Egress Transport Congestion

The Egress Transport Congestion feature manages the egress message traffic flow on a Diameter Peer Connection when the Connection's TCP/SCTP send buffer is exhausted, as indicated by the TCP/SCTP socket being "blocked" (the Diameter Transport Function attempts to write new data to the TCP/SCTP socket fail due to insufficient send buffer space). This can happen for variety of reasons such as under-engineered TCP or SCTP buffers or the inability of the adjacent Diameter Peer to handle the rate of egress message traffic currently being offered on a Connection. In general, this condition should not occur during normal traffic loads, or during abnormal or peak traffic loads if the Per Connection Egress Message Throttling feature is enabled and properly configured for a Connection.

Egress Transport Congestion detection and abatement are solely the responsibility of the Diameter Transport Function.

Message traffic reduction is managed through the use of 5 Egress Transport Congestion Levels: CL-0. CL-1, CL-2, CL-3, and CL-4.

The Connection Congestion Levels CL-0, CL-1, CL-2, CL-3 and CL-4 are mapped to Connection Priority Level (CPL) values 0, 1, 2, 3, 4 respectively, as shown in Table 1.

Mapping Congestion Levels to CPL Values
Attribute	Value	CPL Value
Diameter Connection Operational Status	Available Degraded Unavailable	0 3 99
Diameter Connection Remote BUSY Congestion	CL-0 through CL-3	0-3
Diameter Connection Egress Transport Congestion	CL-0 through CL-4	0-4
Diameter Connection Egress Message Throttling	CL-0 through CL-3	0-3

Diameter messages initiated by the Diameter Transport Function are not impacted by Egress Transport Congestion Levels CL-0, CL-1, CL-2 or CL-3. This includes Peer-to-Peer messages such as DPR/DPA, DWR/DWA and any non-Peer-to-Peer messages such as Diameter Transport Function-initiated Answer responses associated with DA-MP Overload.

The Diameter Transport Function suppresses the creation and attempt to forward any Diameter messages to a Diameter Peer Node when the Egress Transport Congestion Level is CL-4. This includes Peer-to-Peer messages such as DPR/DPA, DWR/DWA and any non- Peer-to-Peer messages such as Diameter Transport Function-initiated Answer responses associated with DA-MP Overload.

The Egress Transport Congestion feature behaves as follows:

Messages that are already committed to the Connection by the Diameter Routing Function when a Connection initially becomes transport congested will be discarded.
When a Diameter Connection socket becomes blocked (such as when a TCP or SCTP socket becomes full), the Diameter Transport Function sets the Egress Transport Congestion Level to CL-4 to prevent the Diameter Routing Function from forwarding any further Request or Answer messages to the Connection.
Any messages received while the Egress Transport Congestion Level is set to CL-4 are automatically discarded by the Diameter Transport Function. This would normally occur for messages that the Diameter Routing Function has already forwarded to the Diameter Transport Function before receiving notification that the Connection Priority Level (CPL) was changed to a value of 4.
When the Diameter Transport Function is notified that the socket is no longer blocked, the Diameter Transport Function sets the Egress Transport Congestion Level to CL-3, and starts Egress Transport Congestion abatement.
The Transport Congestion Abatement Timeout that is configured for each Diameter Connection defines the time spent abating each Congestion Level during abatement. For example, if the Transport Congestion Abatement Timeout value is set to 5 seconds when the Egress Transport Congestion Level enters CL-3, it will remain in CL-3 for the full 5 seconds before the Egress Transport Congestion Level can be reduced to CL-2.

If the TCP or SCTP socket becomes full while the Diameter Transport Function is in Egress Transport Congestion abatement, the entire abatement procedure is restarted.
A throttled event with "Egress Transport Congestion" as the reason for the event is logged every time the Connection Priority Level (CPL) changes due to Egress Transport Congestion.
When the Diameter Transport Function successfully establishes an IPFE TCP or SCTP connection, it sets the Egress Transport Congestion Level for the Diameter Connection to CL-0.

When Egress Transport Congestion occurs, the "Connection degraded" alarm is raised, indicating "Egress Transport Congestion" and the CL.

Note: The "Connection degraded" alarm can be raised by DSR for other reasons, and will not be raised for Egress Transport Congestion if it is already asserted.

When the Connection CL is 0 upon decrementing (the Egress Transport Congestion condition and all other conditions that could raise the alarm are mitigated), abatement is complete and the "Connection degraded" alarm is cleared.

Diameter Configuration for the Egress Transport Congestion Feature

The Egress Transport Congestion feature is always enabled on all DSR Diameter Connections and cannot be disabled by the operator.

For the Egress Transport Congestion feature, the Transport Congestion Abatement Timeout element can be configured for each Diameter Connection, using the Diameter Configuration Connections GUI page. The Transport Congestion Abatement Timeout value is the time period (in seconds) spent by the Connection in abating each Congestion Level during abatement.

The Transport Congestion Abatement Timeout value cannot be modified when the Connection is in service (Connection Admin State=Enabled).