Re: [rtcweb] Why SDP answer needs and ack … was Re: SDP Offer/Answer draft-jennings-rtcweb-signaling

Harald Alvestrand <harald@alvestrand.no> Wed, 19 October 2011 05:00 UTC

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Date: Wed, 19 Oct 2011 07:00:20 +0200
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Subject: Re: [rtcweb] =?utf-8?q?Why_SDP_answer_needs_and_ack_=E2=80=A6_was_Re?= =?utf-8?q?=3A__SDP_Offer/Answer_draft-jennings-rtcweb-signaling?=
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[Admin note: Please keep discussion of ROAP details on the rtcweb list, 
not on both lists.]

On 10/18/2011 09:44 PM, Iñaki Baz Castillo wrote:
> 2011/10/18 Cullen Jennings<fluffy@cisco.com>;:
>> Hi - glad to hear you see this going the right direction. On the topic of why we have an OK, let me provide a bit of the motivation.
>>
>> When one side sends and answer that says it wants to receive VP8 instead of H.264, it's probably useful to know when the other side got that information. This might impact the timing of when o send things or user interface that provides feedback about the status of the other side. We are also dealing with a web transaction model where transaction are not guarantee to happen even if they are sent over TCP. So you need to get back a response to request. It also helps with mapping to SIP but even if you were not mapping to another protocol, I suspect you would still need to be able to have an confirmation than and answer was received.
> Hi Cullen. Let me put an example:
>
> - alice: JS client implementing pure SIP over WebSocket.
> - A proxy implementing SIP over UDP and WebSocket.
> - bob: SIP UDP client.
>
> The flows:
>
> - alice sends INVITE to bob with an empty body (no SDP).
> - bob replies a 200 with the SDP offer.
> - alice receives the 200. Its JS code internally generates a ROAP Offer.
> - alice generates a ROAP Answer and inserts its SDP into an ACK to be
> sent to bob.
> - In SIP we have ended. So how is supposed alice will receive the ROAP
> OK message?
The entity that knows about the mismatch between ROAP and SIP message 
sequences will have to generate it. In your hypothetical scenario - alice.
> I don't understand yet if such ROAP OK message is mandatory to be sent
> to the RTCweb stack. If so, the JS code in alice could auto-generate
> it after sending the ACK, so the RTCweb stack becomes happy. But
> obviously the advantage of such auto-generated ROAP OK is null.
>
>
> Regards.
>
>