upstream property
The hops this message's stream crossed before the sender, when the sender is passing it on rather than producing it. Null otherwise.
sourceClock is then the origin's clock, and clockOffset and uncertainty are already totals for the whole way (these hops plus the sender's), so transitSeconds runs from the origin's timestamp. The hops themselves are here for finding which one is slow; one object is shared by every message of the stream until the sender updates it.
Implementation
final ClockChain? upstream;