Space communications are continuously challenged by extreme conditions that include large propagation delays, intermittent connectivity, and random losses. To combat these problems, the Licklider Transmission Protocol (LTP) splits data blocks into small segments that are radiated independently and retransmitted as needed, through a process that can be paused during long link disruptions. Given the extreme delays involved, the end performance of this protocol is driven by the number of transmission rounds needed to successfully deliver each block. LTP links are defined as overlays with one or more physical channels in the underlay, therefore with sections that may be on different administrative domains and experiencing different conditions. The question of how to select the length of the segments has received negligible attention and the use of improper values can easily lead to suboptimal performance. The segmentation process used by LTP is examined in this paper to determine the role that segmentation parameters and the conditions of the underlay have on the block delivery times. This goal is achieved through the definition of a basic model of LTP’s transmission process that allows deriving the optimal segmentation parameter. Simulation results provide additional evidence of LTP’s performance contrasting the results of the optimal segment length with fixed-length segments. The results provide a theoretical performance reference for practical parameter optimization methods.