First GET request is made to: http://youtube.com/ptracking

examples

http://www.youtube.com/ptracking?cpn=55R5PlsVt02J6NG4&plid=AATqqvrLRH98vIJm&ptk=youtube_multi&pltype=contentugc&video_id=jl3xdLsNjlg&oid=GoednyIs730UDQtrvI_P-Q.8Td-asijjguOS4RL5HexHg.TsT5ztlcfu8EsWy2ztsauA.9b8naKhAjYHfDP3yKaWKtg.4T9AFQqrJgWpTDFrmG6JDA&ei=a_F8UvveLqeDiAbH8YDwBQ

http://www.youtube.com/ptracking?pltype=contentugc&ptk=youtube_multi&ei=a_F8UvveLqeDiAbH8YDwBQ&cpn=ypjw23rgmNUBjcnz&video_id=jl3xdLsNjlg&plid=AATqqvrLRH98vIJm&oid=GoednyIs730UDQtrvI_P-Q.8Td-asijjguOS4RL5HexHg.TsT5ztlcfu8EsWy2ztsauA.9b8naKhAjYHfDP3yKaWKtg.4T9AFQqrJgWpTDFrmG6JDA


Explanation
tk = TrackingToken (but its used for partnerTrackingToken) (example: RjVFmrTkPMarn9hVOZdtlL8YC5-cerE8rqcCpzdXorGx69xF40-VeQ==)
plid = playbackIdToken (example: AATOaQfOQVFg13p5)
cpn = clientPlaybackNonce (example: ??)
oid = partnerTrackingOid (example: Tyef72w-Df2Y65DjX6xIsQ)
ptchn = partnerTrackingChannelToken (example ??)
pltype = partnerTrackingPlaybackType (example: content)
content_v = contentVideoIdentifier (example ??)

?? means I didn't find the example