[SIPForum-discussion] How should B2BUA maintain the SDP.

Mon Feb 20 18:24:37 UTC 2012

Hi,

I would say it depends on the way your B2B is implemented. What I mean is
if the 3 dialogs towards TERM1, TERM2, TERM3 are mapped to only one towards
ORIG(Gateway model as per RFC3960) then there is not even a need to forward
the 180 ringing response received from TERM2 as ORIG has already received
180 ringing. And even if you send it, it should not have SDP as you didn't
receive it.

But if it is not the case (Application Server model as per RFC3960) then as
you said it should be forwarded to ORIG to tell about the new dialog
establishment and that too without SDP.

Regards,
Hemant

On Sat, Feb 18, 2012 at 10:56 PM, Pavel Gavronsky <kamm555 at hotmail.com>wrote:

> Thank you, Hemant,****
>
> ** **
>
> B2B in my case is ,actually, true softswitch, and all originator and
> terminator are subscribers. In our implementation some subscribers supply
> the ringback tone to the originator (let's not discuss if it is correctly
> to do). ****
>
> Your description is very helpful, but we cannot change the B2B logic and I
> still need to clear one point:  if after the 2nd INVITE the terminator
> replies with 180/183 (no SDP) – how should B2B act? For sure it should send
> 180/183 in backward direction, indicating the call progress to the
> ORIGINATOR. In our current implementation B2B does send 180/183 to the
> ORIGINATOR, but uses wrong SDP (despite the SDP is not included into the
> 180/183 from the TERM). I'd like to be sure that from the STANDARD point of
> view B2B has to pass 180/183 as is (specifically for that case – without
> the SDP). Am I right?****
>
> ________________________________________ ****
>
> Thank you in advance,****
>
> Pavel Gavronsky****
>
> ** **
>
> *From:* Hemant Kumar [mailto:uniqhemant at gmail.com]
> *Sent:* Saturday, February 18, 2012 4:46 PM
> *To:* Pavel Gavronsky
> *Cc:* Keyur Amin; discussion at sipforum.org
>
> *Subject:* Re: [SIPForum-discussion] How should B2BUA maintain the SDP.***
> *
>
> ** **
>
> Hi,
>
> I feel it depends on the exact usecase you are looking for. I am not sure,
> but I am just making a wild guess(ignore if it does not match with your
> usecase. Sorry to modify the name of parties involved) -
>
> 1. A calls B through B2B.
> 2. B2B redirects the call to media server to play ringback(in early media).
> 3. Once some timer expires @B2B, it terminates the dialog towards media
> server and ringback stops.
> 4. B2B initiates one more call leg towards B but B does not answer so B2B
> terminates that leg also.
> 5. Now B2B initiates one more call leg towards C (I think again media
> server to play some music).
>
> If atleast first 2 steps are correct then 180/183 received from media
> server(TERM1 in your case) must be forwarded to A otherwise ringback tone
> can not be played.
>
> As 180 ringing from B(TERM2) does not have any SDP then B2B can simply
> ignore it and there is no need to forward it to A.
>
> But with this, the following in your scenario will not work -
> *  >> B2BUA -> INVITE (to:TERM3, SDP=ORIG) (According to its internal
> logic B2BUA makes new call to TERM3)
>   >> 180 Ringing (SDP=TERM3) <- B2BUA <- 180 Ringing (SDP=TERM3) (Here the
> B2BUA correctly passes the SDP=TERM3)*
>
> as towards A, SDP offer-answer is already done. So once the call is
> terminated with B(TERM2), I suggest to do following changes in your flow  -
>
>  B2BUA -> INVITE (to:TERM3, no SDP)
>  B2BUA <- 180 Ringing (SDP=TERM3) (A new SDP offer)
>  UPDATE (to: ORIG, SDP=TERM3) <- B2BUA (Send Update to ORIG as a new SDP
> offer)
>  200 OK(SDP=ORIG) -> B2BUA (Receive ORIG's SDP in 200 for UPDATE)
>  B2BUA --> PRACK(to: TERM3, SDP=ORIG) (Send ORIG's SDP answer to TERM3 in
