Song Dynasty (960-1279) waterbird-shaped porcelain incense burner.[Photo by Yu Jing/China News Service/China Daily]

As merchants and messengers (for the kingdoms they passed by en route), the Sogdians were not taken lightly by the Tang emperors, who set up specific departments within their government to handle their matters.

Rong Xinjiang, one of China's leading Silk Road experts, says it was common for the Sogdian merchants to come in the name of diplomatic envoys.

"Tang-era documents unearthed along the Silk Road show huge mission groups that could include up to a thousand people. Needless to say, only a few of them were real diplomats, although it is important to note that commerce and diplomacy were never quite separable. To trade in the name of paying tribute - since the opening of the Silk Road, this practice had allowed many businessmen smooth entree into China."