this section, we discuss some of the assumptions that are the
   basis for this architectural proposal.

   The data payload is IP packets.

   Satellites are active participants in the control and data planes for
   the network, participating in protocols and forwarding packets.

   There may be a terrestrial network behind each gateway that may
   interconnect to the broader Internet.  The architecture of the
   terrestrial network is assumed to be a typical IS-IS and BGP
   deployment [RFC4271] and is not discussed further in this document.

   The satellite network interconnects user stations and gateways.
   Interconnection between the satellite network and the satellite
   networks of other network operators is outside the scope of this
   document.

2.4.1.  Traffic Patterns

   We assume that the primary use of the satellite network is to provide
   access from a wide range of geographic locations.  We also assume
   that providing high-bandwidth bulk transit between peer networks is
   not a goal.  It has been noted that satellite networks can provide
   lower latencies than terrestrial fiber networks in [Handley].  This
   proposal does not preclude such applications but does not articulate
   the mechanisms necessary for user stations to perform the appropriate
   traffic engineering computations.  Low-latency, multicast, and
   anycast applications are not discussed further in this document.

   As with most access networks, we assume that there will be
   bidirectional traffic between the user station and the gateway but
   that the bulk of the traffic will be from the gateway to the user
   station.  We expect the uplink from the gateway to the satellite
   network to be the bandwidth bottleneck and that gateways will need to
   be replicated to scale the uplink bandwidth, as the satellite
   capacity reachable from a gateway will be limited.

   We assume that it is not essential to provide optimal routing for
   traffic from user station to user station.  If this traffic is sent
   to a gateway first and then back into the satellite network, it might
   be acceptable to some operators as long as the traffic volume remains
   very low.  This type of routing is not discussed further in this
   document.

   We assume that traffic for a user station should enter the satellite
   network through a gateway that is in some close geographic proximity
   to the user station.  This is to reduce the number of ISLs used by
   the path to the user station.  Similarly, we assume that user station
   traffic should exit the satellite network through the gateway that is
   in the closest geographic proximity to the user station.
   Jurisdictional requirements for landing traffic in certain regions
   may alter these assumptions, but such situations are outside the
   scope of this document.

   This architecture does not preclude gateway-to-gateway traffic across
   the satellite constellations, but it does not seek to optimize it.

2.4.2.  User Station Constraints

   The user station is an entity whose operation is conceptually shared
   between the satellite constellation operator and the operator of the
   cluster of end stations it serves.  For example, the user station is
   trusted to attach MPLS label stacks to end-user packets.  It gets the
   information to do so from some combination of its direct satellite
   and its local gateway via protocols outside the scope of this
   document.  Equally, it bootstraps communication via an exchange with
   the current local satellite so that it can find and communicate with
   its local gateway -- again with the details of how that is done being
   outside the scope of this document.

   User stations that can concurrently access multiple satellites are
   not precluded by this proposal but are not discussed in detail.

2.4.3.  Stochastic Connectivity

   We assume that links in general will be available when scheduled.  As
   with any network, there will be failures, and the schedule is not a
   guarantee, but we also expect that the schedule is mostly accurate.
   We assume that at any given instant, there are enough working links
   and aggregate bandwidth to run the network and support the traffic
   demand.  If this assumption does not hold, no routing architecture
   can magically make the network more capable.

   Satellites that are in the same orbit may be connected by ISLs.
   These are called "intra-orbit" ISLs.  Satellites that are in
   different orbits may also be connected by ISLs.  These are called
   "inter-orbit" ISLs.  Generally, we assume that intra-orbit ISLs have
   higher reliability and persistence than inter-orbit ISLs.

   We assume that the satellite network is connected (in the graph
   theory sense) almost always, even if some links are down.  This
   implies that there is almost always some path to the destination.  In
   the extreme case with no such path, we assume that it is acceptable
   to drop the payload packets.  We do not require buffering of traffic
   when a link is down.  Instead, traffic should be rerouted.

2.5.  Problem Statement

   The goal of the routing architecture is to provide an organizational
   structure to protocols running on the satellite network.  This
   architecture must convey topology information to relevant portions of
   the network.  This enables path computation that is used for data
   forwarding.  The architecture must also scale without global changes
   to the organizational structure.

