Applications on mobile computers must adapt to high variability in wireless network performance. Extending the semantics of transport protocols to offer more control over communication to the user allows applications to adapt their behavior to bandwidth variability. We examine adding bandwidth notifications, priorities and timeliness guarantees to a network API as a method for achieving greater application control over bursty traffic. Experiments demonstrate that the extended API allows applications to adjust to bandwidth variations effectively. We also compare three different implementations of the API: two which run on top of TCP, and one new protocol, ATP, which performs comparably to the TCP extensions, but has better performance for some workloads, including a workload simulating remote file system traffic.