Time Variant Routing D. King Internet-Draft Lancaster University Intended status: Informational K. Shortt Airbus Expires: July 18, 2023 January 17, 2023 Time Variant Challenges for Non-Terrestrial Networks draft-king-tvr-ntn-challanges-00 Abstract Advanced networks, including the Internet, will utilise an increasing amount of Non-Terrestrial Network (NTN) infrastructure. NTNs include Low Earth Orbit (LEO) satellites, High Altitude Long Endurance (HALE) aviation, and High-Altitude Platform Stations (HAPS). In addition, NTN infrastructure will facilitate the deployment of advanced 5G use cases and services. NTNs infrastructure is typically mobile, with various links and nodes operating at different altitudes and latencies. Some NTN nodes and links are temporal and need to be scheduled and established at specific times based on line-of-sight availability, traffic demand and power budgets. This document summarises time variant NTN requirements and challenges not met by existing routing and traffic engineering techniques. 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 18 July 2023. King & Shortt Expires July 18, 2023 [Page 1] Internet-Draft Time Variant Challenges for NTNs January 2023 Copyright Notice Copyright (c) 2023 IETF Trust and the persons identified as the document authors. All rights reserved. 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 Terminology . . . . . . . . . . . . . . . . . . . . .5 2. 3GPP NTN Use Cases and Requirements . . . . . . . . . .6 2.1 Architecture . . . . . . . . . . . . . . . . . . . . .7 2.1.1 Physical Layer Control . . . . . . . . . . . . . . .7 2.1.2 Uplinks and Downlinks . . . . . . . . . . . . . . .8 2.1.3 Feeder Links . . . . . . . . . . . . . . . . . . . .8 2.2 Satellite Service Continuity . . . . . . . . . . . . .9 2.3 Satellite-based NG-RAN Architectures . . . . . . . . .9 2.4 NB-IoT and eMTC Support . . . . . . . . . . . . . . .10 3. Routing and Traffic Engineering Challenges for NTNs. . .10 3.1 Link and Routing Resilience for NTNs. . . . . . . . . .12 3.2 Multi-layer Networking in NTNs. . . . . . . . . . . . .13 4. NTN Management and Operation . . . . . . . . . . . . . .13 5. Security Considerations . . . . . . . . . . . . . . . . .14 6. IANA Considerations . . . . . . . . . . . . . . . . . . .14 7. Acknowledgements . . . . . . . . . . . . . . . . . . . .14 8. Contributors . . . . . . . . . . . . . . . . . . . . . .14 9. Informative References . . . . . . . . . . . . . . . . .14 Authors' Addresses . . . . . . . . . . . . . . . . . . . . .15 1. Introduction Exponential increases in Internet speed have facilitated an entirely new set of applications and industry verticals underpinned by evolving fixed network infrastructure. However, the costs of deploying new fixed fibre networks are a limiting factor. Therefore, as 5G and Internet infrastructure build-out continues, we must look up, both figuratively and physically, to our next networking enabler. King & Shortt Expires July 18, 2023 [Page 2] Internet-Draft Time Variant Challenges for NTNs January 2023 Non-Terrestrial Network (NTN) infrastructure, including Geosynchronous Equatorial Orbit, Low Earth Orbit (LEO) satellites Starlink, Kuiper, and OneWeb), High Altitude Long Endurance (HALE) aviation, and High-Altitude Platform Stations (HAPS) and Unmanned Aerial Vehicles (UAV), are providing a significant role in Internet communications in terms of both access and backhaul services. These new networks will continue to increase in size and scale. The NTN definition has become an umbrella term for a network that may involve non-terrestrial flying objects. Approximate altitudes and latencies for NTN nodes, include: o GEO 36,000 km (600-800 ms) o MEO 20,000 km (120-300 ms) o LEO 400 km (30-50 ms) o HAPS 20km (<3 ms) o HALE 10 km (<3 ms) o UAV 1 km (<3 ms) As defined in 3GPP [TR38.82118], a satellite-based NTN typically feature the following elements: o One or several satellite gateways that connect the Non-Terrestrial Network to a public data network; o A feeder link or radio link between a satellite-gateway and the satellite; o A service link or radio link between the user equipment (UE) and the satellite. Satellite nodes may have one of two orbits. Firstly, moving in a circular orbit around the Earth. Secondly, keeping a notional station with its position fixed in terms of azimuth to a given Earth point. NTN infrastructure provides tremendous potential in benefiting the augmentation of terrestrial infrastructures in providing flexible connectivity for a wide variety of use cases, including: NG-Radio Access Network (NG-RAN), Enhanced Mobile Broadband (eMBB) NTN, Internet of Things (IoT) NTN, Massive Machine-type Communications (mMTC) NTN. NTNs must also cooperate with the current terrestrial network infrastructure (Integrated Space and Terrestrial Networks - ISTNs) and exploit existing heterogeneous devices, systems and networks. Thus, providing much more effective services than King & Shortt Expires July 18, 2023 [Page 3] Internet-Draft Time Variant Challenges for NTNs January 2023 traditional Earth-based infrastructure, and