The outbound service is used for initial registration between the branch PBX and the SIP server as well as making all outbound calls to remote endpoints. Outbound Service and Config Auto Generation The plugin automatically generates the ingress service and associated routes using the egress service and plugin configuration. The user is responsible for configuring the egress service and corresponding routes. For each of the inbound and outbound calls the plugin requires two services - an ingress service to route the traffic to the KNI and an egress service to route the traffic to the other SIP endpoint. The SSR uses kamailio 3 to then fix all SIP messages and perform additional routing to manage both inbound and outbound calls. The sip-alg plugin operates on the concept of SFC where all traffic is routed using KNIs to and from Linux. For the outbound calls, the proxy-ip is used to replace the private branch PBX IP address with a non-private address that represents the branch from the perspective of the data center. This information is also useful for fixing all the relevant IP addresses in the SIP messages. In the above example, the mappings are a way to indicate that any inbound calls on the 172.16.2.128 from the remote endpoint will be routed to the 192.168.2.128 address. The ip-mappings on the other hand provide the translation to the private IP address that can be used to reach the branch PBX. The proxy-ip represents a non-private address for the branch PBX that the remote endpoint (such as a data center server) can use for all inbound and outbound calls. There is a limit of 10 sip-alg configurations per router.
The 128T-sip-alg plugin supports multiple branch PBXs by letting the user configure multiple sip-alg instances on the router.
The SIP ALG function can be enabled by installing and configuring the 128T-sip-alg plugin. This enables both signaling and media traffic to traverse the NAT and makes the communication possible between the client behind the NAT and the remote SIP endpoint(s). When the SSR is present at the edge of the NAT boundary, it is capable of performing a Application Layer Gateway (ALG) function for SIP protocol by doing packet inspection and re-writing the private IP address information in the SIP messages for both SIP headers and Session Description Protocol (SDP) 2. When the SIP clients are behind NAT this poses a particular challenge since the addresses in the SIP messages for such clients will be private IPs which are not reachable beyond the NAT boundaries. SIP is a text based protocol and exchanges IP address information within its messages as a means to establish signaling and media sessions between endpoints. The Session Initiation Protocol (SIP) 1 is a signaling protocol used for initiating, maintaining, and terminating real-time sessions that include voice, video and messaging applications.
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