The Kite Runner - Shahnameh

In the novel, The Kite Runner, references are made to the Persian epic, the Shahnameh, which Amir would read to Hassan. Hassan's favourite story from the Shahnameh was the tale of the archenemies Rostam and Sohrab. It would be Hassan's influence in naming his son Sohrab. In Farsi, it was an epic (the Epic of Kings), but translated into English, it loses its poetic flow, but still maintains the story.
"Give ear unto the combat of Sohrab against Rostam, though it be a tale replete with tears."
There are several connections to this tale throughout the course of the novel, both subtle and directly suggested. Hassan and Sohrab's birth being defined by their smiles is one such example. "...A babe whose mouth was filled with smiles, wherefore men called him Sohrab."
Additionally, in the novel there is secrecy and uncertainty regarding who has a blood connection to whom. The moment in which Amir realizes that Hassan is his brother (making Sohrab Amir's nephew), relates to the tale, when Rostam realizes Sohrab is his son after dealing him his death blow. Also tied to this part of the tale is the moment in which Amir accidentally causes Sohrab to try to kill himself.
"If thou art indeed my father, then hast thou stained thy sword in the life-blood of thy son."
Although the story ended with the main character, Rostam in grief ("My heart is sick unto death." ), Khaled Hosseini chose to make his parallel tale more optimistic. Because Amir learned of his connection to Sohrab in time, he was able to save his life in the end, and end it all on a lighter note for the modern reader.
"Then Rostam promised to do the desires of Sohrab," as in a quest of redemption like that which Amir had taken in the novel. Assuming that Hosseini wanted Amir's character to reflect Rostam, the courageous warrior, then it would be more fitting given the meaning of the name Rostam, "I am delivered of my pains."
http://www.iranchamber.com/literature/shahnameh/08rostam_sohrab.php