RTGWG Working Group P. Liu
Internet-Draft T. Jiang
Intended status: Informational China Mobile
Expires: 3 September 2025 2 March 2025
Routing in Satellite Networks: Challenges & Considerations
draft-lj-rtgwg-sat-routing-consideration-00
Abstract
The SDO 3GPP has done tremendous work to either standardize or study
various types of wireless services that would depend on the satellite
constellation network. While the ISLs, or Inter-Satellite Links,
along with the routing scheme(s) over them are critical to help
fullfil the satellite services, the 3GPP considers them out-of-scope.
This leaves the significant work to be explored in the IETF domain.
This draft stems from the 3GPP satellite use cases that have been
studied for many years up to now, across a couple of releases, and
lands on summarizing the challenges & considerations of the
satellite-based routing. Based on some unique & advantageous
characteristics associated with satellite networks, the draft raises
briefly the general routing considerations for the integrated Non-
Terrestrial & Terrestial Networks.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Terminologies . . . . . . . . . . . . . . . . . . . . . . 3
1.2. 3GPP Rel-18: Satellite as Transparent Relay . . . . . . . 3
1.3. 3GPP Rel-19: Satellite with Regenerative Forwarding . . . 3
1.3.1. Regenerative forwarding & ISLs in Satellite
Network . . . . . . . . . . . . . . . . . . . . . . . 3
1.3.2. Challenges from Store & Forward . . . . . . . . . . . 4
1.4. 3GPP Rel-20: More Use cases & More Challenges . . . . . . 6
2. Multi-orbit Satellite Networks：Problems & Challenges . . . . 7
2.1. Challenge#1: The very dynamics of routing topology . . . 7
2.2. Challenge#2: The limited bandwidth of peering links . . . 8
2.3. Challenge#3: The HW limitation & reduced capabilities . . 8
3. Satellite Routing Considerations . . . . . . . . . . . . . . 9
3.1. Uniqueness of Satellite Movement: Ephemeris . . . . . . . 9
3.2. Routing Considerations for Multi-Orbit Satellite
Networks . . . . . . . . . . . . . . . . . . . . . . . . 9
4. Security Considerations . . . . . . . . . . . . . . . . . . . 11
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 11
6. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 11
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 11
7.1. Normative References . . . . . . . . . . . . . . . . . . 11
7.2. Informative References . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 13
1. Introduction
For the last couple of years and until now, the satellite-based
constellation network has gained significant tractions. There are
more and more stakeholders, spanning satellite service providers,
mobile operators, telecom equipment & chip vendors, OTT cloud
providers, etc., engaging, collaboratively and via various sorts of
standardization development organizations (i.e, SDO's), in the
exploration and research upon how to offer advanced mobile services
over satellite networks. Out of all the mattered SDO's, the 3GPP,
via its 5G and future 6G normative work, is currently demonstrating
the most prominent progresses.
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1.1. Terminologies
* TN: Terrestrial Network; refers to networks providing connectivity
through communication lines that travel on, near, and/or below
ground.
* NTN: Non-Terrestrial Network; refers to networks providing
connectivity through spaceborne satellites.
1.2. 3GPP Rel-18: Satellite as Transparent Relay
The 3GPP Rel-18 has completed two satellite related working items
(WIDs), i.e., the Sat-access [TR.23.700-28] and the Sat-backhaul
[TR.23.700-27]. While the Sat-access WID investigates and
standardizes how 5G mobile devices (or UEs) could access 5G systems
and PLMNs (i.e., Public Land Mobile Networks) via wireless access
networks whose transport services are provided by satellite networks,
the Sat-backhaul WID focuses its standardization work upon utilizing
satellite connectivity for the wireless backhaul service. However,
both the Rel-18 WIDs are based on the satellite 'transparent mode'
[TR.38.821], which concentrates on the deployment architecture of
only one satellite. In both WIDs, the RAN, i.e., eNB for LTE and gNB
for 5G, is situated on the ground. The on-board (i.e., on-satellite)
equipment does only fairly simple functionalities, e.g., frequency
conversion, signal amplification, etc., which makes it act like a
simple reflector, or so-called the 'bent pipe' mode as in
[TR.38.821]. A satellite in this mode is restricted to function only
as a transparent relay. There does not exist any implication from
inter-satellite links or ISLs, nor does it have (layer-2) switching &
(layer-3) routing intelligence invovled.
