RtpParameters

There are two types of RTP parameters (RtpSendParameters and RtpReceiveParameters), both sharing the following definition:

RtpSendParameters

@inherits RtpParameters

The RTP send parameters describe a media stream received by mediasoup from an endpoint through its corresponding mediasoup Producer.

• These parameters may include a mid value that the mediasoup transport will use to match received RTP packets based on their MID RTP extension value.
• mediasoup allows RTP send parameters with a single encoding and with multiple encodings (simulcast). In the latter case, each entry in the encodings array must include a ssrc field or a rid field (the RID RTP extension value).
• If a single encoding is given, RTP send parameters must include mid value or the encoding must indicate the ssrc of the stream.
• If no encoding is given (so this is a simple stream without layers), then RTP send parameters must include mid value.

RtpReceiveParameters

@inherits RtpParameters

The RTP receive parameters describe a media stream as sent by mediasoup to an endpoint through its corresponding mediasoup Consumer.

• There is a single entry in the encodings array (even if the corresponding producer uses simulcast). The consumer sends a single and continuous RTP stream to the endpoint and spatial/temporal layer selection is possible via consumer.setPreferredLayers().
• As an exception, previous bullet is not true when consuming a stream over a PipeTransport, in which all RTP streams from the associated producer are forwarded verbatim through the consumer.
• The RTP receive parameters will always have their ssrc values randomly generated for all of its encodings (and optional rtx: { ssrc: XXXX } if the endpoint supports RTX), regardless of the original RTP send parameters in the associated producer. This applies even if the producer's encodings have rid set.

RtpCapabilities

The RTP capabilities define what mediasoup or an endpoint can receive at media level.

RtpCodecParameters

Provides information on codec settings within the RTP parameters. The list of media codecs supported by mediasoup and their settings is defined in the supportedRtpCapabilities.ts file.

RtcpFeedback

Provides information on RTCP feedback messages for a specific codec. Those messages can be transport layer feedback messages or codec-specific feedback messages. The list of RTCP feedbacks supported by mediasoup is defined in the supportedRtpCapabilities.ts file.

RtpEncodingParameters

Provides information relating to an encoding, which represents a media RTP stream and its associated RTX stream (if any).

RtpHeaderExtensionParameters

Defines a RTP header extension within the RTP parameters. The list of RTP header extensions supported by mediasoup is defined in the supportedRtpCapabilities.ts file.

RtcpParameters

Provides information on RTCP settings within the RTP parameters.

RtpCodecCapability

Provides information on the capabilities of a codec within the RTP capabilities. The list of media codecs supported by mediasoup and their settings is defined in the supportedRtpCapabilities.ts file.

Exactly one RtpCodecCapability will be present for each supported combination of parameters that requires a distinct value of preferredPayloadType. For example:

• Multiple H264 codecs, each with their own distinct “packetization-mode” and “profile-level-id” values.
• Multiple VP9 codecs, each with their own distinct “profile-id” value.

RtpHeaderExtension

Provides information relating to supported header extensions. The list of RTP header extensions supported by mediasoup is defined in the supportedRtpCapabilities.ts file.

MediaKind

Codec Parameters

When a producer includes codec parameters into its RTP send parameters, those parameters are passed verbatim to the RTP receive parameters of the consumers associated to that producer.

Some of those parameters are part of the codec settings and are used for codec matching. Some other codec parameters affect the operation of mediasoup for the corresponding producer and consumers.

Parameters for Codec Matching

These parameters are part of the codec settings, meaning that their values determine whether an entry in rtpParameters.codecs matches or not an entry in the remote rtpCapabilities.codecs. These parameters are codec-specific:

Parameters Affecting mediasoup Operation

These parameters influence the mediasoup operation by enabling or disabling some features. These parameters are codec-specific:

Simulcast

Simulcast involves sending N separate video RTP streams (so N different SSRCs) representing N different qualities of the same video source. If RTX is used, there would also be N additional RTP RTX streams, one for each media RTP stream. Each media RTP stream may also contain M temporal layers.

When creating a simulcast producer, the associated rtpParameters given to transport.produce() must conform to the following rules:

• There must be N > 1 entries in the encodings array or a single entry with scalabilityMode indicating 1 spatial layer and more than 1 temporal layer.
• If there are more than 1 encoding, each encoding must include a ssrc field or a rid field (the RID RTP extension value) to help the mediasoup producer identify which RTP stream each packet belongs to.
• Each encoding represents a “spatial layer”. Entries in encodings must be ordered from lowest to highest resolution (encodings[0] means “spatial layer 0” while encodings[N-1] means “spatial layer N-1”, being N the number of simulcast streams).
• If the streams have M temporal layers, those must be signaled in each encoding within the scalabilityMode field:
  • Since each stream has a single spatial layer, the number of spatial layers indicated in scalabilityMode in each encoding must be 1.
  • If there are not temporal layers, the scalabilityMode field can be omitted (it defaults to “L1T1”, this is, one spatial layer and one temporal layer).
  • All encodings must have same number of temporal layers.

Simulcast consumers will just get a single stream and hence a single entry in their rtpParameters.encodings array. Such a encoding entry has a scalabilityMode value that determines the number of spatial layers (number of simulcast streams in the producer) and the number of temporal layers.

Examples

The following examples just show the rtpParameters.encodings field and, for simplicity, do not include RTX information.

encodings :
[
  { ssrc: 111110 },
  { ssrc: 111111 },
  { ssrc: 111112 }
]

encodings :
[
  { ssrc: 222220, scalabilityMode: 'L3T1' }
]

encodings :
[
  { rid: 'r0', scalabilityMode: 'L1T3' },
  { rid: 'r1', scalabilityMode: 'L1T3' },
  { rid: 'r2', scalabilityMode: 'L1T3' },
  { rid: 'r3', scalabilityMode: 'L1T3' }
]

encodings :
[
  { ssrc: 222220, scalabilityMode: 'L4T3' }
]

SVC

SVC involves sending a single RTP stream with N spatial layers and M temporal layers. If RTX is used, there would also be an additional RTP RTX stream.

mediasoup implements two types of SVC, full SVC and K-SVC. The main difference is that, in K-SVC, a RTP key frame is required in order to up/down switch the maximun spatial layer that mediasoup forwards to a consumer. For more information about SVC in WebRTC check the webrtc-svc specification (work in progress).

encodings :
[
  { ssrc: 111110, scalabilityMode: 'L3T2' }
]

encodings :
[
  { ssrc: 222220, scalabilityMode: 'L3T2' }
]

encodings :
[
  { ssrc: 111110, scalabilityMode: 'L4T5_KEY' }
]

encodings :
[
  { ssrc: 222220, scalabilityMode: 'L4T5_KEY' }
]