- Media Gateway Control Protocol (MGCP)—MGCP is a client-server call control protocol, built on centralized control architecture. All the dial plan information resides on a separate call agent. The call agent, which controls the ports on the gateway, performs call control. The gateway does media translation between the Public Switched Telephone Network (PSTN) and the VoIP networks for external calls. In a Cisco-based network, CallManagers function as the call agents.
MGCP is an IETF standard that is defined in several RFCs, which includes 2705
and 3435 . Its capabilities can be extended by the use of packages that include, for example, the handling of dual-tone multifrequency (DTMF) tones, secure RTP, call hold, and call transfer.An MGCP gateway is relatively easy to configure. Because the call agent has all the call-routing intelligence, you do not need to configure the gateway with all the dial peers it would otherwise need. A downside is that a call agent must always be available. Cisco MGCP gateways can use Survivable Remote Site Telephony (SRST) and MGCP fallback to allow the H.323 protocol to take over and provide local call routing in the absence of a CallManager. In that case, you must configure dial peers on the gateway for use by H.323.
- H.323—H.323 inspection provides support for H.323 compliant applications such as Cisco CallManager and VocalTec Gatekeeper. H.323 is a suite of protocols defined by the International Telecommunication Union for multimedia conferences over LANs. The security appliance supports H.323 through Version 4, which includes H.323 v3 feature Multiple Calls on One Call Signaling Channel.
With H.323 inspection enabled, the security appliance supports multiple calls on the same call signaling channel, a feature introduced with H.323 Version 3. This feature reduces call setup time and reduces the use of ports on the security appliance.
These are the two major functions of H.323 inspection:
- NAT the necessary embedded IPv4 addresses in the H.225 and H.245 messages. Because H.323 messages are encoded in PER encoding format, the security appliance uses an ASN.1 decoder to decode the H.323 messages.
- Dynamically allocate the negotiated H.245 and RTP/RTCP connections.
- Skinny (or Simple) Client Control Protocol (SCCP)—SCCP is a simplified protocol used in VoIP networks. Cisco IP Phones that use SCCP can coexist in an H.323 environment. When used with Cisco CallManager, the SCCP client can interoperate with H.323-compliant terminals. Application layer functions in the security appliance recognize SCCP Version 3.3. The functionality of the application layer software ensures that all SCCP signaling and media packets can traverse the security appliance by providing NAT of the SCCP Signaling packets.
There are 5 versions of the SCCP protocol: 2.4, 3.0.4, 3.1.1, 3.2, and 3.3.2. The security appliance supports all versions through Version 3.3.2. The security appliance provides both PAT and NAT support for SCCP. PAT is necessary if you have limited numbers of global IP addresses for use by IP phones.
Normal traffic between Cisco CallManager and Cisco IP Phones uses SCCP and is handled by SCCP inspection without any special configuration. The security appliance also supports DHCP options 150 and 66, which allow the security appliance to send the location of a TFTP server to Cisco IP Phones and other DHCP clients
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