Related for Sukkah 2:4
הָעוֹשֶׂה סֻכָּתוֹ בֵּין הָאִילָנוֹת, וְהָאִילָנוֹת דְּפָנוֹת לָהּ, כְּשֵׁרָה. שְׁלוּחֵי מִצְוָה פְּטוּרִין מִן הַסֻּכָּה. חוֹלִין וּמְשַׁמְּשֵׁיהֶן פְּטוּרִין מִן הַסֻּכָּה. אוֹכְלִין וְשׁוֹתִין עֲרַאי חוּץ לַסֻּכָּה:
If one makes his succah between trees, the trees serving as walls for it, it is kasher. [This, on condition that the trees are thick and strong and do not sway back and forth in a normal wind. And he must also fill the spaces between the branches with straw so that the wind not move them. For any partition that cannot withstand a normal wind is not a partition.] "Messengers of a mitzvah" are exempt from (the mitzvah of) succah. [For one who is occupied with a mitzvah is exempt from a (different) mitzvah. And not only when he is engaged in the performance of the mitzvah, but even when he is not thus engaged, e.g., as when he is on his way to receive his rabbi or to redeem captives. He is exempt even while resting (on the road).] Those who are ill and their attendants are exempt from succah, [even those who are slightly ill. The rationale for non-performance of a positive commandment of the Torah because of slight illness, or because of the discomfort caused by foul odors or bed bugs or fleas, in which instances one is exempt (from the mitzvah of succah), as well as the rabbis' exempting travelers and keepers of gardens and orchards from succah is (Leviticus 23:42): "In succoth shall you sit," similar to "shall you dwell." For whatever reason one would leave his dwelling, he may also leave his succah. But if one makes his succah ab initio in a place where he is bound to eat or to sleep in discomfort, as in a place where he is in fear of being robbed in his sleep, even if he is not in fear of thieves or robbers while eating — he does not fulfill his succah obligation even when eating, in that the succah does not serve all his needs: eating, drinking, and sleeping. For "shall you dwell" must be satisfied, and such a succah fails to satisfy it.] It is permitted to eat and drink "casually" outside the succah [a snack to ally one's hunger, with the intent of eating a regular meal (in the succah) afterwards.]