Re: [rtcweb] When are ICE candidates added to the SDP

"Rauschenbach, Uwe (NSN - DE/Munich)" <uwe.rauschenbach@nsn.com> Tue, 13 May 2014 06:06 UTC

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From: "Rauschenbach, Uwe (NSN - DE/Munich)" <uwe.rauschenbach@nsn.com>
To: ext Martin Thomson <martin.thomson@gmail.com>
Thread-Topic: [rtcweb] When are ICE candidates added to the SDP
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Subject: Re: [rtcweb] When are ICE candidates added to the SDP
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> -----Original Message-----
> From: ext Martin Thomson [mailto:martin.thomson@gmail.com]
> 
> On 12 May 2014 20:46, Rauschenbach, Uwe (NSN - DE/Munich)
> <uwe.rauschenbach@nsn.com> wrote:
> >
> > A question on step 6 and your statement below the sequence: What would
> be the difference between an SDP created by pc.createOffer in step 6 and
> one obtained by getting pc.localDescription in step 6?
> 
> The difference is that the second contains candidates.

Uwe: Then, let me elaborate a bit more.

Obviously, we need an SDP reflecting the local configuration for two purposes:
1) To feed it to setLocalDescription in order to start the ICE gathering
2) To send it as an offer to the other peer

The name "createOffer" implies that the SDP delivered as the result of createOffer is the one to use as an offer in negotiation. Looking at the way this mechanism has evolved, this is probably only true anymore if one is prepared to send the offer immediately, and then to trickle ICE candidates. To obtain an SDP to use as an offer in negotiations without trickle ICE, one has to call setLocal, wait for "end of candidates" and then read pc.localDescription.

So "createOffer" in fact does not create an offer, but an initial (or updated) local description that is then fed into setLocalDescription to start the ICE gathering.

Do you agree with this observation? If you do, shouldn't we rename createOffer to something more generic like "createLocalDescription" or similar? "createOffer" does not create the offer (at least it does not do so in case of non-trickling) but the input to setLocal...
 

> > Why do you consider multiple pc.createOffer calls nasty?
> 
> They aren't convenient.  You risk mistakes.