1.3. 3GPP Rel-19: Satellite with Regenerative Forwarding
1.3.1. Regenerative forwarding & ISLs in Satellite Network
The 3GPP 5G Rel-19 standardization work has a satellite related work
item (WID), i.e., 5GSat_Ph3 [TR.23.700-29]. It studied the
requirements of various kinds of satellite-based services, e.g., SMS,
CIoT, etc., along with the associated challenges to accomplish the
mobile registration, connection management, session establishment,
and policy provisioning, etc. Different from the 'transparent mode'
as described in Section 1.2, this work is standardizing the
'regenerative payload forwarding mode', for which RAN nodes (i.e.,
eNB for LTE and gNB for 5G) will be deployed on-board satellites.
Depending also on the characteristics of the offered mobile services,
there might be other 4G/5G core network functions (NFs) to be
deployed on-board satellite(s). Evidently, the regenerative mode
with multiple satellites and with multiple NF entities on-board
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satellites will certainly go beyond the layer-1 'transparent relay',
and move toward the layer-2 or even layer-3 based switching and/or
routing.
The regenerative mode guarantees the involvement of multiple
independent satellite entities. This leads to naturally the
introduction of the very critical topic for a satellite constellation
network, i.e., the existence of inter-satellite links or ISLs along
with their impact on providing network connectivity among satellites.
1.3.2. Challenges from Store & Forward
The Rel-19 satellite use-case, store & forward or S&F [TR.23.700-29],
features the receiving of a message or datagram at an on-board (i.e.,
on-satellite) RAN from an on-ground UE. However, if the on-board
RAN's connecting link to the on-ground core network is unavailable
(i.e., the so-called unavailability of a feeder link), then the RAN
will be delegated to store the message or datagram. The on-board RAN
continues moving with the (hosting) satellite until the feeder link
can provide the accessibility toward a ground-station (GS). At that
moment, the stored message or datagram (at the on-board RAN) is
delivered to the terrestrial network (TN). For the other direction
of data delivery via the same satellite to the same UE, the satellite
(along with the RAN) will have to rotate one or more rounds until the
RAN (via the coverage of the hosting satellite) can catch the UE
again.
At the first glance, someone might wonder that, even if the rotation
time of one round is indeed long, the satellite will still be able to
orbit back to the same geolocation (relative to Earth) after one
round, at which the UE was previously located. Unfortunatley, this
is not true thanks to Earth's self-rotation. For example, Earth is
self-rotating at approximately 460 meter/sec at the equator.
Assuming a LEO satellite could rotate the Earth one-round in 95 mins
(of course, depending on the satellite's rotation track), then based
on the following formula,
Shift-distance on Earth = Earth-self-rotation-speed * Self-rotation-
period
we have, 460 m/s * (95 mins * 60 sec/min) ~ 2600 KM. This means the
geolocation-shifting at the equator (relative to Earth) after one
round could be more than 2000 Km. This significant shifting is way
beyond the coverage of a RAN that is on-board a LEO satellite,
assuming optical based transmission [Optical-transmission-range].
Therefore, we can inherently draw the conclusion that the multi-
satellite deployment with inter-satellite links (or ISLs) is the most
feasible solution for satellite-based services.
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The Figure 1 shows the multi-satellite constellation network that
serves as the hosting infrastructure for the 4G/5G satellite-based
S&F (Store & Forward) service. In the figure, the wireless network
functions (or NFs) RANs, MMEs and AMFs, etc., are on-board different
satellites, which together provide wireless services to on-ground
UEs. The satellites, with inter-satellite links or ISLs, form a
connected network thru which wireless NFs can exchange operation
context, transport data, sync-up states, and etc. Evidently, the
previously-discussed geolocation-shifting challenge could be
effectively addressed by a multi-satellite network.
