WebRTC ではさまざまなプロトコルが相互作用してピア間の接続を確立し、データやメディアの転送を行いますが、この記事ではその仕組みを解説します。
Note: This page needs heavy rewriting for structural integrity and content completeness. Lots of info here is good but the organization is a mess since this is sort of a dumping ground right now.
残念なことに、WebRTC は中間に何らかのサーバーがなければ接続を作成できません。このサーバーをシグナルチャンネル、またはシグナリングサービスと呼びます。接続を確立する前に情報を交換する伝達手段はどんなものでも構いません。Eメール、はがき、伝書鳩でも...決めるのはあなたです。
交換する必要のある情報はオファーとアンサーと呼ばれ、その中身は下記で説明する {{Glossary("SDP")}} です。
ピア A が接続を初期化する側とすると、ピア A がオファーを作成します。それから選択されたシグナルチャンネルを使ってピア B にオファーを送ります。ピア B はシグナルチャンネルからオファーを受け取ると、アンサーを作成します。それからピア B はピア A にシグナルチャンネルを使ってアンサーを送り返します。
WebRTC 接続のエンドポイント設定は session description と呼ばれます。そこに含まれる情報は、送られるメディアの種類、形式、使用される転送プロトコル、エンドポイントの IP アドレスとポート、またその他メディア転送エンドポイントを記述するのに必要な情報です。この情報を Session Description Protocol ({{Glossary("SDP")}}) を使って交換し、保存します。SDP データ形式の詳細は {{RFC(2327)}} にあります。
ユーザーが WebRTC コールを他のユーザーに開始するとき、オファーと呼ばれる特別な記述を作成します。コールする側がコールに必要な設定を提案し、そのすべての情報をオファーの記述に盛り込みます。受け取る側はアンサーを返します。アンサーは受け取る側が用意する記述です。このようにして、両デバイスがお互いにメディアデータの交換に必要な情報を共有します。この交換は Interactive Connectivity Establishment ({{Glossary("ICE")}}) を使って行われます。ICE とは二つのデバイスが Network Address Translation ({{Glossary("NAT")}}) によって隔てられていてもオファーとアンサーを交換するために媒介を利用できるようにするプロトコルです。
各ピアは二つの記述を手に入れます。 local description が自分側の記述で、 remote description が相手側の記述です。
オファー／アンサーの交換はコールを最初に確立する際に実行されますが、それだけでなくフォーマットや他の設定に変更が必要なときにも随時実行されます。コールの新規作成時でも既存の設定変更時でも、いずれにしてもオファーとアンサーを交換するために以下のような基本的なステップが実行されます。なお、ここでは ICE レイヤーは除外しています。
RTCPeerConnection を作成し、 {{domxref("RTCPeerConnection.addTrack()")}} を実行する。(addStream が非推奨であるため)Taking one step deeper into the process, we find that localDescription and remoteDescription, the properties which return these two descriptions, aren't as simple as they look. Because during renegotiation, an offer might be rejected because it proposes an incompatible format, it's necessary that each endpoint have the ability to propose a new format but not actually switch to it until it's accepted by the other peer. For that reason, WebRTC uses pending and current descriptions.
The current description (which is returned by the {{domxref("RTCPeerConnection.currentLocalDescription")}} and {{domxref("RTCPeerConnection.currentRemoteDescription")}} properties) represents the description currently in actual use by the connection. This is the most recent connection that both sides have fully agreed to use.
The pending description (returned by {{domxref("RTCPeerConnection.pendingLocalDescription")}} and {{domxref("RTCPeerConnection.pendingRemoteDescription")}}) indicates a description which is currently under consideration following a call to setLocalDescription() or setRemoteDescription(), respectively.
When reading the description (returned by {{domxref("RTCPeerConnection.localDescription")}} and {{domxref("RTCPeerConnection.remoteDescription")}}), the returned value is the value of pendingLocalDescription/pendingRemoteDescription if there's a pending description (that is, the pending description isn't null); otherwise, the current description (currentLocalDescription/currentRemoteDescription) is returned.
When changing the description by calling setLocalDescription() or setRemoteDescription(), the specified description is set as the pending description, and the WebRTC layer begins to evaluate whether or not it's acceptable. Once the proposed description has been agreed upon, the value of currentLocalDescription or currentRemoteDescription is changed to the pending description, and the pending description is set to null again, indicating that there isn't a pending description.
The pendingLocalDescription contains not just the offer or answer under consideration, but any local ICE candidates which have already been gathered since the offer or answer was created. Similarly, pendingRemoteDescription includes any remote ICE candidates which have been provided by calls to {{domxref("RTCPeerConnection.addIceCandidate()")}}.
