Hi Dushyant,
It is because two participating entities in a session have their own
logical view of the session which is independent of each other's view.
Unlike a multicast session for which a SDP was primarily designed
where there is a single view of the sessions across all the
participating entities,in the offer - answer model of SIP the different
entities have different views of the session,from their perspectives.To
faciliate the recognition of this two distinct views we have two
different session ids.
Thus for example
two entities have the values 1 and 2 for the session ids,there is a
two-way speechpath between them,now A puts the call on hold,so he sends
a hold SDP
With session number 1 and version 1+old version number of A,B responds
with sending a new SDP with version number incremented by one which
changes the streams from sendrecv to send only or send only to
inactive,thus the two views of the SDP session have different states
now.So the o line is used to identify the current SDP state and to make
the necessary state transition.Other than the session id the other
fields of the o-line may not be unique for two entities.
Thus if we have a
single session id we will end up having a single view of the SDP
session,which will be inconsistent with the offer-answer model,which is
based on the different entities mutually agreeing on a set of codecs for
their purpose,the atrributes associated with this codecs like
directionality can be different in either direction..Each entity can
alter his view of the session after negotiation with the other end
without affecting the far ends view of the session.
Thanks and regards,
Shamik Saha
Project Engineer
Voice Protocols
Cell : +91-9886704155
-----Original Message-----
From: [email protected]
[mailto:[email protected]] On Behalf Of
Dushyant Dhalia
Sent: Tuesday, June 23, 2009 11:55 AM
To: [email protected]
Subject: [Sip-implementors] same session-id in "o=" line of offer-answer
SDP
Hi All,
I have a basic query regarding offer-answer model.
Why can't/don't UAC & UAS use the same session-id in "o=" line of the
SDP? I agree that RFC 4566 define session-id as <sess-id> is a numeric
string such that the tuple of <username>, <sess-id>, <nettype>,
<addrtype>, and <unicast-address> forms a globally unique identifier for
the session, which makes it different for UAC & UAS but i want to know
what advantage does give it over same session-id as we have same
call-id.
Dushyant P S Dhalia
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