Real Interview Transcript: ISRO ICRB Scientist "SC" (Computer Science) Bengaluru

Panelist 1 (OS Specialist): "Great. Suppose a satellite onboard computer has dual redundant CPUs running an RTOS. How will you prevent priority inversion between a high-priority telemetry transmitter task and a low-priority sensor logger sharing a shared memory buffer?" Candidate Answer: "Sir, I will implement the Priority Inheritance Protocol or Priority Ceiling Protocol. When the low-priority sensor logger acquires the mutex for the shared buffer, and a high-priority telemetry task requests the same mutex, the OS temporarily elevates the sensor logger's priority to match the telemetry task. This prevents intermediate medium-priority tasks (e.g. housekeeping tasks) from preempting the sensor logger, ensuring it finishes and releases the mutex promptly."

Candidate Answer: "Sir, standard TCP will fail due to high RTT and slow-start timeouts. For deep-space communications, Delay-Tolerant Networking (DTN) using the Bundle Protocol (RFC 5050) is employed. It utilizes Custody Transfer: intermediate orbital relay nodes store the bundle persistently in non-volatile storage and acknowledge custody back to previous node. Transmission uses Licklider Transmission Protocol (LTP) with large block aggregations and proactive selective acknowledgements instead of reactive TCP handshakes."