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


   Therefore gateways MAY support more sophisticated early media systems
   as they come to be better understood.  One mechanism that provides a
   way of initiating a fully-featured early media system is described in
   [20].

   Note that in SIP networks not just switches but also user agents can
   generate the 18x response codes and initiate early backwards media,
   and that therefore some gateways may wish to enforce policies that
   restrict the use of backwards media from arbitrary user agents (see
   Section 15).

5.6 Mid-Call Transactions which do not change SIP state

   When interworking with the PSTN, there are situations when gateways
   will need to send messages to each other over SIP that do not
   correspond to any SIP operations.

   In support of mid-call transactions and other ISUP events that do not
   correspond to existing SIP methods, SIP gateways MUST support the
   INFO method, defined in RFC2976 [6].  Note that this document does
   not prescribe or endorse the use of INFO to carry DTMF digits.

   Gateways MUST accept "405 Method Not Allowed" and "501 Not
   Implemented" as non-fatal responses to INFO requests - that is, any
   call in progress MUST NOT be torn down if a destination so rejects an
   INFO request sent by a gateway.

5.7 Privacy Protection

   ISUP has a concept of presentation restriction - a mechanism by which
   a user can specify that they would not like their telephone number to
   be displayed to the person they are calling (presumably someone with
   Caller ID).  When a gateway receives an ISUP request that requires
   presentation restriction, it must therefore shield the identity of
   the caller in some fashion.

   The base SIP protocol supports a method of specifying that a user is
   anonymous.  However, this system has a number of limitations - for
   example, it reveals the identity of the gateway itself, which could
   be a privacy-impacting disclosure.  Therefore gateways MAY support
   more sophisticated privacy systems.  One mechanism that provides a
   way of supporting fully-featured privacy negotiation (which interacts
   well with identity management systems) is described in [9B].








Camarillo, et. al.          Standards Track                     [Page 9]