NFD

After check the code, I think the bug may be caused by the udp-face.cpp code.

In UdpFace::getFaceStatus(), the time left to timeout is calculated as:

time::milliseconds left = m_idleTimeout - time::duration_cast<time::milliseconds>(
  time::steady_clock::now() - m_lastIdleCheck);

The m_lastIdleCheck should be replaced using lastUsedTime (when the face is used).
Correct me if I'm wrong.

Please ignore the note-2. It is not the major issue that introduces the inconsistent UDP face timeouts.

The logic in the code is a timeout face is removed when it is not used recently (in the last timeout period). Therefore, a face will be in the FIB for 2*timeout seconds.

Since this task requires the timeout must be exactly as the nfd.conf specified, the code logic is wrong.

I don't think we need exact timeout for UDP faces. This issue can be closed if we simply put comment about the UDP timeout parameter in the config file. The intention for non-precise timeout value is to reduce bookkeeping/scheduling overhead.

If the timeout value is only to reduce the bookkeeping/scheduling overhead, I agree that put UDP timeout in the config file is good enough to let operators understand it.

Anyone has different opinions?

face_system.udp.idle_timeout option represents the interval between idle checks. If an on-demand face has no incoming message between two checks, it's closed due to idle.

It's too expensive to reset the timer upon each incoming message, so the actual timeout would be anywhere within [idle_timeout, 2*idle_timeout).
Since the timer starts when the face is created, in the extreme case where only one message is received ever, the actual timeout would be close to 2*idle_timeout.

I agree with note-3 that this problem only needs a documentation update.

note-2 points out a related but different issue: the expiration time reported in Face Dataset.

I'm unsure what's the current algorithm, but the accurate algorithm is:

duration computeRemainingLifetime()
{
  if (face is not on-demand) {
    return infinite;
  }

  if (no incoming message since last idle check) {
    return duration until next idle check;
  }
  else {
    return duration until next idle check + interval between idle checks;
  }
}

@Junxiao, the current algorithm did exactly as you suggested.

Therefore, I'll just do the config file update for this bug.

Just to alert everybody. As part of this task, we are changing the default value for UDP timeout: from 30 to 600 seconds. For a long time we had inconsistency between effective "default" value in sample configuration file (600) and actual default value in face manager. I'll send alert to nfd-dev mailing list.