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All protocol suites have one or more transport protocols to mask the corresponding application protocols from the service provided by the different types of network protocols. • In the TCP/IP suite, the transport protocols used are the transmission control protocol (TCP) and the user datagram protocol (UDP). • When the application involves the transfer of streams of audio and/or video in real time, the timing information required by the receiver to synchronize the incoming streams is provided by the real-time transport protocol (RTP) and its associated real-time transport control protocol (RTCP)
CYH/MMT/TP/p.1 Transport protocols 12.1 Introduction • All protocol suites have one or more transport protocols to mask the corresponding application protocols from the service provided by the different types of network protocols. • In the TCP/IP suite, the transport protocols used are the transmission control protocol (TCP) and the user datagram protocol (UDP). • When the application involves the transfer of streams of audio and/or video in real time, the timing information required by the receiver to synchronize the incoming streams is provided by the real-time transport protocol (RTP) and its associated real-time transport control protocol (RTCP) CYH/MMT/TP/p.2 12.2 TCP/IP protocol suite • Applications such as file transfers and electronic mail require a reliable service. • TCP converts the best-effort service provided by IP into a reliable service. • For other applications such as interpersonal applications that involve the transfer of streams of audio or video, a simple best-effort service is acceptable and hence they use UDP. CYH/MMT/TP/p.3 • With most networked applications, the client-server paradigm is used. • All protocol data units (PDUs) relating to the protocols that use the services of the IP layer are transferred in an IP datagram. • The protocol field in each IP datagram header is used for IP to identify the protocol to which the contents of the datagram relate. • The source and destination port numbers that are present in the header of the PDUs of TCP/UDP are used to identify the application protocol to which the PDU contents relate. • Both the source port number and the source IP address from the IP datagram header are used to identify the client by the server. • Client port numbers are called ephemeral ports as they are short lived. • A new port number is allocated for each new transfer request (1024~5000). • The port numbers of the peer application protocols in the server application protocols are fixed and are known as well-known port numbers (0~1023). CYH/MMT/TP/p.4 • The protocols in both the application and transport layers communicate on an end-to-end basis. • The IP protocol in each host has local significance and it talks to its peer protocol in the destination host indirectly. CYH/MMT/TP/p.5 12.5 RTP and RTCP • When an application involves the transfer of a real-time stream of audio and/or video over a packet network, the timing information that is required by the receiver to output the received packet stream at the required rate is provided by the real-time transport protocol (RTP). • For applications that involve both audio and video streams, the real-time transport control protocol (RTCP) is used to synchronize the two media streams prior to carrying out the decoding operation. 12.5.1 RTP • Time is critical in real-time application, so UDP instead of TCP is used. • Consequences of using UDP: packet lost, jitter, not in sequence. • Missing packets must be detected and compensated for. • Delay variations in the packet arrival times must be allowed for. CYH/MMT/TP/p.6 • Each of the participants that contributes to a multicast call is called a contributing source (CSRC) and is typically identified by the IP address of the source. • Streams from different CSRC may be multiplexed together for transmission purposes. • To enable the receiver to relate each block/frame to the appropriate participant, the CSRC identifier for each CYH/MMT/TP/p.7 block/frame is included in the header of the mixed packet. • The number of CSRC identifiers present in the packet is given in the CSRC count (CC) field. • Associated with the marker (M) bit is a profile which enables the receiver to interpret the packet data on the correct block/frame boundaries. • The payload type field indicates the type of encoder that has been used to encode the data in the packet. • The sequence number is used for the destination host to detect lost or out-of-sequence packets. • How to deal with lost or out-of-sequence packets: • Replacing the lost packet with the last correctly received packet. • Buffering a number of packets before playout starts • The time-stamp indicates the time reference when the packet was created. • Use of time-stamp: • Determine the current mean transmission delay, the level of jitter that is being experienced. CYH/MMT/TP/p.8 • This information, together with the number of lost packets, forms the current QoS of the path through the network. • This information is periodically returned to the sending RTP by the related RTCP. • The synchronization source (SSRC) identifier identifies the source device that has produced the packet contents. 12.5.2 RTCP • The RTCP adds additional system-level functionality to its related RTP such as the means for a receiving RTP to integrate and synchronize the individual packet streams together and for a sending RTP to be informed of the currently-prevailing network QoS. CYH/MMT/TP/p.9 • The RTCP operates alongside of RTP and shares information with it. • Each RTCP has a different (UDP) port number associated with it so that it can operate independently of RTP. • The RTCP in all the systems involved in a call periodically exchange messages with one another. • Info that is exchanged: • Integrated media synchronization: Let all involved applications use a common clock for synchronization • QoS reports: analyzed result of the network's QoS • Participation reports: who's in and who's out during a conference call • Participation details: information of each participant CYH/MMT/TP/p.10 . CYH/MMT/TP/p.1 Transport protocols 12. 1 Introduction • All protocol suites have one or more transport protocols. its associated real-time transport control protocol (RTCP) CYH/MMT/TP/p.2 12. 2 TCP/IP protocol suite • Applications such as file transfers and electronic