▁a ▁label ▁to ▁the ▁head ▁of ▁each ▁packet ▁and ▁transm its ▁it ▁on ▁the ▁network . ▁ ▁D iffer ences ▁exist , ▁as ▁well , ▁in ▁the ▁nature ▁of ▁the ▁connections . ▁An ▁M PL S ▁connection ▁( L SP ) ▁is ▁un id irection al — allow ing ▁data ▁to ▁flow ▁in ▁only ▁one ▁direction ▁between ▁two ▁end points . ▁Est ab lish ing ▁two - way ▁communic ations ▁between ▁end points ▁requires ▁a ▁pair ▁of ▁L SP s ▁to ▁be ▁established . ▁Because ▁ 2 ▁L SP s ▁are ▁required ▁for ▁connect ivity , ▁data ▁flow ing ▁in ▁the ▁forward ▁direction ▁may ▁use ▁a ▁different ▁path ▁from ▁data ▁flow ing ▁in ▁the ▁reverse ▁direction . ▁AT M ▁point - to - point ▁connections ▁( virtual ▁circ uits ), ▁on ▁the ▁other ▁hand , ▁are ▁bid irection al , ▁allowing ▁data ▁to ▁flow ▁in ▁both ▁directions ▁over ▁the ▁same ▁path ▁( B oth ▁S VC ▁and ▁P VC ▁AT M ▁connections ▁are ▁bid irection al . ▁Check ▁IT U - T ▁I . 1 5 0 ▁ 3 . 1 . 3 . 1 ). ▁ ▁Both ▁AT M ▁and ▁M PL S ▁support ▁t unn eling ▁of ▁connections ▁inside ▁connections . ▁M PL S ▁uses ▁label ▁stack ing ▁to ▁accomplish ▁this ▁while ▁AT M ▁uses ▁virtual ▁paths . ▁M PL S ▁can ▁stack ▁multiple ▁labels ▁to ▁form ▁t unn els ▁within ▁t unn els . ▁The ▁AT M ▁virtual ▁path ▁indicator ▁( V PI ) ▁and ▁virtual ▁circuit ▁indicator ▁( VC