/*
 * Rationale:
 * this code implements the en50221 transport layer.
 * The transport layer is created with gst_dvb_cam_transport_new and destroyed
 * with gst_dvb_cam_transport_destroy.
 *
 * The transport layer is connection oriented.
 * A connection can be requested by the module but in any case it's always
 * created by the host.
 * A connection is created with gst_dvb_cam_transport_create_connection () and
 * closed with gst_dvb_cam_transport_delete_connection (). When the module
 * requests a connection, the transport->connection_requested callback is
 * invoked.
 *
 * On a connection, transport protocol data units (TPDUs) can be written and
 * read. A TPDU is a Tag-Lenght-Value object, meaning that each TPDU is
 * identified by a tag and has a lenght-prefixed value. TPDUs can be classified
 * in two classes: control TPDUs, which are issued by both the host and the
 * module to control the  connection and data TPDUs, which carry data produced
 * by the host or the module.
 * The module never sends TPDUs first, it only sends them in response to host
 * TPDUs.
 * Each host TPDU receives at least one response TPDU from the module.  The last
 * TPDU of every response TPDU sequence from the module is a control TPDU that
 * signals whether the module has data TPDUs to send or not. The value of this
 * TPDU can be accessed with gst_dvb_cam_transport_has_data (). To always get an
 * updated value for this TPDU, gst_dvb_cam_transport_poll_has_data () can be
 * used.
 *
 * Data TPDUs are written with gst_dvb_cam_transport_write_data. Segmentation is
 * done in the link layer. TODO: figure out if there's a maximum TPDU size.
 * To read data TPDUs gst_dvb_cam_read_data is used. 
 *
 * Also the implementation exposes some low levels calls to read and write TPDUs
 * directly on a connection:
 *
 * gst_dvb_cam_transport_tpdu_read_header returns the header of a TPDU (ie.  its
 * tag and length) and then gst_dvb_cam_transport_read_tpdu_value, that reads
 * the body of the TPDU.
 * gst_dvb_cam_transport_tpdu_write writes a TPDU.
 */