## 4 GeoTag-Based Forwarding Strategy  
As previously explained, an interest forwarding strategy is the logic that decides in each NDN node whether, when and where to forward an incoming interest, among all the possible outgoing interfaces, according to the FIB for a given name or, alternatively, a forwarding hint. Already existing strategies are the best route strategy, that follows the longest prefix match principle; the multicast strategy; and the self-learning strategy, which broadcasts interests for the first time to learn a single path toward data. In [23], the authors surveyed the main forwarding strategies proposed in the literature for NDN-based wireless networks. In this paper, we propose a new location-based forwarding strategy for NDN-based LEO satellite networks that takes advantage of the knowledge of the relative position of the satellite nodes and the grid structure formed by the ISLs to perform the forwarding of the interest packets without the need of interchanging information between nodes, as is the case with conventional routing protocols. We consider the most common scenario where each satellite keeps four ISLs connected to its four immediate neighbors: two intra-plane ISLs with the immediate previous and next satellites in the same orbital plane, and two inter-plane ISLs with the closest satellites of both immediate adjacent planes, as shown in Figure 1. That is, for our purposes, we can consider the satellite network as a grid covering the Earth and, therefore, we propose to identify each satellite by two Cartesian coordinates. The horizontal coordinate represents the ordinal number of the orbital plane while the vertical one represents the number of the satellite within the orbital plane, as shown in Figure 2.  
Ground nodes that generate interest packets are referred to as _consumers_, and ground nodes that have the original data are called _producers_, although recall that it is also possible to maintain copies at intermediate NDN node caches. Both consumer and producer ground nodes must choose periodically the _best_ (closest) access satellite to connect to so that they direct their beams to only one satellite for upstream communication, whereas downstream messages come from only that satellite. However, the constant mobility of LEO satellites forces the ground nodes to perform a handover every five minutes approximately. As pointed out in [24], there are two alternatives to solve the producer mobility problem in NDN: either the mobile producer makes the data available at a reachable named location, or it reports constantly its point of attachment (PoA), that is, the address of the satellite to which it is connected to, similarly to how the DNS system works for IP. In this paper, we choose this second option and, thus, the PoA will be the two Cartesian coordinates of this satellite. The NDNLPv2 (NDN Link Adaptation Protocol version 2) protocol [25] offers the possibility to send a GeoTag into the interest packets, that is, a predefined 3-byte header that is meant to carry Cartesian 3D coordinates (GPS coordinates, usually). We propose using the first two bytes to encode the orbital plane and the ordinal position within the orbital  
Figure 1: Part of a constellation showing the ISLs between neighboring satellites. Orange arrows show links between adjacent planes and pink arrows show links between consecutive satellites in the same orbital plane. The non-dashed links show a possible path between Western Europe and the Great Lakes region in the USA. Figure 2: Representation of the grid structure showing both the ISL links and the satellite notation. plane as two Cartesian coordinates, while the third remaining byte is reserved for possible future expansion. In this way, the consumer, using the name of the desired data, obtains the PoA of the satellite connected to the producer (i.e., its two Cartesian coordinates), and sends these coordinates into the GeoTag field of the corresponding interest packet. Then, it forwards this packet to its own access satellite.