## 2 Related Work  
Utilizing multiple network interfaces has always been an attractive feature because it provides many benefits like increased reliability, availability and bandwidth to communication sessions [19; 20]. This has been recognized in wired communications, for example, the SCTP (Stream Control Transmission Protocol), which is very popular for the transport of signaling messages in telecom operator network support multi-homing, increasing availability and reliability [21]. In the case of wireless communications, this feature provides additional benefits, like greater coverage and cell traffic offloading, providing users a better QoS (Quality of Service) [22]. Considering that satellite communications are advancing very quickly with already available commercial solutions like Starlink and OneWeb, a hybrid combination of satellite and terrestrial communications is becoming more and more popular [11; 23; 24; 25]. Multi-access support has been recognized by the 3 GPP by defining the ATSSS paradigm that covers basic functions that comprise multi-access: steering, switching and splitting [26]. Steering defines how to select access at the start of the session--it can be based on a number of factors like user preferences, available bandwidth, latency, application requirements, etc. During the session, there is a possibility that the Quality of Service of the initially selected network access will deteriorate or even that the connection will be completely lost. This usually happens when the user is moving out of the coverage zone. In such cases, the session needs to seamlessly move to another available network access, and this function is called switching. Splitting enables the usage of multiple network access simultaneously. The typical use of the splitting function is to increase the used bandwidth, which can be important for some applications. There are two ways to support ATSSS--at the data link layer (low level) and at the transport layer (high level). In this paper, we consider high-level support. The 3 GPP has recognized the potential of the MP-TCP (MultiPath TCP) and considers it as a good candidate for a high-level ATSSS solution [27]. In addition, the MP-TCP and MP-QUIC (MultiPath QUIC) are also recognized as potential ATSSS high-level solutions by the 3 GPP [28]. Both the MP-TCP and MP-QUIC have their pros and cons. For example, the MP-TCP does not support UDP sessions, which we consider in this paper. Thus, there is room for improvements of existing multipath protocols or the development of new ones that can better cover some specific use cases. Hybrid terrestrial-satellite access can be considered as a specific use case as both accesses are wireless, meaning there are specific conditions compared to wired access like fading, shadowing, interference, weather condition influence, etc. These conditions make wireless links less reliable, which should be considered to avoid the erroneous detections of link congestion [29]. Numerous papers cover the topic of wireless multiple network access and ATSSS support and optimization. Since smartphones usually have a 4G/5G interface as well as WiFi, some papers investigate the performance of transport multipath protocols in such environments [30, 31]. A survey on multipath transport protocols for 5G ATSSS is given in [4]. A survey on multi-connectivity in 5G is given in [10]. A performance evaluation of combining 5G-NR, WiFi and LiFi for Industry 4.0 is addressed in [32]. Given the advancements in satellite communications, many papers cover the topic of multipath in satellite-terrestrial environments. The use of network coding in a satellite-terrestrial multipath environment is proposed in [33]. A study regarding the integration of a non-3 GPP satellite interface in a 5G multiple-access configuration is covered in [34]. Some papers analyze specific use cases like in [35], where multipath analysis for a case of a satellite higher-bandwidth link and terrestrial lower-bandwidth link is given.