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Version 5.1 |
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The "basic" communication model assumes that endpoints can communicate directly, i.e. that all "entities", both clients (phones, softphones, PBX applications) and servers have "real" Internet IP Addresses. In this situation the Server is needed only to establish a call. Media data and (in case of SIP) in-call signaling requests are sent directly between the endpoints:
In the real life, many clients are located in remote LANs ("behind a NAT"), or in different LANs so they cannot communicate directly. CommuniGate Pro supports automatic "NAT traversal" for the standard-based Real-Time communications.
The CommuniGate Pro SIP and XIMSS Modules detect the session initiation requests that are sent from one side of NAT to the other side (a request from a LAN client to a party on the Internet/WAN and vice versa). In this case, the Server uses some local server port (or a set of ports depending on the media protocol(s) used) to build a media stream proxy. The Server then modifies the session initiation request to direct the traffic from both sides to that proxy.
The media proxy relays media data between the "LAN leg" and the "WAN leg" of the media connection:
The CommuniGate Pro SIP and XIMSS Modules detect session update (SIP re-INVITE) and session close (SIP BYE) requests and update and remove the Media Proxies accordingly. The time-out mechanism is used to remove "abandoned" Media Proxies.
The CommuniGate Pro provides NAT proxy services for:Note: If you need the Media Proxy functionality, make sure that the LAN and NAT data is specified correctly on the LAN IPs settings page.
Note: The Server automatically builds Media Proxies when it relays requests from IPv4 addresses to IPv6 addresses and vice versa.
The CommuniGate Pro SIP and XIMSS Modules also provide the "far-end" NAT traversal capabilities by detecting requests
coming from clients located behind remote Firewall/NATs.
The Modules add appropriate Record-Route and Path headers to these requests and they build
Media Proxies to relay traffic to and from those clients.
Note: modern SIP clients support various NAT traversal methods (STUN, etc.). Many of these implementations are quite buggy, so it is often more reliable to switch the client-side NAT traversal methods off, and rely on the CommuniGate Pro SIP Module far-end NAT traversal capabilities instead.
Note: due to the nature of the TCP protocol and the Firewall concept, it is not possible (in general) to open a TCP connection to a client behind a far-end NAT ("near-end" NAT configurations do not have this problem). This means that clients behind a far-end NAT cannot initiate TCP (T.120) sessions.
To solve this problem, you may want to:The CommuniGate Pro SIP Module can be used as an "Edge Service" or ALG ("Application Level Gateway"), providing NAT traversal functionality for users registered on other servers.
The CommuniGate Pro SIP Module detects "media loops", when a call placed from within LAN is proxied to WAN, and then proxied back to the same LAN. In this case the Media Proxies are removed, eliminating unnecessary overhead, and allowing SIP clients to communicate directly within one LAN, while proving registrar services outside that LAN.
The SIP module can detect much more complex loop cases, either avoiding Media Proxies altogether, or minimizing the number of Media Proxies used.