Application-Layer Gateway (ALG) for SIP
Keenetic routers feature Application-Layer Gateway (ALG) for the SIP protocol.
SIP ALG is an application-level gateway that allows VoIP voice traffic (RTP) to pass through a device with NAT (Network Address Translation) without hindrance. If SIP ALG is present and enabled in the router, there is no need to configure port forwarding for this type of traffic additionally.
To enable SIP ALG, install the corresponding component in the router:
As a rule, IP addresses of SIP telephony clients are assigned from a range of internal (local) addresses, and their IP addresses may overlap. In most cases, users connect to the network via a device with NAT (via a router). If this device does not support SIP ALG, a SIP server will not be able to distinguish between clients.
VoIP clients can be registered on a SIP server using one of the following methods to bypass NAT: Outbound proxy, STUN, or SIP ALG. However, if one of the first two methods is configured on a VoIP phone or other device with a SIP client located behind NAT, SIP ALG must be disabled on the device.
For unimpeded passage of SIP traffic through a device with NAT, it is necessary to translate the addressing in SIP packets. A SIP packet is examined at the application level, and IP addresses in it are modified. Thus, SIP ALG allows you to implement SIP telephony services in a network with NAT address translation without using auxiliary external devices.
Example 1. There is no device with NAT in the path of SIP packets.
Example 2. There is a device with NAT and SIP ALG support on the path of SIP packets.
In this case, the device with NAT (router) replaces the source IP address 192.168.1.33 in IP packets with its own WAN IP address 210.243.66.215.
Importante
All devices with NAT must support the SIP ALG mechanism. If there is a device with NAT on the path of SIP packets that does not support SIP ALG, then SIP telephony will not function.
