A Routing Architecture for Satellite Networks
draft-li-arch-sat-09
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| Last updated | 2024-08-30 | ||
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draft-li-arch-sat-09
Network Working Group T. Li
Internet-Draft Juniper Networks
Intended status: Informational 30 August 2024
Expires: 3 March 2025
A Routing Architecture for Satellite Networks
draft-li-arch-sat-09
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, 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 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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and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 3 March 2025.
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
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
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provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Related Work . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Terms and Abbreviations . . . . . . . . . . . . . . . . . 3
2. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . 5
2.1. Topological Considerations . . . . . . . . . . . . . . . 5
2.2. Link Changes . . . . . . . . . . . . . . . . . . . . . . 6
2.3. Scalability . . . . . . . . . . . . . . . . . . . . . . . 6
2.4. Assumptions . . . . . . . . . . . . . . . . . . . . . . . 7
2.4.1. Traffic Patterns . . . . . . . . . . . . . . . . . . 7
2.4.2. User Station Contraints . . . . . . . . . . . . . . . 8
2.4.3. Stochastic Connectivity . . . . . . . . . . . . . . . 9
2.5. Problem Statement . . . . . . . . . . . . . . . . . . . . 9
3. Forwarding Plane . . . . . . . . . . . . . . . . . . . . . . 9
4. IGP Suitability and Scalability . . . . . . . . . . . . . . . 11
5. Stripes and Areas . . . . . . . . . . . . . . . . . . . . . . 12
6. Traffic Forwarding and Traffic Engineering . . . . . . . . . 12
7. Scheduling . . . . . . . . . . . . . . . . . . . . . . . . . 15
8. Future Work . . . . . . . . . . . . . . . . . . . . . . . . . 16
9. Deployment Considerations . . . . . . . . . . . . . . . . . . 17
10. Security Considerations . . . . . . . . . . . . . . . . . . . 17
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 17
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
13.1. Normative References . . . . . . . . . . . . . . . . . . 18
13.2. Informative References . . . . . . . . . . . . . . . . . 18
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 20
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.
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This document proposes a scalable routing architecture for satellite
networks based on existing routing protocols and mechanisms, 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 providing 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]
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* 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, each serving 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 routing
protocol. 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
2,000km or less.
* Local gateway: Each user station is associated with a single
gateway in its region.
* LSP: IS-IS Link State Protocol Data Unit. An 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 orbits and has an altitude between 2,000km and 35,786km.
* SID: Segment Identifier [RFC8402]
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* 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 planet.
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 gradually or quite rapidly. Respectively, these are
typically known as Geostationary Earth Orbits (GEO), Medium Earth
Orbit (MEO), or Low Earth Orbit (LEO), depending on 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 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 and
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 meet service density
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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 traditional 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 IGP can be used to improve
scalability, but there are limitations to traditional approaches.
The IETF currently actively supports two link-state Interior Gateway
Protocols (IGPs): OSPF [RFC2328][RFC5340] and IS-IS.
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OSPF requires that the network operate around a backbone area, with
subsidiary areas hanging off of the backbone. While this works well
for traditional 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). Traditional 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 IS-IS-specific considerations 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 plane 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
[RFC4271] deployment and is not discussed further.
The satellite network interconnects user stations and gateways.
Interconnection between the satellite network and the satellite
networks of other network operators is outside of 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 [Handley]. This
proposal does not preclude such applications but does not articulate
the mechanisms necessary for user stations to perform the appropriate
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traffic engineering computations. Low-latency, multicast, and
anycast applications are not discussed further.
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 that 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, this
might be acceptable to some operators as long as the traffic volume
remains very low. This type of routing is not discussed further.
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 of 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 Contraints
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 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.
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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. We assume that, in general, 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 such that
topology information is conveyed through relevant portions of the
network, that paths are computed across the network, and that data
can be delivered along those paths, and the structure can scale
without any 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 to use MPLS as the
fundamental forwarding plane architecture [RFC3031]. Specifically,
we propose to use a Segment Routing (SR) [RFC8402] based approach
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
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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 TI-LFA alternate paths
[I-D.ietf-rtgwg-segment-routing-ti-lfa], or 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 into 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 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.