MME/AMF: 4G/5G Contro NFs GS: ground-station
TN: terrestrial Network CN: 4G/5G Core Network
: :
: On-board Satellites : On-ground
: :
: +---+ +-------+ :
+---->|RAN|--|MME/AMF|----------------+
| : +---+ +-------+ : |
| : : v
| : +---+ +-------+ : +-----------------+
+---->|RAN|--|MME/AMF|------->| GS / TN / CN |
| : +---+ +-------+ : +-----------------+
| : : ^
+----+ : : |
| UE | : : |
+----+ : : |
| : +---+ +-------+ : |
+---->|RAN|--|MME/AMF|----:-----------+
: +---+ +-------+ :
Figure 1: Multi-SAT Architecture for 4G/5G S&F Service
Another advantage of a multi-satellite network is the latency
reduction in data transfer & delivery. The work in
[UCL-Mark-Handley] has demonstrated thru simulation the better
latency via the use of satellite constellation than purely using the
underground fiber.
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We have to point out that, while ISLs play certainly a very important
role in the Rel-19 satellite work, the architectural assumption and
the corresponding solution proposals of the WID claim that the
network connectivity as provided by ISLs is out of the 3GPP scope
[TR.23.700-29]. While we tend to agree from the 3GPP perspective,
this does leave us an interesting routing topic to explore in the
IETF domain.
1.4. 3GPP Rel-20: More Use cases & More Challenges
The satellite based use cases continue gaining tractions in the 3GPP
Rel-20 study. In [TS.22.887], two use cases have been proposed to
study either the delay- or disruption-tolerant service, i.e.,
resilient notification upon the temporary network unavailability, or
the service continuity in remote areas via multi-orbit satellite
networks.
For the communication between satellites and UEs, the possibly poor
conditions of reception channels and sometimes the lack of LoS (Line
of Sight) might lead to UEs missing important messages. The
resilient notification service specifies a reliable and effective
notification mechanism that delivers alerts (e.g., beacons) to UEs
such that UEs could adjust their spots of signal reception for
(delay-tolerant) critical messages. [TS.22.887] defines resilient
operation mode when either the backhaul link between a LEO satellite
and its corresponding ground station is temporarily unavailable or
the core newtork of the LEO satellite was temporally unaccessible,
for any unusually unexpected reason(s). When a disruption event
occurs, the resilient operation mode of a LEO satellite network helps
(satellite-service) users continue their communication via UE-
Satellite-UE paths.
The same document [TS.22.887] also strives to achieve seamless
network connectivity and service continuity via a multi-orbit
satellite constellation network, with which the (limited) coverage as
experienced by LEO satellites would be complemented by services from
medium-Earth-Orbit (MEO) and/or Geo-stationary (GEO) satellites.
Taking the resilient operation as the example: for some rare case, if
there is no available communication path from a serving LEO to the
ground station via LEO-based inter-satellite links (ISLs), the
satellite system may continue searching via MEO or even GEO
satellites to achieve the service continuity.
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2. Multi-orbit Satellite Networks：Problems & Challenges
A satellite constellation network is generally comprised of tens of
thousands of (satellite) nodes. This implies the application of pre-
configured switching is impractical, nor is the static routing with
certain intelligence. This also means the transparent payload mode
shall be out of the picture. This leaves the only feasible candidate
the dynamic routing scheme. However, a non-terrestrial network (or
NTN) in the space bears some uniqueness to be considered for the
adoption of dynamic routing protocol. We will analyze the special
challenges of running dynamic routing over the integrated NTN & TN.
