Integrated Services Digital Network (ISDN) User Part (ISUP) to Session Initiation Protocol (SIP) Mapping

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RFC 3398                  ISUP to SIP Mapping              December 2002


   translation is not necessary for these parameters.  Of the
   aforementioned 29 optional parameters, only the following are
   immediately useful for translation: the Calling Party's Number (CIN,
   which is commonly present), Transit Network Selection (TNS), Carrier
   Identification Parameter (CIP, present in ANSI networks), Original
   Called Number (OCN), and the Generic Digits (known in some variants
   as the Generic Address Parameter (GAP)).

   When a SIP INVITE arrives at a PSTN gateway, the gateway SHOULD
   attempt to make use of encapsulated ISUP (see [3]), if any, within
   the INVITE to assist in the formulation of outbound PSTN signaling,
   but SHOULD also heed the security considerations in Section 15.  If
   possible, the gateway SHOULD reuse the values of each of the ISUP
   parameters of the encapsulated IAM as it formulates an IAM that it
   will send across its PSTN interface.  In some cases, the gateway will
   be unable to make use of that ISUP - for example, if the gateway
   cannot understand the ISUP variant and must therefore ignore the
   encapsulated body.  Even when there is comprehensible encapsulated
   ISUP, the relevant values of SIP header fields MUST 'overwrite'
   through the process of translation the parameter values that would
   have been set based on encapsulated ISUP.  In other words, the
   updates to the critical session context parameters that are created
   in the SIP network take precedence, in ISUP-SIP-ISUP bridging cases,
   over the encapsulated ISUP.  This allows many basic services,
   including various sorts of call forwarding and redirection, to be
   implemented in the SIP network.

   For example, if an INVITE arrives at a gateway with an encapsulated
   IAM with a CPN field indicating the telephone number +12025332699,
   but the Request-URI of the INVITE indicates 'tel:+15105550110', the
   gateway MUST use the telephone number in the Request-URI, rather than
   the one in the encapsulated IAM, when creating the IAM that the
   gateway will send to the PSTN.  Further details of how SIP header
   fields are translated into ISUP parameters follow.

   Gateways MUST be provisioned with default values for mandatory ISUP
   parameters that cannot be derived from translation(such as the NCI or
   TMR parameters) for those cases in which no encapsulated ISUP is
   present.  The FCI parameter MUST also have a default, as only the 'M'
   bit of the default may be overwritten during the process of
   translation if the optional number portability translation mechanisms
   described below are used.

   The first step in the translation of the fields of an INVITE message
   to the parameters of an IAM is the inspection of the Request-URI.






Camarillo, et. al.          Standards Track                    [Page 20]