Hi Nitin,

ACK is mandatory for 200 response as it guarantees that UAC/caller has received 
your 200OK.
More answers inline!!

Thanks,
Puneet

-----Original Message-----
From: sip-implementors-boun...@lists.cs.columbia.edu 
[mailto:sip-implementors-boun...@lists.cs.columbia.edu] On Behalf Of NK
Sent: Tuesday, August 8, 2017 5:22 AM
To: sip-implementors@lists.cs.columbia.edu
Subject: [Sip-implementors] No ACK for 200 OK

Dear All,

I need the seniors help to understand the problem I am facing right now.
The problem is where one of my client doesn't send the ACK for 200 OK in the 
correspondence of initial invite.

But at the same time when after sending multiple 200 OK, my switch sent the BYE 
then immediately client UA replied with 200 OK.

Doubt I have.

1) Is that really mandatory to send ACK for 200 OK to start the call?
Because once my switch sent the 200 OK then logically the billing is started 
from my end, doesn't matter whether I received ACK or not?
>>>[Puneet Kumar] In case of UDP network you can't guarantee that 200 is 
>>>received by your UAC. Starting billing immediately after sending 200 is 
>>>logically incorrect as caller might have not received this call answer 
>>>indication.
On non-receipt of ACK your switch should retransmit 200 again. <<<

2) Even though if my understanding is not correct on point 1, but then client 
UA replied to BYE message immediately then how my switch will treat this, is 
that treat this as a connected call with duration?
>>>[Puneet Kumar] BYE might have used a different path as your 200. Can you 
>>>check on that? <<<

3) Do we have any document, I did research but cannot find any clear document 
who can state this along with 5407 etc.

Thank you in advance.

Regards,
Nitin
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