LDP Session Hold time vs Discovery Hello

Hello,

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May someone of your guys explain to me the difference between Session hold time :180; keep alive interval:60 vs Discovery hello: holdtime 15 sec; interval: 5 sec?

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router#show mpls ldp parameters

Protocol version: 1
Session hold time: 180 sec; keep alive interval: 60 sec
Discovery hello: holdtime: 15 sec; interval: 5 sec
Discovery targeted hello: holdtime: 90 sec; interval: 10 sec
Downstream on Demand max hop count: 255
LDP for targeted sessions
LDP initial/maximum backoff: 15/120 sec
LDP loop detection: off