[VoiceOps] No RTP stream during 183 w/SDP via Peerless?

Alex Balashov abalashov at evaristesys.com
Wed Jun 3 15:22:12 EDT 2015

On 06/03/2015 03:02 PM, April Jones wrote:

> SDP on 183 is always missing the a=sendrecv attrib. RFC is kind of
> unclear of whether sendrecv is necessary, but it feels like sendrecv
> is assumed. Is that correct?

Correct. Per RFC 3264 Section 5.1 ("Unicast Streams"):

    If the offerer wishes to both send and receive media with
    its peer, it MAY include an "a=sendrecv" attribute, or it
    MAY omit it, since sendrecv is the default.

But note, as I said in my previous reply, that any SDP answer present in 
the 183 is merely a declaration of how the terminating gateway would 
like to receive media, and says nothing about sending it. In the same 

    For recvonly and sendrecv streams, the port number and
    address in the offer indicate where the offerer would
    like to receive the media stream.

Moreover, RFC 3960, which deals with early media in SIP explicitly, states:

    Therefore, by only looking at the SIP signalling, a UAC cannot be
    sure whether or not there will be early media for a particular
    session.  The UAC needs to check if media packets are arriving at a
    given moment.

-- Alex

Alex Balashov | Principal | Evariste Systems LLC
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