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diff --git a/pc/g3doc/dtls_transport.md b/pc/g3doc/dtls_transport.md new file mode 100644 index 0000000000..65206dff5d --- /dev/null +++ b/pc/g3doc/dtls_transport.md @@ -0,0 +1,53 @@ +<?% config.freshness.reviewed = '2021-05-07' %?> +<?% config.freshness.owner = 'hta' %?> + +## Overview + +WebRTC uses DTLS in two ways: + +* to negotiate keys for SRTP encryption using + [DTLS-SRTP](https://www.rfc-editor.org/info/rfc5763) +* as a transport for SCTP which is used by the Datachannel API + +The W3C WebRTC API represents this as the +[DtlsTransport](https://w3c.github.io/webrtc-pc/#rtcdtlstransport-interface). + +The DTLS handshake happens after the ICE transport becomes writable and has +found a valid pair. It results in a set of keys being derived for DTLS-SRTP as +well as a fingerprint of the remote certificate which is compared to the one +given in the SDP `a=fingerprint:` line. + +This documentation provides an overview of how DTLS is implemented, i.e how the +following classes interact. + +## webrtc::DtlsTransport + +The [`webrtc::DtlsTransport`][1] class is a wrapper around the +`cricket::DtlsTransportInternal` and allows registering observers implementing +the `webrtc::DtlsTransportObserverInterface`. The +[`webrtc::DtlsTransportObserverInterface`][2] will provide updates to the +observers, passing around a snapshot of the transports state such as the +connection state, the remote certificate(s) and the SRTP ciphers as +[`DtlsTransportInformation`][3]. + +## cricket::DtlsTransportInternal + +The [`cricket::DtlsTransportInternal`][4] class is an interface. Its +implementation is [`cricket::DtlsTransport`][5]. The `cricket::DtlsTransport` +sends and receives network packets via an ICE transport. It also demultiplexes +DTLS packets and SRTP packets according to the scheme described in +[RFC 5764](https://tools.ietf.org/html/rfc5764#section-5.1.2). + +## webrtc::DtlsSrtpTranport + +The [`webrtc::DtlsSrtpTransport`][6] class is responsŃ–ble for extracting the +SRTP keys after the DTLS handshake as well as protection and unprotection of +SRTP packets via its [`cricket::SrtpSession`][7]. + +[1]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/pc/dtls_transport.h;l=32;drc=6a55e7307b78edb50f94a1ff1ef8393d58218369 +[2]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/api/dtls_transport_interface.h;l=76;drc=34437d5660a80393d631657329ef74c6538be25a +[3]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/api/dtls_transport_interface.h;l=41;drc=34437d5660a80393d631657329ef74c6538be25a +[4]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/p2p/base/dtls_transport_internal.h;l=63;drc=34437d5660a80393d631657329ef74c6538be25a +[5]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/p2p/base/dtls_transport.h;l=94;drc=653bab6790ac92c513b7cf4cd3ad59039c589a95 +[6]: https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/pc/dtls_srtp_transport.h;l=31;drc=c32f00ea9ddf3267257fe6b45d4d79c6f6bcb829 +[7]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/pc/srtp_session.h;l=33;drc=be66d95ab7f9428028806bbf66cb83800bda9241 |