, ▁adv ances ▁in ▁V L SI ▁have ▁made ▁such ▁devices ▁possible . ▁Therefore , ▁the ▁advantages ▁of ▁M PL S ▁primarily ▁revol ve ▁around ▁the ▁ability ▁to ▁support ▁multiple ▁service ▁models ▁and ▁perform ▁traffic ▁management . ▁M PL S ▁also ▁offers ▁a ▁robust ▁recovery ▁framework ▁that ▁goes ▁beyond ▁the ▁simple ▁protection ▁rings ▁of ▁synchron ous ▁optical ▁networking ▁( SON ET / SD H ). ▁ ▁Operation ▁M PL S ▁works ▁by ▁prefix ing ▁packets ▁with ▁an ▁M PL S ▁header , ▁containing ▁one ▁or ▁more ▁labels . ▁This ▁is ▁called ▁a ▁label ▁stack . ▁Each ▁entry ▁in ▁the ▁label ▁stack ▁contains ▁four ▁fields : ▁ ▁A ▁ 2 0 - bit ▁label ▁value . ▁A ▁label ▁with ▁the ▁value ▁of ▁ 1 ▁represents ▁the ▁router ▁alert ▁label . ▁ ▁a ▁ 3 - bit ▁Tra ffic ▁Class ▁field ▁for ▁Q o S ▁( quality ▁of ▁service ) ▁priority ▁and ▁E CN ▁( Exp licit ▁Cong estion ▁Notification ). ▁Prior ▁to ▁ 2 0 0 9 ▁this ▁field ▁was ▁called ▁EX P . ▁ ▁a ▁ 1 - bit ▁bottom ▁of ▁stack ▁flag . ▁If ▁this ▁is ▁set , ▁it ▁sign ifies ▁that ▁the ▁current ▁label ▁is ▁the ▁last ▁in ▁the ▁stack . ▁ ▁an ▁ 8 - bit ▁T TL ▁( time ▁to ▁live ) ▁field . ▁ ▁These ▁M PL S - l abeled ▁packets ▁are ▁switched ▁after ▁a ▁label ▁lookup / switch ▁instead ▁of ▁a ▁lookup ▁into ▁the ▁IP ▁table . ▁As ▁mentioned ▁above , ▁when ▁M PL S ▁was