RFC 4838: Delay-Tolerant Networking Architecture

"The DTN architecture depends on time synchronization among DTN nodes (supported by external, non-DTN protocols)" (Section 3.12)

This is rather surprising for a protocol intended to be used in interplanetary networking and got me wondering about the expected solution to have DTN nodes sitting on Earth, on Mars, on a probe traveling beyond Saturn agree on time synchronization.

https://www.eecis.udel.edu/~mills/ipin.html makes for interesting follow-up reading.