Technical Background

Integrated Space and Terrestrial Networks (ISTNs)

Figure 1 plots a high-level architecture of emerging ISTNs upon LEO satellite constellations, and this architecture has been used by today’s operational ISTNs such as Starlink [1]. In particular, LEO satellites are moving rapidly related to the earth surface. Each satellite can be equipped with laser intersatellite links (ISLs) to interconnect with each other, and radio ground-satellite links (GSLs) to communicate with ground facilities such as satellite terminals and ground stations (GSs). When users access Internet services via the ISTN, packets from the user side are first sent to a ground station through one or multiple satellites (i.e., by “bent-pipe” transparent forwarding if the user is close to the ground station, or by multi-hop routing for remote users), and then forwarded to the Internet by terrestrial fibers [17]. Emerging ISTNs are aimed at providing Internet services to diverse users, such as residential customers [1], and mobile users in recreational vehicles (RVs) [18], airplane [19] and maritime scenarios [9]. In practice, recently Starlink releases its global roaming service [10] to enable ubiquitous access on the earth via ISLs, and provides low-latency in-flight Internet across the globe [20].

图1展示了基于低地球轨道（LEO）卫星星座的新兴ISTN的高层次架构，这种架构已被如今的运营ISTN（如Starlink）所采用。特别地，LEO卫星相对于地球表面移动迅速。

每颗卫星都可以配备激光星间链路（ISLs），以相互连接，并使用无线地面-卫星链路（GSLs）与地面设施（如卫星终端和地面站GSs）进行通信。

当用户通过ISTN访问互联网服务时，用户端的数据包首先通过一颗或多颗卫星发送到地面站（如果用户靠近地面站，则通过“弯管”透明转发；对于远程用户，则通过多跳路由），然后通过陆地光纤转发到互联网。

新兴ISTN旨在为多样化的用户提供互联网服务，包括住宅客户、休闲车辆（RVs）中的移动用户、飞机和海上场景中的用户。在实践中，Starlink最近推出了全球漫游服务，以便通过ISLs在全球范围内实现无处不在的访问，并提供全球范围内的低延迟飞行互联网服务。

Network-layer Mobility Management (MM)

In global roaming scenarios, since users are continuously changing their locations in the network, it is important to track where users are, and deal with handovers to keep connections active. Network-layer mobility management (MM), in which mobility-related features are deployed at the IP layer and signaling messages for mobility purposes are carried by IP traffic, is a classic approach to handle mobility issues, and has been well studied over the past decade in terrestrial Internet [11], [13]–[15], [21]–[23]. As plotted in Figure 2, the core idea of network-layer MM is to exploit anchor points to manage the locations and handovers for mobile users. Specifically, an anchor is a fixed node (e.g., a router or switch) maintaining the mobility status of users. When a mobile user connects to the network through an access point (Acc 1 ), it first registers with a specific anchor (Anc 1 ) and acquire an initial address. Traffic from or to the mobile user is first forwarded to the anchor and then to the final destination (Path 1 ). When the user changes location in the network (i.e., connects to Acc 2 ) within the management area of the registered anchor (Acc 1 ), Acc 2 sends messages to Anc 1 and notifies the location change in real time. Correspondingly, the traffic routes according to Path 2 based on the new location information in Anc 1 . In particular, once the user is managed by a new anchor (Anc 2 ), it needs to re-register and acquire a new address. Similarly, the traffic will pass through the new anchor point (Path 3 ).

在全球漫游场景中，由于用户在网络中不断改变其位置，因此跟踪用户位置并处理切换以保持连接活跃至关重要。网络层移动性管理（MM）是一种经典的方法，用于处理移动性问题，其特点是将移动相关功能部署在IP层，并通过IP流量传递移动目的的信令消息。这种方法在过去的十年中已在陆地互联网中得到充分研究[11][13]–[15][21]–[23]。

如图2所示，网络层MM的核心思想是利用锚点来管理移动用户的位置和切换。具体来说，锚点是一个固定节点（例如路由器或交换机），它维护用户的移动状态。

当移动用户通过一个接入点（Acc 1）连接到网络时，它首先在一个特定的锚点（Anc 1）上注册，并获得一个初始地址。

来自或发往移动用户的流量首先被转发到锚点，然后到达最终目的地（Path 1）。

当用户在网络中改变位置（即连接到Acc 2）时，如果仍在注册锚点（Acc 1）的管理范围内，Acc 2会向Anc 1发送消息并实时通知位置变化。

相应地，根据Anc 1中的新位置信息，流量会根据Path 2路由。特别是，当用户被一个新锚点（Anc 2）管理时，它需要重新注册并获得一个新地址。同样，流量将通过新的锚点（Path 3）。

Research scope. In this paper, we focus on exploring the network-layer mobility management for ISTNs for three key reasons. First, to efficiently integrate satellite constellations into existing terrestrial Internet, emerging ISTNs like Starlink adopts IP-based networking architecture [6] with heterogeneous wireless access technologies. Second, unlike link-layer solutions (e.g., [24]), a network-layer solution does not rely on any assumption about the underlying wireless access technologies. Finally, as compared to high-layer approaches (e.g., QUIC’s connection identifier [25]), a network-layer solution achieves higher handover efficiency (e.g., shorter disruption time). Collectively, solutions in other layers complement our work.

研究范围。在本文中，我们专注于探索ISTN中的网络层移动性管理，理由有三：

首先，为了高效地将卫星星座集成到现有的陆地互联网中，新兴ISTN（如Starlink）采用基于IP的网络架构[6]，支持异构的无线接入技术。

其次，与链路层解决方案（例如[24]）不同，网络层解决方案不依赖于对底层无线接入技术的任何假设。

最后，与高层方法（例如QUIC的连接标识符[25]）相比，网络层解决方案实现了更高的切换效率（例如更短的中断时间）。总的来说，其他层的解决方案与我们的工作相互补充。