Internet DRAFT - draft-hou-tvr-satellite-network-usecases
draft-hou-tvr-satellite-network-usecases
TVR D. Hou, Ed.
Internet-Draft M. Xiao
Intended status: Standards Track F. Zhou
Expires: 16 March 2024 D. Yuan
ZTE Corporation
13 September 2023
Satellite Network Routing Use Cases
draft-hou-tvr-satellite-network-usecases-02
Abstract
Time-Variant Routing (TVR) is chartered and proposed to solve the
problem of time-based, scheduled changes, including the variations of
links, adjacencies, cost, and traffic volumes in some cases. In a
satellite network, the network is in continual motion which will
cause detrimental consequences on the routing issue. However, each
network node in a satellite network follows a predefined orbit around
the Earth and represents an appropriate example of time-based
scheduled mobility. Therefore, TVR can be implemented to improve the
routing and forwarding process in satellite networks. This document
mainly focuses on the use cases in this scenario.
Status of This Memo
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Copyright Notice
Copyright (c) 2023 IETF Trust and the persons identified as the
document authors. All rights reserved.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 2
2. Requirements Language . . . . . . . . . . . . . . . . . . . . 5
3. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 5
4. Problem Analysis . . . . . . . . . . . . . . . . . . . . . . 5
5. Scheme Analysis . . . . . . . . . . . . . . . . . . . . . . . 6
6. Use Cases . . . . . . . . . . . . . . . . . . . . . . . . . . 7
6.1. Scenario 1: Dynamic connectivity relationships . . . . . 8
6.2. Scenario 2: Time-varying link characteristics . . . . . . 10
7. Future Considerations . . . . . . . . . . . . . . . . . . . . 13
8. Security Considerations . . . . . . . . . . . . . . . . . . . 14
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . 14
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 14
11. Normative References . . . . . . . . . . . . . . . . . . . . 14
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 15
1. Introduction
Since the beginning of the 21st century, the satellite network has
become a significant part of information and communication
infrastructure. The large-scale sallite network composed of
thousands or even tens of thousands of LEO satellites, MEO satellites
and GEO satellites can overcome the limitations of the conventional
terrestrial network, achieving global signal coverage, and providing
large broadband as well as low-latency network services for global
users. The global communications ecosystem believes that satellite-
based communication will become an important part of 5G-advanced and
6G.
In a satellite network, satellites move along the orbit, which can be
divided into circular orbit satellites and elliptical orbit
satellites. Different orbits can be described by Keplerian
parameters, including inclination, longitude of the ascending node,
eccentricity, semimajor axis, argument of periapsis, true anomaly.
At present, the mainstream of satellite networks basically adopt
circular orbit.
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When links between satellites are established for end-to-end
communication, each satellite usually has a fixed number of links
which communicate with neighboring nodes, and considering the cost of
satellite links and power restrictions of satellites, satellite links
are generally limited to direct connections between adjacent nodes.
In a single-layer satellite constellation, each satellite may have
four types of contiguous neighbour satellites and each type refers to
a direction. The number of neighbor satellites distributed in one
direction is determined by the number of antennas deployed on the
satellite for communication. If the satellite contains a single
antenna in each direction, the connection relationship between the
satellite N5 and its two satellites in the same orbit and two
satellites in different adjacent orbits is shown in Figure 1. N2 and
N8 are front and rear adjacent satellites in the same orbit plane
which includes N5. N4 and N6 are left and right adjacent satellites
which are adjacent to N5 locate in different orbit planes. In a
multi-tier satellite constellation, each satellite may have two
additional types of adjacent satellites, upper level satellites and
lower level satellites in different tiers.
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^ ^ ^
| | |
| | |
v v v
.--. .--. .--.
<---> ####-| N1 |-#### <---> ####-| N2 |-#### <---> ####-| N3 |-#### <--->
\__/ \__/ \__/
^ ^ ^
| | |
| | |
v v v
.--. .--. .--.
