Layer-4　(L4)　load　balancers　(LBs)　are　essential　for　data　centers,　dispatching　incoming　connections　among　thousands　of　servers.　There　are　two　critical　requirements　for　L4　LBs:　i)　load　balancing　fairness,　i.e.,　the　ability　to　assign　the　load　to　servers　in　proportion　to　their　capacity;　ii)　per-connection　consistency　(PCC),　i.e.,　all　the　packets　belonging　to　the　same　connection　should　be　forwarded　to　the　same　server.　Howbeit,　existing　LBs　at　large　sacrifice　load　balancing　fairness　to　mitigate　PCC　violations,　which　cannot　satisfy　both　requirements　in　the　meantime.　In　this　paper,　we　present　Libra,　a　stateful　L4　LB　that　supports　fair　load　distribution,　PCC,　memory　efficiency,　a　nd　r　esilience　t　o　resource　depletion　attacks.　Libra　makes　load　balancing　decisions　resorting　to　the　proposed　Weighted　M-Least-Connection　First　(WMLCF)　mechanism　considering　the　real-time　load　and　available　processing　capacity　of　servers,　hence　enabling　load　balancing　fairness.　We　prototype　Libra　in　a　programmable　software　switch-BMv2　using　P4　language　and　conduct　extensive　flow-level　simulation　to　evaluate　the　performance.　The　evaluation　indicates　that　Libra　significantly　i　mproves　l　oad　b　alancing　f　airness　(　over　9　5%),　fully　ensures　PCC,　and　reduces　the　average　flow　c　ompletion　t　ime　by　17.27−42.55%　compared　to　existing　mechanisms.　©　2022　IEEE.