SBC Doesn't Forward ACK

Asset ID:	1-72-2244257.1
Update Date:	2018-03-01
Keywords:

Solution Type Problem Resolution Sure

Solution 2244257.1 : SBC Doesn't Forward ACK

Applies to:

Acme Packet 3820 - Version E-Cz7.3.0 to E-Cz7.3.0 [Release E-Cz7.0]
Information in this document applies to any platform.

Symptoms

It has been observed that SBC sometimes not forwarding the ACK which is sent for 200OK. We see in a trace that the SBC gets the ACK from UserA, the SBC forwards it to the SIP Server, the SIP Server returns the ACK to the SBC and now the SBC is supposed to send the ACK to UserB but it doesn't send it.

USER-A SBC SIP-SERVER USER-B

----INVITE------->
------INVITE----->
<-----INVITE-----

---------------INVITE------------->

                         <--------------180 RINGING----------
                          --------180 RINGING->
<---------180 RINGING-
                           <--------------200Ok---------------
                         --------200OK->
                         <---------200OK-

<------200OK------
-------ACK------->
                        ------ACK------->
                        <----ACK---------
                         -------ACK(DROPPED)--------------->
( THIS ACK was dropped by SBC DUE to duplicate branch i.d)

Cause

It has been observed when there was a call made we see normal transaction with Normal INVITE , 180 RINGING , 200 OK , ACK followed with BYE .

Where ACK "VIA" header is associated with a Branch I.D which has to be unique .

Via: SIP/2.0/UDP 10.10.10.10;branch=z9hG4bKcydzigwkX------->
Via: SIP/2.0/UDP 10.10.10.100:5060;branch=z9hG4bKvqbsoq00908detkku400.1

Now there was second call made where we see normal transaction with Normal INVITE , 180 RINGING , 200 OK except 'ACK' followed with BYE .But if we observed here ACK "VIA" header is associated with a Branch I.D which is same as in above call.

Via: SIP/2.0/UDP 10.10.10.10:5060;branch=z9hG4bK0kplcp0000hecrgpu100.1
Via: SIP/2.0/UDP 10.10.10.100;branch=z9hG4bKcydzigwkX-------->

Hence SBC dropped the ACK and stop it in getting forwarded to far end device (USER B). We can also observe the error in sipdebug log as show in below log which show DUP error.

Mar 10 13:06:07.246 [TRANS] (1) ACK 1 B<S-Completed> GOT DUP 4 but Call-ID does not match
Mar 10 13:06:07.246 [TRANS] (1) new=94944773-2668-4b2a-b2c6-b4736ee4e039
Mar 10 13:06:07.246 [TRANS] (1) old=c6bdd2f7-38a3-420e-b172-edabf6032d7c

Solution

As per RFC 3261,

The Via header field value MUST contain a branch parameter. This parameter is used to identify the transaction created by that request. This parameter is used by both the client and the server.

The branch parameter value MUST be unique across space and time for all requests sent by the UA.

The exceptions to this rule are CANCEL and ACK for non-2xx responses As discussed below, a CANCEL request will have the same value of the branch parameter as the request it cancels.

RFC 3261

Created from <SR 3-14469528151>

Applies to:

Acme Packet 3820
1-914CU

References

<NOTE:166650.1> - Working Effectively With Oracle Support - Best Practices
<NOTE:1598273.1> - What Log Files are Required when Opening a Service Request with Acme Packet Regarding Call Flow Issues

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