[SIPForum-discussion] Regarding Early Media and Preconditions.

[Fred Cherny]

For IMS, I believe you'd need to send a 183 response (including PRACK) with the p-early-media header (P-Early-Media: sendrecv). That will result in an ACM being sent back to the caller (assuming it's coming in SS7). At that point, the caller should be able to hear your announcement.

From: discussion-bounces at sipforum.org [mailto:discussion-bounces at sipforum.org] On Behalf Of Stephen James
Sent: Monday, August 13, 2012 3:17 PM
To: discussion at sipforum.org
Subject: [SIPForum-discussion] Regarding Early Media and Preconditions.

The scenario is an MGCF (SIP - ISUP interworking IMS - PSTN). SIP preconditions are in use.

We receive an INVITE with SDP as follows (modified for security)

v=0
o=- 3553598070 3553598070 IN IP4 10.0.0.10
s=-
c=IN IP4 10.0.0.10
t=0 0
m=audio 59068 RTP/AVP 0
a=sendrecv
a=curr:qos local none
a=curr:qos remote none
a=des:qos mandatory local sendrecv
a=des:qos mandatory remote sendrecv

Now, according to 3264, once this offer is sent, the UAC should be ready to receive media; however
the preconditions seem to contradict this. And in actuality the bearer is not set up and the UAC cannot
receive the early media.

In the MGCF we need to play an announcement due to network conditions. Since the SDP is indicating
sendrecv, that should be possible. The UAC does not receive our packets.

- Should we complete the preconditions sequence prior to playing early media?
  - This is 183, PRACK, 200 OK (Prack), UPDATE, 200 OK (UPDATE)
- Should the UAC not put a=sendrecv in the SDP when in fact it cannot receive early media?
  - This would be more explicit in the need to complete the preconditions.

Stephen James
sjames_1958 at yahoo.com<mailto:sjames_1958 at yahoo.com>

We are not princes of the earth, we are the descendants of worms, and any nobility must be earned.


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