[SIPForum-discussion] [E] SIP Flow with a 200OK(without SDP) to the INVITE

Thu Jun 20 19:42:13 UTC 2019

Thanks a lot Tim,

Very clear.

Really good.

Regards,
Guillermo

De: timothy.dwight at verizon.com [mailto:timothy.dwight at verizon.com]
Enviado el: Thursday, June 20, 2019 2:02 PM
Para: Zuñiga, Guillermo; discussion at sipforum.org
Asunto: RE: [E] [SIPForum-discussion] SIP Flow with a 200OK(without SDP) to the INVITE

Guillermo,

This pattern of responses is valid but sometimes problematic.  Many VoLTE devices will respond in this way, which is recommended in GSMA’s IR.92 specification.

Handling of ringback has become more complicated over time.  In the early days of VoIP, we saw 2 main behaviors.

1)      Devices would respond to an INVITE with a 180 without SDP, then provide the SDP answer in the 200 OK response to the INVITE.   A UAC would generate ringing locally when it received a 180.

2)      Switches would [usually] respond to an INVITE with a 183 with SDP, and later send the 200 OK response without SDP.  The UAS would send ringback tone as early media, and the UAC would play that media to its user.

In mobile networks this was not optimal.  Option #1 provides the SDP answer very late, which can cause audio clipping because the access networks may not have time to establish a bearer to carrier the audio before the audio starts to arrive.  Option #2 is expensive since media has to be played across the network in order for the caller to hear ringing;  and it isn’t supportable unless the UAS is a switch.  So, a 3rd option was introduced.  That 3rd option is what you refer to below.  The 183 response with SDP is sent early in the call flow (before beginning to alert the called user) in order to communicate the SDP answer to both access networks.  The information in the SDP answer is needed in order to create the bearer.  Sending it “early” allows time for both access networks to allocate resources.  A new header called P-Early-Media (RFC-5009) may be added to the 183, to clarify to the UAC whether to expect to subsequently receive early media.  The default is “no”, so usually that header is not sent in the scenario you describe.  The 180 response is sent by the UAS when it begins to alert its user (i.e., begins to ring).  Unless the 183 or 180 (or some other message such as an UPDATE) contained a P-Early-Media header “authorizing” early media, he UAC is expected to generate ringback locally when it receives the 180.

A problem has been seen in some networks, when the UAC is an older device that follows “the old rules”.  Under the old rules the UAC could expect to receive ringback tone as early media, after receiving a 183 with SDP.  Actually that expectation is based on a misunderstanding of the Gateway model in RFC-3960;  but unfortunately a common one.  The bottom line is that there are devices that will in the scenario you describe that will provide the calling user no ringback indication.

Some networks implement interworking to resolve this.  You can probably imagine how to do that.  I’ll warn you, though, that there are some complicated corner cases.

Good Luck!

Tim

From: discussion-bounces at sipforum.org [mailto:discussion-bounces at sipforum.org] On Behalf Of Zuñiga, Guillermo
Sent: Thursday, June 20, 2019 10:26 AM
To: discussion at sipforum.org
Subject: [E] [SIPForum-discussion] SIP Flow with a 200OK(without SDP) to the INVITE

Hi fellows,

I would like to know the following:

I am having the following flow and I would like to know if is a valid flow and if the remote SIPGW should be able to handle or complete this dialog when receive a 200OK without SDP to the INVITE.

Flow:
A ->INVITE with SDP
B<-TRYING
B<-183 WITH SDP
A->PRACK
B<-200OK TO THE PRACK
B<-180 RINGING WITHOUT SDP
B<-200OK WITHOUT SDP TO THE INVITE.
A->ACK TO THE 200OK WITHOUT SDP TO THE INVITE.

My question comes, cause I am having audio issue with the client is calling us and here I would like to know if could be related that I am sending the 200OK without SDP. Or he should be able to complete the dialog cause previous to the 200OK without SDP I sent a 183 with SDP?

Please let me know any comments or where in the RFC I  can look for that describe this behavior.

Regards,
Guillermo

Guillermo Zuniga

Especialista de Soporte Técnico

Gerencia de Soporte Técnico

P:

+507 263-6671

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