2.2.3.1 Transmitting Copies of RTP Packets

When this RTP payload format is used to transmit a copy of an RTP packet, the RTP payload format does not insert an RTP payload format header of its own into the RTP packet.

The fields in the RTP payload format headers used in the original RTP packet MUST NOT be modified. Also, the RTP header in the copied RTP packet MUST be identical to the RTP header of the original RTP packet.

RTP packets using this RTP payload format SHOULD be transmitted on an RTP session that is different from the one used for the original RTP packets. RTP sessions are different if the RTP packets are sent to different UDP port numbers (see [RFC3550] section 3). RTP sessions are negotiated through the SelectStream request that uses the RTSP SETUP method, as specified in section 2.2.7.10.1. This allows a receiver to distinguish between the original RTP packets and retransmitted copies of the RTP packets.

Also, because the RTP header is not changed, the value of the Sequence Number field (defined in [RFC3550] section 5.1) in the RTP header of the retransmitted RTP packets does not necessarily increment monotonically. Transmitting the copied RTP packets on a separate RTP session avoids any confusion that might be caused by the Sequence Number field.