A Routing Architecture for Satellite Networks
draft-li-arch-sat-06
| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Author | Tony Li | ||
| Last updated | 2024-02-21 | ||
| Replaces | draft-li-tvr-architecture | ||
| RFC stream | Independent Submission | ||
| Intended RFC status | Informational | ||
| Formats | |||
| Reviews | |||
| Stream | ISE state | In ISE Review | |
| Consensus boilerplate | Unknown | ||
| Document shepherd | (None) | ||
| IESG | IESG state | I-D Exists | |
| Telechat date | (None) | ||
| Responsible AD | (None) | ||
| Send notices to | (None) |
draft-li-arch-sat-06
Network Working Group T. Li
Internet-Draft Juniper Networks
Intended status: Informational 21 February 2024
Expires: 24 August 2024
A Routing Architecture for Satellite Networks
draft-li-arch-sat-06
Abstract
Satellite networks present some interesting challenges for packet
networking. The entire topology is continually in motion, with links
that are far less reliable than what is common in terrestrial
networks. Some changes to link connectivity can be anticipated due
to orbital mechanics.
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
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
This Internet-Draft will expire on 24 August 2024.
Copyright Notice
Copyright (c) 2024 IETF Trust and the persons identified as the
document authors. All rights reserved.
Li Expires 24 August 2024 [Page 1]
Internet-Draft Routing for Satellites February 2024
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents (https://trustee.ietf.org/
license-info) in effect on the date of publication of this document.
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document. Code Components
extracted from this document must include Revised BSD License text as
described in Section 4.e of the Trust Legal Provisions and are
provided without warranty as described in the Revised BSD License.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
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 . . . . . . . . . . . . . . . 8
2.5. Problem Statement . . . . . . . . . . . . . . . . . . . . 9
3. Forwarding Plane . . . . . . . . . . . . . . . . . . . . . . 9
4. IGP Suitability and Scalability . . . . . . . . . . . . . . . 10
5. Stripes and Areas . . . . . . . . . . . . . . . . . . . . . . 11
6. Traffic Forwarding and Traffic Engineering . . . . . . . . . 12
7. Scheduling . . . . . . . . . . . . . . . . . . . . . . . . . 14
8. Future Work . . . . . . . . . . . . . . . . . . . . . . . . . 16
9. Deployment Considerations . . . . . . . . . . . . . . . . . . 16
10. Security Considerations . . . . . . . . . . . . . . . . . . . 16
11. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 16
12. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 17
13. References . . . . . . . . . . . . . . . . . . . . . . . . . 17
13.1. Normative References . . . . . . . . . . . . . . . . . . 17
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
that are far less reliable than what is common in terrestrial
networks. Some changes to link connectivity can be anticipated due
to orbital mechanics.
Li Expires 24 August 2024 [Page 2]
Internet-Draft Routing for Satellites February 2024
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 coupled with the extreme scale
are unprecedented in terrestrial networking. Links between
satellites can utilize free-space optics technology that allows
liberal connectivity, but 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.
This document proposes one approach to provide a routing architecture
for such networks utilizing current routing technology and providing
a solution for the scalability of the network while incorporating the
rapid rate of topological change.
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.
* Gateway: An Earth station that participates as part of 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, with each serving a
portion of the network.
Li Expires 24 August 2024 [Page 3]
Internet-Draft Routing for Satellites February 2024
* 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.
* Earth station: A node in the network that is on or close to the
surface of the planet and has a link to a satellite. This
includes ships, aircraft, and other vehicles below LEO. [ITU]
* 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]
* Stripe: A set of satellites in a few adjacent orbits. These form
an IS-IS L1 area.
* SR: Segment Routing [RFC8402]
Li Expires 24 August 2024 [Page 4]
Internet-Draft Routing for Satellites February 2024
* 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 the rotation
of the planet, 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 mechanics. 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 always be connected.
User +-----------------+ Local
Stations --- | Satellites |--- Gateway --- Internet
+-----------------+
Figure 1: Overall network architecture
Earth stations can communicate with one or more satellites that are
in their region. User stations are Earth stations that have 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 that are in their geographic
proximity and are replicated across the globe as necessary to provide
coverage and 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.
Li Expires 24 August 2024 [Page 5]
Internet-Draft Routing for Satellites February 2024
2.2. Link Changes
Like conventional network links, ISLs and ground links can fail at
any time. However, unlike conventional 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 a guarantee: a link may not
connect for a variety of reasons. Predictions of a link
disconnecting due to orbital mechanics are effectively guaranteed, as
the underlying physics is extremely unlikely to 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.
