add dtls docs
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}
diff --git a/g3doc/sitemap.md b/g3doc/sitemap.md
index b9d913e..51b27e1 100644
--- a/g3doc/sitemap.md
+++ b/g3doc/sitemap.md
@@ -9,7 +9,7 @@
* [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
diff --git a/pc/g3doc/dtls_transport.md b/pc/g3doc/dtls_transport.md
new file mode 100644
index 0000000..331e5b2
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+++ b/pc/g3doc/dtls_transport.md
@@ -0,0 +1,29 @@
+<?% 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 responsŃ–ble 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).
