How the SIP ALG performs NAT

How the SIP ALG performs NAT

In most Network Address Translation (NAT) configurations, multiple hosts in a private network share a single public IP address to access the Internet. For sessions originating on the private network for the Internet, NAT replaces the private IP address of the PC in the private subnet with the public IP address of the NAT device. The NAT device converts the public IP address for responses from the Internet back into the private address before sending the response over the private network to the originator of the session.

Using NAT with SIP is more complex because of the IP addresses and media stream port numbers used in SIP message headers and bodies. When a caller on the private network sends a SIP message to a phone or SIP server on the Internet, the SIP ALG must translate the private network addresses in the SIP message to IP addresses and port numbers that are valid on the Internet. When the response message is sent back to the caller, the SIP ALG must translate these addresses back to valid private network addresses.

In addition, the media streams generated by the SIP session are independent of the SIP message sessions and use varying port numbers that can also change during the media session. The SIP ALG opens pinholes to accept these media sessions, using the information in the SIP messages to determine the pinholes to open. The ALG may also perform port translation on the media sessions.

When an INVITE message is received by the SIP ALG, the FortiGate unit extracts addressing and port number information from the message header and stores it in a SIP dialog table. Similar to an IP session table the data in the dialog table is used to translate addresses in subsequent SIP messages that are part of the same SIP call.

When the SIP ALG receives a response to the INVITE message arrives, (for example, an ACK or 200 OK), the SIP ALG compares the addresses in the message fields against the entries in the SIP dialog table to identify the call context of the message. The SIP ALG then translates addresses in the SIP message before forwarding them to their destination.

The addressing and port number information in SDP fields is used by the ALG to reserve ports for the media session and create a NAT mapping between them and the ports in the SDP fields. Because SDP uses sequential ports for the RTP and RTCP channels, the ALG provides consecutive even-odd ports.


Source address translation

When a SIP call is started by a phone on a private network destined for a phone on the Internet, only source address translation is required. The phone on the private network attempts to contact the actual IP address of the phone on the Internet. However, the source address of the phone on the private network is not routable on the Internet so the SIP ALG must translate all private IP addresses in the SIP message into public IP addresses.

To configure the FortiGate for source address translation you add security policy that accepts sessions from the internal network destined for the Internet. You must enable NAT for the security policy and add a VoIP profile.

When a SIP request is received from the internal to the external network, the SIP ALG replaces the private network IP addresses and port numbers in the SIP message with the IP address of the FortiGate interface connected to the Internet. Depending on the content of the message, the ALG translates addresses in the Via:, Contact:, Route:, and Record-Route: SIP header fields. The message is then forwarded to the destination (either a VoIP phone or a SIP server on the Internet).

The VoIP phone or server in the Internet sends responses to these SIP messages to the external interface of the FortiGate unit. The addresses in the response messages are translated back into private network addresses and the response is forwarded to the originator of the request.

For the RTP communication between the SIP phones, the SIP ALG opens pinholes to allow media through the FortiGate unit on the dynamically assigned ports negotiated based on information in the SDP and the Via:, Contact:, and Record-Route: header fields. The pinholes also allow incoming packets to reach the Contact:, Via:, and Record-Route: IP addresses and ports. When processing return traffic, the SIP ALG inserts the original Contact:, Via:, Route:, and Record-Route: SIP fields back into the packets.


Destination address translation

Incoming calls are directed from a SIP phone on the Internet to the interface of the FortiGate unit connected to the Internet. To receive these calls you must add a security policy to accept SIP sessions from the Internet. The security policy requires a firewall virtual IP. SIP INVITE messages from the Internet connect to the external IP address of the virtual IP. The SIP ALG uses the destination address translation defined in the virtual IP to translated the addresses in the SIP message to addresses on the private network.

When a 200 OK response message arrives from the private network, the SIP ALG translates the addresses in the message to Internet addresses and opens pinholes for media sessions from the private network to the Internet.

When the ACK message is received for the 200 OK, it is also intercepted by the SIP ALG. If the ACK message contains SDP information, the SIP ALG checks to determine if the IP addresses and port numbers are not changed from the previous INVITE. If they are, the SIP ALG deletes pinholes and creates new ones as required. The ALG also monitors the Via:, Contact:, and Record-Route: SIP fields and opens new pinholes as required.

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