Parameters

To provide screening, you do not need to read every field comprising a message. Instead, you read only the fields (parameters) that can cause a security threat. The parameters that contain the message's origin and destination and those used in GTT have particular security importance.

Table 15-2 lists the main SCCP messages that should be screened.

Table 15-2. SCCP Messages to Be Screened

Message	Parameter	Reason for Screening
UDT and XUDT	Calling Party Address	Verifies that the message is received from a specified remote subsystem (such as a specified combination of SSN+SPC).
	Called Party Address	For routing on SSN, verifies that the message is destined for a local subsystem.
		For routing on GT, verifies that the message uses a valid translation table (such as a table allowed for the origin).
	Results of the translation	Verifies that the new values of DPC and SSN match values allowed by the originating node.
SSP and SSA	Calling Party Address	Verifies that the message is received from a specified remote subsystem (such as a specified combination of SSN+SPC).
	Called Party Address	Verifies that the message is destined for the management of SCCP (SSN = 1).
	Affected point code	Verifies that the affected node is inside the originating network.
	Affected subsystem number	Verifies that the affected subsystem is known.
SST	Calling Party Address	Verifies that the message is received from a valid remote subsystem (such as a valid SSN+SPC).
	Called Party Address	Verifies that the message is destined for the management of SCCP (SSN=1).