greater reach and coverage than proprietary and isolated NTN environments. An NTN-based system will be compromised of end devices at different altitude layers, each with a corresponding set of link characteristics. For example, GEO satellites provide stable and continuous links to ground devices with a considerable propagation delay. In contrast, LEO satellites may be characterised by low-delay interfaces but may suffer service discontinuity depending on the constellation density. The type of service provided by each layer will require specific link management and scheduling. By their nature, GEO satellites differ from LEO satellites in terms of location, connectivity, redundancy capabilities, antenna designs, transceivers, operational frequency, and internal resources (e.g., hardening, storage, processing, and power availability). The variance in design and capabilities of Unmanned Aerial Vehicles (UAVs) is apparent with crewless aerial vehicles (HALE and HAPS), as they are conceived for different purposes. In addition, they are designed for varying use cases and environments and terminals whose antennas range from small and isotropic to active ones capable of tracking. The above further exacerbates the need for efficient link and time management to guarantee a near-optimal use of resources while leveraging overall heterogeneity. Beyond current power-triggered procedures for link management, specific NTN and asymmetric approaches will be required, which must consider the handover direction, e.g., within a vertical layer (within an LEO constellation or inter-HAPS) or across technologies (ground-to-air/space or vice versa). Furthermore, network topologies will be created based on anticipated traffic patterns. Finally, links will be planned and scheduled based on node liveliness, line-of-sight availability and link energy costs, prioritising node energy conservation over link data rates. In addition, link management policies must trade off reliability, spectral and energy-efficient operation and load balancing, and signaling overhead caused by conditional handover preparations, planned outages, and radio or optical link failures. In summary, NTN consists of mobile nodes, where the topology is dynamic as nodes and links are removed and re-established due to the nature of the devices. In space and aerial networks, without fixed power sources, such as battery-operated or powered by wave, wind and solar, node aliveness, and link availability will be restricted and planned for in advance of traffic being forwarded. King & Shortt Expires July 18, 2023 [Page 4] Internet-Draft Time Variant Challenges for NTNs January 2023 This document summarises time-variant NTN topology problems; it outlines the use cases and key requirements, for link management and topology creation and routing, when network connectivity is temporal, where nodes and links must be managed to maximise power efficiency. 1.1 Terminology ATG: Air to Ground eNodeB: A 4G base station e-MTC: enhanced Machine Type Communication FSO: Free Space Optics GEO: Geosynchronous orbit with the altitude 35786 km gNB: A 5G base station HAPS: High Altitude Platform System IGP: Interior gateway protocol ISL: Inter Satellite Link ISLL: Inter Satellite Laser Link ISTN: Integrated Space Terrestrial Network LEO: Low Earth Orbit with the altitude from 180 km to 2000 km. MEC: Multi Edge Computing MEO: Medium Earth Orbit NG-RAN: Next Generation Radio Access Network NGSO: Non-Geostationary Satellite Orbit NTN: Non Terrestrial Networks NTN-Gateway: An earth station for accessing NTN nodes RSRP: Reference Signal Receive Power SNO: Satellite Network Operator King & Shortt Expires July 18, 2023 [Page 5] Internet-Draft Time Variant Challenges for NTNs January 2023 SRI: Satellite Radio Interface TN: Terrestrial Networks 2. 3GPP NTN Use Cases and Requirements Discussion on Non-Terrestrial Networks (NTN) started in 3GPP with a Study Item in Release-15 in 3GPP RAN WG1 in 2018. 3GPP is involved in investigating the NTN physical layer aspects, protocols, and architecture, as well as the radio resource management, link requirements, and frequency bands to be used. Work continued, and in 2019 in Release-16 [TR 38.821] detailed deployment scenarios and channel models for NTN. 3GPP Release-17 has also introduced new network topologies into the specifications for NTN. Follow-up work in the 3GPP Technical Specification Groups (TSGs) SA (Systems Aspects) provided use cases for satellite-based NTN in Release-17. The work identified three main use cases for satellite-based NTN: o Service Continuity: Use cases where 5G services cannot be offered by Terrestrial Networks (TN) alone. A combination of terrestrial and nonterrestrial networks, such as commercial or private jet, and maritime platforms, would be required; o Service Ubiquity: Use cases address unserved or under-served geographical areas where terrestrial networks may not be available. Use cases include industrial agriculture, asset tracking, emergency networks, and smart home; o Service Scalability: Use cases that maximise the satellite's extensive