I think the title is pretty descriptive.

I cannot seem to find anything in the standard about what kind of packets could trigger the transition from the state STALE to the state DELAY, in the Neighbor Cache Entry.

I site the corresponding text from the RFC4861 standard:

DELAY: More than ReachableTime milliseconds have elapsed since the last positive confirmation was received that the forward path was functioning properly, and a packet was sent within the last DELAY_FIRST_PROBE_TIME seconds. If no reachability confirmation is received within DELAY_FIRST_PROBE_TIME seconds of entering the DELAY state, send a Neighbor Solicitation and change the state to PROBE.

So, when it says "When a packet is sent...".
What kind of packets can trigger this transition ?
Can all possible Packets, even a response to a Neighbor Solicitation, trigger that?

For example:

When a Host A sends for the first time a NS request to another Host B, host B answers with a NA.
In Host A Neighbor Cache, the state Entry for host B gets from INCOMPLETE to REACHABLE, when NA comes back.
And Host B is supposed to add a new entry with Host's A address and in STALE state.
Could it be, since the entry is in STALE and then a NA packet is sent to Host A, that the Neighbor Cache entry state is updated to DELAY and host B starts sending prob packets after some time ?


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