Is it illegal to modify sdp between createOffer and setLocalDescription?

[y-i]

I read "4.4.2 Interface Definition" and have a question about [[LastCreatedOffer]].

const offer = await pc.createOffer();
// modify sdp
await pc.setLocalDescription(offer);

Considering the behavior of the above code, according to step 5 of createOffer, the value will be set in [[LastCreatedOffer]] when createOffer is called.

If setLocalDescription is called after editing the SDP, it appears to be rejected with an InvalidModificationError because an sdp different from [[LastCreatedOffer]] is passed in step 4.2.
However, when I actually run it, all modern browsers (Chrome/Firefox/Safari) resolve it.

Am I misunderstanding something about this?
Or do all browsers violate the spec?

[henbos]

All browsers are violating the spec. Modifying SDP is not allowed.

[henbos]

Historically, features have been shipped via SDP modifications rather than APIs. The API side of things have been playing catch-up, e.g. setCodecPreferences() to allow modifying codecs that way instead of "SDP munging" or the recent API to modify header extensions to negotiating in the extension spec. So the list of reasons for SDP munging is quite small these days, but I'm not sure all holes have been plugged just yet.

[henbos]

@jan-ivar @alvestrand Should we make an official list of "holes that needs to be plugged"?

[y-i]

Thank you for explaining the detail.
In my case, I modified a SDP directly to use L16 to use Lyra codec over WebRTC.

[alvestrand]

Enabling encoding/decoding of non-default-enabled but implemented codecs is a missing piece of API, so yes. I *think* you can get it by manipulating the remote SDP instead of the local SDP (which is allowed) but that may require that the remote end sends the offer, which may be a nonstarter for your particular use case.

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On Tue, Nov 7, 2023 at 10:44 AM y-i ***@***.***> wrote: Thank you for explaining the detail. In my case, I modified a SDP directly to use L16 to use Lyra codec over WebRTC. — Reply to this email directly, view it on GitHub <#2907 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AADVM7IUXE65JAY7KOQ2EEDYDH7I7AVCNFSM6AAAAAA7AZTS62VHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMYTOOJYGE2DQNBVGE> . You are receiving this because you were mentioned.Message ID: ***@***.***>

[henbos]

We should add a test for this (that would fail)

[fippo]

Discussion how to enable codecs like L16 should now happen in #2925