Despite the notable progress that has been achieved on long-haul radio and optical communications, which are expected to improve the data transfer rates of future space exploration missions, both scheduled and random link outages remain possible. To improve reliability, NASA's delay-tolerant networking (DTN) architecture offers the Bundle Protocol (BP) and the Licklider Transmission Protocol (LTP) that implement bundle custodial transfer and automatic repeat query retransmissions. We analyze the performance of these protocols considering the default flow control method of LTP, which limits the number of concurrent sessions. We derive the combined BP and LTP throughput and provide approximations for model parameters that are difficult to find exactly. Event-driven simulations suggest that our analysis is reasonably accurate under different channel conditions and protocol settings. The proposed model provides insight into the role of the channel conditions, the workload, and the choice of BPLTP protocol parameters on the performance of a reliable space link.