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21.2 THE H.323 PROTOCOL 675TABLE 21.2-2 H.225 RAS MessagesAcronym Message Function Acronym Message FunctionACF Admission confirm IRR Information request responseARQ Admission request LCF Location confirmBCF Bandwidth confirm LRQ Location requestBRQ Bandwidth request RAC Resource availability confirmationDCF Disengage confirm RAI Resource availability indicationDRQ Disengage request RCF Registration confirmIRQ Information request RRQ Registration requestBRQ is sent by an endpoint to request, and BCF by the GK to grant, a change ofbandwidth for the call.DRQ is sent by an endpoint to notify the GK that it is dropping out of the call,and by the GK to an endpoint to force the termination of the call. DCF is theconfirmation response.IRQ is sent by the GK to an endpoint to request a status report, and IRR is sent bythe endpoint to respond to the request.LRQ is sent by an endpoint to request address translation from a GK, and LCF issent by the GK to respond to the request.RAI is sent by a GW to inform the GK of available resources, and RAC is sent bythe GK as the response. The GK uses the information in its routing decisions.RRQ is sent by an endpoint to request registration with a GK, and RCF is sentby the GK to respond to the request.All request messages (ending in RQ, such as ARQ, BRQ, etc.) can also berejected with a corresponding message ending in RJ (such as ARJ, BRJ, etc., notshown in Table 21.2-2).The most significant parameters, and the messages they appear in, are listed inTable 21.2-3.Recommendation H.225 specifies message formats using the ASN.1 notation.Detailed formats are found in [2].Q.931 Messages. These messages use the call signaling channel and aredescribed in Chapter 10. Not all Q.931 messages are used by H.323: for example,of the messages listed in Table 10.3-1, CONACK, DISC, and RLSE are not used.The deviations from Rec. Q.931 are specified in Rec. H.225 [2].H.245 Messages. These messages use the control channel and come in four types:1. Request requires an action and a response from the recipient.2. Response is a response to a request.3. Command requires an action but no response from the recipient.4. Indication does not require any action or response.