RTCRtpCapabilities dictionary is a data type used to describe the capabilities of an
RTCRtpReceiver in response to a call to the
RTCRtpReceiver.getCapabilities() static functions, both of which return an array of
RTCRtpCapabilities object contains an array of objects conforming to
RTCRtpCodecCapability (each describing the capabilities of one codec) and an array of the supported RTP header extensions for that codec.
An array of
RTCRtpCodecCapabilityobjects, each describing one of the codecs supported by the
RTCRtpReceiver. There are some special entries in this array, described below in the section The codecs array.
codecs array is an array of objects conforming to the dictionary
RTCRtpCodecCapability. Each of these objects describes a single codec and its basic capabilities. Its properties are:
An unsigned integer value indicating the maximum number of channels supported by the codec; for example, a codec that supports only mono sound would have a value of 1; stereo codecs would have a 2, etc.
An unsigned long integer specifying the codec's clock rate in Hertz (Hz). The IANA maintains a list of codecs and their parameters, including their clock rates.
DOMStringgiving the format specific parameters field from the
a=fmtpline in the SDP which corresponds to the codec, if such a line exists. If there is no parameters field, this property is left out.
The browser will only report distinct capability combinations separately. If two sets of capabilities can be described as one, they will be. That means that, for instance, if there are two entries for the H.264 codec (as identified by the
mimeType being "video/H264"), there are other values in the capabilities objects indicating how they're different in some way.
Note: The MIME type strings used by RTP differ from those used elsewhere. See RFC 3555, section 4 for the complete IANA registry of these types.
There are three special entries that should always be present, representing underlying components of the transport. Those components are:
- RED (REDundant Audio Data)
The media type of an RED entry may vary due to there being several versions of it, but it will end with
red, such as
video/fwdred. The base RED standard can be found in RFC 2198. There may be multiple entries for RED if different forms are supported; each will have a unique media type in that case.
- FEC (Forward Error Correction)
An FEC component handles error correction data; its media type may also vary due to there being advanced versions of the standard available, but it will always end with
fec. One possible value is
video/ulpfec(a generic error connection model). There may also be multiple FEC entries if more than one form is supported.
- RTX (Retransmission)
This component is responsible for retransmission of data; it's media type should be
video/rtx. There will only be one entry for RTX, and it will not have an
Don't be surprised to see these show up ijn the list; be prepared to ignore them if you're only interested in the actual codecs used for the media itself.
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