coverage and capability, and use multicasting or broadcasting techniques to distribute content. According to the architecture outlined in [TR 38.821], the satellite payload implements frequency conversion and a radio frequency amplifier in both uplink and downlink direction. The 3GPP 5G system is expected to support service continuity between terrestrial 5G access networks and 5G satellite access networks owned by the same operator, or owned by two different operators having an agreement. Connectivity is implied between TN and NTN nodes, a GEO, MEO, or LEO satellite will communicate with HAPS or UAV nodes, or terrestrial Next Generation NodeB (gNB or ng-eNB), or King & Shortt Expires July 18, 2023 [Page 6] Internet-Draft Time Variant Challenges for NTNs January 2023 satellite-enabled 5G User Terminals (UE). It is expected that the next generation (NG) based mobility should work to transition between NTN, TN and B5G NTN. It is anticipated that NTN can interact with 5G, and 4G terrestrial networks via legacy inter-RAT (Radio Access Technology) procedures. 2.1 Architecture Typically, the NTN architecture comprises one or several satellite gateways that connect the NTN to a public data network. In addition, several link elements exist: o A feeder link or radio link between a satellite gateway and the satellite or the UAS platform; o A service link or radio link between the user equipment (UE) and the satellite or the UAS platform; A satellite or a UAS platform may implement either a transparent or a regenerative (with onboard processing) payload. The satellite or the UAS platform typically generates several beams over a given service area bounded by its field of view. The footprints of the beams are typically of an elliptic shape. The field of view of the satellite or the UAS platform depends on the onboard antenna diagram and the minimum elevation angle. Additionally, Inter-Satellite Links (ISL) exist in a constellation of satellites; their interfaces have traditionally been RF-based, but increasingly Free-Space-Optics (FSO) are being deployed. This will require regenerative payloads on board the satellites. The ISL may then operate in an RF or optical band. The logical architecture described in [TS 38.401] may be used as a baseline for NTN scenarios, which include but are not limited to: o Transparent Satellite Based NG-RAN; o Regenerative Satellite Based NG-RAN; o Regenerative Satellite with gNB on Board; o Regenerative Satellite with gNB-DU on Board. 2.1.1 Physical Layer Control King & Shortt Expires July 18, 2023 [Page 7] Internet-Draft Time Variant Challenges for NTNs January 2023 The propagation delays in terrestrial mobile systems are usually less than 1 ms. In contrast, the propagation delays in NTN are much longer, ranging from several milliseconds to hundreds of milliseconds depending on the altitudes of the spaceborne or airborne platforms and payload type. 2.1.2 Uplinks and Downlinks Several NTN uplink power control methods have been proposed in Release-16: o Beam-specific configuration for power control parameter and common parameter for all beams; o A UE prediction of its own transmission power using other available information such as satellite ephemeris and UE trajectory; o Adaptive uplink power control based on adaptive UE configuration of Layer 3 filter coefficients (i.e., configuring multiple Layer 3 filter coefficients and letting UE select one of the Layer 3 filter coefficients based on measured Reference Signal Receive Power (RSRP); o A UE can be configured with different uplink power control parameters such as P0 and alpha parameters for disabled and enabled HARQ processes; o The transmission power of different UEs can be adjusted as a group with a reference UE transmission power. 3GPP Release-17 work develops on earlier studies performed in Release-16, where NTN channel models and necessary adaptations of the NR technology to support NTN were identified. The main challenges identified are related to the mobility and orbital height of the satellite. The height causes a high path loss and a large RTT. The mobility of an LEO satellite introduces a very high Doppler offset on the radio link, and it also inevitably requires all devices to change their serving nodes frequently. Furthermore, Release-17 establishes basic mechanisms to manage these challenges and provides a first set of specifications to support NTNs based on NR, NB-IoT and LTE-M. 2.1.3 Feeder Links During the satellite movement in the NTN, the switch-over of the feeder link between the different NTN gateways will be needed, especially for non-GEO satellites. The switch-over may happen when King & Shortt Expires July 18, 2023 [Page 8] Internet-Draft Time Variant Challenges for NTNs January 2023 the satellite moves out of the vision of the current NTN gateway. A feeder link switchover will occur when the existing feeder link is changed from a source NTN Gateway, to a target NTN Gateway for a specific NTN payload. The feeder link switch-over happens at the transport network layer. In a soft-feeder link switch-over, an NTN payload can connect to more than one NTN gateway during a given period, i.e., a temporary overlap can be ensured during the transition between the feeder links. A hard-feeder link switch-over, is when an NTN payload only connects to one NTN gateway at any given time, i.e., a radio link interruption may occur during the transition between the feeder links. 