Subject: [Sipping] "Early Media"

From: "Yadavalli, Satyamurthy" <[email-masked]>
Date: Sun, 3 Nov 2002 00:55:33 -0500

Hi,
Would like to know how to achieve the following using any of the SIP related
RFCs:

UA1 invites UA2. Say UA2 generates a local ringing tone (or some audio) to
alert its user. However, UA2 woud like to feed some (ringing) audio back to
UA1 (in addition to the 180 RINGING response). And also say UA2 may like its
early media to be overriding any locally generated ringback that UA1 may be
generating to its user about the call in progress.

This 'early media' may have its own characteristics such as codec
requirements etc., different from those of the final dialogue. How (ofcourse
via an SDP but attached to which SIP response: 180?) are these to be
described by UA2 to UA1 so that UA1 may load the necessary codecs etc., so
as to be able to present the early media (to be received from UA2) to its
user. And also how to indicate when the audio path (RTP) for this early
media is to be established/released.

Is there any RFC that describes how to provide this kind of 'subdialogue'
before the actual SIP dialogue is established? A quick glance at the UPDATE
RFC appeared to somewhat support this kind of requirement but appeared (at
first glance) to specify the main dialogue itself and didnot appear to
clearly say when to establish/release this 'subdialogue'.

Appreciate any help in this regard,
Thanks,
Satya
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