2.1. Challenge#1: The very dynamics of routing topology
The rotation variations of satellites result in two types of routing
dynamics [ICNP23-6G.SQSC-Sat.Comm]. They are the dynamics thanks to
the intermittent & varied connectivities between on-ground nodes and
satellites, and the dynamics as caused by the ever-lasting satellite
movements & thus the ISLs/neighborship flappings.
* Dynamics between on-ground routing nodes and satellites: because
of the versatile satellite parameters, e.g., height, inclination
angle, azimuth angle, elevation angle, etc., the neighborship
between a ground node and a satellite varies dramatically.
Moreover, even if for the short period that a neighborship is
maintained, the ever-changing distance (due to the orbital
movement) between the two peering entities impacts the 'routing
protocol cost' of a link, e.g., in the case of OSPF link-cost
computation.
For example, assuming a LEO satellite orbits at the 500 km
altitude. Therefore, the orbital period is roughly 95 minutes.
Thanks to the choice of an evevation angle, a specific spot on
Earth could access the satellite approximately for 7 minutes
during one satellite round. This indicates not only the link-
flapping (i.e., a dramatic routing event) after a 7-min service
duration, but also the very fluid 'routing link cost' within the 7
minutes. The situation would be much challenging if considering
the size of a satellite constellation network, along with the
potentially large scale of on-ground routing nodes that might be
intermittently connected to satellites.
* Dynamics among satellite nodes: In the ideal scenario, there would
be tens of thousands of satellites in a satellite constellation
network. Each satellite orbits around a pre-determined track.
Depending on the coverage requirements, every track has some
number of satellites. For the same height and same inclination
angle, but with varied azimuth angles, there would be a lot of
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tracks forming a 'shell' around the Earth. Then, different height
can yield different 'shell' [IETF-Draft.SAT-PR]. With this multi-
orbit satellite topology in mind, we can project potentially the
very complicated 'routing peers' as formed by satellites on the
same track, between neighboring tracks, and between neighboring
'shells' [IETF-Draft.SAT-PR].
All satellites are moving, on the same direction, on the opposite
directions, or on angled directions. They all move fairly fast.
So, a well-established routing-peer may break up in a short
period, and then either of them may form a new peering with other
satellite nodes. The scenario is extremely dynamic, which will
definitely de-stablize any existing routing protocol(s).
When compared to scenarios in the TN, both types of extreme dynamics
will collaboratively cause the frequent flapping of routing
neighborship. The successive large amount of routing database
updates & sync-up events thus impair the efficiency of any adopted
routing protocols.
2.2. Challenge#2: The limited bandwidth of peering links
Normally, the links between peering satellites and between satellites
and ground-stations or (on-ground) mobile equipment use either the
radio or optical transports, either of which renders the fairly
limited link bandwidth (BW). For example, in one case regarding the
direct satellite service as offered by some mobile-phone providers,
the measured uplink/downlink data-plane transmission rate via a GEO
satellite is only @ 10 Kbps. In another field-trial published by a
tier-1 MNO last year, with a LEO at the orbit height 550 Km, the
measured rate is approximately 5 Mbps for Uplink, 1 Mbps for
downlink, and 230 Mbps for ISLs. Therefore, for the satellite
constellation network with a potentially large routing database
(LSDB), the frequent control-plane activities, e.g., LSP exchanges,
LSDB sync-up, etc., as elucidated in the Section 2.1, will certainly
consume quite some percentage of the precious link capacities. This,
in our opinion, must be avoided.
2.3. Challenge#3: The HW limitation & reduced capabilities
Because of the harsh environment in the space, HW specifications of
routing equipment on-board satellites must conform to very strict
standards to accommodate challenging scenarios. Plus, it is also
very expensive to carry loads in rocket launches. Therefore, the on-
board routing equipment must be as effective as possible and may only
have the minimally-required capabilites to fulfill the intra- and
inter- satellite switching. On-board routing nodes must save energy
due to power constraint. All these together lead to the on-board
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deployment of the capability-reduced routing entities that would not
be able to run a full-fledge routing protocol.
