[Twisted-Python] A question about ITransport.write()

Hi, all As a Twisted newbie, I read the Twisted documentation, and found that: 1. ITransport.write() doesn't return a deferred. 2. IProtocol doesn't have a dataSent() callback or something alike. Since ITransport.write() is a non-blocking call, how can I be informed when data sent by ITransport.write() is actually sent successfully? Cheers Cheng