When the Chinese troops invaded Tibet in 1949, it had been an independent entity for over 150 years. Even prior to that it was unilateral declarations by Beijing about Tibet being its "Vassal State" but without Beijing having a governor or other representative in Lhasa. Tibetans never acknowledged Chinese suzereignty until 1950 when at the point of a gun, the Dalai Lama had to sign coerced agreements.
In 1913, the British-Indian Govt., had no option but to conclude a border agreement, "McMahon Line" with Lhasa. Beijing was not in control of Tibet. China was not interested in Tibet since the 17th century - no clear evidence of control. That is why they had to send troops in 1949 and again in 1959. Majority of Tibetans do NOT want the Chinese to rule them. Neither history nor gound realities support China's occupation of Tibet. The ordinary people of this world know this. Hence the unpopularity of the Beijing Olympics. Beijing has no business discussing the Indo-Tibetan border with New Delhi.
