[Freeswitch-dev] Handling of multiple 183 responses with different SDP

Irina Ivanova i.ivanova at mastervoice.it
Wed Oct 27 04:15:03 PDT 2010

Hi! We are getting the same problem as described in the jira ticket with 
key FS-715:

Our provider is also sending us more than one 183 Session progress 
response, each of them with SDP where destination IP address is the same 
but the UDP ports are different.

If is needed I can provide the pcap file where you can see that after 
receiving of second 183 Session progress response (with updated SDP) 
Freeswitch continues to listen on an old port. As a consequence, no 
audio can be heard.

Our provider assures us that sending of more than one 183 Session 
progress responses is completely standard behavior that does not violate 
SIP RFC 3261 and is handled correctly by any ATA VoIP.
We verified the Asterisk behavior in this case, and can say that it 
handles receiving of multiple 183 Session progress responses without any 
problems and changes the UDP port each time it gets changed in session 
description inside of 183 response.

Our provider gave us a reference to SIP RFC 3261:

13 Initiating a Session
13.1 Overview
"Before sending a final response, the UAS can also send provisional 
responses (1xx) to advise the UAC of progress in contacting the called user.
After possibly receiving one or more provisional responses, the UAC will 
get one or more 2xx responses or one non-2xx final response."

"one or more provisional responses" was underlined by our provider and 
in addition to this was said that there is no any place in RFC from 
which you can conclude that you can not send more than one SIP 183 
Session Progress response.

We also found a document regarding 183 response:

Here are some references to the parts talking about temporary media and 
session description change:

4.2. Change of Temporary Media

   After a temporary media stream has been established, its parameters
   can be changed by sending further provisional responses that also
   contain session descriptions. Upon receipt of such a response, the
   client MUST immediately cease transmission of media relating to the
   old temporary stream. As before, the new temporary media stream is
   established after acknowledgement of the provisional response.

   Provisional responses which contain no session description SHOULD NOT
   have an effect on any currently established temporary media stream.

5.11.7 Caller Receives 183 Response

   When the calling UA receives a 183 response that contains a session
   description and an indication that the session description is for
   early media, it SHALL setup the associated media session and present
   any media received from the called UA to the user.

Again nothing is said about multiple 183 responses specifically, only 
about multiple provisional responses in general.
So, is there anything that can be done to resolve this issue?

Distinti saluti

Irina Ivanova
Settore Sviluppo MasterVoice

tel: +39 0522 1846007
fax: +39 0522 331673
mob: +39 334 6449290
e-mail: i.ivanova at mastervoice.it
web: www.mastertraining.it - www.registroelettronico.com

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