Advertisement of SIP Support by H.323 Gateways

As the Session Initiation Protocol (SIP) gains wider acceptance, it is important to build gateways that provide an interworking function between the two protocols. It is our recommendation that we add SIP to the list of supported protocols advertised by an H.323 gateway to its gatekeeper."

The semantics of existing desired/supported protocol information elements were not appropriate for an H.323 gateway to indicate SIP support to an H.323 gatekeeper.  Currently, the best practice for a H.323 gateway (supporting SIP) implementation is to use the h323 information element in the SupportedProtocols choice for this purpose.  The use of such a field means that this H.323 gateway (supporting SIP) is a pure H.323 proxy, which is not semantically correct.  

SupportedProtocols ::= CHOICE {
    nonStandardData NonStandardParameter,
    h310 H310Caps,
    h320 H320Caps,
    h321 H321Caps,
    h322 H322Caps,
    h323 H323Caps,
    h324 H324Caps,
    voice VoiceCaps,
    t120-only T120OnlyCaps,
    ...,
    nonStandardProtocol NonStandardProtocol,
    t38FaxAnnexbOnly T38FaxAnnexbOnlyCaps,
    sip SIPCaps
 }

SIPCaps ::= SEQUENCE
{
    nonStandardData NonStandardParameter OPTIONAL,
    dataRatesSupported SEQUENCE OF DataRate OPTIONAL,
    supportedPrefixes SEQUENCE OF SupportedPrefix,
    ...
}

CallCapacityInfo ::= SEQUENCE {
    voiceGwCallsAvailable SEQUENCE OF CallsAvailable OPTIONAL,
    h310GwCallsAvailable SEQUENCE OF CallsAvailable OPTIONAL,
    h320GwCallsAvailable SEQUENCE OF CallsAvailable OPTIONAL,
    h321GwCallsAvailable SEQUENCE OF CallsAvailable OPTIONAL,
    h322GwCallsAvailable SEQUENCE OF CallsAvailable OPTIONAL,
    h323GwCallsAvailable SEQUENCE OF CallsAvailable OPTIONAL,
    h324GwCallsAvailable SEQUENCE OF CallsAvailable OPTIONAL,
    t120OnlyGwCallsAvailable SEQUENCE OF CallsAvailable OPTIONAL,
    t38FaxAnnexbOnlyGwCallsAvailable SEQUENCE OF CallsAvailable OPTIONAL,
    terminalCallsAvailable SEQUENCE OF CallsAvailable OPTIONAL,
    mcuCallsAvailable SEQUENCE OF CallsAvailable OPTIONAL,
    ...,
    sipGwCallsAvailable SEQUENCE OF CallsAvailable OPTIONAL
 }