3.  Forwarding Plane

   The end goal of a network is to deliver traffic.  In a satellite
   network where the topology is in a continual state of flux and the
   user stations frequently change their association with the
   satellites, having a highly flexible and adaptive forwarding plane is
   essential.  Toward this end, we propose using MPLS as the fundamental
   forwarding plane architecture [RFC3031].  Specifically, we propose
   using an approach based on Segment Routing (SR) [RFC8402] with an
   MPLS data plane [RFC8660], where each satellite is assigned a node
   Segment Identifier (SID).  This allows the architecture to support
   both IPv4 and IPv6 concurrently.  A path through the network can then
   be expressed as a label stack of node SIDs.  IP forwarding is not
   used within the internals of the satellite network, although each
   satellite may be assigned an IP address for management purposes.
   Existing techniques may be used to limit the size of the SR label
   stack so that it only contains the significant waypoints along the
   path [Giorgetti].  The label stack operates as a loose source route
   through the network.  If there is an unexpected topology change in
   the network, the IGP will compute a new path to the next waypoint,
   allowing packet delivery despite ISL failures.  While the IGP is
   converging, there may be micro-loops in the topology.  These can be
   avoided by using Topology Independent Loop-Free Alternate (TI-LFA)
   paths [SR-TI-LFA]; otherwise, traffic will loop until discarded based
   on its TTL.

   We assume that there is a link-layer mechanism for a user station to
   associate with a satellite.  User stations will have an IP address
   assigned from a prefix managed by its local gateway.  The mechanisms
   for this assignment and its communication to the end station are not
   discussed herein but might be similar to DHCP [RFC2131].  User
   station IP addresses change infrequently and do not reflect their
   association with their first-hop satellite.  Gateways and their
   supporting terrestrial networks advertise prefixes covering all its
   local user stations throughout the global Internet.

   User stations may be assigned a node SID, in which case MPLS
   forwarding can be used for all hops to the user station.
   Alternatively, if the user station does not have a node SID, then the
   last hop from the satellite to the end station can be performed based
   on the destination IP address of the packet.  This does not require a
   full longest-prefix-match lookup, as the IP address is merely a
   unique identifier at this point.

   Similarly, gateways may be assigned a node SID.  A possible
   optimization is that a single SID value could be assigned as a global
   constant to always direct traffic to the topologically closest
   gateway.  If traffic engineering is required for traffic that is
   flowing to a gateway, a specific path may be encoded in a label stack
   that is attached to the packet by the user station or by the first-
   hop satellite.

   Gateways can also perform traffic engineering using different paths
   and label stacks for separate traffic flows.  Routing a single
   traffic flow across multiple paths has proven to cause performance
   issues with transport protocols, so that approach is not recommended.
   Traffic engineering is discussed further in Section 6.

4.  IGP Suitability and Scalability

   As discussed in Section 2.3, IS-IS is architecturally the best fit
   for satellite networks but does require some novel approaches to
   achieve the scalability goals for a satellite network.  In
   particular, we propose that all nodes in the network be L1L2 so that
   local routing is done based on L1 information and then global routing
   is done based on L2 information.

   An interesting property of IS-IS is that it does not require
   interface addresses.  This feature is commonly known as "unnumbered
   interfaces".  This is particularly helpful in satellite topologies
   because it implies that ISLs may be used flexibly.  Sometimes an
   interface might be used as an L1 link to another satellite, and a few
   orbits later, it might be used as an L1L2 link to a completely
   different satellite without any reconfiguration or renumbering.

   Scalability for IS-IS can be achieved through a proposal known as
   "Area Proxy" [RFC9666].  With this proposal, all nodes in an L1 area
   combine their information into a single L2 Link State Protocol Data
   Unit (LSP).  This implies that the size of the L1 Link State Database
   (LSDB) scales as the number of nodes in the L1 area and the size of
   the L2 LSDB scales with the number of L1 areas.

   With Area Proxy, topological changes within an L1 area will not be
   visible to other areas, so the overhead of link-state changes will be
   greatly reduced.

   The Area Proxy proposal also includes the concept of an Area SID.
   This is useful because it allows traffic engineering to construct a
   path that traverses areas with a minimal number of label stack
   entries.