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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.
IS-IS has the interesting property 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 [I-D.ietf-lsr-isis-area-proxy]. 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.
Suppose, for example, that a network has 1,000 L1 areas, each with
1,000 satellites. This would then 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; numbers that are easily achievable
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.
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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
Forwarding in this architecture 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.
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\
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, consider a packet from an Internet source S to a user
station D. A local gateway L has injected a prefix covering D into
BGP and advertised it globally. The packet is forwarded to L using
IP forwarding. When L receives the packet, it performs a lookup in a
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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 oversubscription 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 just
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 (PCE) [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 another delivery mechanism such as
BGP-LS [RFC9552]. User stations do not participate in the IGP.
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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 of 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 of the scope of this document
but could be done through a YANG model [I-D.ietf-tvr-schedule-yang].
Scheduling information needs to be accessible to all of the nodes
that will make routing decisions based on the topological changes in
the schedule, so 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, plus 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.
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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, and that
information is not publicly available currently.
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
34,000mph (55,000kph). Links in this situation may not be possible
or may be so short-lived as to be impractical.
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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. 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.
12. IANA Considerations
This document makes no requests for IANA.
13. References
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13.1. Normative References
[I-D.ietf-lsr-isis-area-proxy]
Li, T., Chen, S., Ilangovan, V., and G. S. Mishra, "Area
Proxy for IS-IS", Work in Progress, Internet-Draft, draft-
ietf-lsr-isis-area-proxy-12, 18 January 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-lsr-
isis-area-proxy-12>.
[ISO10589] International Organization for Standardization,
"Intermediate System to Intermediate System Intra-Domain
Routing 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://standards.iso.org/ittf/
PubliclyAvailableStandards/
c030932_ISO_IEC_10589_2002(E).zip>.
[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>.
13.2. Informative References
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[Bell] Bell, A. G., "On the Production and Reproduction of Sound
by Light", American Journal of Science Third Series. XX
(118): 305-324., DOI 10.2475/ajs.s3-20.118.305, October
1880, <https://zenodo.org/records/1450056>.
[Cao] Cao, X., Li, Y., Xiong, X., and J. Wang, "Dynamic Routings
in Satellite Networks: An Overview", Sensors (Basel,
Switzerland), 22(12), 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", 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", IEEE 2015 IEEE Global Communications Conference
(GLOBECOM), DOI 10.1109/GLOCOM.2015.7417097, December
2015, <https://doi.org/10.1109/GLOCOM.2015.7417097>.
[Handley] Handley, M., "Delay is Not an Option: Low Latency Routing
in Space", ACM Proceedings of the 17th ACM Workshop on Hot
Topics in Networks, DOI 10.1145/3286062.3286075, November
2018, <https://doi.org/10.1145/3286062.3286075>.
[I-D.ietf-rtgwg-segment-routing-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-
17, 5 July 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-rtgwg-segment-routing-ti-lfa-17>.
[I-D.ietf-tvr-schedule-yang]
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-
02, 22 July 2024, <https://datatracker.ietf.org/doc/html/
draft-ietf-tvr-schedule-yang-02>.
[ITU] "ITU Radio Regulations, Article 1", 2016,
<https://search.itu.int/history/
HistoryDigitalCollectionDocLibrary/1.43.48.en.101.pdf>.
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[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>.
[Westphal] Westphal, C., Han, L., and R. Li, "LEO Satellite
Networking Relaunched: Survey and Current Research
Challenges", October 2023,
<https://arxiv.org/pdf/2310.07646.pdf>.
[Zhang] Zhang, X., Yang, Y., Xu, M., and J. Luo, "ASER: Scalable
Distributed Routing Protocol for LEO Satellite Networks",
IEEE 46th Conference on Local Computer Networks (LCN),
Edmonton, AB, Canada, 2021,,
DOI 10.1109/LCN52139.2021.9524989, 2021,
<https://doi.org/10.1109/LCN52139.2021.9524989>.
Author's Address
Tony Li
Juniper Networks
Email: tony.li@tony.li
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