Hi,
The following are a few RFC snippets. As shown within RFC 3725 section
10.2, 3PCC is one of the users of re-INVITE without SDP. If a device does
not support re-INVITE without SDP or doesn't offer all codecs that the UA
is currently willing and able to use, it hinders interoperability during
3PCC situations. For instance within RFC 3725 figure 13, the Media Server
may not be unable to communicate with the Pre-Paid User if the prepaid
device limits the offered codecs to be whatever was recently negotiated
with the Called Party.
RFC 3261 section 14.2:
"A UAS providing an offer in a 2xx (because the INVITE did not contain an
offer) SHOULD construct the offer as if the UAS were making a brand new
call, subject to the constraints of sending an offer that updates an
existing session, as described in [13] in the case of SDP. Specifically,
this means that it SHOULD include as many media formats and media types
that the UA is willing to support."
RFC 6337 section 5.1:
"A UA should send an offer that indicates what it, and its user, are
interested in using/doing at that time, without regard for what the other
party in the call may have indicated previously. This is the case even
when the offer is sent in response to an INVITE or re-INVITE that contains
no offer. (However, in the case of re-INVITE, the constraints of
[RFC3261] and [RFC3264] must be observed.)"
RFC 6337 section 5.2.5:
"When the new offer is sent in response to an offerless (re-)INVITE, it
should be constructed according to the General Principle for Constructing
Offers and Answers (Section 5.1 ): all codecs the UA is currently willing
and able to use should be included, not just the ones that were negotiated
by previous offer/answer exchanges. The same is true for media types --
so if UA A initially offered audio and video to UA B, and they end up with
only audio, and UA B sends an offerless (re-)INVITE to UA A, A's resulting
offer should most likely re-attempt video, by reusing the zeroed "m=" line
used previously."
RFC 6337 section 5.3:
"If a UA has occasion to send another offer in the session, without any
desire to change the hold status (e.g., in response to a re-INVITE without
an offer, or when sending a re-INVITE to refresh the session timer), it
should follow the "General Principle for Constructing Offers and Answers"
(Section 5.1). If it previously initiated a "hold" by sending
"a=sendonly" attribute or "a=inactive" attribute, then it should offer
that again. If it had not previously initiated "hold", then it should
offer "a=sendrecv" attribute, even if it had previously been forced to
answer something else. Without this behavior it is possible to get "stuck
on hold" in some cases, especially when a 3pcc is involved."
-----Original Message-----
From: sip-implementors-boun...@lists.cs.columbia.edu [mailto:sip-
implementors-boun...@lists.cs.columbia.edu] On Behalf Of Ramesh Babu
Kuppili
Sent: Monday, January 18, 2016 3:06 PM
To: sip-implementors@lists.cs.columbia.edu
Subject: [Sip-implementors] Codec negotiation when incoming re-INVITE
has no
SDP
Hello SIP experts,
I have a question about codec negotiation when a UA receives re-INVITE
with
no SDP.
Lets say, the codec negotiation between UA and Proxy ended up with G711
and
RFC 2833 (m=audio 60146 RTP/AVP 0 101). However after the call gets
established, if the proxy sends re-INVITE with no SDP, what should be
the
codec list in the "200 OK" response for the reINVITE. My proxy vendor
claims
that the "200 OK" should contain the list of the codecs supported by the
UA.
Not the codec list already negotiated in the dialogue. Can someone
point me
in the right direction for this scenario as to what the RFC says and if
possible a section number in the RFC.
UA Proxy
----------------------------------------->
(INVITE SDP: codecs 0 18 101)
<------------------------------------------
(200 OK SDP: codecs 0 101)
-------------------------------------------->
ACK (no SDP)
<-------------------------------------------
(re-INVITE no SDP)
---------------------------------------------->
(200 OK SDP: codecs ?????)
- ramesh
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