Background

A. PDU Session in Mobile Networks

In the 5G network, a PDU session refers to the logical channel between a user and a data network (e.g. Internet) through a specific base station and user plane function (UPF). The UPF manages the user session context and fixes the transmission path of user traffic, thus it is referred to as the anchor point of the PDU session in mobile networks. As the sole network function in the core network that handles user traffic, the UPF is responsible for executing all user plane policies according to packet detection rules (PDRs).

在5G网络中，PDU会话（PDU Session） 是指用户通过特定的基站和用户平面功能（User Plane Function, UPF） 连接到数据网络（如互联网）的逻辑信道。UPF管理用户的会话上下文并固定用户流量的传输路径，因此它被称为移动网络中PDU会话的锚点（anchor point）。作为核心网中唯一处理用户流量的网络功能，UPF负责根据分组检测规则（Packet Detection Rules, PDRs） 来执行所有用户平面策略。

Specifically, Fig. 1 illustrates the process by which the UPF handles incoming user traffic [11]. Taking the uplink direction as an example, upon receiving the user packet from the base station (BS), the UPF identifies the specified packet forwarding control protocol (PFCP) session to which the packet corresponds. Then, it selects the highest precedence PDR within the matching PDR of the PFCP session. Next, the UPF processes the data packet based on the associated rules specified by the selected PDR, including forwarding action rules (FARs), buffering action rules (BARs), QoS enforcement rules (QERs), and usage reporting rules (URRs). Finally, the packet is forwarded, with its direction determined by the matching FAR of the selected PDR.

具体而言，图1展示了UPF处理上行用户流量的过程[11]。以上行方向为例，当从基站（BS）收到用户数据包后，UPF首先识别出该数据包所对应的指定分组转发控制协议（Packet Forwarding Control Protocol, PFCP） 会话。然后，它在该PFCP会话的匹配PDR中选择优先级最高的PDR。接着，UPF根据所选PDR中指定的关联规则来处理该数据包，这些规则包括转发行为规则（Forwarding Action Rules, FARs）、缓冲行为规则（Buffering Action Rules, BARs）、QoS执行规则（QoS Enforcement Rules, QERs） 以及使用情况上报规则（Usage Reporting Rules, URRs）。最后，数据包被转发，其去向由所选PDR中匹配的FAR决定。

3GPP has defined an intermediate UPF (I-UPF) that does not serve as an anchor point but is deployed as a traffic classifier between the base station and multiple UPFs [12]. It achieves traffic classification by using different PDRs to match packets with various target IP addresses or data network names. These PDRs are often associated with different FARs, which forward the packets to different UPFs. In terrestrial networks, the I-UPF is primarily used in private networks requiring high reliability (such as vehicular networks and smart factories) or multi-access edge computing (MEC) services to classify private and public network traffic.

PDU session with an I-UPF is established using an insertion-based process [13]. For example, when a user moves into the service range of a specific MEC service, the core network inserts an I-UPF into the user’s current PDU session and instructs the I-UPF to establish a connection with the additional UPF associated with the MEC service. When the user leaves the MEC service range, the I-UPF and the additional UPF are removed.

3GPP定义了一种 中间UPF（Intermediate UPF, I-UPF），它不作为锚点，而是作为流量分类器部署在基站与多个UPF之间[12]

它通过使用不同的PDR来匹配具有不同目标IP地址或数据网络名称的数据包，从而实现流量分类。这些PDR通常与不同的FAR相关联，后者会将数据包转发至不同的UPF。在地面网络中，I-UPF主要用于需要高可靠性的专网（如车联网和智能工厂）或多路访问边缘计算（Multi-access Edge Computing, MEC） 服务中，以对私有和公共网络流量进行分类。

带有I-UPF的PDU会话是使用一种基于插入的流程（insertion-based process） 来建立的[13]。例如，当用户移动到某个特定MEC服务的服务范围内时，核心网会将一个I-UPF插入到用户当前的PDU会话中，并指示该I-UPF与和MEC服务关联的附加UPF建立连接。当用户离开该MEC服务范围时，I-UPF和附加UPF则被移除。

I-UPF背景知识

I-UPF 这个“基于插入的流程” 的精髓在于它的灵活性和无感化

1. 不会中断原有的PDU会话，而是在需要时，动态地、像“插件”一样把一个智能分流点（I-UPF）“插入”到现有的数据路径中
2. 只有当您进入特定服务区域时，这个“分流员”才会上岗
3. 可以在不改变主要上网通道的前提下，为访问那些近在咫尺的边缘服务（MEC）提供一条宝贵的“捷径”，从而实现超低延迟

如图:

B. Mobile Satellite Networks

3GPP has defined two architectures for mobile satellite networks: the transparent mode (also known as bent-pipe) and the regenerative mode. In the transparent mode, the satellite acts as a transparent relay node between the user and the ground base station, whereas in the regenerative mode, base stations are deployed on the satellite, and user traffic can be forwarded through ISLs.

