H.245 Call Control Messages

After a connection has been set up via the call signaling procedure, H.245 messages (there are many of these) are used to resolve the call media type, to exchange terminal capabilities, and to establish the media flow before the call can be established. H.245 also manages call parameters after call establishment. H.245 messages also are encoded in ASN.1 PER syntax. The messages carried include notification of terminal capabilities, and commands to open and close logical channels. The H.245 control channel is permanently open, unlike the media channels.

Note

Table 1 lists various types of messages and the H.323 ports used to transport them.

Table 1: H.323 Ports 

Message	Protocol/Port
H.245 messages	Dynamically assigned ports
RTP messages	Dynamically assigned ports
Gatekeeper	UDP Discovery Port 1718
Gatekeeper	UDP Registration and Status Port 1719
Endpoint	TCP Call Signaling Port 1720
Gatekeeper	Multicast 224.0.1.41
DNS	UDP 53
TFTP	UDP 69
SNMP	UDP 161, 162

H.245 negotiations usually take place on a separate channel from the one used for H.225 exchanges, but newer applications support tunneling of H.245 PDUs within the H.225 signaling channel. There is no well-known port for H.245. The H.245 transport address always is passed in the call-signaling message. In other words, port information is passed within the payload of the preceding H.225/Q.931 signaling packets. The media channels (those used to transport voice and video) are similarly dynamically allocated. Figure 1 is an example of H.245 call control.

 

Figure 1: H.245 Call Control

The called party opens the TCP port for establishing the control channel after extracting the port information from the H.225/Q.931 signaling packet. During this exchange, terminal capabilities such as codec choice and master/slave determination are negotiated. Media channel negotiations begin with the OpenLogicalChannel Request packet. When the called party is ready to talk, it responds with an OpenLogicalChannel Ack, which contains the dynamic port information in the payload. As an aside, this use of dynamic ports makes it difficult to implement security policy on firewalls, NAT, and traffic shaping. In some cases, a special H.323-aware firewall or firewall component called an Application Layer Gateway (ALG) is required to reliably pass H.323 signaling and associated media. Once both RTP/RTCP channels are opened, communications proceeds (see Figure2).

 

Figure 2: RTP/RTCP Media Streams