[Webkit-unassigned] [Bug 198402] New: iOS 12.3 and 12.4 emit PeerConnection SDP offer m-lines in random order

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=198402

            Bug ID: 198402
           Summary: iOS 12.3 and 12.4 emit PeerConnection SDP offer
                    m-lines in random order
           Product: WebKit
           Version: Safari Technology Preview
          Hardware: iPhone / iPad
                OS: iOS 12
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WebRTC
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: webkit at xylil.com
                CC: youennf at gmail.com

In iOS 12.2 and prior, WebRTC PeerConnection offers generated from iOS always structured the SDP such that the audio m-lines came first, and the video m-lines came second.

iOS 12.3 and 12.4 no longer works this way, instead it structures the SDP audio/video m-lines seemingly randomly, sometimes putting the audio m-lines first, sometimes the video m-lines first.

Is this a regression? I'm asking because it breaks compatibility with a popular WebRTC server, which always sends its SDP answers with the audio m-lines first, and video m-lines second. In the case of iOS sending its video m-lines first, this results in the following error when setRemoteDescription() is called with the answer SDP: InvalidAccessError: Failed to set remote answer sdp: The order of m-lines in answer doesn't match order in offer. Rejecting answer.

If this is not a regression, then it would be helpful if you could describe how to munge the answer SDP properly to get around the error message above. I've tried simply extracting all related video m-lines and moving them after the audio m-lines, as in this before/after example, but it fails with "SyntaxError: Invalid SDP line": https://pastebin.com/ZGv7mSqL

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