ITU TFR Procedures

Receiving TFR Messages

If ITU TFR procedures have been enabled for the linkset and a TFR message is received on that linkset, the EAGLE marks the route to the destination as restricted and performs controlled rerouting of the messages that are destined for the destination specified in the TFR message.

If ITU TFR procedures have not been enabled for the linkset and a TFR message is received on that linkset, the TFR message is converted to a TFA (transfer allowed) message and traffic is routed to the destination specified in the TFR message. When this condition is present and a TFR is received on this linkset, UIM 1233 is displayed showing that a TFR was received on a linkset that does not support the TFR procedure.

When a TFR message is received for a route that is already prohibited, and no alternative route exists, the traffic to the concerned node is restarted toward the signaling point from which the TFR message was received.

Invalid TFR messages

The TFR message is ignored under any of these conditions:

• The TFR message is not from an adjacent point code.
• The point code specified in the TFR message is being sent from that same point code.
• The TFR message is from an unknown destination.
• The TFR message is from an adjacent point code, but the adjacent point code is not the route for concerned point code.
• If the route to the concerned point code is already restricted.
• The route to concerned point code not found or is unavailable.

Sending TFR Messages

The EAGLE must send a TFR message containing the affected point code (restricted destination) to all accessible adjacent nodes, whose linkset has the TFR procedure enabled, when the following conditions are in effect:

• When long term failure occurs on the ITU-N linkset (primary) used to route messages to the affected point code. Long term failure occurs when all links of a linkset remain unavailable for more than the amount of time specified by level 3 timer T11.
• While waiting for “long term failure” to be determined, if congestion (or “danger of congestion”) is detected on an alternate linkset used to route messages to the affected point code, then TFRs are sent immediately without waiting for level 3 timer T11 to expire. For example: level 3 timer T11 is set to 30 seconds, the links of the linkset to the adjacent node fail and MSUs are now sent out the alternate linkset. Within 10 seconds of the failure, congestion is detected on the alternate linkset, so TFR messages are sent to each adjacent point code (if linkset has ITUTFR procedures enabled) for each destination (affected point code) routed through that node.
• When an adjacent node becomes accessible by an alternate route, the EAGLE sends a TFR for each destination that is restricted to the node.
• During restarts, TFRs are broadcast to all accessible adjacent nodes for each restricted destination.

Unlike the ANSI network, the ITU national network does not use response method TFR messages. The ITU national network only uses broadcast method TFR messages that are sent to all adjacent nodes under the conditions described above.

Note:

In ANSI networks, response method TFRs are sent to adjacent nodes in response to a MSU, when that node continues to send MSUs after a broadcast method TFR has already been sent.

The EAGLE maintains the status (allowed, restricted, or prohibited) for all destinations. XREF shows the type of message sent when a destination transitions from one status to another.

Table 3-7 Route Management Messages Sent on Status Transition

Status Transition	ITUTFR Procedures Enabled	ITUTFR Procedures Disabled
Prohibited to Restricted	TFR	TFA
Allowed to Restricted	TFR	None
Restricted to Prohibited	TFP	TFP
Restricted to Allowed	TFA	None