Sarf is a bureau of the Orgota government meant to control the flow of information in and out of Orgoreyn (10.72). Ai meets a few members of Sarf, such as Gaum, but otherwise the bureau stays hidden in the shadows for the entire story.
Tibe is Karhide's prime minister. He uses radio communication to spread his overly patriotic message, gearing Karhide up for war with Orgoreyn.
Both perform as Ai's antagonists in the novel. Ai's mission is to bring the Gethen into the Ekumen. To do that, he must establish communication first. Sarf's job is to limit communication or shut it down completely to keep power for certain Orogreyn Commensals. Tibe uses communication to promote his own interests and not the interests of others.
This puts the goals of each in direct conflict with Ai. And that, friends, is the definition of an antagonist.