CODEWITHSUNDEEP
webrtc
Sunny
Sunny

Reputation: 10085

WebRTC: Why "CreateAnswer can't be called before SetRemoteDescription"?

Browser: Chrome

I am trying to debug a webRTC application which works fine on three out of four fronts! I cannot get video from the receiver to the caller. I can get video and audio from the caller to the receiver and audio from the receiver to the caller. The problem is that the receiver does not fire a video (sdpMid="video") ICE candidate. While desperately trying to solve this problem, I tried to use pc.CreateAnswer before setting pc.remoteDescription and it gave the error quoted in the title.

My question is to understand the reason behind this. An answer SDP would just be the SDP based upon the getUserMedia settings/constraints. So, why do we have to wait for setting remoteDescription. I thought that a createAnswer would start firing the gathering of ICE candidates and this can be done earlier without waiting to set remoteDescription. That is not the case. Why?

Upvotes: 1

Views: 2076

Answers (1)

jib
jib

Reputation: 42500

Offers and answers aren't independent, they're part of an inherently asymmetric exchange.

An answer is a direct response to a specific offer (hence the name "answer"). Therefore the peer cannot answer before it has an offer, which you set with setRemoteDescription.

An offer contains specific limitations, or envelope (like m-lines), that an answer has to abide by/answer to/stay within. Another way to say it is that the answer is an iteration of the offer.

For instance, an offer created with offer options offerToReceiveVideo: false can only be answered with recvonly for video (meaning receive video from offerer to answerer only), never sendrecv.

Upvotes: 4

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