H.323 Version 7 - Overview

ITU-T SG16 completed work on H.323 version 7 in November 2009. As indicated previously, changes to the base specifications H.323, H.225.0, and H.245 were intentionally kept to a minimum. Nonetheless, a number of new important features were introduced. Additionally, the ITU continued to evolve the world's most widely-used multimedia communication standard with new feature enhancements through the use of the Generic Extensibility Framework. This approach allows both the ITU and third-parties to create new features for H.323 without requiring changes to the base specifications. (We will note that the H.323 Forum is also producing specifications for H.323, extending the capabilities of this rich multimedia system.)

The first new feature introduced in H.323v7 is something called "single transmitter multicast", which essentially allows an endpoint in a two-party call open a multicast stream to another endpoint. This is useful, for example, for such things as enabling a "music on hold" service where the audio is sourced from a known multicast IP address and port.

Perhaps even more significant is a new feature that allows a H.323 network device, such as an MCU, to contact H.323 terminals (e.g., a videoconferencing or telepresence system) and deliver a conference list to the terminal. This allows a user to then connect to a conference with the proper address information and conference identifier without having to know the specific contact details in advance: they are delivered directly to the terminal, allowing a single button press to join a conference. This feature is utilized today in a proprietary way by some telepresence systems.

Another new enhancement introduced in H.225.0 is the ability to transmit calling party information in multiple languages simultaneously. This feature will be useful, as an example, for companies that have international offices, allowing employees to see names in their preferred language, but also transmitting their names in multiple languages so that called users can display caller identification information in their preferred language.

H.323 also introduced procedures to allow one to transport Common Alerting Protocol (CAP) messages within H.323 systems. The Common Alerting Protocol is defined in Recommendation ITU-T X.1303 and may be used for a variety of purposes, but perhaps most importantly as a means of transmitting public warning and emergency messages.

In addition to enhancements to the core specifications, we also want to report on the new specifications that have been published since the last revision of H.323. There have been three new H.460-series Recommendations published: H.460.22, H.460.23, and H.460.24.

H.460.22 allows for negotiating security procedures between two H.323 endpoints prior to actual call establishment. Specifically, it allows an H.323 device to know in advance whether the called party supports a desired security signaling mechanism (e.g., IPSec or TLS).

H.460.23 is a new procedure that allows H.323 devices behind a NAT/FW device to coordinate with a Gatekeeper and a STUN server to discover the type of NAT/FW devices that is employed. The purpose is to then allow that information to be used during call establishment following procedures in H.460.24.

H.460.24 defines how to enable direct point-to-point media between two communicating H.323 endpoints, even if both devices are behind NAT/FW devices. By using information learned through H.460.23, a Gatekeeper is able to negotiate a strategy with the communicating endpoints to ensure that both devices are able to get media flowing properly through their respective NAT/FW devices. By doing this, media flows directly between the devices, thus reducing the latency and cost associated with media relay functions required by H.460.18 and H.460.19. While not all NAT devices are designed to allow for direct media flows, this fact is taken into consideration and an H.460.19 media proxy is employed when required.

What is particularly important to understand about H.460.23 and H.460.24 is that the media strategy is worked out in advance of the call, thus reducing call establishment time and enabling H.323 devices to communicate from behind a NAT/FW device without the assistance of an ALG, media proxy, or other device in most cases. Most importantly, for the first time ever, H.323 devices are able to easily traverse most NAT/FW devices and communicate over the Internet — something that has thus far been impossible for H.323 without some type of assistive device.