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How does SIP work behind NAT?
The devices behind the NAT use private IP addresses that are not routable in the public Internet. The Session Initiation Protocol (SIP) has established itself as the de facto standard for voice over IP (VoIP) communication. In order to establish a call, a caller sends a SIP message, which contains its own IP address.
What is NAT in VoIP?
A technology called Network Address Translation (NAT) is a solution that fixes this problem until IPv6 is widely deployed. NAT basically shares the public IP address of a router among multiple users through private IP addresses, largely extending IP space and allowing more devices to connect to the Internet.
What is Rport in SIP?
This extension defines a new parameter for the Via header field, called “rport”, that allows a client to request that the server send the response back to the source IP address and port from which the request originated.
Should I enable ALG on my router?
Its purpose is to prevent some of the problems caused by router firewalls by inspecting VoIP traffic (packets) and if necessary modifying it. Many routers have SIP ALG turned on by default. If the SIP proxy doesn’t provide a server-side NAT solution, then an ALG solution could have a place.
Where does the Nat come from on a router?
Allocates an IP address from a pool of inside global addresses. If a new packet arrives from yet another inside host, and it needs a NAT entry, but all the pooled IP addresses are in use, the router simply discards the packet.
How does the RTP port work in SIP?
The RTP ports go into a “listening” state whereby they can accept a new connection from a remote device. The IP address and port number for RTP are sent to the other device within the SIP INVITE message. This is part of the call setup. The end-points should then connect to each other on the advertised ports to establish a two-way audio connection.
How are SIP calls routed through the NAT router?
The SIP call’s packets are routed through NAT, their IP headers and ports being translated from 192.168.1.19 to the Internet IP and the remote end by the NAT router / gateway to the Internet.
Can a NAT device forward port to an IP phone?
If your IP phone specifies in the SIP INVITE that it will be listening for RTP on Port 10005 then it is easy to set up the NAT device to forward Port 10005 to the IP phone. Typically, most User Agents (IP phones) can be configured to use a preset range of port numbers for the RTP Media session.