[SIPForum-discussion] which timer UAC start on 491 response ?

[Ivan, Martin]

I attached pcap file taken from this call flow. I was talking about incoming call from tdm gateway via ip2ip SBC [b2bua(1)] which is ringing on phone(1) registered on b2bua(2).
 
tdm/gw------------b2bua(1)-------------------b2bua(2)---------------phone(1)
 
Both b2bua's are rtp proxies.
If pickup feature is activated on other phone(2) , phone(2) sends invite with feature access code *22 to  b2bua(2) and it receives 200 OK with SDP of tdm/gw.
b2bua(2) then send 200 OK SDP of phone(2) to b2bua(1) and b2bua(1) confim it with ACK.
Now b2bua(1)  should send 200 OK(offer)  but due to race condition with received re-INVITE  (time between received 200 OK SDP and just received re-INVITE is 5ms.) 
 
Vendor which implemented b2bua(1) says  b2bua(2) sends too early re-INVITE message. I wanted them to send  ACK later from b2bua(2) , but they were unable to do it  :-(
That is why they proposed another solution with 491 response. If min time interval for sending 491 is max 1sec from b2bua(1)  and time interval is min. 2.1 seconds on b2bua(2) to send second time re-INVITE , then pickup feature takes plus 3 seconds more than usually. If randomely chosen value from T timer whould be taken from T2 , i could adjust it to be lower. The point is to make pickup feature as fast as it could.
 
Martin
 
 
 
 
 

________________________________

From: AMIT ANAND [mailto:amiit.anand at gmail.com]
Sent: Sun 6/15/2008 8:26 AM
To: Ivan, Martin
Cc: discussion at sipforum.org
Subject: Re: [SIPForum-discussion] which timer UAC start on 491 response ?


What will be the call flow in this case can you please explain a little.
 
Amit Anand

 
On 6/13/08, Ivan, Martin <martin.ivan at siemens.com> wrote: 

	Taken From 3261

	"If a UAC receives a 491 response to a re-INVITE, it SHOULD start a
	   timer with a value T chosen as follows:
	 
	      1. If the UAC is the owner of the Call-ID of the dialog ID
	         (meaning it generated the value), T has a randomly chosen value
	         between 2.1 and 4 seconds in units of 10 ms.
	 
	      2. If the UAC is not the owner of the Call-ID of the dialog ID, T
	         has a randomly chosen value of between 0 and 2 seconds in units
	         of 10 ms."

	 

	 

	Is randomly chosen value taken from interval of sip T2 timer which is 4s ?

	 

	 

	Regards,

	 

	Martin Ivan

	Senior Systems Engineer

	 

	Siemens Enterprise Communications, a.s.

	Stromová 9

	831 01 Bratislava

	Slovak Republic

	 

	 

	 

	 


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