2.2 Satellite Service Continuity Satellites in Earth orbit move at relatively high speed to a fixed position on Earth. The satellite beam towards the Earth determines the area coverage that the satellite provides to the user. There are typically two modes of satellite beam operation: o Moving-beam: This is the case of a satellite with fixed beams, which yields a moving footprint on the Earths ground. In this case, the beam is moving relative to a fixed position on Earth; o Fixed-beam: This is the case of a satellite with steerable beams. As the satellites orbit the Earth, the satellite beams are adjusted so that it can continue to cover the same geographical area. As long as the satellite is above the horizon relative to the given geographical area, the beams can be adjusted to cover that area. The second scenario, fixed-beam, yields the maximum time a user may remain under the coverage area of the same satellite. This time is the time the satellite remains above the horizon relative to the user's location, which is approximately seven to ten minutes. 2.3 Satellite-based NG-RAN Architectures The NG-RAN logical architecture is described in [TS 38.401] and is used as a baseline for NTN scenarios. The satellite payload implements the regeneration of the signals received from Earth. o The radio interface (NR-Uu) on the service link between the UE and the satellite; King & Shortt Expires July 18, 2023 [Page 9] Internet-Draft Time Variant Challenges for NTNs January 2023 o Satellite Radio Interface (SRI) on the feeder link between the NTN gateway and the satellite. Architecture aspects for using satellite access in 5G [TR 23.737] Specified enhancements for RF and physical layer, protocols, radio resource management, and frequency bands. Identified a suitable architecture, addressed TN-NTN roaming and timing-related issues, enhanced conditional handover, and location-based triggering 2.4 NB-IoT and eMTC Support A topic discussed in 3GPP Release-17, [TR 36.763] focused on IoT applications by highlighting issues related to Long Term Evolution (LTE) timing relationships, uplink synchronization, and HARQ (Hybrid automatic repeat request). These use cases may impact requirements for time-variant networking and will require further study. 3. Routing and Traffic Engineering Challenges for NTNs 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 NTN and integrated space and terrestrial network environment, especially given the district characteristics of the two types of networks and 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 [1]. Energy efficiency policies may need to be enforced based on the node or link type, traffic type and their QoS requirements or other contexts such as the distance NTN nodes and power transmission cost. For instance, it may be argued that routing through a chain of LEO satellites using a currently available topology is sub-optimal. Instead, a new topology should be created for the users' end-to-end delay or to meet application or service bandwidth expectations. 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 separate protocols King & Shortt Expires July 18, 2023 [Page 10] Internet-Draft Time Variant Challenges for NTNs January 2023 (the case today) or with a unified protocol suite. Routing and signaling across emerging NTN infrastructure is far from static [2]; satellite-to-satellite connectivity changes frequently, space-based ISL link latencies will vary, and links from space-to-ground will change regularly. The satellite that is overhead a particularly ground station changes frequently, the RF or laser links between space-based satellites change often, and link latencies for satellite-to-ground links will vary based on atmospheric conditions [3]. Satellites will also have to contend with predictive routing capabilities, as links will only be established when optical alignment is possible or powered and in service. Given that meshes of 100s and 1000s of satellites are also expected, techniques that use per-hop Dijkstra calculation will be extremely inefficient [3]. Several link management and control plane challenges have been identified for NTN infrastructure, 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 may necessitate frequent routing updates, unless an SDN-based centralised controller is used; 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 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 Unmanned aerial aircraft link and time availability; o Resilience: overall, the network must be resilient to failures, and capable of routing within bandwidth and latency thresholds, even when traffic levels are significant enough to oversubscribe the preferred paths. King & Shortt Expires July 18, 2023 [Page 11] Internet-Draft Time Variant Challenges for NTNs January 2023 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) [3]. 