EMI/UCP bug

[Andreas Fink]

There's a bug in EMI which shows up under the following conditions:

a) Kannel is under heavy load.
b) the throughput of the EMI/UCP smsc is limited to a low value


The effect occurs like this:

The EMI/UCP SMSC does throttle to enforce the speed limitations. So the ACK's/NACK's will not come in immediately but a bit later.
For example if you have 5 messages a seconds set up, the EMI/UCP SMSC will take 200msg to get you the ACK/NACK.

Now Kannel does send additional packets down the same pipe because of windowing. And they get delayed even further.
The problem starts when those cumulated delays become larger than 60 seconds (or whatever timeout you define). Then Kannel presumes your SMS didnt get there and sends it again. Usually it was getting there but no ACK/NACk was received yet due to enforced speed limit of the SMSC. So the result of all this is SMS duplications in masses.

There's a speed limit factor in Kannel which lets it wait after a submission a certain time. However this one doesn't work because new incoming SMS on bearerbox do wake up the thread to continue. This is relatively easy to fix. 

But ther's another issue. Windowing. Usually if you set windowing to 4 or similar value, kannel should not submit more than 4 outstanding messages and then wait for ACK/NACK. In this scenario, Kannel should never ever get to the point where the ACK/NACK's cumulate to values above 60 seconds but only to 4x the throughput delay, in our 5msg/sec example  with windowing set to 4 that would be 0.8s.

So there's definitively something wrong. We have seen this problem in our setup and I've heard from other Kannel users having similar problems with other SMSC's from other carriers.

I will look into this bug more seriously now. If anyone has a few good hints, let me know.



Andreas Fink
Global Networks, Inc.

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