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



   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.

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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

   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

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

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

   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.

   [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

   Email: d.king@lancaster.ac.uk

   Ning Wang
   University of Surrey

   Email: n.wang@surrey.ac.uk