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SIP TLS processing causes Audio drop outs

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Some weeks ago we notice that ongoing calls have random audio drop outs.

I've managed to track this down to the PBX not sending audio for some 100 milliseconds, sometime up to one second. Upon closer inspection it turned out that this always happens when a new SIP TLS connection is established. Depending on the cipher, the delay between the TLS client hello and the server hello can be something from 100ms up to 800ms. This delay corresponds to audio dropout.

I can trigger this manually by using something like:

openssl s_client -connect pbx:5061

I have verified that this is not an issue with insufficient entropy, the kernel has sufficient.

System details:

  • Intel(R) Core(TM)2 CPU 6300 ¬†@ 1.86GHz)
  • PBX release 57.0
  • Debian 7.9 (wheezy)

I know the system is somewhat slow and the release is old. But it should still not need that long to build the TLS server hello and the SIP TLS processing should not block the normal audio path.

Has this been addressed in newer releases or will they have the same issue? Or in other words, would upgrading solve this issue?



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That problem was solved in version 61. Unfortunately this is no easy fix, porting it back to 57 is not an option. If you upgrade to 61.1 (Debian64 is available) it will also automatically convert the button profiles to the new button profile scheme. 

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