Transmission of Multimedia Data Streams

Figure visualises the data flow inside a session and the relevant method invocations.

 
Figure:  The method invocations relevant to the inter-application data flow.

     Session objects are designed in resemblance to RTP sessions; however, they can work with other transmission protocols as well. A Session comprises two Connection objects (one for payload data transmission, one for control information transmission), a ProtocolCoDec object, which encapsulates media elements in protocol data units of a particular transmission protocol, and a Profile object, which is used to configure the ProtocolCoDec and the Session object. Similar to RTP, only one payload type is conveyed in one session that is stored in the Profile.

  The two Connection objects assigned to a Session object correspond to the two network access points of an RTP session; they are each bound to a local address and send data to a remote address. The only requirement to Connection objects is that their send() and receive() methods must support framing. That means,

a data block which was the argument of one send() method invocation must always be retrieved entirely by a single invocation of the corresponding receive() method, or not at all in case the packet got lost during the transmission. Objects that make use of Connection objects must be prepared to deal with packet loss or a disordered packet sequence. A Connection object can be constructed around a UDP multicast socket joining a multicast host group, for example.

  A Session object interacts with the underlying system's network QoS control entities, by means of a ConnectionControl object. The ConnectionControl object is responsible for negotiating and maintaining network resource reservations for flows that are received or forwarded by means of the Session's Connection object.
ConnectionControl objects are highly dependent on the particular operating system interfaces and networking techniques used by a Session's Connection object. A ConnectionControl implementation could for example use the RAPI mentioned in Section on page .

  A ProtocolCoDec implements a particular transport protocol suitable for the transmission of multimedia data stream. Hence, the ProtocolCoDec is the central element of a Session. Its tasks include:

• Creation and Initialisation of new LocalSource objects for every locally generated multimedia data stream that is intended to be transmitted over the network.
• Creation and Initialisation of RemoteSource objects for all remote senders whose multimedia data streams the ProtocolCoDec receives by means of its data Connection object.
• Encapsulation of media elements in protocol data units of the implemented transport protocol and forwarding of the encoded packets by means of the data Connection object.
• Reception of packets from remote sources using the data Connection object and decoding of protocol data units. Extraction of header information, such as timestamps or source identifiers.

  A Profile object is used to configure the ProtocolCoDec and Session objects. Most of all, the Profile specifies the payload type that is conveyed in a session. The Profile also lays down the maximum packet rate and the bandwidth share of a session. Besides, the profile contains protocol-specific parameters.

  Generally, a Session object manages the operation of its ProtocolCoDec, Connections, LocalSources, and RemoteSources. In particular, the management comprises the following tasks:

• Bandwidth division. The Session object divides the session's network bandwidth among all Producers associated with the Session's LocalSource objects.
• Session startup. On request, the Session object performs all steps necessary to start the session. This includes starting the threads of control that generate or process multimedia data streams, as well as triggering the threads that are involved in the operation of the ProtocolCoDec.
• Session termination and cleanup. On termination of a session, the Session object is responsible for terminating all threads of control associated with objects operating within the context of the session. Besides, the termination code must release all resources held by the Session or associated objects. In addition, protocol- or connection-specific clean-up actions must be triggered, such as sending RTCP bye packets, or leaving multicast host groups.

  SessionManagers administrate the inter-application data flow. They maintain lists of Session, ProtoSession, LocalSource, and RemoteSource objects mainly; besides, they induce the SessionAgent to export session descriptions on special occasions, such as Session creation or termination, or to notify remote session participants of changes in the application's session membership.