How sessions are handled in MVC?

• Is it safe to pass Primary key in sessions?
• what are the most scenarios in which sessions are used?
• and to pass a whole tuple including PK?
• What should we pass and what should be avoided?
• Does Request Object contains session in it?
• Session evaluation should be done in Controller or View?

I found answers of some of these questions and but i want some experienced suggestions.

I will be Thankful for your suggestions.