2. Terminology

The following terms are used in this document:

CSRC: contributing source. See [3].

Original packet: an RTP packet that carries user data sent for the first time by an RTP sender.

Original stream: the RTP stream of original packets.

Retransmission packet: an RTP packet that is to be used by the receiver instead of a lost original packet. Such a retransmission packet is said to be associated with the original RTP packet.

Retransmission request: a means by which an RTP receiver is able to request that the RTP sender should send a retransmission packet for a given original packet. Usually, an RTCP NACK packet as specified in [1] is used as retransmission request for lost packets.

Retransmission stream: the stream of retransmission packets associated with an original stream.

Session-multiplexing: scheme by which the original stream and the associated retransmission stream are sent into two different RTP sessions.

SSRC: synchronization source. See [3].

SSRC-multiplexing: scheme by which the original stream and the retransmission stream are sent in the same RTP session with different SSRC values.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [2].