Common SIP VoIP configurations

Common SIP VoIP configurations

This section describes some common SIP VoIP configurations and simplified SIP dialogs for these configurations. This section also shows some examples of how adding a FortiGate affects SIP processing.

Peer to peer configuration

In the peer to peer configuration shown below, two SIP phones (in the example, FortiFones) communicate directly with each other. The phones send SIP request and response messages back and forth between each other to establish the SIP session.

SIP peer to peer configuration

Peer to peer configurations are not very common because they require the SIP phones to keep track of the names and addresses of all of the other SIP phones that they can communicate with. In most cases a SIP proxy or redirect server maintains addresses of a large number of SIP phones and a SIP phone starts a call by contacting the SIP proxy server.

SIP proxy server configuration

A SIP proxy server act as intermediary between SIP phones and between SIP phones (for example, two FortiFones) and other SIP servers. As shown below, SIP phones send request and response messages the SIP proxy server. The proxy server forwards the messages to other clients or to other SIP proxy servers. Proxy servers can hide SIP phones by proxying the signaling messages. To the other users on the VoIP network, the signaling invitations look as if they come from the SIP proxy server.

redirect server configuration

SIP in proxy mode

SIP proxy server

A common SIP configuration would include multiple networks of SIP phones. Each of the networks would have its own SIP server. Each SIP server would proxy the communication between phones on its own network and between phones in different networks.

SIP redirect server configuration

A SIP redirect server accepts SIP requests, maps the addresses in the request into zero or more new addresses and returns those addresses to the client. The redirect server does not initiate SIP requests or accept calls. As shown below, SIP clients send INVITE requests to the redirect server, which then looks up the destination address. The redirect server returns the destination address to the client. The client uses this address to send the INVITE request directly to the destination SIP client.

Common SIP VoIP configurations                                                                                    SIP registrar configuration

SIP in redirect model

SIP redirect server

SIP registrar configuration

A SIP registrar accepts SIP REGISTER requests from SIP phones for the purpose of updating a location database with this contact information. This database can then become a SIP location service that can be used by SIP proxy severs and redirect servers to locate SIP clients. As shown below, SIP clients send REGISTER requests to the SIP registrar.


SIP registrar and proxy servers

SIP with a FortiGate

Depending on your security requirements and network configuration FortiGates may be in many different places in a SIP configuration. This section shows a few examples.

The diagram below shows a FortiGate installed between a SIP proxy server and SIP phones on the same network. The FortiGate is operating in transparent mode so both the proxy server and the phones are on the same subnet. In this configuration, called SIP inspection without address translation, the FortiGate could be protecting the SIP proxy server on the private network by implementing SIP security features for SIP sessions between the SIP phones and the SIP proxy server.

Common SIP VoIP configurations                                                                                             SIP with a FortiGate

SIP network with FortiGate in transparent mode

call by proxy server. the INVITE request to Phone B.

The phone rings.

The phones and server use the same SIP dialogs as they would if the FortiGate was not present. However, the FortiGate can be configured to control which devices on the network can connect to the SIP proxy server and can also protect the SIP proxy server from SIP vulnerabilities.

The following diagram shows a FortiGate operating in NAT/Route mode and installed between a private network and the Internet. Some SIP phones and the SIP proxy server are connected to the private network and some SIP phones are connected to the Internet. The SIP phones on the Internet can connect to the SIP proxy server through the FortiGate and communication between SIP phones on the private network and SIP phones on the Internet must pass through the FortiGate.

SIP network with FortiGate in NAT/Route mode

  1. 1. SIP phone B registers with SIP Phone B
  2. SIP phone A registers with SIP proxy server

(PhoneB@ SIP proxy server.       using the SIP proxy server virtual IP.

2. Phone A dials Phone B    
by sending an INVITE request to the SIP proxy server. 3. The proxy server looks up the SIP address of Phone B and forwards 4. Phone B is notified of an incoming

the INVITE request to Phone B.      call by proxy server – phone rings.

  1. RTP Media session opens between

Phone A and Phone B when Phone B answers

The phones and server use the same SIP dialog as they would if the FortiGate was not present. However, the FortiGate can be configured to control which devices on the network can connect to the SIP proxy server and can also protect the SIP proxy server from SIP vulnerabilities. In addition, the FortiGate has a firewall virtual IP that forwards packets sent to the SIP proxy server Internet IP address ( to the SIP proxy server internal network IP address (

Since the FortiGate is operating in NAT/Route mode it must translate packet source and destination IP addresses (and optionally ports) as the sessions pass through the FortiGate. Also, the FortiGate must translate the addresses contained in the SIP headers and SDP body of the SIP messages. As well the FortiGate must open SIP and RTP pinholes through the FortiGate. SIP pinholes allow SIP signaling sessions to pass through the FortiGate between phones and between phones and SIP servers. RTP pinholes allow direct RTP communication between the SIP phones once the SIP dialog has established the SIP call. Pinholes are opened automatically by the FortiGate. Administrators do not add security policies for pinholes or for RTP sessions. All that is required is a security policy that accepts SIP traffic.

Opening an RTP pinhole means opening a port on a FortiGate interface to allow RTP traffic to use that port to pass through the FortiGate between the SIP phones on the Internet and SIP phones on the internal network. A pinhole only accepts packets from one RTP session. Since a SIP call involves at least two media streams (one from Phone A to Phone B and one from Phone B to Phone A) the FortiGate opens two RTP pinholes. Phone A sends RTP packets through a pinhole in port2 and Phone B sends RTP packets through a pinhole in port1. The FortiGate opens the pinholes when required by the SIP dialog and closes the pinholes when the SIP call is completed. The FortiGate opens new pinholes for each SIP call.

Each RTP pinhole actually includes two port numbers. The RTP port number as defined in the SIP message and an RTCP port number, which is the RTP port number plus 1. For example, if the SIP call used RTP port 3346 the FortiGate would create a pinhole for ports 3346 and 3347.


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