3GPP为移动卫星网络定义了两种架构： 透明模式（transparent mode）（也称“弯管模式”，bent-pipe）和再生模式（regenerative mode）。

1. 在透明模式中，卫星扮演用户与地面基站之间的透明中继节点
2. 在再生模式中，基站被部署在卫星上，用户流量可以通过星间链路（ISLs）进行转发

However, regardless of whether base stations are deployed on satellites, user traffic must be sent to the specific ground-based anchor point before being forwarded to the target server. Considering that users’ access targets are randomly distributed globally, the anchor point often deviates from the path to the user’s access server, causing significant detours.

Fig. 2 plots a typical example of circuitous routing in a mobile satellite network. For the current PDU session of UE U, the UPF at point F serves as its anchor point. User traffic must first pass through the satellite network to reach the anchor point, and then proceed through the ground network to reach the server. Consider a scenario where the user accesses server S, then the user traffic follows the path U − A − B − C D − F − S. It is evident that this path is not the fastest path in the network. In fact, U −A−B −E −S is a more optimal path, which could significantly reduce latency.

然而，无论基站是否部署在卫星上，用户流量都必须被发送到特定的地面锚点，然后才能被转发至目标服务器。考虑到用户的访问目标在全球范围内随机分布，锚点的位置常常会偏离用户访问服务器的路径，从而导致显著的绕行（detours）。

图2描绘了移动卫星网络中一个典型的路由迂回（circuitous routing） 示例。对于用户UE U当前的PDU会话，位于F点的UPF是其锚点。用户流量必须首先经过卫星网络到达该锚点，然后再通过地面网络到达服务器。考虑用户访问服务器S的场景，其用户流量遵循路径 U-A-B-C-D-F-S。显而易见，这条路径并非网络中的最快路径。实际上，U-A-B-E-S 是一条更优的路径，它能够显著降低时延。

As a specific example, consider the scenario of a user located in the Atlantic Ocean (42.2 ◦ N, 60.0 ◦ W) accessing a server in Paris through different paths within the Starlink constellation, as shown in Table I. When establishing a PDU session, the user selects the nearest ground station (GS) located in Ashburn, USA. Consequently, the user’s traffic is first transmitted via satellite to Ashburn and then through the terrestrial network to the server in Paris, with a total latency of 50.3ms. If the user selects the ground station in London as the anchor point, the total latency can be reduced to 26.8ms, a reduction of 44%.

举一个具体例子，考虑位于大西洋（42.2° N, 60.0° W）的用户通过Starlink星座内的不同路径访问巴黎服务器的场景，如表I所示。当建立PDU会话时，用户选择了位于美国阿什本（Ashburn）最近的地面站（GS）。因此，用户的流量首先通过卫星传输至阿什本，再通过地面网络到达巴黎的服务器，总时延为50.3毫秒。如果用户选择位于伦敦的地面站作为锚点，总时延可以降至26.8毫秒，降幅达44%

To address the issue of circuitous routing in mobile satellite networks, a straightforward method is to place the anchor point at the user satellite access point. This involves deploying a fully functional UPF on each satellite in regenerative mode. Users are provided with user plane services by the anchor point on the connected satellite, thereby avoiding detours caused by anchor points deviating from the shortest path.

为解决移动卫星网络中的路由迂回问题，一种直接的方法是将锚点置于用户的卫星接入点。这涉及到在再生模式下的每颗卫星上都部署一个全功能的UPF。用户由其所连接卫星上的锚点提供用户平面服务，从而避免了因锚点偏离最短路径所导致的绕行。

However, this design faces frequent anchor point reselection. When a base station handover occurs due to user or satellite movement, the anchor point is also reselected. Since the anchor point remains unchanged throughout the PDU session, this reselection means that users need to release the current PDU session and establish a new one. During the session reestablishment period, users experience an average service interruption of several hundred milliseconds, in addition to the interruption caused by the base station handover. More critically, session reestablishment can lead to the reassignment of the user’s IP address [12], causing interruptions in services that rely on connections. Considering the high-speed movement of LEO satellites, the reselection of anchor point occurs every 25 minutes [14], significantly impacting the service continuity for users. Therefore, this method is not a reasonable solution to the circuitous routing problem of mobile satellite networks.

然而， 这种设计面临着频繁的锚点重选问题。当由于用户或卫星的移动而发生基站切换时，锚点也会随之重选。由于锚点在整个PDU会话期间保持不变，这种重选意味着用户需要释放当前的PDU会话并建立一个新的会话。

在会话重建期间，除了基站切换导致的中断外，用户还会经历平均数百毫秒的 服务中断 。更关键的是，会话重建可能导致用户的 IP地址被重新分配 [12]，从而中断依赖于连接的服务。考虑到LEO卫星的高速移动性，锚点的重选大约每25分钟就会发生一次[14]，这严重影响了用户的服务连续性。因此，该方法并非解决移动卫星网络路由迂回问题的合理方案。