   For example, suppose that a network has 1,000 L1 areas, each with
   1,000 satellites.  This would mean that the network supports
   1,000,000 satellites but only requires 1,000 entries in its L1 LSDB
   and 1,000 entries in its L2 LSDB, which are easily achievable numbers
   today.  The resulting MPLS label table would contain 1,000 node SIDs
   from the L1 (and L2) LSDB and 1,000 area SIDs from the L2 LSDB.  If
   each satellite advertises an IP address for management purposes, then
   the IP routing table would have 1,000 entries for the L1 management
   addresses and 1,000 area proxy addresses from L2.

   In this proposal, IS-IS does not carry IP routes other than those in
   the satellite topology.  In particular, there are no IP routes for
   user stations or the remainder of the Internet.

5.  Stripes and Areas

   A significant problem with any link-state routing protocol is that of
   area partition.  While there have been many proposals for automatic
   partition repair, none has seen notable production deployment.  It
   seems best to avoid this issue and ensure areas have an extremely low
   probability of partitioning.

   As discussed above, intra-orbit ISLs are assumed to have higher
   reliability and persistence than inter-orbit ISLs.  However, even
   intra-orbit ISLs are not sufficiently reliable to avoid partition
   issues.  Therefore, we propose to group a small number of adjacent
   orbits as an IS-IS L1 area, called a "stripe".  We assume that for
   any given reliability requirement, there is a small number of orbits
   that can be used to form a stripe that satisfies the reliability
   requirement.

   Stripes are connected to other adjacent stripes using the same ISL
   mechanism, forming the L2 topology of the network.  Each stripe
   should have multiple L2 connections and never become partitioned from
   the remainder of the network.

   By using a stripe as an L1 area, in conjunction with Area Proxy, the
   overhead of the architecture is greatly reduced.  Each stripe
   contributes a single LSP to the L2 LSDB, completely hiding all the
   details about the satellites within the stripe.  The resulting
   architecture scales proportionately to the number of stripes
   required, not the number of satellites.

   Groups of MEO and GEO satellites with interconnecting ISLs can also
   form an IS-IS L1L2 area.  Satellites that lack intra-constellation
   ISLs are better as independent L2 nodes.

6.  Traffic Forwarding and Traffic Engineering

   The forwarding architecture presented here is straightforward.  A
   path from a gateway to a user station on the same stripe only
   requires a single node SID for the satellite that provides the
   downlink to the user station.

                \
    Gateway -->  X
                  \
                   \
                    X
                     \
                      \
                       X ---> x  User Station
                        \

                       Figure 2: On-Stripe Forwarding

   Similarly, a user station returning a packet to a gateway need only
   provide a gateway node SID.

   For off-stripe forwarding, the situation is a bit more complex.  A
   gateway would need to provide the area SID of the final stripe on the
   path plus the node SID of the downlink satellite.  For return
   traffic, user stations or first-hop satellites would want to provide
   the area SID of the stripe that contains the satellite that provides
   access to the gateway as well as the gateway SID.

    Source S
       |
       |
       V
    Internet
       |
       |
       V          \
    Gateway L -->  X
                    \
                     \     \
                      X --- X
                       \     \
                              \     \    Area A
                               X --- X
                                \     \
                                       \
                                        U ---> D   User Station
                                         \

                      Figure 3: Off-Stripe Forwarding

   As an example (Figure 3), consider a packet from an Internet source
   (Source S) to a user station (D).  A local gateway (Gateway L) has
   injected a prefix covering D into BGP and has advertised it globally.
   The packet is forwarded to L using IP forwarding.  When L receives
   the packet, it performs a lookup in a pre-computed forwarding table.
   This contains a SID list for the user station that has already been
   converted into a label stack.  Suppose the user station is currently
   associated with a different stripe so that the label stack will
   contain an area label (A) and a label (U) for the satellite
   associated with the user station, resulting in a label stack (A, U).

   The local gateway forwards this into the satellite network.  The
   first-hop satellite now forwards based on the area label (A) at the
   top of the stack.  All area labels are propagated as part of the L2
   topology.  This forwarding continues until the packet reaches a
   satellite adjacent to the destination area.  That satellite pops
   label A, removing that label and forwarding the packet into the
   destination area.

   The packet is now forwarded based on the remaining label U, which was
   propagated as part of the L1 topology.  The last satellite forwards
   the packet based on the destination address (D) and forwards the
   packet to the user station.

   The return case is similar.  The label stack, in this case, consists
   of a label for the local gateway's stripe/area (A') and the label for
   the local gateway (L), resulting in the stack (A', L).  The
   forwarding mechanisms are similar to the previous case.

