H323 call failures when q931 multiplexing enabled

Discussion related to the ITU-T Recommendation H.323
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ucp
Posts: 2
Joined: Wed Jul 17, 2013 5:46 am

H323 call failures when q931 multiplexing enabled

Post by ucp » Wed Jul 17, 2013 6:35 am

Hi,
I have seen an issue at a customer site wherein h323 calls were failing when q931 multiplexing was enabled.
Customer has two sites A & B connected via h323 trunks.

When q931 multiplexing on:

site A -----------tcp(syn)---------------> site B
<---------tcp(syn/ack)-----------
----------tcp(ack)--------------->

---------h.225 setup------------>
<--------connect-----------------
----------h.225 setup----------> (not reaching site B)
----------h.225 setup---------->
----------h.225 setup---------->
----------h.225 setup----------> (re transmission)

As you can see, when q931 mux is ON, only the first setup is responded back with connect and call is successful.The subsequent h.225 setups are not seen in the sniffer logs captured at site B !!
Site A keeps re-transmitting h.225 setups and calls started to fail.

When q931 mux is OFF, no issues are seen.

The question is:

1) why does the tcp packet go through the packet network when q931 mux is OFF?
2) Does the router check the q931 mux parameter value before routing?
3) Are there any restrictions imposed by router when we reuse the same tcp sockets(q931 mux is ON)??

Thanks
Unni

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paulej
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Location: Research Triangle Park, NC, USA
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Re: H323 call failures when q931 multiplexing enabled

Post by paulej » Wed Jul 17, 2013 1:37 pm

Unni,

There is a field in H.225.0 called "multipleCalls". If the remote end does not set that to TRUE, then the calling device should not attempt to send subsequent SETUP messages over that connection.

Can you check to see what the response messages look like? Do they contain the multipleCalls parameter set to TRUE?

Routers in the path should have no effect on the H.225.0 signaling. However, if there are any H.323-related devices in the path (e.g., Gatekeeper or SBC or ALG in a NAT/FW), any of those devices might be the culprit. For example, if you have a NAT/FW device in the path that translates H.323 addresses, it might be failing to properly reset the "multipleCalls" value, but it might be discarding those additional SETUP messages. It's hard to say what is happening without seeing a debug trace at each node in the path. For sure, though, a vanilla router or switch in the path that knows nothing about H.323 should not affect signaling whatsoever.

ucp
Posts: 2
Joined: Wed Jul 17, 2013 5:46 am

Re: H323 call failures when q931 multiplexing enabled

Post by ucp » Wed Jul 17, 2013 5:29 pm

Thanks a ton Paul :)
I will check the response messages and let you know.

Thank you for your time..appreciate that.

Cheers
Unni

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