"Reference value soup" (Q17)

The information in this article applies to:

There area several tags, sequence numbers, identifiers, etc. in H.225.0. Here is a list of them along with in which protocol they are used, who assigns them, their uniqueness, and how they are used.

Field	RAS	Call signaling	Assignment	Uniqueness	Purpose
endpointIdentifier	X	X	GK	GK	This is the tag by which the GK may identify the EP
gatekeeperIdentifier	X	X	EP; determined out-of-band	Not necessarily	Used in auto discovery (multicast GRQ) to identify GK preferred by EP. Semantics are undefined for static discovery (GK may or may not reject discovery if it does not recognize the gatekeeperIdentifier).
requestSeqNum	X		Requesting entity	EP	All RAS messages have a requestSeqNum field that is used to associate request messages with their corresponding responses, e.g., ARQ with ACF/ARJ.
callReferenceValue		X	Originating EP	EP	All call-signaling messages have a callReferenceValue (CRV) field in the Q.931 part of the message that is used to associate messages to a call. If an "ARQ sequence" takes place prior to sending Setup for the same call, then the callReferenceValue in the Setup shall be the same as the one in the ARQ. The call-signaling and RAS callReferenceValues are otherwise independent.
callReferenceValue	X		Originating EP	EP	All RAS request messages that are associated with a call, e.g., ARQ, BRQ, DRQ, and IRQ, have a callReferenceValue field that is used to associate these messages with that call.
callIdentifier	X	X	Originating EP	Globally	All call-signaling messages, except for the User Information message, and all the RAS request messages that are associated with a call, e.g., ARQ, BRQ, DRQ, and IRQ, have a callIdentifier field that is used to associate the call-signaling messages and the RAS messages associated to that call. Note that callIdentifier was added to H.225.0 once it was realized that callReferenceValue was not sufficiently unique to uniquely identify messages belonging to a call. Note that although the intent was for callIdentifier to uniquely identify a call to the GK, it is actually possible for a GK to have more than call with the same callIdentifier. Therefore, a GK must augment callIdentifier was other information to uniquely identify a call. For example, for Call Forward No Answer or Call Deflection, H.450.3 states that the diverted-to call should be established BEFORE the original call is released. Only on receipt of the first response from the diverted-to endpoint is the original call released. This is done so that if the originally called party can still answer the ringing phone while the forwarding is in progress.
conferenceID	X	X	Originating EP	Globally	The call-signaling messages, Connect, Setup and Facility, as well as the RAS request messages that are associated with a call, e.g., ARQ, BRQ, DRQ, and IRQ, have a conferenceID field that is used to associate calls in the same conference.