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Global Call IP Technology Guide — November 2007 415
Dialogic Corporation
request modification of call attributes — gc_ReqModifyCall( )
To set one or more message header fields in the re-INVITE request, the application inserts into the
GC_PARM_BLK a parameter of the following form for each header field to be set:
IPSET_SIP_MSGINFO
IPPARM_SIP_HDR
• value = string representing the complete header field, including field name
Most SIP header fields that are valid in an INVITE request can be modified in a re-INVITE request
without restriction. The most notable exceptions to this generalization are the Call-ID header and
the URI and Tag in the To and From headers, which RFC 3261 specifies must match the headers in
the original INVITE request. The following table specifies the header fields that are subject to
restrictions or that are automatically populated by the SIP stack.
To request a change in the attributes of a media session, the application uses the same parameter
mechanism that is used when offering a session invitation via gc_MakeCall( ). The application
inserts into the GC_PARM_BLK one or more parameter of the following form:
GCSET_CHAN_CAPABILITY
IPPARM_LOCAL_CAPABILITY
• value = IP_CAPABILITY structure containing the details of the proposed media session,
including capability (transcoder type) and direction
To modify the media attributes for a full-duplex connection, the application must insert at least two
of these parameters, one for each direction, with the appropriate value set in the direction field of
each IP_CAPABILITY structure. All fields in each IP_CAPABILITY structure must be fully
specified even if only one characteristic is actually being changed (for example, if only the
direction field is being modified to place a call on hold). If no media capability parameters are
inserted into the GC_PARM_BLK, the library automatically includes the last SDP answer from the
dialog when it constructs the re-INVITE request.
Header Field Modifiable Parameters
Restricted
Parameters
Automatically Populated Information
Call-ID None All All
Contact All None If not specified, copied from last INVITE or
2xx response transmitted in current dialog
CSeq None All All
From Display, URI parameters
except: user, ttl, method,
maddr
URI, Tag URI, Tag
Max-Forwards All None If not specified, 70
To Display, URI parameters
except: user, ttl, method,
maddr
URI, Tag URI, Tag
Via All None If not specified, copied from last INVITE or
2xx response transmitted in current dialog