   Very frequently, access networks congest due to over-subscription and
   the economics of access.  Network operators can use traffic
   engineering to ensure that they get higher efficiency out of their
   networks by utilizing all available paths and capacity near any
   congestion points.  In this particular case, the gateway will have
   information about all of the traffic it is generating and can use all
   of the possible paths through the network in its topological
   neighborhood.  Since we're already using SR, this is easily done by
   adding more explicit SIDs to the label stack.  These can be
   additional area SIDs, node SIDs, or adjacency SIDs.  Path computation
   can be performed by Path Computation Elements (PCEs) [RFC4655].

   Each gateway or its PCE will need topological information from the
   areas it will route through.  It can do this by participating in the
   IGP directly, via a tunnel, or through another delivery mechanism
   such as BGP-LS [RFC9552].  User stations do not participate in the
   IGP.

   Traffic engineering for packets flowing into a gateway can also be
   provided by an explicit SR path.  This can help ensure that ISLs near
   the gateway do not congest with traffic for the gateway.  These paths
   can be computed by the gateway or PCE and then distributed to the
   first-hop satellite or user station, which would apply them to
   traffic.  The delivery mechanism is outside the scope of this
   document.

7.  Scheduling

   The most significant difference between terrestrial and satellite
   networks from a routing perspective is that some of the topological
   changes that will happen to the network can be anticipated and
   computed.  Both link and node changes will affect the topology, and
   the network should react smoothly and predictably.

   The management plane is responsible for providing information about
   scheduled topological changes.  The exact details of how the
   information is disseminated are outside the scope of this document
   but could be shown through a YANG model [YANG-SCHEDULE].  Scheduling
   information needs to be accessible to all of the nodes that will make
   routing decisions based on the topological changes in the schedule
   (i.e., data about an L1 topological change will need to be circulated
   to all nodes in the L1 area and information about L2 changes will
   need to propagate to all L2 nodes) and to the gateways and PCEs that
   carry the related topological information.

   There is very little that the network should do in response to a
   topological addition.  A link coming up or a node joining the
   topology should not have any functional change until the change is
   proven to be fully operational based on the usual IS-IS liveness
   mechanisms.  Nodes may pre-compute their routing table changes but
   should not install them before all relevant adjacencies are received.
   The benefits of this pre-computation appear to be very small.
   Gateways and PCEs may also choose to pre-compute paths based on these
   changes but should not install paths using the new parts of the
   topology until they are confirmed to be operational.  If some path
   pre-installation is performed, gateways and PCEs must be prepared for
   the situation where the topology fails to become operational and may
   need to take alternate steps instead, such as reverting any related
   pre-installed paths.

   The network may choose not to pre-install or pre-compute routes in
   reaction to topological additions, at a small cost of some
   operational efficiency.

   Topological deletions are an entirely different matter.  If a link or
   node is to be removed from the topology, the network should act
   before the anticipated change to route traffic around the expected
   topological loss.  Specifically, at some point before the topology
   change, the affected links should be set to a high metric to direct
   traffic to alternate paths.  This is a common operational procedure
   in existing networks when links are taken out of service, such as
   when proactive maintenance needs to be performed.  This type of
   change does require some time to propagate through the network, so
   the metric change should be initiated far enough in advance that the
   network converges before the actual topological change.  Gateways and
   PCEs should also update paths around the topology change and install
   these changes before the topology change occurs.  The time necessary
   for both IGP and path changes will vary depending on the exact
   network and configuration.

   Strictly speaking, changing to a high metric should not be necessary.
   It should be possible for each router to exclude the link and
   recompute paths.  However, it seems safer to change the metric and
   use the IGP methods for indicating a topology change, as this can
   help avoid issues with incomplete information dissemination and
   synchronization.

8.  Future Work

   This architecture needs to be validated by satellite operators, both
   via simulation and operational deployment.  Meaningful simulation
   hinges on the exact statistics of ISL connectivity; currently, that
   information is not publicly available.

   Current available information about ISLs indicates that links are
   mechanically steered and will need to track the opposite end of the
   link continually.  The angles and distances that can be practically
   supported are unknown, as are any limitations about the rate of
   change.

