In one example, an intermediate network device sends packets that advertise a transmission control protocol (TCP) window size of zero bytes to a client device and a server device. The device, after sending the packets, receives a first zero-window probe packet from the client device including data representing a first current sequence number for a client-to-server packet flow of an established network session, and a second zero-window probe packet from the server device including data representing a second current sequence number for a server-to-client packet flow of the network session. The device also initializes a TCP state based on the first and second current sequence numbers, and acts as a TCP proxy for packets following the first zero-window probe packet of the client-to-server packet flow based on the TCP state and packets following the second zero-window probe packet of the server-to-client packet flow based on the TCP state.