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.
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.