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Understanding VoIP ALG Types

Junos OS supports voice-over-IP Application Layer Gateways (VoIP ALGs) and basic data ALGs. (Note that supported ALG types vary depending on which hardware device you are using.)

VoIP ALGs provide stateful Application Layer inspection and Network Address Translation (NAT) capabilities to VoIP signaling and media traffic. The ALG inspects the state of transactions, or calls, and forwards or drops packets based on those states.

Junos OS supports the following VoIP ALGs:

Table 1 lists the VoIP ALG types.

Table 1: VoIP-ALG-Types




The H.323 ALG provides support for the H.323 legacy VoIP protocol. The ALG lets you secure VoIP communication between terminal hosts, such as IP phones and multimedia devices. In such a telephony system, the gatekeeper device manages call registration, admission, and call status for VoIP calls. Gatekeepers can reside in the two different zones or in the same zone.


The MGCP ALG provides support for Media Gateway Control Protocol (MGCP). MGCP is a text-based Application Layer protocol used for call setup and call control between the media gateway and the media gateway controller (MGC).


The SCCP ALG provides support for Skinny Client Control Protocol (SCCP). SCCP is a Cisco proprietary protocol for call signaling. Skinny is based on a call-agent-based call-control architecture. The control protocol uses binary-coded frames encoded on TCP frames sent to well-known TCP port number destinations to set up and tear down RTP media sessions.


The SIP ALG provides support for the Session Initiation Protocol (SIP). SIP is an Internet Engineering Task Force (IETF)-standard protocol for initiating, modifying, and terminating multimedia sessions over the Internet. Such sessions might include conferencing, telephony, or multimedia, with features such as instant messaging and application-level mobility in network environments.

For information about enabling and configuring each of these ALGs through J-Web, select the Configure>Security>ALG page in the J-Web user interface and click Help.