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add dtls docs

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BUG=webrtc:12551
NOTRY=true

Change-Id: I702bc2054edb28446109874746f8c62c77246615
Reviewed-on: https://webrtc-review.googlesource.com/c/src/+/217887
Reviewed-by: Harald Alvestrand <hta@webrtc.org>
Commit-Queue: Harald Alvestrand <hta@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#34001}
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Philipp Hancke 2021-05-11 08:51:29 +02:00 committed by WebRTC LUCI CQ
parent 99261af5a4
commit ada00128fe
2 changed files with 30 additions and 1 deletions
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g3doc/sitemap.md
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* [ICE](/p2p/g3doc/ice.md)
* STUN
* TURN
* DTLS
* [DTLS](/pc/g3doc/dtls_transport.md)
* [SCTP](/pc/g3doc/sctp_transport.md)
* [Pacing buffer](/modules/pacing/g3doc/index.md)
* Congestion control and bandwidth estimation

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pc/g3doc/dtls_transport.md Normal file
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<?% 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`](https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/pc/dtls_transport.h;l=32;drc=6a55e7307b78edb50f94a1ff1ef8393d58218369) class
is a wrapper around the `cricket::DtlsTransportInternal` and allows registering observers implementing the `webrtc::DtlsTransportObserverInterface.
The [`webrtc::DtlsTransportObserverInterface`](https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/api/dtls_transport_interface.h;l=76;drc=34437d5660a80393d631657329ef74c6538be25a) 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`](https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/api/dtls_transport_interface.h;l=41;drc=34437d5660a80393d631657329ef74c6538be25a).
##cricket::DtlsTransportInternal
The [`cricket::DtlsTransportInternal`](https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/p2p/base/dtls_transport_internal.h;l=63;drc=34437d5660a80393d631657329ef74c6538be25a) class is an interface. Its implementation is [`cricket::DtlsTransport`](https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/p2p/base/dtls_transport.h;l=94;drc=653bab6790ac92c513b7cf4cd3ad59039c589a95). 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`](https://source.chromium.org/chromium/chromium/src/+/master:third_party/webrtc/pc/dtls_srtp_transport.h;l=31;drc=c32f00ea9ddf3267257fe6b45d4d79c6f6bcb829) class
is responsіble for extracting the SRTP keys after the DTLS handshake as well as protection and unprotection of SRTP packets via its [`cricket::SrtpSession`](https://source.chromium.org/chromium/chromium/src/+/main:third_party/webrtc/pc/srtp_session.h;l=33;drc=be66d95ab7f9428028806bbf66cb83800bda9241).