[SIPForum-discussion] Regarding Early Media and Preconditions.

[Aniella Juverdeanu]

Hi,

So MGCF is receiving the Invite with SDP as below - that means G.711, right?
a=sendrecv is just extra information for the default send/receive (when a=sendrecv is missing) unless otherwise specified by a=sendonly or a=recvonly.

The UAC is just presenting in INVITE SDP its media information for the UAS to know where (its media server) to send the packets. The early media will be open when UAS will reply with 183 and SDP with information about  the media server that plays the announcement (sure PRACK will follow if set between UAS and UAC). This SDP could open one direction only from UAS Media end point to UAC media end point or just leave it two way.
UPDATE is necessary only if UAS indicated one SDP in 183 and for whatever reason that SDP changed, when the new SDP has to be communicated (could be another 183 as well - depends on vendor).

Regards,
Aniella


From: discussion-bounces at sipforum.org [mailto:discussion-bounces at sipforum.org] On Behalf Of Stephen James
Sent: August 13, 2012 1:17 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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