<---> ####-| N4 |-#### <---> ####-| N5 |-#### <---> ####-| N6 |-#### <--->
\__/ \__/ \__/
^ ^ ^
| | |
| | |
v v v
.--. .--. .--.
<---> ####-| N7 |-#### <---> ####-| N8 |-#### <---> ####-| N9 |-#### <--->
\__/ \__/ \__/
^ ^ ^
| | | N
| | | ^
v v v |
Orbit plane 1 Orbit plane 2 Orbit plane 3 |
S
Moving
Direction
Figure 1: N5 and its adjacent satellites
The satellite orbit velocity is related to the satellite orbit
altitude (in a circular orbit), and satellites at the same altitude
move at the same speed. Therefore, the relative position between
satellites in the same orbit plane is stable and the intra-satellite
links can always be connected, and the link distance is basically
unchanged, such as N2 and N5. The relative position between
satellites in different orbit planes changes dynamically, inter-
satellite links may be interrrupted due to antenna tracking
difficulties or limited communication range, and the physical
distance of the link is constantly altering, such as N4 and N5.
As one of the indispensable issues for communication, routing
strategies directly affects the transmission efficiency and the
quality of network services. However, due to the particularity of
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the satellite network, such as the high frequency and intensity
changes in network topology, the relatively mature terrestrial
network routing technologies can not be directly applied to the
satellite network. In view of the mentioned characteristics, and
considering the combination of satellite networks and TVR, this
document includes the following information:
1. The core problems of routing issues in satellite networks are
stated and analyzed.
2. This paper discusses the unique time-based predictable network
information in satellite networks, and proposes a routing
optimization method based on this information.
3. The relevant application scenarios are given and illustrated.
2. Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in BCP
14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Terminology
* LEO: Low Earth Orbit.
* MEO: Middle Earth Orbit.
* GEO: GEostationary Orbit.
* Intra-satellite links: Links between adjacent satellites in the
same orbit.
* Inter-satellite links: Links between adjacent satellites in the
different orbits.
* SGP4: Simplified Perturbations Models
4. Problem Analysis
The dynamic nature of nodes is the most significant feature of
satellite networks compared to conventional terrestrial networks. In
LEO mega-constellations, this feature becomes more obvious and
prominent. Typical phenomena in satellite networks are listed here:
1. LEOs move at a relatively high speed for over 7km/s.
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2. Half of LEOs in the network move in the same direction which is
the opposite to the other half.
3. A great number of links between satellites or between satellites
and ground-stations.
4. A large part of above links may be interrupted at specific areas.
5. All metrics of inter-satellite links are constantly changing.
6. All metrics of links between satellites and ground-stations are
constantly changing.
Existing routing protocols are designated to maintain contemporaneous
end-to-end connections across a network. Once the network topology
or connection of links changes, corresponding operations and
procedures are adopted to recover and maintain the reachability
between various pairs. Representative procedures in traditional
protocols may consist of attempting to re-establish lost adjacencies
,recalculating or rediscovering a valid path. The dynamic changes of
network topologies and links in satellite networks will constantly
trigger the process of routing re-convergence process with existing
routing protocols, resulting in routing shocks, which makes it
inappropriate for existing routing protocols to be directly applied
in satellite networks.
5. Scheme Analysis
The process of satellite motion along the orbit is periodic and
predictable. Predictable information in a satellite network includes
satellite real-time positions in the space, satellite link
connectivity, and satellite link real-time metrics. The satellite-
ground link also has similar characteristics, which have been
described in [I-D.birrane-tvr-use-cases] and will not be repeated
here.
(1) The real-time position of a satellite is predictable.
Satellites move around the earth in a predetermined orbit and are
endowed with a unified and accurate count of time by a ground network
control center or some specific designated nodes. Thus, the real-
time position in the space of a satellite can be predicted in advance
according to the satellite orbit parameters, the orbit injection
moment and the satellite operation time.
