VoIP Protocols

In This Section: Signaling and Media Protocols Supported Standards and Protocols

The Security Gateway secures VoIP traffic in H.323, SIP, MGCP, and SCCP environments.

VoIP calls use a series of complex protocols, each of which can transmit potentially malicious data through many ports.

The Security Gateway makes sure that:

• Caller and recipient addresses are where they claim to be
• Caller and recipient are allowed to make and receive VoIP calls

In addition, the Security Gateway examines the contents of the packets passing through all allowed ports to make sure the packets contain the correct information.

Full stateful inspection on H.323, SIP, MGCP, and SCCP protocols makes sure that:

• All VoIP packets are structurally valid
• The packets arrive in a valid sequence

Signaling and Media Protocols

A phone call on an ordinary digital phone network and on a VoIP network is made up of media signals and control signals. The voice conversation is the media stream.

Dial tones and ringing tones, for example, are an indication that call control processes are occurring.

The different VoIP protocols use very different technologies, though they have the same aim. VoIP protocols handle these call control (or gateway) control and media functions:

• Call Control (signaling): Responsible for:
  • setting up the call
  • finding the peer
  • negotiating coding protocols
  • making the connection
  • ending the call
• Gateway Control: Responsible for control signals between VoIP gateways, rather than between endpoint phones. These gateways negotiate VoIP traffic on behalf of the phones.
• Media: The voice or video payload. VoIP networks and ordinary phone networks use RTP/RTCP for the media. RTP carries the actual media and RTCP carries status and control information.

Control signals open fixed (known) ports and dynamic ports. The parties of a call then use control signals to negotiate dynamically assigned ports that each side opens to receive the RTP/RTCP media stream.