[SIPForum-discussion] Proper RFC 3264 Hold Behavior in SDP

[Andrea Frontini]

Hi,

according to rfc 3264 section 6.1:
"If a stream is offered as sendonly, the corresponding stream MUST be marked
as recvonly or inactive in the answer."
So the IP-PBX is certainly wrong in sending "a=sendonly" in the following
answers.

I can't tell why the B2BUA send an SDP with sendonly after REFER, to my
knowledge this parameter is often used to provide music on hold. If it wants
to re-estabilish the rtp flow between the endpoints it should probably put
a=sendrecv in the SDP

2009/7/21 Matt Vlasach <matt.vlasach at pacificswell.com>
>
> SIPForum,
> I have a B2BUA (SIParator) sitting between a Sonus SBC and ShoreTel PBX.
> During external transfers, one-way audio occurs.
> It has been found that prior to the REFER, reINVITES are sent by the PBX
setting the "a=" attribute in the SDP header to "inactive".  This behavior
is OK.
> The REFER occurs, which is handled by the B2BUA by sending reINVITES to
the Sonus SBC with the new SDP information.  Problem is after the REFER, the
B2BUA sends a reINVITE to the PBX with the SDP media attribute to
a=sendonly.  In turn, the PBX sends all following reINVITEs with the
a=sendonly attribute setting, resulting in one way audio.
> So where is the problem, and what is the correct behavior?  Below are two
ideas of correct behavior:
> 1. The PBX should send a=sendrecv independent of the B2BUA's a=sendonly
SDP header in the reINVITE because the PBX user wants to initiate a
bi-directional call after explicitly setting the a= header to inactive when
the call was put on hold.
>
> 2. The B2BUA should send a=inactive on reINVITE after REFER because that
is what the PBX sent to the B2BUA originally, not a=sendonly.  Sending
sendonly results in the PBX being "confused", causing it to simply mirror
the a= header on subsequent reINVITEs.
> I look forward to your help and thoughts!
> Best,
> Matt Vlasach
> Pacific Swell Networks, Inc.
> Innovating IP Telephony and Unified Communications Solutions
> http://pacificswell.com
> email | matt.vlasach at pacificswell.com
> tel | 310.598.3017
> fax | 310.426.9586
> twitter | http://www.twitter.com/mattvlasach
> sip | sip:matt.vlasach at pacificswell.com<sip%3Amatt.vlasach at pacificswell.com>

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