 is only used by mixers that take multiple RTP streams and output a single RTP stream. Marker (M): This single bit is used to indicate the start of a new frame in video, or the start of a talk-spurt in silence-suppressed speech. Payload Type (PT): This 7 bit field defines the codec in use. The value of this field matches the profile number listed in the SDP. Sequence Number: This 16-bit field is incremented for each RTP packet sent and is used to detect missing/out of sequence packets. Timestamp: This 32-bit field indicates in relative terms the time when the payload was sampled. This field allows the receiver to remove jitter and to play back the packets at the right interval assuming sufficient buffering. Synchronization Source Identifier (SSRCI): This 32-bit field identifies the sender of the RTP packet. At the start of a session, each participant chooses a SSRC number randomly. Should two participants choose the same number, they each choose again until each party is unique. CSRC Contributing Source Identifier: There can be none or up to 15 instances of this 32-bit field in the header. The number is set by the CSRC Count (CC) header field. This field is only present if the RTP packet is being sent by a mixer, which has received RTP packets from a number of sources and sends out combined packets. A non-multicast conference bridge would utilize this header. RTP allows detection of a lost packet by a gap in the Sequence Num- ber. Packets received out of sequence can be detected by out-of-sequence Related Protocols Sequence Numbers. Note that RTP allows detection of these transport- related problems but leaves it up to the codec to deal with the problem. For example, a video codec may compensate for the loss of a packet by repeating the last video frame, while an audio codec may play background noise for the inter- val. Variable delay or jitter can be detected by the Timestamp field. A con- tinuous bit rate codec such as PCM will have a linearly increasing Timestamp. A variable bit rate codec, however, which send