https://en.wikipedia.org/wiki/Replay_attack

A replay attack (also known as playback attack) is a form of network attack in which a valid data transmission is maliciously or fraudulently repeated or delayed. This is carried out either by the originator or by an adversary who intercepts the data and re-transmits it, possibly as part of a masquerade attack by IP packet substitution.

Replay Attack potentially allows adversaries to identify a Shadowsocks server by observing and replaying the data streams between a valid client and the server. Ideally we want to recognize that a data stream is replayed, and react accordingly.