ult imate ▁hop ▁( the ▁hop ▁before ▁the ▁e gress ▁router ). ▁This ▁is ▁called ▁pen ult imate ▁hop ▁po pping ▁( PHP ). ▁This ▁may ▁be ▁interesting ▁in ▁cases ▁where ▁the ▁e gress ▁router ▁has ▁lots ▁of ▁packets ▁leaving ▁M PL S ▁t unn els , ▁and ▁thus ▁sp ends ▁in ordinate ▁amounts ▁of ▁CPU ▁time ▁on ▁this . ▁By ▁using ▁PHP , ▁trans it ▁rou ters ▁connected ▁directly ▁to ▁this ▁e gress ▁router ▁effectively ▁off load ▁it , ▁by ▁po pping ▁the ▁last ▁label ▁themselves . ▁In ▁the ▁label ▁distribution ▁protocol s , ▁this ▁PHP ▁label ▁pop ▁action ▁is ▁advert ised ▁as ▁label ▁value ▁ 3 ▁« ▁implicit - null » ▁( which ▁is ▁never ▁found ▁in ▁a ▁label , ▁since ▁it ▁means ▁that ▁the ▁label ▁is ▁to ▁be ▁po pped ). ▁ ▁This ▁optim isation ▁is ▁no ▁longer ▁that ▁useful ▁( like ▁for ▁initial ▁rational es ▁for ▁M PL S ▁– ▁easier ▁operations ▁for ▁the ▁rou ters ). ▁Several ▁M PL S ▁services ▁( including ▁end - to - end ▁Q o S ▁management , ▁and ▁ 6 PE ) ▁imply ▁to ▁keep ▁a ▁label ▁even ▁between ▁the ▁pen ult imate ▁and ▁the ▁last ▁M PL S ▁router , ▁with ▁a ▁label ▁disposition ▁always ▁done ▁on ▁the ▁last ▁M PL S ▁router : ▁the ▁« U lt imate ▁Hop ▁Po pping » ▁( U HP ). ▁Some ▁specific ▁label ▁values ▁have ▁been ▁not ably ▁reserved ▁for ▁this ▁use : ▁▁▁ 0 : ▁« exp licit - null » ▁for ▁IP