RTG Working Group D. King
Internet-Draft University of Lancaster
Intended status: Informational N. Wang
University of Surrey
Expires: September 8, 2022 March 7, 2022
Routing and Addressing Challenges Introduced by
New Satellite Constellations
draft-kw-rtgwg-satellite-rtg-add-challanges-00
Abstract
Future networks, including the Internet, will utilize an increasing
amount of space-based transport infrastructure. Control and
transport between Earth-based and space-based networks present
several problems - high dynamicity, spatial connectivity, continual
movement tracking and prediction, ocular obstruction, integration
with existing Internet infrastructure, all of which challenge
existing architectures, routing mechanisms and addressing schemes.
This document summerises near-to-mid-term space-networking problems;
it outlines the key components, challenges, and requirements for
integrating future space-based network infrastructure with existing
networks and mechanisms. Furthermore, this document highlights the
network control and transport interconnection, and identify the
resources and functions required for successful interconnection of
space-based and Earth-based Internet infrastructure.
Status of This Memo
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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . .2
1.1 Terminology . . . . . . . . . . . . . . . . . . . . . . .3
2. Routing and Forwarding Challenges for ISTNs . . . . . . . .4
3. Network Control and Addressing for ISTNs . . . . . . . . . .5
4. System Resilience for ISTNs . . . . . . . . . . . . . . . .5
4.1 Routing Resilience . . . . . . . . . . . . . . . . . . .5
5. Multi-layer Networking in ISTNs . . . . . . . . . . . . . . .6
6. ISTN Traffic Engineering . . . . . . . . . . . . . . . . . .6
6.1 ISTN Resource Slicing . . . . . . . . . . . . . . . . . .7
7. Semantic Routing . . . . . . . . . . . . . . . . . . . . . .8
7.1 Applicability of Semantic Routing . . . . . . . . . . . .8
8. Security Considerations . . . . . . . . . . . . . . . . . .9
9. IANA Considerations . . . . . . . . . . . . . . . . . . . . .9
10. Acknowledgements . . . . . . . . . . . . . . . . . . . . . .9
11. Contributors . . . . . . . . . . . . . . . . . . . . . . .9
12. Informative References . . . . . . . . . . . . . . . . . .9
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . .10
1. Introduction
Exponential increases in Internet speed have facilitated an entirely
new set of applications and industry verticals underpinned by
evolving fixed network infrastructure. The costs of deploying new
fixed fibre networks are a limiting factor. As 5G and Internet
infrastructure build-out continues, we must now look up both
figuratively and physically, for our next networking opportunity.
In the future, space communication [1] will play a significant role
in providing ubiquitous Internet communications in terms of both
access and backhaul services.
Future space networks will also need to cooperate with the existing
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terrestrial network infrastructure (Integrated Space and Terrestrial
Networks - ISTNs), exploiting heterogeneous devices, systems and
networks. Thus, providing much more effective services than
traditional Earth-based infrastructure, and greater reach and
coverage than proprietary and isolated space-based networks.
Several challenges are outlined in the bullets below:
o As LEO satellites orbit the Earth at relatively high-speed, the
space-based path latency and bandwidth will fluctuate as routes
shift across the satellite topology.
o Future LEO satellites will support multiple link types, air
interfaces, and frequencies, including high-bandwidth
free-space optical links and low-speed radio interfaces.
o Atmospheric conditions and weather severely degrade communication
between satellites over space-ground links, significantly
reducing throughput or requiring new routing
paths to be selected.
o The ISTN links will become bandwidth-constrained, and it be
necessary to compute alternative paths around those congested
links.
o Dynamic path selection based on current and predicted demands
will need to be factored in, thus traditional Dijkstra techniques
for path routing will not be sufficient.
Existing Internet architecture and protocol mechanisms will likely
apply to converged space-based and Earth-based network
infrastructure, however, there will be limitations [2]. This section
outlines some of the challenges, requirements, and potential
strategies to pursue for future ISTNs.
This document summarises near-to-mid-term space-networking problems
and challenges, phrased as research questions. This document does
not propose solutions or techniques, or elaborate on specific
protocols themselves.
