Inside FortiOS: Voice over IP (VoIP) protection

Inside FortiOS: Voice over IP (VoIP) protection

The FortiOS SIP Application Layer Gateway (ALG) allows SIP calls to pass through a FortiGate by opening SIP and RTP pinholes and performing source and destination IP address and port translation for SIP and RTP packets.

There are a large number of SIP-related Internet Engineering Task Force (IETF) documents (Request for

Comments) that define behavior of SIP and related applications. FortiOS completly support RFC 3261 for SIP, RFC 4566 for SDP and RFC 3262 for Provisional Response Acknowledgment (PRACK). FortiOS also supports other SIP and SIP-related RFCs and performs Deep SIP message inspection for SIP statements defined in other SIP RFCs.

Advanced voice over IP protection

The FortiOS SIP Application Level Gateway (ALG) protects Voice over IP (SIP and SDP) services in Unified Communication and NGN/IMS networks with the following advanced VoIP defense mechanisms.

Deep SIP message inspection (also called deep SIP header inspection)

Verifies SIP and SDP header syntax and protects SIP servers from potential SIP Fuzzing attacks. When a violation is detected, FortiOS can impose counter measures and can also send automatic SIP response messages to offload processing from the SIP server.

SIP message rate limiting

Allows rate limiting of SIP messages per SIP request method. This prevents a SIP server from overload or from DoS attacks using particular SIP methods. For example, FortiOS can protect SIP servers from a flood of SIP REGISTER or INVITE messages, which can be caused by a DoS attack or a flash crowd.

RTP and RTCP pinholing

RTP pinholing only forwards RTP/RTCP packets that conform to the particular session description of the associated SIP dialog. If a SIP dialog is finished, FortiOS automatically closes the pinhole. RTP/RTCP pinholing is supported by FortiASIC acceleration and achieves high packet throughput at low jitter and delay.

Stateful SIP dialog tracking

FortiOS tracks SIP message sequences and prevents unwanted SIP messages that are not related to a particular SIP dialog. For instance, FortiOS can detect malicious SIP BYE messages that do not conform with the associated context of the SIP dialog.

Inspecting SIP over SSL/TLS (secure SIP)

Some SIP phones and SIP servers use SSL or TLS to encrypt SIP signalling traffic. To allow SIP over SSL/TLS calls to pass through the FortiGate unit, the encrypted signalling traffic has to be unencrypted and inspected. FortiOS intercepts and unencrypts and inspects the SIP packets. Allowed packets are then re-encrypted and forwarded to their destination.

Carrier grade

Inspecting SIP on multiple ports

FortiOS can detect and inspect SIP and SDP UDP and TCP sessions and SIP SSL sessions and ou can configure the ports that the SIP ALG monitors for these sessions. In addition you can configure two different ports for SIP UDP sessions and two different ports for SIP TCP sessions. The port configuration can be changed without affecting other parts of the SIP configuration.

Carrier grade protection

To protect VoIP infrastructure in carrier networks, FortiOS complies with typical carrier requirements for availability and robustness.

High availability

FortiOS supports a hot failover configuration with an active and a standby FortiGate device. FortiOS dynamically updates the context on the standby unit with SIP and RTP related data. This enables the standby unit to takeover stable voice calls in case of a planned or unplanned outage or failover of the active unit.

Geographical redundancy of SIP servers

In FortiOS SIP server cluster configurations the active and standby units can be deployed in different geographical locations. This configuration prevents a total outage of a SIP server infrastructure if one location goes offline. FortiOS supports the detection of SIP server outages (loss of heartbeats) and a redirect of SIP messages to the redundant SIP server location.

Logging and Reporting

FortiOS can log call related information internally or to an external SYSLOG or FortiAnalyzer unit. This includes event logs that show particular SIP-related attacks or syntax violations with SIP messages or logs that summarize call statistics.


FortiOS performs configurable network address translation for IP addresses in the SIP and SDP header. The SIP ALG follows the configured NAT addresses in firewall virtual IPs and changes SIP header IP addresses accordingly. RTP NAT is controlled by SIP/SDP and the firewall policy. This allows translating an unlimited number of IP addresses without adding specific RTP policies.

Header manipulation

FortiOS SIP and SDP header manipulation supports SIP Network Address Translation (NAT) through FortiGate units configured as NAT firewalls.


Hosted NAT traversal (HNT)

In many service provider networks, CPE firewall devices provide NAT without application awareness. This causes issues for SIP/SDP and RTP traffic, since UAC IP address information references to the internal network behind the far end firewall. VoIP calls cannot be connected successfully. FortiOS mitigates far end NAT issues (called Hosted NAT traversal) by probing the first RTP packet from the UAC and learning the far end NA(P)T binding.

FortiOS then updates the internal NAT binding for RTP accordingly.

IP address conservation for NAT

In case of SIP and RTP NAT IP the original address information can get lost after translating to the provisioned IP addresses. This IP address information is sometimes required for detailed billing records or debugging purposes. FortiOS can maintain the original IP address information in a translated SIP header by adding it to the SIP/SDP info line (i=) or by adding it to the original attribute (o=). Either option can be selected depending on the SIP billing environment.

SIP ALG activation

The FortiOS SIP ALG is applied to SIP traffic accepted by a firewall policy that includes a VoIP profile. The VoIP profile controls how the SIP ALG processes SIP sessions. FortiOS also includes a high-performance SIP session helper that provides limited SIP functionality. In most cases the SIP ALG should be used because the SIP ALG supports the complete range of FortiOS SIP features.



IP routing and forwarding
IPsec VPN encryption, decryption

Rate limiting and message blocking
Stateful SIP tracking
Message, header, and SDP syntax checking
Network surveillance
NAT and IP topology Hiding
Logging and debugging

Intrusion detection and prevention
Defined by Fortinet and enterprise signatures
SIP decoder identifies SIP sessions

Security policy
IPsec VPN encryption, decryption
Access control

Native (D)DoS prevention
Anomaly detection and prevention

SIP over IPv6

FortiOS, operating in NAT/Route and in transparent mode supports SIP over IPv6. The SIP ALG can process SIP messages that use IPv6 addresses in the headers, bodies, and in the transport stack. The SIP ALG cannot modify the IPv6 addresses in the SIP headers so FortiGate units cannot perform SIP or RTP NAT over IPv6 and also cannot translate between IPv6 and IPv4 addresses.

Platform support and hardware acceleration

FortiOS supports VoIP protection with the SIP ALG on all FortiGate hardware platforms. Whenever a FortiGate unit provides FortiASIC or SPM HW acceleration, the SIP ALG will use this option to fast-path RTP/RTCP traffic.

As well, since the SIP ALG is proxy-based, SIP control packets are not offloaded to NP4 or NP6 processors. But actual voice or other media traffic can be offloaded to NP4 or NP6 processors after the SIP session is established. Many FortiGate units also support low latencey hardware acceleration configurations that also enhance SIP voice transmission.

FortiGate hardware acceleration provides a high throughput solution at very low jitter and delay. FortiOS provides efficient and highly scalable protection for VoIP in emerging Enterprise and Carrier network. This complements Fortinet’s NGFW and UTM offerings. VoIP protection can be easily added to any firewall policy just by adding a VoIP profile.

Platform support and hardware acceleration

VoIP protection is supported in FortiAnalyzer and FortiManager. Centralized logging and management are essential for carrier and MSSP service provider and are influencing business case calculations.


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