What are the attitudes of the schools to the experience of physical pain during sitting meditation?

Both the Theravada and the Zen traditions appear to regard pain, even extreme physical pain, as a necessary part of sitting, whereas Ngakpa Chogyam in *Journey into Vastness*, speaking for the Dzogchen point of view, seems to imply that pain is an indication that one is not sitting properly and that there are ways of sitting that do not produce pain. The only references to pain in the context of meditation in the Pali Canon appear to refer to the Buddha's six-year period of self-mortification, which he repudiated (correct me with a specific reference if I am mistaken). Therefore, I am interested in the attitude of different Buddhist schools to pain in the context of sitting and meditation. Is it a sign of poor sitting practice or is it part of the experience, perhaps even something important that should be cultivated (e.g., ancient shamans used pain to induce altered states of consciousness, an example being the famous Sun Dance in which in which practitioners hung themselves by the pectoral muscles from a post for several days). What do the different Buddhist schools and traditions say about this?