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H323 RAS

Posted: Wed May 11, 2011 2:12 pm
by mcherkadi

I am working on H323 RAS.. Just wanted a clarification.
Does the H323 RAS uses well known unicast UDP ports other than 1719 and UDP multicast ports other than 1718 ?

Cherkadi, M

Re: H323 RAS

Posted: Wed May 11, 2011 3:46 pm
by paulej
By default, H.323 uses port 1718 (multicast) and 1719 (unicast) for RAS traffic. However, those are just the recommended default values. If two communicating devices agree, they could use different addresses.

H.323 Annex O describes how to use DNS with H.323 systems. So, if an endpoint or Gatekeeper is trying to locate the Gatekeeper that manages a particular domain, for example, then a DNS query is performed, perhaps looking for the "h323ls" service. This DNS record contains port numbers and they do not have to be port 1719. In fact, the examples in H.323 Annex O do not use port 1719.

One other important thing to know is that years ago, there was some confusion as to which port was used for multicast and which was used for unicast. For that reason, some devices may still send unicast RAS requests to 1718. So, it's recommended to list on both ports. A gatekeeper should advertise port 1719 when signaling a RAS address.


Re: H323 RAS

Posted: Mon May 16, 2011 3:46 pm
by mcherkadi
Thanks for the information.

From Annex O of H.323 spec, I understood that DNS query is used to get the gatekeeper IP address, port (which may be different from Well know port) and the service information from the H323 SRV RR and A RR, if the h323 URL is provisioned in the h323 endpoint/gateway.

But If there is no provisioning, at the h323 endpoints, they still have to rely on h323 multicast address and well known port 1718 for discovery of Gatekeepers?

The service information mentioned are h323ls, h323rs, h323cs and h323be. Is there any places and clauses that provides what are the h323 Request/Response messages mapped to these services?

Cherkadi, M

Re: H323 RAS

Posted: Mon May 16, 2011 8:43 pm
by paulej
Usually, H.323 endpoints are provisioned with the address of a Gatekeeper. Usually, it's provisioned as the IP address, not a domain name.

However, it is possible to provision an endpoint with a domain name. As an example, if you create an account at and download the Spranto soft client, you'll see that you just enter the domain "" and the client figures out the rest. What it looks for is the h323rs SRV record. That provides the IP address and domain of the Gatekeeper that provides the service.

The services defined in H.323 Annex O are
h323rs - Registration service, used to find the Gatekeeper with which an endpoint can register. To scale this for carriers, this might point to a number of different Gatekeepers and/or alternate Gatekeeper signaling might force endpoints to a "home" gatekeeper
h323ls - This is used by a Gatekeeper (primarily) to determine where to send an LRQ for a remote domain, used by carriers and larger enterprises
h323cs - This is used by a Gatekeeper (primarily) to know where to direct calls, usually used only by smaller enterprises that have a single access point into which all calls are directed

I hope that helps.