How are timestamps deemed invalid?

Question

The Stellar Consensus Protocol orders block candidates (via ballots) by the maximum timestamp and union of their transactions. It says that "values [transactions/ballots] with invalid timestamps will not receive enough nominations".

What makes a timestamp invalid? Is there a range relative to a node's own clock it must be in?

Do nodes sync their clocks with each other, or do they sync through other means (like NTP)?

Thanks!

Answer 1

If you look at the stellar-core codebase, under the HerderSCPDriver's validateValueHelper function, there is this code snippet:

    // Check closeTime (not too old)
    if (b.closeTime <= lastCloseTime)
    {
        return SCPDriver::kInvalidValue;
    }

    // Check closeTime (not too far in future)
    uint64_t timeNow = mApp.timeNow();
    if (b.closeTime > timeNow + Herder::MAX_TIME_SLIP_SECONDS.count())
    {
        return SCPDriver::kInvalidValue;
    }

And in herder, I see that:

std::chrono::seconds const Herder::MAX_TIME_SLIP_SECONDS(60);

So I believe if a timestamp is less than the previous close time, or the timestamp is a minute ahead (a hard coded value) then the timestamp is deemed invalid.

And to answer your second question, there is a NTP sync check. The role of this is, according to the this file:

[To] periodically check if local time is synchronized with NTP server.