The sender transmit timestamp is T$1,$ the reflector receive timestamp is T$2,$ the reflector transmit timestamp is T$3,$ and the sender receive timestamp is T$4.$ How does the sender calculate the delay measurement in the PM one$-$way mode?

$($T$4-$ T$1)-($T$3-$T$2)$

$[($T$4-$ T$1)-($T$3-$T$2)]/2$

T$1-$ T$2$

T$2-$ T$1$