The described technology is generally directed towards a transport protocol for latency sensitive applications. The disclosed transport protocol is “semi-reliable” in that it allows for specification of an importance of data being transmitted, thereby allowing important data to be sent reliably, while other data can be dropped if necessary, e.g., under bad network conditions. A deadline can be specified for such other data, and if the other data cannot be sent prior to the deadline, it can be dropped. Furthermore, the disclosed transport protocol can allow for early discovery of network jitter. A client device can send regular acknowledgments which identify most recently received data packets as well as a number of “heartbeat transmissions” received at the client device. A server device can use the acknowledgments to discover and respond to jitter.