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 [3]. 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, especially when power consideration, and expected link loss, or link activation is planned. 3.1 Link and Routing Resilience for NTNs 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 NTN nodes inoperable, and planned connectivity is lost. 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., uplink and 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 an NTN node, such as a satellite or ISL forwarding, or degradation of satellite orbit, and loss of communication sight to neighbouring node. In general, node failures or components of the forwarding plane are problematic but as the latest generation of NTN infrastructure is highly meshed, routing around node failures is feasible. Once a failure occurs, the centralized controller, King & Shortt Expires July 18, 2023 [Page 12] Internet-Draft Time Variant Challenges for NTNs January 2023 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. 3.2 Multi-layer Networking in NTNs Low-altitude UAVs, HALE and HAPS nodes, and LEO satellite provide latency benefits, but will typically have more dynamic connectivity and oscillating link characteristics, and therefore more planned or expected link outages and re-activations. They may also connect to higher-altitude nodes, such as the Medium Earth Orbit (MEO) and Geostationary Orbit (GEO) satellites, which also 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 between satellite layers, akin to multi-layer networking in terrestrial networks. 4. NTN Management and Operation In 2019, 3GPP SA5 started a study on management and orchestration aspects with integrated satellite components in a 5G network. The main objective is to study business roles and service, network management, and orchestration of a 5G network with integrated NTN components. The scope includes both NTN RAN based satellite access, and non-3GPP defined satellite access, as well as HALE and HAPS aspects. An entity, distributed or centralised, will be required that dynamically manages planned resources at NTN nodes according to their availability, power consumption and recharge rate, mobility patterns, architecture hierarchy, incoming and expected traffic, ensuring seamless service continuity to the end-user despite of intermittent link availability, topology changes and possible link disruptions. Ultimately, link utilisation, across TN and NTN nodes will need to be optimally allocated and leveraged, based on the time-variant requirements outlined in this document. Further discussion of management and operation will be included in future versions of this document. King & Shortt Expires July 18, 2023 [Page 13] Internet-Draft Time Variant Challenges for NTNs January 2023 5. Security Considerations Several existing 5G security requirements and procedures will need to be considered, including the impact on confidentiality and integrity protection of user traffic and control plane traffic. The control plane traffic between UE and the radio access network (i.e. the gNB) may be protected in two ways: o Integrity protected, i.e., it cannot be tampered with; o Confidentiality protected, i.e., it cannot be eavesdropped. The user plane (i.e. data) traffic between UE and the radio access network (i.e. the gNB) will also need to be protected. Additionally, there may also be encryption requirements for NTN interfaces between NTN nodes, such as a satellite node and the gNB, to prevent man-in-the-middle attacks. Further discussion of security will be included in future versions of this document. 6. IANA Considerations This document makes no requests for IANA action. 7. Acknowledgements To be added. 8. Contributors To be added. 9. Informative References [TR 38.821] Solutions for NR to Support Non-Terrestrial Networks (NTN), document TR 38.821, Release 16, 3GPP, Jan. 2020. [Online]. Available: https://www.3gpp.org/ [TS 38.401] 5G NG-RAN Architecture Description, document TR 38.401, Release 16, 3GPP, Nov. 2020. [Online]. Available: https://www.3gpp.org/ King & Shortt Expires July 18, 2023 [Page 14] Internet-Draft Time Variant Challenges for NTNs January 2023 [TR 23.737] Study on Architecture aspects for using Satellite Access in 5G, document TR 23.737, Release 17, 3GPP, Mar. 2021. [Online]. Available: https://www.3gpp.org/ [TR 36.763] Study on Narrow-Band Internet of Things (NB-IoT) / enhanced Machine Type Communication (eMTC) support for Non-Terrestrial Networks (NTN) [1] Curzi, Giacomo & Modenini, Dario & Tortora, Paolo. (2020). Large Constellations of Small Satellites: A Survey of Near Future Challenges and Missions. Aerospace. 2020. [2] 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. [3] 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. Authors' Addresses Daniel King Lancaster University UK Email: d.king@lancaster.ac.uk Kevin Shortt Airbus Germany Email: kevin.shortt@airbus.com King & Shortt Expires July 18, 2023 [Page 15] Internet-Draft Time Variant Challenges for NTNs January 2023