SCA does not Support Alternate IP Fail-Over for the A-Leg (Doc ID 1558712.1)

Last updated on JANUARY 05, 2023

Applies to:

Symptoms

SCA (Session Control Agent) supports remote address redundancy if alternate addresses are configured for a SIP (Session Initiation Protocol) client.

The process is explained below:

1. Users can define a list of destination URLs to be monitored in the SCA Rules file.
2. The SCA sends a request message and retransmits it at intervals configured in the transaction timer.
3. The timer is canceled when an appropriate response is received before the timer expires.
4. If the timer expires and no response is received, the SCA will mark the interface as DOWN and notify the IN.
5. The SCA will send the message to the next alternate address provided in the User-defined list.
6. The SCA continue through the list until an address is successful or all URLs have been tried.
7. If all addresses fail, the SCA will notify the IN to release all active calls to the remote SIP entity and sends an error message to the A-party.

In the event of a remote address failure, the SCA will try alternate destination IP addresses defined for that client in the list of alternate destination URLs specified in a DNS entry or in the SCA Rules file. When the transaction timer expires the SCA will send the message to the next URL in the provided list. The SCA will continue through the list until an address is successful or all URLs are exhausted.

Here's an example of a successful call transmission to the B-party's alternate address:



This mechanism is described in the Session Control Agent Technical Guide (See Doc ID 1341844.1 for information on how to download the documentation). 

However, this mechanism is valid for the B-leg only. If a timeout is detected on the A-leg, the failover will never happen.

Cause

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