What injustice and discrimination is done to Hassan in The Kite Runner?

In The Kite Runner, Hassan is the
victim of discrimination because he is Hazara and the caste system in Afghanistan is set
up to look down on people who are Hazara.  When Hassan runs Amir's kite, the boys in the
alley gang up on him and figure that it is okay to abuse him because of his social
position.  They say that he is "just a Hazara," and so feel justified in their abuse. 
Amir does not want to reveal that he has witnessed the abuse, and he also tries to
convince himself that since Hassan is a Hazara it does not matter that he was hurt. 
Further, Baba does not admit to anyone that Hassan is in fact his son for the same
reasons.  So instead of living a richer lifestyle like Amir, Hassan must live on the
margins.  These are some the instances in which Hassan experiences
discrimination.