126.96.36.199.4 "X-AV" Bandwidth Modifier
The "X-AV" bandwidth modifier MUST be specified for each media description that corresponds to a stream in the ASF content, if that stream has an average bit rate that is different from the peak bit rate. In this case, the bandwidth that is specified by the "b=" field MUST correspond to the average bit rate of the stream in kilobits per second. If the average bit rate is identical to the peak bit rate, a "b=" field with the "X-AV" modifier SHOULD NOT be specified.
The following example shows the "b=" field with the "X-AV" bandwidth modifier: