We have had an ambiguity show up during interoperability testing, which I would like clarified by means of the implementer's guide and future versions of the standard. Any objections, please shout in the usual way!

The problem comes in 19.1.1.1 where UDP port numbers are defined. The standard currently (I'm looking at version 2) says: Gatekeeper UDP Discovery Port 1718 Gatekeeper UDP Registration and Status Port 1719

There are two problems with this. The first is that it implies that the LRQ message (which is nothing to do with discovery) always needs to be sent to port 1719, which is a problem for multicast LRQs, because only one IP address and port number pair has been registered with the IETF - 224.0.1.41:1718.

The second problem is, if anything, more subtle... The standard also seems to imply by the above that GRQ, which is a discovery message, should always be sent to port 1718, whether it is sent to the multicast discovery port or not. The reason this is a problem is that gatekeepers need to know whether these messages (GRQ and LRQ) have arrived by multicast or by unicast, because the failure response is different in the two cases (no response to a multicast xRQ, but xRJ response to a unicast xRQ). The only indication that most IP stacks give as to how a message has arrived is by a port number, however, so if all GRQs appear to port 1718 (and all LRQs to port 1719), there is no way for the gatekeeper to tell whether the message arrived by unicast or multicast.

Common (but not universal - at least in products under development!) usage, driven by these practical considerations, seems to be that the most common implementations send both sorts of multicast addresses to the registered address - 224.0.1.41:1718, and unicast GRQ and LRQ messages to port 1719 (unless there is some private arrangement specifying other special usage).

My suggestion is that this common usage is standardised, initially via the implementer's guide, and then when appropriate in future versions of H.225.0.

Regards, Chris