In order to achieve efficient device fail-over to a backup/secondary Application Server, the SBC uses the Address Reachability Service (ARS) to determine if a server is reachable. This allows the SBC to “blacklist” a server IPv4 or IPv6 address if it is deemed unreachable, and address subsequent requests to that destination appropriately. In summary, the ARS interface provides following configurable capabilities:
Internal to the SBC, multiple call processing modules exist; traffic is shared across these modules. An IP Peer is blacklisted only when the same call processing module reaches the configured number of INVITE timeouts within the configured timeout duration (this is not an absolute value). For a single shot call scenario (i.e. lab testing), it may be necessary to gather additional timeouts to see an IP Peer get blacklisted. Once the peer is blacklisted, the same data is available across all call processing modules.
Similarly, when a peer is not blacklisted but is in the process of being blacklisted because of time outs, the ‘show’ command to check ARS Status may give inconsistent results depending upon the call processing module which response first to the command.
An example of CLI command to set an ARS profile is shown below. Each parameter is entered individually for demonstration purposes. It is not necessary to enter these commands separately.
% set profiles services sipArsProfile TestARSProfile blkListAlgRetryAfterType sip-503 % set profiles services sipArsProfile TestARSProfile blkListAlgTimeoutsNumTimeouts 45 % set profiles services sipArsProfile TestARSProfile blkListAlgTimeoutsType sip-invite % set profiles services sipArsProfile TestARSProfile blkListAlgorithms timeouts % set profiles services sipArsProfile TestARSProfile blkListAlgTimeoutsDuration 34 % set profiles services sipArsProfile TestARSProfile recoveryAlgorithm probe % set profiles services sipArsProfile TestARSProfile recoveryAlgTimerDuration 456 % set profiles services sipArsProfile TestARSProfile recoveryAlgProbeMethod sip-options % set profiles services sipArsProfile TestARSProfile recoveryAlgProbeInterval 78 % set profiles services sipArsProfile TestARSProfile recoveryAlgProbeNumResponses 6 % set profiles services sipArsProfile TestARSProfile recoveryAlgProbeDuration 546 % set profiles services sipArsProfile TestARSProfile blkListAlgNoRetryAfterDuration 10 % set profiles services sipArsProfile TestARSProfile blkListAlgNoRetryAfterNum503 5 % commit
Refer to SIP ARS Profile - CLI or Service Profiles - SIP ARS Profile (EMA) for configuration details.
In the example show status...sipArsStatus
command below, the endpoint IP address is an IPv6 address:
> show status addressContext ac-1 zone SipZone sipArsStatus sipArsStatus 2 0 { sigPortNum 2; endpointIpAddress FC00::3200:0:0:241:2; endpointIpPortNum 20302; endpointArsState blacklisted; endpointStateTransitionTime 2011-12-09T16:25:21.000009Z;
From 6.2.0 release onward, the SBC determines the SRS reachability only through the SIP ARS status. The pathCheckProfile
must be configured to determine the reachability of the SRS peers and treat failure response such as 501 as OPTIONS failure.
In the event of loss of connection with the Application Server (AS), the SBC switches to Local Survivable Mode and handles the call processing with limited functionality. It provides local registration support and limited local calling capabilities.
The feature uses the Address Reachability Service (ARS) functionality to detect and blacklist the AS. The ARS is enhanced to support per request type blacklisting on 503 retry-after response for the following SIP requests:
The ARS blacklists the server for the specific SIP request type for the duration specified in the retryAfter
header while allowing the other request types.
The SBC switches to the local survivable mode under the following conditions:
Registration in local survivability mode works as follows:
The SBC routes the calls in local survivability mode as follows:
Route local calls using user-configured alternate overflow routes that is added in ERE/PSX.
These routes may need digit/prefix manipulation with appropriate trunk group and profiles configured to support various call types and endpoint categories
Currently, the SBC uses a common ARS recovery algorithm for all the selected blacklist algorithms . If the trunk group had more than one peers and one of the peers did not support the selected recovery algorithm, it became difficult to recover that particular peer.
The SBC is enhanced with a new flag to select the recovery mechanism for each blacklist algorithm selected.
The SBC supports selecting of ARS recovery algorithms for each of the following blacklist algorithms.
Selection of ARS Recovery Algorithm
Blacklist Applied | Recovery Algorithm Type Per Blacklist | Description |
---|---|---|
INVITE Time out | probe / timer / none | Select Probe, Timer, or None as the recovery mechanism. |
503 without Retry-After | probe / timer / none | Select Probe, Timer, or None as the recovery mechanism. |