Added on my notes, here is the list of H.225 RAS (Registration, Admission, and Status) signal messages which are initiated by a gateway and gatekeeper.
- Gateway-to-gatekeeper signaling is H.225 RAS signaling. This signaling is User Data Protocol (UDP) based. Signaling messages between gateways are H.225 call control, setup, or signaling messages.
- H.225 call control signaling is used to set up connections between H.323 endpoints. The ITU H.225 recommendation specifies the use and support of Q.931 signaling messages.
- If no gatekeeper is present, H.225 messages are exchanged directly between the endpoints.
- After call signaling is set up between the gateways, H.245 is negotiated. H.245, a control signaling protocol in the H.323 multimedia communication architecture, is for the exchange of end-to-end H.245 messages between communicating H.323 endpoints or terminals. The H.245 control messages are carried over H.245 control channels. The H.245 control channel is the logical channel 0 and is permanently open, unlike the media channels. The messages carried include messages to exchange capabilities of terminals and to open and close logical channels.
- After a connection has been set up via the call signaling procedure, the H.245 call control protocol is used to resolve the call media type and establish the media flow, before the call can be established. It also manages the call after it has been established.
H.225 RAS Messages
Gateway Discovery Messages (See the graphics illustration)
- GRQ-Gatekeeper Request: Message that a gateway sends during the gatekeeper discovery process.
- GCF-Gatekeeper Confirm: Reply from the gatekeeper to the gateway, which indicates the transport address (port) of the gatekeeper RAS channel.
- GRJ-Gatekeeper Reject: Reply from the gatekeeper to gateway that rejects the gateway request for registration. Usually due to gateway or gatekeeper configuration error.
Gateway Registration Messages
- RRQ-Registration Request: Message sent from a gateway to the gatekeeper.
- RCF-Registration Confirm: Message acknowledging that the gatekeeper has allowed gateway registration.
- RRJ-Registration Reject: Message acknowledging that the gatekeeper has not allowed the gateway to register.
Gateway Unregistration Messages
- URQ-Unregister Request: Message sent from a gateway or gatekeeper requesting cancellation of the registration.
- UCF-Unregister Confirm: Message sent from a gateway or the gatekeeper to confirm unregistration.
- URJ-Unregister Reject: Response to a URQ when the gateway was not registered.
Admission Control Messages
- ARQ-Admission Request: Message that a gateway sends to initiate a call.
- ACF-Admission Confirm: Reply from the gatekeeper to the gateway admitting the call. This message also contains the IP address of the destination gateway so that the originating gateway can begin call control signaling.
- ARJ-Admission Reject: Reply from the gatekeeper denying the call request. This can be for many reasons, including a number that could not be resolved to an IP address, insufficient available bandwidth, and so on.
Location Request Messages
- LRQ-Location Request: Message sent between gatekeepers to find a gateway in a different zone.
- LCF-Location Confirm: Message sent between gatekeepers to provide the IP address of the requested gateway.
- LRJ-Location Reject: Message sent between gatekeepers in response to an LRQ when the requested gateway is unknown or not registered.
Status Information Messages
- IRQ-Information Request: Message sent from the gatekeeper to a gateway.
- ICF-Information Confirm: Sent from gateway to gatekeeper to confirm the status.
- IRR-Information Response: Message sent from the gateway to tell the gatekeeper about active calls.
- IACK-Information Request Acknowledgement: Response from the gatekeeper to a successfully handled IRR.
- INACK-Information Request Negative Acknowledgement: Response from the gatekeeper for an unsuccessful IRR.
- RIP-Request in Progress: Message sent from a gatekeeper to a gateway when the gatekeeper must use an LRQ to resolve an ACF in a different zone.
Bandwidth Control Messages
- BRQ-Bandwidth Request: A request for an increase/decrease in call bandwidth that the gateway sends to the gatekeeper.
- BCF-Bandwidth Confirm: Message that the gatekeeper sends to confirm the acceptance of the bandwidth change request.
- BRJ-Bandwidth Reject: Message that the gatekeeper sends to reject the bandwidth change request.
Resource Availability Messages
- RAI-Resource Availability Indication: Message that gateways use to inform the gatekeeper whether resources are available in the gateway to take on additional calls.
- RAC-Resource Availability Confirm: Response from the gatekeeper to the gateway that acknowledges the reception of the RAI message.