Understanding ALG Types

JUNOS Software 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 Software supports the following VoIP ALGs:

H.323—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.
SIP—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.
SCCP—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.
MGCP—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). .

JUNOS Software also supports the following data ALGs:

DNS—Provides an ALG for the Domain Name System. The DNS ALG monitors DNS query and reply packets and closes session if the DNS flag indicates the packet is a reply message.
FTP—Provides an ALG for the File Transfer Protocol (FTP).The FTP ALG monitors PORT, PASV, and 227 commands. It performs NAT on the IP, port, or both in the message and gate opening on the device as necessary. The FTP ALG supports FTP put and FTP get command blocking. When the FTP_NO_PUT or FTP_NO_GET command is set in the policy, the FTP ALG sends back a blocking command and closes the associated opened gate when the FTP STOR or FTP RETR command is observed.
TFTP—Provides an ALG for the Trivial File Transfer Protocol (TFTP). The TFTP ALG processes TFTP packets that initiate the request and opens a gate to allow return packets from the reverse direction to the port that sends the request.
PPTP—Provides an ALG for the Point-to-Point Tunneling Protocol (PPTP). The PPTP is a Layer 2 protocol that tunnels PPP data across TCP/IP networks. The PPTP client is freely available on Windows systems and is widely deployed for building Virtual Private Networks (VPNs).
REAL—Provides an ALG for the Real-Time Streaming Protocol.
MSRPC—Provides an ALG for the Microsoft Remote Procedure Call.
SUNRPC—Provides an ALG for the SUN Remote Procedure Call.
RSH—Provides an ALG for the Remote Shell (RSH). The RSH ALG handles TCP packets destined for port 514 and processes the RSH port command. The RSH ALG performs NAT on the port in the port command and opens gates as necessary.
SQL—Provides an ALG for the Structured Query Language (SQL). The SQLNET ALG processes SQL TNS response frame from the server side. It parses the packet and looks for the (HOST=ipaddress), (PORT=port) pattern and performs NAT and gate opening on the client side for the TCP data channel.
TALK—Provides an ALG for the TALK Protocol. The TALK protocol uses UDP port 517 and port 518 for control channel connections. The talk program consists of a server and a client. The server handles client notifications and helps to establish talk sessions. There are two types of talk servers: ntalk and talkd. The TALK ALG processes packets of both ntalk and talkd formats. It also performs NAT and gate opening as necessary.

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.

Understanding ALG Types

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