[VoiceOps] 503 usage

Alex Balashov abalashov at evaristesys.com
Wed Dec 14 00:28:27 EST 2016

On Tue, Dec 13, 2016 at 09:14:13PM -0600, John S. Robinson wrote:

> There is another potential gotcha' especially with some of the bottom-tier
> service providers.   In addition to frequency of 503 response from your
> providers, watch the /average /and /peak/ interval from *100 TRYING* from a
> provider and 503 Service unavailable.  I have seen some providers take up to
> 20 seconds to decide that they just can't handle a call.   If a provider
> can't handle a call, it shouldn't take very long for them to "figure that
> out."

Aye, this kind of bad behaviour--sending a 100 Trying and then sitting
on the call for a while--is unfortunately common from the
bottom-feeders. In some cases it can indicate that a provider is being
DoS'd and don't have the capacity to process the call, but often it's
just poor service. 

It seems to be especially common on garbage international routes. When
they're not giving you a lump of FAS, they send 100 Trying and then give
you tons of PDD and uncertainty as they fail through 80 upstream vendors
on their side, only to ultimately tell you they can't complete the call.
A lot of their upstreams are equally trashy and do the same thing,
resulting in a rather worst-case scenario,.

The only solution there is to use aggressive PDD-based failover. Don't
let any vendor send you 100 Trying without some sort of progress
indication feedback after ~3 sec, and at most ~5 sec. If that happens,
CANCEL the call leg and move on. This is something that came to be
supported in our product at the request of many customers, even those
(the majority) who do traditional wholesale SIP termination and/or
retail or enterprise SIP trunking.

-- Alex

Alex Balashov | Principal | Evariste Systems LLC

Tel: +1-706-510-6800 (direct) / +1-800-250-5920 (toll-free) 
Web: http://www.evaristesys.com/, http://www.csrpswitch.com/

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