TRANSACTION CAPABILITIES APPLICATION PART

Transaction Capabilities Application Part, from ITU-T recommendations Q.771-Q.775 or ANSI T1.114 is a protocol for Signalling System 7 networks. Its primary purpose is to facilitate multiple concurrent dialogs between the same sub-systems on the same machines, using Transaction IDs to differentiate these, similar to the way TCP ports facilitate multiplexing connections between the same IP addresses on the Internet.
TCAP is used to transport INAP in Intelligent Networks and MAP in mobile phone networks.
TCAP 'messages' are sent over the wire between machines. TCAP 'primitives' are sent between the application and the local TCAP stack, all TCAP messages are primitives but there are primitives that are not messages, i.e. some are only transferred inside the local machine. A TCAP primitive is made up of one or more TCAP components.
A TCAP primitive may be one of the following types:

'Unidirectional'	A single primitive with no subsequent primitives. Sometimes referred to as a 'Notice'.
'Begin'	Start a dialog, further primitives will follow.
'Continue'	Send a subsequent primitive on an existing dialog, further primitives will follow.
'End'	The last primitive on an existing dialog, Close an existing dialog.
'Abort'	An error has causing the dialog to close.
'Cancel'	The invoke timer has expired without a response being received (this is a primitive but not a message)

A Begin primitive has an, up to 32 bit, Originating Transaction ID, Continues also have a Destination Transaction ID, Ends and Aborts only have a Destination Transaction ID.
Each TCAP component may be one of the following types:

'Invoke'	A new operation is being requested, this may or may not solicit a response
'Return Result Last'	A final response to an Invoke
'Return Result Not Last'	A response to an Invoke, further responses will be sent
'Return Error'	An error occurred
'Reject'

Invoke components have a signed 7 bit InvokeID which is present in all the other components to identify which invoke they relate to.
TCAP can be regarded as light weight implementation of the OSI defined ROSE, Remote Operations Services Element protocol

Contents

Transaction ID

Invoke ID

Transaction ID

The transaction ID is a TCAP reference for a set of TCAP operations that are performed within a single dialog. When machine 'A', starts a TCAP dialog with another machine 'B', the machine 'A' sends a 'Begin' message to machine 'B'. This 'Begin' message contains an Originating Transaction ID, which is the Transaction ID reference for 'A'. When the machine 'B' replies 'A' with a 'Continue' message it includes the 'A''s Transaction ID as the Destination Transaction ID. Furthermore 'B' includes its own Transaction ID as the Originating Transaction ID.
As the TCAP dialog goes on each 'Continue' message includes the Transaction ID of the destination machine as the Destination Transaction ID and the Transaction ID of the originating machine as the Originating Transaction ID. When any of the machines wants to close the dialog it sends an 'End' message or an 'Abort' message to the other machine. This message contains the Destination Transaction ID only.

Invoke ID

Invoke ID is a TCAP reference for a specific TCAP operation.