3. Satellite Routing Considerations
3.1. Uniqueness of Satellite Movement: Ephemeris
Even if the multi-orbit satellite nework faces many challenges (as
laid out in Section 2), there exists a fairly unique characteristic
in the satellite constellation, i.e., the trajectory and velocity of
a satellite is predictable and can be pre-determined. This will help
design more efficient routing mechanism.
The periodic movement of a satellite could be well predicated based
on track parameters, peering projection, and operational information
of the satellite. These data can be, e.g., satellite height,
inclination & azimuth angles, time-based link changes (flapppings),
peering adjacencies, peering distance (i.e., link costs), and even
traffic volumes. These satellite footprints are termed 'ephemeris',
which bode well for more 'predictable' routing path selection. For
example, the 5G standard [TS.23.501] demonstrates a ‘predictable’ QoS
probing optimization upon using satellites to provide mobile backhaul
service. In its description, the 5G control-functions (NFs like AMF,
SMF, PCF, etc.) apply 'ephemeris' to predicting the availability of
NFs in future. Then they engage with themselves via the 'scheduled
changes' to guide the probing frequency of QoS monitoring. It is
certainly more effective.
3.2. Routing Considerations for Multi-Orbit Satellite Networks
The challenges in Section 2 and the advantageous ephemeris
information together indicate that it is not effective, if not
infeasible, to run the traditional dynamic routing scheme over on-
board satellite nodes. Moreover, for a potential routing scheme that
could be tailored to satisfy the requirements of a satellite
constellation, it has to be associated with somewhat innovational
satellite-based addressing semantics. For example, the IETF draft
[IETF-Draft.SAT-SemAddressing] has provided a plausible satellite-
based addressing scheme, which proposes the concepts of 'shell-,
track- & sat- indices' to exclusively position (i.e., address) a
satellite in the sky.
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* Consideration #1: No full-set routing intelligence on satellites:
There would not be dependent on dynamic routing, nor would there
have distributed routing database (LSDB) via peering neighborship
& LSP exchanges. Fundamentally, we propose to relieve the
conventional routing burden from intermediary nodes (i.e.,
satellites) which do not need to rely on complex dynamic routing
intelligence.
* Consideration #2: Adoption of layered routing structure: The
satellite constellation or non-terrestrial network (NTN) is
integrated with the on-ground terrestrial network (TN) to offer
the end-to-end connectivity. While the design consideration#1
suggests not considering a full-set routing scheme over the on-
board satellites, there would not be the similar restriction on
the TN nodes. The TN nodes can just run any existing routing
protocol(s).
This could naturally lead to a two-layer routing structure to
differentiate the capability variations between the NTN and TN:
- a traditional routing scheme running for the 'overlay' TN, and
- a novel switching scheme operating exclusively for the
'underlay' NTN
Note this two-layer routing architecture bears the analogue of
SRv6, MPLS, etc. However, unlike them, this scheme does not
require any dynamic routing on the underlay NTN (e.g., the
satellite networks)
* Consideration #3: Impact of the 'multi-orbit' objective: Satellite
network is a multi-hierarchy, multi-track-per-hierarchy and multi-
satellite-per-track, or so-called 'multi-orbit', constellation
network. When an existing routing protocol (of course, with
extension) or a new one is applied, the different roles of
different satellites, i.e., LEO, MEO or GEO satellites, may play
different factors that would impact the topology design and the
selection of routing logics.
A multi-orbit satellite network with LEO, MEO and/or GEO implies
the existence of multiple comparable routing paths, or so-called
equal-cost or non-equal-cost multi-path. This bodes well for the
almost-given sporadic, compromised or even failed communication
between satellites and on-ground devices becuase of the re-route
mechanism from the multi-path nature.
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* Consideration #4: Simplified traffic forwarding logics on-board
satellites: The switching logics should be as straightforward as
they could get. They should not rely on dynamically-generated
routing tags, nor do they stick to the ubiquitious longest-prefix
matching scheme. It would be best if they are predictable and
deterministic given the existence of satellite ephemeris.
