At the application layer, you can think of DataChannel as sending and receiving over SCTP. In the PPID (Payload Protocol Identifier) field of the SCTP header, Datachannel sets value 0x51 for indicating that it’s sending UTF-8 data and 0x52 for binary data.

Yes, you are right. RTCDataChannel uses SCTP over DTLS and UDP. DTLS is used for security. However, SCTP has problems traversing most NAT/Firewall setups. Hence, to overcome that, SCTP is tunneled through UDP. So the overall overhead to send data would be overhead of:

SCTP + DTLS + UDP + IP

and that is:

28 bytes + 20-40 bytes + 8 bytes + 20 – 40 bytes

So, the overhead would be rougly about 120 bytes. The maximum size of the SCTP packet that a WebRTC client can send is 1280 bytes. So at max, you can send roughly 1160 bytes of data per SCTP packet.