On 5/29/15 12:11 PM, Ambrish Kumar wrote:
Dear Team,
I am working on Lawful interception and integrating our IMS nodes (SBC)
with LEA. We have got a very unusual ask form IB/RAW where they want to
listen the RTP flow before call is actually answered by “B”
We need your help and confirmation from any RFC which clarifies when does
Media should start flowing in network :
1. Is it possible RTP media should start flowing right after offer-answer
model is completed say after 18x response?
2. Or RTP Media should start flowing after 200 OK ACK response.
From a SIP perspective, media can start flowing to the offerer as soon
as the offer is sent. (Of course, from a practical perspective it also
can't be sent until the offer has been received by the device that will
be sending.) Note that this can be long before the O/A has been completed.
And then media can start flowing to the answerer as soon as the answer
has been sent. Practically this means when the offerer has received the
answer SDP. (Note that technically the O/A isn't complete until the
answer is sent in a *reliable* response. But for the purposes here it is
sufficient if it was sent in an unreliable response that actually
reaches the offerer.)
But not all deployments are so lenient. In particular, IIUC, IMS "gates"
the media streams until the answer reaches the caller's outbound proxy.
So, if you understand the details of your deployment you can take
advantage of knowledge.
Note that there are some extreme cases of early media that are deployed.
Some call centers may send media (not just ringback) in both directions
for an extended period of time, until the call is assigned to an agent.
The entire IVR interaction, with DTMF from the caller, may be conducted
using early media.
I know most of implementation start pushing media after 200 OK ACK. But
does standard mandates to start flow TWO WAY RTP right after offer-answer
model is completed.
It doesn't mandate that it MUST start flowing. But it does mandate that
it MAY start flowing.
Thanks,
Paul
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