   It is expected that intra-orbit and inter-orbit ISL links will have
   very different properties.  Intra-orbit links should be much more
   stable but still far less stable than terrestrial links.  Inter-orbit
   links will be less stable.  Links between satellites that are roughly
   parallel should be possible but will likely have a limited duration.
   Two orbits may be roughly orthogonal, resulting in a limited
   potential for connectivity.  Finally, in some topologies there may be
   parallel orbits where the satellites move in opposite directions,
   giving a relative speed between satellites around 34,000 mph (55,000
   kph).  Links in this situation may not be possible or may be so
   short-lived that they are impractical.

   The key question to address is whether the parameters of a given
   network can yield a stripe assignment that produces stable, connected
   areas that work within the scaling bounds of the IGP.  If links are
   very stable, a stripe could be just a few parallel orbits, with only
   a few hundred satellites.  However, if links are unstable, a stripe
   might have to encompass dozens of orbits and thousands of satellites,
   which might be beyond the scaling limitations of a given IGP's
   implementation.

9.  Deployment Considerations

   The network behind a gateway is expected to be a normal terrestrial
   network.  Conventional routing architectural principles apply.  An
   obvious approach would be to extend IS-IS to the terrestrial network,
   applying L1 areas as necessary for scalability.

   The terrestrial network may have one or more BGP connections to the
   broader Internet.  Prefixes for user stations should be advertised to
   the Internet near the associated gateway.  If gateways are not
   interconnected by the terrestrial network, then it may be advisable
   to use one autonomous system per gateway as it might simplify the
   external perception of the network and subsequent policy
   considerations.  Otherwise, one autonomous system may be used for the
   entire terrestrial network.

10.  Security Considerations

   This document discusses one possible routing architecture for
   satellite networks.  It proposes no new protocols or mechanisms and
   thus has no new security impact.  Security for IS-IS is provided by
   [RFC5304] and [RFC5310].

   User stations will interact directly with satellites, potentially
   using proprietary mechanisms, and under the control of the satellite
   operator, who is responsible for the security of the user station.

11.  IANA Considerations

   This document has no IANA actions.

12.  References

12.1.  Normative References

   [ISO10589] ISO/IEC, "Information technology - Telecommunications and
              information exchange between systems - Intermediate System
              to Intermediate System intra-domain routeing information
              exchange protocol for use in conjunction with the protocol
              for providing the connectionless-mode network service (ISO
              8473)", ISO/IEC 10589:2002, November 2002,
              <https://www.iso.org/standard/30932.html>.

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

   [RFC5304]  Li, T. and R. Atkinson, "IS-IS Cryptographic
              Authentication", RFC 5304, DOI 10.17487/RFC5304, October
              2008, <https://www.rfc-editor.org/info/rfc5304>.

   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, DOI 10.17487/RFC5310, February
              2009, <https://www.rfc-editor.org/info/rfc5310>.

   [RFC8402]  Filsfils, C., Ed., Previdi, S., Ed., Ginsberg, L.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing Architecture", RFC 8402, DOI 10.17487/RFC8402,
              July 2018, <https://www.rfc-editor.org/info/rfc8402>.

   [RFC8660]  Bashandy, A., Ed., Filsfils, C., Ed., Previdi, S.,
              Decraene, B., Litkowski, S., and R. Shakir, "Segment
              Routing with the MPLS Data Plane", RFC 8660,
              DOI 10.17487/RFC8660, December 2019,
              <https://www.rfc-editor.org/info/rfc8660>.

   [RFC9666]  Li, T., Chen, S., Ilangovan, V., and G. Mishra, "Area
              Proxy for IS-IS", RFC 9666, DOI 10.17487/RFC9666, October
              2024, <https://www.rfc-editor.org/info/rfc9666>.

12.2.  Informative References

   [Bell]     Bell, A. G., "On the Production and Reproduction of Sound
              by Light", American Journal of Science, vol. S3-20, no.
              118, pp. 305-324, DOI 10.2475/ajs.s3-20.118.305, October
              1880, <https://ajsonline.org/article/64037>.

   [Cao]      Cao, X., Li, Y., Xiong, X., and J. Wang, "Dynamic Routings
              in Satellite Networks: An Overview", Sensors, vol. 22, no.
              12, pp. 4552, DOI 10.3390/s22124552, 2022,
              <https://www.mdpi.com/1424-8220/22/12/4552/
              pdf?version=1655449925>.

