a=x-mediabw attribute

The a=x-mediabw attribute is a declarative session-level attribute that specifies the send and receive bandwidth limits for a particular modality. These values are given from the perspective of the user agent that created the SDP message containing the attribute.

The syntax of the attribute is:

 "a=x-mediabw:" label SPACE "send=" bandwidth ";recv=" bandwidth
 label = token
 bandwidth = unsigned-integer

The bandwidth value MUST be a decimal integer in the range 0 to 4294967295 inclusive. The bandwidth value is interpreted in kilobits-per-second (1000 Kbps). For example, a bandwidth value of 1500 represents 1,500,000 bits per second. A value of zero means no bandwidth is available for that stream direction.

The value following the "send=" token is the send stream bandwidth limit; the value following the "recv=" token is the receive stream bandwidth limit.

The syntax of the label token is as defined in [RFC4566] section 9.

The x-mediabw attribute is a session-level only attribute. It MUST NOT be present within a media description. If present within a media description, it MUST be ignored by the receiving user agent.

There MUST NOT be more than one session-level x-mediabw attribute specifying the same label value. If there are multiple x-mediabw attributes with the same label value, the receiving user agent will pick an arbitrary attribute and ignore the others.

The x-mediabw attribute(s) of an SDP message MUST NOT be modified in a subsequent renegotiation. If the bandwidth values change in a renegotiation, the new values MUST be ignored. If a renegotiation introduces a new modality, the SDP offer SHOULD<53> add a new x-mediabw attribute for it.