1.1 Terminology
LEO: Low Earth Orbit with the altitude from 180 km to 2000 km.
GEO: Geosynchronous orbit with the altitude 35786 km
IGP: Interior gateway protocol
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ISL: Inter Satellite Link
ISLL: Inter Satellite Laser Link
ISTN: Integrated Space Terrestrial Network
MEC: Multi Edge Computing
2. Routing and Forwarding Challenges for ISTNs
Routing and signaling across emerging next generation satellite
networks is far from static [3]; satellite-to-satellite connectivity
changes frequently, space-based link latencies, and links from
space-to-ground will change regularly. Satellites will also have
to contend with predictive routing capabilities, as links will
only be established when optical alignment is possible. Given
that meshes of 100s and 1000s of satellites are also expected,
techniques that use per-hop Dijkstra calculation will be
extremely inefficient.
Next generation space networks are not static. The satellite that is
overhead a particularly ground station changes frequently, the laser
links between space-based satellites change often, and link
latencies for satellite to ground links will vary based on
atmospheric conditions [4].
Several control plane challenges have been identified for
space-based networks [5], these include:
o New link acquisition, predicted link availability, and link
metric dynamicity: as the acquisition and tracking of satellites
and links change, there is a need to adjust basic link and TE
metrics (delay, jitter, bandwidth) and update the existing
routing traffic engineering database.
o Space-based path computation: selection of the best path across
ISLs and direct uplinks and downlinks, consideration of cloud
cover, air turbulence and external object occlusion.
o Temporal routing: consideration of the time-varying topology of
the space network will necessitate frequent routing updates.
o Predictive routing: time-scheduled routing paths based on
expected satellite orbits and air-interface alignment.
o Rerouting of paths: which may be required in the event of
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projected space-based debris orbits that prevent line-of-sight
between adjacent nodes, interface and node failures, and adverse
weather which may affect space-to-ground communication points.
o Resilience: overall, the network must be resilient to failures,
and capable of routing with low latencies, even when traffic
levels are significant enough to oversubscribe the preferred
paths.
3. Network Control and Addressing for ISTNs
Integrating the space-based infrastructure with an existing network
might be achieved using traditional Internet routing techniques
and identifying the extra-terrestrial portion of the network as
a specific domain (such as an IGP area or an AS) [6]. The space-
domain might run a traditional routing control plane, likely
logically within an Earth-based representation which programs the
path via an SDN-programming technique [7]. However, this approach
would not be capable of computing paths based on the unique space
connectivity dynamics. Furthermore, if the space-domain was
connected to traditional Earth-based Internet domains (including
ASes via BGP), it might create unwanted route flapping, causing
routing instability.
Due to the unique characteristics of the space-based nodes (which
may have multiple interfaces and lines of sight to next-hop
satellite nodes or ground stations, may fluctuate), other network
control methods may be needed.
4. System Resilience for ISTNs
Legacy satellites might typically operate independently from their
orbiting counterparts. However, next generation space-based
infrastructure will be utilizing multiple links between
satellite nodes and ground-stations, which leaves potential
network paths susceptible to the consequences of node and link
failures or anomalies. Loss of node payload, communication link,
or other sub-system components might render the entire satellite
node inoperable.
4.1 Routing Resilience for ISTNs
Legacy satellites might typically operate independently from their
orbiting counterparts. However, next generation space-based
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infrastructure will be utilizing multiple links between
satellite nodes and ground-stations, which leaves potential
network paths susceptible to the consequences of node and link
failures or anomalies. Loss of node payload, communication link,
or other sub-system components might render the entire satellite
node inoperable.
In a satellite network, there several types of failures a routing
system might be concerned with; these include:
o Failures of components in the forwarding plane, e.g., ISL
communication failure.
o Control plane malfunction, if the central controller is
destroyed or disconnected, or the distributed control plane
suffers a catastrophic failure or attack.
o Misconfiguration of satellite node or ISL forwarding, or
degradation of satellite orbit and loss of communication
sight to neighbouring node.
In general, satellite node failures or components of the forwarding
plane are problematic but as the latest generation of space
infrastructure is highly meshed, routing around node failures
is feasible. Once a failure occurs, the centralized controller,
or distributed control plane, would have to respond and update
the forwarding state in devices to route traffic around the
failed nodes or links. As failure may be seen as an extreme
case of an unexpected change in traffic level, a traffic
reoptimization mechanism would likely be required.
5. Multi-layer Networking in ISTNs
The Low Earth Orbit (LEO) satellite uses a lower physical orbit,
which provides latency benefits, but this orbit will incur more
dynamic connectivity and oscillating link characteristics [9]. The
Medium Earth Orbit (MEO) and Geostationary Orbit (GEO) satellites
provide more physical stability, and reduced dynamicity of the links
as the satellites remain static. The current GEO satellite system
mostly provides relay function; however, in the next generation,
satellite systems could interact providing multi-layer routing and
forwarding functions [8] between satellite layers, akin to
multi-layer networking in terrestrial networks.
6. ISTN Traffic Engineering
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Traffic Engineering (TE) has been well investigated for more than
two decades in the context of the traditional terrestrial Internet.
However, TE has not been systematically understood in the integrated
space and terrestrial network environment, especially given the
district characteristics of the two types of networks and also the
mega-constellation behaviors of LEO satellites. It is generally
understood that the inter-satellite link capacity is not compared to
the optical fiber links in the terrestrial Internet. As such, the
traffic injected into the space network has to be selective.
Policies can be enforced either based on the traffic type and their
QoS requirements or based on other contexts such as the distance
between source and destination pairs. For instance, in it has been
argued that routing through a chain of LEO satellites will outperform
the usage of terrestrial Internet in terms of end-to-end delay if the
distance of the source and destination is beyond 3000 kilometres. It
is also worth noting, the capability of TE in the space network also
largely depends on the specific routing mechanisms that are deployed,
which has been the case in terrestrial network environments, e.g.,
IP/MPLS/SDN. As mentioned above, the capability of TE in integrated
space and terrestrial network infrastructures will also depend on the
routing mechanisms deployed in the two network environments, either
with separated protocols (the case today) or with a unified protocol
suite.
6.1 ISTN Resource Slicing
In the context of 5G, network slicing has been deemed as a promising
feature for operators to provision network resources and functions to
tailor for heterogeneous requirements of emerging applications and
services. While the business model for network slicing on the
traditional network operator side has been relatively clear, a more
complex scenario of involving satellite operators has not yet been
previously elaborated. As a starting point, a terrestrial network
operator can rent virtual network resources provided by a satellite
operator to build a dedicated backhaul link for connecting its point
of presences (PoPs). In this case the terrestrial network operator
can create end-to-end slices for supporting different application
types, and the backhaul component of a selected subset of slices
(e.g., eMBB (Enhanced Mobile Broadband) for video content delivery)
can leverage on the satellite capability.
On the other hand, a satellite operator could also slice its own
satellite link resources and lease to multiple terrestrial network
operators for backhauling or extended access services, by applying
intelligent beamforming techniques to cater for different
geographical areas. As shown in Figure 1 (for simplicity only one
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satellite is shown, but it can be a chain of LEO satellites), sliced
satellite link capabilities can be leased to terrestrial network
operators (e.g., mobile operators) in order for them to build their
own service-tailored slices provided that the sliced satellite
capability is able to fulfil the targeted service requirements. For
instance, once a terrestrial network operator has deployed a
MEC-based content prefetching/caching network function within its
network slice for transmitting 4K/8K video content, then it can use
leased satellite capability for backhauling 4K/8K video in that
slice. From the business point of view, we can envisage a cash flow
from end customers (subscribers of terrestrial network slices) to the
terrestrial network operators and further to the satellite operator.
7. Semantic Routing
The current architecture for IP networking is built using a best-
effort philosophy. Several techniques exist that offer
better-than-best-effort delivery, but require additional hardware and
software overhead. The start-point and end-point of a path are
identified using IP addresses, and traffic is steered along the path
that does not necessarily follow the "shortest path first" route
through the network. Furthermore, the path might not run all the way
from a packet's source to its destination. The assumption is that a
packet reaching the end of a path is forwarded to its destination
using best-effort techniques.