* Consideration #5: Incorporate more intelligence into the routing
scheme & path selection, e.g., the theme & objectives of IETF CATS
or Compute Aware Traffic Steering WG: as argued in Section 2, the
satellite HW normally bears the capability restriction, battery
insufficiency, on-board processing limitation, and etc. All these
compute-like factors should be combined with the traditional
routing metrics (i.e., BW, delay, load, loss, reliability, etc.)
to form a CATS-like network for integrated NTN + TN routing
consideration.
Further, the novel routing scheme should avoid unbalanced density
of the number of satellites, especially in the polar area when the
inclination angle of all orbital tracks are 90-degree
[IETF-Draft.SAT-PR].
4. Security Considerations
Generally, this function will not incur additional security issues.
5. IANA Considerations
This document makes no request of IANA.
6. Acknowledgements
The authors of the document appreciate the valuable inputs and
contributions from Lin Han, the numerous discussions with whom have
instigated the work of the authors.
7. References
7.1. Normative References
[IETF-Draft.SAT-PR]
Han, L., "Problems and Requirements of Satellite
Constellation for Internet", draft-lhan-problems-
requirements-satellite-net-06, January 2024.
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[IETF-Draft.SAT-SemAddressing]
Han, L., "Satellite Semantic Addressing for Satellite
Constellation", draft-lhan-satellite-semantic-addressing-
04, September 2023.
[TR.23.700-27]
"3GPP TR 23.700-27 (V18.0.0): Study on 5G system with
Satellite Backhaul", 3GPP TR 23.700-27, December 2022.
[TR.23.700-28]
"3GPP TR 23.700-28 (V18.1.0): Study on Integration of
satellite components in the 5G architecture; Phase
2", 3GPP TR 23.700-28, March 2023.
[TR.23.700-29]
"3GPP TR 23.700-29 (V19.2.0): Study on integration of
satellite components in the 5G architecture; Phase
3", 3GPP TR 23.700-29, February 2024.
[TR.38.821]
"3GPP TR 38.821 (V16.2.0): Solutions for NR to support
non-terrestrial networks (NTN)", 3GPP TR 38.821, March
2023.
[TS.22.887]
"3GPP TS 22.887 (Rel-20, V0.1.0): Feasibility Study on
satellite access - Phase 4", 3GPP TS 22.887, June 2024.
[TS.23.501]
"3GPP TS 23.501 (V18.2.1): System Architecture for the 5G
System (5GS)", 3GPP TS 23.501, June 2023.
[TS.23.503]
"3GPP TS 23.503 (V18.2.0): Policy and charging control
framework for the 5G System (5GS); Stage 2", 3GPP TS
23.503, June 2023.
7.2. Informative References
[ICNP22-NIB-LEO.Routing]
Han, L. and et al., "New IP based semantic addressing and
routing for LEO satellite networks", https://newip-and-
beyond.net/presentations/W_S3_Han.pdf, October 2022.
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[ICNP23-6G.SQSC-Sat.Comm]
Han, L. and et al., "Evolution to 6G for Satellite NTN
Integration: From Networking
Perspective", https://qualitativesemantic.wordpress.com/,
October 2023.
[Optical-transmission-range]
Duarte, F. and T. Taylor, "Interferometry: Quantum
entanglement physics secures space-to-space
interferometric communications.",
https://www.laserfocusworld.com/optics/article/16551652/
interferometry-quantum-entanglement-physics-secures-space-
to-space-interferometric-communications/, April 2015.
[UCL-Mark-Handley]
Handley, M., "Using ground relays for low-latency wide-
area routing in megaconstellations",
https://discovery.ucl.ac.uk/id/eprint/10090242/1/hotnets-
ucl.pdf, November 2019.
Authors' Addresses
Peng Liu
China Mobile
Email: liupengyjy@chinamobile.com
Tianji Jiang
China Mobile
Email: tianjijiang@chinamobile.com
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