v 4 ▁▁▁ 2 : ▁« exp licit - null » ▁for ▁IP v 6 ▁ ▁Label - switch ed ▁path ▁A ▁label - switch ed ▁path ▁( L SP ) ▁is ▁a ▁path ▁through ▁an ▁M PL S ▁network , ▁set ▁up ▁by ▁the ▁N MS ▁or ▁by ▁a ▁signal ing ▁protocol ▁such ▁as ▁L DP , ▁R SV P - TE , ▁B GP ▁( or ▁the ▁now ▁deprecated ▁CR - LD P ). ▁The ▁path ▁is ▁set ▁up ▁based ▁on ▁criteria ▁in ▁the ▁F EC . ▁ ▁The ▁path ▁begins ▁at ▁a ▁label ▁edge ▁router ▁( L ER ), ▁which ▁makes ▁a ▁decision ▁on ▁which ▁label ▁to ▁prefix ▁to ▁a ▁packet , ▁based ▁on ▁the ▁appropriate ▁F EC . ▁It ▁then ▁for wards ▁the ▁packet ▁along ▁to ▁the ▁next ▁router ▁in ▁the ▁path , ▁which ▁sw aps ▁the ▁packet ' s ▁outer ▁label ▁for ▁another ▁label , ▁and ▁for wards ▁it ▁to ▁the ▁next ▁router . ▁The ▁last ▁router ▁in ▁the ▁path ▁removes ▁the ▁label ▁from ▁the ▁packet ▁and ▁for wards ▁the ▁packet ▁based ▁on ▁the ▁header ▁of ▁its ▁next ▁layer , ▁for ▁example ▁IP v 4 . ▁Due ▁to ▁the ▁forward ing ▁of ▁packets ▁through ▁an ▁L SP ▁being ▁op aque ▁to ▁higher ▁network ▁layers , ▁an ▁L SP ▁is ▁also ▁sometimes ▁referred ▁to ▁as ▁an ▁M PL S ▁tunnel . ▁ ▁The ▁router ▁which ▁first ▁prefix es ▁the ▁M PL S ▁header ▁to ▁a ▁packet ▁is ▁called ▁an ▁ing ress ▁router . ▁The ▁last ▁router ▁in ▁an ▁L