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Understanding the MGCP ALG
The Media Gateway Control Protocol (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).
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Before You Begin
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For background information, read
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The protocol is based on a master/slave call control
architecture: the media gateway controller (call agent) maintains
call control intelligence, and media gateways carry out the instructions
from the call agent. Both signaling packets and media packets are
transmitted over UDP. JUNOS software supports MGCP in
route mode and Network Address Translation (NAT) mode.
The MGCP ALG performs the following procedures:
- Conducts VoIP signaling payload inspection. The payload
of the incoming VoIP signaling packet is fully inspected based on
related RFCs and proprietary standards. Any malformed packet attack
is blocked by the ALG.
- Conducts MGCP signaling payload inspection. The payload
of the incoming MGCP signaling packet is fully inspected in accordance
with RFC 3435. Any malformed-packet attack is blocked by the ALG.
- Provides stateful processing. The corresponding VoIP-based
state machines are invoked to process the parsed information. Any
out-of-state or out-of-transaction packet is identified and properly
handled.
- Performs NAT. Any embedded IP address and port information
in the payload is properly translated based on the existing routing
information and network topology, and is then replaced with the translated
IP address and port number, if necessary.
- Manages pinholes for VoIP traffic. To keep the VoIP network
secure, the IP address and port information used for media or signaling
is identified by the ALG, and any needed pinhole is dynamically created
and closed during call setup.
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