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.
Li Expires 24 August 2024 [Page 6]
Internet-Draft Routing for Satellites February 2024
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 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 also does not articulate the
mechanisms necessary for user stations to perform the necessary
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
Li Expires 24 August 2024 [Page 7]
Internet-Draft Routing for Satellites February 2024
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
first to a gateway 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.
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, then no routing
Li Expires 24 August 2024 [Page 8]
Internet-Draft Routing for Satellites February 2024
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 all the time, even if some links are down. This
implies that there is almost always some path to the destination. In
the extreme case where there is 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 to a
very large network 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 are frequently changing 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 be then 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] This implies that the label stack operates as a form
of loose-source routing through the network. If there is an
unexpected topology change in the network, then 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 through the use of TI-LFA
Li Expires 24 August 2024 [Page 9]
Internet-Draft Routing for Satellites February 2024
alternate paths [I-D.ietf-rtgwg-segment-routing-ti-lfa], or traffic
will loop until it is 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
that is 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 of 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 by using different
paths and label stacks for different 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.
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
Li Expires 24 August 2024 [Page 10]
Internet-Draft Routing for Satellites February 2024
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 the use of a proposal
known as Area Proxy [I-D.ietf-lsr-isis-area-proxy]. With this
proposal, all of the 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 any IP routes other than those
that are 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 significant production deployment.
It seems best to simply avoid this issue altogether and ensure that
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
Li Expires 24 August 2024 [Page 11]
Internet-Draft Routing for Satellites February 2024
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 should 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 of 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.
\
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.
Li Expires 24 August 2024 [Page 12]
Internet-Draft Routing for Satellites February 2024
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
pre-computed forwarding table. This contains a SID list for the user
station that has already been converted into a label stack. Suppose
that 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 that is 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.
Li Expires 24 August 2024 [Page 13]
Internet-Draft Routing for Satellites February 2024
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 are getting 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 that 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 all of
the areas that it will route through. It can do this by being a
participant in the IGP either directly, via a tunnel, or another
delivery mechanism such as BGP-LS [RFC9552]. User stations do not
participate in the IGP.
Traffic engineering for traffic into a gateway can also be provided
by an explicit SR path for the traffic. 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
then 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.
Li Expires 24 August 2024 [Page 14]
Internet-Draft Routing for Satellites February 2024
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.united-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 information 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 reaction 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 liveness mechanisms
found within IS-IS. Nodes may pre-compute their routing table
changes but should not install them before all of the 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 be careful to 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 does not become operational and may need to take
alternate steps instead, such as reverting any related pre-installed
paths.
The network may choose to not do any pre-installation or pre-
computation 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, then 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 takes place. The time
necessary for both IGP and path changes will vary depending on the
exact network and configuration.
Li Expires 24 August 2024 [Page 15]
Internet-Draft Routing for Satellites February 2024
Strictly speaking, changing to a high metric should not be necessary.
It should be possible for each router to simply exclude the link and
recompute paths. However, it seems safer to also 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 at present.
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
into 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, and Dean Bogdanovic for their comments.
Li Expires 24 August 2024 [Page 16]
Internet-Draft Routing for Satellites February 2024
12. IANA Considerations
This document makes no requests for IANA.
13. References
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>.
Li Expires 24 August 2024 [Page 17]
Internet-Draft Routing for Satellites February 2024
13.2. Informative References
[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-
13, 16 January 2024,
<https://datatracker.ietf.org/doc/html/draft-ietf-rtgwg-
segment-routing-ti-lfa-13>.
[I-D.united-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-united-tvr-schedule-
yang-00, 11 October 2023,
<https://datatracker.ietf.org/doc/html/draft-united-tvr-
schedule-yang-00>.
Li Expires 24 August 2024 [Page 18]
Internet-Draft Routing for Satellites February 2024
[ITU] "ITU Radio Regulations, Article 1", 2016,
<https://search.itu.int/history/
HistoryDigitalCollectionDocLibrary/1.43.48.en.101.pdf>.
[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>.
Li Expires 24 August 2024 [Page 19]
Internet-Draft Routing for Satellites February 2024
Author's Address
Tony Li
Juniper Networks
Email: tony.li@tony.li
Li Expires 24 August 2024 [Page 20]
