8. Retransmission Payload Format MIME Type Registration

8.1. Introduction

The following MIME subtype name and parameters are introduced in this document: "rtx", "rtx-time", and "apt".

The binding used for the retransmission stream to the payload type number is indicated by an rtpmap attribute. The MIME subtype name used in the binding is "rtx".

The "apt" (associated payload type) parameter MUST be used to map the retransmission payload type to the associated original stream payload type. If multiple original payload types are used, then multiple "apt" parameters MUST be included to map each original payload type to a different retransmission payload type.

An OPTIONAL payload-format-specific parameter, "rtx-time", indicates the maximum time a sender will keep an original RTP packet in its buffers available for retransmission. This time starts with the first transmission of the packet.

The syntax is as follows:

a=fmtp:<number> apt=<apt-value>;rtx-time=<rtx-time-val>

where

• <number>: indicates the dynamic payload type number assigned to the retransmission payload format in an rtpmap attribute.

• <apt-value>: is the value of the original stream payload type to which this retransmission stream payload type is associated.

• <rtx-time-val>: specifies the time in milliseconds (measured from the time a packet was first sent) that a sender keeps an RTP packet in its buffers available for retransmission. The absence of the rtx-time parameter for a retransmission stream means that the maximum retransmission time is not defined, but MAY be negotiated by other means.