Media before early media

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Andrew Benham

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Dec 31, 2021, 5:40:01 PM12/31/21
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I've been looking at some Wireshark capture files over the holiday
season, and I'm puzzled.
I have several VoIP accounts with Sipgate and Andrews & Arnold. I'm
making test calls to an 0800 PSTN number which provides a convenient
pre-recorded message.
I can see Sipgate working as I expect, but A&A are sending me RTP
packets *before* the "183 Session Progress" which enables early media.
In fact, they are sending me RTP packets before they've sent me an SDP
packet telling me their IP address and port - so they presumably aren't
expecting packets back.

On Sipgate, I see:

INVITE (with SDP) ->
-> 100 Trying
-> 183 Session Progress (with SDP)
Bidirectional RTP
-> 200 OK (with SDP)
ACK ->
Bidirectional RTP
BYE ->
-> 200 OK

With Andrews and Arnold, I see:

INVITE (with SDP) ->
-> 100 Trying
Loads of incoming RTP packets (seen 30 and 92 packets)
-> 183 Session Progress (with SDP)
Bidirectional RTP
-> 200 OK (with SDP)
ACK ->
Bidirectional RTP
BYE ->
-> 200 OK


Is this expected behaviour ? I don't think the phones I'm using
are doing anything with these "ultra early" RTP packets, but is
there some use for the packets which I'm not aware of ?

--
Andrew Benham Southgate, London N14, United Kingdom

The gates in my computer are AND OR and NOT, not "Bill"
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