> PRACK)
>
>
> Regards,
> Hemant****
>
> On Wed, Feb 15, 2012 at 1:49 PM, Pavel Gavronsky <kamm555 at hotmail.com>
> wrote:****
>
> Thank you, Keyur,****
>
>  ****
>
> The reason for the SDP in our 180/183 from the TERMINATOR is to provide
> the RingBack tone from the remote party (early media).****
>
> For the case when the TERMINATOR doesn’t send the SDP in its 180/183,
> should the B2BUA use the "remembered" SDP value from the previous
> TERMINATOR leg or just pass the 180/183 (without SDP) unchanged to the
> ORIGINATOR?****
>
>  ****
>
> ________________________________________ ****
>
> Thank you in advance,****
>
> Pavel Gavronsky****
>
>  ****
>
> *From:* Keyur Amin [mailto:fossil0681 at yahoo.com]
> *Sent:* Tuesday, February 14, 2012 6:18 PM
> *To:* Pavel Gavronsky; discussion at sipforum.org
> *Subject:* Re: [SIPForum-discussion] How should B2BUA maintain the SDP.***
> *
>
>  ****
>
> SDP is required for the B2BUA from the terminator. It it doesnt get it
> 180/183, it must in 200 OK. The old SDP wont work because its a different
> end point. Unless the terminator is sending 183, you dont need SDP for 180.
> ****
>
>  ****
>
> *Thanks,*****
>
> *Keyur** **Amin*****
> ------------------------------
>
>  ****
>
>  ****
>
>  ****
> ------------------------------
>
> *From:* Pavel Gavronsky <kamm555 at hotmail.com>
> *To:* discussion at sipforum.org
> *Sent:* Monday, February 13, 2012 6:22 AM
> *Subject:* [SIPForum-discussion] How should B2BUA maintain the SDP.****
>
>  ****
>
> Hello, everyone,****
>
>  ****
>
> Setup:****
>
>  ****
>
> B2BUA – tandem SIP softswitch (tandem between softswitch_1 and
> softswitch_2)****
>
> ORIGINATOR: Subscriber connected to softswitch_1 ****
>
> TERMINATOR1: Subscriber connected to softswitch_1****
>
> TERMINATOR2: Subscriber connected to softswitch_2****
>
> TERMINATOR3: Subscriber connected to softswitch_1****
>
>  ****
>
> Scenario (only important messages shown):****
>
> Call comes from the ORIGINATOR and is routed to TERMINATOR1:****
>
>  ****
>
> INVITE (to:TERM1, SDP=ORIG) -> B2BUA -> INVITE (to: TERM1, SDP=ORIG)****
>
> 100 trying <- B2BUA <- 100 Trying****
>
> 180 Ringing (SDP=TERM1) <- B2BUA <- 180 Ringing (SDP=TERM1)****
>
> _____________B2BUA -> Cancel (According to its internal logic B2BUA
> releases the TERM1 leg in order to make new call to TERM2)****
>
> _____________B2BUA -> INVITE (to:TERM2, SDP=ORIG) (According to its
> internal logic B2BUA makes new call to TERM2)****
>
> _____________B2BUA <- 100 Trying****
>
> 180 Ringing (SDP=TERM1) <- B2BUA <- 180 Ringing (no SDP) ( ??? Is it
> correctly to provide the old SDP=TERM1 toward the ORIG in that case ???)**
> **
>
> _____________B2BUA -> Cancel (According to its internal logic B2BUA
> releases the TERM2 leg in order to make new call to TERM3)****
>
> _____________B2BUA -> INVITE (to:TERM3, SDP=ORIG) (According to its
> internal logic B2BUA makes new call to TERM3)****
>
> _____________B2BUA <- 100 Trying****
>
> 180 Ringing (SDP=TERM3) <- B2BUA <- 180 Ringing (SDP=TERM3) (Here the
> B2BUA correctly passes the SDP=TERM3)****
>
>  ****
>
>  ****
>
> Question:****
>
> If the 180/183 from the TERM leg does not include the SDP, should the
> B2BUA use the "remembered" old SDP value for the 180/183 toward ORIG leg?
> ****
>
>  ****
>
> ________________________________________ ****
>
> Thank you in advance,****
>
> Pavel Gavronsky****
>
>  ****
>
>
> _______________________________________________
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>
>
> _______________________________________________
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>
>
>
>
> --
> - Hemant****
>

-- 
- Hemant
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