S ▁header . ▁The ▁packet ▁is ▁then ▁passed ▁on ▁to ▁the ▁next ▁hop ▁router ▁for ▁this ▁tunnel . ▁ ▁The ▁M PL S ▁Header ▁is ▁added ▁between ▁the ▁network ▁layer ▁header ▁and ▁link ▁layer ▁header ▁of ▁the ▁O SI ▁model . ▁ ▁When ▁a ▁l abeled ▁packet ▁is ▁received ▁by ▁an ▁M PL S ▁router , ▁the ▁top most ▁label ▁is ▁exam ined . ▁Based ▁on ▁the ▁contents ▁of ▁the ▁label ▁a ▁swap , ▁push ▁( im pose ) ▁or ▁pop ▁( dis pose ) ▁operation ▁is ▁performed ▁on ▁the ▁packet ' s ▁label ▁stack . ▁Rou ters ▁can ▁have ▁pre built ▁lookup ▁tables ▁that ▁tell ▁them ▁which ▁kind ▁of ▁operation ▁to ▁do ▁based ▁on ▁the ▁top most ▁label ▁of ▁the ▁incoming ▁packet ▁so ▁they ▁can ▁process ▁the ▁packet ▁very ▁quickly . ▁ ▁In ▁a ▁swap ▁operation ▁the ▁label ▁is ▁sw apped ▁with ▁a ▁new ▁label , ▁and ▁the ▁packet ▁is ▁forward ed ▁along ▁the ▁path ▁associated ▁with ▁the ▁new ▁label . ▁ ▁In ▁a ▁push ▁operation ▁a ▁new ▁label ▁is ▁pushed ▁on ▁top ▁of ▁the ▁existing ▁label , ▁effectively ▁" enc aps ulating " ▁the ▁packet ▁in ▁another ▁layer ▁of ▁M PL S . ▁This ▁allows ▁hier arch ical ▁routing ▁of ▁M PL S ▁packets . ▁Not ably , ▁this ▁is ▁used ▁by ▁M PL S ▁V PN s . ▁ ▁In ▁a ▁pop ▁operation ▁the ▁label ▁is ▁removed ▁from ▁the ▁packet , ▁which ▁may ▁reve al ▁an ▁inner ▁label ▁below . ▁This ▁process ▁is ▁called ▁" dec aps ulation ". ▁If