Behavior during Service unavailable

A proxy server is considered Active once the system has received a response to an INVITE, REGISTER or OPTIONS.

In the case of the proxy server responding with 503 - Service Unavailable, it should be considered Active - In Maintenance. In this case, the following should occur:

• If the response 503 - Service Unavailable was in response to an INVITE request:

  • If calls are tied to registrations (Calls Route via Registrar enabled) and there are other proxies available, the tied registrations should issue an Un-REGISTER and try to REGISTER with a different proxy. The call should fail with cause = Temporary Fail.

  • If calls are not tied, the INVITE should be immediately tried to a different proxy.

• If the response 503 - Service Unavailable was in response to a REGISTER request:

  • If there are other proxies available, this registration only should issue an Un-REGISTER and try to REGISTER with a different proxy.

  • If Explicit DNS Server(s) are configured, a DNS request should be sent out to see whether the proxy server has disappeared from those being offered.

An Active-InMaintenance proxy server should not be used for a new transactions (INVITE or REGISTER) until:

• There is a change in DNS responses indicating the proxy has become active.

• The configuration does not leave any better option available. In this case, there should be a throttle so that no more than 5 failures (without successes) in 1 minute should be allowed.

• A configuration merge has occurred where the ITSP Proxy Address has been changed.

• 10 minutes has expired.

Behavior during Not Responding

A proxy server that is not-responding (UDP) is indicated when 3 requests are sent and no replies are received. This would normally occur during a single INVITE transaction.

Consideration should be given whether this is caused by a local network fault or is caused by the Proxy being out of service. Since it is likely to be local, no action should be taken unless traffic is received from an alternative proxy while this proxy is actually not responding. The state should be "Possibly non responding".

If explicit DNS servers are configured, a DNS request should be sent out to see whether this Proxy server has disappeared from those being offered.

If possible, an alternative proxy should be stimulated simultaneously with stimulating the suspect server.

The server should be considered non-responding if it is persistently non-responding while other proxies are responding or if it is non-responding and has disappeared from the DNS advertisement.

While in the "possibly not responding" state, it would be better to send an INVITE to an alternative proxy while simultaneously sending any appropriate message to this proxy. This will help to resolve whether it is really not responding rather than there being local network problems. However, there is no requirement to blacklist the proxy.

Once in the "definitely not responding" state:

• If there are other proxies available: this registration only issues an Un-REGISTER, and try to REGISTER with a different proxy. Calls do not automatically clear.

• If a SIP message is received from it, the state should immediately go"Active".

• This proxy should be blacklisted unless there are no better options available. While blacklisted, only one transaction per 10 minutes is allowed.

• Even if not blacklisted, there should be a throttle so that no more than 5 failures (without successes) in 1 minute should be allowed.

Configuration settings

The ITSP Proxy Address and Calls Route via Registrar settings are mergeable. Changing the remaining settings requires a “merge with service disruption”. When the configuration file is sent to the system, the SIP trunk is restarted and all calls on the line are dropped.

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