Modeling and Routing for Predictable Dynamic Networks This paper is only for copyright protection, and unpublished to the top-level version. Zengyin Yang, Qian Wu, Hewu Li, Zhize Li and Jianping Wu, Fellow IEEE Tsinghua University, Beijing, China f yang-zy14, zz-Li14 [email protected], f wuqian, lihewu, jianping [email protected] Abstract—Currently, building a Satellite-Based Internet (SBI) permanently fly along some fixed orbits from a polar to providing global Internet service has become a trend, and other. As satellite routers are extremely far away from each the Iridium-like constellation is considered as the promising other, the propagation delay of link is so large that is in topology. However, such topology is dynamic in terms of node most cases taken as the link metric for routing selection position, network connectivity and link metric, which lead to algorithm [8–12]. drastic end-to-end path changes in polar region and overly Actually, the Iridium-like constellation topology is dy- frequent routing updates. In this paper, we propose a link namic. The link metric and network connectivity continu- lifetime assisted routing (LLAR) scheme that incorporates life- ously changes with the position of routers, which brings time of links into their metrics to solve the routing oscillation great challenges for SBI to achieve routing stability. The in polar region. However, it is unpractical to implement the main problems are two-fold. routing scheme into dynamic network directly because these 1) Polar routing oscillation. In polar region, link metric dynamics will require routing to be updated continuously and link connectivity are conflicted. That is, when a link for keeping consistent with these dynamics. Furthermore, in moves closer to the polar region, it will disconnect but its order to implement the routing scheme into such network and propagation delay will become smaller. And it is opposite meanwhile decrease routing updates, we establish a topology when the link leaves the polar region. When the metrics model to formulate the dynamic topology into a series of static of nearly disconnecting links and newly reconnecting links relatively vary, many end-to-end paths will be forced to snapshots, and propose a dynamic programming algorithm to switch forth and back on these links. In fact, the polar determine the timing of routing update. Extensive experiments routing oscillation overly occurs about once per 136.8s and show that our methods can significantly decrease changed impacts 15% ∼ 30% of total paths for each oscillation, paths and affected satellite nodes, greatly improving routing which incur extremely poor network performance, such as stability. They can also achieve better network performance, severe packet drops and low throughput. less packets loss and higher throughput Besides, few routing 2) Frequent routing updates. As the topology of such updates are triggered to keep consistent with network dynam- network continuously changes, it will require the routing ics. to be updated frequently for keeping consistent with the Index Terms—Satellite-Based Internet, routing stability, dy- topology changes. If routing updates fall behind the topology namic topology changes, some path may become invalid or sub-optimized. However, in each update, at most 41% of total paths will arXiv:1704.00885v3 [cs.NI] 17 Aug 2018 1. Introduction be re-routed and many limited on-board resources will be wasted. Hence, a trade-off between the timeliness of keeping With increases in demands of human activities and consistent with topology dynamics and the cost of routing capacities of satellites, building a satellite-based Internet updates is needed. (SBI) providing global Internet services has become at- These routing stability problems are essentially different tracted lots of attentions from multiple points, including tra- from current researches in ground network or space network. ditional space network providers [1, 2], the Internet content First, the polar routing oscillation is absent in the stable providers [3, 4], and networking equipment providers [5]. routing schemes specific to traditional Internet and ad-hoc Among them, the Iridium system is a well-known practical network [13, 14]. Previous schemes for routing stability network providing global service, and also it will be updated in space network mostly focused on the convergence issue to provide IP-based services for global users [2]. And the of traditional routing protocols (e.g., BGP) suffering from Iridium-like constellation is considered as the promising topology dynamics [15, 16], but the issue of polar rout- topology of SBI due to easier implementation and smaller ing oscillation incurred by route selection have not been communication delay [2, 6, 7]. In the Iridium-like constel- proposed yet. Second, the cost of routing update has not lation topology, there are two polar and satellite routers been considered into previous methods of maintaining the consistent with topology dynamics, like PLSR [17, 18] and 2.1. Iridium-like Constellation DT-DVTR [11, 19]. Hence, the goal of minimizing rout- ing updates and meanwhile maintaining the consistent with In the world of satellites, the Low Earth orbit (LEO) topology dynamics can not be achieved in these methods. constellation offers significantly smaller propagation delays In this paper, we design a link lifetime assisted routing as well as lower signal attenuation than geostationary (GEO) (LLAR) scheme to mitigate the polar routing oscillation. satellites, therefore, the LEO constellation equipped with The proposed scheme firstly defines a link lifetime in LLAR intersatellite link (ISL) has been considered as a good way to locate the links where oscillation occurs. Then it adds the to form a global communication environment in space. As lifetime of these severely dynamic links into their metrics the use of ISL greatly complicates the design of system, the to eliminate the variations of link metric. Furthermore, LEO constellation with circular polar orbits is prone to be by taking advantages of the Dijkstra’s greedy property, it chosen. Up to now, most proposed LEO constellations with decreases the amount of paths sharing these dynamic links ISL are based on the circular polar orbits [2, 7, 21]. Among and to avoid routing switching back and forth between them. them, the Iridium system is a only well-known practical Furthermore, we adopt the Divide-and-Merge methodology commercial network, and also it will be updated to provide [20] to avoid routing updates when the topology keeps IP-based services for global users [2]. Thus, Iridium-like unchanged or slightly changed. First, we divide the dynamic constellation is considered as a representation for analysis topology into sequences of static snapshots. Second, for and simulation in this paper, whose parameters are given in decreasing the number of static snapshots, we propose a TABLE 1. dynamic programming algorithm to merge those unchanged Fig.1 shows that the Iridium-like system is seen from or only slightly changed contiguous static snapshots. Con- the North Pole. The constellation has 6 orbits, all of which sequently, routing updates will only be triggered at the start intersect at the pole. In each orbit, there are 6 or 7 satellites time of each merged snapshot. painted in the figure. However, for the overall system, there Extensive experiments show that our methods can signif- are totally 11 evenly distributed satellites in each orbit. And icantly improve the routing stability issues in SBI. For polar all satellites predictably fly along their own orbits from one routing osculation issue, it can decrease 50% of changed polar to the other. The time of all satellite flying a circle paths and affected satellite nodes than PLSR and 32% around the earth is called the system period, about 100min. than DT-DVTR. That results in a decrease of more than Each satellite is equipped with 2 to 4 ISLs. The ISL in- 90% packet loss and a increase of almost 20% throughput terconnects neighboring satellites within line-of-sight. Two compared with others. And for routing update issue, our of these ISLs (intra-orbit ISL) are used to link the ahead methods can achieve few routing updates and meanwhile and behind satellite in the same orbit, while the other two keep consistent with the SBI dynamics. (inter-orbit ISL) are used to interconnect adjacent satellites The rest of this paper is organized as follows. Section in adjacent orbits. For an inter-orbit ISL, the viewing angle 2 presents the background about iridium-like constellation for maintaining it varies with its latitude. When an inter- and routing concept. In section 3, the routing stability in orbit ISL is above a certain latitude (boundary of polar SBI is deep analyzed. The corresponding routing scheme region), the angular rates for tracking antennas become is presented in section 4 and a detailed presentation about so high that the inter-orbit ISL will be dropped [7]. In our topology model is provided in section 5. In section other words, when a satellite enters the polar region, its 6, we present the experiments used to verify our methods. adjacent inter-orbit ISLs will disconnect; when it leaves the Finally, we review the related work in section 7 and draw a region, its adjacent inter-orbit ISLs will reconnect. Hence, conclusion in section 8. the reconnection/connection of inter-orbit ISLs will jointly change in the polar region. 2. Background The simplified topology with only two adjacent orbits is shown in Fig.1. For simplicity, the adjacent satellites in the Currently, the IP-based applications of ground network adjacent orbits are painted in the same horizontal direction. and space network are integrating. Building a satellite-based We can clearly get that all of intra-orbit ISLs have the same Internet (SBI) providing global Internet services has become delay, and the inter-orbit ISLs disconnect in the polar region. an important part of the future network.