See the individual articles on these properties and methods for more specifics, and Codecs used by WebRTC for information about codecs supported by WebRTC and which are compatible with which browsers. The codecs guide also offers guidance to help you choose the best codecs for your needs.
As well as exchanging information about the media (discussed above in Offer/Answer and SDP), peers must exchange information about the network connection. This is known as an ICE candidate and details the available methods the peer is able to communicate (directly or through a TURN server). Typically, each peer will propose its best candidates first, making their way down the line toward their worse candidates. Ideally, candidates are UDP (since it's faster, and media streams are able to recover from interruptions relatively easily), but the ICE standard does allow TCP candidates as well.
Generally, ICE candidates using TCP are only going to be used when UDP is not available or is restricted in ways that make it not suitable for media streaming. Not all browsers support ICE over TCP, however.
ICE allows candidates to represent connections over either {{Glossary("TCP")}} or {{Glossary("UDP")}}, with UDP generally being preferred (and being more widely supported). Each protocol supports a few types of candidate, with the candidate types defining how the data makes its way from peer to peer.
UDP candidates (candidates with their {{domxref("RTCIceCandidate.protocol", "protocol")}} set to udp) can be one of these types:
hostprflxsrflxrelay"srflx"), but using {{Glossary("TURN")}} instead of {{Glossary("STUN")}}.TCP candidates (that is, candidates whose {{domxref("RTCIceCandidate.protocol", "protocol")}} is tcp) can be of these types:
activepassivesoThe ICE layer selects one of the two peers to serve as the controlling agent. This is the ICE agent which will make the final decision as to which candidate pair to use for the connection. The other peer is called the controlled agent. You can identify which one your end of the connection is by examining the value of {{domxref("RTCIceTransport.role", "RTCIceCandidate.transport.role")}}, although in general it doesn't matter which is which.
The controlling agent not only takes responsibility for making the final decision as to which candidate pair to use, but also for signaling that selection to the controlled agent by using STUN and an updated offer, if necessary. The controlled agent just waits to be told which candidate pair to use.
It's important to keep in mind that a single ICE session may result in the controlling agent choosing more than one candidate pair. Each time it does so and shares that information with the controlled agent, the two peers reconfigure their connection to use the new configuration described by the new candidate pair.
Once the ICE session is complete, the configuration that's currently in effect is the final one, unless an ICE reset occurs.
At the end of each generation of candidates, an end-of-candidates notification is sent in the form of an {{domxref("RTCIceCandidate")}} whose {{domxref("RTCIceCandidate.candidate", "candidate")}} property is an empty string. This candidate should still be added to the connection using {{domxref("RTCPeerConnection.addIceCandidate", "addIceCandidate()")}} method, as usual, in order to deliver that notification to the remote peer.
When there are no more candidates at all to be expected during the current negotiation exchange, an end-of-candidates notification is sent by delivering a {{domxref("RTCIceCandidate")}} whose {{domxref("RTCIceCandidate.candidate", "candidate")}} property is null. This message does not need to be sent to the remote peer. It's a legacy notification of a state which can be detected instead by watching for the {{domxref("RTCPeerConnection.iceGatheringState", "iceGatheringState")}} to change to complete, by watching for the {{domxref("RTCPeerConnection.icegatheringstatechange_event", "icegatheringstatechange")}} event.
During negotiation, there will be times when things just don't work out. For example, when renegotiating a connection—for example, to adapt to changing hardware or network configurations—it's possible that negotiation could reach a dead end, or some form of error might occur that prevents negotiation at all. There may be permissions issues or other problems as well, for that matter.
When renegotiating a connection that's already active and a situation arises in which the negotiation fails, you don't really want to kill the already-running call. After all, you were most likely just trying to upgrade or downgrade the connection, or to otherwise make adaptations to an ongoing session. Aborting the call would be an excessive reaction in that situation.
Instead, you can initiate an ICE rollback. A rollback restores the SDP offer (and the connection configuration by extension) to the configuration it had the last time the connection's {{domxref("RTCPeerConnection.signalingState", "signalingState")}} was stable.
To programmatically initiate a rollback, send a description whose {{domxref("RTCSessionDescription.type", "type")}} is rollback. Any other properties in the description object are ignored.
In addition, the ICE agent will automatically initiate a rollback when a peer that had previously created an offer receives an offer from the remote peer. In other words, if the local peer is in the state have-local-offer, indicating that the local peer had previously sent an offer, calling setRemoteDescription() with a received offer triggers rollback so that the negotiation switches from the remote peer being the caller to the local peer being the caller.
For now, see {{SectionOnPage("/en-US/docs/Web/API/WebRTC_API/Session_lifetime", "ICE restart")}}.