If one makes his succah between the trees, and the trees serve as its walls, it is valid.  Those who are engaged in the performance of a mitzvah are exempt from the [mitzvah of] succah. Ill people and their attendants are exempt from the [mitzvah of] succah.  We may eat and drink casually outside the succah.

Once they brought Rabban Yochanan ben Zakkai some cooked food to taste, and to Rabban Gamliel [they brought] two dates and a pail of water, and they said, “Bring them up to the succah.” But when they gave R’ Tzaddok food smaller than the volume of an egg, he took it with a cloth and ate it outside the succah, and did not recite the benediction after it.