[SIPForum-discussion] Should ACK sent by SS1 before receiving from UAC?

siddharth sharma sid77sharma77 at gmail.com
Thu Aug 29 11:51:34 UTC 2013

Hi All,

I have one issue here.

Call scenario is like below.

SIP phone ---> SS1 ----> SBC ----> SS2 ---> MGW ---> PSTN user.

In the above scenario, call is originated by SIP phone and sent to PSTN
user. 180 ringing message was sent from SS2 and send to SS1 and SS1 sent it
to SIP phone.
PSTN user answered the phone. So, 200 Ok was sent by SS2 to SS1. Here, SS1
is sending ACK message first to SS2 and then it is sending 200Ok to SIP

During this time, connectivity was lost between SS1 to SIP phone. So, SS1
keep sending 200Ok message and after certain timer, call released by SS1 by
sending BYE message.

In this case, CDR was not generated at SS1 end because it was failed call
for SS1. But SS2 generated CDR because it has received ACK message.

I want to know is SS1 behaving properly?
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