A 20 year old male presented with painless mass in the neck for 3 years, which has been progressively increasing in size. O/E the mass is seen just anterior and medial to the right sternocleidomastoid. The mass becomes larger and tender during upper respiratory infections. USG shows a single round cystic mass with uniform low echogenicity and lack of internal septations. CECT scan shows a homogenous mass with low accentuation centrally and smooth rim enhancement. Most probable diagnosis