Implement StreamWriter/Reader writeTag and readTag methods
The current implementation of the StreamWriter/StreamEncrypter write a tag, which allows the recipient to identify the connection. This tag is written automatically when the first message is written (or the stream is flushed).
Writing this tag explicitly before sending any message simplifies the implementation of mailbox session management. A public writeTag method should be added to the 'StreamWriter'/'StreamEncrypter'.
Additionally, the readTag method, which is currently part of the ConnectionManager, is also required to manage mailbox connections. To avoid redundancy we could add a StreamReader interface and implement a readTag function ('StreamReader' currently extends an InputStream).
As @akwizgran suggested, a workaround is to simply flush the StreamWriter, but this also writes an empty frame.