Real-Time Transport Control Protocol

The RTCP is based on the periodic transmission of control packets to all participants in the session, with the same distribution mechanism as that for the data packets. The underlying protocol must provide multiplexing of the data and control packets, for example, separate port numbers with UDP.

The format of the header is shown in Figure 1.

Figure 1: Format of the header.

Version: Identifies the RTP version, which is the same in RTCP packets and RTP data packets. Version 2 is defined by this specification. P (padding): When set, this RTCP packet contains some additional padding octets at the end, which are not part of the control information. The last octet of the padding is a count of how many padding octets should be ignored. Padding may be needed by some encryption algorithms with fixed block sizes. In a compound RTCP packet, padding should be required only on the last individual packet because the compound packet is encrypted as a whole. Reception report count: The number of reception report blocks contained in this packet. A value of zero is valid. Packet type: Contains the constant 200 to identify this as an RTCP SR packet. Length: The length of this RTCP packet in 32-bit words minus one, including the header and any padding. (The offset of one makes zero a valid length and avoids a possible infinite loop in scanning a compound RTCP packet, and counting 32-bit words avoids a validity check for a multiple of four.)

Figure 2 shows the complete packet header for IP, UDP, and RTP. The headers of the three payload-carrying protocols are sent sequentially before the digitized voice samples, which are actually the payload of the RTP header. The result is a 40-octet overhead for every information data packet.

Figure 2: Packet header for IP, UDP, and RTP.

Figure 3 shows an H.323 call setup between two H.323 terminals. The gatekeeper server in the diagram could represent an IP-PBX call telephony server if it were an IP-PBX system, and the H.323 terminals could just as well be IP telephones. The gatekeeper and H.323 terminals reside on a LAN. The first steps in the call set-up process are terminal registration and admission with the gatekeeper. The calling terminal establishes a TCP signaling connection with the called terminal and receives a connection acknowledgment. Bandwidth requirements and management are controlled by TCP-based H.245 signaling. UDP voice packets are transmitted across the LAN between the terminals under the control of RTP and RTCP protocols.

Figure 3: H.323 protocol and call setup.

It shows the precise control messages that are exchanged between terminals from call set-up to call termination. The originating terminal (1) initiates a call to the destination terminal (2) directly, without any intermediate gateway or gatekeeper. H.225 and H.235 messages are indicated. Some messages overlap each other (Messages 4/5 and 9/10). H.225 messages are Messages 1, 2, 3, and 12. 12; the remaining messages are H.245.