Semantic Routing is the process of routing packets that contain IP
addresses with additional semantics, possibly using that information
to perform policy-based routing or other enhanced routing functions.
Thus, facilitating enhanced routing decisions based on these
additional semantics and provide differentiated paths for different
packet flows, distinct from simple shortest path first routing.
In a satellite network, a path might be comprised of mainly FSO
links to meet latency and bandwidth requirements, or use specific
ground-stations, gateways, or follow a designated orbital direction.
The process of known as Semantic Routing is discussed further in the
document [9].
7.1 Applicability of Semantic Routing
Strategies for implementing and operating IP routing effectively
within LEO satellite constellation networks and ISTNs, given known
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constraints on the constellation, may include semantec routing [9]
and addressing [10] techniques.
Typically, in an IP-based network packets are forwarded using the
least-cost path to the destination IP address. Service Providers may
also use techniques to modify the default forwarding behavior based
on other information present in the packet and configured or
programmed into the routers.
As outlined in this I-D numerous challenges exist for network control
of space-based infrastructure, and addressing ISTN issues. Semantic
routing facilitates path decisions based solely on the address and
without the need to find and process information carried in other
fields within the packets, reducing node computational power and
complexity.
We will continue to discuss the applicability of semantic
techniques in further detail, in future versions of this document.
8. Security Considerations
To be discussed.
9. IANA Considerations
This document makes no requests for IANA action.
10. Acknowledgements
To be discussed.
11. Contributors
To be discussed.
12. Informative References
[1] I. d. Portillo, B. G. Cameron, and E. F. Crawley, "A
technical comparison of three low earth orbit satellite
constellation systems to provide global broadband," Acta
Astronautica, vol. 159, pp. 123 135, 2019.
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[2] Curzi, Giacomo & Modenini, Dario & Tortora, Paolo. (2020).
Large Constellations of Small Satellites: A Survey of Near
Future Challenges and Missions. Aerospace. 2020.
[3] Kaushal, H., Kaddoum, G.: 'Optical communication in space:
challenges and mitigation techniques', IEEE Commun. Surv.
Tutor., 2017, 19, (1), pp. 57 96, 2018.
[4] H. Yao, L. Wang, X. Wang, Z. Lu and Y. Liu, "The
Space-Terrestrial Integrated Network: An Overview," in IEEE
Communications Magazine, vol. 56, no. 9, pp. 178-185, Sept.
2018.
[5] D. King, A. Farrel and Z. Chen, "An Evolution of Optical
Network Control: From Earth to Space," 2020 22nd
International Conference on Transparent Optical Networks
(ICTON), 2020.
[6] M. Handley, "Delay is not an option: Low latency routing
in space," in Proceedings of the 17th ACM Workshop on Hot
Topics in Networks, 2018, pp. 85-91.
[7] G. Zheng, N. Wang, R. Tafazolli, X. Wei and J. Yang,
"Virtual Data-Plane Addressing for SDN-based Space and
Terrestrial Network Integration," 2021 IEEE 22nd
International Conference on High Performance Switching
and Routing (HPSR), 2021.
[8] King, D. and Wang, N. "Integrated Space-Terrestrial
Networking and Management", Future Networks,
Services and Management: Underlay and Overlay, Edge,
Applications, Slicing, Cloud, Space, AI/ML, and Quantum
Computing, Springer International Publishing, 2021.
[9] King, D. and Farrel, A. "Challenges for the Internet
Routing Infrastructure Introduced by Semantic Routing",
draft-king-irtf-challenges-in-routing-07 (work in
progress), November 2021.
[10] Han, L. and Li, R. "Satellite Semantic Addressing for
Satellite Constellation", draft-lhan-satellite-semantic-
addressing-01 (work in progress), March 2022.
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Authors' Addresses
Daniel King
Lancaster University
UK
Email: d.king@lancaster.ac.uk
Ning Wang
University of Surrey
UK
Email: n.wang@surrey.ac.uk