| <?% 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/+/main:third_party/webrtc/pc/dtls_transport.h;l=32;drc=6a55e7307b78edb50f94a1ff1ef8393d58218369 |
| [2]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/api/dtls_transport_interface.h;l=76;drc=34437d5660a80393d631657329ef74c6538be25a |
| [3]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/api/dtls_transport_interface.h;l=41;drc=34437d5660a80393d631657329ef74c6538be25a |
| [4]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/p2p/base/dtls_transport_internal.h;l=63;drc=34437d5660a80393d631657329ef74c6538be25a |
| [5]: https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/p2p/base/dtls_transport.h;l=94;drc=653bab6790ac92c513b7cf4cd3ad59039c589a95 |
| [6]: https://source.chromium.org/chromium/chromium/src/+/main: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 |
