Multimedia over IP - 04

RTCP -- Real-Time Control Protocol

The future Integrated Services Internet will provide means to transmit real-time multimedia data across networks. RSVP, RTP, RTCP and RTSP are the foundation of real-time services. RTCP is the control part of RTP that helps with quality of service and membership management. RTCP is the control protocol designed to work in conjunction with RTP. It is standardized in RFC 1889 and 1890. In an RTP session, participants periodically send RTCP packets to convey feedback on quality of data delivery and information of membership.

1. Protocol functions -
   

RTCP provides three basic functions expected to be implemented in all RTP sessions:

• The primary function of RTCP is to gather statistics on quality aspects of the media distribution during a session and transmit this data to the session media source and other session participants. Such information may be used by the source for adaptive media encoding (codec) and detection of transmission faults. If the session is carried over a multicast network, this permits non-intrusive session quality monitoring.
  
• RTCP provides canonical end-point identifiers (CNAME) to all session participants. Although a source identifier (SSRC) of an RTP stream is expected to be unique, the instantaneous binding of source identifiers to end-points may change during a session. The CNAME establishes unique identification of end-points across an application instance (multiple use of media tools) and for third-party monitoring.
  
• RTCP reports are expected to be sent by all participants, even in a multicast session which may involve thousands of recipients. Such traffic will increase proportionally with the number of participants. Thus, to avoid network congestion, the protocol must include session bandwidth management. This is achieved by dynamically controlling the frequency of report transmissions. RTCP bandwidth usage should generally not exceed 5% of total session bandwidth. Furthermore, 25% of the RTCP bandwidth should be reserved to media sources at all times, so that in large conferences new participants can receive the CNAME identifiers of the senders without excessive delay.
  
• A fourth, optional feature, is the provisioning of session control functions, because RTCP is a convenient means to reach all session participants, whereas RTP itself is not. RTP is only transmitted by a media source.
  

1. Message types
   

RTCP distinguishes several types of packets: sender report, receiver report, source description, and bye. In addition, the protocol is extensible and allows application-specific RTCP packets. A standards-based extension of RTCP is the Extended Report packet type introduced by RFC 3611 RFC 1889 defines five RTCP packet types to carry control information. These five types are:

1. RR: Receiver Report
   

Receiver reports are generated by participants that are not active senders. They contain reception quality feedback about data delivery, including the highest packets number received, the number of packets lost, inter-arrival jitter, and timestamps to calculate the round-trip delay between the sender and the receiver.

1. SR: Sender Report
   

Sender reports are generated by active senders. In addition to the reception quality feedback as in RR, they contain a sender information section, providing information on inter-media synchronization, cumulative packet counters, and number of bytes sent.

1. SDES: Source Description
   

The Source Description message is used to send the CNAME item to session participants. It may also be used to provide additional information such as the name, e-mail address, telephone number, and address of the owner or controller of the source. They contain information to describe the sources.

1. BYE: End of participation
   

A source sends a BYE message to shut down a stream. It allows an end-point to announce that it is leaving the conference. Although other sources can detect the absence of a source, this message is a direct announcement. It is also useful to a media mixer. It indicates end of participation.

1. APP: Application Specific Functions
   

The application-specific message provides a mechanism to design application-specific extensions to the RTCP protocol. It is now intended for experimental use as new applications and new features are developed.

1. Services
   

Through these control information packets, RTCP provides the following services:

1. QoS Monitoring And Congestion Control
   

This is the primary function of RTCP. RTCP provides feedback to an application about the quality of data distribution. The control information is useful to the senders, the receivers and third-party monitors. The sender can adjust its transmission based on the receiver report feedback. The receivers can determine whether congestion is local, regional or global. Network managers can evaluate the network performance for multicast distribution.

1. Source Identification
   

In RTP data packets, sources are identified by randomly generated 32-bit identifiers. These identifiers are not convenient for human users. RTCP SDES (source description) packets contain textual information called canonical names as globally unique identifiers of the session participants. It may include user's name, telephone number, email address and other information.

1. Inter-Media Synchronization
   

RTCP sender reports contain an indication of real time and the corresponding RTP timestamp. This can be used in inter-media synchronization like lip synchronization in video.

1. Control Information Scaling
   

RTCP packets are sent periodically among participants. When the number of participants increases, it is necessary to balance between getting up-to-date control information and limiting the control traffic. In order to scale up to large multicast groups, RTCP has to prevent the control traffic from overwhelming network resources. RTP limits the control traffic to at most 5% of the overall session traffic. This is enforced by adjusting the RTCP generating rate according to the number of participants.