Systems and methods are directed towards network data leakage prevention (DLP). More specifically, the systems and methods are directed towards using TCP (Transmission Control Protocol) data packets in conjunction with the DLP monitor. The network DLP utilizes TCP data packets to carry source user identity. With the source user identity, the DLP monitor can determine if sensitive data can be transmitted based on the provided user information and corresponding DLP policies for each user. Furthermore, the DLP monitor can determine if sensitive data can also be transmitted for particular users in situations where multiple users share the same IP address.