lightser.blogg.se - Application level gateway is also known as

The main problem is the poor implementation at SIP protocol level of most commercial routers and the fact that this technology is just useful for outgoing calls, but not for incoming calls:

Replacing SDP media address with public IP and port.

Replacing “Contact” with mapped public IP and port.

Replacing IP in “Via” header with the public IP and port.

The ALG has fixed the NAT related problem by: The same INVITE modified by the ALG router: Anyway some ALG implementations also change this value. The text in blue doesn’t need to be fixed since SIP already handles it (the server adds the parameter “received=REAL_SOURCE_IP” to the “Via” header and sends the replies to that address).

Unidirectional audio since the caller told the callee to send audio to a non-routable address and port (so the caller won’t hear the callee).

The caller couldn’t receive in-dialogue/sequential messages (ACK for the INVITE, BYE, REFER, re-INVITE…) since the address in “Contact” is not routable outside their network.

If not, the proxy reply will not arrive at the client (caller): Note that text in red needs to be fixed before it arrives to the proxy (in case our proxy doesn’t provide us a NAT server solution). Via: SIP/2.0/UDP 192.168.1.33:5060 branch=z9hG4bKjyofoqmpĪllow: INVITE,ACK,BYE,CANCEL,OPTIONS,PRACK,REFER,NOTIFY,SUBSCRIBE,INFO,MESSAGE INVITE from the LAN client (with private IP)

caller behind NAT with private IP 192.168.1.33.A NAT router with a built-in SIP ALG can re-write information within the SIP messages (SIP headers and SDP body) making signalling and audio traffic between the client behind NAT and the SIP endpoint possible. If the SIP proxy doesn’t provide a server side NAT solution, then an ALG solution could have a place.Īn ALG understands the protocol used by the specific applications that it supports (in this case SIP) and does a protocol packet-inspection of traffic through it.

In some scenarios some client side solutions are not valid, for example STUN with symmetrical NAT router. ALG works typically in the client LAN router or gateway. There are various solutions for SIP clients behind NAT, some of them in client side ( STUN, TURN, ICE), others in server side (Proxy RTP as RtpProxy, MediaProxy). While ALG could help in solving NAT related problems, the fact is that many routers’ ALG implementations are wrong and break SIP.

Many of today’s commercial routers implement SIP ALG (Application-level gateway), coming with this feature enabled by default.