| // Copyright 2022 Parity Technologies (UK) Ltd. | |
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| //! Implementation of the [`libp2p_core::Transport`] trait for WebRTC protocol without a signaling | |
| //! server. | |
| //! | |
| //! # Overview | |
| //! | |
| //! ## ICE | |
| //! | |
| //! RFCs: 8839, 8445 See also: | |
| //! <https://tools.ietf.org/id/draft-ietf-rtcweb-sdp-08.html#rfc.section.5.2.3> | |
| //! | |
| //! The WebRTC protocol uses ICE in order to establish a connection. | |
| //! | |
| //! In a typical ICE setup, there are two endpoints, called agents, that want to communicate. One | |
| //! of these two agents can be the local browser, while the other agent is the target of the | |
| //! connection. | |
| //! | |
| //! Even though in this specific context all we want is a simple client-server communication, it is | |
| //! helpful to keep in mind that ICE was designed to solve the problem of NAT traversal. | |
| //! | |
| //! The ICE workflow works as follows: | |
| //! | |
| //! - An "offerer" determines ways in which it could be accessible (either an | |
| //! IP address or through a relay using a TURN server), which are called "candidates". It then | |
| //! generates a small text payload in a format called SDP, that describes the request for a | |
| //! connection. | |
| //! - The offerer sends this SDP-encoded message to the answerer. The medium through which this | |
| //! exchange is done is out of scope of the ICE protocol. | |
| //! - The answerer then finds its own candidates, and generates an answer, again in the SDP format. | |
| //! This answer is sent back to the offerer. | |
| //! - Each agent then tries to connect to the remote's candidates. | |
| //! | |
| //! We pretend to send the offer to the remote agent (the target of the connection), then pretend | |
| //! that it has found a valid IP address for itself (i.e. a candidate), then pretend that the SDP | |
| //! answer containing this candidate has been sent back. This will cause the offerer to execute | |
| //! step 4: try to connect to the remote's candidate. | |
| //! | |
| //! ## TCP or UDP | |
| //! | |
| //! WebRTC by itself doesn't hardcode any specific protocol for media streams. Instead, it is the | |
| //! SDP message of the offerer that specifies which protocol to use. In our use case (one or more | |
| //! data channels), we know that the offerer will always request either TCP+DTLS+SCTP, or | |
| //! UDP+DTLS+SCTP. | |
| //! | |
| //! The implementation only supports UDP at the moment, so if the offerer requests TCP+DTLS+SCTP, it | |
| //! will not respond. Support for TCP may be added in the future (see | |
| //! <https://github.com/webrtc-rs/webrtc/issues/132>). | |
| //! | |
| //! ## DTLS+SCTP | |
| //! | |
| //! RFCs: 8841, 8832 | |
| //! | |
| //! In both cases (TCP or UDP), the next layer is DTLS. DTLS is similar to the well-known TLS | |
| //! protocol, except that it doesn't guarantee ordering of delivery (as this is instead provided by | |
| //! the SCTP layer on top of DTLS). In other words, once the TCP or UDP connection is established, | |
| //! the browser will try to perform a DTLS handshake. | |
| //! | |
| //! During the ICE negotiation, each agent must include in its SDP packet a hash of the self-signed | |
| //! certificate that it will use during the DTLS handshake. In our use-case, where we try to | |
| //! hand-crate the SDP answer generated by the remote, this is problematic. A way to solve this | |
| //! is to make the hash a part of the remote's multiaddr. On the server side, we turn | |
| //! certificate verification off. | |
| pub mod tokio; | |