1<?% config.freshness.reviewed = '2021-05-07' %?> 2<?% config.freshness.owner = 'hta' %?> 3 4## Overview 5 6WebRTC uses DTLS in two ways: 7 8* to negotiate keys for SRTP encryption using 9 [DTLS-SRTP](https://www.rfc-editor.org/info/rfc5763) 10* as a transport for SCTP which is used by the Datachannel API 11 12The W3C WebRTC API represents this as the 13[DtlsTransport](https://w3c.github.io/webrtc-pc/#rtcdtlstransport-interface). 14 15The DTLS handshake happens after the ICE transport becomes writable and has 16found a valid pair. It results in a set of keys being derived for DTLS-SRTP as 17well as a fingerprint of the remote certificate which is compared to the one 18given in the SDP `a=fingerprint:` line. 19 20This documentation provides an overview of how DTLS is implemented, i.e how the 21following classes interact. 22 23## webrtc::DtlsTransport 24 25The [`webrtc::DtlsTransport`][1] class is a wrapper around the 26`cricket::DtlsTransportInternal` and allows registering observers implementing 27the `webrtc::DtlsTransportObserverInterface`. The 28[`webrtc::DtlsTransportObserverInterface`][2] will provide updates to the 29observers, passing around a snapshot of the transports state such as the 30connection state, the remote certificate(s) and the SRTP ciphers as 31[`DtlsTransportInformation`][3]. 32 33## cricket::DtlsTransportInternal 34 35The [`cricket::DtlsTransportInternal`][4] class is an interface. Its 36implementation is [`cricket::DtlsTransport`][5]. The `cricket::DtlsTransport` 37sends and receives network packets via an ICE transport. It also demultiplexes 38DTLS packets and SRTP packets according to the scheme described in 39[RFC 5764](https://tools.ietf.org/html/rfc5764#section-5.1.2). 40 41## webrtc::DtlsSrtpTranport 42 43The [`webrtc::DtlsSrtpTransport`][6] class is responsіble for extracting the 44SRTP keys after the DTLS handshake as well as protection and unprotection of 45SRTP packets via its [`cricket::SrtpSession`][7]. 46 47[1]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/pc/dtls_transport.h;l=32;drc=6a55e7307b78edb50f94a1ff1ef8393d58218369 48[2]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/api/dtls_transport_interface.h;l=76;drc=34437d5660a80393d631657329ef74c6538be25a 49[3]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/api/dtls_transport_interface.h;l=41;drc=34437d5660a80393d631657329ef74c6538be25a 50[4]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/p2p/base/dtls_transport_internal.h;l=63;drc=34437d5660a80393d631657329ef74c6538be25a 51[5]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/p2p/base/dtls_transport.h;l=94;drc=653bab6790ac92c513b7cf4cd3ad59039c589a95 52[6]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/pc/dtls_srtp_transport.h;l=31;drc=c32f00ea9ddf3267257fe6b45d4d79c6f6bcb829 53[7]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/pc/srtp_session.h;l=33;drc=be66d95ab7f9428028806bbf66cb83800bda9241 54