(2) The connectivity of satellite links are predictable.
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Due to the influence of the relative position changes between
adjacent satellites in different orbits and the restrictions of
current communication technologies, the inter-satellite link will be
interrupted when entering a specific area and restored after leaving
this regions. According to the satellite orbit parameters, satellite
operation time, satellite attitude, and the communication range of
the satellite antenna, combined with some specific algorithms, SGP4
(Simplified Perturbations Models) for instance, the connectivity of
satellite links can be predicted in advance.
(3) The characteristics of satellite links are predictable.
Affected by the change of relative position between adjacent
satellites in different orbits, the communication distance between
satellites is constantly changing. This distance reaches the largest
near the equator and declines to the smallest while moving to the
pole. The changes in inter-satellite communication distance will
further lead to the time-varying characteristic of inter-satellite
links, such as propagation delay and bit error rate. According to
satellite orbit parameters, satellite operation time, antenna
transmission power, space propagation loss and so on, combined with
proper algorithms, the characteristics of inter-satellite links can
be predicted in advance.
As analyzed aboved, the management plane, the control plane and the
forwarding plane of the network can be adaptively improved by
utilizing time-based predictable information and combining the
characteristics of inter satellite and satellite to ground
transmission conditions, so as to ensure a stable and optimal end-to-
end reachable path between a pair of satellites, such as:
(1) By improving the control plane protocol based on the
predictability of the interruption/recovery of the satellite links,
on one hand, the flooding of routing convergence information caused
by network topology changes can be avoided, and on the other hand,
the routing re-calculation is able to be fulfilled in advance before
the satellite network topology changes, and thus the calculated
results can be updated immediately and timely.The same methods can
also apply to predictable changes in the characteristics of satellite
links.
(2) By using the predictability of satellite spatial location, the
routing algorithm can be improved, such as Dijkstra algorithm, which
could screen relay nodes in the path without traversing all possible
choices, and further reduce the complexity of the routing algorithm.
6. Use Cases
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6.1. Scenario 1: Dynamic connectivity relationships
As shown in Figure 2, N1, N2 and N3 are adjacent satellites in
different orbit planes at the same altitude, moving from south to
north. At T2 and T3, N3 and N2 enter a specific area (such as the
polar region) in turn, and inter-satellite links are interrupted due
to the difficulty in alignment of the on-board antenna. When the
node leaves the specific area, the on-board antenna is re-aligned and
the inter-satellite link is restored. The Walker constellation also
has the similar characteristic.
.--. .--. .--.
t1 ####-| N1 |-#### <---> ####-| N2 |-#### <---> ####-| N3 |-####
\__/ \__/ \__/
.--. .--. .--.
t2 ####-| N1 |-#### <---> ####-| N2 |-#### ####-| N3 |-####
\__/ \__/ \__/
.--. .--. .--.
t3 ####-| N1 |-#### ####-| N2 |-#### ####-| N3 |-####
\__/ \__/ \__/
Figure 2: Changes in connectivity between adjacent satellites in
different orbits.
For any satellite in the network, the change of the connectivity
failure/recovery state of the satellite links can be predicted in
advance through pre-calculation. Therefore, N2 and N3 do not need to
perform the flooding notification of the link state changes, and the
nodes in the network can calculate the route in advance according to
the predicted network topology, and timely complete the route update
procedures when the topology changes.
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N2-N3 N1-N2
| |
|--------+ |-------------+
| | | |
| | | |
| +-------- | +---
| |
+---++---++---++-- +---++---++---++--
t1 t2 t3 t1 t2 t3
Time Time
Figure 3: Inter-satellite link connectivity.
At time T1, both links between N1 and N2 and between N2 and N3 are
connected, and the end-to-end path from N1 to N3 will be forwarded
through N2, as shown in Figure 4. As the nodes move, the links
between N1 and N2 and between N2 and N3 will predictably fail at time
T3, as shown in Figure 3. In response to this predictable change in
network topology, the relevant satellite nodes may perform routing
calculations in advance, and the end-to-end path from N1 to N3 will
be forwarded through N4, N5, N6 as shown in Figure 5.
