The RTCSctpTransport interface provides information which describes a Stream Control Transmission Protocol (SCTP) transport. This provides information about limitations of the transport, but also provides a way to access the underlying Datagram Transport Layer Security (DTLS) transport over which SCTP packets for all of an RTCPeerConnection's data channels are sent and received.
You don't create RTCSctpTransport objects yourself; instead, you get access to the RTCSctpTransport for a given RTCPeerConnection through its sctp property.
Possibly the most useful property on this interface is its maxMessageSize property, which you can use to determine the upper limit on the size of messages you can send over a data channel on the peer connection.