Session Agent Groups
Session agent groups contain individual session agents. Members of a session agent group are logically equivalent (although they might vary in their individual constraints) and can be used interchangeably. You can apply allocation strategies to session agent groups.
Examples of session agent groups include the following:
- application server cluster
- media gateway cluster
- softswitch redundant pair
- SIP proxy redundant pair
- gatekeeper redundant pair
Session agent group members do not need to reside in the same domain, network, or realm. The Oracle® Enterprise Session Border Controller can allocate traffic among member session agents regardless of their location. It uses the allocation strategies configured for a SAG to allocate traffic across the group members.
Allocation strategies include the following:
| Allocation Strategy | Description |
|---|---|
| Hunt |
Oracle® Enterprise Session Border Controller selects the session agents in the order in which they
are configured in the SAG. If the first agent is available, and has not
exceeded any defined constraints, all traffic is sent to the first agent.
If the first agent is unavailable, or is in violation of constraints, all traffic is sent to the second agent. And so on for all session agents in the SAG. When the first agent returns to service, the traffic is routed back to it. |
| Round robin | Oracle® Enterprise Session Border Controller selects each session agent in the order in which it is configured, routing a session to each session agent in turn. |
| Least busy | Oracle® Enterprise Session Border Controller selects the session agent with the least number of active sessions, relative to the maximum outbound sessions or maximum sessions constraints (lowest percent busy) of the session agent. |
| Proportional distribution | Session agents are loaded proportionately based upon the respective maximum session constraint value configured for each session agent. |
| Lowest sustained rate | Oracle® Enterprise Session Border Controller routes traffic to the session agent with the lowest sustained session rate, based on observed sustained session rate. |
You apply allocation strategies to select which of the session agents that belong to the group should be used. For example, if you apply the Hunt strategy session agents are selected in the order in which they are listed.
Request URI Construction as Sent to SAG-member Session Agent
The Oracle® Enterprise Session Border Controller constructs the request URI for a session agent selected from a session agent group by using the value of the selected session-agent target. This default behavior enables features such as trunk groups and ENUM to work properly. However, care must be given when the hostname parameter is not a resolvable FQDN. The sag-target-uri=<value> option can be used to overcome the default behavior.
- ip – request URI constructed from session-agent, ip-address
- host – request URI constructed from session-agent, hostname
This option is global and is configured in the sip-config configuration element.
Request URI Construction as Forwarded to SAG-member Session Agent
The Oracle® Enterprise Session Border Controller constructs the request URI for a session agent selected from a session agent group by using the session-agent, hostname value of the selected session-agent target. This default behavior enables features such as trunk groups and ENUM to work properly. However, care must be given when the hostname parameter is not a resolvable FQDN. Use the sag-target-uri=<value> option to override the default behavior.
The value can be set to either:
- ip - request URI constructed from session-agent, ip-address
- host - request URI constructed from session-agent, hostname
This option is global and is configured in the sip-config configuration element.
SIP Session Agent Group Recursion
You can configure a SIP session agent group (SAG) to try all of its session agents rather than to the next-best local policy match if the first session agent in the SAG fails.
With this feature disabled, the Oracle® Enterprise Session Border Controller performs routing by using local policies, trunk group URIs, cached services routes, and local route tables. Local policies and trunk group URIs can use SAGs to find the most appropriate next-hop session agent based on the load balancing scheme you choose for that SAG: round robin, hunt, proportional distribution, least busy, and lowest sustained rate. When it locates a SAG and selects a specific session agent, the Oracle® Enterprise Session Border Controller tries only that single session agent. Instead of trying other members of the SAG, the Oracle® Enterprise Session Border Controller recurses to the local policy that is the next best match. This happens because the Oracle® Enterprise Session Border Controller typically chooses a SAG based on the fact that it has not breached its constraints, but the Oracle® Enterprise Session Border Controller only detects failed call attempts (due to unreachable next hops, unresolved ENUM queries, or SIP 4xx/5xx/6xx failure responses) after it has checked constraints. So the Oracle® Enterprise Session Border Controller only re-routes if there are additional matching local policies.
When you enable SIP SAG recursion, the Oracle® Enterprise Session Border Controller will try the additional session agents in the selected SAG if the previous session agent fails. You can also set specific response codes in the SAG configuration that terminate the recursion. This method of terminating recursion is similar to the Oracle® Enterprise Session Border Controller’s ability to stop recursion for SIP interfaces and session agents.
Session agents are selected according to the strategy you set for the SAG, and these affect the way that the Oracle® Enterprise Session Border Controller selects session agents when this feature enabled:
- Round robin and hunt—The Oracle® Enterprise Session Border Controller selects the first session agent according to the strategy, and it selects subsequent session agents based on the order they are entered into the configuration.
- Proportional distribution, least busy, and lowest sustained rate—The Oracle® Enterprise Session Border Controller selects session agents based on the list of session agents sorted by the criteria specified.
You can terminate recursion based on SIP response codes that you enter into the SAG configuration. You can configure a SAG with any SIP response code in the 3xx, 4xx, and 5xx groups. Since you can also set such a list in the session agent configuration, this list is additive to that one so that you can define additional codes for a session agent group with out having to repeat the ones set for a session agent.