^ ^ ^
| | |
| | |
v Src v v Dst
.--. .--. .--.
<-> ####-| N1 |-#### <-> ####-| N2 |-#### <-> ####-| N3 |-#### <->
\__/ ------> \__/ ------> \__/
^ ^ ^
| | |
| | |
v v v
.--. .--. .--.
<---> ####-| N4 |-#### <---> ####-| N5 |-#### <---> ####-| N6 |-#### <--->
\__/ \__/ \__/
^ ^ ^
| | | N
| | | ^
v v v |
Orbit plane 1 Orbit plane 2 Orbit plane 3 |
S
Moving
Direction
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Figure 4: Path from N1 to N3 at T1.
^ ^ ^
| | |
| | |
v Src v v Dst
.--. .--. .--.
####-| N1 |-#### ####-| N2 |-#### ####-| N3 |-####
\__/ \__/ \__/
^| ^ ^^
|| | ||
|| | ||
vv v |v
.--. ------> .--. ------> .--.
<---> ####-| N4 |-#### <---> ####-| N5 |-#### <---> ####-| N6 |-#### <--->
\__/ \__/ \__/
^ ^ ^
| | | N
| | | ^
v v v |
Orbit plane 1 Orbit plane 2 Orbit plane 3 |
S
Moving
Direction
Figure 5: Path from N1 to N3 at T3.
6.2. Scenario 2: Time-varying link characteristics
As shown in Figure 6, N1 and N2 are adjacent satellites at the same
altitude and in different orbit planes, moving from the equator to
the polar region from south to north. At time T1, the distance
between N1 and N2 is the largest, and at time T3, the distance
between N1 and N2 is the smallest. For any satellite in the network,
the changes in satellite communication distances will influence the
characteristics of satellite links, including delay and error rate.
Each satellite in the network can predict these changes in advance
through pre-calculation, and update the link cost correspondingly.
The change of link characteristics is a process from quantitative
change to qualitative change, that is, only when link characteristics
between nodes increases or decreases to a certain extent, it is
necessary to re-computate the path between nodes. In other words, in
an interval range, link characteristics between nodes can be
expressed by an order of magnitude, rather than a precise specific
value. Therefore, N1 and N2 do not need to perform the flooding
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notification of the link state changes, and the nodes in the network
can calculate the route in advance according to the predicted link
cost change and switch the routing path at an appropriate time.
.--. .--.
t1 ####-| N1 |-#### <------------> ####-| N2 |-####
\__/ \__/
.--. .--.
t2 ####-| N1 |-#### <---------> ####-| N2 |-####
\__/ \__/
.--. .--.
t3 ####-| N1 |-#### <------> ####-| N2 |-####
\__/ \__/
Figure 6: Changes of communication distance between adjacent
satellites in different orbits.
At time T1, N7 and N3 are symmetrically located on both sides of the
equator, and N4, N5 and N6 are located in the equatorial region.
Therefore, the communication distance between N4 and N5 and between
N5 and N6 is the largest, and the corresponding link cost is also
higher. Therefore, the end-to-end path from N7 to N3 does not
include the N4, N5, and N6, but forwards through N8, N5, and N2 which
is shown in Figure 7.
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^ ^ ^
| | |
| | |
v v v Dst
.--. .--. .--.
<--> ####-| N1 |-#### <--> ####-| N2 |-#### <--> ####-| N3 |-#### <-->
\__/ \__/ \__/
^ ^^ -------> ^
| || |
| || |
v v| v
.--. .--. .--.
<---->####-| N4 |-####<---->####-| N5 |-####<---->####-| N6 |-####<---->
\__/ \__/ \__/
^ ^^ ^
| || |
| || |
v -------> v| v
.--. .--. .--.
