## 2 Principles and Methods
### The Label Switching-Based ISTNs  
#### 2.2.1 Principles of Label Switching  
In the data plane of ISTNs, various packets are categorized into different flows according to their information, e.g., source/destination IPv4 and/or IPv6 address. The packets that are transmitted over the same path are considered as a forwarding equivalence class (FEC). It may be feasible to employ a single label for a set of FECs, which is called link aggregation. A local unique fixed-length label will be appended to the packets in the same type of FEC. Once receiving an incoming packet, the label switching router extracts the in-label of the packet, replaces it with the out-label, and forwards the packet to the egress port according to the local label forwarding information base (LFIB) table, and thus transmits the packet to the neighbor label switching router. The packet is switched by different label switching routers until the egress label edge router is reached. In some ways, the ISTNs  
Figure 1: The typical application scenario of ISTNs. act similarly to a multi-protocol label switching (MPLS) network. The uni-directional data path, along which a data packet is transmitted in the MPLS domain, is defined as a label switching path. Currently, there are three typical methods of routing path calculation and label switching path setup:  
1. Distributed routing and label switching path setup. All the label edge routers and label switching routers maintain their own link-state databases, and conduct routing computation and label switching path setup by themselves, without having terrestrial controller involvement;
2. Centralized routing, distributed label switching path setup. With the global knowledge of network status, a terrestrial controller is able to cause computer routing paths to meet multiple QoS requirements from the ingress label edge routers. Meanwhile, the ingress label edge router constructs a label switching path according to the related routing paths calculated by the terrestrial controller;
3. Centralized routing and label switching path setup. The terrestrial controller would calculate both routing paths and label switching paths and deliver them to routers. Owing to the frequent topology changes of satellite networks, directly utilizing terrestrial distributed routing protocols (e.g., OSPF) would lead to constant route convergence, thus consuming valuable inter-satellite link (ISL) bandwidth. Hence, by following the idea of Method 1 above, we propose an optimized OSPF routing protocol and constraint label distribution protocol for a GEO/IGSO and MEO satellite network by considering the QoS requirements that are characterized as multiple dimensionless metrics. Here, QoS requirements can be measured by uni-directional link, cost, priority, duration time, forward/backward bandwidth and latency. In addition, link-state update flooding is restrained by periodic updates of the on-satellite local link-state database according to the network topology. The optimized OSPF routing algorithm is described in Algorithm 1. Furthermore, as a type of large-scale and heterogeneous network, whose network states dynamically vary in both time and space, any single node in the ISTNs will be capable of performing both the access function (e.g., multiple data granularity classifications and link aggregation) and the switching function (i.e., multi-layer data distribution, such as a traditional label edge router and label switching router). The aforementioned Method 1, therefore, is more capable of low overhead controlling and high scalability. Given the time-precise synchronization of the link-state database and label switching path, and a new signaling protocol needed for spreading labels among the label switching routers, Methods 2 and 3 may only be able to conduct a small number of label switching paths. According to [14], the label distribution protocol (LDP) is designed to distribute labels to all the label edge routers and label switching routers along a label switching path.