   [CNN]      Wattles, J., "Elon Musk's SpaceX now owns about a third of
              all active satellites in the sky", CNN Business, 11
              February 2021, <https://www.cnn.com/2021/02/11/tech/
              spacex-starlink-satellites-1000-scn/index.html>.

   [Giorgetti]
              Giorgetti, A., Castoldi, P., Cugini, F., Nijhof, J.,
              Lazzeri, F., and G. Bruno, "Path Encoding in Segment
              Routing", 2015 IEEE Global Communications Conference
              (GLOBECOM), DOI 10.1109/GLOCOM.2015.7417097, December
              2015, <https://ieeexplore.ieee.org/document/7417097>.

   [Handley]  Handley, M., "Delay is Not an Option: Low Latency Routing
              in Space", HotNets '18: Proceedings of the 17th ACM
              Workshop on Hot Topics in Networks, pp. 85-91,
              DOI 10.1145/3286062.3286075, November 2018,
              <https://dl.acm.org/doi/10.1145/3286062.3286075#>.

   [ITU]      ITU, "Radio Regulations - Articles", 2024,
              <https://search.itu.int/history/HistoryDigitalCollectionDo
              cLibrary/1.49.48.en.101.pdf#search=radio%20regulation>.

   [RFC1195]  Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
              dual environments", RFC 1195, DOI 10.17487/RFC1195,
              December 1990, <https://www.rfc-editor.org/info/rfc1195>.

   [RFC2131]  Droms, R., "Dynamic Host Configuration Protocol",
              RFC 2131, DOI 10.17487/RFC2131, March 1997,
              <https://www.rfc-editor.org/info/rfc2131>.

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

   [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>.

   [RFC4655]  Farrel, A., Vasseur, J.-P., and J. Ash, "A Path
              Computation Element (PCE)-Based Architecture", RFC 4655,
              DOI 10.17487/RFC4655, August 2006,
              <https://www.rfc-editor.org/info/rfc4655>.

   [RFC5340]  Coltun, R., Ferguson, D., Moy, J., and A. Lindem, "OSPF
              for IPv6", RFC 5340, DOI 10.17487/RFC5340, July 2008,
              <https://www.rfc-editor.org/info/rfc5340>.

   [RFC9552]  Talaulikar, K., Ed., "Distribution of Link-State and
              Traffic Engineering Information Using BGP", RFC 9552,
              DOI 10.17487/RFC9552, December 2023,
              <https://www.rfc-editor.org/info/rfc9552>.

   [SR-TI-LFA]
              Bashandy, A., Litkowski, S., Filsfils, C., Francois, P.,
              Decraene, B., and D. Voyer, "Topology Independent Fast
              Reroute using Segment Routing", Work in Progress,
              Internet-Draft, draft-ietf-rtgwg-segment-routing-ti-lfa-
              19, 22 November 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-rtgwg-
              segment-routing-ti-lfa-19>.

   [Westphal] Westphal, C., Han, L., and R. Li, "LEO Satellite
              Networking Relaunched: Survey and Current Research
              Challenges", arXiv:2310.07546v1,
              DOI 10.48550/arXiv.2310.07646, October 2023,
              <https://arxiv.org/pdf/2310.07646.pdf>.

   [YANG-SCHEDULE]
              Qu, Y., Lindem, A., Kinzie, E., Fedyk, D., and M.
              Blanchet, "YANG Data Model for Scheduled Attributes", Work
              in Progress, Internet-Draft, draft-ietf-tvr-schedule-yang-
              03, 20 October 2024,
              <https://datatracker.ietf.org/doc/html/draft-ietf-tvr-
              schedule-yang-03>.

   [Zhang]    Zhang, X., Yang, Y., Xu, M., and J. Luo, "ASER: Scalable
              Distributed Routing Protocol for LEO Satellite Networks",
              2021 IEEE 46th Conference on Local Computer Networks
              (LCN), DOI 10.1109/LCN52139.2021.9524989, 2021,
              <https://doi.org/10.1109/LCN52139.2021.9524989>.

Acknowledgements

   The author would like to thank Dino Farinacci, Olivier De jonckere,
   Eliot Lear, Adrian Farrel, Acee Lindem, Erik Kline, Sue Hares, Joel
   Halpern, Marc Blanchet, and Dean Bogdanovic for their comments.

Author's Address

   Tony Li
   Juniper Networks
   Email: tony.li@tony.li
