Wireless communications systems and methods related to uplink buffer management are provided. In some aspects, a user equipment receives a synchronization acknowledgment message destined for a device tethered to the user equipment and transmitted by an application server at a network to which the user equipment is connected. In some aspects, the synchronization acknowledgment message includes an application server receiver window size indicating available buffer space in a receive buffer of the application server. The user equipment can modify the application server receiver window size in the received synchronization acknowledgment message prior to transmitting the received synchronization acknowledgment to the tethered device.