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<!--* freshness: {owner: 'hta' reviewed: '2021-05-07'} *-->
## Overview
WebRTC uses DTLS in two ways:
* to negotiate keys for SRTP encryption using
* as a transport for SCTP which is used by the Datachannel API
The W3C WebRTC API represents this as the
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
## 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].