How Gateway discover gatekeeper

We can use gatekeeper to register different gateway to a particular zone.Each zone will have only one active gatekeeper

Now how the gateway find s its gatekeeper ???

Gateway/Gatekeeper works on h323 protocol suite and h323 protocol suite is split into 3signalling

1. h225 (RAS-Registration,admission and Status)
2. h225 control signalling 
3. h245 (Media negotaiation and logical channel or transport signalling )

H225 RAS: RAS signalling work between gateway and gatekeeper on UDP port 1719 and for multicast work on UDP port (1718). In case of multicast it used the IP 224.0.1.41 to discover the gatekeeper

H225 control signalling:This is used  to setup connection between two H323 endpoints,This signalling works on TCP port 1720

H245 Media control and transport: this signalling is used to negotiate media capabilities and parameter fpor establishing logical channel,Adminssion control is also done by h245 signalling.

Gatekeeper discovery process:

1. The gateway or h323 endpoint send a GRQ(Gatekeeper request packet ) on port 224.0.1.41 of udp port 1718
2. the gatekeeper  either respond using a GCF (Gatekeepr_ confirm) or GRJ (Gatekeeper Reject ), In case of rejection it also include the reason of rejection .