<--> ####-| N7 |-#### <--> ####-| N8 |-#### <--> ####-| N9 |-#### <-->
\__/ \__/ \__/
^ Src ^ ^
| | | N
| | | ^
v v v |
Orbit plane 1 Orbit plane 2 Orbit plane 3 |
S
Moving
Direction
Figure 7: Path from N7 to N3 at T3.
With the continuous movement of the node, at time T3, the source
satellite N7 and the destination satellite N3 both move across the
equator and enter the northern hemisphere, while N1, N2 and N3 are in
a relatively near-polar region. Therefore, the communication
distance between N1, N2, and N3 is the smallest compared to other
inter-satellite links, and the corresponding link cost is also lower.
Thus, the end-to-end path from N7 to N3 includes N4, N1, N2 which is
shown in Figure 8.
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^ ^ ^
| | |
| | |
v v v Dst
.--. .--. .--.
<-> ####-| N1 |-#### <-> ####-| N2 |-#### <-> ####-| N3 |-#### <->
\__/ \__/ \__/
^^ ----> ^ -----> ^
|| | |
|| | |
v| v v
.--. .--. .--.
<---> ####-| N4 |-#### <---> ####-| N5 |-#### <---> ####-| N6 |-#### <--->
\__/ \__/ \__/
^^ ^ ^
|| | |
|| | |
v| v v
.--. .--. .--.
<----->####-| N7 |-####<----->####-| N8 |-####<----->####-| N9 |-####<----->
\__/ \__/ \__/
^ Src ^ ^
| | | N
| | | ^
v v v |
Orbit plane 1 Orbit plane 2 Orbit plane 3 |
S
Moving
Direction
Figure 8: Path from N7 to N3 at T1.
7. Future Considerations
To provide a stable and reliable end-to-end service in a dynamic
satellite network communication environment, the network technologies
of the management plane, the control plane, and the forwarding plane
should be innovated in future works by utilizing time-based
predictable information and combining the characteristics of inter
satellite and satellite to ground transmission conditions. The
details are as follows.
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The management plane is responsible for monitoring network status and
scheduling network resources, so as to meet the changeable demands of
network services. For example, a new Yang model containing time
predictable information can be implemented to realize the pre-control
of the network.
The control plane is constructed by network elements which makes
forwarding decisions, including routing protocols, source/segment
routing protocols, routing strategies, and so on. For example, based
on the predictability of the interruption/recovery of the satellite
links, the flooding of routing information caused by network topology
changes can be avoided, and the routing re-calculation is able to be
fulfilled in advance before the network topology changes.
The forwarding plane is the part that performs the forwarding
decisions of the control plane, including data encapsulation and
decapsulation, high-speed forwarding chips, and so on. For example,
the label format.
8. Security Considerations
TBA
9. Acknowledgements
TBA
10. IANA Considerations
This document has no IANA actions.
11. Normative References
[I-D.birrane-tvr-use-cases]
Birrane, E. J., "TVR (Time-Variant Routing) Use Cases",
Work in Progress, Internet-Draft, draft-birrane-tvr-use-
cases-00, 24 October 2022,
<https://datatracker.ietf.org/doc/html/draft-birrane-tvr-
use-cases-00>.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/info/rfc2119>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/info/rfc8174>.
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Authors' Addresses
Dongxu Hou (editor)
ZTE Corporation
No.50 Software Avenue
Nanjing
Jiangsu, 210012
China
Email: hou.dongxu@zte.com.cn
Xiao Min
ZTE Corporation
No.50 Software Avenue
Nanjing
Jiangsu, 210012
China
Email: xiao.min2@zte.com.cn
Fenlin Zhou
ZTE Corporation
No.50 Software Avenue
Nanjing
Jiangsu, 210012
China
Email: zhou.fenlin@zte.com.cn
Dongyu Yuan
ZTE Corporation
No.50 Software Avenue
Nanjing
Jiangsu, 210012
China
Email: yuan.dongyu@zte.com.cn
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